Rare-Earth Silicides a Holistic Study

Transcription

1 Rare-Earth Silicides a Holistic Study Matt Probert & Chris Eames mijp1@york.ac.uk web: 1

2 Outline Motivation Surface Physics experimental and theoretical approaches Example 1 Structure of Sm on Si surface resolving STM and LEED conflicts Example 2 Ho nanowire on Ge surface structural and electronic model 2

3 Why study Rare-Earth Silicides? A potentially very useful metal-silicon contact low Schottky barrier (~0.4 ev) on n-type Si sensor applications Bulk rare-earth silicides have good lattice match to silicon Novel interface/surface structures Novel electronic properties Fundamental interest 3

4 Surface Physics Much current interest - $150 bn/year industry! Understanding essential for growth, catalysis, etc Major impact on electronic and atomic structure Surfaces may reconstruct in order to remove the effect of dangling bonds etc and hence attain lower energy state Unreconstructed Si(111) Surface 4

5 Silicon Surface Reconstruction Brommer et al PRL 68, 1355 (1992) Si (111)-7x7 Takayanagi Reconstruction 5

6 Surfaces and Overlayers Overlayer periodicity related to bulk periodicity - in this case adatoms form 2x2 overlayer 6

7 Low Energy Electron Diffraction Electrons with energies ~40-300eV diffracted from periodic surface mesh Collect elastically scattered electrons Surface sensitive Complementary to other experimental surface techniques such as STM Can be qualitative (e.g. indicating overlayer periodicity and quality) or quantitative Si (111)-7x7 70eV 7

8 Quantitative LEED Intensity of each spot in the LEED pattern depends on energy Intensity vs. Energy curves for each spot gives a unique fingerprint of the structure A difficult inverse problem to solve for the 3D structure which best fits the observations Need phase shift of each scattering event Monte Carlo simulation with many beams and look at yields, e.g. CAVLEED code Kitayama et al Surf. Sci. V , 1481, (2001) 8

9 Rare-Earth Silicide Preparation Start with Si(111) or Ge(111) and clean under UHV to give 7 7 or c(2 8) reconstruction Deposit 1 ML of rare-earth onto surface and anneal to about 550 C LEED shows formation of ordered silicide/germanide with a 1 1 reconstruction MEIS shows that trivalent rare-earth ions do not sit on top of silicon/germanium get an ordered, flat layer underneath a reverse-buckled silicon/germanium overlayer a 2D silicide/germanide 3D silicides prepared under different conditions. 9

10 LEED for Ho on Si(111) Generate clean Si(111)-7x7 Check quality with LEED 40 ev Deposit 1ML of Ho Anneal at 500 o C for ~15mins Check quality with LEED Acquire I(V) curves 150eV Try to find structure that best fits measured I(V) 10

11 Generic 2D Structure Trivalent Lanthanides e.g. Dy, Ho, Er, etc. RE in flat layer on T4 sites under 1x1 reverse-buckled overlayer Hydrogen passivation converts reverse to normal buckling Kitayama et al Surf. Sci. V , 1481, (2001) 11

12 Materials Studied York Surface Physics group has studied Heavy Rare Earth elements (Lanthanides) on Si and Ge Divalent = Sm, Eu, Yb Trivalent = La, Ce, Pr, Nd, Pm, Gd, Tb, Dy, Ho, Er, Tm Done all except La, Ce, Pr, Pm See Trends and strain in the structures of 2D rare-earth silicides studied using medium-energy ion scattering, PRB 72, (2005) Also Fe, Pb, Pd on Si etc With a variety of techniques primarily STM and LEED in York also MEIS at Daresbury and now adding ab initio electronic structure calculations as a complementary tool using CASTEP a holistic approach 12

13 CASTEP for Surface Physics Simple case: Si(100)-2x1 Small area surface so few atoms required Si atoms cheap to include in calculation Many experimental and theoretical results to compare against Unreconstructed Si(100) supercell 13

14 Si(100)-2x1 CASTEP convergence => 9KP, 360eV sufficient => 10Å Vacuum gap Typical calculation time: 15 hours on 9 nodes of a Beowulf 14

15 Si(100)-2x1 Relaxed Structure Asymmetric dimerisation 15

16 Si(100)-2x1 Calculated Properties STM profile +2.0V Electron density contours Angular Momentum channel resolved density of states 16

17 Si(100)-c4x2 Structure 40 atoms, 8 k-points, 260eV cutoff energy 16 days on 8 nodes of the White Rose Grid 17

18 Si(111)-1x1-Ho Experiment Theory Flatter top bilayer and more relaxed in theoretical result 18

19 Example 1 Geometry Optimisation of Samarium on Silicon Surface

20 STM of Si(111)-3x2-Sm STM shows 3x2 reconstruction and a 1D chain of Sm atoms Sm is divalent on Si Ab-Initio Calculation done by Palmino et al using VASP and 150eV cutoff to get STM structure 20

21 The Honeycomb Chain Channel (HCC) 1/6 ML alkali earth metals (Ca, Mg, Ba) and divalent rare earth metals (Sm, Eu, Yb) form a 3x2 reconstruction and 1D chain of metal atoms 3x2 unit cell from above 3x2 unit cell side view See PRL 81, 2296 (1998) and PRL 87, (2001)

22 The Honeycomb Chain Channel (HCC) 1/3 ML alkali metals (Li, Na, K, Rb) form a 3x1 reconstruction 1/6 ML alkali earth metals (Ca, Mg, Ba) and divalent rare earth metals (Yb, Eu, Sm) form a 3x2 reconstruction Common silicon structure responsible 3x1 unit cell from above 3x2 unit cell from above

23 Problem with LEED of Si(111)-Sm Expected 3x2 Observed 3x1 LEED shows 3x1 pattern not 3x2 like STM! Known problem for many metals on Si(111) eg Ba 23

24 Structural Models and Registry Shifts Si(111) 3x2-Ba structure suggested by Wigren et al, PRB 48, (1993). Qualitative calculation of effect of registry shift on LEED pattern by J. Schafer et al, PRB 67, (2003) interference of amplitudes from two registry shifted cells (1/2 unit cell shift) proposed to explain cancellation of x2 spots 24

25 Structural Model with a Registry Shift 25

26 Quantitative LEED Experiment 3 x 1 LEED pattern 40eV 3 x 1 LEED pattern 80eV Deposit 1ML Sm on clean Si(111) Anneal 700 C 15 mins Clean, thermally resilient structure

27 LEED I(V) Curves I(V) curves gathered from many runs on freshly made surfaces Compare curves using the Pendry R-Factor (sensitive to peak positions) Reproducible: variation in Rp ~ 0.1 Averaged to reduce noise Uppermost layer produces fractional and integer spots All surface layers produce integer spots LEED beam typically penetrates ~ 5 layers Fractional spots are sensitive to Sm and Si in honeycomb chain

28 VASP Ab Initio Structure vs Experiment Earlier ab initio calc structure to fit STM experiment VASP DFT geometry optimisation by Palmino et al PRB 67, (2003) R-Factor comparison Good is <0.4 Acceptable is <0.5 Overall R-Factor is 0.78 Poor R-Factors when compared to LEED I(V) Suggests HCC model and/or Palmino is wrong R-Factor calculated from structure using CAVLEED Where next?

29 CASTEP: Basis Set Optimisation Convergence w.r.t. basis set size and Brillouin zone sampling Check forces and total energy

30 Competing HCC Structures Two possible sites for Sm in HCC structure H3 shown in (a) and (b) T4 shown in (c) and (d) CASTEP geometry optimisation shows T4 is more stable than H3 by ~1eV Silicon atoms are grey, samarium is black and the hydrogen atoms are white. 30

31 CASTEP Geometry Optimisation Top view of T4 T4 structure relaxation performed on 32 processors of HPCx Main relaxation found in interlayer spacings Side view of T4

32 CASTEP Structure vs Experiment Pendry R-Factors from the CASTEP suggested structure Overall R-Factor is 0.48 (c.f. VASP Rp=0.78) Good is <0.4 Acceptable is <0.5 Fractional spots better than integer spots Suggests T4-HCC structure is valid but some further tweaking is still needed

33 Independent check structure fitting Calculate Rp as function of interlayer spacing and so map R-factor surface CASTEP result (white cross) found to be in Rp minimum (missing thermal expansion) 256 structure runs of CAVLEED on White Rose Grid parallel computer 1.5 hours per run Step size 0.05Å

34 Sm on Si Summary VASP ab initio result not refined enough to compare to quantitative surface science data: R-factor of 0.78 CASTEP ab initio result agrees well with quantitative surface science data Careful optimisation of basis set parameters New structure gives much better fit to experiment Independently verified by R-factor surface mapping Need effects of thermal vibration/expansion Further improved if include 60% T4 + 40% H3 mixture Best overall R-factor now 0.42

35 Example 2 Holmium Nanowires on a Germanium surface

36 Ge(111)1x1-Ho Structure Empty states STM 1.3V 2nA experiment Image is ~5x5nm Empty states STM 1.3V CASTEP with Tersoff- Hamann scheme Image ~5x5nm Structure common to trivalent RE of flat layer buried below reverse buckled Ge bilayer Details sensitive to Ho spin-state 36

37 Ho Nanowires on Ge(111) : STM Observations Nanowires: self-assembled lines of atoms HOT topic Never before seen on a (111) surface (due to 3-fold symmetry) Depending on coverage they can be parallel => 5x1 environment MEIS shows Ho is not subsurface but no more info 0.25ML Ho on clean Ge(111)-c2x8 grown at 250 C not 500 C What is the structure? Too dilute for LEED!

38 Nanowires on Ge(111) Structural Model Structure suggested by Chris Bonet based on STM dimensions nanowire is 2 atoms wide! Is it stable? Does it agree with STM?

39 Nanowires on Ge(111) CASTEP Top view Side view electronic structure CASTEP stable structure Delicate balance of forces/bonding CASTEP can get accurate surface structures: Need high quality convergence High cutoff energy, vacuum gap, 4 k-points Use Ultra-Soft Pseudopotentials and GGA-PBE Took 5 hours on HPCx (128 processors)

40 Comparison with STM experiment Image areas 4.2 x 3.3 nm Empty states Experiment +1.5V, 2nA Empty states +1.5V Theory Atomic resolution and agreement Filled states Experiment -2.0V, 2nA Filled states -2.0V Theory Simulate STM from CASTEP calculated electronic structure using Tersoff-Hamann scheme

41 Comparison with STS experiment Ab initio LDOS Experimental STS Conducting properties of the nanowires Theoretical DOS shows states at Fermi Level Scanning Tunneling Spectroscopy shows nanowire has conducting states at Fermi Level (compare to Germanium with band gap)

42 Ho Nanowires on Ge Summary Novel observation of nanowires on Ge(111) Model structure proposed on basis of STM measurements but could not be validated Structure confirmed by CASTEP geometry optimisation and both filled and empty states STM simulation Conducting character of nanowire seen experimentally in STS measurement and also in large LDOS at E F with CASTEP 42

43 Acknowledgements York Surface Physics Group especially Steve Tear, Chris Bonet, Ed Perkins (now Nottingham) EPSRC funding STM and ab initio study of holmium nanowires on a Ge(111) Surface, C. Eames, C. Bonet, E.W. Perkins, M.I.J. Probert, S.P. Tear, PRB 74, (2006) Quantitative LEED I-V and ab initio study of the Si(111)-3x2-Sm surface structure and the missing half order spots in the 3x1 diffraction pattern, C. Eames, M.I.J. Probert, S.P. Tear, PRB (accepted 07) 43