K.K. Parat

Indium doping of n-type HgCdTe layers grown by organometallic vapor phase epitaxy

n‐type doping of mercury cadmium telluride was achieved using trimethylindium as the dopant source. The layers, grown by the alloy growth technique, were doped to ∼5×1018 cm−3. The donor concentration in these layers was found to exhibit a linear dependence on the dopant partial pressure over the carrier concentration range from 5×1016 to 3×1018 cm−3. Reasonably high electron mobility values were observed in these indium‐doped layers. Typically, layers with a Cd fraction x=0.23, doped to 3.5×1016 cm−3, exhibited a mobility value of 7.5×104 cm2/V s at 40 K. High electron mobility values, measured over the entire doping regime, suggest a high electrical activity of indium in these layers. The optically measured band edge in these indium‐doped layers was observed to shift to higher energy with increasing doping. The band‐edge energy values measured in 1×1017 and 3×1018 cm−3 doped layers correspond to x=0.23 and x=0.3, respectively. This increase can be due to an increase in the Cd fraction, or to a Burstein–...

The mercury pressure dependence of arsenic doping in HgCdTe, grown by organometallic epitaxy (direct alloy growth process)

Abstract The effect of Hg partial pressure on arsenic doping of HgCdTe is studied. It is found that control of Hg partial pressure is very important in obtaining reproducible doping, and use of high Hg pressure is the key to obtain heavily doped layers. Typically, a factor of 4 increase in the partial pressure of Hg is found to increase the acceptor concentration by about this same magnitude. In addition, arsine doping results in almost uncompensated layers, even though high concentration of Hg vacancies are present. A mechanism is proposed by which As is incorporated as a Cd-As complex, so that it substitutes preferentially on Te sites.

Solar cells in bulk InP, made by an open tube diffusion process

Abstract This paper describes a simple open tube diffusion process for the fabrication of n+-p solar cells in indium phosphide. Large area cells (>0.25 cm2) have been made by this process, with a photovoltaic conversion efficiency of 16.3% under simulated AM1 conditions of illumination. The external quantum efficiency of these cells has been measured, and is approximately 68% without an antireflective (AR) coating, and 100% with a 750 A coating of silicon monoxide. An ideality factor of 1.1 and a saturation current density of 8×10−15 A/cm2 have been observed for these cells. These are the lowest (best) values reported to date for diffused structures in bulk InP.

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.

Extrinsic p‐type doping of HgCdTe grown by organometallic epitaxy

In this letter, we report on the extrinsic p‐type doping of mercury cadmium telluride (MCT), during growth by organometallic vapor phase epitaxy. Arsine gas in hydrogen was used as the dopant source. The layers were characterized by Hall effect and by resistivity measurements over the temperature range from 20 to 300 K. The acceptor ionization energy was obtained for different doping concentrations from the Hall coefficient data. Its value decreases with doping concentration, and is about 8.5 meV for a doping of 1.1×1016 cm−3. This is a factor of 2 lower than the ionization energy of mercury vacancies, for layers of this (27–30% cadmium) composition and doping level. Annealing at 205 °C for 15 h in a Hg‐rich ambient 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, grown by organometallic epitaxy.

Annealing and electrical properties of Hg1-xCdxTe grown by OMVPE

The annealing behavior of Hg1−xCdxTe layers, grown by the conventional organometallic vapor phase epitaxy (OMVPE), is reported. Some of the as-grown layers, which are p-type with a concentration around 4 × 1016 cm-3 of Group II vacancies, become light p-type with carrier concentrations around 1×1015 cm-3 after Hg saturated annealings at temperatures in the range of 200–230°C. These conditions are typically expected to result in complete annealing and n-type conversion of the layer. These layers can be converted to n-type with a carrier concentration of approximately 5×1014 cm-3 by a higher temperature anneal at 290°C, followed by a low temperature anneal at 220°C. Hall effect measurements were made under variable temperature as well as variable magnetic field conditions. Bulk carrier concentrations and mobilities were evaluated by considering the effect of the surface inversion/accumulation layer on the Hall data. It is proposed that p-type conduction in the partially annealed layers is due to the persistence of Group II vacancies in the Hg1−xCdxTe layers, which are not completely annihilated during the low temperature anneal. Conversion to n-type is probably due to residual donor impurities in the as grown Hg1−xCdxTe layer.

The influence of accumulation on the hall-effect in n-type Hg1−xCdxTe

Abstract The presence of an electron accumulation layer on the surface of n-type Hg1−xCdxTe causes the measured Hall mobility and carrier concentration to be significantly different from the actual bulk values. This discrepancy is not readily apparent in the temperature dependence of the Hall coefficient, RH, as is the case with p-type layers. However, it is observed in the magnetic-field dependence of the RH. The B-field dependence of RH was analyzed to extract the actual concentration and mobility of the bulk and surface carriers in Hg1−xCdxTe layers grown by organometallic vapor phase epitaxy. The bulk parameters thus calculated were verified by passivating the surface of these layers using an anodic sulfide to reduce the concentration of surface electrons.

Extrinsic p‐doped HgCdTe grown by direct alloy growth organometallic epitaxy

This paper describes the doping behavior of arsenic in HgCdTe, grown by organometallic epitaxy using the direct alloy growth process. It is shown that arsenic readily incorporates into HgCdTe during this growth process, to a doping concentration of 1×1017 cm−3. Secondary‐ion mass spectroscopy (SIMS) data clearly establishes the presence of arsenic in these layers. Moreover, excellent stability under annealing conditions (16 h at 270 °C followed by 10 h at 220 °C) indicates the suitability of arsenic as an extrinsic dopant source for HgCdTe. The net acceptor concentration is shown to be linearly proportional to the Hg overpressure. This fact, combined with mobility values which are comparable to those of bulk grown material, indicates that these layers are relatively uncompensated. High mobility values are preserved well into the saturated region of the acceptor concentration versus arsine flow rate characteristic. This is explained by the fact that the doping concentration is limited by surface coverage o...

Shallow n+ diffusion into InP by an open-tube diffusion technique

Very shallow n+ layers have been obtained in InP by using gallium sulfide as a source for sulfur diffusion, and chemically vapor deposited SiO2 as a cap. Diffusions were carried out from 585 to 725 °C in an open‐tube system with a nitrogen ambient. The doping profile of sulfur in InP is estimated to be of the complementary error function type with a surface concentration of 5.6×1018/cc and a diffusion constant of 1.1×10−14 cm2/s at 670 °C. Diodes made on n+‐p junctions obtained by this diffusion technique show ideality factors close to unity and saturation current densities as low as 3.4×10−15 A/cm2, signifying the presence of a defect‐free junction. These diffusions, with junction depths in the 400–700 A range, are ideal for solar cell applications.

Sulphur diffusion into InP by an open tube process

Abstract A technique is described for the diffusion of sulphur into InP by an open tube process, which gives highly reproducible results from run to run, as required for device applications. A vacuum-deposited layer of gallium sulphide (Ga2S3) was used as the source for sulphur diffusion, with a chemically vapor deposited SiO2 cap layer for preventing decomposition of the InP surface during heat treatment. Diffusions were carried out at temperatures ranging from 585°C to 708°C. Diffused layers were characterized by the surface carrier concentration and the diffusion constant. From these measurements the diffusion profile for sulphur in InP is estimated to be of the complementary error function type. The activation energy of the diffusion was estimated to be 1.94 eV. The technique described here is ideally suited for the fabrication of shallow n+-p junctions in InP, and has been used successfully for space-borne solar cells.