Transparent complementary thin film transistors with an active oxide layer

Abstract

In the past decades, amorphous oxide semiconductors have been investigated as an active layer in thin-film transistors (TFTs) instead of amorphous silicon and poly-Si TFTs, due to their high on/off current ratio, and high electrical performance. Oxide semiconductors are expected to have good uniformity in TFT performance, resulting from the amorphous phase and an absence of grain boundaries. Also Oxide thin-film transistor (TFT)-based electronic components such as inverters and logic circuits are attractive due to their low-coat, low-temperature fabrication, and mass-production. In addition to exploring high performance oxide TFTs, oxide thin film transistors based on both p-type and n-type active layers in one device with simplified circuit design and fabrication processes are an alternative approach to realizing integrated circuits like radio-frequency identification (RFID) tags or drivers for display applications. The next major challenge in oxide semiconductors TFTs is to realize complementary metal-oxide-semiconductor (CMOS) circuits. In this thesis, we investigated the effect of a rapid thermal annealing process in oxygen ambient on the performance of a-IGZO film transistor fabricated by rf magnetron sputtering deposition. The optimized operation characteristics of a-IGZO thin film transistor (TFTs) were improved to a sub-threshold voltage swing of 4.72 V/decade, a field effect mobility of ~1.16 cm2/Vs, and a threshold voltage (Vth) of -1.7 V by the RTA in oxygen ambient at 300 oC for 5 min. The difference in the electrical properties as a function of the annealing conditions was attributed to the number of oxygen vacancies. From X-ray photoemission spectroscopy (XPS), we confirmed that the time of the annealing can reduce the generation of oxygen vacancy inside the channel and balance the diffusion of oxygen from the channel. Among increasing time, Annealing at 5 min should be optimized for enhanced performance.Also we fabricated a-IGZO TFT with AZO/Ag/AZO (AAA) transparent multilayer source/drain contacts by rf magnetron sputtering. a-IGZO TFT with AZO/Ag/AZO multilayer S/D electrodes (W/L = 400/50 μm) showed a sub threshold swing of 3.78 V/dec, a minimum off-current of 10-12 A, a threshold voltage of 0.41 V, a field effect mobility of 10.86 cm2/Vs, and an on/off ratio of 1.69x108. From the ultraviolet photoemission spectroscopy (UPS), it revealed that the enhanced electrical performance result from the lowering of Schottky barrier between a-IGZO and Ag due to the insertion of AZO layer and thus the AZO/Ag/AZO multilayer transparent electrode would be very promising for an efficient S/D contact material for the fabrication of the high performance TFTs. In order to characteristics of p-type TFTs, Mn(5wt%):SnO precursor solutions were prepared by sol-gel process and their spin-coated thin films on SiO2/Si were annealed at 300oC for 115 min in air. All the crystalline structure of Mn:SnO films were amorphous and showed very smooth surface roughness as much as . All the films show high transmittance in the visible wavelength over 90 %. From Seebeck measurement, electrically they showed that +68 μV/K indicating p-type characteristic conductivity. The electrical resistivities of the grown Mn:SnO films were in the range of 102-104cm measured by four-point probe and the film annealed for 10 min showed the highest value of 104cm. X-ray photoemission microscopy revealed that Mn existed in the several different Mn2+,Mn3+, and Mn4+ valence states and relative ratio of Mn3+ for the film annealed for 10 min showed the largest in Mn3d5/2 core-level spectra. Thin film transistor using Mn: SnO annealed for 10 min was fabricated SiO2/Si and the device had a field-effect mobility of 16.6 cm2/Vs and an on/off ratio 4.14  104. These results give evidence that the Mn element incorporated SnO is a very promising material of high performance p-channel oxide TFT. Finally, Complementary metal oxide semiconductor (CMOS) –like inverters using the p-channel Mn:SnO and n-channel IGZO transistors were demonstrated with a gain up to 1.5. These results also demonstrate that, a simple route in realizing oxide-based TFTs and CMOS-like inverters, provides a robust addition to the existing CMOS technology community.