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 Magdot workshop, Napa, California, September 2008 – p. 2/19

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 Magdot workshop, Napa, California, September 2008 – p. 3/19

Fe/Mo(110) T Monte//Carlo simulations 600 400 300 200 -2 10 200 200 200 200 300 300 comparison with MBE 400 330 350 450 -3 10 interactions calculated by 300 300 400 500 450 550 600 PLD, E b = 0.60 eV 650 N x 500 Mark Asta group PLD, E b = 0.40 eV 400 700 MBE, E b = 0.60 eV -4 10 550 MBE, E b = 0.40 eV 400 500 used only 1st NN 750 550 E d = 0.4 eV 600 interaction 600 -5 10 500 1 2 3 4 5 100/ T [1/K] anomalous behaviour in PLD at intermediate motivation – explanation temperatures of measurement of PLD Fe/Mo(110) a a P .-O. Jubert, O. Fruchart and C. Meyer, Surface Science 522 (2003) 8 Magdot workshop, Napa, California, September 2008 – p. 4/19

MBE vs PLD θ F PLD PLD MBE MBE t t continuous flux vs. pulsed flux steady state vs. far from equilibrium state slightly changing concentration of monomers vs. fast changes in concentration of monomers Magdot workshop, Napa, California, September 2008 – p. 5/19

MBE – Molecular Beam Epitaxy continuous flux lower concentration of monomers constant density of monomers important parameters diffusion coefficient D flux F Magdot workshop, Napa, California, September 2008 – p. 6/19

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 Magdot workshop, Napa, California, September 2008 – p. 7/19

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 N x Magdot workshop, Napa, California, September 2008 – p. 8/19

Diffusion coefficient of clusters -4 10 -3 NN = 1 10 -5 10 NN = 3 NN = 3 NN = 6 -6 10 ← 1000 K D/D 0 D/D 0 -7 -4 10 10 500 K → -8 10 -9 10 -5 10 0 10 20 30 40 0 5 10 15 Cluster size cluster size 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 Magdot workshop, Napa, California, September 2008 – p. 9/19

Interactions six neighbour positions included 0 3 in interaction 1 hamiltonian dominant attractive 5 2 interactions, 4 repulsive also present 6 present simulations - only 1st NN in- cluded Magdot workshop, Napa, California, September 2008 – p. 10/19

Concentration of monomers and dimers -2 1 10 0,00020 1 1 PLD, E b = 0.60 eV monomer, E b = 0.60 eV PLD, E b = 0.40 eV monomer, E b = 0.40 eV 0,00015 MBE, E b = 0.60 eV monomer, MBE, E b = 0.60 eV -3 10 MBE, E b = 0.40 eV dimer, E b = 0.60 eV 2 2 2 dimer, E b = 0,40 eV N x N x 0,00010 dimer, MBE, E b = 0.60 eV 3 3 -4 10 0,00005 1 1 2 2 3 3 0,00000 200 300 400 500 600 700 200 300 400 500 600 700 T [K] T [K] 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 Magdot workshop, Napa, California, September 2008 – p. 11/19

Activation energies 0,8 → E B = − k B T ln D C D 0 0,7 dimers, E b = 0.60 eV activation energy dimers, E b = 0.32 eV monomers 0,6 depends on interaction E d [eV] between adatoms 0,5 0,4 activation energy is given by interaction energy and 0,3 400 500 600 700 800 900 1000 T [K] binding to substrate solid line – simulated do the activation data energies of dimmers depend on temperature? fitted effective activation EB – we are not sure yet energies D C = D 0 e − kBT Magdot workshop, Napa, California, September 2008 – p. 12/19

Diffusion coefficients T 600 400 300 200 -2 1 10 8 10 1 1 PLD, E b = 0.60 eV PLD, E b = 0.40 eV 4 10 MBE, E b = 0.60 eV -3 10 1 MBE, E b = 0.40 eV 1 1 2 2 2 D eff N x 3 0 3 10 monomers dimers, E b = 0.32 eV -4 dimers, E b = 0.6 eV 10 -4 10 2 3 4 5 200 300 400 700 500 600 100/ T [1/K] T [K] 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 Magdot workshop, Napa, California, September 2008 – p. 13/19

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 Magdot workshop, Napa, California, September 2008 – p. 14/19

Future plans detail inspection of stability islands influence of trimmers and larger clusters simulations to model including first three interactions Magdot workshop, Napa, California, September 2008 – p. 15/19

Decay of dimers 1e+08 1e+07 1e+06 t {MCS] 1e+05 10000 1000 400 500 600 700 800 900 1000 T {K} decay of dimers Magdot workshop, Napa, California, September 2008 – p. 16/19

Decay of dimers 0,01 0,0001 t [s] 1e-06 1e-08 200 300 400 600 700 500 T [K] fitted decay of dimers Magdot workshop, Napa, California, September 2008 – p. 17/19

Mean free paths mean free paths of monomers and dimers life time of dimers ... Magdot workshop, Napa, California, September 2008 – p. 18/19

Role of dimers Role of dimers in anomalous behaviour Magdot workshop, Napa, California, September 2008 – p. 19/19