Group III nitride light emitting devices fabricated on semipolar planes have attracted increasing attention due to the potential for significant reduction in the undesirable effects of the built-in field. As the substrates for growth of high quality semipolar Al_xGa_1-xN films, ZnO is a promising candidate because ZnO has the same crystal structure and the small lattice mismatch with Al_xGa_1-xN. However, there still remains one serious problem with the use of the ZnO substrate, which is formation of the interfacial layers at the heterointerface that negate all the advantages of the nearly lattice-matched substrates. This problem stems from the high growth temperatures for Al_xGa_1-xN by the conventional growth techniques. Therefore, it can be solved by development of a low temperature growth technique for Al_xGa_1-xN . Recently, we have found that the use of pulsed laser deposition (PLD) leads to dramatic reduction in the growth temperature of nitrides.

   We have found that RHEED patterns during the growth of AlGaN on ZnO (1-103) at room temperature (RT) remain nearly unchanged from the initial stages, indicating the epitaxial growth of high quality Al_xGa_1-xN (1-103) on this substrate. X-ray reflectivity measurements have revealed that the heterointerface between Al _x Ga _1-x N and ZnO is atomically abrupt. This result indicates that the reduction in growth temperature down to RT is inherently important for suppression of the interfacial reactions. Figure 1 shows XRD 2θ/ω curves of Al_xGa_1-xN with various x values grown at 850°C on the RT AlGaN layers. One can clearly see 1-103 diffraction for all the samples, indicating that high quality semipolar AlGaN can be grown at all the Al concentrations with this technique. We also found the small tilt of the [1-103]_AlGaN relative to that of ZnO from reciprocal space map near 1-103 diffraction shown in Fig. 2. This phenomenon can be attributed to the formation of the small tilt angle boundary at the heterointerface.

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1. Room temperature epitaxial growth of group III nitrides