Drastically enhanced cation incorporation in the epitaxy of oxides due to formation and evaporation of suboxides from elemental sources

TL;DR

This paper uncovers how suboxide formation and evaporation significantly enhance cation incorporation in oxide epitaxy, challenging traditional vapor pressure expectations and providing a quantitative model for flux behavior.

Contribution

It introduces a rate-equation model explaining suboxide effects on flux enhancement during oxide epitaxy, supported by experimental validation and applicable to various elemental sources.

Findings

Suboxide formation causes non-linear flux temperature dependence.

The model accurately predicts flux behavior across temperature regimes.

Suboxide effects are significant for multiple elemental sources in oxide growth.

Abstract

In the molecular beam epitaxy of oxide films, the cation (Sn, Ga) or dopant (Sn) incorporation does not follow the vapor pressure of the elemental metal sources, but is enhanced by several orders of magnitude for low source temperatures. Using line-of-sight quadrupole mass spectrometry, we identify the dominant contribution to the total flux emanating from Sn and Ga sources at these temperatures to be due to the unintentional formation and evaporation of the respective suboxides SnO and Ga$_{2}$O. We quantitatively describe this phenomenon by a rate-equation model that takes into account the O background pressure, the resulting formation of the suboxides via oxidation of the metal source, and their subsequent thermally activated evaporation. As a result, the total flux composed of the metal and the suboxide fluxes exhibit an \textsf{S}-shape temperature dependence instead of the expected linear one in an Arrhenius plot, in excellent agreement with the available experimental data. Our model reveals that the thermally activated regimes at low and high temperatures are almost exclusively due to suboxide and metal evaporation, respectively, joined by an intermediate plateau-like regime in which the flux is limited by the available amount of O. An important suboxide contribution is expected for all elemental sources whose suboxide exhibits a higher vapor pressure than the element, such as B, Ga, In, La, Si, Ge, Sn, Sb, Mo, Nb, Ru, Ta, V, and W. This contribution can play a decisive role in the molecular beam epitaxy of oxides, including multicomponent or complex oxides, from elemental sources. Finally, our model predicts suboxide-dominated growth in low-pressure chemical vapor deposition of Ga$_{2}$O$_{3}$ and In$_{2}$O$_{3}$.