In this paper, we study the limiting mechanisms and design criteria of HgCdTe photodetectors for extended shortwave infrared applications with ultra-high quantum efficiency (QE) in both
n
-on-
p
and
...p
-on-
n
technologies. Numerical and analytical models are employed in order to study the possibility of achieving ultra-high QE eSWIR detectors for the operational wavelengths of approximately 2.0 μm, and our study shows that by proper design of absorber layer and doping density, such a detector can be engineered. Furthermore, we demonstrate that the Shockley–Read–Hall (SRH) lifetime, absorber layer doping density and absorber layer thickness all have an impact on the quantum efficiency whether the detector is used as a small-area pixel element in a focal plane array or as a discrete large-area detector for sensing applications.
In this work, GaSb is proposed as a new alternative substrate for the growth of HgCdTe via molecular beam epitaxy (MBE). Due to the smaller mismatch in both lattice constant and coefficient of ...thermal expansion between GaSb and HgCdTe, GaSb presents a better alternative substrate for the epitaxial growth of HgCdTe, in comparison to alternative substrates such as Si, Ge, and GaAs. In our recent efforts, a CdTe buffer layer technology has been developed on GaSb substrates via MBE. By optimizing the growth conditions (mainly growth temperature and VI/II flux ratio), CdTe buffer layers have been grown on GaSb substrates with material quality comparable to, and slightly better than, CdTe buffer layers grown on GaAs substrates, which is one of the state-of-the-art alternative substrates used in growing HgCdTe for the fabrication of mid-wave infrared detectors. The results presented in this paper indicate the great potential of GaSb to become the next generation alternative substrate for HgCdTe infrared detectors, demonstrating MBE-grown CdTe buffer layers with rocking curve (double crystal x-ray diffraction) full width at half maximum of ∼60 arcsec and etch pit density of ∼10
6
cm
−2
.
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-10
6
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.
Demand for high-performance HgCdTe infrared detectors with larger array size and lower cost has fuelled the heteroepitaxial growth of HgCdTe on CdTe buffer layers on lattice-mismatched alternative ...substrates such as Si, Ge, GaAs and GaSb. However, the resulting high threading dislocation (TD) density in HgCdTe/CdTe limits their ultimate application. Herein, strained CdZnTe/CdTe superlattice layers have been used as dislocation filtering layers (DFL) to reduce the TDs in CdTe buffer layers grown on GaAs (211)B substrates (14.4% lattice-mismatch) by molecular beam epitaxy (MBE). Cross-sectional microstructure characterization indicates that the DFLs suppress the propagation of TDs. For optimal Zn content combined with thermal annealing, the DFLs effectively reduce the defect density of the upper-most CdTe layer from low-10
7
cm
−2
to the critical level of below 10
6
cm
−2
. In comparison to conventional buffer CdTe layers, the in-plane lattice of the CdTe layers in/near the DFL region is compressively strained, leading to a spread in x-ray double-crystal rocking curve full-width at half-maximum values but better in-plane lattice-matching with HgCdTe. The combined advantages of lower dislocation density and better lattice-matching with HgCdTe indicate that the DFL approach is a promising path towards achieving heteroepitaxy of high-quality HgCdTe on large-area lattice-mismatched substrates for fabricating next-generation infrared detectors.
HgCdTe has dominated the high performance end of the IR detector market for decades. At present, the cost to fabricate HgCdTe based advanced infrared devices is relatively high. One approach to ...address this problem is to use cost effective alternative substrate, mainly Si and GaAs. Recently, GaSb has emerged as a new alternative with better lattice matching. In this paper, recent progress in molecular beam epitaxial (MBE) growth of HgCdTe infrared material at UWA is reported. HgCdTe has been grown on GaSb substrates by MBE, and has shown a lower Etch Pit Density (EPD) and higher minority carrier lifetime in comparison to other alternative substrates. This result makes GaSb an interesting and promising alternative substrate material for HgCdTe epitaxy.
•HgCdTe material is epitaxially grown by MBE on GaSb substrate.•Growth technologies for HgCdTe on GaSb are developed.•In comparison to other alternative substrates, higher HgCdTe material quality is observed.
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.
In this paper, a systematic study of interdiffusion in (112)B oriented HgTe/CdTe superlattice (SL) structures has been undertaken in order to investigate the viability of employing SL as the absorber ...layer for very long wavelength infrared (VLWIR, 15
μ
m and longer) applications in imaging focal plane arrays. Using numerical analysis, the optimal superlattice parameters and annealing time at the growth temperature (∼ 180°C) are found, which result in better control of the cut-off wavelength in superlattice absorbers compared to the corresponding HgCdTe alloy absorber. Simulations show that, by appropriate adjustment of annealing time, it is possible to achieve the same cut-off wavelength with a larger HgTe well width while the sensitivity of the SL to well width variations remains at 15 meV/nm. Furthermore, the electron effective mass in a SL absorber is larger than a HgCdTe alloy absorber, which results in lower tunneling dark current. This work focuses on optimization of the superlattice absorber using the stationary Schrödinger equation. A complete photodetector device design based on a SL absorber structure will require a comprehensive numerical modeling using a Schrödinger–Poisson solver and drift–diffusion solver, or a combination of both approaches, which will be undertaken in the future.
CdTe epitaxial layers were grown by MBE on GaSb (211)B substrates from two different suppliers in order to determine the influence of as-received substrate quality on the material quality of ...subsequently-grown CdTe epilayers. It is observed that GaSb substrates with smooth surface and lower degree of surface roughness can lead to CdTe epilayers with superior material quality, as evidenced by a lower dislocation density, lower etch pit density, lower degree of epilayer surface roughness, narrower XRD FWHM, and lower strain. It is concluded that in comparison to substrates with a relatively smooth surface (RMS surface roughness ∼ 0.62 nm), substrates with a high degree of surface roughness (RMS surface roughness ∼ 2.24 nm) lead to effects that generate misfit dislocations during the MBE growth process. This results in approximately an order of magnitude increase in measured etch pit density (from ∼9 × 105 to 7 × 106 cm−2) and the calculated dislocation density (from 6.51 × 105 to 6.87 × 106 cm−2, as determined from X-ray diffraction reciprocal space mapping), indicating that a smooth substrate surface is critical in achieving high quality CdTe epilayers on GaSb.
•Quantitative structural analysis of CdTe epilayers grown on a novel alternative substrate GaSb.•Quality of epitaxial materials, grown on substrates obtained from different manufacturers, is analyzed and compared.•X-ray reciprocal space mapping proved the correlation of material quality with substrate quality.•Quality of substrate is critical to achieve high quality single crystal epilayers.
Design of practically realizable unipolar HgCdTe nBn photodetectors has been studied in detail by numerical analysis. The simulations reported herein reveal that, by optimization of barrier doping, ...dark current levels can be reduced and collection efficiency substantially improved. It is shown that
p
-type doping of the barrier layer can significantly reduce the effective potential barrier arising from the valence band offset between the absorber and barrier regions, thus enabling HgCdTe nBn detector operation under near zero-bias conditions. However, relatively high electric fields in the space charge regions near the barrier/absorber interface result in enhanced trap-assisted Shockley–Read–Hall thermal generation. Our calculations indicate that nBn HgCdTe detectors with barriers engineered by use of HgTe/Hg
0.05
Cd
0.95
Te superlattices have, potentially, substantially better valence band alignment without the need for
p
-type doping.
In this paper the recently developed high-resolution mobility spectrum analysis is demonstrated. In a number of simulations the high resolution of the algorithm is demonstrated in the high and low ...mobility ranges. The effect of random noise, maximum magnetic field limit, and the number of magnetic field points used in the experiment is also demonstrated. Also discussed are requirements critical for obtaining high-quality experimental data. The application of this new algorithm to complex semiconductor structures to study lateral and vertical transport is also demonstrated, resulting in insight into previously unavailable details.
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