The relation between contact potential and planar conduction as a-Si : H films undergo gas adsorption or temperature changes

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The relation between contact potential and planar conduction as a-Si : H films undergo gas adsorption or

temperature changes

J. Abelson, G. de Rosny

To cite this version:

J. Abelson, G. de Rosny. The relation between contact potential and planar conduction as a-Si : H films undergo gas adsorption or temperature changes. Journal de Physique, 1983, 44 (8), pp.993-1003.

�10.1051/jphys:01983004408099300�. �jpa-00209683�

The relation between contact potential ^and planar conduction as a-Si : H films undergo gas adsorption ^or temperature changes

J. Abelson

and G. de Rosny

Equipe Synthèse ^{de Couches} Minces pour l’Energétique, LPNHE, ^Ecole Polytechnique, 91128 Palaiseau Cedex, ^France

(Reçu ^le 15 février ^1983, révisé le 18 avril, accepté le 28 avril 1983)

Résumé.

Quand la surface libre d’une couche mince de silicium amorphe ^est exposée à de la vapeur d’eau, ^le potentiel ^de contact et la conduction planaire varient simultanément. Nous corrélons les deux effets

trois échantillons et nous interprétons ^les

terme de modifications, induites par l’adsorption, ^du potentiel

de surface, de manière similaire

analyses ^des expériences ^{d’effet de} champ. Pendant les cycles thermiques,

observe

forte corrélation entre les portions

linéaires de la fonction liant le logarithme de la conduction à

1/T ^et la variation du potentiel ^{de contact}

Abstract.

When the free surface of an amorphous ^silicon thin film is exposed ^{to water} vapour, the ^contact poten- tial and planar conductance simultaneously change. ^{For three} samples,

correlate the two measurements and

interpret ^{them in terms of adsorbate induced} changes ^of the surface potential, ^{similar to} field effect experimental analysis. During temperature cycles, ^{there is}

strong correlation between non-linear portions ^{of the} log ^conduc-

tance

1/T plots ^{and the} variation of the contact potential.

Classification Physics ^Abstracts

72.80N - 73.30

73.60

1. Introduction

The planar conductance of a-Si : _{H may} change by

orders of magnitude upon adsorption of certain _spe- cies. Fitsche and Tanielian [1, 2] attribute this effect to a donor or acceptor like behaviour of the adsorbed molecules which accumulate or deplete carriers in a

conduction channel beneath the surface. However

they ^{do not} quantify the action of adsorbed species by measuring the induced surface charge density ^or

surface potential. ^Here, by simultaneously monitoring

the change ^{in contact} potential ^and planar ^conduc-

tance during gas adsorption, ^we demonstrate that the effect is similar to what is observed in field-effect

experiments [2-5], ^which supports ^the interpretation

of the above authors. The a-Si : H surface was expos- ed to H20 ^and Br vapour, ^water acting ^{as an electron}

donor and Br2 ^{as an electron acceptor. The} adsorption experiments ^were performed ^{at room} temperature

on samples ^either simply heat-dried or heat-dried and then illuminated by strong ^white light ^{to induce}

Staebler-Wronski effects [8]. ^{We show that, on our} samples, the difference between the ^two sample pre-

*

Present address : Material Science Department, ^Stan-

ford University, Stanford, ^CA. 94305, ^U.S.A.

parations originates partly from surface or interface modifications and that volume effects are also _pre- sent

The determination of activation energies ^{in a-Si : H}

films is sometimes problematic ^{as the} logarithm ^of

conductance versus 1/T plots ^{do not} always ^exhibit unique slopes ^and furthermore, ^{even in} the favourable situations where an activation energy may be measur-

ed, ^the preexponential ^factor exhibits undesirable variations of several orders of magnitude. ^The origin

of these observations has been attributed to interface effects [5] ^{or to more} complex phenomena [7]. ^We

show that measuring ^contact potential ^{as well as}

conductance during ^thermal cycles ^{enables one to}

know if surface effects are present and whether the deduced activation _energy and preexponential ^factor

are reliable in each specific ^case.

2. ExperimentaL

The experimental arrangement ^{is shown} schematically

in figure ^{1. The} planar conduction of a-Si : H takes

place ^{across a «} gap » in the contact metallization of width d and height ^h

^{2.5 cm. The} applied ^electric

field was smaller than 35 V/cm ^{and ohmic conduction}

was verified to take place. ^The sample ^contact poten- tial is measured by ^{a Kelvin} probe ^{located over the}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01983004408099300

Fig. ^1.

^{Schematic of the} experimental ^{set up.}

current collecting electrode. The substrate is mount- ed on a copper heater block. A vacuum enclosure

permits evacuation by ^a mechanical _pump to 10 mtorr.

The adsorbate _gases H20 ^or Br2 may be injected ^into

the chamber by ^{a metered flow of} argon carrier _gas which bubbles through ^their liquids. ^A viewport

allows illumination of the sample.

The Kelvin probe ^or vibrating capacitor ^method

functions on the basis of establishing ^{an a.c. null} [9, 10]. The difference in work function between the Au

grid reference electrode and a-Si : H surface creates

an electric field between the two, which will induce

an a.c. current if the spacing and thus the capacity ^is

modulated by vibration. The applied d.c. level V c

is that needed to cancel the work function difference, making ^{the a.c. current} disappear : q. V, = OAu - Øa-Si. ^{Thus an} increase of Øa-Si ^as the bands bend _up at the semi-conductor surface, ^{causes a} decrease in Vc

and vice ^{versa. If} the a-Si is not in thermodynamic equilibrium ^so that its Fermi level and therefore its work function cannot be defined, ^{the a.c.} null condi- tion implies ^{that the} following relation is verified :

q. Vc

’PAu - X. ⁺ qvs where X. is the a-Si surface electron affinity and vs ^{is the} potential ^of the conduction band edge ^{at the} surface, evaluated with respect ^to the back electrode Fermi level. Of course this last relation holds when thermodynamic equilibrium in the a-Si film is achieved for, ^{in this} situation, Oa-si

X. - qvs.

On _gas adsorption and/or temperature changes,

the quantities oAu, Xr and vs may vary. We studied the

q5Au variations by evaporating ^{1 000 A of} gold ^onto

the probe ^{and a} polished ^{stainless steel} plate, mounting

the plate ⁱⁿ place of the a-Si and monitoring V c

for different treatments of the two surfaces. At the

beginning Vc ^{was found null, then we} rotated the

Kelvin probe away from the substrate and annealed the Au coated plate ^{to 160° for 1 hour in} N2. ^{When the} probe ^was brought ^into position Vc ^was equal ^to

200 mV, ^a reasonable difference considering ^that l/J Au

can vary by ^{1 V} [11]. ^{In one hour} Fc equaled ¹²⁰ mV,

as the probe ^{heated over} the hot substrate. One atmo-

sphere ^{of water} saturated air was admitted, producing Vc

^{12 mV 15} min. later. Pumping ^{out the atmo-} sphere yielded Vc

^{80 mV. Then we} repeated ^the procedure ^of checking the evolution .in Vc ^{when the}

room temperature probe ^was positioned ^{over the}

hot Au plate. ^{In vacuum no} further evolution was

found as the temperature changed between the Au surfaces. This indicates that the probe work function had been stabilized by the successive thermal cycles.

The adsorption ^of H20 in saturated atmosphere ^on

the cold probe, ^but presumably ^{not on the hot Au} plate, ^decreased Vc by = ^{30 mV and was} reversible.

Three a-Si : H samples have been studied : ^one obtained from an RF glow discharge ^of pure silane

provided by ^Proí Solomon, ^{at Ecole} Polytechnique,

one obtained under similar conditions at the Solar

Energy Research Institute (SERI) and the last one

prepared ^{at SERI from a silane} BF3 ^{mixture. For}

detailed parameters ^{see table I.}

3. Results.

3.1 GAS ADSORPTION. - We did not try ^to repeat the detailed study of Tanielian [2] ^on the effects of exposures of the a-Si surface to H20 partial pressures

as a function of _pressure and time. Instead we used the _gas as a tool to vary simultaneously ^{the contact} potential ^{and the} planar electrical conductance, ^seek- ing correlations between these two quantities. ^The complex chemistry relating the variation of the semi- conductor surface charge ^{to the} presence of a given

amount of water in the _gas phase is then in some way

bypassed Figures ^{2 to 7} show the effect of water

adsorption ^{on the these} samples. Before the _gas injec-

tion the samples had been either annealed to 160 OC

(state A) ^or annealed and then illuminated (state B).

The correlation between the contact potential V c

and the electrical conductance G is demonstrated by

the figures, ^each sample behaving ^{in a} specific ^way

which will be analysed ^{in more} details. We must note that the correlation between V c - ^{and G is not} strictly unique ^{for a} given sample, ^{as can be seen on} figures ^{6 or 7 where water} desorption ^{was initiated}

Table I.

Sample characteristics.

Fig. ^2.

Sample I : conductance

contact potential

under exposure ^to H20 vapour, for

annealed initial state (state A). ^{The time} elapsed ^is quoted in minutes. The

curve

is

fit to data

explained ^{in the text}

by gas evacuation. The adsorption ^and desorption

curves do not coincide. However shapes ^{are similar}

and at a given ^{G the} discrepancy ⁱⁿ Fc ^{is at most}

50 mV. Possible explanations ^{for this} effect will be discussed later.

The sample properties ^are analysed, using ^the following simplifying assumptions :

-

The adsorbed species ^act only ^{on the film sur-}

face and do not initiate _any change in the bulk struc- ture which is furthermore assumed homogeneous.

They ^affect the film electrical properties only ^via

the induced surface charge ^or potential.

- The film electrical properties ^are however also controlled by the a-Si substrate interface charges ^or potential. The studied films are assumed thick enough

so that there exists a flat band region ^{in the bulk.}

Fig. ^3.

Sample ^{I :}

^data

^for figure ²

semi-log

scale. The straight ^lines slopes s

^{such that}

The widths of the _space charge regions may be estimat- ed from formula 6 of reference 6 : for a 1017 cm- 3 eV-1 uniform density ^{of states in the} gap, the space charge Debye length ^{is 8.5 x 10-2} Jlm. The films under study ^{are thick} enough ^to insure the exis- tence of a flat band region, provided ^{that the} density

of states is not much smaller than the above value.

In this situation, the interfacial region ^{is not affected} by the modifications arising ^at the free surface. The observed conductance changes may be attributed to

changes only in the free surface space-charge region.

The surface potential VS may then be referenced with respect ^to the flat band region : flat bands at the sur-

face imply Vs

⁰ by definition.

-

A quantitative analysis ^{similar to the one} per- formed in field-effect experiments ^is possible ^{at the}

expense of the same assumptions ^discussed by ^Good-

man and Fritzsche [4]. Furthermore, ^{in the} present

case, the observed contact potential variations _may arise from a variation of both the a-Si affinity X.

and the surface potential V. (the OAu ^{variation was}

shown to be small and is neglected). ^{We shall assume}

that the variation of _X. is negligible ^and identify ^the V,

variations with the variations of the surface potential.

The validity ^{of this} assumption ^{is not evident as} H20

is polar and Helmoltz dipoles may easily ^build up at the surface, modifying X. [12]. ^The resulting ^rela-

tions between the conductance and the surface poten- tial, assuming ^{0 K} statistics, ^are developed ^{in the} appendix. They ^{involve a} few unknown parameters that can be adjusted ^to fit with the experimental ^data.

See the appendix ^for the definition of the conduc- tance used in the following analysis.

Sample ^I.

^In figure 2, the initial conductance decrease is followed by ^an increase, ^{while the contact} potential ^increases monototically. This behaviour reflects that the bands, initially ^curved upward ^{at the} surface, ^are progressively lowered and are finally

curved downward : H20 ^is acting ^{as an} electron donor and the carriers are initially holes and finally ^elec-

trons.

For large ^band bendings, the conductance is domi- nated by ^the transport ^{in the} region ^{close to} the surface

[4, 6]. ^{It varies} exponentially with the surface potential

if the density ^{of states is constant in the} gap region swept by the Fermi level : G oc exp ± qVs/kT [6].

Figure 3 shows the same data as figure ^{2 but on a semi-} log scale, ^{and one} verifies that at large ^{G the} plot ^is

linear. The slope ^{at small} Vs ^is

q/kT ^however

at large Vs ^the slope ^{is 0.5} q/kT. ^{Such a} behaviour is

expected ^{if the} density ^{of states increases} exponen-

tially ^{as a} function of _energy around the flat band E-E

Fermi level: N(E) - N(EF) exp E y F ^{where N(EF)}

is the density ^of states at the flat band Fermi level

(formula A. 4). From the observed slopes ^{of Ln} G/ VS

one deduces that EY N ^kT

^{26 meV (see for-}

mula A. 4 and A. 4’ of the appendix). ^This slope

Fig. ^4.

Sample ^{I :} conductance

contact potential

under exposure ^to H20 vapour, for

illuminated initial state (state B). ^{The time} elapsed ^is quoted in minutes.

is somewhat larger ^{than the ones} deduced from field effect experiments [4, 5]. Moreover the 0 K statistics

approximation ^is poor in this situation.

One can try ^to describe the whole curve using ^for-

mula A. 3 of the appendix :

G(Vs) ^{is the} conductance, Vs the surface potential

referenced to the flat band position. G(0) ^{is the con-}

ductance for flat band conditions at the free surface,

it includes a possible contribution due to band bend-

ing ^at the silicon-substrate interface. In ^and Ip ^are

mathematical functions depending on Vs ^and E.,

as detailed in the appendix, N(EF) ^{is the} density ^of

states at the Fermi level in the bulk region, Qn and up

are the bulk conductivities of electrons and holes

respectively.

The conductance is minimum at a value of the

n’

h

qvm

surface potential VS satisfying the relation : kT

Ln P (A.2) (this ^{holds for} any density ^{of states} an

distribution). ^The experiment provides ^{the value} V’

of the contact potential ^at which G is minimum. Vr

has been related to V. assuming ^{that these} potentials

are shifted by ^{a constant} quantity Y° ^{so that} Vs

Vc-

Vf, yo ^{is the contact} potential for flat band condi- tion at the free surface.

In our experiment, Vf is deduced from a fit to the whole G(yS) ^function using formula A.3.

(Ey

^{26 meV is used as discussed above). Some}

authors [13] ^{use the} hypothesis that under strong ^illu- mination the bands should flatten thus providing directly V0c ^{from a Kelvin} probe measurement under illumination. The results we obtained by this method

were not reliable so we discarded its

The fit (curve ^of Fig. 2) yields ^the following ^{values :}

-

The flat band contact potential Y° ^{is 280 mV}

and the value of the conductance G(0) ^{is 4.5 x} 10-140-1.

-

V7, the value of contact potential ^{at the con-}

ductance minimum is 175 mV, yielding Vm

-

105 mV. From (A. 2), ^one gets Gp/Gn

^0.02.

This implies ^that the bulk of the sample ^under study

is slightly n-type. ^More precisely the Fermi-level is located - 50 meV above the position which would lead to an equal contribution of electrons and of holes to the conductivity.

Note that in contrast to conventional field effect

experiments, ^the up/an ^{ratio is} obtained without the need of _any extra information. See reference 4 for a discussion on this subject.

An _upper limit for an may be deduced as follows : when the bands are flat at the free surface, ^{the con-}

ductance G(0) ^{is the sum} of the flat band conduc- tance GFB ^{in the whole} sample and of the conduc- tance at the semi-conductor substrate interface GI.

The latter _may be expressed ^{as a} function of the inter- face potential by ^{the same formula as the one esta-}

blished for the conductance at the free surface, ^since the sample has been assumed homogeneous. ^{It is}

then _easy to show GFB ² G(0) - Gm ^where Gm =

2.8 x 10-14 Q-1 is the value of the conductance

minimum, ^{so that} GFB ^{6.2 x} lO-14 Cl - ’. Now

GFB

e( Qn ⁺ Qp) N ^{eGn where e, the sample thick-}

ness is 0.5 _pm, so a. ^{1.2 x} 10- 9 0-1 cm-1.

-

uan it 1 c F’o _{is found}

e ual to

-

The

quantity - V N(EJ ^{Un is found equal to}

q N(EF)

1.1 ^x 10-14 0-1. One then deduces ^an upper limit for N(EF), ^{the Fermi level} density ^{of states :}

This ^{is a} typical ^value [6].

Figures ⁴ and 5 show the conductance dependence

with contact potential ^of sample ^I initially prepared

Fig. ^5.

Sample ^{I : same data}

^for figure ^{4 on}

semi-log

scale. The straight ^lines slopes

^{such that}

in state B (Staebler-Wronski), in linear and semi-log plots respectively. The initial conductance is slightly

lowered compared ^{to state A and the} shapes ^{of the}

curves are not the same. The slopes ^at large ^G (Fig. 5)

now suggest ^{a flat} density of states, however ^one does not find _any satisfying ^{fit to the data.} Light ^induced

thickness inhomogeneity ^{is a} possible origin ^{of the} discrepancies ^{between states A and B.}

Sample ^II.

Figure ⁶ shows the conductance as a

function of contact potential ^under exposure ^to H20

vapour. Curve A corresponds ^{to an} annealed initial state (state A) ^{whereas curve B} corresponds ^{to an}

illuminated initial state (state B). ^{Curve B has been}

shifted upward by ^{the amount indicated} by ^{the arrow.}

The two are approximately superposable, although

state B exhibits a slightly higher initial conductance. A noticeable feature is the lower asymptotic ^conduc-

tance and contact potential ^induced by ^{water on}

state B as compared ^{to state} A. This effect has been observed by ^Tanielian [2], ^{who uses it as an} argument in favour of light induced bulk defects. On the present

sample, ^the similarity ^{of the two curves} suggests rather a light induced surface modification that limits the H20 activity.

The constant conductance increase with contact

potential indicates that the bands are initially ^either

flat or curved downward, however if aplan ^{1 they}

may also be slightly ^curved upward ^At large G, ^the log G/ VS slope ^{is too small to be} compatible ^{with the}

conductance model in the appendix.

Fig. ^6.

Sample II : conductance

contact potential

under exposure to H20 vapour, elapsed time in minutes.

Curve A : annealed initial state (state A). Section 0-11 : exposure ^to H20, sections 16-20 and 0-28 : effect of pump-

ing. ^{These two sections}

separated by

unrecorded time.

Curve B : illuminated initial state (state B). ^For clarity ^the

data have been shifted upward by ^{the amount indicated} by

the

Figure ⁷ shows the correlation between conduc- tance and contact potential ^under exposure ^to Br vapour. The evolution is _very fast and difficult to

control, ^{so the data are of} poor quality. The conduc- tance increases by ^{two orders of} magnitude ^{while the}

contact potential ^decreases by - ^{300 mV : Br acts as}

an electron acceptor ^{and the bands curve} upward. ^The

action of Br on the probe ^{has not been} verified, ^so there _may be also an appreciable variation of the gold

work function under _exposure to Br.

From figures ^{6 and 7 ones} deduces that the conduc- tance minimum Vm lies between V c

^{150 and 250 mV.}

It appears that, within the caveats on the interpretation

of figure 7, ^{G increases more} slowly ^{as a} function of

Vc ^when V, increases from V7 than when it decreases.

This is similar to the situation displayed ^on figure ²

and it indicates that up/ an ¹ and that the flat band

position Vf ^is higher ^than Ym. The lack of a reliable model prevents ^{however a} quantitative determination

of Vco.

From the lowest measured conductance - 2 x

lO-14 Q-1 and using ^the arguments developed ^for sample I, ^one gets ^an upper limit for an of the ^same order as for sample ^{I :} an ^{10-9 Q-1 cm-1. No} quantitative evaluation of the density ^{of state is} reliably achieveable, however the large conductance sensitivity

to the contact potential variations indicates that it must be around or less than 1017 cm-3 eV-1. It _may indeed be so low that there is no flat band region ^{in the} sample. ^Then changes in surface potential ^would

induce changes ^at the interface which would explain

the failure of the conductance model developed ^{in the} appendix. ^This interpretation should be ckecked

using ^{a thicker} sample.

Sample ^{III. - This} sample ^was prepared ^{with a} 3 % BF3 in SiH4 ^{mixture so- it is} presumably p-doped Figure ^{8 shows the} change ⁱⁿ conductance ^{as a} function of contact potential ^{when the} sample ^is exposed ^to H20

vapour. Curve A corresponds ^{to an} annealed initial state (state A), ^{curve B to an} illuminated initial state

(state B). ^{There is a net} shift of the conductance

Fig. ^7.

Sample II : conductance

contact potential

under exposure ^to Br, vapour, for

annealed initial state

(state A). ^{The time} elasped ^is quoted in minutes.

Fig. ^8.

Sample III : conductance

contact potential

under exposure ^to H20 vapour. Curve A : annealed initial state (state A). ^{The time} elapsed ^is quoted in minutes. The

curve

is

fit to data

explained ^{in the text Curve B :} illuminated initial state (state B). ^Section 0-20 exposure ^to

H20- Section 20-120 : effect of pumping.

between the two that _may be attributed to an illumi- nation induced change ^{of the} substrate interface

potential. ^Moreover they ^{do not} exhibit the same

curvatures, which indicates ^a modification of bulk states. The initial and final contact potentials ^are only

shifted by N ^{50 mV} between the two curves, suggesting

a minor effect of illumination on the action of H20 ^at

the sample ^surface.

The conductance decrease when Vr increases indi- cates an initial hole transport This decrease is slower than exponential ^{and tends to a} minimum. This indi- cates that the flat band condition is in the vicinity ^{of the}

initial V c value and minimum conductance not far

beyond ^{the final} Vc value. Indeed curve A _may be

fairly well described using ^{a flat band} approximation (formula ^A. 5, ^{curve on} Fig. 8); Vm ^{was fixed at}

650 mV and Vc0 ^{is found at 200 mV.}

Using ^{the same} arguments ^{as in the} study ^of sample I,

one finds a ratio Qp/ Qn

^{3.3 x 10’ at flat band, while}

the Fermi level is located - 225 mV under the position leading ^to equal electron and hole conductivities. This demonstrates that the sample ^is slightly p-doped

From distribution A one gets ^a up ^upper limit of the

order of ^{2.5 x} 107 g-1 cm-1. The quantity

1 880 - . ^{cr is found equal to 1.5 x io-12 c)-l}

and the corresponding upper limit for N(EF) ^{is then}

1.6 x 1017 CM-3 eV- 1.

3. 2 THERMAL CYCLES - After _exposure of the sample

to H20 ^vapour the enclosure is pumped out, then the conductance and contact potential changes ^{are moni-}

tored during temperature cycles. Figures ^{9 to 11} show

their variations as a function of 103/T ^for samples ^I

to III respectively. ^Each figure shows the effect of the first heating ^and cooling, which differ because of the water desorption during heating. ^On subsequent cycles, ^either heating ^or cooling ^{induce G} and

variations that are close, ^{but not} identical, ^{to the first} cooling cycle.

The effects of surface and interface potentials ^{on the}

T dependence of the conductance have often been stressed [5, 6]. ^{In the} present experiment, measurement of the contact potential brings ^some information on

the surface, ^but contrary ^to the fixed temperature adsorption experiments, the effect of the interface cannot be separated

Sample ^I (Fig. 9).

The values swept by ^{the contact} potential show, by comparison ^with figure 2, ^{that the} surface is highly p-type during the thermal cycles. ^If

the interface potential ^{does not} vary ^too much during

the experiment, ^{one may} infer that most of the conduc- tance takes place ^{at the free} surface. Indeed ^one observes ^on figure ^{9 that at a} given T, ^the higher

conductance corresponds ^to the lower potential ^as expected. Moreover, in the surface potential region

under consideration, ^one expects ^{that the} following

relation between the conductance and the surface

potential ^{holds at fixed T : Ln G}

q Vs/k T ^{+ C,}

where C does not depend upon V. (formula ^{A. 4 of the} appendix). This formula is used to find the expected Vc

upper branch, shown _{in open} circles, using ^{at each T}

the measured conductances and the lower branch value of Vr. ^The origins of the obvious discrepancy ^of

Fig. ^9.

Sample ^{I : contact} potential (upper figure) ^and log conductance (lower figure)

103/T. ^The

indicate the direction of evolution from

temperature.

Before the thermal cycle, the surface had been exposed ^to

water vapour and then put ^{under vacuum condition} (- ^{10- 2} torr). ^{Circles :} position ^{of the} expected V c upper values computed ^from conductance and lower Vc ^measure-

ments

explained ^{in the text}

the order of 70 mV may be multiple : the variation of the gold ^{work function} resulting ^{from water} desorp-

tion could account for 30 mV or a sample ^{too thin} compared ^{to the} Debye length, ^{so that no flat band} region ^exists, would invalidate the above formula.

These possible ^{effects are now under} study.

Sample ^II (Fig 10). ^{- Here} again the variation of conductance with surface potential ^{at fixed T is in} qualitative agreement with the data of figure ^{6. Notice}

the straight line in the Log G/( 1 / T) (activation energy 0.63 eV) plot associated with a constant surface poten- tial during ^the cooling cycle. ^The V, value - 1 20 mV

corresponds ^{to a} position ^{close to} the conductance minimum at room temperature (see Fig. 6). ^A straight

line is expected when the surface conductance domi- nates, which corresponds ^{to a} strong bending ^{of the}

bands. The measured activation _energy is then

Eac - I e V s I [6] ^where E3£ is the bulk activation _energy of the carrier type accumulated at the surface. It is however presumably ^not the situation encountered here since V. ^{is close to} the conductance minimum,

where both carriers contribute to the surface conduc- tance. More probably, ^{one is in a} situation where the free surface conductance is negligible compared ^{to the}

conductances in the bulk and/or ^at the substrate inter- face. A _pure bulk conductance would imply ^a pre- factor ao

3.8 fl-’ cm’ which is too small and indi- cates an appreciable contribution of the conductance at the substrate interface [6]. ^The potential ^{at the}

interface would then be independent ^{of T to account}

for the observed straight line in the Log G/(1/T) plot

Sample ^III (Fig. 11).

^The qualitative ^{behaviour of} t e Log G/(1/T) plot may again be understood using figure ^{8. Note that the} Log ^{G curve does not cross at}

the position ^{where the} Vc curve crosses. This effect _may be due to a strong modification at the interface, ^as already stressed when discussing figure ^{8. The} Log GI(I IT) plot ^{is difficult to} interpret quantitatively

as it involves surface potentials where both holes and electrons contribute to the conduction. The last part ^of the cooling cycle ^is performed ^{with an} approximately

constant Vc ’" ^{200 mV, which} correspond ^{to a flat}

band condition at the surface. The corresponding

activation _energy is E.

^{0.3 eV, the} pre-exponential

factor is G = 1.18 x 10-6 fl-’ and they yield

G

10- 11 Q - 1 at room temperature, ^{a value} compa- tible with the one found on figure ^{8. If one} may neglect

the contribution of the substrate interface, ^{the measur-}

ed Ea ^{is to} be identified with Eap, the activation _energy for holes. One then deduces that the level Eo ^corres- ponding ^to equal electron and hole conductivities is located 525 meV above the valence band conduction

edge, ^as it has been shown previously that the Fermi level is located ^{225 mV below} Eo. Remembering ^that

the conductivity ^gap width is in the 1.6-1.8 eV _range [6],

one sees Eo is much closer to the valence band than to the conduction band It implies that the hole mobility

is much lower than the electron mobility. ^Here again

one must take care of a possible underevaluation of the activation _energy due to the conductance at the subs- trate interface, ^{so that on this} particular sample ^the

most reliable information comes from the adsorption experiment

Fig. ^10. - Sample ^{II : contact} potential (upper figure)

and log conductance (lower figure)

103/T. ^{The arrows}

indicate the direction of evolution from room temperature.

Before the thermal cycle, the surface had been exposed ^to

water vapour and then put ^under

^conditions (- ^10-2 torr).

Fig. 11. - Sample ^{III : contact} potential (upper figure)

and log conductance (lower figure)

103/T. ^The arrows, indicate the direction of evolution from

temperature.

Before the thermal cycle, the surface had been exposed ^to

water vapour and then put ^{under vacuum conditions}

(- ^{10 - 2} torr).

4. Discussion.

The assumptions ^{used to} analyse the data include several commonly applied ^to field effect experiments [4]. ^The objections ^{raised to} the latter are also pertinent

here. In particular, ^the strong sensitivity ^{to a} possible inhomogeneity ^{close to} the surface and the invalidity

of the conduction model under strong ^band bending

are often pointed ^out They ^{throw some doubt on the}

deduced Fermi level density ^{of states. However other}

methods sensitive to the bulk density ^{of states} give

values only slightly ^lower compared ^{to the ones}

deduced from field effect data [14]. ^{Two extra} hypo- theses, specific ^{to the} present experiments, ^{are the}

identification of the contact potential Vc ^{with the}

surface potential V. (given ^{a constant} offset) ^{and the} assumption ^{that the} gas is only ^adsorbed superficially

and does not diffuse into the sample. The observation of a non unique ^relation between Vc ^{and G} during

fixed temperature adsorption experiments may be due to the invalidity ^{of one or of} both of these assumptions.

It _may also imply ^{that the} sample ^{is not} really ⁱⁿ equili-

brium and that long ^{time constant} evolution takes

place ^under vapour exposure. A _way to check this last

point ^{would be to} perform ^{the same} experiments ^while insuring ^a much slower evolution of the parameters.

The repartition ^{of the contact} potential between xS, the electron affinity and Vs the surface potential ^{would be}

accessible if it can be proved ^that strong illumination induces flat bands at the surface without changing

other properties ^of the material. Measuring Vc ^under

such a condition then allows one to determine xs [ 13].

Independent measurements of xs would require ^ultra high ^vacuum conditions which are not compatible

with the _presence of _vapour in the vessel.

The present experiment ^{shows that the} planar

conductance measurements are extremely ^{sensitive to}

the value of the surface potential ^as already stressed for instance in reference 6. Here we are able for the first time to demonstrate directly ^{the effect In} particular, ^it

is possible ^{to have a dominant} surface channel conduc- tion by ^{holes in} samples ^{that are otherwise} slightly n-type. These observations emphasize ^{the care that}

must be taken when interpreting planar ^conduction experiments. Similar difficulties are expected ^{to arise}

from the interface with the insulator. An experiment combining field effect technique and Kelvin probe

measurements would allow one to control both surface and interface potentials.

The surface potential ^{is observed to} change during

thermal cycles. ^This change may be correlated with a

variation of the slope ^{of the} Log G/(1/T) distributions.

In the temperature intervals where the contact poten- tial is observed to be approximately constant, ^the

corresponding slope ^{is also constant It} would however be meaningless ^to interpret ^the slope ^{in terms of activa-}

tion _energy without evaluating ^and subtracting ^the

surface channel contribution to the conductance. This evaluation is in principle possible ^{if one is able to} interpret quantitatively ^the gas adsorption experi-

ments, however nothing ^{is known} about the interface

potential. Combination of Kelvin probe ^{and field}

effect would again allow control of both surface and interface potentials during ^thermal cycles.

Light soaking ^{is shown to induce} changes ^{at the} surface, the interface and in the bulk. The three samples

behave in distinct ways. For sample ^{II there is a}

reduction of the band bending variation under water

adsorption, ^{which is} interpreted ^{in terms of} light

modification at the surface. For samples ^{I and} III, ^the bulk _appears to be modified, ^{as evidenced} by ^{the modi-}

fication of the relation between the contact potential

and the conductance. Moreover on sample III, ^there is also evidence for ^a light ^induced change ^{of the inter-}

face potential. ^The present results have to be contrasted to the ones obtained by ^Tanielian [2] who finds that the major ^{effect of} light illumination is a reduction of the band bending variation induced by gas adsorption.

His explanation of the effect by ^a modification of the bulk density ^{of states is} questionable given present

contact potential measurements.

Finally, ^the experiment ^{is now} being ^{redone on}

thicker samples ^{to be sure of} getting ^{a flat band} region

in the bulk, ^so that the modifications arising ^{at the free}

surface do not modify ^the potential profile ^{at the}

substrate interface.

5. Conclusion.

It has been shown on three samples ^{that a} measurement of contact potential ^{as well as} conductance during

both _gas adsorption and thermal cycles may bring original information about the sample. The inter-

pretation ^of adsorption measurements is similar, ^but simpler, than in field effect experiments. Strong light

illumination modifies both the film surface and bulk

(Staebler-Wronski effect). ^The validity of activation energy measurement, ^often problematic ^{due to surface}

or interface effects _may be checked if one simultaneous-

ly follows the contact potential variations. Moreover, in certain situations, the Log G/(1/T) distributions may possibly be corrected to yield ^{the correct activa-}

tion _energy.

Acknowledgments.

The authors wish than Prof. I. Solomon for stimulating

discussions and for providing ^one sample, ^{Dr. R. Kerns}

of SERI for providing ^{the two other} samples, ^and

G. Benet for his skilled construction of the Kelvin

probe.

Appendix.

RELATION BETWEEN CONDUCTANCE AND SURFACE POTENTIAL.

The following approximations, frequently

used in field-effect or capacitance analysis [4] ^{are assumed to be valid :}

-

homogeneous ^material

-

activated electron and hole transport taking place ^{at the} respective mobility edges

-

conductivity prefactors independent of the band bending

-

zero temperature statistics.

Furthermore the sample is assumed thick enough ^{to ensure} the existence of a region ^where space charge

effects _may be neglected (flat ^{band or bulk} region).

DEFINITION OF THE CONDUCTANCE.

Let d be the conductivity ^« gap » width (Fig. 1), ^{h its} height, ^{V the} applied potential ^{across the «} gap » and I the measured current The conductance G used in this article is defined as

G

I/V ^x dlh.

It is such that G

= ^{u(x) dx, u being the} sample conductivity ^{at a} distance x from the surface, e being ^the sample thickness.

RELATION BETWEEN G AND Vs.

6(x) ^{is a} function of x through V(x), the value of the potential ^{at the distance}

x (see Fig. 12) :

an and Qp ^are the bulk conductivities for electron and hole transport respectively Ean, Eap ^{are the} corresponding

activation energies ^and U.nl up ^the corresponding conductivity prefactors.

V(x) is related to the charge density ^{and hence to the} density ^{of states} by ^Poisson’s equation. ^{For a} given YS (the ^{value of Y(x) at the} surface) the solution V(x) ^is unique in the surface _space charge region. Moreover, ^the assumed film homogeneity implies that the solution Y(x) corresponding ^{to various} V. ^{are obtained} by ^transla-

tions along ^{x of a} unique ^curve.

Let xl ^{be a} value of x lying ⁱⁿ the flat band region ^{then :}

The second integral ^{does not} depend ^on V..

The first integral may be written :

is the solution such that V

Vs ^at the surface.

Now let Vs be incremented by AVs, and let åx be the position ^{at which} V(x, V

⁺ AVs) takes the initial value Vs. ^The translational property of the solutions implies ^that

Fig. ^12.

Schematic of the electronic band curvature

function of thickness.

so that

assuming small increments the preceding integrals may be written :

V(0, Vs) = VS by definition and V(xl, Vs)

^{0 as} xl is in the bulk so

or

from (A. .1) ^one gets

so that

may be computed ^as follows : Poisson’s equation yields : ^{, where p is the} charge density

and N(E) ^{is the} density ^{of states.} This relation assumes 0 K statistics and that filled states under the Fermi level are

neutral and unfilled states above the Fermi level are also neutral. The important point ^{here is that p} depends ^{on x} only through V(x). This is also true for finite temperature statistics (Ref ^{5 formula} 3). ^One may then easily perform ^{a first} integration of Poisson’s equation :

where

so

G is minimum at this value of Vs.

G(V.) may be expressed ^{as follows :}

the plus sign being associated with In and the minus sign ^with Ip :

These integrals may be evaluated numerically.

G(o) ^{is the} conductance for flat band condition at the surface.

When

then

when then then

1 one gets ^{a constant} density ^{of states} approximation, ^then

the + sign is associated to In and the minus sign ^to Icp.

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