Transcript Slide 1

Photoemission spectroscopy study of SnZrCh3
[Ch=S, Se] and related materials
Andriy Zakutayev
November 2009
Experimental details
• SnZrSe3, SnSe, ZrSe2 - prepared by Annette
Richard (add preparation details)
• SnZrS3 - prepared by Daniel Harada (add details)
• XPS measured by Andriy Zakutayev (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
SnZrSe3 doping (2% and 5%): Sb on Sn, Bi on Zr site
Sn
Zr
Se
O
Sb
Bi
Si
Intensity [a.u.]
Sb 5%
Sb 2%
Bi 5%
Bi 2%
SnZrSe3
Sn Auger
1200
Se Auger
1000
O Auger
800
600
binding energy [eV]
400
200
•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
0
Core level spectra of the elements
Sn-O
Sn-Se
Sb 5%
Sb 2%
Bi 5%
Bi 2%
SnZrSe3
495 490 485 480
binding energy [eV]
Intensity [a.u.]
Se 3d
Se-O
Sn-Se
and
Zr-Se
Sb 5%
Sb 2%
Bi 5%
Bi 2%
SnZrSe3
60
58 56 54 52
binding energy [eV]
50
Intensity [a.u.]
500
Zr-Se •Degree of the oxidation of
Sn is random – the
Sb 5%
difference most likely comes
Sb 2%
from not-reproducibility of
surface polishing.
Bi 5%
•Zr is oxidized similarly for all
Bi 2%
the samples – it is likely that
SnZrSe3
this indicates oxidation of Zr
in the grain boundaries
•Se peaks ratios is not
360
350
340
330
consistent with d-nature of
binding energy [eV]
the states. This means there
O-C and
O 1s
is a large Se-O component in
O-Zr/Se/Sn
the signal
Sb 5%
• O1s signal also has 2 peaks
– most likely due to
Sb 2%
difference in (1) C-O and (2)
Bi 5%
Zr-O, Se-O, Sn-O bonds
Bi 2%
•Position of the peaks does
SnZrSe3
not change as a function of
doping %. This means that
the EF-EVBM is fixed
536
532
528
Zr 3p
Intensity [a.u.]
Intensity [a.u.]
Sn 3d
Zr-O
binding energy [eV]
Zr 3d and
Se LMM
SnZrSe3
Intensity [a.u.]
Intensity [a.u.]
Other core level spectra that interfere
Intensity [a.u.]
188
184
180
176
binding energy [eV]
C 1s and
Se LMM
SnZrSe3
290 288 286 284 282 280
binding energy [eV]
Zr 4p and
Sn 4d
SnZrSe3
32
28
24
binding energy [eV]
•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
20
•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)
Sb 3d and
O 1s
Sb doping
Intensity [a.u.]
Intensity [a.u.]
Core level spectra of dopands
Sb 5%
Sb 2%
Bi 4f and
Se 3p
SnZrSe3
545 540 535 530 525 520
binding energy [eV]
Sb 5%
Sb 2%
545 540 535 530 525 520
binding energy [eV]
165 160 155 150
binding energy [eV]
Bi 4f
Sb doping
Intensity [a.u.]
Intensity [a.u.]
Sb 3d
170
Bi doping •Both Bi and Sb most
intense peaks (arrows) coinside with peaks of the
other elements (upper
panel), but I subtracted the
Bi 5%
backgrounds (lower panel).
Bi 2% •Signals of Bi and Sb are
SnZrSe3 obvious for the 5% doping
less clear for 2% doping
Bi doping
Bi 5%
Bi 2%
170
165 160 155 150
binding energy [eV]
Composition
atomic
%
Sn+Sb Zr+Bi Se
O
Bi
Sb
SnZrSe3 0.1963 0.1237 0.4405 0.2396 0.0000 0.0000
2%Bi
0.1316 0.1621 0.4714 0.2349 0.0047 0.0000
5%Bi
0.1775 0.1363 0.4532 0.2330 0.0064 0.0000
2%Sb
0.1389 0.1402 0.4332 0.2878 0.0000 0.0037
5%Sb
0.1523 0.1300 0.4268 0.2909 0.0000 0.0078
(Sn+Sb) Anion/ Se/
O/
Bi/
Sb/
Ratios /(Zr+Bi) Cation (Zr+Bi) (Zr+Bi) (Zr+Bi) (Sn+Sb)
SnZrSe3 1.5877 2.1253 3.5622 1.9373 0.0000 0.0000
2%Bi
0.8119 2.4051 2.9084 1.4494 0.0291 0.0000
5%Bi
1.3028 2.1866 3.3257 1.7096 0.0469 0.0000
2%Sb
0.9908 2.5840 3.0910 2.0531 0.0000 0.0264
5%Sb
1.1710 2.5427 3.2828 2.2375 0.0000 0.0511
•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.
Valence band spectra and band bending
Intensity [a.u.]
Sb 5%
Sb 2%
Bi 5%
Bi 2%
SnZrSe3
20
15
10
5
0
binding energy [eV]
Surface
2% Doped Bulk
Not doped Bulk
0.7 eV
•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)
Surface
Surface
0.65 eV
0.7 eV
5% Doped Bulk
0.65 eV
0.7 eV
0.65 eV
UPS VB spectra and secondary electron edge
Intensity [a.u.]
WF=4.5 eV
Intensity [a.u.]
20
6
5
•Totally useless
Full UPS spectra
•UPS is much more surface
Sb 5%
sensitive compared to XPS,
because the kinetic energy of
Bi 5%
the escaping electrons is lower
•The 1/x background slope in
Bi 2%
UPS spectra at high BE –
secondary electrons
SnZrSe3 •The distance between the
secondary electron edge
15
10
5
0 (16.7) and HeI energy (21.2
binding energy [eV]
eV) is a work function
•WF is 4.5 eV for all samples –
VB UPS spectra
characteristic for organic
atmospheric contaminants
•The EF-EVBM is 1.0-1.2 eV –
1.0-1.2 eV
this is also quite typical for
Sb 5% organic molecules
Bi 5% •Two broad peaks in the VB
Bi 2% UPS spectra correspond to
SnZrSe3HOMO and some deeper state
in organic atmospheric
4
3
2
1
0
contaminants
binding energy [eV]
Comparison of SnZrSe3 with SnSe and ZrSe2
Intensity [a.u.]
Sn
Zr
Se
O
Si
O Auger
Sn Auger
Se Auger
SnSe
ZrSe2
SnZrSe3
1200
1000
at % Sn
Zr
Se
O
SnSe 0.4706 0.0000 0.3750 0.1544
ZrSe2 0.0000 0.2174 0.3925 0.3902
(Se+O)
ratios Se/M /M
Se/O O/M
SnSe 0.7970 1.1251 2.4288 0.3281
ZrSe2 1.8058 3.6008 1.0060 1.7951
800
600
binding energy [eV]
ZrSe2:
400
200
•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
0
• Delete bad datapoints
LINE
Average
STDEV
O AT% Zr AT% Se AT% Sn AT% TOTAL
19.602 -0.01895 37.22513
43.117 94.6173
14.94924 0.031107 11.07834 4.595691 7.861138
LINE
Average
STDEV
O AT% Zr AT% Se AT% Sn AT% TOTAL
10.57689 29.41728 59.93863 0.000214 91.24688
2.819077 2.041569 3.543106 0.012913 2.572845
LINE
Average
STDEV
O AT% Zr AT% Se AT% Sn AT%
18.12335
25.935 55.87345 0.000992
8.61367 5.546111 13.59622 0.008969
LINE
Average
STDEV
O AT% Zr AT% Se AT% Sn AT%
16.58395 17.71895 55.08005 10.51583
6.522282 4.192083 21.72282 11.05346
Comparison of core peaks of SnZrSe3, SnSe and ZrSe2
Intensity [a.u.]
Intensity [a.u.]
Sn 3d
Sn-O
SnSe
SnZrSe3
500
495 490 485 480
binding energy [eV]
SnSe
Intensity [a.u.]
Intensity [a.u.]
Se 3d
Se-O
ZrSe2
58 56 54 52
binding energy [eV]
ZrSe2
SnZrSe3
SnZrSe3
60
Zr 3p •Degree of oxidation of Sn is
surface-related and not
conclusive based on SnZrSe3
results
Zr-O
•Zr oxidized the same in
ZrSe2
ZrSe2 and SnZrSe3
•Se oxidizes more in ZrSe2
SnZrSe3
•Amount of oxygen is
ZrSe2>SnZrSe3>Se.
•It correlates with the
360
350
340
330
reactivity of the elements
binding energy [eV]
SnSe
O 1s with O.
•Also it correlates with the
nominal anion/cation ratio:
2>1.5>1
50
536
532
528
binding energy [eV]
Valence band spectra of SnZrSe3, SnSe and ZrSe2
(b)
0.36 eV
Intensity [a.u.]
SnSe
CB
1 eV
EF
0.36 eV
(c)
CB
0.9 eV
ZrSe2
1.2 eV
SnZrSe3
0.64 eV
VB
EF
0.9 eV
VB
CB
(d)
1 eV..?
EF
20
15
10
5
binding energy [eV]
0
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.
SnZrS3 grinded in air and in a glove-bag
Intensity [a.u.]
Sn
Zr
S
C
O
Si
Sn Auger
O Auger
air
bag
1200
1000
800
600
binding energy [eV]
400
200
•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
0
SnZrS3 surface composition
At.%
Ar
air
O 1s
Zr 3p3 Sn 3d5 S 2p
0.19
0.17
0.18
0.46
0.21
0.16
0.19
0.43
O 1s
Zr 3p3 Sn 3d5 S 2p
Ratios An./Cat. O/Sn
Sn/Zr S/Zr
Ar
1.86
1.08
1.07
2.70
air
1.79
1.08
1.19
2.63
•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…
SnZrS3 spectra of core levels
Zr 3p
Sn-O
air
Intensity [a.u.]
Intensity [a.u.]
Sn 3d
Zr-O
air
495 490 485 480 360
350
340
330
binding energy [eV]
binding energy [eV]
S-O..?
air
Intensity [a.u.]
O 1s
Intensity [a.u.]
•Zr is oxidized the same way
as in SnZrSe3
bag
bag
500
•Sn is less oxidized than in
SnZrSe3
bag
170 168 166 164 162 160
binding energy [eV]
O-C and
O-Zr/S/Sn
air
bag
•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!!!!
536
532
528
binding energy [eV]
SnZrS3 valence band spectra
Intensity [a.u.]
EF-EVBM=1.2eV
air
bag
20
(b)
Bulk
1.4 eV
15
10
5
binding energy [eV]
0
•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?
Surface •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
1.2 eV bulk-sensitive).
•Under these assumptions, the bands of SnZrS3 must bend down
towards the surface.
SnZrS3 EPMA - AR
• Delete bad datapoints
LINE
Average
STDEV
O AT% Zr AT% Sn AT% S AT%
TOTAL
1.658943 20.06218 21.61608 56.63195 83.17313
1.980805 0.646271 0.308424 2.10158 1.736416