Importance of lattice matching and surface arrangement for the helicon-wave-excited-plasma sputtering epitaxy of ZnO

Importance of lattice matching and proper surface arrangement of the substrate was shown to obtain better epilayer qualities of ZnO grown by an epitaxy method, the helicon-wave-excited-plasma sputtering epitaxy. The a-axis-locked single-domain (0001) ZnO epitaxy was accomplished on the uniaxially nearly lattice-matched (112̄0) Al2O3 substrate. The epilayer had approximately 50-100-nm-diam atomically smooth two-dimensional terraces with 0.26-nm-high monolayer steps, and exhibited a predominant near-band-edge photoluminescence peak, whose full width at half maximum value was 107 meV at room temperature. The a-axis-locked epitaxy was also realized on a lattice-mismatched (0001) Al2O3 substrate, indicating the compatibility of oxide substrates. However, the ZnO epilayer grown on the (0001) AlN epitaxial template prepared on (0001) Al2O3 had an in-plane 12-fold multidomain structure, though the effective lattice-mismatch between [112̄0] ZnO and [112̄0] AlN is smaller than that between [112̄0] ZnO and [101̄0] Al2O3. Proper purification and polarity-control techniques for the AlN surface, and/or insertion of an appropriate buffer layer are required to form single-domain heterovalent ZnO/AlN interfaces.