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

Changes of morphology during the growth of ZnO on Al2O3 (0001) 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 degrees C, no meaningful ZnO forms. At 200 degrees C < T-g <= 260 degrees C, columnar grained, textured ZnO films having a large amount of crystalline defects grow. At higher growth temperatures of 260 degrees C < T-g <= 320 degrees C. arrays of vertically well-aligned ZnO nanorods grow. The nanorods are aligned epitaxially with a 30 degrees rotation of ZnO basal planes with respect to Al2O3 basal planes. At further higher growth temperatures of 320 degrees C < T-g <= 380 degrees C, ZnO of nanoneedle type grows. However, at T-g > 380 degrees 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. (c) 2004 Elsevier B.V. All rights reserved.