Oxide semiconductor films: epitaxial growth and thickness-dependent properties

Oxide semiconductors play important roles in optoelectronics, catalysis, sensing, energy harvesting, etc. In this talk, I will present our recent work on wide-bandgap semiconductor SnO₂ and thermochromic VO₂.

Realization of epitaxial growth of high-quality SnO₂ films is of primary importance for using SnO₂ in short wavelength optoelectronic devices. We prepared high-quality epitaxial SnO₂ films on c-plane sapphire substrates with varying thickness by pulsed laser deposition (PLD), and systematically studied the effects of film thickness on the structural and optical properties of epitaxial SnO₂ films. With the help of high-resolution X-ray diffraction (HRXRD), we determined the lattice parameters of SnO₂films, and illustrated in detail their evolution with growing film thickness. Optical transmission measurements indicate that the bandgap of SnO₂ varies significantly with the film thickness. When the film is thinner than 6.1 nm, the bandgap widens gradually with decreasing film thickness, due to the quantum size effect. For films thicker than 6.1 nm, the bandgap widens gradually with increasing film thickness, which is attributable to strain effects in the films.

In the vicinity of 341 K, VO₂ undergoes a reversible metal-to-semiconductor transition (MST), which attracts interest not only for fundamental mechanism study but also for practical applications. To explore lattice strain effects on the MST, we grew high-quality epitaxial VO₂(110) films with various thicknesses on TiO₂(110) substrates by PLD. We used HRXRD including RSM to determine the crystal structure of the films, and performed four-probe measurements to monitor the variation of the films resistivity with temperature. It turns out that the MST of VO₂ films show remarkable changes, including the transition temperature T_c and the abruptness, with increasing film thickness. We correlate the T_c to c/a of the films. More interestingly, while VO₂ exhibits a MST co-driven by Peierls and Mott mechanisms, we found alloying with little Ti can lead to pure Mott transition in Ti_xV_1-xO₂.