Photoemission studies of amorphous semiconductor heterojunctions

Journal of Non-CrystallineSolids 77 & 78 (1985) 969-978 North-Holland, Amsterdam

969

Secn'on 14. Surfaces, Interfaces

PHOTOEMISSION STUDIES Of AMORPHOUS SEMICONDUCTOR HETEROJUNCTIONS * F. EVANGELISTI Dipartimento di F i s i c a , U n i v e r s i t a ' "La Sapienza", 00185 Roma, I t a l y

Recent results on amorphous semiconductor heterojunctions are reported. The main focus is on the discussion of valence band d i s c o n t i n u i t i e s between amorphous s i l i c o n and silicon-based a l l o y s . I t is suggested t h a t the compositional disorder is responsible for the n e g l i g i b l e disc o n t i n u i t i e s found. i.

INTRODUCTION Recently promising h e t e r o j u n c t i o n devices have been r e a l i z e d with amorphous

semiconductors: etc. the

solar c e l l s , m u l t i l a y e r s t r u c t u r e s , f i e l d - e f f e c t

transistors,

Since the behavior of a h e t e r o j u n c t i o n device is strongly microscopic

dependent

parameters of the i n t e r f a c e (band d i s c o n t i n u i t i e s ,

on

interface

states, d i f f u s i o n p o t e n t i a l , e t c . ) there was an immediate i n t e r e s t for microscopic

investigations

of these q u a n t i t i e s .

Photoemission spectroscopy is the

most d i r e c t way of measuring these parameters, as shown by the large amount of work on the c r y s t a l l i n e heterojunctions done in recent years 1 The f i r s t the

photoemission study of an amorphous i n t e r f a c e

a-Sil_xCx:H/a-Si:H

heterojunction 2

evidence that the i n t r o d u c t i o n of t h i s h e t e r o j u n c t i o n devices

was

performed

on

This choice was motivated by the in

p-i-n

photovoltaic

by the Hamakawa group 3 represented a real breakthrough in the e f f o r t

of increasing the e f f i c i e n c y of amorphous solar c e l l s .

Later on, the i n v e s t i -

gation has been extended to several other interfaces 4-7 The amorphous heterojunctions studied u n t i l tegories

(see

Table

I).

The f i r s t

amorphous s i l i c o n and silicon-based a l l o y s . for

the

They have an

immediate

a p p l i c a t i o n in solar c e l l s and m u l t i l a y e r s t r u c t u r e s .

category belong the amorphous close

now can be divided in three ca-

one includes the heterojunctions between

silicon/germanium

interfaces,

interest

To the second which

p a r a l l e l with w e l l - c h a r a c t e r i z e d c r y s t a l l i n e counterparts.

allow

the t h i r d category we can group pseudo-heterojunctions obtained by growing amorphous

an

o v e r l a y e r of a given material (Si and Ge in our case) on a c r y s t a l -

l i n e or hydrogenated-amorphous substrate of the same m a t e r i a l . should

a

F i n a l l y , in

In t h i s way i t

be possible to i n v e s t i g a t e fundamental problems l i k e the e f f e c t of d i -

sorder and/or hydrogenation on the e l e c t r o n i c s t r u c t u r e of the m a t e r i a l s .

Work p a r t i a l l y supported by PFE II-ENEA. 0022-3093/85/$03.30 © Elsevier Science Publishe~ B.V. (No~h-HoUand PhyficsPublishing Dwifion)

1~ Evangelisti / Photoemission studies

970

A brief review of this f i e l d , which is s t i l l at its f i r s t stages of ment,

can be found in Ref.8.

develop-

For lack of space in the present paper the het-

erojunctions of the f i r s t group only will be discussed. 2. EXPERIMENTAL TECHNIQUE The surface sensitivity of

U.V.

photoemission spectroscopy makes this

technique an ideal probe for interface studies.

The physical information car-

ried out by the emitted electrons is related to the outermost region solid.

ergy of the electrons according to the using

of

the

The thickness of this region can be varied by changing the kinetic enescape depth curve.

Therefore,

by

a tunable photon source like the synchrotron radiation, i t is possible

to modulate the surface sensitivity and to enhance surface effects. A photoemission study of a heterojunction is performed by growing in and step by step one semiconductor on top of the other. levels spectra are measured at each step. termine

parameters like

situ

Valence band and core

As a result, i t is possible to

de-

the valence band discontinuity, the position of the

Fermi level at the interface and the magnitude as well

as

the

evolution

of

band bending. Moreover, i t is often possible to detect interface states, interdiffusion of the constituent species, etc.

A discussion of the

d a t a ana-

lysis procedure can be found in many papers concerning crystalline heterojunctions, while a

brief

mentioning of

the

problems is

reported

in

Ref.8.

Nevertheless, there is a point worth a brief discussion here. The most important piece of study

is

information

is necessary to locate the position in Ev in

obtained

the valence-band discontinuity a Ev • energy of

from a

heterojunction

In order to obtain the

aE v

it

valence-band maximum

the experimental energy distribution curve (EDC). Usually this is done

by a linear extrapolation to zero of the valence-band leading edge. This procedure

is easily affected by an uncertainty of 0.1-0.15 eV and represents the

major source of error in the value of tions

the

AEv.

Under usual experimental

valence-band spectral edge merges into the noise at a signal level

0.05-0.1 times the intensity of the highest-in-energy or

4p

states for Si or Ge, respectively), i . e .

valence-band feature (3p

Ev corresponds approximately

to 0.5-1xi021 states cm-3 e V-1 in tetrahedral semiconductors. talline

case this (f.e.

In

the

crys-

procedure does not cause any ambiguity because the top of

valence band is a well defined concept and the density of states ases rapidly

in

c-Si

g(E) varies from 0 to 1021 in

over an energy distance smaller than the experimental the

condi-

g ( E ) incre0 . 0 5 eV, i . e .

uncertainty).

As

for

amorphous semiconductors the definition of valence-band maximum i t s e l f is

ambiguous due to the continuum of localized states t a i l i n g level.

toward the

Fermi

Therefore, we can only say that the experimentally defined Ev locates

F Evangelisti/Photoemission studies the

energy

where the

cm-3 eV - I .

It

density

was

of

states

971

decreases

estimated 9 that

a

below

state

1020-1021cm-3eV -1

range is a reasonable

between

and localized states in amorphous s i l i c o n .

extended

number for

therefore, to assume that Ev , as defined above, uncertainty,

the

valence-band

~10 21 states

density

the

locates,

in

demarcation

the level

I t is temptinB,

within

0.1-0.2 eV

m o b i l i t y edge, at least in amorphous s i l i c o n ,

both hydrogenated and not. TABLE I Substrate/Overlayer

Eg(Subst.)

Eg(Overl.)

(eV)

(eV)

a Ev

A Ec

(eV)

(eV)

a-Sil,xCx:H/a-Si

2.2 (a)

1.26 (b)

0 (c)

0.9

a-Sil_xCx:H,B/a-Si

2.05 (a)

1.26 (b)

O(c);O.15(d)

0.65-0.8

a-Sil_xCx:H/a-Si:H

2.2 (e)

1.75 (e)

0 (e)

0.45

a-Sil_xCx:H,B/a-Si:H

2.05 (a)

1.7 (a)

0 (d)

0.35

1.9 (e)

1.75 (e) -

1.4 (e) 1.2 ( f )

0.75

0 (f)

0.5

a-SiNx:H/a-Si:H .2 (f) a-Si:H/a-Sil_xGex:H

1.7 (f) -

<0.2 (h) for a l l values of x

a-Si:H/a-Ge

1.7 (a)

0.7 (g)

0.2 (h)

0.8

a-Si/a-Ge

1.26 (b)

0.7 (g)

0 (h)

0.54

a-Si:H/a-Si

1.7 Ca)

1.26 (b)

0 (c)

c-Si/a-Si

1.15

1.26 (b)

0 - 0 . i (c)

c-Ge/a-Ge

0.65

0.7 (g)

0 (i)

(a) Ref.lO; Ref.12;

(b) Ref.11;

(h) Ref.6;

(c) Ref.4;

(d) Ref.2;

(e) Ref.5;

0.44 -(0.1-0.2) 0 ( f ) Ref.7;

(g)

( i ) Ref.13.

3. AMORPHOUSSILICON/SILICON ALLOYS HETEROJUNCTIONS In Table I the measured valence-band d i s c o n t i n u i t i e s gaps

Eg of

the

Also reported are the conduction-band d i s c o n t i n u i t i e s AEv and

Eg .

the

optical

AEc estimated by

using

From Table i i t is seen that most of these heterojunctions ex-

h i b i t a small valence-band d i s c o n t i n u i t y . with x up to

AEv and

two semiconductors forming the heterojunctions are reported,

This is the

~0.5 and a-SiNx:H with x = 0.4.

case

for

a-Sil_xCx:H

Notice, however, that the o p t i -

cal gap of these alloys is s u b s t a n t i a l l y larger than that of amorphous s i l i c o n

F. Evangelisti /Photoemission studies

972

P~ L2 X, X3 L~ J

a)

I

]

f]

Xs L3 ~s [

I

I

b)

-20

-15

-10

~ E,](ev)

-5

0

FIGURE 1 Valence band spectra of c r y s t a l l i n e SiC (a) and of a-Sil_xCx:H for x ~I0 at.% (b) and ~50 at.% (c), respectively

or hydrogenated amorphous silicon giving rise, therefore, to a tion-band

discontinuity.

large

conduc-

As for a-Si1_xGex:H, the discontinuity is also ne-

g l i g i b l y small but this is to be expected owing to the small discontinuity a l ready present at the interface between Ge and Si. In order to understand at least q u a l i t a t i v e l y these results with

for two different carbon contents 14. of

let

us

begin

a discussion of the a-Sil_xCx:H valence EDC's. They are shown in Fig. 1 crystalline

Also shown is the valence band spectrum

SiC obtained by soft X-Ray emission 15.

Finally, at the top

are reported the theoretical energies at the high symmetry points of the B r i l louin

zone as calculated by empirical t i g h t binding 16.

amorphous spectra exhibit a well resolved structure at s-like

states

of

the

Both c r y s t a l l i n e and ~ - 1 5 eV due to

the

Si-C bond. This structure, due to the lowest valence

band, is well resolved in all tetrahedral semiconductors with polar

bond be-

cause p o l a r i t y gives rise to the removal of degeneracy and to the opening of a gap at the symmetry point X (X1-X3).

This feature is a convenient

reference

in order to compare the spectra of amorphous and c r y s t a l l i n e materials as well as the theoretical energies.

1~ Evangelisti / Photoemission studies

973

Turning now to the strongest s t r u c t u r e s of the s p e c t r a , a large peak is evident

in

a-Sil_xCx:H at

between p and s-p s t a t e s . From

~-6.5 eV which f i l l s

the " v a l l e y " present in c-SiC

This peak is c e r t a i n l y due to the

bonds.

a comparison of spectrum (a) with spectra (b) and ( c ) , an important fea-

ture can be a p p r e c i a t e d , namely the valence band of tially

wider

than that of c-SiC.

2 eV l a r g e r than the l a t t e r . on

hydrogen

the

chosen

is

substan-

While the actual value is s t r o n g l y

dependent

alignment and should be taken with c a u t i o n , there is no doubt

that in a-Sil_xCx:H there is a t a i l move the

a-Sil_xCx:H

From the alignment of Fig. i the former is

top

to higher energy.

of states t h a t widen the valence band

and

The shoulder due to these states depends on

the concentration and is Stronger at higher Si c o n t e n t , as can be reckoned a

comparison

of spectra (b) and ( c ) .

t h a t the uppermost p o r t i o n of the valence band in a-Sil_xCx:H ( f o r formed

by states of tne Si-Si bond, in p a r t i c u l a r 3p s t a t e s .

membering here that the same s i t u a t i o n

by

We are induced to conclude, t h e r e f o r e ,

for

x
as

The occurrence of an appreciable number of Si-Si bonds even

for

x=O.5

in

a-Sil_xCx:H

present

in

is

a-SiNx

shown by Kaercher et a l .

is

x
I t is worth re-

17.

points to a large compositional d i s o r d e r .

The presence of such a

d i s o r d e r was already reported f o r s i l i c o n - c a r b o n a l l o y s and confirmed more recently

in

glow-discharge-deposited

Auger spectroscopies. maximum number

of

The f i n a l

x < 0.6-0.7

by

Lee 18 by using XPS and

Si-C bonds accurs at x between 0.6 and 0.7 rather than at

x=O.5 as would be expected for a t r u l y for

a-Sil_xCx:H

conclusion reached by t h i s author is that the random t e t r a h e d r a l

creases with increasing x, but probably never reaches a silicon

atom

network. situation

where

is surrounded by more than one or two carbon atoms 18

c l u s i o n , there is ample evidence not only for the occurrence of disorder

Moreover,

the amount of s i l i c o n s u b s t i t u t e d by carbon and hydrogen i n one

In con-

compositional

but t h a t t h i s d i s o r d e r is even l a r g e r than expected f o r a t r u l y

ran-

dom network. We turn now to the discussion of the h e t e r o j u n c t i o n formation. the

In

Fig. 2

e v o l u t i o n of the uppermost region of the valence band f o r increasing a-Si

o v e r l a y e r thickness is shown. first

structure

Two features are e v i d e n t :

the increase of

the

(extending in the range 0-5 eV) due to Si 3p bonding states of

the o v e r l a y e r and the s l i g h t movement of the onset

Fermi

level.

From the a n a l y s i s of the Si 2p core level i t was demonstrated -4 t h a t :

a) the

i n t e r f a c e between the two components is abrupt;

toward

the

b) the displacement of

Ev is

simply due to a change of the i n i t i a l

band bending and no valence band discon-

tinuity

The same conclusions

the

is present at the i n t e r f a c e .

case of a - S i l _ x C x : H / a - S i : H by Abeles et a l . 5

dure, i . e .

by f i t t i n g

were

obtained

by using a d i f f e r e n t

in

proce-

the valence EDC of the h e t e r o j u n c t i o n at small coverage

F Evangelisti / Photoemission studies

974

EDC h~ =40eP

x,.~A a-S

I ,

-8

-6

I I -4 (~E~) ,

I -2

~

C~.H E~=O (eV)

FIGURE 2 Valence band spectra of a clean a-Si~ C :H substrate ~-XX and of the same substrate covered by an a-Si overlayer of increasing thickness. Energy scale is referred to the Fermi level EF

by a l i n e a r superposition of EDC's of clean a-Sil_xCx:H and a-Si:H. From the data we conclude therefore, that the formation of the tion tail

proceeds

states which form the top of the silicon-carbon

hand t h i s

observation

confirms

the

Si 3p

in

the

composition

of

the

Notice

The

that

exactly

As a consequence, i f

the

morphology

and/or

from

band discontinuity

should

the t h e o r e t i c a l point of view, using Harrison's SiC

should

recede

as

case

of

~3 eV compared to that of the c-Si. appearance

of

silicon-carbon/silicon

a

valence

band discontinuity

heterojunctions

has

not

However, a s i m i l a r e f f e c t has been demostrated genated

On one states in

aEv is n e g l i g i b l e

model 16, the valence band maximum of the c r y s t a l l i n e much as

band. these

a l l o y change in such a way that the number of Si-Si

bonds becomes n e g l i g i b l e we expect that a valence develop.

of

silicon-carbon a l l o y there are enough Si-Si bonds to push up

the top of valence band i t s e l f . the

valence

character

a-Sil_xCx:H, on the other hand i t demostrates that because

heterojunc-

with the Si 3p states developing in the energy i n t e r v a l of the

amorphous s i l i c o n - n i t r i d e / s i l i c o n

d i s c o n t i n u i t y of the

a-SiNx:H/a-Si:H

in

the

b e e n investigated very recently for the

heterojunctions.

heterojunction was

as y e t . hydro-

The valence-band first

measured

F. Evangelisti / Photoemission studies

975

EDC h~=4OeV

]-SiNo4:H~'~

/

>- ~-SiNl(

i

}---

, I , l , l l l l l l l , l l

-24

-16

,1~

-8

0

EJ(evl FIGURE 3 Valence band spectra of two a-SiNx:H samples Abe]es e t a ] . 5 optical

for si]icon-nitride

gap of 3.9 eV.

samples with x large

mainly N 2pz character 1 7 accordingly urements

were

repeated

give

The meas-

in the f i r s t

case the s i l i c o n n i t r i d e

was

found.

In the second case no detectable valence band d i s c o n t i n u i t y was

those used by Abe]es e t a ] .

predominant

character,

with

found in a-SiN x dence

= 1.2 eV was meas-

of states

respectively.

extending

toward

have In the higher

which s h i f t the valence band maximum of an amount e x a c t l y equal to the

valence band d i s c o n t i n u i t y found for the x=I.5 case. samples

AEv

Peaks A,B and C have been i d e n t i f i e d by Kaercher e t a ] . 17 and

N 2Px,y,N 2Px,y +Si 3s

x = 0.4 case the spectrum e x h i b i t s a t a i l energy

and

In Fig. 3 the valence band EDC's of the two s i l i c o n n i t r i d e samples are

reported. N 2pz,

an

using two very d i f f e r e n t s i l i c o n

sample ured.

to

to

AEv was found to be 1.4 eV.

by Co]uzza et a l , 7

n i t r i d e compositions (x=1.5 and x=0.4), similar

enough

At large x the states at the top of valence band have

These

tail

states

in

low hydrogen content have Si3p character in agreement with what f o r x < 0.9.

In conclusion there is at present a strong e v i -

that the compositional disorder is responsible for the small

aEv found

in most heterojunctions between s i l i c o n and s i l i c o n based a l l o y s . As mentioned p r e v i o u s l y , these findings are of immediate design

and analysis of amorphous s i l i c o n solar c e i l s .

interest

in

the

I t was already pointed

~ Evangelisti/Photoemiss~n studies

976

out2 that the band alignment could explain, at least p a r t i a l l y , the ment

improve-

in efficiency obtained in the solar cells with the p-doped layer made by

a-Sil_xCx:H. tinuity

As a matter of fact, the resulting large conduction-band discon-

at the heterojunction prevents electrons from diffusing back into the

p-doped layer while not hindering hole collection.

More recently i t was even

suggested 19,20 that the degradation of cell performances upon prolonged i l l u mination is to be ascribed to the disappearance of this barrier for backdiffusion of electrons. Since the absence of any valence band discontinuity seems to be a

general

tendency involving also silicon-germanium alloys and s i l i c o n - n i t r i d e s at low N content, a general rule can be derived according to heterojunction solar cells realized with

a combination of these materials should have the p-doped layer

and not the n-doped one made with the larger-gap compound in properly.

order

to

work

In fact, i f the n-doped layer were made by the larger-gap material

the conduction band discontinuity would oppose the carrier collection and decrease

the

efficiency.

This effect has been demonstrated quite recently by

Niemann et a l . 21 who made solar cells with n-doped a-Sil_xCx:H

and

found a

barrier for electrons in the I-V characteristics. CONCLUSIONS We have seen that silicon-based

alloys

too high a value of x. positional

disorder

the

heterojunctions

between amorphous silicon

and

exhibit negligible valence band discontinuities for not This behavior is l i k e l y to result from the large present

in the alloys.

com-

F i n a l l y , i t is pointed out that

these band alignments must be properly taken into account in the device design and analysis. ACKNOWLEDGMENTS I am greatly indebted to M.Capozi, P. F i o r i n i , M.K. Kelly, G. Margaritondo, F. Patella,

P. Perfetti and C. Quaresima who contributed considerably to most

of the material discussed in the present paper. REFERENCES i)

G. Margaritondo, Solid State Electron. therein.

26

(1983) 4 9 9 , and

2)

F. Evangelisti, P. F i o r i n i , C. Giovannella, F.Patella, P. P e r f e t t i , C. Quaresima, and M. Capozi, Appl. Phys. Lett. 44 (1984) 764.

3)

Y. Tawada, H. Okamoto, and Y. Hamakawa,Appl. 237.

P h y s . Lett.

references

39

(1981)

1~ Evangelisti / Photoemission studies

977

4)

F. P a t e l l a , F. Evangelisti, P. F i o r i n i , P. P e r f e t t i , C. Quaresima, M.K. Kelly, R.A. Riedel, and G. Margaritondo, Photoemission studies of amorphous s i l i c o n heterostructures, in: Optical Effects in Amorphous Semiconductors, AIP Conf. Proc. No.120, Ed. P.G. Taylor and S.G. Bishop, 1984, pp.402-409.

5

B. Abeles, I . Wagner, W. Eberhardt, J. Stohr, M. Stasiewski and F. Sette, Electronic structure of amorphous semiconductor heterojunctions by photoemission and photoabsorption spectroscopy, i b i d , pp.394-401.

6

F. Evangelisti, S. Modesti, F. Boscherini, P. F i o r i n i , C. Quaresima, M. Capozi, and P. P e r f e t t i , MRS Spring Meeting, San Francisco, 1985.

7

C. Coluzza, G. Fortunato, C. Quaresima, M. Capozi, and P. P e r f e t t i , Photoemission study of a-SiNx:H/a-Si:H heterojunctions, t h i s volume.

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