Transmission Electron Microscopy Study of Room Temperature Lasing Epitaxial ZnO Films on Sapphire
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Reflection high energy electron diffraction (RHEED) patterns showed sharp streaks from the
start of deposition, indicating epitaxial growth. X-ray diffraction and low energy electron diffrection (LEED) patterns show the following epitaxial relationship of the ZnO films grown on (0001) sapphire: (0001)zno H (0001),.P, [1OTO]- o HI[11 0],P. Atomic force microscopy (AFM) images of the ZnO film surface show close array of hexagonal-shaped {10T0} faceted nanocrystals. Planar and cross-sectional specimens for transmission electron microscopy (TEM) study were prepared by the usual mechanical polishing followed by Ar+ ion-milling to electron transparency. TEM and high resolution electron microscopy (HREM) images were taken in a Philips CM-20 and a JEM 2010 electron microscope at 200 kV. RESULTS The epitaxial relationship: (0001)z,0 // (0001).p , [1010],o // [1120].P, is readily confirmed by electron diffraction. Fig. 1 shows the [0001] electron diffraction from two ZnO films, Z39 and Z194. Stimulated emission has been observed only from film Z39. The corresponding plan-view images of the films are shown in Figs. 2 and 3. The diffraction patterns are composites patterns from overlapping ZnO and sapphire layers, plus extra spots due to double diffraction. The pattern in Fig. 1(a) is dominated by reflections from the thick (300 nmn) ZnO film, reflections from the sapphire substrate are quite weak, weaker than the double diffrac-
Fig. 1. [0001] diffraction pattern of(a) film Z39 and film 194. Note the arcing of ZnO and double diffraction spots in (b). tion spots. The ZnO film in Fig. 1(b) is thinner (50 nm) and the reflections from the sapphire substrate is as strong as those of the ZnO film. The 10 10zoa reflections in Fig. 1(b) are short arcs while the 1120., reflections are sharp spots. The arcs are about 50. Arcing of ZnO reflections is not discernible in Fig. 1(a). The lattice mismatch for the orientation relationship above obtained from the diffraction patterns is 18%. The polycrystalline grain structures of the films is quite evident in both films. But regular hexagonal grains have not been observed, although 1200 boundaries can sometimes be seen. Moire fringes are clearly visible in Fig. 2. The grains are rather small, ranging from less than 10 nrn to larger than 30 nm in size. The angle of rotation of the moire fringes across adjacent grains varies over quite a large range. Irregularly 424
Fig. 2. [0001] plan-view image of thin ZnO film Z194 showing moire fringes depicting grains with various angle of misorientations. Note the presence of nanotubes of different sizes. shaped and hexagonally shaped pinholes or nanotubes, which are often observed in GaN [8], are readily visible. Nanotubes are only observed in the highly misoriented film Z194. The grains of film Z39 in Fig.3, taken from a substrate free region, are separated by dislocation boundaries. The rather regularly spaced dislocations are clearly visible when the specimen is tilted off the (0001) axis (Fig. 3(b)). This is similar to the columnar grain structur
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