When 10:30 AM - 11:30 AM Mar 01, 2019
Where 1670 Beyster
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Exploiting in situ interfacial reactions for oxide semiconductor power devices


Prof. Becky Peterson
Department of Electrical Engineering & Computer Science, University of Michigan

Redox reactions have always been at the heart of electronics: silicon’s commercial dominance is due, in large part, to facile formation of a high-quality dielectric interface via thermal annealing in an oxidizing environment, and to the use of metal interface reactions to form high-quality ohmic contacts. As silicon CMOS transistors reach fundamental scaling limits, further system performance benefits require heterointegration of novel materials with silicon.

Oxide semiconductors’ unique properties – a wide bandgap, reasonably high electron mobility, and ease of bulk and thin film preparation – make them a prime material candidate for this task. Already, in the last decade, amorphous indium gallium zinc oxide has replaced amorphous silicon in the active matrix backplane of displays made by Apple, Microsoft, and Sharp, to enable large-scale, high frame rate products, and metal oxide thin films form the functional layer in novel memory devices to alleviate the memory bottleneck. Metal oxides pose a unique challenge for interface stabilization, due to the availability of oxygen to react with adjacent thin films. In this talk, I will describe three recent studies in which we exploit the thermodynamics of interfacial reactions with amorphous and crystalline oxide semiconductors to improve the electrical properties of dielectric and metal interfaces. 

First, using amorphous zinc tin oxide (a-ZTO) semiconductor thin films deposited by an air-stable solution process, we have shown that in situ reduction, oxidation and diffusion at metal-ZTO interfaces can be exploited to tune the contact properties. Using the Gibbs free energy of oxide formation and ionic radii of the bottom metal thin films as selection criteria, we are able to form quasi-ohmic contacts (Pd-ZTO), rectifying contacts (Mo-ZTO), or contacts with conductive-bridge resistive memory behavior (PdOx-ZTO). Second, we study the formation of Ti/Au ohmic contacts to crystalline gallium oxide, an ultra-wide bandgap semiconductor of interest for multi-kV power devices. The Ti/Ga2O3 interface is not predicted to be thermodynamically stable even at room temperature. Using high-resolution electron microscopy and chemical mapping, we show that after a brief (1-min) anneal at 470oC, the titanium layer oxidizes and forms nanocrystallites, while Au and Ti interdiffusion and Ti incorporation into Ga2O3 may facilitate formation of the ohmic contact. Third, we explore the interface trap density of a novel high-k dielectric, Y-Sc-O, for use in the gate stack of Ga2O3 field effect transistors. The large negative formation energy of Y-Sc-O creates an interface with very low trap density and the Y-Sc-O’s ternary composition increases its crystallization temperature, which should enable high-temperature operation of future power devices.

 

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