Kyoto University · 재료과학
켄코 히로카와 교수의 연구실은 초광역역대 반도체 소재인 코르룬드형 산화물(α-Ga₂O₃, α-Ir₂O₃, α-Fe₂O₃ 등)을 중심으로 고성능 전력 소자 및 페로마그네틱 소재의 개발을 주요 연구 방향으로 삼고 있습니다. 특히, 미세구조 분석과 전자현미경, 광전자 스펙트로스코피를 활용한 고정밀 표면 및 인터페이스 분석을 통해 고순도 박막의 성장과 전기적·자기적 특성 제어를 추구하고 있습니다. 또한, p형 도핑을 통한 pn 헤테로재조합 소자 구현과 함께, 열적 안정성과 전기적 성능이 뛰어난 TO220 봉제 패ackaging 기반 소자 구현에도 성공했습니다.
표시된 성과는 수집된 데이터 기준으로 산출되며, 일부 차이가 있을 수 있습니다.
Corundum-structured α-Ga 2 O 3 epitaxial thin films were grown on c -plane α-Al 2 O 3 (sapphire) substrates by a mist chemical vapor deposition method. To reveal the defect structures, the α-Ga 2 O 3 film was observed by high-resolution transmission electron microscopy (TEM). We found that the α-Ga 2 O 3 thin film was in-plane compressive stressed from the α-Al 2 O 3 substrate. Although misfit dislocations were periodically generated at the α-Ga 2 O 3 /α-Al 2 O 3 interface owing to the large lat
Corundum-structured iridium oxide (α-Ir2O3), showing p-type conductivity, is a strong candidate to form high-quality pn heterojunctions with α-Ga2O3. We fabricated α-Ir2O3/α-Ga2O3 pn heterojunction diodes and they showed well-defined rectifying current-voltage (I-V) characteristics with the turn-on voltage of about 2.0 V. The band alignment at the α-Ir2O3/α-Ga2O3 interface was investigated by X-ray photoemission spectroscopy, revealing a staggered-gap (type-II) with the valence- and conduction-b
Corundum-structured oxides have been attracting much attention as next-generation power device materials. A corundum-structured α-Ga2O3 successfully demonstrated power device operations of Schottky barrier diodes (SBDs) with the lowest on-resistance of 0.1 mΩ cm2. The SBDs as a mounting device of TO220 also showed low switching-loss properties with a capacitance of 130 pF. Moreover, the thermal resistance was 13.9 °C/W, which is comparable to that of the SiC TO220 device (12.5 °C/W). On the othe
Highly crystalline corundum-structured α-(Ga1-xFex)2O3 alloy thin films were fabricated on c-plane sapphire substrates by using a mist chemical vapor deposition method. The full-widths at half maximum of X-ray diffraction rocking curves were smaller than 100 arcsec for the entire range of x from 0 to 1. Optical band gaps were artificially tuned to a value between those of α-Ga2O3 and α-Fe2O3, that is, 2.2 and 5.3 eV with changing the Fe content x in the films. Magnetic measurements revealed ferr
Highly crystalline corundum structured α-(Ga0.42Fe0.58)2O3 alloy thin film showed magnetic properties at room temperature. Microstructure analysis of cross-sectional transmission electron microscope (TEM) observation and TEM energy dispersive X-ray spectroscopy measurement indicated that different crystal phase could not be detected as well as there is no remarkable phase separating area, that is, Fe and Ga ions are distributed uniformly in the film. Magnetic measurements were performed on α-(Ga
Ultra-wide bandgap p-type α-(Ir,Ga)2O3 films with bandgaps of up to 4.3 eV have been obtained by unintentional doping or Mg doping. For Mg-doped films, Hall-effect measurements revealed a hole concentration of 9.9 × 1018 to 8.1 × 1019 cm−3 and a mobility of 0.13 − 0.92 cm2/V s, respectively. A preliminary test of a pn junction diode composed of p-type α-(Ir,Ga)2O3 and n-type α-Ga2O3 did not show catastrophic breakdown in the reverse direction until 100 V and the current on/off ratio at +3 V/−3V
Routes to semi-stable phases of Ga2O3 are the subject of extended discussions based on the review of growth methods, growth conditions, and precursors in works that report semi-stable phases other than the thermally stable β phase. The focus here is on mist chemical vapor deposition because it has produced single-phase Ga2O3 of α, γ, and ε (or κ) in terms of the substrate materials, and features of this growth method for phase control are emphasized. Recent reports of phase control by other grow
Recently, rutile germanium dioxide (r-GeO2) has emerged as a novel ultra-wide bandgap semiconductor due to its theoretical excellent properties, that is, high thermal conductivity, ambipolar dopability, and high carrier mobility, in addition to its wide bandgap (4.44–4.68 eV). In this study, r-GeO2 thin films were grown on (001) r-TiO2 substrates by mist chemical vapor deposition. To optimize the growth conditions, we analyzed the decomposition processes of the Ge source (C6H10Ge2O7) by thermogr
Abstract We propose a novel alloy system of oxide semiconductors promising for unique multifunctions, the corundum structured Ga 2 O 3 ‐Cr 2 O 3 ‐Fe 2 O 3 alloys on sapphire substrates. With this system the optical band gap can be widely tuned from 2.2 to 5.3 eV while keeping the acceptable lattice mismatch. A key issue was to fabricate α‐Ga 2 O 3 thin films, which has been recognized as a sub‐stable phase and there is a strongly tendency of growing at β‐Ga 2 O 3 with traditional growth techniqu
Abstract Single-phase rocksalt-structured Mg x Zn 1− x O ( x > 0.5) alloy films were grown on MgO substrates using the mist chemical vapor deposition method. A specular surface with a step and terrace structure was obtained. The bandgap was tuned from 5.9 to 7.8 eV as x varied from 0.5 to 1. Deep ultraviolet cathodoluminescence, stemming from near band edge transitions, was observed for Mg 0.57 Zn 0.43 O in the 4.8–5.5 eV range, peaking at ∼5.1 eV (∼240 nm) in the 12–100 K range.
Abstract Recent progress in α -phase gallium oxide ( α -Ga 2 O 3 ) grown on sapphire for low-cost and practical device applications is reviewed. This review focuses on (i) dislocations formed by heteroepitaxy, (ii) p-type conductivity (a common issue with β -Ga 2 O 3 ), and (iii) thermal instability due to the metastable phase of α -Ga 2 O 3 , and discusses efforts aimed at overcoming these issues. The results reveal guidelines for the dislocation density (<1 × 10 8 cm −2 ) so that the disloc
Coating of stainless steel (SUS) separators of polymer electrolyte fuel cells (PEFCs) with tin oxide (SnO2) thin films by nonvacuum-based mist chemical vapor deposition technology has been proposed for cost-effective and highly endurable cells. Fluorine-doped SnO2 resulted in low electrical resistivity of the films and high corrosive resistance of the SnO2-coated SUS separators. The novel pretreatment of SUS substrates successfully enhanced the strength of SnO2 adhesion to the substrates. The PE
Rocksalt-structured MgZnO thin films of optical band gaps as large as about 5.8–6.6 eV were grown on MgO substrates by a mist chemical vapor deposition method using carbon-free precursors. MgZnO thin films with the dislocation density of less than 107 cm−2 were obtained. Pure deep-ultraviolet cathodoluminescence was observed without noticeable emission at the longer wavelengths, being attributed to improved crystallinity as well as reduced carbon impurity. The highest emission energy (the shorte
Abstract (‐201)‐oriented beta gallium oxide (β‐Ga 2 O 3 ) thin films were grown on yttrium‐stabilized zirconia (YSZ) substrates using a mist chemical vapor deposition (CVD) method. The lowest full‐width at half maximum value in ω‐scan X‐ray diffraction was 0.50°, obtained for the growth temperature of 650 °C on a YSZ(100) substrate. Observation by a secondary electron microscope (SEM) revealed flat surface including large‐scale precipitates. Electron diffraction patterns suggested that thin film
