Electrochemical characterization and preparation of semiconducting materials

Solid State lonics 18 & 19 (1986) 873-877 North-Holland, Amsterdam

ELECTROCHEMICAL

873

CHARACTERIZATION

AND PREPARATION

OF SEMICONDUCTING

MATERIALS

Werner WEPPNER Max-Planek-Instltut

fflr Festk6rperforsehung,

D-7000 Stuttgart

80, Fed. Rep. Germany

Important relations exist between solid state electrochemistry and semiconductor science. Three different areas of interaction are presented and illustrated by examples: Solid state electrochemical techniques are employed to characterize and control the electronic properties of semiconducting materials. The composition or the dopant concentration is precisely established by coulometric titration. - Electrochemical measurements have disclosed semiconductors with the potential to form extremely high internal electrical fields which drastically enhance the ionic motion to similar orders of magnitude like in the liquid or gaseous state. This effect makes new applications of semiconductors visible in combination with solid state galvanic cells and in other fields. The application of (small) electrical voltages to semiconductors with mobile ionic species will result in compositional inhomogeneities which affect the electronic conductivity. p-n junctions and more complex semiconductor devices may be generated at ambient temperature or formed at high temperature and quenched to room temperature. Also, junctions of the electronic minority charge carriers may be created in solid ionic conductors by applying small voltages.

I. INTRODUCTION Semiconductors point

of

view

are mostly

of

physics

looked at from the

using

the

principles

developed by physicists to describe the electronic structure ducting

the solid

state.

depend,

however,

properties

on chemical structure, and

of

the

parameters impurity

chemical

The semiconsensitively

such as the composition,

and

dopant

reactivity

at

for

the future

have

larger

of

development

materials

improved

number

present

2. New

tial

of

formation

applications

of

fields and extend

fast

interface

types

of

high

or novel

processing

devices

which

properties.

semiconductors

paper

Reaction

rates

may

extremely

in the solid state which results

in a fast

than

A much

presently

3. The

application

semiconductors tional

of

(small)

influences

distribution.

be formed to produce

the

three

sion.

techniques and

with otherwise

fields

of

convenient

electrical

provide a concontrol

defined

unknown high preci-

Simply the application signals

to

composi-

transitions

may

tunable electronic devices

in a readily controllable manner.

important.

describes

prepare

voltages

local

Electronic

2. PREPARATION

AND

CHARACTERIZATION

One

I. Electrochemical

compositions

electrical

become

conductor technology:

to

semiconduc-

internal

TIONS AND ELECTRONIC PROPERTIES

tool

of

the present use in electronic

impact of solid state electrochemistry on semi-

venient

properties

is obtained.

tors become visible which make use of the poten-

the

and

and

used may become technologically The

the semiconductor

variation of various materials properties.

semiconductor

will

and kinetic

thermodynamic

circuits.

Solid state chemical aspects may play a major role

information on phase equi-

libria,

concentration,

with other phases.

of

addition, fundamental

and measurement is

required.

0 167o2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

of

the

major

concerns

OF COMPOSI-

since

the

early

days of semiconductor research is the control of composition

(stoichiometry),

dopant

concentra-

tion and impurity content.

Solid state galvanic

of

cells

may

act

In

pumps

to

be

employed

control

the

to

as

composition

sensor~

and

sensitively.

874

W. Weppner / Electrochemical characterization and preparati(m o f semiconductiltg materials

An

electrical

galvanic

cell

current

across

is related

a

solid

2.5

state

to a m a t e r i a l s

, •Li2 Sb, Li3Sb I Li=;Si04 - Li3P041 lnSb(.l.i

trans-

amounts

may

The

controlled.

galvanic

activity that

be

a or

cell

2

voltage

concentration

is mobile

in the

relates

c of

to

the

the component

electrolyte

according

_J

to I.d

N e r n s t ' s equation.

T

The r e l a t i o n s h i p between the e q u i l i b r i u m cell voltage

E

and

the

composition

is

obtained

by

C o u l o m e t r i c T i t r a t i o n I. W i t h this knowledge, composition applying

may

the

be

in

reverse

corresponding

voltage

T

1

any

established

cell

W

by

~

C

until

As an example,

Li~Si04 -40

lithium

~

,

the preparation of InSb with a

very well defined amount of Li illustrated.

,

- 6 x 10 2 in U61nSb

the current flux becomes zero.

solid

T: 365" C

,=

port according to Faraday's law. Extremely small

15

m/o

LiBP04 2

electrolyte

(as a dopant) is

in the

used

is as

following

FIGURE I Relation between the e q u i l i b r i u m voltage of the indicated galvanic and the c o n c e n t r a t i o n of the dopant Li in InSb.

galvanic cell at 365 °C motion

L i 2 S b , L i 3 S b Li~SiO4-Li3P04 InSb(+Li) The e q u i l i b r i u m the dopant plateau

cell

concentration

indicates

the

equilibrium. Other electrochemical the

systems

voltage

is plotted against

in Fig.

I. The voltage

formation

of

a

semiconductors

to

which

technique

Li3 Sb3,

was

Li3Bi 4

3-phase

applied

the

include

LixA15, LixFeSy 6

of

species gradient which

ions

i

is

and

electrons

moving

under

(or holes) 9. Any

the

of the e l e c t r o c h e m i c a l

combines

the

of the activity

driving

influence

of

potential,

forces

of

(a) or c o n c e n t r a t i o n

a

VTIi,

gradients

(c) and the

e l e c t r o s t a t i c potential

(}). The following equal0 tion holds for the flux of ions and electrons :

L i x S i 7 a n d Ti028. Additionally,

kinetic

fusion

of

voltage

polarizations

lithium

in

parameters InSb

are

of t h e d i f -

obtained

during the current

g.

Ji -

from

3- HIGH INTERNAL ELECTRICAL FIELD EFFECT

high

effective

typical ductors

for

8

studies

diffusion

liquids

. This

or

have

gases

very

which are

for many

semicon-

is a very unique feature of the

local

Any

view

of

its

technological

poten-

of semicon-

becomes visible.

This

may

be

of

atomic

virtually

divided

species

into

the

in

a

solid

simultaneous

partial

Boltzmann's

conductivity,

constant,

absolute

neutrality of all

is m a i n t a i n e d

e l e c t r o n e u t r a l J t y condition,

transport

that

is

by a cor-

charged species according

required

in

~

ziji= O.

addition

to

the diffusion in order to fulfill this condition is

transport

number,

charge

the

ductors in other fields than electronic circuits

The

q are the

related motion

recognized

in

k, T and

Except for the formation of space charge regions,

to

A variety of new applications

z,

temperature and elementary charge,respectively.

solid s e m i c o n d u c t i n g state which has hardly been

tial.

z 2 2 (kT 71nai+ z i q V@) (1) iq

]

charge

indicated

coefficients

]

V~i=

f l u x 9. o,

Atomic transport

0.

1

~

provided

by

field

the result

is

an due

internal to

the

electrical

charge

of different mobilities

ous species.

field.

separation

as

of the vari-

W. Weppner / Electrochemical characterization and preparation of semiconducting materials

The following culated

local electrical

from eqn.

field is cal-

(I) and the electroneutrality

The slope of the Coulometric Titration indicates tration

condition:

875

the electrical

gradient.

curve

field per unit concen-

If the maximum

stoichiometric

width of a compound such as Li3Sb occurs over a t.

V¢

: - ~kT [[~.. . Vlnai); ]

distance of I pm, the field is as large as -10 ~

(2)

ti: oi/[oi

V/em.

]

The effect of the electrical t. is the t r a n s f e r e n c e number of s p e c i e s i 1 ([ti~1). The a c t i v i t y of the ionic species may be

replaced

by

the

(a;)

component

activities

of

and t h e e l e c t r o n s

the

neutral

(a e) a c c o r d i n g

ai.a e / a . i~ = c o n s t . :

field

concentration

over

the

gradient

and electronic

V¢ = ~'-k___i~[[ _! Vlna~ - Vlnae] q i~e,h zi 1

flux

as

a

conducting

are predominantly

plifying assumptions.

electronically

(ti~ 0) and one type of ions is more

as

tained

a

small

is

results

the activity

determined

of

the electrons

large electronic In

contrast,

electrical generally The

by

potential small

field

of

like

may

gradients

number

gradient

and is small

concentrations

semiconductors

electrical

the

is

for

in metals.

build

up

because

electrons

E=-V¢

of

of

the

but

conditions

case

driving

if

mobile than the other ones. The electrical field in this

species

caused

function

by

the

of

the

of the mobile

ionic

under the indicated sim-

(3) field

Semiconductors

is

(1) by Vlna~° Fig. 2 l shows the enhancement of the flux of ions by the

ratio of the concentrations t.

field as a dri-

to the activity gradient

determined by dividing eqn.

electrical

Z. 1

to the ionization equilibrium,

ving force relative

mobility common

were employed

properties

of

solids.

ponse

of

the

time

for

concentration

their are

A high ~nternal electrical force

the of

ions

is ob-

electrons

is

is high (te-1). These

for

semiconductors.

The

to optimize the transport As

an

example,

electrochromic

the

material

resW03

high the

or

holes.

directed

in the

W= jI7~o) • jIVc) jlvcJ 6

opposite

direction

of

the

electronic

activity

gradient and acts as an additional driving force for the mobile ions. The field

magnitude may

coulometric

be

of

the

estimated titration

internal

from

the

curve.

If

electrical

slope the

of

the

12

0.5 ° ° I k ~

material

shows

high ionic disorder, Vlna. may be neglecl z.Vlna equals approximately Vlna~ and the i e l electrical field is given by ted.

vdiffusion

~

-2 -6 V¢ = k T 71na; (te~ I; high ionic disorder) ziq

(4)

lna~ is measured by the cell voltage E according to Nernst's

law if an electrolyte

for species

i

Js used: V¢ = ~dE Vc; (te~ I; high ionic disorder)

(5)

-4

-2 •

0 log

2

ce

FIGURE 2 Enhancement of the flux j of ions by the internal electrical field over the flux under the influence of a concentration gradient as a function of the ratio of the electronic and ionic concentrations and the electronic transference number.

~7~

W Weppt~er / Electrochemical characterizatiott and preparation o f semiconducting materiaA'

was ~.nhanced by moving the ratio of the electro-

tremely

nie

state

and

ionic

concentrations

into

a

favorable

ra~ige.

fast

may

in semiconductors,

be

reached

within

and

the

a short

steady

period

of

time, especially in m i c r o e l e c t r o n i c devices with

Other

important

raldq!y

new

fields

equilibrating

devel(Jpment of fast

of application of

semiconductors

is

solid

battery electrodes 11 for s e n s o r s

~]as : e n s i t J ve l a y e r s

the or

short

diffusion

stoichiometry

lengths.

may

be

The

not

variation

of

the

switched

on

and

only

off but also tuned by the applied The

local

variation

of

the

voltage.

composition

of a

semiconductor by an applied voltage is illustra-

4.

FORMATION OF S E M I C O N D U C T O R JUNCTIONS AND

ted for F e - d o p e d TiO 2 single crystals

DEVICES BY E L E C T R I C A L FIELDS it is generally of

(small)

assumed

voltages

ted temperatures that the a p p l i c a t i o n

to p r e d o m i n a n t l y

electronic

conducting solids has no influence on the composition

of

applied

the

material.

In

voltage

E produces

electrochemical

potential

ween

the right

This

causes

fact,

mobile component.

of

~d the electronic properties occurs. As

a

result

electrons,

of

the

component A,

the

equilibrium

mobile

(Fig.

A

voltage

the

ions A z+ and the neutral

expressed by:

_ I (nl +- r z--q Az nAZ+)

in

a

closed

zirconia

probe

partial

pressure

across

the

quartz

serves

to

change

zirconia

vessel determine

of

the

gas.

electrolyte

the The

gene-

rates a r e v e r s i b l e steady state variation of the oxygen

partial

pressure,

kd-

or

desorpt~on

effects are not responsible since the integrated amount

between

qA z+ + zn e = UA, the voltage may be

i_ ~ ) E = z~q (PA

in an

mixture

3).

a local variation

imperme-

Ar/O2

oxygen

(i) hand electrode.

Accordingly,

is applied by one permeable and another

the

of the activity of the

at eleva

A voltage of 0-] V

able Pt electrode after being e q u i l i b r a t e d

the

n e of electrons bet-

(r) and left

a difference

however,

a difference

(600-900°C).

of

exchanged

oxygen

corresponds

to

the

loss of oxygen of several atomic layers. Another

indication

between

Ti02

tion

funnel

of

samples shaped

of and

an the

oxygen gas

cavities

exchange

is the forma-

if a current

is

(6)

z+ Since

the

ions

A

may

not

be t r a n s f e r r e d

in

steady state because of the ionic blocking behaviour

of

the

electrodes,

any

driving

forces

or

ZrO2(*Y203) QUARTZ

differences

in

the

have to disappear. a

difference

of

electrochemical The

the

potentials

applied voltage produces

chemical

potential

of

the

mobiie component A in steady state in analogy to N e r n s t ' s law:

E = z~qq (~i_ ~ ) ( s t e a d y

A transport

state)

(7)

of ions within the sample is requi-

red to reach this steady state. This may be slow in many

solids.

however,

that

The previous

the

transport

section has shown, of

ions may

be ex-

FIGURE 3 Experimental arrangement to demonstrate the local variation of the c o m p o s i t i o n of a semiconductor by the effect of an applied voltage.

W. Weppner / Electrochemica/ characterization and preparation of semiconducting materials

passed and

along

the

the

crystallographic

formation

of

grooves

in

c-direction, perpendicular

crystallographic directions. A device with an oxide semiconductor

or

applied

sink

which

voltage.

may

Also,

voltage.

If necessary,

between

be

the

controlled

described

by the

method

of

a new

technique

to prepare

semiconductor

junc-

Earlier investigations have demonstrated that p-n junctions

may also be formed by electroche-

mical

applied to electrochromic

conducting solids 2 .

in order to

and

This would be

tions and devices.

variation of the stoichiometry might be possibly materials

the junc-

tions may be formed at elevated temperature then quenched to room temperature.

two metallic electrodes may be used as an oxygen source

of an applied

877

techniques

within

predominantly

ionically

produce color changes in a very simple way without auxiliary electrolytes

and counter

electro-

des. Problems of degradation would not be longer relevant in this case.

5. OUTLOOK The described

Probably most important is the potential for-

a first approach

techniques to numerous

conducti-

tions

between

vity and of p-n junctions by applying small vol-

state

electrochemistry.

mation of gradients

tages

to originally homogeneous

The reduction ductor

of the electrical

process

semiconductors.

of an ambivalent

semicon-

The

important physics

provide interac-

and

relation

of

solid both

fields allows several new practical applications of semiconductors.

at one hand side may produce n-type con-

ductivity

whereas

the oxidation

process

at

the

other hand side may result in p-type conductivity

semiconductor

and effects

REFERENCES

(Fig. 4). Another procedure to form p-n junc-

tions

is

to

distributed

separate donors

originally

homogeneously

and acceptors

by the effect

I. C. Wagner,

J. Chem. Phys. 21 (1953) 1819

2. Y.-W. Hu, I.D. Raistrick, and R.A. Huggins, Mater. Res. Bull. 11 (1976) 1227; J. Electrochem. Soc. 124 (1977) 1240

Fe-doped

Ti02

Poi = 10"2atm, 8 ~ ' C O X O X O X O X O X O X O X OXO O X X O l X O

3. W. Weppner and R.A. Huggins, Soc. 125 (1978) 7

4. C.J. Wen, B.A. Boukamp et al., J. Electrochem. Soc. 126 (1979) 2258

X O O l X O

5. J.A. Schmidt and W. Weppner, 6. W. Weppner,

IV

--

:.

..... iL

, , .o

OIx

o

o

0

X X

X X

X X

OLI o XiX

X o

0 o

0 o

X

X

X

X Ii o

0

o

o

X

X

J. Electrochem.

to be published

unpubl, work

7. W. Weppner and R.A. Huggins, Ann. Rev. Matls. Sci. 8 (1978) 269 8. W. Weppner, 9. C. Wagner,

Solid State Ionics 5 (1981) 3 Z. f. Phys. Chem.

B21 (1933) 25

'I

'IV o:

acceptor

x : donor

Io.H. Rickert, Electrochemistry of Solids (Springer Verlag, Berlin, 1982) 11.O. HOtzel and W. Weppner,

FIGURE 4 Formation of a p-n junction in Fe-doped TiO 2 by the application of I V.

12.W. Weppner,

this volume

J. Solid State Chem.20 (1977) 305