Theoretical study on the high-energy electron energy loss spectroscopy compared with X-ray absorption spectroscopy

566

PhysicaB 158(1989)566-567 North-Holland, Amsterdam

THEORETICAL

STUDY ON THE HIGH-ENERGY

SPECTROSCOPY

ELECTRON

COMPARED WITH X-RAY ABSORPTION

ENERGY LOSS

SPECTROSCOPY

Shuji Takatoh, Takashi Fuj ikawa and Seiji Usami Faculty of Engineering, Yokohama

the alternative

phenomena

EXELFS

have

methods

in the reflection

by the use of

(EELS) spectra

In this work, we have performed and energy dependence

electrons have

Hodogaya,

are excited

derived

the

the general

study

surface

calculation

for

EELS in which

from deep core orbitals.

energy

particular,

[2].

numerical

of high-energy

high

[I]. In

mode has been widely used to

because of its high surface sensitivity

angular

applied

University,

for the study of EXAFS and XANES

been developed

energy loss spectroscopy

geometry

National

240, Japan

Recently, type electron

Yokohama

For this

formula based on many-body

it in the lowest order to atomic excitation

problem,

theory [4].

t31,

we

diffraction

In this paper, elastic after core excitation approximation

Bethe

and of thermal vibration

scatterings

are completely

However, we can expect the interference

waves.

those

the inelastic

be described

by the momentum

the

wave

vector

amplitude

electron

scattering

in terms of that for one atom system m(a,g;As) damping

this

from

As. Let z be a

M(a,z;A$)

deep

core given

is now

and of phenomenological

factor AA as follows:

property

orbital

of m(a,z;A$)

The

paper(31.

of scattering

Here, we consider and

has been discussed

in detail

of Ath atom is given by sA

position

CY is localized.

properties -1x1

the

electron

M(u,~;A$)=kexp(-iA$"$A)m(~,~;A~)exp{-AA}. The

and

from deep core ~1 can

excited

amplitude

to

sites. In

transfer of a fast electron,

of a secondary

CY. The inelastic

orbital

scattering

the

scattering

two

between

waves which excite the same kind of cores on different approximation

before

It corresponds

where plane waves are used for

and

of atoms.

of fast electrons

neglected.

we

consider

two kinds of crystal effects on EELS spectra in the lowest order; effects of inelastic

the

secondary

Exp(-iA$*sA)

in (1) is due to

the EELS from diamond

in a

where

electron waves in poly-atomic

-2x1 surfaces.

(1) previous the

core

coherent

the

systems.

(111) and silicon

(100)

for

these

We use the cluster approximation

surfaces which include about 5Os60 atoms per one layer. Only

when

coincides

with some surface

0921-4526/89/$03.50 @ El sevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

reciprocal

lattice

567

S. Takatohetal./High-energy electron energy loss spectroscopy

the

vector,

intensity

scattered

large. We should note that

becomes

energy

A$ depends on the incident E

incident (scattered) the i' take-off angle ei (e,), and so on.

Figure

work.

geometrical

considered

Figure

(=k2/2)

the

shows

1

parameters

the

shows

2

this

in

Fiq.l The geometrical

parameters

ck

of the loss intensity at several spots.

dependence

energy region, we can expect to measure and 10.0'. At these points, A?

I/

to some surface reciprocal

strong loss signals at @f=3.0"

be

however,

lattice vectors,

purpose.

strong enough over wide energy shows

that

whereas

it is not the case intensity

calculation,

prospective

only special points are

transmission

at A6 ,,=O. Therefore,

for

that

factor

forthcoming

mode EELS always measures

the loss intensity

EELS spectra should be strong for any Ei. Figure

shows this situation

loss

This

range.

The results for Si(100) surface also show similar results.

However,

3

close

can arrive at the points quite

at 1.0' and 8.0'. In order to get good EXAFS data, the loss should

some

In

clearly,

it decays rapidly because

[43. Other results will be discussed

in

of

detail

atomic in

paper. DIAMOND

(111) K-edge

Si(100)

Fiq.Z(left) The loss intensity from diamond (111) with C surface K-edge excitation as function of E k at various spots.

2X1 (L2,3-edge)

Transmission

mode

I

Fig.3criqht) The intensity from Si L2,3-edge transmisin the sion mode. The film considered here is composed of 10 layers.

SECONDARY [ll R.Egerton:Electron (Plenum

Energy Press,New

Loss

ELECTRON

Spectroscopy

ENERGY

in the Electron

Microscope.

York,London,l986)

[2l J.Derrien,E.Chainet,M.DeCresenzi

and C.Noguera:Surf.Sci.l89(1987)590.

[31 T.Fujikawa:J.Phys.Soc.Jpn.57(1988)306. [4l T.Fujikawa,S.Takatoh

\eV)

and S.Usami:Jpn.J.Appl.Phys.27(1988)348.

the