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We present four new types of III-V quantum well infrared photodetectors (QWIPs) operating in photoconductive (PC) and photovoltaic (PV) modes for the wavelength range from 2 to 14 μm. These dual-mode (DM) operation QWIPs were grown by the MBE technique using GaAs/AlGaAs, AlAs/AlGaAs, and InGaAs/InAlAs material systems. Based on the bound-to-miniband (BTM) and the enhanced bound-to-continuum (BTC) intersubband transition schemes, these detectors provide the features of large absorption coefficient, low dark current, and high detectivity in the wavelength of interest, and show promising for use in large area IR focal plane array image sensor applications.
Ternary and quaternary III-V alloys are important for many optical device applications, and a precise control of the composition is required. Molecular beam epitaxy (MBE) is generally considered a non-equilibrium or kinetically controlled process but most of these models are too computationally intensive for real time control. We report on using a precursor state growth model 1,2 for the growth of GaAsSb to control the growth conditions and hence the film composition. The activation energies and the parameters appearing in the relationship are determined by fitting the calculated compositions to experimental ones as determined by x-ray diffraction. The effect of substrate temperature, growth rate and flux intensities on composition is discussed.
In this paper, we discuss a differentially strained p-doped quantum well infrared photodetector that achieves high performance specifications. We examine key device specifications for a 9 and 18 μm infrared detector. We calculate that through differential strain, these novel detectors have improved gain and substantially reduced dark current over previous quantum well infrared photodetectors, while being able to detect normal incident light.
Short period InAs/InxGa1−xSb superlattices (SLs) may allow strong optical transitions in the long wavelength infrared (> 10 μm) spectral region. Absorption calculations can be difficult, however, because of the strongly type - II interface and because of the large lattice mismatch. We propose that a long wavelength response can be achieved for substantially thinner layers of SLs if the In composition in InxGa1−xSb is properly chosen. This will misalign the bands through strain effects and further reduce the superlattice bandgap. Band structure calculations are reported for InAs/InxGa1−xSb type - II SLs grown on GaSb substrate by using an empirical tight-binding model (ETBM). All of the structures considered here are assumed to be well within the critical strain thickness. Particular care is taken to incorporate the strain effects accurately in the ETBM formalism by modifying the overlap integrals according to the bond lengths and bond angles. We compute the band structure and the cutoff wavelengths of InAs/InxGa1−xSb (001) SLs and compare the results with the existing magnetooptical and photo conductivity data. In addition, we compare the ETBM with the k.p and effective bond orbital models.
Undoped LEC GaAs grown with very low residual acceptor concentration is often low resistivity with an activation energy for conduction of 0.43 eV. We demonstrate that this material can form a very sensitive bolometer element and show how large two dimensional bolometer arrays might be fabricated from this material.
A new photoluminescence spectrometer has been developed for the characterization of optical emission in the 2.5 to 14.1 micron wavelength range. This instrument provides high sensitivity for the detection of interband and defect luminescence in a variety of infrared detector materials. The spectrometer utilizes a solid state photomultiplier detector and a circular variable filter, which serves as the resolving element. The entire spectrometer is cooled to 5K in order to decrease thermal radiation emission. Band-edge luminescence at 10.1 microns from HgCdTe samples has been readily detected with argon-ion laser excitation powers less than 70 mW/cm2. Representative spectra from HgCdTe and other infrared detector materials are presented.
We have investigated the low temperature (4.5 K) photoluminescence (PL) spectra of GaSb and GaInAsSb layers. The layers were grown by liquid phase electro-epitaxial (LPEE) technique. Several bound excitomc transitions were observed both in GaSb and GaInAsSb layers. Shift in the PL peak energy corresponding to the band to band transition with temperature was determined. The linear part of the shift above 100K, exhibited a slope of -0.3 meV/K.
We have measured the Shubnikov-de Haas (SdH) effect in a HgTe/CdTe superlattice (SL) with a tilted magnetic field in the absence of an external electric field. We found that the peaks of the SdH oscillation changed with Bcosθ, indicating the two-dimensional character of the electron gas. The carrier concentration of the two-dimensional electron gas (2DEG) is equal to 2.95×1011 cm−2. The 2DEG shows the existence of a Stark ladder, which is caused by the internal electrostatic field, near the interface between the substrate and the SL. From the temperature dependence of the SdH measurements, we also show that the 3DEG in the SL miniband contributes to the non-oscillatory magnetoresistance and that the mobility of 3DEG in the miniband is lower than that of 2DEG in the Stark ladder.
Intersubband transitions in a series of well-doped ([Si] = 2.0×1018cm−3) In0.07Ga0.93As/Al0.4Ga0.6As multiple quantum well samples were studied as a function of the well width by using the optical absorption technique. A single intersubband transition is observed in samples in which the Fermi energy level is between the ground and the first excited states in the quantum well. On the other hand, two intersubband transitions were recorded in samples where the Fermi energy level lies between the first and the second excited states. These two intersubband transitions were attributed to ground-to-first excited states and first-to-second excited states transitions. The energy separation between the latter two intersubband transitions was found to increase as the well width is increased. The fact that two intersubband transitions were observed in certain samples may suggest that specially designed quantum wells can be used for two color long wavelength infrared detectors.
The photomodulated reflectivity (PR) spectroscopy of modulation-doped diluted magnetic semiconductor Cd0.72 Mn0.28 Te:In/CdTe multiple quantum wells has been measured at 20 K – 300 K. Several spectral features associated with intersubband transitions have been found. The feature associated with Fermi level is first reported in PR spectra of II–VI heterostructures. In addition, a abnormal transition intensity ratio of 22H to 11H caused by electron filled effect has been reported. The Photoluminescence(PL) spectra of the sample have also been measured at 10 K – 300 K. A strong enhancement of the photoluminescence intensity towards the electron Fermi energy, which is caused by multiple electron-hole scattering processes, is reported. The temperature dependence of the Fermiedge singularity has been measured and discussed. The mechanism of the Fermi-edge singularity can be explained by hole localization.
In this paper we have studied the photoemission from super-lattices of III-V semiconductors under magnetic quantization by formulating a new dispersion law. It is found, taking InAs/GaSb super-lattice with graded interfaces as an example that the photoemission, increases with increasing electron concentration in an oscillatory way and increases with decreasing magnetic field in the magnetic quantum limit. Besides, the photoemission in superlattices is much greater than that of the constituent materials and the well-known results for wide-gap materials have also been obtained from our generalized analysis. In addition, the theoretical analysis is in agreement with the experimental datas as given elsewhere.
Fe- doped InP is studied by a spatially resolved photocurrent technique, probing electronic transitions related to the iron impurity levels. This scanning technique allows to get information on bulk homogeneity regarding to the electrically active iron impurity distribution. The presence of doping growth striations is revealed with a high sensitivity. Also results about the local photocurrent at the Cottrell atmospheres of the dislocations are shown. The role played by other defect levels in the photocurrent contrast is also discussed.
In this paper we have investigated the Burstein-Moss shift in quantum wires and dots of III-V and II-VI materials on the basis of Kane and Hopfield models for the appropriate carrier dispersion laws. It is found taking Hg1−xCdxTe, In1−xGax AsyP1−y lattice matched to InP and CdS as examples that the Burstein-Moss shift exhibits oscillatory dependences for quantum wires and dots of the said materials with respect to doping and film thickness respectively. Besides, the numerical value of the same shift is greatest in quantum dots and least in quantum wires. In addition, the theoretical analysis is in agreement with the experimental datas as given elsewhere.
We are developing two-layer LPE P-on-n HgCdTe photovoltaic detector arrays with cutoff wavelengths out to 17 μm for a NASA spaceborne infrared radiometer. These bilinear multiplexed arrays will operate at 60 K, and must achieve sensitivities approaching the background limit for a background photon flux of 2×1015 photons/cm2-sec. The detectors must operate at reverse bias voltage to interface with silicon CMOS multiplexer circuits, and must exhibit low 1/f noise.
This paper reviews progress toward these demanding requirements. The limiting junction current mechanisms for HgCdTe photodiodes at these very long cutoff wavelengths are reviewed. Data are presented for both CdTe-passivated and ZnS-passivated arrays at 60 K with cutoff wavelengths of 15.4−16.9 μm. Average R0A products of 13 ohm-cm2 and quantum efficiencies of 89% have been achieved for cutoff wavelengths of 15.4 μm at 60 K. These array data demonstrate the potential for VLWIR PV HgCdTe to meet the requirements for advanced NASA applications.
Recent progress in the growth of Hg1−xCdxTe (MCT) by metal organic vapor phase epitaxy (MOVPE) is reviewed. The preferred diode structure for LWIR detectors is the p/n heterostructure, which requires extrinsic doping of both n and p-type layers and good compositional control of the base and cap layers. Uniform n-type doping has been demonstrated using a new precursor, TIPIn, with Auger-limited lifetime down to a doping concentration of 1 × 1015 cm−3. p-type doping has been more difficult to control because Group V dopants can occupy either Group II or Group VI sites, leading to autodoping. Some encouraging progress has been made by doping under metal rich conditions. An alternative approach to p-type doping during growth is the Rockwell-developed process of As implantation, diffusion and activation annealing, which has been used to demonstrate near diffusion-limited LWIR diodes at 77K. Major strides in the reproducibility of the MOVPE process have been achieved by in situ monitoring. Laser reflectometry has been used to monitor growth rates and morphology throughout the growth of multiple layer structures. This wafer monitoring has been complemented by system monitoring, using Epison concentration monitors and pyrometry to measure temperature.
Non-alloyed ohmic contacts of HgTe on Hg1−xCdxTe (x = 0.60) with metallisations of Ti, In and Au have been investigated. Layers of HgTe with thickness in the range from 0.1 μm to 1.0 μm were grown by organometallic epitaxy either as an abrubt or a graded junction, depending on the in-situ annealing conditions. The layer thickness and the extent of interdiffusion with the Hg1−CdxTe were determined using Rutherford backscattering spectrometry (RBS). The results have shown that an abrupt rather than a graded structure was essential in order to achieve the minimum value of specific contact resistance, ρc, of ≈5 × 10−5 Ωcm2. In addition, a critical thickness of HgTe (≥0.2 μm) was required in order to obtain a substantial reduction in ρc. For these metal/HgTe/Hg1−xCdxTe contacts, the metal Ti has produced the lowest values of ρc and greatest adhesion to the HgTe. Both of these properties have been attributed to the strong interfacial reaction of the overlayer of Ti with HgTe.
Dry passivation of HgCdTe with ZnS or CdTe using physical or chemical vapor deposition can be improved by incorporating an in situ plasma cleanup of the HgCdTe surface prior to the deposition. Contamination at the HgCdTe/ dielectric interface from ambient oxide and hydrocarbon residues may lead to fixed charge in capacitor or diode device structures. In addition, the oxides of HgCdTe are known to be thermally unstable. Removal of the surface contamination layer is advantageous for producing a consistent and electrically reliable interface. We describe the interaction of a remotely generated H2 or H2/Ar plasma (2.45 GHz, 600W) with HgCdTe, using ex-situ and in-situ ellipsometry, and atomic force microscopy. This work represents the first effort to characterize a low damage HgCdTe surface cleanup process which is compatible with vacuum in-situ passivation.
The thermal diffusivities of (Hg1−xCdx)1−yTey and (Hg1−xZnx)1−yTeywith 0.55 ≤ y ≤ 1.0 and 0.0125 ≤ x ≤ 0.05465 and of pure Te are measured over a wide temperature range by the laser flash technique. The diffusivity of near pseudobinary Hg1−xCdxTe solids decrease more rapidly with temperature approaching the melting point than pseudobinary solids previously reported. The solid diffusivity for x=0.02817 is 0.83 mm2/s at 371°C, decreasing to 0.22 mm2/s at 614°C. The diffusivity of Te rich (Hg1−xCdx)1−yTey melt increases with x and with temperature. The melt diffusivity for x=0.03934 is 0.91 mm2/s at 485°C, increasing to 4.93 mm2/s at 851°C. For Te rich (Hg1−xZnx)1−yTey melt with x=0.0125 and y=0.7944 there appears to be a minimum diffusivity of about 2.6 mm2/s near 700°C. The thermal diffusivity of pure Te solid is 0.97 mm2/s at 300°C and decreases to 0.64 mm2/s at 439°C. The melt diffusivity is 1.52 mm2/s at 486°C, increasing to 3.48 mm2/s at 584°C.
The ellipsometry and RHEED study of MCT grown on (112) CdTe and GaAs by MBE was carried out. The dependence of ellipsometric parameter on composition is evaluated. As shown we can measure the growth rate, the roughness changing, initial temperature and composition by ellipsometry in situ. We investigated the evolution of roughness of film surface. We observed the appearance of surface roughness at initial stage of MCT growth under various composition (XcdTe0÷0.4). The following growth in optimal growth condition (including constancy of substrate temperature) gives us the smoothing of the surface and supplies us the high-quality MCT films. It is found that under constant temperature of substrate heater we can not grow the thick, perfect film of MCT. The concentration, mobility and life time of carriers in MCT films were respectively: n=1.8*1014 ÷8.2*1015cm−3, μn=44000÷370000cm2 V−1 s−1, τ=40÷220ns;p=1.8*1015÷8.4*1015 cm−3, μp=215÷284 cm2V−1 s−1 τ=12÷20ns.
Single crystals of CdTe or dilute alloys of Cd1−yZnyTe (y≤0.04) and CdTe1−zSez (z ≤0.04) with low defect density, high purity and large single-crystal area (>30 cm2) are required as substrates for high-quality epitaxial Hg1−xCdxTe thin films in the infrared (IR) detector industry. Bridgman or gradient freeze is the most common technique used for commercial production of these materials because of its success in producing large area substrates of good quality and reproducibility. For epitaxial growth of Hg1−xZnxTe, which has been of considerable interest in recent years as an IR detector material, the substrate of choice has been Cd0.80Zn0.20Te, for lattice matching with long wavelength Hg1−xZnxTe epitaxial layers (x = 0.13–0.14). The primary focus of this paper is on CdZnTe which is currently the preferred substrate material and most widely used for both HgCdTe and HgZnTe epitaxy. This paper reviews the current status of bulk substrate technology for IR detector applications, highlighting critical issues and essential research areas for further improvement of these materials.