ELECTRON-BEAM INDUCED CENTERS IN HYDROGENATED AMORPHOUS SILICON

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ELECTRON-BEAM INDUCED CENTERS IN HYDROGENATED AMORPHOUS SILICON

H. Schade, J. Pankove

To cite this version:

H. Schade, J. Pankove. ELECTRON-BEAM INDUCED CENTERS IN HYDROGENATED AMORPHOUS SILICON. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-327-C4-330.

�10.1051/jphyscol:1981469�. �jpa-00220926�

JOURNAL DE PHYSIQUE

CoZZoque C4, suppZ6rnent au nOIO, Tome 42, octobre 1981 page C4-327

H. Schade and J . I . Pankove

RCA Laboratories, Princeton,

NJ

08540, U. S. A.

A b s t r a c t . - Photoluminescence was used t o compare t h e e f f e c t s of low-energy e l e c t r o n bombardment and l a s e r i r r a d i a t i o n i n a-Si:H. The main e f f e c t s a r e s i m i l a r : 1 ) d e c r e a s e of t h e main luminescence peak a t 1 . 2 eV, 2) enhancement of luminescence a t 0.8 eV, and 3) complete recovery of t h e o r i g i n a l proper- t i e s a f t e r a n n e a l i n g a t 200°C. The d e c r e a s e a t 1.2 eV however, is much more pronounced w i t h e l e c t r o n t h a n w i t h photon i r r a d i a t i o n .

I. I n t r o d u c t i o n . - E l e c t r i c a l and o p t i c a l b u l k p r o p e r t i e s of hydrogenated amorphous s i l i c o n (a-Si:H) a r e a f f e c t e d by photon- (1,2,3), a s w e l l a s by e l e c t r o n - and ion- i r r a d i a t i o n ( 4 ) . With r e g a r d t o electron-induced p r o p e r t y changes, so f a r o n l y 1 NeV e l e c t r o n i r r a d i a t i o n h a s been s t u d i e d ( 4 ) . We have r e p o r t e d r e c e n t l y (5) t h a t undoped a-Si:H can be damaged by e l e c t r o n s a t much lower e n e r g i e s than c r y s t a l l i n e s i l i c o n , w i t h a damage t h r e s h o l d energy below 1 keV. The p e n e t r a t i o n d e p t h of e l e c - t r o n s w i t h a few keV i s of t h e o r d e r of 100 nm ( 6 ) , and i t i s t h u s comparable t o t h e p e n e t r a t i o n of photons i n t h e v i s i b l e range. With about e q u a l p e n e t r a t i o n of photon- and e l e c t r o n - i r r a d i a t i o n , a c l o s e comparison of t h e i r e f f e c t s can b e made; based on

-L y,,otslaninescence and a n n e a l i n g measurements, we w i l l p r o v i d e evidence t h a t t h e na- t u r e of d e f e c t s g e n e r a t e d by e i t h e r photons o r e l e c t r o n s i s very s i m i l a r .

*

Research r e p o r t e d h e r e i n was supported by S o l a r Energy Research I n s t i t u t e , under C o n t r a c t No. XJ-9-8254, and RCA L a b o r a t o r i e s .

11. Experiments.- The samples were undoped a-Si:H d e p o s i t e d on s t a i n l e s s s t e e l o r c r y s t a l l i n e s i l i c o n by glow d i s c h a r g e decomposition of s i l a n e . The hydrogen con- c e n t r a t i o n determined by secondary-ion mass-spectroscopy (7) was 5 t o 7 x 1 0 21 &3 The e l e c t r o n i r r a d i a t i o n was performed a t room temperature i n a scanning Auger m i - croprobe system, using e l e c t r o n e n e r g i e s between l and 5 keV, and an e l e c t r o n dose of t y p i c a l l y 1 x 10'' e l e c t r o n s / c m 2

.

The p h o t o n - i r r a d i a t i o n occurred a t room tem- p e r a t u r e o r e l e v a t e d temperatures (up t o 100°C) u s i n g an argon l a s e r (488 nm) a t a d o s e of about 3 x 1021 photons/cm 2

.

Both t y p e s of

i r r a d i a t i o n were a p p l i e d t o s i m i l a r samples, o r even side-by-side t o t h e same sample. Damage pro- f i l i n g , by u s i n g a combination of scanning Auger

+

^{10' -}

microprobing and A r - s p u t t e r i n g ( 5 ) , c l e a r l y shows t h a t i n b o t h c a s e s t h e i r r a d i a t i o n h a s a f f e c t e d

-

^-

t h e b u l k of t h e sample, r a t h e r t h a n j u s t t h e s u r -

'

^-

f a c e . The depth of damage a s a f u n c t i o n of t h e

5 ⁸

^!A

e l e c t r o n energy i s shown i n Fig. 1; t h e e l e c t r o n Z - - o p e n e t r a t i o n a g r e e s w e l l w i t h t h a t o b t a i n e d from ^{I- I I} range-energy r e l a t i o n s ( 6 ) . The photon penetra-

t i o n was determined by damage p r o f i l i n g t o b e t

- 1 0 ~ 4 -

z

Fig. 1. Maximum d e p t h of damage a s a f u n c t i o n of - X

a t h e e l e c t r o n i r r a d i a t i o n energy f o r f o u r d i f f e r e n t

undoped a-Si:H samples. The dashed c u r v e i n d i c a t e s

z

a n average. I ; 2 3

ELECTRON ENERGY

[K~v]

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981469

JOURNAL DE PHYSIQUE

Fig. 2. Photoluminescence intensities at 1.2 eV (upper two curves) and 0.8 eV (lower two curves) across a photon- and electron-irradiated samples. Solid lines:

after irradiation; dashed lines; after partial anneal.

200 nm; this value closely corresponds to the inverse absorption constant of a-Si:H at 488 nm (8).

The photoluminescence (PL) was measured at 77 K, with the exciting beam at a sufficiently low dose to avoid irradiation damage during the measurement. The sample could be translated in the focal plane of the spectrometer to scan the lumi- nescence intensity and compare the irradiated and non-irradiated regions on the same sample. Figure 2 shows traces through electron- and laser-irradiated regions, at the luminescence peak at 1.2 eV, and at 0.8 eV, following irradiation and a par- tial anneal (see below). Figure 3 shows the spectral dependence of the PL before and after electron-irradiation. The main peak at 1.2 eV always drops in intensity after irradiation, typically by 15% after photon-irradiation, and by about 90% after electron-irradiation. At about 0.8 eV, the changes in PL are similar for both pho- ton- and electron-irradiation. While after photon exposure one always finds an in- crease at 0.8 eV, electron exposure may result in either an increase (as shown in Fig. 3) or a decrease of the PL intensity. As shown in Fig. 2 and further discussed below, the 0.8 eV emission can be turned into an enhancement by a slight anneal.

At room temperature, some annealing of the irradiation-induced defects occurs over a period of several months. Isochronal annealing (5 min periods) of a sample that contained both photon- and electron-damage resulted in a gradual disappearance of the irradiation-induced effects. Thus, Fig.

4

shows that the original intensi- ties at 1.2 and 0.8 eV are recovered at about the same annealing temperature of 230°C. In the case of electron-irradiation, we observe a transition of the 0.8 eV intensity from a decrease to an increase before the final recovery. A similar in- crease of PL at about this energy has also been observed after low-temperature anneals at about 100°C on samples irradiated with 1 MeV electrons

(4).

Secondary-ion mass-spectroscopy shows that the irradiation-induced changes are not related to a loss of hydrogen, and this is further evidenced by the full re- covery of the original PL intensity after annealing.

o i I

A

0-

-

- 0 5 -

w>:: ^d

o---dA

" A \ ' / ,

,(-ELECTRON- INDUCED

*--*

^---Y*

-I 0 I I I

40 80 120 1 6 0 200

ANNEALING TEMPERATURE PC]

Fig. 3 . S p e c t r a l dependence of photolumi- F i g . 4. R e l a t i v e photoluminescence nescence b e f o r e and a f t e r 5 keV e l e c t r o n - change a t 1 . 2 and 0.8 eV a s a f u n c t i o n i r r a d i a t i o n . of i s o c h r o n a l a n n e a l i n g of photon- and

electron-induced i r r a d i a t i o n damage.

111. Discussion.- Our experiments cannot g i v e d i r e c t i n f o r m a t i o n on t h e atomic s t r u c t u r e of t h e d e f e c t s i n t r o d u c e d by e i t h e r photons o r e l e c t r o n s . However, t h e following c o n c l u s i o n s w i t h r e g a r d t o t h e i r p r o p e r t i e s can b e reached.

For photon-induced d e f e c t s we must p o s t u l a t e t h a t t h e y a r e caused by e l e c - t r o n i c e x c i t a t i o n s , s i n c e no d i r e c t momentum t r a n s f e r can occur. For e l e c t r o n i r r a d i a t i o n , energy and momentum can g i v e r i s e t o d i r e c t atomic displacements. But t h e Icy?-damage-thresh018 energy found f o r a-Si:H ( 5 ) would account a t most f o r t h e atomic displacement of hydrogen. Experiments w i t h a-Si:D, however, have l e d t o t h e same low-damage-threshold energy ( 9 ) . T h e r e f o r e , i n t h e c a s e of e l e c t r o n - i r r a d i a - t i o n , we can r u l e o u t d i r e c t atomic displacements of hydrogen. I n s t e a d , e l e c t r o n i c e x c i t a t i o n s may l e a d t o changes i n c e r t a i n bonding c o n f i g u r a t i o n s .

Our t e n t a t i v e model f o r t h e mechanism of d e f e c t formation i s t h a t i r r a d i a t i o n breaks t h e weaker bonds between S i atoms. Those a r e t h e bonds between atoms t h a t a r e f a r t h e r a p a r t t h a n i n a c r y s t a l , o r bonds t h e s t e r i c conformation of which is n o t o p t i m a l l y t e t r a h e d r a l . Such bonds a r e a l r e a d y p r e s t r e s s e d and r e q u i r e l e s s energy f o r d i s s o c i a t i o n .

For electron-induced d e f e c t s , t h e depth of damage a f t e r a given e l e c t r o n energy and dose i n c r e a s e s w i t h t h e hydrogen c o n t e n t of t h e sample ( 9 ) . Hydrogen t h u s p l a y s a r o l e i n t h e n a t u r e of t h e damage. But we cannot d i s t i n g u i s h , whether Si-H bonds a r e d i r e c t l y involved, o r whether Si-Si bonds c l o s e t o Si-H bonds a r e broken more e a s i l y , a s h a s been suggested (3,lO). Note t h a t s i n c e t h e Si-H bond is s t r o n g e r t h a n t h e Si-Si bond ( l l ) , i t i s l e s s l i k e l y t h a t i r r a d i a t i o n would i o n i z e t h e Si-H. This e x p l a i n s t h e s t a b i l i t y of t h e H c o n c e n t r a t i o n , a s evidenced by t h e secondary-ion mass-spectroscopy r e s u l t s and by t h e complete recovery of PL upon a n n e a l i n g .

I n an amorphous m a t e r i a l , t h e r e i s a l a r g e r a n g e of b i n d i n g e n e r g i e s . Blue l i g h t p r o v i d e s a t most 2.5 eV t o t h e d i s s o c i a t i o n p r o c e s s . The low-energy e l e c - t r o n i r r a d i a t i o n , on t h e o t h e r hand, p r o v i d e s m u l t i p l e , and p o s s i b l y more e n e r g e t i c e x c i t a t i o n s . T h e r e f o r e , provided t h e c r o s s - s e c t i o n s f o r e l e c t r o n - and photon-pro- duced d e f e c t s a r e n o t v a s t l y d i f f e r e n t , comparable damage can b e generated a t a much s m a l l e r e l e c t r o n t h a n photon dose. The l a r g e r drop i n PL a t 1.2 eV a f t e r e l e c t r o n i r r a d i a t i o n , compared t o photon i r r a d i a t i o n , may b e due t o more e n e r g e t i c e x c i t a - t i o n s t h a t r e s u l t i n a b r o a d e r range of bond breaking. From t h e PL r e s u l t s -ye con- c l u d e t h a t a t l e a s t two d i f f e r e n t d e f e c t c e n t e r s a r e involved: 1) n o n - r a d i a t i v e c e n t e r , causing a r e d u c t i o n of t h e main peak a t 1.2 eV, and 2) deep g a p - s t a t e s ,

C4-330 JOURNAL DE PHYSIQUE

opening new r a d i a t i v e r e c o m b i n a t i o n p a t h s a t e n e r g i e s around 0.8 eV, and t h u s ac- c o u n t i n g f o r t h e observed i n c r e a s e s i n PL.

It i s b e l i e v e d t h a t t h e n o n - r a d i a t i v e c e n t e r s a r e r e l a t e d t o t h e S t a e b l e r - Wronski e f f e c t (2) which g r e a t l y lowers t h e room t e m p e r a t u r e d a r k c o n d u c t i v i t y a f t e r e x p o s u r e t o l i g h t . Presumably, t h e c a r r i e r l i f e t i m e i s g r e a t l y reduced by t h e newly formed r e c o m b i n a t i o n c e n t e r s . Such recombination c e n t e r s would b e e x p e c t e d t o ad- v e r s e l y a f f e c t t h e performance of s o l a r c e l l s .

As t o t h e n a t u r e of t h e 0.8 eV l w n i n e s c e n t c e n t e r , i t s i d e n t i f i c a t i o n w i l l r e - q u i r e f u r t h e r s t u d y . Its energy s u g g e s t s a mid-gap s t a t e (E

;

1 . 6 eV). Deep

g

c e n t e r s , by v i r t u e of t h e i r s t r o n g l o c a l i z a t i o n , can b e e f f i c i e n t r a d i a t o r s . It a p p e a r s t h a t t h e s e c e n t e r s r e s p o n s i b l e f o r t h e PL i n c r e a s e a t 0.8 e V a r e t h e r e s u l t of a t e m p e r a t u r e - a s s i s t e d rearrangement of i r r a d i a t i o n - a f f e c t e d bonds t h a t o c c u r s e i t h e r d u r i n g t h e i r r a d i a t i o n , o r t h e r e a f t e r . Thus t h e enhancement of t h e 0.8 eV PL i s observed always a f t e r p h o t o n - i r r a d i a t i o n and sometimes a f t e r e l e c t r o n - i r r a d i a - t i o n . However, when t h e e m i s s i o n a t 0.8 eV i s d e c r e a s e d a f t e r e l e c t r o n - i r r a d i a t i o n , a s l i g h t a n n e a l t r a n s f o r m s t h e d e c r e a s e i n t o a n i n c r e a s e ( s e e F i g . 4 and ( 4 ) ) . I V . Conclusion.- Both e l e c t r o n - and p h o t o n - i r r a d i a t i o n produce two t y p e s of c e n t e r s : n o n - r a d i a t i v e c e n t e r s t h a t a r e dominant, and r a d i a t i v e c e n t e r s t h a t emit a t about 0.8 eV. The b e h a v i o r of t h e s e c e n t e r s i s a s f o l l o w s : 1 ) The s p e c t r a l dependence of t h e PL i s a f f e c t e d s i m i l a r l y by e l e c t r o p - and p h o t o n - i r r a d i a t i o n , namely, a d e c r e a s e of PL i n t e n s i t y a t t h e e m i s s i o n peak and an i n c r e a s e of t h e PL i n t e n s i t y around 0.8 eV. 2) The t e m p e r a t u r e h i s t o r y l e a d i n g t o t h e PL i n t e n s i t y i n c r e a s e a t 0.8 eV i s s i m i l a r , namely, a moderate h e a t i n g ( t o %lOO°C) d u r i n g i r r a d i a t i o n ( I ) , o r l a t e r d u r i n g a n n e a l i n g ; enhances t h e i n c r e a s e . 3) F u l l r e c o v e r y of t h e PL i n t e n s i t y i s o b t a i n e d a t t h e same a n n e a l i n g t e m p e r a t u r e a f t e r b o t h e l e c t r o n - and p h o t o n - i r r a d i a - t i o n .

V. R e f e r e n c e s

.-

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(1980) L357.

4. STREET R.

,

BIEGELSEN D., and STUKE J., P h i l . Mag.

B-40

(1979) 451.

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7. ^{MAGEE C.} W. and CARLSON D. E., S o l a r C e l l s 2 (1980) 365.

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⁽¹⁹⁷⁷⁾

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2

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