Hiromu Kato

Photoluminescence study of InxAl1−xAs‐GaAs strained‐layer superlattices

We present the strain‐induced effects of the InxAl1−xAs‐GaAs strained‐layer superlattices grown by molecular beam epitaxy. The evaluation of the effects of biaxial strain in the planes perpendicular to the [001] superlattice direction was made by conventional photoluminescence measurements. The observed optical transition energies were evaluated by a Kronig‐Penny model involving strain‐induced band structure. Comparison between the observed transition energies and the calculated energies suggests that the optical transition of strained‐layer superlattices is explained by the band‐gap shift and the valence‐band splitting, which are induced by the biaxial strain.We present the strain‐induced effects of the InxAl1−xAs‐GaAs strained‐layer superlattices grown by molecular beam epitaxy. The evaluation of the effects of biaxial strain in the planes perpendicular to the [001] superlattice direction was made by conventional photoluminescence measurements. The observed optical transition energies were evaluated by a Kronig‐Penny model involving strain‐induced band structure. Comparison between the observed transition energies and the calculated energies suggests that the optical transition of strained‐layer superlattices is explained by the band‐gap shift and the valence‐band splitting, which are induced by the biaxial strain.

X‐ray study of misfit strain relaxation in lattice‐mismatched heterojunctions

High‐resolution x‐ray diffraction measurements have been carried out in AlxGa1−xAs and InxGa1−xAs grown by the molecular beam epitaxy method on (001) GaAs substrates. The thin epitaxial layers in these lattice‐mismatched semiconductor single heterojunctions are uniformly distorted and there is an elastic limit for large x. The epitaxial layer is affected by a thick substrate even over the elastic limit, i.e., the epitaxial layer still shows a strained state beyond the elastic limit. The relationship between the misfit strain and the lattice distortion is discussed.

Mono- and Bi-Layer Superlattices of GaAs and AlAs

The synthesis and X-ray diffraction study of superlattices grown by exactly alternate depositions of monolayers of GaAs and AlAs are reported. The mono- and bi-layer superlattices were synthesized by molecular beam epitaxy utilizing the intensity oscillations in the specularly reflected beam in the RHEED pattern during growth. The X-ray diffraction profile implies that the synthesized monolayer crystal has a simple cubic symmetry. The satellite diffractions for the synthesized bilayer superlattice were clearly detected at L=0.5, 1.0, 1.5, 2.5, 3.0 and 3.5.

Raman scattering from GaAsAlAs monolayer-controlled superlattices

Abstract Measurements of Raman scattering were performed on (GaAs)n−(AlAs)n monolayer-controlled superlattices with n = 1, 2, 3 and 4 grown by molecular beam epitaxy. The zone-folding effect for longitudinal acoustic phonons and frequency shift of longitudinal optic phonons were observed. The experimental results agree well with the calculated ones according to the elastic and linear chain models.

Raman study of GaAs‐InxAl1−x As strained‐layer superlattices

Raman spectroscopy has been used to study the lattice‐mismatch strains in GaAs‐InxAl1−xAs strained‐layer superlattices grown by molecular beam epitaxy with the layer thicknesses of 10–200 A and In content x of 0.11, 0.20, and 0.35. The strain‐induced shifts of the longitudinal optic phonon modes indicate that the GaAs and InxAl1−xAs layers have the tensile and compressive strains, respectively, along the interfaces. The strain calculated from the observed frequency shift agrees with the lattice‐mismatch strain given by the elastic theory.

Temperature Dependence of Molecular Beam Epitaxial Growth Rates for InxGa1-xAs and InxAl1-xAs

The temperature dependence of the growth and evaporation rates in GaAs, InxGa1-xAs and InxAl1-xAs grown by molecular-beam epitaxy (MBE) was measured by reflection high-energy electron-diffraction (RHEED) oscillations. The growth rate and the evaporation rate of GaAs were fitted using a thermodynamical approach. For ternary alloys the calculated results were in agreement with the observed growth rates.

Zone-Folding Effects on Phonons in GaAs-AlAs Superlattices

Raman spectroscopy has been used to study the zone-folding effects on both longitudinal acoustic (LA) and longitudinal optic (LO) phonons in (GaAs)n-(AlAs)n superlattices with integral n (2 ~12) grown by molecular-beam epitaxy. All the observed frequencies of the zone-foled LA phonon modes up to the sixth order agree well with those calculated using the elastic model, while the observed frequencies of the zeroth- and first-order zone-folded LO phonon modes agree with those calculated using the linear-chain model. However, since long-range interactions are neglected in the linear-chain model, this model is not a good approximation for the higher-order modes. In contrast to the LA phonon modes, all the GaAs- and AlAs-like LO phonon modes are confined in the GaAs and AlAs layers, respectively.

X-ray studies of semiconductor superlattices grown by molecular beam epitaxy

X-ray diffraction measurements of semiconductor superlattices grown by molecular beam epitaxy (MBE) have been carried out. Alternating monolayer growth of GaAs and AlAs has been confirmed by detecting the reflections at Brillouin zone boundaries in the reciprocal space. In strained GaAs–Ga 1- x In x As superlattices the tetragonal distortion is found and the tetragonality is independent of the thickness of the strained layer but increases with decreasing thickness of the GaAs layer. The X-ray study of the interdiffusion in the superlattice has been also carried out.

Raman scattering from GaAs-InxGa1−xAs strained-layer superllatices

Abstract Measurements of Raman scattering were performed on GaAs-InxGa1−xAs strained-layer superlattices, grown by molecular beam epitaxy, with lattice periods ranging from 30 ∼ 250 A and In concentrations x, 0.22 and 0.37. Only one GaAs-like longitudinal optical phonon peak was observed in each strained-layer superlattice, in contrast to the well-known result that two peaks were observed in GaAs-AlxGa1−xAs superlattices. The GaAs-like phonon frequencies shifted from those of bulk GaAs to those of bulk InxGa1−xAs alloys as the ratio of the one-layer thickness of InxGa1−xAs to the lattice period increases from zero to one. We conclude that the GaAs-like phonon mode is a uniform mode of the whole strained-layer superlattice and the phonon frequency is determined by the averaged In concentration.

Γ - X crossover in GaAs/AlAs superlattices

The electronic band structure of the conduction band of (GaAs)m(AlAs)m superlattices (SLs) with ranging from m=4 to 18 monolayers was investigated using the photoluminescence and absorption measurements at 20 K. In the SLs with m<14 the fundamental emission is attributed to the transition between a Γ hole confinement state localized in GaAs and a X electron state in AlAs. The energy separation between the Γ and X states is minimum at m=13. The abrupt change in Γ-emission intensity and decay time between m=13 and 14 provides a clear evidence of the Γ-X crossover in this range of m.