Photodarkening in AsS films and its application in grating fabrication

JOURNA L OF

ELSEVIER

Journal of Non-Crystalline Solids 198-200 (1996) 769-773

Photodarkening in As-S films and its application in grating fabrication A. Zakery

a,*,

P.J.S.

Ewen b, A.E.

Owen b

a Department of Physics, College of Sciences, Shiraz University, Shiraz 71454, lran Department of Electrical Engineering, Edinburgh University, Edinburgh EH93JL, Scotland, UK

Abstract Of the many photoinduced effects which occur in chalcogenide glasses, photo-darkening is of prime importance for applications. In order to determine our film compositions we have used an electron microprobe analysis. This analysis shows that in the A s - S system for compositions richer in As than As30S70 there is good agreement between the film and source compositions. For sulphur-rich compositions however the films seem to be more As rich than the source. This difference has been attributed to the preferential evaporation of sulphur. The results show that, in the A s - S system, the extent of photodarkening increases with increasing As concentration. This increase can be explained if photo-darkening involves the formation of A s - A s bonds. The photo-darkening becomes negligible around the composition As30S70. By knowing the extent of photo-darkening, it is possible to work out the change in absorption coefficient and the change in the film refractive index due to illumination. Our analysis shows that for As-rich compositions, changes in the refractive index as high as 0.1 are induced by photo-darkening. Such a large change in the refractive index makes A s - S thin films promising media for optical recording.

1. Introduction Chalcogenide glasses exhibit a wide variety of photoinduced phenomena that enable them to be used as optical imaging or storage media [1]. They are also well-known IR transmitting materials [2] having passbands (depending on composition) from the visible to beyond 15 ~xm. Thus by making use of the photoinduced phenomena that occur in these glasses, it should be possible to produce diffractive

* Corresponding author. Tel.: +98-71 24 609; fax: +98-71 20 027.

optical elements for use at IR wavelengths. The photodarkening effect is one of the various photoinduced effects that occur in these glasses and produces a large change in the optical and chemical properties o f the chalcogenide. This paper is concerned with the measurement of the changes in the optical constants of A s - S thin films due to photodarkening. Because of the large changes that are induced in the optical constants of A s - S films due to photo-darkening, the applicability of this effect in grating fabrication is also considered. Measurements of diffraction efficiency of gratings made in As30S70 films using the metal photodissolution of silver in these glasses have been made previously [3]. In the

0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022-3093(96)001 21-4

770

A. Zakery et al. / Journal of Non-Crystalline Solids 198-200 (1996) 769-773

present work modeling of transmissive gratings using photo-darkening in As40S60is considered.

45o oo

o

o

2. Experimental oo

30--

Samples were prepared by evaporation onto cleaned Corning 7059 glass substrates at a pressure of 2 × 103 Torr. The A s - S film thickness was approximately 1 ~ m for photodarkening experiments and 1.25 I~m for the compositional analysis. A microbeam instrument ( C A M E C A Camebax) was used for the electron microprobe (EM) compositional analysis of the films and of the source materials. The A s - S evaporation sources were powdered meltquenched glasses of the appropriate composition. FeS 2 and As2S 3 were used as sulphur and arsenic standards respectively. The films were carbon coated prior to the analysis in order to avoid charging effects. An accelerating voltage of 10 kV with a beam current of 10 nA was found suitable for these measurements and the Aseo, SK, lines were used for the analysis. Nine different glass compositions were used in this study and these measurements were accurate to within + 1 at.%. The technique used to monitor photodarkening was based on simultaneous transmissivity and reflectivity measurements. The transmittance of the as-deposited film decreases and goes to saturation after a few minutes due to illumination. The illumination intensity at 514.5 nm was 200 W / c m 2 and was provided by an argon ion laser.

o

¢:x

E

o

o

o o

E

.-m-[a-

15

I

I

[

15

I

I

30

I 45

Source c o m p o s i t i o n

et. •

As

Fig. l. The variation of film composition as a function of the As-S source composition. values of reflectance and transmittance at each instant of time, it is possible to work out the change in absorption coefficient or the extinction coefficient and also the change in the refractive index due to illumination. The relation (1)

R+T+A=I

should hold at each wavelength, where R is the reflectance, T is the transmittance and A is the percentage of light being absorbed in the film. One can write: R+(1-R)exp(-ad)+A=l.

(2)

Thus, a = (1 - R ) ( 1 - e x p ( - c ~ d ) ) .

(3)

40-3. Results 30--

Fig. 1 shows the measured A s - S film composition versus the source composition. For compositions richer in As than As30ST0there is a good agreement between the film and source compositions. For sulphur-rich compositions however, the films seem to be more As rich than the source. Fig. 2 shows the variation of the percentage decrease in the transmittance for different film compositions. The decrease in the transmittance was obtained by subtracting the saturated value of the transmittance from its original value. The interesting feature is that clearly for the A s - S system the extent of photodarkening increases with increasing As concentration. By knowing the

o o 0 O~ cO cO

0

2010--

0

0--10 60

o

0

I

I

I

I

I

65

70

75

80

85

C o m p o s i t i o n ot. •

S

Fig. 2. Variation of the percentage change in transmittance versus As-S composition at 514.5 nm. Illumination intensity was 200 mW/cm 2.

771

A. Zake~ et al. / Journal of Non-Crystalline Solids 198-200 (1996) 769- 773

By calculating A from the experimental values one can obtain the absorption coefficient. Knowing the values of a one can find the extinction coefficient K and by using equations for the reflectance and transmittance of a single absorbing film on a transparent substrate [5] the refractive index of the film and its change due to illumination is obtained. Table 1 lists for four A s - S compositions, the refractive index, the absorption coefficient and the change in these quantities as a result of illumination.

4. Discussion Results of our compositional analysis agree well with the results of Tanaka et al. [4] for As-rich compositions. Tanaka et al. report a value of _+ 2 at.% difference between the film and source compositions. In sulphur-rich glasses, excess sulphur exists probably in chain-like form but in the range where the sulphur content is greater than that in the range As17S83-As20S80, both chain-like and ring-type forms coexist. The bond strengths of the three bonds A s - A s , A s - S and S - S are 43.5, 52.8 and 55.0 kcal mol -~, respectively. Hence for sulphur-rich compositions some of the sulphur is not evaporated at the same rate as the other units during film deposition. The photodarkening becomes negligible around the composition As30870. It is possible to explain this if photodarkening involves the formation of A s - A s bonds. As a result of illumination some A s - S bonds are replaced by A s - A s bonds [1]. Since there are more A s - S bonds in arsenic-rich compositions, the extent of photodarkening increases with increasing As concentration. The change of ~- 0.1 in the refractive index of As40860 is in good agreement with the results of Ohmachi et al. [6]. The observation of refractive-index changes as large as 0.1 in these measurements make As40S60 thin films promising media for optical recording.

5. Modeling of transmissive gratings produced by photodarkening The grating is assumed to b e phase-modulated, lossless, unslanted and uniform with depth. A coupled wave approach is used [7,8]. The grating is taken to be in form of a parallel sided slab of thickness d in the y - z plane. The input and output surfaces of the slab are bounded by homogenous regions. An infinite monochromatic plane wave, polarized perpendicular to the grating vector and of amplitude A 0 is incident upon the slab. The rectangular modulation profile can be written as a Fourier series

~.(r) = 6.0 -[- E ~i c o s ( i K . r ) i=1 with 'fro = ~'min q'- A ~/.Z,

ei = ( 2 / i ' r r ) A e s i n ( i / z ' r r ) ,

1000 1100 1000 1000

(5)

where G0 is the bulk relative dielectric constant of the slab and e i is the phase modulation of the ith harmonic of the grating profile, K is the grating vector with K = 2 7 r / A where A is the grating period, A e : e m a x - - 6min and /x is a fill ( m a r k / s p a c e ) parameter. For a square profile /z is 0.5. The coupled wave equations describing propagation inside the grating can be written as: (for m = - N . . . . . - 1 , 0 , 1. . . . . N ) Kj

1

d2Am --

cos20o 2 j / 3 d~c2

dA m +--jmI2(m+P)A,,,

d~:

:¢

+j~_,

(6)

Ki ( Z m + i + Z m _ i ) = 0 ,

i=1 KI

where A m is the amplitude of the mth diffraction order: Ki =/3Ei/4E0, [3 = 27rx/~o/A , A is the freespace wavelength, ~:= K I x / c o s 00 is a modulation

Table I Compositional dependence of the photoinduced effects in the As-S system Composition As-S Thickness (nm) no An As40S60 As 35$65 As 33867 AS28S 72

(4)

a 0 (cm- l )

A a (cm- 1)

AK

2.53 0.11 800 1912 0.0029 2.53 0.095 551.7 307.7 0.0012 2.50 0.1 525 247.3 0.0012 No detectable change in either transmittance or reflectance was observed.

A. Zakery et al./ Journal of Non-Crystalline Solids 198-200 (1996) 769-773

772

parameter, P = (sin O o ) 2 f l / K is a Bragg parameter and the angle which the incident wave makes with the normal to the grating is 00. Each diffraction order A m is coupled to the m _+ i order A m + i and A m - i by the ith coupling coefficient Ki.x The diffracted amplitudes A m a r e obtained by solution of these equations subject to grating boundary conditions. It is possible to simplify the calculations by neglecting second derivatives in Eq. (6). So for the bulk case Eq. (6) becomes (for m = - N . . . . . - - 1 , 0 , 1. . . . . N )

dA m --jm12(m

1.0 0.8 O

0

•,~ 0.4 0.2 o.o

V--v--T - r---r--r---V--v-

1- T - r ° - r - - f

0

4

I

2

3

5

6

+ P) A m

dE

Fig. 4. Diffraction efficiency versus ~ for ~ = 10.

o~ +j~

O.6

Ki ( a m + i + a m _ i ) = 0 . i=l K1

(7)

The transmission grating boundary conditions are

Am(x=O ) =0

(mv~0),

A 0 ( x = 0 ) = 1.

(8)

Analytic solutions of Eq. (7) are possible for the thin (12 << 0.1) and volume (12 > 10) diffraction regimes. For the multiwave regime, numerical solution of Eq. (7) is necessary. A fourth-order R u n g e - K u t t a method is used in this work.

6. N u m e r i c a l results

Fig. 3 shows how the behaviour of a grating with a square profile depends on the thickness parameter

12. For 12 = 1 we see that many diffracted orders are present. This is the characteristic of a grating with a thin behaviour. For large values of 12 (i.e., 12 = 10) only one diffracted order in addition to the zero order is seen in the replay of the grating with a red wavelength. This grating shows a volume character and i s shown in Fig. 4. The calculated results of diffraction efficiency versus the replay angle are shown in Fig. 5. This is the behaviour of a grating with nearly a volume character (i.e., 12 = 2.76 and = 1.50). The peak seen in the Fig. 5 for + 1 order is due to the fact that when the Bragg condition

1.0--

1.0

0

0.8

0.8 O9 ,t-4 0

~ 0.8

0.6

•~ 0.4

°i,,,~ O

•,.~ 0.4

0.2

0.2

o.o

0.0

--0

'

-30 1

2

3

4

5

Fig. 3. Diffraction efficiency versus ~: for ~ = 1.

6

I

-20 Replay

i

I

-10

.....

i-

-

T

0

angle

Fig. 5. Calculated diffraction efficiency versus the replay angle for a nearly volume ( ~ = 2.76) grating. The theoretical first Bragg angle occurs at 14.3 °.

A. Zake~ et al. / Journal of Non-Crystalline Solids 198-200 (1996) 769-773

holds the efficiency is a maximum. A disturbing effect is the presence of the 2nd order in Fig. 5 at around the replay angle of - 2 4 ° which is due to the slightly low value of 12 for this grating (for a true volume behaviour 12 should be as high as 10).

Acknowledgements This work was sponsored by the financial support of the Shiraz University research council under grant No. 71-SC-757-415. A.Z. wishes to thank M. Hatami for his assistance with computer modeling.

773

References [1] A.E. Owen, A.P. Firth and P.J.S. Ewen, Philos. Mag. B52 (1985) 347. [2] J.A. Savage, IR Optical Materials and their Antireflection Coatings (Adam Hilger, Bristol, 1985) p. 79. [3] A. Zakery, P.J.S. Ewen, C.W. Slinger, A. Zekak and A.E. Owen, J. Non-Cryst. Solids 137&138 (1991) 1333. [4] K. Tanaka and Y. Ohtsuka, Thin Solid Films 57 (1979) 59. [5] O.S. Heavens, Optical Properties of Thin Solid Films (Butterworth Scientific, London, 1955). [6] Y. Ohmachi and T. Igo, Appl. Phys. Lett. 20 (1987) 506. [7] H. Kogelnik, Bell Syst. Tech. J. 48 (1969) 2909. [8] M.G. Moharam and T.K. Gaylord, J. Opt. Soc. Am. 71 (1981) 811.