The aim of the paper is to review the main achievements in the research of HgCdTe ternary alloy and InAs/InAsSb type-II superlattice material to indicate the Polish contribution to the development of medium and long wavelength infrared photodetectors. Research and development efforts in the WAT-VIGO joint laboratory have focused on the metal-organic chemical vapor deposition (from 2003) and molecular beam epitaxy (from 2015). At present stage of development, the photoconductive and photovoltaic HgCdTe detectors are gradually replaced with novel device designs based on III-V material system. T2SL devices complement the offer of MCT ones in applications where it is necessary to ensure, among others: higher resistance to difficult operating conditions and high uniformity of parameters of multi-element detectors.
Plasmonic enhancement has a great potential for performance improvement of high operating temperature (HOT) photodetectors, especially those optimized for long-wavelength infrared (LWIR). Conventional HOT photodetectors exhibit poor quantum efficiency (QE) due to short carrier diffusion lengths of narrow bandgap semiconductors and relatively low absorption coefficients within the LWIR range. Plasmon-driven subwavelength light confinement enables high absorption even in a very thin absorber that provides efficient carrier collection, boosting the detector QE. We propose a photovoltaic detector equipped with a two-dimensional subwavelength hole array (2DSHA) in gold metallization on InAs/InAsSb type-II superlattice (T2SL) heterostructure. Our numerical study utilizing the finite-difference time-domain (FDTD) method predicts five times increased absorption in comparison with a conventional, back-side illuminated device. The simulated behavior of the plasmonic structure was confirmed experimentally by transmittance measurements, which revealed resonant features corresponding to various plasmonic modes.
We report on LWIR multi-stage thermoelectrically cooled cascade photodiodes with InAs/InAsSb superlattice absorbers and contact layers and bulk quaternary wide-gap regions. The aim is to reach high detectivity in conditions where the conventional IR photodiodes suffer from a very low quantum efficiency and extremely low resistances due to a high thermal generation of charge carriers. The heterostructures were grown by MBE on GaAs substrates buffered with GaSb. The connections between stages are made using heavily doped narrow-gap p+/n+ tunnel junctions. The room-temperature detectivities of the devices are close to that of immersed MCT multijunction detectors offered by VIGO (PVMI series).
Most photodetectors offered by VIGO are made of mercury-cadmium telluride compound (MCT) by Metalorganic Chemical Vapour Deposition (MOCVD) technology. Despite many advantages of MCT compound, including lattice parameter being almost independent on composition, nowadays in some applications, detectors containing mercury, cadmium and lead are successively removed from the consumer market through norms and directives (e.g. RoHS) due to their toxicity. The abovementioned limitations connected to MCT encouraged the company to find alternative material system and technology as a replacement. Inspired by literature, VIGO decided to develop Ga-free InAs/InAsSb superlattices which are a great candidate, operating in a similar wavelength regime from MWIR to VLWIR. We continue our idea of backside-illuminated devices using substrate material (GaAs) as an immersion lens. We still take advantage of detector material lattice-matched to the buffer layer, replacing CdTe and HgCdTe by GaSb and InAs/InAsSb SL, respectively. The architecture of SLs-based heterostructures originates from MCT photovoltaic devices and utilizes wide bandgap depletion layers for dark current reduction. The detectivity of SL devices is similar to MCT (Fig. 1). Currently, VIGO efforts are focused on the development of HOT LWIR photodiodes including thin absorber devices.
A Peltier cooled long wavelength infrared (LWIR ) position sensitive photon detector (PSD) based on the lateral effect is reported for the first time. It is a modified PIN LWIR HgCdTe photodiode forming the tetra lateral PSD of the photosensitive area 1×1 mm2, cooled to 205 K, optimized for the 4-11 µm wavelength band and reverse biased. The position resolution close to 1 µm was achieved with 32x averaging of 100 ns QCL laser pulses of the 10.5 µm wavelength and the 0.3 nJ energy focused on the spot of 240 µm 1/e2 diameter, with the box-car integration times of 1 µs and correlated double sampling.
The InAs/InAsSb superlattices are attractive materials for the replacement of both RoHS restricted bulk HgCdTe and strongly Shockley-Read (SR) generation limited InAs/GaSb superlattices. Two main factors limit the performance of InAs/InAsSb photodiodes: the rate of the SR processes, especially in the depletion region, which is the source of the large dark current and a short vertical diffusion length of charge carriers in superlattice absorbers which results in poor responsivity. In this paper, we report on the status of HOT LWIR detectors based on InAs/InAsSb superlattices at VIGO System S.A. The uncooled and Peltier cooled LWIR photoconductors are the most successful devices developed so far. The practical InAs/InAsSb SL-based photoconductors have been fabricated by MBE heteroepitaxial growth on buffered 3” wafers. The design of the devices, material composition and doping, has been optimized for operation at temperatures from 200 to 300 K at a spectral range up to 18 μm. Some of the detectors were supplied with immersion microlenses formed in the GaAs substrates. The devices were characterized by measurements of the spectral responsivity and frequency-dependent noise density. The measured spectral detectivities of the best SL devices were found to be close or better compared to the HgCdTe counterparts operating at the same conditions. The devices are now offered as commercial products. Vigo present efforts are focused on the development of HOT LWIR photodiodes including monolithic cascade devices and thin absorber devices with the plasmonic enhancement of absorption. The development roadmap of advanced HOT devices is also sketched.
We report on the investigation of the long term stability study of InAs1-xSbx (x=0.09) high operation temperature (HOT) photodiode grown on GaAs substrate. The electrochemical passivation technique was proposed to modify the mesa sidewalls properties and obtain anodic sulphur coating covered by SU-8 negative photoresist. The dark current densities of sulphur anodic film, SU-8 photoresist and unpassivated devices was compared. Obtained results indicates that the surface leakage current was not fully supressed by unipolar electron barrier. The most stable behaviour after an exposure of 6 months to atmosphere and annealing at 373 K for 72 h was observed for sulphur anodic passivation. This technique turned to be effective also in reduction of oxygen (O) 2s peak in X-ray photoelectron spectroscopy (XPS) in comparison with only etched sample.
The highly Be-doped InAs layer has been grown on semi-insulating GaAs (100) substrate by Molecular Beam Epitaxy. Very good quality of the layer has been attested by high resolution scanning electron microscope (HR-SEM), X-ray diffraction (XRD) and the Raman spectra. The parallel and perpendicular residual strain are determined to be – 1.17 × 10-3 , and 1.12 × 10-3 , respectively. Moreover, the absorbance (ABS) and photoluminescence (PL) spectra were collected in order to estimate the bandgap narrowing. The 10 meV bandgap shrinking for 1.7×1018 cm-3 acceptor concentration suggests necessity of reexamining the Jain et al model [Jain, S. C., et al. - JAP 68(7): 3747-3749] in the context of actual values of InAs valence-bands effective-masses.
We investigate the high-operating temperature performance of InAsSb/AlSb heterostructure detectors with cutoff wavelengths near 5 μm at 230 K. The devices have been fabricated with different types of absorbing layers: nominally undoped absorber (with n-type conductivity), and both n- and p-type doped. The results show that the device performance strongly depends on absorber layer type. Generally, the p-type absorber provides higher values of current responsivity than the n-type absorber, but at the same time also higher values of dark current. The device with the nominally undoped absorbing layer shows moderate values of both current responsivity and dark current. Resulting detectivities D * of nonimmersed devices vary from 2 × 109 to 5 × 109 cm Hz1/2 W ? 1 at 230 K, which is easily achievable with a two-stage thermoelectric cooler. Optical immersion increases the detectivity up to 5 × 1010 cm Hz1/2 W ? 1.
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.
In this work we investigate the high-operating temperature performance of InAsSb/AlSb heterostructure detectors with cut-off wavelengths near 5 μm at 230 K. The devices have been fabricated with different type of the absorbing layer: nominally undoped absorber, and both n- and p-type doped. The results show that the device performance strongly depends on absorber layer doping. Generally, p-type absorber provides higher values of current responsivity than n-type absorber, but at the same time also higher values of dark current. The device with nominally undoped absorbing layer shows moderate values of both current responsivity and dark current. Resulting detectivities D° of non-immersed devices varies from 2×109 to 7×109 cmHz1/2/W at 230 K, which is easily achievable with a two stage thermoelectric cooler.
We report on the quadrant photon HgCdTe detectors optimized for 2-11 μm wavelength spectral range and Peltier or no cooling, and photosensitive area of a quad-cell of 1×1 to 4×4 mm. The devices are fabricated as photoconductors or multiple photovoltaic cells connected in series (PVM). The former are characterized by a relatively uniform photosensitive area. The PVM photovoltaic cells are distributed along the wafer surface, comprising a periodical stripe structure with a period of 20 μm. Within each period, there is an insensitive gap/trench < 9 μm wide between stripe mesas. The resulting spatial quantization error prevents positioning of the beam spot of size close to the period, but becomes negligible for the optimal spot size comparable to a quadrant-cell area. The photoconductors produce 1/f noise with about 10 kHz knee frequency, due to bias necessary for their operation. The PVM photodiodes are typically operated at 0 V bias, so they generate no 1/f noise and operation from DC is enabled. At 230 K, upper corner frequency of 16 to 100 MHz is obtained for photoconductor and 60 to 80 MHz for PVM, normalized detectivity D*~6×107 cm×Hz1/2/W to >1.4×108 cm×Hz1/2/W for photoconductor and >1.7×108 cm·Hz1/2/W for PVM, allowing for position control of the radiation beam with submicron accuracy at 16 MHz, 10.6 μm wavelength of pulsed radiation spot of ~0.8 mm dia at the close-to-maximal input radiation power density in a range of detector linear operation.
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, the results of the initial work on determining the photoelectric properties of graphene detector operating in a photoconductive mode are presented. Graphene is considered as a material for uncooled fast detectors. The investigation has been done by electrical and optical characterization. Two values of substrate resistivity have been used in the project – below 1 Ωcm and higher than 1.6 kΩcm. Measurements of detectors response to short, strong light pulses were conducted. Presented studies show that the use of high resistivity substrates is necessary to prevent capacitive shorting of the signal to the substrate, causing signal losses and increasing response time.
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.
High-speed, high-sensitivity, avalanche photodiodes operating at 1.55 μm spectral range have been utilized in modern
long-haul and high-bit rate optical communication systems. Related research was focused on developing detectors with
minimized excess noise and maximized gain-bandwidth product.
Recently imaging and critical sensing applications stimulated development of modified avalanche photodiode structures
operating in 1.55 μm spectral range. For these devices speed is no more critical. Instead, very low current densities and
low multiplication noise are the main requirement.
In the present work the performance of uncooled InGaAs/InAlAs/InP avalanche photodiodes operating near 1.5 μm has
been studied. Device modeling based on advanced drift and diffusion model have been performed with commercial
Crosslight APSYS simulator. Conventional separate absorption, charge and multiplication (SACM) avalanche
photodiode as well as devices with a relatively thick undepleted p-type InGaAs absorption region and thin InAlAs
multiplication layer have been considered. The latter type of APD structure enables to increase device quantum
efficiency, reduce dark current and eliminate impact ionization processes within absorbing layer.
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.
This book presents approaches, materials, and devices that eliminate the cooling requirements of IR photodetectors operating in the middle- and long-wavelength ranges of the IR spectrum. It is based mainly on the authors' experiences in developing and fabricating near room temperature HgCdTe detectors at Vigo Systems Ltd. and at the Institute of Applied Physics Military University of Technology (both in Warsaw, Poland).
The text also discusses solutions to other specific problems of high-temperature detection, such as poor collection efficiency due to a short diffusion length, the Johnson-Nyquist noise of parasitic impedances, and interfacing of very low resistance devices to electronics.
Suitable for graduate students in physics and engineering who have received a basic preparation in modern solid state physics and electronic circuits, this book will also be of interest to individuals who work with aerospace sensors and systems, remote sensing, thermal imaging, military imaging, optical telecommunications, IR spectroscopy, and lidar.
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.
Resonant cavity enhanced photodetectors are promising candidates for applications in high-speed optical communications due to their high quantum efficiency and large bandwidth. This is a consequence of placing the thin absorber of the photodetector inside a Fabry-Perot microcavity so the absorption could be enhanced by recycling the photons with resonance wavelength.
The performance of uncooled resonant cavity enhanced InGaAs/InAlAs photovoltaic devices operating near 1.55 μm has been studied both theoretically and experimentally. The analyses include two different types of structures with cavity end mirrors made of semiconducting and metallic reflectors as well as semiconducting and hybrid (dielectric Si3N4/SiO2 + metal) Bragg reflectors. Optimization of the device design includes: absorption layer thickness, position of absorption layer within the cavity and number of layers in distributed Bragg reflectors.
Dependence of absorption on wavelength and incidence angle are discussed. Various issues related to applications of resonance cavity enhanced photodiodes in optical systems are considered.
Practical devices with metallic and hybrid mirrors were fabricated by molecular beam epitaxy and by microwave-compatible processing. A properly designed device of this type has potential for subpicosecond response time.
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.
At present, uncooled thermal detector focal plane arrays are successfully used in string thermal imagers. However, the performance of thermal detectors is modest, they suffer from slow response and they are not very useful in applications requiring multispectral detection. In the paper, a number of concepts to improve performance of photon detectors operating at room temperature are presented. Several types of detector materials are considered: HgCdTe, Sb-based III-V ternary alloys, and type-II InAs/GaSb superlattice. Initial efforts were concentrated on photoconductors and photoelectromagnetic detectors. Recently, advanced heterojunction photovoltaic detectors have been developed. It is shown that uncooled HgCdTe photovoltaic detector can achieve detectivity of 109 cmHz1/2W-1 at the 8-9 μm range. Potentially the devices can be assembled in large focal plane arrays. This will enable obtaining of NEDT of less than 0.1 K for staring thermal imagers operating with f/1 optics and 30s-1 frame rate.
Fast refractive microlenses are increasingly important as optical concentrators for uncooled infrared photodetectors. They are used in purpose to improve performance and speed of response. Refractive microlenses formed directly onto semiconductor materials draw much attention because they facilitate monolithic integration with active element of infrared photodetector. Gallium arsenide due to its superior optical and mechanical properties is a material of choice for fabrication of microlenses. We have developed process for fabrication of GaAs refractive microlenses monolithically integrated with InGaAs and HgCdTe photodetectors, both as single element devices and two-dimensional arrays. Specially designed machine tool has been used for preparation of relatively large single spherical GaAs microlenses with 0.5 mm-10 mm diameter. The microlens-detector arrays were prepared using a combination of ion milling and wet chemical etching. The typical process involves one photolithography, one ion milling and one or two chemical etching steps. More advanced procedures have also been proposed to improve quality of the lenses. The lenses can be optimized as optical concentrators for IR photodetectors with circular, square, rectangular and other geometries. This process is especially convenient for fabrication of lenses with size less than 50 μm, but larger lenses with size exceeding 300 μm can be prepared as well with some modifications of the fabrication procedures.
Research, development and fabrication of advanced optoelectronic devices in the spin-off company operating on the open market are discussed. Technological solutions, which secure company competitive position on the world market, are presented.
We report here uncooled and thermoelectrically cooled InAs photodetectors designed for fast and sensitive detection of IR radiation. This has been achieved by the use of a complex architecture of the device that ensures reduced thermal generation of charge carriers, fast diffusion and drift transport of photogenerated carriers across the absorber region, a low series resistance, and a low capacitance. In addition, the device are monolithically immersed to GaAs hyperhemispherical microlenses that reduces capacitance by more than two orders of magnitude in comparison to non-immersed devices of the same optical area. As a result, the optimized devices are characterized by picosecond response time.
InAs-based alloys has been the subject of intense studies over the past few years, due to its importance as a material for photovoltaic detectors that can be tuned to optimum performance at any wavelength within 1 - 3.6 μm spectral range. We report here new results concerning electrical and photoelectrical characteristics of epitaxial InAs photovoltaic detectors optically immersed to GaAs microlenses and operating at near room temperatures. The devices are based on epitaxial heterostructures grown directly on thick GaAs substrates by molecular beam epitaxy. The performance of the detectors has been significantly improved by the use of multilayer heterostructures with optimized doping and band gap profiles, improved crystalline perfection of the heterostructure, improved processing resulting in reduction of parasitic series resistance and increase of shunt resistance of the device. Additionally, monolithic optical immersion to GaAs substrates has been applied. The devices have been characterized by current and capacitance measurements performed at steady-state and time dependent conditions at temperatures 77-300 K. The measurements provide an indication that tunneling and defect- assisted rather than band-to-band processes limit performance of the devices. The measurements of capacitance and series resistance of devices show picosecond RC time constant even for devices with relatively large apparent area (1 mm2). This is due to the reduction of junction capacitance by two orders of magnitude in comparison with non-immersed devices of the same optical area. The unique properties make possible more widespread applications of the optically immersed InAs devices in various infrared systems.
The paper reports on the design and fabrication of LPE-grown (formula available in paper) heterojunction photodetectors operating in the 2-2.4 micrometers wavelength region. Experiments on LPE growth of high-x- content quaternaries as well as optimization of device processing has been carried out. LPE growth at Tapproximately equals 530DEGC enabled obtaining lattice matched heterostructures with 19% indium in the active layer In (formula available in paper) and photodetectors with (lambda) co=2.25micrometers . By increasing the temperature of epitaxial growth to 590DEGC In (formula available in paper)heterostructures (with 23%indium content suitable for photodetectors with (lambda) co=2.35 micrometers have been obtained. Mesa-type photodiodes were fabricated by RIE in Ccl (formula available in paper) plasma and passivated electrochemically in (formula available in paper). These devices are characterized by differential resistance up to (formula available in paper) and the detectivity in the range (formula available in paper), in dependence on the photodiode active area cutoff wavelength.
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.
The performance of uncooled InxGa1-xAs photovoltaic devices operating in the 2-3.6 micrometers spectral range has been studied both theoretically and experimentally. Various multilayer homo- and heterojunction devices have been considered. Band-to-band Auger processes were taken into account as the dominant mechanisms of thermal generation and recombination. Calculations show that the best performance can be obtained in devices with lightly doped n-type absorber region as a result of the significant role of Auger S (spin-off band) process. The optimized heterostructures with near intrinsic n-type narrow gap InGaAs absorber were grown using molecular beam epitaxy. The performance of the present InGaAs devices remains significantly below theoretical limits. Better agreement can be observed for devices with longer cutoff wavelength as the result of increasing role of fundamental limitations and improved quality of materials with x approximately equal to 1. Improvement of performance by almost an order of magnitude has been achieved by the use of monolithic optical immersion. This has been accomplished by growth of the InGaAs heterostructures on thick SI GaAs substrates and by formation of the immersion lenses directly in the substrate. These devices are expected to find important applications in infrared systems which require better performance than existing uncooled detectors.
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.
The present high quality HgCdTe photodetectors are based on heterostructures with complicated band gap and doping profiles. Such structures can be obtained by combination of Isothermal Vapor Phase Epitaxy (ISO VPE) and RDF sputtering of CdTe. Cadmium Telluride exhibits unique properties - transparency to IR light, low conductivity, nearly lattice match and chemically compatibility to HgCdTe. Thus, low defect densities and hence low surface state density at the CdTe/HgCdTe interface are expected, which in turn lead to low surface recombination velocities and long effective lifetimes. The immediate use of CdTe epilayer is passivation of HgCdTe. High quality layers HgCdTe have been grown on CdTe substrates by open tube ISO VPE. CdTe layers have been obtained by RF magnetron sputtering on to previously deposited HgCdTe layers in 400 LS Leybold AG System. The substrate temperature can be varied form 20 to 90 degrees C. Thermal treatment has been performed in Hg/H2 atmosphere in reusable chamber at temperatures 200-400 degrees C. The resulting heterostructures have been characterized by visual microscopy with Nomarski contrast, Hall measurements and IR transmittance. This technic and photolithography has been extensively used for successful fabrication of heterojunction contact photoresistors and heterostructure photodiodes.
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.
The performance of near-room temperature long-wavelength photovoltaic IR detectors has been analyzed. The ultimate performance of the devices is limited by the noise due to the statistical nature of thermal generation and recombination processes. Another practical limitation arises from the Johnson-Nyquist noise due to parasitic impedances and contribution of preamplifier noise. This is particularly significant for large area devices. Possible ways to overcome the difficulties are discussed.
The ultimate signal-to-noise performance of the semiconductor photodetector is limited by the statistical fluctuations of the thermal generation and recombination rates in photodetector material. Cooling is an effective but impractical way of suppression of the thermal processes. The performance of uncooled detectors can be improved by minimizing the thermal generation and recombination rates and reducing the actual volume of photodetector. This can be realized in 3D heterostructure devices. In these devices, the incident radiation is absorbed in small regions of narrow gap semiconductor, buried in wide gap volume and supplied with wide gap electric contacts and radiation concentrators. The practical near room-temperature 1 - 12 micrometers IR heterostructure photodetectors are reported. The devices are based on variable gap Hg1-xCdxTe. The 3D heterostructures have been obtained by Isothermal Vapor Growth Epitaxy in a reusable growth system which enables in situ doping during growth with foreign impurities. Ion milling was extensively used in preparation of the devices. Monolithic optical immersion has been applied for further improvement of performance. The 3D heterostructure devices exhibit performance exceeding that of conventional photodetectors.
A brief description of construction, as well as investigation of properties of the multispectral non-cooled infrared detector, is being presented below. The detector's construction enables its sensitivity control in a wide spectrum range from 0.6 up to 11 micrometers .
The major drawback of IR photodetectors is the need for cooling to suppress thermal generation of free carriers resulting in noise. New ways to improve the performance of infrared photodetectors operated without cryogenical cooling are discussed including the optimum design of the devices, the use of optical immersion of photodetectors to high refraction index lenses and optical resonant cavity. Another and very promising way, however, is the suppression of thermal generation which is governed by Auger mechanism by depletion of semiconductor in charge carriers. The combination of various methods would eventually enable achievement near-BLIP performance of IR detectors without cooling.
The ultimate signal-to-noise performance of the semiconductor photodetector is limited by the statistical fluctuations of the thermal generation and recombination rates in the semiconductor. Various ways to improve the performance of photodetectors operated at near room temperature including: optimized gap/doping profile, reduced volume of the device, and the use of non- equilibrium depletion of semiconductor have been proposed. Practical 2 to 12 micrometers devices with improved performance are reported.
The performance of thermoelectrically cooled p+ -n medium-wavelength infrared (MWIR) HgCdTe photodiodes is analyzed. The effect of doping profile on the photodiode parameters (R0A product, quantum efficiency) is solved by forward-condition steady-state analysis. Results of calculations are compared with experimental data. The p+ -n homojunctions are formed by arsenic diffusion in HgCdTe epilayers. MWIR photodiodes effectively operate at elevated temperatures around 200 K and exhibit background-limited photodetection (BLIP) performance when monolithic optical immersion is used.
The major drawback of IR photodetectors is the need for cooling to suppress thermal generation of free carriers resulting in noise. New ways to improve the performance of infrared photodetectors operated without cryogenical cooling are discussed, including the optimum design of the devices, the use of optical immersion of photodetectors to high refraction index lenses, and the optical resonant cavity. Another and very promising way, however, is the suppression of thermal generation, which is governed by the Auger mechanism by depletion of the semiconductor in charge carriers. The stationary depletion can be achieved by the use of exclusion, extraction, and magnetoconcentration effects. The combination of various methods would eventually enable us to achieve near-background-limited photodetection (near-BLIP) performance of IR detectors without cooling.
The p+-n homojunctions were formed by arsenic diffusion in the HgCdTe monocrystals and epilayers. Photodiode performance was established by measuring the current-voltage and spectral response characteristics. LWIR photodiodes are background limited at 77 K. MWIR photodiodes effectively operate at elevated temperature around 200 K and exhibit near BLIP performance when optical immersion is used.
The influence of blocking contacts on IR MCT photoconductor parameters is studied. The photoelectric gain, current responsivity and detectivity are investigated by numerical approach. It is shown that the best performance should be expected in n+v devices.
A method is presented for thermal noise reduction in a near room-temperature intrinsic IR photodetector. The method is based on suppression of the Auger generation-recombination processes using the electro-magnetic carrier depletion (EMCD) of a narrow gap semiconductor. The device is a lightly doped narrow gap semiconductor flake with a high backside surface recombination velocity, supplied with electrical contact band and placed in a magnetic field. Due to the action of the Lorentz force most of the device depletes charge carriers, which results in suppression of the Auger generation and recombination processes. As a result, the I-V characteristic becomes nonlinear, exhibiting regions of high positive and negative resistance. Thermal noise can be dramatically reduced, leading to a substantial improvement in performance. The ultimate detection may be determined either by background radiation or by Shockley-Read generation, depending on the ratio of the background photon flux to the recombination center concentration. Near-BLIP performance is predicted for 10.6-μm (Hg, Cd) Te devices, prepared from high-quality materials and operated at 225-250 K.
A method is presented for the thermal noise reduction in a near room-temperature intrinsic IR photodetector. The method is based on suppression of the Auger generation-recombination processes using the Electro-Magnetic Carrier Depletion (EMCD) of a narrow gap semiconductor. The device is a lightly doped narrow gap semiconductor flake with a high backside surface recombination velocity, supplied with electrical contact and placed in a magnetic field. Due to action of the Lorentz force, most of the device depletes in charge carriers, resulting in suppression of the Auger generation and recombination processes. As a result, the I-V characteristic becomes nonlinear, exhibiting regions of high positive and negative resistance. The thermal noise can be dramatically reduced, leading to a substantial improvement of performance. The ultimate detectivity may be determined either by the background radiation or by the Shockley-Read generation, in dependence on the ratio of the background photon flux to the recombination center concentration. The near-BLIP performance is predicted for 10.6 micrometers (Hg,Cd)Te devices, prepared from high quality materials and operated at 225 - 250 K.
The analysis of a magneto-concentration effect carrier-depleted IR photodetector is reported. The device is a lightly doped narrow-gap photoconductor with a high backside surface recombination velocity, placed in a magnetic field. Due to action of the Lorentz force, the carrier concentration in the most parts of the device is highly reduced. As a result the I-V characteristics exhibit saturation and negative dynamic resistivity. The Auger generation and recombination processes are highly suppressed, resulting in a decrease of noise current. This makes it possible to improve dramatically the performance of IR devices. For example, the background limited performance is predicted for 10.6 micrometers (Hg,Cd)Te devices operated at 230-250 K.
A novel electro-optic configuration active modelocked laser oscillator is described. A regenerative amplifier system with an electrooptical shutter is used to the amplification of subnanosecond and nanosecond light pulses. k
There were carried out numerical investigations of the possibility of applying the self-injection technique for generation of short pulses jil rare-gas halide excimer lasers. We investigated a case in which the electrooptical switch is placed in a resonator and is contrulled by two half-wave voltage gates and active medium has the parameters typical for the KrF laser. The model regarded vibrational relaxation and collisional mixing of the B states of excimer molekule. The influence of parameters characterizing the course of voltage applied to the Q-switch upon energy power temporal shape and contrast of generated pulses was analysed. It was stated that there is a possibility of a controlled generation of pulses of time duration from a few hundreds of picoseconds to a few nanoseconds and power of on order higher than in case of free oscillation.
The paper presents the results of preliminary investigations of electronic system built using three krytrons. Results of applying this system to generation laser pulses by self -injection method are also presented.
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