Masao Tabuchi

Mesoscopic Structure in Lattice-Mismatched Heteroepitaxial Interface Layers

Low dimensional mesoscopic structures are important for the realization of electron wave and quantum optoelectronic devices. In this study, we propose to utilize three-dimensional (3D) growth, which occurs at the initial stage of the heteroepitaxial growth of a lattice-mismatched system, for the realization of a mesoscopic structure. InAs layers grown 3-dimensionally on (001) just-oriented and on 3.5° off-angled GaAs substrates were investigated by photoluminescence (PL) measurement and by transmission electron microscopy. A strong PL emission was observed from the 3D grown InAs samples. Both TEM observation and PL measurement suggest that the strong PL emission would be caused by the quantum effect of the 3D InAs layer sandwiched by the GaAs layers.

Strain energy and critical thickness of heteroepitaxial InGaAs layers on GaAs substrate

Abstract When a grown crystalline layer is as thin as one or two monolayers, the lattice constant in a grown layer is coherent with the substrate lattice even in a highly lattice-mismatched crystalline system. In this study, we calculate the strain energy in deviated bond lengths and bond angles for individual atoms by valence force field (VFF) method and find the atom position with the minimum strain energy. The critical thickness below which no dislocation occurs is derived. The thickness is thinner as compared with those predicted by previous conventional theories. However, the experimental results agree fairly well with our theoretical results.

Rheed and x-ray characterization of InGaAs/GaAs grown by MBE

InxGa1-xAs layers with various alloy compositions (0 ≦ x ≦ were grown on GaAs substrat es by MBE, and grown layers and heterointerfaces were characterized by RHEED and X-ray diffraction. The RHEED patterns indicated that the crystal nucleation is two-dimentional throughout the epitaxial growth for 0 ≦ x < 0.4, but for 0.5 < x ≦ 1, it is three-dimensional at the early stage of the growth followed by two-dimensional growth. All the experimental results consistently showed that the crystalline quality degrades with the increase of x from 0 to about 0.5., it tends to become rather improved with x.

MBE growth of lattice-mismatched layers: InxGa1−xAs/InAs and InxGa1−xAs/InP from x=1 to x=0

We investigate growth phenomena of InxGa1−xAs (0≦x≦1) layers on various substrates such as InAs and InP. The layer quality depends not simply on the degree of lattice-mismatching, but also on the other factors such as two- or three-dimensional growth near the heterointerface, compound or alloy grown layers, and compressed or tensile strain.

Micro-Raman study of heteroepitaxial InGaAs layers

Abstract Heteroepitaxial InGaAs layers on (100) GaAs and InAs substrates are characterized by micro-Raman spectroscopy. The samples are angle-lapped to observe depth profiles of phonon frequency. The GaAs-like LO frequency is shifted upward near the interface in InGaAs / GaAs and downward in InGaAs / InAs. The shift is believed to be due to residual misfit strain, and it is larger in InGaAs / InAs than in a GaAs / InAs structure where the mismatch degree is larger. The results of Raman measurements are compared with results of X-ray diffraction and transmission electron microscope.

Characterization of GaAs heterolayers by micro-Raman spectroscopy

GaAs heterolayers on (100) InAs and InP substrates were characterized by micro-Raman spectroscopy. The layers were 2μm thick and grown by molecular beam epitaxy at 480°C. A bevel was formed on each sample, and Raman scattering from the exposed interface region was collected. In close vicinity to the GaAsInAs interface, the frequency of the GaAs longitudinal-optical (LO) phonon was lower by about 3 cm−1 than at the as-grown surface. The LO shift decreased with increasing distance from the interface, but a small shift (about 0.5 cm−1) was still observed even at distances larger than 0.2 μm. The shift was believed to be due to residual misfit strain, and the value of biaxial strain was estimated to be about 0.6% at the interface. Almost the same results were obtained for GaAsInP, where the mismatch degree is smaller by a factor of two than in GaAsInAs. This would indicate that the residual strain in a thick heterolayer depends little on the initial misfit degree.

Increase of overlayer critical thickness by off-angled substrates

Abstract Molecular beam epitaxial (MBE) grown InAs layers on off-angled GaAs substrates are experimentally investigated by transmission electron microscopy (TEM), and the critical thickness of the lattice-mismatched grown layers on the off-angled substrates are theoretically discussed based on calculations using the valence-force field method. Dislocation densities observed by TEM in the InAs layers grown on the just-oriented and the off-angled GaAs substrates are compared with each other. In the 4 ML grown InAs layer on a 3.5°off-angled substrate, no dislocations are observed. On the other hand, a high density of dislocations is observed in the 4 ML grown InAs layer on the just-oriented substrate. This result indicates that the off-angle of the substrate changes the critical thickness. The theoretical calculation interprets the increase of the overlayer critical thickness on the off-angled substrate.

Raman scattering of ultrathin InAs layers inserted in GaAs

Raman spectra of ultrathin InAs layers inserted in GaAs are measured. The structures are grown on both (001) just-oriented and misoriented GaAs substrates. Transmission electron micrograph observation shows that the misorientation delays the introduction of misfit dislocations. When the InAs thickness is less than 6ML (monolayers), optical phonon modes in InAs are not observed. This would be due to coupling of the InAs vibration to that of the GaAs host lattice. At a thickness of 6ML, the InAs mode begins to be observed, and its intensity is stronger for the just-oriented sample than for the misoriented one. Thus, the misfit dislocations enhance the intensity of the InAs mode.

COMPUTER SIMULATION OF STRAIN RELAXATION MECHANISM OF MBE GROWN LATTICE MISMATCHED InGaAs/GaAs LAYERS

To investigate influence of lattice mismatching on crystalline quality of heteroepitaxialy grown layer, we have grown and characterized InGaAs on several different substrates. In this work, to disclose growth mechanism of lattice-mismatched crystal layer, we attempted to calculate theoretically the strain energy accumulated in the grown layer. We use the valence force field(VFF) method to calculate the strain energy microscopically. When the size of crystal is so small as atomic scale, the VFF method is more exact than other macroscopic methods. Calculated critical layer thickness of InAs/GaAs is not different from experimental results.