X-ray diffraction techniques have proved invaluable in the characterization of infrared materials, particularly those prepared by thin film deposition techniques such as molecular beam epitaxy, MBE. The techniques are sufficiently sensitive and rapid to provide the information feedback required for efficient optimization of the growth process. They are nondestructive and permit the correlation with results on the same sample obtained by other characterization techniques such as those being described at this Symposium. Depending on the development status of the growth technology, the information to be acquired includes presence of twinning, quality, and type of epitaxial orientation, strains, and compositional variations. A critical issue in the application of these materials in detector arrays is the question of uniformity control, both laterally and in depth. The techniques to be described include not only modern x-ray topographic and multiple crystal diffractometric techniques but particularly for the early stages of growth process development, classical photographic ones such as the oscillation and Weissenberg methods. Examples of these various aspects are presented with emphasis placed on the characterization involved in MBE growth of HgCdTe films.

Using high resolution transmission electron microscopy (HRTEM) it is possible to directly image the projected structure of semiconductors with point resolutions at the atomic level. This technique has been applied to the study of interface and defect structures associated with molecular beam epitaxial (MBE) growth of HgCdTe layers on GaAs substrates. The structure of the CdTe/GaAs interfaces is described for (100) and (111) epitaxy. From the atomic structure, a model for the early stages of epitaxial growth is presented. The structure of HgTe-CdTe superlattices is discussed from the HRTEM point of view.

Defect morphology and distribution up to depths of 20um have been shown to be critical to device performance in micro-electronic applications. A unique and novel x-ray diffraction method called DARC (Digital Automated Rocking Curve) topography has been effectively utilized to map crystalline micro-lattice strains in various substrates and epitaxial films. The spatial resolution of this technique is in the the order of 100um and the analysis time for a 2cm2 area is about 10 secs. DARC topography incorporates state-ofthe- art 1-dimensional and 2-dimensional X-ray detectors to modify a conventional Double Crystal Diffractometer to obtain color x-ray rocking curve topographs.

This technique, being non-destructive and non-intrusive in nature, is an invaluable tool in materials’ quality control for IR detector fabrication. The DARC topographs clearly delineate areas of microplastic strain inhomogeniety. Materials analyzed using this technique include HgMnTe, HgCdTe, BaF2, PbSe, PbS both substrates and epitaxial films. By varying the incident x-ray beam wavelength the depth of penetration can be adjusted from a 1–2 micron up to 15–20um. This can easily be achieved in a synchrotron.

A transmission electron microscope study of HqTe-CdTe multilayer structures grown by molecular beam epitaxy (MBE) on (100) Cd Zn Te is presented. Both cross-sectional and plain-view observations show highly reaular structures of superlattices and tunnel structures. Dislocation densities estimated by Plan-view observations are of the order of 104 cm−2 in these multilayer structures. A quantitative characterization of interface sharpness of superlattices has been carried out by intensity analysis of satellite spots in electron diffraction patterns. It is shown that interfaces in these superlattices are hichly abrupt with a width of one or two monolayers. These observations suggest the effectiveness of the use of lattice-matched substrates to qrow high quality HgTe-CdTe multilayer structures.

Simulated rocking curves for a light-emitting diode structure are presented. Results for a structure containing uniform layers are compared to rocking curve data for a wafer grown by vapor phase epitaxy (VPE), and we conclude from the comparison that the VPE wafer closely approached the hypothetical ideal assumed in the simulations. Simulations illustrating difficulties in analyses and the effects of a graded active layer are also presented.

In this work, we report the use of Rutherford backscattering(RBS) measurements and computer simulations to provide accurate stoichiometry information and semi-quantitative defect densities for the near surface region of Hg1−xCdxTe (MCT). The accuracy of the Hg1−xCdx Te x-values determined by our method is found to be comparable to other commonly used methods, such as FTIR or the electron microprobe. The data obtained as structural defects from RBS channeling measurements are in basic agreement with other techniques, such as chemical etching. The sensitivity of the channeling measurement to uniformly distributed dislocations is found to be about 107−108 cm−2, however, for dislocations forming subgrains, the detectable level of dislocation comes to 105 – 106 cm−2. The depth profiles of lattice disorder resulting from ion implantation into MCT are also extracted from RBS channeling measurements using these simulation programs. These profiles are found to closely match the calculated profiles for the displaced atoms calculated using an implantation modeling program (TRIM). We also report on the use of channeling-in-grazing-angle-out technique for evaluating the stoichiometry of the first few monolayers of the MCT surface.

We have used two complementary techniques to study deep trapping levels in CdTe single crystals. Samples of undoped, semi-insulating material and p-type material doped with phosphorus or cesium have been examined using transient spectroscopic techniques. Both capacitance transients (DLTS) and photocurrent transients (PITS) have been measured. The DLTS measurements showed several trapping levels in all of the specimens, while the PITS data usually revealed only a single level.

High purity In.53 Ga.47 As and InP with carrier concentrations [ND–NA] < 5×1015 cm−3 has been grown by the LPE technique on both n-type and semi-insulating substrates to detect and identify trace donor and acceptor impurities. Acceptor impurities have been detected in low temperature photoluminescence spectra where LPE melt baking and growth programs indicate a melt origin for two of these species, one of which is zinc. Data from semiconductor profiles provides evidence for sulfur and tin donor impurities, which comes from the rinse melt used to etch back substrates doped with the respective contaminants. Silicon and sulfur contaminants have been detected by SIMS measurements; and may arise not only from the indium and III-V materials, but also the graphite boat used to grow the epilayers. Volatile sulfur-containing compounds have been detected during high temperature bake-out of high purity graphite boats.

The optical properties of a GaAs-AlAs superlattice have been examined using the non-destructive technique of infrared reflectance spectroscopy. Through this technique, the absorption edge, the effective superlattice refractive index, the thickness, and the optical grading of the superlattice-substrate were determined. Location of the absorption edge was made from the reflectance spectrum and showed general agreement with photoluminescence measurements. A more detailed analysis of the infrared spectra indicated the presence of a transition region between the substrate and the superlattice. Based on a non-linear least squares method for fitting the experimental data, a dispersion relation for the dielectric function was obtained. This dielectric function yielded a value for the superlattice refractive index that was lower than that of the corresponding, homogeneous, AlGaAs alloy film for wavelengths between 1.0 and 2.5 micrometers.

High purity epitaxial silicon samples, grown on indium doped and on ultrahigh resistivity silicon substrates, were analyzed for impurity content using photoluminescence spectroscopy (PL) and spreading resistance analysis (SRA). Calibrated SRA indicated typical net carrier concentrations of < 3×1012cm-3 in the epitaxial layers, and about 7×1011 cm-3 in the substrates. Impurities were identified by collecting highly resolved, very clean no-phonon and TO-phonon replica PL spectra at liquid helium temperatures. Spectra were taken on the substrate material alone and on substrates with epitaxy. Ga, As, A1, B and P contamination was evident in the epitaxy. Correlation of SRA and PL results on samples with various levels of contamination at the epitaxy substrate interface identified Al as the main interfacial impurity.

N-type and p-type (100) CdTe films have been grown on (100) CdTe substrates by photoassisted molecular beam epitaxy, using indium and antimony as n-type and p-type dopants, respectively. The application of this growth technique to substitutionally dope another II-VI material is demonstrated by the successful n-type doping of (100) CdMnTe films with indium. Modulationdoped superlattices consisting of barrier layers of CdMnTe:In alternating with CdTe have also been grown. The point defect nature of these in situ doped films and multilayers is studied with low temperature (1.6–5 K) photoluminescence and excitation photoluminescence measurements. The introduction of the dopant atoms using this new growth technique produces immediate changes in the photoluminescence spectra of the epilayers. Photoluminescence studies of the superlattices show the effects of quantum well confinement and band filling due to free carriers.

The influence of boron ion implants on the optical and physical properties of photochemically deposited SiO2 films on HgO 7 CdO 3Te and silicon has been investigated. The distributions of the boron atoms between the SiO2 film and substrate have been determined by a nondestructive neutron depth profiling method. The implants produce an apparent densification of the SiO2 films, which is accompanied by an increase in refractive index and changes in the infrared vibrational spectra for these films.

Minority carrier lifetime measurements at 77K were carried out in ptype liquid phase epitaxial (LPE) Hg 1-xCdx Te/CdTe (x = 0.22) using the photoconductive decay technique. Lifetimes of 20 to 7000 ns were obtained in samples with hole concentrations, p, in the range 1014 to 1016 cm-3. The hole concentrations were determineg by analyzing the Hall data using a double-layer model. It was found that the minority carrier lifetime is inversely proportional to p01.86. This result demonstrates that the Auger mechanism may be the dominant recombination process in p-type LPE Hg0.78 Cd0.22Te/CdTe. The temperature dependence of minority carrier lifetime was also measured between 10 and 200K for several samples.

The results of an in-depth study of HgCdTe epilayers and HgTe-CdTe superlattice structures are summarized. In particular, both spectral and transient photoconductance measurements have been made on samples which were also characterized by x-ray diffraction, Hall, IR transmission, IR photoluminescence, and transmission electron microscopy measurements. Selected HgCdTe samples grown at General Electric during this investigation exhibit some of the best structural and electrical properties reported to date for MBE-grown HgCdTe. Sharp photoconductance spectra have been obtained for HgTe-CdTe superlattice structures grown at North Carolina State University. This study is part of an on-going collaborative effort between General Electric's Electronics Laboratory and North Carolina State University.

The frequency response in the modulation of the excess electron concentration, produced by a modulated photogenerating pump beam, is used to determine bulk life-time and the surface or interface recombination velocity. The depth-wise integrated excess electron concentration is contactlessly monitored by the proportional absorption/ transmission modulation of a second probe beam. Using this approach over the 20 kHz to 3 MHz frequency range recombination velocities up to 104 cm/sec have been measured in n-type epitaxial HgCdTe films.

The MBE growth of HgTe on CdTe is examined over a hundred-fold range in Hg/Te2 flux ratio and over four separate substrate surface orientations [(111)Te, (111)Te-4 degrees, (112)Te and (001)]. The 77K Hall mobility of the (112)Te and (001) oriented HgTe layers approaches the best bulk values reported to date, although our (111)Te and (111)Te-4 deg oriented HgTe films yield much lower values. The growth process is shown to be very far from thermodynamic equilibrium at our optimal substrate temperature and calculated equivalent Hg beam pressure. Important clues to help understand the kinetics governing the HgTe growth process are uncovered by studying the defects that form under Hg- or Te-rich conditions with cross-sectional transmission electron microscopy (XTEM). Since multilayered structures are an important application for MBE growth of Hg-based semiconductors, we have also examined the interfacial roughness present in HgTe-CdTe superlattices (SL) as a function of growth orientation.

Over the past few years, liquid-phase epitaxy (LPE) has become an established growth technique for the synthesis of HgCdTe. This paper reviews one of the most successful LPE technologies developed for HgCdTe, specifically, “infinite-melt” vertical LPE (VLPE) from Hg-rich solutions.

Despite the very high Hg vapor pressure (> 10 atm) and the extremely low solubility of Cd in the Hg solution (< 10−3 mol%), this approach was believed to offer the best long-term prospect for growth of HgCdTe suitable for various device structures. Since the initial demonstration of LPE growth of HgCdTe layers from Hg solution in experiments conducted at SBRC in 1978, the VLPE technology has advanced to the point where epitaxial HgCdTe can now be grown for photoconductive (PC) and photovoltaic (PV) as well as monolithic metal-insulator-semiconductor (MIS) and high-frequency laser-detector devices with state-of-the-art performance in the entire 2–12 μm spectral region.

A historical perspective and the current status of VLPE technology are reported. Particular emphasis is placed on the important role of the ther-modynamic parameters (phase diagram) and on control of stoichiometry (defect chemistry) and impurity doping (distribution coefficient) for growth of HgCdTe layers from Hg solution. Critical material characteristics, such as transport properties, minority-carrier lifetime, morphology and crystal structure, are also discussed. Finally, a comparison with the LPE technol-ogy using Te solutions, which has been the mainstay of the remainder of the IR community, is presented.

The intrinsic semiconductor mercury cadmium telluride (Hg1−xCdxTe), a solid solution of HgTe and CdTe, has assumed an ever increasing role in the fabrication of infrared (IR) detectors because its energy gap (0-1.5 eV) can be tailored to match the specific needs of IR detection and fiber optic systems. In photovoltaic focal plane array (FPA) applications, low power consumption, as well as excellent sensitivity at elevated temperatures, have made Hg1−xCdxTe the material of choice for both the midwave IR (MWIR) and longwave IR (LWIR) region.

The Interdiffused Multilayer Processing (IMP) technique has been developed as a way of growing uniform layers of cadmium mercury telluride (CMT). The principle of the technique is discussed in the context of an interdiffusion coefficient so as to assess the potential applicability of the technique to the growth of other II–VI or III–V alloys. The development of IMP for the growth of CMT layers, together with the properties of the layers and their suitability for use in the fabrication of IR detectors, are reviewed.

A qualitative stability model established for hydrocarbon molecules and extended to organometallic compounds is applied to alkyl organometallic compounds used in thin film growth. For hydrocarbon molecules the strength of a carbon-hydrogen bond is reduced by neighboring alkyl groups, The mechanism for this effect is delocalization of the unpaired electronic charge in the resultant free radical by neighboring alkyl groups. The delocalization effect is present in organometallic compounds and is illustrated here for several alkyl organometallic systems. Two applications of the delocalization effect in thin film growth are discussed. First the thermal stability of organotellurium compounds is substantially reduced in branched compounds which permits significantly lower HgCdTe metalorganic growth temperatures. Also carbon incorporation in GaAs and GaAlAs is reduced with ethyl instead of methyl organometallic compounds. An important factor contributing to this result is the weaker carbon-metal bonds in the ethyl compounds.