Transport phenomena in high performance nanocrystalline ZnO:Ga films deposited by plasma-enhanced chemical vapor deposition
Introduction
Zinc oxide is a versatile wide bandgap semiconductor that is widely used in numerous applications including solar cells, displays, surface acoustic wave devices, and sensors. Zinc oxide is a promising alternative to indium tin oxide in transparent conducting oxide (TCO) applications, due to its low cost and ease of patterning. As a TCO, ZnO is commonly doped with aluminum and deposited by physical vapor deposition techniques such as sputtering [1] or pulsed laser deposition [2]. Plasma-enhanced chemical vapor deposition (PECVD) has found limited use for ZnO synthesis, with most work focused on intrinsic zinc oxide for structural properties [3] and more recently for UV emission [4]. The electrical properties of PECVD ZnO were the focus of a recent report [5], and ZnO:Al has been fabricated by injecting metallorganic precursor downstream of a cascaded thermal plasma [6]. However, some of the best zinc oxide films have been gallium doped, usually produced by chemical vapor deposition [7], [8].
In this paper, we discuss the fabrication of ZnO:Ga by PECVD. A statistical design of experiments (DOE) was used to optimize film conductivity. High conductivity films have a unique orientation and morphology relative to films deposited by other techniques. Excellent transmission in the visible spectrum and low resistivity were obtained. Both optical and electrical performances were related to microstructure. The nanocrystalline morphology resulted in exceptionally smooth films; however, electrical conductivity was limited in part by grain boundary and structural defects.
Section snippets
Experimental details
The films were deposited in a parallel plate, capacitively coupled RF plasma that has been described previously [5]. Corning 1737 glass was used as substrates. Diethyl zinc (DEZ, Strem, >95% purity) and trimethyl gallium (TMG, Strem, 99% purity) were carried into the reactor using argon and reacted with oxygen. The total metallorganic precursor flow rate was 2 sccm. This work derives from a study that interrogated the entire phase space from ZnO to Ga2O3[9]. In that work, it was found that both
Synthesis and characterization
Previous work on intrinsic ZnO PECVD demonstrated that high conductivity was achieved under highly oxidizing conditions [5]. The increased conductivity was attributed to higher surface mobility during growth, which yielded preferentially (002) oriented films. This study found that fuel-rich conditions (low O2/DEZ ratio) produced insulating films with random orientations. A recent study of gallium incorporation showed that the electrical conductivity and rate were optimized at a DEZ/TMG ratio of
Conclusions
Transparent and uniform conducting ZnO:Ga films with a fine grain size in the range of 50–100 nm were fabricated by PECVD. Statistically designed experiments used to optimize electrical conductivity demonstrated the strong interactions among plasma operating parameters. Optimized films had a unique crystal orientation as well as a smooth morphology. Optimized films had resistivity values of 7.5×10−4 Ω cm and 93% transmission across the visible. The electrical performance was limited by electron
Acknowledgements
The authors gratefully acknowledge the National Science Foundation for support for this work through award CTS-0093611.
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