of Tin-dopedBismuth Telluride-based
Propertiesof the Electron and PhononSub-systems
Solid Solutions
M. K. Zhitinskava',S. A. Nemov',M. Yu. Nikulina',
r. e. sveciliiouu'.E. Mtill#. D. Platzekl
42-2-99,5t.Petersburg,194356
1 . StatePolltechnicalUniversity,pr.Lunacharskogo
Russia,e-mail:nym@online.
ru
Moscow,Russia
2. A.A.BaikovInstituteof MetallurgyandMaterialsSciences,
-t- GermanAerospaceCenter(DLR), lnstitute of MaterialsResearch,
Kdln, Germaly
Abstract
We report the results of an experimentalstudy of the thermal conductivity and other basic transport
coefficients:electrical, Hall, Nernst-Ettingshausen
and Seebeckcoefficients and their anisotropy in Bi2,Sn Te3-rSe,single crystalsdopedwith Sn. All crystallswere grown by the Czochralskitechnique.The pq?e samplesshow similar peculiaritiesin the temperaturedependenceof tlle transport coefficientsthat
can be explainedby the presenceof resonantstatesof Sn. lmproved thermoelectricpropertiesof n-type
Bi2-,Sn"Te3-rSe,
werefound:a maximumof Z:3.3'10""K' at 340-370K whichis a largervaluecompared
to solid solutionswithout Sn. We observeda different influenceof Sn atomson the electronand phonon
sub-systems:an increaseof the thermal resistivity, whereasthe electronmobility doesn't changesimultaneously.Moreover, improved homogeneityof the carrier concentrationdistribution was detectedby
locally resolvingmeasurementofthe Seebeckcoefficient by the scanningthermo-probeon thesecrystals.
The obtainedexperimentaldatasupportto the point ofview that the incorporationofSn atomsinto Bi2Te3,Se, is accompaniedby the formation of a band of Sn statesagainstthe backgromd of the allowed
valence-band
spectrum
that
interact
with
basic
electron
states
ofthesecompounds.
obtain material with high homogeneity.one of them
Introduction
is to usethe Czochralskitechnique[6]. Anotherway
Traditionalapplicationof Bi2Te3-based
solid soluis to use doping impurities (Sn) creating resonant
tions in thermoelectricconvertersgives rise to look
states in the compound. We used both of these
for methodsto improve theil properties.To create
approachesin our investigation.
materialshaviag a high thermoelectricpropertiesit is
It is known that the thermal conductiyity is the
used the doping of these materials with various
important parameter in the expression for the
impuities. But the addition of impurities usually
thermoelectricfigure of merit. So an experimental
leads to a significant increase in fluctuations of
study on the influence of Sn doping on the thermal
thermoelectricpropertiesthat is associatedwith the
conductivity of. crystals lattices is important an
statistical character of the spatial impurity
object. Thecrystallatticeofbismuthtellurideaswell
distribution.
as the crystal lattices of lead chalcogenides,have a
According [l-3] it was establishedthat on
high polarizability. So the influence of irnpurities on
introducing Sn impurity into Bi2Te3quasilocalstates
the lattice thermal conductivity dependsstrongly on
appearwithin the valenceband near of the top of the
their charge state. The effective phonon scattering
additional extremum. The presenceof resonant
cross-section <D of charged impurities in tlese
states provides the positive effect of producing
compounds is several fimes greater than that of
crystals with high electrical homogeneity.Resonant
neutralimpurities[5]. This fact we will use in this
stateslead to pinning of the Fermi level. Since the
work to elucidatethe chargestateof an tin impurity
thermoelectricpower is sensitiveto variation of the
and to veri! the model of quasilocalimpudty states
Fermi level, the resonant states lead to its
in Bi2Te3basedsolidsolutions.
stabilizationand to enharcementof the homogeneity
Experiment
that was testified by investigationsof the dishibution
All single crystals were grown by Czochralski
of the Seebeckcoefficient on the swface of crystals
technique with replenishmentof the melt from a
with scanningmicroprobe[3] and by experienceson
liquid phase [6]. The single crystals were grown in
microrentgenoshucture analyses l4]. This is
the [1010] direction normal to the main
particularly important for crystals of the AvBvI
crystallographicaxis c. Samplesfor measurement
had
trigonal systemthat exhibit a pronouncedanisotropy.
different composition:1. samplesof BirTe3doped
Under real grorth conditions, even the best single
only with tin were describedby the chemicalformula
crystals of this compoundcontain a large amountof
Bi2-*SnTe3,werex : 0; 0.002;0.004;0.005;0.007;
inhomogeneities.It is known that the thermoelectric
0.01 and 0.02 ( x : 0.01 corresponds
to a carrier
figure of merit is higher in homogeneouscrystals.
density of 6.l0iecm-3);2. samplesBi2Te3codoped
Hence, different possibilities may be consideredto
with tin and iodine havethe formula of Bi2-*SnTe3+
ySbJ3,wherex : 0.005;0.01;0.02andy : 0.05;0.1;
0.15 wtYo;3. samplesBi2Te3codopedwith tin and
lead have the formula Bir-*",SnJb,Tq (x = 0.005;
0.01; 0.02 and z = 0.005;0.01; 0.02; 0.03);4.
samplesof solid solution p-Bi2-,SnTe3-"Sgwith x :
0; 0.01 and y : 0.06 and 0.12; 5. samplesof solid
solutionn - Bi2-,SnTe3-rSe"
with x : 0; 0.002;0.004
and y:0.15. The tin contentwas determinedby
plasmaatomic-absorptionspectroscopy.
Measurementsof thernal conductivity were carried
out at room temperatureusing a normal kind of
system. As well as thermal conductivity K1r we
measuredthe following independentcomponentsof
the transport tensors: the Hall coefficient R321,the
thermoelectric power S11 and the elechical
conductivity or. In this notation the number 3
indicates the trigonal axis of a crystal. The
measurementsof transport coefficients were carried
'17
out mainly in the temperaturerange - 420 K. T\e
carrier density was determinedfrom the expression
-'.
n, p = lq Rzzt(11K)l The lattice resistivity Wl was
determined from the lattice contribution to the
thermalconductivity
-'
W : xl : ( ( - K")-'. The elecfonic componentof
the tlermal conductivity was calculatedfrom formula
r<. = (ko/e)'LcT, whereL is the Lorenz number.Thw
Lorenz numberwas calculatedtaking accountof the
parabolic electron spectmm
and dominant
contribution from acoustic phonons to electron
scatteringat room temperature.
The spatialdistribution of thermoelectricpower over
the surface of Bi2Te3-,Se,:Snsingle crystals was
investigated by a 30 pm-resolution microprobe
methoddescribedin [7]. The main advantagesofthis
technique are the simple measuringconditions and
high lateral resolution. In this method the applied
temperaturedifference was 3-5 K. The accuracyof
thermoelectricpower measurementswas better than
1o/o.
Bi2Te3
Investigationsof thermal conductivity, carried oul on
specimensof Bi2Te3,doped only with tin [8],
showed that the additional lattice scattering per
impurity atom is 3-4 times less than for an iodine
impwity. It can be assumedthat this is due to the
absenceof electrical charge on the majority of Sn
atoms.The exceptionis the part of curve at a light
guantity Sn. At Nso: 0.2 at. o/othe thermalresistivity
W, increasesthe samemanneras for scatteringon a
chargeimpurities.May be,itis dueto the light partof
Snatomsplaceson the Te''' positions,wheretheyare
elechically active. As the concentration of tin is
increased, the thermal resistivity decreasesup to
value of undopedBizTe:. Probably.the Sn atoms
praceson ue le' posruonsaner n mg ol le'-'.
Accordinglyconclusionof [2], atomsSn substituting
for Te''' in Bi'Te" createa resonantstates.There are
atomsSn neutralrelatively of lattice at this positions.
The results of measurements
of thermoconductivity
support this assumption.Thetlermal resistivity of
lattice follows the low of phonon scattering on
neutralimpurity
(Fie. l).
t--- rGn'l
E
v
ll
5
| ^ dt/v\rs' I
I x dwi/vrtt I
L ^ d ' . w i nI
- o
5
0
a
2
q
4
s
tlsn,mol%
q
E
r
c
Fig.l. Dependences
ofthe thermalconductivrtyr"o, (1)
andrelative valueofadditional latticethermalresistivity
dWWr (2-4) BizTe3 on the amountof dopedimpurity:
1,2-Sn; 3 chargedI impurity,4- neutralSe
impurb (3,4 - aaxsne pa6orn [?].
Results on the additional lattice resistivity W1 are
more interesting. Consider the change of lattice
thermalresistivity by codopingBi2Te3with tin and
with acceptor@b) or donon (I, Cl). As seen from
Fig.2 W1 is practically constant at codoping with
acceptor,but wl increasessharply at codoping with
donor imouriw.
* 2
* 3
I
E
=
* 5
* 6
a 7
D 8
r 9
r 1 0
G
=
N
-mol.%
Fig.2. Dependencesof lattice thermal resistivity W
of BizTe: ( 1-6) and BirTe3-rSe"(7- I 0) on the arnount
of dopedimpurity: only Sn (1-3,7); without Sq only
with acceptorB(Pb) (8) or with donor I (10); and
codopedwith Sn ard acceptor (4) or with Sn and
donor(5-6,9).
We shall analyze the Wr : f (Ni.p) dependence
obhined. We will considerthat the changein W1at
doping with donorsand acceptors is associatedwith
featuresof phonon scatteringin BirTe3 at additional
doping. Then tlte phonon scatteringcross-sectionO
can be estimated from results on the thermal
conductivityusingthe loffe's formula
(I)
t r ' t = W r / W o : 1+ ( N / N o ) O ( l o / l )
where N is the impurity concentation, No is the
numberof atomsin the materialper cmr, a is nearest
neighbordistance,lois the phononmeanfree path h
the impudty - free crystal, O is the coefficient in tBectr
relation S : @a' ( S is the phononscatteringcrosssectionof an impurity), k and lq , Wr ard W0are the
lattice tlermal conductivity andresistivity in a crystal
with and without impurity, respecrively.
It tum out tiat the value of O is approximately
constantand equal to 1.3 over tJle whole area of
additional doping Bi2Te3:Snwith aceptor impurity
(atoms Pb) and at codoping with donor impurity
(atomsCl or I) up to 0.5 mol.o/o.
At Ni.D : ( 0.5- I )
mol.7o phonon scatteringcross-sectionincreasesup
to O - 8. We note that the valuesof O, obtainedin
this worlg agreewell with literaturevaluesfor Bi2Te3
for phonon scattering on neutral and charged
impurities.
The result obtainedare similar to tle dataobservedin
PbTe:Tl at additional doping with Na The impurity
Tl producesthe resonantstatesin valenceband of
PbTe, and a strong acceptor impurity Na give
possibility to emptythesestatesandto take out Fermi
level from tle zone of resonant states [9]. The
likenessof the experimentalresults obtainedin this
work on Bi2Tq:Sn lies in the fact that weak
dependence
W1&om additionalimpurity, whenFermi
level is in a mne of resonantstates:this is a zone of
relatively stability of hole concentrationand strong
dependenceW; , when Fernri level is out a zone of
resonantstates.
Electricalproperti€s
p - type
A resonancestatesin the valencebandofBi2Te3are
leadingto modifting the temperaturedependences
of
tle aansportcoefficientson, R32r,,Srlin these
crystalsdrastically,similarly to asit wasreporfedfor
Bi2Te3.
We canseethis from the experimentaldatashownin
Fig. 4, We observedthe following changes:
l. The temperaturedependences.R(l)ofthe samples
containingSn changeAom a rising pattemwith a
maximumtypical for Bi2Te3andBi2Te3-,Se,
to a
saonglydecreasing
curve(Fig.l)
2. The Seebeckcoefficientfor tin containing
samplesis somewhatlargerin the temperatureregion
at T400 K andat T>380 K andtheir deoendence
becomesmoregentlyslopingwith tempirarure
(Fie.4).
3.Considerablefalloff of the electroconductivityo
(hole mobility) andNernst is observed(Fig.4 a,b)
n - type
The doping with tin n-type solid solutiondoesnot change
the characterofthe kinetic coeffrcients.But the anisotropy
of the Hall and Nemst coefficientsare reducedin Sn
doping samples. Consequently, inhomogeneity in the
distributionofthe dopingdefectsis reducedwhich resultsin a
high electricalhomogeneity.
BitTe2.E5seo.t
5
At rcplace of the part of Te atoms on Se atoms in
Bi?Te3, doped with Sn, we watched the same
electrical properties as Bi2Te3:Sn.For example, we
can see on the Fig.3 the dependencesof hole
concentration(p) on of doped Sn impurity amount.
TherearepresenteddataofBizT% andofBi2Te3-ysey,
doped only with donor I(Cl)
or acceptor Pb
impurities and codopedwith theseimpurities and Sn.
We observethe low changesof hole concentrationin
tin-dopedcrystals.And we can seedifferent action of
donor impurities I (Cl) atoms at presenceSn and
without it. The electronconcentrationchansesmuch
smaller
presence
a) p-type
-l.
sl
__-r--
r
'
-q 0,4
E_g
o.:
'
"*:.,,*.
Sn.
Fig.3. The dependenceofhole concentration@1)
againstof impurity concentration(N 6") of BirTe3(1-7) and
Bi2Te3ySE(8-10):only Sn (1,8); without Sn, only acceptor
Bi,Pb (2) or donor I (3,9) impurity; and codopedwith Sn and
acceptor(4) or donor(5-7,10)impurity.
b) n-type
Fig.4 a) &2r : f (T) for p-q?e Bi2Te3(l), doped
with Sn (2),underreplace Sb --- Bi (3,a) and under
replace Se --- Te (5,6); open symbol - without Sn,
frrll ryrnbot - dopedwith Sn. b): R321 : f(T) for nt)?e Bi2Te3-ysey
with different electronconcentration.
s
0
1
&
6
:
E
)
O
&
IK
IK
Fig.5a.Sij : f (T) for p-typeBirTe3(l), dopedwitl
Sn (2), rmder replace Sb ---' Bi (3,4) and under
replace Se -- Te (5,6); open symbol - without Sn,
Fig.5.bSij : f (T) for n-typeBi2Te3(l), dopedwith
Sn andwith different electronconcenfation.
tull
symbol- dopedwith Sn.
70001
6000
5000
4000
3000
2000
IU
1000
t
1 r
{-5
\
1
2
o 3
4-2
i
i-6
"l
\\\
\
J=.-La-
100
200
TK
J-.,
+
0
300
li*
400
Fig.6a, Electroconductivityfor p-type BizTe3(1),
dopedwith Sn (2), underreplaceSb--r Bi (3,4) and
under
replace Se --- Te (5,6); open symbol without Sn,full symbol- dopedwith Sn.
T,K
Fig.6b Electroconductivity for n-type Bi2Te3 (l),
dopedwith Sn (2), under replaceSb -"+Bi (3,4) and
under
replace Se --- Te (5,6); open symbol
without Sn,full symbol- dopedwith Sn.
IK
Fig 7a. The components of tensor NemstEttingshauseneffect Qft= f (T) for p-tlpe: Q321- I
- 3 without Sn
and Qp3- 2 for crystal with Sn; Q123
Fig 7b. The components of tensor NemstEttingshauseneffect Qijr: f (T) for n-type: Q321- I
- 3 without Sn
andQ13- 2 for crystalwith Sn; Q123
The spatialSeeb€ckcoef{icientdistribution.
A resonancestatesin the valencebandof Bi2Te3are
leading to the pinning of the Fermi level, to
modiling the tempemture dependence of the
electrical properties and to exfeme elecfrical
homogeneity[3]. Similarbehavioris observedin pq?e BizTe3-ySE[10]. However, in spite of the
absenceof any unusual transport effect in n-type
ofnBi2Te3-"SE,
but extremeelectricalhomogeneity
t)?e crystalshasbeenfould aswell [11] Fig.8.
electricalhomogeneityof single crystalsBizTe: and
Bi2Te3-ysey
arguefor essentialinJluenceof Sn
impurity states,placed within the valence band, on
thermoelectricpropertiesboth on p-type and n-type
crystals. It should be remarked that thermoelectric
parametersofn-q?e crystalsare moreimprovedand
reachesa valueofZ : 3.3*10- K-' in thetemDeratue
mnseof340 - 370K.
.
,t
Fig. 8. The abundanceplot of the Seebeckvaluescontained
in the 2D Seebeckscanof p-type(on the left) and
n-t)?e (on the right) Bi?Te2.s8seo.rz
without Sn(upper)
ard Bil eestuorTe2.sSe6
12dopedwith Sn(atthebottom).
We can see extremely sharp distribution peak, indicating
extremehomogeneityin the samplesdopedwith Sn.
Tin is supporting improved crystallinity of Bi2Tq-"Se,
thus suppressingthe formation of scatteringcentersand
improving the electron mobility in n-type solid solution.
Sn addition leads to a decreaseof the lattice
tlrermal
conductivity.
On the Fig.2 are also presentedthe results Wr for
Bi2Te3-"Sqdopedandundopedwith Sn togetherwith
data for Bi2Te3. W1 for Bi2Te3-rSe"is practically
constantai codopingwith acceptor,but W1increases
sharply at codoping with donor impurity the same
way asfor Bi2Te3.
We proposethat there are only small part of atoms
Sn chargedin p type solid solution.On the whole Sn
atoms are neutral. So a phononesare scatteringon
neutral impurities and W1 depends from amount
impurity weakly. In the contrary,therearethe change
of charge states of atoms Sn takes place under
influenceof interactionwith atoms iodine in n-q?e
crystals.And the mostpart of Sn and all atomsiodine
are charged in. A phonons are scattering on tle
chargedimpuritiesstrongly.
Conclusion
The resultsobtainedin this work on the influence of
impurity Sn on lattice thermoconductivity ard on
:"*?a i:;-
I
|
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