Growth of Ga-face and N-face GaN films using ZnO Substrates
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 16
Growth of Ga-face and N-face GaN films using ZnO Substrates E. S. Hellman, D. N. E. Buchanan, D. Wiesmann, I. Brener Bell Laboratories, Lucent Technologies This article was received on June 2, 1996 and accepted on October 10, 1996.
Abstract We have used plasma molecular beam epitaxy on (0 0 0 1) and (0 0 0 ) ZnO substrates to induce epitaxial growth of GaN of a known polarity. The polarity of the ZnO substrates can be easily and unambiguously determined by measuring the sign of the piezoelectric coefficient. If we assume that N-face GaN grows on O face ZnO and that Ga-face GaN grows on Zn face ZnO, then we can study the growth of both Ga and N faces. The most striking difference is the doping behavior of the two faces. Growth on the Ga-face is characterized by a higher carrier concentration and a lower threshold for Ga droplet formation.
1. Introduction Surprisingly little is known about the polarity of GaN epitaxial films. GaN has the wurtzite structure, and typically grows with its hexagonal basal plane parallel to the substrate. The orientation normal to the substrate is either 〈0 0 0 1〉 or 〈0 0 0 〉. The two orientations are not equivalent; the Ga-N bonds along the hexagonal axis are all oriented in one direction, resulting in faces that are either nitrogen terminated or gallium terminated. The two faces, along with the terminology conventions used in this article, are illustrated in Figure 1. The surface chemistry, epitaxial growth, doping, and defect structure on the two faces should be quite distinct. The GaN research community has by and large neglected the distinction between these two orientations, both because it is hard to measure, and because most growth technechniques should select one or the other orientation. One exception is the work of Sasaki and Matsuoka, [1] who studied the growth of GaN on the carbon and silicon terminated faces of SiC. Using x-ray photoemision spectroscopy to measure the oxidation of Ga atoms on the surface, they determined that Ga-face GaN was grown on C-face SiC, and that N-face GaN was grown on Si-face SiC. It is almost a surprise, given the differences between the two surfaces, that GaN of reasonable quality could even be grown with both polarities. Recently, the polarity identification problem has attracted the attention of electron microscopists [2] [3] [4], but there have been conflicting results. ZnO has the same crystal structure as GaN, and a reasonably close lattice match. It is therefore expected that a GaN film grown epitaxially on a ZnO substrate should have the same polarity. Because both materials are reasonably polar, electrostatic considerations indicate that N-face GaN should grow on O-face ZnO and Ga-face GaN should grow on Zn-face ZnO. The polarity of a substrate can be measured quite easily and unambiguously by measuring the sign of the piezoelectric coefficient, or by measuring etch rates in HCl or H2NO3. [5] Although ZnO has been touted as a promising substrate for GaN and InG
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