Kinetics, thermodynamics, and catalysis of the cation incorporation into GeO2, SnO2, and (SnxGe1-x)O2 during suboxide molecular beam epitaxy

TL;DR

This study investigates the kinetics and thermodynamics of suboxide incorporation during molecular beam epitaxy of GeO2, SnO2, and their alloy, revealing insights into growth control and cation exchange effects for oxide semiconductor development.

Contribution

It provides the first comprehensive analysis of suboxide MBE growth mechanisms for GeO2, SnO2, and their alloy, highlighting the catalytic role of SnO in promoting GeO incorporation.

Findings

Suboxide MBE simplifies growth kinetics and improves control over growth rates.

SnO incorporation is kinetically favored in binary growth due to oxidation efficiency and vapor pressure.

In alloy growth, GeO incorporation is preferred, with SnO suppressed, indicating a catalytic cation exchange effect.

Abstract

Rutile GeO$_2$ is a promising ultra-wide bandgap semiconductor for future power electronic devices whose alloy with the wide bandgap semiconductor rutile-SnO$_2$ enables bandgap engineering and the formation of heterostructure devices. The (Sn$_x$Ge$_{1-x}$)O$_2$ alloy system is in its infancy and molecular beam epitaxy (MBE) is a well-suited technique for its thin-film growth, yet presents challenges in controlling the alloy composition and growth rate. To understand and mitigate this challenge, the present study comprehensively investigates the kinetics and thermodynamics of suboxide incorporation into GeO$_2$, SnO$_2$, and (Sn$_x$Ge$_{1-x}$)O$_2$ during suboxide MBE, the latest development in oxide MBE using suboxide sources. We find suboxide MBE to simplify the growth kinetics, offering better control over growth rates than conventional MBE but without supporting cation-driven oxide layer etching. During binary growth, SnO incorporation is kinetically favored due to its higher oxidation efficiency and lower vapor pressure compared to those of GeO. In (Sn$_x$Ge$_{1-x}$)O$_2$ growth, however, the GeO incorporation is preferred and the SnO incorporation is suppressed, indicating a catalytic effect, where SnO promotes GeO incorporation through cation exchange. The origin of this cation exchange is likely thermodynamic yet calls for further theoretical studies. Our experimental study provides guidance for controlling the growth rate and alloy composition of (Sn$_x$Ge$_{1-x}$)O$_2$ in suboxide MBE, highlighting the impact of the substrate temperature and active oxygen flux besides that of the mere SnO:GeO flux stoichiometry. The results are likely transferable to further physical and chemical vapor deposition methods, such as conventional and hybrid MBE, pulsed laser deposition, mist- or metalorganic chemical vapor deposition.