Transparent Conducting Oxides (TCOs) have unique optoelectronic properties which allow visible light to pass through while having reasonably high electrical conductivity. TCOs find a variety of applications ranging from uses in solar cells, optical displays, reflective coatings, light emission devices, low-emissivity windows, electrochromic mirrors, UV sensors, and windows, defrosting windows, electromagnetic shielding, and transparent electronics. The conductivity of TCOs can be tuned from insulating via semiconducting to conducting as well as their transparency adjusted depending on the donor/acceptor levels as well as the bandgap of the material. This enables the realization of both n-type and p-type TCOs which make them highly attractive for transparent opto-electrical circuitries and technological applications. Most research activities have focused on the optimization of n-type TCOs, but many transparent electronic applications require the necessity of p-type TCOs as well. There is a need to realize p-type TCOs which offer high yield, scalability, and low cost. RF magnetron sputtering of TCO sources can lead to high uniformity and homogeneity, along with the capability to control the film thickness and deposition rate. It also allows for large-area deposition at a relatively low cost and optimum thermal budget. In this work, we investigate the realization of p-type CuInOx thin films by RF magnetron sputtering using Cu2O: In2O3 target and study the effects of (i) post-deposition annealing and (ii) substrate heating during a deposition for controlling the optoelectronic and morphological properties. In the first part of the study, post-process annealing of the deposited films was performed at temperatures ranging from 100-900°C in O2 ambiance. The X-ray diffraction (XRD) analysis performed on the samples identified the presence of Cu2In2O5 phases along with CuInO2 or In2O3 for the films annealed above 500°C. A morphological study performed using SEM shows the crystallization and the grain growth with an increase in the annealing temperatures. Optical studies carried out on the films indicated a small bandgap change in the range of 3.4-3.6 eV during annealing. In the second part of the study, the effect of substrate heating during the deposition was investigated to reduce the thermal budget of realizing the p-type CuInOx thin films and also to increase the throughput. It is seen that substrate heating influences the material characteristics more significantly than post-deposition annealing as we can tailor the thin film characteristics in situ to initiate crystalline growth and control the proportion of indium oxide or copper oxide phases to improve the transparency while retaining the p-type characteristics of the thin film. Copper Indium Oxide (CuInOx) thin films were deposited by the RF magnetron sputtering technique using a Cu2O: In2O3 target at varying substrate temperatures up to 400°C. A morphological study performed using SEM further confirmed the crystallization and the grain growth (95-135 nm) with increasing substrate temperatures resulting in superior conductivity and enhanced transparency of more than 70% in the 400-700 nm range. Optical studies carried out on the films indicated a bandgap change in the range of 2.61-2.99 eV as a function of substrate heating. XPS analysis of the thin films has also been carried out to identify the oxidation state and bonding configurations of Cu, In, and O in copper indium oxide films. Mutually exclusive requirements of having a p-type thin film along with increased conductivity and high transparency were achieved by controlling the migration of indium oxide phases during the sputtering process as verified by the XPS studies. This is due to the controlled replacement of copper sites with indium while maintaining the p-type characteristic of the thin film. Junction studies of the p-type CuInOx have been investigated with n-Si and ITO for demonstrating heterojunction behavior which can potentially find applications in transparent electronics, photodetectors, and solar cells. A p-CuInOx/n-Si heterojunction was fabricated with a measured knee voltage of 0.89V. The photovoltaic behavior of the device was investigated and initial solar cell parameters are reported.
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Doctor of Philosophy (Ph.D.)
College of Engineering and Computer Science
Electrical and Computer Engineering
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Skaria, Giji, "Studies on Copper Indium based Oxide Transparent Semiconducting Thin Films." (2022). Electronic Theses and Dissertations, 2020-. 1091.