Semiconductor Detectors and Focal Plane Arrays for Far-Infrared Imaging

OPTO−ELECTRONICS REVIEW 21(4), 406–426

DOI: 10.2478/s11772−013−0110−x

Semiconductor detectors and focal plane arrays for far-infrared imaging

A. ROGALSKI*

Institute of Applied Physics, Military University of Technology, 2 Kaliskiego Str., 00–908 Warsaw, Poland

The detection of far−infrared (far−IR) and sub−mm−wave radiation is resistant to the commonly employed techniques in the neighbouring microwave and IR frequency bands. In this wavelength detection range the use of solid state detectors has been hampered for the reasons of transit time of charge carriers being larger than the time of one oscillation period of radiation. Also the energy of radiation quanta is substantially smaller than the thermal energy at room temperature and even liquid ni− trogen temperature. The realization of terahertz (THz) emitters and receivers is a challenge because the frequencies are too high for conventional electronics and the photon energies are too small for classical optics. Development of semiconductor focal plane arrays started in seventies last century and has revolutionized imaging sys− tems in the next decades. This paper presents progress in far−IR and sub−mm−wave semiconductor detector technology of fo− cal plane arrays during the past twenty years. Special attention is given on recent progress in the detector technologies for real−time uncooled THz focal plane arrays such as Schottky barrier arrays, field−effect transistor detectors, and micro− bolometers. Also cryogenically cooled silicon and germanium extrinsic photoconductor arrays, and semiconductor bolome− ter arrays are considered.

Keywords: THz detectors, focal plane arrays, Schottky barrier diodes, semiconductor hot electron bolometers, extrinsic photodetectors, performance limits.

1. Introduction medical and security applications, have led to increased activity by the mainstream physics and engineering commu− Far−infrared (far−IR) range of electromagnetic spectrum still nity in recent times. Because diffraction is important at presents a challenge for both electronic and photonic tech− these frequencies, THz systems cannot be successfully de− nologies and is often described as the final unexplored area signed using traditional optical techniques alone. In the low of spectrum. Terahertz (THz) band bordered by far−IR infra− frequency THz region (below 0.5 THz), the radiation can red and millimeter−wave bands of the spectrum spans the penetrate most nonmetalic and nonpolar substances and be− transition range from radio−electronics to photonics. cause of it the imaging applications are the substantial part THz radiation is frequently treated as the spectral region of technological development in this direction. within frequency range n » 1–10 THz (l»300–30 μm) The past 20 years have seen a revolution in THz sys− [1–3] and it is partly overlapping with the loosely treated tems, as advanced materials research provided new and submillimeter (sub−mm) wavelength band n » 0.1–3 THz higher−power sources, and the potential of THz for ad− (l » 3 mm – 100 μm) [4]. Even a wider region n » 0.1–10 THz [5,6] is treated as THz band overlapping thus with vanced physics research and commercial applications was sub−mm wavelength band. As result, frequently both these demonstrated. Numerous recent breakthroughs in the field notions are used as equal ones (see, e.g., Ref. 7). Here THz have pushed THz research into the centre stage. As exam− range is accepted as the range within n » 0.1–10 THz. ples of milestone achievements can be included the devel− The THz region of the electromagnetic spectrum has opment of THz time−domain spectroscopy (TDS), THz ima− proven to be one of the most elusive. Being situated bet− ging, and high−power THz generation by means of nonlin− ween infrared light and microwave radiation, THz radiation ear effects [1–3]. Researches evolved with THz technolo− is resistant to the techniques commonly employed in these gies are now receiving an increasing attention, and devices well established neighbouring bands. Historically, the THz exploiting this wavelength band are set to become increas− technology development was driven by astronomers and ingly important in a diverse range of human activity appli− planetary scientists, but other potential uses, particularly in cations (e.g., security, biological, drugs and explosion de− tection, gases fingerprints, imaging, etc.). The interest in the THz range is attracted by the fact that this range is the place *e−mail: [email protected] where different physical phenomena are revealed which fre−

406 Opto−Electron. Rev., 21, no. 4, 2013 quently calls for multidisciplinary special knowledge in this Progress in THz detector sensitivity has been impressive research area. Nowadays, the THz technology is also of within a period of more than half century what is shown in much use in fundamentals’ science, such as nanomaterials Fig. 1(a) in the case of bolometers used in far−IR and sub− science and biochemistry. This is based on the fact that THz −mm−wave astrophysics [12]. The NEP value has decreased frequencies correspond to single and collective excitations by a factor of 1011 in 70 years, corresponding improvements in nanoelectronic devices and collective dynamics in bio− by factor of two every two years. Individual detectors achie− molecules. In 2004 Technology Review’s editors selected ved photon noise limited performance for ground−based THz technology as one of “10 emerging technologies that imaging in the 1990s. The photon noise from astrophysical will change your world” [8]. sources, achievable in space with a cold telescope, ~10–18 W/ÖHz, is now within demonstrated sensitivities. In present 2. Trends in developments of far-infrared decade, the studies of inflation via cosmic microwave back− detectors ground (CMB) polarization will be driven not by detector sensitivity but by array formats. Far−infrared spectroscopy The detection of THz radiation is resistant to the commonly from a cold telescope however requires sensitivity of ~10–20 employed techniques in the neighbouring microwave and in− W/ÖHz to reach the astrophysical photon noise limit. Achie− frared (IR) frequency bands. In THz detection the use of solid ving this sensitivity in working detector arrays remains state detectors has been hampered for the reasons of transit a challenge for the coming decade, as the number of the time of charge carriers being larger than the time of one oscil− detectors in the array is a key parameter that determines the lation period of THz radiation. Also the energy of radiation information capabilities of the system and the speed impro− quanta is substantially smaller than the thermal energy at room vement in obtaining a complete imaging or spectrum when temperature and even liquid nitrogen temperature. observing galactic objects. Detector development is at the heart of all current plants. The development of pixel arrays has been comparably There is a large variety of traditional deeply cooled mm and revolutionary [12]. Figure 1(b) shows increasing in number sub−mm wavelength detectors (mainly bolometers) as well of pixels in the period over last two decades. Detector arrays as new propositions based on optoelectronic quantum de− have doubled in format every 20 months over the past vices [9], carbon nanotube bolometers, plasma wave detec− 10 years producing arrays with pixels now numbering in the tion by field effect transistors, and hot electron room tem− thousands. Steady increase in overall observing efficiency perature bipolar semiconductor bolometers [10,11]. Espe− is expected in near future, which by now has reached factors cially astronomy has greatly benefited from the advances in in the range of 1012 in comparison to capabilities in the early semiconductor technology over the past few decades. The 1960s. development of far−infrared arrays has yielded orders of NASA has historically been the leading US agency for magnitude increases in the observational capability. promoting the development of long wavelength detector

Fig. 1. Trends in development of THz detectors: (a) improvement of the bolometer NEP−value for more than half a century (sensitivity dou− bled every 2 years over the past 70 years), (b) detector arrays have doubled in format every 20 months over the past 10 years; green symbol in− dicates current expectation (adapted after Ref. 12).

Opto−Electron. Rev., 21, no. 4, 2013 A. Rogalski 407 Semiconductor detectors and focal plane arrays for far−infrared imaging

Fig. 3. Fundamental optical excitation processes in semiconductors: (a) intrinsic absorption, (b) extrinsic absorption, (c) free carrier absorption.

The photon detectors show a selective wavelength depend− ence of response per unit incident radiation power (see Fig. 4). They exhibit both good signal−to−noise performance and a very fast response. But to achieve this, the photon detectors require cryogenic cooling. This is necessary to prevent the thermal generation of charge carriers. The ther− mal transitions compete with the optical ones, making Fig. 2. Sensitivity of far−IR spectroscopy platforms (after Ref. 12). non−cooled devices very noisy. Depending on the nature of the interaction, the class of photon detectors is further sub−divided into different types. technologies. Figure 2 shows the sensitivities of currently− The most important are: intrinsic detectors, extrinsic detec− −planned or active far−IR/sub−mm spectroscopic facilities in tors, photoemissive (Schottky barriers). Different types of near future. detectors are described in details in monograph Infrared As the last figure shows, the James Webb Space Tele− Detectors [13]. Figure 5 shows spectral detectivity charac− scope (JWST) operates at wavelengths below about 27 μm. teristics of different types of detectors [14]. The Atacama Large Millimeter/submillimeter Array (ALMA) Photoconductors that utilize excitation of an electron operating through a number of submillimeter atmospheric from the valence to a conduction band are called intrinsic windows as well as ~650 μm, will have sensitivities at least detectors. Instead of those which operate by exciting elec− 100 times higher than Herschel spanning the intervening trons into the conduction band or holes into the valence 60–650 μm wavelength range. ALMA and JWST are cur− band from impurity states within the band (impurity−bound rently scheduled to start operations within the next few years. states in energy gap, quantum wells or quantum dots), are The Space Infrared Telescope for Cosmology and Astrophys− called extrinsic detectors. In comparison with intrinsic pho− ics (SPICA), with launch envisioned in 2017, will provide toconductivity, the extrinsic photoconductivity is far less two−to−three orders of magnitude increase in sensitivity in efficient because of the limits in the amount of impurity that comparison with Herschel that will bring far−IR/sub−mm sen− can be introduced into semiconductor without altering the sitivity into line with those of JWST and ALMA. The ambi− nature of the impurity states (see Fig. 6). Intrinsic detectors tious requirements of future space missions are summarized in Ref. 11.

3. General classification of far-infrared detectors

The majority of detectors can be classified in two broad cat− egories: photon detectors and thermal detectors. 3.1. Photon detectors In photon detectors the radiation is absorbed within the material by interaction with electrons either bound to lattice atoms or to impurity atoms or with free electrons. The observed electrical output signal results from the changed electronic energy distribution. The fundamental optical ex− citation processes in semiconductors are illustrated in Fig. 3 Fig. 4. Relative spectral response for a photon and thermal detector.

408 Opto−Electron. Rev., 21, no. 4, 2013 © 2013 SEP, Warsaw Fig. 5. Comparison of the D* of various available detectors when operated at the indicated temperature. Chopping frequency is 1000 Hz for all detectors except the thermopile (10 Hz), thermocouple (10 Hz), thermistor bolometer (10 Hz), Golay cell (10 Hz) and pyroelectric detec− tor (10 Hz). Each detector is assumed to view a hemispherical surrounding at a temperature of 300 K. Theoretical curves for the back− ground−limited D* (dashed lines) for ideal photovoltaic and photoconductive detectors and thermal detectors are also shown. PC – photoconductive detector, PV – photovoltaic detector, PEM – photoelectromagnetic detector, and HEB – hot electron bolometer. are most common at the short wavelengths, below 20 μm. It comparison with intrinsic detectors. Low−temperature oper− is interesting to note the presence of a marked gap in the ation is associated with longer−wavelength sensitivity in wavelength coverage around 40 μm. order to suppress noise due to thermally induced transitions A key difference between intrinsic and extrinsic detec− between close−lying energy levels. The long wavelength tors is that extrinsic detectors require much cooling to achie− cutoff can be approximated as ve high sensitivity at a given spectral response cutoff in 300K T = . (1) max lm c []m

The general trend is illustrated in Fig. 7 for five high per− formance detector materials suitable for low−background applications: Si, InGaAs, InSb, HgCdTe photodiodes, and Si:As blocked impurity band (BIB) detectors; and extrinsic Ge:Ga unstressed and stressed detectors. Terahertz photo− conductors are operated in extrinsic mode. One advantage of photoconductors is their current gain which is equal to the recombination time divided by the majority−carrier tran− sit time. This current gain leads to higher responsivity than is possible with nonavalanching photovoltaic detectors. However, series problem of photoconductors operated at low temperature is nonuniformity of detector element due to Fig. 6. Typical quantum efficiencies of semiconductors used in the recombination mechanisms at the electrical contacts and its wavelength range between 1 and 300 μm. dependence on electrical bias.

Opto−Electron. Rev., 21, no. 4, 2013 A. Rogalski 409 Semiconductor detectors and focal plane arrays for far−infrared imaging

Fig. 7. Operating temperatures for low−background material systems with their spectral band of greatest sensitivity. The dashed line indi− cates the trend toward lower operating temperature for longer wavelength detection.

3.2. Thermal detectors face coating, the spectral response can be very broad. Atten− tion is directed toward three approaches which have found The second class of detectors is composed of thermal detec− the greatest utility in infrared technology, namely, bolom− tors. In a thermal detector shown schematically in Fig. 8, the eters, pyroelectric and thermoelectric effects. In pyroelectric incident radiation is absorbed to change the material tem− perature, and the resultant change in some physical property detectors a change in the internal electrical polarization is is used to generate an electrical output. The detector is sus− measured, whereas in the case of thermistor bolometers pended on lags, which are connected to the heat sink. The a change in the electrical resistance is measured. signal does not depend upon the photonic nature of the inci− Usually bolometer is a thin, blackened flake or slab, dent radiation. Thus, thermal effects are generally wave− whose impedance is highly temperature dependent. Bolom− length independent (see Fig. 4); the signal depends upon the eters may be divided into several types. The most com− radiant power (or its rate of change) but not upon its spectral monly used are the metal, the thermistor, and the semicon− content. Since the radiation can be absorbed in a black sur− ductor bolometers. The fourth type is the superconducting bolometer. This bolometer operates on a conductivity tran− sition in which the resistance changes dramatically over the transition temperature range. The key trade−off with respect to conventional unco− oled thermal detectors is between sensitivity and response time. The detector sensitivity is often expressed by noise equivalent different temperature (NEDT) represented by the temperature change, for incident radiation, that gives an output signal equal to the rms noise level. The thermal conductance is an extremely important parameter, since 1/2 NEDT is proportional to (Gth) , but the thermal response time of the detector, tth, is inversely proportional to Gth. Therefore, a change in thermal conductance due to impro− vements in material processing technique improves sensi− tivity at the expense of time response. Typical calculations of the trade−off between NEDT and time response carried out in Ref. 15 are shown in Fig. 9. If the NEDT is dominated by a noise source that is pro− Fig. 8. Schematic diagram of thermal detector. portional to Gth, what has place when Johnson and 1/f noises

410 Opto−Electron. Rev., 21, no. 4, 2013 © 2013 SEP, Warsaw rection as a result of the change in its spontaneous polariza− tion with temperature. The choice of pyroelectric materials is not an obvious one as it will depend on many factors including the size of the detector required, the operating temperature and the frequency of operation. An ideal mate− rial should have large pyroelectric coefficient, low dielectric constant, low dielectric loss and low volume specific heat. The possibility of satisfying these requirements in a single material is not promising. While it is generally true that a large pyroelectric coefficient and a small dielectric con− stant are desirable, it is also true that these two parameters are not independently adjustable. Thus, we find that materi− als having a high pyroelectric coefficient also have a high dielectric constant, and materials having a low dielectric Fig. 9. Trade−off between sensitivity and response time of uncooled constant also have a low pyroelectric coefficient. This me− thermal imaging systems (after Ref. 15). ans that different detector−preamplifier sizes and configu− rations will be optimized with different materials. are dominated, and since tth = Cth/Gth, then the Figure of Merit given by 4. Detectors for room temperature THz imaging =´t FOM NEDT th (2) Particular attention in development of THz imaging systems is devoted to the realization of sensors with a large potential for can be introduced for long wavelength IR bolometers [16]. real−time imaging while maintaining a high dynamic range and Users are interested not only in the sensitivity, but also in room−temperature operation. CMOS process technology is es− their thermal time constants and the FOM described by pecially attractive due to their low price tag for industrial, sur− Eq. (2) recognizes the tradeoffs between thermal time con− veillance, scientific, and medical applications. However, stant and sensitivity. CMOS THz imagers developed thus far have mainly operated Many types of thermal detectors operated in LWIR and single detectors based on a lock−in technique to acquire ras− far−IR regions (including bolometers, pyroelectric detectors ter−scanned imagers with frame rates on the order of minutes. and Golay cells) are also used in THz band. The operation With this mind, much of recent developments are directed principles of thermal detectors are described in Ref. 13. towards three type of focal plane sensors: Bolometers, as other thermal devices, for a long time l Schottky barrier diodes (SBDs) compatible with CMOS traditionally were treated as slow devices and their perfor− process, mance is limited by a trade−off between speed and sensitiv− l field effect transistors (FETs) relay on plasmonic rectifi− ity. For conventional uncooled microbolometers operated in cation phenomena, and a wavelength range of 10–100 μm at room temperature, the l adaptation of infrared bolometers to the THz frequency typical value of heat capacity is about of 2×10–9 J/K (for range. bolometer with dimensions 50×50×0.5 μm) and thermal An important issue for a FPA is pixel uniformity. It –7 conductance of 10 W/K (a−Si or VOx bolometers). Both appears however, that the production of monolithically−in− t parameters define time constant, th, equal about 20 ms. The tegrated detector arrays encounters so many technological upper NEP limit of thermal detector defines intrinsic tem− problems that the device−to−device performance variations perature fluctuation noise. Then, the upper limit of NEP for and even the percentage of non−functional detectors per chip a bolometer limited only by radiation exchange with the tend to be unacceptably high. To solve the problems a num− » –13 1/2 environment is NEPR 2.7×10 W/Hz . The upper limit ber of research groups are working on. » –19 1/2 value of NEPR 1.7×10 W/Hz can be achieved only at The perofmance of monolithically integrated detector the expense of large t values (t » 3.5×104 s) [10]. arrays with other room temperature THz detectors is sum− If the bolometer is to be used as a THz mixer, it has to be marized in Table 1. It is difficult to make clean comparison fast enough to follow the IF, i.e., the overall time constant of since the differences in the tuned frequency are consider− the processes involved in the mixing has to be a few tens of able. In general, the NEP of SBD and FET detectors is better picoseconds at maximum. In other words, high heat conduc− than that of Golay cells and pyroelectric detectors around tivity and small heat capacity are required [17]. These requi− 300 GHz. Both the pyroelectric and the bolometer FPAs rements can be fulfilled by such subsystem as electrons in with detector response times in the millisecond time range semiconductor or superconductor interacting with the lattice are not suited for heterodyne operation. FET detectors are (phonons). Electron heat capacity is many orders lower clearly capable in heterodyne detection with improving sen− compared to the lattice one. sitivity. Diffraction aspects predicts FPAs for higher fre− Whenever a pyroelectric crystal undergoes a change of quencies (0.5 THz and above) and in conjunction with large temperature, surface charge is produced in a particular di− f/# optics.

Opto−Electron. Rev., 21, no. 4, 2013 A. Rogalski 411 Semiconductor detectors and focal plane arrays for far−infrared imaging

Table 1. Comparison of uncooled THz detector performance. NEP Responsivity Frequency Monolithic Technology Optics Ref. (pW/Hz1/2) (kV/W) (THz) integration 2×2 array, 130−nm digital 66* 0.063* 0.28 Yes – 18 CMOS, 450×450 μm pitch 73** 20** 4×4 array, 130−nm digital 29* 355 0.28 Yes Without Si 19 SBD CMOS, 500×500 μm pitch lens GaAs 20 0.4* 0.8 No – 20 1.5 3.8* 0.15 ErAs/InAlGaAs 1.4 6.8* 0.1 No – 21 Heterojunction InAs/AlSb/AlGaSb 0.18 49.7 0.094 No – 22 backward diode Silicon 100* 0.033* 0.7 Yes – 23 3×5 array, 250−nm CMOS, 300** 80** 0.65 Yes Si lens 24 MOSFET 150×200 mm pitch 32×32 array, 65−nm SOI 100** 140** 0.856 Yes Si lens 25 CMOS, 80×80 μm pitch 320×240 array, 23.5×23.5 < 100 – 3 Yes Lens 26 μm pitch, NbN 320×240 array, 49×49 μm < 30 – 2.5 Yes Lens 27 Bolometer pitch, a−Si 160×120 array, 52×52 μm 70 – 3 Yes Mirrors 28 pitch, VOx and lens Gollay cell – 200–400 0.1–45 0.2–30 No – 29 Pyroelectric – 400 150** 0.1–30 No – 30

*without amplifier, **with amplifier

Below, a short description of three kinds of uncooled tacts to n−Ge, n−GaAs, n−InSb were used (see, e.g., Ref. 32). THz detectors is presented. In the mid 1960s Young and Irvin [33] developed the first lithographically defined GaAs Schottky diodes for high fre− 4.1. Schottky barrier diodes quency applications. Their basic diode structure was next replicated by a variety of groups. The basic whiskered diode In spite of achievements of other kind of detectors for THz structure, depicted in Fig. 10(a), greatly improved the qual− waveband, the Schottky barrier diodes (SBDs) are among ity of the diode due to inherently low capacity of the wicker the basic elements in THz technologies. They are used ei− contact. The pointed metal wire has a tip diameter of about ther in direct detection and as nonlinear elements in hetero− 0.5 μm and contacts a single anode in the array. The metal dyne receiver mixers operating in a temperature range of anodes are about 0.2 μm where they contact the GaAs sur− 4–300 K. The cryogenically cooled SBDs were used in mix− face which is located below the passivation layer (usually ers preferably in the 1980s and early the 1990s and then silicon dioxide). A tipped metal whisker provides the elec− they have been replaced widely by superconductor−insula− trical contact to the anode and serves as a long wire antenna tor−superconductor (SIS) or hot electron bolometer (HEB) to couple in an external radiation. mixers [31], in which mixing processes are similar to that The SBD structure shown in Fig. 10(a) is similar to the observed in SBDs, but, e.g., in SIS structures the rectifica− so−called “honeycomb” diode chip design, first produced by tion process is based on quantum−mechanical photon−assis− Young and Irvin in 1965 [33]. Due to limitation of whisker ted tunnelling of quasiparticles (electrons). The nonlinearity technology, such as constraints on design and repeatability, of SBD I−V characteristics (the current increases exponen− starting in the 1980s, the efforts were made to produce pla− tially with the applied voltage) is the prerequisite for mixing nar Schottky diodes [see Fig. 10(b)]. The air−bridged fingers to occur. replace the now obsolete whisker contact. This design has Historically, the first Schottky−barrier structures were been the most important step toward a practical Schottky pointed contacts of tapered metal wires (e.g., a tungsten nee− diode mixer for THz frequency applications, with several dle) with a semiconductor surface (the so−called crystal thousand diodes on a single chip and where parasitic losses detectors). Widely used were, e.g., contacts p−Si/W. At ope− such as the series resistance and the shunt capacitance are ration temperature T = 300 K they have NEP » 4×10–10 minimized. To achieve good performance at high frequen− W/Hz1/2. Also pointed tungsten or beryllium bronze con− cies the diode area should be small. By reducing junction

412 Opto−Electron. Rev., 21, no. 4, 2013 © 2013 SEP, Warsaw Fig. 10. GaAs Schottky barrier diode: (a) whisker contacted diode (after Ref. 34), (b) schematic of a planar diode, and (c) a four-diode chip array (after Ref. 34). area one reduces junction capacities to increase operating frequency. But at the same time one increases the series resistance. The planar Schottky diodes are fabricated on a highly doped GaAs substrate (» 5×1018 cm–3) with an ohmic con− tact on the back side and a thin GaAs epitaxial layer, with the thickness of 300 nm to 1 μm, is grown on the top of the substrate. Holes filled with a metal (Pt) in the SiO2 insulat− ing layer on top of the epitaxial layer define the anode diam− eter (0.25–1 μm) [35]. The state−of−the−art devices have anode diameters about 0.25 μm and capacitances 0.25 fF. In the case of a discrete diode chip, the diodes are flip−chip mounted into the circuit with either solder or con− ductive epoxy. Using advanced technology, the diodes are integrated with many passive circuit elements (impedance matching, filters and waveguide probes) onto the same sub− strate. By improving the mechanical arrangement and redu− cing loss, the planar technology is pushed well beyond 300 Fig. 11. Dependence of SBD voltage sensitivity on radiation fre− GHz up to several THz. For example, Figure 10(c) shows quency dependence for diodes produced by VDI, Virginia Diode, photographs of a bridged four−Schottky diodes’ chip arra− Inc. (after Ref. 22). yed in a balanced configuration to increase power handling and Figure 11 presents frequency dependent voltage sensi− tivity characteristics of SBDs at room temperature. VDI 4.2. Field effect transistor detectors offers zero biased detectors with full waveguide band opera− The channel of a field effect transistor (FET) can act as tion, high sensitivity, and high responsivity for a variety of a resonator for plasma waves with a typical wave velocity THz applications. 108 cm/s. The fundamental frequency of this resonator de− The typical Schottky diodes usually have high low−fre− pends on its dimensions and, for gate length of a micron or quency noise levels due to introduction of oxides, contami− less, can reach the THz range. The use field effect transis− nants, and damage to the junction in the fabrication process. tors (FETs) as detectors of THz radiation was first proposed More recently, an alternative method of Schottky barrier by Dyakonov and Shur in 1993 [36] on the basis of formal formation has been elaborated by molecular beam epitaxy analogy between the equations of the electron transport in (MBE) in−situ deposition of a semimetal (ErAs) on semi− a gated two−dimensional transistor channel and those of conductor (InGaAs/InAlAs on InP substrates) to reduce the shallow water, or acoustic waves in music instruments. As imperfections that give rise to excess low−frequency noise, a consequence, hydrodynamic−like phenomena should exist particularly l/f noise [21]. Excellent NEP performance for also in the carrier dynamics in the channel. Instability of this this III−V semiconductor SBD has been reported (1.4 flow in the form of plasma waves was predicted under cer− pW/Hz1/2 at 100 GHz). By using interband tunnelling, a he− tain boundary conditions. Figure 13 schematically shows terojunction backward diode presented in Ref. 22 demon− the resonant oscillation of plasma waves in gated region of strated 49.7 kV/W responsivity and 0.18 pW/Hz NEP at 94 FET. GHz. Recently Han et al. have demonstrated fully func− Nonlinear properties of plasma wave excitations (the tional CMOS imager operating near or in the sub−millime− electron density waves) in nanoscale FET channels enable ter−wave frequency range (see Fig. 12). The 4×4 array incre− their response at frequencies appreciably higher than the ases the imaging Speer by 4 to 8 times, due to fewer mecha− device cutoff frequency, what is due to electron ballistic nical scan steps. transport. In the ballistic regime of operation, the momen−

Opto−Electron. Rev., 21, no. 4, 2013 A. Rogalski 413 Semiconductor detectors and focal plane arrays for far−infrared imaging

Fig. 12. CMOS SBD 280−GHz imager: die photos of the array (a) and an image of music greeting card (b) obtained (after Ref. 13). tum relaxation time is longer that the electron transit time. are used in fabrication FET, HEMT, and MOSFET devices The FETs can be used both for resonant (resonant case of including: Si, GaAs/AlGaAs, InGaP/InGaAs/GaAs, and high electron mobility, when plasma oscillation modes are GaN/AlGaN. Currently, the most promising application ap− excited in the channel) and non−resonant (broadband) THz pears to be the broadband THz detection and imaging in the detection [37] in the case of low mobility, where plasma non−resonant regime. oscillations are overdamped. The physical mechanism sup− The large−scale interest in using FETs as THz detectors porting the development of stable oscillations lies in the started around 2004 after the first experimental demonstra− refection of plasma waves at the borders of transistor with tion of sub−THz and THz detection in silicon−CMOS FETs subsequent amplification of the wave’s amplitude. Plasma [38]. Two years later it was shown that Si−CMOS FETs can excitations in FETs with sufficiently high electron mobi− reach a NEP value competitive with the best conventional lity can be used for emission as well as detection of THz room temperature THz detectors [23]. At present the advan− radiation. tages of Si−CMOS FET technology (room temperature oper− The detection by FETs is due to nonlinear properties of ation, very fast response time, easy on−chip integration with the transistor, which lead to the rectification of the ac cur− read−out electronics and high reproducibility) lead to the rent induced by the coming radiation. As a result, a photo− straight−forward array fabrication. response appears in the form of a dc voltage between source At present, focal plane arrays in silicon technology have and drain. This voltage is proportional to the radiation inten− been designed for imaging at frequencies reaching 1 THz sity (photovoltaic effect). Even in the absence of an antenna, range. Si FETs are clearly capable of multipixel parallel the THz radiation is coupled to the FET by contact pads and detection. For example, Figure 14 shows 3×5 FPA of Si bonding wires. A big progress in sensitivity can be obtained NMOS FET detectors for 0.65 THz fabricated in a commer− by adding a proper antenna or a cavity coupling. cial quarter−micron technology [24]. Each pixel consisted of The transistor receivers operate in a wide temperature a narrow−band patch antenna including an integrated gro− range up to room temperatures. Different material systems undplane, a differential NMOS FET pair, and a 43−dB volt−

Fig. 13. Plasma oscillations in a field effect transistor.

414 Opto−Electron. Rev., 21, no. 4, 2013 © 2013 SEP, Warsaw A camera with 32×32 pixel array fully integrated in a 65−nm CMOS process technology has been demonstrated [25] (see right side of Fig. 15). Each 80−μm array pixel consists of a differential on−chip ring antenna coupled to NMOS direct detector operated well−beyond its cut−off frequency. The camera chip has been packed together with a 41.7−dBi sili− con lens in a 5×5×3 cm3 camera module. In continuous− −wave illumination (video mode) the camera achieves the responsivity of 100–200 kV/W and the total NEP of 10–20 nW/Hz1/2 up to 500 fps at 856 GHz. In a non−video mode the maximum responsivity is 140 kV/W and the minimum NEP was reached at 856 GHz for a 5−kHz chopping frequency. Fig. 14. Left side: micrograph of a wire−bonded 3×5−pixel 0.65−THz CMOS FPA. Top right side: one of the differentia patch antenna (the transmission lines guiding the THz signals to the FETs are visible). 4.3. Microbolometers Bottom right: cross−section through a patch antenna (after Ref. 24). An impressive promising technology is also coming from commercially available microbolometer arrays. Adaptation age amplifier with a 1.6 MHz bandwidth. The radiation is of infrared microbolometers to the THz frequency range shielded from the doped and hence loosely substrate by after the successful demonstration of active THz imaging in a metal layer which represents the groundplane of the patch 2006 [39] entailed that in the period of 2010–2011 three dif− antennas. It is only open to the substrate at the locations of ferent companies/organizations announced cameras optimi− the transistors and the protection diodes. For these detectors, zed for the >1−THz frequency range: NEC (Japan) [26], the measured responsivities were between 70–80 kV/W at INO (Canada) [28] and Leti (France) [27]. The number of gate voltage around 0.2 V, while the best NEP values were vendors is expected to increase soon. 300 pW/ÖHz between 0.4 and 0.5 V. The experimental active THz imaging arrangement is Recently, the first CMOS FPA used to capture transmis− shown in Fig. 16. The most of papers presented character− sion−mode THz video streams in real−time without need for izations of FPAs using monochromatic THz sources such as raster scanning and source modulation has been fabricated. quantum cascade lasers (QCLs) or far infrared optically

Fig. 15. Development status of uncooled THz focal plane arrays.

Opto−Electron. Rev., 21, no. 4, 2013 A. Rogalski 415 Semiconductor detectors and focal plane arrays for far−infrared imaging

Fig. 16. Experimental setup of THz imaging system. Cutaway depicts alternative reflection mode setup. pumped lasers delivering mW−range powers. As shown in standard IR bolometer. The membrane is suspended over Fig. 17 the reflected beam backlights an object with a maxi− the substrate by arms and pillars. In order to enhance the mum area and the transmitted light is collected by a camera antenna gain, an equivalent quarter−wavelength resonant lens. The focal plane is positioned behind the camera lens, cavity is realized under antennas with an 11−μm thick SiO2 making the object plane in front of the lens. Also shown is layer deposited over the metallic reflector. To ensure elec− the modified reflection mode setup, where a specular reflec− tric contact between the bolometer pillars and CMOS metal tion is collected by the repositioned lens and camera. upper contacts, the vias are etched through the 11−μm cavity Different designs of THz bolometer pixels have been and then metalized. proposed. NEC’s pixel is divided into two parts (see Figure 18 summarized the NEP values for bolometer Fig. 17), silicon Si readout integrated circuit (ROIC) in the FPAs fabricated by three vendors. The FPAs optimized for lower part and suspended microbridge structure in the upper 2–5 THz exhibit impressive NEP values below 100 part. The microbridge has a two−storied structure. The 1st pW/Hz1/2 (also the order of tens of pW/Hz1/2 at video rate floor is composed of a diaphragm and two legs, while the 20–30 fps). It can be seen that wavelength dependence of eaves structure is formed on the diaphragm to increase the NEP is quite flat below 200 μm. Further improvement of sensitive area and fill factor. The diaphragm and the eaves performance is possible by increasing number of pixels, absorb THz radiation. The diaphragm is composed of VOx modification of antenna design while preserving pixel pitch, bolometer thin film, SiNx passivation layers and TiAlV ROIC and technological stack. electrodes, while the eaves structure is composed of SiNx Recently new microbolometeric approach for design of layer and TiAlV thin film THz absorption layer. array pixel has been proposed – using Si FET and exploiting A schematic of one Leti’s pixel of an amorphous silicon the dependence of the channel’s conductance on tempera− microbolometer array is shown in top centre of Fig. 16. The ture [40]. It should be mentioned about Tracer Systems Inc. 50−μm pitch is associated with quasi−double−bowtie anten− nas to a thermometer microbridge structure derived from the

Fig. 17. Schematic diagrams of pixel structure of uncooled bolo− meter−type THz−FPA (after Ref. 26). Fig. 18. Spectral dependence of NEP for bolometer THz FPAs.

416 Opto−Electron. Rev., 21, no. 4, 2013 © 2013 SEP, Warsaw FPA design and fabrication based on antenna−coupled tun− nelling diodes for the frequency regime from 0.6 to 1.2 THz [41]. FPAs with the size up to 120×120 pixels, are specified with the NEP of 5 nW/Hz1/2. Heating by radiation in HgCdTe layers can be also used for designing uncooled THz/sub−THz detectors with appropriate characteristics for active imaging and possibility to be assembled into arrays [42].

5. Extrinsic detectors Historically, an extrinsic photoconductor detector based on germanium was the first extrinsic photodetector. After it, photodetectors based on silicon and other semiconductor materials, such as GaAs or GaP, have appeared. Fig. 19. Relative spectral response of some germanium extrinsic Extrinsic photodetectors are used in a wide range of the photoconductors (after Ref. 45). IR spectrum extending from a few μm to approximately 300 μm. They are the principal detectors operating in the range of l > 20 μm. The spectral range of particular photo− germanium photoconductors doped with Zn, Be, Ga and of detectors is determined by the doping impurity and by the stressed gallium doped germanium [45]. Despite a large material into which it is introduced. For the shallowest amount of effort recently in the development of very sensi− impurities in GaAs, the long wavelength cutoff of photores− tive thermal detectors, germanium photoconductors remain ponse is around 300 μm. Detectors based on silicon and ger− the most sensitive detectors for wavelength shorter than manium have found the widest application as compared 240 μm. with extrinsic photodetectors on other materials. The achievement of low NEP values in the range of Research and development of extrinsic IR photodetec− a few parts 10–17 WHz–1/2 was made possible by advances in tors have been ongoing for more than 60 years [43,44]. In crystal growth development and control of the residual mi− the 1950s and 1960s, germanium could be made purer than nority impurities down to 1010 cm–3 in a doped crystal. As silicon; doped Si then needed more compensation than a result, a high lifetime and mobility value and thus a higher doped Ge and was characterized by shorter carrier lifetimes photoconductive gain have been obtained. than extrinsic germanium. Today, the problems with pro− Ge:Be photoconductors cover the spectral range from ducing pure Si have been largely solved, with the exception » 30 to 50 μm. Be doping concentrations of 5´1014 cm–3 of boron contamination. Si has several advantages over Ge; to 1´1015 cm–3 give significant photon absorption in for example, three orders of magnitude higher impurity sol− 0.5–1 mm thick detectors, while at the same time keeping ubilities are attainable, hence thinner detectors with better dark currents caused by hopping conduction. Responsivities spatial resolution can be fabricated from silicon. Si has > 10 A/W at l = 42 μm and quantum efficiency 46% have lower dielectric constant than Ge, and the related device been reported at low background [46]. technology of Si has now been more thoroughly developed, Ge:Ga photoconductors are the best low background including contacting methods, surface passivation and ma− photon detectors for the wavelength range from 40 to ture MOS and CCD technologies. Moreover, Si detectors 120 μm. Since the absorption coefficient for a material is are characterized by superior hardness in nuclear radiation given by the product of the photoionization cross−section environments. and the doping concentration, it is generally desirable to maximize this concentration. The practical limit occurs 5.1. Extrinsic germanium photoconductors when the concentration is so high that impurity band con− duction results in an excessive dark current. For Ge:Ga the Silicon detectors have largely supplanted germanium ex− onset of impurity banding occurs at approximately 2×1014 trinsic detectors for both high and low background applica− cm–3, resulting in an absorption coefficient of only 2 cm–1 tions where comparable spectral response can be obtained. and typical values of quantum efficiency range from 10% to However, for wavelengths longer than 40 μm there are no 20% [47]. Consequently, the detectors must either have appropriate shallow dopants for silicon; therefore germa− long physical absorption path lengths or be mounted in an nium devices are still of interest for very long wavelengths. integrating cavity. Germanium photoconductors have been used in a variety of However, there are a number of problems with the use of infrared astronomical experiments, both airborne and space− germanium. For example, to control dark current the mate− −based at wavelength ranging from 3 to more than 200 μm. rial must be lightly doped and, therefore, absorption lengths Very shallow donors, such as Sb, and acceptors, such as B, become long (typically 3–5 mm). Because the diffusion In or Ga, provide cut−off wavelengths in the region of 100 lengths are also large (typically 250–300 μm), pixel dimen− μm. Figure 19 shows the spectral response of the extrinsic sions of 500–700 μm are required to minimize crosstalk. In

Opto−Electron. Rev., 21, no. 4, 2013 A. Rogalski 417 Semiconductor detectors and focal plane arrays for far−infrared imaging space applications, large pixels imply higher hit rates for An innovative integral field spectrometer, called the cosmic radiation. This in turn implies very low readout Field Imaging Far−Infrared Line Spectrometer (FIFI−LS) noise for arrays operated in low background limit, what is that produces a 5×5 pixel image with 16 spectral resolution difficult to achieve for large pixels with large capacitance elements per pixel in each two bands was constructed at the and large noise. A solution is using the shortest possible Max Planck Institut für Extraterrestrische Physik under the exposure time. Due to a small energy band gap, the germa− direction of Albrecht Poglitsch. This array was developed nium detectors must operate well below the silicon “freeze− for the Herschel Space Observatory and SOFIA – see −out” range – typically at liquid helium temperature. Fig. 20 [51]. To accomplish this, the instrument has two Application of uniaxial stress along the [100] axis of 16×25 Ge:Ga arrays, unstressed for the 45–110 μm range Ge:Ga crystals reduces the Ga acceptor binding energy, and stressed for the 110 to 210 μm range. The low−stressed extending the cutoff wavelength to » 240 μm [48,49]. At the blue detectors has a mechanical stress on the pixels which is same time, the operating temperature must be reduced to reduced to about 10% of the level needed for the long−wave− less than 2 K (see Fig. 7). In making practical use of this length response of the red detectors. Each detector pixel is effect, it is essential to apply and maintain very uniform and stressed in its own subassembly, and a signal wire is routed controlled pressure to the detector so that the entire detector to preamplifiers housed nearby what obviously limits this volume is placed under stress without exceeding its break− type of array to much smaller formats than are available ing strength at any point. A number of mechanical stress without this constraints. modules have been developed. The stressed Ge:Ga photo− The Photodetector Array Camera and Spectrometer conductor systems have found a wide range of astronomical (PACS) is one of the three science instruments on ESA’s far and astrophysical applications [50,51]. infrared and sub−millimetre observatory – Herschel Space The standard planar hybrid architecture, commonly used Laboratory. Apart from two Ge:Ga photoconductor arrays, to construct near and mid−infrared focal−plane arrays [13], is it employs two filled silicon bolometer arrays with 16×32 not suitable for far IR detectors where readout glow, lack of and 32×64 pixels, respectively, to perform integral−field efficient heat dissipation, and thermal mismatch between spectroscopy and imaging photometry in the 60–210 μm the detector and the readout could potentially limit their per− wavelength regime. Figure 21 shows the spectral response formance. Usually, the far−infrared arrays have a modular of the filter/detector chain of the PACS photometer in its design with many modules stacked together to form a 2−di− three bands. Median NEP values are 8.9×10–18 W/Hz1/2 for mensional array. the stressed and 2.1×10–17 W/Hz1/2 for the unstressed detec− The Infrared Astronomical Satellite (IRAS), the Infrared tors, respectively. The detectors are operated at ~1.65 K. Space Observatory (ISO), and for the far−infrared channels The readout electronics is integrated into the detector mod− the Spitzer−Space Telescope (Spitzer) have all used bulk ules – each linear module of 16 detectors is read out by germanium photoconductors. In Spitzer mission a 32×32− a cryogenic amplifier/multiplexer circuit in CMOS techno− −pixel Ge:Ga unstressed array was used for the 70−μm band, logy but operates at temperature 3–5K. while the 160 μm band had a 2×20 array of stressed detec− As mentioned above, the standard hybrid focal plane tors. The detectors are configured in the so−called Z−plane to array (FPA) architecture is not generally suitable for far−IR indicate that the array has substantial size in the third dimen− arrays (although this architecture is used as well) primarily sion. The poor absorption of the Ge:Ga detector material because glow from the readout is sensed by the detector, requires that the detectors in this array are huge – 2 mm degrading its performance. In response, a new layered−hy− long. brid structure was introduced to alleviate these problems

Fig. 20. PACS photoconductor focal plane array. The 25 stressed and low−stress modules of PACS instrument (corresponding to 25 spatial pixels) in the red and blue arrays are integrated into their housing (after Ref. 52).

One of the major problems in the design of extrinsic photo− conductors is that the doping concentration is driven by con− flicting requirements: the doping concentration needs to be as high as possible to get high photon−absorption coeffi− cients (the doping concentration is limited by hopping con− duction induced by direct transfer of charge carriers from one impurity site to the next in heavily−doped semiconduc− tors). In contrast a low doping concentration is also desir− able to achieve a low electrical conductivity which in turn reduces Johnson noise. In 1979 M. Petroff and D. Stapelbroeck, working at the Rockwell International Science Center, invented what they Fig. 21. Effective spectral response of the filter/detector chain of the referred to as Blocked−Impurity Band (BIB) detectors [57]. PACS photometer in its three bands (after Ref. 53). These detectors were developed to provide significantly reduced nuclear radiation sensitivity and improved perfor− and make possible the construction of large format far−IR mance compared to extrinsically doped silicon photocon− FPAs (see Fig. 22) [54–56]. In this design, an intermediate ductors. BIBs have some excellent properties which make substrate is placed between the detector and the readout, them extremely useful for astronomical applications. which is pixelized on both sides in a format identical to that BIB detectors overcome the limitation of the doping of the array and the electrical contact between correspond− density present in a standard extrinsic photoconductor by ing pixel pads are made through embedded vias. The sub− placing a thin intrinsic (undoped) silicon blocking layer strate material must be chosen to have sufficient IR−block− between a heavily doped IR active layer and a planar con− ing property, high thermal conductivity, and expansion coe− tact (see Fig. 24). The active region of detector structure, fficient that is between that of germanium and silicon. Alu− usually based on epitaxially grown n−type material, is sand− mina (Al2O3) and aluminium nitrite (AlN) have these prop− wiched between a higher doped degenerate substrate elec− erties and are possible choices as substrate materials. Block− trode and an undoped blocking layer. Doping of active layer ing of the readout glow from reaching the detector provides is high enough for the onset of an impurity band in order to more efficient heat dissipation, improves temperature uni− display a high quantum efficiency for impurity ionization formity across the array, and mitigates the thermal mis− (in the case of Si:As BIB, the active layer is doped to match between the detector and the readout. In addition, the »5´1017 cm–3). The device exhibits a diode−like character− substrate serves as a fanout board providing a simple and istic, except that photoexcitation of electrons takes place robust way to connect the FPA to the external electronics between the donor impurity and the conduction band. The with no additional packaging requirement. Figure 23 shows heavily doped n−type IR−active layer has a small concentra− an assembled Ge:Sb FPA (lc » 130 μm) using the lay− tion of negatively charged compensating acceptor impuri− ered−hybrid architecture. For low bias voltage photocon− ties. In the absence of an applied bias, charge neutrality ductor operated at low temperatures, a capacitive transimpe− requires an equal concentration of ionized donors. Whereas dance amplifier (CTIA) design offers an effective readout the negative charges are fixed at acceptor sites, the positive solution. It is predicted, that using this structure, very large charges associated with ionized donor sites (D+ charges) are format FPAs with sensitivities better than 10–18 W/Hz1/2 mobile and can propagate through the IR−active layer via could be realized, fulfilling the technology goals of the the mechanism of hopping between occupied (D0)and upcoming astronomical instruments. vacant (D+) neighbouring sites. A positive bias to the trans−

Fig. 22. The layered−hybrid design suitable for large format far IR and sub−mm arrays.

Opto−Electron. Rev., 21, no. 4, 2013 A. Rogalski 419 Semiconductor detectors and focal plane arrays for far−infrared imaging

Fig. 23. Ge:Sb FPA (lc » 130 μm): (a) the fully assembled layered−hybrid with SB349 readout (CTIA readout). – shown is the readout side; the detector is located on the other side of the fanout board, and (b) typical spectral response of Ge:Sb phtoconductor (after Ref. 54). parent contact creates a field that drives the pre−existing D+ length of about 28 μm. Thermal excitation of electrons charges towards the substrate, while the undoped blocking across the narrow bandgap leads to the dark current and the layer prevents the injection of new D+ charges. A region detectors must be operated at temperatures sufficiently low depleted of D+ charges is therefore created, with a width (T < 13 K) to limit the dark current. depending on the applied bias and on the compensating The design of BIB detectors offers a number of advan− acceptor concentration. tages over conventional extrinsic photoconductors: the high BIB detectors effectively use the hopping conductivity absorption coefficient of the absorbing layer means that associated with “impurity banding” in relatively heavily detectors with comparatively small active volumes can be doped semiconductors. Because of the presence of the blo− made, providing low susceptibility to cosmic rays without cking layer, BIB detectors do not follow the usual photo− compromising quantum efficiency. Also, due to the heavy conductor model. The behaviour of BIB detectors is closer doping of the active layer, the impurity band increases in to that of a reverse−biased photodiode except that photo− width, therefore effectively decreasing the energy gap be− excitation of electrons occurs from the donor impurity band tween the impurity band and the conduction band. As to the conduction band. The gap between the impurity band a result, BIB detectors typically offer spectral responses and the conduction band is narrow; therefore, the response extended towards longer wavelengths compared to bulk− of a BIB detector extends to the VLWIR region of the spec− −type photoconductors with the same dopant. Ultimately trum. Silicon−based BIB detectors doped with arsenic and BIB devices also provide better noise performance com− antimony have the materials of choice at wavelengths from pared to conventional photoconductors (the positive and 5 μm to 40 μm [see Fig. 25(a)]. Conventionally designed negative charge carriers separate after generation and, there− and processed Si:As BIB detectors have a cut−off wave− fore, no recombination occurs in the depletion region. Since

Fig. 24. Si:As BIB detector: (a) layer structure and (b) energy band structure of a positively biased detector.

420 Opto−Electron. Rev., 21, no. 4, 2013 © 2013 SEP, Warsaw Fig. 25. Si:As BIB detectors: (a) examples of extrinsic silicon detector spectral response. Shown are Si:In, Si:Ga, and Si:As bulk detectors and a Si:As IBC (after Ref. 58); (b) cut−off wavelength vs donor doping (after Ref. 59). generation and recombination of the charge carriers in this mance is strongly affected by background levels. Extrinsic case are statistically independent processes, only a single silicon arrays for high background applications are less de− noise event is associated with the detection of each photon, veloped than that for low background applications. leading to a decrease in detector noise by a factor of »2 The largest extrinsic infrared detector arrays are manu− compared to conventional photoconductors. factured for astronomy owing to investments from NASA DRS Technologies has demonstrated a longer cut−off and the National Science Foundation. At present Raytheon wavelength of the BIB detectors [see Fig. 25(b)]. The far Vision Systems (RVS), DRS Technologies, and Teledyne infrared extended BIB (FIREBIB) detectors are fabricated Imaging Sensors (formerly Rockwell Scientific Company) by further increasing a donor doping. Average overlap of supply the majority of IR arrays used in astronomy between donor wavefunctions becomes large, resulting in the impu− them the most important are BIB detector arrays. An im− rity conduction band becoming wider and the gap between pressive progress has been achieved especially in Si:As BIB the impurity band and the conduction band becoming nar− array technology with formats as large as 2048×2048 and rower. The narrowed bandgap results in a longer wave− pixels as small as 18 μm. Figure 26 shows the evolution of length response. The points according two models are repre− BIB detector arrays at RVS. sented in Fig. 25(b). The proposed broadband detector in the Nowadays, attempts have been also made to provide 10−to 50−μm (goal 3− to 100−μm) is based on the As doped Si a detector technology for operation in the far−IR by switching BIB detectors to operate at 10 to 12 K. from silicon to a semiconductor material that would provide The main application of BIB arrays today is for ground− a shallower impurity band [61]. Both Ge−based and GaAs−ba− and space−based far−infrared astronomy – Si:As BIB perfor− sed BIB systems have been attempted, with greater success mance are gathered in Table 2. The arrays should be oper− achieved in germanium [62]. The smaller binding energy of ated under the most uniform possible conditions, in the most shallow donors in GaAs compared to Ge results in response benign and constant environment possible. Array perfor− at wavelengths exceeding 300 μm without uniaxial stress.

Table 2. Performance of Si:As BIB FPAs fabricated in several formats for both ground and space based applications (after Ref. 60). Parameter Si:As BIB Phenix MIRI Aquarius−1k Applications/Users Ground−based telescopes Space telescopes JAXA Space telescopes JWST, Ground−based telescopes ESO, Univer. of Tokyo NASA ESO, Univer. of Arizona 1024×1024 1024×1024 Format 320×240 1024×1024 2048×2048 Pixel size 50 μm 25 μm 25 μm 30 μm ROIC type DI SFD SFD SFD Fill factor ³ 95% ³ 95% ³ 98% ³ 98%

ROIC input referred noise < 1000e−RMS 6–20e−RMS < 10–30e Low gain < 1000e−RMS High gain < 100e−RMS Integration capacity 7 or 20×106 e– 3×106 e– 2×105 e– 1 or 11×106 e– Max. frame rates 100 to 500 Hz 0.1 Hz 0.1 Hz 120 Hz Number of outputs 16 or 32 4 4 16 or 64 Packaging LCC LCC Module Module – 2 side buttable

Opto−Electron. Rev., 21, no. 4, 2013 A. Rogalski 421 Semiconductor detectors and focal plane arrays for far−infrared imaging

Fig. 26. Evolution of BIB FPAs at RVS. From left to right: SIRTF 256×256, CRC774 320×240, Aquarius−1k 1024×1024, and Phoenix 2048×2048 devices (after Ref. 60).

7. Semiconductor bolometers The thermistors are typically fabricated by lithography on membranes of Si or SiN. The impedance is selected to a The classic bolometers contain a heavily doped and com− few MW to minimize the noise in JFET amplifiers operated pensated semiconductor which conducts by a hopping pro− at about 100 K. Limitation of this technology is assertion of p cess that yields the resistance R(T)=Roexp(T/To) , where R thermal mechanical and electrical interface between the is the resistance at the temperature T, and To and Ro are the bolometers at 100–300 mK and the amplifiers at » 100 K. constants which depend on the doping and , for Ro, on the Usually, JFET amplifiers are sited on membranes which iso− thermistor dimensions [63]. The exponent p is a constant, late them so effectively that the environment remains at and it is often assumed that p = 0.5. The thermistors are much low temperature (about 10 K) – see Fig. 28. In addi− made by ion implantation in Si, or by neutron transmuta− tion, the equipment at 10 K is itself thermally isolated from tions doping (NTD) in Ge. Figure 27 shows the experimen− nearby components at 0.1–0.3 K. There are not practical tal results at the temperature range from 70 mK to 1 K for approaches to multiplexing many such bolometers to one some of the NTD Ge samples together with fitting curves, JFET amplifier. Current arrays require one amplifier per where p−value for all samples is about 0.5 [64]. NTD con− pixel and are limited to a few hundred pixels. 70 71 74 75 verts Ge to Ga (acceptor) and Ge to As (donor). Dop− In a bolometer metal films that can be continuous or pat− ing level depends on neutron flux, instead compensation terned in a mesh absorb the photons. The patterning is ratio can be changed by altering isotope ratios. designed to select the spectral band, to provide polarization Cooled silicon bolometers demonstrate broadband and sensitivity, or to control the throughput. Different bolometer nearly flat spectral response in the 1–3000 μm wavelength architectures are used. In close−packed arrays and spider range. They are easier to fabricate with high operability, web, the pop−up structures or two−layer bump bonded struc− good uniformity, and lower cost, but has low operating tem− tures are fabricated. Agnese et al. have described a different perature (4.2–0.3 K). During fabrication of bolometers, array architecture, which is assembled from two wafers by their area, operating temperature, thermal time constant, and indium bump bonds [185]. Other types of bolometers are thermal conductance are adjusted to meet the specific de− integrated in horn−coupled arrays. To minimize low fre− sign requirements. The present day technology exists to pro− quency noise an AC bias is used. duce arrays of hundreds of pixels that are operated in many With the development of low−noise readouts that can experiments including NASA pathfinder ground based in− operate near the bolometer temperature, the first true high− struments, and balloon experiments such as BOOMERANG, −performance bolometer arrays for the far IR and sub−mm MAXIMA and BAM. spectral ranges are just becoming available. For example, the Herschel/PACS instrument uses a 2048−pixel array of bolom− eters [67] and is an alternative to JFET amplifiers. The archi− tecture of this array is vaguely similar to the direct hybrid mid−infrared arrays, where one silicon wafer is patterned with bolometers, each in the form of a silicon mesh, as shown in Fig. 29. The development of silicon micromachining has enabled substantial advances in the bolometer construction generally and is central to making large−scale arrays. To achieve appropriate response and time constant characteris− tics, the rods and mesh are designed carefully. The mesh is blackened with a thin layer of titanium nitride with sheet resistance matched to the impedance of free space (377 W/ section of film) to provide an efficiency of 50% over a broad spectral band. Each bolometer located at the centre of the mesh, containing a silicon−based thermometer doped by ion Fig. 27. Zero−bias resistivity as a function of temperature for several implantation, is characterized by appropriate temperature− different NDT Ge samples (after Ref. 64). −sensitive resistance. Their large resistances (> 1010 W)are

Fig. 29. The Herschel/PACS bolometer array with pixel size 750 μm (after Ref. 68).

Opto−Electron. Rev., 21, no. 4, 2013 A. Rogalski 423 Semiconductor detectors and focal plane arrays for far−infrared imaging well adjusted to MOSFET readout amplifiers. In the final crete and low pixel arrays operated at low or sub−Kelvin step of hybrid array fabrication, the MOSFET−based readouts temperature are close to ultimate performance at low back− and the silicon bolometer wafer are joined by indium bump ground. Detector development is at the heart of all current bonding. Performance is currently limited by the noise in the plants. There is a a large variety of traditional deeply cooled MOSFET amplifiers to the NEP » 10–16 W/Hz1/2 regime, but mm and sub−mm wavelength detectors, as well as new prop− this technology allows for the construction of very large ositions based on novel optoelectronic quantum devices. arrays suitable for higher background applications. Further The leading position of far−infrared semiconductor−ba− details are in Billot et al. [67]. sed is challenged by the promise of large−format, high−per− In the normal bolometer, the crystal lattice absorbs formance arrays and will undoubtedly continue to play an energy and transfers it to the free carriers via collisions. important role in the future. Particular attention in a devel− However, in a hot electron bolometer the incident radiation opment of far−infrared/THz imaging systems will be devo− power is absorbed directly by free carriers, the crystal lattice ted to the realization of sensors with a large potential for temperature remaining essentially constant. Note that this real−time imaging, while maintaining a high dynamic range mechanism differs from photoconductivity in that free−elec− and room−temperature operation. CMOS process technol− tron mobility rather than electron number is created by an ogy is especially attractive due to their low price tag for incident light. 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