Optical constants of thermally evaporated arsenic triselenide using only transmission spectrum

Solid State Communications, Printed in Great Britain.

OPTICAL

Vo1.59,No.5,

CONSTANTS

TRISELENIDE

pp.271-274,

OF THERMALLY

USING

Department

of

M. Department

of

EVAPORATED

ONLY TRANSMISSION M.

?hysics, (Received

+ .OO Ltd.

ARSENIC

SPECTRUM

HAMMAM

University Puysics, Mansoura, Egypt

Abdel

0038-1098/86 $3.00 Pergamon Journals

1986.

Harith

and

Cairo March

W.

of

H.

Mansoura,

Osman

University,

Giza,

31 1986 by A.A.

Egypt

Maradudin)

ABSTRACT -r\ procedure to calculate the optical constants and Thickusing only data from the transmission ness of thin films, spectrum has been employed in the case of arsenic triselenide films prepared by thermal evaporation. Using this technique, the Calculated thickness was found to an accuracy ranging between 1.0 to 3.5 % compared to other methods used. The refractive index and absorption coefficient, as a function have been calculated using this technique of photon energy, The surface roughness and showed to be accurate enough. calculated is compared to that determined using stylus profile technique.

Introduction __----_thickness inhomogeneity of the layer the variation of film thickness within td cross-section of the incident beam. This effect is not negligible when the corss-sectin of the incident beam is large. It is easy to show that the variation of the thickness Ad if a wedge-shape dtransparent layer in the regin of incident beam should obey the inequality Ad
i.e.,

One of th? m,ithods proposed in literature to make the problem of determining the optical constants easier is to deduce these constants by simple straightforuard cal.culations using the (181 transmission spectrum alone . This author has determined the optical constants and thickness of amorphous silicon using this technique and accuracy of the order of 1% has been obtained. In this report, we will check the validity of using this methode in determining the optical constants of amorphous arsenic triselenide evaporated films,

The thickness of the films can be determined using a surface profiling stylus or by various interferometric methods. In this work a multiple beam Fizeau fringes method (16) was used to determine the thickness of our films for the sake of comparison with the data obtained from the tramsmission spectra. It is transmission

also

well known that the spectra are strongly

reflection influenced

(3)

the

The amorphous form of the chalcogenide arsenic triselenide has been widely investigated during the last decade as one of the most important chalcogenide semiconductors either for their important applications or as a direct reference in the discussion of the optical properties of these materials (7). From both the theoretical and experimental points of view, it is important to kriow the optical constants of the considered material (2). In the case of plane parallel and optically homogeneous thin solid films deposited oh transparent or slightly absorbing substrates, the optical constants were usually calculated from measiurecl values of the transmittance T, the reflectance R and the thickness d (3,‘1). From it is known that the relations literature (5-B), describing R and T are very complicated functions, even for normal incidence of light. It is found that a small experimental error of T,R or d evaluations influences the optical constants significantly (2). Nevertheless, calculated, several researchers (9-15) have reported the determination of the optical constants of thin films of metals, semi ccnductors and dielectrics, using this technique.

Sample

Preparation

Arsenic triselenide thin film samples were prepared by evaporating crushed bulk material onto The bulk material was prepared glass substrates. from 99.999% pure As and Se. The melt was quenched in air.

and by 27 f

THERMALLY EVAPORATED ARSENIC TRISELENIDE USING ONLY TRANSMISSION SPECTRUM

272

The evaporation process was carried out in a -5 c o a t i n g unit at a p r e s s u r e better than 10 torr from a quartz crucible. Prior to the e v a p o r a t i o n process, the glass substrates have been washed in distilled water and then placed in an u l t r a s o n i c c l e a n e r for about 20 min. A flow of hot air was used to dry the substrates before putting them in the c o a t i n g unit. During the evaporation process, the substrates were kept at room temperature. The d i s t a n c e from the c r u c i b l e to the substrate was adjusted so the t e m p e r a t u r e of the latter could have been increased only slightly during deposition. The rate of e v a p o r a t i o n was put to yield a few microns per hour.

n I = { N + ( N 2 - n~ )I/2 }I/2

2 TM -

Tm

n2 + 1 +

N

= 2n2

TM T m

2

(2)

where T M and T m are the t r a n s m i s s i o n

maximum

and

corresponding m i n i m u m , on the envelope, at a certain wavelength and n 2 ( = 1.51 in our case ) is the r e f r a c t i v e index of the substrate. Using equation (I), n(A) can be calculated. Figure (2) r e p r e s e n t s the t r a n s m i s s i o n s p e c t r a of t h r e e s a m p l e s , FI, F2 a n d F3 of

1.0

The experimental arrangement shown in figure (I) has been used in discussing the s i t u a t i o n for

0.8

-~-._--_-----~-_-~_-----_---------!

!

|

,

i

,

!

!

.

!

,

,

0.6

IoCX)

d

(1)

where

Results and Discussions

oir

Vol. 59, No. 5

0.4

~

n°:l

film

n.ik

s u bstrate

n2

0.2 1.0 0.8 0.6

(glass)

air

o3 b3

~

no:l

I(X)

0.2

rr

1.0

arsenic trise!enide films evaporated on transparent substrates. The thin film is of thickness d and complex refractive index n c = n I - ik n I is the r e f r a c t i v e

i/ /..~.

?

index

and

k is

can be neglected (19) The films prepared in this work have proved to be u n i f o r m in t h i c k n e s s s h o w i n g the interference effects, o t h e r w i s e , the f r i n g e s s h o u l d have been d e s t r o y e d showing smooth transmission curves (18). The interference fringes will be used to c a l c u l a t e the optical constants as mentioned before.

0.4

of

the

Refractive

Tm

.

0.2

Tm

,f t 600

I

I I 800

I I I 1000 1200

WAVELENGTH

i

I 1400

I

I J 1600

(nm}

Figure (2) The Transmission Spectra of the Three Thin Film Samples FI, F2 and F3 of Arsenic Triselenide.

e v a p o r a t e d a r s e n i c triselenide° The dotted lines trace the interference m a x i m a T M and m i n i m a T m. F i g u r e (3) r e p r e s e n t s the calculated refractive index n I as a function of p h o t o n e n e r g y (eV) for the three measured films. These values were found to be in good a g r e e m e n t with those p u b l i s h e d in (20)

Index

Following Swanepoel (18), the refractive index of the films, nl, will be c a l c u l a t e d using the equation

.

IY"

literature Determination

.

0.6

the

extinction coefficient. The substrate is transparent and has a thickness several orders of m a g n i t u d e larger than d and index of refraction n 2. If the thin film is reasonably transparent, k I

(I)

.

0,8

(I)

The Experimental Arrangement Used For Discussing The Arsenic Triselenide Films Evaporated on Transparent Substrates.

where

to z <~ I--

Figure

0.4

(2) Determination of the Thickness d The thickness of the prepared films has been determined using three d i f f e r e n t t e c h n i q u e s and

Vol.

59,

No.

5

273

THERMALLY EVAPORATED ARSENIC TRISELENIDE USING ONLY TRANSMISSION SPECTRUM 3-

F3

F2

F’

0 0

L

X

3. t

.

l

-0

l

F2

0

' 1

Figure (4) The Plot of l/2 versus n/A For The Evaporated Arsenic Triselenide Films Films FI, F2 and F3.

Table (1) summarises the results thickness determination using the three methods mentioned. Table (1 )

Fl

Sample Technique

I

I

0.7

I

I

I

I

1.3 1.5 ENERGY

0.9 1.1 PHOTON

1

1.7 1.9 (eV)

2.1

F2

579.57 589.62 569.74

Transmission d Graphical (nm) Interferometric

I

F3

386.25 400.00 380.78

457.76 464.28 470.50

16.85 23.00

Transmission Ad (nm) Stylus

of the different

16.70

16.41 18.00

Figure (3) The Calculated Refractive Index and Absorption Coefficient as a Function of Photon Energy, for the Three

Films

Fl,

F2 and F3.

the results were compared. First, it has been calculated from the transmission curves of figure (2) using the relation

Al A2

d =

2 (n;

A,-

n;

(3) Determlnatlon

(3)

A,)

From the inspection of these data, it was found that the valuse of d (nm) determined from the graphical method deviates by about 1.7 - 3.5 % from those determined using the transmission spectra while the d values obtained from the interferometric method deviate only by about 1.0 1 .7 $ from those determined by the transmission method.

n(A) where

n’ and n’,’ are the refractive indices at two 1 adjacent maxima ( or minima ) A, and A2. Second, a simple graphical basic equation for

method was interference

2nd = m A where m is an integer for minima. Equation l/2 1

employed fringes

using

for maxima and half integer (4) can be written as 3,

m

a(A)

(5)

.. .

Plotting l/2 versus (n/A), yields a straight line with slope 2d and cut-off on the y-axis of -m. Figure (4) shows this plot for the three samples m was found to be Fl t F2 and F3. From this figure 2.0 and 2.0 for the three samples 2.5, the multiple beam Fizeau respectively. Third, fringes method(16) has been used to determine the only for the sake of comparison. film thickness,

x is the in figure

interference x = P + T is

the

-

(-u(A)

using

2 3

geometric

for given

transmission

I P2 + 2QT [ 1-X X

mean of

index coeffi(18) formula

the

d 1

absorbance (1) and is free

Coefficient

of the refractive d, the absorption

be calculated

x = exp where shown

the Absorption

the values thickness

can

the

(4)

= 2d (n/A) = 0, 1) 2,

cient

Knowing and the

of

(6) a system in terms (18) T by

1 I 1'2/

Q

as that of the

(7)

TM and Tm and is

by T = ( T14 Tm )1’2 and

P = ( x,

-

Q-2T(X,X2+XX X={(l-n 1

(8)

1 )( x2 - 1 )( x3

13 1 )/(l+n

-2X,X2X3) 1 )I

- 1 1

given

274

THERMALLY EVAPORATED ARSENIC T R I S E L E N I D E U S I N G O N L Y T R A N S M I S S I O N S P E C T R U M

X2= { (n I - n 2 )/(n I + n 2 ) } X3= { (n 2 - I

)/(n 2 + I

A

a =

b=

B + D

) }

Vol. 59, No. 5 C B+C

where o A = 16 n I n 2

T h e a b s o r p t i o n coefficient a as calculated from these equations, for the three samples FI, F2 and

F3, as a function of photon energy (eV), is shown in figure (3) along with the data on the refractive index.

B = (n I + I) 3 (n I + n 2) C = 2(n~-I)(n~ D = (n I

- n~)

I) 3 (n I - n~)

(4) D e t e r m i n a t i o n of the S u r f a c e R o u g h n e s s

Under certain conditions of preparation of the t h e r m a l l y e v a p o r a t e d f i l m s , s o m e f o r m of variation in composition and structure may cause a variation in the optical constants of the films. This may change the transmission spectrum, and the formulae u s e d for u n i f o r m f i l m s h a v e to be

The results for gd after solving these e q u a t i o n s are shown in table (I) with those obtained by the stylus profile for comparison.

modified. A c c o r d i n g to Swanepoel (21), the actual v a r i a t i o n in thickness, Ad, f r o m the a v e r a g e t h i c k n e s s d will represent the surface roughness. To evaluate d from the transmission spectrum, the following two e q u a t i o n s h a v e to be s o l v e d simultaneously

(18,19) The technique presented by Swanepoel to use the transmission spectra only to determine the optical constants and thickness of thin films has proved to be useful, easy and accurate in the case of our arsenic triselenide evaporated films. This t e c h n i q u e should be applied also in other chalcogenide amorphous s e m i c o n d u e t i n g films to check its validity.

1

a

TM = 2~niAd

Conclusions

(1_b 2) I/2 I + b .2~nAd. tan -I {(i--q--_~)I/2 tan ( ~ ) ~

(9) Acknowledgements

1 T

m

2znigd

a

The authors are indebted to Prof. E1-Semary, D e p a r t e m e n t of Physics, Cairo U n i v e r s i t y , for i n v a l u a b l e d i s c u s s i o n s and continuous interest. The help of the l a b o r a t o r y of C r y s t a l G r o w t h , Department of Physics, Cairo University, Egypt, in the c o u r s e of s a m p l e p r e p a r a t i o n is h i g h l y appreciated.

(I-b2) I/2

tan

-I

1-b

tan(2~nlAd ) (10)

{ (i_b2) I/2

These equations are valid only Ad< I/4n I. a and b follows from

in the range

0<

References

(I)

(2) (3) (4) (5) (6) (7) (8) (9) (10) (11)

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