Authors: Kishwar Mashooq
Source: Deep Blue (University of Michigan) (article, 2023-01-01)
Field: Materials Science — Polymers and Plastics

Advances in p-type and Ambipolar Oxide Semiconductor Materials and Devices for Thin Film Electronics As artificial intelligence, neural networks, internet of things, and autonomous robots continue to emerge, the need for high performance computing technology becomes increasingly vital. Meeting this demand requires not only the continuous advancement of traditional Si-based computing technology, but also the flexibility to support new technologies. Thin film electronics, particularly thin film transistors (TFTs), are being actively explored to enable ongoing development in computing technology. The key requirements for TFT technology include uniform large area deposition, low temperature processing, low complexity, and low cost, which allow TFTs to be used in monolithic integration as back-end-of-line (BEOL) devices. Oxide semiconductors are promising materials for TFT applications due to their balanced performance in mobility, robustness, and spatial uniformity. As a result, n-type oxide semiconductors have already been successfully commercialized in display backplanes and are a fairly mature technology. However, the absence of p-type oxide semiconductor TFTs with performance equivalent to n-type TFTs limits further development of oxide semiconductor technology. Tin monoxide (SnO) is a well-known p-type oxide. Unlike other p-type oxides, the thermal budget of SnO TFTs is low, making it more suitable for BEOL device integration. This thesis builds on this context by investigating p-type and ambipolar SnO and Ta2SnO6 (a SnO alloy) film devices for complementary metal-oxide semiconductor device technologies. The first part of my work focused on identifying the process window for p-type SnO using RF sputtering. I discovered that Sn droplets that appear to be on SnO films do not negatively impact the electrical performance of p-type SnO thin film transistors. I investigated the factors limiting p-type SnO TFT performance using numerical simulation. I found that the high density of interface donor-like tail states and the high contact resistance are the dominant factors limiting p-type SnO TFT on current, while the high density of shallow, bulk Gaussian acceptor-like defects is responsible for poor on-off current ratio. To address these limitations, I introduced a floating metal capping layer on top of the p-type SnO channel and demonstrated record performance for p-type SnO TFTs. By incorporating a metal capping layer and surface passivation, I was able to significantly improve both TFT mobility and the on-off current ratio in SnO TFTs. SnO is one of very few oxide semiconductors that show ambipolar behavior. I developed ambipolar SnO TFTs with symmetric n-type and p-type conduction by implementing appropriate post-passivation annealing. This led to the development of an ambipolar SnO inverter with the highest inverter gain among any ambipolar inverters. An analytical model was developed to corroborate our experimental results and provide a pathway for further advancement of ambipolar inverter performance. Finally, I worked on identifying the process boundaries of p-type Ta2SnO6 deposited by co-sputtering. The electrical and material properties of the films and TFTs were analyzed as a function of deposition and annealing process conditions. This thesis provides insights to guide future research to realize high performance p-type stannate oxide semiconductor materials and devices. Thin-film transistor ; Electronics ; Thin film ; Semiconductor ; Transistor ; Ambipolar diffusion ; Semiconductor device ; Context (archaeology) Transition Metal Oxide Nanomaterials ; Ga2O3 and related materials ; Semiconductor materials and devices