Jozef Piotrowski

Intrinsic infrared detectors

2. Semiconductor Detectors 2.1. General classification of infrared detectors 2.2. Photon noise and fundamental limits 2.3. Photoconductive detectors 2.3.1. Photoconductivy theory 2.3.2. Noise mechanisms in photoconductors 2.3.3. Quantum efficiency 2.3.4. Ultimate detectivity of infrared photoconductors 2.35. Sweep-out effects 2.3.6. Influence of background 2.3.7. Influence of surface recombination 2.4. Photovoltaic detectors 2.4.1. Photovoltaic effect 2.4.2. Dark current in p-n junctions 2.4.3. Photocurrent in p-n junctions and quantum efficiency 2.4.4. R, A product 91 91 94 95 95 97 97 98 100 102

New generation of infrared photodetectors

HgCdTe remains the most important material for infrared photodetectors despite numerous attempts to replace it with alternative materials such as closely related mercury alloys (HgZnTe, HgMnTe), Schottky barriers on silicon, SiGe heterojunctions, AlGaAs multiple quantum wells, GaInSb strain-layer superlattices, high-temperature superconductors and especially two types of thermal detectors: pyroelectric detectors and silicon bolometers. It is interesting, however, that none of these competitors can compete in terms of fundamental properties. In addition, HgCdTe exhibits nearly constant lattice parameters, which is of extreme importance for new devices based on complex heterostructures. Examples of novel devices based on three-dimensional heterostructures are presented.

Uncooled photovoltaic Hg1-xCdxTe LWIR detectors

We report an advanced Hg1-xCdxTe photovoltaic detector based on monolithic Hg1-xCdxTe heterostructure with 3-dimensional architecture. It consists of a narrow gap, p-type Hg1-xCdxTe small size (approximately equals 10x10x7 micrometers ) absorber of infrared radiation buried in a graded gap Hg1-xCdxTe layer surrounding absorber and heterojunction contacts obtained by selective doping of the graded gap Hg1-xCdxTe layer surrounding the absorber region. The heterostructure is passivated with a ZnS layer and coated with contact metallization to nPLU and p-type regions. The device is supplied with 50x50 micrometers immersion microlens formed directly in the CdZnTe substrate. These two layers also play a role of a mirror that improves quantum efficiency for weakly absorbed infrared radiation. In addition, the mirror eliminates backside incidence of thermal radiation, which prevents generation of dark current. The design of the device is optimized to achieve the best compromise between requirements of good absorption and collection efficiency; low thermal generation; and low parasitic impedance. Test devices have been prepared using the modified isothermal vapor phase epitaxy of Hg1-xCdxTe on profiled CdZnTe substrates, negative epitaxy of Hg1-xCdxTe to widen band gap of surface regions, selective doping, multiple chemical etching and ion milling, vacuum deposition of dielectric and metal layers.

Numerical analysis of longwavelength extracted photodiodes

Abstract The electrical and photoelectrical properties of the longwavelength n + πp + Hg 1− x Cd x Te structures have been analyzed numerically. The band diagram, electrical field, photoelectrical gain, responsivity, noise and detectivity have been calculated as a function of voltage applied. The calculations have been performed for devices prepared from Hg 1− x Cd x Te, optimized for optimum performance at λ = 10.6 μm , and operated at a temperature of 230 K. Reverse bias results in suppression of Auger generation current in the lightly doped region of the junction. The main contribution to the total noise current comes mostly from the heavily doped p + region and then from regions close to l-h junctions.

MOCVD HgCdTe heterostructures for uncooled infrared photodetectors

Recent progress at VIGO/MUT (Military University of Technology) MOCVD Laboratory in the growth of Hg1-xCdxTe (HgCdTe) multilayer heterostructures on GaAs/CdTe substrates for uncooled infrared photodetectors is presented. The optimum conditions for the growth of single layers and complex multilayer heterostructures have been established. One of the crucial stages of HgCdTe epitaxy is CdTe nucleation on GaAs substrate. Successful composite substrates have been obtained with suitable substrate preparation, liner and susceptor treatment, proper control of background fluxes and appropriate nucleation conditions. The other critical stage is the interdiffused multilayer process (IMP). The growth of device-quality HgCdTe heterostructures requires complete homogenization of CdTe-HgTe pairs preserving at the same time suitable sharpness of composition and doping profiles. This requires for IMP pairs to be very thin and grown in a short time. Arsenic and iodine have been used for acceptor and donor doping. Suitable growth conditions and post growth anneal is essential for stable and reproducible doping. In situ anneal seems to be sufficient for iodine doping at any required level. In contrast, efficient As doping with near 100% activation requires ex situ anneal at near saturated mercury vapors. As a result we are able to grow multilayer fully doped (100) and (111) heterostructures for various infrared devices including photoconductors, photoelectromagnetic and photovoltaic detectors. The present generation of uncooled long wavelength infrared devices is based on multijunction photovoltaic devices. Near-BLIP performance is possible at ≈ 230 K with optical immersion. These devices are especially promising as 7.8-9.5-μm detectors, indicating the potential for achieving detectivities above 109 cmHz1/2/W.

Uncooled or minimally cooled 10μm photodetectors with subnanosecond response time

We report fast and sensitive long (10 μm) wavelength photodetectors operating at near room temperature. The devices are based on HgCdTe multilayer heterostructures grown by MOCVD on (211) and (111) GaAs substrates. Device-quality heterostructures are obtained without any post growth anneal. The recent improvements of MOCVD growth were: optimized design of the device architecture to increase speed of response, better IMP growth parameters selection taking into account interdiffusion time changes during growth, stoichiometry control during growth by the layer anneal at metal rich vapors during each IMP cycle, precursor delivery to the growth zone monitored with IR gas analyzer, additional metal-rich vapor anneal at the end of growth and passivation of detector structures with wide gap HgCdTe overgrowth deposition. Monolithic optical immersion of the detectors to GaAs microlenses has been applied in purpose to improve performance and reduce RC time constant. The response time of the devices have been characterized using 10μm quantum cascade laser, fast oscilloscope with suitable transimpedance amplifier as a function of detector design, temperature and bias. Detectivity of the best thermoelectrically cooled optically immersed photodiodes approaches 1⋅1010 cmHz1/2/W at ≈10 μm wavelength. The response time of small area decreases with reverse bias to response achieving <100 ps with weak reverse bias.

Mid and long infrared detection modules for picosecond range measurements

Sensitive and broadband detection of MWIR and LWIR radiation with any wavelength within the 2 to 16 μm spectral range and bandwidth from DC to GHz range is reported. Recent efforts have been concentrated on the extension of useful spectrum range above 13 micrometers. This was achieved with improved architecture of the active element, use of monolithic optical immersion technology, enhanced absorption of radiation, dedicated electronics, series connection of small cells and applying more efficient Peltier coolers.

Isothermal vapor phase epitaxy as a versatile technology for infrared photodetectors

We report here the use of isothermal vapor phase epitaxy to grow 3D Hg1-xCdxTe heterostructures for photoconductive, photovoltaic and photoelectromagnetic infrared detectors operated at near room temperatures. A reusable two-zone atmospheric pressure growth system has been developed.the system makes it possible not only to grow epilayers but also to perform in situ other processes such as high temperature annealing to control the compositional grading, the low temperature annealing for reduction of native acceptor concentration, and doping with foreign impurities. The required various composition profiles have been theoretically predicted and then implemented changing the temperature and mercury pressure during growth and subsequent thermal treatment. In addition, post-growth etching, substrate shaping, selective epitaxy, and negative epitaxy have been used to achieve 3D band gap profiles. The photoconductors were based on lightly p-type doped epilayers. Low diffusion length, weak absorption of radiation and a very low junction resistance makes it difficult to obtain useful performance of longwavelength photovoltaic devices operating at near room temperature. This was overcome with development of multiple heterojunction photovoltaic devices in which short elements were connected in series. To improve the performance of any type of heterostructure photodetector, monolithic optical immersion has been used. Detectivities as high as 1 X 108 cmHz1/2/W and 1 X 109 cmHz1/2/W were obtained at (lambda) equals micrometers and temperatures of 300 K and 220 K, respectively.

Computer simulation of HgCdTe photovoltaic devices based on complex heterostructures

We analyze numerically properties of small-size infrared photovoltaic devices based on complex two-dimensional Hg1- xCdxTe heterostructures. An original iteration scheme was used to solve the system of nonlinear continuity equations and the Poisson equation. All quantities are expressed as functions of electric potential and Fermi quasi-levels. The results of calculations are presented as the maps showing spatial distribution of sensitivity and density of noise generation for 4 types of heterostructures. In addition, resulting parameters of the devices are summarized in the table. This approach may help to understand specific features of the heterostructural devices and optimize their performance. The simulations show viability of constructing devices with active region buried inside a wide gap material where existing potential barriers prevent adverse effects of both recombination of photogenerated carriers and thermal generation at surfaces, interfaces and contacts.

Calculation of the carrier lifetime in Hg1−xZnxTe

Abstract The carrier lifetimes in Hg 1− x Zn x Te for radiative and band-to-band Auger recombination are calculated for the temperature range 77–300 K and the composition range 0.10 ⩽ x ⩽ 0.24. The Auger rates are calculated according to Petersen-Casselman theory and Gelmont theory. Comparative study of these theories is made. The discrepancy between them is considerable especially for the Auger 7 recombination process. It is shown that band-to-band Auger recombination is an important non-radiative process especially at higher temperatures in material with high carrier concentrations. The calculations are performed for the most important compositions of Hg 1− x Zn x Te material which can be used for production of IR detectors.