fluorinated a-Si1−xGex alloys

Journal of Non-Crystalline Solids 97&98 (1987) 507-510 North-Holland, Amsterdam

507

ELECTRONIC S T R U C T U R E OF HYDROGENATED/FLUORINATED a-Si 1-xGex ALLOYS Bal K. AGRAWAL and Savitri AGRAWAL P h y s i c s D e p a r t m e n t ) Allababad University, A l l a h a b a d 211002, India. A very good description of the e l e c t r o n e n e r g y s t a t e s in a-Sil_xGex:F(H) alloys h a s been obtained throughout the entire energy region by t h e i n c l ~ slon of a high e n e r g y e x c i t e d s* s t a t e ha the basis of the atomic orbital set in the c l u s t e r B e t h e l a t t i c e m e t h o d (CBLM) for the first time. For a-Sil_xGe x alloys, t h e c o m p u t e d v a r i a t i o n s of the m a g n i t u d e of the fundam e n t a l energy gap for t h e random and c h e m i c a l l y ordered s e q u e n c e s are seen to be in e x c e l l e n t a g r e e m e n t with the available photoemission data. G r e a t i m p r o v e m e n t in t e r m s of the n u m b e r and location of F- and H - i n d u c e d peaks in a-Si alloy h a s been obtait~ed where the calculated r e s u l t s are in very good a g r e e m e n t with t h e photoemlsslon data. The F(H)-induced peaks remain u n a f f e c t e d by t h e p r e s e n c e of the different c o n c e n t r a t i o n s of the c o n s t i t u e n t h o s t a t o m s in a-Sil_xGe x alloys both for the random and chemically ordered sequences. A conversion e f f i c i e n c y of 11-12% has r e c e n t l y been r e a c h e d in Sl-based devices

in t e x t u r e d

solar cell

structures.

can have

fluorinated

Multi-band gap a-Si solar cells n a m e d tandem

an e s t i m a t e d value of 21-24%.

silicon-germanium

alloys

can

have

Amorphous hydrogenated/

variable

energy

gaps

smaller

t h a n a-Si:H or a-Si:F:H alloys. In drawback

all

the c l u s t e r B e t h e

h a s been

the

lattice

appearance

calculations reported of an electronic

so far,

density

of

the main

states

(DOS)

gap which is much wider as c o m p a r e d to the e x p e r i m e n t a l l y m e a s u r e d semiconducting

gap 1"3. Also,

satisfactorily

the

electron

states

of

the conduction

band were

not

described by a sp3-Hamlltoniar~ A s u c c e s s f u l a t t e m p t to r e m o v e

t h e s e s h o r t c o m i n g s h a s been made in t h e p r e s e n t article. We now find a variation

of

the DOS gap

with c o n c e n t r a t i o n

x in e x c e l l e n t

a g r e e m e n t with t h e

available e x p e r i m e n t a l d a t a 4-6. The c a l c u l a t i o n s are t h e n e x t e n d e d to the hydrog e n a t e d and t h e fluorinated a-Sil_xGe x alloys where the e f f e c t s of the occurrence

of

the

mono-hydride

(fluoride)

and

di-hydride

(fluoride)

complexes

are

investigated. In the c a l c u l a t i o n s Py" Pz' s*)

at

each

we consider

atomic

a five

a t o m i c orbital

basis s e t

(s, Px'

site in the LCAO s c h e m e . The inclusion of the

high errergy s* s t a t e s not m e r e l y makes a surprising i m p r o v e m e n t ha t h e values of

the DOS gaps but

induced

by the

also

difluoride

the n u m b e r of complexes

which

the c a l c u l a t e d F(H)-induced peaks are

t h e photoemission data. 0022-3093]87]$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

in e x c e l l e n t

a g r e e m e n t with

508

B.K. Agrawal. S. Agrawal / Hydrogenated/fluorinated a-Si I _ xGex alloys

In binary AxBI_ x alloys, there are

a - S i : fie alloys .~

I "~,~ -~[--- Binary alloys ~'"[--Random alloys zl.8r~r-)u-_A.

Photoemission expt A RIf • Ref'5 - * Ref" 6

A

various

probabilities

~

i

0.0 Si

A

i

0.2

I

i

0.4

i

I

,

0.6

I

B

B

A,

and

consider Cayle)t

a

1.0

CONCENTRATION (x)

-

B

reference

tree.

bonds. site

We

in

the

The

decendants

site

will be deter-

of

mined by the level of the valence

,

0B

the

the A - A, A - B ,

this r e f e r e n c e ~

of

occurrence of

Ge

saturation its

Figure 1 Comparison of variation of the fundamental ~jap in a-Sil_xGe x alloys calculated for the binary and random sequences with the photoemission data.

already

parent

atom.

present For

due to alloy

having

the

for the

two kinds of atoms, e.g.,

like

Sil_xGe x

that

Pa

like

(Qa)

(unlike)

same

an

coordination

alloys,

we

assume

is the probability of atom

decendants

of an A atom. The branching ratios of the

descendants should be con-

s i s t e n t with the saturation condition and the cluster averages for the different values of average probabilities. We consider two different sequences for describIng the different bonding tendencieg random and chemically ordered.

A.

a-Si l=xGex ALLOYS The values of the calculated fundament'd gaps for Ge and Si are 1.1 and

1.8 ev, respectively which are the experimentally m e a s u r e d values. The calculated variation

of E with concentration x for the random and the chemically g ordered alloys which has been compared with the experimental points in Fig. 1 is seen to be in excellent agreement with the experimental d a t a 4-6. The variation of E is seen to be slightly different for the random and the chemically g ordered sequences.

B.

a=Si1_xGex:F(H) ALLOYS

(i}

SiF(H) and SIF'2(H2) units The e f f e c t of the presence of SiF and SiF 2 units on the electronic structure

of the a-Sll_xGe x alloys has been studied both for the random and chemically ordered sequence~

Here

we p r e s e n t the

results only for the random

sequen-

ce in Fig.2. For SiF in pure a-Si, the salient f e a t u r e s in the local DOS remain the

same.

However,

for the

case

of dl-flaorlde,

the

features

are drastically

B K Agrawal, S Agrawal / Hydrogenated~fluorinated a S i t ~Gex allovs

509

altered. In place of the five peaks

in a-Si~F alloys (a)

I--

obtained

SiF2 /

(c)

SiF~

orbital

Z

three

a5 tv"

-8.5 ev.

li--Caicul~ed

5o.c

i

triO.4 0.2

='

peaks

R 0.6 U, 0~ I.IJ -J UJ 0.2

a

four

at

These

only

-12.6,

-10.3

and

values

are

very

ed

peaks

at

-12.5,

-10.2

and

-9.2 ev in the photoemission data 7. The

l.) 7

with

we now obtain

close to the experimentally observ-

i!iii

, ion darn I

earlier 3 basis,

(b)

SiF

(d) I SiF2

/

results

The been

the

"chemically

calculations

performed

SiF 2

-12-1~ -4 0 4 - 1 2 - 8 -4 0 4 ENERGY (eV)

for

ordered sequences are similar.

complexes

alloy

for

have

for

the

in

different

then

SiF

and

a-Sil_xGe x

concentrations.

The locutions of the peaks induced by

Figure 2

SiF

much

Local electronic density of states (DOS} for SiFn (n=1,2) units in a-SiS alloys.

of

and

SiF 2

disturbed

other

units

by

kind of

are

the

not

presence

atoms

Le`,

Ge

atoms present as the other nearest neighbours of the Si atom coupled to

Si0.1Geo.9:F Z ;D

(a) 0.6 0.~

0.2

SiF

SiF 2 J

(c) /

at

F

iI

Sill

::

after

local

atoms for the alloy

in

and

Sill2

units

three -12-8-4

0

4

ENERGY (eV}

the data

Figure 3 Local electronic DOS for SiFn (n=l,2} units in random a-Sio,lGeO.~:F alloys.

inclusion of

the

high

energy s* state. However, for Sill unit, the local D O S reveals peaks

-4.0 ev 4

Si

typical

the

a-Si0.1Ge0. 9

For

~, 0.2 hi 0

and

a

the main features remain unchang-

I "~

-12-8-4

here

Fig. 3.

L(d) sir2

0.~

As

present

random

ed

siv

F-atoms.

we

DOS

,,ii.2

(b) O.E

the

case,

and

at

-9.3, -7.2 and

in good

peaks

in

agreement the

appearing -5.3

ev.

peaks

at

very

close

at

For

-10.4 to

with

photoemisslon -10.3,

Sill2,

-7.6

the

two

and

-6.4 ev

are

the

experimental

peaks at -11.3 and -6.3 ev.

510

B.K. Agrawal, S. Agrawal / Hydrogenated/fluorinated a-Si I xGex alloys _

Si09.Ge0:F .1 I(c) 5eF2/, < 0.6

u~ 0.4

much similar to those seen earlier for the SiF

,I % 11

and StF 2 complexes

The three peaks induced by GeF 2 unit

appear

at

-12.5,

-10,1

and

-8.5 ev in contrast to the appea-

g a2 ~ 0.6 0,~

(1i) GeF(H) and GeF2(H 2) units The new features are very

rance of five peaks in a four orbi-

(b)

5eF

I (d)

tal basis 5. Again, the presence of

5eF 2

Si

atoms

as

the

neighbours

of

the Ge atom coupled to F atoms do

-12-,

-4

-12-8-t, ENERGY (eV) 0

t,

0

t,

not

affect

peaks in Fig. 4). The

Figure t,

the

F(H)-induced

St0.9Ge0.I:F present

ahoy

study

(see

reveals

that the CBLM is quite capable of

Electronic DOS for OeFn (n=l,2) units in random a-Si0.gGe0.1:g alloys.

furnishing a very reliable description of the electron-energy states throughout

the

regiom

considerations of

The

entire

energy the

high-energy s* s t a t e s in the elemental and compound semiconductors reproduce the experimentally measured values for the energy gap. Photoemlssion measure, ments need to be performed on the a-Sll_xGex:F(H) alloys. ACKNOWLEDGMENTS The

authors

acknowledge

the

flnandal

assistance from

Department of

Science and Technology, New Delhi. REFERENCES 1)

J.D. Jaonnopoulos and F. Ynduraln, Phys. Rev. B10 (1974) 5164.

2)

Bal K. Agrawal and Savltrl Agrawal, Phys. Rev. B29 (1984) 6870.

3)

S. Agrawal and Bal K. Agrawal, Phys. Rev. B31 (1985) 5355; J. Phys. C. 19 (1986) 2741.

4)

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5)

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6)

K.D. Mackenzie, J. Hanna, J.R. Eggert, Y.M. LI, Z.L. Sun and W. Paul, J. Non-Cryst. Solids 77 and 7._88(1985) 881.

7)

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