BROADBAND SPECTROSCOPY OF ULTRATHIN LAYERS BY OPTICAL GUIDED WAVES TECHNIQUES, APPLICATION TO A 80 Å THICK a - Si : H LAYER

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BROADBAND SPECTROSCOPY OF ULTRATHIN LAYERS BY OPTICAL GUIDED WAVES

TECHNIQUES, APPLICATION TO A 80 Å THICK a - Si : H LAYER

M. Olivier, J. Peuzin, A. Chenevas-Paule

To cite this version:

M. Olivier, J. Peuzin, A. Chenevas-Paule. BROADBAND SPECTROSCOPY OF ULTRA- THIN LAYERS BY OPTICAL GUIDED WAVES TECHNIQUES, APPLICATION TO A 80 Å THICK a - Si : H LAYER. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-123-C10-126.

�10.1051/jphyscol:19831026�. �jpa-00223483�

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JOURNAL DE PHYSIQUE

Colloque CIO, supplément au n°12, Tome W, décembre 1983 page C10-123

BROADBAND SPECTROSCOPY OF ULTRATHIN LAYERS BY OPTICAL GUIDED WAVES TECHNIQUES, APPLICATION TO A 80 A THICK a - Si : H LAYER

M. Olivier, J.C. Peuzin and A. Chenevas-Paule

LETI,CEN. G, CEA, 85 X, 28041 Grenoble Cedex, Frame

Résumé - A l'aide d'une technique de perturbation, les méthodes de spectro- scopie par ondes guidées sont étendues au cas de couches ultraminces non forcément guidantes.

Abstract - By means of a perturbation technique, guided wave spectroscopy methods are extented to the case of non-guiding ultrathin layers.

Recently there has been a great deal of interest in using surface electromagnetic waves (SEW) to probe ultrathin films, overlayers or adsorbed species located at the surface of SEW supporting media /l/. The high sensitivity of such probes as compared to bulk waves is mainly due to the field confinement near the surface involved in SEW Comparatively, optical guided waves (GW) despite giving rise to a similar near surface field confinement have undergone a very much lower development as surface characterization probes. Moreover interesting spectroscopic studies using GW have been precluded apparently because of widespread belief that GW optics requires laser light.

In fact it has been shown by the authors / 2 / that GW optics is readily feasible by using conventional monochromatic sources. This result has opened the possibility of several new powerfull spectroscopic techniques for thin films study. We designate all these techniques as guided wave spectroscopy (GWS). So far GWS has been only used to study light-guiding layers /3,4/ and the aim of the present work is to show that GWS can be extended by a perturbation method to the case of non-guiding layers.

The paper is set up as follows : in section I GW and SEW are compared from the point of view of their usefulness as surface optical probes. In section It a brief recall of GWS technique is given. The proposed GW perturbation is described and discussed in section III. Finally application of the technique to an ultrathin a-Si H layer is presented in section IV.

I-COMPARISON BETWEEN GDIDED WAVES AND SURFACE EI^ECTROMAGNBTIC WAVES

GW and SEW belong to a same class of localized optical modes namely modes with one- dimensional energy confinement near a surface. As far as their use for probing sur- faces is concerned, GW and SEW look like quite similar. In both cases the surface property to be studied is probed by mean of the evanescent field associated to the particular optical mode which is considered. Besides these similarities, GW and SEW have several differences which are summarized in Table 1.

Remarks : 1. Despite the two following facts i) field amplitude of GW not maximum at the surface ii) mode energy of GW not mainly located in the air region (as for SEW), it can be shown that GW probes compare favourably with SEW probes from the point of view of sensitivity. This is mainly due to a better confinement of the wave energy in the air region close to the surface in the GW case as compared to the SEW ones.

This is especially true in the IR region where SEW are only loosely bound to the surface due to an effective index very close to one. On the contrary GW which exhibit effective indexes significantly greater than one have evanescent tails much more tightly bound to the surface. Further it can been shown (to be published) that a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831026

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C10-124 JOURNAL DE PHYSIQUE

certain class of waveguides called "surface impedance waveguides" can be used to in- crease up to 10 % the amount of energy located in the air region. This is done wi- thout lossing the previous mentioned good confinement. As a result the sensitivity of the GW probe is significantly increased.

2. GW offer several other advantages as compared to SEW. Firstly TE as well as TM modes can be used to probe overlayers or adsorbates. Thus dichroic properties may be detected. Secondly the GW spectral range is much wider than the SEW one. Particu- larly the near W region ( 0.2

<

^(0.4 m) is attainable (for instance by using a SiO /LiF waveguide). The infrared range is also attainable at least up t o r = 15

2 .

r

(by uslng a Ge on GaAs waveguide for instance).This limit arises because of phonon absorption bands.

3 . GW allow surface optical studies to be extended to a wider range of materials.

Until now such studies have only been possible on surface of SEW supporting media. Ey using GW several surface phenomena such as oxidation, adsorption, overlayer deposi- tion

...

should be studied quite similarly atthe surface of various dielectrics (91s- ses, oxides, semiconductors, organics . . . I

Table 1 I1 GUIDED WAVE SPECTROSCOPY

Waveguiding structures which support one or several guided modes may be studied by guided wave spectroscopy using two rather different techniques. The first one which deals only with mode excitation and does not make use of propagation over macroscopic distances is called guided wave excitation spectroscopy (GWES). GWES allows the gui- ded-modes effective-index dispersion laws N

(1)

to be determined. The second one which actually uses the GW propagation phengmenon is reminiscent of conventional transmission spectroscopy. We thus call it guided wave transmission spectroscopy

(GWTS). By GWTS one obtains the modes attenuation spectra A

( A )

^(m⁼^0.1.^...^{) .}

I1 1. GWES In order to perform GWES experiments,amsingle prism coupler

-

- -

-

- - -

-

which allows mode excitation in the whole studied spectral range is used. At a given Supporting

structure

Spectral range of existence

Polarization Propagation dis..

tance (L~)

Field repar- tition

--

wavelength, coupling to a particular mode occurs only at a specific "synchronous angle" between the incident beam and the prism-coupler entrance face. GWES method basically consists in recording the varations of synchronous angle versus wavelength i.e: i

( A ) .

This can be actually done in two different ways (Fig.1). In the first one a $idely-open converging beam is used. Mode excitation gives rise to a dark m-line in the reflected beam. The dark m-line angular displacement versus wavelength isthen recorded which yields i(a)/5/. In the second method which makes use of a parallel beam,

/ 6 / , one records as a !?unction of wavelength the incident-beam angular position for which a dip appears in the reflected beam. From i

( A )

it is then possible to deduce the effective index dispersion law N

(1)

ⁱⁿ^ast?aightforward manner (see formula at the bottom of Fig.1). m

GW

Dielectric films generally on dielectric substrates.

Whole optical range(W, ViS, IR )

TE or TM

In the whole optical range one car find optical waveguides with

~ p > l c m

Field amplitude maximum inside the film. Surface field amplitude increases with mode order,

SEW

Metals (surface plasmons-polaritons. Ionic crystals-

(surface phonons-polaritons)

Limited range in the IR for surface phonons. ViS,IR for surface-

~lasmons

.

TM

Lp << 1 cm in the VIS range.

Lp > 1 cm in the IR range.

Field amplitude maximum at the surf ace.

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IIILI-iES

^In^GWTSexperiment the guided mode is launched in the structure by a first prism (the in-coupler) and extracted by a second one (the out-coupler) after a given propagation length (Fig.2). By using different propagation lengthes and taking into account in-and out-coupling efficiency variations as a function of wavelength, quantitative attenuation spectra, A ( > ) , may be obtained (see formula at the bottom of Fig. 2)

.

^Remark^:Unavoidable coupying efficiency changes due to prisms dis- placements has also to be considered to actually ensure quantitative measurements,

(to be published)

.

For simplest guiding structures i.e. composed of a single high-index thin-film on a substrate of lower-index, knowledge of N ( A ) ^andA

( A )

then generally allows the refractive-index dispersion n ( ) and tRe absorption spectrum m ( A) of the film to be determined /2/. The tecffnique is not restricted, however, t6 such simple structures. For instance knowledge of N

(1)

^{and A}( X ) for one mode of a bilayer guiding structure enables the optical propErties of one of the layers to be determined pro- m vided optical properties of the other are well known. If the unknown layer is very thin, a perturbation analysis can be used as discussed in the following section.

F i g . 1. GWES

-

Ci C: : I n - c o u p l i n g e f f i c i e n c y Co ^:Out-coup1 i n g e f f i c i e n c y

F i g . 2. GWTS

.

I11 GUIDED WAVE PERTURBATION SPECTROSCOPY

Although being a quite versatile method, guided wave perturbation spectroscopy is discussed here in the particular case of ultrathin layers characterization. The layer to be studied is deposited on part of the surface of a low-loss and well characteri- zed waveguide. Great care has to be taken in choosing the waveguide to ensure that its surface properties do not influence the ultrathin film intrinsic properties.

Where deposited, the overlayer slightly modifies the guided-modes propagation constan- t s . ~ e t A ~ , andA&,,the effective index and attenuation coefficient modifications ex- perienced by moae "m". Knowledge

O ~ A N

and &A as a function of wavelength enables the refractive-index n( ) as well as ?!he overlgyer absorption spectrum& ( A ) to be determined

AN^(

^)and^A^Am( ) are obtained in a straightforward manner by performing GWES and GWTS experiments successively on uncoated and coated areas of the waveguide.

As a low-loss waveguide is used, sharp mode resonances are detected in GWES experiments. Thus very small difference (1' of arc ) between synchronous angles of uncoated and coated part of the waveguide may be detected-This corresponds to effective index variationsa~ of the order of 10-4- A low-loss waveguide also means long propagation lengthes iB GWES experiments. Large interactions lengthes between GW probes and overlayer can thus be achieved. So, weak attenuation changes A A can be

deter^^

mined. For instance with a typical 1 cm propagation length,

A A

as Tow as 0.05 cm can be readily measured. As the overlayer causes both intrinsicm(na ) and scatterinq

A

) losses, the measured induced-absorption is the sum of two contributions m

:Ahm=

A Q . ~ A ~ .

Characteristic spectral dependance o f b r or salient features in

ACL

^ge-

nerally allow a clear distinction between the two cgntributions to be made. ThuE the intrinsic induced-attenuation,Aa

( A

) , can be obtained. 0. (

h )

and &N

( A

^have

then to be linked to the optical ?!onstants n(X) or O<

( A )

mo£ the overlzyer.

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CIO-126 JOURNAL DE PHYSIQUE

For an ultrathin layer of thickness d much smaller than

h / ( ~ r ( m )

^(the^{mode de-}

theory can be used. We find that

&A,= 2noldS,

on the uncoated wavegug(Fe parametes aEd particular mode used. S values typically range between 10-3 to 10 A.E@gi Taking into acczynf: the pre?iously indicated sensitivity performances,,it is seen that a Sm = 10 A guided probe enables

-

overlayer with (n2

-

^1)d^N1A to be detected

-

absorption coefficients as low as 20 cm-' to be measured £05 less than 10 A, n = 1.5, layers. This last result means sensitivity enhancement > 10 as compared to conventional transmission spectroscopy.

IV APPLICATION TO AN a-Si : H ULTRATHIN FILM

A 80 B - L ¹⁰A r-f. sputtered a-Si : H film has been deposited on part of the surface

of a LPCVD Si N /A1203 waveguide. The guiding layer is 0.61 )lm thick and its refrac- 3 4

tive index is about 2. Fig.3 shows the TEo attenuation spectra for both uncoated (curve a) and coated (curve b) parts of the waveguide. Scattering contribution in curve b is not yet clearly konwn and only an upper limit of scattering losses can be estimated. A lower limit of intrinsic absorption is thus obtained (Fig.4 curve a).

Fig.4 indicates that the ultrathin a-Si : H layer absorption is significantly higher than the bulk one (curve b). Intetpretation of this result is out of the scope of this paper and obviously requires further work. It is rather stressed here that GW pergrbat5on tfchnique have succeedeg in measuring low absorption coefficients

(10

-

^{10 cm} ⁾ in a less than 100 A a

-

^Si^:H layer, a task definitely impossible by conventional techniques.

-

⁷

^.-.

4

WAVELENGTH ( pm ) PHOTON ENERGY ( eV )

Fig. 3. TE attenuation spectra. Fig. 4 a-Si : H absorption spectra

CONCLUSION : A new spectroscopic method which uses guided waves for probing dielectric surface properties has been presented. As conventional light sources are used, con- tinuous spectra can be obtained in the whole optical range. The method shows several advantages as compared to SEW spectroscopy and has proved particularly useful forlow absorption measurements in ultrathin media.

RGfBrences :

/I/. F. a l e s , J. Physique

38

(19771, suppl. C 5 , 67.

/2/. M. Olivier, "New directions in guided waves and coherent optics", Cargsse (1982).

to be published in "NATO Advanced Study Series".

/3/. M. Olivier, J.C. Peuzin, J.S. Danel and D. Challeton, Appl. Phys. Lett.

38

(1978) 79.

/4/. M. Olivier, J.C. Peuzin and A. Chenevas-Paule, J. Phys. Soc. Japan

49

⁽¹⁹⁸⁰⁾

Suppl. A. 1201.

/5/.M. Olivier, J.S. Danel, J.C. Peuzin, D. Challeton and P. Bouchut,Thin Solid Films 89 (1982) 295.

-

/6/. K. Tanaka, Appl. Phys. Lett.

2

(1979) 672.