ENHANCEMENT OF THE PHOTOCURRENT SIGNAL IN a-Si:H BY MEAN OF A WAVEGUIDING TECHNIQUE, APPLICATION TO THE OPTICAL ABSORPTION SPECTROSCOPY BELOW THE BAND GAP

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ENHANCEMENT OF THE PHOTOCURRENT SIGNAL IN a-Si:H BY MEAN OF A WAVEGUIDING

TECHNIQUE, APPLICATION TO THE OPTICAL ABSORPTION SPECTROSCOPY BELOW THE

BAND GAP

M. Olivier, P. Bouchut

To cite this version:

M. Olivier, P. Bouchut. ENHANCEMENT OF THE PHOTOCURRENT SIGNAL IN a-Si:H BY

MEAN OF A WAVEGUIDING TECHNIQUE, APPLICATION TO THE OPTICAL ABSORPTION

SPECTROSCOPY BELOW THE BAND GAP. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-

305-C4-308. �10.1051/jphyscol:1981464�. �jpa-00220921�

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C o Z k o q u e C4, s u p p Z 6 m e n t au nOZO, T o m e 42, o c t o b r e 2981 p a g e C4-305

ENHANCEMENT OF THE PHOTOCURRENT SIGNAL IN a-Si:H BY MEAN OF A WAVEGUIDING TECHNIQUE, APPLICATION TO THE OF'TICAL ABSORPTION SPECTROSCOPY BELOW THE BAND GAP

M. Olivier and P. Bouchut

L E T I , CEN. G, CEA, 85X, 38041 G r e n o b Z e C e d e x , F r a n c e

Abstract.- A waveguiding technique intended for photoconductivity measurements in thin films is described. This new method which offers at least one order of magnitude enhancement over conventional photoconductivity technique is used to determine the optical absorption of a a-Si:H film at photon energies down to 0.65 eV. These results, combined with optical and conventional photoconductivity measurements, allow to draw the optical absorption spectrum of the sample between 0.65 and 2.2 eV.

1. Introduction.- In order to gain information about the distribution and the density of states in the forbidden gap of a-Si:H, there has been recently a great deal of in- terest in determining the optical absorption of this material for photon energies below the energy band gap. Optical transmission spectroscopy (OTS) and conventional photoconductivity (CPC) measurements have been generally used for chis purpose (1,2,3,4).

However, due to the thin-film geometry, both methods suffer limitations which can be summarized as follows : for a film-thickness of the order of 1 pm OTS is limited to absorption coefficients a 3 100 cm 'while CPC allows to measure a down to 1 cm-'(2,s).

Another technique based on the light-guiding properties of a-Si:H films deposited on SiOz substrates has been proposed recently(6). This method called guided-wave optical spectroscopy (GWOS) which is not limited by the thickness of the film, allows to measure absorption coefficients in the range 0.5-50 cm-l. Another attractive feature offered by the light-guiding properties of a-Si:H is the possibility of studying photoconductivity (PC) with an enhanced sensitivity. The aim of this paper is to describe this new method called waveguiding photoconductivi~y(WPC) and to outline its advanta- ges over CPC. In the case of a particular a-Si:H film, this method is used to perform photocurrent measurements between 1.6 and 0.65 eV. These measurements are combined with results obtained by OTS, GWOS and CPC, and the optical absorption spectrum of the sample is drawn in the range 0.65 to 2.2 eV

2. Waveguiding technique.- 2.1 Princi~ie. A usual way of studying thin film photoconductivity consists in illuminating the gap of a coplanar-electrodes structure with a light beam nearly perpendicular to the surface of the sample. In this geometry, the interaction length between the light beam and the material is given by the thickness t of the film ( case a in Fig. 1). An alternative way consists in injecting the light beam in the material through a cross-section of the film in a direction parallel to the surface. In this geometry, the interaction length is no longer limited by the thickness of the film but rather by the length 1 of the electrodes ( case b in Fig.1).

he possibility of such a longitudinal propagation s offered by the light-guiding properties of a-Si:H ilms deposited on SiOz substrates that have been escribed elsewhere(7). For simplicity in comparing he two configurations, we assume that in both geom etries, despite different possible insertion los- es, the same available photon flux N ( in pho- 0ns.s-') is allowed to interact with the structure.

further assume that in spite of different inci- ent photon flux densities, the drift mobility pd, he quantum efficiency for photocarrier generation

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981464

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C4-306 JOURNAL DE PHYSIQUE

0

,

and the response time for photoconductivity

-rr

are the same in both configurations. Consequently, the photocurrents I, and Ib which, in each case, are proportional to the total number of photons absorbed per second in the structure, can be ex- pressed as :

Ia = CNat (1) for conventional geometry, and

Ib = CNal (2) for waveguiding geometry

where C is a common constant factor.

As we are concerned only with low

a,

these expressions have been derived assuming that at << a1 << 1. From

(1) and (2) we may write Fig. 2 Experimental arrangement. Ib /Ia = l/t (3) which indicates that for electrodes of 1 mm in length, and a 1 pm-thick film, the photocurrent intensity may be enhanced by a factor

lo3

when using WPC. Actually the insertion losses are not the same in the two geometries and must be included in a more rigourous treatment. In the WPC case, the prism coupling technique is used in order to inject the light in the film (Fig. 2). As a consequence, losses firstly come from reflection on the prism entrance face and secondly from the coupling efficiency which may differ markedly from unity. It can be shown that the enhancement factor has to be reduced by a factor ,< 2 if a laser beam is used and 6 100 by using a conventional focused beam (6,8). But even in the latter case, a gain of one order of magnitude remains observed, as shown hereafter.

2.2 Example.

-

A photocurrent spectrum obtained by WPC is shown in Fig.3. It consists in a succession of photocurrent peaks, each of them corresponding to the excitation of a particular guided mode of the film. Curves a and b in Fig. 4 show the photocurrent intensity spectra of a particular a-Si:H sample, respectively obtained by CPC and WPC. Geometrical parameters given in Fig. 2 have been used. An enhancement factor of the order of 20 is clearly visible on the whole wavelength range. The sensitivity offered by WPC may be used to extend the measurement of the optical absorption to photon energies fairly below the energy gap, cs shown in the following paragraph.

3. Results.

-

By using OTS, GWOS, CPC and WPC, the optical absorption spectrum was derived in the energy-range 0.65 to 2.2 eV for a typical a-Si:H sample. The layer was deposited by r.f. sputtering onto a SiOn substrate. Details concerning the elaboration have been published elsewhere (9). The sample which is 1.12 pm thick has the following optical and electrical properties : optical tap = 1.9 eV ; refractive index at

?, = 1.15 pm : n = 3.08 ; dark-resistivity = 3.10' Rcm. In order to perform PC measurements, ohmic contacts were evaporated onto the a-Si:H layer. Geometrical parameters are given in Fig. 2. The absorption spectrum of the sample is shown in Fig. 5. Bet- ween 1.75 and 2.20 eV, absolute values of the absorption coefficient were obtained by OTS ( stars in Fig. 5). GWOS meascrements were carried out in the range 1.05 to 1.65 eV. The corresponding spectrum ( open circles 2 Fig. 5) has been normalized by using

E

1 bWPC

51d3 -

a CPC

1.2 14 1.6 2.0

A ( p l Fig. 4 Comparison between CPC (a)

waveguidlng technique and WPC (b) speCtra

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a (1.08 eV) = 2.5 cm-', a value deduced from preliminary sliding-prism measurements.

For photon energies in the range 0.65 to 1.8 eV, a was measured by using PC techniques : CPC from 1.8 to 1.15 eV ( dots in Fig. 5) and WPC from 1.1 to 0.65 eV ( cros- ses in Fig. 5). In order to extract ci from PC data, we are submitted to the same as- sumptions widely discussed before (2) i.e. : we suppose that pd, T, and 11 are inde- pendant of the photon energy. In paragraph 2.1 it has been assumed for demonstration purposes that the photocurrent is a linear function of the photon flux density F.

However it is well known (10) that the photocurrent is rather a sublinear function of F of the type

I =

c

(4)

where 6 is a factor which depends on F ( as in Eq. (1) and (2) it is assumed that at << a1 << 1). Thus, in order to determine a we rather used Eq.(4). For CPC measurements F was kept at a constant value, namely 10" photons.cm-2s-', independent of the photon energy. In the WPC case, F was ranging from 3.10'~ to 1018 photons.cm-2s-' It has been actually checked that 6 keeps the same value of 0.73 at three different wavelengths (A = 0.8, 1 and 1.15

urn)

and above five decades of photon flux density

(from 10" to 1 0 1 9 photons.cm-2s-'). Therefore, assuming that 6 remained the same for

X

> 1.2 um, the above 6 value was used for both techniques on the whole photon energy

range studied. In the case of CPC the constant C in Eq.(4) was found by normalizing the photocurrent in the energy range common with OTS( for which ci was already known (Part. I in Fig. 5)). Similarly, WPC has been normalized in the energy range common with CPC ( Part. I1 in Fig. 5).

gem_ark

: For some WPC measurements carried out at photon energies such as al>,l,Eq.(4) was no longer valid and a special treatment was necessary in order to derive ci.(Un- published work).

Fig. 5 Spectral dependence of the optical absorption

2..

1..

8..

-

-2.-

4. Discussion

. -

On the spectrum shown in Fig.5, three distinct regions are clearly visible. The region I ( hv > 1.5 eV) has the shape of an exponential absorption edge described by the law a a exp(hv/%) with Eo = 0.06 eV in agreement with values generally found in both glow-discharge (4) and sputtered a-Si:H ( 3 ) . Although the origin of such an absorption edge is not yet clear, it is generally attributed to transitions involving exponential tailing of band states into the energy gap (11).

In region I1 (1.5 > hv > 1 eV) the exponential decrease of a is no longer present and a plateau appears as clearly seen by both GWOS and PC techniques. Such a behaviour,

t*** f- wmepldlng pho-tlrlty

'.:' I

0 0

..:

0 00 0 .

0 0

II ..+.-

. ^*

*

+*

+ +

ENERGIE (eV)

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C4-308 JOURNAL DE PHYSIQUE

which has also been observed in a-Si:H by other authors (2,3) may be attributed to transitions from a distribution of gap-states, close or below the Fermi level, into states in the conduction band close to the conduction level. The higher values of a given by GWOS are due to a scattering contribution in the attenuation of guided waves.

By using different guided modes with various spatial confinements, surface scattering rather than bulk scattering has been evidenced ( unpublished work ). After deducing this scattering part from the total measured attenuation, it remains however an in- trinsic absorption greater than the one obtained by PC techniques. A possible expla- nation for such a residual discrepancy may be that PC measurements give an inferior limit of a due to a reduced quantum efficiency at low hv (2). Finally in region 111, a (hv < 1 eV) almost exponential decrease of a is observed. By fitting the curve with a law of the previous type, a caracteristic energy Eo = 0.07 eV is found. Transitions from gap-states above the Fermi level, whose occupancy is determined by the Fermi function, into states located near the conduction band edge may explain the optical absorption in this spectral range.

In conclusion, WPC can be considered as a potentially useful method to extend optical absorption measurements down to 0.01 cm-' even in the case of film thicknes- ses as low as 1 pm.

We wish to thank A. Chenevas-Paule, R. Cuchet and H. Boucher for supplying the a-Si:H layer. Many thanks are also due to I. Meyer and J.S. Dane1 for their helpful advice in preparing the paper.

References.

(1) R.S. Crandall, J. Non.-Cryst. Solids

2-36

(1980) 381.

(2) G. Moddel, D.A. Anderson and W. Paul, Phys. Rev.

B22

(1980) 1918.

(3) T.D. Moustakas, Solid State Comm.

5

(1980) 745.

(4) B. Abeles, C.R. Wronski, T. Tiedje and G.D. Cody, Solid State Comm.

36

(1980) 537.

(5) P. ~oichut and A. Chenevas-Paule, J. Physique

42

(1981) 439.

(6) M. dlivier, J.C. Peuzin and A. Chenevas-Paule, J. Phys. Soc. Japan

2

(1980), Suppl. A, 1201.

(7) M. Olivier and J.C. Peuzin, Appl. Phys. Letters

32

(1978) 386.

(8) F. Zernike : in "~ntegrated Optics", edited by T. Tamir ( Springer Verlag, 1975 )

,

^Chap.5, pep. 216-231.

(9) Contrat DGRST 77.06-223 ( 1977 ) .

(10)A. Rose : in " Concept in photoconductivity and allied problems", (Interscience 1963 ) p. 38.

(11)J. Tauc : in "Optical properties of solids", edited by F. Abeles (North-Holland, 1975) p. 297.