Application of Ga2O3-based Thin Film Transistors and UV detection sensors

Abstract

The main goal of this dissertation is fabrication of the Ga2O3-based oxide semiconductors via co-sputtering method By the co-sputtering method the bandgap energy of the Ga2O3-based oxide semiconductors become tunable and controllable Additionally the Ga2O3 oxide semiconductor has the wider bandgap energy (i e 4 9 eV) and deep UV absorption (i e 250 nm) Hence it is expected that the Ga2O3-based oxide semiconductors can be used in the deep UV detection application Firstly we choose the use of a Ga2O3/ZnO alloying material as the active layer for applying in ultraviolet photodetectors and thin film transistors For UV PDs application under a 10-V applied bias the dark current the photo current and the corresponding IPhoto/IDark ratio are 2 9 × 10^-12 A 4 2 × 10^-9 A and 1 4 × 10^3 respectively When the fabricated device measured at their corresponding cutoff wavelengths with a 0 2-V applied bias the responsivity value is 2 7 × 10^-6 A/W Also the UV-to-visible rejection ratio is calculated as 2 5 × 10^2 Moreover we also apply Ga-Zn-O thin film as the active layer of thin film transistors At operation drain voltage of 4 V (VDS=4V) the threshold voltage (VT) filed-effect mobility (μFE) subthreshold swing (S S) and ION/IOFF ratio are 0 35 V 25 8 cm2/Vs 0 2 V/decade and 1 1 × 10^5 respectively Additionally we applied the Ga-Zn-O TFTs upon UV illumination to demonstrate the optical properties It can be found that the responsivity under cutoff wavelength and DUV-to-visible rejection ratio are 0 19 A/W and 1 1 × 10^2 On the other hand we demonstrate MSM UV photodetectors on glass substrate with Ga-In-O oxide semiconductors by co-sputtering using Ga2O3 and In2O3 targets at room temperature With 5 V applied bias it is found that measured the dark current the photo current and the corresponding IPhoto/IDark ratio are 2 3 × 10^-11 A 1 9 × 10^-9 A and 8 3 × 10^2 respectively For the optical property of MSM UV PDs under 5 V applied bias the responsivity measured at their corresponding cutoff wavelength is 6 9 × 10^-5 A/W while the UV-to-visible rejection ratio is 3 7×10^3 respectively The Ga-In-O thin films are also applied as the active layers of thin film transistors The fabricated device presents better electrical characteristics with a μFE of 7 6 cm2/Vs an SS of 0 2 V/decade and an on/off current ratio of ~10^5 We also use the fabricated device as the UV sensing detector under external UV excitation The responsivity under cutoff wavelength is 0 18 A/W and the DUV-to-visible rejection ratio is 4 × 10^4 For the electrical properties improvement of thin film transistors the bilayers channel structure has been proposed to carry out both the lower threshold voltage and the higher mobility The fabricated device presents excellent electrical characteristics with a μFE of 53 2 cm2/Vs an SS of 0 19 V/decade and an on/off current ratio of ~10^7 However when we applied the device in UV sensing the responsivity under cutoff wavelength and DUV-to-visible rejection ratio are 35 8 A/W and 1 1 × 10^2 respectively Compared with the single channel ones it can be obviously found that the rejection ratio is degraded due to the relative high density of oxygen vacancies for front channel Hence it indicates that the deposited condition of front channel needs to be fine-tuned for optical properties improvement

Date of Award	2018 Jun 6
Original language	English
Supervisor	Shoou-Jinn Chang (Supervisor)