Semiconductor polytype heterostructures, which consist of chemically homogeneous structures formed via an abrupt change in crystal structure, offer opportunities for performance exceeding those of composition-based semiconductor heterostructures. Of particular interest are heterostructures formed via an abrupt change in atomic plane stacking sequence, such as the transition from the wurtzite (WZ) polytype, with AB stacking, to the zincblende (ZB) polytype, with ABC stacking. At a fixed chemistry WZ/ZB heterojunction, the lattice mismatch and thermal expansion coefficient mismatch are typically < 0.1%, leading to a negligible interfacial defect concentration. Meanwhile, the WZ/ZB band offset and polarization discontinuity are expected to lead to the confinement of a two-dimensional electron gas (2DEG) at the interface, without the need for impurity doping and/or alloying. Depending on the type I (nested) vs. type II (staggered) band offset at the WZ/ZB polytype junction, such heterostructures would also be promising for high power electronics and single photon emitters. Recently, we discovered a metal-mediated growth process to nucleate the metastable ZB polytype during molecular-beam epitaxy of GaN . Interestingly, for both nanowires and films, a transformation from the ZB to WZ polytype occurs after 1 hour of growth, at a thickness of 150 nm. Due to the consistency in the timing of the transformation, we hypothesize that the ZB-to-WZ transformation is not due to a random event but instead due to an extrinsic effect associated with the growth process. We will discuss the relative influences of effusion-cell-induced surface heating and electron beam-induced surface polarization on polytype selection and transformation, along with prospects for realization of GaN QDs-in-NWs single-photon emitters and lattice-matched polarization-doped WZ/ZB GaN high-electron mobility transistors.
 H. Lu, S. Moniri, C. Reese, S. Jeon, A. Katcher, T. Hill, H. Deng, and R.S. Goldman "Influence of gallium surface saturation on GaN nanowire polytype selection during molecular-beam epitaxy", Appl. Phys. Lett. 119, 031601 (2021).
Rachel S. Goldman is a professor of MSE, Physics, and EECS at UM. She received degrees from UM (B.S. Physics, 1988), Cornell (M.S. Applied Physics, 1992), and UC-San Diego (PhD Materials Science, 1995). Following her PhD, she was a postdoctoral fellow in Physics at Carnegie Mellon. She is currently Associate Director of Applied Physics, and has served as MSE Graduate Chair, Associate Director of the DoE Energy Frontiers Research Center, and Education Director of the NSF Materials Research Science and Engineering Center. Her research emphasizes the role of local solute configurations on novel functionalities of semiconductor alloys and nanostructures. She has published > 135 papers on processing-structure-property correlations in semiconductors; she holds a U.S. patent on “ion-cut-synthesis”, a novel approach for simultaneous synthesis and integration of nanocomposite materials with virtually any substrate. Recently, she pioneered the incorporation of Writing to Learn pedagogies into introductory materials science and engineering courses. Prof. Goldman is a Fellow of the American Physical Society (APS), the American Association for the Advancement of Science, and the American Vacuum Society. She is Associate Editor of the Journal of Applied Physics and Past Chair of the APS Division of Materials Physics.