Temperature-induced morphological changes of ZnO grown by metalorganic chemical vapor deposition

Changes of morphology during the growth of ZnO on Al₂O ₃ (0 0 0 1) substrates by metalorganic chemical vapor deposition (MOCVD) were investigated as a function of growth temperature (T_g). The morphology of ZnO changes dramatically with T_g. At very low growth temperatures of T_g≤200 °C, no meaningful ZnO forms. At 200 °C<T_g≤260 °C, columnar grained, textured ZnO films having a large amount of crystalline defects grow. At higher growth temperatures of 260 °C<T_g≤320 °C, arrays of vertically well-aligned ZnO nanorods grow. The nanorods are aligned epitaxially with a 30° rotation of ZnO basal planes with respect to Al₂O₃ basal planes. At further higher growth temperatures of 320 °C<T_g≤380 °C, ZnO of nanoneedle type grows. However, at T_g380 °C, ZnO nanowires start to grow on top of a continuous ZnO layer. Both the nanoneedles and the nanowires are not only well aligned both in the out-of- and in the in-plane direction, but also show a very low density of crystalline defects. Our results suggest that T_g is one of the key processing parameters and need to be optimized in a narrow regime in order to grow a desired type of ZnO in MOCVD process.