Transport phenomena in high performance nanocrystalline ZnO:Ga films deposited by plasma-enhanced chemical vapor deposition

doi:10.1016/j.tsf.2004.06.154

Thin Solid Films

Volume 473, Issue 1, 1 February 2005, Pages 35-40

https://doi.org/10.1016/j.tsf.2004.06.154 Get rights and content

Abstract

Nanocrystalline gallium doped zinc oxide (ZnO:Ga) thin films were synthesized by plasma-enhanced chemical vapor deposition (PECVD). A statistical design of experiments (DOE) was employed to optimize electrical conductivity. A carrier concentration of 5.5×10²⁰/cm³ and a mobility of 15 cm²/V s yielding a resistivity of 7.5×10⁻⁴ Ω cm resulted from the conditions of high pressure, rf power, and electrode gap. X-ray diffraction showed that gallium doping had a profound impact on film orientation. Atomic force microscopy (AFM) revealed that the films were nanostructured, with an average grain size of 80 nm and a surface roughness of ∼2 nm. This unique morphology benefited optical transmission, but limited electrical performance. Average transmission across the visible spectrum was ∼93% as scattering losses were minimized. Temperature dependent Hall and optical transmission measurements demonstrated that structural defects and ionized impurities were equal contributors to electron scattering.

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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Transport phenomena in high performance nanocrystalline ZnO:Ga films deposited by plasma-enhanced chemical vapor deposition

Abstract

Introduction

Section snippets

Experimental details

Synthesis and characterization

Conclusions

Acknowledgements

Thin Solid Films

Thin Solid Films

J. Cryst. Growth

Thin Solid Films

Thin Solid Films

J. Vac. Sci. Technol., A

Appl. Phys. Lett.

J. Appl. Phys.

J. Electrochem. Soc.