Electron and ion transfer processes at insulator surfaces

Journal of Electrostatics, 10 (1981) 107--114 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

107

ELECTRON AND ION TRANSFER PROCESSES AT INSULATOR SURFACES

C. BARNES, P. G. LEDERER, T. J. LEWIS

School of Electronic Engineering Bangor,

and R. TOOMER

Science, University College of North Wales,

Gwynedd, LL57 IUT (U.K.).

ABSTRACT Transfer of charge to the surface and thence through to the bulk of insulating solids occurs in many electrostatic examined

in new experiments

situations.

The physical processes

on synthetic and biopolymers

on the surface can be monitored by a non-contacting definition.

Deductions

about surface states

through the bulk followed carrier mobility

are re-

where charge behaviour

induction probe of high

can be made and charge transport

as a function of time.

Some examples

from which

and charge trapping and release parameters may be deduced are

given to illustrate

the methods.

INTRODUCTION The origin of conduction currents whether isothermal

in insulating

or thermally-stimulated

separation of true conduction

solids continues

studies are made.

from dipole relaxation

of the mechanism of transfer

across the metal-solid

of scanning electrostatic

interface.

This subject has

induction probes of good definition has allowed the (ref. 4).

synthetic and biopolymers

Since details of the experimental only the essential

to the need

as sources of charge and

I, 2 and 3 are typical) but the recent development

studies to be made in more detail than hitherto of this work on insulating

The reliable

leads inevitably

for specific knowledge of the role of metal electrodes

been studied for some time (refs.

to be obscure

A brief survey of some

is given here.

method have been described

features will be repeated here.

already in ref. 4,

Samples to be investigated

in the form of thin discs or film and mounted on a grounded metal turntable pass under a calibrated

and fully guarded metal induction probe of 80~m diameter.

Signals from probe scans of the sample surface are interpreted surface potential V(x,y,t) charges

as measures

at a point x,y on the surface at time t generated by

in surface states of density o(x,y,t)

and in bulk states of density

p(x,y,z,t) V(x,y,t)

of the

at a depth z below the surface. It can be shown that fd = -i~ o(x,y,t) + jo(d-z)p(x,y,z,t)dz]

0 3 0 4 ~ 8 8 6 / 8 1 / 0 0 0 0 - - 0 0 0 0 / $ 0 2 . 5 0 © 1981 Else~er ScientfficPubHshing Company

108 where c is the permittivity of the sample of thickness d.

To follow charge

evolution on and in the sample successfully it is arranged to be charge free or at least neutral prior to initiating an experiment at t = O. and O(x,y,O)

Thus @(x,y,z,O)

= O

is then the charge initially deposited on the surface as tbe result

of a contact or some such process.

In the present simple argument,

the materials

are assumed to be non-polar but the inclusion of dipole orientation processes

in

response to the appearance of O(x,y,O) while complicating the treatment considerably does not alter the principle of measurement. After the initial charge deposition, o and p will evolve in a complex way depending on charge transfer rates from surface to bulk states and on the field and charge carrier mobility in the bulk.

There is also the possibility of charge

spreading laterally across the surface. complicated time-dependence

Thus V(x,y,t) will,

in general, have a

(ref. 5) which will be discussed later.

TRANSFER AT METAL CONTACTS If charge is to transfer when a metal contact is made to an insulating solid there must be accessible states in both.

At normal temperatures the most likely

process is quantum tunnelling between states of like energy and separated by a distance of iO-9m or less.

The charge transferred to the insulator on a single

contact may easily be monitored both in magnitude and spatial resolution by the scanning probe and is typically lO-6Cm -2. biopolymers

For insulating synthetic polymers and

in the dry state the charge transfer is quite selective and remains

highly localised over relatively long times, a behaviour reinforcing the argument of Duke and Fabish (ref. 6) that the electronic states in these polymers are localised molecular states.

The product of the tail of the Fermi distribution

function for the metal and the tunnelling probability function for the inter-state barrier create an energy

'window' for charge transfer about Ef (Fig. 1).

contact

meta I E state I

Fig. i.

,

I II

E

u lator state metal state density

I/> tunnelling probability

transfer probability 'window '

Tunnelling and transfer window concepts.

In a practical

situation charge transfer is made more complicated by local

mechanical deformation at the contact.

This is especially so in the present

109 instance where metal biopolymers.

contacts are made to easily deformed compressed

Several authors have com~nented on this and Lowell

suggested that deformation

of contacts grow.

(ref. 3) has

leads to stirring in which new molecular

brought from below to within tunnelling

range on the surface.

discs of

sites are

In a succession

to the same surface region the total charge transfer will tend to

Modifying Lowell's

treatment

in certain respects,

the local charge density

after N contacts becomes ~N = eftnt

E1 + ~/~ - ~/~ (I-B)N~

where n

is the concentration of sites in the surface region of thickness t, f is t the fraction contacted according to the 'window' ideas and ~ and ~ are the

proportions

of charge which move out of and into the surface region as a result

of the deformation.

Fig. 2 shows how well this law is obeyed for a synthetic

1"0:

O'B

o-G

/o

./

o .

./

¢.,

v

b7

o 0"2

0

0 0 0

(a)

/

/

E o 0-4.

o/ e/

/ i

I

200 1 O0 10

400 200 20

I

600 300 30 N

I

600 400 40

t

1000 500 50

(a) (b) (c )

Fig. 2. Increase of charge with number of contacts N; experiment compared with theory. O~ normalised to unity. Contacts: (a) nickel to dry collagen at a pressure of O.15MPa, positive charging; (b) aluminium to PET, negative charging (ref. 3); (c) mercury to PET, negative charging (ref. 3). (a) (b) (c) ~ 0.2 0.15 O.i 1.6xlO -3 0.O19 O.214 o/~ 1 126 9 1.47

110

polymer polyethylene terephthalate biopolymer collagen.

(PET) and a highly compressed disc of the

Since maximum stirring might be expected for the easily

deformable collagen and hardly any stirring at all for a mercury contact, seen how the parameter B is a sensitive indicator of this.

it is

The greater the degree

of polymer stirring the smaller is $ and the greater is the ratio of the ultimate limiting charge to the first contact charge (~ /oi). it has been possible to estimate fn

.9 when the contact metal is changed.

t

From ~I and particularly

and to determine how the 'window' alters

Figure 2 also introduces another important feature.

Whereas contact to PET is

negative signifying electron transfer from the metal, the opposite occurs for collagen. windows

In the latter case there must be donor states for which the tunnelling

lie above the Fermi energy of the metal which in this case was nickel of

work function 5.O3eV. Donor and acceptor sites may coexist in the surface as illustrated by Fig. 3.

/

j

x (c)

x (b) ~

10

0.---- 0 ~ 0 ~

)

oJ

i

0

o

0

o --

•

O / . ~ (a

E LJ -I

5 ~)

0

I

IO 0

I

2-00

N

,o ,,..,. o " •

Fig. 3. Contact to collagen sample containing chloranil impurity. (b) nickel and (c) platinum.

(a) niobium,

A succession of contacts was made to a disc of collagen containing Chloranil as an impurity.

The initial charging was negative when a niobium contact was used

suggesting that chloranil-induced niobium (3.7eV) were being filled.

acceptor states below the work function of Continued contact stirring led to a nett

positive charge as expected for collagen alone. with higher work functions

Electrodes of nickel and platinum

(5.03 and 5.32eV) on the other hand did not allow

Iii initial negative charging of chloranil sites.

On some occasions the probe was

able to resolve areas of both positive and negative charge coexisting stably on the surface.

These impurity effects taken with those reported by Lowell (ref. 7)

indicate the complexity of the charging process. Since an imposed electric field will alter the relative positions of the energy states on either side of a contact and so change the 'window' for transfer (Fig. i), it may be anticipated that a voltage applied across a contact alter the charge transferred.

will

Indeed this effect has already been reported.

For

example (ref. 8), an aluminium contact to PET causes negative charging which is practically unchanged at 3×10-4Cm -2 for imposed external fields between -30 and +21×106Vm -I.

Only when the field exceeds +21×106Vm -I does positive charging

occur and then only weakly, indicating that PET has relatively few and deep-lying donor states.

Collagen behaves similarly but, in this case, it is very difficult

to induce negative charging (Fig. 4).

Chloranil may also be produced in compressed

4 / I 3

•

I

/

I

"

/

'E

J

I iI

(a)

•

°

1, l l ~ a . - , , . . - . . - . , - l l . ~ -1ooo

-soo

•

/_ o/~°~

.,,~'/I

0 o J

___~t "

'

°

o

I soo

I ~ooo

bias voltage CV)

-1 -2

Fig. 4. Effect of bias voltage on contact charging after 3 contacts (a) collagen, and (b) chloranil (values scaled by 0.i). Limits according to eqn. (i) shown by broken lines.

112 disc form and, as expected since the molecule is strongly electronegative, negatively very readily (Fig. 4).

charges

Indeed the charging follows the high surface

state density limit law predicted by Hays

(ref. 9) namely,

for an impressed field F

o(F) = ~(o) - ~: F. Col]agen,

also accepts surface

(l)

as can be s e e n from F i g . positive

state

limit

4 charges at

c h a r g e when t h e o p p o s i t e

less bias

than this is applied

limit.

Chloranii

but the high

is not reached.

SURFACE CHARGE DECAY The surface potential V(x,y,t)

due to deposited charge will, in general, decay

with time for a variety of reasons.

We consider only the one manifest in the

experiments to be described which is transport of charge from the surface states into and through the bulk states to reach the grounded electrode. (see ref. 5) are illustrated in Fig. 5.

surface

Carriers

bulk

~~ --

- -

Y~

~_--

I/e~

rt~ -

~molecular .J states lraps

----~Jb~accepior

--

-

F i g . 5. S u r f a c e - b u l k t r a n s p o r t mode[ f o r e l e c t r o n but through donor molecular states. move, probably by activated hopping, (ref.

The processes

in the bulk (electrons or holes)

motion°

Hole motion similar

through a series of localised molecular states

iO) with a mobility ~b' but can be trapped at sites of imperfection.

simplicity a single trap characteristic

For

is adopted and seems adequate with trapping

and release rates r t and rr respectively.

Transfer

from the surface to bulk states

may proceed differently such that a proportion 7i(Y 7i = i) of the charge deposited initially transfers It is possible

to bulk states at a rate 8..

to show that, up to the transit time t d for charge to cross the

sample,

2}

f~ ½ (V(O)/d) 2 I t L~(t){l ~j 5 i e x p ( - 0 i t ) ] dt ,o w h e r e s p a t i a l c o o r d i n a t e s a r e o m i t t e d f o r s i m p l i c i t y and t h e e f f e c t i v e

V(t) =V(O)

dependent,

-

trap-controlled

time-

mobility

~J(t)=~%l! r + r t exp (-Rt)]/R_

where R = rr + rt.

(3)

We have made experiments based on this time-of-flight relevant surface and bulk parameters biopolymers.

(2)

for polyethylene

theory to determine the

(ref. 5) and for some

The initial charge d(O) may be deposited instantaneously either by

metal contact as above or in a completely non-contacting way from a grid-controlled corona in air, which is a very convenient source of positive or negative ions

113

(ref. 4).

The conditions

ions to insulator

for tunnelling

transfer of charge from states of these

states will be similar to those for metal contacts

except that the Fermi distribution

of electron states of the metal is replaced

by a narrow Gaussian one of ion states.

Some examples of experimentally

V(t) are shown in Fig. 6 together with the predictions

I'N,°

800 -

(Fig. i)

based on equations

determined (2)

2.4

Xo

1ONO%

(a)

X

>2-0

(b)

0%

\.

O,~

* o"~-.,

700

~

1-2

I

I

I

I00

200

300

t.d

"O"'O,.o ~ I 4

0

t (s)

I

Igo 8

I

t (s)

24 0

110

(c)

(d) 90

*~ 2 0 0

~o\

~>

>

70

°'e.o 160

• • • •. ~p4°

0

I

L

5

10

50 OO

15

I

0

40

t (s)

I

I

I

80 120 t (s)

160

Fig. 6. Experimental curves of V(t). (a) PE film, positive corona ion charging (ref. 5); (b) bovine serum albumin, negative corona ion charging; (c) collagen, negative corona ion charging; (d) collagen, single metal contact charging. For (a), (b) and (c) the theoretical predictions are also shown. and (3).

Note particularly

in the case of the biopolymers

(i) that the limiting

value of ~ =~brr/(rr+rt ) is attained when the decay enters a linear range and before t d is reached and (ii) that just after the charge front reaches the ground electrode

at td, there is a characteristic

illustrative

values of the parameters

are given in the Table below.

disturbance

deducible

of the decay.

from such experiments

A few and theory

114 TABLE I Some typical transport parameters deducible from experimental employing equations (2) and (3) and the model of Fig. 5. Field (iO6Vm -I)

@I

+104

40

c~o

-84

0

II0

Biopolymer

(I) -1.7

O

~

Biopolymer

(II)-4.13

O

~

Biopolymer % water.

(1), Collagen + 8.9 wt % water.

PE (ref. 5)

G2 @3 (s-i)

~

Y1

Y2

0.8

0.2

Y3

curves of V(t) by

~b (m2V-is -I )

rr (s -1)

rt (s -1)

4.2xlO -15

O.O1

5.7xlO -5

0.35 0.25 15xlO -15 -IO 0.48 0.52 2.2xlO -ii 0.7 0.3 2xlO

0.04

2.1xlO -4

0.4

0.07

0.5

0.03

O.16

(II), Bovine serum albumin + 3.9 wt

CONCLUSIONS Techniques

for detailed

study of the behaviour

now exist which can yield important and about the dynamics contact,

transfer

direct information

solids

about carrier mobilities

of charge states in the surface and the bulk.

In a

from metal to surface states and thence to the bulk appears to

be highly selective

and localised

will always play an important not reported

of charge in insulating

and it is likely,

role in regulating

therefore,

conduction.

here in detail, have shown that metal contacts

to be strongly blocking when compared with equivalent

that the contact Indeed experiments,

to polymers

electrodes

are like]y

produced by ion

deposition.

REFERENCES i 2

D. K. Davies, Adv. Stat. Elec., 1 (1970) 10-2]. T. J. Fabish, H. M. Saltsburg and M. L. Hair, J. Appl. Phys., 47 (1976) 930-939 and 940-948. 3 J. Lowell, J. Phys. D: Appl. Phys., 9 (1976) 1571-1585. 4 E. A. Baum, T. J. Lewis and R. Toomer, J. Phys. D: Appl. Phys., iO (1977) 487-497; J. Phys. D: Appl. Phys., Ii (1978) 703-716 and 963-977. 5 R. Toomer and T. J. Lewis, J. Phys. D: Appl. Phys., 13 (1980) 1343-1356. 6 C. B. Duke and T. J. Fabish, J. Appl. Phys. 49 (1978) 315. 7 J. Lowell, J. Phys. D: Appl. Phys., 12 (1979) 2217-2222. 8 R. Toomer and T. J. Lewis, Electrostatics 1979, Inst. Phys. Conf. Set. 48 (1979) 225-232. 9 D. A. Hays, J. Chem. Phys., 61 (1974) 1455-1462. IO N. Krupp, Static Electrification 1971, Inst. Phys. Conf. Set. Ii (1971) 1-15.