The goal of DARPA's research program in Infrared Detector Materials is to develop a science base for production of affordable large area high performance focal plane arrays. Intrinsic materials (HgCdTe and related Hg-based alloys) are preferred, principally because of their large absorption coefficient. A summary of progress and issues in bulk and epitaxial growth of II-VI materials, II-VI superlattices, contacts, ion implantation, and low frequency noise is given.

Melt-growth of bulk material at high temperatures leads to stress-and diffusion-induced dislocations, precipitation and non-uniform alloy composition. These defects can be reduced by epitaxial growth at lower temperatures, but epitaxial processes are prone to surface and substrate-layer interface defects and the propagation of line defects from the substrate. Results from the UK R&D programme are used to illustrate the ways in which many of these problems are being solved.

The electronic properties of Hg_1-xCd_xTe (MCT) are analysed within the theoretical frame given by ETBM band structure calculation method. Properties emphasized are those of interest for device application i.e. band gap bowings and the variation of electronic properties throughout the composition range. A special attention is focused on inter-valence band optical transitions and on resonant Auger processes.

IC-VI superlattices, recently grown by molecular beam epitaxy, exhibit far more diverse characteristics than the conventional GaAs-Al_xGa_{1_x}As heterostructures, in particular because of the zero-gap band strcuture of some binary and ternary mercury compounds. The superlattices band Structure can be calculated as a function of the layers thicknesses and the temperature, and the calculations are confirmed by optical and magneto--optical experiments in the case of HgTe-CdTe saperlattices. Finally, these new systems present interesting potential applications as infrared materials.

A systematic study of the electrolyte electroreflectance lineshape in semiconducting alloys rich in defects has shown the necessity of generalizing the theory to include effects resulting from the interaction of the modulating electric field with local lattice instabilities and with polarizable charged defects. This generalization permits one to understand and use fully lineshapes previously considered pathological and incomprehensible. It has extended considerably the usefulness of electroreflectance and now provides detailed information on the role of specific defects such as stacking faults or antisites in the behavior of materials in device structures. This is particularly attractive, given the simplicity of the technique.

An optical reflectance method is utilized to determine the effective acceptor concentra-tion of p-type Hg_{l_x}Cd_xTe (o.2≤ x≤ 0.3). This method is based on the analysis of the spectral reflectance in the plasmon-phonon region of the samples at room temperature, and is capable of determining acceptor densities as low as 5.10¹⁵ cm^-3. Experimental results of p-type bulk crystals and of p-type layers on n-type bulk crystals are presented, and the prac-tical limits of the method are discussed.

Cathodic behavior of n-Type Cadmium Telluride is investigated in aqueous acidic and basic solutions by means of cyclic voltametry and in situ reflectanance measurements. The formation of cadmium layers is demonstrated in basic media wohereas no cadmium deposits are observed in acidic solution (pH = 0) under simular treatments. These results are explained by considering the variation with the pH value of the interface parameters (decomposition and hydrogen evolution potentials, band edges positions). It is shown that very uniform cadmium films of various thicknesses up to one micron can be grown on CdTe by this method.

Hall and resistivity data are presented for epitaxial mercury cadmium telluride layers grown by the MOVPE IMP technique. The effect of variation of substrate material and its orientation upon the Hall characteristics of the layers is discussed and a correlation between substrate orientation and anomalous p-character is reported. This correlation is explained in terms of a skin effect, which may be present as a surface or an interfacial layer. The effect of such a skin when annealing material from p-type to n-type has been investigated. The conditions favouring the observation of skin effect have been considered. The Hall parameters presented for CMT grown on sapphire indicate layers with low carrier concentration (n ~ 2 x 10¹⁴ cm^-3) can be grown by this MOVPE process. Parameters on both n and p type material are reported.

An experimental comparison of Hg_{l_x}Cd_xTe (CMT) and Hg_{l_x}Zn_xTe (ZMT) alloys for 0 < x < 0.5 is carried out using electrochemistry and the electrolyte electroreflectance technique. The current versus potential characteristics of CMT and ZMT show clearly that the latter is the more chemically stable. This electrochemical behavior is correlated with a good definition of the electroreflectance spectra of ZMT in the El transition region. For the first time, the relation between El and the alloy composition of ZMT is determined in the whole composition range. The whole set of results tends to indicate that this new alloy is a promising material for infra red detection.

Characterization is an important activity in the field of semiconductors. Many different methods are available using various physical effects. We focus this paper on the transport phenomena and an example of study is given on HgTe/CdTe super lattices.

The aim of this paper is to demonstrate that transport experiments under high magnetic fields and under hydrostatic pressure are a very convenient tool to characterize impurity levels in II-VI compounds. The method has been used in the cases of bulk . semiconductors and of implanted thin films. The existence of resonant impurity states has been clearly pointed out. In the particular case of boron implanted thin films the experiments demonstrate clearly the existence of a resonant level induced by implantation and located approximately at 150 meV above the conduction band minimum.

Rutherford backscattering analysis was used to study the damage due to In implantation into Hg_{l_x}Cd_xTe (x = 0.7). We show that the defects can be removed successfuly by rapid thermal annealing (T = 440°C, t = 10s).

Some physical and electrical properties of liquid phase epitaxial (LPE) layers of Hg_0.78Cd_0.22Te grown on CdZnTe substrates are presented in this paper. In order to qualify these epilayers, 64 x 64 arrays of backside illuminated photovoltaic detectors were successfully manufactured. The high performances obtained on these detectors demonstrate that LPE of Hg_0.78Cd_0.22Te remains the most suitable material for such high density arrays.

Samples of HgCdTe with negative interaction gap determined by the pressure transition are studied in S.D.H. at 4.2K. Due to the density of states quantization with B, the Fermi level oscillations, taking into account the strong non parabolicity, are calculated. When an acceptor level lies near the Fermi level (E_F - E_A < 1 meV) an oscillation transfert of population between the C.B. and the acceptor states takes place. So the giant 0 oscillation seen in TMR and LMR configurations is clearely due to this transfert, because the LMR cannot exhibit mobility oscillation at low temperature without a spin flip. The calculation of the part due to this transfert agrees well in the case of 0 oscillation when the distance E_F - E_A is changed by mean of hydrostatic pressure. The contribution of the oscillations and average parts of C.B. concentration is in good agreement with the experimental results of the Hall coefficient.

A description is given of the use of the Shubnikov-de Haas effect, parallel-field magnetoresistance and cyclotron resonance to give a rather complete characterisation of (Hg,Cd)Te photodetectors. The Shubnikov-de Haas and parallel-field magnetoresistance enable one to distinguish the separate contributions to conductivity from surface layers and bulk which bedevil normal characterisation methods such as the Hall effect. In addition, the surface carrier densities can be very accurately measured, and the number of populated surface subbands deduced. Cyclotron resonance is used to make an accurate determination of the x-value of the bulk material, and to measure the effective masses of the surface electrons, thus indicating whether the surface composition differs from that in the bulk. It is shown that these measurements are not only useful in the characterisation of detectors and material, but are of fundamental interest in the study of the two-dimensional electron gas: the surface electrons exhibit many novel modes of behaviour, such as enhanced or reduced polaronic effects and skipping orbits, the latter of which have only been previously observed in metals.

Hg_l-xCd_xTe-layers have been grown on CdTe- and Cdi__y^Zn_yTe-substrates by liquid-phase-epitaxy from a Te-rich solution in a vertical dipping system. This technique allows a series of layers to be grown simultaneously. Throughput and quality of the layers obtained are well suited for production of photovoltaic IR-detectors and renders this technique a promising alternative to bulk growth.

Epitaxial layers of cadmium telluride and cadmium mercury telluride are currently being grown by use of two vertical 'infinite melt' liquid phase epitaxy dipping furnaces. In one system, CdTe is being grown from mercury solvent at 250-300°C under an atmosphere of pure hydrogen at one bar pressure. In the second furnace, CdHgTe layers are produced from a mercury-rich melt at 460-500°C under a hydrogen atmosphere at up to fifteen bar pressure. Both furnaces are remotely controlled and feature extremely accurate and sensitive pressure control systems. The combination of a dipping furnace and large (10 kg) mercury-rich melt offers many potential advantages over more conventional small scale LPF systems employing tellurium-rich melts. For example - 1) Melt wipe-off problems are drastically reduced. 2) Impurity levels can be cut since mercury is readily available in an extremely pure form. 3) Etch-back of CdTe substrates is much slower and more easily controlled, giving good interfaces. 4) Both n- or p-type layers can, in principle, be grown without recourse to annealing. 5) The melt is maintained at temperature over many months aiding both run-to-run uniformity and a high through-put of material. The design of the furnaces will be described, and results obtained from measurements of the layers produced will be presented and discussed.

Cadmium Telluride and Cadmium Zinc Telluride are still the most widely used substrate materials in CMT epitaxy. Much of the material which is available commercially has unsuitable electrical properties for on-substrate device fabrication. Other material contains impurities or defects which can lead to the growth of inferior quality epitaxial layers. Precipitates can be a major problem where they intersect the surface of the polished substrate. They may lead to surface defects in the epitaxial layer, or to the release of any impurities which may be present if dissolved by the growth solution in the case of growth from the liquid phase. Removal of these precipitates and control of the electrical properties of the substrate is therefore desirable so that optimum quality epitaxial material may be grown. Epitaxial growth of CMT from either the liquid phase or from the vapour phase involves heating the substrate in mercury vapour. This has an effect on the surface regions of the substrate which can be revealed using a suitable etch, and using a sectioning technique which enables the electrical properties through the thickness to be determined. A high temperature anneal in a Cadmium overpressure has been shown to modify the electrical properties and to reduce defect densities in commercial Cadmium Telluride material. (Ref. 1).

The intrinsic semiconductor alloy system Cdx Hgl-x Te is an important material for I.R detection. By changing the alloy composition x, the energy gap of Cdx Te can be varied and may be optimized for various wavelengths through the I.R spectrum. The most important wavelength ranges are the atmospheric windows 8-12 μ (x≈ 0,2) and 3-5 μ (x≈ 0,3) for thermal imaging and the 1,3-2,5 μ range (x ≈0,5 to 0,7) for optical fiber applications.

Crystals of Hgl-xCdTe were grown by the Travelling Heater Method (THM) with Te as a solvent zone and aiameers up to 15 mm. Microprobe measurements show that the new developed furnace is suitable for the growth of HgCdTe-crystals. A new method to produce feed material with larger diameters than 10 mm is presented.

II-VI semiconductor materials are receiving increasing attention for optical applications in the near infrared from 1 to 3 μm. This interest is driven by the wide range of optical and electronic characteristics offered by II-VI compositions for band structure design as well as the attractive band structure characteristics of certain alloys for avalanche detection. New approaches to the growth of high quality II-VI epitaxial layers and super-lattices are discussed. These include the low temperature growth of material by MBE and MOCVD using both pyrolysis and photolysis. In parallel with improved epitaxial techniques, improved substrates are also being developed. Ternary compounds such as Cd1-xMnxTe are found to be of higher crystalline quality than CdTe grown by the same technique. These crystals can be grown with uniform composition of controlled lattice parameter. Epitaxial layers can now be grown either on lattice matched substrates of improved quality or alternatively on lattice mismatched GaAs substrates. In the latter case, a detailed under-standing of the growth of interface phases for high quality epitaxy will be described.

Photovoltaic detectors have been made from (Hg,Zn)Te crystals grown by THM. Their first characteristics are presented and discussed.

CdTe and ZnCdTe layers have been grown on (001) GaAs substrates between 150-350°C for growth rates of 0.5-2 μm/h. For CdTe, (001) and (111) orientations were observed, whereas only the (001) orientation was obtained for ZnCdTe layers. The layers, structural and optical properties were determined from SEM, double-crystal rocking-curve and photoluminescence studies.

Preliminary experimental results of epitaxial growth of CdZnTe on sapphire, silicon and single crystal CdZnTe via a low pressure chemical vapor deposition process are reported. The CdZnTe layers grown were smooth and specular in appearance. The infrared transmission trace of CdZnTe grown on silicon and sapphire showed good transmission through the film. The X-ray diffraction scan of CdZnTe on silicon showed that the film is epitaxial with a lattice constant of 6.45 Å.

On indium doped silicon monolithic integrated focal plane arrays with 64 x 64 picture elements were developed. The chip size is 56 mm2, the pixel size 551μm x 65 μm, the pitch 85/μm, and the fill fator 50 %. The black body (500K) detectivity of the image sensor is 2 x10¹¹ cm W^-1 sec^-1/2 and the inhomogenity of the responsivity less than 5 %. The significant features of the array are - picture elements operating with punch through technique (inherent antiblooming) - resistive gate structure for the read-out in the vertical direction - 4 phase CCD for serial read-out in the horizontal direction Using this device a camera system has been built to demonstrate the possibilities for thermal imaging with extrinsic silicon. The output signals are processed with the double correlated sampling method. To fully utilize the performance capability of the device it is necessary to compensate offset and gain nonuniformities. The demonstration system is built up with a pixel-by-pixel offset compensation with 10 bit correction factors.

±We have performed a detailed study of the barrier height of PtSi/p-Si Schottky diodes with thicknesses ranging from 25 to 200Å. The hole photocurrent was measured by using irradiation through cooled (70K) narrow band (▵λ : 0,14μm) irinterference filters in the spectral range from 3,0 to 5,4 μm. The photocurrent is analysed as a function of the reverse bias voltage to determine the barrier height by the Schottky effect. The flat band barrier is determined to (1)BFB = 0,257 eV ± 8 meV independent of PtSi thickness. The scatter due to fabrication processes (sputtering or evaporating of Pt and annealing) is only + 7meV. The temperature dependence of the barrier follows the temperature variation of the Si bandgap in the range 20 to 100K. The tight pinning of the barrier height and the photo-excitation process are discussed in the light of interface states at the PtSi-Si interface.

The next generating of thermal imaging systems probably will use 2-dimensional FPAs without mechanical scanners. This means to develop starring infrared sensor arrays. The limitations of present systems will be discussed. As a result, the detector array has to be combined with a suited readout multiplexer. Within the infrared transparent wavelength range from (3-5) μm the Si:In-IR detector seems to be one of the favourite candidates, especially for the monolithic integration with read out electronics. As multiplexer, the charge coupled device, CCD, is a very useful tool.

The standard poling procedure of a PVDF biaxially oriented film implies for simplicity of the equipment, the combined application of a DC electric field between the metallic electrodes deposited on both faces on the film, and a temperature ranging between the ambiant and the Curie point. So far, no scientific approach has been advanced to justify the values of the poling parameters : the applied voltage Vp, the temperature Tp, the duration t , usually adopted during the poling. The interdependence between the three parameters effEcts should be considered, as the maximum dipole orientation is achieved for an applied voltage value depending on the other two parameters. (1000 V for 90°C and 30 mn, whereas 1200 V for 70°C and 30 mn in our experiments).

Ge:Ga and Ge:Be extrinsic photoconductors are used in the 50 - 200 and 30 - 50 μm wave-length range respectively as optimized detectors for far-infrared astronomy applications. Reproducible materials synthesis and comprehensive materials characterization are required for the development of focal plane detectors which reach the high sensitivity which is theoretically possible under the very low background flux conditions of space-based observation. Ge:Ga, Ge:Be, and Ge:Zn photoconductor materials have been grown using the Czochralski technique under high purity conditions. Residual impurities can be controlled to levels of~10¹⁰ cm ^-3, leading to greater control of free carrier lifetime and mobility, and more reproducible detector performance. Characterization of the material with variable temperature Hall effect and infrared spectroscopy shows that Be and Zn doped crystals grown under a H2 atmosphere can contain hydrogen related complexes which affect detector performance by decreasing the resistivity of the material at low temperatures. The growth of Ge:Be presents a special case in which the oxygen content of the melt environment is a critical factor in determining reproducibility of Be doping and device performance.

Pb1-xEuxSe is a promising material for IR devices. The band gap becomes larger with the relative content of Europium. The preparation of Pb_1-xEu_xSe by molecular beam epitaxy and the properties of these layers will be described.