We present in this review a brief appraisal of the current status of HgCdTe layers grown by Molecular Beam Epitaxy (MBE). Although this technique has produced unique material structures such as HgTe:CdTe superlattices and HglxCdxTe/Hgl-yCdyTe heterostructures, there are still significant problems in 'obtaining high quality detector materials. In this paper an attempt is made to identify some of the possible causes of these problems. These difficulties are divided into two classifications: problems related to the control over the growth technique, and the basic material science issues concerning the metallurgical properties of these alloys and the surface nucleation mechanisms responsible for epitaxy by MBE. Following this review, some of the work performed at Georgia Tech to provide answers to the identified problems is presented. An argument is then made for the growth of these alloys by a photon-assisted chemical beam epitaxy (PACBE) technique which is shown to provide the capability for both fundamental studies of surface nucleation kinetics, and we believe the growth of high-quality materials.

Recent developments in Metalorganic Vapour Phase Epitaxy (MOVPE) of CMT are reviewed and the potential of the technology for producing advanced IR cadmium mercury telluride (CMT) detectors, particularly planar diode arrays, are assessed. The assessment covers the current status of layer production, including compositional uniformity, surface quality and crystalline perfection as well as the electrical properties and doping. In addition, the role of the newer, low temperature precursors as a means of producing advanced detector structures are considered. Recent diode results are reported and it is shown that MOVPE CMT, either on CdTe or GaAs substrates, is capable of producing state-of-the-art performance for diode arrays. The future scope of photolysis for in situ device fabrication is also considered, together with some recent results on photo-patterning of epitaxial CdTe.

HgTe and HgCdTe layers have been grown by organometallic vapor phase epitaxy at low temperature by using methylallyltelluride (MATe), dimethylcadmium (DMCd) and elemental mercury. Use of MATe enabled us to grow layers in the 250-320°C range, which is 50°C lower than the growth temperature when diisopropyltelluride (DIPTe) is used. The layers were characterized by double crystal x-ray diffraction, low temperature Hall measurements and Fourier transform infrared spectroscopy (FTIR). Growth below 340°C resulted in featureless HgTe layers. Layers grown on CdTe are misoriented with respect to the substrate by about 60 to 150 arc-seconds whereas such tilting was not observed when lattice matched CdZnTe substrates were used. The high quality of HgTe grown at low temperature is demonstrated by the very narrow (29 arc seconds) full width at half maximum of the x-ray diffraction curve. HgCdTe layers grown at 320°C showed sharp interference fringes even for thin layers, indicating the presence of a sharp interface.

The advantages predicted for (Hg,Zn)Te over (Hg,Cd)Te are stressed. The(Hg,Zn)Te alloys are then situated, according to their properties in IR detection at 10.6 μm, among the other II-VI ternary alloys and some low dimensional III-V's. The different techniques used so far for MZT bulk and epitaxial growth are reviewed. The problem of substrates for the growth of MZT by liquid phase epitaxy is discussed. Finally recent results on MZT device performances, and mainly on the reliability of photodiodes, confirm the advantages predicted for MZT over MCT.

The poor thermal conductivity of CdTe has prevented controlled directional solidification and, therefore, completely single crystal growth is not achieved on a routine basis. The Heat Exchanger Method (HE(TM)) has been adapted for the growth of 600 g, 5.5 cm diameter and 1300 g, 7.5 cm diameter CdTe ingots. Emphasis was placed on achieving controlled directional solidification. Unseeded crystal growth was carried out using presynthesized CdTe as meltstock and without the use of a separate Cd or Te reservoir to control stoichiometry. It was demonstrated that only two grains were formed and the structure was maintained during growth. Twinning was minimized and large twin-free samples could be obtained. The EPD values of CdTe were in the range of 103 to 5x105/cm2. Most of the material was p-type, but CdTe with 105 ohm-cm resistivity was grown. An absorption coefficient as low as 0.07/cm was measured showing that the crystals were suitable for CO2 laser modulator applications.

High quality HgCdTe epitaxy is essential to the development and producibility of infrared focal plane arrays (IRFPA' s) for emerging strategic and tactical aerospace applications. The lattice mismatch between an epitaxial layer and its substrate is known to effect various layer characteristics, especially morphology and crystallinity, which impact downstream device fabrication yields. Lattice matching is thus an important issue in the continuing development of large scale HgCdTe epitaxy. This paper reviews recent progress in the development of the HgCdTe lattice matching alloys CdZnTe and CdTeSe. Improvements in growth methods, lattice parameter control, substrate size, stoichiometry control and crystallinity are presented.

N-type, 5LPE2 HgCdTe, with carrier concentrations of 2 x 10 14 cm-3 and mobilites greater than 1 x 10 5 cm /V-s, at 20 K, were produced by a Hg-vapor anneal of the as-grown, p-type starting material, followed by diffusion of an evaporated indium layer. When the Hg-vapor anneal follows the indium diffusion, the material remains p-type, indicating that the two steps are not commutative. Experimental techniques used to explore these differences were Hall effect, capacitance-voltage measurements on MIS structures, photoconductive lifetime, and SIMS. Possible causes of the differences in electrical properties between the two cases are discussed. The low temperature mobilities obtained for the n-type LPE layers compare well with bulk data from other workers. A calculation of the transport properties for the n-type case is also presented to show the effects of compensation.

Successful Cadmium Mercury Telluride (CMT) thermal imaging systems have been in production for some time now, but critical materials problems are still present which need to be overcome in order to facilitate development of the next generation of detectors. Currently, there is much interest and effort in the use of mercury and impurity diffusion for diode fabrication. The importance of the understanding and characterisation of diffusion processes to the development of devices and the problems associated with gaining the required fundamental and applied knowledge is discussed. Our current state of knowledge of impurity and self-diffusion is reviewed. It is concluded that a limited amount of diffusion data, useful for identification of possible stable dopants and conversion through mercury annealing, is available for bulk material. There is an urgent need for similar data in epitaxial material along with parallel electrical measurements.

Photoluminescence (PL) and electron paramagnetic resonance (EPR) are powerful techniques for both fundamental studies and potential materials screening of CdTe substrates for HgCdTe growth. Certain extended defects that are common in epitaxial CdTe have a distinctive PL signature that correlates with X-ray measurements of crystallinity. Bulk samples with prominent subgrain structure also have this PL feature, and cathodoluminescence images show that the defect is localized to the subgrain boundary regons. PL and EPR are very sensitive techniques, and specific impurities such as Fe or Ag have been observed in some nominally pure samples. PL and EPR spectroscopy can also detect changes associated with thermal annealing treatments, which alter the stoichiometry of CdTe by varying the number of Cd vacancies and interstitials. These findings illustrate the versatility of PL and EPR as nondestructive techniques to assess the quality of substrates for IR-detector materials.

The far infrared reflectance technique has been used to characterize ZnTe and CdTe films grown using Metal Organic Chemical Vapor Deposition (MOCVD) on GaAs substrates. Carrier concentrations, derived from the reflectance measurements, decreased with increasing ZnTe film thickness as a result of the large lattice mismatch between ZnTe and GaAs. This mismatch produces a high strain, high dislocation region near the interface. The uniformity of CdTe films grown on GaAs substrates in two different types of reactors are compared. The thickness and carrier concentration is found to be far more uniform for films grown in a vertical flow, rotating susceptor reactor than for films grown in a conventional linear flow reactor.

A strong reduction of the n+-accumulation layer in n-Hgl-xCdxTe (MCT) samples after UV-irradiation was recently reported l 2 3 4. This effect produced by electron trapping in deep levels of the passivation oxide, lasts several hours at 77 K and even more at 4 K. The sample surfaces are driven into a "quasi" depletion state. Upon UV-illumination we now observed a ten-fold decrease in the carrier lifetime. Obviously this is a consequence of the "quasi" depletion state, in which the electron-holes pairs recombine now with a much shorter time constant characteristical of the depletion regions 5 8.

We present results of our calculations of the equilibrium barrier formation in graded Hg1_xCdxTe heterojunctions using a highly accurate numerical model. Results for wide gap-p on narrow gap-n structures (Pn) are presented together with a review of our earlier results on narrow gap-p on wide gap-n (pN) heterojunctions [J. Appl. Phys. 62, 3267 (1987); 64, 6373 (1988)1 in which the barrier forms only in the conduction band. All band profiles are calculated with and without (common anion rule) a valence band offset; clear trends are observed. In the case of the narrow gap-p on wide gap-n heterostructures, the band profiles calculated with and without the valence band offset do not differ significantly. On the other hand, for the wide gap-p on narrow gap-n heterostructures, and using the common anion rule, the valence band tends to bow down on the n-side for the larger grading widths. The band bowing acts as a potential barrier for the minority carriers. When the valence band offset is included, though, the existence of a barrier to minority carriers depends upon the grading width: for the most narrowly graded junctions, a potential well for the minority carriers is present. In our calculations, we assume a valence band offset of 300 meV for HgTe:CdTe. We cannot make general predictions with regard to conditions needed to support the formation and growth of a barrier either in the conduction or valence band; instead; we find the band profiles to be a complex function of all the junction design parameters.

Pyroelectric infrared detectors are serious competitors for intrinsic semiconductor detectors, their main advantages being a simpler and a cheaper fabrication process and operation at room temperature. Their handicap, however, is the slow signal response. As believed, the detectivity decreases significantly in consequence of their long dielectric relaxation time. Here we demonstrate that there are no theoretical arguments for such a frequency dependent roll-off. In addition, we propose a special doping process to speed-up the detectivity by optimizing in this way the AC resistance-matching between detector element and load.

Y1Ba2Cu30x films, screen printed grom submicron powders have demonstrated sensitivity to . lc radiation, with a responsivity of 103V/W. Based upon detailed calculations the response is consist.ent with a bolometer mode of operation. The response, to chopped radiation with lmw/cm input power, was determined as a function of temperature and dc bias current. Calculations were made on the use of antenna-like elements as detectors based on using their macroscopic properties. Modeling using lossy array elements predicted a marginal improvement compared to conventional flat plate design.

Lattice matched In 53Ga.47As alloys, epitaxially grown on InP substrates are now standard photodetectors for fiber optic and instrumental applications in the 800 - 1700 nm spectral range. Among their chief advantages are high sensitivity, low noise, high speed and room temperature operation. The advent of low loss optical fibers operating at 2500 nm and of near infrared spectroscopy in the 1000 - 2500 nm spectral range makes the extension of responsitivity beyond the standard 1700 nm highly desirable. In this paper, we will discuss the growth and characterization of epitaxial layers of materials such as InxGa1-xAs (x > 0.53) and InAsvP1-y for detectors responding to 2600 nm. Since these materials are not lattice matched to InP substrates, compositional grading epitaxial growth must be employed. Such grading technique enables one to prepare epitaxial layers with different band gaps and thus regulate the responsivity range of detectors produced from these alloys. The grown wafers have been processed into planar detectors with active area of approx 100 μm diameter, which are suitable for fiber optic and other applications. These detectors have been fully characterized for spectral responsivity, quantum efficiency, noise and speed of reponse abd results will be presented. Room temperature values of D* exceeding 1 x 10 11 cmHz1/2W were obtained.

The MBE growth and electronic structure of a-SnxGe i_x alloy films were studied with RHEED and angle resolved synchrotron radiation photoemission spectroscopy. Doe to an increased interfacial reactivity, CdTe substrates are not suitable for the epitaxial growth of homogeneous films with x>0.1. Alloy films with good crystalline quality can be grown on Ge(100) substrates at approx 400 C. The expected closing of the alloy band gap is confirmed by photoemission data which show the shift of the T8 valence band maximum from -0.6 eV in Ge(100) to -0.16 eV below EF in a-Sn0.48 Ge0.52

Planar heterojunctions were fabricated on mercury cadmium telluride layers grown by Molecular Beam Epitaxy. P-type (111)B Hgi_,Cd,Te having an x value gradually increasing to the surface have been grown. The sputtering technique has been used to form the junctions. For the first time, electrical and optical characterizations of a representative junction are presented in this report. The resistance-area product at zero bias and at 80 K is 2.4X103 ohm-cm 2 for x=0.27. The spectral response of this device shows that two junctions, one with x=0.27 and the other with x=0.33, exist. However, Responsivities as high as 7.2 A/W at 80 K were recorded due to the electrically reflecting boundary on the p-type surface.

The potential of angle-resolved x-ray photoelectron spectroscopy (ARXPS) for characterizing the surface region of pro-cessed compound semiconductor materials is explored with respect to depth-compositional profiling and with respect to assessing the crystalline surface structure. It is demonstrated by analyzing the sputtered surface region of a (Hg, Cd)Te wafer that ARXPS can readily distinguish between three different compositional zones regarding their sequence and their chemical nature in a depth region of about so A. The quantitative scaling of the depth profile is accomplished using a theoretical model of photoemission from a planar sample covered with two uniform overlayers. It is further demon-strated, by analyzing a (Hg, Cd)Te sample grown by liquid phase epitaxy on (111) oriented (Cd, Zn)Te, that azimuthal and polar variations in the anige-resolved photoemission of Hg, Cd and Te occur which are indicative of the phenome-non of x-ray photoelectron diffraction (XPD). The prominent maxima of the respective photoemissions were observed at three azimuthal angles (120° periodicity) for a polar angle of about 35°. A shift of 60° between the azimuthal maxima of the photoemission of Te and the metal atoms (Cd or Hg) is attributed to a reconstruction of the examined (111) surface.

The unusual properties of the HgTe-CdTe superlattice (SL), and its potential as an infrared detector material, make it worth study. Growth of this SL is a complex process so that good characterization is essential. Important structural and electronic properties appear in the region 10-250 cm-1, making far infrared spectroscopy a powerful probe. We show how reflectivity spectra analyzed by a simple, effective theory give Hg content in the nominal CdTe layers, layer thicknesses, effective mass, and the HgTe-CdTe valence band offset. We get m*/m° = 0.010 and 0.022 for dHgTe/dcdTe = 80 Å/40 A, and 0.013 and 0.044 for dHgTe/d CdTe = 64 Å/60 Å, at 78 and 300K respectively. We obtain a valence band offset of 300 meV for the 80 Å/40 Å SL.

Various surface passivations of p-type Hg1-xCdxTe were studied to understand their interface properties and and their potential for device technology. Anodic oxide forms an inverted layer near the interface. This n-type skin layer exhibits extremely good n-type properties, which equal, and even surpass, bulk properties. The high electron mobility may be explained by quantization of the electron levels in the space-charge region, and the formation of a two dimensional electron gas near the interface. Thick (approx 500 Å) anodic sulfide generates a similar inversion layer. The charge density is proportional to the sulfide thickness. Carefully prepared thin (approx 100 Å) anodic sulfide films as well as ZnS coating on freshly etched sur-faces, form nearly Hatband conditions which are suitable for n+ on p diode technology. The surface recombination velocity, determined for these two passivations using the photoelectromagnetic effect, is shown to be similar at low temperatures, increasing with decreasing temperatures. The dominant trapping mechanism at the surface is similar to that in the bulk, and is probably mostly due to vacancies.

The validity of the assumption that the contribution of the n region to the current of Hg1-xCdxTe diodes is negligible was examined. By placing such diodes in a magnetic field we were able to separate between the current component originating in the p -type substrate, and that from the graded n region. Experimental results show that the ratio between these currents is 0.5 - 3, depending on temperature. The theoretical analysis reveals the influence of the electric field present outside the depletion region on the current generated by the graded region. This field not only produces a drift component, which drives the minority carriers into the junction, it also greatly modifies the excess carrier distribution, thereby changing diffusion part of the current. The analysis shows the importance of the lifetime profile in the graded region, which is a function of the specific recombination mechanism and its dependence on the local dopant concentration. The effect of parameters such as substrate concentration, surface concentration, and junction depth on this current is discussed.