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Photoemission spectroscopy study of SnZrCh 3 [Ch=S, Se] and related materials

Photoemission spectroscopy study of SnZrCh 3 [Ch=S, Se] and related materials. Andriy Zakutayev November 2009. Experimental details. SnZrSe 3 , SnSe , ZrSe 2 - prepared by Annette Richard (add preparation details) SnZrS3 - prepared by Daniel Harada (add details)

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Photoemission spectroscopy study of SnZrCh 3 [Ch=S, Se] and related materials

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  1. Photoemission spectroscopy study of SnZrCh3 [Ch=S, Se] and related materials AndriyZakutayev November 2009

  2. Experimental details • SnZrSe3, SnSe, ZrSe2 - prepared by Annette Richard (add preparation details) • SnZrS3 - prepared by Daniel Harada (add details) • XPS measured by AndriyZakutayev (August 2009, TU Darmstadt) • Stored in food-grade vacuum and polished with sand paper before XPS (Escalab 250, monochromated Al anode x-rays, calibrated with clean Ag sample) • ZrSe2 pellets degassed strongly in 10-9 mbar UHV

  3. Sb 5% SnZrSe3 doping (2% and 5%): Sb on Sn, Bi on Zr site Sb 2% Bi 5% Bi 2% SnZrSe3 • All the peaks in the survey spectra are due to either photoemission or Auger electrons from Sn, Zr, Se, O, Bi, and Sb . • There is no Si or C after polishing with SiC sand paper

  4. Core level spectra of the elements Sn 3d Sn-O Sn-Se Zr 3p Zr-Se • Degree of the oxidation of Sn is random – the difference most likely comes from not-reproducibility of surface polishing. • Zr is oxidized similarly for all the samples – it is likely that this indicates oxidation of Zr in the grain boundaries • Se peaks ratios is not consistent with d-nature of the states. This means there is a large Se-O component in the signal • O1s signal also has 2 peaks – most likely due to difference in (1) C-O and (2) Zr-O, Se-O, Sn-O bonds • Position of the peaks does not change as a function of doping %. This means that the EF-EVBM is fixed Zr-O Sb 5% Sb 5% Sb 2% Sb 2% Bi 5% Bi 5% Bi 2% Bi 2% SnZrSe3 SnZrSe3 O-C and O-Zr/Se/Sn Se 3d Sn-Se and Zr-Se O 1s Se-O Sb 5% Sb 5% Sb 2% Sb 2% Bi 5% Bi 5% Bi 2% Bi 2% SnZrSe3 SnZrSe3

  5. Other core level spectra that interfere Zr 4p and Sn 4d Zr 3d and Se LMM • The most intense Zr 3d (179 and 181 eV) peak coincides with Se LMM Auger line (181 eV + weaker satellites at lower BE) • Zr 4p (28eV) coincides with Sn 4d (25 eV)– apparent ratios of Sn 4d intensities is reverse because of it SnZrSe3 SnZrSe3 C 1s and Se LMM • There is not much C around – broad peak is from Se LMM Auger line (C 1s should be sharper) • Polishing with sand paper removes most of C-O atmospheric contamination, but not all. It is also obvious form useless UPS spectra (few slides down) SnZrSe3

  6. Core level spectra of dopands • Both Bi and Sb most intense peaks (arrows) co-inside with peaks of the other elements (upper panel), but I subtracted the backgrounds (lower panel). • Signals of Bi and Sb are obvious for the 5% doping less clear for 2% doping Sb 3d and O 1s Sb doping Bi 4f and Se 3p Bi doping Sb 5% Bi 5% Sb 2% Bi 2% SnZrSe3 SnZrSe3 Sb 3d Bi 4f Sb doping Bi doping Sb 5% Bi 5% Sb 2% Bi 2%

  7. Composition • The atomic % of the elements is obtained from the integrated area under the photoelectron peaks from XPS. The accuracy of this method is 2-3 at%. • Table 2 is just different interpretations of the data (Table1) • In all samples Se/(Zr+Bi)=3.0…3.5, all the samples are Se-rich • Sn/Zr fluctuates. Is it related to Sn low melting T? Need to be controlled better if we want reliable transport properties • Sb-doped samples and 2%Bi sample have Sn/Zr=1. The two other samples are Sn-rich (Sn/Zr=1.5) • Bi/Zr and Sb/Sn are consistent with 2% and 5% doping of the samples. • O/Zr=2 in most samples. It is likely that ZrO2 is present at the surfaces (compare to Ba/O=1 in BaCuSeF where BaO might be present) • Anion/Cation=2-2.5 (should be 1.5 in SnZrSe3). The surfaces are oxidized.

  8. Valence band spectra and band bending • EF-EVBM is the same for all doping levels – the Fermi level at the surface is pinned at 0.65 eV above VB – almost in the middle of the gap. This agrees with no shift in core levels. • VBM shape compares qualitatively well with the DFT predictions for DOS in SnZrS3. Calculate DOS for SnZrSe3 • Assume that the Bi and Sb substitute for Sn or Zr and cause EF in the bulk to shift towards CBM (this might not be the case if the Fermi level is pinned in the bulk by the defects) • Under these assumptions, the bands of SnZrSe3 must bend up towards the surface (see figures below) Sb 5% Sb 2% Bi 5% Bi 2% SnZrSe3 Surface Surface Surface 2% Doped Bulk 5% Doped Bulk Not doped Bulk 0.65 eV 0.65 eV 0.65 eV 0.7 eV 0.7 eV 0.7 eV

  9. UPS VB spectra and secondary electron edge • Totally useless • UPS is much more surface sensitive compared to XPS, because the kinetic energy of the escaping electrons is lower • The 1/x background slope in UPS spectra at high BE – secondary electrons • The distance between the secondary electron edge (16.7) and HeI energy (21.2 eV) is a work function • WF is 4.5 eV for all samples – characteristic for organic atmospheric contaminants • The EF-EVBM is 1.0-1.2 eV – this is also quite typical for organic molecules • Two broad peaks in the VB UPS spectra correspond to HOMO and some deeper state in organic atmospheric contaminants Full UPS spectra WF=4.5 eV Sb 5% Bi 5% Bi 2% SnZrSe3 VB UPS spectra 1.0-1.2 eV Sb 5% Bi 5% Bi 2% SnZrSe3

  10. Comparison of SnZrSe3 with SnSe and ZrSe2 SnSe ZrSe2 SnZrSe3 • ZrSe2: • Surface are slightly Se deficient and strongly oxidized • Se/Zr=1.8; O/Zr=1.8; Se/O=1 • SnSe: • Surfaces are slightly Se deficient and slightly oxidized • Overall surfaces are anion-rich – oxidation • Very many Se Auger lines in the spectra… bad element

  11. Delete bad datapoints

  12. Sn 3d Zr 3p • Degree of oxidation of Sn is surface-related and not conclusive based on SnZrSe3 results • Zr oxidized the same in ZrSe2 and SnZrSe3 • Se oxidizes more in ZrSe2 • Amount of oxygen is ZrSe2>SnZrSe3>Se. • It correlates with the reactivity of the elements with O. • Also it correlates with the nominal anion/cation ratio: 2>1.5>1 Zr-O Sn-O Comparison of core peaks of SnZrSe3, SnSe and ZrSe2 ZrSe2 SnSe SnZrSe3 SnZrSe3 Se 3d SnSe O 1s Se-O SnSe ZrSe2 ZrSe2 SnZrSe3 SnZrSe3

  13. (b) CB 1 eV SnSe EF Valence band spectra of SnZrSe3, SnSe and ZrSe2 0.36 eV VB (c) CB ZrSe2 EF 1.2 eV 0.9 eV SnZrSe3 VB (d) CB 1 eV..? EF 0.64 eV VB • EF-VBM is different in 3 materials: Based on the known band gaps, the surfaces are: for SnSe is p-type; for ZrSe2 is n-type (less n-type than expected though); for SnZrSe3 EF is in the middle of the gap. Might be nice for i-absorber in pin cell if the bulk Fermi level is the same • d-states in ZrSe2 VB are obvious. Need DFT DOS to assign the features of the VB – calculate it.

  14. SnZrS3 grinded in air and in a glove-bag air bag • All the peaks in the survey spectra are due to either photoemission or Auger electrons from Sn, Zr, S, O and C. • There is no Si after polishing with SiC sand paper • Related thoughts: Sn, Zr and Te have similar atomic weight. Might be easy to make SnZrTe3 films by sputtering

  15. SnZrS3 surface composition • The atomic % of the elements is obtained from the integrated area under the photoelectron peaks from XPS. The accuracy of this method is 2-3 at%. • Table 2 is just different interpretations of the data (Table1) • Sn/Zr=1 – nice! What is the difference in synthesis procedure compared to SnZrSe3? • S/O=2, just like Se/O in SnZrSe3; • S/Zr<3 and ZrS2 impurity is present – if add extra S in synthesis, will get phase-pure SnZrS3. • Anion/cation>1.5 – oxidized surface. • O/Sn=1 – SnO on the surface? That would be nice to have…

  16. SnZrS3 spectra of core levels Sn 3d Zr 3p • Sn is less oxidized than in SnZrSe3 • Zr is oxidized the same way as in SnZrSe3 • Oxidation shoulders of Sn and Se peaks are the same for the glove bag and air samples – because of the same surface preparation with sand-paper • There is less oxygen signal for the glove bag sample, hence higher oxygen 1s signal should come from the grinding in the air • Grind in glovebags!!!! Zr-O Sn-O air air bag bag O 1s O-C and O-Zr/S/Sn S-O..? air air bag bag

  17. SnZrS3 valence band spectra air bag • EF-EVBM is the same for both samples – EF is 1.2 eV above VB – closer to the CB. Why is it p-type than according to Seebeck? • EF-EVBM=1.2 eV is much larger compared to 0.65 eV in SnZrSe3 • Fixed EF agrees with no shift in core levels. • VBM shape agrees well with the DFT predictions for DOS in SnZrS3. Calculate DOS deeper. • Assume that SnZrS3 is p-type in bulk (if Seebeck measurements are bulk-sensitive). • Under these assumptions, the bands of SnZrS3 must bend down towards the surface. (b) Bulk Surface 1.4 eV 1.2 eV

  18. SnZrS3 EPMA - AR • Delete bad datapoints

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