Solution-processed Amorphous Oxide Semiconductors for Thin-film Power Management Circuitry

Abstract

Thin-film electronics has opened up new applications not achievable by wafer-based electronics. Following commercial success in displays and solar cells, the future industry sectors for thin film devices are limitless, and include novel wearable electronics and medical devices. Such new applications enabled by human-size electronics have been widely investigated, but their potential use in power-management circuitry has been seldom addressed. The key strengths of thin-film electronics are that they can be deposited on various substrates at a large-area scale, and they can be additively deposited on existing device layers without degrading them. These advantageous features can be used to overcome the current barriers facing silicon (Si) electronics in power-management applications. Namely, thin film electronics can be used to directly deposit circuits including power harvesters on RFID tags to reduce the current tag cost based on Si IC. Furthermore, they can be directly heterointegrated with Si chips to enhance their voltage handling capability. Finally, thin film electronics can be deposited onto solar cell arrays to improve efficiency by managing partial shading conditions. Among thin-film materials, we explore the scope of solution-derived amorphous oxide semiconductor (AOS) due to its high carrier mobility, wide band-gap, and in-air deposition capability. In this thesis, we push the boundaries of AOS by (i) developing an air-stable, ink-based deposition process for high-performance amorphous zinc-tin-oxide semiconductor. We choose a deposition process based on metal-organic decomposition, such that the film properties are independent of relative humidity in the deposition ambient, enabling future large-area roll-to-roll processing. (ii) Second, by exploiting in situ chemical evolution, namely reduction and oxidation, at the interface of zinc-tin-oxide and various metal electrodes (primarily Pd, Mo, and Ag), we intentionally manipulate the electrode contact properties to form high-quality ohmic contacts and Schottky barriers. We explain the results based on competing thermodynamic processes and interlayer diffusion. (iii) Third, we combine these techniques to fabricate novel devices, namely vertically-conducting thin-film diodes and Schottky-gated TFTs, and we investigate the impact of the contact formation process on the resulting device physics using temperature-dependent current-voltage measurements. (iv) Finally, we demonstrate the use of these devices in several novel thin-film power electronics applications. These circuits include thin-film RFID energy harvesters, thin-film heterointegrated 3D-IC on Si chip for voltage bridging, and thin-film bypass diodes for future integration on solar cells to improve efficiency under partial shading conditions.