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

Andre Zeumault

EECS Department
University of California, Berkeley
Technical Report No. UCB/EECS-2017-70
May 12, 2017

http://www2.eecs.berkeley.edu/Pubs/TechRpts/2017/EECS-2017-70.pdf

Primarily used as transparent electrodes in solar-cells, more recently, physical vapor deposited (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 displays 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 attractive emergent technology.

Unfortunately, the benefits of solution-processing are often overshadowed by a dramatic reduction in material quality relative to films produced by conventional PVD methods. Consequently, 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 substrates.

Mobility is a commonly used metric for assessing the electronic performance of semiconductors in terms of charge transport. It is commonly observed that TCO materials exhibit significantly higher field-effect mobility when used in conjunction with high-k gate dielectrics (10 to 100 cm2/Vs) as opposed to conventional thermally-grown SiO2 (0.1 to 20 cm2/Vs). Despite the large amount of empirical data documenting this bizarre effect, its physical origin 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 doping 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 negative 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.

Advisor: Vivek Subramanian


BibTeX citation:

@phdthesis{Zeumault:EECS-2017-70,
    Author = {Zeumault, Andre},
    Title = {Understanding the Enhanced Mobility of Solution-Processed Metal-Oxide Thin-Film Transistors Having High-k Gate Dielectrics},
    School = {EECS Department, University of California, Berkeley},
    Year = {2017},
    Month = {May},
    URL = {http://www2.eecs.berkeley.edu/Pubs/TechRpts/2017/EECS-2017-70.html},
    Number = {UCB/EECS-2017-70},
    Abstract = {Primarily used as transparent electrodes in solar-cells, more recently, physical vapor deposited (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 displays 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 attractive emergent technology. 

Unfortunately, the benefits of solution-processing are often overshadowed by a dramatic reduction in material quality relative to films produced by conventional PVD methods. Consequently, 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 substrates. 

Mobility is a commonly used metric for assessing the electronic performance of semiconductors in terms of charge transport. It is commonly observed that TCO materials exhibit significantly higher field-effect mobility when used in conjunction with high-k gate dielectrics (10 to 100 cm2/Vs) as opposed to conventional thermally-grown SiO2 (0.1 to 20 cm2/Vs). Despite the large amount of empirical data documenting this bizarre effect, its physical origin 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 doping 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 negative 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.}
}

EndNote citation:

%0 Thesis
%A Zeumault, Andre
%T Understanding the Enhanced Mobility of Solution-Processed Metal-Oxide Thin-Film Transistors Having High-k Gate Dielectrics
%I EECS Department, University of California, Berkeley
%D 2017
%8 May 12
%@ UCB/EECS-2017-70
%U http://www2.eecs.berkeley.edu/Pubs/TechRpts/2017/EECS-2017-70.html
%F Zeumault:EECS-2017-70