This article reports the parameters and characteristics of recently introduced mid Infrared (3-12um) detection modules for gas sensing applications. In Mid infrared range one can detect almost every simple or complex compound existing on earth. Currently a driving factors for development of gas sensors are related to air/water quality, explosive material detection and medical applications, especially breath analyzers. Gas sensors require source (thermal, diode or laser), sampling compartment and detection module. At VIGO System we are concentrated on designing and manufacturing high operating temperature detectors, fast, sensitive, affordable and reliable required for development of such platforms. We are using active, absorber elements based on complex HgCdTe or InAsSb heterostructures monolithically integrated with optical immersion lens. Additional collective optics, signal amplification, temperature control and heat dissipation will be also discussed in this article. Those functions are critical for ultimate performance of gas sensors.
Theoretical and experimental investigations on the response time improvement of unbiased long-wave infrared (LWIR) HgCdTe detectors operating at temperatures T=230 K 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 2 ns was achieved for unbiased 30×30 μm HgCdTe structures with λ50%=10.6 μm operating at T=230 K.
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.
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.
In this paper we present progress in 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 in fabrication of different HgCdTe detector structures with a wide range of composition, donor/acceptor
doping and without post grown annealing.
Particular progress has been achieved in the growth of (100) HgCdTe epilayers for long wavelength infrared
photoconductors operated in HOT conditions. The (100) HgCdTe photoconductor optimized for 13-μm attain detectivity
equal to 6.5x109 Jones and therefore outperform its (111) counterpart.
The paper also presents technological progress in fabrication of MOCVD-grown (111) HgCdTe barrier detectors.
The barrier device performance is comparable with state-of-the-art of HgCdTe photodiodes. The detectivity of HgCdTe
detectors is close to the value marked HgCdTe photodiodes. Dark current densities are close to the values given by “Rule
07”.
The nominally sharp interfaces in layered HgCdTe heterostructures are affected by interdiffusion for growth at a
temperature of above 300 K. Significant composition and doping grading always occur in layered HgCdTe
heterostructures grown with MOCVD (360°C), LPE (480°C), and ISOVPE (500°C) epitaxial techniques. MBE (170°C) is
the only technique that practically does not introduce significant diffusion grading, but it can be introduced by post
growth processing, especially during dopants activation. The purpose of this paper was to explain how the grading
affects performance of photodetectors operating at near room temperatures (190-300 K). Influence of the growth related
and intentional grading on dark currents and response time was studied with numerical calculations and experiments.
Practical infrared devices with controlled grading were grown with programmed MOCVD and characterized. The studies
revealed interesting properties of the N+pP+ devices with graded interfaces. Controlled grading minimizes Auger,
Shockley-Read and tunnel currents, increases responsivity and linearity range. The grading is also important for high
frequency performance of the devices.
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.
We have developed various types of photodetectors operating without cryocooling. Initially, the devices were mostly
used for uncooled detection of CO2 laser radiation. Over the years the performance and speed of response has been
steadily improved. At present the uncooled or Peltier cooled photodetectors can be used for sensitive and fast response
detection in the MWIR and LWIR spectral range. The devices have found important applications in IR spectrometry,
quantum cascade laser based gas analyzers, laser radiation alerters and many other IR systems. Recent efforts were
concentrated on the extension of useful spectral range to >13 μm, as required for its application in FTIR spectrometers.
This was achieved with improved design of the active elements, use of monolithic optical immersion technology,
enhanced absorption of radiation, dedicated electronics, series connection of small cells in series, and last but not least,
applying more efficient Peltier coolers. Practical devices are based on the complex HgCdTe heterostructures grown on
GaAs substrates with MOCVD technique with immersion lens formed by micromachining in the GaAs substrates. The
results are very encouraging. The devices cooled with miniature 4 stage Peltier coolers mounted in TO-8 style housings
show significant response at wavelength exceeding 16 μm.
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.
The present generation of uncooled infrared photon detectors relies on complex heterostructures grown by low temperature epitaxial techniques. We report recent results on MOCVD grown Hg1-xCdxTe photodetectors and their applications. Special modifications to the interdiffused multilayer process (IMP) has been applied for the in-situ control of stoichiometry, improved morphology and minimized consumption of precursors. As a result we are able to grow fully-doped multiple layer heterostructures without any post-growth thermal anneal. The heterostructures have been used for fabrication of IR photodetectors optimized for any wavelength within the 1 to 15 μm range and operating at temperatures 200-300 K. Variable bandgap absorbers have been used for detectors with tuned spectral response and multicolor devices. The uncooled photodetectors have been applied in sub-ppb gas analyzers, laser warning devices, free space optical communications, Fourier Transform IR Spectroscopy, and many other IR systems.
The performance of multi-layer hetrojunction (MLHJ) HgCdTe photodiodes at high temperatures is presented. The effect of inherent and excess current mechanisms on quantum efficiency and R0A product is analyzed. The diodes with good R0A operability and high quantum efficiency at 200-300 K have been demonstrated at cutoff wavelengths up to 5 μm. The temperature dependence of the differential resistance is discussed. The experimental results show that proper surface passivation and low series/contact resistance are major issues relating to fabrication of HgCdTe detectors with high performance.
We report here the recent progress at VIGO/MUT (Military University of Technology) MOCVD Laboratory in the growth of Hg1-xCdxTe multilayer heterostructures for various types of uncooled infrared devices. The detectors are optimized for any wavelength within 1-12 μm spectral range. Hg1-xCdxTe growth with interdiffused multilayer process (IMP) technique has been improved. The total flow of the carrier gas was optimized to improve lateral uniformity of the composition and doping. The parasitic transient stages between the CdTe and HgTe phases were reduced to reasonable minimum. As a result, we were able to grow layers with homogeneous composition and doping, characterized by steep interfaces. The additional benefits were improved morphology, reduced dislocation density, and minimized consumption of precursors. The other issues addressed in this work were growth of heavy As-doped low-x and heavy Idoped high-x materials. Special modification to IMP process has been applied for in-situ control of stoichiometry. To maintain low vacancy concentration, special growth finish procedure has been developed. No post-growth thermal anneal was necessary for device-quality material. The MOCVD grown heterostructures have been successfully used for advanced uncooled infrared photodetectors such as multiple heterojunction photodiodes, multicolor and specially shaped spectral response multiabsorber devices.
The history and present status of the middle and long wavelength Hg1xCdxTe 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 vapor 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 vapor 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. Actually, 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.
The performance of uncooled photodetectors operating in the middle and long wavelength spectral range is limited by the noise originated from thermal generation and recombination processes in semiconductors. The noise level exponentially increases with decreasing band gap of the semiconductor. Therefore, the uncooled short wavelength devices are characterized by good performance while the long wavelength ones are much less sensitive. The consequence is very poor performance of long wavelength devices at short wavelength range. We report here two-lead multilayer photoconductors that operate over a wide spectral band with performance improved by a large factor at short wavelength ranges. The devices consist of several stacked active regions (absorbers) with their outputs connected in parallel so the resulting output signal current is the sum of the signals generated at all active regions. Due to a high photoelectric gain in the wider gap absorbers and low thermal generation and recombination, the devices offer significantly better performance at short wavelengths while the long wavelength response remains essentially unaffected. The practical devices have been obtained using complex Hg1-xCdxTe heterostructures grown on CdTe or GaAs substrates by ISOVPE, MOCVD or combination of the two epitaxial techniques. An example is an uncooled photoconductor operating up to 11μm, with response at 0.9-4μm increased by ≈3 orders of magnitude in comparison to the conventional 11 μm device.
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.
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