LATTICE POSITION OF SI IN GAAS DETERMINED BY X-RAY STANDING-WAVE MEASUREMENTS

The x-ray standing wave (XSW) technique was applied to determine the l attice location of Si impurity atoms in GaAs(100) crystals. The synchr otron radiation of X24A at the national synchrotron light source was u tilized to set up backreflection XSW, an experimental geometry which d rastically relaxes the otherwise stringent requirement on the lattice perfection. Specifically, the lattice sites were determined with respe ct to the [311] reflection planes which differentiate a Ga site from a n As site. With the aid of an appropriate choice of the x-ray fluoresc ence filter, we were able to study GaAs(100) samples with very low lev els of Si impurities. On a sample doped with 4 X 10(18) cm-3 Si during the molecular-beam epitaxy growth, we found that the Si atoms predomi nantly occupied the Ga sites. On both an ion-implanted sample after an nealing and a sample with Si impurities introduced by thermal diffusio n, about 30% of the Si atoms occupied the Ga sites, and the rest occup ied random sites. The As site occupation was less than 6%. Suggestions are made for further experiments with improved sensitivity.