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 SnO2 films, 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 Tc and the abruptness, with increasing film thickness. We correlate the Tc 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 TixV1-xO2.

Institution: 

Faculty of Materials Science and Engineering, Wuhan, Hubei University, China

Date: 
Friday, August 18, 2017 - 11:00
Speaker: 
Prof. Yunbin He