X-ray photoelectron spectroscopy - An introduction Spyros Diplas SINTEF Materials & Chemistry, Sector of Materials and Nanotechnology, Department of Materials Physics-Oslo & Centre of Materials Science and Nanotechnology, Department of Chemistry, UiO spyros.diplas@sintef.no spyridon.diplas@smn.uio.no 1
Material Characterisation Methods 2
XPS-Basic Principle Photoelectron Auger electron Vacuum Fermi valence band 2p 1/2, 2p 3/2 L 23 2s L 1 hν Internal transition (irradiative) 1s K 3 E kin = hν E B - ω E KL2,3L2,3 (Z) = E K (Z) [E L2,3 (Z) + E L2,3 (Z + 1)] Excitation De-excitation
Auger electron vs x-ray emission yield 1.0 Auger Electron Emission Probability 0.8 0.6 0.4 0.2 0 5 X-ray Photon Emission 10 15 20 25 30 35 40 Atomic Number B Ne P Ca Mn Zn Br Zr It is easier to detect light elements with Auger than with EDS Elemental Symbol 4
80 70 60 50 x 10 4 Auger peaks XPS spectrum ITO In 3s In 3s Sn 3p In 3p Sn 3d O 1s In 3d Photoelectron peaks CPS 40 30 Sn MNN In MNN In/Sn 4s 20 O KLL C 1s In/Sn 4p 10 5 1200 1000 800 600 400 200 0 Binding Energy (ev)
Peak width (ΔE) ΔE = (ΔE n2 + ΔE p2 + ΔE a2 ) 1/2 Gaussian broadening: Natural width X-ray source contribution Analyser contribution -Instrumental: There is no perfectly resolving spectrometer nor a perfectly monochromatic X-ray source. -Sample For semiconductor surfaces in particular, variations in the defect density across the surface will lead to variations in the band bending and, thus, the work function will vary from point to point. This variation in surface potential produces a broadening of the XPS peaks. -Excitation process such as the shake-up/shake-off processes or vibrational broadening. Lorentzian broadening. The core-hole that the incident photon creates has a particular lifetime (τ) which is dependent on how quickly the hole is filled by an electron from another shell. From Heisenberg s uncertainty principle, the finite lifetime will produce a broadening of the peak. Γ=h/τ Intrinsic width of the same energy level should increase with increasing atomic number 6
Examples of XPS spectrometers 7
Schematic of an XPS spectrometer
Instrument: Kratos Axis Ultra DLD at MiNaLab Analyser e - Detector e - Monochromator X-ray source X-ray source Sample 9
Parallel angle resolved XPS instrument-theta Probe Spectroscopy Source-defined small area XPS 15 µm to 400 µm Snapshot spectrum acquisition Up to 112 channels Faster serial mapping Faster profiling Unique parallel ARXPS with up to 96 channels Large samples (70 mm x 70 mm x 25 mm) Sputter profiles Mapping possible up to full size of sample holder ISS included Target applications Thickness measurements Surface modification, plasma & chemical Self assembly Nanotechnology Ultra thin film technologies Shallow interfaces 10
Sample requirements Has to withstand high vacuum ( 10-7 Torr). Has to withstand irradiation by X-rays Sample surface must be clean! Reasonably sized. 11
XPS Depth of Analysis The probability that a photoelectron will escape from the sample without losing energy is regulated by the Beer-Lambert law: Where λ e is the photoelectron inelastic mean free path Attenuation length (λ) 0.9 IMFP IMFP: The average distance an electron with a given energy travels between successive inelastic collisions 12
Primary structure Features of the XPS spectrum - Core level photoelectron peaks (atom excitation) - Valence band spectra - CCC, CCV, CVV Auger peaks (atom de-excitation) Secondary structure - X-ray satellites and ghosts - Shake up and shake off satellites - Plasmon loss features - Background (slope) 13
Quantification Unlike AES, SIMS, EDX, WDX there are little in the way of matrix effects to worry about in XPS. We can use either theoretical or empirical cross sections, corrected for transmission function of the analyser. In principle the following equation can be used: I = J ρ σ K λ I is the electron intensity J is the photon flux, ρ is the concentration of the atom or ion in the solid, σ s is the cross-section for photoelectron production (which depends on the element and energy being considered), K is a term which covers instrumental factors, λ is the electron attenuation length. In practice atomic sensitivity factors (F) are often used: [A] atomic % = {(IA/FA)/Σ(I/F)} Various compilations are available. 14
Chemical shift ΔE (i) = kδq + ΔV M ΔR Initial state: Before photoemission-ground state Final state: After photoemission Initial state contribution Δq: changes in valence charge ΔV M : Coulomb interaction between the photoelectron (i) and the surrounding charged atoms. final state contribution ΔR: relaxation energy change arising from the response of the atomic environment (local electronic structure) to the screening of the core hole. 15
Chemical shift - Growth of ITO on p c-si Intensity arbitrary units Si In oxide Sn oxide Si 2p In 3d 5/2 Sn 3d 5/2 3/2 In 3/2 SiO x 1.5 nm 1.5 nm 1.5 nm 0.5 nm BHF 15 sec + 500 o C 3.0 nm Sn 3.0 nm 0.5 nm 0.5 nm Binding Energy (ev)
Chemical shift 17
Shake-up satellites in Cu 2p 2p 3/2 2p 1/2 Cu CuO Shake-up satellites CuSO 4 970 960 950 940 930 Binding energy (ev) 18
Plasmons They describe the interaction (inelastic scattering) of the PE with the plasma oscillation of the outer shell (valence band) electrons Plasmons in their quantum mechanical description are pseudoparticles with energy E p =hω ω = (ne 2 /ε 0 m) 1/2 /2π n =valence electron density, e, m electron charge and mass ε 0 =dielectric constant of vacuum Pure elements Mo-Si-Al Compound 19
Peak asymmetry Zn Arbitrary Units ZnO 1055 1050 1045 1040 1035 1030 1025 1020 1015 1010 Binding Energy (ev) ZnO Zn Arbitrary Units 16 14 12 10 8 6 4 2 0-2 Binding Energy (ev) Peak asymmetry in metals caused by small energy electron-hole excitations near E F of metal 20
Depth profile with ion sputtering SnO 2 Sn Use of an ion gun to erode the sample surface and re-analyse Enables layered structures to be investigated Investigations of interfaces Depth resolution improved by: Low beam energies Small ion beam sizes Sample rotation 500 496 492 488 484 480 Depth 21
Probing different depths by choosing different areas in the XPS spectrum Ge3d Ge(0) Ge(IV) Ge2p3/2 Ge(0) Ge(IV) GeLMM Ge(IV) Ge(0) 8.0 ev Ge(IV) Ge(0) 4.0 ev 4.0 ev 8.2 ev 30 Binding Energy (ev) Ge3d: E K = 1453 ev λ = 2.8 nm 1220 Binding Energy (ev) Ge2p: E K = 264eV λ =0.8 nm 1140 1150 1160 1170 1180 Kinetic Energy (ev) GeLMM: E K = 1140-1180 ev 22
Angle Resolved XPS (ARXPS) for non-destructive depth profile I (d) = I o* exp(-d/λ) OH oxide θ I (d) = I o* exp(-d/λcos θ) RT surface Substrate Film Arbitrary Units AR bulk λ=attenuation length (λ 0.9 IMFP) 536 534 532 530 528 526 524 Binding Energy (ev) λ=538α A /E A2 +0.41α A (α A E A ) 0.5 (α A3 volume of atom, E A electron energy) 23
XPS-Check list Depth of analysis ~ 5nm All elements except H and He Readily quantified (limit ca. 0.1 at%) All materials (vacuum compatible) Chemical/electronic state information -Identification of chemical states -Reflection of electronic changes to the atomic potential Compositional depth profiling by -ARXPS (ultra thin film <10 nm), -change of the excitation energy -choose of different spectral areas -sputtering Ultra thin film thickness measurement Analysis area mm 2 to 10 micrometres 24 XPS gives information of what elements exist on the sample surface, how much of each element, at what chemical state, how much of each chemical state.
Some application examples
d=λ Si cosθ ln(1+r/r ) Interfacial studies of Al 2 O 3 deposited on 4H-SiC(0001) Avice, Diplas, Thøgersen, Christensen, Grossner, Svensson, Nilsen, Fjellvåg, Watts Appl. Physics Letters, 2007;91, 52907, Surface & Interface Analysis,2008;40,822 Si 4+ Si 3+ SiC/Si 2+ Al 2p plasmon contribution d: SiOx film thickness λ Si :inelastic mean free path for Si, Θ: the angle of emission, R: the Si 2p intensity ratios ISiox/ISiC, R the Si 2p intensity ratios I Siox/I SiC where I is the intensity from an infinitely thick substrate. R =(σ Si,SiO 2. λ Si,SiO 2 ) / (σ Si, Si. λ Si,Si ) where σ Si,SiO2 and λ Si,SiO2 are the number of Si atoms per SiO 2 unit volume and the inelastic mean free path respectively The σ Si,SiO2 / σ Si,Si ratio is given by σ Si,SiO2 / (σ Si,Si = (D SiO2. F Si ) / D Si. F SiO2 where D is the density of the material and F the formula weight. Si + Si 0 Ar 1273 K 60 mins Ar 1273 K 30 mins Ar 1273 K 15 mins, N2/H2 873 K 30 mins N2/H2 873 K 15 mins as-grown Al 2 O 3 For the calculations we also assumed that the Si 2p photoelectrons from both SiC and Si oxide film will be attenuated by the same amount as they travel through the Al2O3 film therefore, their intensity ratio will reflect the attenuation of the Si 2p electrons coming from the SiC through the Si oxide film. From XPS d= 1nm at RT, d=3nm at 1273 K 26 SiC SiO x
XPS on ITO e-beam deposited prior and after annealing (SINTEF internal) Sn 3d In 3d Air annealed 300 C e-beam deposited O 1s Valence band Air annealed 300 C e-beam deposited 27
Nitrogen in ZnO NO 2 N-C=O or N-C N outermost AG Arbitrary Units 200 sec sputtering outermost Arbitrary Units 900 o C 410 408 406 404 402 400 398 396 394 392 390 388 Binding Energy (ev) 200 sec sputtering 414 412 410 408 406 404 402 400 398 396 394 Binding Energy (ev) 28
Band bending in ZnO R. Schifano, E. V. Monakhov, B. G. Svensson, and S. Diplas, 2009, Appl. Phys. Lett. 94, 132101 491,6 Zn 2p - O 1s energy difference Arbitrary Units Zn 2p - O 1s energy difference (ev) 491,4 491,2 491 490,8 490,6 490,4 490,2 LR4 Zn2p-O1s LR3 Zn2p-O1s 1050 1045 1035 1030 1025 1020 1015 Binding Energy (ev) 490 0 20 40 60 80 100 Etching time (sec) Zn 2p-LM45M45 Auger parameter comparison 2011 Arbitrary Units Zn 2p-LM 45 M 45 Auger parameter (ev) 2010,5 2010 2009,5 2009 LR4 2p-LM45M45 LR3 2p-LM45M45 980 982 984 986 988 990 992 994 996 998 1000 Kinetic Energy (ev) 2008,5 0 20 40 60 80 100 Etching time (sec) 29
3 SEM of a Cu(In,Ga)Se 2 solar cell (cross-section) and its mode of operation CIGS solar cell Energy/environmental application Solar cells based on Cu(In, Ga)Se 2 (CIGS) Thin-film stack on glass Mo and Zn oxide layer form electrical contacts p-type CIGS film (sunlight absorber) and n-type CdS film form p-n junction Excellent efficiency Low cost compared to thicker silicon-based solar cells Practical problem Controlling film composition and interfacial chemistry between layers (affects electrical properties) XPS solution XPS sputter depth profiling Elemental and composition information as a function of depth Identify chemical gradients within layers Investigate chemistry at layer interfaces Acknowledgement: Thermo Electron Corporation 30
Depth profile of CIFS film stack CIGS solar cell Mo Depth profile of CIGS film stack Demonstrates standardless quantification of XPS Excellent quantification agreement between XPS and Rutherford BackScattering (RBS) Both techniques show cross-over of In and Ga close to 1.6µm depth XPS tool is able to analyze product solar cell device Zn Se O CdS In Cu C Ga Sputter depth profile of CIGS film stack Rutherford BackScatter profile of CIGS film stack Acknowledgement: Thermo Electron Corporation 31
Interfaces in Solar cells 32
Interface between p-si/zno: Si HF with and without Ar etched (SINTEF SEP 09) Ar etching No Ar etching Si Zn mixed oxide SiO 2 Si Depos at RT + Ar etching Depos at 500 C + Ar etching Depos at 500 C RT depos RT depos + 300 C annealing 33
Elemental distribution and oxygen deficiency of magnetron sputtered ITO films A. Thøgersen, M.Rein, E. Monakhov, J. Mayandi, S. Diplas JOURNAL OF APPLIED PHYSICS 109, 113532 (2011) XPS depth profiling reveals metallic In and Sn at the interface High resolution TEM showing metallic In at the interface 34
Initial stages of ITO/Si interface formation studied with XPS and DFT O.M. Løvvik, S. Diplas, A Romanyuk, A. Ulyashin, JOURNAL OF APPLIED PHYSICS 115, 083705 (2014) Presence of pure In and Sn, as well as Si bonded to oxygen at the ITO/Si interface were observed. The experimental observations were compared with several atomistic models of ITO/Si Interfaces. These results support and provide an explanation for the creation of metallic In and Sn along with the growth of SiOx at the ITO/Si interface. 35
Cu 2 O as a Potential Solar Cell Material NFR project "Heterosolar" (SINTEF-UiO) Theoretical efficiency of Cu 2 O/ZnO based solar cell: [1-3] ~10-20 % Highest experimental efficiency reported: ~ 2 % without buffer layer [2] ~ 1-4% with buffer layer [4,5] [1] S. Jeong et al., Electrochim. Acta (2008), 53, 2226. [2] A. Mittiga et al. Appl. Phys. Lett. (2006), 88, 163502. [3] T. Gershon, et al., Sol. Energy Mater. Sol., C. (2012), 96,48 [4] Z. Duan et al. Solar Energy Materials & Solar Cells 96 (2012) [5]T. Minami et al., Appl. Phys. Exp.4, 062301(2011) 36
Samples Investigated with XPS Top film nominal thicknesses: 1, 3, 5, 10 nm 37
AZO/Cu 2 O AZO/CuO XPS on ultrathin AZO deposited on Cu 2 O. The satellite indicates formation of CuO at the interface in agreement with TEM (see next slide). Variations in the satellite intensity as the interface is approached indicate that ZnO influence the electronic structure of the interfacial CuO 38
STEM HHADF and FFT STEM-HAADF imaging of Cu 2 O/ZnO cross-section interface. (A) The overview STEM HAADF image of the Cu 2 O film grown on O-polar ZnO substrate. (B) The HRSTEM HAADF image of the boxed region indicated in (A). The different layers in the cross-section are marked by false coloring. (C) and (D) The FFT patterns from the top and the interfacial layer in (B) respectively were identified as Cu 2 O and CuO phases. 39