Electrical Properties of a Synthetic Pyrite FeS2 Non Stoichiometric Crystal

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Electrical Properties of a Synthetic Pyrite FeS2 Non Stoichiometric Crystal

M. Morsli, A. Bonnet, Linda Cattin, A. Conan, S. Fiechter

To cite this version:

M. Morsli, A. Bonnet, Linda Cattin, A. Conan, S. Fiechter. Electrical Properties of a Synthetic

Pyrite FeS2 Non Stoichiometric Crystal. Journal de Physique I, EDP Sciences, 1995, 5 (6), pp.699-

705. �10.1051/jp1:1995161�. �jpa-00247095�

J.

Phys.

I France 5

(1995)

699-705 JUNE

1995,

PAGE 699

Classification

Physics

Abstracts

72.20Dp

72.20Pa 71.25Rk 71.30+h

Electrical Properties of

_a

Synthetic Pyrite FeS2 Non Stoichiometric Crystal

M.

Morsli(~),

A.

Bonnet(~),

L.

Carlin(~),

A.

Conan(~)

and S.

Fiechter(~)

(~) Laboratoire de

Physique

des Matériaux _pour

l'Electronique(*),

Faculté des Sciences et des

Techniques,

2 _rue de la

Houssinière,

44072 Nantes Cédex 03, France

(~)

Hahn Meitner Institut Berlin

Gmbh,

Postfach 390128, ^{Glienicker Strabe} 100, 14109

Berlin, Germauy

(Received

20

July

1994, ^{revised 13} December 1994,

accepted

2 March

1995)

Abstract. FeS2 attracts interest _{as a} navet

semiconducting

materai for

photovoltaic

_euergy

conversion. Electrical

conductivity

_a and thermoelectric _power

(TEP)

S have been measured in

a wide temperature _range

(80

500

K)

_{on a} FeS2-x

(~

₌

0.Il) single crystal

which _was

prepared by

_vapour transport with bromine _as transport reagent

(CVT).

Trie

experimeutal

results

are

analysed

with the

help

of _a u-type semiconductor model with _a donor level

originating

tram

(S-Br)~~

_centres. The random

potential

due to the

charged

lacunar aud impurity ^sites iuduces the

broadening

of trie donor level into

a narrow band. It is shown that the current carriers

which take part in the conductiou _processes

are

only

electrons _in the conductiou bond where

they

_are scattered by acoustical phonons and neutral

impurities

and electrons _in the bromiue

narrow band in the form of

thermally

activated

hopping.

1. Introduction

Dichalcogenides

^of transition elements

TX2

bave

recently

been trie

subject

of _an

increasing

interest

owing

to their

particular photoelectrochemical properties

which enable _a

large

vari-

ety

of _new

applications

to be

explored.

One of the

important advantages

associated with these materials _over other

chalcogenides

and III-V semiconductors is that for the former _case the

photogenerated

carriers

belong

to bands

having nonbonding

character and hence carnet

participate

in corrosion reactions

Il, _2].

FeS2

is

particularly interesting

since it consists of _{a non-toxic} and abundant element and _con be

prepared

in the form of

single crystals

and thin films. Ii

crystallizes

in the

pyrite

structure

and

is,

in nature, a

frequently occurring

minerai. It has been shown to have _a

high absorption

coefficient

(6

x

10~ cm~~

_at 800

nm),

_a

high quantum efficiency

of

90%

and _an

adequate

bond gap of 0.95 eV

[3,4).

(*)E.A.

l153

©

Les Editions de

Physique

1995

700 JOURNAL DE

PHYSIQUE

I N°6

io~l_T, r'

_

Fig.

1. Electrical

Napierian logarithrnic conductivity

variations _~ersus

10~/T.

The theoretical

curve is drawn _m fuit fine.

Its

photovoltaic properties obviously depend

_upon the band structure and the energy values involved in the electronic transfers. A

possible approach

is to

study

the behaviour of the

transport phenomena (electrical conductivity

_a, thermoelectric _power S and Hall effect

RH)

in a wide

temperature

range.

Then,

details of the band _structure of the material _near the Fermi level _can

easily

be deduced from this

study.

TO this

end,

measurements of the electrical

conductivity

_a and thermoelectrical _power S have been

performed

on a

Fesi.89 crystal prepared

by

_{a vapor}

phase transport

method

(CVT) using

bromine _as the

transport agent

[Si. ^Ail

these

expenmental

results _are discussed in terms of

impurity

and acoustical

phonon scattenng

mechanisms in the conduction band and

hoppmg

mechamsms _{m a narrow}

band,

which is located

just

below the conduction band and

originates

from bromine

and/or

lacunar sites.

Hall effect

measurements,

which have been

recently performed by

lt. Schieck et ai. [Si on

crystals being

issued from trie _same

bath,

_are fitted and have been used to scale the _carriers concentrations and

mobilities, respectively.

2.

Experimental

Procedure and Electrical Measurements

The

expenmental technique

which has been used for the electrical

conductivity

and TEP

is based upon an automatic data

acquisition system [6j.

^The

ohmicity

has been tested

by drawing

the I-V charactenstics and the

room-temperature conductivity

_is measured

by

the Van der Pauw method.

The measurements have been

performed

with _{an accuracy} of

2%

for _a and

5%

for S _on the

non-stoichiometric

FeS2 crystal.

The

expenmental

variations of

log

_a

(~~~cm~~)

and S

(/LV/K)

_uer8ics 10~

/T

_are

plotted

in

Figures

1 and 2

respectively.

We _can notice the _two

following

features the electrical

conductivity (m ^0.7~~~cm~~

at 300

K)

is

thermally

activated

(activation

_{energy m} 90

mev),

whereas the

TEP,

which is

negative,

decreases with the

temperature

^down to about -loo

/LV/K

at 300 K.

N°6 NON STOICHIOMETRIC FeS2 ELECTRICAL PROPERTIES 701

2 4 6 ₈ 10 12

10~IT K~

--

Fig.

2. Thermoelectric _power variations S _~ersics

10~/T.

Trie theoretical _curve

is drawn in fuit fine.

3. Tl~eoretical

Approacl~

The

experimental

results

reported

here _can be well fitted

by using

_a twc-baud model of _a

compensated

semiconductor. At

least,

two

"impurity" levels,

which

originale

tram lacunar and bromine

sites,

are found in the gap: a donor level

ED

which is located

just

below the conduction bond and an

accepter

^level

EA

which is located much lower than trie donor _one and is

completely

filled

by

electrons from this level.

However,

it _was

impossible

to fit

simultaneously

trie electrical

conductivity

and thermoelectric _power

experimental

results with the

only

contribution of the extended states in trie conduction baud.

Then,

_we have assumed that

stoichiometry

deviation leads _{to a}

broadening

of the donor

ED

level into _{a narrow} bond in which

thermally

activated

hopping

conduction mechanisms take

place.

Trie electrical

neutrality

is _as

usually

written _{as: n} +

NA

#

N(

where

ND, N(, _NA

_are trie concentrations of the

donor,

of the ionized donor and of the

accepter

states,

respectively.

In trie whole

temperature

_range

investigated,

the best fit for trie electrical

conductivity

and trie

thermoelectric _power is obtained

by

the summation of the

following

terms

a = an + aH and S

=

(ansn

+

aHSH) la

with

~ T ^~~

an = ne/Ln = ne/L~

To

and

Sn

₌ ^~

[fl(Ec EF)

+

2.5j

Hopping

conduction _is written in the form used to describe adiabatic

jumps

_I?1

aH "

(ND N()e/LH

with _/LH

= iLhop

l~ c(1- c) exp[-fIWH(T)j

T

where

fl

is

1/kT, To

is the _room temperature ^and c is the relative

quantity

of the

partiales

for which

hopping

_{can occur : c}

=

(ND N()/ND.

m2 JOURNAL DE

PHYSIQUE

I N°6

9

~,

E .

à

~

.

«

3

2

0~I , _-'

Fig.

3. Hall eifect

expenmental

variations of RH _~ersus 10~

/T

from Schieck et ai. [si

(the Napierian logarithm

of the absolute value of RH is plotted _as a function of the reciprocal

temperature).

Dur

theoretical _{curve is} drawn _m fui] hne.

WH(T)

which is the

hopping

_energy

presents

the

following T.dependence

_[8]:

WH(T)

=

WH

^~~~ with _x

=

fl~~°

x 4

where huJ~ is an

optical

vibration

quantum

^which has been found _to be about 35 mev.

The

expression

of the associated T.E.P. is [9j S

=

fin (fi)

_which

assumes that the width

of the band is less than _or of the order of kT.

The Hall

mobility

due to

hopping being generally

much smaller than that observed in the extended states of the conduction

band, RH

bas been therefore written in the

following

_ap-

proximation Il

₀₎

RH

" -en < /L$ >

la~

_{m -en < /Ln}

>~ la~

for

electron-phonon scattering

mechanisms and where the

subscnpt

_<> is the _mean value.

In order _to

verify

the

approximation

which has been retained for

RH,

the _term

/L( (ND N(

has been calculated at 300 K

: it is round to be two orders of

magnitude

less than the term n <

/L(

>.

However,

the

hopping

contribution _is

non-negligible

in the low

temperature

_range which

explains

the

discrepancy

observed _on the theoretical _curve

(Fig. 3)

at low

temperatures.

4. Discussion

The values of the

physical

parameters ^which

give

the best fit to the

expenmental

curves are

listed in Table I. The theoretical _{curves are} drawn in fuit fine _in

Figures

and 2. The Hall effect

measurements have been deduced from Hall

mobility

measurements

performed by

Schieck et

ai. [Si and trie fit of the Hall constant

RH

is shown in

Figure

3.

The semiconductor

Fesi.89 single crystal

is

compensated

with _a

compensation

ratio

ND INA

close _to o-à- The _presence of at least two levels _in the _gap has to be attributed _{to the existence} of bromine and lacunar sites. The donor level

ED

would

onginate

from

(S-Br)~~

centres which

replace (S-S)~~

dumbbells [Si while different kinds of

accepter

lacunar and

impurity

sites should

lead to the _occurrence of

compensated

levels

acting electrically

_{as an}

equivalent single

_accepter

level

EA

in the

temperature

range

investigated.

N°6 NON STOICHIOMETRIC FeS2 ELECTRICAL PROPERTIES 703

Table I. Varices

of physicai

constants obtained _on

Fesi.89.

Nco 5.3 x1016

N~ (cm-3) 7.7

_x

10'8

NA, (cm-3) 3.7 x1018

(eV) 80

_x

10-3

elecUons

lÀ$

(cm~/Vs) ^~°°

le 96 xl 0-3

(300 Ki (cm2/vs _o,34

It has been shown

by

transmission electron _microscopy and

X.ray powder

diffraction _per- formed _on

pyrite FeS2-x (11]

^{that the}

crystal

would net contain _a

significant population

of disorder defects which _may account for the sulfur deficit.

Consequently,

trie S-vacancies _are

proposed by

trie authors to be

homogeneously

distributed

throughout

trie lattice

and,

electron-

ically,

should lead _to the _occurrence of _a rather

high

concentration of donor defect-states in the forbidden _{gap (+~}

lo~~ cm~~

for

Fesi.8g).

However,

ii should be

kept

in mind that natural

non-stoichiometry

is known to introduce lacunar

accepter

^{levels in the} gap of _most of transition menai

dichalcogenides [12,13].

Con-

ceming FeS2,

Willeke et ai. have

prepared FeS2

films

by magnetron sputtering

which _are round _to be

p-type

with _a hale concentration

practically temperature-independent

ai about 5 _x

10~~ cm~~ [14j. Moreover,

Cu and P

impurities

bave been found in trie

FeS2 crystal

stud- ied here

by inductively coupled plasma spectrometry

[Si and could

eventually

act _as

accepter

centres

(respectively Cufe

and

Ps). Thus,

in order to

interpret

trie

high

sulfur deficit and trie low

NA

value obtained

by

trie

fit,

_a

possible explanation

would be the

following

_:

S-vacancies should lead to trie _occurrence of _a

high

concentration of donor states much lower than trie donor level

ED

in trie forbidden _gap

(or, eventually,

in trie valence

band).

trie natural

Fe-deficit,

trie

Cufe

and

Ps

centres should lead to trie _occurrence of _{a con-} centration of

acceptor

states (+~ 4 x 10~~

cm~~).

All these states _are assumed to act

together

_{as an}

equivalent single

level located much lower than trie bromine donor level.

The

Fe-vacancies,

the

Cufe

and

Ps

centres _are

fully

ionized in the whole

temperature

range

investigated

and

they participate indirectly

in trie conduction

by making possible hopping

conduction in the donor _narrow band which is

partially

filled from _{0 K.} Trie

broadening

of the

ED

level into _{a narrow} band could result from trie random

potential

of trie

fully

ionized centres.

Regarding

^{the conduction mechanisms in the conduction}

bond,

the

exponent

in

mobility (-1),

as well _as the kinetic term in thermoelectric _power

(-2.5),

mdicate _a main combination of acoustical

phonon scattenng

_{(/L +w}

T~~.~)

and a

temperature-independent

neutral

impurity scattenng

which is in

good agreement

^with trie _presence of neutral lacunar sites.

However,

trie

high

value of the _carrier

mobility

_m the conduction band is in

general agreement

with

an acoustical

phonon

and

for impurity scatterings

with _a low concentration of neutral lacunar

704 JOURNAL DE

PHYSIQUE

I N°6

sites.

Therefore,

^the

assumption

of the _presence of disorder defects _cannot be

completely

ruled off and could also

explain

the

broadening

of the

ED

level.

Electrons

participate

_m the conduction in the

ED

band

by thermally

activated

hopping

of small

polarons

with _a

polaronic

_energy

nearly equal

to 100 mev. The

mobility

_/LH is found to be about 0.3

cm~/Vs

at 300 K. That

is,

the

probability

that the

charge

carrier will follow atomic

motions _{so as} to

produce

_a

hop

_is

likely

_m and adiabatic

hopping

is mdicated

Il Si.

Trie term

WH(T)

is found to be about loo mev at 300 K which _means that the _energy of distorsion around each _centre is

greater

^{than the}

optical

vibration

quantum

and

implies strong-coupling hops.

The _term

hure

in the

T.dependence

of

WH(T)

is found

by

the fit to be

nearly equal

to 35 mev. Ii must be noticed that this value is in

good agreement

with the

energies

of

phonons

which have been

reported by

Sounsseau et ai. from Raman results [161.

Moreover,

the _energy difference

Ec ED

is found to be about 80 mev which

explains

the small electronic concentration at _room

temperature (n

+~

lo~~ cm~~). Consequently.

the

hopping

contribution to the

conductivity

is still about rive times _more than that of the extended states in this temperature _range.

For the

fitting,

the activation

energies

^which have been found _are those which _can be

directly

deduced from the

experimental

_curves. Hall effect results [Si have been used to scale the _carrier concentrations and mobilities

respectively. Therefore, only

the

mobility

and concentration ratios have been

directly

deduced from the

fit,

whereas exponents m

mobilities,

_as well _as

m kinetic terms in the thermoelectric _power, have net been allowed _to

depart

much from theoretical values.

5. Conclusion

A donor band which

originates

from the bromine

transport agent

^{has been found}

just

below the bottom of the conduction band of _a

Fesi

89

single crystal prepared by

_{a vapor}

phase _transport

method.

Ail the conduction mechanisms _can be

explamed

on the basis of _a two-band model

conduction _m the extended _states of the conduction band

thermally

activated

hopping

conduction _m the widened bromine donor level which is

partially

filled from o K.

The low concentration of acceptor ^{states which is found}

by

the fit has been related to natural Fe-vacancies and

impurity

centres.

The

good agreement

^between

expenmental

and theoretical results _{over a} wide

temperature

range

(electrical conductivity

_a and thermoelectric _power S and Hall

effect),

without

using

any

asymptotic

behaviour for the calculation of the _carrier

densities,

confirms trie

vahdity

of the

simple

model which has been retained.

References

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A.D.,

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N°6 NON STOICHIOMETRIC FeS2 ELECTRICAL PROPERTIES 705

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