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Control of the polarity and crystal face of GaN epilayers grown by MBE
|S. Pezzagna , Nicolas GRANDJEAN 1, A. Dussaigne , P. Vennegues|
1. CRHEA-CNRS, rue B. Gregory, Sophia Antipolis, 06560 Valbonne, France, France
GaN epilayers are grown by molecular beam epitaxy using NH3 as nitrogen precursor on sapphire substrates. When a low temperature buffer layer is used, the GaN films grow with the Ga polarity. However, the polarity can be changed from Ga to N by the deposition of a Mg monolayer during growth interruption. Such a polarity inversion may also occur when a large Mg flux is supplied during growth. Secondary ion mass spectroscopy shows that the critical Mg concentration for the polarity inversion must be larger than 3x1020 cm-3. The interface between the two polarities has been characterized by transmission electron microscopy (TEM). It demonstrates a faceted morphology. The polarity inversion is perfectly controlled over a 2 inch wafer and no dislocation is introduced. Actually, the quality of the N-polarity GaN layer replicates that of the Ga-polarity underlayer. Despite some indications about the role of the Mg, the actual mechanism responsible for the polarity inversion is still unclear. Furthermore, if the growth proceeds on the N-polarity with a large Mg flux, the GaN layer undergoes a crystal phase transition from hexagonal to cubic. Surprisingly, this is achieved for standard growth temperatures (800oC) for which the cubic phase should be unstable. Moreover, high-resolution TEM images show a perfectly smooth interface between h- and c-GaN.
Presentation: invited oral at E-MRS Fall Meeting 2003, Symposium C, by Nicolas GRANDJEAN
See On-line Journal of E-MRS Fall Meeting 2003
Submitted: 2003-05-13 09:39 Revised: 2014-10-06 14:01