Journal of Molecular Structure (Theochem), 163 (1988) 63-18 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands zyxwvutsrqponmlkjihgfe SURFACE ,n BONDING IN THE (2 X 1) RECONSTRUCTION OF Si (ill)* A. CHATTOPADHYAY, P. V. MADHAVAN and J. L. WHI’ITEN Department of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794 (U.S.A.) C. R. FISCHER Department of Physics, Queens College of City University of New York, Flushing, NY 11367 (U.S.A.) I. P. BATRA IBM Almaden Research Center, 650 Harry Road, San Jose, CA 95120 (U.S.A.) (Received 7 April 1987) ABSTRACT The role of n bonding in the (2 x 1) reconstruction of Si (111) is examined. The dif- ference in total energy (AE) between the planar and twisted Si-Si n systems in a single n bond (S&H,), a pair of n bonds (S&H,) and an Si, chain with three n bonds embedded in an Si,, cluster show a definite trend: zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG AE decreases as the number of n bonds increases. For the largest system studied, an Si,, cluster model of the Si (111) surface, A,?Z is neg- ligibly small. The corresponding AE values for C,H, and for models of the (2 x 1) re- construction of the diamond (111) surface are larger than the values for silicon. Such differences between silicon and carbon can be understood in terms of smaller energy needed for atomic rehybridization (s2p2 + sp” transition) for silicon. The results sug- gest that the partial breaking of Si-Si n bonds does not energetically inhibit surface reconstruction. INTRODUCTION Si (111) has been one of the most studied surfaces. Of the various models proposed for the (2 X 1) reconstruction of the surface, the most important ones are the buckling model [l] , the n-bonded chain model [2], and several closely related structures. The justification for each model strikes a fine balance between minimizing elastic energy through buckling and lowering of total surface energy through interaction (n-bond formation) of surface dangling bonds. This paper describes a study of the role of A bonding in the reconstruction. Restricted HF and CI methods were employed along with an embedding scheme to delineate the effects of B bonding. The systems studied were SizH4, Si4H6 and an Silz cluster based on a previous LEED study [3]. *Dedicated to Prof. M. J. S. Dewar on the occasion of his 70th birthday. 0166-1280/88/$03.50 o 1988 Elsevier Science Publishers B.V.