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Plasma-assisted epitaxial growth of nitrogen-doped and high-quality ZnO thin films

Plasma-assisted epitaxy has been demonstrated as one possible process for the ZnO growth, which is processed at low temperatures below 400oC using oxygen-plasma excited by radio-frequency power at 13.56MHz. This paper is focused on the plasma-assisted epitaxial growth of ZnO layer concerned with shallow acceptor doping using N2+O2 gas plasma and high-quality undoped-ZnO growth using a novel buffer layer consisted of very thin (below 1nm-thick) strained Ti2O3 on A-sapphire. Donor-acceptor pair emission was clearly observed at 3.273eV in low temperature photoluminescence spectrum of nitrogen-doped PAE-ZnO layer grown in Zn-rich supply condition and the activation energy of the shallow acceptor was determined about 132meV. The doping feature of nitrogen related with the oxygen vacancy is also expected by the photoluminescence study. High-quality ZnO layer with smooth surface including fine hexagonal pyramids was grown on the buffer layer at the growth temperature as low as 340oC with the photoluminescence spectrum dominated by free-exciton emissions at 10K. RHEED observations indicated the ZnO on the thin buffer layer was epitaxially grown toward c-axis with an in-plane relation ship of [-12-10]ZnO//[1-104]Al2O3 without any rotational domains.