A. Piotrowski

Uncooled infrared photodetectors in Poland

The history and present status of the middle and long wavelength Hg1-xCdxTe infrared detectors in Poland are reviewed. Research and development efforts in Poland were concentrated mostly on uncooled market niche.Technology of the infrared photodetectors has been developed by several research groups. The devices are based on mercury-based variable band gap semiconductor alloys. Modified isothermal vapour phase epitaxy (ISOVPE) has been used for many years for research and commercial fabrication of photoconductive, photoelectromagnetic and other devices. Bulk growth and liquid phase epitaxy was also used. At present, the fabrication of IR devices relies on low temperature epitaxial technique, namely metalorganic vapour phase deposition (MOCVD), frequently in combination with the ISOVPE.Photoconductive and photoelectromagnetic detectors are still in production. The devices are gradually replaced with photovoltaic devices which offer inherent advantages of no electric or magnetic bias, no heat load and no flicker noise. Potentially, the PV devices could offer high performance and very fast response. At present, the uncooled long wavelength devices of conventional design suffer from two issues; namely low quantum efficiency and very low junction resistance. It makes them useless for practical applications. The problems have been solved with advanced 3D band gap engineered architecture, multiple cell heterojunction devices connected in series, monolithic integration of the detectors with microoptics and other improvements. Present fabrication program includes devices which are optimized for operation at any wavelength within a wide spectral range 1–15 μm and 200–300 K temperature range. Special solutions have been applied to improve speed of response. Some devices show picoseconds range response time. The devices have found numerous civilian and military applications.

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.

Control of acceptor doping in MOCVD HgCdTe epilayers

The acceptor doping of mercury cadmium telluride (HgCdTe) layers grown by MOCVD are investigated. (111)HgCdTe layers were grown on (100)GaAs substrates at 350°C using horizontal reactor and interdiffused multilayer process (IMP). TDMAAs and AsH3 were alternatively used as effective p-type doping precursors. Incorporation and activation rates of arsenic have been studied. Over a wide range of Hg1−xCdxTe compositions (0.17 < x < 0.4), arsenic doping concentration in the range from 5×1015 cm−3 to 5×1017 cm−3 was obtained without postgrowth annealing. The electrical and chemical properties of epitaxial layers are specified by measurements of SIMS profiles, Hall effect and minority carrier lifetimes. It is confirmed that the Auger-7 mechanism has decisive influence on carrier lifetime in p-type HgCdTe epilayers.

Uncooled MWIR and LWIR photodetectors in Poland

The history, status, and recent progress in the middle and long wavelength Hg1−xCdxTe infrared detectors operating at near room temperatures are reviewed. Thermal generation of charge carriers in narrow gap semiconductor is a major limitation or sensitivity. Cooling is a straightforward way to suppress thermal generation of charge carriers and reduce related noise. However, at the same time, cooling requirements make infrared systems bulky, heavy, and inconvenient in use. A number of concepts to improve performance of photodetectors operating at near room temperatures have been proposed and implemented. Recent considerations of the fundamental detector mechanisms suggest that near perfect detection can be achieved without the need for cryogenic cooling. This paper, to a large degree, is based on the research, development, and commercialization of uncooled HgCdTe detectors in Poland. The devices have been based on 3D-variable band gap and doping level structures that integrate optical, detection and electric functions in a monolithic chip. The device architecture is optimized for the best compromise between requirements of high quantum efficiency, efficient and fast collection of photogenerated charge carriers, minimized thermal generation, reduced parasitic impedances, wide linear range, wide acceptance angles and other device features. Recent refinements in the devices design and technology have lead to sensitivities close to the background radiation noise limit, extension of useful spectral range to > 16 μm wavelength and picosecond range response times. The devices have found numerous applications in various optoelectronic systems. Among them there are fast scan FTIR spectrometers developed under MEMFIS project.

Two-colour HgCdTe infrared detectors operating above 200 K

The performance of dual waveband HgCdTe photodiodes fabricated using metaloorganic chemical vapour deposition operated at high temperatures is presented. The effect of additional separating layer on the quantum efficiency and cross-talk of the photodiodes is analyzed. The photodiodes with cutoff wavelengths up to 6 μm, good R0A product, and high quantum efficiency at 200 K have been demonstrated. The temperature dependence of the differential resistance is discussed. It is shown that the multilayer heterojunction P-n-N-n-P structure operating in a simultaneous mode has better performance than a structure operating in a sequential mode.

Response time improvement of LWIR HOT MCT detectors

Theoretical and experimental investigations on the response time improvement of biased and unbiased long-wave infrared (LWIR) HgCdTe detectors operating at temperatures T = 230K were presented in this paper. MOCVD technology is an excellent tool in fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor doping and without post grown ex-situ annealing. Donor doping efficiency in (111) and (100) oriented HgCdTe layers has been discussed. The time constant is lower in biased detectors due to Auger suppression phenomena and reduction of diffusion capacitance related to wider depletion region. The relatively high bias currents requirements and excessive low frequency noise which reduces the detectivity of biased detectors inspire researches on the time constant improvement of unbiased detectors. The response time of high-operating temperature (HOT) LWIR HgCdTe detectors revealed complex behavior being dependent on the applied the reverse bias, the operating temperature, the absorber thickness and doping, the series resistance and the electrical area of the devices.

Uncooled middle wavelength infrared photoconductors based on (111) and (100) oriented HgCdTe

Abstract. We present progress in metal organic chemical vapor deposition (MOCVD) growth of (100) HgCdTe epilayers achieved recently at the Institute of Applied Physics, Military University of Technology and Vigo System S.A. It is shown that MOCVD technology is an excellent tool for the fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor doping, and without post grown ex-situ annealing. Surface morphology, residual background concentration, and acceptor doping efficiency are compared in (111) and (100) oriented HgCdTe epilayers. At elevated temperatures, the carrier lifetime in measured p-type photoresistors is determined by Auger 7 process with about one order of magnitude difference between theoretical and experimental values. Particular progress has been achieved in the growth of (100) HgCdTe epilayers for medium wavelength infrared photoconductors operated in high-operating temperature conditions.

Response time study in unbiased long wavelength HgCdTe detectors

Theoretical and experimental investigations on the response time improvement of unbiased long-wave infrared (LWIR) HgCdTe detectors operating at temperatures T=230u2009u2009K were presented. Metal–organic chemical vapor deposition technology is an excellent tool in fabrication of different HgCdTe detector structures with a wide range of composition and donor/acceptor doping and without postgrown ex-situ annealing. The time constant is lower in biased detectors due to Auger-suppression phenomena and reduction of diffusion capacitance related to a wider depletion region. The relatively high bias current requirements and excessive low-frequency noise, which reduces the detectivity of biased detectors, inspire research on the time constant improvement of unbiased detectors. The response time of high-operating temperature LWIR HgCdTe detectors revealed complex behavior being dependent on the applied reverse bias, the operating temperature, the absorber thickness and doping, the series resistance, and the electrical area of the devices. The response time of 2xa0ns was achieved for unbiased 30×30u2009u2009μm HgCdTe structures with λ50%=10.6u2009u2009μm operating at T=230u2009u2009K.