Detection of light

Photodetectors Lecture Outline • Introduction to Photodetectors • Photodiodes – General – p-i-n and p-n – Avalanche • Quantum Well Infrared Photodetector • Quantum Dot Infrared Photodetector Introduction • Photodetectors are semiconductor devices that can detect optical signals through electronic processes – Three main processes: • Carrier generation by incident light • Carrier transport and/or multiplication by current-gain mechanism • Extraction of carriers as terminal current (or voltage) to provide the output signal

– Desired: High sensitivity, high response speed, minimum noise, compact size, low biasing voltage and current Introduction • Wavelength relation to transition energy

hc 1.24 Often minimum  E EeV() wavelength for detection • ∆E is the transition of energy levels – Depending on photodetector type can be: • Energy gap of the semiconductor • Barrier height as in a metal semiconductor photodiode • Transition energy between impurity level and band edge as an extrinsic photoconductor • Intersubband transition, etc. Introduction • Important Factors/Parameters – Absorption Coefficient – Response Speed – Quantum Efficiency – Responsivity –Gain –Noise – Detectivity Introduction

• Absorption coefficient – Determines whether light can be absorbed for photoexcitation – Determines where light is absorbed • High value means near surface • Low value means deeper penetration Photodiodes: General • Photodiodes have depleted region with a high electric field that separates photogenerated electron-hole pairs – Tradeoff between speed of response and quantum efficiency (depletion layer: transit time, absorbance area) – Reverse biasing often employed to reduce carrier transit time and lower diode capacitance – All photodiodes except Avalanche have a maximum gain of one a) pn photodiode b) p-i-n photodiodes c) Metal-i-n photodiode d) Metal-semiconductor photodiode e) Point contact photodiode

Photodiodes: General Important characteristics WD ~ 1/α – Quantum efficiency • Absorption coefficient strong dependence on wavelength • Long wavelength cutoff given by energy gap of semiconductor • Short wavelength cutoff given by large value of α (surface where recombination is likely) – Response Speed • Limited by – Drift time in the depletion region – Diffusion of carriers – Capacitance of detection region • Optimized when the depletion layer is chosen so the transit time is on the order of one half the modulation period • Depletion layer thickness (intrinsic layer) can be tailored to optimize the quantum efficiency and frequency response

• Total current density is the sum of Idr inside the depletion region and Idiff outside the depletion region

Introduction • Responsivity: Photocurrent generated per incident optical power I ph q  m A R P opt hv 1.24 W • Gain and response time for common photodetectors Introduction • Noise ultimately determines minimum detectable signal strength – Sources of noise • Dark current • Thermal noise • Shot noise • Flicker noise • Generation recombination noise – Figure of Merit: Noise Equivalent Power (NEP) NEP = incident rms optical power required to produce a signal-to-noise ratio of one in a 1 Hz bandwidth (minimum detectable light power) Introduction A is the Area • Detectivity AB B is the Bandwidth D * NEP – The signal-to-noise ratio when one watt of light power is incident on a detector of area 1 cm2 measured over 1 Hz bandwidth – Normalized to area, noise is generally proportional to the square root of area – Detectivity depends on • Detector sensitivity • Spectral Response •Noise – Is a function of wavelength, modulation frequency and bandwidth Photodiodes: p-n • p-n photodiode – Thin depletion layer means some light can be absorbed outside – Light more than a diffusion length outside does not contribute at all to photocurrent • Reduces quantum efficiency • Diffusion process is slow – Time required to diffuse a 2 4x distance x t 2  D p • Lower response speed than p-i-n • Neutral region contributes to noise pin Photodetector

w

The high electric field present in the depletion region causes photo-generated carriers to separate and be collected across the reverse –biased junction. This gives rise to the photocurrent. Energy-Band diagram for a pin photodiode

Responsivity vs. wavelength Photodiodes: p-i-n and p-n • Frequency Response – Phase difference between photon flux and photocurrent will appear when incident light intensity is modulated rapidly – Assume light is absorbed at surface, applied voltage is high enough to ensure saturation velocity – Response time is limited by the carrier transit time through the depletion layer – Compromise for high frequency response and quantum efficiency – Absorption region of thickness 1/α to 2/α • Illustrates trade off between – Large portion of light is absorbed within the response speed and quantum depletion region efficiency at various wavelengths by adjusting the depletion width

• Smaller WD, shorter transit time, higher speed, but reduced η Photodiodes: Heterojunction • Advantages – Large bandgap material can be transparent and used as a window for transmission of incoming optical power • Quantum efficiency is not dependent on distance of junction from surface – Unique material combinations so quantum efficiency and response speed can be optimized for a given optical wavelength – Reduced dark current

J.H. Jang et al., Journal of Lightwave Technology, Vol. 20, No. 3, March 2002.

Structures for InGaAs APDs

• Separate-absorption-and multiplication (SAM) APD

light

InP substrate InP buffer layer

INGaAs Absorption layer

InP multiplication layer

Metal contact

• InGaAs APD superlattice structure (The multiplication region is composed of several layers of InAlGaAs quantum wells separated by InAlAs barrier layers.

Comparison of photodetectors Infrared detectors Electromagnetic Spectrum

http://www.nasa.gov/centers/langley/science

Visible Near-IR Mid-IR Far-IR Micro Wave

0.8 – 5 m 5 - 30 m 30 - 300 m

Wavelength 54 Applications

http://www.netcast.com.hk/Products.htm Infrared Body Remote controller and receiver Temperature Thermometer

Visible Light Infrared

56 Applications brain imaging Blood Flow

www.medphys.ucl.ac.uk/research/borl/

Transverse, coronal, and sagittal views across the 3D absorption image of the infant, acquired at 780 nm.

Human suspect climbing over a fence at 2:49 AM in total darkness Night vision helmet Infrared image of Orion 57 Applications

Thermal analysis of a fluid tank level detection Close up image of a Intel Celeron chip

www.x20.org ºF

www.x20.org Faulty connection at power station Bad Insulation spots 58 Different Types of Infrared Detectors

IR Detectors

Photon Thermal

Photo- Bolometric Thermoelectric Photovoltaic conductive

Photoemissive Pyroelectric

59 Quantum Well Infrared Photodetector (QWIP) Infrared Photodetection QWIP Bulk Crystal

Wavelength

Photons +- CB

61 VB Quantum Well Infrared Photodetectors

• Structure of QWIP using GaAs/AlGaAs heterostructure – QW layers 5nm doped to n-type in 1017 range – Barrier layers are undoped and have a thickness 30-50nm – Periods 20 to 50 Quantum Well Infrared Photodetectors

• Incident light normal to surface has zero Figure 716 absorption – Intersubband transition require electric field have components normal to QW plane – Two methods • Polished facet • Grating to refract light Quantum Well Infrared Photodetectors • Intersubband excitation – Three types of transitions • Bound to bound (escape well by tunneling) • Bound to continuum (escape well because first state is above barrier: easier) • Bound to miniband in superlattice Quantum Well Infrared Photodetectors

• I-V of QWIP is similar to photodetectors G is optical gain I ph q ph G a a • Quantum efficiency is different since light absorption and carrier generation occur only in quantum wells

 ()1expN1R   op N wL wE pP –Nop number of optical passes, Nw number of quantum wells, Lw is the length, P is the polarization correction factor

–Ep is the escape probability and is a function of bias

1 Cp is the capture probability t p t t G C p a of electron traversing quantum well  N  N wC p w

tp transit time across single period of structure tt transit time across entire QWIP active length L Quantum Well Infrared Photodetectors

• Dark current is due to thermionic emission over the quantum well barriers and thermionic field emission (thermally assisted tunneling) near the barrier peaks – To limit dark current, the QWIP has to be operated at low temperatures in the range of 4-77 K

• Can be applied in focal plane arrays for 2D imaging – High speed capability and fast response – Coupling of light to the photodetector is difficult QWIP Drawbacks

• High Intensity / Low Temp

• Polarization requirement: scattering grating

Grating Quantum Dot Infrared Photodetector (QDIP) Quantum Dots

Boron doped Ge quantum dots growth sample Producing using molecular-beam epitaxy (MBE) method in a thin layer of semi-conductor materials. Quantum Dots

Self-Assembly (a.k.a Stranski- Krastanow Method): Mismatched lattice constants cause surface tension which results in Qdot formation with surprisingly uniform characteristics. GaAs  5.6533 Å InAs  6.0584 Å

http://cqd.eecs.northwestern.edu/research/qdots.php The different between quantum well & quantum dot Theoretical advantages of QDIPs

• 3D Confinement: Sharper wavelength discrimination • QDIP allow direct incident normal to wafer surfaces. • Avoid fabricating grate coupler as in QWIP. • “Photon Bottleneck” : e- stays excited for a longer time (i.e. less recombination), resulting in a more efficient detector and resistance to temperature. • Higher temperatures and lower intensity • It has lower dark current & high detection sensitivity than QWIP.

72 Possible Applications

High speed infrared detection Infrared image application—possible use in security systems to produce image of various objects. Possible use in IR Spectrophotometer Possible use in Cell Sorter Could be used in Infrared Camera QDIP summary

There are still many challenges to overcome such fabrication or manufacturing process that will produce quantum dot to meet design requirement Current manufacturing process limit to size and dot density that it is impractical for commercial used Due to complex fabrication process and limited size it is expensive to manufacture Needs better doping control Summary • Photodiodes have depleted region with a high electric field that separates photo-generated electron-hole pairs • Width of depletion layer determines tradeoff between speed and quantum efficiency • P-n photodiodes have lower response speed and higher noise than a p-i-n photodiode • Heterojunction photodiodes can move light absorption region away from the surface due to transparence of larger bandgap materials • Avalanche photodiodes have high gain but at the cost of noise, better for signals of low light intensity • QWIPs and QDIPs use various intersubband transitions for electrons, and are often operated at low temperatures