D. D. Edwall

GaAs charge‐coupled devices

A Schottky‐barrier‐gate buried‐channel GaAs CCD has been successfully demonstrated. A 10‐cell (30 gates) three‐phase device was operated at room temperature. The device employs a natural channel stop formed by the transfer gates extending from an n‐type active region onto a semi‐insulating GaAs substrate.A Schottky‐barrier‐gate buried‐channel GaAs CCD has been successfully demonstrated. A 10‐cell (30 gates) three‐phase device was operated at room temperature. The device employs a natural channel stop formed by the transfer gates extending from an n‐type active region onto a semi‐insulating GaAs substrate.

High‐efficiency AlGaAs/GaAs concentrator solar cells

Efficiencies of 25% have been obtained with 1‐cm‐diam AlGaAs/GaAs heteroface concentrator solar cells utilizing an ultrathin AlGaAs window layer design. A low specific resistance (<0.005 Ω cm2) Ohmic contact is achieved by direct contact to the p‐GaAs active layer. Liquid phase epitaxy has been developed to grow <500‐A thick window layers on large‐area (3.3×3.3 cm) GaAs substrates. Four 1‐cm‐diam cells are produced from each wafer and demonstrate the potential for larger‐scale production.

Material characteristics of Hg1−xCdx > Te grown by organometallic vapor phase epitaxy

Abstract We compare the material properties of Hg 1- x Cd x Te grown by conventioanal organometallic vapor phase epitaxy (OMVPE) with those obtained interdiffused multilayer OMVPE. Both are compared with material properties of state-of-the-art LPE Hg 1- x Cd x Te grown from Te-rich solutions. The Hg 1- x Cd x Te layers were grown on bulk CdTe, sapphire (Al 2 O 3 ) and GaAs substrates. Layers were characterized by optical microscopy, IR transmission, Hall effect, double crystal X-ray diffraction, electron microprobe and secondary ion mass spectroscopy (SIMS). Better crystallinity and smoother morphologies are obtained by conventional OMVPE; interdiffused multilayer OMVPE currently results in better compositional uniformity and reproducibility. The observed differences can be interpreted on the basis of growth kinetics. Both techniques yield Hg 1- x Cd x Te epitaxial layers with LPE-like qualities.

p on n ion‐implanted junctions in liquid phase epitaxy HgCdTe layers on CdTe substrates

The first demonstrated achievement of p on n‐type activated junctions in HgCdTe material by arsenic ion implantation is reported. The junctions were formed by treating the implant as a finite diffusion source in the post‐implant anneals. The materials employed for this study were n‐type indium impurity‐doped liquid phase epitaxy HgCdTe grown on CdTe. Arsenic was selected as the candidate acceptor impurity since it activated during post‐implanted anneals in Hg vapor. The arsenic concentration profile determined by secondary ion mass spectroscopy showed that during post‐implant anneal a complex diffusion mechanism redistributes the arsenic. The activation efficiency appears to be mechanism dependent. In the junction region the implant activation efficiency is about 50%. Junction depth can be controlled by varying arsenic diffusion and background carrier concentration. Junction depths determined by the electron beam induced current technique were consistent with the differential Hall electrical profiles. A p...

p‐type doping of metalorganic chemical vapor deposition‐grown HgCdTe by arsenic and antimony

Extensive data are presented on the arsenic doping characteristics of Hg1−xCdxTe layers grown by metalorganic chemical vapor deposition using the dopant source tertiarybutylarsine (TBAs). The incorporation of arsenic in the layer is proportional to the II–VI alkyl ratio during TBAs injection. High arsenic concentrations significantly increase the layer dislocation density. Acceptor activation efficiency (taken to be the 77 K carrier concentration divided by the layer arsenic concentration) is ∼50% for a wide range of conditions, but decreases for arsenic concentrations approaching 1018 cm−3. For arsenic concentrations below low‐1017 cm−3, the 77 K carrier concentration is stable even for sample annealing at 400 °C on the Te‐rich side of the phase boundary. 77 K hole mobility strongly depends on composition x. Variable temperature Hall effect measurements show that one layer exhibits the expected carrier freeze‐out with an ionization energy of 4.8 meV, but three other layers exhibit a much smaller degree o...

High performance photovoltaic infrared devices in Hg1−xCdxTe on sapphire

A combination of organometallic and isothermal vapor phase epitaxy was used sequentially to grow CdTe and Hg1−xCdxTe on GaAs substrates. Photodiodes in the Hg1−xCdxTe show properties comparable to the best Hg1−xCdxTe grown by liquid phase epitaxy. Resistance‐area products were ≥107 Ω cm2 and >104 Ω cm2 at 77 K for Hg1−xCdxTe with cut‐off wavelength of 3.73 and 5.54 μm at 77 K, respectively. The backside‐illuminated spectral response was broadband with peak external quantum efficiencies typically >50% (without antireflection coating).

Gamma ray detectors with HgCdTe contact layers

A new device structure for room‐temperature gamma ray spectrometry has been developed and demonstrated. The device is a heterojunction p‐i‐n, HgCdTe/CdTe/HgCdTe structure. The p layer is Au doped with NA=1.9×1016 cm−3, μp=35 cm2/Vs, and a Cd composition xCd=0.6. The n layer is In doped with ND=2×1017 cm−3, μn=1880 cm2/Vs, and xCd=0.31. Ohmic contacts were achieved using electron beam evaporated Au (p layer contact) and In (n‐layer contact). Devices (with approximately 2‐mm2 area, 2‐mm thickness) exhibited reverse leakage currents of 50 pA–4 nA, good photovoltaic response with visible light, and voltage breakdowns in excess of 1000 V. At 250‐V reverse bias, the energy resolutions at the principal photopeaks of Am‐241 (60 keV) and Co‐57 (122 keV) were 12.5% and 8.4%, respectively.

Arsenic doping in metalorganic chemical vapor deposition Hg1−xCdxTe using tertiarybutylarsine and diethylarsine

P‐type arsenic doped epitaxial layers of HgCdTe have been grown by metalorganic chemical vapor deposition using two alkyl sources, tertiarybutylarsine and diethylarsine. Data are presented on Hall characteristics and arsenic concentration profiles. High activation efficiencies and hole mobilities have been obtained over the range mid‐1015 to low‐1017 cm−3.

Variable temperature Hall effect on p-Hg1−XCdXTe grown on CdTe and sapphire substrates by liquid phase epitaxy

Variable temperature Hall effect measurements have been made down to 9–10K on p-type Hg1−xCdxTe grown by liquid phase epitaxy on both CdTe and sapphire substrates. Carrier freeze-out was usually observed throughout the measured temperature range. For most samples, the hole mobility was well-behaved and exhibited a maximum at ˜ 35K. Values of acceptor ionization energy EA and donor concentration ND were estimated from the data, using a model assuming significant compensation, which provided a good fit to the low temperature data. In addition, values of ND were also estimated from an analysis of the low temperature mobility using the hole effective mass as a parameter to provide reasonable agreement between the ND values calculated from the Hall coefficient and mobility data. The measured carrier concentration is a result of close compensation between stoichiometric acceptors and donors, with ND usually in the low-1017 cm−3 range. Average values of EA for as-grown, undoped x = 0.32 layers on CdTe and sapphire substrates are 7.4 and 6.6 meV, respectively. An activation energy of 0.84 meV was determined for a Cu-doped x = 0.32 layer that was annealed in Hg vapor to reduce the number of Hg vacancies. The average EA for undoped Hg-annealed x = 0.22 layers on CdTe substrates is 2.35 meV. Layers with x = 0.32 grown on sapphire substrates have average carrier concentrations of 2.92 (σ = 0.54) × 1016 cm−3, compared with 4.64 (θ = 1.26) × 1016 cm−3 for the same composition layers grown on CdTe substrates.

Study of microinhomogeneities in midwave infrared mercury cadmium telluride grown by metalorganic chemical vapor deposition‐interdiffused multilayer process onto GaAs and GaAs/Si substrates

The causes for variations in quantum efficiency (QE) for boron‐implanted midwave infrared diodes formed in mercury cadmium telluride on CdTe buffered GaAs/Si substrates have been investigated. Smooth layers have QE≳30% with many of the diodes in the 50%–60% range. Layers having a faceted morphology produce diodes with QE<30%. The faceted layers have been shown by infrared absorption measurements and secondary ion mass spectrometry depth profiles to possess microinhomogeneities in composition. Carrying out comparative growths on GaAs substrates have shown that these microinhomogeneities are not due to the growth process or fundamentally due to the interdiffused multilayer process used to grow the alloy layers. A study of the origins of the faceting behavior on GaAs/Si substrates, using in situ laser reflectance, has shown that they form from three‐dimensional island growth, nucleating on the substrate and growing outwards to dominate the growth mode after 2 000 A of buffer growth.