Oxidation of lead by low energy O2+ bombardment

Abstract

Evaporated Sn layers on Si and sapphire substrates were used to demonstrate the applicability of the backscattering teehnique to measurement of the fraction of covered area, the average thickness of the layer, and the sticking factor for temperatures over the range oK. The sticking factor at 293"K was found to be ! of that at 77 and 393°K, and 19% higher at roorn temperature (r. t.) for Sn on Sn as compared with Sn on Si. At 393"K the fraction of covered area was found to be about 60% and the average island thickness was in the range 450-1000 Ä.Backscattering of 4He+ ions in the MeV energy range was previously used to study diffusion processes in thin metal films. 1 In the course of Gur study of diffustön processes at the Cu-CdTe mtertace" we were able to demonstrate that this technique is suitable for study of the intermediate stages of growth of thin films. The intermediate stages" follow nucleation and precede the formation of a continuous film. During the coalescence stage we can measure the fraction of covered area, the average thickness of the layer, and the sticking factor as determined by the substrate material and the substrate temperature. We reporl he re results which demonstrate this application. ENERGY (CHANNEL)proportional to the fraction of uncovered area. Also, inhomogeneity in film thickness will change the shape of the backscattered spectrum. Both effects are illustrated in Fig. 1 in terms of spectra obtained for l-MeV~e+ ions scattered from a discontinuous Cu layer on top of a continuous Cu layer evaporated on Si at r. t, The yield at higher values of energy is the same as that for a thick Cu layer, whereas the. discontinuous film gives a reduced height toward lower-energy values , In addition, a second spectrum is shown for a single discontinuous film on silicon. The wires of the screen do not provide a sharp mask,and a shadow effect is visible which corresponds to a higher fraction of covered area at smaller layer thickness. The observed dependence of the fraction of covered area on the layer thickness corresponds to the scattering distribution of neutral atoms from the wire, for which analytical expressions could be derived. Thus, Fig. 1 I .:°+ We evaporated equal quantities of Sn onto Si at substrate temperatures of 77, 293, and 393°K. The results are shown in Fig. 2 in terms of the backscattered spectra. There is a significant reduction in the yield for layers evaporated at the higher temperature, and an increase in thickness obtained at this same temperature. Further, the total number of backscattered particles is substantially lower at room temperature, which shows that the sticking factor is greatly reduced at this temperature. By peak-area integration we find that the sticking factor is the same, to within 6%, at 77 and 393 "K, but at 293°K it is 1-of that at 77°K.The growth of Sn films was studied in terms of the quantity of Sn evaporated. As an example, we show in Fig. 3 the film growth process at substrate temperatures of (a) 293 and (b) 393°K as the area and thickness in...