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.