KEYWORDS: Dark current, Design and modelling, Astronomical imaging, Mercury cadmium telluride, Astronomy, Telescopes, Avalanche photodetectors, Silicon, Readout integrated circuits, Signal to noise ratio
The linear-mode, avalanche photodiode array (LmAPD) based on bandgap-engineered HgCdTe, grown by Metal Organic Vapour Phase Epitaxy (MOVPE) is an important product type at Leonardo UK. High-value instruments often employ LmAPDs where the photon count is low and conventional detectors cease to be sensitive. Applications now split into three main categories. Firstly, for applications with intrinsically short integration time, such as: wavefront sensors, fringe trackers and devices for rapid-time-domain astronomy, LmAPDs arrays are established in 320x256 and 515x512 formats, with an avalanche gain of >100x at 15V bias and 80-100K operating temperature. Secondly, for free-space telecoms, LIDAR and gated arrays a GHz version of the LmAPD is available. Thirdly, there is a class of applications with very low photon arrival rates and these provide the most demanding of challenges for LmAPDs. The main field is astronomical imaging and interferometry. In a collaboration with the University of Hawaii (UH) a 1kx1k/15μm device been developed and together with a low dark current version of the LmAPD is under detailed characterisation at UH. Initial results show dark currents below 0.001ph/s/pixel and usable avalanche gain up to 10x. A 2kx2k/15μm format device funded by ESA is currently in trial manufacture. This paper provides an update on the technology and the status of our developments and collaborations.
Leonardo are currently involved in a number of projects within the space and astronomy sector for MCT cryogenically cooled detectors and DLATGS pyroelectric products. Leonardo MCT avalanche photodiodes have had significant success for both imaging and wavefront sensing applications, demonstrating sub photon sensitivity in the short wave. The latest results on the development of arrays in a number of configurations for a range of products are presented. We report on the progress of our latest COTS MCT IDCA flight programme following BIRD. Data is also presented on Leonardo’s current work in extending MCT into the visible band and creating high speed devices suitable for GHz, deep space communication.
Linear-mode, avalanche photodiode arrays (LmAPDs) based on bandgap-engineered HgCdTe, grown by Metal Organic Vapour Phase Epitaxy (MOVPE) are used in low-flux applications, where the signal-to-noise ratio would otherwise be very low. The LmAPD mesa-device architecture provides 100% fill factor, low crosstalk and minimal interpixel capacitance. Saphira (320×256/24 μm) devices operating at an avalanche gain of 50-100 and temperature of 80-90 K are deployed in 12 telescopes as wavefront sensors and notably control the four 8.2 m telescopes in the Very Large Telescope (VLT) interferometer. Some of these devices have operated for many years at full gain. Applications now split into three main categories. Firstly, those with intrinsically weak infrared sources that need moderate avalanche gain but very low dark current – 1k×1k/15 μm and 2k×2k/15 μm arrays are currently in development to service this requirement. Secondly, future adaptive optics (AO) systems, associated with 30 m class telescopes, require larger arrays and frame-rates over 2000 frames/s. A 512×512/24 μm device, specifically for pyramid wavefront sensors, is currently under development for the Extremely Large Telescope (ELT). The third category covers high speed (GHz) APDs mainly for free-space optical communications and LIDAR. This paper provides an update on the technology and status of the developments.
SAPHIRA (320x256/24μm) is the first of a family of HgCdTe APD infrared sensors for NIR/SWIR sensing in low flux conditions for scientific applications. The paper will present the status of the latest developments of detectors suitable for a range of ground and space applications including wavefront sensing, low flux imaging, Lidar and high speed optical communications. The next generation SAPHIRA (512x512/24μm) array has an architecture specific to pyramid wavefront sensors, supported by a consortium comprising European Southern Observatory, Max Planck Institute, NRC Herzberg Institute and Potsdam University. A 1kx1k/15μm, 3-side buttable sensor called Ike Pono after the Hawaiian for ‘far-vision’ is aimed at extreme, low-flux imaging, supported by NASA through the University of Hawaii, Institute of Astronomy. A large area device suitable for LIDAR and supported by NASA also demonstrates a performance suitable for high speed optical communication applications. Most recently the development of a 2k x 2k/ 15μm device for scientific imaging has started with the support of ESA.
Linear-mode avalanche photodiode arrays, LmAPDs, based on bandgap engineered HgCdTe, grown by Metal Organic Vapour Phase Epitaxy, MOVPE, can produce virtually noise-free infrared sensors. These are required for applications in big science, security and biochemistry. A custom device called SAPHIRA (320x256/24μm) has been designed specifically for LmAPDs. SAPHIRA has been deployed as a wavefront sensor for adaptive optic systems in nine major telescopes and notably five devices control the four 8.2 metre telescopes of the VLT interferometer. These demanding applications have driven frames rates up to 200 kframes/s and avalanche gains to x600 achieving read noise as low as 0.26 e- rms and enabling single photon counting. The detector is baselined for time-domain astronomy vital for exoplanet spectroscopy and understanding the physics of active stellar objects. The three 30 metre class telescopes currently under construction and the three candidate space telescopes, HabEx, Luvoir and EZE will depend on noise-free infrared detectors at very low dark current. Work at the University of Hawaii and European Southern Observatory has demonstrated dark currents in the 4-10 electrons/hour range and with avalanche gain offers the prospect of higher science return from these instruments. A 1kx1k/15μm 3-side buttable array is currently in development to service extreme low background applications especially spectroscopy. A 512x512/24μm SAPHIRA array with 64 parallel video outputs is in development for pyramid wavefront sensing on the European Extremely Large telescope, ELT, mirror co-phasing and rapid time-domain astronomy.
Mercury Cadmium Telluride (MCT) grown by Metal Organic Vapour Phase Epitaxy (MOVPE) on GaAs substrates is a mature technology at Leonardo MW used for the production of high-performance infrared detectors in the short, mid and long wavebands. Recently reported developments of single waveband devices have seen pixel densities increase by almost an order of magnitude, driven by system requirements for increased resolution together with reduced size, weight and power. High resolution MWIR detectors with 8μm pixels are now in volume production. The mesa structure of MOVPE grown MCT pixels controls most sources of inter-pixel crosstalk with optical scattering, carrier diffusion and other blurring mechanisms reduced to negligible levels, resulting in characteristically sharp infrared images from systems which use these devices. More than a decade ago, Leonardo MW pioneered the development of Dual Waveband Infra-Red (DWIR) technology with devices using 30μm, 24μm and 20μm mesa pixels. Both MWIR-LWIR and MWIR-MWIR devices have been successfully fabricated. These devices use a “back-to-back” diode arrangement in which the waveband is selected by changing the bias polarity across the diode stack. This ensures the spatial correlation between the two wavebands, which is essential for many applications. Such DWIR structures are significantly thicker than single waveband devices, posing challenges to increasing pixel density whilst retaining the highly desirable benefits of mesa isolation. This paper discusses the evolution of the DWIR technology to high density, 12μm MWIR-LWIR pixels, enabling the manufacture of higher resolution, lower cost DWIR devices.
Many branches of science require infrared detectors sensitive to individual photons. Applications range from low background astronomy to high speed imaging. Leonardo in Southampton, UK, has been developing HgCdTe avalanche photodiode (APD) sensors for astronomy in collaboration with European Southern Observatory (ESO) since 2008 and more recently the University of Hawaii. The devices utilise Metal Organic Vapour Phase Epitaxy, MOVPE, grown on low-cost GaAs substrates and in combination with a mesa device structure achieve very low dark current and near-ideal MTF. MOVPE provides the ability to grow complex HgCdTe heterostructures and these have proved crucial to suppress breakdown currents and allow high avalanche gain in low background situations. A custom device called Saphira (320x256/24μm) has been developed for wavefront sensors, interferometry and transient event imaging. This device has achieved read noise as low as 0.26 electrons rms and single photon imaging with avalanche gain up to x450. It is used in the ESO Gravity program for adaptive optics and fringe tracking and has been successfully trialled on the 3m NASA IRTF, 8.2m Subaru and 60 inch Mt Palomar for lucky imaging and wavefront sensing. In future the technology offers much shorter observation times for read-noise limited instruments, particularly spectroscopy. The paper will describe the MOVPE APD technology and current performance status.
SELEX Galileo Infrared Ltd has developed a range of 3rd Generation infrared detectors based on HgCdTe grown by Metal Organic Vapour Phase Epitaxy (MOVPE) on low cost GaAs substrates. There have been four key development aims: reducing the cost especially for large arrays, extending the wavelength range, improving the operating temperature for lower power, size and weight cameras and increasing the functionality. Despite a 14% lattice mismatch between GaAs and HgCdTe MOVPE arrays show few symptoms of misfit dislocations even in longwave detectors. The key factors in the growth and device technology are described in this paper to explain at a scientific level the radiometric quality of MOVPE arrays. A feature of the past few years has been the increasingly sophisticated products that are emerging thanks to custom designed silicon readout devices. Three devices are described as examples: a multifunctional device that can operate as an active or passive imager with built-in range finder, a 3-side buttable megapixel array and an ultra-low noise device designed for scientific applications.
SELEX Galileo in collaboration with the Astronomy Technology Centre (ATC) undertook an activity to develop near
infrared (NIR) and short wave (SWIR) sensor arrays as a precursor to a large format array in future phases of work. In
this study, SELEX grew wafers of mercury cadmium telluride (MCT) material (cut off wavelengths ranging from 1.9μm
to 2.7μm) using metal organic vapour phase epitaxy (MOVPE) on GaAs substrates. With substrate sizes up to 150mm
available, this technology is ideal for very large arrays. Mesa structure arrays were processed and hybridised to
multiplexers with a floating gate input.
MOVPE requires the growth of buffer layers which would absorb the shortest wavelengths. Results will be presented
showing how the cut-on wavelength can be controlled by thinning these buffer layers and the subsequent achievement of
a response to radiation shorter than 0.8 μm. Data will be presented showing sub 0.1 e/s/pix dark current at 80K, quantum
efficiencies of 75% in H-band, and less than 3 minutes persistence after spot illumination into "double saturation".
This paper describes the fabrication and performance of our LW Hawk arrays. These are Full-TV (640x512) LW infrared
detectors at small pitch (16 μm) made from HgCdTe grown by Metal Organic Vapour Phase Epitaxy (MOVPE).
The detectors are staring, focal planes consisting of HgCdTe mesa-diode arrays bump bonded to silicon read-out
circuits. The HgCdTe structure is grown on GaAs and consists of an absorber layer sandwiched between wider band-gap
cladding layers. Device processing is wafer-scale. This is an extension of the work reported in previous years with the
innovation of dry etching for mesa isolation. The GaAs substrate is removed after bump bonding to minimise the
thermal stress on cooling.
The technology will be described. Results will be presented which show operability of 99.96% with a median NETD of
32 mK, reducing to 22 mK in binning mode. The results of various imaging trials will also be presented.
Selex Sensors and Airbourne Systems has been active in developing Very Long Wave arrays for space applications
under a contract of the European Space Agency. Arrays have been demonstrated with a 15 μm cut-off operating at 55 K.
The technology is an extension of our standard LW, described elsewhere, using MOVPE layers grown on GaAs to
provide a low cost, large area capability with state-of-the-art performance. The test vehicle for the VLW development is
a direct injection 320 x 256, 30 μm pitch ROIC with a well capacity of 20 million electrons. While it may be considered
that direct injection is not ideal for typical diode impedances expected in the VLW, and alternatives are in design, it is a
testament to our technology that the diodes have sufficient dynamic resistance to allow this approach.
Our diode design provides low diffusion currents such that at these operating temperatures the arrays are largely limited
by trap assisted tunnelling (TAT). Results of dark current as a function of voltage and temperature will be presented
along with the array electro-optical performance.
This paper describes the design, fabrication and performance of dual-band MW/LW infrared detectors made from
HgCdTe (MCT) grown by Metal Organic Vapour Phase Epitaxy (MOVPE). The detectors are staring, focal plane arrays
consisting of HgCdTe mesa-diode arrays bump bonded to silicon read-out circuits. Each mesa has one connection to the
ROIC and the bands are selected by varying the applied bias.
Arrays of 320x256 pixels on a 30 μm pitch have performed exceedingly well. For example, arrays with a cut-off
wavelength of 5 μm in the MW (mid-wave) band and 10 μm in the LW (long-wave) band have median NETDs of 10 and
17 mK and defect levels of 0.3% and 0.05%, in the MW and LW bands respectively. Interestingly the LW defect level is
often lower than the MW defect level and the defects are not correlated; i.e. a pixel that is defective in the MW band is
usually not defective in the LW band.
Arrays of 640x512 pixels on a 24 μm pitch have been developed. These use a read-out integrated circuit (ROIC) that has
two capacitors per pixel and the ability to switch bands during a frame giving quasi-simultaneous images. The
performance of these arrays has been excellent with NETDs of 14mK in the MW band and 23mK in the LW band. Dual
band-pass filters have been designed and built into a detector.
This paper describes long wavelength (LW) infra-red detectors made from HgCdTe grown by Metal Organic Vapour
Phase Epitaxy (MOVPE) and the performance in a low photon flux background compatible with a multispectral
requirement. The detectors are staring, focal plane arrays consisting of HgCdTe mesa-diode arrays bump bonded to
silicon read-out circuits. The HgCdTe structure is grown on GaAs and consists of an absorber layer sandwiched between
wider band-gap cladding layers. Device processing is wafer-scale. Wet etching is used to define the mesas and the mesa
sidewalls are passivated with inter-diffused CdTe. The GaAs substrate is removed after bump bonding to minimise the
thermal stress on cooling.
The technology is sufficiently advanced to enable production not only of LWIR detectors but also dual band
MWIR/LWIR detectors, as reported last year. Cameras for both types have been developed.
There is now increasing interest in using the technology for LWIR multispectral imaging. Due to the requirement for
narrow bandwidths, resulting in low radiant flux, the diode quality, in terms of dark current and resistance, must be
exceptionally good. This requirement has been difficult to achieve in many technologies, however MOVPE grown
MCT has consistently provided LWIR arrays with the necessary low dark current and high resistance. Performance from
arrays of size 640x512 with 24 μm pixels and having a cut-off of 10 μm will be described. These achieve diode
impedances of several GΩ's with less than 1 nA dark current at 90K.
The drive towards improved target recognition has led to an increasing interest in detection in more than one infrared band. This paper describes the design, fabrication and performance of two-colour and three-colour infrared detectors made from HgCdTe grown by Metal Organic Vapour Phase Epitaxy (MOVPE). The detectors are staring, focal plane arrays consisting of HgCdTe mesa-diode arrays bump bonded to silicon read-out integrated circuits (ROICs). Each mesa diode has one connection to the ROIC and the colours are selected by varying the applied bias. Results will be presented for both two-colour and three-colour devices.
In a two-colour n-p-n design the cut-off wavelengths are defined by the compositions of the two n-type absorbers and the doping and composition of the p-type layer are chosen to prevent transistor action. The bias polarity is used to switch the output between colours. This design has been used to make MW/LW detectors with a MW band covering 3 to 5 μm and a LW band covering 5 to 10 μm.
In a three-colour n-p-n design the cut-off wavelengths are defined by the compositions of the two n-type absorbers and the p-type absorber, which has an intermediate cut-off wavelength. The absorbers are separated from each other by electronic barriers consisting of wide band-gap material. At low applied bias these barriers prevent photo-electrons generated in the p-type absorber from escaping and the device then gives an output from one of the n-type absorbers. At high applied bias the electronic barrier is pulled down and the device gives an output from both the p-type absorber and one of the n-type absorbers. Thus by varying the polarity and magnitude of the bias it is possible to obtain three-colours from a two-terminal device. This design has been used to make a SW/MW/MW detector with cut-off wavelengths of approximately 3, 4 and 6 μm.
This paper describes the fabrication and performance of affordable LW infrared focal plane arrays (IRFPAs) made from
HgCdTe (MCT) grown by Metal Organic Vapour Phase Epitaxy (MOVPE) bump bonded to silicon read-out integrated
circuits (ROICs). The growth substrate is GaAs, being readily available from several sources and suitable for wafer
scale processing. Arrays of size up to 640x512 at 24 μm pixel pitch have been produced, encapsulated, and
demonstrated in a camera system. Arrays of this size are produced in n-on-p material, that is, the common layer is p-type.
This orientation is chosen from a contact technology viewpoint. It is shown that at higher biases trap-assisted
tunnelling (TAT) can limit the performance of arrays. This becomes an issue for large arrays at high infrared flux with a
p-type common layer due to its inherent higher sheet resistance compared to n-type, this can result in debiassing of the
central elements. The key is found to be the control of the MCT structure and quality to ensure good diode performance
with minimal TAT, allowing the higher biases needed to overcome debiassing.
The drive towards improved target recognition has led to an increasing interest in detection in more than one infrared band. Many groups have demonstrated two-color detection, typically by employing two back-to-back junctions, one for each color. In this paper we describe a method for introducing a third color via an absorber of intermediate wavelength placed between the two junctions. Electronic barriers are used to isolate this intermediate region. The design and location of the barriers in the structure are such that the barrier height is readily controlled by the applied bias, enabling the intermediate color to be turned on by applied bias. To provide the positional and doping control needed in the materials structure, MOVPE growth of MCT is used. Both FPA's hybridised to a read-out chips with switchable inputs, and test diodes for direct assessment, have been produced. This paper concentrates on the test diode assessment, as this provides the greater insight into the operation of the device. It is envisaged that such a device will be used with sequential framing of the different colors to provide quasi-temporal imaging.
The successful demonstration of the 3-color concept is described.
C. Jones, L. Hipwood, C. Shaw, J. Price, R. Catchpole, M. Ordish, C. Maxey, H. Lau, R. Mistry, M. Wilson, A. Parsons, J. Gillespie, L. Baggaley, M. Wallis
This paper describes the fabrication and performance of MW and LW infrared focal plane arrays (IRFPAs) made from HgCdTe (MCT) grown by Metal Organic Vapour Phase Epitaxy (MOVPE) bump bonded to silicon read-out integrated circuits (ROICs). MOVPE of HgCdTe is possible on CdTe, CdTe:Si, GaAs or GaAs:Si substrates. When choosing the substrate an important factor is the difference in thermal expansion coefficient between the array and the ROIC; if it is large the hybrid will delaminate when cooled to its operating temperature. GaAs:Si substrates provide a simple solution to the thermal stress problem so these were used initially and several hundred MW 640x512 arrays were made. The NETDs were in the range 10 to 14 mK and the defect levels could be as low as 0.1%. However, HgCdTe grown on GaAs:Si suffers to varying degrees from short-range non-uniformity in cut-off wavelength and the ability of these devices to withstand storage at elevated temperatures is also variable. Recently, the thermal stress problem for arrays on GaAs substrates has been solved and small quantities of MW and LW arrays have been made; they have excellent uniformity and bake stability. For MW 384x288 arrays with a cut-off wavelength of 4.95 μm the NETD is in the range 15 to 18 mK and the defect level can be as low as 0.05%. For LW 320x256 arrays with a cut-off wavelength of 10.0 μm the NETD is in the range 20 to 25 mK and the defect level can be as low as 1.3%. These devices will withstand temperature excursions up to 70°C and higher while in storage. The ability of the devices to withstand temperature cycling is being assessed. A 384x288 array has survived 1800 cycles between room temperature and 80 K with no change in performance. Thus GaAs is the preferred low cost substrate for MOVPE growth of HgCdTe.
Gavin Bowen, Ian Blenkinsop, Rose Catchpole, Neil Gordon, Mark Harper, Paul Haynes, Les Hipwood, Colin Hollier, Chris Jones, David Lees, Chris Maxey, Daniel Milner, Mike Ordish, Tim Philips, Richard Price, Chris Shaw, Paul Southern
Conventional high performance infrared (IR) sensors need to be cooled to around 80K in order to achieve a high level of thermal sensitivity. Cooling to this temperature requires the use of Joule-Thomson coolers (with bottled gas supply) or Stirling cycle cooling engines, both of which are bulky, expensive and can have low reliability. In contrast to this, higher operating temperature (HOT) detectors are designed to give high thermal performance at an operating temperature in the range 200K to 240K. These detectors are fabricated from multi-layer mercury cadmium telluride (MCT) structures that have been designed for this application. At higher temperatures, lower cost, smaller, lighter and more reliable thermoelectric (or Peltier) devices can be used to cool the detectors. The HOTEYE thermal imaging camera, which is based on a 320x256 pixel HOT focal plane array, is described in this paper and performance measurements reported.
Infrared detectors based on Hg1-xCdxTe and grown by the MOVPE process can be designed to have very low dark currents, even for temperatures above 200K. These low dark currents are compatible with achieving background-limited performance at a temperature of 200K in f/2. However, in practice the detectors suffer from high 1/f noise. In this paper, a novel approach is explored in which most of the low frequency noise can be eliminated by operating the arrays at near zero bias. Using this technique, imaging arrays have been demonstrated at temperatures up to 220K giving a NETD of around 60mK in f/2.
Recent advances in MOVPE growth and heterostructure fabrication technology mean that infrared detector arrays based on Hg1-xCdxTe now have the potential to produce high performance imagery when operated in the temperature range 150-200 K. This has a number of system advantages including reduced cooler power consumption and increased cooler life. This paper reports the fabrication and assessment of a MW staring array with a cut-off of 4 μm at 150 K for intermediate temperature operation. Near background limited (BLIP) performance was achieved at temperatures up to 180 K with a median NETD better than 12 mK. Above this temperature, the array still operates normally however there is an exponential increase in the number of noisy pixels, and the median NETD degrades more rapidly than predicted from Shot noise. This behavior is consistent with increased low frequency or 1/f noise at the higher temperatures. This excess noise is not a fundamental limitation and if it could be eliminated, the array would remain close to BLIP up to 200 K.
Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including for use as IR sources in gas sensing systems and as thermal radiation shields in IR cameras. Additionally these devices can be used as calibration sources for very large IR focal plane arrays and have many potential advantages over conventional calibration sources, including high speed operation and low power consumption. For many of these applications a large area device which displays as large area device which displays as large as possible apparent temperature range is required. However, under reverse bias significant currents are required to reduce the carrier concentrations to the levels needed for maximum dynamic range. We have therefore used a novel micromachining techniqe to fabricate integrated optical concentrators in InSb/InAlSb and HgCdTe NL devices. Smaller area diodes can then be used to achieve the same absorption and the required currents are thus reduced. To fabricate the concentrators, spherical resist masks are first produced by resist reflow. Inductively coupled plasma etchign is then used to alternatley etch the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with almost parabolic profiles. Recent results from large area medium wavelength devices with integrated optical concentrators are presented, together with a description of the continuing optimization of the process and progress towards the fabrication of large area long wavelength devices.
Negative luminescent (NL) devices, which to an IR observer appear colder than they actually are, have a wide range of possible applications, including for use as thermal radiation shields in IR cameras, and as IR sources in gas sensing systems. For many of these applications a large area (>1cm2) device is required, together with as large as possible apparent temperature range. However, under reverse bias significant currents are required to reduce the carrier concentrations to the levels needed for maximum possible absorption. These may lead to current heating of the device, which in turn reduces the apparent temperature range. We have therefore used a novel micromachining technique to fabricate integrated optical concentrators in InSb/InAlSb and HgCdTe NL devices. Smaller area diodes can then be used to achieve the same absorption (e.g. for InSb an area reduction of 16 is possible) and the required currents are thus reduced. To fabricate the concentrators parabolic resist masks are first produced, which are approximately 10 μm high and approximately 53 μm wide, by resist reflow at 120 degrees C. Inductively coupled plasma (ICP) etching is then used to alternately etch the resist mask and the semiconductor, with oxygen and methane/hydrogen respectively, producing concentrators with almost parabolic profiles. Currently, the concentrators are typically 30 μm high, with a top diameter of approximately 15 μm. Continuing optimization of the process to reach the theoretical limits of optical gain is described.
We describe uncooled mid-IR light emitting and negative luminescent diodes made form indium antimonide based III-V compounds, and long wavelength devices made from mercury cadmium telluride. The application of these devices to gas sensing, improved thermal imagers and imager testing is discussed.
Cadmium mercury telluride (Hg1-xCdxTe or MCT) non- equilibrium detector structures which allow room temperature operation have been grown by metal-organic vapor phase epitaxy (MOVPE). These devices suppress the auger generation by reducing the intrinsic electron and hole concentrations in the active region of the device. The MCT characteristics in this region should then be determined by the extrinsic doping concentration. In order to minimize the remaining generation processes within this so called (pi) -region, it is best formed from low acceptor doped (low X1015 cm-3) MCT, with as low a trap density as possible. The p+(pi) n+ device structure which is required to achieve the non-equilibrium phenomena requires stringent control on acceptor and donor doping, as well as composition. Acceptor doping studies with trisdimethylamino arsine (DMAAs) have been performed using GaAs and CdZnTe substrates. Minority carrier lifetime results have been obtained which are near rotatively limited and comparable to As-doped, Hg-rich liquid phase epitaxy (LPE) grown layers on CdZnTe substrates. Ambient temperature, auger-suppressed devices have levels of 1/f noise which currently limit their use in imaging applications. However, they are of great interest in other applications such as approximately equals 10 micrometer negative luminescence emitter devices and heterodyne detection of 10.6 micrometer infrared (IR) radiation from carbon-dioxide lasers. Reduction in the series resistances has been achieved by utilizing a device design with a n+ MCT common which should improve the frequency response of these devices. Another design modification, predicted to reduce the leakage current, has been the introduction of low doped, wide band gap regions either side of the (pi) -region. In practice these structures have produced over an order of magnitude improvement in the leakage current characteristics.
Recent advances in the growth of cadmium mercury telluride (Hg1-xCdxTe or MCT) by metal organic vapor phase epitaxy (MOVPE) allow the fabrication of advanced device structures where both the alloy composition x and the doping concentration can be accurately controlled throughout the epitaxial layer. For p-type doping using arsenic, the acceptor concentration can be varied from 5 X 1015 cm-3 to 4 X 1017 cm-3 and for n-type doping using iodine, the donor concentration can be varied from 1 X 1015 cm-3 to 2 X 1017 cm-3. A number of diode arrays have been fabricated in this material and their properties assessed at 77 K, 195 K and 295 K. It has been found that the diffusion currents are at least ten times lower than in homojunctions. In addition, the devices exhibit negative resistance at temperatures above 190 K due to auger suppression. The successful demonstration of auger suppression in these structures has greatly improved the diode leakage currents at room temperature and will enable the development of new devices such as a room temperature laser detector.
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