Tomonori Ito

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.

Ab initio-based study for adatom kinetics on AlN(0001) surfaces during metal-organic vapor-phase epitaxy growth

The kinetics of Al and N adatoms on reconstructed AlN(0001) surfaces under growth conditions is investigated by performing first-principles pseudopotential calculations. Our calculations reveal that the adsorption of Al adatom strongly depends on the surface reconstruction while its diffusion is not affected by the reconstruction: the adsorption of Al adatom on the surface under N-rich conditions is much easier than that under H-rich conditions. These results indicate that the growth of AlN during metal-organic vapor-phase epitaxy is prominent under N-rich conditions rather than H-rich conditions, consistent with experimentally reported growth rate difference.

Reconstructions on AlN Nonpolar Surfaces in the Presence of Hydrogen

Structures and stability of nonpolar AlN(100) and (110) surfaces under hydrogen rich conditions are theoretically investigated by performing total-energy calculations within the density functional theory. The calculated surface energies demonstrate that several hydrogen incorporated structures are favorable depending on the chemical potentials of constituting elements. However, H atoms desorb and the relaxed ideal surfaces are stabilized even under the metal–organic vapor-phase expitaxy growth. These results suggest that the growth processes on AlN nonpolar surfaces are quite different from those of polar surfaces.

Progress in theoretical approach to InGaN and InN epitaxy: In incorporation efficiency and structural stability

The surface stability, growth process, and structural stability of InGaN and InN are reviewed from a theoretical viewpoint. In 2001, a new theoretical approach based on an ab initio calculation was developed. This theoretical approach enables the investigation of the influence of growth conditions such as partial pressure and temperature on the surface stability. The theoretical approach is applied to the research on the In incorporation efficiency in InGaN grown on nonpolar and semipolar surfaces. The calculation results suggest that the N?H layer formed on such surfaces has a crucial role in In incorporation. Moreover, the structural stability of InN grown by pressurized-reactor MOVPE is reviewed. It was found by the theoretical approach that facet formation causes the spontaneous formation of islands with the zinc-blende structure.

Empirical interatomic potential approach to the stability of graphitic structure in ANB8−N compounds

Empirical bond order potential (BOP) with the aid of ab initio calculations is applied to investigate the structural stability of 3-fold coordinated graphitic structure for binary octet ANB8−N compounds including C, BN, BeO, Si, and AlP. The graphitic structure is favored in C and BN with small ratio of equilibrium bond length and large bond order p(3) where superscript corresponds to the coordination number. Employing simple electrostatic interaction, the structural phase diagram is successfully obtained as functions of and p(3) using critical bond order for stabilizing the graphitic structure.

Electronic bands and excited states of III-V semiconductor polytypes with screened-exchange density functional calculations

The electronic band structures and excited states of III-V semiconductors such as GaP, AlP, AlAs, and AlSb for various polytypes are determined employing the screened-exchange density functional calculations implemented in the full-potential linearized augmented plane-wave methods. We demonstrate that GaP and AlSb in the wurtzite (WZ) structure have direct gap while III-V semiconductors in the zinc blende, 4H, and 6H structures considered in this study exhibit an indirect gap. Furthermore, we find that inclusion of Al atoms less than 17% and 83% in the hexagonal AlxGa1−xP and AlxGa1−xAs alloys, respectively, leads to a direct transition with a gap energy of ∼2.3 eV. The feasibility of III-V semiconductors with a direct gap in WZ structure offers a possible crystal structure engineering to tune the optical properties of semiconductor materials.

Diffusion of carbon oxides in SiO2 during SiC oxidation: A first-principles study

The diffusion mechanisms of CO and CO2 molecules in SiO2 during SiC oxidation are theoretically investigated by means of total-energy calculations within the density-functional theory. We find characteristic features of the stable structures of carbon oxides depending on polymorph of SiO2. The calculated formation energies and diffusion energy barriers of CO and CO2 in SiO2 also reveal that the CO2 can be a dominant species of product gas caused by SiC oxidation. On the basis of calculated results, we propose that the outward diffusion of carbon oxides as well as the reaction processes at SiO2/SiC interface is rate-limiting during SiC oxidation on the Si-face.