Transport Properties and Finite Size Effects in $\beta$-Ga$_2$O$_3$ Thin Films

TL;DR

This study investigates how thin film thickness affects electron transport in $eta$-Ga$_2$O$_3$, revealing quantum size effects that limit mobility more than classical models predict, with implications for electronic device design.

Contribution

It introduces a quantum mechanical size effect model based on Bergmann's electron wave theory to explain mobility suppression in thin $eta$-Ga$_2$O$_3$ films, surpassing classical explanations.

Findings

Electron mobility decreases significantly with film thinning.

Classical Fuchs-Sondheimer model is insufficient to explain surface scattering.

Quantum effects due to large de Broglie wavelength limit transport properties.

Abstract

Thin films of the wide band gap semiconductor $\beta$-Ga$_2$O$_3$ have a high potential for applications in transparent electronics and high power devices. However, the role of interfaces remains to be explored. Here, we report on fundamental limits of transport properties in thin films. The conductivities, Hall densities and mobilities in thin homoepitaxially MOVPE grown \mbox{(100)-orientated $\beta$-Ga$_2$O$_3$} films were measured as a function of temperature and film thickness. At room temperature, the electron mobilities ((115$\pm$10) $\mathrm{\frac{cm^2}{Vs}}$) in thicker films (> 150 nm) are comparable to the best of bulk. However, the mobility is strongly reduced by more than two orders of magnitude with decreasing film thickness ((5.5$\pm$0.5) $\mathrm{\frac{cm^2}{Vs}}$ for a 28 nm thin film). We find that the commonly applied classical Fuchs-Sondheimer model does not explain the contribution of electron scattering at the film surfaces sufficiently. Instead, by applying an electron wave model by Bergmann, a contribution to the mobility suppression due to the large de Broglie wavelength in $\beta$-Ga$_2$O$_3$ is proposed as a limiting quantum mechanical size effect.