Polarization fields in III-nitride nanowire devices.

Control of the polarization fields is the most important parameter in designing III-nitride thin-film devices, and herein we show that the polarization fields may be equally, if not more, important in devising III-nitride nanowire devices. One common approach to produce III-nitride nanowires is via a vapor-liquid-solid approach that, in general, yields nanowires with the major (growth) axis in the <1120> direction. The cross section of this wire is an isosceles triangle with two {1101} facets and one {0001} facet. In this work, we analyze the polarization fields that arise in two distinct sets of crystal planes that can manifest in this triangular nanowire geometry: (0001), (1101), (1101) or (0001), (1101), (1101). Calculations show that the polarization field at the {0001} facet is much larger than at the two opposing {1101} facets, although the sign of the field at each facet has a complicated dependence on the orientation and structure of the nanowire. An undoped nanowire transistor was fabricated that displayed p-type operation based solely on polarization-induced hole carriers at the (0001) AlGaN/GaN interface, consistent with our field calculations.