Microstructure and secondary phase segregation correlation in epitaxial/oriented ZnO films with unfavorable Cr dopant
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Low solubility dopant-host systems are well suited to study secondary phase segregationmicrostructure dependence. We discuss the effect of microstructure on secondary phase segregation in epitaxial/oriented ZnO thin films with Cr as an unfavorable dopant (Cr: ZnO). Since differences in thin film microstructure are a function of the substrate and its orientation, simultaneous chemical vapor depositions were carried out on single crystals of Si (100), c-axis oriented Al2O3 (c-ALO), and r-axis oriented Al2O3 (r-ALO) resulting in epitaxial film growth on r-ALO and c-axis oriented film growth on Si and c-ALO, with a difference in vertical grain boundary density. To enhance the analysis sensitivity to the microstructure difference, the thickness of Cr:ZnO films was maintained at 50 nm. High-resolution transmission electron microscopy (HRTEM) analysis indicates uniform stress distribution in Cr:ZnO grown on r-ALO. Surface sensitive x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) techniques were utilized for analysis of the data. We observe that a higher grain boundary density and the presence of an amorphous layer at the interface for films grown on Si (100) single crystal led to interfacial Cr-based secondary phase segregation as opposed to lower grain boundary density and epitaxial films grown on c-ALO and r-ALO single crystals, respectively. We also discuss the effects of trace carbon solubility on the film microstructure/secondary phase segregation relationship. I. INTRODUCTION
ZnO is an extensively studied material in the literature due to the relative ease in its growth and a variety of applications utilizing wide-band gap semiconductors. It is a multifunctional semiconducting oxide spanning its cross disciplinary usage in photonics, transparent electronics, sensors, buffer layers, dielectrics, and magnets. Thus, it is important to know about the secondary phase segregation microstructure dependence. Due to modulation in physical and chemical properties in the host system caused by dopant incorporation in ZnO, an understanding of dopant solubility and secondary phases related to excess dopant can provide valuable insight. As an example, a review by Triboulet et al.1 and a report by Jin et al.2 discuss the availability and solubility limit of various dopants in ZnO. It is clear from such studies that Co and Mn seem to have more solubility in ZnO than dopants such as Cr and Ni. The solubility of a particular dopant in ZnO replacing a cation depends upon three major components (i) the atomic radii of dopants compared to atomic radius of Zn, (ii) the ionic radii of dopants compared to ionic radius of Zn, and (iii) the a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0054
506
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valence state of the ions in the molecule (ZnO). There is a large difference between atomic radii of Zn (0.153 nm) and Cr (0.185 nm). Even by considering the 2+ valence state, this difference remains large (Zn2+ = 0.0
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