| Thin film transistors(TFTs) have important application in active matrix organic light emitting diode display(AMOLED) as switch drive semiconductor devices. According to the different properties of channel material, TFT devices mainly consist of amorphous silicon(a-Si) based and amorphous oxide semiconductor(AOS) based TFT. Currently, a-Si TFTs have been considered as one of the most successful commercialization TFTs owing to their advantages of being low temperature processing, low stress, good large area uniformity and low cost. However, the field effect mobility for a-Si TFT is less than 1 cm2/V.s and can’t be used in the AMOLED display. Nowadays, AOS TFT based on amorphous indium zinc oxide(a-IZO) and amorphous indium gallium zinc oxide(a-IGZO) materials have offered an alternative to a-Si based devices and been expected to become the core element for new generation display due to their high electron mobility, room temperature processing, high transmittance in visible light range and good uniformity. In this dissertation, we focus on the properties of amorphous silicon germanium(a-SiGe) and a-IZO thin films and their application in TFT devices. The major works are summarized as follows:1. The nanocrystallization, structural evolution and photoelectrical properties of hydrogenated amorphous silicon germanium(a-Si1-xGex:H) thin films deposited by plasma enhanced chemical vapor deposition(PECVD) diluted with argon gas have been investigated. The results show that argon dilution plays a significant role in the growth of nanocrystalline grains in a-Si1-xGex:H thin films. It is found that the nanocrystalline grains can be observed in amorphous network when the argon dilution ratio is more than 4. Moreover, with the rise of argon dilution ratio, nano-grain size and crystalline volume fraction of thin films increase while the dark conductivity and optical band gap decrease. Boron doped nanocrystalline Si0.78Ge0.22:H thin film is assessed for use as resistive sensing layer in uncooled infrared bolometer and as channel layer in TFT devices.2. The effect of microwave annealing on the field effect mobility and threshold voltage of a-IZO TFT is investigated. A control device with traditional hotplate annealing at 200 °C for 1 hour was applied for comparison. The results show that both microwave annealing and hotplate annealing increase the field effect mobility from 12.3 cm2/V.s in as-deposited state to ~19 cm2/V.s in annealed state. However, the negative shift in threshold voltage with microwave annealing(from 0.23 V to –2.86 V) is smaller than that with hotplate annealing(to –9 V). An oxygen vacancy mechanism related with the electrical properties of a-IZO material is proposed. This rapid low-temperature annealing technology makes a-IZO TFTs promising for use in flexible, transparent electronics.3. A new approach to reducing the contact resistance at compositional conducting/semiconducting IZO homojunctions used for contacts in thin film transistors is developed. By introducing silver nanoparticles(Ag NPs) at the homojunction interface between the conducting IZO electrodes and the amorphous IZO channel, we reduce the specific contact resistance, obtained by transmission line model(TLM) measurements, down to 4.4×10-2 ?.cm2, ~3 orders of magnitude lower than either NP-free homojunction contacts(36 ?.cm2) or solid Ag metal contacts(68 ?.cm2). The resulting back-gated TFTs with Ag NP contacts exhibit good field effect mobility of ~27 cm2/V.s and an on/off ratio > 107. We attribute the improved contact resistance to electric field regulation and reconfiguration by the Ag NPs.4. The thickness of in-situ oxide layer as function of annealing time and temperature for a-IZO/Ti/a-IZO and a-IZO/Al/a-IZO sandwich structures is investigated by high resolution transmission electron microscope(HRTEM). It is found that the thickness increases with the increase of annealing time and temperature. We fabricated top-gated IZO TFT with a 3 nm layer of aluminum between the IZO active layer and high-k HfO2 gate insulator. A series of anneals at 300 oC was used to convert the Al metal into Al2O3. The results show that 8-hour-annealed TFT with Al layer has an effective field mobility of 115 cm2/V·s, an on/off ratio > 107, a threshold voltage |Vth| < 0.5 V and a subthreshold slope of 0.14 V/decade in devices with LG = 50 μm gate length. For smaller devices with LG = 5 μm, the threshold voltage, subthreshold slope and on/off ratio are similar, but the effective field mobility are lower. Cross-sectional HRTEM images and the capacitance-voltage characteristics with little hysteresis confirm that the thin Al layer, converted in-situ into Al2O3, can protect the IZO channel during processing and produce a good high-k dielectric gate stack.
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