Public Abstract

DE-SC0023222: A Unified Model for Epitaxy of Metastable Crystal Polytypes at the Nanoscale

Award Status: Active

Institution: Regents of the University of Michigan, Ann Arbor, MI
UEI: GNJ7BBP73WE9
DUNS: 073133571

Most Recent Award Date: 05/20/2025
Number of Support Periods: 3
PM: Kidd, Timothy

Current Budget Period: 09/01/2024 - 08/31/2026
Current Project Period: 09/01/2022 - 08/31/2026
PI: Goldman, Rachel

Supplement Budget Period: N/A

Public Abstract

A Unified Model for Epitaxy of Metastable Crystal Polytypes at the Nanoscale Rachel S. Goldman, University of Michigan (Principal Investigator) Liang Qi, University of Michigan (co-Investigator) Judith C. Yang, University of Pittsburgh (co-Investigator) Stephen House, University of Pittsburgh (co-Investigator) Interest in nanoscale crystal growth has exploded in recent years due to the extraordinary properties that emerge at the nanometer scale. Since the first demonstration of micro-wire formation in the 1960s, there have been several reports on metal catalyst-based growth of nanowires (NWs) ranging from metals to semiconductors to complex oxides. Generally, liquid metal droplets act as catalysts, which allow gaseous species to be transformed into a nanoscale crystal in the vicinity of the vapor-liquid-solid triple junction. In the case of epitaxy, NW growth often occurs via a self-catalyzed process, in which liquid metal droplets are consumed during the nucleation and/or transformation process. However, for complex materials, the mechanisms for polytype selection at the nanoscale remain elusive. In contrast to bulk systems whose polytype selection is determined by thermodynamics, it has been hypothesized that metastable 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 zincblende (ZB) or wurtzite (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 would predict ZB polytype selection, independent of contact angle. Indeed, for WZ-polytype-preferring materials – such as GaN, InN, AlN, and ZnO – an approach for controlled switching between WZ and ZB has yet to be identified. We recently discovered an approach to coax self-catalyzed GaN NWs to form the ZB polytype. In this project, we will build upon these results to determine the underlying fundamental mechanisms for ZB vs. WZ polytype selection during nanoscale epitaxy of WZ-polytype preferring materials. Building upon our findings to date, we aim to move beyond existing empirical models to develop a unified physics-based atomistic model for nanoscale epitaxy of metastable crystal polytypes. Our predictive model will include the effects of surface polarity and other atomistic processes not captured by classical nucleation theory. We will examine WZ-polytype preferring materials, including wide-bandgap semiconductors, InN, AlN, and ZnO, that find energy applications ranging from solid-state lighting to high-power electronics. This project consists of a combined computational-experimental-statistical approach to develop a predictive framework for epitaxy of metastable crystal polytypes in confined geometries. We will use ab initio calculations and atomistic simulations of interfacial structures, energetics, and kinetics to reveal the nucleation/growth mechanisms related to polytype selection and morphological evolution. A feedback loop between atomistic simulations and in situ characterization will be used to predict the growth rates of WZ and ZB polytypes. In another feedback loop, models for polytype selection during nucleation will be verified and improved by machine-learning-aided characterization of NW polytype selection. We explore three main hypotheses: (1) Polytype selection is controlled by WZ vs. ZB nucleation rates in given environments rather than by adatom diffusion, (2) The energy barrier for nucleation of ZB GaN inside liquid Ga droplets or at solid-liquid interfaces is lower than that for WZ III-N at triple-phase junctions. Therefore, the ratio of ZB to WZ III-N nucleation rates may be controlled by Ga (or In)/N flux ratios, growth rates, and temperatures, and (3) An atomistic understanding of WZ vs. ZB nucleation mechanisms in given environments will inform strategies for nanoscale epitaxy of stable/metastable polytypes, heterostructures (WZ/ZB/WZ QDs-in-NWs), and high-precision NW polytype superlattices. This project directly addresses Basic Research Needs for Transformative Manufacturing, with a tightly integrated feedback loop between epitaxy processing (*Goldman), theory and modeling (Qi), and machine-learning aided characterization via in situ* and ex situ TEM (*House, Yang*).

A Unified Model for Epitaxy of Metastable Crystal Polytypes at the Nanoscale

Rachel S. Goldman, University of Michigan (Principal Investigator)

Liang Qi, University of Michigan (co-Investigator)

Judith C. Yang, University of Pittsburgh (co-Investigator)

Stephen House, University of Pittsburgh (co-Investigator)

Interest in nanoscale crystal growth has exploded in recent years due to the extraordinary properties that emerge at the nanometer scale. Since the first demonstration of micro-wire formation in the 1960s, there have been several reports on metal catalyst-based growth of nanowires (NWs) ranging from metals to semiconductors to complex oxides. Generally, liquid metal droplets act as catalysts, which allow gaseous species to be transformed into a nanoscale crystal in the vicinity of the vapor-liquid-solid triple junction. In the case of epitaxy, NW growth often occurs via a self-catalyzed process, in which liquid metal droplets are consumed during the nucleation and/or transformation process. However, for complex materials, the mechanisms for polytype selection at the nanoscale remain elusive.

In contrast to bulk systems whose polytype selection is determined by thermodynamics, it has been hypothesized that metastable 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 zincblende (ZB) or wurtzite (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 would predict ZB polytype selection, independent of contact angle. Indeed, for WZ-polytype-preferring materials – such as GaN, InN, AlN, and ZnO – an approach for controlled switching between WZ and ZB has yet to be identified.

We recently discovered an approach to coax self-catalyzed GaN NWs to form the ZB polytype. In this project, we will build upon these results to determine the underlying fundamental mechanisms for ZB vs. WZ polytype selection during nanoscale epitaxy of WZ-polytype preferring materials. Building upon our findings to date, we aim to move beyond existing empirical models to develop a unified physics-based atomistic model for nanoscale epitaxy of metastable crystal polytypes. Our predictive model will include the effects of surface polarity and other atomistic processes not captured by classical nucleation theory. We will examine WZ-polytype preferring materials, including wide-bandgap semiconductors, InN, AlN, and ZnO, that find energy applications ranging from solid-state lighting to high-power electronics.

This project consists of a combined computational-experimental-statistical approach to develop a predictive framework for epitaxy of metastable crystal polytypes in confined geometries. We will use ab initio calculations and atomistic simulations of interfacial structures, energetics, and kinetics to reveal the nucleation/growth mechanisms related to polytype selection and morphological evolution. A feedback loop between atomistic simulations and in situ characterization will be used to predict the growth rates of WZ and ZB polytypes. In another feedback loop, models for polytype selection during nucleation will be verified and improved by machine-learning-aided characterization of NW polytype selection.

We explore three main hypotheses: (1) Polytype selection is controlled by WZ vs. ZB nucleation rates in given environments rather than by adatom diffusion, (2) The energy barrier for nucleation of ZB GaN inside liquid Ga droplets or at solid-liquid interfaces is lower than that for WZ III-N at triple-phase junctions. Therefore, the ratio of ZB to WZ III-N nucleation rates may be controlled by Ga (or In)/N flux ratios, growth rates, and temperatures, and (3) An atomistic understanding of WZ vs. ZB nucleation mechanisms in given environments will inform strategies for nanoscale epitaxy of stable/metastable polytypes, heterostructures (WZ/ZB/WZ QDs-in-NWs), and high-precision NW polytype superlattices.

This project directly addresses Basic Research Needs for Transformative Manufacturing, with a tightly integrated feedback loop between epitaxy processing (Goldman), theory and modeling (Qi), and machine-learning aided characterization via in situ and ex situ TEM (House, Yang).