Nb-doped ZrxSn1−xO2: Experimental and first-principles study

Ultra-wide bandgap semiconductors with exceptional advantages have potential use in ultrahigh power, ultrahigh frequency devices, and other applications. In this paper, a series of high-quality Nb-doped ZrxSn1−xO2 (Nb:ZrxSn1−xO2) alloy epitaxial films were prepared on c-plane sapphire substrates by pulsed laser deposition. A greater proportion of Zr successfully widened the optical bandgap of SnO2 up to 4.70 from 4.28 eV. Interestingly, although Nb is a common n-type dopant for SnO2, the conductivity of Nb:ZrxSn1−xO2 decreased with increasing Zr content. The greater activation energy Ea of the films with more Zr contents was determined by variable resistance measurements and rationalized by the first-principles calculations. The higher Zr content leads to a lower conductivity in the films. This is because the electronegativity of Zr is smaller than that of Sn and Nb, making it easier for O to attract electrons from Zr and Nb donating less electrons with increasing Zr content. It leads to more electrons filling the Nb 4d orbital and brings the donor level further down from the conduction band minimum. However, Nb:ZrxSn1−xO2 with a low Zr content of x = 0.1 has good electrical conductivity, with a carrier density of 5.426 × 1020 cm−3 and a resistivity of 7.89 × 10−3 Ω cm, and simultaneously a broadened bandgap of 4.4 eV. Therefore, Nb can act as an effective n-type dopant for ZrxSn1−xO2 with proper Zr content, making Nb-doped ZrxSn1−xO2 promising for developing ultraviolet-transparent conductive electrodes.