The temperature gradient at the growth interface is as important as the growth temperature and the growth rate for growing CdTe single crystals by the Traveling Heater Method (THM). This article presents the results of an experimental study of the influence of the growth temperature gradient on THM growth of CdTe single crystals. CdTe crystals were grown at about 900°C with the growth rate of 10 mm/day and the rotation rate of 3 rpm. With the growth temperature gradient of about 30 °C/cm even a single-grain structure became a multi-grain structure in the final stage of growth. On the other hand, with the growth temperature gradient of about 50 °C/cm, even if the crystal started with multi-grains, it became a single crystal eventually. The constitutional supercooling criterion was used to interpret these results.
We report the growth of HgCdTe by metal organic vapor phase epitaxy (MOVPE), using (211)B CdTe/Si substrates grown by molecular beam epitaxy (MBE). The surface morphology of these films is very smooth. Morphological defects are primarily oval type void defects with the density of 500cm-2. The etch pit densities (EPD) and full widths at half maximum (FWHM) of x-ray rocking curves exhibit that the grown epilayers replicate exactly the structural properties of substrate. The Hall parameters of undoped HgCdTe layers show anomalous n-type behavior with temperature. Long time annealing under Hg-saturated condition found to deteriorate the transport properties of HgCdTe layers resulting from the possible in corporation of impurities in the layers. Mid wave infrared (MWIR) photovoltaic devices have been fabricated from n on p HgCdTe films. The MOVPE grown films were processed into mesa type discrete devices with wet chemical etching employed for mesa delineation and ZnS surface passivation. The dynamic resistance-area product at zero bias voltage for a temperature of 77K is 4000 ohm-cm2. But RoA values are scattered drastically. In some detectors, RoA values are less than 100 ohm-cm2. It is thought that these results are related with localized defects or anomalous transport properties of epilayer.
We report the growth of short wave infrared (SWIR) HgCdTe on (001) GaAs by metal organic vapor phase epitaxy (MOVPE). KOH dissolved in water used as final substrate rinse produce a mirror - like surface and with a hillock density of less than 10cm−2. It is shown that K element diffuses during layer growth and heat treatment for metal vacancy filling from the GaAs substrate/buffer interface into the MCT with a surface concentration of around low 1015cm−3. The transport properties of undoped MCT layers are dominated by residual K element in the layer. Short wave infrared (SWIR) photovoltaic devices have been fabricated from n on p HgCdTe films on GaAs substrates. The MOVPE grown films were processed into mesa type discrete devices with wet chemical etching employed for mesa delineation and ZnS surface passivation. The photodiode forward and reverse current-voltage characteristics, as well as the temperature dependence of the zero-bias dynamic resistance, were measured in the temperature range of 200-300 K. The zero bais dynamic resistance-area product at 200K and 300K were 5×105 and 3.0×102 ohm-cm2, respectively. The relative spectral response cut off wavelength of device at 300K was 2.5μm.
ZnS and CdTe layers are deposited by thermal evaporation on HgCdTe epilayers with xequals0.23 grown by MOCVD. Thermal evaporation is performed from commercial effusion cell in UHV chamber. The evaporation sources are ZnS and CdTe. Electrical properties of the ZnS/HgCdTe and CdTe/HgCdTe are determined by capacitance-voltage characteristics of metal- insulator-semiconductor test devices. It is found that the HgCdTe surface is slightly accumulated and the interface charge density of the order of low 1011cm-2 in both ZnS/HgCdTe and CdTe/HgCdTe structure. In the case of CdTe deposition, best results are obtained when the deposition rate is extremely slow of around 0.2A/sec. With this slow deposition rate, there is possibility of unintentional contamination from the residual gas in vacuum chamber into growing films. Thus, evaporation in a UHV environment is necessary for a sufficiently clean layer. The effect of Cd overpressure on surface charge of HgCdTe is presented. The main features of the ZnS and CdTe deposited from effusion cell in UHV chamber are low fixed surface charge density, and small hysteresis. The good electrical properties of the interface, thermal stability and chemical properties of ZnS and CdTe grown with very slow rate under ultra-high vacuum suggest that these layers can be applied for improving the surface passivation of photovoltaic devices fabricated on HgCdTe.
We report in-situ growth of MWIR P-on-n HgCdTe on GaAs by Metal Organic Vapor Phase Epitaxy. HgCdTe epi layers were grown by interdiffused multilayer process (IMP). Tris- dimethylarminoarsenic (DMAAs) was used as a precursor for arsenic doping (p-type layer) and isoprophyliodide (IPI) was used for iodine doping (n-type layer). Standard bubbler configuration was used for both precursors. Doping concentration could be controlled accurately in the range of 2 X 1015 to 7 X 1016 cm-3. After growth, HgCdTe layers were annealed in Hg-atmosphere at 415 degrees Celsius and 260 degrees Celsius consecutively. This Hg- annealing is for activating dopants and then reducing Hg- vacancy concentration. The layers doped with iodine in low 1015 cm-3 concentration showed higher Hall mobility than undoped layers. The Hall mobility of iodine doped layers decreased with increasing doping concentration. Secondary ion mass spectroscopy (SIMS) analyses for the iodine-doped layer showed sharp decrease of iodine concentration after IPI precursor being turned off, indicating negligible memory effect and very slow diffusion of iodine during growth and Hg-annealing. SIMS analyses for the arsenic doped layer showed that arsenic diffused by about 1 micrometer during growth and Hg-annealing. These results show that IPI and DMAAs could be used as stable precursors for in-situ growth of HgCdTe heterojunction. A P-on-n structure was grown. The P-layer has x composition of 0.32 and acceptor concentration of 6 X 1016 cm-3. The n-layer has x composition of 0.30 and donor concentration of 2 X 1015 cm-3. SIMS depth profile for the structure shows well-defined regions of doping concentration and alloy composition. After Hg-annealed, P-on-n structures were fabricated into MESA structure diodes. Electron beam evaporated CdZnTe was used as a passivation layer. This MWIR diode had RoA value of about 3 X 104 (Omega) .cm2.
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