Renjie Gu

MBE Growth of Mid-wave Infrared HgCdTe Layers on GaSb Alternative Substrates

GaSb has been studied as a new alternative substrate for growing HgCdTe via molecular beam epitaxy (MBE). Cross-sectional transmission electron microscopy (TEM) studies indicate that MBE-grown CdTe buffer layers on GaSb have much lower misfit dislocation density than comparable layers grown on GaAs. The MBE-grown mid-wave infrared (MWIR) HgCdTe layers on GaSb substrates present material quality comparable to those grown on GaAs substrates, which is one of the state-of-the-art alternative substrates currently used to grow HgCdTe for the fabrication of MWIR detectors and focal plane arrays. Typically, HgCdTe materials grown on GaSb are found to have a rocking curve (double crystal x-ray diffraction) full width at half maximum of ~122 arcsec and an etch pit density of ~mid-106 cm–2. Electron backscatter diffraction mapping shows that the lattice misorientation/misfit dislocations near the HgCdTe/CdTe interface are negligible for GaSb substrates in comparison to GaAs substrates, and that the material quality of the HgCdTe layer on GaSb is determined primarily by the material quality of the CdTe buffer layer. These preliminary results are very encouraging considering that this is a relatively recent research effort, and higher quality MBE-grown HgCdTe materials are expected on GaSb substrates with further optimization of HgCdTe growth conditions as well as further improvements in the growth conditions for CdTe buffer layers.

Recent progress in MBE grown HgCdTe materials and devices at UWA

HgCdTe has dominated the high performance end of the IR detector market for decades. At present, the fabrication costs of HgCdTe based advanced infrared devices is relatively high, due to the low yield associated with lattice matched CdZnTe substrates and a complicated cooling system. One approach to ease this problem is to use a cost effective alternative substrate, such as Si or GaAs. Recently, GaSb has emerged as a new alternative with better lattice matching. In addition, implementation of MBE-grown unipolar n-type/barrier/n-type detector structures in the HgCdTe material system has been recently proposed and studied intensively to enhance the detector operating temperature. The unipolar nBn photodetector structure can be used to substantially reduce dark current and noise without impeding photocurrent flow. In this paper, recent progress in MBE growth of HgCdTe infrared material at the University of Western Australia (UWA) is reported, including MBE growth of HgCdTe on GaSb alternative substrates and growth of HgCdTe nBn structures.

Minority carrier lifetime in iodine-doped molecular beam epitaxy-grown HgCdTe

The minority carrier lifetime in molecular beam epitaxy grown layers of iodine-doped Hg1−xCdxTe (x ∼ 0.3) on CdZnTe substrates has been studied. The samples demonstrated extrinsic donor behavior for carrier concentrations in the range from 2 × 1016 cm−3 to 6 × 1017 cm−3 without any post-growth annealing. At a temperature of 77 K, the electron mobility was found to vary from 104 cm2/V s to 7 × 103 cm2/V s and minority carrier lifetime from 1.6 μs to 790 ns, respectively, as the carrier concentration was increased from 2 × 1016 cm−3 to 6 × 1017 cm−3. The diffusion of iodine is much lower than that of indium and hence a better alternative in heterostructures such as nBn devices. The influence of carrier concentration and temperature on the minority carrier lifetime was studied in order to characterize the carrier recombination mechanisms. Measured lifetimes were also analyzed and compared with the theoretical models of the various recombination processes occurring in these materials, indicating that Auger-1 ...

Investigation of Substrate Effects on Interface Strain and Defect Generation in MBE-Grown HgCdTe

Si, Ge, and GaAs have been extensively investigated as alternative substrates for molecular-beam epitaxy (MBE) growth of HgCdTe and, at present, are widely used for HgCdTe-based infrared focal-plane arrays. However, the problem of high dislocation density in HgCdTe layers grown on these lattice-mismatched substrates has yet to be resolved. In this work, we investigated another alternative substrate, GaSb, which has a significantly smaller lattice mismatch with HgCdTe in comparison with Si, Ge, and GaAs, and is readily available as large-area, epiready wafers at much lower cost in comparison with lattice-matched CdZnTe substrates. The resultant stress due to lattice and thermal mismatch between the HgCdTe epilayer and various substrates has been calculated in this work using the elasticity matrix, and the corresponding stress distribution simulated using ANSYS. The simulated structures were matched by experimental samples involving MBE growth of HgCdTe on GaAs, GaSb, and CdZnTe substrates, and were characterized via reflection high-energy electron diffraction and x-ray diffraction analysis, followed by etch pit density (EPD) analysis. In comparison with other alternative substrates, GaSb is shown to have lower interface stress and lower EPD, rendering it an interesting and promising alternative substrate material for HgCdTe epitaxy.

Superlattice Barrier HgCdTe nBn Infrared Photodetectors: Validation of the Effective Mass Approximation

Implementation of the unipolar barrier detector concept in HgCdTe-based compound semiconductor alloys is a challenging problem, primarily because practical lattice-matched materials that can be employed as the wide bandgap barrier layer in HgCdTe nBn structures present a significant valence band offset at the n-type/barrier interface, thus impeding the free flow of photogenerated minority carriers. However, it is possible to minimize the valence band offset by replacing the bulk HgCdTe alloy-based barrier with a CdTe-HgTe superlattice barrier structure. In this paper, an 8 × 8 k.p Hamiltonian combined with the nonequilibrium Greens function formalism has been employed to numerically demonstrate that the single-band effective mass approximation is an adequate numerical approach, which is valid for the modeling, design, and optimization of band alignment and carrier transport in HgCdTe-based nBn detectors incorporating a wide bandgap superlattice barrier.

Structural and electrical properties of Iodine doped Hg 1−x Cd x Te films grown by MBE

Iodine (I) doping in mercury cadmium telluride (Hg_1-xCd_xTe) grown by molecular beam epitaxy (MBE) on CdZnTe substrates with cadmium-iodide (CdI₂) as the dopant source was investigated. I doping concentration in the samples was controlled by CdI₂ source temperature that varied in 110°C-150°C range. Depending upon I doping concentration, the electrical conductivity at 77K for as grown films varied in the 3×10³ Ω^-1m^-1 - 6×10⁴ Ω^-1 m^-1 range and was about of six to ten orders of magnitude higher than those of non-doped HgCdTe films. The Hall coefficient showed classical n-type extrinsic behavior. The electron mobility for lower doping level was observed to be as high as that in an indium-doped material reported in literature [1]. The x-ray diffraction (XRD) studies revealed that there was no prominent change in crystal structure with increasing doping concentration. However, atomic force microscopy (AFM) measurements showed that dislocation densities and consequently defect concentration and size increased with increasing doping concentration.

Characterisation of SiN x -HgCdTe interface in metal-insulator-semiconductor structure

In this paper, we report results of a study of SiN_x films deposited on HgCdTe epitaxial layers. Hydrogenated amorphous SiN_x films were deposited by inductively-coupled plasma-enhanced chemical vapour deposition at relatively low substrate temperatures (80°C-100°C). The capacitance-voltage characteristics of SiN_x/n-Hg_0.68Cd_0.32Te metal-insulator-semiconductor structures indicated that Si-rich SiN_x films deposited at 100°C can be employed as electrical passivation layers.

Strain simulation and MBE growth of CdTe on GaSb substrates

In this paper, we present a study on ANSYS strain analysis and MBE growth of CdTe epilayers on GaSb substrates. The ANSYS simulation result shows that GaSb performs much better than other alternative substrates, e.g. GaAs. To verify the simulation results, CdTe buffer layers were grown by MBE on GaSb, which show material quality comparable to or slightly better than that on GaAs substrate. Furthermore, TEM study on the CdTe layers grown on GaSb indicates that fewer dislocations are generated around the interface between CdTe and GaSb, which accords with the simulation results. This preliminary study shows the great potential of GaSb to be next generation alternative substrates for growing high quality HgCdTe infrared materials.