THE EFFECT OF TEMPERATURE ON NH3-SI(111) INTERACTION

Clean Si(111) 7 x 7 surfaces have been exposed to ammonia once heated to a constant temperature until saturation of the nitrogen uptake. The temperatures ranged from 25 degrees C to 500 degrees C, upon which sa turation coverage theta increased from 0.2 to 1 ML. The N-saturated su rface has been studied by low-energy electron diffraction, Auger elect ron spectroscopy and photoemission yield spectroscopy as a function of the reaction temperature. The interaction appears as a three-step pro cess: from 25 degrees to 200 degrees C, ammonia dissociates into H and NH2 which occupy at random the Si dangling bonds, keeping the 7 x 7 o rder and theta = 0.2 ML; then up to 320 degrees C, the only change is a re-ordering of the same adsorbed species driven by electronic and ge ometric smoothing of the surface; the third step occurs around 350 deg rees C and corresponds to an increased N uptake to theta = 1 ML and a growing surface disorder in correlation with the continuous removal of dangling bonds resulting from hydrogen loss. (C) 1998 Elsevier Scienc e B.V. All rights reserved.