| Summary: | 博士 === 國立交通大學 === 材料科學與工程學系 === 101 === As a wide direct bandgap wurtzite semiconductor, zinc oxide (ZnO) is an attractive material for potential applications in optoelectronic devices. However, the built-in electrostatic field due to piezoelectric polarization in wurtzite structure makes the lower efficiency of carrier recombination which degrades the optical emitting properties. In order to avoid the so-called quantum confined Stark effect, nonpolar films without polarity along the growth direction is required. Hence, nonpolar ZnO film growth in a-plane and m-plane has been intensively studied in recent years.
A systematic study of pulse laser deposited (PLD) a-plane ZnO grown on r-plane sapphire at different temperature has been done by using in-situ reflection high-energy electron diffraction (RHEED), atomic force microscopy (AFM), high-resolution x-ray diffraction (HRXRD) and transmission electron microscopy (TEM). The significant effects of growth temperature and Al-doping on the growth characteristics were observed. The misfit accommodation including strain evolution and interface structure at various growth stagehave been carefully characterized.
For initial growth in PLD, ZnO grown at 750°C (HT-ZnO) shows step morphology, while ZnO grown at 450°C (LT-ZnO) exhibits island growth mode. For thick films, both HT- and LT-ZnO surfaces develop into stripe morphology. The crystallinity is shown to be improved with film thickness for both HT-ZnO and LT-ZnO.
Along ZnO[1-100] of large lattice mismatch, TEM examination shows that a-type misfit dislocations are spaced in a distance of 1.3 - 2.2 nm on HT-ZnO/sapphire interface, whereas dislocation pairs in spacing of 2.8 - 3.5 nm are observed for the LT-ZnO/sapphire one. Also, tensile strains are present in LT-ZnO films in ZnO[1-100] direction, but residual compressive strains are observed for 10 nm and 50 nm HT-ZnO films.
For smaller lattice mismatch along ZnO c-axis direction, reciprocal space maps of XRD show that HT-ZnO is nearly fully strained without much relaxation and has a highly coherent interface with sapphire, in contrast with partial relaxation in LT-ZnO.
Finally, it is observed that the streaky RHEED patterns obtained at growth temperature may evolve to spotty RHEED patterns during cooling. It may be related with the thermal mismatch between ZnO and sapphire. High cooling rate can result in crystallinity deterioration, which is apparent for Al-doped ZnO epitaxial films.
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