Advanced device applications of oxide semiconductors by physical vapor deposition

Abstract

Metal-oxide semiconductors are being studied as a next-generation semiconductor. The typical materials are IGZO(InGaZnO), ZnO, Ga2O3, CuxO, and CoOx. The metal-oxide semiconductors are multi-element compounds. Therefore, it could be possible to optimize the electrical characteristics as the density of states, carrier mobility and carrier density by changing the element compositions, crystal structures, etc. Peripheral environment sensitivity can also be attained, for instance, by including an optional revision. In addition, it is used in the fabrication of devices such as photodetectors, power devices, memory devices (ReRAM, Memristor, etc.), and synapse-mimetic devices. Chapter 1 describes the introductions with a background of metal-oxide semiconductors. Experimental and analysis in our result are addressed in Chapter 2. Here, detailed explanation of the device application, fabrication process, and analysis methods will be provided. Chapters 3 to 5 will demonstrate our research results of metal-oxide semiconductors. In Chapter 3, we have studied the effect of barrier controlled electrodes on characteristics of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) using an interlayer with modulated oxygen defects. Interlayers of a-IGZO with different electrical resistivity were deposited with various oxygen ratios during the RF sputtering. As the ratio of O2/(O2 + Ar) was increased from 0 to 20%, the carrier concentration decreased from 2.84 × 1018 to 1.56 × 1014 cm-3 and the electrical resistivity increased from 0.12 to 9600 Ω·cm. Using this result, a-IGZO thin layers with different resistivity (low and high) to control the contact barriers were inserted between the a-IGZO TFT channel and both the source and drain electrodes. In the case of a-IGZO TFT with a low resistivity interlayer, the threshold voltage (Vth) was shifted by -4.1 V compared to the reference device without an interlayer. In addition, on/off ratio, the subthreshold swing, and the mobility of the devices were also enhanced by achieving ohmic contact. In contrast, the a-IGZO TFT with a high resistivity interlayer showed a positive Vth shift of 1.5 V and also improved device performance, while maintaining a mobility of ~84% of the reference device due to the energy barrier. In Chapter 4, we have studied the resistive switching behaviors of cobalt oxide films with structural changes by a post-thermal annealing. During post-annealing at temperatures of 500-700 oC for the CoOx films deposited by ultra-high-vacuum RF magnetron sputtering, two structural phases of CoO and Co3O4 were modulated by selection of ambient N2 or O2 gases. As a device application of these films, the resistive random-access memory (ReRAM) structures of p+-Si/CoOx/Ti/Au were fabricated with CoO or Co3O4 films by post-annealing at 700 ℃ under N2 and O2 atmosphere, respectively. The resistive switching characteristics of the CoO device appeared much better than that of the Co3O4 device, also showing good uniformity from a cumulative probability distribution, long non-volatile retention characteristics of 104 s, good endurance of ~ 100 cycles, and an excellent current ratio of 104. These results demonstrate the method to provide stable and good resistive switching properties of sputtered CoO films using the phase selection during post-thermal annealing. In Chapter 5, we described metal–semiconductor–metal-structured β-Ga2O3 photodetectors using a plasma-assisted pulsed laser deposition system with various oxygen plasma radio frequency (RF) powers ranging from 0 to 100 W. All optoelectronic properties of the material were enhanced as the RF power increased. β-Ga2O3 photodetector with RF power of 100 W showed the best optoelectronic characteristics, such as photoresponsivity of 0.39 A/W, external quantum efficiency of 192.61%, and detectivity of 9.09 × 1013 cm Hz1/2/W. In addition, photo-switching analysis revealed the fastest photo-response speeds (1.46 s and 0.21 s) for on/off switching. These results originate from the decrease in the oxygen vacancy defect concentration in the β-Ga2O3 films by the oxygen RF power. Our results suggest that β-Ga2O3 photodetectors fabricated with oxygen plasma can optimize and improve the photodetection performance and can be applied for future deep ultraviolet detectors.