Surface Characterization i LEED Photoemission Linear optics
Surface characterization with electrons MPS M.P. Seah, WA W.A. Dench, Surf. Interf. Anal. 1 (1979) 2
LEED low energy electron diffraction De Broglie wavelength: λ= h/(mv) g g /( ) For electrons: λ = 150 / E0 E0 in ev, λ in Å. For 100 ev electrons: λ(100) = 1.22 Å (low energy) Corresponds to atomic dimensions, similar to XRD
Analogy aogyto optical grating. gat g Constructive interference: Enhancement of intensity only in certain directions: n λ = d sinθ For 2D arrangement (plane lattice): scattering conditions have to be fulfilled in both directions. h,k (order)
20 ML Ge Ge on Si(111)7x77 Transition from 7x7 to 5x5 Si(111)7x7
Synchrotron radiation photoemission spectroscopy ISA Aarhus University Maximum Energy 580 MeV Max Current 250 ma Lifetime 15h hours SGM1 30 400 ev 10 10 photons/sec @130 ev
Core levels Photoelectron spectroscopy Chemical reactions/mixing Growth modes Valence bands Electronic levels relevant for optics hν=130 ev hν Intensity (a arb. units) Al2p sp band Secondary electrons E F E vac Binding Energy(eV) Kinetic Energy (ev)
Wedge shaped metal film AFM Sample moved into shadow of shield. Evaporation rate ~1 ML per minute 18. 18. μ m Triangular domains ~200 nm Ag(111) LEED-pattern - only one type of domains 2 Scanning film thickness by moving wedge through laser or synchrotron beam ght (arb. units) Si i2p peak hei 25 Convolution fit of 20 Gauss beam and step 15 function d=150 μm 10 5 0 38,80 38,85 38,90 38,95 39,00 Sample position (mm) Width of synchrotron beam
The Si(111) 7 7 structure Structure known fromstm and various spectroscopies 12 adatoms 6 rest atoms Surface states: Occupied S 1 S 2 S 3 Empty U 1 U 2 0.3 03eV 0.8 ev 1.7 17eV 0.5 ev 17eV 1.7
Si2p core levels 24 Si2p 130 ev 7x7 Inte nsity (a arb. un nits) 20 16 12 8 0 Si(IV) Si(III) Si(II) Si(I) 100 L O 2 Decomposition of spectra Sum Bulk Ad-atoms Rest atoms 4 Background Experiments 106 104 102 100 98 96 Binding energy (ev) 7x7: Bulk Ad atom Rest atom Si surface oxide: Bulk Oxidation stages
The Si(111) 7 7 structure Structure known from STM and various spectroscopies: 12 adatoms + 6 rest atoms Surface states: Occupied: S 1 03eV 0.3 ev, S 2 08eV 0.8 ev, S 3 17eV 1.7 Empty: U 1 0.5 ev, U 2 1.7 ev 20 Si(111)7x7 38 ev Inte ensity (arb b. units) 15 10 5 θ=10 o S 3 S 2 300K 450K S 1 0 6 4 2 0 Binding Energy (ev)
Film growth Si2p spectra In ntensity (arb. units s) 10 8 6 4 2 Growth at 170 K No annealing 130 ev 7x7 Growth at 170 K Annealing at room temperature ) (arb. units Intensity 10 1 0,1 Bulk 300 K Surface 300 K Bulk 170 K Surface 170 K 0 102 101 100 99 98 Binding Energy (ev) 101 100 99 98 97 0,01 0 2 4 6 8 10 12 Coverage (ML) Growth at 170 K leads to exponential decay of Si2p levels with ~5Å decay rate. Room temperature annealing of the film leads to growth of large atomically flat domains. Areaswith low Agcoverage areformed. Annealing
Au on 6 ML Cu/Si(111) Au4f spectra Inte ensity (a arb. uun its) 12 16 ML Au on Si(111) 10 8 6 4 2 0 16 12 8 4 0 Au4f Reacted Bulk 82 81 80 79 78 77 76 75 20 ML Au on 6ML Cu/Si(111) Surface 82 81 80 79 78 77 76 75 Binding Energy (ev) Au on Si(111): Reacted component at higher h BE Au Si Reacted layer Au on Cu/Si(111): Surface component at lower BE Au Si 6 ML Cu
Valence band spectra Excitation energy typically 30 100 ev Binding energy: 0 10 ev sp electrons in Ag, Au, Cu, Al Intens sity (arb b. units) 25 20 ML Au 20 15 3 10 15 10 5 4 2 1 ML d bands 47 ev SP bands Surface state 5 6 ML Cu/Si(111) 0 8 6 4 2 0 Binding Energy (ev)
STM Difference STM images of occupied surface states on a Si(111)7x77 surface: a) topographic image b) adatom states at 0.35eV c) dangling gbond states at 0.8eV d) back bond states at 1.7eV [15]. Topographic image of Si(111)7x7 surface as recorded using Scanning Tunneling Microscope (STM) with a bias voltage of + 2 V [8].
Reflection-Anisotropy Spectroscopy (RAS) Wolfgang Richter, TU Berlin
RAS - Probe for solid-liquid interface UHV Au(110) Th l d l Three-layer model McIntyre and Aspnes, Surf. Sci. 24, 417 (1971 Ambient Surface Bulk Electrolyte ΔR R exp = d λ [ AΔε '' B Δε '] 8π s s Surface states at 2.5 and 3.7 ev Sheridan et al. PRL 85, 4618 (2000)
Surface optics - Differential reflection f 1 ~100 Hz f 2 ~10 Hz Selci et al. JVST A 5, 327 (1987)
Ellipsometry - Optical properties of Si E 0 +E₁ E 2 0 ε R ε I Aspnes and Studna, PRB 27,, 985 (1983)
Depth sensitivity of linear optics The probe depth can be varied through the wavelength
SDR Si(111)2 1 Difference between clean and oxygen exposed sample Excitation of surface states Anisotropy confirm chain model
SDR on Si(111)7 7 Experiments 1.7 ev: Ad atom dangling bonds 2 3 ev: Ad atombackbonds to bulk states Tight binding model Noguez et al. PRL 76, 4923 (1996)