Study of diffusion and stability of clusters in submonolayer growth - - PowerPoint PPT Presentation

Study of diffusion and stability of clusters in submonolayer growth of Fe on Mo

Martin Maˇ s´ ın and Miroslav Kotrla Institute of Physics, Academy of Science of the Czech Republic Department of Thin Films

Magdot workshop, Napa, California, September 2008 – p. 1/19

Outline

Introduction MBE and PLD Diffusion of monomers and dimers Explanation of anomalous behaviour Conclusions

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Introduction

studied processes during deposition comparison of MBE and PLD study focused to deep understanding of processes during PLD diffusion of clusters, namely dimers stability of dimers

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Fe/Mo(110)

200 300 400 450 500 550 600 650 700 750 200 300 330 350 400 450 500 550 600 200 300 400 500 550 600 200 300 400 500

1 2 3 4 5 100/T [1/K] 10

• 5

10

• 4

10

• 3

10

• 2

Nx

PLD, Eb = 0.60 eV PLD, Eb = 0.40 eV MBE, Eb = 0.60 eV MBE, Eb = 0.40 eV

200 300 400 600 T

Ed = 0.4 eV

motivation – explanation

• f measurement of PLD

Fe/Mo(110)a Monte//Carlo simulations comparison with MBE interactions calculated by Mark Asta group used only 1st NN interaction anomalous behaviour in PLD at intermediate temperatures

aP

.-O. Jubert, O. Fruchart and C. Meyer, Surface Science 522 (2003) 8

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MBE vs PLD

PLD t MBE F t θ MBE PLD

continuous flux vs. pulsed flux steady state vs. far from equilibrium state slightly changing concentration of monomers vs. fast changes in concentration of monomers

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MBE – Molecular Beam Epitaxy

continuous flux lower concentration of monomers constant density of monomers important parameters diffusion coefficient D flux F

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PLD – Pulsed Laser Deposition

deposition in pulses period without deposition, only diffusion higher concentration of monomers after puls fluctuating concentration of monomers important parameters diffusion coefficient D flux F chopping frequency f length of pulse d

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MBE vs PLD

• ← MBE

PLD →

• at low temperatures – the same behaviour (like PLD)

at intermediate and high temperatures – MBE always gives lower density of islands Nx

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Diffusion coefficient of clusters

10 20 30 40 Cluster size 10

• 5

10

• 4

10

• 3

D/D0 NN = 1 NN = 3 NN = 6

← 1000 K 500 K →

5 10 15 cluster size 10

• 9

10

• 8

10

• 7

10

• 6

10

• 5

10

• 4

D/D0

NN = 3

normalised diffusion coefficients – lattice parameter and vibrational frequency set to unity diffusion coefficients calculated for different sets of neighbouring positions low diffusion coefficient for compact clusters

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Interactions

6 3 2 4 5 1

six neighbour positions included in interaction hamiltonian dominant attractive interactions, repulsive also present present simulations

• only 1st NN in-

cluded

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Concentration of monomers and dimers

200 300 400 500 600 700 T [K] 0,00000 0,00005 0,00010 0,00015 0,00020 Nx

monomer, Eb = 0.60 eV monomer, Eb = 0.40 eV monomer, MBE, Eb = 0.60 eV dimer, Eb = 0.60 eV dimer, Eb = 0,40 eV dimer, MBE, Eb = 0.60 eV 1 2 3 1 2 3

200 300 400 500 600 700 T [K] 10

• 4

10

• 3

10

• 2

Nx

PLD, Eb = 0.60 eV PLD, Eb = 0.40 eV MBE, Eb = 0.60 eV MBE, Eb = 0.40 eV 1 2 3 1 2 3 1 2

concentration of monomers and dimers in the case of PLD – before next puls coming decay of monomers at T = 300 K decay of dimers for MBE at T = 300 K slow decay of dimers for PLD, depending on interaction

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Activation energies

400 500 600 700 800 900 1000 T [K] 0,3 0,4 0,5 0,6 0,7 0,8 Ed [eV]

dimers, Eb = 0.60 eV dimers, Eb = 0.32 eV monomers

solid line – simulated data fitted effective activation energies DC = D0e−

EB kBT

→ EB = −kBT ln DC

D0

activation energy depends on interaction between adatoms activation energy is given by interaction energy and binding to substrate do the activation energies of dimmers depend on temperature? – we are not sure yet

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Diffusion coefficients

2 3 4 5 100/T [1/K] 10

• 4

10 10

4

10

8

Deff

monomers dimers, Eb = 0.32 eV dimers, Eb = 0.6 eV

200 300 400 600 T 1 1 1

200 300 400 500 600 700 T [K] 10

• 4

10

• 3

10

• 2

Nx

PLD, Eb = 0.60 eV PLD, Eb = 0.40 eV MBE, Eb = 0.60 eV MBE, Eb = 0.40 eV 1 2 3 1 2 3 1 2

effective diffusion coefficients of monomers and dimers they are obtained using fitted activation energies diffusion coefficients for dimmers in case of MBE are similar to diffusion coefficients for dimers in case of PLD point 2 corresponds to decay of monomers in case of MBE and dimers in case of PLD

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Conclusions

dimers play key role in PLD island density in PLD depends on interaction energy between adatoms key role plays also stability of dimers and larger clusters respectively anomalous behaviour of PLD is real, generic phenomenum diffusion of small clusters is strongly dependent on size diffusion coefficients of islands which are able to create compact shape (i.e. 4 × 4) are significantly smaller then

• dif. coef. of other clusters

diffusion coefficients using first 3 and firs 6 interaction parameters are almost same → we can use only first 3 parameters in simulations

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Future plans

detail inspection of stability islands influence of trimmers and larger clusters simulations to model including first three interactions

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Decay of dimers

400 500 600 700 800 900 1000 T {K} 1000 10000 1e+05 1e+06 1e+07 1e+08 t {MCS]

decay of dimers

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Decay of dimers

200 300 400 500 600 700 T [K] 1e-08 1e-06 0,0001 0,01 t [s]

fitted decay of dimers

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Mean free paths

mean free paths of monomers and dimers life time of dimers ...

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Role of dimers

Role of dimers in anomalous behaviour

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