Akihiro Wakahara

COMPOSITIONAL INHOMOGENEITY AND IMMISCIBILITY OF A GAINN TERNARY ALLOY

Compositional inhomogeneity in a GaInN ternary alloy layer is investigated. A theoretical estimation of the interaction parameter based on the delta lattice parameter suggests that the immiscibility of InN in a nitride alloy is very strong. We investigate the compositional splitting and the existence of InN inclusion in the GaInN epilayer grown on sapphire (0001) substrates. The mechanism of compositional inhomogeneity is discussed.

Morphological transition of InAs islands on GaAs(001) upon deposition of a GaAs capping layer

The interaction between a GaAs cap and InAs islands grown on vicinal GaAs(001) has been studied by transmission electron microscopy and atomic force microscopy. Samples were prepared by molecular beam epitaxy at 480 °C. Upon GaAs cap deposition, it was found that the previously grown InAs islands undergo a novel type of morphological transition, i.e., a transition from disk‐shaped to ring‐shaped islands. InAs becomes depleted or entirely absent in the central area of what had been a disk‐shaped InAs island. The GaAs cap was also shown to be virtually absent within the same central region, resulting in the formation of crater‐like surface depressions.

Strong photoluminescence from AlP/GaP disordered superlattice grown by atmospheric pressure organometallic vapor phase epitaxy using tertiarybutylphosphine

AlP/GaP disordered superlattices (d‐SLs) are grown for the first time by atmospheric pressure organometallic vapor phase epitaxy using tertiarybutylphosphine as the phosphorous source. Strong photoluminescence (PL) is observed from the d‐SL, comparing with the ordered superlattice (o‐SL) and the bulk alloy (b‐AL). The PL intensity of the d‐SL is 158 times stronger than that of the o‐SL and 114 times than that of the b‐AL. The strong PL from the d‐SL is attributed to the localized states created by the artificial disordering.

OMVPE Growth of AIP/GaP Superlattices Using Tertiarybutylphosphine as a Phosphorus Source

Abstract AlGaP layers and AlP/GaP superlattices were grown by atmospheric pressure OMVPE at low temperature on GaP substrates using tertiarybutylphosphine (TBP) as a phosphorus source for safety and using (NH 4 ) 2 S x solution for surface stabilization. High crystalline quality AlGaP epi-layers with specular surfaces were obtained at relatively low growth temperatures ( 4 ) 2 S x surface treatment. The AlP/GaP superlattices were grown at 725°C with a V/III ratio of 20 by using the (NH 4 ) 2 S x treatment. The X-ray diffraction profile showed satellite reflection peaks up to as high as fifth order for the (AlP) 17 /(GaP) 17 superlattice.

CRITICAL THICKNESS OF INAS GROWN ON MISORIENTED GAAS SUBSTRATES

We grow InAs layers on (001) on‐axis and misoriented GaAs substrates by molecular beam epitaxy, respectively. The critical thickness of each InAs layer is investigated with photoluminescence spectroscopy and transmission electron microscopy (TEM). We show that the critical thickness is significantly influenced by the substrate misorientation. The critical thickness of the InAs layer grown on the GaAs substrate misoriented toward the [110] direction becomes thicker [5 monolayers (ML)] than that (3 ML) of the InAs layers grown on the GaAs substrates (001) on‐axis or misoriented toward the [110] direction. The plan‐view TEM images show that the islands grown coherently do not coalesce even beyond the on‐axis critical thickness (3 ML). The strain energy is calculated based on valence‐force‐field model to investigate the interaction between dislocation and step in case on misoriented substrate. As results, we show that dislocation has the minimum length above which dislocation can stably exist and that the ex...

Self-assembled InP islands grown on GaP substrate

InP islands are grown on a GaP substrate by organometallic vapor-phase epitaxy using tertiarybutylphosphine and characterized by atomic force microscopy and transmission electron microscopy. InP grows two-dimensionally at first and then begins to grow three-dimensionally at 1.2 ML. The island sizes are 400 nm in lateral dimension and 100 nm in height at 1.8 ML with the growth temperature of 550°C. The island density increases with increasing InP layer thickness while the island size remains the same. Misfit dislocations are observed in the islands at 1.8 ML growth. By lowering growth temperature to 420°C, the island size becomes as small as 40 nm in the lateral direction, moreover smaller islands without dislocations are obtained. The formation mechanism of large islands is discussed.

Substrate nitridation effect and low temperature growth of GaN on sapphire (0 0 0 1) by plasma-excited organometallic vapor-phase epitaxy

Abstract Nitridation of sapphire (0 0 0 1) substrate by plasma-excited N 2 for organometallic vapor-phase epitaxy (OMVPE) of GaN is investigated. The surface structure of nitrided layer and the relationship between the nitridation conditions and the crystalline quality of GaN epitaxial layer are investigated. The substrate surface becomes amorphous by the nitridation. An optimized nitridation improves the crystalline quality of the GaN epitaxial layer and is effective to decrease the growth temperature. A clear and transparent GaN epitaxial layer with a mirror-like surface can be grown at temperature as low as 550°C. Room temperature photoluminescence indicates no deep level emission but only a strong band-edge emission from the GaN layers grown at 680°C.

Enhanced electroluminescence of AlP/GaP disordered superlattice

Electroluminescent diodes with active layers of Al0.5Ga0.5P bulk alloy (b‐AL), (AlP)5/(GaP)5 ordered superlattice (o‐SL), and (AlP)m/(GaP)n (m,n=3,6,9) disordered superlattice (d‐SL) are fabricated by organometallic vapor phase epitaxy using tertiarybutylphosphine. Stronger injection electroluminescence is observed from the d‐SL diode as compared with the b‐AL and the o‐SL diodes. The total light output of the d‐SL diode is about 4–5 times stronger than that of the b‐AL and the o‐SL diodes. The result indicates the remarkable luminescence capability of the d‐SL.

Plasma-excited organometallic vapor phase epitaxy of GaN on (0 0 0 1)sapphire

Abstract GaN epitaxial thin films are grown on (0 0 0 1)sapphire by plasma-excited organometallic vapor phase epitaxy (OMVPE) in which RF-excited N 2 is utilized as reactive nitrogen source. Dependence of growth rate, surface morphology, and crystalline quality on growth conditions is systematically investigated. GaN epitaxial layer with a mirror-like surface is obtained without any buffer layer technique. This is attributed to extremely high nucleation density by the plasma-excited OMVPE at an early stage of the growth. Nucleation process of the plasma-excited OMVPE is compared with that of a conventional OMVPE using NH 3 .

Surfactant effects of Sn on SiGeSi heteroepitaxy by molecular beam epitaxy

Surfactant effects of Sn on SiGeSi heteroepitaxy by molecular beam epitaxy are investigated. The Sn adlayer strongly segregates onto both Si and Ge overgrowth layers. Both, observations of the reflection high energy electron diffraction and the transmission electron microscope indicate that Sn facilitates the formation of SiGe islands which is opposite to the effect reported for other surfactants. This effect leads to the formation of large islands. As a result, the critical thickness for misfit dislocations is reduced by Sn-mediated growth. However, surface segregation of Ge can be suppressed by using Sn. The surfactant effect of Sn on Ge island formation is discussed.