Recent developments in low-energy ion scattering spectroscopy (ISS) for surface structural analysis

Nuclear Instruments and Methods in

264

Physics Research

B37/38

(1989)

North-Holland,

Section IV. Ion implantation

and ion beam interactions

RECENT DEVELOPMENTS IN LOW-ENERGY FOR SURFACE STRUCTURAL ANALYSIS M. AONO, M. KATAYAMA, RIKEN

Recent include possible

E. NOMURA

years have seen significant

the development

progress

of impact-collision

and the use of alkali ions rather

probability. pulsed-beam

The latest progress

ion source and a time-of-flight

at 180 O. Various

characteristics

for the energy analysis,

in low-energy

than noble-gas

ION SCATTERING

ion scattering spectroscopy

Low-energy ion scattering spectroscopy (ISS), which was initiated by Smith [l] in 1967, was first expected to be a promising method for the composition analysis of solid surfaces. However, it was later found that the composition analysis of surfaces by ISS is only qualitative because the neutralization probabilities of noble-gas ions, which are used in usual ISS experiments, are so sensitive to surface chemical and physical (structural) conditions that we cannot predict them without serious ambiguity. Instead, it was proved that ISS is rather useful for the structural analysis of surfaces. In fact, ISS is a convenient method for analyzing surface structures qualitatively. In the structural analysis, shadowing and blocking effects, which are simple “classical” phenomena are effectively utilized. We can refer to several review articles [2-61 on the structural analysis of surfaces by ISS. It is, however, not easy for usual ISS to obtain quantitative information on surface structures, i.e., the coordinates of surface atoms. In order to make the structural analysis of surfaces quantitatively by ISS, a specialization of ISS, impact-collision ion scattering spectroscopy (ICISS), was developed in 1981 [7-91. In ICISS, the experimental scattering angle is taken to be 180° or close to it, so that each atom is “seen” at its center via impact-collision (head-on collision) of ions. Because of this, in a critical situation that an atom NEC

Corp.,

Miyamae,

Kanagawa

213, Japan. ** On leave from Karl-Marx ***

On leave from Kangwon

University, University,

coaxially

Leipzing,

GDR.

Seoul, Korea.

0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

(ISS) for surface

to make quantitative

(ALICISS)

impact-collision

tube for scattered

1. Introduction

* Permanent address:

spectroscopy

to avoid

ion scattering

structural surface

the ambiguity spectroscopy

scattering

analysis.

structural

They

analysis

of ion neutralization (CAICISS)

so as to take the experimental

which are due to (a) the 180 o experimental

and (c) the use of an acceleration

(ISS)

**, D. CHOI ** * and M. KATO

(ICISS)

ions in ICISS

of coaxial

energy analyzer are arranged

of CAICISS,

SPECTROSCOPY

Wake, Saitama 351, Japan

ion scattering

is the development

with solids

*, T. CHASSk

(The Institute of Physical and Chemical Research),

264-269

Amsterdam

scattering

in which a angle just

angle, (b) the time-of-flight

mode

ions, are discussed.

begins to shadow another atom, the edge of the shadow cone of the shadowing atom passes the center of the shadowed atom. By using this simple geometrical relationship, we can analyze the structures of surfaces quantitatively if we know the shape of the shadow cone of the shadowing atom previously [6,7]. ICISS has been successfully used to study various surfaces [lo-231. It is sometimes helpful to use alkali ions rather than noble-gas ions in ICISS. This method is called alkali-ion impact-collision ion scattering spectroscopy (ALICISS) [13,24,25]. Alkali ions are characterized by their small (and trajectory-independent) neutralization probabilities during scattering at surfaces. Because of this, the intensity of scattered ions in ALICISS is much larger than that in usual ICISS in which noble-gas ions are used. If we use a time-of-flight energy analyzer rather than an electrostatic energy analyzer, not only ions but also neutrals can be detected (and if necessary, ions and neutrals can be distinguished in measured time-of-flight spectra by accelerating only ions electrostatically) [25,30]. This time-of-flight impact-collision ion scattering spectroscopy (TOF-ICISS) therefore has essentially the same effect as ALICISS even if noble-gas ions are used. Various surfaces have been studied by ALICISS [13,24,26-291 and TOF-ICISS [25,30,31]. Recently, some of the present authors and co-workers [32] have developed a novel ion scattering spectrometer, in which an ion source and an energy analyzer are arranged coaxially so as to take the experimental scattering angle to be 180 O. In usual ICISS, the experimental scattering angle is actualy not 180° but smaller than that by some loo because it is difficult to put an ion source and an energy analyzer in the same direction. In the mode of the novel ion scattering spectrometer, which we call coaxial impact-collision ion scattering

hf. Aono Edal. / Low-energy

spectroscopy (CAICISS), the experimental scattering angle is just 180°. This results in several striking advantages as compared with usual ICISS and ISS mentioned above. In this paper, we discuss the characteristics of CAICISS which is the latest development in the field of ISS in a broad sense. In section 2, the apparatus of CAICISS will be described briefly. The characteristics of CAICISS will be discussed in section 3 using examples of experimental data if necessary. A summary will be given in section 4.

2. Apparatus of CAICISS

Since the apparatus of CAICISS is described in detail elsewhere [32], only an outline is given in this paper. Fig. 1 is a schematic of the apparatus of CAICISS. The apparatus consists of a pulsed-beam ion source and a time-of-flight energy analyzer that are arranged coaxially so as to take the experimental scattering angle at 180 O. The ion detector of the time-of-flight energy analyzer, which consists of double microchannel plates and an anode, has a small hole in the center so that incident ions pass through it and impinge on a goniometer in a sample chamber. One of the improvements which we have made recently is an acceleration tube [25,30] which is put between the sample chamber and the detector (see fig. 1). A high voltage is applied to the tube in such a pulsed mode that the voltage is turned off when pulsed inci-

FLIGHT

t\

BEAN SCANNING DEFLECTOR

265

ion scattering speciroscopy

dent ions go through the tube and turned on when scattered particles from the sample come back through it. The tube therefore accelerates only ions of scattered particles. In this way, we can distinguish between ions and neutrals in measured CAICISS spectra. 3. Characteristics of CAICISS The characteristics of CAICISS, which result from (a) the 180 o experimental scattering angle, (b) the timeof-flight mode of energy analysis, and (c) the use of the acceleration tube for ions, are summarized as follows: (1) Quantitative structural analysis of solid surfaces can be made easily. (2) Not only one or two atomic layers at the surface but also several atomic layers below the surface can be observed. (3) Layer-by-layer composition analysis is possible over several atomic layers from the surface. (4) A surface-sensitive mode (in which only one or two atomic layers at the surface are observed) or a bulk-sensitive mode (in which several atomic layers below the surface are also observed) can be selected by simple electronic switching. (5) Information on the chemical states (charge states) of surface atoms at different sites is obtained. is convenient for monitoring various (6) CAICISS surface processes (e.g., epitaxial growth of atomic layers on a surface) in situ. (7) CAICISS is suitable for the time-resolved analysis of dynamical processes at surfaces.

PATH

-I

LENS 6 ACCELERATION TUBE \

OETECTOR =%!fi,

.”

.”

.“-

IMCP) , CHOPPING PLATES

CHOPPING APERTURE

n

SAWLt

I

I

Fig.

1.

I

Schematic of the apparatus of CAICISS. IV. ION

IMPLANTATION

266

M. Aono

et al. / Low-energy

ion scattering

(b)

Fig. 2. Schematic

figures indicating

spectroscopy

010 (BOND

DIRECTION)

how the bond direction and length of neighboring two atoms A and B are determined at the same time by CAICISS.

In what follows, these characteristics will be explained in some detail. The apparatus of CAICISS has axial symmetry around the incidence/detecting direction of ions. Because of this, any variation in CAICISS spectra due to the rotation of a sample directly reflects the symmetry of atomic arrangement at the sample surface. In fig. 2a, A and B represent two neighboring atoms at a surface, which may be bonded to each other. Suppose that we measure the intensity of particles (ions and neutrals) scattered from atom B as a function of the incidence/ detecting angle of ions, (Y (measured from the surface plane), in the CAICISS mode. The observed intensity variation will be something like fig. 2b. The zero inteneffect in sity around (Y= OL,, is due to a shadowing which atom A shadows atom B. At LY’Sthat largely deviate from (me, the shadowing effect does not occur, and hence the intensity is constant. The intensity enhancement on both sides of a = a0 occurs because in general the flux of ions is necessarily concentrated just outside a shadow cone and hence a shadowed atom is bathed in the concentrated ion flux when it goes out from the shadow cone of a shadowing atom (actually, the intensity enhancement is partly due to another effect called “self-blocking” [33], but we do not go into details of it in this paper). Indicated by (Y,, and (Y,,- in fig. 2b are two critical angles corresponding to the beginning and the end of the shadowing effect (see fig. 2a). The difference between (Y,, and Q, A(Y,, corresponds to the distance between the two atoms A and B, d. As d increases, Aa, decreases. It is thus found that by measuring the intensity variation like fig. 2b experimentally, we can determine both the bond direction and the bond length of the two atoms A and B at the same time. In this way, quantitative structural analysis of surfaces can be done easily by CAICISS as mentioned in (1). As illustrated in fig. 3a, it is in general difficult to observe low energy ions backscattered from subsurface

atomic layers because of blocking effects. However, CAICISS can observe backscattering from subsurface atomic layers by virtue of the 180” experimental scattering angle as illustrated in fig. 3b schematically. This is demonstrated by the experimental data shown in figs. 4a and 4b. Fig. 4a shows the intensity of He particles (ions and neutrals) scattered from a Ag(ll1) film epitaxially grown on a Si(ll1) 7 X 7 surface as a function of the incidence/detecting angle of the ions, OL,in the CAISISS mode in which the experimental scattering angle is 180 O. On the other hand, fig. 4b shows the intensity of Li+ ions scattered from the same sample as a function of the incidence angle of ions, (Y, in an ALICISS mode in which the scattering angle is 163O (an electrostatic energy analyzer was used). The energy of incident ions was 1 keV and the azimuth of measurements was (110) in both cases. Because of the similarity of TOF-ICISS to ALICISS described in section 1, the experimental conditions of the two cases are essentially the same except the scattering angle; note that He and Li are close in atomic number. In fig. 4a for the 180° scattering angle, peak B is due to ions

;‘-: -:-$6 0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

0

(a) ICISS,

e =1t30°

(b) CAICISS,

9 =180°

Fig. 3. Comparison between (a) usual ICES in which the experimental scattering angle 19 is not just 180° and (b) CAICISS in which 8 is just 180”. Ions scattered from subsurface layers can be observed in CAICISS.

M. Aono et al. / Low-energy ion scattering spectroscopy angle.

This clearly

not only

He'(lkeV)-Ag(lll)

indicates

atomic

layers

the 180 o scattering By combining scattering

from

elemental

already, below

layers

are

used.

scattered

150

01 (deg)

Fig. 4. (a) Intensity of He particles (ions and neutrals) scattered from a Ag(ll1) film epitaxially grown on a Si(111)7 X 7 surface as a function of the incidence/detecting angle of ions, LY,in the CAIClSS mode in which the experimental scattering angle is 180 “. (b) Intensity of Li’ ions scattered from the same sample as in (a) as a function of the incidence angle of ions, a, in an ALICISS mode in which the scattering angle is 163O (an electrostatic energy analyzer was used). The energy of the incident ions was 1 keV and the azimuth of the measurements was (110) in both cases.

found peak

from Ag atoms

in the fourth

with the aid of computer is not observed

atomic

simulations

in fig. 4b for the 163’

Agl

layer

as

can

we

in section

simple

electronic

characteristic

observe

several

However,

this holds

when

noble-gas

only

ions

noble-gas

of

ions

is

layers at the surface. a pulsed

high

tube between

the sample

and

we can distinguish spectra. mode

the

2, by applying

to the acceleration

sensitive

is

to one or

using

As described

surface

This

observe

to one or two atomic

between

Therefore,

or the bulk

switching

is very useful

ions and neu-

we can select sensitive

as mentioned

in (4).

in the structural

the

mode

by This

analysis

of

surfaces. The

fact

neutrals sure

that we can distinguish

in CAICISS

the

scattering

spectra

neutralization along

means

probability

a selected

between that of

ions

an

ion trajectory.

ion

during

It is known

that the neutralization

of a He+ ion during scattering (with a work function

solid surfaces

4 eV) occurs

by an Auger mechanism

of a target

atom that collides

and

we can mea-

ordinary electron

scattered

if

to over

in addition

are detected

CAICISS

trals in CAICISS

18C

by CAICISS

surface.

CAICISS

fact,

particles,

the detector, 120

In

ions

in (3).

very sensitive voltage

90

the

the surface

neutrals

between

it will be possible

at the surface.

only if scattered

of

and the fact that ion

analysis

from

but

because

in (2).

distinguish

elements,

layers

layers

two atomic

60

the surface

of what we mentioned

As we saw

30

can

different

atomic

meaning

below

can observe

of the surface

this characteristic

make layer-by-layer several

layers

angle, as mentioned

spectroscopy

scattered

ions

that CAICISS

one or two atomic

also several

atomic

267

by

as large as

in which a valence with the He+ ion

falls into the open Is level of the He+ ion and another

[34]. This

valence

scattering

Auger

electron electron

of the target [35-381.

atom

Therefore,

is kicked the

out as an

neutralization

Sit1111

Fig. 5. CAICISS spectra of a Si(ll1) surface with vacuum-deposited Ag on the surface, which were measured at regular time intervals during the deposition of Ag, the temperature of the surface being kept at about 300 o C. IV. ION IMPLANTATION

268

M. Aono et al. / Lmv-energy ion scattering spectroscopy

resolution is not good at present because ion current obtainable from the ion source estimation

shows

tion of the order

of the small used, simple

that it is easy to have a time resoluof 0.5 s if a better

ion source

is used.

4. Summary Recent for

years

surface

velopment Ag

DEPOSITION

TIME

(s)

tural

Fig. 6. Intensity of the Ag peak of fig. 5 as a function Crosses and open circles show background-subtracted -unsubtracted data, respectively.

of time. and

gress

of the He+ ion is sensitive

state (charge can obtain atoms

state)

of the target

information

at different

to the chemical

atom.

In this way, we

on the chemical

sites

states of surface

by CAICISS

as mentioned

in

(5). Because

of the coaxial

arrangement

and the energy analyzer, a “bolt-on” tached

structure,

i.e.,

epitaxy

(MBE)

chamber

with a 34 or 70 mm flange. convenient the

for monitoring

epitaxial

mentioned equipped

with

for MBE.

three

surface

CAICISS

can

vacuum-deposited

Each

figure.

tracted

spectrum

consists

respectively).

of two peaks

solved

(crosses

and

in slope observed of the Si(lll)fi

by low-energy

ions,

analysis

of dynamical

is suitable processes

5 and 6, which were already this ability

mode

the whole

elec-

of CAICISS.

for the

of a spec-

at the same time without

CAICISS

demonstrate

in the in fig. 6

and -unsub-

uses the time-of-flight

of scattered

Therefore,

(7). Figs.

of

due to

as indicated

A change

as confirmed

trum can be measured scan.

5

with

(LEED).

CAICISS analysis

on

Fig.

were mea-

of the Ag peak is plotted

structure,

Since energy

growth

surface

of the time of Ag deposition

tron diffraction

shroud position

being kept at about

at about 270 s is due to the completion x &Ag

of pyrolytic

the deposition

show background-subtracted

data,

is

which

during

of the surface

The intensity circles

as

in situ.

Si(ll1)

from Si and Ag atoms

as a function open

a

is such

situ

to the sample

Ag on the surface, time intervals

Ag, the temperature ions scattered

in

made

monitored of

at-

apparatus

so that epitaxial be

spectra

sured at regular

processes

layers

cells

for

acceleration ous

coaxially

the am-

The latest

CAICISS

in

de-

strucpro-

which

a time-of-flight

a

energy

so as to take the experi-

angle just at 180 O. By virtue of (a) the scattering

the energy

angle, (b) the time-of-flight

analysis,

tube for scattered

characteristics

cussed

and

surface

of

of section

interest 3. The

in such a manner

as a tool for monitoring

and ions, as

(c)

the use of an

CAICISS

characteristics

that the potential the structures

has vari-

enumerated

at were

the dis-

of CAICISS

of surfaces

in situ

was emphasized.

a single port

and a liquid-nitrogen-cooled

measurements

sample

300°C.

atomic

Knudsen

be

of this, CAICISS

our CAICISS

The cells are directed

for CAICSS shows

of

has

such as a molecu-

Because

In fact,

can

through

various surface

growth

in (6).

BN with a shatter

the

apparatus

easily to any other equipment

lar-beam

as

the

of CAICISS

are arranged

180 o experimental mode

source

the

to avoid

probability. of

in ISS

include

and ALICISS

development ion

progress

They

to make quantitative

possible

scattering

beginning

of the ion source

the apparatus

the

pulsed-beam analyzer

probability

significant

of ion neutralization is

mental

seen

analysis.

of ICISS

analysis

biguity

have

structural

energy

for the time-reas mentioned discussed Although

in

above, the time

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IV. ION IMPLANTATION