a-Si:H heterojunctions

Journal of Non-CrystallineSolids 77 & 78 (1985)999-1002 North-Holland, Amsterdam

999

PHOTOEMISSION STUDIES OF a-SiNx:H/a-Si:H HETEROJUNCTIONS C. COLUZZA, P. PERFETTI~*

G. FORTUNATO*,

C. QUARESIMA**,

M. CAPOZI**,

and

Dipartimento di F i s i c a , Univ. di Roma, P.le A. Moro 2, 00185 Roma * I.E.S.S.-CNR, Via Cineto Romano 42, 00156 Roma ** I.S.M.-CNR, Via E. Fermi 28, 00044 F r a s c a t i , Roma Samples at d i f f e r e n t nitrogen concentrations were produced by ion-bombardment of the substrate - in presence of d i f f e r e n t ammonia pressures in the experimental chamber - during the deposition of the amorphous s i l i c o n or by external Glow-Discharge. For a-SiNx:H at low nitrogen content (semiconducting substrates) no valence bands d i s c o n t i n u i t y was observed w i t h i n the experimental accuracy (0.15 eV) while for the near s t o i c h i o m e t r i c nitrogen content ( i n s u l a t i n g substrates) the d i s c o n t i n u i t y was l a r g e r than 1.2 eV. I . INTRODUCTION The i n t e r e s t of comparing results for s i n g l e - c r y s t a l phous

interfaces

has

and

detectors 3.

In

eg.,

superstructures i

solar

c e l l s 2 and

p a r t i c u l a r heterojunctions between wide gap amor-

phous semiconductors (a-SiNx:H or a-Sil_xCx:H) and a-Si:H are of great tance

for

amor-

been stimulated by the recent development of promising

amorphous h e t e r o j u n c t i o n devices, chemical

interfaces

impor-

the possible blocking e f f e c t s on electrons at the p-i i n t e r f a c e in

p - i - n solar c e l l s 4 or for the charge confinement in superstructures.

On these

heterojunctions an i n t e r e s t i n g information is the valence band d i s c o n t i n u i t y . Photoemission spectroscopy is a most r e l i a b l e technique to i n v e s t i g a t e understand

the

interface

parameters

in

semiconductor

Valence band d i s c o n t i n u i t y ( a E v ) can be measured d i r e c t l y by growing o v e r l a y e r of a semiconductor on the other one:

and

heterojunctions 5. a

thin

AEvis obtained by the d i f f e r -

ence between the e x t r a p o l a t i o n to zero of the two leading edges (see F i g . I ) . 2. EXPERIMENTAL The s i l i c o n - n i t r o g e n a l l o y s were prepared both by RF glow-discharge capacitive

techniques, a d i f f e r e n t

concentration in the a l l o y s . were ion

a

reactor in SiH 4 +NH3 atmosphere, and in s i t u by evaporation of Si

in presence of ammonia bombardment by an ion gun, focused on the sample. both

in

obtained

by

bombardment.

using

For

ammonia p a r t i a l pressure gave d i f f e r e n t nitrogen The hydrogenated

amorphous

silicon

overlayers

the same evaporation set-up in presence of hydrogen

Photoemission data were obtained at the Synchrotron Radia-

0022-3093[85/$03.30 © Elsevier Science Pubfishe~ B.V. (No~h-HoUandPhyficsPubfishing Division)

1000

C Coluzza et aL / Photoemission studies

tion f a c i l i t y of Frascati National Laboratory. Photoemission spectra of the valence bands were taken at photon energy of 60 eV and 40 eV, with a total resolution of 0.6 eV and 0.2 eV respectively. The energy distribution a-SiNx:H/a-Si:H

for

ported in Fig. the

in-situ

curves (EDC's) of

the

valence band of

the

x=1.5 at hu =40 eV and hu =60 eV heterojunctions are re-

2 and in Fig.

3.

The bottom spectrum is the valence band of

grown a-SiNx:H. The N concentration was estimated by Auger ana-

l y s i s . The other curves obtained for thin amorphous silicon films of increasing thickness are shifted of respect to each other.

0.6 eV to align the nitrogen 2s core levels with

These corrections are necessary to compensate possible

band bending and charging effects.

This procedure is j u s t i f i e d by a consis-

tent shift of the nitrogen lone-pair emission peaked at ergy

(peak A of Ref. 6).

valence band discontinuity ( the

~-2 eV of binding en-

A linear estrapolation of the valence band gives a A Ev ) of 1.2 ± 0.15 eV in good agreement with

v a l u e reported by Abeles et al. 7 , and with that predicted by K~rcher et

al. 6

11"•

N(E}

ol

EDC h~ :60 eV

o-SaN~sH/o-Si H

d(il 35 /

Ec,

z

~Ec

15

2

5 .........

F..f

L_ ~ _

AEv : . Semiconductor 1 IS~N x )

.

Ev~

.

.

.

.

.

.

2L

-16

Ei

-8

{eV)

~ Semiconductor 2

FIGURE I

FIGURE 2

O-V~

j

0

C Coluzza et al.

/Photoemission studies

We note the absence of any double edge in the VB

for

1001

intermediate

coverages

as one has to expect for abrupt h e t e r o j u n c t i o n s . This means t h a t a probable i n t e r m i x i n g occurs at low level of that

a

a-Si:H. tion

residual To f i t

of

the

NH3 in

the

chamber

the observed valance bands we t r i e d to use a

or

layers of

linear

combina-

a-Si:H (top curve) and a-SiNx:H (bottom curve) spectra with the

weighting f a c t o r s and energy s h i f t s as a d j u s t a b l e procedure

coverage

could contaminate the f i r s t

described

in Ref.

7:

parameters,

according

the

the r e s u l t i n g curve is q u i t e d i f f e r e n t

from

the i n t e r m e d i a t e one f u r t h e r supporting the hypotesis of i n t e r m i x i n g . To show the dependence of strate

of

a-SiN×:H

with

~ E v on the N concentration we have used a different

value

of

x(x=O.4).

This

glow-discharge grown and a f t e r a soft cleaning by Ar s p u t t e r i n g at Ar

partial

analysis. ined

pressure of

in

~lO-5mbar, the N concentration was estimated by Auger obta-

d e e p - p r o f i l i n g ESCA measurements performed on a s i m i l a r sample grown

at the same time. between

was

200 eV

The in s i t u determinated x value agrees q u i t e well with t h a t

by

surface

surface. hu =40 eV.

In Fig. The

This bombardement, i f doesn't

cause

a

strong

4

are

shown the

spectra

obtained

with

photon

energy

low nitrogen content is c o n s i s t e n t with the low i n t e n s i t y of

EDC ED[

difference

and bulk N c o n c e n t r a t i o n , could decrease the H amount in the

the nitrogen induced peak at the top of the valence band in the 6 trum h~=~O eV

bottom h~=~O eV

dIi) ~'~

35

28

Z

Z

~

15

,.

,P .

-16 - 8 Ei (eV) FIGURE 3

spec-

o-SiN~, H/Q-S, H a- St NI~ H/o-SL

-24

sub-

film

O~V~

.

.

.

.

.

-2~

.

-16 -8 Ei (eV) FIGURE 4

0

O~V~

C Co~zza etaL /Photoemiss~n studies

1002

We have used the above procedure to align the valence bands of the yers

and

as

uncertainty.

a result we f i n d , in t h i s case,

The energy gap Eg in a-SiNx:H varies with x from 1.7-1.8 eV

x=O to 3.9 eV in a-SiNo.7:H. near 8 for

until

value;

0.4

and

the

for

This dependence, however, is far from being l i -

In effect the value of E9 is slowly varying (from 1.8 eV to

x

overla-

AEv=O within the experimental

2.3 eV)

largest v a r i a t i o n occurs for x greater than that

for x=O.4 the 0.5 eV gap difference between a-SiN x

:H and

a-Si:H

is

completely accomodated by the conduction band d i s c o n t i n u i t y . CONCLUSIONS I t is well known that the e f f i c i e n c y of thin f i l m solar c e l l s and ticular

par-

of amorphous s i l i c o n c e l l s can be increased by heterostructures using

wide gap semiconductors as window 2. this

in

Photoemission data are able

to

explain

phenomenon, by revealing that the conduction band d i s c o n t i n u i t y prevents

the back-diffusion of the photoexcited achieved

electrons.

This

result

is

usually

by using the frontal heterojunction a-Sil_xCx:H/a-Si:H but our meas-

urements have shown that the structure a-SiNx:H/a-Si:H could produce the effect

i f x~0.4.

same

Increasing the N concentration i t is also possible to modu-

late the band d i s c o n t i n u i t i e s according to any device necessity. REFERENCES i)

B.Abeles and T.Tiedje, Phys.

2)

Y.Tawada, H.Okamoto and Y.Hamakawa, Appl.

Rev.

Lett.

3)

A.D'Amico, G.Fortunato, G.Petrocco and C.Coluzza, Appl. 964 (1983)

4)

F.Evangelisti, P.Fiorini, C.Giovannella, F.Patella, P.Perfetti, C.Quaresima and M.Capozi, Appl. Phys. Letters 44, 764 (1984)

5)

G.Margaritondo, Soi.

6)

R.Karcher, L.Ley and R.L.Johnson, Phys.

7)

B.Abeles, I,Wagner, W.Eberhardt, J.Stohr, H.Stasiewski and F.Sette, Proc. Int. Conf. on "Optical Effects in Amorphous Semiconductors", Snowbird, Utah (1984)

8)

D. d e l l a Sala, C.Coluzza, G.Fortunato and F . E v a n g e l i s t i , 11th I n t . on Amorphous and Liquid Semiconductor, Rome (September 1985)

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51, 2003 (1983) Phys.

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