H. Ehsani

Independently accessed back-to-back HgCdTe photodiodes: a new dual-band infrared detector

We report the first data for a new two-color HgCdTe infrared detector for use in large dual-band infrared focal plane arrays (IRFPAs). Referred to as the independently accessed back-to-back photodiode structure, this novel dual-band HgCdTe detector provides independent electrical access to each of two spatially collocated back-to-back HgCdTe photodiodes so that true simultaneous and independent detection of medium wavelength (MW, 3–5 μm) and long wavelength (LW, 8–12 μm) infrared radiation can be accomplished. This new dual-band detector is directly compatible with standard backside-illuminated bump-interconnected hybrid HgCdTe IRFPA technology. It is capable of high fill factor, and allows high quantum efficiency and BLIP sensitivity to be realized in both the MW and LW photodiodes. We report data that demonstrate experimentally the key features of this new dual-band detector. These arrays have a unit cell size of 100 x 100 μm2, and were fabricated from a four-layer p-n-N-P HgCdTe film grown in situ by metalorganic chemical vapor deposition on a CdZnTe substrate. At 80K, the MW detector cutoff wavelength is 4.5 μm and the LW detector cutoff wavelength is 8.0 μm. Spectral crosstalk is less than 3%. Data confirm that the MW and LW photodiodes are electrically and radiometrically independent.

Ternary and quaternary antimonide devices for thermophotovoltaic applications

Thermophotovoltaic (TPV) devices have been fabricated using epitaxial ternary and quaternary layers grown on GaSb substrates. GaInSb ternary devices were grown by metalorganic vapor phase epitaxy (MOVPE) with buffer layers to accommodate the lattice mismatch, and GaInAsSb lattice-matched quaternaries were grown by MOVPE and by liquid phase epitaxy (LPE). Improved devices are obtained when optical absorption occurs in the p-layer due to the longer minority carrier diffusion length. Thick emitter p/n devices are limited by surface recombination, with highest quantum efficiency and lowest dark current being achieved with epitaxially grown surface passivation layers on lattice-matched MOVPE quaternaries. Thin emitter/thick base n/p devices are very promising, but require improved shallow high-quality n-type ohmic contacts.

Growth and characterization of In0.2Ga0.8Sb device structures using metalorganic vapor phase epitaxy

In0.2Ga0.8Sb epitaxial layers and thermophotovoltaic (TPV) device structures have been grown on GaSb and GaAs substrates by metalorganic vapor phase epitaxy (MOVPE). Control of the n-type doping up to 1×1018 cm−3 was achieved using diethyltellurium (DETe) as the dopant source. A Hall mobility of greater than 8000 cm2/Vs at 77K was obtained for a 3×1017 cm−3 doped In0.2Ga0.8Sb layer grown on high-resistivity GaSb substrate. The In0.2Ga0.8Sb epilayers directly grown on GaSb substrates were tilted with respect to the substrates, with the amount of tilt increasing with the layer thickness. Transmission electron microscopy (TEM) studies of the layers showed the presence of dislocation networks across the epilayers parallel to the interface at different distances from the interface, but the layers above this dislocation network were virtually free of dislocations. A strong correlation between epilayer tilt and TPV device properties was found, with layers having more tilt providing better devices. The results su...

The organometallic epitaxy of extrinsic p‐doped HgCdTe

Extrinsic p‐doped mercury cadmium telluride (MCT) layers have been grown by organometallic vapor phase epitaxy, using arsine in hydrogen as the dopant gas. Controllable doping in the range 3.5×1015–4.3×1016 was obtained when grown on GaAs with CdTe buffer layers. Consistently higher doping concentration (a factor of 2 to 4) was observed when a CdTe substrate was used. This is believed to be due to higher dislocation densities present when grown on GaAs, around which As may segregate. The cadmium fraction fell at very high flow; we believe that this is due to the prereaction of dimethylcadmium with arsine. Isothermal annealing under a Hg‐rich ambient of MCT grown on CdTe substrates did not produce significant changes in the measured doping concentration. This indicates that the acceptor level is extrinsic in nature and that arsenic behaves as a stable acceptor dopant in MCT. The activation energy of this acceptor was determined as a function of doping, and is about one‐half the value of the acceptor due to...

Improved CdTe layers on GaAs and Si using atomic layer epitaxy

In this paper, we report on the atomic layer epitaxy (ALE) of CdTe on GaAs and Si by the organometallic vapor phase epitaxial process at atmospheric pressure. Self-limiting growth at one monolayer was obtained over the temperature range from 250°C to 320°C, under a wide range of reactant pressure conditions. A study of growth mechanism indicates that DMCd decomposes into Cd on the surface and the Te precursors react catalytically on the Cd covered surface. We have used this ALE grown layer to improve the crystal quality and the morphology of conventionally grown CdTe on GaAs. Improvement in the crystal quality was also observed when ALE CdTe nucleation was carried out on Si pretreated with DETe at 420°C. Atomic layer epitaxy grown ZnTe was used to obtain (100) oriented CdTe on (100) silicon.

p-Type and n-type doping in GaSb and Ga0.8In0.2Sb layers grown by metalorganic vapor phase epitaxy

P-type and n-type GaSb and GA{sub 0.8}In{sub 0.2}Sb layers have been grown on GaSb and GaAs substrates by metalorganic vapor phase epitaxy (MOVPE) using silane and diethyltellurium (DETe) as the dopant precursors, respectively. Hall measurements show that the concentration and mobility of holes and electrons in GaSb and GA{sub 0.8}In{sub 0.2}Sb are higher when the layers are grown on GaSb substrates than when grown on GaAs substrates. Secondary ion mass spectrometry (SIMS) results show that the incorporation of Si and Te is higher when GaSb substrates are used. The electron concentration increased from 5 {times} 10{sup 16} cm{sup {minus}3} to 1.5 {times} 10{sup 18} cm{sup {minus}3} as the Te concentration was increased from 1 {times} 10{sup 17} cm{sup {minus}3} to 5 {times} 10{sup 18} cm{sup {minus}3}. As the Te concentration was increased further, the electron concentration decreased, with only about 1% of the Te electrically active at a Te concentration of 2 {times} 10{sup 20} cm{sup {minus}3}.

Low‐temperature growth of HgTe and HgCdTe using methylallyltelluride

HgTe and HgCdTe (MCT) layers have been grown by organometallic vapor phase epitaxy at low temperature by using methylallyltelluride (MATe), dimethylcadmium (DMCd), and elemental mercury. Use of MATe enabled the growth of layers in the 250–320 °C temperature range, which is 50 °C lower than the growth temperature when diisopropyltelluride is used as the tellurium alkyl, for the same growth rate. The layers were characterized by optical microscopy, double crystal x‐ray diffraction, and Fourier transform infrared spectroscopy. Growth at 320 °C resulted in featureless surfaces for both HgTe and HgCdTe layers. The high quality of HgTe layers grown at 320 °C is demonstrated by the very narrow full width at half maximum of x‐ray diffraction (29 arcsec), which is comparable to that of the CdTeZn substrates used in this study. MCT layers grown at 320 °C showed sharp interference fringes even for very thin layers, indicating the presence of a very sharp interface with the substrate.

The growth and characterization of HgTe and HgCdTe using methylalylltelluride

HgTe and HgCdTe layers have been grown by organometallic vapor phase epitaxy at low temperature by using methylallyltelluride (MATe), dimethylcadmium (DMCd) and elemental mercury. Use of MATe enabled the growth of layers in the 250–320 °C range, which is 50 °C lower than the growth temperature when diisopropyltelluride (DIPTe) is used, for the same growth rate. The layers were characterized by double crystal x‐ray diffraction, Fourier‐transform infrared (FTIR) spectroscopy, and by low temperature Hall measurements. Growth below 340 °C resulted in featureless HgTe layers. HgTe layers, grown on CdTe, are misoriented with respect to the substrate by about 60–150 arc s. This tilting was not observed when CdZnTe substrates, to which there is a better lattice match, were used. The high quality of low temperature grown HgTe is demonstrated by the very narrow (27 arc s) full width at half‐maximum (FWHM) of x‐ray diffraction for layers grown on CdZnTe substrates. HgCdTe layers, grown at 320 °C, showed well resolve...

Atomic Layer Epitaxy of CdTe on GaAs by Organometallic Vapor Phase Epitaxy

Abstract In this paper, we report on the atomic layer epitaxy (ALE) of CdTe on GaAs by the organometallic vapor phase epitaxial process. Self-limiting growth at one monolayer was obtained over the temperature range from 250°C to 320°C, under a wide range of reactant pressure conditions. A study of growth mechanism indicates that the decomposition of DMCd into Cd is the rate limiting step at and below 250°C, and the Te precursors decompose catalytically on the Cd covered surface. We have used this ALE grown layer to improve the properties of conventionally grown CdTe on GaAs. A significant improvement in the morphology and crystal quality was observed when the initial CdTe nucleation step was carried out using the ALE method, prior to the deposition of CdTe by the conventional method.

High quality planar HgCdTe photodiodes fabricated by the organometallic epitaxy (Direct Alloy Growth Process)

Hg1−xCdxTe, grown by the alloy organometallic vapor phase epitaxy technique, was used in the fabrication of p‐n junction photodiodes. Hg1−xCdxTe layers, capped with a CdTe cap, were grown in a continuous run by the direct alloy growth process. These layers were p type due to column II vacancies, with a concentration of 3–4×1016/cm3. n‐type regions were obtained by selectively annealing the Hg1−xCdxTe layer after opening windows in the CdTe cap layer. Vertical p‐n junction diodes, with CdTe as the junction passivant, were thus formed in a planar configuration. Photodiodes, with cutoff wavelengths of 4.5 μm at 77 K, had R0 A products in excess of 9×107 Ω cm2.