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SIMULATION MODEL The effects of R-modulation on growth were simulated using a solid-on-solid kinetic Monte Carlo simulation model without vacancies and overhangs, based on Refs. [6,7]. Surface migration is modelled by introducing an isotropic nearest neighbour hopping rate n(Ts)=voexp((nEn + mEnn - lEedge) / (kBTs)) where vo = temperature-independent vibration frequency; kB = Boltzmann's constant; En = nearest neighbour binding energy; Enn = next nearest neighbour binding energy and Eedee = edge site related reduction in binding energy. The (11) surface of the diamond structure implies that there is one nearest neighbor bond to the substrate for adatoms in odd layers (n = 1) and not more than three nearest neighbor bonds to the substrate for adatoms in even layers (n < 3). The modelling of the diamond structure has been simplified in such a sense that the atomic planes are stacked in an AevenAoddAevenAodd... sequence. m equals the number of neighbours in the plane, which is not greater than 6 (m _ 6) for the (111) surface and 1is the number of empty surface lattice sites adjacent to an edge site, which are situated in a layer below the edge site (1 < 6).

We have calculated the evolution of the reversed step density (RSD), since its features correlate very well with experimental reflection high-energy electron diffraction (RHEED) oscillations for certain diffraction conditions [8,9]. The following definition and normalization was used: RSD = 1 - (no. of steps)/(no. of scanned surface lattice sites). Periodic boundary conditions were employed at the edges of a 300x 150 lattice sites surface matrix. The initial surface had one BL step at the centre parallel to the short side of the matrix and one BL step at the short sides. The following parameters were used in the simulations: E, = 1.5 eV; En = 0.2 eV; Eedge = 0.02 eV; vo = 5.1012 Hz and T, = 500'C. EXPERIMENTAL AND SIMULATION RESULTS In order to define the R-modulation related parameters we refer to Fig. 1, where the deposited thickness, s, versus time, t, normalized with the modulation period, T,, is shown. As indicated with the full drawn curve in Fig. 1, two growth rates are employed: the high growth rate R1 is used during td•- t < td+ tj and the low growth rats R 2 is used during td+ tj < t < td + t] +t2 where tj + t 2 = Tm. The delay time,td, can be chosen arbitrarily between 0 and rm and R 1 and R 2 can be expressed as R1 = 0 am / tj and R 2 = (1 - 0) am / t2 , respectively, where 0 is the fraction of am (the total thickness deposited during one modulation cycle) deposited during t1 . The deposition period, To, at constant low growth rate is defined as TO= aBL/R2. In order to relate am to aBL, we defined a•-= am / aBL. A qualitative estimation of the modulation strength is thus given by the ratio .R1 /R 2 . ".....

. 2 . . .. .. ....

.

. . .

2, ::(.

Fig. 1. Deposited thickness, s, vs. time, t, normalized with the R-modulation

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period, rm. The heavy line shows Rmodulation with the an initial delay tdlrm, before applying high growth rate,

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