Theory of the Sulphur-Passivated InP(001) Surface
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have two S sublayers separated by 0.22 A; the top S sublayer contains monomer pairs sitting close to bridge sites, while the lower sublayer contains strongly-dimerized S pairs. Thus the S atoms exist in two types of chemical environments. This structure is quite different from the currently-accepted description of the more extensively studied GaAs-S surface [8], which our calculations confirm. Core-level (CL) spectroscopy can distinguish an atom in different bonding situations since the core-level positions of atomic lines shift as a function of the chemical environment [9]. The method is a powerful tool for probing surface structure; this however requires theoretical predictions of the CL excitation energies (CL-E), in particular when different structures "compete" because, e.g., of annealing, surface preparation, etc., which may lead to structures other than the ground state. We have calculated the CL-E from first-principles in order to follow the evolution of the InP(001)-S surface upon annealing. The theory predicts a number of stable structures besides the ground which become energetically accessible upon annealing. In particular, surface S atoms exchange with bulk P atoms, forming new strong S-P bonds while dissociating pre-existing S-S dimers. Thus, we are lead to a picture wherein the InP(001)-S surface is actually a system which could contain a mixture of S, S and P, or P-terminated InP(001) domains, depending on the kinetics imposed by the annealing conditions. Our results are confirmed by calculations of the photoemission (PE) and inverse photoemission (IPE) spectra. We give full details of these surface configurations below, but first present the theoretical framework underlying the present calculations.
COMPUTATIONAL FRAMEWORK Total-energy minimization The total-energy minimization calculations were performed within the framework of density-functional theory (DFT) in the local-density approximation (LDA), using plane waves (PW) to expand the electron wavefunctions, together with non-local, norm-conserving pseudopotentials (PP) [10]. The electron exchange-correlation energy is taken to be of the Ceperlev-Alder form [11]. Only the F point was used to sample the reciprocal space and a 10 Ry energy cutoff was employed in the plane-wave expansions. The validity of our pseudopotentials for In, P and S was verified on various molecular configurations [12, 13, 14]. The semi-infinite crystal with with the InP(001) surface exposed to vacuum is modeled, for the total-energy minimization calculations, as a supercell slab containing six layers, each with four atoms. The first (topmost) layer contains four S (or P) atoms. The second contains four In atoms, as implied by the chemistry of the material and by recent experimental studies [4, 5]. Subsequent layers follow the zinc-blende structure, except the bottom one, which consists of four In atoms and two H atoms positioned in a (I x2) pattern such as to saturate the dangling bonds [13]. The atoms in the bottom three layers are held fixed in their bulk positions, w
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