Takayoshi Maeda

Freestanding GaN Substrate by Advanced Facet-Controlled Epitaxial Lateral Overgrowth Technique with Masking Side Facets

We propose here an advanced facet-controlled epitaxial lateral overgrowth (FACELO) technique for obtaining a GaN layer with a low threading dislocation (TD) density. In the selective-area-growth (SAG) GaN with (11–22) facets, TDs from the underlying GaN are bent horizontally, i.e., toward the or direction. In the advanced FACELO technique, TDs are terminated by SiO2 masks on (11–22) facets. Except for coalescence regions, the TD density of the surface was less than 106 cm-2 for the GaN layer grown by MOVPE using the advanced FACELO technique. The side masks have an additional benefit of contributing to the void formation which facilitates the separation of the thick GaN films from the sapphire substrate.

Transmission Electron Microscopy Investigation of Dislocations in GaN Layer Grown by Facet-Controlled Epitaxial Lateral Overgrowth

We investigated the propagation of the threading dislocations in the GaN layer grown by facet-controlled epitaxial lateral overgrowth (FACELO). The mixed-type dislocations were bent toward the mask areas and they were terminated at the voids on the SiO2 masks. On the other hand, the pure edge dislocations were bent in the direction of the mask stripe. No dislocations originating from the GaN/sapphire interface propagated to the surface. As a result, it was confirmed that a large reduction of dislocation density was achieved. Therefore, FACELO seems to be a promising technique for the realization of a GaN wafer of low dislocation density.

Buried Tungsten Metal Structure Fabricated by Epitaxial-Lateral-Overgrown GaN via Low-Pressure Metalorganic Vapor Phase Epitaxy

A buried tungsten (W) structure with GaN is successfully obtained by epitaxial lateral overgrowth (ELO) technique via low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). The selectivity of GaN growth on the window regions is excellent. GaN with a striped W metal pattern is easily decomposed above the low temperature of 500°C by the catalytic effect of W. It is difficult to bury the W mask because severe damage occurs in the GaN epilayer under the mask. It is found that employing an underlying AlGaN/GaN heterostructure with a narrow W stripe mask width (L/S=2/2 µm) leads the epilayer to be free from damage, resulting in a good W buried structure.

Crystal Orientation Fluctuation of Epitaxial-Lateral-Overgrown GaN with W Mask and SiO2 Mask Observed by Transmission Electron Diffraction and X-Ray Rocking Curves

Fluctuation of the crystal orientation of epitaxial-lateral-overgrown (ELO) GaN with a tungsten (W) mask is compared to that with a SiO2 mask by means of selected-area transmission electron diffraction (TED) and X-ray rocking curves (XRCs). XRCs of (0004) reflection exhibited three peaks for ELO-GaN on a SiO2 mask, and were confirmed to originate from domains of different c-axis orientation by TED. On the other hand, fluctuation of the c-axis was not observed for ELO-GaN with a W mask. These results indicate that the crystalline quality of ELO-GaN with a W mask is better than that with a SiO2 mask.

Investigation of Thermal Annealing Process of GaN Layer on Sapphire by Molecular Dynamics

Molecular dynamics (MD) calculations have been performed to simulate the heteroepitaxial growth process of GaN on sapphire (0001) substrates and the thermal stability of the low-temperature GaN layer during the annealing process has been investigated. The MD simulations indicated that the surface morphology and the crystallinity of GaN thin layers grown on sapphire at low temperature of 800 K was amorphouslike and very rough. The surface atomic morphology of the low-temperature GaN layer was improved after the thermal annealing MD simulations at 1100 and 1200 K and showed atomically hexagonal structures. The crystallinity of the GaN epitaxial layer on the sapphire (0001) substrate is found to be strongly dependent on the temperature of annealing.

Computational Studies on GaN Surface Polarity and InN/GaN Heterostructures by Density Functional Theory and Molecular Dynamics

We have performed periodic density functional calculations and molecular dynamics (MD) simulations to investigate the reconstructions of the GaN (0001) surface and the heteroepitaxial growth process of InN thin films on the GaN surface. Grown GaN planes have a polar configuration. Surface energy calculations predict that the reconstruction of the N-terminated GaN (0001) surface is energetically more favorable than that of the Ga-terminated surface. MD results suggest that the growth of InN thin films on Ga- and N-terminated surfaces is different. On the N-terminated surface, the surface morphology of the grown InN layer is three-dimensional and rough. On the other hand, on the Ga-terminated surface, it is observed that the InN molecules have adequate migration mobility for growth and this suggests that the growth follows the two-dimensional growth mode.

Selective Area Growth (SAG) and Epitaxial Lateral Overgrowth (Elo) of GaN Using Tungsten Mask

Selective area growth (SAG) and epitaxial lateral overgrowth (ELO) of GaN using tungsten (W) mask by metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) have been studied. The selectivity of the GaN growth on the W mask as well as the SiO 2 mask is excellent for both MOVPE and HVPE. The ELO-GaN layers are successfully obtained by HVPE on the stripe patterns along the crystal axis with the W mask as well as the SiO 2 mask. There are no voids between the SiO 2 mask and the overgrown GaN layer, while there are triangular voids between the W mask and the overgrown layer. The surface of the ELO-GaN layer is quite uniform for both mask materials. In the case of MOVPE, the structures of ELO layers on the W mask are the same as those on the SiO 2 mask for the and stripe patterns. No voids are observed between the W or SiO 2 mask and the overgrown GaN layer by using MOVPE.

X-Ray Analysis of Twist and Tilt of GaN Prepared by Facet-Controlled Epitaxial Lateral Overgrowth (FACELO)

The tilt and twist of GaN prepared by facet-controlled epitaxial lateral overgrowth (FACELO) were evaluated by the X-ray rocking curve measurement of symmetric and asymmetric reflections. Compared to the underlying GaN, the tilt and twist values became substantially smaller at 130 and 250 arcsec, respectively, which are consistent with published results of transmission electron microscopy and cathode luminescence analyses. Moreover, FACELO-GaN is almost free of small angle tilt boundaries in contrast to GaN prepared by simple epitaxial lateral overgrowth.

Time-resolved nonlinear luminescence of excitonic transitions in GaN

Excitonic optical properties of GaN have been studied by means of time-resolved nonlinear luminescence spectroscopy, which was based on an excitation correlation technique under excitation at which a transition of the dominant radiative recombination process occurred from donor-bound excitons to biexcitons. Nonlinear luminescence observed at a donor-bound-exciton line resulted from the superposition of three components: a faster-decay component of a superlinear signal, a slower-decay component of a sublinear signal, and a positive base; that is, a much slower-decay component of a superlinear signal as compared with the time range employed in the present measurement (∼600 ps). The sublinear signal was attributed to the state-filling effect of donor-bound excitons, and the positive base was attributed to the saturation of nonradiative recombination centers. In addition, the superlinear signal was attributed to the stimulation of biexciton luminescence, which appeared close to the donor-bound-exciton lumines...

Investigation of Initial Growth Process of GaN Film on Sapphire Using Computational Chemistry

We have performed molecular dynamics (MD) simulations and periodic density functional calculations to investigate the interatomic interactions of the GaN/sapphire (0001) interface and the energetically stable site of GaN molecule on the sapphire (0001) surface. The MD simulations indicate that the mobility of the Ga atoms is larger than that of the N atoms on the sapphire (0001) surface, as the Al–N interaction is stronger than the O–Ga interaction. The density functional calculations also suggest that the formation of the Al–N bond is most energetically stable on the sapphire (0001) surface. Since the sapphire (0001) substrate is terminated with Al atoms in the reductive atmosphere, we assumed that the formation of Al–N bonds is dominant at the GaN/sapphire (0001) interface.