Structual and thermoelectric properties of undoped IV-VI epitaxial films alloyed with tin
J. Kdnig, J. Nurnus, R. Glatthaar, H. Bitttner, A. Lambrecht
Fraunhofer Institute for Physical Measurement Techniques (IPM)
Heidenhofstr. 8, 791 10 Freiburg, Germany
email: jg4qbi4lllugiplll$g.dg
Abstract
Iv-Vl-compounds
are well known materials with
favourable thermoelectric properties at elevated temperaft[es
(-700 K). Using related mixed crystals the thermoelechic
properties can be optimised due to alloy scattering which leads
to a decreaseof tJIethermal conductivity without changing the
other thermoelectric properties significantly. This method is
well known as Joffe concept. The Joffe concept is about 50
years old and successfi.rllyproved several times using bulk
crystals. But only few data exist about semiconductor thin
fihns. Here we report on stuctural (SEM-, EDx-, XRD-, FTIR-analysis) and in particular thermoelectric properties
(Seebeck coefficient and Hall-Effect measuremenlsfor canier
concentration, conductivity and mobility) of molecular beam
"Joffe-systems"
epitaxy (MBE) grown thin film based on dle
(Pb,Sn)Se and (Pb,Sn)Te. Special care was taken to evaluate
the in-plane thermal conductiyity of insulated fre€ standing
(Pb,Sn)Sethin films.
With increasing tin concenEations increasing charge
carrier concentration as well as increasing thermopower values
were observed. The bandgap decreased with increasing tin
concenhation - shifting the optimum operating temperatures
towards room temperature - and also the lattice thermal
conductivity decreasedas expected due to alloy scattedng. All
these effects strongly improve the thermoelectric properties of
Sn-alloyed IV-VI epitaxial layers in the room temperatue
region.
FOM will be determined by the dlstribution of the
crystaltographic orientations of the grains weighted wift tle
respective thermoelectric properties in the direction of the
force (t€mperature gradient for
respective driving
thermogenerators or electric current for Peltier coolers).
Although high FOM-values for V-Vl-superlattices ul were
reported up to now no devices exploiting these enhanced
matedal properties are known. For the case of polycrystalline
V-VI materials with an reduced average FOM miniaturized
Peltier coolers [2] were successfiily realized.
For IV-VI - based superlattices a high FOM has been
reported by Harman [3], however no practical devices with
this material are realised so far.
In contrast to the V-Vl-compounds IV-VI-based
compounds possess the cubic NaCl-crystal structue and
almost isofiopic physical properties. Therefore in this rxaterial
system polycrystalline thin films could be as good as - or due
to a reduced thermal conductivity becauseof grain boundary
scattering even better than - polycrystalline v-VI thin films.
An epitaxial map (energy gap vs. laftice constant) of the
lead salts compared to conmonly used substrate materials for
lead chalcogenide epitaxial gro\a'th and possible temary
components is shown in figure 1.
Introduction
For applications in the region of 300 K normally V-VI
compounds based on solid solutions of Bi2Te3, Bi2Se3 and
Sb2Te3are used. Up to now thesecompounds show the highest
figures of merit (FOM) and therefore have the best
efficiencies in thermoelectric devices designed for room
temperature use. A striking disadvantage of these V-Vlcompounds is the lamellar crystal structure leading to shong
anisotropies of alrnost all physical properties relevant to
thermoelectric (e.g. thermal and el€ctric conductivity) and thin
film gowth (e.g. linear ihennal expansion coefficient). These
anisotropies complicate the use of V-VI compounds in thin
film thermoelectric devices. Either the favourable proPerties
of single crystalline hlms are found parallel or perpendicular
to the substrate. Therefore - depending on the crystalline
orientation of the substratesused to deposit epitaxial thin filrns
- only applications exploiting in-plane (e.9. thermopiles) or
cross-plane (e.g. Peltier coolers) will show optimum
performance. In tlrc case of a polycrystalline thin fihn the
l.+ti&
an.t.nt
.
Ihml
Fig. l: Epitaxial map of the lead chalcogenides, possible
temary components and common substxatematerials.
In the fotlowing the struchral and thermoelectric
properties of undoped epitaxial Pbr-.Sn'Se and Pb1-,Sn*Te
thin films grown by MBE are be investigated.
IV-VI epitaxial thin filn growth and characterisation
The investigatedfilms were grown in an EPI 930 MBE
systemequippedwith a PbS€anda PbTecompotmdsource,a
tellurium source and a seleniumcracker cell necessaryio
the Se--or Te-lossin the compoundsource.Prior
compensate
to the gro&'th1 cfif BaF2substrateswerefreshly cleavedand
fixed to the substrateholder using an InGa eutectic.In the
load chamber the substrates\ /ere heated to 180'C for
18 minutesto remove water from the subshatesurface.An
additionalthermalcleaningstep (425 'C for 25 minutes)was
donein the grox'tl chamber,Thenthe substrateswerecooled
down to grora{h temperature of 370'C (thermocouple
reading).The flux ratiosof the PbSe(PbTe) andSe(Te) were
chosenas 10:1 to guaranteea sufiicient stabilization.The
gro\trthratewasabout1.4pm per hour.
were used for tlte
X-ray diffractometry-measurernents
determination of lasice constants and O/2Gscans are
(figure
performedfor phaseanalysisfor the Pb1-,Sn*SeJayers
2). In Figure 3 lhe decreaseof the lattice constanta0 with
increasinstin contentin PbSnSeis shown.
.q,
E 10'
';
g
,",
-
)t
70
80
90
2s l'1
Fig. 2; X-ray diffraction @/2@scan (Cu Kar) of a
on a (111) BaF2substrat€measued at
Pb6.eaSn6.q6Se-filn
Fraunhofer-lAF.
?
szsH
E
g
.9
,?5E
o
g
I
8
bulk PbS€
a
a
Fig. 3: Lattice constantao and full width at half maximum
(FWHM; of PbSnSefilrns on BaF2(l1l) at 300K det€rmined
by X-ray difftaction rneasurementsversus Sn-contentin
data[15], [16].
comparison
with literature
After the gro*th the layerswereinvestigatedusingFomier
transforminftared specfoscopy(FTIR) in order to determine
of the absorptionedgeof the Pbr-"Sn"Seand
tlrc dependence
Pb1,"Sn"Telayers on the tin concentration.Figure 4 shows
measuredinfrared transmissionspectra of the Pbl-*Sn*Se
layers and of an uncoated BaF2 substrate.Besides the
t)
absorptiorsdue to BaFz(- 700 cm the interferencefringes
with an almost equidistantspacingdo = n d/2 (n = reftactive
index, d = frlm thickness)can be seenclearly. Knowing the
refractiveindex of the thin filn|, the film thicknesscantllus be
d€terminedin principle from the FTIR spectrum.Since the
reftactiveindex of the filrns dependson the Sn concentration
the film thicknesswere determinedby secondaryelecton
microscopy(SEM) as shown in figure 5. Besidesthe film
thicknessalso surfacemorphologywas determinedby SEM
wasinvestigatedusingEDX up
analysis.The Sn-concentration
to frs
at% accuacy. Using these Sn-cont€nts the
compositionsof the sampleswerecalculated.
1.0
0.9
0.8
0.7
E 0.4
0.3
o.2
0.1
800 1000 1200 1400 1600 1&0 M
2N
24cp 2ffi
2ffi
wavenumber
[cru-']
with
samples
of
Pb1-"Sn*Se
FTIR
transmission
specEa
Fig.4:
andof an uncoatedBaFzsubstrate.
differentSn concentrations
The measuredcharge carrier concentrationsare aknost
temperatureindep€ndent,with^ increasing Sn-content the
risesup to 4.l0r8cm3 for PbSnSe(x=0.08)
concentration
cmr.
andfor PbSnTe(x = d.12)up to 6.101E
-150
I
)
=
-zm
3
3 -zso
f
l,/
-3m
of a 4.16 Fm thick Pbr-.Sn"Se
Fig.5: SEM measurement
samplegrownon a (111)BaF2substrate.
The thermopowersof the sampleswere measuredat room
a temperaturegadient
temperature.During the measurement
of 10K was applied to generatethe Seebeckvoltage. The
resultsare shownin figure 6. It is obvious,that the measued
with increasingSn-content.
thermopowervaluesdecrease
The electricalconductivitiesandchargecarier concentrations
were determined by temperatue depetrdent Hall
using the van-der-Pauwmethod,The samples
measurements
werethennally contactedto a cooled copperheat sink using
silver glue,the electricalcontactswererealizedby gold wires
solderedonto the corners of the samplesusing In-solder.
During the measuerEnt electrical curents in the region of
*5 rnA and magneticfrelds of 0.2 Tesla were used. The
measuredmobilities and chargecarrier concentrationsin the
terperature rangefrom 40 K to 320 K are shownin figure 7a
and b. With increasingSn content the mobility at room
t€mperatureincreaseswhereastlre mobility rise towardslower
derreaseswith increasingSn content.
iemperahnes
-
d
Pb",Sn;Se
Pbt' SnJe
/
o.@
0.04
0.06
o.04
0.10
0.12
Sn clntent x
Fig. 6: Room temperature tlermopower values of the
investigatedPb1-,SnSeandPbl-"Sn'Tesamples.
In order to conplete the thermoelectricproperties,in-plane
using a bridge method
thermal conductivity measurements
were performed on the PbSnSe samples.Details on the
are given elsewhere[4],
thennal conductivity measurements
t51.
10000
g
z
2io'o
E
ll
5
E
E
E
5 rxroll
8no"
5do't
100
temperaturc[K]
150
2@ 2500s0
o Pbe.e76Sn632aSe;
Fig.
7: Temperature dependentmobilities (a) and charge carrier concentrations O) r Pbq.e57Sno.oa3Se;
-Pbo.rrsoo.ors";
v
Pb9.$Sne.q62Te
a Pb6.q6Sne31aTe;
o Pb691sSne.662Te;
o Pbs376Sn6.12aTe;
' Pbq.913Sno.0E2Se;
^
For the evaluation of the measurementsan emissiyity of the
samplesof e=0.3 - independentof tlle Sn content of the
samples - was assumed. The resulting itr-plane therrnal
conductivity values are shown in figure 8. Within the
experimental error the total thermal conductivity l, decreases
with increasine Sn content of tle films.
grown under non-equilibrium conditiors significantly differ
from those single crystals.
To=3nX
aPbse SrierdL
t
.q 0.26
uJ
0.24
aPbsf- Rnvichet al,
v -!
r.l
n-PbSei
p-PbSe
'Uox"'
-+ "
1.3
1.1
1.0
-1.0 {.5
Ph
- -orr- Sn
^;'
l-horrrSno*"S"
,
Pbo.-s\r,S" i*
0.0
0.5
1.0
Sc
0.0.3--
1.5
2.o
2.5
3.O 3.5
4.0
4.5
at% Sr
Fig. 8: Measured in-ptane trermil conductivities of the
investigated Pbr "Sn Se samples. [7], [8]
Results and discussion
The absorption edge of the investigated IV-Vl-samples
decreases with increasing Sn-content @g. 9). Although the
determined absorption edges are not conected for the MossBurstein-Shift [6] - which would qualitatively result in
slightly lower energies at higher charge carrier concentrations
- a good agrcement with literafire data [9] - [13] can be
found.
It is well knowr! that materials with a band gap of E - 4 fu T
(kB: Bohzmann constant) are well suited for applications at a
temperatureT (e.9.Bi2Te3:E" - 150 mev - 6 kB'300K)
Therefore the decreased bandgap in the investigated Sn-rich
samples should result in an enhanced room te[perature
thermoelectric efficiency. It is known, that for binary PbTe
and PbSe epitaxial films the optimum charge carrier
concenfiations for high FOMs are in the region ftom
8.1018cm'3 to l'10recm3. Since on the one hand side
increasing Sn-contents result in increasing charge carrier
concenfations and on the other hand increasing Sn-contents
result in decreased band gaps, Sn has a double function in
undoped lead chalcogenide films: It reduces the band gap
and at the sane time acts as a dopand. According to phase
diagams that can be found in the literature [14], [i5] valid for
crysials grown under equilibrium conditions p-type doping is
found for Se (Te)-stabilized Pb1-*SnSe (Pbr-"SnTe). The
non-equilibrium MBE grown samples show n-type doping for
Se-stabilized thin epitaxial films. The same effect was fotmd
for undoped, Te-stabilized Pbr-.Sn'Te epitaxial films and
confirmed by thermopower and Hall-measurements.Therefore
the emplacement of Sn in our IV-VI compound thin films
,9, 0.26
Lil
o.24
PbSnTeHarman
-PbSnTeAnderson
--'... PbsnTeHewes
r
P D S nt € l . r 6 t e r
PbSnTeShani
PbSnTe this work
Fig. 9: Energy of the absorption edge of Pb1,"Sn*Seand Pb1*Sn"Te samplesrespectively in comparison with literature data
tel- t13l
Using the experimentally determined charge carrier
concentrations the electronic contibution l. of the thermal
conductivity was determined under the assumption of
parabolic energy bands. The effective masses m* used to
calculate the reduced Fermi energies and Lorentz-numbe$ L
were taken from [14]. The lattice thermal conductivities
calculated bV l,r = l,-l'" for the Pby *Sn Se thi! films are
shown in figrue 10. The effect of reduced 7,,yvalues due to
alloy scattering is obvious. For the sample with the highest Sn
a lattice thermal conductivity of
content Pbo92Sno.oESe
- 1 wmrlf
is found. Compared to tlle binary PbSe
I this corresponds to a
compounds with l,r - 1.4 WmtI(
reduction of }'l by approxirnately 30 percent.
asfor low Sncontentsin this regionZT valuesof 0.8 at 300K
are expectedfrom the exfapolateddata within the systemof
unintentionallydopedepitaxial Pbl *Sn*Setlin films.
Similar results are expected for Pbr-*SnJe. The
powerfactorfor Pb1,*Sn"Te
is higher than that for Pbr,"Sn"S€
with a comparabletin cont€nt,but the thermalconductivityis
expected to be higher too. Thermal conductivity
frJmsarein progress.
measurements
of thePb1-,Sn*Te
Further improvementsof the FOM are expectedwith
PbSnSe/PbSnSeTe- or PbSnTe/PbSnSeTe-superlasices,
similar to the results with PbSeTe-PbTe- SLt19l and
PbTeSe-PbTe
Quantum-Dot-Sl[3].
To=300
K
V r.s
h*"tS\rrr&
Pbors?sno4.3se
-
t.l
1.0
Pborrsno"rse
Pboersqirrse
0.8
0.7
-0.010.00 0.01 0.@ 0.0€ 0.(X 0.05 0.06 0-07 0.04 0.09 0.10
Sum.maryand outlook
Sn cootrat x
Fig. 10; Lattice thennal conductivities calculated from the
measuredtotal thermal conductivities by 1,1= l,-1,".
The room temperature properties of the investigated
"undoped" IV-VI
epitaxial films alloyed with tin are
sumrnarized in table I and compared with other for high FOM
optimised IV-VI thin films. Binary PbSe filrns with carrier
concentrations optimised for room temperahle (- 1 10tecn3)
have FOM < 0.3. Witi increasing Sn content the FOM of the
samples increases strongly even in these "undoped" samples.
For Pbe.elsSne.$2Sea very high value of ZT = 0.6 was
determhed.
A number of details are very promising that even higher
FOMs can be realized within those rmintentionally doped Pb1"Sn Se epitaxial films: Optimised FOMS are expected for
canier concenftations in the region of 1.101ecm-3. The
experimental data presented indicate tlat these concentrations
(s. table 1). For this
could be reached for Pbe325Sne.175Se
compound a band gap of - 100 meV - 4.kB.T - that means
almost ideal for room temperatue applications - is expected.
The extrapolated Seebeckcoefficient for this concentation is
- 135 pV/K. Finally due to the almost constant mobilities
conductivities of 1400 O'rcm'r may be anticipated. Even
though the lattice thermal conductivity is not reduced as much
n-type
Material
x
EDX
PbSnse
0-o2l
PbSnSe
o.o24
PbSnSe
PbSnSe
0.043
0.082
0.014
It has been shown, tlat epitaxial Pbr,"Sn"Seand Pbr*Sn*Tequasibinaryfilms can reachthermoelecbicfigures of
merit ashigh as0.6 at room temperature.Main reasonfor this
is the multifunctionaleffect of Sn in the thin film: Sn reduces
the band gap towards values ideal for room temperature
applications,Sn acts at the sametime as a dopandincreasing
the chargecarier concentrationtowardsl0r9 crl3 andlastbut
not least reduces the thermal conductivity due to alloy
scattedng.The emplacementmechanismsfor Sn in IV-VIbasedcompoundnon equilibriumMBE samplesis not known
up to now.
Exhapolatingthe present€dresultsshows,that ZT = 0.8 could
thin films.
be reachedwithin this systemfor Pbq.s25Sno.175Se
This value is still smaller than those known ftom standard
BizTe.:-basedV-Vl-compounds, neverthelessthe expected
isotropy of the Pbr-"Sn"Secompoundscould be a striking
advantage.Due to isotopy suchhigh - or due to enhanced
phonon scatteringat grain boundaries- even higher FOMs
should be found in polycrystallinematerials.Thereforethe
complicateddepositionof single crystalline compoundsfor
theusein high efficientthin film thermoelectricdevicescould
be overcomeby tlre materialsystempresentedin this paper.
T
-'
[w K']
Power
z:f
lpwcmK?l
1.14
10.57
16.35
0.28
t.33
20.o9
0.45
0.6
23.69
PbSnTe
9.87
23.57
PbSnTe
o.062
PbSnTe
25.54
0.082
PbSnTe
26.59
0.124
PbSe* [17]
2.87
21.0
x
PbTe [181
4.15
31.6
-SL U9l
Pbseo.rTeo.rPbTe
1.87
28.0
Table 1: Experimentaldataof Pbr-rsnrseandPbr-xsn*Se
samplesitr comparisonwith literaturedata.
x ootimisedcarrierconcentration
l.1E
0.37
0.22
0.23
0.45
Acknowledgements
This work was pardy supportedby BMBF grant no.
03N1084.
wereperformedby Dr. L. Khste
The XRD-measuements
at FraunhoferIAF.
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