Vertical α-Ga2O3 Device Applications Enabled by Same Crystal Structured Indium Tin Oxide Electrode via Mist Chemical Vapor Deposition

Extensive research has been conducted on gallium oxide (Ga₂O₃), an ultra-wide bandgap material, in the field of power electronics and deep UV optoelectronic devices. The band gap of Ga2O3 ranges from 4.4 to 5.3 eV, surpassing the bandgap of Si, SiC, and GaN, thus indicating its potential for enhanced device performance. Among the polymorphs of Ga₂O₃, namely α, β, δ, γ, and κ, α-Ga₂O₃ is characterized by the highest bandgap of 5.3 eV. [1] This α phase exhibits promising potential in power-switching and deep-UV optoelectronics devices, such as Schottky barrier diodes and photodetectors, owing to its ultra-wide bandgap. [2-3] A vertical device structure can provide low on-resistivity and rapid response. One way to realize vertical structure is by inserting a conductive layer under the active layer. However, the growth of metastable α-Ga₂O₃ necessitates an epitaxial growth on other substrates, and an insulating α-Al₂O₃ substrate is usually used, making realizing vertical structure challenging. To address this issue, we featured high-conductive rhombohedral indium tin oxide (rh-ITO) as the bottom electrode. This rh-ITO exhibits the same crystal structure as α-Ga2O3; therefore, it is expected that α-Ga₂O₃ is grown epitaxially on the rh-ITO electrode. This study demonstrates heteroepitaxial growth of α-Ga₂O₃ on rh-ITO electrode and its device applications.<br/>Mist CVD was utilized for the epitaxial growth of oxide thin films. Initially, we investigated the growth of α-Ga₂O₃ on rh-ITO, however, orthorhombic κ-Ga₂O₃ was grown on bare rh-ITO thin films because of the large lattice mismatch between α-Ga₂O₃ and rh-ITO (-9.2%). Therefore, to decrease the lattice mismatch, α-Fe₂O₃ buffer layers with a small lattice mismatch (-1.0 %) were utilized for the growth of α-Ga₂O₃ thin films. XRD 2θ-ω scans determined that the α-Fe₂O₃ buffer layer allowed the growth of α-Ga₂O₃ thin films on rh-ITO thin films. Furthermore, XRD phi scans revealed that α-Ga₂O₃ thin films were successfully grown in the same in-plane orientations as rh-ITO and c-plane sapphire substrates. These XRD measurements indicated that α-Ga₂O₃ thin films were epitaxially grown on the rh-ITO/c-plane sapphire substrates.<br/>Next, we conducted device operations using α-Ga₂O₃ thin films with rh-ITO thin films as bottom electrodes, and fabricated two device architectures, deep-UV photodetectors, and Schottky barrier diodes. For photodetectors, PEDOT:PSS thin films, exhibiting high transparency in the deep UV region to the visible light region, high conductivity, and large work function were utilized for top electrodes. PEDOT:PSS thin films were deposited by mist deposition[4]. The n-type α-Ga₂O₃ and PEDOT:PSS showed Schottky behavior enabling the separation of the photo-exited hole-electron pairs. The spectral photoresponsivity of the photodetector was measured using a short-circuit current and demonstrated the ability to detect UV-C region light without any bias voltage. The photoresponsivity at 220 nm was 9.7×10^-4 A/W, exceeding that of a self-powered lateral structured α-Ga2O3 photodetector, thereby suggesting its association with the vertical structure, which can reduce the recombination of photogenerated carriers[3]. Furthermore, we investigated the Schottky barrier diode based on α-Ga₂O₃ with Ni/Au electrode deposited by thermal evaporation. Current-voltage measurements confirmed the rectification characteristics of the diode, which can be attributed to the Schottky contact between Ni and α-Ga₂O₃. These results demonstrate the device applications of α-Ga₂O₃ grown on rh-ITO bottom electrodes using mist chemical vapor deposition, including deep-UV photodetectors and Schottky barrier diodes.<br/>References<br/>[1] D. Shinohara, and S. Fujita, Jpn. J. Appl. Phys. 47, p. 7311(2008)<br/>[2] M. Oda, et al., Appl. Phys. Express 9, p.021101(2016)<br/>[3] J. Bae, et al., APL Mater. 9, p. 101108(2021)<br/>[4] T. Ikenoue, et al., Thin Solid Films, 520, p.1978(2012)