PHOTOVOLTAIC EFFECT IN INTERSTITIALLY DOPED a-Si:H

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PHOTOVOLTAIC EFFECT IN INTERSTITIALLY DOPED a-Si:H

S. Kumar, S. Agarwal

To cite this version:

S. Kumar, S. Agarwal. PHOTOVOLTAIC EFFECT IN INTERSTITIALLY DOPED a-Si:H. Journal

de Physique Colloques, 1981, 42 (C4), pp.C4-527-C4-530. �10.1051/jphyscol:19814113�. �jpa-00220730�

PHOTOVOLTAIC EFFECT IN INTERSTITIALLY DOPED a-Si:H

S. K u m a r and S.C. A g a r w a l

Department o f Physics, Indian I n s t i t u t e o f Technology, Kanpur 208016, India

Abstract.- a-Si:H f i l m s prepared by d.c. glow discharge of s i l a n e a r e doped with l i t h i u m by thermal d i f f u s i o n a t 450K.

The dark conductivity a t room temperature i s found to i n c r e a s e by 4 o r d e r s of magnitude upon doping, whereas t h e conductivity a c t i v a t i o n energy (AE) decreases from 0.62eV to 0.17eV. The thermopower(S) f o r t h e doped f i l m s i s found t o be negative and a p l o t of S v s l/T has a slope (E o f about 0.17eV. The photo- conductivity (a h) f o r t h e doped e i l m s i s found t o i n c r e a s e by a f a c t o r of 5. !in some sanples, having a coplanar geometry of electrodes, an e l e c t r i c f i e l d

=

103v/cm i s applied between the e l e c t r o d e s immediately a f t e r L i evaporation a t 450K and t h e s a q l e s a r e allowed to cool down t o room terrrperature i n t h e presence of t h e e l e c t r i c f i e l d , This r e s u l t s i n a gradient of L i i o n s along t h e length of t h e sanple, a s evidenced by a photovoltage which appears upon s h i n i n g white l i g h t onto these

samples, I n c o n t r a s t , no photovoltage i s observed in samples, which a r e n o t subjected t o the e l e c t r i c f i e l d .

I. Introduction.- Lithium a c t s a s a donor i n c r y s t a l l i n e

c

l ] a s well a s i n hydrogenated amorphous s i l i c o n (a-SirH) [ 23. Beyer et. al. [ 2

1

achieved i n t e r s t i t i a l doping of L i i n glow discharge deposited a-Si:M by thermal d i f f u s i o n a t 400°C, a s well a s by i o n Inplantation. Sang

La

^{reported L i} doping of vacuum evaporated a-Si which were hydrogena- t e by annealing i n a hydrogen plasma before c a r r y i n g o u t t h e in- d i f f u s i o n of Li a t t e n p e r a m r e varying between 200 and , 4 0 0 ~ ~ . Both t h e s t u d i e s show an i n c r e a s e I n e l e c t r i c a l conductivity by s e v e r a l o r d e r s of magnitude upon doping. Further, t h e t h e m p o w e r measurements of

Beyer e t . a l . show t h a t litihium a c t s a s a donor i n a-Si:H.

We have c a r r i e d o u t t h e l i t h i u m doping of glow dtscharge depo- s i t e d a-Si:H by thermal d i f f u s i o n a t 450K. The room temperature con- d u c t i v i t y i n c r e a s e s by about 4 o r d e r s of magnitude and t h e photocon- d u c t i v i t y (a ) i n c r e a s e s by a f a c t o r of 5. The thennopower i s nega- t i v e f o r theP$ped sanples, i n agreement with t h e e a r l i e r work 121.

I n c r y s t a l l i n e semiconductors a gradient of impurities along t h e length of the sample i s known t o give rise to a photovoltage[4,5j.

We have found t h a t a s i m i l a r e f f e c t i s observed i n L i doped a-Si :H, i f t h e doped sample i s subjected t o an e l e c t r i c f i s l d a t high temperature and cooled down i n t h e presence of t h e e l t q t r i c f i e l d , thus g i v i n g r i s e t o a g r a d i e n t of L i i o n s along t h e length of t h e sample. I n a sample, with coplanar geometry of electrodes, w e observe V Z 100mV upon shining white l i g h t . The samples without g r a d i e n t of

%f

i o n s show no photovoltage.

The n e x t section d e s c r i b e s t h e tenpera t u r e dependence of con- d u c t i v i t y and thermopower i n Li doped a-Si:H. The photovoltaic e f f e c t

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814113

JOURNAL DE PHYSIQUE

observed i n gradient doped samples i s described i n section 111.

Section IV contains t h e discussion of t h e results,

11, Conductivitv and Themmower,- Thin films of a-Si: H a r e prepared by d,c, glow discharge of 3% s i l a n e i n Argon, onto corning 7059 g l a s s substrates, a t T s 570K A l l samples a r e heat dried a t 500K f o r two hours i n a v a c u a of 1 0 ~ ' Torr. Samples used a r e 0.5 to lpm thick, For doping, a t h i n film of L i i s deposited on top of t h e a-Si:H samples a t 450K and kept a t t h a t temperature f o r about 2 hours, The samples a r e then allowed to cool down t o room temperature i n about 4 hours, After L i diffusion the sanples a r e cleaned by d i l u t e W and by vapours of isopropyl alcohol t o remove any excess lithium remaining on t h e top surface,

For d,c. conductivity (a) measurements nichrome o r chrome gold electrodes a r e used i n planar geometry with gaps varying between Uan and 2cm, on top as well a s the b o t t o m of the sample. Conductivity is found to be independent of these parameters. I n f i g 1 curve shows a (T) f o r undoped a-Si:?I. U a t r o o m tenperatwe i s 10-&-lcm-f and the

a c t i v a t i o n energy i s 0.62eV. The photoconductivity U f o r undoped a-Si:H films i s about f o r a white l i g h t f l u x ph 6 X lp14 pbtons/aa2/s.

aph

tor the doped samples i s about 5 X

lo-*&-l~i~- ,

Curve b i n f i g 1 shows t h e temperature dependence of U f o r a L i doped s?mple. We see t h a t a f t e r doping a t 3OOK increases by a f a c t o r of 105 u a s thermally activated between 150 and 400K, with activation energy

( AI) --, 0 17eV. Ebr comparison we have a l s o shown the r e s u l t s of B e y e r et, al.

1 2

^(c- c ) f o r a sample with 4 a t % L i , whose slope i s a l s o 0,17eV, and i s i n q u a l i t a t i v e agreement with the curve b obtained by

us, Although, we have not measured the concentration of L i i n our films independently, t h i s comparison suggests t h a t our films also contain about 4 a t % of lithium.

For themPower(S) measurements nichrome electrodes of width lnnr are deposited on top of the sarnple with a gap of 2cm, Fig 2 shows t h e temperature dspendence of S f o r t h e same sample whose o(T) i s shown i n f i g 1. S i s negative and varies l i n e a r l y as a function of l/T between 150 and 400K with a slope (E ) of 0.17e~ which i s t h e same a s

as,

The thennopower data f r a n ~ e f e r et. al. 1 2

g

i s a l s o shown

t h i s figurf:$, who obtain ES 0.04eV.

200 150

M O 300 T ( K )

O J I 200 I

a-S8 H + 4.1.1. L, -0 5-

-1 5 1

l

-2 0 In,,#. &"M e, 0 , >

l. 5

1 0 ' / 1 1 ~ ' 1

~ i g 1. d,c.conduc=tivity a s a Fig 2, Thermopower a s a func- function of l/T f o r ( a ) undoped t i o n of l/T f o r t h e L i doped

(b) L i doped a-Si:& Curve (C) a-Si:R whose

c

i s shown i n f i g l.

i s from Ref.2. Al o shown a r e t h e r e s u l t s from

~ e f .2,

i s

a-Si:H sample immediately a f t e r lithium evaporation a t 450K, This field i s kept on during t h e e n t i r e cycle of in-diffusion, i.e., f o r 2 hours a t 450K and during t h e cool down Such samples, upon shining white l i g h t ( f l u x r= 6 x l ~ l ~ ~ h o t o n s / c m ~ / s e c ) show an open-circuit photo-

voltage (V ) which i s t y p i c a l l y IOOmV, The short-circuit-current ( 1 2

O? 8 2

i s about 10 A (current density =. 1 0 - ~ ~ / c r n 1, Electrode of t h e sample t o which p o s i t i v e f i e l d had been applied becomes p o s i t i v e upon shining l i g h t , No photovoltage i s observed i n samples which a r e not subjected t o t h e e l e c t r i c field, Fig 3 shows I-V c h a r a c t e r i s t i c s f o r a gradient doped sample i n dark and i n presence of l i g h t , I-V curve i s symmetric i n dark, but has the form shown 'in f i g 3 i n presence of white light, The applied voltage V i s consf dered to be p o s i t i v e when the electrode which becomes positive upon shining l i g h t i s positively biased. For V > ^(-V 1 t h e current through t h e s a t q l e under l i g h t i s p o s i t i v e and f o r V(%voc) it i s negative.

Voc i s found t o be unchanged upon s t o r i n g the samples i n a desiccator f o r a period of several months and upon annealing i n vacuum a t 450K f o r 2 hours, However, annealing a t 450K f o r 4 hours i n

presence of an e l e c t r i c f i e l d with reversed p o l a r i t y (E =.

~ o ~ v / Q ~ )

reverses t h e sign of V

.

^V remains unchanged when only the l i g h t of energy less than 1,5e~?s aftowed t o f a l l on the sample using a c u t off f i l t e r , I i s however, found t o reduce to about half of i t ' s o r i g i n a l value

To check the d i s t r i b u t i o n of photovoltage inside the sample, Voc i s measured a t d i f f e r e n t p o i n t s along t h e length of the sanple. I n this m e r i m e n t sanple i s deposited on a substrate having 4 electrodes of width 0.5mm and with a gap of 5 m between two adjacent electrodes, A L i gradient is obtained using an e l e c t r i c f i e l d

-

100V/cm applied between thetwo electrodes a t t h e extreme ends, Fig 4 shows Voc a s a function of distance (X) measured from one of the extreme electrodes along the length of t h e sample, We see t h a t t h e d i s t r i b u t i o n of photovoltage i s alrmst l i n e a r along the length of t h e sample,

IV, Results and Discussion,- The c o n d w t i v i t y data f o r the L i doped

100

-

_>

E 50-

-

_0"

>

I

0'

0 5 10 1 5

X (cm1

Fig 3, W c h a r a c t e d s t i c s of Fig 4, Photovoltage i n gradient a-Si:H (Li) sample i n dark and doped a-Si:H(Li) a s a function of i n presence of white l i g h t distance X from bne of the

electrodes.

I I

a - SI H (L11

L+.

3-

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and undoped a-Si: H f i l m s a r e i n good agreement with t h e published work[g. Comparison of t h e d.c. conductivity r e s u l t s with those of Beyer e t . a l . f o r t h e Qped samples show t h a t o u r f i l m s contain about 4 a t % L i . Thus it appears t h a t a l a r g e f r a c t i o n of L i atoms remains n e u t r a l i n t h e sample. A small f r a c t i o n a c t a s donors, a s i s c l e a r from o u r thermopower data, The s l o p e of S v s 1/T i s found to be ES '0.17eV which i s t h e same a s AE. This i s a t variance with t h e r e s u l t s of Beyer e t . a l . who r e p o r t a much smaller ES (=0.04e~). This may be caused by t h e d i f f e r e n c e s i n t h e s q l e preparation. Thus, o u r thennopower d a t a can be s p l a i n e d i n terms of conduction i n t h e extended s t a t e s .

The observation of a photovoltage i n s a q l e s subjected t o an e l e c t r i c f i e l d during L i doping, can be understood q u a l i t a t i v e l y i n terms of a gradient of L i i o n s along the sample. Since i d e n t i c a l e l e c t r o d e s i n a coplanar geometry a r e used, t h e c o n t a c t e f f e c t s and Dember e f f e c t are r u l e d out, a s the cause of the photovoltage, Since t h e mobility of l i t h i u m i n s i l i c o n i s high ( p a r t i c u l a r l y a t high temperatures a t which t h e f i e l d i s applied), we expect a g r a d i e n t of L i ions, which might give rise to a b u i l t - i n p o t e n t i a l i n t h e sample.

This b u i l t i n p o t e n t i a l , which although helps i n separating t h e charged p h o t o c a r r i e r s , i s q e c t e d to be q u i t e small, s i n c e the I-V c h a r a c t e r i s t i c s i n dark i s l i n e a r and symmetric. Further, it appears t h a t i t i s l i n e a r l y d i s t r i b u t e d throughout t h e sample a s evidenced by by t h e l i n e a r i t y of t h e photovoltage along t h e length of t h e sample, A p h o t o s t r u c t u r a l change a s t h e cause of photovoltage does not appear p l a u s i b l e , a s t h e l i g h t of low energy

K

1 . 5 e ~ ) g i v e s t h e same value of

Vo, a s t h e white l i g h t . Purther, t h e s t a b i l i t y of VOc with time and with annealing a l s o supports t h i s p o i n t of view. Moreover* t h e s i g n of t h e photovoltage i s c o n s i s t e n t with t h e above hypothesis, We might mention, however, t h a t t h e l a r g e amount of l i t h i u m p r e s e n t i n t h e f i l m s might change not only 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 b u t a l s o a£ f e c t the d e n s i t y of conckaction ,and valence band s t a t e s , h i e h n j g h t further: c o n t r i b u t e t o t h e photovoltage i n t h e c a s e of g r a d i e n t doped samples. B u t a q u a n t i t a t i v e treatment o f t h e s e e f f e c t s i s not p o s s i b l e a t t h i s stage,

I n conclusion, we have been a b l e t o show that L i a c t s l i k e a donor i n a-Si:H and t h a t a g r a d i e n t of L i i o n s produces a photovoltage, a s i s observed i n t h e gradient doped c r y s t a l l i n e semiconductors.

Acknowledaements,- We are g r a t e f u l to Professor R. Sharan and D r B. B h a d c k i?or some u s e f u l discussions and t o Professor T. ^M, Srinivasan and Ik P.N. D i x i t f o r help i n t h e experimental work. ^This work i s supported by a grant: from t h e Department of Science and Technologya Governaaent of India.

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