M. Djafari Rouhani

BEYOND THE SOLID ON SOLID MODEL : AN ATOMIC DISLOCATION FORMATION MECHANISM

We investigate the growth of mismatched thin films by a kinetic Monte Carlo computer simulation. The strain is introduced through an elastic energy term based on a valence force field approximation and stress is relaxed along “atomic chains” at each step of the simulation. The calculations use a set of elementary atomic processes including, besides well-known standard processes, the collective incorporation of atoms. This leads us to introduce a new “hanging” position with only one bond created toward the substrate contrary to solid on solid models. This position plays a role of defects initiation, and thus an atomic dislocation nucleation mechanism is described. Finally, we present the influence of a step in the dislocations creation.

Localized vibrational states in amorphous silicon

Using a valence-force-field model as interatomic potential, vibrational eigenstates have been computed in the harmonic approximation. The density of vibrational states and their inverse participation ratio are compared for crystalline silicon, fully-coordinated amorphous silicon (a-Si) and a-Si with voids. Voids of various sizes and concentrations have been introduced into an a-Si structure that was generated with a vacancy model. The presence of voids increases the local strain in the a-Si network and causes substantial changes in the vibrational density of states. Localization occurs not only for high frequency modes but also for band edge states. At low frequencies, the deviation from a Debye density of states is due to such localized extra modes depending on void size and concentration.

Simulation of GaAs/CdTe heteroepitaxial growth

Abstract Atomic scale simulation of the epitaxial growth of two monolayers of CdTe on (100) GaAs substrate has been carried out by associating an elastic model to the Monte Carlo technique. The lattice mismatch is 14.7%. Mechanisms of misfit dislocation formation and the (100) or (111) orientation of the deposited layer have been examined. Extrapolation of results allow the evaluation of critical thickness.

Atomic scale simulation of crystal growth applied to the calculation of the photoemission current

Abstract A kinetic model and atomic scale simulations have been used to study the variations of the photoemission current during the in situ growth of compound semiconductors, and GaAs in particular. The effect of the atomic and molecular nature of the incoming species and the influence of the flux ratio of the two components are investigated. Results are compared with RHEED intensity calculations. Both models lead to results in agreement with experimental data on GaAs, provided the As incorporation rate is high.

Simulation of thin film growth and in situ characterization by RHEED and photoemission

Simulations of epitaxial growth of GaAs associated with RHEED intensity and photoemission current, as in situ characterization techniques, have been performed. The nature of incoming species which can be of atomic or molecular form is taken into account. The simulations show in phase RHEED and photoemission oscillations, in agreement with experimental results.

Atomic-scale simulation of interface defect formation in the initial stages of thin film growth

Abstract We have performed an atomic-scale simulation of lattice-mismatched heteroepitaxial growth of semiconductors with a zincblende structure by associating the Monte Carlo technique with an energy model based on the valence force field approximation. We have observed the formation of misfit dislocations by alignment of point defects. We have evaluated the critical thickness for their creation which are in good agreement with experimental results for high lattice mismatches (greater than 6%), while previous macroscopic models give good results for low lattice mismatches (less than 4%). The deposition of alloy films is investigated to study the variations in the density and the nature of interface defects, leading to misfit dislocations.