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DENSITY OF STATES STUDY IN SPUTTERED a-Si : H : EFFECT OF IMPURITIES AND H RELATED
DEFECTS
P. Viktorovitch, G. Moddel, J. Blake, S. Oguz, R. Weisfield, W. Paul
To cite this version:
P. Viktorovitch, G. Moddel, J. Blake, S. Oguz, R. Weisfield, et al.. DENSITY OF STATES STUDY
IN SPUTTERED a-Si : H : EFFECT OF IMPURITIES AND H RELATED DEFECTS. Journal de
Physique Colloques, 1981, 42 (C4), pp.C4-455-C4-458. �10.1051/jphyscol:1981495�. �jpa-00220952�
JOURNAL DE PHYSIQUE
CoZZoque C4, suppldment m nO1O, Tome 42, octobre 1981 page C4-455
DENSITY OF STATES STUDY I N SPUTTERED a-Si:H
:
EFFECT OF I M P U R I T I E S AND H RELATED DEFECTSP. viktorovitch*, G. Moddel, J . Blake, S. Oguz, R.L. Weisfield and W. Paul Division of Applied Sciences,
02138, U.S.A. Harvard University, Cambridge, Massachusetts A b s t r a c t . - We o b s e r v e two regimes i n t h e v a r i a t i o n of qap s i z e (Eo4) and den- s i t y of s t a t e s (DOS) a s a f u n c t i o n of H-content (c?) i n s p u t t e r e d a-Si:H. I n t h e f i r s t regime, an i n c r e a s e i n c H from 5 t o 17% 1s accompanied by a n exponen- t i a l f u n c t i o n a l d e c r e a s e of t h e DOS near midgap, which v a r i e s a l s o d i r e c t l y with E04 o v e r a wide range o f p r e p a r a t i o n c o n d i t i o n s . I n t h e second regime, a d d i t i o n a l i n c o r p o r a t i o n of H beyond 17% f a i l s t o f u r t h e r d e c r e a s e t h e DOS
( w i t h CH and E O 4 ) , which shows a s a t u r a t i o n w i t h s c a t t e r . This behavior i s i n t e r p r e t e d i n terms o f H-induced and r e s i d u a l i m p u r i t y r e l a t e d d e f e c t s . Vari- a t i o n o f t h e DOS i n t h e lower h a l f o f t h e gap, r e s p o n s i b l e f o r h o l e t r a p p i n g , i s observed v i a t h e h o l e m o b i l i t y - l i f e t i m e product, which e x h i b i t s a d r a m a t i c drop w i t h i m p u r i t y i n c o r p o r a t i o n (P,O) and d e t e r i o r a t e s f o r i n c r e a s i n g c H i n t h e second regime.
I n t r o d u c t i o n . - A s t u d y o f t h e change i n t h e o v e r a l l d e n s i t y o f s t a t e s (DOS) versus H-content ( c H ) i n a-Si:H produced by s p u t t e r i n g r e v e a l s two d i s t i n c t regimes i n t h e v a r i a t i o n of t h e DOS, which a r e i n t e r p r e t e d i n terms of H-induced and impurity- r e l a t e d d e f e c t s . The CH i s o b t a i n e d from t h e i n t e g r a t e d i n t e n s i t y o f t h e Si-H wag mode v i b r a t i o n a l a b s o r p t i o n band.l A d d i t i o n a l i n f o r m a t i o n a b o u t H-bonding i s d e r i v e d from thermal e v o l u t i o n experiments. D i f f e r e n t t y p e s o f measurement a r e used t o i n v e s t i g a t e t h e DOS d i s t r i b u t i o n : n e a r IR and v i s i b l e (1.4-2.5 eV) absorp- t i o n measurements p r o v i d e i n f o r m a t i o n about t h e v a l e n c e band edge
DOS^
and g i v e a measure o f t h e o p t i c a l gap E04 ( t h e energy a t which a = 104cm-l); deeper s t a t e s a r e sampled by lower energy a b s o r p t i o n measurements (1 -1-1.4 eV) which complete and confirm t h e d e t e r m i n a t i o n of t h e DOS d i s t r i b u t i o n between t h e midgap and t h e Fermi l e v e l o b t a i n e d from Schottky d i o d e conductance/capacitance [G(w)/C(w)l measurements v e r s u s frequency a t z e r o b i a s ; 5 r 6 f i n a l l y , t h e Schottky d e v i c e s p e c t r a l r e s p o n s e i s a p p l i e d t o g i v e t h e h o l e m o b i l i t y - l i f e t i m e product [ ( u T ) ~ ] from which i n f o r m a t i o n i s i n f e r r e d r e g a r d i n g t h e s t a t e s i n t h e lower h a l f o f t h e gap r e s p o n s i b l e f o r t h e t r a p p i n g of h o l e s .Sample p r e p a r a t i o n . - a-Si:H was prepared8 by r f s p u t t e r i n g of a c-Si t a r g e t i n an Ar-H2 atmosphere. The i n c o r p o r a t i o n o f i m p u r i t i e s (P,O) was achieved by i n t r o d u c i n g phospfiine o r oxygen t o t h e plasma.
Results.- F i g u r e 1 shows t h e v a r i a t i o n of t h e DOS n e a r midgap9 VerSUs cH. Two d i s t i n c t regimes a r e o b s e r v a b l e . I n t h e f i r s t regime an i n c r e a s e i n c H up t o about 17% r e s u l t s i n a n e x p o n e n t i a l f u n c t i o n a l d e c r e a s e i n t h e DOS. I n t h e second regime, a d d i t i o n a l i n c o r p o r a t i o n o f H, beyond 17%, f a i l s t o f u r t h e r d e c r e a s e t h e DOS, which shows a s a t u r a t i o n w i t h s c a t t e r . The H e v o l u t i o n s p e c t r a f o r cH <17% and f o r c H > 1 7 % a r e shown i n F i g . 2 . The two regimes a r e a l s o c l e a r l y r e f l e c t e d i n t h e var- i a t i o n of t h e DOS versus Eo4 shown i n F i g . 3. I n t e n t i o n a l i n c o r p o r a t i o n of impuri- t i e s (P,O) r e s u l t s i n a l a r g e i n c r e a s e i n t h e DOS magnitude and a s i g n i f i c a n t modi- f i c a t i o n o f i t s d i s t r i b u t i o n near midgap.
The most r e l e v a n t r e s u l t t o t h i s paper o f a d e t a i l e d s t u d y o f (UT) p u b l i s h e d elsewhere7 concerns t h e second regime: it was found t h a t , f o r a g i v e n P ~ O s , ( V T ) ~ i s
*Ecole C e n t r a l e de Lyon, ERA CNRS ~ d n i e e ~ e c t r o n i q u e (no. 6 6 1 ) . 36 Route d e D a r d i l l y , 69130 E c u l l y , France.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981495
JOURNAL DE PHYSIQUE
F i g u r e 1: Midgap d e n s i t y of s t a t e s DOS
versus cH. Two d i s t i n c t regimes a r e c l e a r l y obser- v a b l e corresponding respec- t i v e l y t o an e x p o n e n t i a l d e c r e a s e of t h e DOS f o r C ~ < 1 7 % and t o a s a t u r a t i o n o f t h e DOS w i t h s c a t t e r f o r c > 1 7 % .
H
F i g u r e 2 : Evolution s p e c t r a showing t h e r a t e of hydrogen e v o l u t i o n , aP/aT Versus-T, f o r a f i l m with c H < 1 7 a t . % and one w i t h c H > 1 7 a t . % , both depo- s i t e d o n t o 200°C s u b s t r a t e s
( T S ) .
F i g u r e 3: Midgap d e n s i t y of s t a t e s DOS versus E 4 t h e energy where a = 108 ;m-l. The two regimes i n t h e c h a r a c t e r i s t i c DOS (cH) a r e a l s o i n evidence h e r e .
lowest for the highest cH and decreases with impurity content, at any cH. Typical results illustrating this are given in Table 1.
sample number SIS S I S S I S S I S S I S S I S 140 183 I1 174 I11 174 I 197 IV 197 111
Table I: Results obtained on some typical samples illustrating the change in the midgap DOS and ( U T ) ~ with cH (in the second regime) and with impurity incorporation (detarls regarding the errors in the determination of vari- ous parameters are given in references 5 and 10).
midgap DOS (cm-3ev-1)
100 NH cH(%) =-
N ~ f N ~ i impurity incor- poration (par- tial pressures of phosphine or oxygen)
(PT) ( C I ~ ~ V - ~ )
I
PDiscussion.. First regime.: the midgap DOS is far larger than that determined (1012 -1013cm-3e~-~) from extrapolation of the band tails to midgap using the qeas- ured slope of the exponential absorption tail,3 not unexpectedly, since the former is presumably caused by defects and the latter by disorder in the completely-coor- dinated network. Nevertheless the midgap DOS varies directly and decreases exponen- tially with Eo4 over a wide range of deposition conditions, indicating the simulta- neous actions of H of removal of gap states and shifting of band edge states. Evi- dently the low DOS required for photovoltaic applications can be achieved only at the expense of an increased gap size (although the relationship DOS -Eo4 may depend on the details of preparation).
Second regime.: the increase in the DOS with impurity incorporation indicates that the saturation of the DOS with cH may be associated with residual impurities whose presence is revealed when most of the dangling bond-related states are compensated by H-incorporation. The failure to further decrease the DOS for cH larger than 17%
can be caused also by H-induced defects. This second interpretation seems to be confirmed by H-evolution measurements: the evolution spectra show that at most 17%
of H can go to tightly bound sites and evolve at temperatures higher than 450°C.
Any additional H goes into more weakly bound sites and evolves at about 300°C.
Therefore we suggest that: (i) only the tightly bound H reduces the midgap DOS;
(ii) once these sites are saturated, the additional incorporation of H either has no effect and saturation in the DOS is due to impurities, or H actually induces new defect states. A combination of the two may also occur.
7x10 l5
28.5
-
5.5x10-~O
The onset of the second regime coincides with the cH where other properties are known to undergo a transition. The conductivity shows a scatter in both the preex- ponential factor and the activation energy for high cH.ll The photoconductivity magnitude also shows a scatter.12 The photoluminescence exhibits a second peak near 0.9 eV besides the usual peak at about 1.3 eV, and the photoconductivity spectrum exhibits an enlarged shoulder near 1.2 eV; the material is then similar to a doped material .l3
The behavior of ('1~)~ summarized in Table I shows that the incorporation of impurities or an excess H beyond 17% affects the states responsible for hole trapping in the lower half of the gap5 more than midgap states: the sample with an excess H
9x10 l5
17.1
-
2.5x10-~
5x10 l7
9.4
-
1. 5x10-l0
1.5X10 18
9.6
P = ~ o - ~ T 0
1.3X10-l' 3X1016
21
-
4.7x10-~O
2x1~17
20.2
pPH = ~ x ~ o - ~ T 3
9x10-l~
C4-458 JCURNAL DE PHYSIQUE
of about 11% (cH = 28.5%: first column of Table I) has about the same midgap DOS as the sample with cH = 17.1% (second column), whereas its (PT) is significantly smal- ler. The incorporation of impurities (third to sixth column? also reduces (PT), more dramatically than it increases the midgap DOS.
It is uncertain whether residual impurities or excess H are primarily responsi- hle for the behavior observed in the second regime. Given the similar consequences of incorporation of impurities and excess H, it is plausible that they both catalyse the same type of defects which affect the gap state distribution.
Acknowledgments.- We thank B. Yacobi and J. Blake for carrying out some of the opti- cal measurements. This research was supported by the U.S. Department of Energy under Subcontract No. xW-g-9358-1 of Prime Contract No. EG-77-C-01-4042 and by the Joint Services Electronics Program under Contract No. N0014-75-C-0648.
References
(1) The calibration of the H-content, CH = NH/(NH;tNSi), to the integrated area of the IR Wag mode was deduced from a comparison of H evolution, nuclear reaction and IR measurements on sputter-deposited films.
(2) S. oguz and M.A. Paesler, Phys. Rev. B
22,
6213 (1980).(3) G. Moddel, J. Blake, R.W. Collins, P. Viktorovitch, D.K. Paul, B. von Roedern and W. Paul, Proc. Int. Conf. on Tettahedrally-Bonded Semiconductors, Carefree, Arizona, March 1981 (to be..published R.A. Street, ed.).
(4) G. Moddel, D.A. Anderson and W. Paul, Phys. Rev. B
z,
1918 (1980).(5) P. Viktorovitch, G. Moddel, J. Appl. Phys.
51,
4847 (1980).(6) P. Viktorovitch, J. Appl. Phys.
52,
1392 (1981).
(7) P. Viktorovitch, G. Moddel, J. Blake and W. Paul, to be published in J. Appl.
Phys
.
(8) D.A. Anderson, G. Moddel, M.A. Paesler and W. Paul, J. Vac. Sci. Technol.
16,
906 (1979).
(9) The quoted numbers, derived from Schottky diode G(w) and C ( w ) measurements, are an average value of the DOS between the surface and bulk Fermi levels of the device. They are found to vary linearly with the low energy (1.1 to 1.4 eV) optical abosrption coefficient derived from photoconductivity.
(10) S. Oguz, thesis, unpublished.
(11) D.A. Anderson and W. Paul, to be published in Phil. Mag.
(12) G. Moddel, thesis, unpublished.
(13) D.A. Anderson, G. Moddel and W. Paul, J. Non-Cryst. Solids
