Structural Evolution During the Initial Epitaxial Growth of Moon on Sapphire
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Mat. Res. Soc. Symp. Proc. Vol. 584 © 2000 Materials Research Society
thickness calibrated ex-situ using x-ray reflectivity, Rutherford backscattering spectroscopy and profilometer measurements. The evolution of the Mo surface was characterized in real-time using RHEED and after deposition by room temperature in-situ STM. RESULTS AND DISCUSSIONS i) Growth of Mo (11)
on A120 3 (0001)
The initial growth of Mo on sapphire (0001) is 3D. This is indicated by a monotonic drop of RHEED intensity, including the specular reflection, at the onset of Mo deposition and through the first few A [6], and is confirmed by the subsequent appearance of spotty and broad RHEED patterns. The 3D RHEED patterns along three different azimuths are illustrated in Figure 1, and they also confirm that the Mo film has a three-fold symmetry and is grown along the [111 ] direction. Specifically, the two RHEED patterns shown in Figure 1 (b) and (c) correspond to the respective azimuths that are rotated + 300 with respective to the [112] azimuth shown Figure 1 (a). The mirror images shown by the two patterns with respect to each other indicate that there is no crystallographic twinning.
Figure 1. RHEED images of Mo (111) along the [112] (a) and (b) and (c) azimuths. Images (b) and (c) are mirror images, indicating the crystal is not twinned. The bright dot with concentric circular rings near the top of each image is the direct beam and the associated artifact of the phosphorous RHEED screen. The intensity of the RHEED patterns, and the full width at half maximum (FWHM) and the separation of the diffraction peaks together give a quantitative measure of the surface structure for the growth of Mo (111) on A12 0 3 (0001). Their evolution as a function of film thickness is illustrated in Figure 2 for a growth at 800 °C with rate - 0.2 A/s. The RHEED patterns including the specular reflection appear at - 3 A coverage from the diffused background. Their intensities increase monotonically with deposition indicating a continuous enhancement of the ordering, while their FWHM show a slight sharpening reflecting a rather constant crystal coherence length. The observed FWHM corresponds to surface structures that are several nanometers wide, and the observed narrowing indicates that these surface features grow in size with increasing coverage. The RHEED intensity increases more rapidly for higher temperature growth. This is consistent with the notion that higher growth temperature promotes higher structural ordering. The observed evolution of lattice spacing indicates that lattice relaxation occurs mainly within the first 2 -3 A before the onset of RHEED intensity, and it is fully relaxed before reaching 10 A. The observed rapid relaxation is consistent with the presence of a large lattice mismatch and 3D growth. In the full temperature range of the study, the growth is 3D. Furthermore, surface becomes faceted for growth at temperatures greater than 1000 'C or at high coverages. This observation indicates that (111) face of Mo is not energetically favored.
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