Toshihiko Takebe

Pregrowth treatment dependence of surface morphology for GaAs grown on exactly oriented (111)A substrates by molecular‐beam epitaxy

We investigate the dependence of growth layer surface morphology on surface treatment processes, i.e., etching, mounting, and loading wafers before molecular‐beam epitaxial growth. The surface morphology of a GaAs growth layer on exactly oriented (111)A GaAs is found to be strongly dependent on stoichiometry and roughness of the surface before growth. Device‐quality layers with a good surface morphology were obtained only on the exactly oriented (111)A GaAs wafers etched with NH4OH:H2O2:H2O (2:1:96) etchant, loaded into a growth chamber with little residual As4, and mounted without In. It is very important to use the NH4OH etchant (2:1:96) with (111)A GaAs, because it maintains the surface stoichiometry and smoothness.

Electrical Characterization of Lateral P-N Junctions Grown on (111)A GaAs Nonplanar Substrates by Molecular Beam Epitaxy

Electrical characteristics of lateral p-n junctions grown on (111)A GaAs nonplanar substrates by molecular beam epitaxy are investigated. According to current-voltage and capacitance-voltage measurements, it is clarified that uniformly Si-doped lateral p-n junctions have graded doping profiles at the intersection between the (111)A flat plane and (311)A side slope, and the lateral p-n interface becomes steeper with decreasing growth temperature. In the δ-doped lateral p-n junction, negative resistance is observed in the current-voltage characteristics. Assuming it is due to interband tunneling, we consider that the δ-doped lateral p-n junction has a steeper interface than the uniformly doped lateral p-n junctions. These junction characteristics are discussed in relation to the surface migration of Ga adatoms.

Electroluminescence from Lateral P-N Junctions Grown on (111)A GaAs Patterned Substrates

Lateral p-n junctions were grown on (111)A GaAs patterned substrates by a plane selective doping method and electroluminescence (EL) properties of the lateral p-n junctions were investigated. EL was observed along the line, and the line formed a triangular pattern on the substrate. The emission spectrum had a sharp peak at 870 nm at room temperature. Even though these p-n junctions were not optimized for LED structures, the emission intensity of EL was higher than that from p-n junctions of conventional structure. These characteristics are advantages of lateral p-n junctlons grown on (111)A GaAs patterned substrates. These are the first observations of EL from (111)A GaAs lateral p-n junctions.

Element-Selective Observation of Electronic Structure Transition between Semiconducting and Metallic States in Boron-Doped Diamond Using Soft X-ray Emission and Absorption Spectroscopy

The electronic structure transition between the semiconducting and metallic states in boron (B)-doped diamonds was element-selectively observed by soft X-ray emission and absorption spectroscopy using synchrotron radiation. For lightly B-doped diamonds, the B 2 p-density of states (DOS) in the valence band was enhanced with a steep-edge feature near the Fermi level, and localized acceptor levels, which are characteristic of semiconductors, were clearly observed both in B 2 p- and C 2 p-DOS in the conduction bands. For heavily B-doped diamonds, the localized acceptor levels developed into extended energy levels and the new energy levels generated formed an extended conduction band structure that overlapped with the valence band. Thus, the metallic energy band structure is actually formed by heavy boron doping. These valence and conduction band structures observed by soft X-ray emission and absorption spectroscopy accounted for the electrical properties of B-doped diamonds.

Demonstration of Lateral p-n Subband Junctions in Si Delta-Doped Quantum Wells on (111)A Patterned Substrates

The lateral subband p-n junction (LSJ) is demonstrated through a Si delta-doped GaAs/AlGaAs multiple quantum well (MQW) structure on patterned (111)A GaAs substrates. The optical properties of the MQW on a (311)A sloped surface and a (111)A flat surface are evaluated by cathodoluminescence. We confirm the subband formation and lateral p-n junctions for Si delta-doped MQWs. The LSJ shows good current-voltage properties when forward currents are injected into it. A recombination emission (791 nm) with an energy level higher than the band gap of bulk GaAs is observed at 300 K; this clearly shows that the potential bending occurs between n-type subbands and p-type subbands. The rise time of light-emitting diodes (LED) with the LSJ (LSJ-LED) is below 5 ns (>200 MHz). The LSJ has a high potential for applications in optical devices.

Heat-Treatment Study of Deep-Level Defects in Semi-Insulating Liquid-Encapsulated Czochralski Gallium Arsenide Substrates

Deep-level defects in semi-insulating (S.I.) liquid-encapsulated Czochralski (LEC) GaAs substrates were studied using the thermally stimulated current (TSC) technique. From the heat-treatment temperature dependence of the TSC signal intensity and electrical properties, the thermal behavior of the detected defects with respect to the heat treatment and their correlation to the electrical properties of the substrate were clarified for the first time. The change in the resistivity was closely related to the changes in the TSC signal intensity of T3 (trap depth: 0.31 eV), T6 (0.58 eV) and T x (0.29 eV). The net concentration of these defects changed by 4×1014 cm-3 after heat treatment at 800°C, leading to a threefold increase of the resistivity. Precise thermal control of substrates is very important for consistently obtaining high-quality GaAs substrates.

Low Diffusivity of Dopants in (111)A GaAs

We investigate the evaporation of As atoms from the surface and the diffusion of Si and Be in δ-doped layers, for (111)A and (100) GaAs. As atoms on the (111)A surface do not easily evaporate compared with those on the (100) surface. The diffusion of dopants in (111)A GaAs is smaller than in (100) GaAs, independent of the presence of As vacancies. The diffusion in (100) GaAs, on the contrary, is enhanced by the presence of As vacancies.

Misorientation dependence of crystal structures and electrical properties of Si-doped AlAS grown on (111)A GaAs by molecular beam epitaxy

Si-doped AlAs layers are grown on (111)A GaAs by molecular beam epitaxy (MBE). The crystal phase is found to change from hexagonal (H) to cubic (C) and the resistivity of AlAs layers is shown to be strongly dependent on the misorientation from exactly oriented (111)A GaAs. It is confirmed that hexagonal AlAs can only be grown on exactly oriented (111)A GaAs and that zincblende AlAs can be grown on misoriented (111)A GaAs, using X-ray diffraction and Raman scattering measurements, respectively. The lattice constants of hexagonal AlAs are determined to be a=4.034 A and c=6.568 A. Si-doped AlAs with a zincblende structure shows n-type conductivity and low resistivity, but Si-doped AlAs with a hexagonal structure exhibits high resistivity (>105 Ωcm). The crystal qualities and electrical properties of Si-doped AlAs on 5°-misoriented (111)A GaAs are equivalent to those of Si-doped AlAs on (100) GaAs.

Substrate Misorientation Effects on Silicon-Doped AlGaAs Layers Grown on GaAs(111)A by Molecular Beam Epitaxy

Silicon-doped Al0.3Ga0.7As layers were grown on GaAs(111)A misoriented 0-19.5° toward (100) by molecular beam epitaxy (MBE). The surface morphology was strongly affected by the off-angle of the substrates; a mirror surface was obtained when the off-angle was over 5°. Occupation sites of Si atoms were dependent on the flux ratio; at a ratio of 2, most Si atoms occupied As sites, whereas at ratios above 5, most Si atoms occupied Ga sites although there was greater compensation. The results show that the off-angle of GaAs(111)A substrates is an important factor for controlling the surface morphology and the impurity concentration of AlGaAs.

Titanium/Gold Schottky Contacts on P-Type GaAs Grown on (111)A and (100) GaAs Substrates Using Molecular Beam Epitaxy

The Schottky barrier heights of Ti/Au contacts on p-type GaAs, grown on (111)A and (100) GaAs substrates by molecular beam epitaxy, have been investigated by I-V and C-V techniques. Higher barrier heights are observed for contacts on (111)A GaAs films. Comparison between our results and the ideal Schottky barrier height for Ti on p-type GaAs shows that Ti/Au barrier heights on p-type (111)A GaAs films are closer to the ideal case than the Ti/Au barrier heights on p-type (100) GaAs films. This suggests that the defect densities of the Ti-GaAs interfaces of Ti/Au contacts on (111)A GaAs films are lower than those of identical Ti/Au contacts on (100) GaAs films.