On the problem of the core level shifts at (110) surfaces of 111-v semiconductor compounds

155

Journal ofElectron SpectroscopyandRelated Phenomena, 52 (1990)155-160

ElsevierSciencePublisbersB.V.,Amsterdam-PrintedinTbeNetherlands

ON THE PROBLEM SENICONDUCTOR

OF THE CORE

P. Rodriguez-HernBndez

We the

E38204

have

core

analyse

the

surfaces

under

out,

level

ideaL a

Fundamental

La Laguna,

carried

surface

and

LocaL

reconstruction

and

caLcuLated

the

surface with

in a

the

some

OF III-V

with

and

the

modify

give

a

a

surface

the

treatment and

of We

semiconductor

how

show

shifts

level

a study compounds.

condition

results

de

SPAIN.

treatment.

CffO>

SeLfconsistent

experimentaL

Universidad

semiconductor

neutraLity

reasonable

results.

INTRODUCTION

Although

the

(110) surface

of several

III-V

determined

(l-6), the physical

semiconductor ideas

about

shifts with

SURFACES

Islands>

reconstructed

Uu.r

core

the

CCanary

tight-binding for

charge

treatment.

1.

AT CllO>

y Experimntal,

Tenerife

shifts

selfconsistent

agreement

SHIFTS

and A. MuKoz

de Fisica

Departamnto

La Laguna,

LEVEL

COMPOUNDS.

with

compound

surfaces these

solely

atoms due

invoked.

to

surface. processes cations

differences

Another

the changes

(7.8). thus

surface

idea connect

in the

This

between

means

change

that

and the

the ionicity

at the surface

surface charge

bonding relaxation

of the

and, thus,

compared

bonds

at

the and

O1990ElsevierSciencePublishersB.V.

the is

Madelung to

shift

be with

semiconductor reconstruction

between

are responsible

in

observed

needs

level

(9).

0368-2048/90/$03.50

as

bulk

transfer

core

are two chemical

charge

and

clean

the

and the shift

the surface

geometry

the

in

There

connect

differences

is the same as in the bulk

and no additional

potential,

shifts

understood.

scheme

shifts

experimentally

been

of these

potential

potential

binding-energy

have

not well One

Madelung

the bulk Madelung

surface

origin

is still

processes.

surface

core-level

semiconductors

anions

of these

and

shifts

2.

THE

MODEL

The purpose Tight-Binding shifts

of this paper

Model,

for different

the complete

potential

III-V

surface

It is clear

binding

energy

surface

rearrangement

surfaces

energies

of these simple where

bond

E(Ec

Eo)/21

heteropolar atom.

are

dangling

including

the

sensitive

The

in

these

(instead

in

the

to

et al.

chemical

the

origin using

(9),

of the III-V

of

binding

the

about

to the electron

l-a

level

ones.

since

neighbors

out by Priester

and

core

to the bulk

shifts

description

different

a

compounds,

population

for the cation,

where

of a

is

: Ec,)' + V21*'2

V

is

(110) surface

bonds

the anion

level

coordination

lower

different

explained

very

of each bond

/ t(E/

The neutral

surfaces,

Let us say a few words

orbital

by

and

core

the

nearest

that

Ec and Ea are the cation

respectively,

to

easily

is l+a for the anion given

Selfconsistent

a

as compared

three

That means

contribution

the ionicity

where

be

as pointed

molecular the

the atom

a=

may

of the atom.

shifts,

a

these

experience

somewhat

a

atoms

of core electrons

environment

due

atoms,

only have

in the bulk).

present to obtain

atoms will

expect

at surface

the atoms

to

(110) semiconductor

surface

than the bulk

so one should

is

in order

reconstruction.

that

number,

four

(SCTBM),

and anion

a

hopping

is created,

surface

and finally

Cl1

correspond there

sp' hybrid integral

one

bond

to the bulk

of

levels, When

the

breaking

per

(10). is

to two electrons

are an excess

(Ea < EC) 6na relative

energy

per pair of

population

on

given by:

6na= l-a

and 6n = -6n shifts:

U an" and U bn

for anion: valence

level

( Upand

Ucare

levels,

shifts

In our model reference

This

respectively),

are not related

by means

cations.

a:d cat&i,

intra-atomic

the core shifts

for the

of anions with

we follow

(11). Swmrarizing of a

that

decimation

electron the

Coulomb the

on

explain

the method

we use a Green technique,

bulk

of calculation

we

function calculate

and of

and shows

and cation

induces

contributions

anion

the change

and cations

the anion

transfer

cation sign

that

of

these

charges. developed

approach the

in

and,

surface

157 components of the Green function of the system, obtained from

the

Dyson's equation. Thus we can project the whole Green function for the semiconductor onto the last

layers

hamiltonian to an effective one,

and

reduce

associated

with

the one

whole

layer

of

semiconductor and finally we include four layer around the surface (this yield a 40x40 effective matrix, counting 10 orbitals in each layer). From this matrix we can analyse the different electronic properties of the surface. In order to calculate the atomic charge near the surface using the method described above, we define the electronic band structure of the different compounds under using a

first

nearest

neighbors

tight-binding

proposed by Vogl, Hjalmarson and Dow (12) with effect of the surface reconstruction means a d-' scaling law

(with

d

proposed by Harrison (10) for

is

the

the

parametrization

sp's* hybrids. The

taken

from

the

atomic

interatomic

interatomics

geometries

of

account

into

distance)

terms,

diagonal elements of the hamiltonian. The surface are taken

study,

the

by as non

reconstructions

(110)

reconstructed

surfaces reviewed by Kahn (13). With this method we study the core level shifts analysing the ideal and reconstructed surfaces condition

for

three

in

cases,

a

local

charge

first

ideal,

neutrality atomic

layer

reconstruction and complete reconstruction (first and second layer displacements).We

obtain identical results to that the founded by

Driester et al. (9) for the first two cases,

but

gives bad results in some cases when the total

this

condition

reconstruction

is

included; so, we conclude that the charge neutrality condition needs to be modified with a selfconsistent treatment similar to that proposed by

MuKoz

heterojunction band

et

offset

al.

for

(14).

the

Let

determination

us

now

discuss

of how

the to

introduce the effect of the change on the coulomb potential due to the charge redistribution near the surface; effect of this potential can be

included

we

assume

only

in

the

terms of the hamiltonian. This potential is the shift the anions and

cations

valence

intra-atomic

that

the

diagonal

induced

in

levels,

but

from

simple electrostatics arguments (151, one can show that

the

core

levels will experience a shift of

order

the

same

sign

and

magnitude. The selfconsistency between this diagonal and the charge transfer is introduced at the remark that in the III-V compound partially ionic and the

charge

transfer

perturbation

surface

semiconductors between

understood as due to the changes in geometry and

of

the

layers.

We

bonds

are

atoms bonding

can at

be the

In a tetrahedally

surface. charge

transfer

A% =

4 Aq

where

Aq,is the charge

transfer

coordinated

in the bulk

can be

III-V

semiconductor

approximateh

the

by:

1.

by

approximation

for

transfer

From

bond.

in bulk

this

it

(110) surface

Aq.

and

is

clear

the

is

\hat

charge

in

a

first

atoms:

AsrD = 3/4 Aq L the

condition

c21 C23 define

tight-binding possibility

our starting

calculations. of charge

transfer

at the same or different these

charge

around

charge

table,

correct

the local

reconstruction the possible role

First

consider

more

Hartree-like

of three

level

shifts

this

case we obtain our simple

also

C31:

layers

around

as compared

with

the correct

approximation

because

the estimated

eV, mainly

because

we don't

the

the importance as

we

of

show

doesn't

in

give the

the

complete

we conclude

can play

that

an

important of charge

(we

don't

give

. We remark of

a total

in our model

possible

need

to

and we impose

a good behavior

sign and order

include

we

the possibility

experiments

of the Madelung

models

and

the surface)

error

the

transfer

when

layers

doesn't

for

and with

condition

we recover

shifts

condition

and,

at the surface

and subsurface

description

We

remark

When we include

agreement,

also a better

the charge

semiconductors

selfconsitency,

core

bulk).

for the

in the calculation:

tranfers

at the surface

them

given by

surfaces

neutrality

is considered

transfer

by

core level

at all we

some

processes.

between

created

to the

reconstruction

charge

for

charge

in these

course

between

of the surface

description

located

consistency

131

our predicted

treatment

displacements).

the inclusion

the

=O

and the reconstructed

the selfconsistent

this

and cations

( with the local charge neutrality

studied

energies

anions

condition

include

we

and we impose

( with respect

L

I we report

unreconstructed have

between

neutrality

6ni+6nz+.............+6n

In table

for our selfconsistent

model

6nL, and the potentials

considered

a global

point

our

planes,

transfers,

the atom

include

In

that

magnitude.

a the in Of

quantitative is around

screening

potential

of

should

0.1

effects; improve

159 our

results.

TABLE

I

Calculated

surface

cations,

AT3 c' reported.

also

Semicond. Compound

in

eV.,

core

Level

Results

from

shifts

Charge neutrality condition Ideal

AEAE Q

c

Priester AEAE 0

anions

for

photoemision

Al3a.

are

Charge Transf.

Real reconstr. Selfconsist AIIAEAEAJIAEAE P c Q

c

ati

experiments

c

Experim. P

c

AlAs

0.53 -0.47

0.40 -0.33

0.33 -0.48

0.31 -0.4

---

---

AlP

0.68 -0.63

0.69 -0.30

0.53 -0.48

0.48 -0.25

---

---

GaAs

0.43 -0.58

0.40 -0.38

0.37 -0.46

0.24 -0.26

0.37 -0.28

GaSb

0.28 -0.40

0.41 -0.35

0.47 -0.30

0.34 -0.10

0.36 -0.30

GaP

0.54 -0.60

0.43 -0.41

0.43 -0.41

0.40 -0.24

0.41 -0.28

InAs

0.06 -0.58

0.28 -0.34 -0.14 -0.46

0.00 -0.39

---

-0.26

InP

0.11 -0.56

0.0

0.0

0.

-0.30

InSb

0.19 -0.46

0.22 -0.28

-0.34 -0.19 -0.51 0.14 -0.35

In conclusion we have showed our origin of the surface core level

-0.47

0.11 -0.22

ideas

about

0.29 -0.22

the

physical

shifts at III-V (110) semiconductor surfaces, pointing that the local charge neutrality

condition proposed to explain these shifts needs to be modified by a selfconsistent model, where we take into account the possibility of charge transfer between planes. Of

course

the

anions

model

can

and be

description of the Madelung potential:

cations

at

improved this

is

the

with

surface a

currently

better being

studied. ACKNOWLEDGMENTS.

We thank to the Consejeria de Educaci6n de1 Gobierno Aut6nomo Canario for partial financial support of this work. REFERENCES.

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