Alla G. Nastovjak

Possibilities of Monte Carlo simulation for examination of nanowhisker growth

The kinetic Monte Carlo (MC) model of nanowhisker (NW) growth is suggested. Two variants of growth are possible in the model—molecular beam epitaxy (MBE) and chemical vapor deposition (CVD). The effect of deposition conditions and growth regimes on the whisker morphology was examined within the framework of the vapor–liquid–solid (VLS) mechanism. A range of model growth conditions corresponding to NW and nanotube formation was determined. The suggested MC model was used for analyses of the morphology of the catalyst–whisker interface and for examination of Si–Ge whisker growth.

Peculiarities of axial and radial Ge–Si heterojunction formation in nanowires: Monte Carlo simulation

The process of axial and radial Si–Ge heterostructure formation during nanowire growth by vapor–liquid–solid (VLS) mechanism was studied using Monte Carlo (MC) simulation. It was demonstrated that radial growth can be stimulated by adding chemical species that decrease the activation energy of precursor dissociation or the solubility of semiconductor material in catalyst drop. Reducing the Si adatom diffusion length also leads to Si shell formation around the Ge core. The influence of growth conditions on the composition and abruptness of axial Ge–Si heterostructures was analyzed. The composition of the GexSi1–x axial heterojunction (HJ) was found to be dependent on the flux ratio, the duration of Si and Ge deposition, and the catalyst drop diameter. Maximal Ge concentration in the HJ is dependent on Ge deposition time owing to gradual changing of catalyst drop composition after switching Ge and Si fluxes. The dependence of junction abruptness on the nanowire diameter was revealed: in the adsorption-induced growth mode, the abruptness decreased with diameter, and in the diffusion-induced mode it increased. This implies that abrupt Ge–Si HJ in nanowires with small diameter can be obtained only in the chemical vapor deposition (CVD) process with negligible diffusion component of growth.

Effect of substrate-drop parameters on nanowhiskers morphology

Si nanowhiskers (NW) morphology was studied by Monte Carlo simulation. It was found out that different substrate-seed wetting conditions can lead to growth of straight, curved or tube-like NWs. Initial substrate-seed interface orientation and orientation of growing NW sidewall facets determine growth directions of model NWs.

Monte Carlo model of GaAs nanowhisker growth

GaAs nanowhisker growth according to vapor-liquid-solid mechanism was realized on the basis of the lattice Monte Carlo model. Different solubility of Ga and As atoms in Au drop was taken into account in the model. Nanowhisker growth on GaAs(111)B surface activated by golden drops was demonstrated.

Peculiarities of vapor-liquid-solid process realization in Monte Carlo model of nanowhisker growth

Vapor-liquid-solid nanowisker growth was considered using the Monte Carlo simulation. Dependences of catalyst drop composition on temperature, flux intensity and nanowhisker diameter were obtained. Periodical oscillation of drop composition near mean nonequilibrium value of whisker material concentration was observed. The mean value of this concentration was found to be practically independent of flux intensity and defined by growth temperature. Oscillation period rises with flux decreasing and diameter increasing. Oscillation results from layer-by-layer growth at the drop-whisker interface and necessity of supersaturation creation for growth to start.

Monte Carlo Simulation of Silicon Nanowhiskers Growth

Examination of silicon nanowhiskers (NWs) growth on Si(111) surface activated by gold was carried out using Monte Carlo simulation. Dependence of NW length on gold drop size was obtained. It was shown that for given temperature and deposition rate there is optimal drop size corresponding to maximal whisker growth rate. Effect of surface wetting by drop material was investigated: for strong wettability whiskers grew curved and for weak - drop became too movable and could slide down from the whisker top. It was demonstrated that combination of two mechanisms of Si incorporation at Si-Au interface: diffusion through the drop bulk with following Si precipitation at interface and Si incorporation into drop perimeter due surface diffusion is the most optimal for vertical Si nanowhiskers growth.

Influence of A III B v substrate morphology on congruent temperature under Langmuir evaporation conditions

Simulation of InAs substrate annealing was carried out using a kinetic lattice Monte Carlo model. Congruent temperature of Langmuir evaporation was estimated for different substrate orientations. Dependence of congruent temperature on substrate morphology was analyzed. Congruent temperature value was demonstrated to be dependent on substrate orientation, step density of vicinal surfaces and surface defect concentration.

Monte-Carlo modeling of Langmuir evaporation of GaAs (111) substrates

A kinetic Monte-Carlo model of the Langmuir evaporation of GaAs (111) surfaces was realized. Transition from congruent evaporation to incongruent one was observed at congruent evaporation temperature Tc. The reason of such transition is liquid Ga droplet formation at temperatures above Tc. The value of model congruent evaporation temperature is in accordance with experiment.

Abruptness of axial Si-Ge heterojunctions in nanowires

The process of Si-Ge heterostructures formation in nanowhiskers (NWs) grown by the vapor-liquid-solid mechanism was investigated using Monte Carlo simulation. It was demonstrated that it is impossible to grow atomically abrupt axial heterojunctions via classical vapor-liquid-solid mechanism due to gradual change of catalyst drop composition when switching the fluxes. Dependences of GexSi1–x composition in transition region on the ratio of germanium and silicon fluxes and deposition duration were investigated. The width of Si-Ge axial heterojunction was found to be dependent on NW diameter d. In adsorption-induced growth mode the width has linear dependence on d and in diffusion-induced growth mode it is either constant (when growth rate ∼1/d) or proportional to 1/d (when growth rate ∼1/d2).

Examination of nanotube growth conditions by Monte Carlo simulation

The dependence of nanowhisker morphology on growth conditions was studied by Monte Carlo simulation. Areas of growth conditions of filled and hollow nanowhiskers (nanotubes) were found. The area of inside dimension of nanotubes was shown to rise with increasing of flux.