Roy Byung Kyu Chung

Remote epitaxy of the corundum phase α-Ga ₂ O ₃ is demonstrated through a domain-free, transfer-free monolayer of MoS ₂ template synthesized on sapphire. Despite the intrinsic nonidealities of MoS ₂ —specifically its thickness and polar bonding characteristics—for remote epitaxy, single-crystalline α-Ga ₂ O ₃ films are epitaxially grown on polycrystalline MoS ₂ with crystallographic fidelity comparable to those grown directly on bare sapphire. This observation substantiates that the substrate lattice potential can still be effectively transmitted through the MoS ₂ layer, facilitating epitaxial registry at the growth front. The resulting α-Ga ₂ O ₃ films exhibit crystal quality and surface morphology on par with those grown directly on bare sapphire. Moreover, a centimeter-scale, crack-free α-Ga ₂ O ₃ membrane was successfully exfoliated using a Ni stressor, underscoring the mechanical compatibility and scalability of the remote epitaxial process. Structural disordering observed with more-than-bilayer MoS ₂ further corroborates the remote epitaxy mechanism. First-principles calculations reveal that interfacial interactions between the bottom S atoms of MoS ₂ and the Al atoms of sapphire induce spatially modulated charge distributions on the MoS ₂ surface, creating energetically favorable adsorption sites for Ga species. Collectively, these findings establish a mechanistic framework for charge-mediated remote epitaxy across polar interfaces and expand the design space for heterogeneous integration of transferable wide bandgap Ga ₂ O ₃ membranes in next-generation power electronics and optoelectronic devices.

Ultrawide-bandgap (UWBG) semiconductors are central to next-generation power electronics and deep-UV optoelectronics, yet the absence of reliable p-type materials remains a critical bottleneck. Here, we demonstrate stable intrinsic p-type conductivity in the spinel UWBG oxide ZnGa2O4, epitaxially grown on c-plane sapphire via mist chemical vapor deposition. The films exhibit high crystalline quality, evidenced by relatively narrow rocking curves and atomically smooth surfaces. Hall measurements confirm stable p-type conduction with a hole mobility of 4.43 cm2 V−1 s−1 and a carrier concentration on the order of 1015 cm−3 in the optimized film at room temperature. By off-stoichiometrically tuning the Zn/Ga precursor ratio, we reveal that intrinsic p-type behavior originates from the antisite defect, establishing cation disorder as an important mechanism governing carrier polarity. Comprehensive structural, optical, electrical, and first-principles analyses indicate the interplay between defect chemistry and charge transport in spinel oxides. These results position ZnGa2O4 as a viable p-type UWBG semiconductor and introduce defect-controlled spinel oxides as promising materials for future high-power and extreme-environment electronics.

Thermal stability and phase transformation of conductive α-(Al0.16Ga0.84)2O3/Ga2O3 heterostructure on sapphire substrates were investigated via in situ high temperature x-ray diffraction, scanning electron microscopy (SEM), and atomic force microscopy (AFM). The conductive α-(Al0.16Ga0.84)2O3/Ga2O3 heterostructure with fluorine (F) doping was grown by mist-chemical vapor deposition on sapphire substrates, achieving a Hall mobility of 28 cm2/(V s). The heterostructure exhibited thermal stability up to ∼550–575 °C before transforming to β-(AlxGa1−x)2O3/Ga2O3. The transformed β-Ga2O3 is mainly polycrystalline rather than a high-quality epitaxial phase. Reciprocal space mapping results reveal that the edge dislocation density remains consistently higher than the screw dislocation density throughout the heating process, indicating that the crystalline imperfection in α-Ga2O3 is dominated by in-plane mosaicity. After the phase transformation from the α phase to the β phase, catastrophic damage to the film and upheaval of the surface were observed by SEM and AFM.

This study examines the effects of 5 MeV proton irradiation, applied at fluences of 1 × 10 ¹¹ , 1 × 10 ¹² , 1 × 10 ¹³ , and 1 × 10 ¹⁴ cm ⁻² , on 20 nm thick SnO _2‐x thin films and field‐effect transistors (FETs) with Sn‐doped In ₂ O ₃ (ITO) electrodes. In the SnO _2‐x films, both carrier concentration and mobility increased steadily with rising fluences. The irradiated ITO layers displayed higher carrier concentrations but reduced mobilities compared with their pristine counterparts. Higher conductivity of the constituent films resulted in a negative shift in threshold voltage (V _th ) and an enhanced drain current (I _DS ) in the SnO _2‐x FETs. The greater the fluence, the more pronounced these changes became. Regardless of the fluence, the on/off ratios of the FETs remained stable at ∼10 ⁶ , indicating minimal structural damage. Moreover, the irradiation‐induced changes are not temporary. When the irradiated devices are stored in air for a month or annealed at 200 °C in air for 2 h, both I _DS and V _th became more stabilized. Collectively, these results demonstrate that SnO _2‐x FETs offer radiation tolerance and electrical stability, making them strong candidates for radiation‐hardened electronics in space environments.

Nonconventional epitaxial techniques, such as van der Waals epitaxy and remote epitaxy, have attracted substantial attention in the semiconductor research community for their capability to repeatedly produce high-quality freestanding films from a single mother wafer. Successful implementation of these techniques depends on creating a robust, uniform two-dimensional (2D) material surface. The conventional method for fabricating graphene on silicon carbide (SiC) is high-temperature graphitization. However, the extremely high temperature required for silicon sublimation (typically above 1500°C) causes step bunching, forming nonuniform multilayer graphene stripes and an unfavorable surface morphology for epitaxial growth. Here, we developed a wafer-scale graphitization technique that allows fast synthesis of single-crystalline graphene at low temperatures by metal-assisted graphitization. In contrast to previous reports, we found annealing conditions enabling SiC dissociation while avoiding silicide formation, producing uniform single-crystalline graphene while maintaining the pristine surface morphology of the substrate. We successfully produce high-quality freestanding single-crystalline III-N (AlN and GaN) membranes on graphene/SiC via the 2D material–based layer transfer technique.

In this work, we investigated the heterointerface formed between metastable α-Ga₂O₃ (corundum) and κ-Ga₂O₃ (orthorhombic). When κ-Ga₂O₃ was grow on c-plane sapphire at temperatures above 570 °C via mist chemical vapor deposition, α-Ga₂O₃ initially formed, followed by a spontaneous phase transition from α- to κ-Ga₂O₃, resulting in an atomically abrupt interface. Despite this abrupt interface, the process was uncontrolled, leading to an uneven interface on a micrometer scale. To address this, the growth process was optimized, identifying strain and temperature as the key control parameters to achieve a uniform and atomically abrupt interface. The band offset at the κ-/α-Ga₂O₃ interface was analyzed using X-ray photoelectron spectroscopy. The results revealed a type-II band alignment, with the valence and conduction bands of κ-Ga₂O₃ positioned approximately 1.00 and 0.80 eV higher, respectively, than those of α-Ga₂O₃. This was further corroborated by density functional theory calculations. The observed band offset at the pristine κ-/α-Ga₂O₃ interface holds promise for advanced device designs, offering potential applications in future electronic and optoelectronic technologies.

Gyeong Ryul Lee, Cheolho Yang, and Roy Byung Kyu Chung. Applied Science and Convergence Technology 2024;33:18-22. https://doi.org/10.5757/ASCT.2024.33.1.18