Defence Science Journal, Vol. 53. No. 3, July 2003, pp. 281-324 O 2003, DESIDOC
REYZEW PAPER
Mercury Cadmium Telluride Photoconductive Long Wave Infrared Linear Array Detectors
Risal Singh and Vandna Mittal Solid State Physics Laboratory, Delhi - 110 054
Mercury cadmium telluride (Hg,.,Cd*Te) (MCT) photoconductive long wave infrared linear arrays are still in demand due to several advantages. The linear array technology is well established, easier, economical and is quite relevant to thermal imaging even today. The scan thermal imaging systems based on this technology offer wider field of view coverage and capacity for higher resolution in the scan direction relative to staring systems that use expensive and yet to mature focal plane array detector technology. A critical review on photoconductive n-Hg,.rCdrTe linear array detector technology for the long wave infrared range has been presented. The emphasis lies on detector design and processing technology. The critical issues of diffusion and drift effects, Hi-Lo and heterostructure blocking contacts, surface passivation, and other related aspects have been considered from the detector design angle. The device processing technology aspects are of vital importance.
Keywords: Photoconductor, infrared detector, LWIR, linear array, processing, passivation, overlap, design, blocking contacts, heterostructure, responsivity, noise, detectivity
NOMENCLATURE p Ambipolar drift mobility h Wavelength p Ambipolar mobility o ~oduldonfrequency p Mobility of majority carriers a Absorption coefficient p, Mobility of minority carriers , Surface electrons mobility a, Diffusion length along the direction of electric field r,Bulk lifetime a, Diffusion length against the direction of electric r Excess carrier lifetime/recombination time field T Sweep-out lifetime '1 Quantum efficiency r,,, Intrinsic Auger I lifetime Internal quantum efficiency re, Effective lifetime considering sweep-out effect
9 Angular view of the background Effective lifetime =el.. Signal photon flux rp Minority carrier lifetime Revised 13 March 2003 281 DEF SCI J, VOL. 53, NO. 3, JULY 2003 t,Dwell time Minority carrier diffusion length , Effective lifetime taking into account surface Thermal equilibrium concentration of electrons recombination Intrinsic carrier concentration h Cutoff wavelength Density of surface electrons in the accumulation h Peak response wavelength layer I' A Detector active area iw Concentration of mercury vacancies initially present b Ratio of pe to p, Thermal equilibrium concentration of holes d Detector thickness Optically generated background hole density D Ambipolar diffusion coefficient Excess minority carrier density D* Normalised detectivitv P, Incident power D: Normalised spectral detectivity blackbody radiant power falling on detector D;, Detectivity at high frequency P,, at a temperature T D;,,,,, Detectivity of background-limited infrared Q, Background flux photodetector Q; 300 K background irradiance Dp Minority carrier diffusion coefficient Q, Fixed-charge density D,, Interface trap density Reflection coefficient from front surface AE Energy barrier r, Reflection coefficient from back surface E Bias electric field r, R, Voltage spectral responsivity f Modulation frequency Rv Voltage responsivity fx., g-r cutoff frequency R,, Blackbody voltage responsivity f# FOV R' Detector resistance per unit length fc Corner frequency Re Contact resistance Af Bandwidth in Hertz R, Detector resistance g Photoconductive gain RdCd Depletion region time constant g b Background generation rate per unit volume R, Series load resistor grh Thermal generation rate per unit volume s Pixel rate G Number of ions striking the MCT per second S Surface recombination velocity I Ion milling beam current Transit time of electron across the detector k Boltzmann constant t, Ion beam milling time I Detector length 'elrh T Absolute temperature in K 1, Length of heavily doped end regions in blocking contacts vdt Drift length L Distance between electrical contacts in overlap vs Velocity at which the target image scans the structure strip L' Length of the detector in SPRITE V Volume of converted material during milling RISAL SlNGH & MITTAL: MCT PHOTOCONDUCTIVE LWIR LINEAR ARRAY DETECTORS
V,, Ilf noise The terrestrial objects at 300 K have peak in the IR region at 10 pm in the 8-14 pm atmospheric Vn Preamplifier noise - window. An IR imaging system also works in V, Bias voltage 3-5 pm atmospheric window by detecting radiation VR., g-r noise emitted in the tail ofthe Planck curve, or by detecting reflected IR sunlight during daytime operation. Since V, Johnson noise the wavelength of the detected IR radiation is Vn Total noise voltage many times that of the visible light, this radiation is scattered much less by water vapour or particulates. VT Photo signal Consequently, IR thermal can provide w Detector width superior performance under conditions of fog, clouds, smoke, and dust. Essentially, a thermal image is a AW Signal power two-dimensional mapping of the change in the IR W Element width (SPRITE) radiance of a scene as a function of angular coordinates. The radiance being imaged is the sum ofthe object's graybody emission plus any energy reflected from 1. INTRODUCTION the object. The atmosphere between the source 1.1 Thermal Imaging and sensor attenuates this total radiance. The wavebands of principal interest for thermal imaging A tremendous effort has been made on developing are mid-wavelength infrared (MWIR) (3pm to 5pm) the capability to see during conditions of darkness and long wavelength infrared (LWIR) (8pm to and obscured visibility. The motivation is to extend 12 pm) bands. The useful infrared energy available all daylight activities to periods of darkness, especially at the sensor depends upon the temperature and for activities involving safety and defen~e'.~.This emittance of the source, the spectral bandwidth to is possible through naturallartificial illumination or which the sensor responds and the atmospheric infrared (IR) thermal imaging. The approach transmittance of the path2'. involving amplification of small amount of natural illumination employs image intensifiers and 1.2 Scanning versus Staring requires lighting equivalent to a bright starlit night and clear weather conditions free from clouds, With the development of photovoltaic 2-D focal fog, smoke, or thick dust. The technique of artificial plane arrays (FPAs), there has been a shift in the illumination, though very versatile, suffers from technology focus from photoconductive linear arrays several shortcomings, such as being ineffective in to FPAs. In a staring system, the entire field of conditions of obscurity created by fog, smoke, and view is imaged onto an array of infrared detectors. dust. Also being of active nature, this method of Video signal processing, including pre-amplification imaging betrays the secrecy of presence and location and multiplexing, is usually required to read the of the observer being detected by the enemy in video data of the array. The 2-D FPAs offer advantages security and military environment. like being staring-type, the complex opto-mechanical scanning is avoided, higher output impedance helps An IR thermal imaging system does not suffer in interfacing, compatibility with readout integrated from the above limitations and can detect the circuits (ROICs) and low power dissipation. Also, electromagnetic radiation emitted by all objects. as the staring systems are able to integrate the The blackbody or thermal emission described by incident energy over a relatively longer time interval, the Planck's law is a function of the body's temperature they exhibit the lowest noise equivalent temperature and emissivity. The intensity of radiation and its difference (NETD) (typically < 0.1 K). However, peak frequency increases with temperature. The the staring arrays offer limited field of view (FOV) sun at 6000 K has a peak at emission wavelength and suffer from image under- ampl ling^,^'. The FPA near 500 nm lying in the middle of visible spectrum. technology is still emerging and has a long way to DEF SCI J, VOL. 53, NO. 3, JULY 2003 mature. Only limited production of the FPAs has Table 1. Comparison of scanning versus staring sensor been undertaken the world over because of the Scanning sensor Staring sensor - low yield (high cost) and other technological Wide field of view FOV limited to arrd) sire difficulties. In satellite surveillance and in other Lower sensitivity due to Higher sensitivity due to longer imaging applications, the FPA's staring concept shorter dwell time dwell time holds good for sensors placed in geosynchronous Optics to accommodate Simple optical design orbit where line of sight is stabilised to a point scanner on earth. As the sensor and the earth move Scanner increases size and No moving part, more reliable insynchrony, the image on FPA remains fixed reduces reliability and the moving targets wrt each can be detected. Requires line-to-line Requires individual detector non However, even small motions induced by drift or equalisation uniformity correction jitter of sensor andlor spacecraft cause limitation High resolution in scan Relatively lower resolution direction of this concept, leading to clutter-induced noise. Well matured and simpler Yet to mature and very difficult Also, the range and resolution dictate the use of detector fabrication detector fabrication extremely large optical systems with FPA elements technology technology greater than 10%. Easier to scale up the amy Difficult to scale up the array size size In a scanned system, opto-mechanical scanning of detector element (or elements) is often used in circumvented using micro scanning, but it adds to IR systems to cover 2-D FOV with fewer detectors system complexity. A summaryZoof comparative than would be required to cover whole FOV merits of the two systems is given in Table 1. simultaneously as in a staring system. The scanning systems [which employ both the photoconductive 1.3 Perspective of IR Detectors as well as linear FPA] can be designed to cover large over-sampled FOVs, but reduced integration The two principal types of semiconductor IR time2' (dwell time) can raise NFTD to 0.2 K or detectors are photon and thermal detectors, 0.3 K. The scanned systems with photoconductive depending on the nature of photon interaction with IR linear arrays are still in demand due to several In the first categoly, the photon absorption advantages. The photoconductive linear array causes generation of electron-hole pairs. The change technology is well established and the routine large in electrical energy distribution resulting from this volume production of relatively high yield (low cost) absorption gives rise to an observable electrical arrays by several manufacturers continues. The signal. These photon detectors can be further divided linear arrays are relatively easier to scale up in into four subgroups: (i) intrinsic direct band gap size. like MCT, (ii) extrinsic type like Si(In, Ge) and Ge (Cu, Hg), (iii) photoemissive-type such as metal Two fundamental issues for any detector system silicides and negative electron affinity materials, are sensitivity and resolution. The staring system and (iv) quantum wells (111-V and 11-VI ternary offers improved sensitivity due to its longer integration1 and quaternary compounds). The photoconductive dwell time and can collect photo generated signal and photovoltaic phenomena are the two widely for as much as a frame time, in contrast to a exploited modes of photon detectors. The photon scanner system where detectors are to be shared detectors are highly sensitive to radiation and operate with multiple points in the image space, lowering in a particular bandwidth only. The thermal detectors the dwell time. On the other hand, the requirement fall in the other category where the radiation absorption, of the sampling frequency (twice that ofthe sensor either directly in the material or by the auxiliary cutoff frequency) can be fulfilled easily in a scanner black surface coating, leads to change in some system providing better resolution in scan direction. temperature-dependent properties such as electrical This is not possible in a staring system and affects resistance for thermistor bolometer and the internal resolution. This shortcoming, though may be polarisation for the pyroelectric detectors, and so RlSAL SINGH & MITTAL: MCT PHOTOCONDIUCTIVE LWIR LINEAR ARRAY DETECTORS on. The thermal detectors are generally wavelength- by vacuum evaporation or chemical deposition from independent. a solution. But the phenomenological understanding of these detector materials was rather extremely The IR detectors are passive devices and have poor. The narrow gap single crystal detectors from gained much use in defence and security in the last the alloys of 111-V, IV-VI and 11-VI compounds five decades. For such applications, high performance were aimed at tailor-made bandgap and the approach and fast response are the essential requirements. proved to be very successful. This lead to the High performance is obtained by cooling the detectors emergence of Hg,_CdZTealloy as one of the most to low temperatures, which also raises their cost. important detector material for MWIRILWIR as However, the improved performance of cooled detectors well as VLWIR region^^^^^.^^. Finally, GaAslGaAIAs justifies their higher unit cost. A high sensitivity quantum well infrared photon (QWIP) detectors and fast response detection of IR radiation is achieved have been developed taking due advantage of the using the semiconductor photon detectors. At present, matured state of Gas te~hnology~~-"-~~.During Hg,_CdxTeis one of the most widely used tunable the 1950s, the single element cooled IR detectors bandgap semiconductor for IR photon detector^^^, were made usually from lead salt materials. With both from fundamental considerations and the the development of integrated circuits in 1960s material flexibility. The specific advantages of and the availability of photolithographic techniques, Hgl_CdxTe are the direct energy gap, ability to multiple element linear array detector technology obtain both low and high carrier concentrations, could become possible, mainly using lead salts and high mobility of electrons, and comparatively low InSb materials. The use of extrinsic Ge:Hg was dielectric constant. In the case of direct narrow made to manufacture forward-looking infrared gap semiconductors, the optical absorption is much (FLIR) system^'^ for VLWIR region but with higher than in the extrinsic detectors. stringent cooling requirement up to 25 K. However, for the intrinsic narrow band gap semiconductors Historically, the photoconductive effect was like HgCdTe, the cooling condition could be relaxed discovered in selenium28by Smith in 1873. The to higher temperatures in the range 80-200 K. very first 1R detectors fabricated, using naturally lnspite ofthe difficulties of growth and fabrication occurring PbS or galena and other materials, were technologies, MCT has the advantage over the reported by Bo~e~~in 1904 in a US patent, in other materials in terms of fast response time and which he had described the detectors in almost all compatibility with Si-ROICs. However, MCT wavelength ranges, 'including IR. The first T12SIR photoconductors were not suitable on focal plane photoconductor of high responsivity was developed arrays due to low input impedance and high power by Case30. The initial work on improved photo- dissipation problems. The 2-D FPAs were, therefore, conductive detectors was done mainly in germ an^^',^^ developed based on photovoltaic detectors for the using PbS. The production of PbS detectors was last more than two decades. Of late since early mostly done in USA and UK" and these detectors nineties, the uncooled IR detectors have seized the were used in World War 11. The recent developments attention of many workers due to their vast potential in IR technology were the results of extensive for commercial and military application^^',^^. work done, earlier on TI2&PbS, PbSe, PbTe, Ge:X, Si:X, InSb, HgCdTe, PbSnTe and G~AsIG~AIAs'~. CT Elliott, a scientist working at the Royal The focus of interest had been on two atmospheric Signals & Radar Establishment, UK, invented a windows, from 3-5 pm (MWIR) and 8-14 pm novel IR detector called SPRITE (signal processing (LWIR). Besides, very long wavelength IR (VLWIR) in the element) extending conventional photoconductive region greater than 15 pm was particularly explored MCT detector technology by incorporating signal for space applications. time delay integration (TDI) concept within a single long element. The simple, three-lead structure with From the point of view of the detector technology, a single preamplifier performs the functions of the early detectors based on lead salts were prepared detection and time delay and integration simply DEF SCI I, VOL. 53, NO. 3, JULY 2003 within a filament ofn-MCT which previously required different materials including 111-V and 11-VI. Reine4' a number of discrete elements in a conventional summarised the status of HgCdTe 1R detector serial or serial-parallel scan system with associated technology, with emphasis on recent developments amplifiers and time delay circ~itry~~,~~. in HgCdTe photoconductor technology for use in the first-generation thermal imaging systems. 1.4 Relevance of Photoconductive Detectors There is a comprehensive review on characterisation For the reasons brought out in Section 1.2, techniques, both for PC and PV detectors by Rei~~e~~, HgCdTe photoconductor linear arrays are still in et al. However, there is a clear lack of detailed demand and further consolidation of this technology description of different technological aspects of of scanned linear arrays is relevant even today. the device processing in the literature. The main Several reviews on photoconductors have appeared reason behind this deficiency could be due to the in the literature from time to time9~'0~12,23.24,26,27,45-48. strategic or secret nature of the IR technology, Long and Schmit') reviewed and concentrated almost developed particularly for military applications, and entirely on the development of HgI_CdxTe alloy therefore, the relevant literature is either classified system as 1R detector material. They have also or has limited accessibility. In this review, a sincere discussed the photoconductive and photovoltaic effort has been made to bridge this gap with emphasis detector theories applicable to this material and the on photoconductive detector design and processing early developments of detector technology in this technology. Therefore, a critical review has been area. The brief review by Reine and Bro~dy'~was presented on the subject, specifically on the linear intended to provide basic information on principles array photoconductive detector technology, based of PC and PV detectors operation, the performance on n-(Hgl.xCdx)Tematerial for thermal imaging in characteristics and the summary of the HgI_CdxTe 8-12 pm spectral range at cryogenic temperatures. detector technology. Broudy and Mazurc~yk~~focussed on Hgl.xCdxTephotoconductive detector theory, device 2. PHOTOCONDUCTIVE INFRARED analysis and design, and briefly discussed the detector DETECTORS processing technology, pointing out key steps involved in array fabrication without elaboration. Elliott9Jo 2.1 Standard Configuration provided one of the most comprehensive reviews on IR semiconductor detectors. The photoconductive detector is essentially a radiation-sensitive resistor. Normally, for large number After a brief survey of the many types of of photoconductive devices, the device is designed semiconductor detectors, significant details of the so that electrons dominate the conductivity due to theory as well as the limitations and operating their high mobility, and holes play a secondary role. characteristics of both photoconductive and photovoltaic The operation of a photoconductor is shown in detectors of different materials, including MWIR Fig. 1. Photons of energy hv greater than the band and LWIR Hgl_CdxTedetectors operating at cryogenic gap energy Er are absorbed to produce electron- and intermediate temperatures have been discussed hole pairs, thereby changing the electrical conductivity in this paper. KnowlesI2 reviewed the developments of the semiconductor. The change in conductivity in detector technology with emphasis on require- is measured by means of electrodes attached to ments and operating conditions of IR imaging systems. the detector. To sense the change in conductivity, A summary of Hgl.rCdrTe properties for both a bias current or voltage is required. Typically, photoconductive and photovoltaic (approaches to detectors are manufactured in a square or rectangular FPA) detectors, detector noise mechanism and configuration to maintain uniform bias current distribution performance is presented. There is another quite throughout the active region. A rigorous theory of comprehensive review on intrinsic IR detectors by photoconductors was given by Rittner22to derive Rogalski and Piotrowskiz6. The review includes and solve a one-dimensional transport equation for detector theory, crystal growth technology, material photogenerated electrons and holes in a photoconductor. properties and detector fabrication technology of The first results on photoconductivity in Hg,.$dxTe RlSAL SlNGH & MITTAL: MCT PHOTOCONDUCTIVE LWIR LINEAR ARRAY DETECTORS
SIGNAL generation ofthe photo carriers in the main detector 1-1 1-1 (i.e. n-type), and detector thickness, d, small wrt. minority carrier diffusion length Lp=(Dprp)"', one- dimensional approach was used to study the INCIDENT RADIATION transport of optically generated electron-hole pairs (Ap = An). When the series load resistor, R,, is large compared to the detector resistance, R, a signal voltage across the load resistor is essentially the open circuit voltage. The voltage spectral responsivity, R,, is given byZ6
R K-llhVb (~+b) 7 a - p, hc iwd (brio + PO) dl,) (1)
Figure 1. Schematic of a standard photoconductor, For low modulation frequency 02r2<< 1, showing infrared radiation falling on a slab of o, detector material having volume 1.w.d and the therefore, can be neglected. signal is measured across a load resistor, R,. The r@o of mobility of majority carriers, pe, were reported by Laws~n~~,et al. in 1959. The to mobility of minority carriers, p,, is taken as b. Rittner equation had been and remains important It is quite clear from this equation that the basic in understanding and predicting n-HgCdTe requirements for high photoconductive responsivity photoconductor performance in one dimen~ion'~.~~-~~.at a given wavelength, h, are high quantum A comparison of detector performance using both efficiency, 11, long excess carrier lifetime, r, the one dimesional and two dimensional models had smallest possible size of detector (length, I, width, been rep~rted~~-~~.Gopals7 discussed the biasing w and thickness, d), low thermal equilibrium schemes for photoconductors. For low resistance concentrations, no, andp,, and the highest possible material, such as Hg,.xCdxTe, the photoconductor bias voltage, V,. is usually operated in a constant current circuit, as shown in Fig. 1. The series load resistor meant to Responsivity varies significantly with active limit the detector current is large compared to volume of the detector. The optimum system detector resistance, and the signal is detected as performance is achieved with the smallest size a change in voltage developed across the load. detector capable of collecting the available incident radiation. Responsivity of PC detector is a function 2.1 .I Figures of Merit of bias. At low bias, the responsivity increases almost linearly with bias. At high bias, self-heating Many different figures of merit are used to of the detector eventually causes the responsivity characterise or quantify the performance of a detector. to fall. The optimum bias may vary from application- These figures of merit enable the user to compare to-application, depending on background radiation relative performance between detectors. The important levels. performance characteristics of photoconductive IR detectors are as follows: Two material requirements of particular interest in Hgl.xCdxTe photoconductors are the electron- Responsivity doping level and the composition. The doping level A basic figure of merit that applies to all detectors is particularly important for the responsivity; lowering with electrical output is spectral responsivity. It the doping level increases the responsivity. Thus, is defined as the ratio of photo signal, V,, of the achieving a particular responsivity with a particular detector to the incident power, Pa,of the radiation detector design requires a doping level less than at wavelength, h, falling on it. Assuming uniform some fixed value, which depends on the specifications DEF SCI I, VOL. 53, NO. 3, JULY 2003 and design. An important detector parameter called detectivity and cutoff wavelength specifications combine to determine a material composition window. Responsivity for even identical detectors may vary Here, qo is the internal quantum efficiency, in range over a factor of 2 due to variation in which can be approximated to unity for direct-band material composition. The specified cutoff wavelength materials. For the ideal situation, the reflection and its tolerance limit determine maximum values coefficients r, and r, from front and back surfaces of x and Ax for the material to have a band gap should have values zero and unity, respectively. small enough to absorb the signal photons. The quantum effi~iency~~is then given by the following relation: In case the total blackbody spectrum is falling on the detector, the blackbody voltage responsivity, R,,, would become Applying anti-reflection coating on the frontside of the detector, q can be made greater than 0.9.
Noise where P,, is the radiant power falling on the detector Noise refers to an electrical output other than from the blackbody at a temperature T (normally the desired signal. It is undesirable and a considerable at 500 K) and modulated at frequency f (normally effort had been put in to reduce it, as it may at 800 Hz). The P is obtained by integrating the obscure or completely hide the small signals. The H'! power over the entlre blackbody radiant spectrum potential noise sources are fluctuations in the detector and the solid angle of FOV. It may be noted here itself, in the radiant energy to which detector responds, that when measuring the blackbody responsivity, or in the electronic system accompanying the detector. the radiant power on the detector contains all the The detectivity of a detector is limited by noise wavelengths of radiation and is independent of the mechanisms. Usually, the signal and background spectral response curve of the detector. Some of fluctuations are considered to be responsible for the wavelengths produce an output and others do radiation noise. In a photoconductor, the free carriers not, but the entire flux incident on the detector always exhibit random thermal motion, and because appears in calculation of blackbody responsivity. of this, there occurs a fluctuation in the velocity In practice, a cold filter with sharp cut-on wavelength of these carriers, which leads to internal noise. and a FOV is desired to reduce the background The fluctuations in density of free carriers due to radiation. The detector material decides the higher randomness in the rates of thermal generation and wavelength cutoff. recombination also contribute to the internal noise. There can be four different types of internal noise Quantum Efficiency in a photoconductive detector. These are Johnson noise V,, llf noise VIPg-r noise Vg+ and preamplifier The quantum efficiency, q, is a very important noise Va. Quadratic noise voltage, V:, of a parameter as it is a measure of how efficiently photoconductive detector is composed of these noise photons are converted to electrical signal. It is factor^^^.'^, as given below: defined as the number of electron-hole pairs generated per incident phot~n'~.It depends on absorption 2 2 2 2 2 coefficient a, thickness dand frontside and backside v,, =v,-, +v, +V,,, +vo (5) reflection coefficient^'^,^^. Assuming a single pass (a) Generation recombination (g-r) noise, V*,, is of the radiation in the detector (shown as a slab due to random fluctuations in the generation of material in Fig.1) and negligible frontside and and recombination rates of carriers within the backside reflection coefficients, the quantum detector volume and consists of thermal and efficiency is background contribution^^^, as given below: RlSAL SlNGH & MITTAL: MCT PHOTOCONDLlCTlVE LWlR LINEAR ARRAY DETECTORS
The measured preamplifier output noise (without a detector at input) of 0.6 ~V/HZ"~is reported recently. This noise is comparable to the 0.5 nV/HzU2Johnson noise of a 15 0 detector resistance60.
Generation-recombination(g-r) noise and Johnson noise are fundamental mechanisms in photocon- ductors. In the absence of non-fundamental noise, the total noise of the detector, Vn represented earlier by the Eqn (5) would becomeI5 In above expressions, it is clear that a long lifetime is important in reducing thermal g-r noise below background g-r noise and determines the highest operating temperature at which detector can remain background limited. Frequency dependence is related to the lifetime, r of carriers in the material, and r depends on The equation for Vq., (in a near intrinsic material composition and operating temperaturet6. photoconductor) by s~mplifyingall the terms All HgCdTe photoconductive detectors exhibit can be expres~ed'~as excess low frequency noise which increases approximately as 11'" below a certain corner frequency fc. As frequency is increased above corner frequency, the noise remains constant (called white noise) up to g-r cutoff frequency,& which is given by f,, = (2%~)-I. (b) Johnson-Nyquist noise46(thermal noise), V,is: The schematic of characteristic noise power spectral density for a photoconductor is shown in Fig. 2. The dominant noise at low frequencies is (c) The frequency-dependentnoise, VIfis dominating at low frequencies. The llfnoise is caused by the electronic transitionsinvolving surface states andlor the electrical cdntacts to the detector. g-r CORNER g-r NOISE FREQUENCY It can often be minimised by appropriate fabrica- tion techniques (by surface treatments or electrical llf CORNER contacting procedures). JOHNSON (d) The noise contribution of the amplifier can be NOISE modelled by the presence of a voltage noise - generator of rms magnitude en and a current noise generator in. The voltage generator is in series with amplifier input, while the current LOG FREQUENCY noise generator is in parallel with the input. When a detector of resistance R, is placed Figure 2. Characteristic noise spectrum of a across the input of the amplifier, the noise, Vg photoconductive detector. The dominant noise generated will be46 at low frequencies is I// noise, at mid frequencies the g-r noise, and at higher frequencies the g-r noise rolls-off and Johnson noise is dominant. DEF SCI J, VOL. 53, NO. 3, JULY 2003 l/f noise and at mid-frequencies is the g-r noise. At higher frequencies the g-r noise rolls-off, and Johnson noise is dominant. Discussion on experi- mentally measured noise of a photoconductive MCT detector is given later. At higher temperatures where the detector Noise Equivalent Power becomes intrinsic satisfying the condition Per>> y' + V,' at sufficiently low biases levels, the The noise equivalent power of a detector is performance at mid-range frequencies f
IE +lo 1 10 100 -(pm) WAVELENGTH Figure 3. Theoretical values of Da,,,at spectral peak Q," is the 300 K background irradiance for photoconductive detectors viewing a (- 6 x 1 O"~m-~slfor 7.5-1 1.7 pm spectral window). hemisphere surrounded at different The geometry of the detector and use of cold background temperatures. shield FOV control the background irradiance on the detector. If q = 0.6, d = 10 pm, .r = 2 ps, combination velocity at the detector surfaces. The = no x n, 10" cm-', = 5 IOl4 cm-', then the technique, which is commonly used to passivate background-limited operation should be observed the surfaces, is anodic oxidation. This results in an when Q, > 1 x IOl5 cnrZsl obtainable using a accumulated surface at which bands bend, giving FOV with 8 > 5" for a background scene temperature9 an electric field which repels minority carriers of 300 K. from surface recombination sites, thus reducing In the advanced infrared detector applications surface recombination velocities. State-of-the-art it is usually necessary to approach D*,,,. One cooled infrared detectors are in production at 80- generally wants to achieve the highest possible 90 per cent of the fundamental limit of D* in both value of D*,,, by having the quantum efficiency LWIR and MWIR spectral, bands6'. near its maximum attainable value; 11 + 1. The The above background-limited values of detectivity, D*,,,, can be expressed asI6 D*,,, are based upon an optical acceptance angle of 2n steradians, otherwise known as a hemispherical FOV [2n FOV]. In general, it can be shown3 that
For BLlP operation, the D* is inversely proportional to the square root of the Q,. Therefore, by controlling the background irradiance, the D* of the detector can be improved. Decreasing the number of background The limiting D* in the zero background flux photons incident on the detector, each of which which can be obtained in large detectors62 is: contributes to the g-r noise, reduces the noise. Figure 3 shows the variation of D*,, as a function of cutoff wavelength at different background temperatures. Q, was calculated using equations for spectral radiance of a blackbodyI9.The achievement where .rAi, is the intrinsic Auger 1 lifetime which of background-limited performance depends critically has a value of approximately 1 ms, and taking all on a suitable technology to produce a low re- other values as above DEF SCI J, VOL. 53, NO. 3, JULY 2003
2.2 SPRITE-Photoconductive Alternate to Focal Plane Array
The agreement is excellent with D* values The signal processing in the element (SPRITE) of well over 1012 cm Hz"ZW-l in the liquid nitrogen detector was originally invented by CT Elliott and temperat~re~~. developed further virtually exclusively by British s~ientists~~~~~~~'-'~.It is a highly sensitive IR detector, Gain which facilitates TDI function in a single strip of n-type Hg,,CdxTe. This device is applicable to a The photoconductive gain, g, is defined as serial or serial-parallel thermal imaging first- the number of carriers passing through the contact generation systems. The major advantage of the per each generated electron-hole pair. This gain device is in considerable simplification of the off- describes how efficiently the generated electron- focal-plane electronics with consequential benefits hole pairs are used to create electrical signal. in size, weight, and reliability of the systems. Since, in general pe > ph, the photogenerated electrons traverse through the semiconductor much The relatively smaller number of leads and faster than holes. To preserve charge neutrality, interconnects needed per unit area of the detector and for current continuity, the external electrode stripes on focal plane makes it attractive for large must, therefore, supply electrons from the opposite arrays. The device was conceived as an alternative side. These new electrons may travel faster across approach to hybrid FPAs employing charged coupled the detector than the original holes and cannot devices (CCDs)/complementary symmetry metal recombine with them. If this happens, a gain is oxide semiconductor (CMOS) readout integrated produced which is proportional to the number of circuits (ROICs). As it does not suffer from charge times an electron can transit the detector within storage limitations of CCDs, the device is particularly its lifetime. Typical gain, g, can be written as a suitable for LWIR band. Also since intrinsic function of lifetime of minority carrier, r, and photoconductive mode is employed, the SPRITE transit time of electron across the detector, t,: detector, in principle, can operate at higher temperatures than extrinsic silicon IRCCDs. .
The detector, as illustrated in Fig. 4, consists of a strip of IR-sensitive material (generally n-type Crosstalk photoconductor) on a sapphire substrate4'; it has only three electrical connections, i.e., two bias In case of an array of detectors, when the image contacts and a readout potential probe. These detectors is focused on one single detector, there should be no are used extensively in high performance thermal signal from other detectors. But practically, some imaging systems. If a small area of the strip is signal will be present on other adjacent detectors, exposed to 1R radiation, excess current carriers although it should be very small. This excess signal are generated within that region. is known as the crosstalk. It is generally measured as a percentage of the output or driving signal. A These carriers drift towards the readout area requirement of crosstalk less than 5 per cent can be of the strip at a velocity determined by the bias easily met but values less than 0.05 per cent are hard current and matched to the velocity at which the to meet. Crosstalk can be due to the optical effects target image scans the strip, vs. Consequently, the (eg, reflections from the detector on which the image excess carriers are swept along the strip together is focused) and the electrical effects (eg, capacitive with the target image. In this way, the accumulated coupling between the signal leads). This effect becomes carriers arrive at the readout area at the same very important when designing an array with closely time, eliminating the need for the external delay spaced detectors and will put a limitation on the and summation circuitry, and reducing the pre- detector size and the pitch". amplifier output connections to one. RlSAL SINGH & MITTAL: MCT PHOTOCONDUCTIVE LWlR LINEAR ARRAY DETECTORS
coherent integration of signal and incoherent integra- IMAGE SCAN VELOCITY vs READOUT tion of the noise. In the background-limited detector, - REGION the excess carrier concentration due to background AMBIPOLAR VELOCITY (V = v~) also increases by the same factor, but corresponding pa noise is proportional only to integrated flux. As a result the net gain in the SNR wrt to a discrete element is increased by a factor TIT^,^^,)"^.
DRIFT REGION D*, can be expressed in terms of pixel rate, CONSTANT CURRENT BIAS s, which, for a nominal resolution size of W x W is v/W, then in the high scan speed limit
The performance of the SPRITE device is t YI thus equivalent to a serial array with the number 5 qo,r[l -exp(-x/vsr)] n -___----- of BLIP-limited elements (N = 2sr). For example, BACKGROUND in a system with a 4 MHz pixel rate and assuming as = a 0 qo,~[l -exp(-xlv T)] r 2 ~s,the simple three lead structure with a 4 0 _-_-----i-- U 0 single preamplifier should give equivalent perfor- 0 V) YI 0 r0 mance to a serial row of 16 background-limited W / cc " / 0- photoconductive detectors with 16 preamplifiers 5 JHO and the associated delay circuits64.
The material requirements for the device are determined by the need for large lifetime r to provide long integration, and low minority carrier Figure 4. Theoperating principle of a SPRITE detector: (a) an MCT element with three Ohmic diffusion length to provide good spatial resolution. contacts, and (b) the build-up of excess carrier The diffusion-limited resolution size is approxi- density in the device as a pixel of the mately twice the diffusion length. image is scanned along it. The movement of image matches the drift of minority carriers Parallel arrays of SPRITE can be substituted along SPRITE filament. for discrete element arrays in serial parallel imaging systems4'. The principal benefit of SPRITE in systems The length of the detector, L, is typically close has been in simplification of the electronics and to or larger than the drift length v,~,where r is gain in detectivity. Two major advantages of these the recombination time. The excess carrier concentration close-packed two-dimensional devices are: (i) signifi- in the material increases during scan. When the cant reduction in the number on interconnects as illuminated region enters the readout zone, the compare to conventional array and (ii) the high increased conductivity modulates the voltage on responsivity levels which facilitate the fabrication the readout contact and provides an output signal. of low power consumption buffer amplifiers. The integration time approximates the recombination time r for long detectors. It becomes much longer 3. DETECTOR DESIGN: SOME CRITICAL than the dwell time T~,~~~on a conventional element ISSUES in a fast scanned serial system. Thus, a proportionally larger (aT/T~,~~,) output signal is observed. Therefore, Quality performance not only demands optimum the SPRITE detector continuously performs a TDI sensitivity, it also mandates product reliability. Surface function and the SNR is improied as a result of passivation, contact metallisation and reliability of
293 DEF SCI J, VOL. 53, NO. 3, JULY 2003 wire bonding are the three main factors affecting the generation-recombination noiselsweep-out limited the stability of the MCT detector. Detectors are detectivity. When the generation-recombination noise designed keeping in view all the unit processes, is reduced below the Johnson noise, the detectivity which are responsible for achieving the desired becomes Johnson noiselsweep-out limited. detector parameters. Some of the important issues related to detector design are discussed below: The effective carrier lifetime can be reduced considerably in detectors where the minority carrier 3.1 Diffusion & Drift Effects diffusion length exceeds the detector length, Lp. At higher values of the applied field, the drift length Equation (1) shows that R,,should increase of the minority carriers is comparable to or greater monotonically with the V,,. However, there are two than Lp. Some of the excess minority carriers are limits on applied bias voltage, thermal conditions, lost at an electrode, and to maintain space charge i.e., Joule heating of the detector element, and the equilibrium, a drop in excess majority carrier density sweep-out of minority carriers. The effects of contacts is necessary. This way the majority carrier lifetime and of drift and diffusion of excess carriers cannot is reduced. The lifetime degradation problem is be ignored. For small detectors used in high resolution associated with Ohmic contacts and can be minimised thermal imaging systems with 1 < 50 pm, the minority using: (i) an extended (overlap) structure and (ii) carriers can drift to Ohmic contacts even at moderate blocking contacts (Hi-Lo or heterojunction). Sweep- bias field (Fig. 5), in a time short compared to out significantly reduces detector responsivity and recombination time in the detector material. Removal increases the material and temperature requirements of minority carriers at an Ohmic contact in this necessary for the detector to achieve background- way is called the sweep-out effect's~50~75-79. limited detectivity. Various ~orkers'~~~~~~~-~~have carried out analysis of the influence of the sweep-out on photoconductor performance. The expressions for voltage responsivity, noise and normalised detectivity are applicable, if /pY& the term for lifetime, T is modifiedIs by r6 --- OHMIC -Li2 - OHMIC X CONTACT CONTACT
Figure 5. Excess carrier distribution in a detector element with Ohmic contacts: At E = 0, it is uniform for L >> Lpand significantly reduced with symmetric bell-shapedfor L S 24.At E >O and L < 2Lp, it gets skewed towards cathode. When the material is close to intrinsic or the This results in reduction of responsivity and background flux density is very high, n m p and generation-recombination noise and also limits the pQ a 0. For n-type photoconductors with a large maximum applied voltage, V,,. Because the rate of mobility ratio (b >> 1) and n >>p, Eqn (25) become^'^, reduction of the generation-recombination noise is lower than that of the responsivity, it results in reduction of detectivity. The loss of detectivity is especially significant when the Johnson noise becomes dominating due Modifications of standard device geometry have to reduction of generation-recombination noise. For been discussed as an approach to reduce the undesirable moderate sweep-out conditions, the detectors exhibit effects of carrier sweep-out on device performance. RlSAL SINGH & MITTAL: MCT PHOTOCONDlJCTIVE LWlR LINEAR ARRAY DETECTORS
Use of asymmetrical overlap on cathode side and 112 the n-n+blocking contacts can reduce these undesirable where alS2=$*[($I + &] ( 29) effects. Both of these features enhance the effective lifetime of the carriers without compromising the resolution. For the purpose of analysis, the effective lifetime may be defined by 3.2 Contact Schemes 3.2.1 Extended/Overlap Contacts The Ohmic contacts are regions of high recombination. It is, therefore, desirable to isolate the contact regions from active area boundary. In For large values of detector length, 1, the the extended/overlapconfiguration, the distance between effective lifetime approaches the bulk lifetime. these electrical contacts is extended to L beyond the However, as I approaches Lp, the effective lifetime active window length I without compromising active decreases rapidly and for I = Lp, the value of window size (and resolution), and thus increasing the r,,/7 is approximately 0.1. minority carrier transit time, ro.An optical mask is Figure 6 illustrates the variation of effective used to define active area to achieve required spatial lifetime versus detector length for various values resolution. The advantages of such a structure compared of bulk lifetime. It is apparent that for lower to a conventional photoconductor can be realised by geometry detectors a serious problem of lifetime a simple analysis of the voltage responsivity of the degradation exists, with values of refi overlap detector. limited to 1 650 ns, even for r as high as 8 ps. For a conventional photoconductor with zero applied bias field, and for detector length, I, much more than minority carrier diffusion length, Lp, the signal photon flux, 4, generates a virtually uniform density of excess minority carriers along the length. As the detector length approaches minority carrier diffusion length, the distribution of excess minority carrier significantly deviates from uniform profile across the detector length to non-uniform profile with zero carriers at the contacts and the density of excess carrier at the centre of the detector element also decreases, as it is now less than the diffusion length away from the contacts. Further deterioration of this situation occurs, when an electric field is applied, and sweep-out of minority carriers takes place. The excess carrier density is further reduced and the distribution is skewed towards the negative electrode. This phenomena was analysed by Rittnerz2and the excess minority carrier density, Ap at any point x along the detector element is I (MILLS) given byT9 Figure 6. Variation of effective lifetime versus detector length for various values of bulk lifetime. T~ is strongly dependent on detector length (< 75 pm). r, is reduced to s 650 ns even for a good lifetime bulk material. DEF SCI I, VOL. 53, NO. 3, JULY 2003
I - OPTICALLY ACTIVE AREA 1 OPTICALLY ACTIVE AREA I - L - DISTANCE BETWEEN k L - DISTANCE BETWEEN I L OHMIC CONTACTS OHMIC CONTACTS L (+I I- (-) (+) k d (-) PASSIVANT METAL , , METAL METAL I I , METAL
n-MCT
I SAPPHIRE 1 SAPPHIRE
Figure 7. (a) Schematic of symmetric overlap structure, the distance between Ohmic contacts, L is more than the length of active are#, I and (b) maximally asymmetric overlap structure, it yields optimum performance enhancement. This problem is further compounded for The detector voltage responsivity A, variation 0.1 eV detectors that are required to operate at with detector element length, L, is of prime importance near-ambient background photon fluxes. Under such while bias power is held constant. For signal power conditions, the Auger mechanisms limit the attainable ~W=,$~hvlwand constant bias power p= J/R', bulk detector lifetime'9 to values of \< 1 ps. For where R' is the detector resistance per unit length, example, for a detector of length 50 pm this results the voltage responsivity is given by in the values for rcn 5 400 ns.
This degradation of lifetime can be reduced by a modified geometry of the detector as shown in the Fig. 7(a) for a symmetric overlap structure. Where the Ohmic contacts, shown at a distance L The effective lifetime increases as a function apart, do not define the optically active area of the of L2 up to values of L satisfying the condition detector, which has a comparatively lower dimension L/Lp-5. Beyond this, effective lifetime increases I. This optically active area is defined by a metal as a function of L'I2. The detector length, therefore, overlay of the insulating passivant layer. Thus, the can be increased up to approximately five times optically active volume is displaced from the Ohmic the minority carrier diffusion length with enhancement contact regions with their associated high recombination of voltage responsivity at constant detector bias velocities. The photo-signal associated with a signal power. flux density of I)> photons/cm2 s for a signal power79 For operation in the sweep-out limit, the effective AW = +,hvlw is given by the equation: lifetime is equivalent to the transit time for photo- generated disturbance across the detector element, and for a symmetric overlap structure ren5L2/2pY. For a grossly asymmetric overlap geometry, as shown in Fig. 7(b), ~~~5L21pV. Then, for near where AN JV= ll+,reflw intrinsic material or detector operating under high background fluxes such thatpo+nothe responsivity and N = n,Lwd will show a further increase. In comparison to standard structures, the overlap ll1 structures offer several advantages, such as: (i) therefore, AV = V- X- ' nod L (32) reduced responsivity saturation with increasing field, RlSAL SlNGH & MITTAL: MCT PHOTOCOIVDUCTIVE LWlR LINEAR ARRAY DETECTORS
(ii) lower D* dependence on field, and (iii) reduced In the case of blocking contact, the photo- Ilf noise corner frequency but only marginally generated electron-hole pairs are not annihilated affecting response time constant. However, the in the immediate vicinity of the metal-semiconductor increased detector length increases both the detector contact. Such a contact may be partially or even power dissipation and noise at a fixed bias. Thus, totally reflecting minority carriers as a result of there is a severe limit to how much the length can built-in electric field opposing the collection of be increased and by what factor an increase in minority carriers at contacts. In the case of total responsivity can be achieved. Kin~h~~,et al. estimated reflection, the electron-hole pairs are virtually forced the critical detector element length associated with to recombine eventually in the bulk of the semiconductor, thermal D* improvement to be approximately three due to finite lifetime, and the responsivity ismodified times the minority carrier diffusion length. Acornparison accordingly. The built-in electric field results from of standard versus overlap structures is summarised a decreasing majority carrier concentration profile, in Table 2. from the metal contact into the semiconductor bulk.
Table 2. Figure of merits for.standsrd versus overlap The detectivity decreases as intrinsic carrier structures concentration, N,, is increased for all values of Figure of Standard Overlap surface recombination velocities, Ssr,,.This decrease merit in D* is the result of increased free-minority carriers, Responsivity Sweep-out saturation No sweep-out saturation leading to increased thermal g-r noise. The rate (Rv) (Rt, 9,d) (1.25 -5.0 x R, ,#) of decrease of D* with N, is much less for detectors Dnectivity Fast decrease with E Very slow decrease with E D' (Dfud) (1.2-2.5 x D:,,,) with small value of S,," than for Ohmic contact Iljcomer Fast increase with E Fast increase with E detector. For the highly blocking contact detector frequency 6 *,d) (< 113 X& d) (also longer effective lifetime) when the background g-rnoise dominates the thermally generated contribution, Resistance Low (r,,,,) High (3.0- 5.0 x r,,) the effect ofN on D* is relatively less. In comparison Response time Low High to Ohmic contacts, the blocking contacts5' enhance responsivity and D*,allow to use material with 3.2.2 Blocking Contacts higher carrier concentration and wider compositional In a conventional photoconductor, two Ohmic variations, Ax. These features have led to improved contacts are applied at the ends ofthe semiconductor D*, R,,, lower llfnoise corner frequency and other slab. According to the definition of an Ohmic contact, benefits in the fabricated array^".^^-^^. Contacts the excess carrier density at the contacts is zero, can be formed on n-MCT by either creating an Hi- or the surface recombination velocity is infinite, Lo n-n+ barrier (by indium diffusions6or ion beam which leads to drastic reduction in effective lifetime miIling87.ss)called Hi-Lo blocking contacts (HLBCs) of the carriers. To enhance the lifetime, the contacts or by forming heterojunction barrier called as of carrier blocking or reflecting type are desirable. heterojunction blocking contact (HJBC). The n-n+ blocking contacts create a barrier to the flow of minority carriers while offering no resistance 3.2.2.1 Hi-Lo Blocking Confacts to the majority ~arriers~"-~~.This decreases the (a) HLBC by Diffusion recombination velocity at the contacts, SS,>,,which effectively increases the lifetime of the carriers. Indium is frequently used as contact metal in K~mar~',~~,et al. examined the relationship between n-type MCT pbotoconductive detectors to form contact recombination velocity and the n'ln blocking Ohmic contacts, During the low-temperature annealing contact structure. For a given carrier recombination cycle of device processing technology the indium velocity at the metal-semiconductor interface, both present at the contacts diffuses into the lattice. the doping ratio of the n'ln region and the thickness Concentration gradients and consequently built-in of the n+ region need to be optimised to improve electric field of 10-20 eV cm-' at 77 K are formeds6. the detector performance. These fields modify the distribution and surface DEF SCI J, VOL. 53, NO. 3, JULY 2003 recombination velocity of photo-excited carriers. Excess photo-excited minority carriers are pushed away from the surface and electrons are attracted towards its9.An n-type photoconductor with heavily The direction of this internal field is such as doped-end regions is shown in Fig. 8. to oppose collection of minority carriers (holes) by the metal contacts (at coordinates 0 and I). Majority carrier electrons are nearly unimpeded by such REGION / REGION ? contact. This results in reduction in the recombination HIGH-LOW velocity of photo-generated holes and increase in effective lifetime. The measured effective lifetime of holes increases approximately by a factor of two. A comparison of various figures of merit of detector for Ohmic versus blocking contacts is presented in Table 3.
Table 3. Figure of merits for Ohmic versus blocking contacts Figure of Ohmid Blocking merit Standard contacts Responsivity (R,) R, ,d 3.0 R, ,d Detectivity (D') D. .%rd 1..5 x DSud Carrier conc. range (cm") < 4 10'' 4 - 20 x lo" Composition variation ? 0.006 (X = 0.2) Response Fast Slow
Figure 8. Impurity profile and corresponding energy However, obtaining reproducible clean MCT diagram of one-dimensional n-type photo- surfaces in the contact regions of an array before conductor with heavily doped n+ contact regions. metallisation is quite a challenging task. Also, it The optical excitation occurs in region 3. The has been observed that indium contacts degrade as n+ regions (I, =I, <.c I,) are approximated by a uniform step junction profile with constant a result of thermal annealing. Gold used for fanout electron concentration N,'. pattern and contact indium form an intermetallic compound, which modifies the contact resistance The excess majority carriers in the vicinity of to the extent that the total detectorlcontact impedance the contacts result from in-diffusion of donor during become unstable with time, leading to long-term the processing of the device. Accordingly, the device reliability problem of the device. Therefore, the is divided into three regions, with optical excitation alternate metallisation schemes such as Cr-Au or taking place in the central region 3 only. It must ZLPf-Au have to be used to address the long-term be emphasised that I, = I, << I,. As a result of this reliability issues. majority carrier profile, an internal electric field E,, must exist in the above regions, in addition to any (b) HLBC by Milling applied field. This is equivalent to an energy barriers9 Work on MCT surface modification and conversion AE, which is given by ofp-type MCT to n-type and n-type to n4-type by low energy ion bombardment was reported earlier by some ~orkers~~,~~.~~-~~.A detailed study of the dependence of the conversion depth profiles on ior RlSAL SINGH & MITTAL: MCT PHOTOCONDUCTIVE LWlR LINEAR ARRAY DETECTORS beam milling conditions was presented recently by but the converted region depth increases linearly Mitta19>,et al. The depth to which these changes with ion dose, for constant beam energy. occur depends strongly on ion milling conditions. The local damage due to impinging ions is restricted Type conversion is found to take place rather to a distance of the order of the ion interaction too fast and deeper than expected from the process range, while the converted zone is much deeper. of chemical diffusion of Hg in HgCdTe, especially The rapid diffusion of mercury freed from the when the sample is cooled to about 13 OC during lattice is believed to be the cause ofjunction formation. milling. The milling process etches out the top Many a~thors~~~~~"~~~~have reported detailed discussions layer from HgCdTe sample by the sputtering process on junction formation for p-type MCT. However, as a result of momentum transfer by bombarding very few reports are available on the effect of ion ions to the lattice atoms. Due to non-stoichiometric beam etching on n-type HgCdTe. To select the ion etching, free Hg atoms are left on the surface. beam current density and etch time for proper The energetic bombarding Ar' ions may transfer junction formation, a detailed study of the carrier their kinetic energy during collision to the free Hg concentration profile of n+lnjunctions induced by atoms (in the direction of lattice), thus resulting in- ion milling process under different process conditions diffusion of these as Hg interstitials at a diffusion was carried out by differential Hall meas~rements~~-~'. rate much higher than predicted thermodynamically. The depth of modified region increases almost In addition, collision ofAr+ions with lattice atoms linearly as a function of ion dose. Formation of a in the sample creates the Frenkel defects (mercury strong n-doped region is observed even for very interstitial-vacancy pair) and loosing their energy small milling doses. The increase in the ion dose in the process. The amount of free Hg atoms (beam current x etch time) results in an increase generated within the etched region during milling in carrier concentration for these samples by would depend on ion dose. The junction depth, in 3-4 order over the base carrier concentration values turn, is a complex function of the amount of free of 2-4 x IOl4 cm-). Hg atoms, their kinetic energy, etc. The qualitative explanations and models presented so far are insufficient Blackmans8, er al. have shown that the volume, to predetermine the depth profile and junction depth V, of converted material follows a simple law: of the converted region.
It is pointed out here that the increased detector length in the extended contact scheme increases both the detector power dissipation and noise at a fixed bias. In the nln' contact scheme, an energy If r is the time of bombardment and A is the barrier to minority carriers is generated in front of area, then the junction depth, d, for large area is metal by heavily doping of this region. The lifetime given by in heavily doped region severely degrades due to Auger recombination, limiting its effectiveness. Thus, both the extended contact scheme and the blocking contact scheme can lead to increased responsivity, but neither scheme eliminates sweep-out nor is where (GI A) = J, is the current density, and G x t close to an optimal approach. In all the three approaches is defined as the ion dose. used to lower sweep-out effect, the maximum increase Mitta195,ef al, have shown experimentally that in responsivity is approximately a factor of the similar theory (as given for p-type material) (ii) Heferostructure Blocking Contacfs holds good for n-type material also, the integrated ion dose is an important parameter, as reported Honeywell Gro~p~~,~~,~~presented an experimental earlier. The change in carrier concendation tends and theoretical study of n-type Hgl.,CdxTe to saturate at an ion dose of 8 x 10Is ions/cm2, heterostructure photoconductors in which a non- DEE SCI J, VOL. 53, NO. 3, JULY 2003
absorbing large band gap alloy was grown on top and detectivity is pushed toward the background of a smaller band gap active region. The contacts limit. At higher applied bias fields, the responsivity were made to the larger gap material. The larger of HBC photoconductors improves in comparison band gap material causes an energy barrier to to Hi-Lo junction photoconductors. holes, which decreases the rate at which they reach the high recombination region of the metal-semi- MuscaR5,et al. have used more practical conductor interface. As a result, this heterojunction approach to analyse HBCs, assuming a finite contact greatly reduces the effects of carrier sweep- recombination rate at metal/semiconductor interface out on device performance and leads to much higher and presence of recombination centres at hetero- detector resp0(lsivities. As an example, for interface (due to lattice stress because of change Hg, =CdrTephotoconductors made from lower narrow in the lattice constant). Also, the surface shunting gap layer with x = 0.2 and a larger band gap top of detector due to the presence of either a wider layer ofx- 0.24, the barrier height with this Ax = 0.04 band gap capping layer and/or passivation layer at is about 9 kT. This barrier is 2-3 times that is the surface of the semiconductor active layer has achieved in the Hi-Lo juncti~n~'.?~-~~.Both the layers been taken into account. It is very much clear that are n-type having approximately similar doping by applying HBC, the recombination velocity decreases densities and heterojunction interface is graded and the effective lifetime increases. Change in over a larger depth compared to depletion width. these two factors in a one-dimensional model The energy barrier, approximately equal to the band [Eqn (I)] can explain both Hi-Lo and heterostructures. gap difference of 0.04 eV, occurs in the valence Responsivity increases by a factor of two for band, as shown in Fig. 9. When an electric field photoconductors with heterostructures as compared is applied, photogenerated minority carrier holes to the ones with Hi-Lo contacts. Also, the improvement encounter a potential barrier at the heterojunction in responsivity is more than an order of magnitude at low-bias fields in a near ideal HBC photoconductors as compared to devices with non-blocking Ohmic x, = 0.22 contacts. HETEROSTRUCTURE x, = 0.24 (x, - XI) ------3.3 Detector Resistance In its simplest form, the n-type MCT photoconductor consists of a rectangular piece of material of thickness d, with an active area defined by the width, w,and the electrode length, I, as shown in Fig. 1. The detector resistance, R,, is given by
Figure 9. Energy band diagram for an n-n+heterostructure with x, = 0.220 and x, = 0.240. Usually there is a surface accumulation layer interface, which prevents these from reaching the placed over the surfaces of the HgCdTe for passivation metal/semiconductor interface, thus effectively reducing purposes. Passivation reduces surface recombination the sweep-out effects. However, virtually no barrier and llfnoise and renders the surface impervious exists for electrons. Thus, the barrier reduces the to environmental conditions. For example, anodic rate at which minority carriers are lost at the metal- oxide can be used for this purpose. The positive contact, which, in turn, results in decrease in surface fixed charge in the oxide layer produces a strong recombination velocity and increase in the effective accumulation layer99,'00on n-type HgCdTe. This minority carrier lifetime. Consequently, responsivity accumulation layer has an electrical resistance that and background generated g-r noise, both increase adds in parallel to that ofthe bulk HgCdTe material. RlSAL SINGH & MITTAL: MCT PHOTOCONDUCTIVE LWlR LINEAR ARRAY DETEC~I-OKs
If n, is the density of surface electrons in the can lead non-uniformity in the responsivity, while accumulation layer and pJ is their mobility, then the high values of contact resistance can lead to degradation detector resistance becomes in the D* and also adds to the cooler heat load.
3.4 Surface Recombination Velocity In a thin HgCdTe photoconductor, the surfaces are very near to the excess carriers, therefore where Re represents contact resi~tance~~.Contact recombination at these surfaces can limit the attainable resistance in n-type HgCdTe photoconductors has lifetime. If is the HgCdTe lifetime, then the two origins: (i) interface resistance between the resulting lifet~mewith surface recombination is4? contact metal and the HgCdTe material, and (ii) the current crowding that occurs when the 1 1 2s,m direction of the current changes from parallel to -=- +- the detector surfaces to perpendicular to the contact net r,, d surface. Usually the contact.resistance term is small (10-1 5 R) compared with the HgCdTe material where S,,,, is the surface recombination velocity. and surface terms and in most cases, the contact A value of SSn,as small as 500 cm s1would limit resistance has no deleterious effect on device the lifetime of carriers in a 10 pm thick detector perf~rmance~~~'~~. to 1 ps. Also, the recombination at the contacts can be important in many cases. Room temperature detector resistance (RTR) measurements can be used as the first criterion in At large values of ST_,the responsivity saturates the selection of good detectors for further with increasing bias field because the effective characterisation at low-temperature measurements. lifetime becomes an average carrier transit time. Uniform resistance values (close to the calculated As the contact recombination velocity is decreased, detector resistance plus two times Re, with the effective lifetime is limited by the rate of hole Rc-10-15 R) at room temperature across the passage into the contact rather than the rate of array indicate no failure at device processing stage. transport to the contact. Smiths1 has shown the Then, the low-temperature resistance (LTR) variation of responsivity, noise and detectivity at measurements give an idea about the quality of low frequency as a function of applied bias field detector material and contacts. with surface recombination velocity as a parameter. Both responsivity and noise increase rapidly as It has been observed from our experience that SJrVis decreased below a few thousand centimeter for detectors showing high responsivity, low noise, per second. Thus, the maximum value of responsivity and high detectivity, the value of LTRIRTR ratio increases as S_, is decreased. If the limit S,,,, -t 0 should be around two. The large increase in this is taken,'the holes cannot pass into the contact at ratio beyond two is an indicator of either the poor all, and the responsivity does not saturate with adhesion of the metal contact to HgCdTe that is increasing bias field but continues to increase linearly. adversely affected by thermal cycling or caused by damage to the material. Also, the improper The generation-recombination noise at large metal step coverage on the HgCdTe slope while Ssrvdoes not saturate with increasing bias but the contact runs down to the epoxylsapphire can increases like d~,when there is a large thermal also contribute to such phenomenon. Pallo2,et al. generated contribution to the g-r noise. This occurs had performed detector-resistance measurements because the thermal contribution to the g-r noise at 77 K for variable incident flux on the detectors depends on the square root of the effective lifetime and from these measurements calculated the values and the background contribution to the g-r noise of contact resistance, shunt resistance, and qr depends linearly on the effective lifetime, [Eqns product. Variations in shunt resistance and qr product (6)-(S)]. As the recombination velocity is decreased, DEF SCI J, VOL. 53, both the thermal and background contributions to are of great importance. It must exhibit excellent the g-r noise increase, but the background contribution transmission in the relevant spectral region. A good increases at a faster rate5'. passivation should also exhibit radiation hardening to avoid degradation of the detectors in harsh As the thermally generatedg-r noise contribution environments, such as space, etc. to the Ohmic contact detector (with Ssp,-+ oo) is significant, the low frequency detectiv~ty,D*, is The HgCdTe surface passivation is complex less than D*,,,,!, and D* decreases with increasing because of the compound nature of the semiconductor, field. Decreastng Ssp,,increases the responsivity the difference in the chemical properties of the and background-generated g-r noise faster than constituents, and also due to the tendency of electrically the thermal g-r noise and D* becomes nearly equal active defect formation in the interface region prior to D*,,,,,when the background-generated to and during passivation process. Defects movement g-r noise dominates the thermal-generated contribution. from bulk to surface and vice versa, even at room At larger bias fields, it is necessary to go to smaller temperature, is a severe problem. This is directly values of ST,,, to reach this condition. related to the weakly bound and highly mobile mercury atom. The HgCdTe surfaces cannot be exposed to 3.5 Surface Passivation Technology temperatures above SO "C, a limit imposed to control the loss of mercury due to weakness of mercury- Development of suitable passivation scheme is bonding in the lattice and the properties of atomic very crucial in fabrication of photoconductive arrays mercury such as its low melting point and high having uniform and high performance. The knowledge vapor pressureio3. The band gap of HgCdTe for of interface and optimum interface properties in a LWlR is of the order of 100 meV, which is almost particular situation is very essential for the development equal to surface band bending and it is very easy of a device. In addition to achieving the excellent to accumulate, deplete, or invert the surface. interface properties, surface passivation technology also needs to be assessed in terms of uniformity Passivants employed for the passivation of HgCdTe and capability of producing multi-element arrays photoconductive detectors result in repulsion of with minimum detector-to-detector variations in photo-generated minority carriers from surface, responsivity and specific detectivity. Several authors'"-'09 thereby reducing surface recombination velocity- have reviewed the surface passivation technology desirable effect. But at the same time, a passivant suitable for MCT material. forms an accumulation layer of majority carriers which acts as a surface shunt resistance parallel The passivant layer plays an important role in to detector resistance-an undesirable effect. controllingthe defects and providing physical, chemical, Pal,IBoet al. developed a model and shown that the and electrical stability to the semiconductor surface. optimum performance of a photoconductive detector High densities of fixed charges and interface states is obtained for the values of fixed charge density, are observed in the disordered surface region. Surface Q,, and interface trap density, D8,,of the order passivation controls the surface electrical properties of 5-10 x 101° and 1 x 10'" cm-2eV-',respectively. of the semiconductor by reducing density of fixed Hence, the requirements for a suitable passivant charges and interface states, thus lowering down layer should be such that it provides passivation to the surface recombination velocity. A good passivation the detector surface. This results in reduction of should yield well-controlled, stable and spatially llfnoise and preservation of bulk lifetime. Also, uniform interface electrical properties. It forms a it encapsulates the MCT material, thus preventing chemically and physically protective layer over the its decomposition and protecting the device from active areaof the deviceand shields it from environmental external contamination. conditions. Passivation layer must have adequate physical, thermal, and chemical stability compatible The single most important mechanism that limits with device processing.- In front illuminated detectors, photoconductor performance is carrier recombination the optical properties of the passivant materials at the device surface, which effectively reduces
302 RISAL SINGH & MITTAL: MCT PHOTOCONDUCTIVE LWIR LINEAR ARRAY DETECTORS - PHOTOSIGNAL
CAPACITANCE
4
- 40 - 30 - 20 - 10 0 -25 -20 -15 GATE VOLTAGE (V,) (V) GATE VOLTAGE (V,) (V)
Figure 10. (a) Photosignal and capacitance versus gate voltage for 0.1 eV n-MCT photoconductor at 77 K and (b) photoconductivity signal versus gate voltage for 0.3 eV n-MCT photoconductor at 77 K. the minority carrier lifetime. It is well known that element -20 pm thick, using n-type Hg,.rCdrTe the surfaces of a semiconductor are regions where (Ex= 0.leV) with a bulk lifetime of - 3 ps, with recombination can proceed at a higher rate than the upper surface completely controlled by a trans- in the bulk. Surface recombination reduces the parent field plate on a 1 pm layer of native oxide total number of steady state excess carriers by plus ZnS. This MIS structure has a flat-band voltage effectively reducing the average recombination time. of -27 V. The photosignal and capacitance versus In fact, much of the technology of the present-day gate voltage for this sample are shown in Fig. lO(a). device fabrication aims at minimising surface The photosignal is observed to be relatively flat in recombination of minority carriers by appropriate the region of surface accumulation. However, on chemical and mechanical surface preparation methods, changing the gate voltage from accumulation to and subsequently passivating the surface with a depletion condition, the photosignal decreases native or a deposited insulator film, or a combination dramatically, being reduced by more than an order of both10s. of magnitude for a strongly inverted surface condition. A typical depletion region time constant (RdC,) for It must, however, be pointed out that the surface this material is 2 x 1W7s at 77 K. potential can have a drastic influence on the bulk photoconductive lifetime even if no surface states The higher signal in the accumulation condition are present'". This becomes obvious if one considers is due to the effect of the built-in transverse electric the dark current flowing in a surface region, which field, which drives the minority carriers away from is under strong inversion. The electron-hole pairs the surface towards the bulk, resulting in increased generated by incident photon flux within the depletion lifetime. The wider gap compositions of Hg,_Cd,Te region or within the diffusion length from it will be at 77 K, on the other hand, can exhibit photosignal physically separated and will recombine at a rate enhancement upon surface inversion, relative to determined by the response time (RdCd) of the the accumulation condition, as shown in Fig. I0 (b), depletion region. Under thermal equilibrium, this because the values of the depletion region time time constant will be controlled by the dominant constant in this case are considerably in excess of dark current source as discussed earlier, that is, the minority carrier lifetime, T~,which is typically1'* by the diffusion, depletion region, surface, or tunnel in the range < T~ < 4 x 10" s at 77 K. currents. The effect of the surface recombination on Surface-controlled photoconductivity has been the performance of a photoconductive detector demonstrated by Kinch'Iz on a photoconductive was investigated theoretically by Bro~dy~~as shown DEF SCI I, VOL. 53, NO. 3, JULY 2003 in Fig. ll for n-type Hgo,Cdo2Te (;LC = 14.2 pm surfaces providing Hi-Lo n+- n junction, which sets at 80 K) with a net carrier concentration of 9 x 1 OI4 cd. a reflecting barrier for minority carriers. It may be Compared with an ideal photoconductive detector, noted here that the same principle of high-low junction is used for reflecting the minority carriers IDEAL at the contacts of these detectors, thus increasing DETECTOR their lifetime and also reducing the sweep-out effects. The effect of fixed charge density (Q/) of the passivant on the performance parameters of HgCdTe photoconductive extended/overlap structures was investigated theoretically by Bhan1I3,et al. extending Smith models3." by including the effect of passivation 50mV BlAS SO WITH SURFACE for a detector with pixel size 35 x pm2, thickness RECOMBINATION 45 mV BIAS 10 ym, x = 0.2 1 and n = 5 x 10" cw3. Their results show that the responsivity dependence on Q,shows a peak for both symmetric as well asymmetric 0- overlap structures for the optimised Q, value of 60 90 70 80 100 -2-3 x 10" ~m'~.The noise shows almost no TEMPERATURE (K) dependence on Qffor lower values up to - 1 x 10" cm2, decreases sharply for values > 4 x 10" ~m-~and Figure 11. Theoretical detectivity as a function of shows a peak at intermediate value -3 x 10" cm2. temperature. ideal case (upper curve) at Q, 2 x 10" cnr2 is found to be optimum for 45 mV for zero surface recombination velocity, max~m~singD* for both the standard and the overlap no sweep-out and ideal thermal interface. With added surface recombination velocity, sweep-out structures. However, there is a stronger dependence and a thermal interface at 45 mV bias (lower on Qj of the detector parameters for overlap curve) and at 150 mV bias (middle curve). structures than for standard structures. with its front and back surfaces having zero surface 4. DEVICE PROCESSING TECHNOLOGY recombination velocity, the detector with surface recombination was assumed to have its back surface The photoconductive detectors can be fabricated with infinite recombination velocity. It can be seen by the methods, many are quite similar to the processes that the addition of surface recombination considerably used in Si-ICs, such as lithography, etching, insulator reduces the detectivity. The performance of the deposition, vacuum metallisation, bonding, etc. ideal detector properties for temperatures above Additionally, some MCT specific techniques such 80 K as thermally generated minority carriers begin as slab mounting on sapphire, thinning 1.y lapping to dominate. and polishing, MCT array structure etching, etc. also have to be applied. As discussed earlier, the Transverse electric field on the surface of a MCT has weak Hg-Te bond and mercury has low photoconductive detector is commonly used to reduee vapour pressure, therefore all the processing steps surface recombination. In this approach, the minority have to be optimised at or below 70 "C and active carriers are prevented from reaching the surface area should never be exposed to chemical etchants. by means of a built-in electric field. For example, The presence of a 10 ym MCT step on sapphire for an n-type MCT photoconductor, a transverse introduces a non-planarity in the process due to field would be provided by an+accumulation layer which the processing becomes critical, particularly induced on the surface of the semiconductor by a the lift-offmetal patterning. The actual photoconductive positive fixed charge present in the anodic oxide Hg,_Cd1Te sensor used in IR systems is a complex passivation layer. Implantation or ion milling can multi-element array requiring a high level of also be used to create a n+ layer on n-type MCT sophistication in analysis, design, and processing. RlSAL SlNGH & MITTAL: MCT PHOTOCONDUCTIVE LWlR LINEAR ARRAY DETECTORS
State-of-the-art processing of MCT detectors mechanical polishing. Lapping is carried out at low must be capable of achieving the following key pressure and low speed. A precision lapping and requirements: polishing machine, such as Logitech PM2A and PM5 machines can be used for this purpose. Surface Bulk MCT crystal properties throughout is checked for grain boundaries. Wafer thickness fabrication process must be preserved. is monitored at each stage of polishing.
All surfaces must be damage-free and well Polishing is a high load process compared to passivated. lapping and is carried out at higher speeds. The principal advantage of chemical polishing over Ohmic/blocking contacts of proper geometric mechanical polishing lies in the fact that it reduces configuration must be provided. sub-surface damage to the crystal structure. In mechanical lapping and polishing, fine scratches Detector's active area must be defined accurately. are introduced in the material which are associated with dislocations and deformation of the material. Material thickness must be controlled to close The damaged region typically comprises: (i) a surface tolerances. layer of amorphised semiconductor and embedded abrasive (Billhy layer), (ii) a polycrystalline zone, Major process steps normally used in the and (iii) a dislocated zone extending deep into the fabrication of a photoconductive array46J"'.115are sampleu6. shown in Fig. 12 in the form of a flow chart. Details of some critical and important processing Sharn~a,"~et al. have presented a detailed study stepsiissues are described below. on the effect of damages generated in MCT material during surface preparation on lifetime of excess 4.1 Substrates Selection carriers. The lifetime measurements were done using microwave reflectance technique. The depth of damaged The choice of substrate material is governed region keeps on reducing with polishing powder grit by physical and optical characteristics, such as size, which leads to corresponding improvement in thermal expansion, inertness, fragility, reflective lifetime. The polishing damage of the previous stage index, conduction as well as availability and cost. is removed in the next stage of polishing These The critical issues are flatness, parallelism, and defects are estimated to a depth of approx. 20 times surface finish. To maintain dimensional control of the grit size"', therefore, care should be taken to the finished device, it is necessary to properly remove the material, at least up to this depth, while specify the substrate to which the device will be using lower grit polishing powder. attached. The most favoured substrate is sapphire because of its physical, optical, mechanical, and Prepared MCT surface is characterised using electrical properties. It has high thennal conductivity ellipsometry. Rhiger1I8 has used ellipsometric and very low electrical conductivity. It is chemically measurenients to characterise the MCT surface inert and can be polished to achieve high surface after it has been subjected to various surface treat- flatness and plane-parallelism of the two faces ments. Bromine-methanol etchant leaves the surface within the tight tolerance of -1 pm. tellurium-rich. Dilute HNO, treatment is used to remove this excess tellurium-rich layer. 4.2 Sample Preparation The MCT slabs of required size are cut after Lot of efforts have been made to prepare the wafers are backside passivated. Sapphire substrates damage-free MCT surface. This involves successive (of required thickness uniformity) are thoroughly removal of surface damage by lapping and polishing cleaned using standard cleaning procedures. Generally, with finer abrasive grit sizes, followed by chemo- a low-temperature epoxy is used for bonding MCT DEF SCI J, VOL. 53, NO. 3, JULY 2003 - BACK MCT SLAB n-MCT CHARACTERISATION SURFACE PASSIVATION * * MOUNTING --t WAFER be, Pe . AX, T) PREPERATION (Iox3 n,, 0 - ON SAPPHIRE ('+',A,T)
DELINEATION PASSlVATlON
n-MCT CHARACTERISATION STRUCTURE PASSIVATION - (n, ,& ,A 7) DELINEATION (tar.",, ')
METALLISATION & PLATING
Figure 12. Major process steps used in the fabrication of a photoconductive array. Starting with bulk-grown wafers, the process after front surface preparation may follow either ofthe two paths: (i) surface passivation first, followed by detector structural delineation or (ii) structural delineation first, followed by surface passivation. Starting with epitaxial wafers, the process eets simplified with no material thinning required. slabs with sapphire using special epoxy mounting Hg,_CdXTeof appropriate thickness on insulating fixtures. Bonding material has two components substrate99, such as CdTelCdZnTe. The process of comprising resin and hardener that are mixed in sample preparation is considerably simplified using the specified ratio and diluted with organic solvent epi layers and the above laborious steps of lapping to get a thin uniform layer. Epoxy curing is done and polishing are totally avoided. at temperatures lower than 70 "C. Front surface preparation is carried out in a way similar to back 4.3 Surface Passivation surface preparation, leaving final MCT thickness - 10 pm. Prepared surface is characterised using The semiconductor-passivant interface as well optical microscope, ellipsometer and scanning electron as the dielectric properties of the passivating layer microscope. It is to be pointed here that the play important and often dominant role in determining photoconductive detectors are also fabricated using device performance. Passivation technologies can infrared absorbing epitaxial layer of n-type be classified into three categories: (i) native films KISAL >INbH MI1 IAL: MC1 PHOTOG(JNDUCTIVE LWlR LINEAR ARRAY DETECTORS
(oxides, sulphides, fluorides), (ii) deposited films 80 - (ZnS, SO2,Si,N,, polymers), and (iii) in-situ grown 0.3 m.4lcmZ heterostructures where wide band gap layer acts 0 1 M KOH in 90 % 60 - EO +IO%H,O as passivant. A native layer, which becomes an - integral part of the semiconductor, is formed when Z the growing layer incorporates the semiconductor 3 40- atoms. Thick native films become porous and do 5 not adhere well to the substrate. Therefore, thick ? deposited dielectric films like ZnSare required for 20 - passivation to achieve the protection ofthe interface from environmental conditions by acting as a protective 0 I I I I coating. These deposited films are required to insulate 0 1000 2000 3000 4000 the metallisation pattern of the contacts and the THICKNESS (A) bonding pads from the substrate. Thus, these films must be mechanically stable, electrically insulating, Figure 13. Variation ofanodic oxide thickness as a function of anodization cell voltnee- for constant current free of traps, transparent in the relevant wavelength nnodization of Hg,.,CdzTe. region, and chemically protective. made to grow by pure chemical oxidation in an Since all surfaces must terminate with damage- aqueous solution of K,Fe(CN), (0.075-0.75 molefl) free passivation, the back surface, i.e., the surface and KOH (0.06-0.6 molell). The Fe(CN);' ions to be epoxied to the substrate, must be passivated. react with tellurium of MCT to form TeO,. The The prepared MCT surface is passivated (to provide speed of oxidation depends on the concentrations environmental protection, control surface leakage retained in the solution and thickness of the oxide currents, and stabilise MCT chemically) using either layer at any instant, i.e., as the layer thickness native oxide, plasma oxidei5, or CdTe. Anodic increases the speed of growth decreases as shown ~xide'~'J~~-'~'is grown on MCT in an electrolyte in Fig.14. Thus, a layer of 40 nm grows in the of 0.1M KOH in 90 per cent ethylene glycol (EG) first five minutes and only 30 nm is added in the and 10 per cent deionised water in an anodisation next 40 min. The passivant oxide layer is capped cell. with complimentary ZnS layer of thickness 300-500 nm. The passivated detector obtained using The MCT wafers serve as the anode and these oxides is endowed with good characteristics, growth is carried out at a constant current density with stability up to 80 "C. 100-300 yAlcm2 followed by a constant voltage anneal. The relationship of anodic oxide film thickness and applied voltage for anodisation at constant current density 300 yAlcm2 is shown"9 in Fig. 13. The thickness and refractive indices of anodic oxide and the MCT surfaces are characterised using ellipsometer. For anodic oxides on MCT grown ai constant current density 200 yAlcm2, moderate fixed positive charge (5-7 x 10" cm2)and small hysteresis are reported, achieving virtually an ideal condition for photocond~ction~~~. TIME (min) Gauthiern5 patented a process for passivation of Hgl.,CdxTe (x = 0.2 or 0.3) photoconductive Figure 14. Variation of chemical oxide thickness and detectors, wherein a native oxide layer is formed growth rate as a function of deposition time by chemical reaction. The native oxide layer is (Data taken from Ref. 125). DEF SCI J, VOL. 53, NO. 3, JULY 2003
Table 4. Passivants producing positive fixed charges at the passivantln-HgCdTe surface
Passivant charge Interface trap Thermal Rekective Dielectric stabilitylimit Chemical Passivant density density Adhesion lnsulator index constant (OC) corrosion Q, (cm-') D,, (cm-' ev-')
Native films Attacked Anodic oxide 2 x 10" 3 x loi0 Excellent Inferior to Si02 2.1 19.3 'O by alkalis Plasma oxide I x 10" 2 x 10" -- Inferior to A.O. -- -- 90 --
Anodic Fluoride 2 x loi0 -- -- Inferior to SIO' -. -- 105 --
Chemical oxide Detectivity (at h,= 4.5 pm and 195 K) > 10" cm Hz "'W-' 80 --
Deposited films
CdTe 5 x 1O'O ' < 1 x 1010 ------
The CdTe is generally deposited by thermal advantageously utilised for passivation of sidewalls evaporation technique. Adequate precautions are of each element. The completed device is dipped taken to avoid heating of HgCdTe surface during into chemical oxide growth solution of K,Fe(CN), evaporation. The evaporation rate of CdTe, chamber and KOH allowing the chemical oxide to grow vacuum at the time of evaporation, substrate temperature, selectively on the exposed MCT sidewalls, as described source-to-substrate distance, etc. are the crucial earlier'25.However, the chemical oxide is relatively parameters, which need to be optimised to achieve a good CdTe layer.
A dethiled comparison of different passivants u:ed for PC device^'^^,"^ is given in Table 4.
as-ally a photoconductive linear array consists of a number of detector elements (such as 60, 120,180, etc.) in a row, which are further subdivided into combs-each containing 5-1 0 elements, as depicted in Fig. 15(a). After the front surface passivation, Br,-NBr solution wet etching or ion-beam milling does the structural delineation of the MCT slab into individual combs with detector elements. This process of passivation prior to structuring has the inherent advantage of starting with a damage-free clean native MCT surface, which is well sealed by the passivant immediately after the surface preparation. However, during the process of etching, the sidewalls of elements are rendered bare and unpassivated. If left under these conditions, these unpassivated sidewalls will add to the detector noise and adversely Figure 15. Schematic diagram ofthe part of a Hg, PHOTORESIST Ill EVAPORATED (a) TAPER (b) METALLISATION (c) ELECTROLESS Au Figure 16. (a) Taper etch profile, (b) improper metal step coverage leading to uncertainty in metal continuity and unstable detector resistances, and (c) electroless Au deposited on tapered edge, covering all the surface with metal. inferior to anodic oxide in passivating properties. 4.4 Metal Step Coverage After this step, the MCT surface is covered by either passivant or metal layer. This results in an The complexity of device processing arises improvement in the detector's figures of merit. due to non-planarity of the device. It has a 10 pm steep MCT step, which is tapered at the ends-a An alternate structural delineation approach step essential for continuous metal coverage. For would be to etch or ion mill individual elements into reliable and repeatable functioning of device, the the MCT slab prior to front surface passivation by metallisation must be continuous over the HgCdTe keeping a common corridor of semiconductor for and epoxy edges onto the substrate surface. However, electrical continuity, as shown in Fig. lS(b). This even minor overetching during tapering may lead is followed by anodic oxide growth. Subsequently to improper metal step coverage on the 10 pm the common corridor is etched off, leaving desired steep MCT edge and run-over to the sapphire comb structure with all surfaces passivated by leads to uncertainty in metal continuity and causing anodic oxide in a single step. flicker in the element resistances [Fig.l6]. This requires optimal control on MCT edge profile etching, This method, however, suffers from the dis- i.e., proper rounding. This can be done by optimising advantage that after completing the delineation the the distance of the tapered edge from the device exposed surface could be laden with some uncontrolled edge, the dilution of etchant (bromine ratio in HBr) growth of oxides or other chemical remnant left on and etch time. the MCT surface. These oxides and residues are extremely hard to be removed. Also, it is found that The metallisation of such steps with reliable the anodic oxide growth solution attacks exposed and reproducible metal step coverage usually requires epoxy under the structure during the growth. Several complex rotating mechanisms and jigs in a thermal workers have tried different chemical treatments to evaporation system. While the requirement for successful obtain the nascent clean MCT surface. For example, lift-off process is that the deposition should be light spray-etch with one sixteenth per cent bromine normal to the surface, which is not possible with solution in ethylene glycol is applied, followed by the above-mentioned systems. In addition, solvent cleaning process in toluene, acetone, methanol, exposure of tapered area to the atmosphere before and isopropanol sequentially sprayed'26. metallisation leads to uncontrolled growth of oxide. However, both of these problems can be solved by In an alternate approach of array fabrication, a much simpler in-situ electroless gold deposition using infrared absorbing epitaxial layer of n-type technique using auric chloride s~lution'~'.After Hg,_CdXTe,the surface is automatically passivated achieving rounded MCT taper edge by etching, the in situ in the epitaxial chamber by further growing gold solution is applied on the exposed edge, which higherx (0.24) material99,followed by ZnSprotective leaves the surface covered by gold due to higher capping. This higherx layer creates a heterostructure affinity ofthe MCT for gold. This process eliminates barrier needed for blocking contacts for high performance the metal continuity failures drastically and reduces detectorss5. element resistance var~ations. DEF SCI J, VOL. 53, NO. 3, JULY 2003 4.5 Hi-Lo Blocking Contacts Area Milling the etch rate is 270 nm/min and increase in carrier concentration is of more than four order. At ion The need of freshly cleaned semiconductor current density of 0.6 mA/cm2, it is observed that surface prior to metallisation to achieve low-contact HgCdTe sample &face has been damaged, as resistance was realised earlierI0'. The use of chemical viewed optically. Also, the carrier concentration is etching for this purpose sometimes leads to undesired low at the surface95. com~lexformation. such as uncontrolled oxide -mowth on the semiconductor surface-an unreliable process. The exposure of photoresist to ion milling is On the other hand, ion-beam milling is a novel etch inevitable during device processing; hence, the technique, which is anisotropic, non-selective, and processing conditions as photoresist treatment and highly reproducible. Also, as discussed earlier, the ion energies play a very crucial role for success main advantage of ion-beam milling is desirable of the step. ~~~i~~ ion bombardment, photoresist modification of electrical properties of semiconductor gets hardened and is very difficult to remove after in contact region from n-or p-type to n+-ty~e~~-~'.the Drocess is com~lete.Generallv, the methods commonly used for removing this hardened photoresist The depth of converted region depends on ion in silicon technology are not suitable for MCT, current density. Figure 17 shows profiles of carrier because of involvement of high temperatures and/ concentration versus etch depth at various ion or stark action of strong chemicals. Mitta1128,et al. current densities, for a fixed ion etch time of 15 min. have studied effects of various alternative treatments At very low ion current density, i.e., 0.3 mA/cm2, to photoresist prior to ion bombardment. UV hardening the etch rate of HgCdTe is almost zero and increase after the development of the photoresist pattern in carrier concentration is approximately two times hardens the developed photoresist. The aim is to its original value. As the ion current density is harden the photoresist prior to exposure to ion increased to 0.34 mA/cmZ,the etch rate is 13 nmlmin - bombardment, so that the further hardening during and increase in carrier concentration is of more ion milling can be slowed and the removal ofphotoresist than three order over its original value. Further in hot acetone spray is possible. increase in the ion current density results in increase in carrier concentration and also the depth of converted The ion bombardment adversely affects the lip region. At the ion current density of 0.6 mA/cm2, structure of photoresist, required for lift-off process. BhanIz9,et al. have studied the effect of ion current density on the desired shapelprofile of lip structure formed in photoresist prior to actual lift-off. At lower ion current densities up to 0.5 mA/cm2,virtually no adverse effect on the lip structure is observed, while at higher current densities (0.7 mA/cm2)the lip structure is significantly damaged (Fig. 18) leading to lift-off failure. 4.6 Fan-out Metallisation No semiconductor device can function without any electrical contact to outside world. The various metallisation schemes used for this purpose have (pm) DEPTH to be suitably patterned. This can be done either Figure 17. Carrier concentration depth profiles at various by selective chemical etching after the complete ion current densities. Etch removal of the metallisation or by lift-off process. The stark chemicals amorphised top material layer up to the depth of the peak of the carrier concentration prior used in wet etching adversely affect anodic oxide to contact matellisation leads to improved passivant and ZnS capping layer which are porous, contacts. and hence, susceptible to chemical seepage and RlSAL SlNGH & MITTAL: MCT PHOTOCONDUCTIVE LWIR LINEAR ARRAY DETECTORS Figure 18. Cross-sectional SEM micrographs of the photoresist lip structure: (a) milling at current density 0.34 mA/cm2-a desired profile and (b) at 0.70 mA/cm2-the lip structure shrinkage due to damage. unwanted reactions. The etching process at this source, a reference detector, a test dewar, and a stage is, therefore, ruled out. The advantages of variable voltage supply for biasing detector. The lift-off process over selective etching have been electronics normally used consists of a preamplifier, disc~ssed"~J~'.For lift-off process, conventional an amplifier, a spectrum analyser, and calibration photolithographic step is modified slightly by soaking instruments. The responsivity measurement requires in chlorobenzene after exposure and prior to the a stable, modulated source of infrared radiation at development of pattern. This soaking modifies the three frequencies up to 1000 Hz to measure detector photoresist profile near the edges to mushroom- signal dependence on modulation frequency. type having negative slope, which helps in lift-off process. In routine practice, the test laboratories operate blackbody temperature at 500 K and the radiation Device yield depends considerably on the success is modulated by chopping at 800 Hz. The test setup of the metal fan-out pattern lift-off. This process for measurements of spectral responsivity basically involves preparing a proper photoresist hangover remains the same except that the blackbody source lip structure using chlorobenzene-soak treatment is replaced by a monochromator. Radiation of a before normal incidence metal evaporation. This specified wavelength, range from the monochromatic step becomes very critical because of highly non- source is allowed to fall on the detector and photosignals planarity of the device topology, leading to photoresist so generated are recorded. As the thermal detectors thickness variation on the detector fingers. For exhibit a flat spectral response, the generated achieving a proper hangover lip structure, all thermosignals will be proportional to the incident photolithographic parameters like pre-bake time, monochromatic power. The ratio of photosignal to chlorobenzene-soak time, UV radiation dose, etc. thermopower when plotted against the wavelength are to be standardised. yields the relative spectral response curve normalised to peak response. Peak response wavelength (hp) 5. DETECTOR CHARACTERISATION and cutoff wavelength (A") are obtained from this The equipment used to measure the necessary curve. At cutoff wavelength, the response drops detector parameters and the test procedures are to 50 per cent of its peak value towards the longer well documented in the literat~re"~"~~"~.Ameasurement wavelength. The rms noise voltage of a detector setup consists of an electronicequipment, a blackbody is measured by the lock-in amplifier generally at source, a variable frequency source, a monochromator 20 kHz central frequency with a 10 per cent equivalent DEF SCI 1, VOL. 53, NO. 3, JULY 2003 noise bandwidth (ENBW), when chopper is off for a system designer to keep FOV smaller in the and blackbody aperture is closed. Normally, the practical FOV range of 180" > FOV > 30". At low measured noise mainly represents the g-r noise, as background, g-r noise varies as reflecting all other noises become negligible. Johnson noise decrease in minority carrier density and constant is very small for low- resistance devices a. cryogenic n and T. At high background, g-r noise varies as temperatures; Ilf noise becomes negligible at 20 $,-'I3. However, for the practical range of FOV kHz; Va is also kept very low by selecting a low (1 80" >FOV> 30°), the g-r noise remains practically noise amplifier and good laboratory techniques of constant. Excess llfnoise is seen to be correlated grounding, shielding, etc. Frequency-dependent to minority carrier density. llf noise comer frequency behaviour of the noise components is estimated by decreases for reduced FOV. D' values for the measuring the rms noise voltages at various cases n>>p and n = p have been estimated. In frequencies in the 0.1-100 kHz. general, D' increases with reduction in FOV, as shown in Fig. 19. In military applications, the MCT infrared detectors are operated at cryogenic temperatures to enhance their performance. For this purpose, the detectors FOV (') 10 15 20 30 45 60 90 180 10'~ are encapsulated in miniatureltactical glass or glass- 7 to-metal dewars. Joule-Thomson cryocoolers, capable of cooling detectors to operating temperatures 80-90 K in a few seconds are in use in seeker1 guidance systems. Thermoelectric and stirling cycle cryocoolers are also used in a variety of night vision, targeting and guidance applications, in both the ground-based and airborne system^"^. 6. TECHNOLOGY ILLUSTRATIONS As cited above, tremendous amount of work has been done in the last couple of decades in the field of HgCdTe photoconductive deteotor technology and it is almost out of place to cover these studies Figure 19. Peak D*,, versus background flux for Hg,.=CdXTe(x=O.l eV) photoconductive detector even briefly and do any semblance of justice to it. when n >>p. Upper limit is D*,,,, x 42 when It was, therefore, thought to be appropriate to n =p. Experimental data points follow calculated discuss only a few selective studies that are in a D* values for no= 6 x 10" cm-', assuming way representative of the developments in the field q = 0.6. at different stages and have bearing on the theme of topics covered. The temperature variations of D',, R,,,and g-r B~rrello"~,et al. have calculated the variatioqs noise over the range 64-190 K were predicted in vital detector parameters with background photon theoretically for the thermally limited case for flux, $, and validated their model by observing n-type photocond~ctor~~with no * 8 x lOI4 cm~' excellent agreement with experimental data. The and heo= 12.5 pm. The temperature variations of photoconductive lifetime, T, is limited by Auger D*,predicted theoretically and measured experimentally recombination at 77 K for flux $,> 1016photons cm2s-I. are illustrated in Fig. 20 (a). The measured D', As the lifetime recovers with decrease in FOV values of well over loi2cm Hzu2W-' in the nitrogen from 180" to 30°, the response time in excess of temperature range were obtained. The FOV used 2 ps becomes observable. In correspondence with in these measurements are estimated to be - 3" the lifetime, the responsivity increases significantly using = 0.6 and Ace = 12.5 pm. The temperature with reduction in FOV. It is, therefore, advantageous dependence of R,,is contained in the factor ~ln~ CALCULATED g-r NOISE 41, CALCULATED AT n = 1 40.0-1 103/T Figure 20(c). Temprature variation of measured noise at Figure 2O(a). Temprature variation of measured and 30 kHz and calculated thermal noise. calculated detectivity. not exhibit any strong dependence on the temperature, suggesting that llfnoise is not due to thermally generated carriers but possibly is a majority carrier phenomena. 0-0-0-0- g AUGER 0 LIMIT The advantage of a totally asymmetric overlap q -0.7 /.' structure is illustrated in Fig. 21 showing the blackbody responsivity dependence on electric field of one of the 60 elements of an n-type photoconductive linear array with x = 0.208, no = 4 x lOI4 cm-'and pixel size'36 of 34 pm x 48 pm . Although the resistance of the detector remains the same in two directions of the field polarity, the overlap advantage is obtained with the detector finger as cathode and Figure 20(b). Temprature variation of measured and calculated responsivity for intrinsic regime. and is given in Fig. 20 (b), showing an excellent agreement between theory and experiment. The R,,is virtually independent of temperature below I00 K and has a slope of 0.16 eV in the intrinsic range. The temperature dependence of g-r noise, Yg-,, set by the factor (T / n,)''2, is shown in A'. Fig. 20 (c). The calculated solld lines for the intrinsic Figure 21. Responsivity versus electric field for standard and extrinsic cases are in perfect agreement with and overlap structures ofone of the 60 elements the experimental plot of points. The 1lf noise does of an array. DEF SCI 1, VOL. 53, NO. 3, JULY 2003 the common contact ofthe comb having five elements mechanism is consistent with g-r noise, where the as anode and, therefore, power remains the same thermal and background contributions to the noise for both the polarities. The responsivity is higher are matching. by a factor of 1.5 at 100 Vlcm for the overlap compared to standard mode of operation. Also, the Bla~kburn~~,et al. realised high performance responsivity keeps on increasing almost linearly 8-element SPRITE detectors in production using even at higher fields with no sign of saturation, as Hg,.xCdxTe wafers with x = 0.21, lifetime of 2 ps, expected of such a structure. ambipolar mobility 390 cm2v-Is1for 8-14 pm band. Standard SPRITE device consists of an array of Figure 22 shows the responsivity and detectivity 8 in-line elements. Each element is 700 pm long versus field at 80 K for a symmetric heterostructure and 62.5 pm wide on a 75 pm pitch. Readout contact photoconductor (HCP) devices1fabricated bifurcated contacts have been brought out parallel on a lightly doped HgI_CdxTe double layer LPE to the length of the element. The mean values of wafer with a compositional difference Ax = 0.03 D'(500 K, 20 kHz, I, 62.5 x 62.5 pm') =[.I x 10" between the lower active region (n - 2 x 1OI4 ~m-~cm Hz'/~W-Iand responsivity = 6.0 x lo4 VIW were and hco= 7.88 pm) and top higher bandgap blocking achieved. The performance variation of such a layer. It can be noticed that there is no sweep-out SPRITE detector (h =11.5 pm) with bias field and effect at the typical fields and responsivity values FOV is shown in :&. 23. in excess of 1 x lo6 VIW were measured on the device. The saturation sets in only for fields -140VIcm. Comparatively, for the device with single layer (Ax = 0) with similar material parameters, the responsivity is one order lower and the saturation sets in at lower fields - 80 Vlcm. The detectivity for the HCP detector remains constant at low-bias fields (7-8 x 10'' cm Hzv2 W-I) and starts to decrease at fields > 100 V/cm. This type of behaviour in D', 2000 - r10" DL - ..o,,.o-u-v-.~.d..a~ A,b-4°h. .b - > 1600- * 0 A~ - - Ax=0.031 3 1200- -1O'O 2- )&, >; - L2 t d* 5 800- ' T=80K - -A ',p 0, A MEASURED DATA aEi ,! ...... V1 CALCULATED -lo9 5 '4 k % 400- A AFO 0 11 I, I, ,I I .- BIAS FIELD (V cm-I) BIAS FIELD (vcrn-l) Figure 22. Responsivity and detectivity versus bias field Figure 23. Variation of the performance of a SPRITE for standard (Ax = 0) and symmetric hetero- detector with bias field and field of view: structure (Ax = 0.031) contact photoconductor. kc =I l pm, T = 77 K. RISAL SI GH & MITTAL: MCT PHOTOCONDUCTIVE LWIR LINEAR ARRAY DETECTORS Figure 24(a). A 6O-element photoconductive array on a PCB Figure 24(c). A 60- element photoconuctive array as a packaged device. responsivity. Ashley", et al. optimised readout geometry and shape of the elements to overcome background effects, improving the spatial resolution in these detectors. Subsequently, several workers carried out further work on improving the performance of SPRITE detectors and their use in system design and implementation69.72.73,137.138. A review on the development ofphotoconductive linear arrays for thermal imaging at this laboratory was reported recently by Singh!". The arrays were fabricated using n-Hg1.;xCd Te with x =0.214 ±0.003, carrier concentration, no ~ 4 x 10 14 em", mobility, Jl, - I-2 X 105 cm2N.s and minority carrier lifetime, 't ~ Ius. Lifetime measurements using photoconductive decay and microwave reflection techniques served both as diagnostic and process-monitoring tools at Figure 24(b). Partial view of a few detector elements of a different stages of device fabrication, i.e., stages 60-element photoconductive array. of lapping and polishing during sample preparation, after wafer passivation, and finally on the completed The detectivity increases with bias field as devices. Process-induced material damage in the E'{2 and also increases with the reduction in FOY. MCT has been estimated using lifetime measurement'". Responsivity shows approximate linear dependence A 60-element array mounted on a PCB along with and noise square root dependence on bias field. In magnified partial view of few elements and a device the wide FOV (j# < I), the detectivity and responsivity dewar package are shown in Fig. 24 (a-c). A yield values are anomalously low. Davis?' studied the of 27 per cent with arrays R y in the range 4 adverse effect of high signal photon fluxes on the I-lOx 10 V/W and, D ' BB in the range 315 DEF SCI J, VOL. 53, NO. 3, JULY 2Ml3 1-4 x 101° cmHzln/W at 80 K using 34" FOV has been achieved. BLIP-dominated performance was attained in several arrays'36. The relative spectral responsivity of an element in an array with peak wavelength at 11.0 pm and cutoff at 12.7 pm is shown139in Fig. 25. WAVELENGTH (p) Figure 25. Relative spectral response of a photoconductive Hg,.,Cd,Te (x = 0.1 eV) detector =11.0 pm and k cutom = 12.7 pm. Figure 27. Thermal images of (a) buildings at - 700 m The peak spectral detectivity of the element with two persons standing shown encircled and the trees in the foreground and (b) a vehicle is 7.6 x 101° cmHzlnlW. A typical noise density at 250 m. spectrum of an element is shown in Fig. 26 having - a corner frequency fc - 540 Hz and frequency- The llf noise corner frequency fc as low as independent noise density of - 6.5 ~V/HZ~/~. 50 Hz has been obtained for some devicestm.Thermal imaging was done using these detector arrays with average D' - 1 X 101° c~Hz'~/W.Several targets in the range of about 2-3 km were viewed. The two microwave towers at - 2 km could also be sighted through a blanket of smog. A thermal image of the buildings at - 700 m and a vehicle on a bridge at - 250 m are shown in Figs 27(a) and 27(b), respectively. 7. SUMMARY 4 ' I I i After presenting a brief account of the historic lo2 1o3 1o4 lo5 perspective of the IR photon detectors, the relative FREQUENCY (Hz) merits and demerits of the scanning versus staring thermal imaging systems have been brought out Figure 26. Qpical noise density spectrum of an element of a photoconductive detector having a corner and the need for further refinement on the subject frequency6 - 540 Hz and trequency-independent of photoconductive detector design and processing noise density of - 6.5 nV/HzH. technology has been emphasised. Subsequently, impor- RISAL SINGH & MITTAL: MCT PHOTOCONDLICTIVE LWlR LINEAR ARRAY DETECIORS tant performance parameters of the photoconductive 7. Hudson, R.D. & Hudson, J.W. lnfrared detectors in standard as well as SPRITE configurations detectors. Dowden, Hutchinson and Ross, and the often used basic formulations have been Stroudsburg, 1975. described. The key issues of detector design have been critically examined. The detector processing 8. Keyes, R.J.(Ed.) Optical and IR detectors. sequence is then presented in the form of a summary Topics in Applied Physics, Ed. 2, Vol. 19, flow chart. Some of the critical processing steps Springer-Verlag, Berlin, 1980. have been dealt in sufficient detail. Finally, a few selective illustrations of photoconductive detector 9. Elliott, C.T. lnfrared detectors. In Handbook technology have been presented. on semiconductors, edited by T.S. Moss, Vol. 4, edited by C. Hilsum, 1981. (Also See Ed. 2, 1993). ACKNOWLEDGEMENTS The authors would like to express their gratitude 10. Elliott, C.T. Photoconductive detectors in to Dr Vikram Kumar, the then Director SSPL, HgCdTe. In Properties of narrow gap Delhi, for the permission to publish this paper. cadmium-based compounds, edited by P. Thanks are also due to Dr V. Gopal and Dr R.K. Capper. 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Infrared No USP-3,977,018, 1976. Phys. Technol., 1999, 40, 101-07. 120. Nemirovsky, Y & Finkman, E. Anodic oxide 11 1. Kinch, M.A. Electronic properties ofHgCdTe. films on Hgl_CdxTe. J. Electrochem. Soc., J. Vac. Sci. Technol., 1982, 21, 215-19. 1979, 126, 768-70. 112. Kinch, M.A. Metal-insulator-semiconductor 121. Janousek, B.K. & Carscallen, R.C. The infrared detectors. In Semiconductors and mechanism of (HgCd)Te oxidation. J. Appl. semimetals, edited by R. K. Willardson and Phys., 1982, 53, 1720-726. RlSAL SlNGH & MITTAL: MCT PHOTOCONDUCTIVE LWlR LINEAR ARRAY DETECTORS 122. Chavada, F.R.; Tewari, G. & Gupta, S.C. Seminar on IR Devices and their Defence Bull. Muter. Sci., 1987, 9, 219. Applications. Delhi, India, 1989. pp.19.1-19.2. 123. Yadav, R.D.S. & Warrier, A.V.R. Anodic 131. Gupta, S.; Mittal, V. & Chhabra, K.C. A polarisation of HgTe, CdTe and Hg,.xCdxTe clean method for growing uniform 4 pm -oxide formation kinetics and omposition. thick In bumps. In Proceedings of the J. Muter. Sci., 1991, 26, 423-28. Conference on Physics and Technology of Semiconductor Devices. Pilani, India, 1992. 124. Singh, R.; Sharma, B.L.; Mittal, V.; Pathak, H.T.; Srivastava, S.; Pal, R.; Singh, J.P.; 132. Limperis, T. & Mudar, J. Detectors. Ch. ll. Dewan, H.S.; Madaria, R.K.; Basu, P.K. & In The infrared handbook. IRIA Series Warrier, A.V.R. Asymmetric overlap and in Infrared and Electro Optics. Optical locking contacts for linear arrays. National Engineering Press, Bellingham, WA, 1993. Conference on Recent Advances in Conductors (NCRAS), IIT DeIhi,1995. 133. Eisenman, W.L.; Merriam, J.D. & Potter, R.F. Operational characteristics of infrared 125. Gauthier, A. Process for passivation of photodetectors. In Semiconductors and photoconductive detectors made of HgCdTe. semimetals, edited by R.K. Willardson and US Patent No. USP - 4,624,715, 1986. A.C. Beer. Vol. 12, 1977. pp.1-38. 126. Wilson, J.A.; Cotton, V.A.; Silberman, J.; Laser, 134. Paugh, R.L. Improving the performance D.; Spicer, W.E. & Morgen, P. (HgCd) of military IR systems with cryogenic Te- SiO,, interface structures. J. Yac. Sci. cooling. Photonic Spectra, 1994, 28, 93-95. Technol.,l983, Al, 1719-722. 135. Borrello, S.R.; Kinch, M. & Lamont, 127. Mittal, V.; Monga, K.; Pathak, H.T.; Singh, D. Photoconductive HgCdTe detector R.; Gopal, V. & Kumar, V. Some critical performance with background variations. processes in the fabrication of PC linear Infrared Physics, 1977, 17, 121-25. array. In Proceedings 1X-IWPSD-1997, edited by V. Kumar and S.K. Aggarwal. 136. Singh, R.; Sharma, B.L.; Mittal, V.; Pathak, Narosa, New Delhi, 1998. pp. 803-05. H.T.; Srivastava, S.; Pal, R.; Singh, J.P.; Dewan, H.S.; Madaria, R.K.; Basu, P.K. & 128. Mittal, V.; Singh, K.P.; Singh, R. & Gopal, V. Warrier, A.V.R. Asymmetric overlap structures A study of UV treatment of photoresist for 0.1 eV MCT PC linear arrays. In Proceedings for dry processing. In Proceedings IX- IWPSD ofthe VIlI International Workshop on Physics -1997, edited by V. Kumar and S.K. Aggarwal. of Semiconductor Devices, (IWPSD-199S), Narosa, New Delhi, 1998. pp.1215-217. edited by K. Lal. Narosa, India, 1996. pp. 208-10. 129. Bhan, R.K.; Mittal, V.; Rawal, D.S.; Naik, A.A.; Sehgal, B.K.; Singh, K.P.; Singh, 137. Yamagata, T.; Oda,N.; Miyamoto, K.& Fujino, B.V.; Aneja, B.R.; Sharma, B.L.; Singh, R. Y. Size optimisation in SPRITE detector. In & Gopal, V. Effect of ion beam milling on Proceedings of the Sixth Sensor Symposium, lips prior to lift-off process. In Proceedings 1986. pp.305-08. IX-IWPSD-2001, edited by V. Kumar and P.K. Basu. Narosa , New Delhi, 2001. 138. Zhijun, X. & Wenqing, F. Optimisation pp. 1049-052. of SPRITE detectors. Infrared Physics, 1990, 30, 489-97. 130. Pathak, H.T.; Gandhi, A,; Khurana, K.; Sharma, A. & Singh, K.P. PLG metal lift-off process 139. Singh, R.; Sharma, B.L.; Mittal, V.; Pathak, for MCT 20-element PC array. In National H.T.; Srivastava, S.; Pal, R.; Singh, J.P.; Dewan, H.S.; Madaria, R.K.; Basu, P.K. & 140. Pal, R.; Singh, J.P.; Mittal, V.; Sharma, Warrier, A.V.R. Fabrication of (HgCd)Te B.L.; Srivastava, S.; Pathak, H.T.; Singh, R.; photoconductive linear arrays for thermal Madaria, R.K.; Basu, P.K. & Warrier, A.V.R. imaging in 8-14 pm range. In Proceedings of Temperature and field-dependence of the Emerging Opto-elecctronic Technologies noise in 0.1 eV MCT PC IR detectors. In (CEOT). McGraw-Hill, Delhi, 1994. National Conference on Recent Advancesi n pp. 252-55. Semiconductors (NCRAS), IIT Delhi, 1995. Contributors DF Rissl Singb obtained his PhD in Physics from the Indian lnstitute of Technology (IIT) Delhi, New Delhi, in 1973 and Advanced Diploma in French from the University of Delhi in 1977. Presently, he is working as Sci F at the Solid State Physics Laboratory, (SSPL) Delhi. He was awarded the Alexander von Humboldt (AvH) senior fellowship for pursuing postdoctoral studies at the Institute of Semiconductor Electronics, Faculty of Electrical Engineering, Technical University of Aachen (Germany) during 1981-83. His areas of interest are: MOS device physics and technology, VLSI processing and materials, and infrared detector technology. He has about 50 research publications1 presentations in international journals and conference proceedings. He is a life member of the Materials Research Society of India. Dr Vandna Mittal obtained her MSc in Physics from the University of Roorkee and received her PhD in thin wear resistant coatings from the IIT Delhi, New Delhi, in 1988. She joined DRDO at the SSPL, Delhi, in 1988 and is presently working as Sci D. Her areas of research are: Infrared detector technology, photolithography, and dry processing technology. She has about 20 research publicationslpresenrations in nationallinternational journals and conference proceedings.