Thermal roughening on stepped tungsten surfaces. I. The zone (011)–(112)

Abstract

The surface self-diffusion behavior of tungsten along the zone (011)–(112) was measured on the orientations corresponding to (011), (257), (123), (235), and (112) directions by the field emission current fluctuation method. These regions correspond to steps and terraces of (011) orientation, with terrace widths shrinking as one goes from (011) to (112), where the ‘‘width’’ is only one atomic spacing. On (011) no signal was seen. On (112) a single regime corresponding to two-dimensional diffusion was seen, as indicated by the t−1 dependence of the correlation function at long time. On the intermediate regions two-dimensional diffusion attributed to W atom diffusion on the terraces, and one–dimensional diffusion identified by t−1/2 behavior at long times was seen. This is attributed to kink motion along the steps. For two-dimensional diffusion E=16 kcal, D0≂10−5 cm2 s−1 with some variation in D0 depending on orientation. For one-dimensional diffusion E=13 kcal, D0≂10−11 cm2 s−1 with some variation in D0 with orientation. The temperature dependence of the mean square fluctuations is used to estimate an enthalpy of formation of single atoms on terraces of ∼7 kcal, and of the diffusing kink entities of ∼19 kcal/mol.At 875 K D, for one-dimensional diffusion decreases sharply, reaching a minimum at 925 K, and then increasing again. For T≥950 K this diffusion becomes two dimensional. The two-dimensional diffusion increases normally with T to 890 K, then decreases sharply. Above 1000 K only a single two-dimensional diffusion regime can be observed. These observations are interpreted as corresponding to increasing step disorder, which initially leads to a decrease in kink diffusion, then global disorder which makes this diffusion two dimensional. Concomitantly single atom diffusion on terraces decreases and eventually disappears. Thus disorder occurs at first gradually, and between 950 and 1000 K becomes complete. A discussion of the effect of free atom creation and annihilation, as well as, of temporary adsorption at steps is given. Some prediffusive fluctuation results from 80–300 K are also presented.