Structure and transformation characteristics of impurity stabilized phases on the Si(111) surface

Abstract

A reversible, second order surface phase transformation between the Si(111)−7 structure and a stable Si(111)−1 structure takes place in the temperature range of 865–890°C. Owing to the large difference in LEED intensity of first order diffracted beams from the two phases, the intensity is a very sensitive and rapidly responding measure of the surface state. A study of intensity, at constant temperature in the transformation range, disclosed fluctuations having a maximum amplitude in the middle of the transformation range. The fluctuations are invariably in the direction of increasing intensity, though never exceeding the intensity characteristic of the high temperature state. The time duration of the fluctuations is of the order of seconds and the fluctuations can be ascribed to impurities of similar life-time on the surface. These impurities are generated at microscopic points of high contact-resistance between the crystal and the tantalum current leads. The effect of a particular impurity was demonstrated by the deliberate addition of chlorine to the surface. The fluctuations, which are observed only in the transformation range, appear to possess characteristics of phenomena associated with the critical point of a second-order phase transformation and, as such, require further study. The high temperature 1 × 1 phase can be retained at room temperature by rapid cooling of the high temperature phase with adsorbed chlorine. A chlorine concentration of ≈ 4–5 % monolayer is necessary to maintain the high temperature phase in a metastable state at room temperature. The small chlorine concentration associated with the Si(111)1−(≈ 4–5 % Cl) structure at room temperature presumably does not contribute significantly to the observed LEED intensities and thereby permits a detailed study of intensity-voltage profiles of the stable 1× 1 structure. The surface structure of the quenched phase, Si(111)−1 (≈ 4−5% Cl), has been determined from the LEED intensity data interpreted with a single-scattering kinematic model. The resultant structure is an unreconstructed silicon surface with a slight contraction of the bond distance (≈4%) between a surface silicon atom and its next nearest neighbors. The similarity between the LEED intensity data for the high temperature 1×1 phase and the quenched room temperature structure suggests that at elevated temperatures, ≈ 900 °C and above, the unreconstructed surface is the stable equilibrium configuration. The mechanism by which chlorine impurities stabilize this structure down to room temperature is unknown, but their presence is necessary. The possibility that other impurities can stabilize this phase at room temperature is considered likely.