Growth kinetics of ZnO prepared by organometallic chemical vapor deposition
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I. INTRODUCTION The preparation of thin-film zinc oxide has been the subject of continued interest due to its potential in numerous applications including electro-optic, piezoelectric, and photoconductive devices. To date the highest quality zinc oxide films reported have been prepared by magnetron sputtering.1 Nevertheless, for applications in which the defect structure plays a major role in determining its properties, such as for solid-state lasers and optical displays, other deposition techniques are needed. One deposition method that shows great promise is organometallic chemical vapor deposition. Using this technique, high-quality films can be deposited at temperatures as low as 275 °C.2"5 The low temperature enables preparation of films that have low concentrations of both impurities and native defects. Despite its potential importance, the deposition mechanisms are not well understood. In this paper the study of the crystal growth kinetics of heteroepitaxial ZnO on sapphire will be discussed. Results of the characterization of the films will also be presented. II. EXPERIMENTAL Zinc oxide films were grown in a cold-wall horizontal quartz reactor. The silicon-carbide-coated graphite susceptor was heated by a 1.3 kW quartz-halogen lamp. All the gas tubing and fittings leading to the chamber were stainless steel. The upstream end of the reaction chamber had two separate inlets for the introduction of the reactants. The source materials were electronicgrade dimethyl zinc (DMZ) and tetrahydrofuran (THF). Palladium-diffused hydrogen served as the carrier gas. The deposition was carried out at atmospheric pressure. Sapphire with a (0001) orientation served as the substrate in this investigation. Prior to deposition the mechanically polished substrates were chemically etched in dilute sulfuric acid solution and then concentrated aqua regia for 5 min followed by a DI water rinse. 740
J. Mater. Res. 3 (4), Jul/Aug 1988
http://journals.cambridge.org
The substrate was then heated in situ in hydrogen at 750 °C for 15 min. Typical growth conditions used are reported in Table I. The growth rates obtained were 2-4 fim h ^ 1 resulting in layers 1-2//m thick. III. RESULTS AND DISCUSSION A. Growth kinetics Deposition of ZnO from the reactants dimethyl zinc, tetrahydrofuran, and hydrogen is described by the following reaction: (CH 3 ) 2 Zn + C4HBO + 5H 2 -*ZnO + 6CH 4 . Figure 1 shows the temperature dependence of the growth rate; four separate temperature dependencies are observed. Below 300 °C no significant growth was observed. In the region labeled 1 in Fig. 1, the growth rate increased up to 4.1 //m/h at 350 °C. In region 2, 350-375 °C, a sharp decrease in the growth rate was observed, with a minimum of 1.8 /um at 375 °C. In region 3, 375-450 °C, an increase in the deposition rate was noted. This is typically indicative of a surface reaction ratelimited regime. For higher growth temperatures (region 4, 450-500 °C) a decrease in the growth rate was noted.
TABLE I. Typical growth conditions for OMVPE growth of ZnO using THF
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