Harunori Sakaguchi

Synthesis of gallium nitride by ammonia injection into gallium melt

Abstract Gallium nitride (GaN) was synthesized by injecting ammonia gas into molten gallium at 900–980°C under atmospheric pressure. A large amount of GaN powder was reproducibly obtained using a simple apparatus. The synthesized powder was characterized by scanning electron microscopy, X-ray diffraction, photoluminescence and energy dispersive X-ray spectroscopy, and was found to consist of fine crystals of hexagonal GaN of good quality. The total of GaN obtained was far more than the amount calculated from expected saturation solubility in the Ga melt at that temperature. We speculate that the GaN crystals were largely formed by direct reaction between Ga and the gaseous N source at the surface of the NH 3 bubbles in the melt. GaN synthesized by this method may be useful as a starting material for bulk growth.

Ordering-induced electron accumulation at GaInP/GaAs hetero-interfaces

Abstract Hetero-junctions between GaAs and GaInP were investigated, which had been grown by metal organic vapor-phase epitaxy (MOVPE) at a wide range of temperatures such that the GaInP had different long-range-order parameter ( η ) values. Electron depth-profiles in the samples were characterized by capacitance–voltage ( C – V ) measurement, and delta-shaped electron accumulation was observed at the interface between GaAs and GaInP, although both layers were grown undoped. It has been proven that the sheet concentration of this unexpected electron accumulation is strongly dependent on the η value of GaInP. Macroscopic polarization, caused by spontaneously polarized gallium-rich clusters in ordered GaInP, is attributed as being the reason for this electron accumulation.

Systematic study on Si and Se doping of MOVPE GaAs

Si and Se doping mechanisms of MOVPE GaAs have been studied systematically over a wide range of [AsH3]/[TMG]. In the region where undoped GaAs in n-type, the Se incorporation depends on [H2Se]/[AsH3] and the thermal decomposition of AsH3, but it is independent of [TMG]; the incorporation depends on [Si2H6]/[TMG] (or[SiH4]/[TmG]) and the thermal decomposition of [Si2H6] (or [SiH4]), but it is independent of [AsH3]. In the region where undoped GaAs is p-type, the incorporation of Si or Se depends on both [TMG] and [AsH3].

Dependence of Deep Level Concentration on Nonstoichiometry in MOCVD GaAs

Investigations have been made of deep levels in undoped GaAs crystals grown by metal organic chemical vapor deposition (MOCVD) on liquid encapsulated Czochralski (LEC) GaAs wafers. The dependence of electron trap level EL2 concentration and a lattice constant on nonstoichiometric composition of epitaxial GaAs crystals has been clarified by changing the mole flux ratio of arsine (AsH3) to trimethyl gallium (TMG), [AsH3]/[TMG], from ten to eighty. The dependence of EL2 concentration and a lattice constant on [AsH3]/[TMG] strongly suggests that an interstitial arsenic atom or group of interstitial arsenic atoms leads to development of EL2.

Mechanism of Carrier Accumulation at the Hetero Interface between InSb and GaAs

Various surface orientations of GaAs were used as substrates for InSb epitaxial growth, and the dependence of the interface carrier density on substrate surface indexes was discovered. The interface carrier density increased in proportion to the dangling bond density at the hetero epitaxial interface, and carrier accumulation was significantly suppressed using a (111) substrate which had the least dangling bond density among all the surface indexes of a zinc-blende structure. A carrier accumulation model based on the estimated band diagrams at the hetero junction was proposed and well explained these experimental results.

Thermal stability of highly Se-doped specular surface of In0.5Ga0.5As grown by low-pressure metalorganic vapor phase epitaxy

We investigated the highly Se-doped n+-In0.5Ga0.5As growth using low-pressure metalorganic vapor phase epitaxy (MOVPE) and the thermal stability of the epilayers. H2Se and triethylgallium (TEG) were used as a dopant gas and a gallium source material, respectively, for high doping at a lower temperature. We found that the surface morphology depended strongly on the growth temperature and that a specular surface could be realized by growth at a temperature as low as 450°C. H2Se enabled high efficiency in doping at this low growth temperature and a high carrier density of 3.6 × 1019 cm-3 could be obtained. We also found that the carrier density of highly Se-doped InGaAs was thermally stable in annealing at 500°C, which was higher than the growth temperature of 450°C, and that this density remained high enough to form good non-alloyed ohmic contacts.

Fourier transformed photoreflectance characterization of interface electric fields in GaAs/GaInP heterojunction bipolar transistor wafers

We performed Fourier transformed photoreflectance (PR) spectroscopy on GaAs/Ga0.5In0.5P heterojunction bipolar transistor wafers. The use of Fourier transformation of the PR spectrum resolves the signals coming from the emitter–base and base–collector interfaces. The evaluated interface electric fields were compared with the capacitance obtained from capacitance–voltage measurements. The result for the base–collector interface is consistent with the Poisson equation. On the other hand, the atomic ordering in the Ga0.5In0.5P emitter plays an important role in determining the characteristics of the emitter–base interface.

Low-temperature growth of GaAs with high quality by metalorganic vapor phase epitaxy

Growth of GaAs by MOVPE (Metal-Organic Vapor-Phase Epitaxy) at a much lower temperature than conventional conditions of 650°C required to produce highly resistive buffer layers and molecular-layer-level abrupt heterojunctions and doped-layer interfaces. It has been widely recognized that growth at 550~600°C causes poor morphological surfaces on the grown layers. This paper describes a fundamental improvement of the low-temperature growth of GaAs, resulting in smooth surfaces with very low impurity concentrations. Excellent GaAs layers can be grown at 500°C by increasing partial pressure of AsH3 during growth. The method has been applied to high quality epitaxial layers for electronic device production.

Influence of n-GaAs/i-AlGaAs heterostructure interfaces on the electrical properties of the n-GaAs channel layer

Abstract The electrical properties of the n-type GaAs channel layer on various buffer layers of GaAs and AlGaAs were investigated, using epitaxial wafer samples grown by metal-organic vapor phase epitaxy (MOVPE) with very low contamination at the epitaxial/substrate interface. The n-GaAs/i-AlGaAs interface was found to enlarge the depletion layer and to reduce the mobility near the interface. This unexpected effect was explained by a model which took into consideration the compensation of the carbon acceptor by the oxygen deep donor in AlGaAs. By decreasing the oxygen concentration, the properties complied well with the basic theory, Poissons equation and continuity equations. These results are very useful in the design of an epitaxial wafer for a device with a GaAs channel and heterostructural buffer layer.

Influence of arsenic flux ratio in the thermal cleaning process on InP (100) surface characteristics

Influence of arsenic flux ratio in the thermal cleaning process on InP (100) surface characteristics was investigated using reflected high-energy electron diffraction (RHEED) and atomic force microscopy (AFM). We observed that the reconstruction temperature of an In-stabilized (4/spl times/2) structure and an As-stabilized (2/spl times/4) structure was dependent on the arsenic pressure. The AFM images showed that the height and number of the InAs islands on the InP surface were determined by the arsenic pressure during the thermal cleaning process.