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PHOTOLUMINESCENCE STUDIES ON a-Si : H : Cl FILMS
F. Evangelisti, P. Fiorini, G. Fortunato, A. Frova, G. Bruno, P. Capezzuto, F.
Cramarossa
To cite this version:
F. Evangelisti, P. Fiorini, G. Fortunato, A. Frova, G. Bruno, et al.. PHOTOLUMINESCENCE STUDIES ON a-Si : H : Cl FILMS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-849-C4-851.
�10.1051/jphyscol:19814187�. �jpa-00220813�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppZe'ment au nO1O, Tome 4 2 , octobre 1981 page C4-849
PHOTOLUMINESCENCE STUDIES ON a-Si:H:Cl FILMS
F. Evangelisti, P. Fiorini, G. Fortunato, A. Frova, G . ~runo*, P.; .capezzuto
*
and F. ~ramarossa*I s t i t u t o d i Fisica "G. Marconi", Universita' d i Roma, Rome, ItaZy.
"Centro d i Studio per Za Chernica de-i PZasmi deZ C. N.R., Bar?, ItaZy.
Abstract. - We report photoluminescence measurements on a-Si:H:Cl. The spec- tra show that three different transitions. can occur at 0.75, 0.95 and 1.3 eV, their individual presence being dependent on growth conditions. It is found that the gas composition in the reactor is the main parameter which influ- ences the overall radiative efficiency by determining the quality of the films.
Introduction.
-
Since it has been realized that a-Si:H can be used for device produc- tion, many efforts have been devoted to obtain better material for photovoltaic con- version. Among several possibilities, little attention (1) has been paid to amorphous silicon deposited by glow-discharge in SiC14+H2. One advantage of this technique is that silicon-tetrachloride is abundant and less expensive than SiH4. Moreover, the inclusion of C1 might result in films more stable both structurally and electrically.Aim of the present work is to investigate in some detail the influence of the glow- discharge parameters on the photoluminescence spectra of a-Si:H:Cl and to compare them with those reported in the literature for a-Si-H. Photoluminescence is used as a main probe of the "quality" of the material, because it is very sensitive to the presence of gap states, in as much as the latter act as recombination centers and reduce carrier lifetime.
Experimental procedure.
-
The amorphous silicon films studied in the present work have been grown by glow-discharge in a mixture of SiC14 and Hz. The deposition apparatus consists of a tubular flow reactor, capacitively coupled to a 35 MHz r.f.generator (2). The ranges of variation of the growth parameters are: r.f. power 20- 200 W, substrate temperature T 200-400°c, pressure 0.8-4.5 torr, feeding gas com- positionR6.e. the ratio betwetn SiC14 pressure and the total pressure) 0.03-0.15.
The hydrogen and chlorine contentsrange from 1 to 3 and from 1 to 5 at.% re- spectively. The amount of both H and C1 decreases upon increasing T whereas, at a given T the C1 content increases for higher values of R. s'
s '
Photoluminescence measurements have been taken at liquid nitrogen temperature by CW excitation with the 5145 A line of an Ar-ion laser (200 to 500 mW' power range).
Films of different thickness have been used to discriminate against structures in the spectra introduced by interference effects.
Results and discussion.
-
The feeding gas composition R is the main parameter which affects the luminescence intensity and lineshape. The behavior of the radiative ef- ficiency as a function of R cah be seen in Fig.1. A reduction of two or more orders of magnitude is found in samples grown at low R. On the contrary, at higher values of RArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814187
C4-850 JOURNAL DE PHYSIQUE
Fig. 1 : Intensity of photoluminescence Fig. 2 : Photoluminescence vs.
vs.R for four samples deposited at photon energy for three samples Ts = 320 C and r.f. power = 75 W grown at Ts = 320 C, r.f. power =
75 W and different values of R.
the efficiency is comparable to that of a-Si:H, as we have deduced from measurements in our apparatus on samples grown at IBM-Yorktown, the University of Dundee and in our laboratory.
Changing R results also in a different spectral distribution of the emitted radiation. Two characteristic behaviors can he singled out. For R ? 0.1, only one band ( A ) with high radiative efficiency is present in the spectra (see Fig. 2). Its features are similar to those of luminescence spectra from a-Si:H, i.e. peak po- sition falling between 1.25 to 1.4 eV and full linewidth between 0.36 to 0.24 e V . For R O.l,band A is missing or very weak. As can be seen in Fig. 2, there appear two new bands at 0.95 eV (B) and 0.75 eV (C), which come either together or isolated
Among the other growth parameters, only the r.f. power seems to have an ap- preciable influence on luminescence, the efficiency becoming higher with power.
Discussion of this dependence is beyond the scope of this paper.
Comparison of the present results with the luminescence behavior known for a-Si:H from the literature, allows drawing some conclusions and proposing a scheme of optical transitions, which accounts for the observed emission bands. Band A results from radiative transitions between conduction and valence band tail states.
This band dominates the spectra of films showing high radiative efficiency. As the quantum efficiency is strongly affected by defect states in the gap, we conclude that the samples grown at the larger values of R have the lower density of such states
.
Reduced r a d i a t i v e e f f i c i e n c y s u g g e s t s a d e f e c t d e n s i t y which s h o u l d exceed 1017 k 3 ( 3 ) . The o c c u r r e n c e o f reduced e f f i c i e n c y , u s u a l l y accompanied by t h e ap- p e a r a n c e o f a new band a t
2
0.8-0.9 eV, h a s been r e p o r t e d i n samples w i t h h i g h doping ( 4 ) o r a f t e r e l e c t r o n bombardment (5) o r grown by r . f . s p u t t e r i n g a t l a r g e hydrogen p r e s s u r e ( 6 ) . The p r e s e n c e of a s i m i l a r band i n o u r c h l o r i n a t e d samples a t lower R-values (band B) s u p p o r t s t h e s u g g e s t i o n by S t r e e t ( 3 ) t h a t i t i s r e l a t e d w i t h some b a s i c f e a t u r e i n t h e d e n s i t y of l o c a l i z e d s t a t e s of e x t r i n s i c o r i g i n , i . e .t h e gap s t a t e s d e n o t e d by E t h a t a r e o b s e r v e d i n f i e l d - e f f e c t measurements. I t i s t h e r e f o r e assumed t h a t t h e Harge d e n s i t y of gap s t a t e s i n c r e a s e s t h e e l e c t n u n c a p t u r e r a t e a t E where t h e r a d i a t i v e t r a n s i t i o n o r i g i n a t e s .
As t o band C , t o o u r knowledge t h i s i s t h e f i r s t r e p o r t of a r a d i a t i v e t r a n - s i t i o n a t energy lower t h a n 0 . 8 eV o c c u r r i n g s i m u l t a n e o u s l y w i t h band B . We r u l e o u t t h e p o s s i b i l i t y t h a t chis new band ( a s w e l l a s band B ) i s a s s o c i a t e d w i t h t r a n s i t i o n s i n v o l v i n g l e v e l s i n t r o d u c e d by c h l o r i n e . T h i s i s b e c a u s e i t s s t r e n g t h d r o p s down by i n c r e a s i n g t h e amount of C 1 . A t t h e moment, s t a r t i n g from t h e o b s e r - v a t i o n t h a t band C i s s e e n i n samples w i t h v e r y weak r a d i a t i v e e f f i c i e n c y , we c a n o n l y s p e c u l a t e t h a t t h i s t r a n s i t i o n becomes p o s s i b l e when t h e d e n s i t y of gap c e n t e r s i s s o h i g h t h a t t h e h o l e s a l s o t e n d t o b e c a p t u r e d by s t a t e s f u r t h e r i n s i d e t h e pseudo-gap, e.g. t h e E s t a t e s d e t e c t e d i n t h e f i e l d e f f e c t measurements. I n t h i s model, t h e new band C would o r i g i n a t e from r a d i a t i v e t u n n e l i n g between E Y and E
.
X Y
I n conitlusion, t h e p r e s e n t d a t a s u g g e s t t h e p o s s i b i l i t y of p r o d u c i n g a-Si films by glow-discharge i n SiC14+H2, which have comparably good p r o p e r t i e s t o t h o s e d e p o s i t e d from s i l a n e . I t i s found, however, t h a t t h e q u a l i t y of t h e m a t e r i a l i s c r i t i c a l l y dependent on t h e gaseous composition i n t h e r e a c t o r . High hydrogen c o n t e n t r e s u l t s i n v e r y d e f e c t i v e a-Si, w h i l e a t S i c 1 p a r t i a l p r e s s u r e s i n e x c e s s of some 20%, no f i l m d e p o s i t i o n seems t o o c c u r . T h i s may e x p l a i n why t h e method of 4 d e p o s i t i o n from SiC14, s o f a r , h a s been t a k e n under c o n s i d e r a t i o n o n l y m a r g i n a l l y . R e f e r e n c e s .
-
1 ) K R ~ L E R W . W . , PLXTTNER R.D., MOELLER M . , RAUSCHER B . , and STETTERW., J . of Non- C r y s t . S o l i d s
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(1980) 333.2 ) BRUNO G., CAPEZZUTO P., CRAMAROSSA F., and D'AGOSTINO R., Thin S o l i d s Films
63
(1980) 103.
3) STREET R.A., Phys. Rev. B
21
(1980) 5775.4) REHM W . , ENGEMANN D . , FISCHER R., and STUKE J . , P r o c . 1 3 t h I n t . Conf. on P h y s i c s of Semiconductors, ed. F.G. Fumi, Rome 1977, p . 525.
5 ) STREET R.A., BIEGELSEN D . K . , and STUKE J . , P h i l . Mag. B
3
(1979) 451.6 ) ANDERSON D.A., MODDEL G . , COLLINS R.W., and PAUL W . , S o l i d S t a t e Commun.
21
(1979) 677.
