Investigation of Tungsten-doped Indium Oxide Thin-film Fabricated by RF Co-sputtering System and Their Optoelectronics Applications

Abstract

In this thesis we deposit tungsten-doped indium oxide (InWO) thin-films by RF magnetron co-sputtering method and discuss the optical and electrical properties of the thin-film under different process conditions At the same time InWO thin-films will be used as an active layer for UV photodetectors (PDs) and channel layer for thin-film transistors (TFTs) Also we optimize the optical and electrical properties of InWO thin-films to improve device performance In the first part of the experiment we use the RF magnetron co-sputter deposition method co-sputtering two targets indium oxide (In2O3) and tungsten oxide (WO3) simultaneously By adjusting the sputtering power of the WO3 target we deposited InWO thin-films with different tungsten doping ratios InWO thin-film properties are also controlled by the thermal annealing process Then we analyze InWO thin-films through the crystal structure elemental composition surface and depth analysis optical and electrical characteristics The results of atomic force microscopy indicate that the root-mean-square value of the surface roughness shows a trend from decline to rise with an increase in the proportion of tungsten doping ratio Transmission electron microscope guarantee the real thickness and interface quality of the stacked thin-films structure The results of the X-ray diffraction spectrum show that the InWO thin-films with different tungsten doping ratios have the rhombohedral structure and their crystallinity decreases with the increase of the tungsten doping ratios and thermal annealing also increases their crystallinity The absorption spectrum shows that InWO thin-films have higher than 80% transmittance in the visible and near-infrared light regions result in excellent transparent conductive oxides (TCOs) characteristics The results of X-ray photoelectron spectroscopy show that as the proportion of tungsten doping ratios increases the oxygen vacancies in the InWO film decrease and the number of free carriers decreases accordingly The energy dispersive X-ray spectroscopy confirmed the actual weight percentage and atomic percentage of tungsten doping ratio Hall effect measurement calculates the carrier concentration of tungsten-doped indium oxide film and indium oxide Finally a secondary ion mass spectroscopy is used to analyze the main ions in the InWO thin-films In the second part of the experiment we apply InWO thin-films to UV PDs We set different sputtering power of the WO3 target and various thermal annealing gas ambient as the changing parameters The influence of the optical and electrical properties of UV PDs was discussed As the proportion of tungsten doping ratio increases the photocurrent and dark current of the photodetector decrease significantly Because of the oxygen bond dissociation energy of tungsten and oxygen is larger which can effectively reduce the number of oxygen vacancies in the InWO thin-films Although it can increase the stability of the film at the same time reduces the number of free carriers Also when InWO thin-films are thermally annealed in oxygen ambient its photo-dark current ratio can be greatly improved Due to the introduction of oxygen gas during the thermal annealing process can further rearrange the lattice and fufill the oxygen deficiency region In conclusion UV PDs with the sputtering power of the In2O3 target 80 Watt and the sputtering power of the WO3 target 5 Watt then thermal annealing for 1 hour in oxygen ambient has the best performances with ON/OFF current ratio greater than 106 the responsivity of 160 A/W and UV-to-visible rejection ratio of 4 71×104 Also the time-dependent switching properties have been demonstrated In the third part of the experiment we demonstrate TFTs with silicon dioxide (SiO2) as the gate insulator and InWO thin-films as a channel layer Again we adjusting the power of the WO3 target to modulate the tungsten doping ratio of the InWO thin-films and controlling the sputtering time to change channel layer thickness The influence of the optical and electrical characteristics of TFTs was discussed When the InWO thin-film at an appropriate tungsten doping ratio the on-current can be boosted while maintaining a small and stable off current and the switching speed of the transistor can be improved due to reducing of subthreshold swing The reason is that when tungsten atoms take place in the lattice of the InWO thin-film properly creating several additional free carriers Also changing channel layer thickness will effectively adjust the threshold voltage of the TFTs By finding the appropriate threshold voltage value the power consumption of the TFTs can be reduced In conclusion we searched for the best optical and electrical properties of the TFTs We found that when the sputtering power of the In2O3 target is 80 Watt and the sputtering power of the WO3 target is 10 Watt with 20 nm channel layer TFTs have the best performance The result calculated from transfer characteristics exhibit field-effect electron mobility of 11 6 cm2/V ? s the threshold voltage of 0 28 V the subthreshold swing of 0 21 V/dec and the ON/OFF current ratio is 1 02 × 107 Also we extend the application of InWO TFTs to phototransistors With a gate bias of -2 V the UV-to-visible rejection ratio gives was 5 82 × 108 Moreover by reducing the drain bias from 8 V to 0 1 V the UV-to-visible rejection ratio remains 2 34 × 107 which agrees with the excellent power-saving characteristics of the InWO TFTs Also the time-dependent switching properties have been demonstrated In the fourth part of the experiment to simplify the fabrication process and improve the interface properties between the top electrode and channel layer we made InWO TFTs with a homojunction structure by using an indium oxide thin-film as the drain and source electrode The channel layer and top electrode are simultaneously deposited by RF magnetron sputtering Due to the homojunction between the channel layer and the top electrode the switching properties of the transistor are improved Also because the In2O3 thin-film has high transmittance in the visible light region the transparency of the TFTs was significantly enhanced In conclusion when using the In2O3 target to deposit the top electrode with 80 Watt the performance of the homojunction InWO TFTs can be further improved We found that when the sputtering power of the In2O3 target is 80 Watt for top electrode TFTs have the best performance The result calculated from transfer characteristics exhibit field-effect electron mobility of 20 1 cm2/V ? s the threshold voltage of 0 89 V the subthreshold swing of 0 17 V/dec and the ON/OFF current ratio is 1 45 × 106 Again we extend the application of InWO TFTs to phototransistors With a gate bias of -2 V the UV-to-visible rejection ratio gives was 5 82 × 108 Also the time-dependent switching properties have been demonstrated

Date of Award	2020
Original language	English
Supervisor	Shoou-Jinn Chang (Supervisor)