Epitaxial thin films of MgIn_(2)O_(4) were grown on MgO(100) single-crystal substrates through the pulsed laser deposition technique. X-ray diffraction measurements revealed film orientations MgIn_(2)O_(4)(h00)//MgO(h00) and MgIn_(2)O_(4)(0k0)//MgO(0k0), respectively. Proton implantation was applied to generate carrier electrons in the films. The electrical conductivity of the as-deposited films is below ～10~(-7) S cm~(-1) at room temperature. The maximum conductivity of ～70 S cm~(-1) was obtained by the implantation. Hall voltage measurements revealed that H~(+) implantation causes carrier generation in proportion to H~(+) fluence without reduction of electron mobility. Following the post-annealing process resulted in further enhancement of the conductivity in each H~(+)-implanted film, as conductivity and generation efficiency were found to increase up to ～2×10~(2) S cm~(-1) and ～95 at the maximum, respectively. This differs from the behavior of polycrystalline films in which conductivity decreased by post-annealing due to a decrease in the Hall mobility of electrons. Thus it is concluded that crystal quality is crucial for heavy carrier doping by ion implantation, especially when utilizing post-annealing treatments to enhance the carrier generation efficiency without reduction of the Hall mobility.
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