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Polarity-controlled ZnO films with an MgO buffer layer were grown on c -plane sapphire by plasma-assisted molecular beam epitaxy (MBE). The effect of the thickness of the MgO buffer layer on the MBE growth of ZnO was investigated. The growth rate of ZnO increased from 250 to 500 nm/h when the MgO layer thickness was increased from 1 to 3 nm. Convergent beam electron diffraction results showed that Zn-polarity (+ c ) growth occurred when the MgO layer was thicker than 3 nm, whereas O-polarity (− c ) growth occurred when the layer was less than 2 nm. Polarity conversion apparently occurred due to the difference in atomic structure between the MgO wetting layer and MgO islands. The minimum line-widths of (0 0 0 2) and (1 0 1¯ 0) ω -rocking curves were 70 and 722 arcsec, respectively, for O-polar ZnO, and 289 and 851 arcsec, respectively, for Zn-polar ZnO. The O-polar ZnO with a 1-nm-thick MgO buffer layer had a residual carrier concentration of 4.6×10 16 cm −3 with an electron mobility of 155 cm 2 /V s, whereas Zn-polar ZnO with an MgO layer thicker than 3 nm showed semi-insulating properties. In conclusion, control of the polarity of ZnO films grown on c -plane sapphire substrates by plasma-assisted MBE was achieved by inserting an MgO buffer layer of specific thickness between a ZnO film and a nonpolar substrate. |
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