Tadao Toda

Growth of undoped, high purity, high resistivity ZnSe layers by molecular beam epitaxy

The growth of undoped, high purity ZnSe layers by molecular beam epitaxy with extremely high purity Se source materials refined by sublimation is described. The resistivity of these layers is greater than 104 Ω cm. In low‐temperature photoluminescence spectra, only free‐exciton emission line becomes dominant and all the bound exciton emission lines are extinguished, which is characteristic of very high purity ZnSe. Detailed behavior of electrical and optical properties of normally undoped layers is remarkably dependent on the purification cycles of Se materials used as source materials. The results indicate that the high resistivity of undoped, high purity ZnSe layers is due to a reduction of residual donor impurities.

The preparation of conductive ZnS films by using MBE with a single effusion source

We have successfully grown ZnS films on GaAs and GaP substrates by using an MBE technique with a single effusion source of ZnS compounds. The epitaxial ZnS films grown on the (001) face of the substrates have a cubic structure free from twins and exhibit n-type conductivity with a maximum carrier concentration of ≈ 2x1014 cm-3 and a maximum Hall electron mobility of ≈ 90 cm2/Vh.s. This was realized under the optimum growth temperature of ≈ 240°C. Their luminescence spectra show that two kinds of deep level luminescence, divided into a high energy blue broad emission band and a low energy red broad emission band, are dominant. Their major origin may be attributed to the Cu impurity which is incorporated during growth by evaporation of the impurities contained in the source materials. On the other hand, epitaxial ZnS films grown on the(111) face of the substrates contain {111} twins 180° rotated about the axis normal to the grown layer surface so that they exhibit a resistivity greater than 104 Ω cm.

Growth and characterization of MBE-grown ZnTe:P

Abstract Heteroepitaxial P-doped ZnTe films with precisely controlled carrier concentrations ranging from 1.2 × 10 16 to 4 × 10 17 cm -3 through the regulation of P-beam pressure were successfully grown on GaAs by means of MBE. The crystallinity of the P-doped films and the activation efficiency of incorporated P-impurities were strongly affected by beam pressure ratios of Te/Zn beams (BPRs). In the Te rich case, where two-dimensional growth was dominant, a high quality film was grown and an efficient carrier activation from shallow P-acceptors took place together with dominant excitonic PL emission. In the Zn rich case, where three-dimensional growth occured, the quality of the grown film was degraded, so that an uncontrollable carrier concentration was confirmed with the existence of a strong deep PL emission. However, from SIMS analysis, the concentration of P-impurities incorporated in both films was controllable by P-beam pressure; BPRs were not as well-defined a parameter for governing the concentration of P-impurities as the P-beam pressure.

Characteristics of Li- and Cl-doped ZnTe grown by molecular beam epitaxy

Abstract We found that the carrier concentration of Li-doped ZnTe, grown at growth temperature ( T g ) of 320°C, increased with rising the Li-cell temperature ( T Li ) below 220 and above 260°C, and decreased between 220 and 260°C. Both the acceptor and donor concentration of the T Li = 300° C sample were estimated to be smaller than those of the T Li = 220° C sample. We also found by PL spectra that only when the film was grown at T Li = 220° C , I Li 1 was replaced by an A Li − Li band. These properties above T Li = 220° C can be explained if we assume that the electrical activity of Li in ZnTe decreases drastically above T Li = 220° C . We also investigated the PL spectra of Cl-doped ZnTe. The VI-II ratio and T g dependence of PL spectra indicated that the films with better crystallinity were grown under Te-rich conditions above T g = 300° C .

Suppression of Cu diffusion from a bulk ZnSe substrate to a homoepitaxial layer by Se‐beam irradiation as a pregrowth treatment

Cu diffusion of homoepitaxial ZnSe was investigated through low‐temperature photoluminescence (PL) spectra and secondary ion mass spectroscopy. Though Cu in ZnSe normally diffuses easily, we found that Cu in a ZnSe substrate did not diffuse into the homoepitaxial layer when the substrate was heated under Se‐beam irradiation prior to growth. Cu‐related emissions, such as Ideep1 and Cu‐green, disappeared from the PL spectrum of the homoepitaxial layer grown on the Se‐beam irradiated substrate. This suppressed Cu diffusion can be explained by site transformation of interstitial Cu atoms in the ZnSe substrate into Cu atoms occupying the Zn lattice site. This treatment is very useful for improving the purity of homoepitaxial ZnSe layers.