arXiv subjects

Chinkyo Kim

Publications and source records attributed to Chinkyo Kim.

9 recordsLinked to original sources

arXiv2026

GaN Nucleation Landscape on Patterned Sapphire Shaped by the Growth Temperature of Directly Grown Boron-Compound Masks

The growth temperature of directly grown boron-compound masks on patterned sapphire can modify the local accessibility of the underlying sapphire surface and thereby alter the subsequent nucleation behavior of GaN. In this work, we investigate how ammonia-borane-derived boron-compound masks grown at different temperatures shape the GaN nucleation landscape within circular SiO$_2$ openings during the initial stage of epitaxial lateral overgrowth. The preferential nucleation position of GaN changes systematically with mask growth temperature: masks grown at 700--750$^\circ$C produce pronounced edge-biased distributions, whereas higher-temperature masks lead to more inward-shifted and spatially sparse GaN domains. Quantitative analysis of the GaN areal fraction, the number of visibly isolated domains, and the radial distribution of domain centers shows that the mask growth temperature affects both the amount of GaN coverage and the spatial arrangement of GaN domains within each opening. The nonmonotonic change in the number of visibly isolated domains is interpreted as a consequence of competition between reduced lateral merging and reduced effective substrate accessibility, rather than as a direct measure of the number of active nucleation sites. Kinetic Monte Carlo simulations reproduce the essential experimental trends by varying the effective density and radial distribution of substrate-accessible sites. These results suggest that the growth temperature of directly formed boron-compound masks provides a practical means of reshaping the intra-opening GaN nucleation landscape by controlling the spatial distribution and effectiveness of local pathways through which GaN precursors can access the underlying sapphire surface.

cond-mat.mtrl-sci↗

arXiv2026

Propagation-mediated amplification of \{11\={2}0\}-biased inversion domain boundary alignment in polarity-mixed GaN lateral overgrowth

GaN polarity inversion and the associated inversion domain boundaries (IDBs) are frequently observed during lateral overgrowth and are often discussed in terms of the small energetic spread among competing IDB structures predicted by first-principles calculations. In circular mask openings, $\{11\bar{2}0\}$-aligned IDBs have previously been explained by geometric closure of a single-polarity hexagonal domain at the circular boundary. Here we examine an experimentally distinct regime in which opposite-polarity domains already coexist within the opening before the later development of long, straight IDB traces. In this mixed-polarity regime, the final trace orientation cannot be attributed solely to the macroscopic circular boundary. Nevertheless, plan-view SEM line-trace statistics show that IDB orientations remain biased toward the $\{11\bar{2}0\}$ family. To quantify how this bias develops during propagation, we perform distance-resolved, length-weighted orientation analysis in concentric annular regions defined from the opening center. The resulting metrics show that $\{11\bar{2}0\}$-biased alignment is progressively amplified with propagation distance, while the orientation distribution becomes narrower, indicating systematic sharpening of the preferred alignment state. We further apply the same ring-resolved statistical operators to minimal two-domain propagation simulations in a circular opening and find that a propagation-mediated anisotropy reproduces the observed radial amplification under fixed circular geometry. Together, these results establish a quantitative phenomenology of $\{11\bar{2}0\}$-biased IDB alignment in polarity-mixed GaN lateral overgrowth on patterned sapphire and indicate that, although mask-boundary-imposed selection may describe single-polarity closure cases, the present mixed-polarity regime is better explained by propagation-mediated amplification.

cond-mat.mtrl-sci↗

arXiv2025

Controlling GaN nucleation via O$_2$-plasma-perforated graphene masks on c-plane sapphire

Atomically thin, perforated graphene on $c$-plane sapphire functions as a nanoscale mask that enables GaN growth through thru-holes. We tune the perforated-area fraction $f_p$ by controlled O$_2$-plasma exposure and quantify its impact on early-stage nucleation: the nucleation-site density scales with $f_p$, while the nucleation-delay time decreases approximately as $1/f_p$. Time-resolved areal coverage and domain counts exhibit systematic $f_p$-dependent trends. A kinetic Monte Carlo (kMC) model that coarse-grains atomistic events -- adatom arrival, surface diffusion, attachment at exposed sapphire within perforations, and coalescence (the first front-front contact between laterally growing domains) -- reproduces these trends using a constant per-site nucleation rate. Fitting the kMC simulation data yields onset times t$_0$ for the nucleation delay that closely match independently observed no-growth thresholds (Set 1: 28.5s vs $\sim$30s; Set 2: 38s vs $\sim$35s), validating the kMC-experiment mapping and highlighting plasma dose as an activation threshold for plasma-induced through-hole formation in 2D materials. Together, experiment and kMC identify $f_p$ as a single, surface-engineerable parameter governing GaN nucleation statistics on perforated graphene masks, providing a quantitative basis and process window for epitaxial lateral overgrowth (ELOG)/thru-hole epitaxy (THE) workflows that employ two-dimensional masks.

cond-mat.mtrl-sci↗

arXiv2025

Scalable thru-hole epitaxy of GaN through self-adjusting $h$-BN masks via solution-processed 2D stacks

Selective epitaxy on 2D-material masks is a promising pathway for achieving localized, defect-suppressed GaN growth, but conventional 2D transfer processes limit scalability and interface control. Here, we demonstrate a thru-hole epitaxy (THE) method that enables vertically connected and laterally overgrown GaN domains through a spin-coated, solution-processed stack of hexagonal boron nitride ($h$-BN) flakes. The disordered $h$-BN mask exhibits a self-adjusting structure during growth, which locally reconfigures to allow percolative precursor transport and coherent GaN nucleation beneath otherwise blocking layers. Comprehensive structural analyses using scanning electron microscopy, Raman mapping, and high-resolution transmission electron microscopy confirm both the presence of epitaxial GaN beneath the h-BN and suppression of threading dislocations. This strategy eliminates the need for patterned 2D mask transfers and demonstrates a scalable route to selective-area GaN growth on arbitrary substrates, relevant to future micro-LED and photonic integration platforms.

cond-mat.mtrl-sci↗

arXiv2025

TBA-enabled spin-coating of a percolatively connected GO nanosieve for thru-hole epitaxy: tuning GO flake stacking and coverage to control GaN nucleation

We report a spin-coating-based approach for forming a percolatively connected graphene oxide (GO) nanosieve on SiO$_2$-patterned sapphire substrates, where the addition of tetrabutylammonium (TBA) to the GO solution significantly improves the uniformity of flake coverage and modulates GaN nucleation behavior. Upon thermal annealing of GO, the resulting reduced graphene oxide (rGO) films exhibit spatially varying coverage, leading to three distinct GaN nucleation outcomes: (i) ELOG-like nucleation on exposed substrate regions, (ii) thru-hole epitaxy (THE)-like nucleation through appropriately thin areas, and (iii) complete nucleation suppression on thickly stacked zones. On spin-coated GO films without TBA, all three behaviors coexist, and undesired ELOG- and no-nucleation modes persist due to uneven coverage. Importantly, these issues cannot be resolved by simply adjusting GO flake concentration, as concentration tuning alone fails to eliminate the formation of locally bare and overly thick regions. In contrast, the addition of TBA results in a more uniform, moderately stacked rGO morphology that suppresses both ELOG- and no-nucleation modes while expanding THE-like nucleation regions. This reshaped nucleation landscape confines GaN growth to areas with engineered percolative transport. The approach offers a scalable, lithography-free route for controlling GaN epitaxy using solution-processable 2D material masks.

cond-mat.mtrl-sci↗

arXiv2024

A new critical growth parameter and mechanistic model for SiC nanowire synthesis via Si substrate carbonization: the role of H$_2$/CH$_4$ gas flow ratio

SiC structures, including nanowires and films, can be effectively grown on Si substrates through carbonization. However, growth parameters other than temperature, which influence the preferential formation of SiC nanowires or films, have not yet been identified. In this work, we investigate SiC synthesis via Si carbonization using methane (CH$_4$) by varying the growth temperature and the hydrogen to methane gas flow ratio (H$_2$/CH$_4$). We demonstrate that adjusting these parameters allows for the preferential growth of SiC nanowires or films. Specifically, SiC nanowires are preferentially grown when the H$_2$/CH$_4$ ratio exceeds a specific threshold, which varies with the growth temperature, ranging between 1200$^\circ$C and 1310$^\circ$C. Establishing this precise growth window for SiC nanowires in terms of the H$_2$/CH$_4$ ratio and growth temperature provides new insights into the parameter-driven morphology of SiC. Furthermore, we propose a mechanistic model to explain the preferential growth of either SiC nanowires or films, based on the kinetics of gas-phase reactions and surface processes. These findings not only advance our understanding of SiC growth mechanisms but also pave the way for optimized fabrication strategies for SiC-based nanostructures.

physics.app-ph↗

arXiv2023

Orientation disparity in GaN/graphene/$m$-sapphire: control-based re-examination of thru-hole epitaxy

The crystallographic orientation of films grown on 2D-masked substrates is often used to infer the pathway among remote, van der Waals, and thru-hole (pinhole-seeded) epitaxy. However, attribution of a specific growth mechanism based on orientation can be ambiguous unless mask continuity and substrate pre-treatment are evaluated within a single process window. We compare GaN grown under identical conditions on four m-plane sapphire templates: (i) bare, (ii) "graphene-grown" (high-temperature Ar/H2 with CH4 on), (iii) "anneal-only" (high-temperature Ar/H2 with CH4 off), and (iv) graphene oxide spin-coated and reduced on pristine sapphire. GaN selects (103) on graphene-grown and anneal-only m-plane sapphire, selects (100) on bare m-plane sapphire, and is predominantly (100) with a minority (103) on graphene oxide spin-coated and reduced/pristine m-plane sapphire. High-resolution TEM shows that, on partly graphene-covered samples, nucleation occurs on exposed sapphire (thru-hole), not on graphene, providing mechanism evidence independent of orientation. Within this window, the substrate surface state set by high-temperature Ar/H2 pre-treatment (rather than mask continuity) primarily governs orientation, while open-area effects can play a secondary role. Thus, preferred orientation alone may not determine the growth mechanism; mask continuity and substrate pre-treatment must be explicitly controlled when using orientation as evidence for mechanism assignment.

cond-mat.mtrl-sci↗

arXiv2022

Establishing Epitaxial Connectedness in Multi-Stacking: The Survival of Thru-Holes in Thru-Hole Epitaxy

Thru-hole epitaxy has recently been reported to be able to grow readily detachable domains crystallographically aligned with the underlying substrate over 2D mask material transferred onto a substrate. [Jang \textit{et al.}, \textit{Adv. Mater. Interfaces}, \textbf{2023} \textit{10}, 4 2201406] While the experimental demonstration of thru-hole epitaxy of GaN over multiple stacks of $h$-BN was evident, the detailed mechanism of how small holes in each stack of $h$-BN survived as thru-holes during multiple stacking of $h$-BN was not intuitively clear. Here, we use Monte Carlo simulations to investigate the conditions under which holes in each stack of 2D mask layers can survive as thru-holes during multiple stacking. If holes are highly anisotropic in shape by connecting smaller holes in a particular direction, thru-holes can be maintained with a high survival rate per stack, establishing more epitaxial connectedness. Our work verifies and supports that thru-hole epitaxy is attributed to the epitaxial connectedness established by thru-holes surviving even through multiple stacks.

cond-mat.mtrl-sci↗

arXiv2021

Thru-Hole Epitaxy: Is Remote Epitaxy Really Remote?

The remote epitaxy was originally proposed to grow a film, which is not in contact but crystallographically aligned with a substrate and easily detachable due to a van der Waals material as a space layer. Here we show that the claimed remote epitaxy is more likely to be nonremote `thru-hole' epitaxy. On a substrate with thick and symmetrically incompatible van der Waals space layer or even with a three-dimensional amorphous oxide film in-between, we demonstratively grew GaN domains through thru-holes via connectedness-initiated epitaxial lateral overgrowth, not only readily detachable but also crystallographically aligned with a substrate. Our proposed nonremote thru-hole epitaxy, which is embarrassingly straightforward and undemanding, can provide wider applicability of the benefits known to be only available by the claimed remote epitaxy.

cond-mat.mtrl-sci↗