Epitaxial growth of metals on Si and Ge investigated by RHEED-TRAXS and UHV-SEM

Abstract

In the study of epitaxy, it is very important to investigate the surface composition during the growth process. In RHEED experiments, strong X-rays are excited from the sample surface hit by the electron beam. The intensity of these adsorbate characteristic X-rays changes drastically with change of the take-off angle (θ t) and it becomes maximum at a critical angle (θ c) corresponding to total reflection of the X-rays. Applying this phenomenon, surface element analysis is possible with high sensitivity. This method is called RHEED-TRAXS (total reflection angle X-ray spectroscopy). The sensitivity of this surface element analysis is generally comparable to that for AES but for heavy atoms it is higher. When the deposited metal atoms are confined to the surface, the glancing angle (θ g) dependence of the emitted X-rays is expressed by I/sin θ g. Thus, by measuring the θ g dependences, the method gives also good information on the in-depth composition. Applying the RHEED-TRAXS method we have studied in situ the adsorption, desorption, and epitaxial growth processes of Au, Ag and Au-Ag alloys on clean Si(111) surfaces. We have developed a new UHV-SEM which has a high resolution of about 5 Å. It was found that by using the UHV-SEM we could observe directly domain contrasts from two-dimensional surface structures such as 7 × 7, 5 × 2-Au, and √3 × √3-Ag which are formed by Ag and Au deposition on Si(111). These contrasts correspond to two-dimensional element mapping at the surfaces.