Metalorganic chemical vapor deposition of carbon-free ZnO using the bis(2,2,6,6-tetramethyl-3,5-heptanedionato)zinc prec
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Bis(2,2,6,6-tetramethyl-3,5-heptanedionato)zinc [Zn(TMHD)2] is a relatively uninvestigated precursor that was used in this work to grow highly c-axis-oriented ZnO films on Si(100). X-ray photoelectron spectroscopy studies before and after Ar ion sputtering indicated that surface carbon on several samples was reduced from as much as 34 at.% to much less than 1 at.% within the first 5 nm, indicating very clean Zn(TMHD)2 precursor decomposition. Microstructural and compositional analysis revealed columnar ZnO grains with domain widths of approximately half the total film thickness and a Zn-to-O atomic percent ratio indicative of stoichometric ZnO. I. INTRODUCTION
The use of environmentally friendly precursors in metalorganic chemical vapor deposition (MOCVD) growth processes is becoming an essential factor for minimizing the hazards associated with highly reactive precursors and for increasing operational safety. ZnO is a popular wide-band gap semiconductor used in solar cells, ferroelectric capacitors, pH sensors, and ultraviolet (UV) detectors. Device fabrication for these technologies requires a growth process like MOCVD because of its superior conformal coverage.1–4 In the case of ZnO film growth by MOCVD, numerous reports in the literature focus on the use of di-methyl zinc (DMZ), di-ethyl zinc (DEZ), zinc acetate, zinc acetylacetonate, and other precursors.5 DEZ and DMZ are particularly popular due to their high volatility, high vapor pressures, and ease of use.6 However, low thermal stability and environmental hazards render these precursors less desirable. As pointed out previously, even minor temperature fluctuations can severely affect the resulting film-growth rates.6 Moreover, the pyrophoric nature of these precursors makes them dangerous and difficult to handle. Additionally, secondary reactions can yield toxic products, and waste management is thus a significant issue. Thus, there is a clear need to investigate new ZnO precursors. One of the important factors in MOCVD precursor decomposition is the complete removal of precursor ligands as volatile species. Incomplete removal results in film contamination. Precursor decomposition studies also help in the design of new precursors with the goal of
II. EXPERIMENTAL
A three-source cold-wall MOCVD reactor built by EMCORE Corporation (Somerset, NJ) and modified by Peak Materials, Inc. (Colorado Springs, CO) at Pacific Northwest National Laboratory (Richland, WA) was
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0146 1230 http://journals.cambridge.org
minimizing impurity incorporation in films. There are relatively few publications on the use of bis(2,2,6,6tetramethyl-3,5-heptanedionato)zinc [Zn(TMHD)2] despite the fact that this precursor minimizes several of the problems mentioned above.7–10 Although Zn(TMHD) exhibits lower volatility than popular precursors like DMZ and DEZ, we show that fully stoichiometric ZnO films can be grown by reducing the growth pressure and maintaining uniform precursor line tempe
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