Carrier generation in polycrystalline MgIn2O4 thin films by proton implantation

Abstract Thin films of polycrystalline MgIn2O4 (spinel structure; bandgap, 3.5 eV) were prepared on silica glass by the rf sputtering technique. The thin films were insulating in the as-deposited state, and proton implantation was carried out in the fluence range from 1 × 1015 to 1 × 1018 cm−2 at room temperature to introduce carrier electrons in the films. Upon implantation at a fluence of 1 × 1015 cm−2, the electrical conductivity increased to as high as approximately 3 × 1015 cm−1 and saturated at this level even for higher fluences. Carrier generation efficiency was about 100 % at a fluence of 1 × 1015 cm−2 but monotonically decreased with increasing fluence. Nuclear reaction analysis on deuteron-implanted specimens revealed that the depth profile of D concentration for all specimens was almost the same in the as-implanted state, and the D fraction retained in the films was about 80 % for a D+ fluence of 1 × 1016 cm−2, about 70 % for 1 × 1017 cm−2, and about 65 % for 1 × 1018 cm−2. After post-annealing at 300°C, the width of the D distribution became broader, and the retained D fraction was reduced to 50 % for a D+ fluence of 1 × 1017 cm−2 or 17 % for 1 × 1018 cm−2. These results strongly suggest that the majority of H+ ions implanted at high fluences are present in the films as neutral species such as H2 molecules. Therefore, the formation of these species, which are inactive for carrier generation, is responsible for the decrease in carrier generation efficiency.