Electrification properties of semiconductor and dielectric layers

Journal of Electrostatics, 23 (1989) 395-400 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

ELECTRIFICATION

PROPERTIES

395

OF SEMICONDUCTOR AND DIELECTRIC LAYERS*

L.S. YOURUKOVA and K.M. KOLENTSOV Institute of Solid State Physics, Blvd. Lenin (Bulgaria)

Bulgarian Academy of Sciences,

1784 Sofia,

72

SUMMARY Basic electrification quanties the surface charge potential V0, the charge half-decay times to -+' to 5- and charge holding power u - of semiconductor and dielectric'~ayers 'are studied. Investigated ZnS, TiO 2 and As-S layers are prepared by binder or vacuum technology on metallic substrates and they are charged by positive and negative corona discharge. On the basis of measurements done and data obtained an evaluation of the possibility to use these layers as active or passive components in AC EL structures is given.

INTRODUCTION The

study

dielectric

of

the

basic

electrification

times

of

the

material's

the

polarity

(ref.

scientific

parameters

- maximum

and

to

material

technological

negative

keep

charges

with

the

i). On the basis

physical-chemical

samples

help

(ref.

a

semiconducting significance.

potential and

to,5-,

polarity

corona

u

discharge

of the thus obtained values

properties

procedures

to,5 +

definite of

layered

practical

surface

charges of

of

and

2),

of and

materials the

half-decay

as well are

as

measured

with

the by

different

one can judge both

prepared

structural

VO,

and Their

using

different

peculiarities

of

the

investigated.

In the present

work,

the electrification

and dielectric binder layers, been studied. useful

properties

is of both

positive

capacity

charging

the

electrification

materials

in

properties

and thin vacuum evaporated dielectric

The data on the basic electrification view

electroluminescent

of

their

structures

of thick semiconducting

application as well

such structures with thin chalcogenide

as

as for

parameters

active

and

elucidating

passive

layers

the peculiarities

in of

passive layers.

* This project has been completed with the financial

support of the Committee

for Science at the Council of Ministers under contract No. 669/1987.

0304-3886/89/$03.50

layers, have

of the layers are

© 1989 Elsevier Science Publishers B.V.

396

PREPARATION OF THE SAMPLES The

semiconductor

binder

technique

and

(refs.

powder based on ZnS:Cu the

dielectric

was

dielectric

binder

3-5).

semiconductor

The

layers

weight, were

ratio

powder

of

purity

"puris"

in a single-component

prepared at

at

a

i:i.

After

a

50

~m

thick

polymerization samples were

aluminium and

foil

subjected

of

size

1-2

Dm.

oligomer

of high

molecular

of

of

1:1,5

the

and

the

prepared

samples

dielectric

heterogeneous

it was deposited using a rake technique on

substrate

cross-linking

particle

6). The semiconductor

ratio

homogenization

semiconducting or dielectric mixture,

organic

size from I0 to 25 Dm and

and

epoxy (ref.

a binder-to-semiconductor

ratio

using

electroluminophore

these substances were dispersed in a

phenol

which served as a binding substance

samples

obtained was

(type EL 570 M) with particle

TiO

Following an appropriate thermal treatment, definite

were used

(ref.

7).

the particles

to suitable heat-treatment.

To

of

ensure

the

the

epoxy

complete

olygomer,

The thickness

the

of the ZnS:Cu

layers was 50 um and that of the TiO 2 layers 30 Nm. The

layers

technique and

from

onto

the

system As-S

aluminium

ultra-sound

cleaning

were

substrates procedures

evaporated

chalcogenide

evaporation

rate were measured

layer

deposited

using

which had been

was

(ref. 1,2

8).

Dm.

The

This

in the process

a vacuum

subjected

evaporation

to both

thickness thickness

of as

chemical

the

well

vacuum as

the

of deposition with the help of a

piezo-quartz measuring device.

EXPERIMENTAL The charging

of the samples was carried out in darkness with

the help of a

d.c. corona of different polarity at a voltage of 6 kV and discharge current of several

uA

using

special

device

the

so-called

universal

"turn-table"

dynamic

tester

technique. (ref.

This

9).

The

carried out in ambient air humidity of 659 and temperature the sample to saturation,

was

done

with

measurements

a

were

25°C. After charging

the corona was switched off. There followed a process

of gradual discharging of the surface of the sample due to neutralization of the charges with ions from the ambient air or leaking of charges to

the

grounded

characterized charges

in the

characteristic

by

substrate. the

sample for

the

The

half-decay to decay discharge

process time,

to half rate

of

discharging

to,5,

i.e.

their

initial

of

the

the

through of

time

the

sample

needed

quanitity.

electrized

the layer

layer.

for

This The

time

is the is

discharge

times (half-decay times) are different for charges of different polarity and one can define the capacity of a given layer to keep positive or negative charges on its surface in the following way:

397

t0,5+

t0, 5-

U

2 (to,5 +

where

to,5+

half-decay

is the half-decay time

of

the

2 t0,5-)/2

time of the positive

negative

charges.

The

charges

parameter

u

and to, 5- is the characterizes

the

prevailing capacity of the layer to keep charges of one definite polarity on its surface.

EXPERIMENTAL RESULTS The

time

dependence

semiconductor

ZnS:Cu

of

the

surface potential

layer while charging

V 0 of a 50 pm

it with a d.c.

polarity,

and after switching off the corona discharge,

is

that

seen

speaks

in

the

favour

half-decay of

different

times

t0,5 +

degrees

of

and

is shown in Fig. i. It

to, 5-

keeping

thick binder

corona of different

of

are

different,

charges

of

which

definite

polarity. The

time

dependence

of

the

surface

potential

V 0 of a 30 pm

dielectric TiO 2 layer under the same charging conditions, The

charging

thick vacuum

and dark-decay

curves

is shown in Fig. 2.

of the surface potential

evaporated As2S 3 layer are shown in Fig.

thick binder

V 0 of

3. Similar

1,2 ~m

dependences

were obtained for thin vacuum evaporated AS2S 5 and AsS 5 layers too.

"~'500

r"

5250 o..

Q.; o L.

!

0

2'0

40

60

Time (s) Fig. i. Surface potential V^ as a function of the charge and discharge of a thick binder semiconductor ZnS:Cu layer.

time t

398

~1000

v0

0 >

I

0

+

Vo

~

C

.,-, 500 0 O.

t 0,5-t

K

0 0

O3

0

20

o,5+_

4o

6o

Time(s) Fig. 2. Surface potential V^u as a function of a thick binder dielectric TiO 2 layer.

of the charge

and

discharge

time

t

time

t

~400" O >

-6

V ~ ~ .

0 0..

t ~ _ t o.s+

200

C.) 0

50

100

Time (s) Fig. 3. Surface potential V^ as a function of a thin v a c u u m evaporated ~s2S 3 layer.

The binder

data

about

the

semiconductor

summarized

negative

in Table corona

basic and

electrification

dielectric

i - namely,

charging

t0, 5- and the capacity

of the charge

VO +

and

the maximum and

to keep charges

V0 ,

parameters

thin

vacuum

surface the

of definite

and discharge

of

the

studied

evaporated

potential

under positive

half-decay polarity

thick

layers,

times u.

to,5 +

are or and

399

TABLE i

Material

V0-

V 0+

t0, 5-

to, 5 +

(v)

(v)

(s)

(s)

450 900 385

300 500 275

13 15 40

7,5 9 53

ZnS:Cu Ti0_u As2~ 3

u

-0,595 -0,485 +0,277

DISCUSSION The

electrification

properties

of

the

dielectric

layers depend on their structural

a

system

complex

dispersed

of

in a polymer matrix

homogeneous lower

consisting

(ref.

depth

either,

and

which

follows

the surface

forming last

The

formations oligomer

from

the

polymerization.

It

complete

lower is

cross-linking

semiconductor

and

polymerization

before

of the capacity

that

by

the

charges

three

of

of the phenol in

the

course

are

linear

place

in

treatment.

on the surface

the

course

The stronger

with

the work

of

dark

decay

the

concerning thickness of

the

dependence

of amorphous

thin vacuum

manifestation

The

importance

and

of

the

As2S 3 layers,

characteristic the

with

processes

of

the

lack

for the binder of

of

layers

the

generation

and

electrification thin

vacuum

passive

104 V/cm layers

is concerned, in

of

and

thickness

(ref.

potential

to 70 ~m.

structural provides

relaxation

of

the

their

of charges

and Neyhart

surface up

layers,

on

d in

15) the

In the case inhomogeneity for the clear the

electric

of the samples in darkness.

properties

evaporated

so far as the possibilities

above

Ing

thickness

in the course of charging and discharging study

dielectric

fields

of

As2Se 3 layers

evaporated

and stratification

charges

in agreement

of

of and

dependence

u on the layer

the

is

epoxy

oligomers

of the role of bulk generation

which

(ref.

structural

among which are dispersed

takes

a

a middle

"aerial"

can be connected with the increase layer,

layer is

sublayers:

density

surface

eopxy

molecules,

and during the thermal

for keeping

and

formations

substrate,

sublayer,

largest

the

phenol

particles,

of

of the solvent

towards the

of the polymer

dielectric

consist

of the aluminium

of diffusion

sublayer

known

The binder

agglomerated

and an upper

is characterized

due to the process

and

structurally

the bulk of structure,

sublayer

semiconductor

12). On the other hand these layers are not

in

13).

binder

inhomogeneity.

polycrystalline

sublayer,

sublayer,

thick

of

thick

dielectric

semiconductor

layers

is

of

and

marked

for using these layers in strong electric when

these

electroluminescent

layers

structures.

play

the role of active

Besides

this,

these

400

investigations characteristics

are

helpful

and peculiarities

SnO2-ZnS:Cu-As2S3-AI

for

elucidating

of electroluminescent

the

electroluminescent

structures

of the type

(ref. 16).

REFERENCES I. J. Strojni, Static electricity, Technics, Sofia, 1981. 2. K. Kolentsov, D. Manova, N. Balchev, S. Balabanov and P. Batev, Univ. Ann. Techn. Phys., 20(1) (1983), 107. 3. Applied Electroluminescence (Edited by M.V. Fok), Soviet r a d i o Moscow, 1974. 4. Electroluminescence (Edited by J.I. Pankove), Springer-Verlag BerlinHeidelberg-New York, 1977. 5. Display Devices (Edited by J.I. Pankove), Springer-Verlag, BerlinHeidelberg-New York, 1980. 6. K. Kolentsov, Bulgarian Patent No. 37219 (priority April 15, 1983) 7. K. Kolentsov and N. Balchev, Bulgarian Patent No. 36760 (priority October 18, 1983). 8. Various arsenic sulphide and its alloys (Edited by B.T. Kolomiets) Shiintsa, Kishinev, 1981. 9. K. Kolentsov and L. Yourukova, Bulgarian Patent No. 40949 (priority November 22, 1985). i0. J. Nikolov, I. Sandrev, K. Kolentsov, U. Indjova and N. Balchev, Bulgarian Patent No. 38131 (priority June 14, 1984). Ii. K. Kolentsov, I. Sandrev, J. Nikolov, I. Radkov, N. Balchev and L. Yourukova, Univ.Ann. Techn. Phys., 25(1) (1988) (in press). 12. N.A. Balchev, K.M. Kolentsov and E.M. Vateva, J.Electrostatics, 14 (1983) I. 13. P.V. Koslov, In Polimer Laminated Materials, Chemistry, Moscow, 1976. 14. A. Knop and W. Scheib, Chemistry and Application of Phenolic Resins, Springer-Verlag, Berlin-Heidelberg-New York, 1979. 15. S.W. Ing and J.H. Neyhart, J.Appl.Phys., 13(6) (1972) 2670. 16. K.M. Kolentsov and L.S. Yourukova, Bulg. J.Phys., 14(3) (1987) 253.