Physical properties of (Zn, Mg)O thin films and related heterojunctions

Abstract

Transparent cubic phase Mg₀.₅Zn₀.₅O (c-MZO) films of about 500 nm thickness were successfully fabricated on single crystal MgO(001) and LAO(001) substrates by Pulsed Laser Deposition (PLD) method. X-ray diffraction (XRD) studies showed that typical cube-on-cube epitaxy was obtained for c-MZO films grown on MgO substrate. Films deposited on LAO substrates, however, exhibited excellent epitaxy with 45℃ in-plane twisted relative to the substrate. Upon introducing an appropriate Indium (In) content, the optical bandgap energy of c-MZO films can be widely tuned from 6.17 to 4.29 eV. All In doped Mg₀.₅Zn₀.₅O (IMZO) films displayed excellent optical transmittance of over 90 % for the whole visible spectrum, while the resistivity of these films can be brought down to 10⁻² Ω-cm when alloyed with the In content of 0.18 in the target. Therefore, these IMZO films can be used as tunable UV filter and transparent conducting oxide (TCO) for various optoelectronics applications. Li₀.₁₅Ni₀.₈₅O (LNO) is a p-type TCO. Epitaxial and transparent IMZO based p-LNO/n-IMZO junctions were fabricated on MgO(100) substrate. They all have demonstrated good rectifying property at room temperature. Moreover, the temperature dependence of I-V characteristic showed that the current transportation mechanism of these heterojunctions was dominated by tunneling through the barrier or the generation-recombination process occurring in the depletion region. In the optical response measurement, responsivity of these photodetectors can be varied from 307 to 315 nm by changing the In content in the n-IMZO thin films. These results showed that such all-oxide heterojunctions with good electrical and optical properties are a promising material for optoelectronic applications. The p-LNO/n-IMZO heterojunctions have been successfully integrated on TiN buffered Si substrate. These p-n junctions exhibited even better rectifying characteristics than those fabricated on MgO substrate, and their structural and electrical properties can be effectively tuned by bandgap engineering of n-IMZO. By increasing the In content in n-IMZO layer, electrical resistivity and bandgap are lowered. These can lead to higher current rectifying ratio, higher threshold voltage and higher leakage current in the bias voltage of -4V. Furthermore, heterojunctions grown on TiN buffered Si possess higher current rectifying ratio, lower series resistance and lower leakage current than those grown on MgO substrate. These results suggested that all-oxide p-LNO/n-IMZO heterojunctions can be integrated on Si with better electrical performances than those grown on MgO substrate.