Search

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.

Minimization of operating bias and generation–recombination dark current in long wavelength infrared (LWIR) strained layer superlattice (SLS) detectors, consisting of a lightly doped p-type absorber layer and a wide band gap hole barrier, are investigated with respect to the band alignment between the wide band gap barrier and absorber layers. Dark current vs. bias, photoresponse, quantum efficiency, lifetime, and modeling are used to correlate device performance with the wide gap barrier composition. Decreases in dark current density and operating bias were observed as the conduction band of the wide gap barrier was lowered with respect to the absorber layer. The device achieved 95% of its maximum quantum efficiency at 0 V bias, and 100% by 0.05 V. This study demonstrates key device design parameters responsible for optimal performance of heterojunction based SLS LWIR detectors.

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.

Design parameters for the heterojunction-based strained layer superlattice (SLS) long-wave infrared (LWIR) detector are investigated so that it operates at a lower bias voltage with lower dark current and higher photo response. At typical operating temperatures ( T ∼ 77 K), the dark current of GaSb/InAs SLS LWIR detectors is dominated by the Shockley–Read–Hall (SRH) generation–recombination (g–r) process in the space-charge (depletion) region. In order to suppress this dark current, a wide bandgap region next to the absorber layer has been included in recent SLS designs. A series of heterojunction-based LWIR SLS detectors with various doping and barrier profiles have been designed and characterized. The significance of the doping profile and thickness of the wide-bandgap layer in optimization of the heterojunction-based SLS detector performance are exhibited from the modeling and experimental results of these devices.

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.

This paper will discuss the desired quantum efficiency and dark current density of type-II strained-layer superlattice detectors (T2SLS), and required read-noise levels for long-wavelength infrared (LWIR) imaging applications at various background levels. The computation begins with noise equivalent irradiance NEI requirements with the focal plane array (FPA) operating at near-background limited infrared photodetection (BLIP) performance conditions. The paper will theoretically analyze the quantum efficiency and the dark current based on various components such as: diffusion, generation–recombination and tunneling, and dependence on the minority carrier lifetime of T2SL detector material. These calculations show appropriate minority carrier lifetime of the detector material for the various flux levels discussed in the article.

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 report on the significance of thermal expansion mismatch in the heteroepitaxial growth of CdTe on highly lattice-mismatched substrates. Such substrates including Si, Ge, or GaAs are desirable for CdTe buffer layer growth and subsequent deposition of HgCdTe infrared absorber layers. Besides lattice misfit, the thermal mismatch associated with these systems can produce additional strain and defects at low or elevated temperatures. However, little work has been done towards documenting and understanding these effects. Experimental evidence of thermal-expansion-dependent residual stress is presented for CdTe/Si(2 1 1), CdTe/Ge(2 1 1), and CdTe/GaAs(2 1 1) films based on X-ray diffraction studies. The experimental results are also compared with residual stress calculations based on known material properties. These results may play a vital role in the design and development of HgCdTe detectors for large-format, infrared focal plane arrays (FPA).

X-ray standing wave and X-ray diffraction measurements were used to determine the structure of nominal 1 monolayer and 1/2 monolayer InAs films buried in GaAs(0 0 1). The films were grown by atomic layer epitaxy using trimethylgallium, tertiarybutylarsine and trimethylindium. For the full monolayer sample the standing wave measurement shows that the indium atoms reside 1.577±0.014 A above the GaAs(0 0 4) substrate planes. A calculation based on the macroscopic elastic theory suggests that this corresponds to a single In x Ga 1− x As layer with x =0.794±0.068. The coherent fraction of 0.766±0.051 indicates a reasonably abrupt interface, as confirmed by the In-excitonic photoluminescence full width at half maximum of 5.74±0.01 meV. The half monolayer sample is less strained, as expected, with the indium atoms at 1.502±0.030 A above the substrate planes, corresponding to an In x Ga 1− x As layer with x =0.446±0.145, and a coherent fraction of 0.88±0.12. This study exemplifies the complimentary nature of XSW and XRD.

Extended x-ray absorption fine structure performed at the Ge K edge has found the Ge-Ge and Ge-Si bond lengths in a series of crystalline Ge{sub x}Si{sub 1{minus}x} alloys (x{le}0.5) to be compositionally dependent. This accurate measurement was made possible by utilizing the {ital experimentally} derived Ge-Si atomic phase shift from the isoelectronic compounds AlAs and GaP. Strain and Coulomb contributions to the bond lengths are also considered. {copyright} {ital 1998} {ital The American Physical Society}

Extended x-ray absorption fine structure measurements performed at $\mathrm{In}\ensuremath{-}K$ edge have resolved the outstanding issue of bond-length strain in semiconductor-alloy heterostructures. We determine theIn-As bond length to be $2.581\ifmmode\pm\else\textpm\fi{}0.004\AA{}$ in a buried, 213 \AA{} thick ${\mathrm{Ga}}_{0.78}{\mathrm{In}}_{0.22}\mathrm{As}$ layer grown coherently on GaAs(001). This bond length corresponds to a strain-induced contraction of $0.015\ifmmode\pm\else\textpm\fi{}0.004\AA{}$ relative to the In-As bond length in bulk ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{As}$ of the same composition; it is consistent with a simple model which assumes a uniform bond-length distortion in the epilayer despite the inequivalent In-As and Ga-As bond lengths.

Raman spectroscopy is used to study phonons in a series of thin (AlAs)m/(GaAs)n superlattices (SLs) grown by molecular beam epitaxy (MBE). The influence of buffer layer type on the interface roughness of heterostructures is carefully evaluated. The accuracy of optical phonons and the degree of peak sharpness of GaAs-like confined modes are examined via off-resonance Raman spectroscopy. Theoretical calculations of phonons in thin (AlAs)m/(GaAs)n superlattices (i.e. samples with m, n ≤ 12) are reported for various directions of propagation by using a rigid-ion model. Optical phonons acquire significant dispersive character when the wavevector q forming an angle θ with the growth axis of the superlattice is changed from θ = 0 to π/2, i.e. from [001] to [100]. The frequency gaps in the angular dispersions due to mode anti-crossing behavior observed recently by Zunke et al. using micro-Raman spectroscopy and studied by a continuum model are found to be in reasonably good agreement with our lattice dynamical model calculations.

The correlation between the structural and transport properties for a series of high‐quality modulation‐doped field‐effect‐transistor (MODFET) structures was made for various growth temperatures. X‐ray reflectivity, x‐ray diffraction, and magnetotransport measurements were used to assess structural quality and transport parameters. Four samples with growth temperatures in the range 500–630 °C were examined. The results show a correlation exists between the measured electron mobility and the quality of the interface width, as measured from satellite peaks of the buffer layer. In addition, these results show, for the first time to the best of our knowledge, that a direct correlation can be made between x‐ray reflectivity structural measurements and the measured electron mobility of high‐quality gallium–arsenide‐based MODFETs. Both x‐ray and transport results suggest a higher‐quality structure was obtained at higher growth temperatures.

Superlattices composed of III–V heterostructures have established applications in high‐speed electronic and optoelectronic devices. As layer thicknesses are reduced, the role of heterostructure interface sharpness becomes more critical to ensuring high quality two‐dimensional growth. In this work, short‐period (less than 1 nm) superlattices with active layer thicknesses of 31 nm were investigated to assess interface roughness in the initial stages of growth. X‐ray diffraction was used to evaluate interface roughness and to calculate superlattice periodicity. Results suggest that surface roughening by islanding may be promoted by GaAs buffer layers that are 10–100 nm thick. Smoother interfaces were obtained in samples with buffer layers 250 nm and greater.

Spectroscopic ellipsometry (SE) has been used to study the effects of interface roughness on the optical properties of ultrathin short‐period 3×3 GaAs/AlAs superlattices grown by molecular‐beam epitaxy (MBE). The complex dielectric function and thickness of the whole superlattice and the thickness of the native oxide overlayer were simultaneously determined by an inversion technique from data in the 1.5–5.0 eV region. The main optical critical points E0, E0+Δ0, E1, E1+Δ1, and E2 were deduced by line‐shape fitting of the second derivative of the complex dielectric function of the superlattice to the analytical line‐shape expression. The interface roughness is found to shift the optical transitions, except E2, to higher energy and broaden their line shapes. A simple interpretation of the shift and broadening is given. The interface roughness and layer thicknesses obtained by SE are found to be consistent with the results of x‐ray diffraction and Raman scattering studies previously reported. The results in t...

In this work the current versus voltage data of a p-n+ junction is converted into minority carrier lifetime data. Space charge recombination currents dominate at modest reverse bias at 80K and taking the dominant recombination centers to be located at the intrinsic Fermi level, the lowest minority carrier lifetime τ0 is determined to be 35ns. This single Shockley-Read-Hall carrier recombination parameter provides an excellent fit to the data over temperature range 40K≤ T≤ 130K; the 35ns minority carrier lifetime also explains the quantum efficiency data. The transition from diffusion to space charge currents occurs for temperatures, T ≤ 100K. For T≤ 40K trap assisted tunneling is the dominant current component. Based on imaging system requirements to be near background limited for photon flux ≈ 1015 ph/cm2-s and detector temperature of 80K, the minority carrier lifetime will need to be increased by one order of magnitude.

The use of molecular beam epitaxy to produce heterostructures has made it possible to better study superlattices with monolayer and submonolayer period spacings. In this work we examine the physical properties for the superlattice system (GaAs) n 1 (AlAs) n 2 /GaAs(001) for low values of n 1 and n 2 , i.e. n 1 = n 2 = 3,6,12. Normal, interrupted growth, and migration-enhanced epitaxy growth techniques were used to grow the superlattice structures and the X-ray diffraction spectra were obtained and the major and satellite peak positions analyzed. An analysis of the major diffraction peaks and their associated satellites exhibited 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 peaks indicates periodicities which are greater than the intended period. One possible cause for these periodicities is variations in growth conditions which occur while the superlattice is being grown. An understanding of low-order superlattices is important for structures which are dependent on interface sharpness.

The strain and relaxation of InAs and InGaAs films grown on GaAs(001) have been examined by the x‐ray standing wave and extended x‐ray absorption fine‐structure techniques. While 1 monolayer (ML) films of both InAs and InGaAs are found to be tetragonally distorted in accordance with the prediction of macroscopic‐elastic theory, thicker InAs films are found to collapse to their natural‐lattice constant past a critical thickness Tc, of ∼2 ML’s. By 8 ML’s, bond‐length strain is no longer evident, and a large degree of structural disorder is observed.

The x-ray standing wave (XSW) and extended x-ray absorption fine structure (EXAFS) techniques have determined the strain and bond distortions in an InAs monolayer embedded in GaAs(001). A unifying picture of bond lengths and elasticity in strained-layer semiconductors is presented.

X-ray standing-wave and extended x-ray-absorption fine-structure measurements have determined the strain and bond distortions in a buried InAs monolayer grown epitaxially on GaAs(001). The In atoms are found to reside 1.64{plus_minus}0.03 A above the last-As plane of the GaAs substrate with an In-As bond length of 2.57{plus_minus}0.02 A. Relative to bulk InAs, this corresponds to an 8% expansion in the In-As planar distance perpendicular to the interface and a 0.05-A compression in the In-As bond length. This experiment indicates that macroscopic-elastic theory describes the distortions in InAs/GaAs(001) films even in the monolayer limit.

Photoreflectance was used for the first time to study silicon films on sapphire (SOS). The film thicknesses ranged from 150 to 40 000 A. The 3.4 eV structure was monitored with bulk silicon as a standard. A shift of this structure toward lower energies was observed for the thinner films. As the film thickness increased from 1000 to 40000 A, the energy shift appeared to oscillate about the energy value associated with the bulk silicon sample. The energy shift with thickness of the photoreflectance structure is consistent with that calculated from the X-ray measured strain for the thicker films. The short optical penetration depth at 3.4 eV (≈ 100 A) is essential to allowing photoreflectance investigation of films of this order of thickness.

For heteroepitaxial systems, such as silicon on sapphire (SOS), microtwins occur in significant numbers and are thought to contribute to strain relief in the silicon thin film. The size of this contribution can be assessed from TEM measurements, of the differential volume fraction of microtwins, dV/dν (the derivative of the microtwin volume V with respect to the film volume ν), for SOS grown by both chemical vapor deposition (CVD) and molecular beam epitaxy (MBE).In a (001) silicon thin film subjected to compressive stress along the [100] axis , this stress can be relieved by four twinning systems: a/6[211]/( lll), a/6(21l]/(l1l), a/6[21l] /( l1l), and a/6(2ll)/(1ll).3 For the a/6[211]/(1ll) system, the glide of a single a/6[2ll] twinning partial dislocation draws the two halves of the crystal, separated by the microtwin, closer together by a/3.

The effect of an applied, time‐varying voltage on the plastic behavior of (Hg,Cd)Te has been investigated. We observe that both positive and negative voltages suppress plasticity, with the negative voltage being more effective. The influence of this voltage polarity is strain dependent and provides information on the charge of the mobile dislocations.