Energy Spectroscopy Spectroscopy gives access to the electronic properties (and thus chemistry, magnetism,..) of the investigated system with thickness dependence Ex.: Fe/MgO Fe O Mg Control of the oxidation at the interface Control of the layer purity Sources of impurities: evaporators, evaporate targets, UHV residual gas (in 10-6 mbar every second each surface atomic site is touched by an atom of the residual gas), etc.
Energy Spectroscopy Excitation by means of a probe Spectral analysis of the incident photon-> Energy spectral analysis of the out coming particles Different probes are possible: Auger -> electrons (2 10 kev) XPS (or ESCA) -> X rays (0.2 2 kev) (x-rays photoelectron spectroscopy) UPS -> Ultraviolet photons (10 50 ev) (UV photoelectron spectroscopy) or Out coming particles: electrons - XAS (XMCD) - EXAFS UPS Electromagnetic spectrum XAS -XPS EXAFS E = hn = hc/l; c = 3 10 8 m/s; h = 6.6 10-34 Js
The Auger process Auger spectroscopy is based upon a single electron in - electron out process. Ejected electron Vacuum level Ejected electron E kin KL 1 L 3 excitation radioactive Auger relaxation E kin E(L 3 ) = E(L 1 ) E(K) E(L i ) and E(K) depend on the atomic structure -> E kin does not depend on E i chemical sensitivity N.B.: the sample must be a conductor and must be connected to ground to avoid charging
Auger experimental setup CMA = Cylindrical Mirror Analyzer electrons sample electron gun Electron energy: in out grids channeltron 1-10 kev 10 1000 ev Auger spectrum: number of emitted electrons as a function of their kinetics energy
Counting mode MgO Auger spectrum Derivative mode The monotone background is due to multi-scattered electrons Auger transitions for the different chemical elements
Fe/MgO/Fe( 001) heterostructures MgO does not wet completely the Fe bottom electrode when grown at 670 K, being porous when grown at RT. However, the annealing treatment of this MgO layer gives rise to a more compact, porous free, and continuous barrier. J. Appl. Phys. 97, 036104 (2005)
Thickness sensibility The thickness of the investigated surface depends: 1) Electron energy 2) Probability of the Auger transition 3) Atomic scattering cross section 4) Abortion of the Auger electrons Electron beam intensity at depth z: J(z) = I(0) r -z/d r = layer attenuation factor; d = atomic layer thickness Auger electrons (detected at surface) coming from depth z: I(z) = I(0) r -z/d s -z/d = I(0) exp(-z/d ln(rs)) = I(0) exp(-z/l) s = attenuation factor for the Auger electrons; l = d/ln(rs) electron mean free path Total Auger electron current measured at surface: I = 0 z I(z) dz = I(0) l (1-exp(-z/l)) l = electron mean free path 1/l = 1/l i + 1/l Auger ~ 1/l Auger l i >> l Auger surface sensibility is given by the reduced mean free path of the out coming electrons Depth sensibility (D): 95% of the signal coming from a film with infinite thickness -> I(D) = 0.95 I(0) l (1-exp(-D/l)) -> D = - l ln(0.05) ~ 3 l
Growth of Ni films on 1ML Co/Pt(111) 848
Alloying during annealing of 2 ML Ni/1 ML Co/Pt(111) Co 53 -> l = 3.9 Å Co 656 -> l = 11.4 Å Ni 102 -> l = 4.6 Å Ni 848 -> l = 13.2 Å Co 53 increases and Ni 102 decreases -> Ni-Co alloying on top of Pt(111) Co 53 and Ni 102 decrease while Pt 237 increases -> Ni-Co alloying with the Pt surface Co 656 and Ni 848 decrease while Pt 237 increases -> Ni-Co alloying with Pt bulk C. S. Shern et al. Phys. Rev. B 70, 214438 (2004)
XPS and UPS Exciting particle -> photons Emitted particle -> electrons X ray photons (0.2 2 kev) -> to investigate core levels UV photons (10-45 ev) -> to investigate valence levels Photoelectron spectroscopy is based upon a single photon in/electron out process. The energy of a photon is given by the Einstein relation : E = h n h - Planck constant ( 6.62 x 10-34 J s ) n - frequency (Hz) of the radiation XPS UPS E kin = hn - E bond
Laboratory photon sources X-rays: electron beam impinging at energies of 10-50 kev on an anode excites the core electron of the anode -> during the relaxation photons are emitted Mg K a E = 1253.6 ev DE = 0.7 ev Al K a E = 1486.6 ev DE = 0.9 ev UV-rays: discharge in a lamp containing rare gas at low pressure (0.1 mbar) -> during the relaxation photons are emitted He I E = 21.2 ev DE = 0.01 ev He II E = 40.8 ev DE = 0.01 ev Ne I E = 16.9 ev DE = 0.01 ev Photon penetration depth > 1-10 mm -> the surface sensibility is given by the reduced mean free path of the out coming electrons
Synchrotron sources A synchrotron is an extremely powerful source of X-rays. These are produced by highly energetic electrons moving with a speed v/c=0.999 in a large circle in the synchrotron European Synchrotron Radiation facility (ESRF) Intensity: more than 10 12 photons/second Mg Kα XPS laboratory source: 10 7 photons/second Tunable energy Also in Switzerland: Swiss Light Synchrotron (SLS)
XPS Spectrum 2 ML MgO/Fe Energy (ev) Sensibility to Auger transition To distinguish between photo-electrons and Auger-electrons is sufficient to take two spectra at different energies: XPS -> the energy of the out coming electron depends on hn Auger -> the energy of the out coming electron depends on the core transition
Intensity (arb. un.) UPS Spectrum MgO is an insulator with a gap of 8 ev Fe electronic structure -> 3d 6 4s 2 d - band He 2 40.8 ev MgO Fe Fe 3d Bonding energy (ev) Shift of the 3d Fe peak following MgO deposition -> Fe oxidation O 2 dose He I UP spectra of the pure Fe film and after its exposure to various oxygen doses at 300 K. Sicot et al. Phys. Rev. B 68, 184406 (2003) K. Ruhrnschopf et al. Surf. Sci. 374, 269 (1997)
UPS Spectrum UV photons (10-45 ev) -> to investigate valence levels ARUPS: You can measure the wave vector k and the energy at the same time (band structure) With STM only access to the DOS (density of state as a function of energy, but no information on k)
ARUPS: Angle Resolved UPS Measure of the dispersion relation (energy vs wave vector) of surface states i.e. the band structure of a surface. Three-step model: 1) The electron is excited from an initial to a final state within the crystal; 2) The electron travels through the solid towards the surface; 3) The electron crosses the surface and is emitted into the vacuum with a certain kinetic energy. Measurement of the dispersion curve requires a determination of the wave vector of the emitted photoelectrons. The wave vector has a component both parallel and perpendicular to the surface, so that the kinetic energy of the photoelectron should be written: Measuring the photoelectron intensity as a function of E and q one gets the dispersion relation
Relationship between the k outside (value of the photoelectron in vacuum outside the crystal) and the value of k inside (of the electron in the solid) Elastic scattering -> E kin = hn - W s - E bond Momentum conservation: the photon momentum is negligible and thus the electron s final momentum must equal its initial momentum in the solid. p e = (2mE) 1/2 -> ~ 4 10-25 for 1 ev electron p hv = E/c -> ~ 3 10-28 for 1 ev photon However, the photoelectron must, after traveling through the solid, traverse the surface into the vacuum. The surface represents a scattering potential with only 2D translational symmetry. As for LEED, the electron will be scattered by a reciprocal lattice vector of the surface. Thus, the relationship between the photoelectron s wave vector in the solid and in vacuum is: k outside = k inside + G s where G s is a reciprocal lattice vector of the surface. Note that, the component of wave vector perpendicular to the surface is not conserved. Thus, from a measurement of the energy and emission angle of the photoelectron the value of k within the solid can be determined.
Surface State The surface states are localized at surface -> k^ ~ 0 E s = E F E 0 + 1/2m* (ħk) 2 m* -> effective mass of the surface state electron Conduction electrons behave likes a 2D gas of free electrons
Surface states on Au(788) // to the steps ^ to the steps L = terrace size = 3.8 nm One dimensional quantum well of size L perpendicularly to the steps Y k (r) exp(ik // y) cos(k^x) k^ = np/l E n = E F E 0 + n 2 /2m* (ħp/l) 2 A. Mugarza et al., Phys. Rev. Lett. 87, 107601 (2001)
Carbon based material Electronic structure: 1s2 2s2 2p2 Diamond: sp3 bonding Graphite: sp2 bonding
Graphene : a monolayer of graphite NPG Asia Mater. 1(1) 17 21 (2009)
Graphene Graphene is an atomic-scale honeycomb lattice made of carbon atoms. Nature Materials 6, 183 191 (2007) graphene: E k Free electron gas: E k 2
Free standing graphene zero gap (D=0) semiconductor for applications 1) Band gap opening 2) Tailoring the group velocity perturbation Band gaps in g nanoribbons Gate controlled band gaps in bilayer g PRB 78, 161409 (2008); PRL 98, 206805 (2007) Nature 459, 820 (2009)
Phys. Rev. B 77, 165419 (2008). Ir/graphene/Ir(111)
Band gap and angular dependent electron mobility