April 22 - 26, 2024
Seattle, Washington
May 7 - 9, 2024 (Virtual)

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2024 MRS Spring Meeting
EL04.04.01

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

When and Where

Apr 23, 2024
3:15pm - 3:30pm
Room 345, Level 3, Summit

Presenter(s)

Co-Author(s)

Abby Liu1,Zhucong Xi1,Meng Li2,3,Dmitri Zakharov3,Fernando Camino3,Judith Yang3,2,Liang Qi1,Rachel Goldman1

University of Michigan1,University of Pittsburgh2,Brookhaven National Laboratory3

Abstract

Abby Liu1,Zhucong Xi1,Meng Li2,3,Dmitri Zakharov3,Fernando Camino3,Judith Yang3,2,Liang Qi1,Rachel Goldman1

University of Michigan1,University of Pittsburgh2,Brookhaven National Laboratory3
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.

Keywords

crystallographic structure | nucleation & growth | transmission electron microscopy (TEM)

Symposium Organizers

Hideki Hirayama, RIKEN
Robert Kaplar, Sandia National Laboratories
Sriram Krishnamoorthy, University of California, Santa Barbara
Matteo Meneghini, University of Padova

Symposium Support

Silver
Taiyo Nippon Sanso

Session Chairs

Jennifer Hite
Sriram Krishnamoorthy

In this Session