• Jang, Hyochang
• Tanaka, Tooru
• Saito, Katsuhiko
• Guo, Qixin

Abstract

The effects of Al doping on the structural, electrical, and optical properties of rock-salt (RS)-Zn_1-xCd_xO (ZnCdO) thin films were investigated. The films were grown by molecular beam epitaxy under various Al cell temperatures. Reflection high-energy electron diffraction and compositional analyses indicated that single-crystal RS-ZnCdO (<i>x</i> ≈ 0.83) layers were obtained with up to 7.8% Al incorporation. The electron concentration in the thin films increased with increasing Al cell temperature and was 1.5 × 10²¹ cm⁻³ at ∼7.8% Al. Al-doped RS- and wurtzite (WZ)-ZnCdO thin films with various Cd concentrations were grown under the same Al cell temperatures. The ZnCdO films had an electron concentration of 10²¹ cm⁻³ for Cd content x ranging over 0.27–1.0, regardless of the crystal structure. Al-doped RS-ZnCdO films had a 3.45-eV optical band gap at <i>x</i> = 0.7 because of the high electron concentration that led to a Burstein-Moss shift. This was the highest-energy optical band gap reported for RS-ZnCdO. Therefore, Al doping was very effective in expanding the optical band gap, as well as increasing the RS-ZnCdO electrical conductivity, making the films suitable as transparent conductors for full-spectrum solar cells.

Topics

• thin film
• electron diffraction
• electrical conductivity