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  • Understanding the Enhanced Mobility of Solution- Processed Metal-Oxide Thin-Film Transistors Having High-

    k Gate Dielectrics

    Andre Zeumault Vivek Subramanian

    Electrical Engineering and Computer Sciences University of California at Berkeley

    Technical Report No. UCB/EECS-2017-70 http://www2.eecs.berkeley.edu/Pubs/TechRpts/2017/EECS-2017-70.html

    May 12, 2017

  • Copyright © 2017, by the author(s). All rights reserved.

    Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission.

    Acknowledgement

    Professors Vivek Subramanian, Sayeef Salahuddin and Oscar Dubon. Fellow researchers Steve Volkman, Dan Soltman, Teymor "Tim" Bakshishev, Huai Yuan "Michael" Tseng, Lakshmi Jagannathan, Feng Pan, Hongki Kang, Eung Seok Park, Kyle Braam, Jaewon Jang, Rungrot "Jack" Kitsomboonloha, Sarah Swisher, Jake Sadie, Gerd Grau, Muhammed "Rumi" Ahosan Ul Karim, Jeremy Smith, Nishita Deka, Seungjun Chung, Will Scheideler, Carlos Biaou, Matt McPhail, Raj Kumar, Jacob Sporrer and Dwipayan Patnaik.

  • Understanding the Enhanced Mobility of Solution-Processed Metal-Oxide Thin-Film Transistors Having High-k Gate Dielectrics

    By

    Andre Zeumault

    A dissertation submitted in partial satisfaction of the

    requirements for the degree of

    Doctor of Philosophy

    in

    Engineering – Electrical Engineering and Computer Sciences

    in the

    Graduate Division

    of the

    University of California, Berkeley

    Committee in charge:

    Professor Vivek Subramanian, Chair Professor Sayeef Salahuddin

    Professor Oscar Dubon

    Spring 2017

  • Understanding the Enhanced Mobility of Solution-Processed Metal-Oxide Thin-Film Transistors Having High-k Gate Dielectrics

    Copyright 2017 by

    Andre Zeumault

  • 1

    Abstract

    Understanding the Enhanced Mobility of Solution-Processed Metal-Oxide Thin-Film Transistors Having High-k Gate Dielectrics

    by

    Andre Zeumault

    Doctor of Philosophy in Engineering – Electrical Engineering and Computer Sciences

    University of California, Berkeley

    Professor Vivek Subramanian, Chair

    Primarily used as transparent electrodes in solar-cells, more recently, physical vapor de- posited (PVD) transparent conductive oxide (TCO) materials (e.g. ZnO, In2O3 and SnO2) also serve as the active layer in thin-film transistor (TFT) technology for modern liquid- crystal displays. Relative to a-Si:H and organic TFTs, commercial TCO TFTs have reduced off-state leakage and higher on-state currents. Additionally, since they are transparent, they have the added potential to enable fully transparent TFTs which can potentially improve the power efficiency of existing displays.

    In addition to PVD, solution-processing is an alternative route to the production of dis- plays and other large-area electronics. The primary advantage of solution-processing is in the ability to deposit materials at reduced-temperatures on lower-cost substrates (e.g. glass, plastics, paper, metal foils) at high speeds and over large areas. The versatility offered by solution-processing is unlike any conventional deposition process making it a highly attrac- tive emergent technology.

    Unfortunately, the benefits of solution-processing are often overshadowed by a dramatic re- duction in material quality relative to films produced by conventional PVD methods. Con- sequently, there is a need to develop methods that improve the electronic performance of solution-processed materials. Ideally, this goal can be met while maintaining relatively low processing temperatures so as to ensure compatibility with low-cost roll-compatible sub- strates.

    Mobility is a commonly used metric for assessing the electronic performance of semiconduc- tors in terms of charge transport. It is commonly observed that TCO materials exhibit signif- icantly higher field-effect mobility when used in conjunction with high-k gate dielectrics (10 to 100 cm2 V−1 s−1) as opposed to conventional thermally-grown SiO2 (0.1 to 20 cm2 V−1 s−1). Despite the large amount of empirical data documenting this bizarre effect, its physical ori-

  • 2

    gin is poorly understood.

    In this work, the interaction between semiconductor TCO films and high-k dielectrics is studied with the goal of developing a theory explaining the observed mobility enhancement. Electrical investigation suggests that the mobility enhancement is due to an effective dop- ing of the TCO by the high-k dielectric, facilitated by donor-like defect states inadvertently introduced into the dielectric during processing. The effect these states have on electron transport in the TCO is assessed based on experimental data and electrostatic simulations and is found to correlate with negative aspects of TFT behavior (e.g. frequency dispersion, gate leakage, hysteresis, and poor bias stability).

    Based on these findings, we demonstrate the use of an improved device structure, analogous to the concept of modulation doping, which uses the high-k dielectric film as an encapsulate, rather than a gate-dielectric, to achieve a similar doping effect. In doing so, the enhanced mobility of the TCO/high-k interface is retained while simultaneously eliminating the neg- ative drawbacks associated with the presence of charged defects in the gate dielectrics (e.g. frequency dispersion, gate leakage, hysteresis, and poor bias stability). This demonstrates improved understanding of the role of solution-processed high-k dielectrics in field-effect devices as well as provides a practical method to overcome the performance degradation incurred through the use of low-temperature solution-processed TCOs.

  • i

    To my partner Shola, for being my best friend and companion especially throughout difficult times. To my mom Lori, for never losing faith in me as a youth and pushing me to

    excel academically. To my dad Nathaniel for teaching me, through example, quality and the meaning of hard work. To my brother Miguel for showing me persistence and

    dedication to my commitments. And to my curious Siberian cat Mouska for fun-filled days of laughter as well as emotional support when appropriate.

  • ii

    Contents

    Contents ii

    List of Figures iv

    List of Tables x

    1 Introduction 1 1.1 Conventional Role of High-k Gate-Dielectrics in Field-Effect Transistors . . . 1 1.2 Thin-Film Transistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.3 Transport in Disordered Systems . . . . . . . . . . . . . . . . . . . . . . . . 6 1.4 Printed Electronics for Large-Area Applications . . . . . . . . . . . . . . . . 13 1.5 Transparent Conductive Oxides . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.6 Method of Investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 1.7 Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

    2 Quantifying Mobility of Disordered-Channel Thin-Film Transistors 20 2.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.2 Description of Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 2.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 2.5 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

    3 Effect of Synthetic Conditions on Density of Localized States in the TCO Channel 37 3.1 Background and Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 3.5 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

    4 Electron Donation from Donor States in High-k Gate Dielectrics 57 4.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 4.2 Establishing a Baseline Comparison to Thermally-Grown SiO2 . . . . . . . . 60

  • iii

    4.3 Correlating ZrO2 Defects to ZnO Mobility . . . . . . . . . . . . . . . . . . . 62 4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 4.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 4.6 Experimental Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

    5 Electrostatic Simulations of TCO TFTs With Donor States in the Gate Dielectric 74 5.1 Numerical Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 5.2 Validation of Model: General MTR Behavior . . . . . . . . . . . . . . . . . . 80 5.3 Effect of Donor-Traps in ZrO2 Dielectric . . . . . . . . . . . . . . . . . . . . 83 5.4 Summary and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

    6 Modulation Doping of TCO Channel by High-k Dielectric Encapsulation 93 6.1 Background and Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 6.2 Thin-Film Characterization of ZnO and Ga2O3/ZnO Heterostructures . . . . 94 6.3 TFT Characterization of ZnO and Zn