EPITAXIAL-GROWTH OF KBR ONTO NACL(001) BY HIGH-RESOLUTION HE ATOM SCATTERING

The vapor deposition and subsequent growth of KBr onto the cleaved (001) surface of single crystal NaCl were studied by measuring the scattering behavior of a nearly monoenergetic (DELTA-E/E congruent-to 2%) He atom beam. Oscillations in the intensity of the specular beam due to layer-by-layer growth were observed for the first several layers. The structure of the surface for each deposited layer was determined by He diffraction via angular distribution measurements. The surface lattice spacings for NaCl and KBr have a mismatch of about 17% (3.99 angstrom for NaCl and 4.66 angstrom for KBr) and, hence, 6 KBr's can fit very nearly onto 7 NaCl's (approximately 28 angstrom). The measured diffraction patterns showed peaks corresponding to a surface corrugation with this length. For the first adlayer, peaks only for NaCl and this "superstructure" were observed. As additional layers were deposited, the NaCl peaks disappeared and the superstructure peaks at first increased and then diminished as the Bragg peaks corresponding to the KBr lattice spacing grew in. By 6 monolayers only the diffraction peaks due to the KBr were observed. Additionally, time-of-flight energy analysis of the inelastically scattered He atoms was employed to determine the surface phonon dispersion curves of the adlayer. After only 4 monolayers, the Rayleigh and crossing modes were found to be almost identical to those measured from a cleaved KBr(001) surface with the exception of a non-zero value for the Rayleigh mode at the zone center. However, possible evidence for strain was seen in the S2 mode, a surface optical mode lying in the gap between the acoustic and optical bulk bands. Similar behavior in the measured diffraction has been found in current complementary experiments for NaCl deposited onto KBr(001).