Investigation of the Physical and Electrical Properties of n-type Nb-doped Ga 2 O 3 Films

This study aims to develop a simple and effective method for synthesizing Nb-doped Ga 2 O 3 films as an n-type semiconductor. Initially, Ga 2 O 3 powder doped with 12 at% Nb 2 O 5 (Ga:Nb=100:12) was used as the starting material and was pre-calcined at 950°C to promote its transformation into the more stable β-phase. The calcined powder was then formed into a pellet-shaped target and deposited onto substrates using electron beam evaporation to fabricate Nb-doped Ga 2 O 3 films. Subsequent annealing of the Nb-doped Ga 2 O 3 films was performed at 500°C under three conditions: without any gas flow, in ambient air, and in a reducing atmosphere composed of 95% N 2 and 5% H 2 . The reducing atmosphere was intended to enhance the film's conductivity by increasing the oxygen vacancy concentration. The electrical properties of the three types of Nb-doped Ga 2 O 3 films-namely, as-deposited, annealed in air, and annealed in the reducing gas-were compared using a semiconductor parameter analyzer. Additionally, Hall effect measurements were carried out to determine the carrier concentration, carrier mobility, and resistivity of the films. The results revealed that both the as-deposited film and the film annealed in air at 500°C remained highly resistive, with resistivity exceeding 10 8 Ω˙cm, indicating insulating behavior. In contrast, the film annealed at 500°C in the reducing atmosphere exhibited significantly improved electrical conductivity. Based on these findings, further analysis focused on the as-deposited film and the film annealed in the reducing atmosphere. Optical transmittance spectra were recorded, and the optical bandgap was estimated using Tauc plot analysis derived from UV-visible spectroscopy. X-ray photoelectron spectroscopy (XPS) was also employed to investigate the chemical bonding states, oxidation states, and the presence of oxygen vacancies in the Ga 2 O 3 films.