LOW-ENERGY ELECTRON-DIFFRACTION STUDIES OF ATOMIC STEP TRANSITIONS ON A NI(111) VICINAL SURFACE

A low-energy electron diffraction system with a two-dimensional position sensitive detector was used to investigate the step clustering phase transition on a Ni surface vicinal to (111), i.e., Ni (111) 5-degrees [112BAR]. The directions parallel and perpendicular to the step edges were [112BAR] and [110BAR], respectively, and the surface was miscut by 5-degrees about the [112BAR] axis. Split beams at high temperature, indicative of a single atomic height step structure, coalesced to single beams as the temperature was lowered. The transition occurred around 560 K, and was reversible with some hysteresis. Scans along the [111] truncation rod were performed by changing the incident beam energy. Beam profile analysis was done to quantitatively analyze the change of step and terrace distributions on the surface. Kinematic calculations of intensity distributions were made for a two phase model, in which the surface was taken to consist of different areas, one type with large terraces and the other with small terraces, and compared with the experimental data. Two gamma functions were used for the terrace distribution and one exponential function for the step distribution. Fitting of experimental results to the calculation showed that step clustering and (111) faceting occurred as temperature decreased. The measured hysteresis in the heating and cooling cycles might be due to nucleation in a first order phase transition involving coexistence of regions with single atomic height steps and regions with double atomic height steps. The present result is consistent with our previous reported observation on a Ni vicinal surface tilted by 5-degrees about the [110BAR] axis, i.e., Ni (111) 5-degrees [110BAR], using glancing incidence x-ray diffraction.