Controlling the polarity of polar semiconductors on nonpolar substrates offers a wealth of device concepts in the form of heteropolar junctions. A key to realize such structures is an appropriate buffer-layer design that, in the past, has been developed by empiricism. Understanding the basic processes that mediate polarity, however, is still an unsolved problem. We present results on the structure of buffer layers for group-III nitrides on sapphire by transmission electron microscopy. We show that it is the conversion of the sapphire surface into a rhombohedral aluminum-oxynitride layer that converts the initial N-polar surface to Al polarity. With the various AlxOyNz phases of the pseudobinary Al2O3-AlN system and their tolerance against intrinsic defects, typical for oxides, a smooth transition between the octahedrally coordinated Al in the sapphire and the tetrahedrally coordinated Al in AlN becomes feasible. Based on these results, we discuss the consequences for achieving either polarity and shed light on widely applied concepts in the field of group-III nitrides like nitridation and low-temperature buffer layers.
Martin Albrecht, Natalia Stolyarchuk, Stefan Mohn, Toni Markurt, Ronny Kirste, Ramon Collazo, Aimeric Courville, Zlatko Sitar, and Philippe Vennéguès, "Polarity control in III-nitrides: new insights into an old problem (Conference Presentation)," Proc. SPIE 10532, Gallium Nitride Materials and Devices XIII, 105320C (Presented at SPIE OPTO: January 30, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2291849.5751531008001.
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