Yoshihiro Kangawa

A new theoretical approach to adsorption–desorption behavior of Ga on GaAs surfaces

Abstract We propose a new theoretical approach for studying adsorption–desorption behavior of atoms on semiconductor surfaces. The new theoretical approach based on the ab initio calculations incorporates the free energy of gas phase; therefore we can calculate how adsorption and desorption depends on growth temperature and beam equivalent pressure (BEP). The versatility of the new theoretical approach was confirmed by the calculation of Ga adsorption–desorption transition temperatures and transition BEPs on the GaAs (0 0 1) - (4×2) β2 Ga-rich surface. This new approach is feasible to predict how adsorption and desorption depend on the growth conditions.

Theoretical approach to influence of As2 pressure on GaAs growth kinetics

Abstract The newly developed first-principles calculation based computational method incorporating chemical potential of As 2 gas is applied to understand the influence of As 2 pressure on GaAs growth kinetics under the molecular beam epitaxy growth conditions with high As 2 pressures where the c(4×4) reconstructed structure appears on the surface. The calculated results suggest that the chemical potential of As 2 gas increases with As 2 pressure, which suppresses As 2 (As-dimer) desorption or extends As 2 surface lifetime. This induces the decrease of GaAs growth rate, because GaAs layer-by-layer growth does not proceed without As 2 desorption on the As-rich c(4×4) surface.

Surface Stability and Growth Kinetics of Compound Semiconductors: An Ab Initio-Based Approach

We review the surface stability and growth kinetics of III-V and III-nitride semiconductors. The theoretical approach used in these studies is based on ab initio calculations and includes gas-phase free energy. With this method, we can investigate the influence of growth conditions, such as partial pressure and temperature, on the surface stability and growth kinetics. First, we examine the feasibility of this approach by comparing calculated surface phase diagrams of GaAs(001) with experimental results. In addition, the Ga diffusion length on GaAs(001) during molecular beam epitaxy is discussed. Next, this approach is systematically applied to the reconstruction, adsorption and incorporation on various nitride semiconductor surfaces. The calculated results for nitride semiconductor surface reconstructions with polar, nonpolar, and semipolar orientations suggest that adlayer reconstructions generally appear on the polar and the semipolar surfaces. However, the stable ideal surface without adsorption is found on the nonpolar surfaces because the ideal surface satisfies the electron counting rule. Finally, the stability of hydrogen and the incorporation mechanisms of Mg and C during metalorganic vapor phase epitaxy are discussed.

Growth of GaN directly on Si(111) substrate by controlling atomic configuration of Si surface by metalorganic vapor phase epitaxy

The direct growth of a GaN epitaxial layer on a Si(111) substrate by metalorganic vapor phase epitaxy (MOVPE) was performed using a low-temperature (LT)-GaN buffer layer with no Al-containing intermediate layer (e.g., AlN or AlGaN). No deterioration in the Si surface caused by the reaction between Si and Ga vapor was observed. However, when there were Ga droplets on the surface, Ga and Si formed a Ga?Si alloy, which caused the generation of numerous holes on the surface by melt-back etching at high temperatures. In addition, it was revealed that the coverage of the LT-GaN buffer layer on Si was strongly affected by the hydrogen (H2) partial pressure in the carrier gas. Using nitrogen (N2) carrier gas, a complete coverage of the LT-GaN buffer layer could be achieved directly over the Si surface. These features can be explained by the facts that the Si surface is partially terminated by hydrogen atoms and the coverage of hydrogen on Si surface depends on H2 partial pressure.

Anomalous behavior of excess energy curves of InxGa1−xN grown on GaN and InN

Abstract We worked out the excess energies for bulk In x Ga 1− x N and In x Ga 1− x N thin films on GaN and InN in order to investigate their thermodynamic stabilities. It has been found that the excess energy maximum shifted toward x ∼0.80 for InGaN/GaN and x ∼0.10 for InGaN/InN due to the lattice constraint in contrast with x ∼0.50 for bulk. Moreover, it has been revealed that the excess energy for InGaN/GaN is larger than that for bulk at x >0.65. This suggests that In-rich films are less stable on GaN than bulk state. These results indicate that the lattice constraint has a significant influence on thermodynamic stabilities of thin films.

Novel solution growth method of bulk AlN using Al and Li3N solid sources

In this work, we developed a solution growth method that uses Li–Al–N solution to epitaxially grow AlN on a self-nucleated, columnar AlN seed crystal. The seed crystal was grown by physical vapor transport, and the solution was obtained by annealing a Li3N–Al mixture. The epitaxial AlN grew ~5 µm in 10 h. Scanning electron microscopy analyses showed that the grown layer had many voids near the epilayer/seed interface, but no evidence of cracks. Using transmission electron microscopy analyses, we found that the growth direction of the AlN was [1100] and the layer had threading dislocation propagating along [1100] with a density of ~4×108 cm-2.

An empirical potential approach to structural stability of GaNxAs1−x

Abstract Structural stability of GaN x As 1− x including zinc blende (ZB)–wurtzite (W) structures and miscibility is systematically investigated based on a newly developed empirical potential, which incorporates electrostatic energies due to bond charges and ionic charges. Using the empirical potential, the system energies of ZB and W forms are calculated for bulk GaN x As 1− x over the entire concentration range. The calculated results predict that the structural phase transition from ZB to W occurs at x ∼0.4, which differs from x ∼0.7 estimated by electrostatic energy contributions. The shift of the ZB–W structural transition concentration toward x ∼0.4 is clarified in terms of difference in bond length between ZB- and W-GaN x As 1− x . Based on these findings, the miscibility of GaN x As 1− x is discussed by excess energy calculations.

Theoretical Investigation of the Effect of Growth Orientation on Indium Incorporation Efficiency during InGaN Thin Film Growth by Metal–Organic Vapor Phase Epitaxy

The effect of growth orientation on In incorporation efficiency in InGaN films grown by metal–organic vapor phase epitaxy (MOVPE) is theoretically investigated. We propose a new theoretical model that explains the role of the surface N–H layer in In incorporation based on first-principles calculations. During III–nitride MOVPE, N-terminated reconstruction with N dangling bonds passivated by H is stable. A surface N–H layer that covers a group-III (In, Ga) atomic layer prevents In atoms from desorbing and being replaced by Ga atoms. In incorporation is therefore more efficient for higher N–H layer coverage and stability. To investigate this relationship, the enthalpy change for the decomposition of a N–H layer was calculated. This enthalpy change which depends on growth orientations is in good agreement with the experimental In content.

Thermal conductivity of SiC calculated by molecular dynamics

We calculated the thermal conductivity of SiC by molecular dynamics simulation and investigated the effects of impurities on the thermal conductivity of SiC. We used Tersoff potential to express the structure of a SiC crystal. Thermal conductivity was obtained using Green–Kubos equation. The results show that the thermal conductivities of perfect 2H-, 3C-, 4H-, and 6H-SiC polytypes were in the range of 260 to 420 W/(m·K) and that the thermal conductivity of 3C-SiC was the largest among the polytypes. The thermal conductivities of 4H-SiC decreased with an increase in impurity concentration above 1.0×1017 to 1.0×1018 1/cm3.

InSb Mid-Infrared Photon Detector for Room-Temperature Operation

We developed a small InSb mid-infrared (2–7 µm wavelength range) photon detector that operates at room temperature. The photodiode was made from (hetero epitaxial) InSb layers that were grown on a semi-insulating GaAs substrate by molecular beam epitaxy. To suppress the effects of the diffusion current of the p–i–n photodiode, we used an AlInSb barrier layer that raises the resistance of the photodiode. We also optimized the devices doping concentration and the infrared incidence window structure. These optimization steps realized high photoelectric current output in a room-temperature environment. We also increased the signal-to-noise ratio of the detector by connecting multiple photodiodes in series. The size of this detector is 1.9×2.7×0.4 mm3 and the detectivity is 2.8×108 cm Hz1/2/W at 300 K. This is a practical IR detector that can be used in general signal amplification ICs.