Space charge limited currents under illumination in a-Si:H

Journal

olNonCrystailioe

Norlh.l-lolland.

SPACE

J.

Solids90(1987)195~

CHARGE

BULLOT,

LIMITED

limited The from

knowledge

problems'

established

part

of

above

the

the

M. GAUTHIER

and

IN a-Si:H

K. HADDAB

des Rayonnements, ORSAY, France

gap.

DOS.

Among

ties

of

it

DOS at

the

(SCLC)

are

UA 75,

below

under level

under

EXPERIMENTAL

at

T

The a-Si:H

B^atiment

490,

under increasing EFo but differ

The

vacuum

i-layer

for

med with uniform

n+ in+ was

at

least

i.e. Dark on the

above

conductivity n+in+ same

0022-3093/87/$03.50 (North-Holland

samples

been

0 Else&r Publishing

an

prepared

optical

gap exp(-

compare

charge

samples

the

on Corning

Science Publishers Division)

limited of

the

bulk

proper-

surface

states. of

the

distribution.

limited

currents

flux

at f.

are

the

quasi

By varying

f we

was

or

by GD of Sn02

annealed

coated at

Irradiations In

order

to

illuminated

silane2

substra-

18O'C

under

were

perfor-

get

with

a quasi 1.77

eV pho-

eV).

Ea/kT) to

the

determined

IT0

were

set-up.

(1.73

rise

Polytechnique on

film

steep

- EFJ.

to measurements.

the

spectroscocharge

the

lower

energies

space

to

of

easily

Ecole

monochromator

the

is

the

determination

shape

main It

DOS in

yields

light

(Ekn

at

less it

space

the

transient or

sensitive

then

of

a-Si:H.

for

level

accurate

the

deposited

profile,

appear

or

incident

All

prior

a high

mainly

the

of

urn thick.

hours

oD = o. to

for

were

lamp

the

which

Ein

1.6-Z

GD parameters

Physics

in

one

give

because an

on

as a function

absorption

just

is gives

The DOS is

have

a quartz-halogen in-depth

effect

is in

effect

a more

2.3

used SCLC

structures

two

of

gap

transport

deep

field

information

illumination

samples

= 25O'C.

little

N(E)

while like

field

excitation.

measure

results

widely

a new method

light

2.

s

only

conflicting

terms

that

the

bonds

particular

been

in

to dangling

in

claimed2

(DOS) electronic

methods

whereas

been

FL but

may directly

tes.

has

material has

In

other

interpreted

SCLC

them

states the

some

(FL).

minimum

of of due

contrast

level

We present measured

density

defects

a deep

the

However

Fermi

In

Fermi

shows

the

understanding that

the

currents

with

ILLUNINATION

currents in a-Si:H have been measured results show that N(E) increases above those obtained by SCLC or photoconductivity.

of

for

well

red

UNDER

INTRODUCTION The

tons

CURRENTS

de Physicochimie Paris-Sud, 91405

Space charge light fluxes. significantly

py

195

P. CORDIER,

Laboratoire Universite

I.

198

Amsterdam

and

photoconductivity

data

obtained

7059

substrates.

B.V.

with

have gap The

cells

results

been

measu-

prepared for

both

Table Comparative

data

oD at

RT

for

( n-1

cm-l)

Oo ( o-1 cm-l)

sandwich

2.3

x 10-B

278

COpla".%r

3.6

x 10-S

x Photoconductivity tron mobility types

of

in

samples

are

given

structure,

the

activation

3.

RESULTS

AND DISCUSSION

Typical

log

i vs

and

29

yield measured with extended states ;

the

in

table

energies

log

I

sandwich

V plots

I.

"UT

x

Ea (4

(cm2

0,58

2.2

x 10

0,57

2.4

x lo-'

Whereas the

presented

oD is ilu~

in ned

i(A)

samples

v-l)

I

lower

light

5.10"

to

The

current

the

dark

region

and

4.10'3

rent.

current

iD

All

the

onset

low

1o-2

VW

the

illuminations

the

dark

i-V

curve

is

identical

been

of

by the

curby

SCLC

regime

in-

the

light

flux

(see

from of

ohmic

the free

regime.

In

we limited

to

damage checked

prevent to

the

that the

original

increaelec-

to relatively

with

the

Vc (indicated

levels

permanent

photo-

followed

expected

the

.

characterizes

experiments

jection

has of

in

the show

the

vestigations

F1GUF.B I log i vs log V curves : A in the dark;B to E under increasing f. Inset : Vc vs log f (in arbitrary units). ries

as

-1

s

: an ohmic

concentration

trons

1

and

voltage

which of

-2

sum of

behaviour

with

inset)

the

curves

The voltage arrows)

sing

cm

increase

the

creases 10-O

now

E

from

photons

a supralinear

obtaiB to

ranging

i is

at

been

curves

fluxes

general

same.

A has

dark

i p.

; IJ elec-

coplanar

the

Curve the

under

same

the

are

fig.1. in

yield

in

products

current

lo-’

-5

2 eV photons. q quantum T electron lifetime.

Ea and

are

coplanar

all

our

in-

low

in-

possible sample. after

a seone.

It

II 0

FIGURE 2 (i/V) vs V curves : A in the ; B to E with the same f as

log dark

In

injection

mely assumed

the

i EFn

when

that

se of fig

experiments

above

- EF,S the

log(i/V)

voltage

nism.

However

trapped is From plots

same

states

by

the

or

given

is in

conditions. that

N(E)

with

increasing

upwards

in

to

formula

given

fig

3 curves The

increases

the

A and

energy

the

large

A of

FO V. Above and

the

this uniform

observe

already

states

increaCurve

a similar

are

na-

may be

).

we still

the

in

a linear

1.5

too

text

levels, it

and

indicating E;,

trapping

by N(E below

is

we know DOS at

illumination.

pushed the

2-4

SCLC to

under

Ein

decreases G” fitted

of

proportional

dark

range

. See

0

injection

governed

(*W

a li-

SCLC

occupied above

mechaby

and

Ei,

the

N(Ein). analysis

inversely

E and FO now fill

will

E-EF

electron

illumination E)

vs

injection),

energy

FL shift

B to

N(E)

low

observed

Under

between

injection

detailed

are

50 regularly

are

the

governed the

in

when

since

the

to At

being

indeed

vs V (curves

due

FL under

small slope

valid.

of

dark).

quasi

this the

is

the

the

because

longer

log(i/V)

in

is

occurs no

photoelectrons,

slope

see

of

in 4

charge

FO

a variation

is

increase

been

uniform

some deviation

near

being

expected

such

DOS approximation

(EFn’

kT

V is

that

space

FL (E

DOS is vs

2 shows

the

equilibrium

0.1

E-E%

Variations for details.

states

J

I 0.05

scale

significantly

gap.

thus

slopes

To obtain

an

just

above

E

of

values

our

DOS.

samples

prepared

from

Ein FO

.

- E

FcJ

such

that

is

2 we see

a uniform

two different obtained5

fig

increase

accurate for

S of FL,

of

indicating

by Weisfield3 is

the

equilibrium

By inspection f

B for

that the

The

at that

S

N(Ebn) data

have

results

under

= kT ln(i/iD).

the We

For the

comparison

sample

we analyzed

used

to obtain

B, curve

C shows

that

the

DOS so obtained

information

from

the

ohmic

photoconductivity.

assumes

an of

rature

the

DOS distribution

by

eV excitation.

This

values

shown

at

the

the

also

Rose's

Y exponent to the

D fig

3.

get

some 697

describing

the

characteristic

same

no

A and

model

We measured

for

with makes

curves

We can to

Y = Tc/(T+Tc).

by curve

with

flat.

According

'1s related

voltage which

contrast

quite

yields,

higher

a procedure3

In is

iP u fy

1.77

N(E)

obtained We used

DOS distribution,

photocurrent

relative

3.

DOS distribution.

the

K with

ve,

the

exponential

the

Tc of

T=300

of

curve

fig

on

dependence

shape

SCLC

A of

assumption

which

the

a dark curve

f

tempeY = 0.6

energy

at

range

The variation

as

is

rather

known

that

abo-

steep. The

reasons

deep

states

fects

can

Okushi end

et of

discrepancies by in

al.'

observed

Indeed

vation

of

need

ductivity

to

light

our

be taken

some

conclusion

mination.

we have

This

method

of

clear.

transient

optical not

It

for

weak

the

after

DOS has states

conventional

a complete

ef-

illumination

disappear

trap

additional

by

irreversible

which

upon

some

well

that Under

states bias

detected

account

is

we checked

conditions.

experiments are

into in

not but

experimental

which

experiments

are

illumination*

be excluded

presence

In

these

created

excitation. The effect IO . Obviously in our

discussed the

for are

also

are

SCLC.

seen This

understanding

the been

of

in obser-

photocon-

a-Si:H. shown

can

that

the

be a tool

DOS can

to

study

be measured

the

defect

bySCLCunder

creation

illu-

by

light.

REFERENCES 1)

H. Fritzsche,

2)

I.

J Non-Cryst

3)

R.L.

4)

M.A. Lampert and New York 1970).

Solomon,

R.

Weisfield

J.

Anderson

Solids

Benferhat Appl.

D.A.

6)

A. Rose, Publishers,

Concepts in New York

and

Phys.

W.E.

and

3,

6401

Spear,

Phil.

E. ArPne M.

Baixeras,

Phys.

9)

H. Okushi, Sol. 59/60,

A. Asano, M. Miyakawa, 393 (1983).

10)

R. Pandya,

E.A.

Jackson

Schiff

and

and

Msg.

K.A.

in 36, and

Rev. C.C.

Rev.

695

E,

2016

Tsai,

Phys.

J.

solids

E,

3422

(1984).

(Acad.

Press,

(1970).

allied

S. Yamasaki Conrad,

(1985). Phys.

(1983).

photoconductivity 1963).

7)

W.B.

273

Tran-Quoc,

injection

8)

Stutzmann,

77/78,

H.

P. Mark,Current

5)

J.

and

problems

(Interscience

(1984). Rev. and

Non-Cryst.

K.

E,

23

Tanaka, Sol.

(1985). J.

66,

Non-Cryst. 193

(1984).