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THE ANISOTROPY OF THE PbTe FILMS
OBLIQUELY DEPOSITED IN VACUUM STUDIED BY ELLIPSOMETRY AND PHOTOVOLTAIC
EFFECT
E. Zamfir, C. Oancea
To cite this version:
E. Zamfir, C. Oancea. THE ANISOTROPY OF THE PbTe FILMS OBLIQUELY DEPOSITED IN
VACUUM STUDIED BY ELLIPSOMETRY AND PHOTOVOLTAIC EFFECT. Journal de Physique
Colloques, 1983, 44 (C10), pp.C10-49-C10-52. �10.1051/jphyscol:19831009�. �jpa-00223458�
JOURNAL DE
PHYSIQUE
Colloque CIO, supplement au n012, Tome 44, d k e m b r e 1983 page C10-49
THE ANISOTROPY OF THE PbTe F I L M S OBLIQUELY DEPOSITED I N VACUUM STUDIED
BY ELLIPSOMETRY AND PHOTOVOLTAIC EFFECT
Faculty of PTzysics, University of Bucharest, P.O. Box MG-11, Bucharest, Romania
*Physical Laboratory, PolytechnicaZ I n s t i t u t e of Bucharest, SpZaiul Independen$ei nr. 313, B u c h e s t , Romania
Resume
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Les couches minces de PbTe deposees sous incidence o b l i q u e sous v i d e montrent un c a r a c t e r e anisotrope, e t u d i e p a r l ' e f f e t p h o t o v o l t a f q u e e t des mesures e l l i p s o m e t r i q u e s . Unemethodegraphiqued'interpolation basee surl e s r e l a t i o n s de Smet a e t e u t i l i s e e pour determiner l e s i n d i c e s de r e f r a c - t i o n p r i n c i p a u x . Les r e s u l t a t s obtenus dans ce cas o n t & t & compares avec ceux obtenus par I ' e t u d e des couches minces de PbTe deposees sous incidence normale.
A b s t r a c t
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The PbTe l a y e r s o b l i q u e l y deposited i n vacuum e x h i b i t an aniso- t r o p i c character, s t u d i e d by photovol t a i c e f f e c t and e l 1 ip s o m e t r i c measu- rements. A g r a p h i c a l i n t e r p o l a t i o n method based on t h e Smet r e l a t i o n s i s a p p l i e d t o determine t h e p r i n c i p a l r e f r a c t i o n i n d i c e s . The r e s u l t s obtained i n t h i s case a r e compared w i t h those obtained w i t h t h e PbTe l a y e r s normally deposited.I n previous papers i t was p o i n t e d o u t t h a t t h i n t e l l u r i u m l a y e r s deposited i n vacuum under o b l i q u e incidence present an o r i e n t e d s t r u c t u r e o f the c r y s t a l l i t e s , as shown by e l e c t r o n microscopy / I / . This s t r u c t u r e determines t h e appearance o f a photovol t a i c e f f e c t due t o l a y e r a n i s o t r o p y /2/.
I n t h i s paper, we present r e s u l t s on t h e a n i s o t r o p i c c h a r a c t e r o f PbTe f i l m s , obtained by s t u d i e s o f t h e e l l i p s o m e t r i c and p h o t o v o l t a i c p r o p e r t i e s o f these f i l m s /3/ /4/. The a n i s o t r o p y o f t h e f i l m s i s i n v e s t i g a t e d by e l l i p s o m e t r i c measurements a t t h e a i r - f i l m surface, t h e p r i n c i p a l i n d i c e s being evaluated i n a complex fonn.
I
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THE PHOTOVOLTAIC EFFECT-
RESULTSThe PbTe f i l m s were deposited on g l a s s s u b s t r a t e s by thermal evaporation i n vacuum o f about 18-5 t o r r under normal and o b l i q u e incidence (60") a t room temperature. The e l e c t r o d e s were formed by the d e p o s i t i o n o f Pb t h i n l a y e r s on t h e e x t r e m i t i e s o f t h e substrates. The p h o t o v o l t a i c e f f e c t was s t u d i e d w h i l e i l l u m i n a - t i o n pulses from a l i g h t spot were scanning t h e whole l e n g t h o f t h e sample. The obtained phototension was v i s u a l i s e d on a screen o f a duoscop a f t e r a previous a m p l i f i c a t i o n . T h i s f a c t p e r n ~ i t s us t o d i s t i n g u i s h t h e two components o f the pho- totension.
I n f i g u r e s 1 and 2, t h e two phototension components
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jump ( x ) and exponen- t i a l ( 0 )-
a r e presented f o r two samples : t h e f i r s t one deposited a t normal i n c i - dence ( f i g . 1 ) , t h e second one deposited a t o b l i q u e incidence ( f i g . 2 ) . The o b l i q u e l y deposited l a y e r s show a p h o t o v o l t a i c e f f e c t along t h e l e n t h o f t h e sample between layers, t h e phototension i s nu1 1 i n t h e same region.7
t h e electrodes,due t o t h e i r a n i s o t r o p i c character, w h i l e o r the normally deposited
Mesdames E. Zamfir e t C . Oancea n ' o n t malheureusement pas e t e en mesure de v e n i r au Colloque. Leur c o n t r i b u t i o n a cependant
e t @
presentee e t d i s c u t e ea
l a Confe- rence, e t l e t e x t e q u ' e l l e s nous o n t f a i t p a r v e n i r e s t p u b l i e dans ces comptes- rendus.Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831009
JOURNAL DE PHYSlQUE
Fig.1 - Phototension along the sample of Fig.2 - Phototension along the sample the normally deposited PbTe layers
:of the obliquely deposited PbTe layer:
(x) jump component and
(0)exponential (x) jump component,
(0)exponential
component. component.
I1
- ELLIPSOMETRIC METHOD AND RESULTS
The ellipsometric measurements have been carried out with a null-ellipsome- ter type PCSA. The azimuths of the polarizer, the compensator and the analizer were read with a precision of
1minute and the incidence angle with a precision of 10 seconds. Optical thick films were selected (~<1%). In this case we considered the light interaction with the bulk material surface.
The optical anisotropy of the obliquely deposited PbTe layers is studied using the Smet relations
/5/and a graphical interpolation method. The Smet rela- tions allowed the calculation of the polarizer and analizer azimuths at the light extinction during the rotation of the sample in its plane. The optical axis of the layer parallel to the substrate surface was also considered.
The graphical interpolation method is based on the use of the diagrams ob- tained with Smet relations for different values of the four optical constants
:no, ne, ko, ke. The variations of the polarizer azimuth
PI- only for the values corresponding to the first ellipsometric quadrant in the Smith convention
/6/-
versus the sample rotation angle
0 ,is presented in figure
3.In this case no
= 1.75,ko
= 1.50,the ne values are indicated on the graphs and the ke values are distinguished by means of different lines. We notice that
:(i) the curves with the same ke and different ne present two intersection points (o), and (ii) the curves with the same ne and different ke present small intersection regions
(6)that can tend towards a point with smaller variations of ke and ne than those consi- dered above.
The intersection points in both cases depend only on three parameters and are independent of the fourth. From these selected points with a given refraction index no, four equiindex curves ko were built up in a coordinate system PI,
0 ,for
O (0°-
180°).Two of these four curves correspond to the ke values, the other ones to the ne values. For the same no, but different ko, each selected curve
generates an equiindex curve famlly as it can be seen in figure
4,where the expe-
rimental curve C is also represented. At the intersections of the experimental
curve C with the equiindex curves, several pairs of ko, ke values are obtained, for
a given no, independently of ne, which can be graphically presented
bya curve
ke
=f (ko).
Fig.3
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The v a r i a t i o n o f t h e p o l a r i z e r azimuthPi
corresponding t o t h e f i r s t e l l i p - sometric quadrant(9nith convention) versus r o t a t i o n angle 0 o f t h e sample (between t h e o p t i c a l a x i s and t h e plane o f i n c i d e n c e ) .I n t h e same way, t h e azimuth values A1 o f t h e a n a l i z e r l e a d t o an o t h e r i n t e r p o l a t i o n curve ke = f ' ( k o ) . The i n t e r s e c t i o n o f these two curves gives t h e s e t o f ko and k, p o s s i b l e values f o r a g i v e n no, independent o f ne. The same procedure i s used f o r t h e i n t e r p o l a t i o n curves ne
=
F (ko) o f t h e second type.I n f i g u r e 5, a l l t h e obtained sets o f index values i n a c o o r d i n a t e system no ko, are g r a p h i c a l l y presented. A t every p o i n t , t h e value o f t h e t h i r d known parameter, ke o r ne, i s marked. Thus, two curves a r e obtained, one depending on ke and t h e o t h e r on ne. T h e i r i n t e r s e c t i o n a1 lows t o determine t h e f o u r o p t i c a l cons- t a n t s : two on t h e coordinate axes and two on t h e i n t e r s e c t e d curves.
The complex p r i n c i p a l r e f r a c t i o n i n d i c e s thus obtained f o r t h e o b l i q u e l y deposited PbTe layer, a t the a i r - f i l m i n t e r f a c e , a r e :
tio = 1.79
-
i 1.20 and A, = 1.75-
i 0.96I n t h e case o f a weakly absorbing a n i s o t r o p i c m a t e r i a l , a simple i n t e r p o l a - t i o n method can be used /7/.
I n t h e case o f t h e normally deposited f i l m t h e sample can be considered as i s o t r o p i c . The measurement o f t h e e l l i p s o m e t r i c parameters Y and A a t t h e two i n - t e r f a c e s leads t o :
f o r t h e f i l m - a i r i n t e r f a c e u s i n g t h e conventional method, and R = 4.26
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i 3.57f o r t h e f i l m - s u b s t r a t e i n t e r f a c e u s i n g t h e i n t e r n a l r e f l e x i o n method /8/.
JOURNAL
DE PHYSIQUEFig.5
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The i n t e r s e c t i o n of t h e i n t e r p o l a t i o n curves t h a t gives t h e f o u r o p t i c a l constants : no, ne, k o s ke.To e x a c t l y evaluate t h e o p t i c a l constants o f normally deposited PbTe l a y e r we e l i m i n a t e t h e atmosphere contamination by t h e i n t e r n a l r e f l e c t i o n method and we take i n t o account t h e s u b s t r a t e roughness, according t o /9/.
Fig.4
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The f a m i l y o f t h e Thus we o b t a i n/ l o /
:equiindex curves and t h e fi = 4.15
-
i 3.55experimental curve C.
I 1 1
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CONCLUSIONSThe behaviour o f PbTe t h i n l a y e r s r e v e a l s t h e a n i s o t r o p i c c h a r a c t e r o f these f i l m s when o b l i q u e l y deposited i n vacuum. This c h a r a c t e r i s p o i n t e d o u t by means of e l l i p s o m e t r i c measurements o f t h e p r i n c i p a l r e f r a c t i o n i n d i c e s and o f t h e non-null phototension on t h e l a y e r .
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1
(1970) K 131/2/ GHEORGHITA-OANCEA C., CRISTEA P., AGARBICEANU I., Brit.J.App1.
Phys. SER.2 (1969) 617
/3/ ZAMFIR E. ,-OANCEA C., Anal .Univ.Bucharest, Fiz., (1973) 5
/4/ ZAMFIR E., OANCEA C., Le Vide, Couches Minces (Suppl )
201
(1980) 482 /5/ D.J .De SMET, J.Opt.Soc.Amer., 63 (1973) 958/6/ SMITH P.H., S u r f .Sci
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,l6 (1969)34/7/ BRATESCU G., ZAMFIR E.TCRAIU C., TUDOR C., RADULESCU L., "Conferinsa N a t i o n a l l de F i z i c a " Bucharest (1982) 141
/8/ CHAN E.C., MARTON J.R., J.Appl
