Probing Polytype Selection during Metal-Mediated Epitaxy of GaN Quantum Dots and Nanowires

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 to the zincblende (ZB) polytype. At a fixed chemistry WZ/ZB heterojunction, the lattice mismatch and thermal expansion coefficient mismatch are typically &lt; 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. Such heterostructures would be promising for high power electronics and single photon emitters.<br/><br/>It has been hypothesized that metastable nanowire (NW) polytype selection is governed by surface/interface energies, surface diffusivities, and/or droplet angles that determine ABC vs. AB stacking of atomic planes, resulting in ZB or WZ polytypes. For ZB-polytype-preferring materials, such as III-As and III-P, ZB vs. WZ polytype selection has been described by empirical “contact angle” models, enabling the design and fabrication of NW polytype superlattices. However, for GaN, a WZ-polytype preferring material, the “contact angle” models for NW polytype selection, using literature values for WZ and ZB GaN surface energies, predict ZB polytype formation, across contact angles, even though WZ GaN NWs are most frequently reported in the literature.<br/>We recently discovered a Ga-mediated molecular-beam epitaxy (MBE) process to nucleate ZB and WZ GaN NWs on Si(001) [1]. Key to this process is a Ga pre-deposition step, in which Ga droplet arrays are formed prior to NW growth. For the ZB NW ensembles, reflection-high energy electron diffraction and x-ray diffraction suggest overall ZB-to-WZ transformations at thickness ~20 nm. High-angle annular dark-field STEM reveals WZ NWs with close-packed (0001) planes oriented along the Si [001] surface normal, i.e. [0001]-oriented WZ NWs. On the other hand, ZB NWs with the close-packed (111) planes oriented ~37° from the Si [001] surface normal, i.e. [001]-oriented ZB NWs. Interestingly, the NW axis orientation remains fixed as it transforms to the WZ polytype. We hypothesize that Si incorporation into Ga droplets influences the polytype selecting during NW growth. Preliminary energy dispersive spectroscopy (EDS) data reveals higher Si concentration within ZB NW in comparison to that within WZ GaN NWs. Correspondingly, density functional theory (DFT) calculations show that ~8 at% Si in the Ga sublattice makes ZB GaN thermodynamically more stable than WZ GaN. Thus, Si appears to be a ZB polytype stabilizer for GaN. Direct observations of GaN nucleation in an in-situ environmental TEM (ETEM) will also be presented.<br/><br/>This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0023222. This research used the Electron Microscopy facility of the Center for Functional Nanomaterials, which is a U.S. Department of Energy Office of Science User Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704.<br/><br/>[1] Lu, H., S. Moniri, C. Reese, S. Jeon, A. Katcher, T. Hill, H. Deng, and R.S. Goldman. 2021. “Influence of gallium surface saturation on GaN nanowire polytype selection during molecular-beam epitaxy.” Appl. Phys. Lett. 119:031601.