As electro-optic sensors increase in size and frame rate, the data transfer and digital processing resource requirements also increase. In many missions, the spatial area of interest is but a small fraction of the available field of view. Choosing the right region of interest, however, is a challenge and still requires an enormous amount of downstream digital processing resources. In order to filter this ever-increasing amount of data, we look at how nature solves the problem. The Advanced Guidance Division of the Munitions Directorate, Air Force Research Laboratory (AFRL/MNG) at Eglin AFB, Florida, has been pursuing research in the area of advanced sensor and image processing concepts based on biologically inspired sensory information processing. A summary of some vertebrate and invertebrate inspired 'neuromorphic' processing efforts will be presented along with a seeker system concept utilizing this innovative technology. Concepts and requirements for future such efforts will also be discussed.

We report on the growth and characterization of InSbBi, InTlSb, InTlP, and the quaternary alloys for uncooled long- wavelength infrared photodetector applications. The layers were grown on InSb and GaAs substrates by low-pressure metalorganic chemical vapor deposition. The incorporation of Bi and Tl in InSb was investigated with high-resolution x- ray diffraction, energy dispersive x-ray analysis, and optical photoresponse measurements. We also demonstrate the photodetectors fabricated from the grown InSbBi and InTlSb alloys. InSb_0.96Bi_0.04 photoconductive detectors exhibited a responsivity of 3.2 V/W at 77 K. The estimated Johnson noise limited detectivity at 7 micrometers was 1.7 X 10⁸ cmHz^1/2/W at 77 K. A room temperature operating InSb_0.95Bi_0.05 photodetector was also demonstrated. Photoresponse up to 12 micrometers was achieved at 300 K. The responsivity and Johnson noise-limited detectivity at 10.6 micrometers were 1.9 mV/W and 1.2 X 10⁶ cmHz^1/2/W, respectively. Photoresponse up to 15 micrometers was achieved at 300 K from quaternary InTlAsSb and InBiAsSb alloys.

We present an overview on the progress of InP/InGaAs based Hi-Lo APD's, which are important for long-haul optical fiber communications. Much of recent research efforts have been focused on improving the operation reliability, the gain- bandwidth (GB) product, and reducing the excess noise factor. To achieve a high GB product and a reliable operation, the reduction of the thickness of the multiplication layer and an optimum design of the internal electric field distribution are essential. The concept of the planar InP/InGaAs APD is very important from this perspective and the Hi-Lo APD's are expected to play an important role.

We present a discussion of a new edge breakdown suppression scheme for use in planar avalanche photodiodes called the joint opening effect avalanche photodiode, JOE-APD. The JOE-APD utilizes a single growth process that achieves center breakdown dominance without the use of guard rings, partial charge sheets or surface etches. Edge breakdown suppression is achieved within the JOE-APD by partially insulating the electric field growth in the active region from the geometry of the primary well. This design methodology allows for the fabrication of a thin multiplication region, which is necessary, for APDs used in Gb/s applications. In addition the electric fields at the surface of the joint opening effect APD are reduced. An advanced drift-diffusion simulation is used to demonstrate the workings of the JOE-APD.

The avalanche multiplication noise characteristics of Al_xGa_1-xAs (x equals 0-0.8) have been measured in a wide range of PIN and NIP diodes. The study includes determining the effect of the alloy fraction, x, as it varies from 0 to 0.8 while the effect of the avalanche width, w, is investigated by varying it from 1 micrometers down to 0.05 micrometers . For x equals 0-0.6, the ratio of the electron to hole ionization coefficients, 1/k, decreases from 3 (for x equals 0) to 1 (for x equals 0.6), leading to higher noise in a local prediction as x increases. Measurements for x equals 0-0.6 in nominally 1um thick diodes indicates that the excess noise factor can be approximately predicted by the local model. However, as the avalanche width reduces, a lower than expected noise factor was measured. This behaviour is associated with the effect of deadspace, whereby carriers have insufficient energy to initiate ionization for a significant region of the device. The presence of deadspace leads to a more deterministic process, which acts to reduce excess noise. For x equals 0.8 however, its 1/k value is surprisingly high in a bulk structure, leading to noise performance that is primarily determined by the 1/k value and is comparable to that of silicon. Similar to the results of thin Al_xGa_1-xAs (x equals 0-0.6) diodes, thinner Al_0.8Ga_0.2As structures exhibit excess noise factor that is significantly reduced by the nonlocal deadspace effects.

There has been a great revival of interest in the area of ultrafast photodetectors after the discovery of low temperature (LT) MBE-grown GaAs [1-3]. These detectors have found numerous applications in picosecond electrical pulse generation and sampling, and as main components of spectroscopic arrangements for terahertz frequency range. The unique properties of LT GaAs such as short carrier lifetimes and high resistivity made it a promising material for ultrafast photoconductive switches and photodetectors. It has been reported that ion implanted material may provide an alternative material for these applications [4-6]. Defects introduced by implantation may act as traps or recombination centres that have large carrier capture cross sections which shorten the carrier lifetimes significantly. The work presented below is an overview of how ion implantation could be used to modify the properties of GaAs and InP for ultrafast photodetector applications. Also, in the area of quantum well infrared photodetectors (QWIPs), there has been much effort in developing the technology for multi-color QWIPs [7,8]. These devices are highly desirable in advanced high performance infrared (IR) systems as it provides not only the spatial information of the image but also the spectral information. Although the functionality of these devices could be designed right from the epitaxy stage, it requires complicated structures and processing steps. In this paper we will demonstrated an alternative method of realizing the multi-color QWIP by a technique known as intermixing. By implantation, the band structure of the quantum wells (QWs) could be modified and hence tune the detection wavelength of the device.

We present a new concept and first results for a photoconducting detector exhibiting extremely high gain and extremely high detectivity. Previously we have demonstrated detectors and optical switches consisting of a n-i-p photodiode whose n-layer is depleted at sufficiently large reverse bias. Under illumination a photovoltage is induced by the photo-generated electrons and holes, accumulated in the n- and p-layer, respectively. We have shown that each photo-generated electron collected in the n-layer contributes about 10 nA to the photoconductive saturation current if the layer is covered with interdigitated `source' and `drain' contacts of about 1 micrometer separation. This signal persists until the photovoltage decays. For low-noise single- or few-electron detection the capacitance of the n-i-p diode has to be minimized. For this purpose we have fabricated n-i-p-structures consisting of crossed p- and n-doped stripes of a few micrometer width. First the (bottom) p-doped stripe is defined by focused Be-ion beam implantation directly into the semi-insulating substrate, followed by MBE-overgrowth with an i- and an n-doped layer. Narrow n-stripes are defined by wet-etching and n-contact fingers are deposited. Room temperature dark currents at a few volts reverse bias are in the low pA- and capacitances in the low fF-range and the expected large photoconductive gain is observed. The photoresponse is independent on the position of the illumination spot on the 50 X 50 micrometer mesa, although the area of crossing stripes is only a few micrometer wide.

We report on an optoelectronic receiver consisting of a special photoconductive n-i-p detector and a n-i-p reference diode fabricated from the same structure. The receiver is illuminated by two surface-normal light beams using a dual rail code. All applied DC biases are compatible with normal silicon CMOS logic, no AC biasing is required. The photoconductive gain of the receiver allows for output currents of more than 10 mA without any further amplification. The active device area of the smallest detectors has been scaled down to 120 micrometers ², resulting in a total optical switching energy for this receiver as low as 348 fJ. This optical switching energy is constant over a wide range of input power, resulting in fast switching times at sufficiently high input power while still retaining well-defined, but slower switching characteristics at lower input powers. Using input beams with 0.6 mW optical power at a wavelength of 787 nm, high speed measurements with a 3 dB frequency in excess of 400 MHz have been made. At these measurements a photoconductive gain of 6 times the p-i-n photocurrent was achieved, but using input powers of about 7 nW a gain in excess of 10⁶ has been demonstrated. Due to their simple design and biasing demands these photoconductive receivers are well suited for smart pixel applications and optical interconnects. For demonstration we present results for new a monolithically integrated smart pixel with a high-efficiency non-resonant cavity LED as emitter.

We report on In(80%)GaAs line scan sensors with 128 pixels on 50 micrometers pitch for use as thermo-electrically-cooled spectroscopic sensor in the short-wave infrared (1 - 2.5 micrometers ).

Extremely thin (50-100nm) polycrystalline silicon germanium (poly SiGe) microbolometers have been realized thanks to structural stiffness enhancement techniques within the pixel and the support legs. The technique involves the definition of U-shaped profiles using surface micromachining. This approach allows to decouple thermal isolation to some extent from thermal time constant. The result is a faster yet sensitive microbolometer compared to its thicker counterparts. Thermal time constants between 5 and 10 ms are achieved in vacuum yet the thermal conductance of the support legs is as low as the radiation limit (3x10^-8 W/K). Apart from the (CMOS compatible) absorber definition and the release of the sacrificial oxide layer, the microbolometer process runs in a 8' Si CMOS pilot line and uses deep submicron stepper capability of the pilot line. The release process using vapor HF does not attack pixel, absorber or metal interconnect and leads to a yield close to or equal to 100%. Linear arrays and small 2D arrays of such microbolometers are demonstrated. To protect the bolometers in an early stage of the packaging, a zero-level (on-chip) flip-chip package based on indent-reflow sealing has been developed. The germanium window material is processed using process steps from multi-chip-module technology.

During the last 4 years, an infrared technology based on amorphous silicon has been developed by the Infrared Laboratory (Leti-Lir) in France. After having made a first demonstrator of 256 X 64 pixels, Lir and Sofradir have decided to develop a larger format of 320 X 240 on an industrial basis which needed of course a transfer of technology and a real production line installation. All this work has been done and after a 6 month transfer of technology the production line is running at Sofradir. The purpose of this paper is to present our special connection between research and production and to update the results from a production point of view.

This paper reports implementation of a low-cost microbolometer focal plane array using n-well layer in a CMOS process as the microbolometer material. N-well microbolometer structures are suspended for thermal isolation by post-etching of fabricated CMOS dies using silicon bulk-micromachining techniques. Although n-well has a moderate TCR of 0.5-0.65%/K at 300K, it still provides a reasonable performance due to its single crystal structure which contributes low 1/f noise. Detailed thermal simulations in ANSYS were performed to obtain an optimized structure. Various prototype FPAs with 16x16 array sizes have been implemented with 80 mm x 80 mm and 50 mm x 50 mm pixel sizes. The measurements and calculations show that the n-well microbolometers can provide a responsivity of 8.5 x 10⁶ V/W, a detectivity of 5.5 x 10⁹ cmHz^1/2/W, and an NETD of 260 mK at 30 frames per second using a simple, fully-serial readout approach with an integrator output. The performance of the array can be increased with advanced readout techniques and improved pixel structures. The CMOS n-well microbolometer approach seems very cost-effective to produce large focal plane arrays for uncooled infrared imaging with reasonable performance.

The light-assisted method of electrochemical etching of n-Si allows the formation of cylindrical macropores with a high ratio of pore depth to pore diameter and with strong periodicity. Such structures are highly promising for modification of the electromagnetic wave spectra in semiconductor devices. In this work, the method of macropore formation on low resistance silicon plates was improved. Photoluminescence (PL), peaking near 600 nm with an intensity of 10 (mu) W/cm², was measured on macroporous silicon, which had evolved microporous layers containing nanocrystals. For higher current densities during pore formation the intensity of the orange light emission decreased, and a blue PL shift was observed due to a reduction in the sizes of the Si nanoparticles. Anomalous coefficients of light absorption allow the development of thermal- and photo-sensors on the basis of macroporous silicon. The investigated macroporous silicon structures are a promising material for multi-element thermal sensor fabrication due to a high temperature coefficient of electrical resistance (1 - 4%), low noise level (2 (DOT) 10^-9 VHz^-1/2) and good radiation absorption in the spectral region (lambda) equals 2 - 20 micrometers .

Quantum wells, especially those made of GaAs and InP related compounds, have enabled several unique infrared devices. Two prime examples are quantum well infrared photodetectors (QWIP) and quantum cascade lasers. This paper discusses a few examples of QWIP related devices: (1) QWIPs are well suited for high speed and high frequency applications--work on achieving high absorption efficiency and high operating temperature has been carried out. (2) A variation of conventional QWIP structures can lead to simultaneous visible and infrared detection, and demonstrations using both GaAs and InP based structures have been made. (3) P- type structures may achieve competitive performance and lend to easy fabricating of large focal plane arrays, and good performance has been achieved in resonant-cavity enhanced p- QWIPs.

Quantum dots are recognized as very promising candidates for the fabrication of intersubband photodetectors in the infrared spectral range. At present, material quality is making rapid progress and some devices have been demonstrated. Examples of mid-infrared quantum dot intersubband photodetectors are presented along with device design and data analysis. Nonetheless, the performance of these devices remains less than comparable quantum well intersubband photodetectors due to difficulties in controlling the quantum dot size and distribution during epitaxy.

We have first investigated three superlattice infrared photodetectors in order to raise their temperature for the background limited performance. Their basic structure is a 20-period GaAs(6nm)/Al_0.32Ga_0.78As(4nm) superlattice sandwiched between two 50nm Al_xGa_1-xAs blocking layers. We changed the barrier height (x) of the blocking layers and the well doping density (Nd) to see their effects on the temperature for the background limited performance. Three samples were grown with molecular beam epitaxy. The associated parameters (x, Nd) of Devices A, B and C are (0.28, 10¹⁸), (0.28, 10¹⁷) and (0.24, 10¹⁷) respectively. The unit for N_d is cm^-3. The comparison between A and B is to see the effect of the doping density while that between B and C is the barrier height. The I-V characteristics of the three detectors at various temperatures have been investigated carefully. Two important experimental results have been concluded. A high doping density not only decreases the activation energy and increases the dark current but also increases the impurity scattering for the photoelectrons. For the dark current, high activation energy puts down the dark current and extends the dominant region of the thermionic emission current. Applying those results, we changed the parameters into (0.32, 5 X 10¹⁶) and successfully fabricated a superlattice infrared photodetector to reach the background limited performance at 77K. The detectivity for the detector at 77K with wavelength 8.81 micrometers is 4.3 X 10⁹ cm(root)Hz/W.

The optoelectronic properties of short-period InAs/(GaIn)Sb superlattices (SLs) grown by molecular beam epitaxy on GaSb substrates are discussed. We report on the optimization of the SL materials properties with special emphasis on the use for infrared detection devices. The materials quality is evaluated by using high resolution x-ray diffraction, atomic force microscopy, and photoluminescence spectroscopy. In- plane magneto-transport investigations were performed applying mobility spectrum analysis. The SL diodes were analyzed performing standard electro-optical measurements. The observation of resonances in the I-V curves in the regime of Zener-tunneling due to Wannier-Stark localization opens a new tool for the electrical investigation of photodiodes with low band gap energy. The status of the processing technology is reported demonstrating the feasibility for the fabrication of 256 X 256 focal plane arrays operating in the 8-to-12 micrometers atmospheric window. In addition, results are given for mid-infrared SL-diodes, grown with lattice matched AlGaAsSb barriers instead in the binary InAs/GaSb SL system.

The design and characteristics of very long wavelength InAs/InGaSb strained layer superlattice photodiodes are presented. These photodiodes have cutoff wavelengths ranging from 12 to longer than 15 microns, and are among the longest wavelengths reported for photovoltaic detectors made using this material system. Structural, optical and electrical properties are reported. Measured quantum efficiencies are as high as 10% at 10 micron for a 0.7 micron thick structure at 77K. The absorption coefficients are excellent, however, the electrical properties still need improvement.

We report on the demonstration of high performance p-i-n photodiodes based on type-II InAs/GaSb superlattices operating in the very long wavelength infrared (VLWIR) range at 80 K. Material is grown by molecular beam epitaxy on GaSb substrates with excellent crystal quality as evidenced by x- ray diffraction and atomic force microscopy. The processed devices with a 50% cutoff wavelength of (lambda) _c equals 22 micrometers show a peak current responsivity about 5.5 A/W at 80 K. The use of binary layers in the superlattice has significantly enhanced the uniformity and reproducibility of the energy gap. The 90% to 10% cut-off energy width of these devices is on the order of 2 kT which is about four times smaller compared to the devices based on InAs/Ga_1-xIn_xSb superlattices. Similar photovoltaic devices with cut-off wavelengths up to 25 micrometers have been measured at 80 K. Our experimental results shows excellent uniformity over a three inch wafer area, indicating the possibility of VLWIR focal plane arrays based on type-II superlattices.

New infrared detector materials with high sensitivity, multi-spectral capability, improved uniformity and lower manufacturing costs are required for numerous long and very long wavelength infrared imaging applications. One materials system has shown great theoretical and, more recently, experimental promise for these applications: InAs/In_xGa_1-xSb type-II superlattices. In the past few years, excellent results have been obtained on photoconductive and photodiode samples designed for infrared detection beyond 10 microns. Far-infrared photoresponse of superlattices with cut-off wavelengths between 15 micrometers and 25 micrometers were studied. The measured photoresponse spectra for both photodiodes and photoconductors are compared to calculated absorption coefficient spectra. The electronic structure and the optical absorption of InAs/In_xGa_1-xSb superlattice infrared (IR) detector structures are calculated, for several values of x, using our implementation of the 8x8 envelope-function approximation (EFA) formalism. Good experimental-theoretical agreement is obtained regarding the long-wavelength threshold and absorption shape.

One of the unique properties of GaN is the polarizability. Also, demonstration of luminescent properties in devices such as light emitting diodes and lasers has been surprising, considering the defect density. The large polarization and inactive defects may be related, as demonstrated here by the measurement of several barriers to electron capture. N-type samples grown by both MOCVD and RMBE showed two adjacent DLTS peaks at 125 - 150 K with energies of 0.190 eV and 0.253 eV and one larger peak at 300 K. The 300 K peak was resolved to two traps, one with emission energy of 0.548 eV in both samples, and one with emission energy of 0.613 eV in the GaN grown by MOCVD. Analysis of the change in amplitude of the emission transients under non-saturating filling pulse conditions gives insight to the capture behavior. The two traps at 300 K have coupled trapping and emission characteristics. Both the rate window plots and the fit of the capacitance transient amplitude showed several traps with barriers to capture of electrons at 0.1 eV, 0.04 eV, 0.14 eV, and 0.38 eV. The capture barriers may be related to the shift in core electrons on ions surrounding the defect.

There has been a growing interest for the development of solar blind ultraviolet (UV) photodetectors for use in a variety of applications, including early missile threat warning, flame monitoring, UV radiation monitoring and chemical/biological reagent detection. The Al_xGa_1-xN material system has emerged as the most promising approach for such devices. However, the control of the material quality and the device technology are still rather immature. We report here the metalorganic chemical vapor deposition, the n-type and the p-type doping of high quality Al_xGa^1-xN thin films on sapphire substrates over a wide range of Al concentration. The quality of this Al_xGa_1-xN material was verified through the demonstration of high performance visible and solar blind ultraviolet p-i-n photodiodes with a cut-off wavelength continuously tunable from 227 to 365 nm, internal quantum efficiencies up to 86% when operated in photovoltaic mode, and a ultraviolet-to-visible rejection ratio as high as six orders of magnitude. Both front and back side illuminated p-i-n photodiodes were realized. Photodetector devices were also demonstrated on GaN material obtained using lateral epitaxial overgrowth. The technology for such Al_xGa_1-xN based devices was improved in an effort to enhance their performance, including the development of ohmic metal contacts to both n-type and p-type Al_xGa_1-xN films with an Al concentration up to 40%.

High performance ultraviolet (UV) detectors have been fabricated using plasma-enhanced molecular beam epitaxy. The realized AlGaN Schottky detectors exhibit high responsivity, sharp spectral cutoff and high shunt resistance of several giga-ohms for 0.5 mm² active area devices. Quantitative measurements have been carried out on these detectors in the photon energy range from < 1 to > 10 eV (from approximately 1200 to 120 nm in wavelength). Very short UV spectral measurement of these AlGaN detectors is reported for the first time using high intensity sources. The detectors exhibited almost eight orders of magnitude in response dynamic range in that spectral span. Reliability of the devices is evaluated after exposure to repeated DUV irradiation.

The optical properties of InGaAsN structures for the fabrication of photodetectors are investigated. An expression for the bulk bandgap as a function of the nitrogen fraction is obtained from x-ray diffraction, photoreflectance and photoluminescence measurements. Optical absorption of undoped MQW structures show that the cutoff wavelength is extended due to the presence of nitrogen. A functioning heterojunction phototransistor was fabricated. Photocurrent spectra show that a responsivity higher than 1.5 A/W is obtained with a cutoff wavelength of 1.16 micrometers . I-V measurements under different light levels show that a peak gain of 5 is obtained with a collector current of 260 (mu) A and a dark current lower than 2 nA with a 10V bias.

The fundamental parameters of IR photon detection are discussed relevant to the meaningful comparison of a wide range of proposed IR detecting materials systems. The thermal generation rate of the IR material is seen to be the key parameter that enables this comparison. The simple materials physics of (1) intrinsic direct bandgap semiconductors, (2) extrinsic semiconductors, (3) quantum well devices, including types I, II, and III superlattices, (4) Si Schottky barriers, are examined with regard to the potential performance of these materials as IR detectors, utilizing the thermal generation rate as a differentiator. The possibility of room temperature photon detection over the whole IR spectral range is discussed, and comparison made with uncooled thermal detection.

This paper reviews recent advances in photovoltaic (PV) HgCdTe infrared detector technology. Recent advances have enabled a new generation of spaceborne multispectral instruments for remote sensing applications, and have led to the practicality of dual-band (or two-color) IR focal plane array technology. The focus of this paper is on the back-illuminated HgCdTe PV arrays that have made this new generation of spaceborne instruments possible.

A long-wavelength large format Quantum Well Infrared Photodetector (QWIP) focal plane array has been successfully used in a ground based astronomy experiment. QWIP arrays afford greater flexibility than the usual extrinsically doped semiconductor infrared arrays. The wavelength of the peak response and cutoff can be continuously tailored over a range wide enough to enable light detection at any wavelength range between 6 - 20 micrometers .

Using Si VLSI technology, we can fabricate various kinds of infrared focal plane arrays (FPAs) which cover spectral bands from short wavelength infrared to long wavelength infrared. The Si-based technology offers many attractive features, such as monolithic integration, high uniformity, low noise, low cost, and high productivity. We have been developing Si-based infrared FPAs for more than 20 years and have verified their usefulness.

Uncooled infrared imaging is growing into a large business for several companies. With hybrid barium strontium titanate (BST) ferroelectric detectors leading the way in lowering cost, the markets are exploding. Rapid advances in VOx bolometers in the last few years have improved NETDs below those achieved to date with BST, and VOx system prices have approached those for BST systems. When these factors are coupled with the inherently superior MTF of monolithic sensors, one would expect the market for BST sensors to be waning. However, such is not the case for two reasons. First, the markets for uncooled IR imaging sensors are critically cost sensitive, and small price differences create large sales differences. Second, the laboratory performance advantage of VOx is often not realized in the fielded systems, because of the `fixed pattern' noise characteristic of DC-coupled systems. Thus, MRTs for BST systems are often better than those of VOx systems having better NETDs. Another technology looms on the horizon, poised to offer a new option for applications with stringent stability requirements coupled with high performance. Recently written off by many because of lack of progress, TFFE technology is now beginning to show why it is such a good idea.

We report on the technology we are developing to product photovoltaic devices of HgCdTe which are sensitive in the short wave region of the solar radiation and exhibiting detectivity performance close to theoretical limits imposed by the fundamental properties of the material.

In the paper the performance of P-on-n double-layer heterojunction HgCdTe photodiodes are temperature 77 K is analyzed theoretically. Calculation has been performed for the backside-illuminated configuration. The effect of photodiode base layer geometry on quantum efficiency and R₀A product is analyzed. The effect of lateral collection of diffusion current and photocurrent on photodiode parameters is also shown. Moreover the dependence of the p-n junction position within heterostructure on the band-gap energy profiles and photodiode performance is presented. Finally, the influence of the composition gradient and p- side doping concentration on photodiode parameters is described briefly.

Mercury-Cadmium-Telluride (MCT) 2 X 64 linear arrays with silicon readouts were designed, manufactured and tested. NCT layers were grown by MBE method on (103) GaAs substrates with CdZnTe buffer layers. 50 X 50 mm n-p-type photodiodes were formed by 80 divided by 120 keV boron implantation. The dark current at 100 mV reversed biased diodes was within 15*30 nA and zero bias resistance-area product was within R₀ approximately equals 20 divided by 50 Ohm X cm². Silicon read-out circuits were designed, manufactured and tested. Read-outs with skimming and partitioning functions were manufactured by n-channel MOS technology with buried or surface channel CCD register. The parameters of LWIR MCT linear arrays with cutoff wavelength (lambda) _co 10.0 divided by 12.2 micrometers and Si readouts were tested separately before hybridization. The HgCdTe arrays and Si readouts were hybridized by cold welding In bumps technology. With skimming mode used for integration time of 24-30 ms for such MCT n-p-junctions, the detectivity D*_(lambda ) approximately equals 4 X 10¹⁰ cmXHz^1/2/W. Dark carrier transport mechanisms in these diodes were calculated and compared with experimental data. Two major current mechanisms were included into the current balance equations: trap-assisted tunneling and Shockley-Reed-Hall generation-recombination processes via a defect trap level in the gap. Other current mechanisms (band-to-band tunneling, bulk diffusion) were taken into account as additive contributions. Tunneling rate characteristics were calculated within k-p approximation with the constant barrier electric field. Relatively good agreement with experimental data for diodes with large zero resistance-area products (R₀A > 10 OhmXcm²) was obtained.

Accurate knowledge of the decay rates of optically generated charge carriers in bulk semiconductor materials is important for various infrared applications. Most of the published decay rates of free carriers generated with above band-gap energy light, in materials such as InAs and InSb are obtained from measurements in thin films. Stronger attenuation of above band gap energy light in these materials prevents the probing in samples thicker than a few microns. To study the decay of free carriers in the bulk semiconductor wafers, we use two-photon absorption of below band gap energy light (obtained from a pulsed CO₂ laser). This method generated charge carriers throughout the bulk of the material used (typically having thickness of 1 - 2 mm). The decay of the charge carriers is then probed with a low power cw infrared laser (also with photon energy below the band gap). The decay rates measured at different temperatures are compared with calculations that include Auger and defect-assisted Shockley-Reed-Hall (SRH) recombination processes. Calculation of various recombination processes indicate that the lifetimes are limited by SRH mechanism in InAs samples.

Results are presented for a novel HEterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) far-infrared detector with a cutoff wavelength of 70 micrometers . A responsivity of 10.5 A/W and a D* of approximately 10¹³ cm(root)Hz/W at 20 micrometers was achieved at 4.2 K. Dark current for the detectors was 2 orders of magnitude better than for homojunction interfacial workfunction internal photoemission (HIWIP) detectors at liquid helium temperatures. Capacitance measurements show similar behavior to other infrared photodetectors such as HIWIPs and QWIPs. The overall superior characteristics of HEIWIP detectors over HIWIP and QWIP detectors at longer wavelengths are of interest for future developments in far-infrared applications.

We experimentally compare the peak responsivity R, gain g, quantum efficiency, and detectivity of GaAs/AlGaAs-QWIPs with devices based on the competing material system InGaAs/GaAs. For this purpose we use a typical n-type GaAs/AlGaAs-QWIP and three n-type InGaAs/GaAs-QWIPs with varying doping densities. R and g of the GaAs/AlGaAs-QWIP show a typical negative differential behavior, while both quantities grow monotonously with increasing bias voltage in the case of the InGaAs/GaAs-QWIPs. For identical nominal doping densities and similar cutoff wavelengths between 8.9 micrometers and 9 micrometers , InGaAs/GaAs-QWIPs show much higher responsivities than GaAs/AlGaAs-QWIPs. The ratio between these responsivities is 2.5 at the bias voltage where the GaAs/AlGaAs-QWIP has its maximum. By making use of the different bias dependence of the responsivity in both types of QWIPs a further enhancement of this factor is achieved. Nevertheless, both types of QWIPs show comparable detectivities. This is due to the fact that the gain has a negligible influence on the detectivity. In conclusion, InGaAs/GaAs-QWIPs are promising if high responsivities and short integration times are required.

It is found that the temperature accuracy (dT), measured by the photocurrent ratio of two linearly independent spectral responses, depends on two factors. One is the dissimilarity of the spectral responses determined primarily by the separation of the long and short wavelength peaks, and is not related with their responsivity magnitude. The other is dominated between the smaller S/N for the two measured photocurrents. The dissimilarity can be improved by moving the short wavelength peak to an even shorter one. However, this decreases the signal magnitude measured by the shorter wavelength peak at low target temperature, and results in the degradation of the S/N and (Delta) T. Therefore, a superlattice structure with absorption peaks at 6.7 micrometers and 9.7 micrometers was designed, and the wavelength separation is less than the conventional two-color QWIP. The spectral response of the detector is voltage-tunable through a current blocking layer that acts as an energy filter for the photo-excited carriers. The measured peak detectivity of this detector is 3.7 X 10⁹ cmHz^0.5/W (9.7 micrometers ) at -0.5 V and 2.2 X 10¹⁰ cmHz^0.5/W (6.7 micrometers ) at -0.1 V at 45K. Our detector is capable of measuring target temperatures ranging from 300K to 1200K and higher. The estimated (Delta) T is 0.8K for 300K target, and 0.17K for 1200K target.

Using low-pressure metalorganic chemical vapor deposition, we have grown GaInAs/InP QWIP structures on GaAs-coated Si substrate. First, the procedure to optimize the epitaxy of the InP buffer layer on Si substrate is given. Excellent crystallinity and a mirror-like surface morphology were obtained by using both a two-step growth process at the beginning of the InP buffer layer growth and several series of thermal cycle annealing throughout the InP buffer layer growth. Second, results of fabricated GaInAs/InP QWIPs on Si substrate are presented. At a temperature of 80 K, the peak response wavelength occurs at 7.4 micrometers . The responsivities of QWIPs on both Si and InP substrates with identical structures are equal up to biases of 1.5 V. At a bias of 3 V, the responsivity of the QWIPs on Si substrate is 1.0 A/W.

By using a simplified time domain modeling approach, the temperature dependent performance characteristics such as multiplication gain and bandwidth are studied for InP/InGaAs separate absorption, grading, charge and multiplication (SAGCM) APDs within the temperature range from -243 to 358 K. The modeling approach is improved to consider the effects of hole diffusion, hole trapping, load circuit RC and gain-bandwidth product limit together with the fast Fourier transformation component of the impulse response from the time domain computation. The modeling results agree with experiments. The effects of changing material parameters on modeling are also discussed. The improved performance characteristics also indicate the potential application prospects of InP/InGaAs SAGCM APDs in low temperature environments.

This article reports the advancement of Hg_1-xCd_xTe epitaxial growth on CdZnTe(111)B substrates. Prior to growth of HgCdTe layers the substrate has been etched to form the bars on 30 micrometers centers and 20-micrometers depth. Next, 20-micrometers thick HgCdTe epitaxial layer has been grown by liquid phase epitaxy from Te-rich solution. The Nomarski microscopy showed that the surface of specially prepared layers were flat and the composition of layers, measured by FTIR microscopy, was homogeneous. Samples were cleaved and examined in cross section by SEM.

Chalcogenide based alloys find applications in a number of devices like optical memories, IR detectors, optical switches, photovoltaics, compound semiconductor heterosrtuctures etc. We have modified the Gurman's statistical thermodynamic model (STM) of binary covalent alloys. In the Gurman's model, entropy calculations are based on the number of structural units present. The need to modify this model arose due to the fact that it gives equal probability for all the tetrahedra present in the alloy. We have modified the Gurman's model by introducing the concept that the entropy is based on the bond arrangement rather than that on the structural units present. In the present work calculation based on this modification have been presented for optical properties, which find application in optical switching/memories, solar cells and other optical devices. It has been shown that the calculated optical parameters (for a typical case of Ga_xSe_1-x) based on modified model are closer to the available experimental results. These parameters include refractive index, extinction coefficient, dielectric functions, optical band gap etc. Ga_xSe_1-x has been found to be suitable for reversible optical memories also, where phase change (a yields c and vice versa) takes place at specified physical conditions. DTA/DSC studies also suggest the suitability of this material for optical switching/memory applications. We have also suggested possible use of Ga_xSe_1-x (x = 0.4) in place of oxide layer in a Metal - Oxide - Semiconductor type solar cells. The new structure is Metal - Ga₂Se₃ - GaAs. The I-V characteristics and other parameters calculated for this structure are found to be much better than that for Si based solar cells. Maximum output power is obtained at the intermediate layer thickness approximately 40 angstroms for this typical solar cell.

For the first time, we reported that Bragg reflector with less than 10 periods designed for near band gap wavelength can reflect IR photons with energy less than 1.42 eV and reduce the solar absorptance of a GaAs/Ge solar cell from 0.889 to 0.809. This reduction would lower down the working temperature of a solar panel on space orbit by 8 degree(s)C. With the consideration that this Bragg super lattice structure can obviously improve the morphology of the hetero-epitaxy grown GaAs on Ge and a thick buffer layer are not needed any more for a good device, the design will not increase the epitaxy cost and can be used in large scale production.

Characteristics of thin-film NTC infrared sensors fabricated by micromaching technology were studied as a function of the thickness of membrane. The overall-structure of thermal sensor has a form of Au/Ti/NTC/SiO_x/(100)Si. NTC film of Mn_1.5CoNi_0.5O₃ with 0.5 micrometers in thickness was deposited on SiO_x layer (1.2 mm) by PLD (pulsed laser deposition) and annealed at 600-800 degree(s)C in air for 1h. Au (400nm)/Ti (100nm) electrode was coated on NTC film by dc sputtering. By the results of microstructural, X-ray and NTC analysis, post-annealed NTC films at 700 degree(s)C for 1h showed the best characteristics as NTC thermal sensing film. In order to reduce the thermal mass and thermal time constant of sensor, the sensing element was built-up on a thin membrane with the thickness of 20-65mm. Sensors with thin sensing membrane show the good detecting characteristics.

The optical fiber video transmission system enables up to 2 km video link for standard video color signal (1 Vp-p/75(Omega) ). The system consists of the following parts : a video camera, an optical transmitter, an optical cable, an optical receiver and a TV monitor. The optical transmitter is build around a laser diode module, controlled through two feedback loops : one stabilizes the laser diode operating temperature while the other stabilizes the laser diode optical power, in order to lock the laser output settings in any operating conditions. The transmitter contains also a modulating stage, enabling the direct internal modulation of the laser diode. The optical receiver consists of a PIN-FET photodiode amplifier module and a bipolar output stage, for impedance matching with the TV monitor. The system operates at a wavelength of 1300 nm, using multi-mode optical fibre.

Currently, photo-cathodes hold the highest promise in the near term (next few years) of being able to detect low light level UV signals at high QE while being nearly blind to visible wavelengths. We briefly discuss the requirements for UV detection for astronomical applications, and then we describe our work on producing GaN based photo-cathodes. The p-type GaN films were grown on sapphire at Northwestern University. The films were then converted into opaque photo-cathodes inside photo-tubes at Hamamatsu. Hamamatsu tested detective quantum efficiencies (DQE) of these detectors to be as high as 30% at 200 nm. The ratio of peak DQE at 200 nm to the minimum DQE at 500 nm was measured to be about 6 X 10³. We found a dramatic increase in the DQE at 200 nm versus the conductivity, with the break point being near 0.13 1/(Ohm-cm). Based on this dramatic increase, we believe that further improvement in photo-cathode quantum efficiencies can be achieved by increasing the conductivity. We have recently achieved more than an order of magnitude increase in conductivity by co-doping techniques. Improvements in the solar blindness of the devices depend both on characteristics of the film and its surface properties. A detailed discussion of decreasing the visible response and producing a sharper wave-length cutoff is beyond the scope of this work, but we briefly discuss the attributes that most likely affect the wavelength dependence of the photo-cathode response.

An efficient theoretical approach incorporating the mechanism of resonant absorption of the multiple reflected lightwaves is presented to model the frequency response of resonant-cavity (RC) avalanche photodiodes (APDs). Although the theoretical expressions are derived with respect to the RC separate absorption, charge and multiplication (SACM) structure, they are actually very general and can be applied to other RC APD structures and many non-RC APDs. As an example, the theoretical approach is applied to the InGaAs/InAlAs RC SACM APD. The computation results of -3 dB bandwidth based on the present theoretical approach are consistent with the experiment.

As predicted theoretically, quantum dot infrared photodetectors (QDIPs) can substantially surpass quantum well infrared photodetectors (QWIPs). Recently, a number of research groups reported fabrication and extensive experimental investigation of various InAs/GaAs, InGaAs/GaAs, and InGaAs/InGaP QDIPs. However, most of the fabricated QDIPs have worse performance than QWIPs. To answer the questions why QDIPs are still inferior to QWIPs and how to improve them, we analyze the QDIP operation using the developed device model of QDIPs with realistic parameters. The model takes into account the main physical factors determining the operation of QDIPs. We calculate the dark current and the responsivity of QDIPs as functions of their structural parameters, the applied voltage, and temperature. The calculated characteristics are in agreement with those of realistic QDIPs studied experimentally. The revealed relations between the QDIP operation characteristics and structural parameters explain the main features of QDIPs observed in experiments. We estimate the QDIP detectivity and find the conditions for its maximum value. We compare the QDIP characteristics with those of QWIPs.