Chinkyo Kim

A crystallographic investigation of GaN nanostructures by reciprocal space mapping in a grazing incidence geometry

Reciprocal space mapping with a two-dimensional (2D) area detector in a grazing incidence geometry was applied to determine crystallographic orientations of GaN nanostructures epitaxially grown on a sapphire substrate. By using both unprojected and projected reciprocal space mapping with a proper coordinate transformation, the crystallographic orientations of GaN nanostructures with respect to that of a substrate were unambiguously determined. In particular, the legs of multipods in the wurtzite phase were found to preferentially nucleate on the sides of tetrahedral cores in the zinc blende phase.

Spontaneous inversion of in-plane polarity of a-oriented GaN domains laterally overgrown on patterned r-plane sapphire substrates

Spontaneously regulated in-plane polarity inversion of a-oriented GaN domains has been demonstrated for the first time. Crystallographic analysis revealed that each domain grown on circular-hole-patterned r-plane sapphire substrates has basal faces with oppositely oriented in-plane polarity. The inverted orientation of in-plane polarity on the opposite basal faces is not due to merging between in-plane polarity-inverted domains nucleated on the patterned r-plane sapphire substrate, but it was found to be due to spontaneous formation of an inversion domain boundary on the growth fronts of existing domains. This result provides new insights into controlling the in-plane polarity of a-oriented GaN, because the nucleation of in-plane polarity-inverted domains of a-oriented GaN on r-plane sapphire is symmetrically not allowed.

Controlled growth and surface morphology evolution of m-oriented GaN faceted-domains on SiO2-patterned m-plane sapphire substrates

m-oriented GaN faceted-domains were grown on SiO2-patterned m-plane sapphire substrates with no low-temperature-grown buffer layers, and their surface morphology evolution was investigated. The preferred crystallographic orientations of GaN domains are found to be sensitively influenced by substrate temperature. The growth rate of m-oriented GaN faceted-domains along the c-direction is found to be significantly suppressed after filling up the circular-shaped window regions. Our simple model calculation reveals that this can be explained by the minimization of surface energy increment per volume increment, and that the growth along the c-direction is energetically not favored until the domain reaches a critical size.

Mechanism of preferential nucleation of [\bf 1{\overline 1}0{\overline 3}]-oriented GaN twins on an SiO2-patterned m-plane sapphire substrate

The mechanism of preferential nucleation of [1{\overline 1}0{\overline 3}]-oriented GaN faceted twins on an SiO2-patterned m-plane sapphire substrate was investigated. Each variant of twins, which were enclosed by c- and m-facets, was observed to be preferentially nucleated over the opposite sides of an SiO2 pattern. It was hypothesized, from the fact that the same method of Legendre transformation is applied to a Wulff plot and a kinetic Wulff plot to determine the growth morphology of a crystalline domain, that the effective surface energy of c- and m-facets would be proportional to the growth rate of the respective facet. On the basis of this hypothesis, minimization of the effective surface energy of a nucleated domain was proposed as a mechanism of preferential nucleation. This proposed mechanism successfully explained the preferential nucleation behaviour.

Analysis of diffuse reflectivity of a highly disordered GaN nanostructure as an antireflection coating.

A highly disordered GaN nanostructure was grown by using hydride vapor phase epitaxy on an Si(100) substrate, and its diffuse reflectivity was measured in the 200-1200 nm spectral range by using an integrating sphere. A modified multiple-scattering-based model successfully explained the spectral distribution of diffuse reflectivity of this highly disordered GaN nanostructure.

Regularly branched InN nanostructures: zinc-blende nanocore and polytypic transition

Regularly branched InN nanostructures were controllably grown on c-plane sapphire substrates by using hydride vapor phase epitaxy. On the basis of the crystallographic analysis of these InN tetrapods, it is proposed that (i) each tetrapod consists of four nanorods in the wurtzite phase protruding from a truncated tetrahedral nanocore in the zinc-blende phase and that (ii) branching occurs at the four {111} faces of the truncated tetrahedral nanocore. Our work suggests that the branching regularity of InN tetrapods is attributed to the polytypic transition at these four faces.

Nanostructural analysis of GaN tripods and hexapods grown on c‐plane sapphire

The crystallographic and structural characteristics of GaN tripods and hexapods grown on c-plane sapphire substrates were investigated using synchrotron X-ray scattering and microscopic analysis. The core structure of a GaN hexapod is revealed to be in the zincblende phase with an inversion domain, and a refined crystallographic analysis of tripods and hexapods with synchrotron X-ray scattering shows the existence of the zincblende phase in wurtzite-based protruding nanorods. The atomistic model combined with this crystallographic analysis reveals that the core size of a hexapod is much smaller than the diameters of the protruding nanorods. This refined structural analysis can be utilized in tailoring the opto-electronic characteristics of GaN multipods.

Polarity-inverted lateral overgrowth and selective wet-etching and regrowth (PILOSWER) of GaN

On an SiO2-patterned c-plane sapphire substrate, GaN domains were grown with their polarity controlled in accordance with the pattern. While N-polar GaN was grown on hexagonally arranged circular openings, Ga-polar GaN was laterally overgrown on mask regions due to polarity inversion occurring at the boundary of the circular openings. After etching of N-polar GaN on the circular openings by H3PO4, this template was coated with 40-nm Si by sputtering and was slightly etched by KOH. After slight etching, a thin layer of Si left on the circular openings of sapphire,but not on GaN, was oxidized during thermal annealing and served as a dielectric mask during subsequent regrowth. Thus, the subsequent growth of GaN was made only on the existing Ga-polar GaN domains, not on the circular openings of the sapphire substrate. Transmission electron microscopy analysis revealed no sign of threading dislocations in this film. This approach may help fabricating an unholed and merged GaN film physically attached to but epitaxially separated from the SiO2-patterned sapphire.

Faceted growth of ({\bf {\overline 1}103})-oriented GaN domains on an SiO2-patterned m-plane sapphire substrate using polarity inversion

Heteroepitaxial growth of ({\overline 1}103)-oriented GaN domains on m-plane sapphire is energetically unfavourable in comparison with that of (1{\overline 1}0{\overline 3})-oriented GaN domains, but the faceted domains with ({\overline 1}103)-oriented GaN reveal a more m-facet-dominant configuration than (1{\overline 1}0{\overline 3})-oriented GaN in such a way that the quantum-confined Stark effect can be more effectively suppressed. It is reported here, for the first time, that semipolar ({\overline 1}103)-oriented and faceted GaN domains can be grown on an SiO2-patterned m-plane sapphire substrate by employing polarity inversion of initially nucleated (1{\overline 1}0{\overline 3})-oriented GaN domains. This polarity inversion of semipolar GaN was found to occur when the domains were grown with a 20–37.5 times higher V/III ratio and 70 K lower growth temperature than corresponding parameters for polarity-not-inverted domains. This work opens up a new possibility of effective suppression of the quantum-confined Stark effect by polarity-controlled semipolar GaN in an inexpensive manner in comparison with homoepitaxial growth of ({\overline 1}103)-oriented GaN on a GaN substrate.

Nanoclustering of vacancies in thin metal films revealed by x-ray diffuse scattering

The authors report the incorporation of unexpectedly large vacancy clusters into homoepitaxial Ag(001) films. These results, which are for a simple noble metal system, have important implications for understanding the atomic-scale kinetics of surfaces where current models have mostly ignored the role of vacancies. For films grown at 150 K, an average vacancy cluster exhibits a local dilatation volume of 750A3, which leads to a 1% compressive strain of the film. Vacancy clusters are observed even for films grown near room temperature. These in situ diffuse x-ray scattering experiments measure the local deformation around the cluster and, therefore, provide conclusive evidence of vacancy clusters.