Photochemical Sciences Ph.D. Dissertations

Development of zinc oxide based flexible electronics

Date of Award

2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Farida Selim (Advisor)

Second Advisor

Brent Archer (Other)

Third Advisor

Malcolm Forbes (Committee Member)

Fourth Advisor

Emily Heckman (Committee Member)

Fifth Advisor

Alexey Zayak (Committee Member)

Abstract

This dissertation work is focused on development of zinc oxide (ZnO) thin films by atomic layer deposition (ALD) and sol-gel processes and investigating its optoelectronic properties and potential applications in flexible electronics.

Through ALD efforts, a unique doping method was developed to incorporate In3+ and Ga3+ donors in ZnO at 250°C. The ALD process allowed us to deposit individual layers of In and Ga precursors, sandwiched between many layers of ZnO. The result is a 160-nm indiumgallium- doped ZnO (IGZO) transparent conductive oxide (TCO), with a sheet resistance of 60.9 Ω·sq-1 and percent transmittance > 93.8% at 550 nm, and figure of merit Φ = 8.66 × 10-3 Ω-1. IGZO is shown to be amorphous or polycrystalline in nature, depending on the substrate, but all IGZO thin films exhibit a low resistivity ρ (≤1.10 × 10-3 Ω·cm), high carrier concentration η (≥2.30 × 1020 cm-3), and high mobility μ (≥15.3 cm²/V·s). IGZO was found to be a fully degenerate semiconductor. Compared to gallium-doped ZnO (GZO), it appears that In3+ is responsible for the improved polycrystalline growth and increased grain size. Based on these results IGZO is a promising replacement for the industry-standard indium tin oxide (ITO) for TCO applications.

A sol-gel approach was used to deposit ZnO and IGZO layers onto a variety of flexible and transparent substrates using spin coating, inkjet printing (IJP), and aerosol jet printing (AJP) tools. In and Ga were introduced as co-dopants in solution and improved conductivity and polycrystalline growth. The polycrystallinity of ZnO improves with temperature from 200– 400°C and is dependent on the substrate. Both ZnO and IGZO thin films exhibit a decrease in resistivity upon UV exposure (103–106 Ω·cm). Increasing the light intensity further shows a nonlinear behavior. This effect is attributed to UV light adsorption and oxygen desorption. Upon bending the substrate at a 4 mm radius of curvature, the photoconductive response is decreased by 104 Ω·cm and cannot be restored.

A spin coating method was used to study the effects of In and Ga as individual dopants and co-dopants (0–3% in solution). Ga-doping gradually decreases the crystallite size D and polycrystallinity at doping levels up to 2%, due to Ga3+ having a smaller ionic radius than Zn2+, which causes in the lattice. A further increase up to 3% shows a slight increase in D, but a less polycrystalline structure, suggesting that Ga interstitials are forming. In-doping is shown to increase D and polycrystallinity at a doping levels of 1%, while doping further to 3% hinders crystallinity. The increase in D is due to In3+ having a larger ionic radius than Zn2+ and the eventual loss in polycrystallinity is due to the stress induced by In.

Sol-gel ZnO was used in prototyping of UV photodetectors and thin-film transistors (TFTs) with silver inks. AJP was used to deposit sol-gel ZnO and silver inks with a minimum line width of ~136 μm and ~109 μm, respectively. A minimum gap of ~65 μm was achieved between silver source and drain contacts. The photoconductive nature of ZnO is shown through time-resolved photocurrent measurements, while there were no TFT I-V characteristic observed.

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