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Submitted on 1 Jan 1981
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TIME RESOLVED PHOTOLUMINESCENCE SPECTRA OF SPUTTERED a-Si:H
R. Collins, W. Paul
To cite this version:
R. Collins, W. Paul. TIME RESOLVED PHOTOLUMINESCENCE SPECTRA OF SPUTTERED a- Si:H. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-591-C4-594. �10.1051/jphyscol:19814129�.
�jpa-00220747�
Abstract.
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We report measurements of the time resolved photoluminescence spectra of sputtered a-Si:H. The sample and excitation energy dependence of E (t), the peak energy of the spectrum as a function of time, for 10-8Pt < 1 0 - ~ s is discussed.Introdqction.
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Time resolved photoluminescence spectra have been used to study thermalization, relaxation, and recombination processes in a-Si:H [1,21. It has been suggested that since the recombination mechanism is radiative tunneling, at a time, t, maximum emission is observed from electrons and holes with a fixed value of R/RO given byt = T 0 exp (2R/R0) (1)
where R is the average electron-hole separation, R.
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1 is the localization parameter of the electron, and -r0-10 ns [l]. Experimental results on glow discharge (g.d.) prepared a-Si:H have suggested two specific processes that contribute to the shift in the peak energy of the luminescence spectrum with time, Ep(t) [l]. The observed de- crease in Ep(t) for t < 1 0 vs was attributed to thermalizing carriers whereas at longer times relaxation into localized band tail states contributed to the decrease in Ep(t). In both processes R/RO increases. An increase in Eg(t) for 200 n S <t < 1 0 0 vs observed in g.d. a-Si:O:H was attributed to the reduct~on, with increasing R, in the Coulomb energy between recombining electrons and holes, observable due to the lower dielectric constant of the heavily oxygenated material [ 3 1 .
We report the results of similar measurements on sputtered a-,Si:H and concen- trate on Ep(t) as a function of excitation energy, hv, and sample preparation.
Experimental.
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The samples were prepared 141 bysputteringin Ar and H using different H partial pressures and/or substrate temperatures to obtain a variation in H-content from 15-30%. A comparison sample of a-Si:H:F was prepared by dc g.d. decomposition of a 2:l ratio of SiH4 and SiF4 [51. Approximately 5% F and 25% H was incorporated.The time resolved photoluminescence measurements were carried out at 77 K using -10 ns, 25 VJ, pulses from a dye laser pumped by an N2 laser. An S1 photomultiplier tube and boxcar integrator were used for detection and signal processing. For measure- ments as a function of hV the excitation intensity was varied such that a constant luminescence output was maintained independent of hv. The density of absorbed photons was kept roughly constant with hV at about 1 X 1017 cm-3.
Results and Discussion.
I. Sample Dependence. - Figure 1 exhibits E (t) for three samples of sputtered a-si:H and a fourth sample of g-d. a-Si:H;F.
T R ~
widest gap sample exhibits a feature in Ep(t) characterized by a fast decrease in Ep with time for t < 5 0 ns, followed by an increase for 50 ns < t < 500 ns. We observe a progressive weakening of the feature with increasing bandgap size. We associate the increase,in Ep with a changing Coulomb interaction between recombining electron-hole pairs as R increases with time for 50 n s < t < 500 ns. The sample dependence of the increase in Ep(t)Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814129
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1.3L I I I I
l
10-8 10-7 10-5 I - 10-3
t (sec)
Fig. 1: E p ( t ) f o r glow d i s c h a r g e a-Si:H:F ( t o p ) and t h r e e s p u t t e r e d a-Si:H samples.
O p t i c a l c o n s t a n t s f o r s p u t t e r e d samples a r e (from t o p t o bottom) EO4(300 K)=
2.16, 2.07, 1.97 eV; n ( 2 . 2 ~ ) = 3 . 1 1 , 3.36, 3 . 5 7 . For t h e glow d i s c h a r g e sample Eo4 2.25 eV. Eo4 i s t h e photon energy f o r which t h e a b s o r p t i o n c o e f f i c i e n t i s 10* cm-l.
s u g g e s t s t h a t a s t h e gap s i z e i n c r e a s e s and t h e index of r e f r a c t i o n , n, of t h e sample d e c r e a s e s , t h e Coulomb i n t e r a c t i o n i s l e s s e f f e c t i v e l y screened. WO p o s s i b l e e x p l a n a t i o n s may b e c o n s i d e r e d f o r t h e sample dependence o f t h e t < 50 ns d e c r e a s e i n E p ( t ) . We might s u g g e s t t h a t a s t h e band gap s i z e i n c r e a s e s , t h e t a i l s t a t e d i s t r x b u t i o n i s broadened, r e s u 1 t i n g . h a s t e e p e r E p ( t ) , a s was r e p o r t e d f o r a-Si:O%
[31. A second a l t e r n a t i v e i s t o a s s o c i a t e t h e entire f e a t u r e (6 n s < t < 5 0 0 n s ) i n E p ( t ) w i t h t h e Coulomb c o n t r i b u t i o n . S i n c e s t e a d y s t a t e measurements of t h e spec- t r a l width i n d i c a t e t h a t t h e t h r e e s p u t t e r e d samples have comparable t a i l s t a t e d i s t r i b u t i o n s , t h e second a l t e r n a t i v e i s f a v o r e d and t h e e n t i r e f e a t u r e i n E p ( t ) i s t e n t a t i v e l y a s s o c i a t e d w i t h a Coulomb i n t e r a c t i o n which i s e f f e c t i v e l y screened i n t h e h i g h e r n m a t e r i a l .
I n t h e absence of t h e Coulomb i n t e r a c t i o n , we s u g g e s t t h a t Ep may be approxi- mately l i n e a r w i t h l o g t a t a l l times w i t h a s l o p e g i v e n by t h e long time ( t > 5 0 0 n s ) b e h a v i o r i n Fig. 1. Using t h i s assumption, t h e Coulomb c o n t r i b u t i o n can be e x t r a c t e d from t h e d a t a o f Fig. 1. For r e p r e s e n t a t i v e d a t a t h e c o n t r i b u t i o n i s roughly
symmetric i n l o g t about t = 2 5 n s ( s e e F i g . 2) w i t h a maximum of 100 meV.
Using E = n2 f o r t h e d i e l e c t r i c c o n s t a n t , F i g . 2 p r e d i c t s R = 15A a t 25 ns and R = 50fj a t b o t h t h e o n s e t of measurement ( 6 ns) and a t 150 ns. Such r e s u l t s s u g g e s t t h a t a f t e r t h e r m a l i z a t i o n , f o r some f r a c t i o n of t h e e l e c t r o n - h o l e p a i r p o p u l a t i o n R d e c r e a s e s w i t h time and t i g h t l y bound p a i r s a r e formed. This popula- t i o n may n o t obey Eq. (1) s i n c e t h e r e l a x a t i o n r a t e r a t h e r t h a n t h e r a d i a t i v e t u n n e l i n g r a t e may determine t h e recombination time. I n t h e l a r g e r n m a t e r i a l recombination a t s h o r t times i s n o t dominated by t h i s p r o c e s s .
11. E x c i t a t i o n Energy Dependence.
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F i g u r e 3 e x h i b i t s E p ( t ) f o r one of t h e s p u t t e r e d samples o f F i g . 1 . ( E o 4 = 2.16 eV) u s i n g f o u r d i f f e r e n t e x c i t a t i o n e n e r g i e s . For hv > 2.14 eV no variation xn E p ( t ) w i t h h V i s observed. I f w e a s s u m e t h a t t h ea larqe difference between the average electron hole separations after thermali-
10-8 .- 10-7 zation for the two different excitation
t (sec) energies would be evident in different
Ep(t) at least in the regime of strong Coulombic effects. The absence of any changes indicates that Rth is nearly inde- pendent of hV-EG. This in turn suggests that the time required for complete thermalization is dominated by the final thermalization events when the energy of the thermalizing electrons approaches EG.
Between hV=2.14 eV and 1.97 eV, changes in Ep(t) begin to occur in the Cou- lombic regime. Figure 4 displays the eircitation energy dependence of Ep at the time of maximum Coulomb contribution (t=25 ns). There appears to be a steady de- crease in Ep with decreasing hV beginning at a well-defined value of hV=2.07eV
f 0.01. It seems likely that in this range of excitation energy we begin to excite electrons into localized states and the function R(t) is altered measurably since relaxation begins from a distribution of electron-hole separations that is peaked at a smaller value of R. We also note that in Fig. 3, electrons recombining at longer times have lost memory of the initial distribution of electron-hole separations. The lower the excitation energy, the longer is required to erase the memory of the initial distribution.
Fig. 3: E (t) for different hV for sample of Fig. 2.
P
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Fig. 4: E (t=25 ns) vs. hv for sample of Fig. 2. Vertical lines indicate estimated and EO4 at 77 K.
Conclusions. - (1) A feature in Ep, the peak energy of the luminescence spectrum vs.
time is observed in a-Si:I4, characterized by a fast decrease in Ep(t) for t < 5 0 n s and an increase in Ep(t) for 50 n s < t < 500 ns. The feature becomes weaker for samples of progressively larger dielectric constant and is thus associated with an electron-hole Coulomb interaction contribution to Ep(t).
(2) For a typical sample of low dielectric constant, using super-bandgap exci- tation the average electron-hzle pair separation, R, at the onset of measurement
(6 ns) is estimated to be 50 A. When the Coulomb contribution is at its maximum (t-25 ns) R-l5
A.
(3) In situations where the Coulomb contribution to Ep is large, we question the applicability of Eq. (1) describing the time dependence of R.
( 4 ) The absence of changes in Ep(t) with excitation energy, hv, well above
the band gap suggests that the electron-hole pair separation after thermalization is independent of hV.
(5) We observe changes in Ep(t) in the Coulombic regime as hv is decreased below a well-defined value (within 0:02 eV) of hV. We suggest that this represents the onset of the excitation of localized carriers, and this influences the initial distribution of electron hole separations.
Acknowledgments. - We thank J. Blake and B.G. Yacobi for performing optical measure- ments, B. von Roedern and D.K. Paul for the g.d. sample and P. Ketchian for the sputtered samples. Funding was provided by National Science Foundation grant number NSF-DMR78-10014.
References
[l] Tsang C. and Street R.A., Phys. Rev. (1979), 3027.
[2] Kurita S., Czaja W., and Kinmond S., Solid State Commun.
2
(1979), 879.[3] Street R.A., Solid State Commun.
2
(19801, 157.[4] Anderson D.A., Moddel G., Paesler M.A., and Paul W., J. Vac. Sci Technol.
16
(1979), 906.
[5] von Roedern B. and Paul D.K., private communication.
[6] Davis E.A., J. Non.-Cryst. Solids
