The development of HgCdTe-alloy and -superlattice array detectors for astronomical applications in the x = 50-150 m band is discussed in light of NASA mission requirements over the next decade. High background observations on the SOFIA aircraft and planetary probes are identified for initial applications.

State-of-the-art large area photovoltaic detectors fabricated in HgCdTe grown by Molecular Beam Epitaxy have been demonstrated for the Crosstrack Infrared Sounder instrument. Large area devices (1 mm in diameter) yielded excellent electrical and optical performance operating at 81K for LWIR band and at 98K for MW and SWIR bands. LWIR and MWIR detectors have near-theoretical electrical performance, and AR-coated quantum efficiency is greater than 0.70. Measured average RoA at 98K is 2.0E7 W-cm2 and near-theoretical quantum efficiencies greater than 0.90 were obtained on SWIR detectors. These state-of-the-art large area photovoltaic detector results reflect high quality HgCdTe grown by Molecular Beam Epitaxy on CdZnTe substrates in all three spectral bands of interest.

This paper provides the design details for a new two-color quantum-well infrared photodetector for use in advanced thermal imaging. The single pixel experimental data used in the evolution of this design is discussed in significant detail. Using the knowledge gained from extensive measurements comparing miniband transport with bound-to-quasi-bound QWIPs and also double and triple coupled-well QWIPs with the more conventional single-well QWIPs, a two-color design is presented. The detector design provides for a lattice-matched growth and superior response in the MWIR spectral region, while keeping the dark currents from the LWIR portion of the detector to a minimum. The architecture will allow for the MWIR and LWIR stacks to be biased separately and also for the signals to be read independently.

The GaAs/AlGaAs based quantum well IR photodetectors (QWIPs) afford greater flexibility than the usual extrinsically doped semiconductor IR detectors because the wavelength of the peak response and cutoff can be continuously tailored over any wavelength between 6-20 micrometers . The spectral band width of these detectors can be tuned from narrow to wide allowing various applications. Also, QWIP offers multi-color IR cameras which is capable of simultaneously acquiring images in different IR bands. Each pixel of such array consists of vertically stacked, independently readable, QWIP detectors sensitive in different narrow IR bands. In this article, we discuss the result of a 10-16 micrometers large format broadband QWIP focal plane array IR bands. In this article, we discuss the results of a 10-16 micrometers large format broadband QWIP focal plane array (FPA). The size of the FPA is 640 X 512 and its pixel pitch is 25 microns. The highest operating temperature of the FPA is 45K, and it was determined by the charge storage capacity and the other features of the particular readout multiplexer used in this demonstration. Excellent imagery, with a noise equivalent differential temperature of 55 mK has been achieved. In addition, we will discus the developments and results of the 640 X 512 dual-band QWIP FPA.

Recently there has been considerable interest in two-color focal plane arrays (FPAs), particularly in mercury cadmium telluride. Single-color FPAs provide only the signal from the target, not its emissivity and temperature separately. The schemes that have been implemented for two-color FPAs involve two bumps per pixel, and two back-to-back diodes in four different layers, which are technologically challenging. We propose a simple scheme that requires only one bump per pixel, in a material that is not a heterostructure, and can be grown by liquid phase or molecular beam epitaxy. The two different cutoff wavelengths are obtained by implanting two different junction depths in a n+-on-p configuration. Since the n+ layer does not contribute, because of the Moss-Burstein shift, the different diodes correspond to different surface composition values, xs1 and xs2, in the graded p-type epilayer. The precise cutoff wavelengths can be chosen by appropriate slope s of the composition in the epilayer. For the purpose of radiometry it is not necessary that the two cutoff wavelengths should differ by large amounts: even 0.3 mm is sufficient - easily achievable, with reasonable junction depths. The two colors could form a checkerboard pattern across the FPA.

Since its initial synthesis and investigation more than 40 years ago, the HgCdTe alloy semiconductor system has evolved into one of the primary infrared detector materials for high-performance infrared focal-plane arrays (FPA) designed to operate in the 3-5 mm and 8-12 mm spectral ranges of importance for thermal imaging systems. Over the course of the past decade, significant advances have been made in the development of thin-film epitaxial growth techniques, such as molecular-beam epitaxy (MBE), which have enabled the synthesis of IR detector device structures with complex doping and composition profiles. The central role played by in situ sensors for monitoring and control of the MBE growth process are reviewed. The development of MBE HgCdTe growth technology is discussed in three particular device applications: avalanche photodiodes for 1.55 +m photodetection, megapixel FPAs on Si substrates, and multispectral IR detectors.

The epitaxial growth of Hg_1-xCd_xTe in the composition range 0.40 < x < 0.17 has been carried out on 3-inch CdTe/Si substrates mounted on indium-free molybdenum substrate holders. Because this mounting configuration prevents the effective use of a direct thermocouple contact to control the sample temperature, and because a dramatic change in the surface emissivity of the sample occurs during the onset of HgCdTe nucleation, an alternative method for controlling the surface temperature is developed. We utilize reflection high-energy electron diffraction (RHEED) and a thermocouple ramping sequence to maintain a constant HgCdTe surface temperature. Due to the narrowness of the HgCdTe growth window, small variations in the surface temperature produce a slight but observable change in the RHEED pattern. Through careful observation of the RHEED images, an optimized thermocouple ramping process is obtained such that the RHEED pattern remained constant from the onset of HgCdTe nucleation. Structural and electrical characterization of these samples demonstrate the usefulness of the temperature ramping methodology. For middle wavelength IR (MWIR) material, mobility measurements made on several n-type samples at 77 K range give values in the 2 X 10⁴ - 4 X 10⁴ cm²/Vsec range with doping levels in the low 10¹⁴ cm^-3. Additionally, preliminary lifetime measurements made on one MWIR sample gives 2.8 microsecond(s) ec. For long wavelength IR material, mobility measurements made on several n-type samples at 77 K give values in the 3 X 10⁵ to 5 X 10⁵ cm²/Vsec range with doping levels in the mid 10¹⁵ cm^-3. Electrical, structural and defect characterization along with device results are presented with a focus on the optimization of the thermocouple ramping process. In addition, the efficacy of Si-based composite substrates for the technological advancement of large format IR focal plane arrays will be discussed.

The annealing and electrical properties of extrinsic in situ doped mercury cadmium telluride epilayers grown by molecular beam epitaxy (MBE) on B CdTe/Si and CdZnTe substrates are studied. The doping is performed with an elemental arsenic source. HgCdTe epilayers of CdTe mole fraction in the range of mid-wavelength IR are grown at substrate temperatures of 175-185 degrees C. The temperature dependent Hall effect characteristics of the grown samples are measured by the van der Pauw technique. A magnetic field of up to 0.8 T is used in these measurements. The analysis of the Hall coefficient in the temperature range of 40-300 K with a fitting based on a three-band non-parabolic Kane model, a fully ionized compensating donor concentration, and tow independent discrete acceptor levels is reported. Both as-grown and annealed samples are used in this study. All of the as-grown samples showed-type characteristics whereas annealed samples showed p-type characteristics. Activation annealing at different temperatures was performed. Conversion to p-type at lower than conventional annealing temperatures was achieved. Theoretical models are utilized to understand the dependence of the activated arsenic concentration on the annealing temperature.

Intersubband transition in 3-Me He⁺-ion or 1 MeV proton irradiated GaAs/AlGaAs multiple quantum wells were studied using optical absorption technique. The intersubband transitions in 3MeV He⁺-ion irradiated were completely depleted in samples irradiated with doses as low as 1 X 10¹⁴ cm^-2. Thermal annealing recovery of intersubband transitions was observed in samples irradiated with lower doses while in heavily irradiated samples intersubband transition show no annealing recovery which could be said irradiation induced defects are so severe that annealing temperature could not repair the damage. On the other hand, intersubband transitions in 1 MeV proton irradiated samples were completely depleted with irradiation doses as low as 4 X 10¹⁴ cm^-2. More than 80 percent recovery was achieved at annealing temperature as low as 650 degrees C. The total integrated area and peak position energy of un-irradiated and irradiated samples were monitored as a function of annealing temperature. Depletion recovery here is noted to depend on the thermal annealing and irradiation dose.

In the presence of a time-dependent external source such as a bias electric field, an incident optical flux, or the temperature, electrons in quantum well devices experience non-adiabatic transport through the barrier layer between two adjacent quantum wells. This non-adiabatic transport process induces charge density fluctuations within each quantum well, resulting in several seemingly unrelated transient phenomena. When a time-dependent electric field is applied to the system, a dynamical breakdown and a zero-bias residual dark current in the quantum-well photodetectors are predicted theoretically. If a chopped time-dependent optical flux is incident on the system, a dynamical drop in the photo-responsivity with increasing chopping frequency and an emission-current spike as the optical shutter is opened are predicted. Finally, as the device temperature is varied with time, a counter-clockwise thermal hysteresis is found theoretically in the dark current curve as a function of the changing temperature. Experimental confirmation of the above theoretical predictions is presented.

GaInAs/InP quantum well IR photodetectors (QWIP) were grown on Si substrate by metalorganic chemical vapor deposition. The growth condition of InP buffer layer on Si was optimized and its crystal quality was evaluated by high-resolution x- ray diffraction and atomic force microscopy experiments. Two different in-situ thermal cyclic annealing techniques were used to reduce the threading dislocation density in the InP- on-Si. The new thermal annealing with larger temperature range was found to improve the quality of InP-on-Si dramatically. QWIP-on-Si samples with these two different thermal annealing were prepared. Important detector properties, like dark current, spectral response, peak responsivity, and specific detectivity were studied and compared with a QWIP-on-InP sample with identical structure. Small blue shift was observed in both QWIP-on-Si detector's spectral responses. Record high detectivity of 2.3 X 10⁹ cmHz/W was obtained for one QWIP-on-Si detector at 77K.

Reactive ion etching (RIE) of HgCdTe using CH4:H2 is known to generate p- to n-type conversion in both intrinsically doped and extrinsically doped p-type HgCdTe. The use of RIE to form n-on-p junctions in planar diodes has a number of advantages including state of the art diode performance, high uniformity, passivation of the junction at the surface throughout processing, the possibility of the formation of deep junctions, and removal of any need for high temperature processing after junction formation. However, it has long been believed that H₂ based plasma junction formation techniques will be long-term unstable. Initial results are presented indicating that surface passivation plays a major role in determining the stability of planar junctions formed using H2 based RIE. Comparisons of ZnS and CdTe passivation for n-on p-junctions formed on x approximately 0.3 Hg_1-xCd_xTe show dramatic differences in 2 to 3 hour, 80 degrees C bake stability tests. Diodes fabricated using either passivant initially exhibit R0A performance close to the theoretical limit, but are degraded after a 2 hour, 80 degrees C bake. Diodes with CdTe passivation have moderate performance as fabricated, but exhibit improvement rather than degradation after 3 hour, 80 degrees C bake. Such results indicate that planar junctions formed using H2 based RIE may offer a viable technology for low cost, highly uniform, large area IR detector arrays if passivation issues are satisfactorily resolved. Finally, a dual layer ZnS/CdTe passivation process is introduced which results in bake-stable devices after a 17 hour, 80 degrees C bake.

Recently, it was reported that p-type, Au-doped HgCdTe epilayers have a carrier lifetime two to three times higher than the Hg-vacancy doped epilayers with the same condition type. Analysis of the temperature dependent Hall measurement results indicates the existence of vacancy complexes in the vacancy doped HgCdTe epilayers but not in the Au-doped epilayers. Therefore, it is very likely that the defect complexes are generation-recombination centers, which reduce the carrier lifetime. Shortwave, midwave, and longwave HgCdTe diodes arrays have been produced in the Au-doped HgCdTe epilayers by the ion implantation technique. The n- type conversion by implantation is explained by the formation of tellurium antisites. Excellent array performances have been observed. Comparing these arrays to the heterojunction HgCdTe arrays, the arrays formed by ion implantation perform similar to or even better than the heterojunction array at liquid nitrogen temperature, but are inferior to the heterojunction arrays at a temperature over 150K.

Dark carrier transport mechanisms in narrow-gap Hg_1-xCd_xTe multilayer structures and Pb_1-zSn_zTe/PbTe_1-yS(Se)_y heterojunctions at T~80 K for applications in IR arrays are analyzed and compared with homojunction mercury-cadmium telluride (MCT) photodiode characteristics in the temperature range T~70-150 K. In the analysis procedure two major current mechanisms were included into the current balance equations: trap-assisted tunneling (TAT) and Shockley-Reed-Hall (SRH) generation-recombination processes for a defect trap level. Other current mechanisms (e.g., band-to-band tunneling, bulk diffusion) were taken into account as additive contributions. For TAT the tunneling rate characteristics were calculated within the k-p-approximation. Using donor and acceptor concentrations, trap level energies and concentrations, and in-junction trap level lifetimes as fitting parameters, good agreement with experimental data for HgCdTe and PbSnTe heterojunction and homojunction diodes was obtained, which allows one to predict the diode parameters from the known material characteristics. Photodiode or array parameters itself, or with CCD readouts, or CCD readouts separately were tested to study the influence of readout cascade on the diodes' properties.

RIO has achieved a significant technical breakthrough in uncooled FPAs by reducing the pixel size by a factor of two while maintaining state-of-the-art sensitivity. Raytheon has produced high-quality 320 by 240 micro bolometer FPAs with 25 micrometers pitch pixels. The 320 by 240 FPAs have a sensitivity that is comparable to micro bolometer FPAs with 50 micrometers pixels. The average NETD value for these FPAs is about 35 mK with an f/1 aperture and operating at 30 Hz frame rates. Good pixel operability and excellent image quality have been demonstrated. Pixel operability is greater than 99 percent on some FPAs, and uncorrected responsivity nonuniformity is less than 4 percent. The micro bolometer detectors also have a relatively fast thermal time constant of approximately 10 msec. This state-of-the-art performance has been achieved as a result of an advanced micromachining fabrication process. The process allows maximization of both the thermal isolation and the optical fill-factor. The reduction in pixel size offers several potential benefits for IR systems. For a given system resolution requirement, the 225 micrometers pixels allow a factor of two reduction in both the focal length and aperture size of the sensor optics. The pixel size reduction facilitates a significant FPA cost reduction since the number of die printed on a wafer can be increased. The pixel size reduction has enabled the development of a large-format 640 by 512 FPA array applicable to wide-field-of-view, long range surveillance and targeting missions, and a 160 by 128 array where applications for miniaturization and temperature invariance are required as well as low cost and low power.

La_0.7Ba_0.3MnO_3-(delta ) (LBMO) has been noticed as a potential candidate for an uncooled IR bolometer material, which has a large temperature dependent electrical resistance, as a potential candidate for an uncooled IR bolometer material. The LBMO thin films were deposited on SrTiO₃ substrates by a laser ablation method. The films were characterized by an x-ray diffractometer. In the observation by an atomic force microscope, the size of the crystal grains and the number of grain boundaries on the films were extremely sensitive on the ambient oxygen pressure during the deposition. The grain size found to have an effect on both the magnitude of the resistivity and the temperature dependence of resistivity. The films deposited under the oxygen pressures of 53.3-80.0Pa, having the temperature dependence of resistivity like a bulk, and it showed a large temperature coefficient of resistance over 3 percent K near room temperature.

Third generation IR focal plane arrays will be required to operate at significantly higher temperatures than utilized today. The ultimate aim is operation at room temperature, for nay desired cutoff wavelength in the complete IR spectral bandwidth of 1 to 14 micrometers , with performance characteristics equivalent to those achieved today at 77K. Thermal detectors offer a limited capability of meeting these requirements, particularly for any system not operating at LWIR with a slow frame rate. However, the HOT detector concept, first proposed by Elliott and Ashley, offers the promise of uncooled photon detector across the complete range of the IR spectrum at high speeds. This paper discusses the materials and device properties that are important to successfully reduce this concept to practice, together with the sate of the art in HOT detectors today.

The advantages of mercury cadmium telluride for 'HOT' IR detector applications are discussed. Molecular beam epitaxy (MBE) is used to grow advanced device structures for this purpose. MBE offers the potential to grow HgCdTe heterostructure layers on large silicon substrates leading to very large format and high performance IR focal plane array sin the future. Preliminary material and device properties achieved p⁺-v-n⁺ device structures grown on 3 inch oriented silicon wafers are discussed.

This paper reports results obtained on mid-wave IR x equals 0.3 Hg_1-xCd_xTe avalanche photodiodes (APDs) that utilize a cylindrical 'p-around-n' front side illuminated n+/n-/p geometry. This 'p-around-n' geometry favors electron avalanche gain. These devices are characterized by a uniform, exponential, gain voltage characteristic that is consistent with a hole to electron ionization ratio, k equals (alpha) _h/(alpha) _e, of zero. At 6 bias and 77 K, gains are typically near 50, and gains of over 100 have been measured at higher biases. Response times have been modeled and measured on these devices. The modeling indicates that the geometry and dimensions of the diode control the diffusion limited device bandwidth. Rise times of less than 0.35 nsec should be possible according to this analysis. TO dat 10 percent to 90 percent rise times as low as 1 nsec have been measured. The gain is approximately noiseless up to gains of over fifty which is consistent with insignificant hole ionization. The noiseless gain behavior reported here is inconsistent with the original theory of McIntyre that predicts an excess noise factor of 2 for the k equals 0 case. The explanation for these results will require application of the modified 'history dependent' theory for excess noise later proposed by McIntyre.

HgCdTe APDs and APD arrays offer unique advantages for high-performance eyesafe LADAR sensors. These include: operation at room temperature, low-excess noise, high gain, high-quantum efficiency at eyesafe wavelengths, GHz bandwidth, and high-packing density. The utility of these benefits for systems are being demonstrated for both linear and area array sensors. Raytheon has fabricated 32 element linear APD arrays utilizing liquid phase epitaxy (LPE), and packaged and integrating these arrays with low-noise amplifiers. Typical better APDs configured as 50-micron square pixels and fabricated utilizing RIE, have demonstrated high fill factors, low crosstalk, excellent uniformity, low dark currents, and noise equivalent power (NEP) from 1-2 nW. Two units have been delivered to NVESD, assembled with range extraction electronics, and integrated into the CELRAP laser radar system. Tests on these sensors in July and October 2000 have demonstrated excellent functionality, detection of 1-cm wires, and range imaging. Work is presently underway under DARPA's 3-D imaging Sensor Program to extend this excellent performance to area arrays. High-density arrays have been fabricated using LPE and molecular beam epitaxy (MBE). HgCdTe APD arrays have been made in 5 X 5, 10 X 10 and larger formats. Initial data shows excellent typical better APD performance with unmultiplied dark current < 10 nA; and NEP < 2.0 nW at a gain of 10.

We model the effects of the electronic band structure on hole- and electron-initiated impact ionization coefficients. Calculations for bulk alloy AlGaSb avalanche photodiodes with alloy compositions near the resonance between the energy gap and the spin-orbit splitting reveal that the hole- to electron-impact ionization coefficient ratio shows no enhancement at high electric fields. This is due to carrier heating spreading the hole distribution in the split-off band. However, an enhancement due to the resonance in the band structure is predicted for weak fields. A strategy to extend this type of an enhancement to high fields in a superlattice involves band engineering the superlattice to place flat bands approximately one energy gap below the top of the valence band. This prevents hot holes from spreading in energy and hence gives rise to strong hole-initiated impact ionization and a large hole- to electron-impact ionization coefficient ratio. Quantitative results are presented for a mid-infrared InAs/GaInSb/AlSb superlattice.

At present, band pass filtered, very high-gain photomultiplier tube (PMT) imaging sensors are employed to detect and image hot, UV emitting objects within the solar blind UV spectral region for wavelengths < 289 nm, either within or through the earth's atmosphere. In this paper we will reviews and summarize the stringent requirements that must be met in order to design AlGaN/GaN solar blind UV photodetector arrays that will have responsivities, detectivities, gain, speeds, and low noise levels comparable to those of present-day, high-gain PMT imaging detectors. Photodiode detectors operating in both the photovoltaic and photoconductive modes will be considered together with signal-gain that may be achieved through hybrid avalanche photodiodes and photodiodes operating in the pulsed Geiger mode.

It has been known that in BIB type, Si:As Mid-IR detectors the internal gain can be strongly related to the internal noise. We prove that by modifying the internal gain it is possible to increase the signal-to-noise ratio to a level which is consistent with poissonian statistics only.

In Korea, Japan and China, the measurement of surface temperature profile shown in abnormalities in neural and vascular functions, facial lesions, changes of blood stream in peripheral tissues (breast cancer, etc.), and psychosomatic problems is widely used for the diagnosis and the progress monitor of disease and symptoms (pains). For this application, single element LWIR Hg0.78Cd0.22Te photo-conductive (PC) detectors were fabricated with the wafers having a cutoff wavelength larger than 12.5 mm. The optical characteristics such as responsivity and detectivity were tested and the operation of the detectors was proved by the thermal imaging system IRIS5000. It was found that the 1/f noise makes lines and seriously degrades the thermal images. MWIR Hg0.70Cd0.30Te photo-voltaic (PV) detectors were also fabricated and tested for the medical application. However, owing to the low signal, the results were far from satisfactory. It is supposed that the integration methods are required for the single element MWIR detector.

We were investigated photoemission properties of porous silicon capsulated by thin films SiOx,ZnS,Al2O3. This films were deposited by RF magnetron sputtering in argon-oxygen atmosphere and had crystalline structure. Light-emission spectra such double structure in visible and infra-red region were investigated.

We were investigated the creating process of porous silicon (porSi) and CdHgTe based heterostructures for IR detectors. We were studied current-voltage characteristics, photosensitivity and photoconductivity in such heterojunctions and heterostructures after Hg ion irradiations. It was shown that studied heterostructures may be used for IR detectors.

Micro-Raman spectra of four annealing Cd_1-yZn_yTe crystal samples with small concentrations were measured within the spectral range of 40cm^-1 to 500cm^-1. Three main Raman scattering peaks were found in all four samples and that the Raman peaks in the samples get higher and narrower with longer annealing time. Strong photoluminescence in the four samples also were observed, which come from the band edge radiation. The composition value y of Cd_1-yZn_yTe crystals can be easily calculated by using these micro-photoluminescence peaks.

Low temperature IR photoluminescence (PL) spectroscopy has been performed on InAs/GaAs epilayers with the thickness of 0.4, 0.5, 0.75 and 1.5 micrometers grown by molecular bema epitaxy for potential use as active layers in InAs-based IR lasers. A bulk InAs grown by Bridgeman technique is included for comparison with InAs epilayers. For the bulk InAs, three broad near-band-edge emission peaks are observed at 417 meV, 403 meV, and 383 meV at 10 K. Temperature and excitation power dependence of PL intensity give evidence that they may be related to band-to-band and/or exciton emission, conduction band-to-acceptor, and DAP recombination, respectively. For InAs/GaAs epilayers, the PL intensities of emission peaks are much smaller and emission peaks appear at higher energy side with maximum shift of about 5 meV, compared with the bulk sample. This blue shift can be attributed to strain effect due to large lattice mismatch between InAs epilayers and GaAs subset rates. As the thickness of epilayer increases the peak shift becomes small, which implies that the strain effect is reduced.

The paper theoretically examines the photocarrier extraction effect taking place in thin variable-gap photoresistors with linear profile of energy gap and Ohmic contacts. monochromatic light. Taking into account quasielectric built-in fields occurring in variable-gap semiconductors, it has been deduced analytical expressions for spatial dependencies of photocarriers, as well as for photocurrent in thin variable-gap photoresistors. When the applied external field has the direction opposite to quasi-electric one, the latter promotes pulling the carriers towards the Ohmic contact where they recombine with an infinite rate what leads to decrease in the carrier effective lifetime. In the case of opposite direction of external field the quasielectric field counteracts moving the photocarriers forward the Ohmic contact what gives rise to the increase in carrier effective lifetime. In these conditions the negative differential photoconductivity can arise with the magnitude of maximum photoconductivity considerably exceeding that of uniform semiconductor layers.