Investigations of the properties of ZnO-based TCO thin films grown via rf sputtering deposition method on flexible substrates for thin-film transistor applications

Abstract

Zinc oxide (ZnO) thin films with n-type conductivity and high transparency have been applied in devices such as transparent electrodes photovoltaic cells and thin-film transistors (TFTs) In this work ZnO-based films were deposited by RF sputtering method and ZnO films were used as the buffer layers to enhance the figure of merit adhesion properties and bending durability properties of Ga-doped ZnO (GZO) transparent conductive oxide thin films on flexible PES substrates With the addition of an optimized 100-nm-thick ZnO buffer layer the transmittance carrier concentration Hall mobility resistivity and figure of merit of GZO films improved from 88 3 to 94 45% -2 89×1021 to -3 39×1021 cm-3 1 76 to 7 97 cm2/V-s 1 32×10-3 to 2 201×10-4 Ω-cm and 2 40×10-2 to 3 20×10-1 Ω-1 respectively With insertion of 100-nm-thick ZnO buffer layers the strain of GZO films without ZnO after outward and inward bending for 2000 cycles decreased from 2 063 × 10-3 and 2 203 × 10-3 to 1 74 ×10-3 and 1 966 × 10-3 respectively Accordingly the GZO/ZnO/PES structure can be a viable alternative for increasing the bending durability of GZO films deposited on PES substrates for use in flexible optic-electrical devices In addition effect of UV-ozone treatment on the performance of ZnO TFTs fabricated by RF sputtering deposition technique was systematically investigated As a deposition method sputtering was applicable to the low temperature process with high deposition rate Moreover The ZnO films were then subjected to 16 minutes of ultraviolet (UV)-ozone treatment which resulted in fewer oxygen vacancies enhanced crystallization lower strain lower surface roughness and higher thin film density as well as improved surface energy and adhesion properties of the gallium zinc oxide (GZO) source/drain electrodes To study amorphous metal oxide thin films InGaZnO (IGZO) films were prepared by RF sputtering deposition technique for application as active layers in TFTs IGZO TFTs are attractive for use in active-matrix organic light-emitting diode (AMOLED) displays due to their high electrical mobility transparency in visible light and high on/off ratios In this research top-gate bottom-contact thin-film transistors (TFTs) made with amorphous indium gallium zinc oxide (?-IGZO) active layers were grown using the radio-frequency sputtering technique Two kinds of source and drain (S/D) electrodes namely bi-layer GZO/100-nm ZnO buffer layer/Corning 1737 and single-layer GZO/Corning 1737 used in the TFT devices and the electric characteristics of the devices were compared The bi-layer GZO/100-nm ZnO buffer layer S/D electrodes were more stable and suitable for fabricating TFTs because of their low surface roughness of 0 817 nm and high thin film density of 5 94 g/cm3 Additionally the bi-layer GZO/100-nm ZnO buffer layer had better adhesion to neighboring ?-IGZO active layers due to its high surface energy of 62 07 mJ/m2 The μsat S S Nt and ION/OFF values of the top-gate bottom-contact ?-IGZO TFT with bi-layer GZO/100-nm ZnO buffer layer S/D electrodes were 13 5 cm2V-1S-1 0 43 V/decade 5 65× 1012 eV-1cm-2 3 56 × 107 respectively Finally the ?-IGZO/5-nm GZO double active layer structure was applied to fabricate low-temperature high-performance sputter-processed ?-IGZO TFTs Generally the ?-IGZO/5-nm GZO double active layer thin films were very stable and suitable than ?-IGZO single active layer for fabricating the thin film transistors because of its low surface roughness of 1 89 nm and high thin film density of 5 87 g/cm3 Additionally ?-IGZO/GZO demonstrated the best adhesion properties to the neighboring thin film layers based on its high surface energy of 60 07 mJ/m2 The μsat S S Nt and ION/OFF of the bottom-gate ?-IGZO/GZO double-active layer devices characteristics were 18 92 cm2V-1S-1 0 33 V/decade 4 25 × 10 12 eV-1cm-2 and 1 33 × 108 respectively The obtained results demonstrate that the ?-IGZO/GZO double active layer TFT could be taken as a TFT structure candidate for application in large-area-flat-panel displays

Date of Award	2014 Aug 11
Original language	English
Supervisor	Sheng-Yuan Chu (Supervisor)