Photoelectron spectroscopy Instrumentation Nanomaterials characterization 2 RNDr. Věra V Vodičkov ková,, PhD.
Photoelectron Spectroscopy general scheme Impact of X-ray emitted from source to the sample penetration of photons a fewµm under the surface absorption of photon energy of absorbed photon is transmitted to internal electron subsequent emission of electron kinetic energy of this electron = difference between photon energy and bond energy electron in the atom Detection of electrons emitted from sample measuring of their kinetic energy 2
Sources of radiation Source UV radiation energy < 40 ev valence electrons excitation resonance discharge tube UPS X-ray energy 100 8000 ev excitation of internal electrons X-ray tube XPS 3
UV sources Resonance discharge tubes: - gas discharge in noble gases - line width 0.005-0.03 ev 4
X-ray sources 1. X-rayX tubes - accelerated electrons are emitted from the cathode to the metallic anode interaction of electrons with metall atoms emission of X-ray photons Materials used for anode 5
Instrumentation tion of the method Sources of X-ray Usually two-anode source: Mg (characteristic line Mg K α ) and Al (characteristic line Al K α ) by reason of possibility of Auger lines identification in spectrum (for better lucidity of spectrum) Mg Kα Al Kα energy 1253,6 1486,6 half-width 0,7 0,85 6
Sources of X-ray 2. Synchrotron radiation Radiation originating in circular particle accelerator 7
Monochromator - quartz crystals - geometry: the Bragg law has to be fulfilled - resolution is enhanced to cca 0.3 ev 8
Kinetic energy of electron analyzer RFA (Retarding Field Analyzer) consists of a series of concentric hemispherical grids electrons pass through the grid, that has a voltage - V applied (time-increasing) so that any electrons higher in energy (E>U B ) can pass through the grid (electrons lower than this energy are reflected back) step by step detection of electrons with defferent energy Elektrostatic analyzers are designed in different configurations (cylindrical, hemisferical) principle: trajectory of electrons in electric or magnetic field depends on their velocity TOF (Time Of Flight) - principle: electrons energy is determined via a time measurement, long detector response (ns) 9
Currently the most popular electron kinetic energy analyzers 1. Electrostatic hemispherical analyzer (CHA) consists of two hemispheres positioned concentrically and having radii, R1 (inner hemisphere) and R2 (outer hemisphere). Negative potentials are applied to both hemispheres and with V 2 being greater than V 1. The median equipotential surface between the hemispheres would have a value, V 0 given by V 0 = (V 1 R 1 + V 2 R 2 )/2R 0 (R 0 - the radius of the median equipotential surface) an electron entering through slit S with a kinetic energy E = ev 0 will follow the trajectory through the analyzer along the median equipotential surface of radius R 0 and will be focused at the exit slit F electrons entering at S with a kinetic energy not equal to ev 0 will follow a different trajectory and will not be focused at F because R 0, R 1 and R 2 are fixed, in principle changing V 1 and V 2 will selectively pass electrons of varying kinetic energies through the analyzer 10
Currently the most popular electron kinetic energy analyzers 2. Cylindrical mirror Analyzer (CMA) CMA consists of two concentric metal cylinders arranged such that their axes are coincident, different voltages are placed on each cylinder such that there is an electric field between the two cylinders electrons are injected from a point on the axis into the gap between the two cylinders. If the electrons are travelling very fast, they will impinge on the outer cylinder. If they are travelling very slowly, they will be attracted to the inner cylinder. hence only electrons in a narrow energy region (called thepassenergy) succeed in getting all the way along the cylinders to the detector the resolution is improved by apertures within the analyser 11
CHA Instrumentation of the method Electrostatic deflection analyzers comparison 5 ±6 CMA 42.3 Small signal Compatible with simple electrostatic aperture and tube lenses Long focal distance Radius 100-150 mm Res. Power about 1000-5000 Working distance about 25-50 mm E B E = 5000 0 2 mev Large signal Non compatible with simple electrostatic aperture and tube lenses Short focal distance Cyl diam 100-150 mm Res. Power about 200 Working distance about 5 mm E B E = 200 0 0.1eV 12
Detectors of electrons Cu-Be dynodes multipliers of electrons signal amplification Chanelltron a "continuous" continuous-dynode" structure is feasible if the material of the electrodes has a high resistance so that the functions of secondary- emission and voltage-division are merged. This is often built as a funnel of glass coated inside with a thin film of semi-conducting material, with negative high voltage applied at the wider input end, and positive voltage near ground applied at the narrower output end. Electrons emitted at any point are accelerated a modest distance down the funnel before impacting the surface, perhaps on the opposite side of the funnel. amplification up to 10 8 Micro-channelplate channelplate 13
Evaluation Photoelectron spectrum - Dependence of number detected photoelectrons on kinetic energy The kinetic energy of electrons emitted from sample surface is measured ( hν - incident photon energy) In solid matters the values of electron binding energy E BF are related to Fermi level, reduced by work function ϕ. Then the binding energy of electron is E B = K hν E ϕ E f photon energy E B binding energy of electron, characteristic for element and binding site E k photoelectron kinetic energy ϕ - work function Chemical analysis 14
Evaluation Photoelectron spectrum structure Except photoelectron lines the detection of Auger electron lines is possible If X-ray source is not monochromatic, secondary lines can be detected in the spectrum another photoelectron are excited satellite structure of spectrum Kinetic energy scale Binding energy scale Photoelectron spectrum lines description per quantum number Auger spectrum convention used for X-ray AES K L 1 L 2 L 3 M 1 M 2 M 3 M 4 M 5 N 1... XPS 1s 2s 2p 1/2 2p 3/2 3s 3p 1/2 3p 3/2 3d 3/2 3d 5/2 4s... 15
Evaluation Photoelectron spectrum structure chemical shift For atoms - bound to atoms of another elements - in different oxidation state - that have not equivalent position in crystal lattice - on the sample surface measurable shifts of the spectral lines chemical shift Degree of chemical shift 0-10 ev Resolution of chemical states Note: similar shift is possible to observe for lines of Auger spectrum too it usually uses to be more marked efficiency for chemical states resolution in case of very small value of photoelectron lines shift (sometimes so called Auger s parameter is used = position difference of Auger a photoelectron line) Chemical shift of Si 2p bond Si-O 3.6 ev shift of peak in comparison with bond Si-Si. (green line -thin layer of SiO 2 on Si) 16
Evaluation Quantitative analysis Element concentration determination per spectral lines intensity Thickness determination of very thin homogenous layers (< 10 nm) - per intensity ratio of spectral lines (at simple emission angle) (Intensity measured in layer and in substrate depends on layers thickness) - measurement of more complicate systems (multilayer systems) is possible measurement at the different emission angles (AR-XPS) - concentration profiles dependence of element concentration on depth 17