Naokatsu Sano

Extremely Flat Interfaces in GaAs/AlGaAs Quantum Wells Grown on GaAs (411)A Substrates by Molecular Beam Epitaxy

GaAs/Al0.3Ga0.7As quantum wells (QWs) grown on (411)A-oriented GaAs substrates by molecular beam epitaxy (MBE) showed extremely flat interfaces over a macroscopic area (about 200 µm ) even for the case of no growth interruption, which is mainly due to the intrinsically large migration of Ga atoms and layer growth in the step-flow mode on the (411)A plane. Photoluminescence linewidths at 4.2K were almost the same as or better than the narrowest linewidths reported for GaAs/AlGaAs and GaAs/AlAs QWs grown on GaAs (100) substrates with growth interruption at each GaAs/AlGaAs(AlAs) interface. Only one sharp luminescence peak was observed for each QW on the (411)A substrates, in contrast with three luminescence peaks for the QWs on the (100) substrates, indicating that extremely flat and uniform interfaces over a macroscopic area of laser excitation (200 µm diameter) are realized in the GaAs/AlGaAs QWs grown on (411)A GaAs substrates.

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.

Extremely high uniformity of interfaces in GaAs/AlGaAs quantum wells grown on (411)A GaAs substrates by molecular beam epitaxy

GaAs/AlGaAs quantum wells (QWs) were grown on (411)A‐oriented GaAs substrates by molecular beam epitaxy (MBE). Photoluminescence linewidths at 4.2 K are almost the same as the narrowest linewidths reported so far for GaAs/AlGaAs QWs grown on (100)‐oriented GaAs substrates with the growth interruption at the heterointerfaces. Furthermore, only one sharp peak was observed for each QW on the (411) substrate over the whole area of the wafer (10 mm×10 mm), in contrast with three splitted luminescence peaks for one kind of GaAs/AlGaAs QW grown on the (100) substrates by MBE with growth interruption. This result implies that effectively atomically flat interfaces over a macroscopic area (about 10 mm×10 mm) has been realized for the first time in GaAs/Al0.3Ga0.7As QWs grown on (411)A GaAs substrates by MBE. This is possibly due to the large migration of Ga and Al atoms on the (411)A plane during MBE growth and the step‐flow growth mode on the atomically corrugated (411)A plane.

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.

Fine Tuning of Metal-Insulator Transition in Al0.3Ga0.7As Using Persistent Photoconductivity

We report experiments on the electrical conductivity of Al 0.3 Ga 0.7 As:Si in the critical region of the metal-insulator transition. The electron concentration is finely controlled in the vicinity of the transition using the persistent photoconductivity effect. The value of the critical exponent turns out to be 1. The metal-insulator transition due to the delocalization effect by the magnetic field is also studied.

Surface Corrugation of GaAs Layers Grown on (775)B-Oriented GaAs Substrates by Molecular Beam Epitaxy

Surface morphologies of GaAs layers grown on (775)B-oriented GaAs substrates by molecular beam epitaxy were studied using atomic force microscopy. The surface of a GaAs layer grown on a (775)B GaAs substrate is flat when it is grown at a substrate temperature (T s) below 580°C, but corrugates regularly for T s ≥640°C. Similar periodic corrugation of the (775)B GaAs surface formed during thermal annealing at T s=640°C under As4 atmosphere (10-6 Torr), which is similar in shape to that of a GaAs layer in the early stage of growth (a GaAs layer thinner than 5 nm). As the thickness of a GaAs layer increases from 5 nm, the lateral period and step height of the corrugation increase and tend to saturate for thick GaAs layers (200 nm thick or more). The precise structure of the surface corrugation of a 3-nm-thick GaAs layer (the lateral period of 12 nm and step height of 1.2 nm) was observed as a corrugated AlAs-on-GaAs interface of a GaAs/(GaAs)5(AlAs)5 quantum well structure with an average well width of 3 nm by cross-sectional transmission electron microscopy observation.

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.

NMR Study in PdCo and PdNiCo Alloys

NMR of Co 59 has been performed in ferromagnetic PdCo alloys. The internal field changes from -215 kOe in pure Co metal to 230±5 kOe in alloys of Co diluted in Pd. The results are consistent with the previous measurements by nuclear polarization and Mossbauer effect experiments. The line width is very broad in alloys with low Co concentration. Even in 0.5% Co alloy, the width still has the value of 50 MHz. T 2 shows rapid decrease with decreasing Co concentration. The experiment has also been performed on Co 59 in ferromagnetic Pd x Ni 1- x Co system. The results are quite similar to those in PdCo alloys.