Ryuichi Hirano

Growth of dislocation free grade Fe doped InP crystals

3-inch Fe doped dislocation-free (DF) grade (dislocation density < 500 cm/sup -2/) InP crystals were grown by the phosphorus vapor controlled liquid-encapsulated Czochralski (PC-LEC) method. By controlling the solid/liquid (S/L) interface shape and reducing the axial temperature gradient, the dislocation density of 3-inch Fe doped InP crystals below 500 cm/sup -2/ was achieved for the whole ingot. The average dislocation density per a wafer was 100 cm/sup -2/ at lowest. The dislocation density of 4-inch Fe doped InP crystals was also reduced to less than 5 /spl times/ 10/sup 3/ cm/sup -2/ by this method.

4-inch InP crystals grown by phosphorous vapor controlled LEC method

4-inch Fe doped and S doped InP crystals have been grown by phosphorous vapor controlled LEC method (PC-LEC). By controlling the solid / liquid (SL) interface shape, the dislocation density of S doped InP crystals was reduced to less than 3 /spl times/ 10/sup 3/ cm/sup -2/ for the whole ingot. The dislocation density of 4 inch Fe doped InP crystals was less than 5 /spl times/ 10/sup 4/ cm/sup -2/. The total thickness variation (TTV) of 4-inch substrates was improved by optimizing the polishing conditions. The minimum TTV was about 1.6 micrometer. There was no significant difference in breakage strength between 4-inch and 3-inch Fe doped InP substrates.

Effects of Antimony Doping in Polycrystalline CdTe Thin-Film Solar Cells

The effects of antimony (Sb) doping of the CdTe layer in the CdTe solar cells were investigated using Sb-doped CdTe powders as source materials for CdTe deposition by the close-spaced sublimation (CSS) method. Conversion efficiency increased with increasing Sb concentration below 1×1018 cm-3, mainly owing to the improvement of the fill factor. Secondary ion microprobe mass spectrometry (SIMS) depth profile revealed that the Sb impurities at a concentration of approximately 1×1016 cm-3 were incorporated into the CdTe layer when using the Sb-doped CdTe source of 1×1018 cm-3. The observation of surface morphology showed that the grain sizes were improved by Sb addition. Therefore, the improved performance upon Sb addition to CdTe solar cells was probably due to the improvements in crystallinity, such as increased grain size.

Epitaxial wafer quality versus substrate quality

The effect of various InP substrates properties on epitaxial layer quality was investigated. From the results of experiments on MOCVD, we found that the average haze of epitaxial layer surfaces measured by surfscan can be reduced by using substrates with optimum surface mis-orientation angles of 0.05-0.10 degrees. Our experiments on MBE also showed a similar tendency. Furthermore it was found that appropriate treatment of substrates prevents slips on the periphery of epitaxial wafers.

Evaluation of InP-Based Epitaxial Layers by Photoluminescence Measurement

The correlation between the photoluminescence (PL) intensity of either InGaAsP or InGaAs epitaxial layer and the properties of InP substrates was investigated. It was found that the PL intensity of epitaxial wafers strongly depended on carrier concentration, the condition of the back surface of the substrate and the structure of epitaxial layers. In the PL intensity measurement, the absorption of luminescence by the substrate itself sometimes causes the reduction in PL intensity. In the case where a pattern was observed at the back of the substrate, which was contaminated by the outgas from a spring in a fluoroware, the measured PL intensity showed an anomalous distribution. However, the quality of epitaxial layers did not change. It is very important to consider the effects of the absorption of the substrate and the roughness of the back of the substrate for evaluating the quality of epitaxial layers by PL measurement.

Electrical conductivity measurement by acoustic attenuation in semi-insulating GaAs crystals

GaAs crystals are both semiconductive and piezoelectric. In such crystals, acoustic waves induce a piezoelectric field which creates an electric current by causing conduction carriers to flow, resulting in Joule heat and attenuation of the acoustic waves. This means that the attenuation of the acoustic waves corresponds to the conductivity in a GaAs crystal. Accurate values of the electrical conductivities in various semi-insulating GaAs crystals were obtained using this principie. Ohmic contacts are sometimes very difficult to form on GaAs crystals, but this method requires no contact electrodes. We therefore expect it to prove an extremely useful method of measuring the conductivity of these materials.