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Infrared focal plane arrays (IRFPA) based on HgCdTe semiconductor alloys have been shown to be ideal for tactical and strategic applications. High density (>1 M pixel), high operability HgCdTe detectors on large area, low-cost composite substrates, such as CdTe-buffered Si or GaAs, are envisioned for next-generation IRFPAs. Thermal expansion mismatch is among various material parameters that govern the structural properties of the final detector layer. It has previously been shown that thermal expansion mismatch plays the dominant role in the residual stress characteristics of these heteroepitaxial structures (Jacobs et al. in J Electron Mater 37:1480, 2008). The wafer curvature (bowing) resulting from residual stress, is a likely source of problems that may occur during subsequent processing. This includes cracking of the film and substrate during post-growth annealing processes or even certain characterization techniques. In this work, we examine dynamic curvature and stress during molecular beam epitaxy (MBE), of CdTe on Si and GaAs substrates. The effect of temperature changes on wafer curvature throughout the growth sequence is documented using a multi-beam optical sensor developed by K-Space Associates. This monitoring technique makes possible the study of growth sequences which employ annealing schemes and/or interlayers to influence the final residual stress state of the heteroepitaxial structures.

Molecular beam epitaxy n-type long-wavelength infrared (LWIR) Hg1−xCdxTe (MCT) has been investigated using variable-field Hall measurement in the temperature range from 50 K to 293 K. A quantitative mobility spectrum analysis technique has been used to determine the role of multicarrier transport properties with respect to epilayer growth on lattice-matched cadmium zinc telluride, as well as lattice-mismatched silicon (Si) and gallium arsenide (GaAs) buffered substrates. Overall, after postgrowth annealing, all layers were found to possess three distinct electron species, which were postulated to originate from the bulk, transitional (or higher-x-value) regions, and an interfacial/surface layer carrier. Further, the mobility and concentration with respect to temperature were analyzed for all carriers, showing the expected mobility temperature dependence and intrinsic behavior of the bulk electron. Electrons from transitional regions were seen to match expected values based on the carrier concentration of the resolved peak. At high temperature, the lowest-mobility carrier was consistent with the properties of a surface carrier, while below 125 K it was postulated that interfacial-region electrons may influence peak values. After corrections for x-value and doping density at 77 K, bulk electron mobility in excess of 105 cm2 V−1 s−1 was observed in all epilayers, in line with expected values for lightly doped n-type LWIR material. Results indicate that fundamental conduction properties of electrons in MCT layers are unchanged by choice of substrate.

The surface kinetics of CdTe (211)B grown by molecular beam epitaxy (MBE) is investigated using spectroscopic ellipsometry (SE) during in situ cyclic annealing. A method of measuring sublimation rates from high-index surfaces without use of reflection high-energy electron diffraction is presented. The effect of Te2 overpressure on the activation energy of sublimation for the CdTe (211)B surface is reported. The sensitivity of SE to surface temperature and film thickness was leveraged to monitor sublimation rates of CdTe stabilized by a Te2 overpressure. The sublimation activation energy was found to increase from 0.45 eV to 2.94 eV under the Te2 beam pressure regime investigated.

We present the results of a detailed study of the changes that occur on CdTe buffer layer surfaces grown on ZnTe/Si(211) and GaAs(211)B during the routine thermal cyclic annealing (TCA) process. Observations indicate that CdTe buffer layer surfaces are Te saturated when the TCA is performed under Te overpressure. In the absence of Te flux during the TCA step, the CdTe surface loses CdTe congruently and the typical CdTe nanowires show the presence of nodules on their surfaces. The observed changes in reflection high-energy electron diffraction patterns during TCA are explained in terms of surface chemistry and topography observations. Overall, the Te overpressure is necessary to maintain a smoother and pristine surface to continue the molecular beam epitaxy (MBE) growth.

This paper correlates measurements of minority carrier lifetime using photoconductivity transient or photoconductive decay (PCD) with measurements and analysis of current–voltage (I–V) on P + n photodiodes of the same layer. This analysis is done at the primary temperature of operation near 77 K. Measurements of minority carrier lifetime in n-type molecular beam epitaxy Hg1−x Cd x Te (MCT) on variable substrates of GaAs and CdZnTe are presented. The minority carrier lifetime from I–V analysis of P + n long-wave infrared (LWIR) photodiodes on CdZnTe (CZT) and GaAs substrates is similar to PCD analysis. Deviations occur for MCT/GaAs in the low-temperature extrinsic region associated with Shockley–Read–Hall (SRH) at 78 K in contrast to MCT/CZT at similar temperatures. The x-values in the formula Hg1−x Cd x Te for samples grown on CdZnTe and GaAs are 0.22 and 0.235, respectively. I–V analysis of variable-area LWIR photodiodes shows a calculated minority carrier diffusion coefficient of 2.3 cm2/s on lattice-matched CZT and 3 cm2/s on non-lattice-matched GaAs with diffusion lengths of approximately 24 μm and 20 μm, respectively.

Reduction of threading dislocation density is critical for improving the performance of HgCdTe detectors on lattice-mismatched alternative substrates such as Si. CdTe buffer layers grown by molecular beam epitaxy (MBE), with thicknesses on the order of 8 μm to 12 μm, have helped reduce dislocation densities in HgCdTe layers. In this study, the reduction of threading dislocation densities in CdTe buffer layers grown on locally thinned Si substrates was examined. A novel Si back-thinning technique was developed that maintained an epiready front surface and achieved Si thicknesses as low as 1.9 μm. Threading dislocation densities, acquired by defect decoration techniques, were reduced by as much as 60% for CdTe buffer layers grown on these thinned regions when compared with unthinned regions. However, this reduction is inconsistent with prior notions that threading dislocation propagation is dominated by image forces. Instead, the thickness gradient of thinned Si may play a larger role.

HgCdTe heteroepitaxy on low-cost, large-lattice-mismatched substrates such as Si continue to be plagued by large threading dislocation densities that ultimately reduce the operability of the thermal imaging detector array. Molecular-beam epitaxy (MBE) of 10 μm- to 15 μm-thick CdTe buffer layers has played a crucial role in reducing dislocation densities to current state-of-the-art levels. Herein, we examine the possibility that growth on locally back-thinned substrates could prove advantageous in further reducing dislocation densities in the CdTe/Si heteroepitaxial system. Using defect decoration techniques, a decrease in dislocation (etch-pit) density of up to ~42% has been measured in CdTe regions where the underlying Si substrate was chemically back-thinned to ~20 μm. A theoretical understanding is proposed, where a substrate-thickness-dependent dislocation image force is a likely cause for the experimentally observed reduction in threading dislocation density. These observations raise the prospect of combining localized substrate thinning with other techniques to further reduce dislocation densities to levels sought for HgCdTe/CdTe/Si and other large-lattice-mismatched systems.

Large-area high-quality Hg 1-x Cd x Te sensing layers for infrared imaging in the 8 μm to 12 μm spectral region are typically grown on bulk Cd 1-x Zn x Te substrates. Alternatively, epitaxial CdTe grown on Si or Ge has been used as a buffer layer for high-quality epitaxial HgCdTe growth. In this paper, x-ray topographs and rocking-curve full-width at half-maximum (FWHM) data will be presented for recent high-quality bulk CdZnTe grown by the vertical gradient freeze (VGF) method, previous bulk CdZnTe grown by the vertical Bridgman technique, epitaxial CdTe buffer layers on Si and Ge, and a HgCdTe layer epitaxially grown on bulk VGF CdZnTe.

It is well known that the large lattice mismatch (>14%) associated with CdTe/Si, CdTe/Ge, and CdTe/GaAs composite substrates, is a great contributor to large dislocation densities and other defects that limit the performance of HgCdTe-based infrared detectors. Though thermal expansion mismatch is another possible contributor to material defects, little work has been done towards documenting and understanding its effects in these systems. Here, we perform studies to determine the relative contributions of lattice and thermal mismatch to CdTe film characteristics, including dislocation density and residual stress. Unannealed and thermally cycled films are characterized using x-ray diffraction, defect decoration, and Nomarski and transmission electron microscopy. For CdTe/Si, the residual stress is consistently observed to be tensile, while for CdTe/Ge and CdTe/GaAs, a compressive residual film stress is measured. We show based on theoretically predicted stress levels that the experimental measurements imply the dominance of thermal mismatch in the residual stress characteristics.

We present the real-time pseudodielectric function E)> of low-temperature-grown GaAs (LT-GaAs) thin films during the growth as a function of growth temperature Tg and thickness. We obtained accurate measurements of the real-time E)> by using in situspectroscopic ellipsometry (SE) in conjunction with active feedback control of the substrate temperature using diffuse reflectance spectroscopy. We show that for epitaxial LT-GaAs layers, the peak in the imaginary pseudodielectric function 2(E)> decreases in amplitude and sharpness systematically with decreasing Tg. We also revealed an abrupt change in E)> near the critical epitaxial thickness hepi, the value of which decreases with decreasing Tg. Above hepi, the LT-GaAs grows polycrystalline (amorphous) above (below) Tg ∼ 190°C. We also simultaneously monitored the surface roughness and crystallinity by using real-time reflection high-energy electron diffraction (RHEED). These results represent progress in obtaining real-time control over the composition and morphology of LT-GaAs

We present comparisons of photoluminescence (PL) data for various vertical-cavity surface-emitting laser (VCSEL) and distributed quantum well structures taken with the pump beam (and the collection path) in two different configurations: normal to the surface of the sample; and perpendicular to a cross-section of the epitaxial layers. We demonstrate that the cross-sectional PL (XPL) technique can resolve individual features in the structures, and that the surface-normal PL (NPL) spectra are perturbed by the multilayer mirrors in the VCSELs. We elucidate a potential method for transforming between the NPL spectra and the non-perturbed XPL spectra and evaluate the sensitivity of this method to various measurement as well as material parameters. This simulation technique is well-suited to wide parametric variations of the dispersion curves for the complex dielectric constant of the materials, the pump field distribution, and the depth profile of the gain medium.

Simulations of reflectance spectra and electric field distributions for vertical-cavity structures were used in the computer aided design of epitaxial mirrors and lasers. The binary GaAs/AlAs superlattice alloys and AlxGa1−xAs random alloys that compose these structures were grown by molecular beam epitaxy. Photoluminescence, photoreflectance, reflectance spectroscopy, scanning electron microscopy, transmission electron microscopy, and double crystal x-ray diffractometry were applied to characterize cavity and Bragg mirror layer thicknesses and alloy composition.

Low-order, monolayer by monolayer (1×1) AlAs/GaAs superlattices grown by MBE using different growth techniques have been studied by X-ray diffractometry. High-resolution multiple-crystal diffractometry was used to study diffraction features near the [004] peaks due to the substrate and epilayers. Using this technique, we have investigated the effect of growth techniques on the strain and tilt of the superlattices. High-resolution X-ray diffraction (HRXRD) results suggest that the sample grown by migration-enhanced epitaxy (MEE) is more highly strained and has more tilt than the same superlattice sample grown using the interrupted growth (IG) technique.

In this work we examine the physical properties for the superlattice system (GaAs)n1 (AlAs)n2/GaAs(100) for low values of n1 and n2, i.e., n1 = n2 = 3, 6, 12. Normal, interrupted growth, and migration enhanced epitaxy (MEE) growth techniques were used to grow the superlattice structures in a molecular beam epitaxy system. X-ray diffraction spectra were obtained, and the major and satellite peak positions were analyzed to obtain the superlattice periodicity. An analysis of the major diffraction peaks and their associated satellites produced superlattice periodicity in good agreement with theory. Diffraction peaks were also observed in regions adjacent to the primary diffraction peaks which did not occur in the expected satellite positions. An analysis of these peaks relative to the primary peak indicate periodicities corresponding to layer thickness greater than the intended period. One possible cause for these periodicities is growth conditions that exist during the growth of the superlattice which result in the deposition of fractional monolayers. In this study we present results which suggest that an arsenic-deficient growth condition may be a contributing factor in the deposition of fractional monolayers.

We report the results of our study of controlled interface roughness in low-order GaAs/AlAs superlattices. Samples were prepared using either the interrupted growth or the migration-enhanced epitaxy (MEE) technique. The samples were prepared with m atomic planes of GaAs and m atomic planes of AlAs (m × m) per modulation wavelength and repeated p times. For this study, m = 1 or 3. The samples were studied using X-ray diffraction. The interrupted growth samples both showed a split in one diffraction line indicating layers were not of integral order while the MEE samples showed no splitting, indicating integral order layers.

Photoreflectance (PR) was used to study crystalline quality and stress in silicon films on insulator. The position and amplitude of the 3.4 eV PR silicon structure was monitored for both MBE silicon on sapphire (SOS) and SIMOX structures. The SOS film thicknesses ranged from 6 to 4000 nm. The shift in energy of the 3.4 eV structure from its bulk position is dependent on the sample thickness and on the strain in the films. In the SIMOX case, the amplitude of the 3.4 eV PR signal provided information about the crystalline quality of the top silicon layer for a set of six samples, each one removed at a different step of a process involving three implantation-anneal cycles. In this case the PR signal appeared to deteriorate with the each cycle undergone by the samples. Further investigations are needed to resolve the discrepancy bet ween these SOS results and the ones obtained in an earlier set of measurements.

Severance of a peripheral nerve leads to a characteristic series of events in the distal stump, including the dissolution of axons and myelin and the proliferation of Schwann cells within their basal lamina. This study examines the relationship between the spatial-temporal pattern of the induction of the Schwann cell S phase, loss of the structural and functional properties of axolemma, and the clearance of myelin debris in the cat tibial nerve. Nerve transection stimulated a monophasic increase in [3H]thymidine incorporation that peaked at 4 days post-transection throughout an 80-mm length of distal stump. Light microscope autoradiography revealed prominent incorporation into Schwann cells of myelinated fibres. Treatment of distal stumps with mitomycin C at the time of nerve transection greatly retarded thymidine incorporation and clearance of myelin debris, but not the time course of axonal degeneration, decline in the synthesis of the major myelin glycoprotein, P0, or the onset of ovoid formation. Nerve transection also greatly reduced the specific uptake of [3H]saxitoxin (STX), a ligand which binds to voltage-sensitive sodium channels. Binding in the distal stump fell precipitously to 20% of the normal at 4 days post-transection, concurrent with the peak of thymidine incorporation. This low level of binding was maintained for periods of up to 70 days, demonstrating that some STX binds to structures other than axons in denervated distal stumps. Prior treatment with mitomycin C delayed the loss of specific STX binding. In conclusion, these studies suggest that: (a) Schwann cell DNA replication- and/or mitosis regulates other events during Wallerian degeneration, including myelin degeneration, catabolism of P0 and the clearance of sodium channels from nodal axolemma; (b) the decline in P0 synthesis and/or shift to synthesis of less extensively processed P0 is independent of the induction of Schwann cell S phase; and (c) Schwann cells enveloping myelinated axons enter S phase within a 24-h period throughout the entire 80-mm length of distal stump.

An x-ray diffraction study is made of the effect of silicon film thickness on the peak broadening associated with the (024) sapphire Bragg reflection peak for MBE—grown SOS material. The predeposition treatment and growth parameters for the samples in the series were identical. The thicknesses ranged from 5500 to 40,000 angstroms and the growth rate was roughly I angstrom/second.SOS films with silicon thicknesses below 5500A did not produce sapphire peak broadening when compared with the corresponding sapphire standard. With reference to standard sapphire reflection peaks, the 5500A film shows slight broadening while a I micron film broadened 14 seconds of arc when compared at FWHM with the sapphire standard. A possible explanation for this is that at overlayers less than 5500A, the film is not sufficiently massive to exert a uniform strain on the sapphire. At 5500A the film attains a critical thickness sufficient to strain the substrate and produce detectable broadening. For thickerfilms the strain would increase proportionally. Results from lattice parameter measurements on the silicon epilayer indicate that the perpendicular lattice parameter relaxes atgreater thicknesses but is still in tension when compared with bulk silicon. One would expect that the strain gradient suggested by these results would be greatest at the silicon/sapphire interface and that the defect concentration at the interface might beaffected by this strained condition.

We have continued our investigation1 of the effects of pre—deposition substrate annealing for the case of MBE grown silicon on sapphire. Three identical series of samples were grown with film thicknesses ranging between 65 and 1500 angstroms. Each series of samples was grown upon sapphire which had been annealed at either 1450ºC, 1300ºC, or 900ºC. The growth temperature and growth rate were the same for each series. The anneal time for each sample was 30 minutes.

Beryllium is an effective p-dopant in GaAs and AlGaAs and plays an important role in device characterizations of hetero bipolar transistors. This work addresses the doping and mobility properties for two series of beryllium-doped samples: GaAs and AlGaAs. Within each series the doping ranged between 3×1015cm-3 to levels of 5×1019cm-3. Mobility and carrier concentrations were obtained through Hall and Polaron measurements. The doping concentration results suggest the onset of carrier compensation at higher doping levels. One possible explanation is that for high doping levels, Be is incorporated as interstitial donors. A thermodynamic model is used to explain the observations.