Mikihiro Yokozeki

Observation of Threading Dislocation Generation Process in Highly Lattice-Mismatched Heteroepitaxy

The generation process of threading dislocations in highly lattice-mismatched heteroepitaxy was investigated by transmission electron microscopy (TEM). In the growth of InAs epilayers on GaAs substrate, the misfit strain was partially accommodated by the formation of coherent three-dimensional islands before the generation of misfit dislocations. However, the misfit dislocation was eventually introduced in each grown three-dimensional island in order to relieve the misfit strain. It was clearly shown that the misfit dislocations propagate towards the growth direction; i.e., threading dislocations originated from the grown islands.

Transparent Oxide Thin-Film Transistors Using n-(In2O3)0.9(SnO2)0.1/InGaZnO4 Modulation-Doped Heterostructures

We investigated transparent oxide thin-film transistors (TFTs) using n-(In2O3)0.9(SnO2)0.1/InGaZnO4 (n-ITO/IGZO) modulation-doped heterostructures, which are effective in achieving high carrier mobilities. From transmittance measurements and UV photoemission spectroscopy, n-ITO/IGZO modulation-doped heterostructures are expected to realize the type-II energy band lineup, in which both the conduction band minimum and the valence band maximum of n-ITO are higher in energy than those of IGZO. Van der Pauw Hall measurements revealed Hall mobility enhancement and two-dimensional behavior of electrons at the n-ITO/IGZO interface. Using the n-ITO/IGZO modulation-doped heterostructures, we obtained TFTs with higher electron mobility than that of IGZO TFTs. We consider that modulation doping is a promising method for performance improvements of TFTs using transparent oxide semiconductors.

Reduction of Point Defects and Formation of Abrupt Hetero-Interfaces in Low-Temperature Molecular Beam Epitaxy of GaAs and GaP under Atomic Hydrogen Irradiation.

The removal effect of excess As and P atoms adsorbed on GaAs (100) and GaP (100) surfaces by atomic hydrogen (H) irradiation was investigated by reflection high-energy electron diffraction and X-ray photoelectron spectroscopy. It was found that the excess As and P atoms were effectively removed by atomic H irradiation at a low temperature of 350°C. Then, we attempted to obtain a high-quality GaAs epitaxial layer and an ordered (GaAs)1(GaP)3 strained short-period superlattice (SSPS) with abrupt GaAs/GaP hetero-interfaces in the low-temperature growth under atomic H irradiation. The quality of the GaAs epitaxial layer and the abruptness of the GaAs/GaP hetero-interfaces in the (GaAs)1(GaP)3 SSPS were evaluated by photoluminescence, deep-level transient spectroscopy and transmission electron microscopy. As a result, it was clarified that the density of point defects in the GaAs epitaxial layer was reduced and an abrupt GaAs/GaP hetero-interface of the (GaAs)1(GaP)3 SSPS was formed in the low-temperature growth under atomic H irradiation.

Defect-Controlled Selective Epitaxial Growth of GaP on Si by Migration-Enhanced Epitaxy under Atomic Hydrogen Irradiation

We investigated the properties of deposition of GaP on dry-SiO2 and SiNx masks using molecular beam epitaxy (MBE) or migration-enhanced epitaxy (MEE) under atomic hydrogen irradiation and attempted the selective epitaxial growth of GaP-on-Si. The critical substrate temperature, below which poly-GaP was deposited on a mask layer, was lower for dry-SiO2 than that for SiNx, and was lowered by MEE rather than MBE. As a result, the selective epitaxial growth of GaP was achieved by MEE using the dry-SiO2 mask. It was found that the formation of large anti-phase domains expanding into the surface was suppressed by forming a P-prelayer at low temperature. It was also confirmed that the density of misfit dislocations at the GaP–Si hetero-interface was remarkably reduced with a decrease in the growth area.

Passivation of misfit dislocations by atomic hydrogen irradiation in lattice-mismatched heteroepitaxy

We investigated the passivation effect of misfit dislocations in an (InAs)1(GaAs)4 strained short-period superlattice grown on GaAs with atomic hydrogen (H) by transmission electron microscopy and electron-beam-induced current. The misfit dislocations in the 〈1 1 0〉 directions are generated at the heterointerface in order to accommodate the misfit strain. They were effectively passivated by atomic H irradiation during the growth and cooling processes. Furthermore, it was found that the misfit dislocation in the [1 1 0] direction was effectively passivated rather than that in the [1 1 0] direction. The desorption temperature of atomic H from the [1 1 0] misfit dislocation was higher than that from the [1 1 0] misfit dislocation. These phenomena could be attributed to the fact that the bonding strength of As-H is larger than that of GaH.

Reduction of threading dislocation density in an (InAs)1(GaAs)1 strained short-period superlattice by atomic hydrogen irradiation

The generation process of dislocations as well as the initial growth mechanism were investigated in the growth of an (InAs)1(GaAs)1 strained short-period superlattice (SSPS) on a GaAs (001) substrate under atomic hydrogen (H) irradiation. A two-dimensional (2D) growth mode was maintained even after lattice relaxation occurred for growth at 350° C. On the other hand, the growth mode changed from 2D to a three-dimensional (3D) one without atomic H irradiation. The threading dislocation density was remarkably reduced and the critical thickness was markedly increased by atomic H irradiation. Misfit dislocations propagating along the and directions were generated at the heterointerface. Such effects disappeared for growth at 500° C, where the atomic H atoms desorb from the growing surface. It was also found that 3D growth was more effectively suppressed in the growth of (InAs)1(GaAs)1 SSPS than in a In0.5Ga0.5As alloy which has the same average In composition as that of the SSPS.

Strain Relaxation Process of (InAs)m(GaAs)n Strained Short-Period Superlattices Grown by Molecular Beam Epitaxy

The strain relaxation process and crystalline quality of (InAs)m (GaAs)n strained short-period superlattices (SSPSs) grown on a GaAs(001) substrate were investigated by transmission electron microscopy and double-crystal X-ray diffraction. The lattice mismatch between the superlattice and the substrate was mainly accommodated by the generation of misfit dislocations propagating parallel to the heterointerface with no threading dislocations. However, the density of misfit dislocations was relatively low in the single SSPS heterostructure. Moreover, the full width at half-maximum of X-ray diffraction was markedly increased with the generation of misfit dislocations. It was found that the residual strain of SSPS was effectively relieved by applying multiple SSPS heterostructures.

Realization of two-dimensional growth and suppression of threading dislocation generation in (InP)1(GaAs)n quaternary strained short-period superlattices grown on GaAs

We investigated the growth mode and the dislocation generation process in the growth of (InP)1(GaAs)n quaternary strained short-period superlattice (SSPS) on GaAs(100) by reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM). The (InP)1(GaAs)n SSPSs with n of 2 and 3 were two-dimensionally grown even after the lattice relaxation. The generation of threading dislocations was suppressed by introducing the misfit dislocations at the hetero-interface.

Increase of Critical Thickness and Optical Emission Range in (InAs)1(GaAs)n Strained Short-Period Superlattices.

The critical thickness and optical emission wavelength range of (InAs)1(GaAs)n strained short-period superlattices (SSPSs) grown on GaAs(100) substrates were investigated by reflection high-energy electron diffraction, transmission electron microscopy and photoluminescence (PL). When the (InAs)1(GaAs)n SSPSs were grown at a substrate temperature of 420°C, the critical thickness was increased up to about ten times as large as that of InGaAs alloys except for the (InAs)1(GaAs)1 SSPS. The wavelength range of the PL peak was markedly increased in the (InAs)1(GaAs)n SSPS/GaAs single quantum wells (SQWs), compared with that in the InGaAs/GaAs SQWs. When the (InAs)1(GaAs)n SSPS was grown at 500°C, the longest experimental wavelength was 1.16 µm at room temperature. A wide range of PL peak wavelengths from 0.87 to 1.23 µm can be expected at room temperature in the (InAs)1(GaAs)n SSPSs, if nonradiative recombinations can be reduced with a low temperature growth of 420°C.