Rutherford backscattering experiments on percolating Pd films

0038-1098/90$3.O(H.O0

Solid State Communications,Vol. 73, No. 4, PP. 285-287, 1990. Printed in Great Britain.

RUTHERFORD Centre

BACKSCATTERING

EXPERIMENTS

ON PERCOLATING

Pergamon Press plc

Pd FILMS

N. Papandreou, P. Nddellec and A. Traverse de Spectrometrie Nucleaire et de Spectrometrie de Masse, Bdtiment 108, Campus Orsay, 91405 Orsay, France

(Received June 14, 1989 by J.Joffrin. Revised form September 22, 1989) Abstract : experiments Rutherford backscattering are performed on ultrathin percolating Pd films during irradiation at 300 K with Xe ions. The spectra are sensitive to the topology of the films, allowing us to follow the appearance of a percolating structure.

may lead

Introduction

to

a perforatlng

process

through

crater

creation as yet observed by scanning microscopy’. 21 mlxlng between target and substrate atoms, due to collisions located at the film-substrate atomic Interface. The R.B.S. measurements allow us to evaluate the amount of mixing .at the interface (which is found to be negligible in the fluence range where sputtering is already Important), and to investigate the topological evolution of the target. To our knowledge, this is the first time that R.B.S. is used to get the surface coverage of a percolating sample. In R. B. S. experiments, a beam of He particles

The understanding of the electronic transport propertles of a sample through the going metal-insulator induced by transition, atomic disorder, 1s not yet achieved ; an interesting approach to the problem consists in studying systems which present both atomic (mlcroscoplc) and topological (macroscopic) disorder, where the latter amplifies the effects of the former 1,s and makes the metal-insulator transltion more easily observable. A macroscopically disordered sample 1s usually film, a composite made of randomly dlstrlbuted metallic and insulating crystallltes. Here we used a system, where the lnsulatlng component Is the vacuum : the holes play the role of macroscopic dlsorder. Such lnhomogeneous samples present resistive properties strongly dependent on the metallic coverage p, with a divergence of the resistance when value p a crltlcal Is reached, the percolation c threshold. This latter corresponds to the breaking continuous of the metallic path, which allows electrical conduction to take place. Recently, lon irradiation has been proposed to

impinges on a target’. Although most of He particles loose their energy by inelastic electronic excltatlons, a number of them undergo elastic nuclear collisions on atoms target with a probabillty to be backscattered : these outcoming particles are detected versus their energy, which depends on the mass of the collided target atom and on the depth where the collision took place. A peak corresponding to each kind’ of target atom is recorded. Its front edge corresponds to the energy of a hellum particle backscattered at the surface of the film, hence is characteristic of the knocked on atom. Its area 1s proportional to the number of atoms via the lnteractlon cross-section, Its width is related to the sample thickness vla the stopplng power (the energy transferred to the target per unlt length) and Its height is related to the atomic concentration. Through these experiments, It has been possible to describe qualitatively and quantltatlvely the processes involved in the evolution from the homogeneous’to the inhomogeneous reglme. The sputtering yield Y (the number of ejected surface atoms per incoming ion), the metallic surface coverage p. the fluence where the first holes appear are deduced from the experimental spectra.

prepare percolating thin films’ : starting from a homogeneous film, irradlatlon with heavy ions at medium energy induces, through the sputterlng effect, a progressive thickness decrease, then a statlstlcal perforatlng process leading to a discontinuous sample. This is indicated by the in situ measured resistance divereence. characterlstlc of a percolation transition. Transmission electron microscopy (TEMI permits a direct measurement of the metallic coverage but it does not bring any lnformatlon on the film thickness and does not allow in situ resistive measurements. Using the llght ions beam delivered by the implanter the where lrradiatlon is performed, Rutherford backscattering (R. B.S. 1 experiments provide the number of atoms In a thln sample but also may glve information on the topological state : they provide a direct measurement of the metallic coverage. In thls letter, we want to show that by performing In sl tu Rutherford backscatterlng experiments at different steps of the lrradlatlng fluence @, wlth simultaneous reslstlve measurements, It IS possible to correlate topological and electronic Information. Through Irradiation, several processes take place, due to the energy transferred from the implnglng particles to the target atoms via elastic colllslons : 1) sputterlng which erodes the fllm and

Experimental Thin Pd (75 A) layers have been deposited by electron gun evaporation either on quartz substrate for reslstlve or (and) R.B.S. in situ measurements, or on NaCl substrate for TEM observations. The substrates have been previously coated with SlO layers 150 A thick. The Pd films deposited on the amorphous SiO sublayer initial topology is 285

are homogeneous6. expected to be

and the ldentlcal

286

Vol.

RUTHERFORDBACKSCATTERINGEXPERIMENTS ON PERCOLATING Pd FILMS

whatever the (monocrystalline The

substrate below SlOa) or NaCl.

samples

have

SlO.

elther

quartz

lrradlated

been

under

normal incidence with a 100 keV Xe beam at 3OOK’. Xe ions are implanted at a depth of 390 A from the surface, hence in the quartz substrate ; the full width at half maximum of the dlstrlbutlon profile Is 205 A, as calculated by the TRIM slmulatlon codea. DC resistive measurements have been made using the usual four point probe technique and in situ R. B. S. spectra have been recorded after each step of fluence. up to 4x10” Xe.cme2. The He beam delivered by the implanter has an incident energy of 380 keV. -2

Another sample, irradiated at 5x10” Xe. cm has been analyzed through R.B.S. some days after irradiation. TEM observatlons have been performed on samples

irradiated

at

fluences

of

3x10”

and 4~10’~

Xe. cme2.

Results Several spectra recorded in situ for various fluences are shown in Fig. 1. The Pd peak height is strongly decreased while the Si front edge Is shifted to higher energles with increasing Xe fluence. We note that the Xe peak corresponding to the Xe atoms lmplanted In the quartz substrate Is located below the Pd peak. different The processes occuring dur lng irradiation, 1. e sputterlng at the surface and mlxlng at the boundary of the SlO and Pd layers modlfy the topology and composltlon of the Pd film, hence affect the shape of the Pd peak and Sl front as leading to a reduction of the edge. Sputtering, number of target atoms, induces a decrease of the Pd peak area with a wldth reduction. Mixing and sputterlng wlll induce a decrease of the Pd peak height and a displacement of the Si front edge to When holes are created In the Pd higher energles. film, part of the He beam lmplnges directly on the SiO sublayer and the other part on the remalning Pd atoms, inducing also a peak height decrease. In order to evaluate mlxlng effects, other experimental results are requlred that we describe below : irradiatlon experiments performed with heavy ions on S102/Pd systems indicate that mixing Is weak

2000

t

thls assumptlon. bllayers have been room temperature,

non

No. 4

of the Si-0 bond’. Hence, we for SIO/Pd. no experimental to our knowledge to support

In another experlment’o. SI/Pd irradiated with Xe at 100 keV at then analyzed

by R.B.S.

For 4~10’~

Xe. cmm2, we measure on the average 1.3~10’~ Si atoms enterlng the Pd layer. This amount compared to the atoms in our number of thin Pd samples peak corresponds to a concentration Si/Pd 4%. The mlxing amount between SlO and Pd should be even lower If chemical binding Is argument of strong the considered. In no case, such a low mlxlng effect may explaln the helght decrease of the Pd peak by a factor of two as seen on Fig. 1. The presence of holes Is more llkely to account for this decrease. Simulations of the peaks have been performed. The difficulty here arises- from two facts : 1) the Incident energy of the He beam is lower than the these for used MeV) often energy (1 to 2 simulation characterization beams, so a specific for account the written to program has been corrections on cross sections and energy straggllng of the occurlng at 380 keV ; 2) due to the thickness the energy resolution. A samples, we are below careful simulation of thick reference samples has been made, with a particular attention to accurately to get a the front edge, In order reproduce determination of the energy calibration and detector enter the resolutlon, those parameters which simulation of a peak. The data treatment 1s carried out in the following way : 1. After substractlon of the Xe contribution the Pd peak area is which has been simulated, compared to the area corresponding to a thick film directly the number of reference sample, providing thickness Pd atoms In our sample, thus Its average ;i.

2. The Pd peak simulations are performed by assuming pure Pd In the film and existence of holes account for the peak helght decrease. The to simulation (an example is shown on Fig. 2) provides the metallic coverage and the real thickness d of clusters, related to the average metallic the values for d thickness by d = d p. The obtained agree with those given by the Pd peak surface. In Table 1, the results are summarized for different Xe

6000

-

J c 1000 : ”

because of the strength expect a slmllar effect results being available

73,

I

3000 -t

irrad

Pd film rxp.

3015 Xs/cmO

spsctrum

Xe/cmB

t

lEi5

x

3E15 Xe/cmB

a

4E15 Xe/cmB

4000

2000

J 5 z ,

0

100

120

0

140

180

i0

200

1 : Fig. Pd/SiO/S102,

R. B.S spectra for dlfferent

of the Pd peak height front edge are clearly

220

240

260

channel

channel

of the fluences.

target layered The decrease

and the displacement vlslble.

of

the Sl

Fig.

2

: experimental

R.B. S.

contributions) at @ = slmulatlon. The slmulatlon also plotted.

spectrum

(Pd

and

3x10t5 Xe. cmm2 and of the Xe contribution

Xe its is

Vol.

73,

No.

RUTHERFORD BACKSCATTERING EXPERIMENTS ON PERCOLATING

4

Table

expression,

1

d (A)

0 (Xe. cm-2)

0

Eq. 1

R. B. S.

75

75

100

5 1o14

73

69

100

1 lOi5

63

64

100

2 lo=

54

54

81

3 lo’s

42

46

81

4 lo’=

39

39

42

5 lo=

31

33

30

The value

for

a fllm

Xe. cm-2 is also reported. with an uncertainty of f within + 8 X.

lrradlated

at

TEM pictures. The directly observed

5~10’~

Thicknesses are determlned 5 %, and metallic coverage

Looking at Table 1, the process of the topological modification may be described as follows: 1) Sputtering takes place in the whole investigated

fluence

range,

Xe. cmm2, as seen by the The measured sputtering 4 = 1~10’~

I.e.

decrease yield

Xe. cme2. For

up

to

5

x1o’5

of the thickness. 1s Yo= 8.5 for a 100 keV Xe lmplnglng

on bulk Pd, a yield of about 17 1s expected *I. The sputtering yield Is proportlonnal to the density of deposited energy. In our layered samples (Pd/S10/S102) with an ultrathln Pd layer (d < 75 A), the

atoms

to the ejection

of

the

collision of the

SlO

and S102 sublayers

participate

cascades, he&e to the process of surface atoms. The Y, value 1s then

accounted for by the density of deposlted energy which 1s 140 eV/A in our samples, much lower than In pure Pd, where it reaches 400 eV/A, as evaluated with the TRIM code. This also implies that the sputtering yield decreases with decreasing Pd layer thickness. The thicknesses given by R.B.S. are consistent wlth a model described In reference 3 which, assuming that Y 1s proportlonnal to d provldes d = do exp(-4/4d) where

do 1s

constant denslty.

n

do/Ye

we

recover

78% for

metallic the as

3~10’~

such

the

lnltlal

characterlstlc In fact, by

(1) thickness fluence, calculating

and

4d = n d/Y

n being d ulth

the

a

the target the above

that

also

seen

coverage decrease fluence Increases.

Xe.cme2

and

the

percolation evacuate the so no R.B.S.

40% for

via Is It

4~10’~

wlth values as deduced Xe . cmm2 In good agreement from R. B. S. and presented In Table 1. Samples irradiated at hlgher fluences, analyzed by R.B.S., but not in situ, display a metallic coverage of 30%. behavlour of the the dlverglng Moreover, induced by the. dlsconnectlon resistance’. remaining Pd clusters 1s also *In agreement perforating process seen In TEM and R.B.S.

Discussion

fluence

=

= 2x10” Xe. cmm2. For Xe fluences 4i the below metallic coverage ls it 1s not possible to threshold, charges induced by He bombardment, spectra can be registered. The occurence of holes at O1 1s

reaches fluences.

4d

experimental values ulthln f: 10%. 2) The perforating process takes place and the metallic coverage 1s no longer 100% for a fluence of

p (X1

R. B. S

with

287

Pd FILMS

of with

the the

Conclusion well known for The R. B. S. technique, determlnlng the number and depth distribution of target atoms In usual experiments, reveals to be a useful too1 for In situ studies of Irradiation-induced topological modlficatlons in ultrathin films. Upon TEM. It has the advantage that can be taken the spectra simultaneously with resistive measurements, hence a set of electronic and topological lnformatlon 1s obtalned. The and perforating processes have been sputtering followed and quantltatlve data have been provided on the metallic coverage and the critical perforating fluence. This kind of lnformatlon 1s very useful to electronic the understanding of the transport propertles near the metal-lnsulator transltlon as the metallic coverage and thickness are lnportant parameters for the reslstlvlty. R.B.S. may be used to get topological information on thin percolating samples prepared by other techniques.

Acknowledgements : M.G. Le Bolt6 1s acknowledged for wrltlng the R.B.S. spectra slmulatlon code. We thank Louls Dumoulln for thin fllm preparation. MO. Ruault for the T.E.M. observations, H. Bernas for frultful dlscusslons and 0. Kal tasov for lrradlatlons.

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7. J. Chaumont. F. Lalu, M. Salome, A.M. Lamolse and H. Bernas, Nucl. Instr. and Meth. 189 (1981) 304 8. J.F. Ziegler, J.P. Blersack and U. Llttmark. The Stopping and Range of Ions In Solids, Vol. 1 of The Stopping and Range of Ions In Matter, edited by J.F. Ziegler (Pergamon, New York, 1985) p. 202 9. G.E. Chapman, S.S. Lau, S. Matteson, J.W. Mayer, J. of Appl. Phys. 50 (19791 6321 10. MC. L.e Bolt& and A. Traverse, unpubllhshed work 11. H.H. Andersen and H.L. Bay In Sputterlng by Particle Bombardment I, Topics in Applled physics (Springer. Berlln, 1981) Vol. 47 p. 145-218