On improvements in metal oxide based flexible transistors through systematic evaluation of material properties

Thin-film metal oxide (MOx) semiconductors have opened the way to a new generation of electronics based on their unique properties. With mobilities, mu, of up to 80 cm2V-1s-1, metal oxides do not rival crystalline silicon (mu~1000 cm2V-1s-1) for complex applications. But such oxides do have three unique characteristics driving great interest: their mobilities persist in the amorphous form, contrary to the thousandfold drop seen in silicon; they are transparent; and they can be processed at, or near, room temperature. Most work on MOx semiconductors, in particular indium gallium zinc oxide (IGZO), has focused on display applications, where MOx thin-film transistors (TFTs) are used to drive individual pixels, reducing power consumption by blocking less light than alternatives, and allowing smaller pixels due to reduced TFT sizes. Such work has seen great advances in IGZO, but has generally not considered the thermal budget during production. By utilising the low temperature processing possible with MOx, a new world of applications becomes possible: flexible electronics. This work aims to improve the characteristics of TFTs based on amorphous IGZO (a-IGZO) through detailed study of the thin-film structure in relation to functional performance, looking at the material structure of three critical layers in an a-IGZO TFT. A study of optimisation of a dielectric layer of Al2O3, deposited by atomic layer deposition (ALD), is presented. This dielectric, between the a-IGZO and the gate electrode, shows a three-layer substructure in what has previously been regarded as a single homogeneous layer. A study of the insulating Al2O3 buffer layer below the a-IGZO compared the properties of Al2O3 deposited by ALD and sputtering. Sputtered material has a more complex structure than ALD, consisting of multiple sublayers that correlate with the sputtering process. The structure of the two materials is discussed, and the impact on device performance considered. A detailed systematic study of the effects of annealing of a-IGZO shows a strong dependence of the density on both time and temperature. A two mechanism model is proposed which consists of structural relaxation of the amorphous material followed by absorption of oxygen from the environment. Finally, investigation of the influence of the buffer material on the a-IGZO, and the structure of this interface showed little difference in the growth of the a-IGZO, but did reveal some changes in the interface, while a systematic study of annealing effects on the a-IGZO-dielectric interface showed some interesting changes in this structure, both of which are likely to significantly impact the operational characteristics of TFT devices

Advances in Amorphous Oxide Semiconductor Devices, Materials, and Processes for Customizable Scalable Manufacturing of Thin-Film Electronics

Electronic circuits comprised of thin-film transistors (TFTs) are essential to nearly every modern display technology. For decades, the TFT industry relied on amorphous silicon, but increasing performance demands required semiconductors with superior electron transport leading to the adoption of amorphous oxide semiconductors (AOS). The superior electron transport and ease of thin-film preparation of AOS has led to a growing interest in developing thin-film electronics for beyond-display technologies. These include monolithic 3D integration on Si complementary metal-oxide-semiconductor integrated circuits (ICs) – to continue Moore’s law, add new functionality, and improve performance – and flexible electronics for electronic skins, textiles, solar cells, and displays. In this thesis we facilitate the adoption of thin-film electronics for beyond-display technologies by: 1) developing uniform and conformal AOS deposition processes with record performance; 2) demonstrating expanded AOS capabilities by exploring new device architectures; and 3) developing a new additive manufacturing technique for customizable scalable manufacturing. First, we meet the performance and thermal budget requirements of AOS for beyond-display applications by using atomic-layer deposition (ALD) – a conformal, uniform, and precise vapor-phase deposition technique – and aggressively optimizing the process conditions. We discovered that improved electrical performance correlated with an increase in film density, which can be achieved by increasing deposition temperature, by post-deposition annealing, and by using plasma enhanced-ALD (PE-ALD). Second, we made innovations in device design to expand the range of circuit applications for AOS TFTs by exploiting the benefit of their wide-bandgap to fabricate high-voltage TFTs (HVTFTs). While the current handling capabilities of these HVTFTs cannot compete with conventional power electronics, the ability to deposit AOS materials directly on Si ICs may enable monolithic 3D integration of HVTFTs, adding new functionality as an HV interface to aggressively scaled low-voltage Si CMOS. Third, we show that ambient instabilities are caused by interactions between the surface of the AOS film and ambient molecules. We eliminate these instabilities by developing an ALD-based passivation layer. Fourth, we study the temporal and bias stress stability of our ALD AOS thin-film transistors and see excellent stability after the first month of aging and improved positive bias stress stability with passivation. Fifth, we investigate several materials to form a Schottky contact to ALD AOS films to enable future rectifier-based circuits and unipolar logic circuits. Finally, we develop an additive manufacturing approach for customizable manufacturing of AOS devices. Further improvement in device performance and reduction of channel length, enabled by the sub-µm precision of EHD, has the potential to yield fully customizable additive manufacturing of high-frequency circuits.PHDElectrical and Computer EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/169721/1/allemang_1.pd

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

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.PHDElectrical and Computer EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/149911/1/ybson_1.pd