Bulk — limited conductivity in germanium monoselenide films

Materials Chemistry and Physics, 21 (1989)

BULK

-

LIMITED

CONDUCTIVITY

C.VODENICHAROV

and

IN

1

455

GERMANIUM

MONOSELENIDE

FILMS

S.PARVANOV

Department

of

1156

(Bulgaria)

Sofia

455-46

Physics,

Higher

Institute

of

C hemical

Higher

Institute

of

Civil

Technology,

M.VODENICHAROVA Department

of

Physics,

Architecture, Received

1421

Sofia

September

Engineering

and

(Bulgaria)

5,1988;

accepted

October

13,1988

ABSTRACT Current-voltage with at

amorphous different

region

characteristic, electric

elertron the

space

as

semiconductor

0.26

to

0.90

x

1024

for

the

up

film to

3.4

amorphous

the the

limited

their gap.

ones

it

is

have 295

the

and

is

for

the

about

In

the

to

been

order

the

of throughout

interval

rom w

within

of

of

observed

varies

be

the

part

pclycrystalline

one

K.

determined

density

found

93

distribution

in

trap

to

investigated

Schottky

been

levels The

system

been

have

energy havse

Trap

me3

Al-GeSe-Al

currents The

established.

10z5

from and

density

thickness x

range

V/m.

IO7

the

films

Ohmic

to

well

are

GeSe

charge

energy

eV

semiconductor 1.2

in

between

fields traps

of

polycrystalline

temperatures

intermediate

at

characteristics

and

th

the

limits

films

while

magnitude

arger.

INTRODLJCTION Germanium

chalcogenides,

three-component increasing

systems interest

application

in

of

in

solar

GeSe, the

view

of

cells,

type

GeSeZ,

GexSel_x,

Ge-Se-Ga

the

are

possibilities

electronic

switching

and

the

objects

for

their

and

memory

the of

devices,

El.0.

The time

by

electric

Asanabe

properties

and

Okazaki

shallow

impurity

levels

gations

on

properties

monoselenide

these were

carried

of [l]

near

the of

out

GeSe

who

valence

crystals by

were

studied

established

other

band. and authors

for

the

thin

the

first

existence

of

Later films

on,investiof

germanium

12-101.

0 Elsevier Sequoia/Prjnt~d

in The Netherlands

456

In a previous characteristics emission fields this

region,

has been

paper

work

[II] when

of thin-film the influence

that

the current-voltage

samples

of space

Experimental

assumed.

showing

investigating

Al-GeSe-Al

charge

results

the currents

in the thermionic at low electric

are presented

in the intermediate

in

region

between the Ohmic and the Schottky part of the current-voltage characteristics of the Al-GeSe-Al system are space charge limited.

EXPERIMENTAL, Samples studied,

RESULTS

with

The procedure

I

and polycrystalline

of the films

of sample

preparation

characteristics

I

I I111b11

varying

GeSe

films

between

and the method are described

have

been

50 and 100 nm. of measuring

in

[IIT.

I I,

10

Fig.1 Current-voltage of Al-GeSe-Al sample

In Fig.1

uvin characteristics with polycrystall ine

the current-voltage

temperatures with

DISCUSSION

amorphous

the thickness

the current-voltage

lDL1

AND

are presented

a polycrystalline

guished

by clearly

voltage

jrvU"

GeSe

expressed

, n )

characteristics

in co-ordinates film. power

1. Their

InU co-ordinat 139 nm thick.

at different

Inj-InU

for a sample

The characteristics dependence

slope

increases

.es

are distin-

of the current

on

continuously

reaching the value n q 20 at low temperatures. The power dependence 2 observed in the intermediate range between the Ohmic part jhlU

457

(j

U)

N

and

the

Schottky

characteristics appearance has Al

of

been

that

the is

fields

film

a

the

space for

does

charge

carrier

not

definite

in

same

of

a

Al-GeSe-

The

an

assump-

bulk-limited

limited

region. is

the

dependence system

[II].

to

the

current

At

higher

electrode-limited

characteristics,

nor

the

method the the

and of

energy

Manfredotti

knowledge it

of

is

the

based

insulator,

distribution

et

current-voltage

nor the

the

al.

[IZ)

was

characteristics.

The

actual

of

upon

mechanism

the

respectively

assumption the

of

semi-

film.

After

this

the

method

the

energy

distribution

the

of

traps

is

given

derivative

_

1

d? (I

edF’

dF where

nt

space

is

charge

Fermi

level,

valid

when

that

of

the

is

in

and

is

free

ones.

low

pos

traps 9

evaluated

N t =4kT

the

trapped

the

electron

For

the

is

energy

The

carriers

whose

particularly

the

charge.

trapped

GeSe,

electrons,

F

sample,

much

gap

the

is

of

the

quasi-

is

(I)

larger 1.16

measurements

=

tion

eV,

this

have

= f(F)

has a maximum

at

(2) and their

the

for

the

by

the

expression

F,,,’

value

density

2

(d”t) dF

is

than

temperatures.

dn,/dF

and

1

the

expression

is

energy since

p

ng

the the

value

at

of the

of

fulfilled,

dependence

F,=Et+kTl from

e

density

performed The

density

density

the

the

condition been

a

observed.

magnitude

of

the

a

charge

conductivity

the

require

related

suggests

the

films

space

part

generation,

Such of

GeSe is

current-voltage

V/m


samples

typical

been

of

structure

conductor

dn, --

study

analysis

method

amorphous

square-law

charge,

the

and

of

the

currents.

studied

thickness

has

to

limited

the

of

e

all

mechanism

neither

order

used

of

the the

jmexpfi)

dependence by

filled-voltage In

by

quadratic

on

where

process,

a

in

(

fields

charge

supported

dependence

of

space

polycrystalline

process

trap

electric

established

with

tion

at

part

degeneration

of N

t

which

the

could

be

factor,

the

energy

level

determined. traps

density

Et Using

the

is

(3)

458

The

assumptions

application

to

and

1111,

of a

moreover

current-voltage interval

to

energy

in

93K,

gap

varies

level

method wi th

the

in

the

at

proves

the

us

to

from

reason

for

be

to

since to

By

measuring

temperatures

possible

0.94

its conductivity

range.

different

semiconductor,

limits

the

electrode-limited

temperature

stirs

it

of

give

proved

low

characteri

295

entire

the

system

probe

the

0.24

the

in

the

almost

the

quasi-Fermi

eV.

no/

go_ ^ ‘;h, 70. % c 50.

30 _

10 _ 21 10 _

93K

,

i53K

0.2 Fig.2 Fermi

In

Fig.2

the

level

F is

position the

of

traps

maxima of

from

one

Table

I

different

are This levels

maxima

of

mean

values

samples

are for

are

given.

the

fluctuation

narrow

and

their

is

considerably obtained

Fig.2

film.

another

full less are

sample

The

deviations

obtained

Only level width than

discrete

at 3.5kT

ones.

the

do

maximum indicating

of

thick-

maxima not

whose into

quasi-

have

position

exceed

measurements

peaks

half

the

different in

and

taken

of From

distribution

with

from

the are

energy

quasi-

= f( F )

samples

small

the

Fig.1.

energy

the

very

Et

the

1,

of

the of

dn,/dF

curves

F for

semiconductor to

position

on

the

using

The

times

thus

for

and

values

on the Fig.1.

of

29sK

0.6

E,. eV ___

ofdn,/dF

determined.

sample the

dn,/dF sample

pr.esented

close

the

of the

,(

/

22?K

I

0.6

dependence

the

is

at

ness

five

189K

0.L The dependence for level F

Fermi

,

IkT.

magnitudes account. is that

exceed The

less

than the

In

on

trap

peaks 0.02

eV.

459 The

traps

density

the

corresponding

the

polycrystalline

varies

in

3.4~10'~ values

the rne3.

of

the

dielectric

and

film

/4

room

given

In

calculations

the

parameters

TABLE Table

is

Et

and

the

electron

hole

free

in

But

II.

2

amorphous

the

trap

density

one order 25 to 9 10

x

levels

polycrystalline

GeSe

film

coincide

of x

in the

Etof

the

electron

and

amorphous

film

GeSe

Et

x

10z4

-

9.6

x

10z4

0.90

x

1oz4

-

1.7

x

1oz5

0.84

0.80

2.4

x

1O24

-

1.6

x

1O25

0.73

7.5

x

1024

0.66

4.0

x

1024

1O24

-

6.7

2.4

x

10z4

1o24

-

3.4

2.0

x

1024

5.0

x

0.26

Some by shed the

other by Hall

of

films

show

maxima

the

authors. Asanabe coefficient

trap So and

at

of

in the

and

and

their

film

IeVl

0.73

x

10z4

0.61 0.46

x

1oz5

0.33 0.26

levels the

thus

level

Okazaki

in GeSe

those

GeSe

1.2

0.33

GeSe

films.

Amorphous

2.4

x

mass),

larger

traps

0.84

1.6

the

samples

magnitude 25 m-3 10

Cm-"1

0.46

for

effective

with

0.90

0.59

to

relative

electron

mobility

positions

about

energy

GeSe

Nt

the 0.3171

q

the

24

following

for

m,

the

for

of

1.2x10

the

=6.3

for

values

density from

[II],

being

the

thickness

Nt

&

0.78117

with

limits

of the

Polycrystalline

[eVl

(Table

the

in

N,

of

used:

( m

whose

GeSe

within

density

for

Al-GeSe-Al

film

1. I. Values

[9]

trap

magnitude,

films

and

(13j.

amorphous

varies

GeSe

I. The

of

been

[11],mp:

m'/V.s

dn+/dF=f(F),

polycrystalline

Et

of

GeSe

=5~10-~

in Table order

have

mass

The samples

with

different

one

crystalline

curveS

eqn.(3)

with

samples

of

temperature

the

from

in

are

effective in

calculated

limits

permittivity

electron mass

is

levels

[I]

single

obtained 0.80

eV

f rom crystals;

are

close

coincides

the the

temperature value

to

with

0.33

those that

found establi-

dependence eV is

of

460 confirmed

by

energy (

(0.34

x=0.5).

by

data

eV)

of

The

Aat

eV)

of the

levels

Borissenko

(0.511 the

the

films

0.46

al

[I51

of

GeSe2

concentration

Mehra

of

et

al.1141

conductivity and in

in

0.33

eV

for

are

close

amorphous

(0.378eV)

In

crystals

.

single

acceptors

is

the

activation

amorphous

evaluated

GexSel_x

films

those

determined

to and

crystalline

Ge l+x to

be

3

Se x

10

(x 24

0.005)

q

mm3191.

CONCLUSION The

investigations

characteristics 295

to

bution for

93

K

of

the

yield

amorphous

The

results

an

out

of the

out

samples

in

complete

GeSe.

and

carried

This

point

SCLC

region

at

of

current-voltage

temperature of

the

proves

to

interval

energy be

distri-

identical

films.

such

that

could

strong

the

picture

out

electric

electrode-limited

on the

picture

polycrystalline

characteristics

where

mechanisms

almost in

obtained

current-voltage

had

Al-GeSe-Al

traps

the

ratures

we

of

be

fields

processes

analysis

of

successfully and

at

are

low

the

performed tempe-

the

dominating

conductivity.

ACKNOWLEDGEMENT The

authors

are

grateful

to

S.Eftimova

for

technical

assistance.

REFERENCES 1

S.Asanabe,

2

A.Mikolaychuk, (1970)

A.Okazaki, A.Kogut

3

L. Fritzsch

and

H.Stb;tzel

and

W.Bobe.

F.Braun,

Wiss.

B.From,

Paul

W.

Y.Ishihara S.Ishida,

and

phys.

M.Ignatiev,

stat.

sol.

-15

(1960)

Izv.VUZ,

(bl, -58

Tagungsber.

989.

FIZIKA,

Z.Techn. and

Univ.

W.

1974

Dresden,

Teubner,

I.Nakada,

phys.

(19731

Conf.

Reinchardsbrunn,

T.Fukunaga.

-7

Thin stat.

T.Kinosada

107,

(1974)

Solid sol.

and

pp.

,

-23

K49

Amorphous

357.

Films, (b),

2,

105

K.Murase,

338

(1979),337

(1981)

Physica

285.

B+C,

70.

9

Y.Ishihara,

IO

D.Kyriakos,

Y.Ohno

and

I.Nakada,

phys.

stat.

sol.

(b),

121,

407.

11

C.Vodenicharov,

12

C.Manfredotti, and

and

A.Kottwitz,

Semiconductors'74,

(1984)

Phys.Soc.Japan,

103.

4

(1981)

J.

A.Tepore,

A.Anagnostopulos, S.Parvanov C.D.Blasi, phys.

stat.

J.Appl. and

P.Petkov

S.Calassini, sol.,(a)

Phys.,

-58

3917.

,Mater.Chem.Phys.(l989). G.Mococci,

-36

(1985)

(1976)

569.

L.Ruggiero

105

461

13.

C.Kanazirski, Compt.

14.

R.Mehra,

15.

T.Borissenko,

(1987)

2

A.Stanchev

M.Traikova,

rend.

Acad.

M.Arora,

Sci.

bulg.,

P.Mathur

and

z

and (1977)

H.Kumar,

C.Vodenicharov, 511. Mater.

Chem.

Phys.,fl

Vesz.

AN

343.

(1986)

N.Koren, 67.

E.Matias

andE.Shrubova,

EISSR,