Phase Evolution and Substrate-Dependent Nucleation of Quartz GeO$_2$ Films Grown by MOCVD on r- and c-Plane Sapphires

TL;DR

This study explores the growth, phase transition, and substrate-dependent morphology of GeO$_2$ films grown by MOCVD on sapphire, revealing how temperature and substrate orientation influence crystallization and pattern formation.

Contribution

It provides the first systematic analysis of the temporal phase evolution and substrate-dependent morphological patterns of GeO$_2$ films during MOCVD growth.

Findings

Quartz patterns grow larger with increased growth time.

Higher temperatures accelerate crystallization.

Distinct quadrangular and hexagonal patterns form on different sapphire orientations.

Abstract

Ultrawide-bandgap (UWBG) semiconductors, such as GeO$_2$, are gaining significant attention for their potential in high-performance applications, particularly in piezoelectric devices. Despite extensive research, a comprehensive understanding of the growth dynamics and phase evolution of GeO$_2$ films via metal-organic chemical vapor deposition (MOCVD) remains insufficient. In this study, we investigate the growth behavior and morphological evolution of GeO$_2$ thin films on r-plane and c-plane sapphire substrates for the MOCVD growth process. The temporal evolution of crystallization and the amorphous-to-quartz phase transition are systematically elucidated for the first time. As growth time increases, the spherulitic quartz patterns expand in size, and elevated growth temperatures are found to enhance the crystallization rate. Distinct morphological symmetries emerge depending on the substrate orientation: quadrangular patterns on r-plane sapphire and hexagonal patterns on c-plane sapphire. Atomic force microscopy reveals that these spherulitic domains exhibit pyramid-like surface topography, consistent with volumetric contraction during the amorphous-to-quartz phase transition. These findings offer new insights into the phase evolution and substrate-dependent crystallization behavior of GeO$_2$ films grown by MOCVD.