Hiroshi Fujiyasu

A proposal for p‐type ZnS1‐xSex–ZnTe superlattices

Energy band structures ZnSe–ZnTe and ZnS0.5Se0.5 –ZnTe superlattices are calculated using the Kronig‐Penney model taking into account strain effects due to the lattice mismatches between the materials. The ZnSe(30 A)–ZnTe(5 A) and the ZnS0.5Se0.5 (30 A)–ZnTe(5 A) superlattices have energy gaps of 2.5 and 2.7 eV, respectively. The ZnS0.5Se0.5 (30 A)–p‐type ZnTe(5 A) superlattice is considered to be a p‐type material of 1017 cm−3 and to be available for a hole injection layer to an n‐type ZnSe layer.

PbTe‐Pb1−xSnxTe superlattices prepared by a hot wall technique

The PbTe‐Pb0.8Sn0.2Te superlattices were made by the hot wall technique on BaF2 substrates. Two superlattices with the period thickness of 100 and 200 A were analyzed by sputtering‐Auger electron spectroscopy and good results were obtained. The interdiffusions of Pb and Sn across the junction were also investigated and the diffusion coefficients of Pb and Sn at 300 °C were found to be 1.8 and 2.9×10−17 cm2/sec, respectively.

Properties of ZnTe-ZnSe and -ZnS superlattices prepared by hot wall epitaxy

Abstract ZnTe-ZnSe and -ZnS superlattices were prepared on GaAs(100) substrates by hot wall epitaxy. Their high-energy electron and θ-2θ X-ray diffraction and photoluminescence at 77 and 300 K were measured. Though the lattice mismatches between the materials are 7% (ZnTe-ZnSe) and 13% (ZnTe-ZnS), strong photoluminescence associated with band edges was observed, i.e. a superlattice consisting of a thin ZnTe layer and a thick ZnSe layer showed strong luminescence associated with band edges and weak luminescence associated with deep level defects. The ZnTe (20 A)-ZnS (50 A) superlattice showed strong but rather broad luminescence near 2.6 eV. Variations of the luminescence photon energy of the ZnTe-ZnSe superlattice with the thickness of the ZnTe or the ZnSe layer can be explained qualitatively by strain effects due to the lattice mismatch on the band gap of ZnTe and on the band offsets of the superlattice. The experimental results show the superlattices are of type II, where the conduction and valence band edges of ZnTe are above the corresponding edges of ZnSe or ZnS.

Strain relaxation of CdTe(100) layers grown by hot‐wall epitaxy on GaAs(100) substrates

The strain relaxation of CdTe(100) layers grown on GaAs(100) substrates by hot‐wall epitaxy was investigated by measurement of optical properties, x‐ray analysis, and transmission electron microscopy. It is considered from transmission electron microscopy observation that relaxation of most of the strain due to lattice mismatch occurred at the interface. However, a small amount of strain, of the order of 10−3, remained in layers thicker than 0.7 μm, and it was relaxed as the layer thickness increased. The residual strain of 4×10−4, which exists in layers thicker than 10 μm, was due to the difference between the thermal expansion coefficients of the layer and the substrate. Moreover, for layers thicker than 17 μm, split ground (n=1) and first excited (n=2) free‐exciton states due to internal strain have for the first time been observed by photoluminescence and reflectance spectroscopy. The results show that CdTe layers with excellent crystallinity and homogeneity in strain are obtained.

Raman scattering from ZnTe‐ZnSe strained‐layer superlattices

Raman scattering measurements have been made to determine the elastic deformation in ZnTe‐ZnSe strained‐layer superlattices grown by hotwall epitaxy. Both ZnSe‐ and ZnTe‐like modes have been observed, which confirms the formation of a superlattice in the multilayer system having a large lattice mismatch between host layers. We observe that the frequencies of the ZnSe‐like longitudinal and transverse phonon modes vary strikingly with the relative thickness of ZnTe and ZnSe layers. This effect is fully accounted for by uniform misfit strains which accommodate the lattice mismatch of the host.

Growth of CdTe on GaAs by hot‐wall epitaxy and its stress relaxation

CdTe(100) layers of the thickness range from 0.7 to 15 μm were grown on GaAs(100) substrates by hot‐wall epitaxy. The crystallinity of the layer was examined by reflection high‐energy electron diffraction, x‐ray rocking curve, and photoluminescence. The lattice relaxation were investigated by x‐ray analysis and optical reflectance spectra. CdTe layers with thickness up to 15 μm were under compressive biaxial stress. In addition to the split exciton lines (n=1), the emission from excited states (n=2) was observed.

Semimetallic Hall properties of PbTe‐SnTe superlattice

The PbTe‐SnTe superlattice was expected to be a type‐II superlattice where the valence band edge of SnTe is higher than the conduction band edge of PbTe. To ascertain the type‐II structure, we prepared the PbTe‐SnTe superlattice by hot wall epitaxy, and performed its Hall measurement. Magnetic‐field‐dependent and relatively small Hall coefficients were obtained for the superlattices, which show coexistence of free electrons and holes in the superlattice. Hall coefficients of the superlattices increased with annealing time owing to the gradual disappearance of the coexistence. Diffusion of Sn was studied using x‐ray diffraction analysis.

(p/n) PbTe multiple‐layer films prepared by a hot wall technique

The PbTe multiple‐layer structures with alternating types of conductivity (p/n) were made by the hot wall technique. The net carrier density of multiple‐layer films (obtained by Hall‐effect measurements) decreased by an order of magnitude compared to singly doped films. These multiple‐layer films indicated sensitive photoconduction and their structures were confirmed by I‐V and C‐V measurements.

Strong luminescence from dislocation-free GaN nanopillars

GaN nanostructures were prepared on Si(111) by a hot-wall epitaxy technique employing the modified two-step growth method. Isolated hexagonal pillar-like GaN nanostructures (GaN nanopillars) with the typical diameter, height, and density of 200–300nm, 0.5–1μm, and 3–4×108cm−2, respectively, are self-organized without intentional pre-processing to the Si substrate. The photoluminescence and cathodoluminescence (CL) measurements show the strong near-band-edge emissions without the yellow band at room temperature. Stronger CL is obtained from the GaN nanopillars in comparison to single-crystalline GaN. The obtained strong CL is related to high crystal quality of the dislocation-free GaN nanopillars.

Raman probing of ZnTe-ZnS strained-layer superlattices

Abstract ZnTe-ZnS strained-layer superlattices grown by Hot Wall Epitaxy are investigated by Raman spectroscopy. The formation of the superlattice structure is clearly observed. The line profile and the frequency shifts of LO and TO Raman modes due to elastic strain in the superlattice layers depend remarkably on the layer thickness and appear to be in compliance with theoretical calculation within the critical values of layer thicknesses. The spectra of ZnTe 10A- ZnS 10A superlattices give an indication of folded LA phonon modes caused by the new periodicity.