Sanghwa Lee

A self-passivated Cu(Mg) gate electrode for an amorphous silicon thin-film transistor

The feasibility of using Cu(Mg) alloy film as a gate electrode for thin-film transistor (TFT) liquid crystal displays has been investigated. When pure Cu was used as a gate electrode, severe interdiffusion occurred between Cu and the gases SiH4, NH3, and CF4 during plasma-enhanced chemical vapor deposition of a gate dielectric, SiNx, and dry etching of the SiNx. On the other hand, the deposition of a Cu(Mg) alloy film gives rise to the formation of a MgO/Cu bilayer structure with low Cu resistivity, good adhesion to SiO2, higher leakage current density, and excellent passivation capability. A hydrogenated amorphous silicon TFT with a MgO encapsulated Cu gate exhibited a gate voltage swing of 0.91 V/dec. and a threshold voltage of 6.8 V, resulting in a reduction of process steps and better performance.

Nitridation- and Buffer-Layer-Free Growth of [1100]-Oriented GaN Domains on m-Plane Sapphire Substrates by Using Hydride Vapor Phase Epitaxy

Over a wide range of growth conditions, GaN domains were grown on bare m-plane sapphire substrates by using hydride vapor phase epitaxy (HVPE), and the relation between these growth conditions and three possible preferred crystallographic orientations ([1100], [1103], [1122]) of GaN domains was investigated. In contrast with the previous reports by other groups, our results revealed that preferentially [1100]-oriented GaN domains were grown without low-temperature nitridation or a buffer layer, and that the growth condition of preferentially [1100]-oriented GaN was insensitive to V/III ratio.

Surface-morphology evolution and strain relaxation during heteroepitaxial growth of GaN films without low-temperature nucleation layers

With no low-temperature nucleation layers, heteroepitaxial GaN films were grown at 1050°C on c-plane sapphire substrates by hydride vapor phase epitaxy. Not like for the films grown on low-temperature buffer layers, upon island merging no immediate smoothing of a surface was observed, but through several intermediate stages the surface morphology evolved from three-dimensional islands to terrace-and-step structures. From synchrotron x-ray diffraction measurements, it was revealed that the density of screw-or mixed-type threading dislocations greatly increased when the islands merged, but that of edge-type dislocations did not (edge-type threading dislocations are known to be dominantly formed in the GaN films grown on low-temperature buffer layers). This implies that the evolution of surface morphology sensitively depends on the type of threading dislocations introduced during island merging. Despite the absence of intentional nucleation layers, the strain was found to be fully relaxed even before the nuc...

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.

A surface flattening mechanism of a heteroepitaxial film consisting of faceted non-flat top twins: [11¯03¯]-oriented GaN films grown on m-plane sapphire substrates

We carried out experiments and computational simulations in order to answer a yet unanswered question about a surface flattening mechanism of a [11¯03¯]-oriented GaN film consisting of faceted non-flat top twins. Our results revealed that an overgrowth of one variant of twins over the other, which was manifested only at a thickness larger than a few microns due to a slight asymmetric crystallographic tilt (1.0° ± 0.4°) of twins, played a key role in a surface flattening mechanism. In addition, we experimentally demonstrated that GaN grown on a SiO2-patterned m-plane sapphire substrate had no asymmetric tilt and that no surface flattening occurred.

Self-regulated in-plane polarity of [11¯00]-oriented GaN domains coalesced from twins grown on a SiO2-patterned m-plane sapphire substrate

In-plane polarity of [11¯00]-oriented GaN domains coalesced from twins grown on a SiO2-patterned m-plane sapphire substrate was observed to be self-regulated in such a way that basal faces of coalesced domains were mainly found to have the (0001¯) polarity only. This self-regulation behavior of in-plane polarity was explained by a computational simulation of plan-view surface morphology evolution during coalescence of twins. Based on a computational simulation, asymmetrically suppressed growth rates of twins near a SiO2 pattern were proposed to be responsible for the survival of the slower growing (0001¯) basal faces instead of the faster growing (0001) basal faces during coalescence of twins.

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.

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.

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.