Clean Semiconductor Surfaces Covered by Ag at Low Temperatures

CLEAN SEMI CO~DUCTOR SURFACES COVERED BY Ag AT LOW TEMPERATURES V. A. G razhuIis Institute of Solid State Physics, Academy of Sciences of the USSl'<, Chernogolovka, lVloscow district, 142432, USSR In thi s paper a brief review of the latest results obtai ned by the LEED dnd Auger-techniques for clean Ge(lll), Si(lll) and InSb(110) surfaces and for the surfaces wi th adsorbed Ag atoms in the low t emperature interval of 8-300 K is presented. It is shown that in the low temperature region one Cdn observe a number of new phenomena that may 1 ead to new notions about the atomic structure and properties of 01 ean and metal covered surfaces of semi conductors. It is found that transiti on from low to "el evated" temperatures can dramati cally change not onl y the structure of cl ean surfaces but al so the structure of interfaces and metal films. Observed physical phenomena are discussed and interpret ations are suggested. INTRODUCTION As shown by first investigations

[1-5]

,interest ing physical

mena may ari se on the semiconductor surfaces at

pheno-

low temp eratures.

In a

number of cases the exi stence of these phenomena is di fficult to predict. We shall consider the latest low temperature results obtained in our group for Ge,

Si

and InSb at "1'=8-400 K.

We shall treat both the properties

of cl ean surfaces and the properti es of the surfaces with adsorbed Ag atoms. Without going into the details of the experimental technique, we shall onI y

note the t

we employed ultra-high-vacuum ESCALAB-5 and LAZ-620

spectrometers, comprising attachments enabling us to monitor the t emperature from 8 to 400 K

[lJ

Now let us consider some experimental temperatures for Ge(111), I.

Clean Ge( 111) In

[1-5J

temperature "1'

Si(111)

and SiC 111)

surfaces at low temperatures

it was found that in UHV ranging from 4 to 300 K

cl e.l ways possesses a

results obtained at low

and InSb(110).

(~10-10

Torr)

at a

2xl superstructure, whereas Ge( 111)

st ructure essentially depends on "1'cl'

cleavage

the cleaved SiC 111)

namel y, at "1'Cl~O K

surface

surface the cleaved

Ge(111) surface also possesses a 2xl superstructure but at T K c l(40 the LEED pi cture may show no superstructural 2xl spots. The latter can be interpreted as a

result of nonreconstructed state of cleaved

Ge( 111)-lxl or strongly disordered Ge( 111)-2xl surface. behaviour of Ge( 111)

Such "unusual"

cl eaved surfaces was related to the overheating

140 21nd

qucnchi ng p li oriom c:

temp"nlturos

gt'OUp

the

c1 Cd"\/,,:-\§tC' prucess o t 10\1\

[(,1

T-hc C~e( 111) UHV vv o r o

d.cun1p<':~111ying

surfaces cl

o t J ow

C'i:Yvc,(j

tCll1peratLlt"CS

(10-~300

(fat' detai Is see ref. [7J ).

face atomi c essential 1 y

Some new results concerning the sur-

t ) All Ge(lll) cleClvages at T

K show c l=(,0-300 structures with isotropic broadening of

the same 2xl LEED

diffract i on spot.,:, and with 1,:::.1"-'1 ~q/(15-20), see fig.la. a (ii) Low-T Ge( 111)

WIll [tIl]

'" !

I[TOI]

f2f1l.... ~\ / -'" [tZIl

/~'-.. /0\ •

• 0/

•0

J

-

•

\0

•

.

t Wf]

t UlZ]

vages, '1' CI~40 K,

, --h 9• 1 • '~ • • 0 0 ·L 0 • 0 • • • »: • • -[ITo] ~(I10~

t iio :

c

fj

some spread in the 1,EED Si( 111) i one can observe Ge( 111) 1,EED pictures of -

three types, fig. 1, with we-

tun:

akly or strongly broadened

1. Three types of diffraction patterns w i th superreflections for low tem-

a

clea-

show

data, as contrasted from

-0- .

[J

Fig.

In

stnAct ures were obtained. These resul ts suggest some i m-

p o r-t o nt conclusicms: all

1'-)

recently investigated again by the LEED technique in our

extra spots of the 2xl

structure or even pictures perature cleavages of Ge crystals: without the ext ra spots. -ordered superstructure; b,c - strongl y disordered superstruct ures (ii i) Strong broadening is (di fferent in the ori entation of the observed only for the extra superrefl ections). Dashed arrows i ndi cate the doubling peri ad direspots. The I atter means that ctions in a (111) plane. o ril y the topmost surface dan-

gl i ng bond (SDB)

layer can be essential! y

di sordered and thus gi ve

ri se to the extra spot broadeni rig, The atomi c do not "feel" signi ficantl y

layers below the surface

even strong di sordering of the topmost layer.

(i v) The extra spot broadeni ng can be essenti all y

anisotropi c wi th the

strongest correl at i on in the surface dangling bond syst em along the [112.1 or

(iOl) directions. The latter is doubling periodicity direction in the

( 111)

cl eavage pI ane, see fig. 1.

(v) The low-T cl eavages may give

ri se to extremely strong disordering of the Ge( 111) with correlation length as "amorphous-I ike".

f

(1,)-::::(3-6)a

topmost layer

so that this layer may be treated

o' (vi) These "amorphous-like" surfaces are sensitive

to the pri mary el ectron beam (giving rise to some electron beam annealing effect)

whereas the thermal

heating does not affect significantly these

surfaces throughout the investigated range (10-300 K). A cl uster model of the Ge( 111) surface has been proposed on the base of the resul ts obtai ned in (1] Fi gs 2 and 3 present the models

1·11 of

:2.x.1 surfclcc supc?rstn.. J .ctul'lil

tJH?

•

1

clustc>rs

o

2

l.wo

@J

.• .

types i:-.l.cl<-?Cju<3te

ill fi g. 1. 'T'l i o

shovvn

•

of tw o

type", of tile LEED

t i on

~

101

'f

igths

of the

Si7:0S

cl ustE::'TS dl'e d c t o rmin e ct

G'Z?l

vvrii c l r,

to

paLtenls corr'ela-

b)~

ill

tun), f o t l o vv

from tile' "'izes of superr'ef]ectioll s.

a

Note that

(J

f u, r=: Kc f / (,

~(t);=C<-oy/t ,f('L}-c:Ove,i/L

where 1"i g. 2. Diffl'action pattern ( al,d the model (b) of a superstructural Ge( t i i ) -2x1 clustel' corresponding to this pa tto r-n, l,2-position of nonoquivalont I e tti ce sites on the surface, 3-supel'structural reflections. t u ro

is not yet known.

maximal

,.1

I e tti o e

0

doubling, q -

p c ri o d

neighbouring m o ii : spots; cl eavages [7]

bef ore

the di stc\llce between

L;;; t::: ec

for high-T

.::; ~ /

(l5-20)

• Figs 2,3 show the position

of equi val ent I attice sites wi thin each cluster.

The real atomic struc-

Note, that in the cluster of the first type

o o r-ro l ation length

f

(e)

(fig.2)

a

in the dangli ng-bond system must arise

ellong the

[ll2J

type direction, and in the cluster of the second type

along the

[lOl]

, see fig. 2,3.

The overheating (and "mel t i ng") of

the dangling bond surface in

the progress of cl eavage at low T

[6,7J

may explain the disorde-

ri ng of the dangling bond system

• •

~

and the superspot broadening, however,

it can not expl ai n the

whol e variety of the phenomena observed for cl eaved Ge( ces.

i )

atomic structure of

(lll)-2xl surfaces in quest-

ion a

(J

surfa-

Evident! y, in order to el uci-

date the real the

ri

numerical cal culations of the

Fig. 3.

Diffraction pattern (a) and the model (b) of a superstructural cluster, corresponding to this pattern. The designation is the same as in fig. 2.

atomic structures and additi onal experi mental investigations are needed [8J • II.

Low temperature studies of Ge(lll), 5i(l1l) In5b( t i

o)

and

surfaces at Ag atom deposition

There are a

lot

of investigations card ed out at "high" T

5i(lll)+Ag and Ge(lll)+Ag, see for instance

[lO-12].

for

In this section

14:: vvo

J.H'icfl")/

OUT l.i:ILC'.sl

disC1LSS

c<::::,sults

O}ltdlJICd

Si(:J.'1L)+Ag,

fo r

Cie(lll)+Ag cJnd 1nSb(11U)+Ag using Auger> CllJd LESD 10\\"

\IVO

lor:npcraturcs.

observe

IIJterestiJlg

sllcdl

shcnv her'e t l io l

p11")'sict.11

ill

vvi i i c li

phcnolnena,

JeH\

lcclmi'lues cd

l(?lnpC'I'{llul~C,S

can

IJc'

not

one

CiJ.ll

o b s o r-vo d

loil

high temperatures. (i)

Experimentul

First of o l I

studies of Si(111)+Ag [9]

note that in the case of Si(I1:1)+Ag we Clssume that

one rn o rro l o yo r of Ag(G =1) >T'ypical Ag( 356 figs. 4, 5

eV) [9]

c o r-ro s.p orid s,

e!ep0ndences of 1 Si(&)

Auger-peclks, to.k o r: ul 8

1

to 7.n.l0]c

allC] lAg(6i) K

atoms/c:m.

f or

'Ule! JOO K

o

Si( 92

eN)

ane!

r o pl'esentee! in

• One can see essential e!ifference between low-T aile!

hi gh-T data. The 1J0haviour of the Si( 111)-Ag system with increasing temperature from

n to JOO K

after the 8

K

de-

position is markedly dependent on (). We have found tho t the ratio 0,5

of the Auger-peak intensities, I Ag/lSi' for

e >~

1 is invCll'iable at

heating within the error of the mea-

o

surement s. However, at ()

ISi(G') experimental curves: 1 - at T=8 Kj 2 - at T=JOOK solid 1 ine - cal cuI ation usi ng formula ls.<-<>exp (-7) wi th 1( E. s'» as fitting param0te r (1~4 A~,C.S> ;::::. 92 eVj 3 - approximafle5n of IS/G) at 61 o.

i rr cv e r s i bl y

at T:;?- 100 K.

rii ng 300 K

this value decreases

by e.p p r-o xi matel y

two orders of mag-

tic temperature T*, at which the changes begin to be noticeabgrows with increasing coverage.

For example for () however for

g

z

Z

lAg

~ >

.! / '

0,4

/

/

/tV"""'

'j~••".

•• -

_

.•••••••••

2

On attai-

nitude for () :;;:. 15. The c he.re ct o r i s-

1 e,

the

lAg/lSi value begins to diminish

Fig. 4.

sai d

>1

4 T* ~ 200 K,

15 T* ~ 300K. The

J

Fig.5.

2

b 8

lA (6) experimental curves: 1 -gT=8 Kj 2 - T=300 Kj soli d 1 i ne - calculation usi ng f o rmulo r LA c:' I-exp(--f-J with l(E.A!I) a"s fitting parameter (lAg%-6A), tAg::::.356eV.

cause of this phenomenon is left unci ear. The Q

dependences of Si and Ag Auger-signal

and

e

e

and their subsequent heating up to

dmplitudes at 300 K

=0-1 are practically linear (see fig.4,5 dnd [10,11,12]

Note, that cool ing of the sampl es (after 300 K K

~

300 K

).

deposition)

down to

do not practically affect

the Auger-spectra ampl itudes. At Ag deposi ti on on a

cl eaved Si (111)

surface a

great variety

of LEED patterns corresponding to structures Si(111)-2xl, Si(111)-lx1,

fi

Si(111)-V7' x

R(':::19.1

was observed. Wi th temperature,

f9 z

0

) ,

three-domain Si(111)-3xl dnd Ag(111)-lxl

1,2-1,3, when the sampl es were deposited at room

we were e bl e

to observe all

the above structures simultaneo-

usly. In

[9J

it was found that deposition of Ag atoms at

affect significantly the Si(111)-2xl superstructure,

e

i.e.

rather than chemi cal adsorption of the Ag atoms is l i kel y In fact, in the process of deposition at

a

beam

£::. 100

si bl e

e

eV)

from

e

K

physical

to take place.

primary el ectron

3-4. We emphasize that with a

growth

val ue both the superstructural and main reflections become i nvi-

p r e c ti cally simultaneously. The disappearance of all the LEED ref-

I ections from the SiC 111)

e~ 3

=0 to () ~

does not

low temperature the Si(lll)-

2xl LEED pat tern can be observed (at the energy of of the

K

at 8

surface at

(J ~ 3 implies that the Ag film with

is nontransparent for electrons with

E.

~100 eV. So, from the LEED

E ~ 100

data we can estimate the electron free path I( energy loss. Clearl y, that l~

e : : . 3.

eV)

without the

This val ue agrees with the electron

free path value obtained from the AES experimental

data

[9J.

There are interesting phenomena related to Si crystal Ag deposition at 10 K and T'>r-T* Z 100 K

[9]. We just note here that at G ~

heating after

e"

~ (0.4-0.5)

an irreversible phase transition of the SiC 111)-2xl-

Si ( 111) -lxl type occurs due, probabl y, to drastic increase of Ag film interaction with the substrate at '1' the "critical" temperature '1'*

essential! y

> '1'*.

di sordered

It was found that at

increases wi th increasi ng

f9 >()*

e

value.

Note, that the strengthening of the Ag-Si interaction can be interpreted as a

resul t of the f ormation of the chemi cal bonding which needs thermal

tivation and, therefore, (Initially,

occurs only at high enough temperatures,

at 10 K, Ag atoms are in physical

details see [9]

adsorption state).

ac-

T~T*.

For other

144 (ii)

Investigution of Ge( 111) +Ag

Let us note, first of 011, that in the case of G0(111)+Ag one mOlIU. . 1-1:2 , ' luyer of Ag(fl =1) corresponds tu 7.2.10 atoms/em. '['he expenmelltal results for Ge(111)+Ag al T~10 K

turned out lo be very differenl Ir-o m

those of Si( 111) +Ag and of Ge( 111) +Ag obtai ned at 300 K No m ory, at 10 K

the supen'efl o c t i ons of Ge( 111) -2x.1 completely dis'-lppe-

ar al ready al 19 - (0.1-0.2)

(nole thal

al 300 K

deposi lion in the case

of Ge( .111) +Ag lhe superrefl ections dis'''lppear only al In the range 19-(1-2) substrale compl etel y Ge(111)

at 10 K

disappear only at

The result of sampl e

e:

(i)

at

heating to 300 K

and (iii)

al

[13,1'IJ).

5

the basic reflections from

Cl

[13J). The reflections from the Ag film

heating (after 10 K

e -(0-1)

8-(1-2)

e~ 2

e >r-

e >r- 2.

e ~ O. 5-1

the basic reflections from lhe Ge(111)

di sappear (at 300 K

itsel f become noti ceable at depends on

03, 1·1J.

deposition)

to 300 K

the LEED pattern is retained,

(u)

at

gives rise to the Ag(111)-lxl reflections

the Ag reflections arising already al 10 K

become.

1 ess broadened at heating. So the Ge(11.1)-2xl+Ag system atrongly differs from the Si(1.11)-2x1.+Ag one.

We observed, thal at 10 K

already with

e ~(0.1-0.2)

the

superreflections from the Ge( 111) -2xl completely disappear. This probabIy

indicat es that a Ag atom at 10 K

Ge(1.11.)

readil y

forms chemical

bonds wi th

surface atom in contrast to Si(ll1.). This causes the Ge(lll)-

2xl

~

ti o n

energy f or chemical bonding of the surface Ge atoms with Ag is very

Ge( 111)-lxl transition.

low or, el se, A Thi s

it is absent.

strong chemical

rance at 10 K

Hence one can conclude that the activa-

int eraction can be the reason for the disappea-

of main LEED

reflections from a

Ge(111)

surface at (J-2.

cannot be attributed to "screening" of the substrate by the Ag film,

since in the Si(111)+Ag system (see above)

obtained in the same con-

di tions, the mai n LEED spots are observable up lo

e ~ 3-5.

Therefore, a

strong chemical interaction of Ge atoms with the disordered system of Ag atoms results at 10 K symmetry in a

in an essential

few I ayers at the Ge( 111)

disturbance of the translation surface.

Other details of the Ge(111)+Ag system studies one can find in [l3] . (ii i )

Investigation of InSb( 1.10 )+Ag

This section presents some results (see, al so, [15J) LEED

t e c l-iril q u o

for InSb( 110)

obtained by

cleaved surface covered by Ag at low

14,i

temperature.

A

mean thickness of the Ag films was varied, like o o rl i or; one monolayer (8 =1)

in the range of (9:::::'0-20 monolayers;

corresponds

now to the number of the InSb( 110) substrate atoms per unit area, i, e. 14 6.74.10 cm-2. The crystal s were cleaved at 300 K. Cool ing the sampl es down to 10K does not change qualitatively the LEED pattern for a O~ e~

0.1, a

clean surface. At Ag deposition at 10 K,

distinct LEED pattern from the InSb(llO)

surface is still observed,

however, at O. 1~

LEED picture grows wi th growing of the InSb( 110)

and, si multaneousl y, the intensity

reflections completely disappear,

no new reflect-

The "screening effect" must arise with 6l?3, see above.

Therefore, the disappearance of the InSb( 110)

reflections at e~ 1-1. 5

may be t.r e e.t.e d , like in the case of Ge(lll)+Ag, as a tial

substrate

1 the background in

reflections decreases. At 19-1-1. 5 the background alone

is seen, the InSb( 110) ions are formed.

e

G~

result of an essen-

disturbance in the translation symmetry of several atomic layers

near the InSb(llO)

surface due to a

strong chemical

interaction of the

disordered system of Ag atoms with the substrate immediately after the adsorption at 10 K We shall

(negligible activation energy for chemi cal bondi ng).

now consi der the behaviour of InSb( 110) +Ag at a

fur-

ther increase of (; and T::::; 10 K. At 1~

e~

4 the InSb(llO)+Ag surface remains strongly disordered

(amorphous). It does not give any LEED reflections.

e = 19-10=4-4. 5

an interesting phenomenon is observed,

si t ion of the Ag system to the ordered state;

However, at namel y, the tran-

the LEED pattern suddenly

starts showing new reflections that do not repeat the earlier disappeared refl ections of the InS b( 110) deal

substrate, see fig,6,

So. we are likely to

with the phase transition in the Ag film from amorphous to crys-

tall ine state at constant temperature (T z 10 K), The critical ThUS, there exist s

a

parameter

..

is the Ag film thickness,

critical value of 61=8",4-4.5 at whi ch the said

transit i on arises, As (J grows the intensity of the arisen LEED grows until

()~ 5; with a

with the cases of (» approxi matel y respect to

e

further increase of

20 here)

e

to

e- 20

reflect ions

(we do not deal

the intensity of the reflections remains

constant. It means that the transi tion is "diffuse" with

in the () range from about

4 to about 5, This, probably,

is associated with large fluctuations in the film thickness, arising at a low temperature of the substrate. From the diffraction pattern analysis it follows

[15.16]

that Ag

146

T' zlO IZ at omi c C1

bee

forms c\ no\v nCllnel)T

stt~ucturef

one

InsteCJ.d

usual fcc. Let us

a

of the

emphi:l-

s1z0 that thi s

" iu iu s.uo !''

.sil'/el~ gl~O\VS

only' at

low-T deposition.

8

So, at

Fig.6.(a)

InSb(110)+Ag diffraction pattern (8 Z 9), obtained by 10 K deposition and heati ng 300 K, E =63 eV; (b) schematic presentationPto expl ain the di ffract ion pattern involved; (1) basic spots of the Ag film, corresponding to the Ag(111) bee-lattice; (2)sat ellites of the basi c spots of silver, (3) the subst rat e spot si tes before the silver deposition (also revealed at heating of the deposited sampl es to T >300 K.

T-=-10 K

"

e >e';'cll1d

new bee modi-

fi cation of Ag (wi th a ::::3,4

A.

[15]

al'ises

on the InSb( 110)

surface

as a

result of the struc-

t u re l

transition.

Note that

the Ag deposition at 300K does not lead to the f ormation of this Ag modification. The other detai Is of the investigations may be found in [15,16] •

CONCLUSION The presented experimental data sugg,2st the conclusion that there exists a tigated,

wide temperature interval, practically very poorly inves-

within which interesting physical

phenomena may arise.

There

are grounds to bel ieve that these studies will be irit ensi vely progressing and I ead to new fi ndi rrg ss, REFERENCES 1

Aristov V.Yu., Golovko N.I., Grazhulis VA., and Ossipyan Yu.A.: presented at 4-th European Conf.

on Surface Sci ence, 1981.

2

Aristov V.Yu., Golovko N.I., Grazhulis V.A., Ossipyan Yu.A. and

3

Aristov V.Yu., Batov I.E.

4

Haneman D.

Talyanskii V.I: Surface Sci. 117

(1983) 337.

(1982)

204.

and Grazhulis V.A.:

Surface Sci.

132

73. and Bachrach R.Z.:

J.Vacuum Sci.Technol.

21

(1982)

147

5

Haneman D.

6

Grazhulis V.A.

7

Golovko ~.I., Grazhulis V.A., Kuleshov V.F., and Talyanskii

(1985)

and Bachrach P.• Z.:

8

Applic.

(1986)

(19t33)

J9:27. :2

Grazhulis V.A.: Exper.Teor.Fiz.

Poverkhnost,

21

V.I.:

76. 6

(1986)

Arlstov V. Yu., Boloti n I.L., G razhulis V.A.

140. and Zhilin V.M.:

Zhurn.

4(10), 1986, 1411.

10.

McKinley A.,

11.

Bolmont D., Ping Chen,

12.

Le Lay G.,

24,

I

of Sudace Sci.

14.

Poverkhnost, 1 9.

Phys.P.ev.B17

and l<.uleshov V. F'.:

Williams R.H., Park A. W. :

J.Phys.C.

1979, 12, 2447.

Sebenne CA., Proix F. Phys.Rev.B.:

19t31,

4552.

1981,

Chouvet A., Mannevill e M., Kern P.• Appl, Surf.Sci.,

9, 190.

13. Aristov V.Yu., Grazhulis V.A., Zhilin V.M., Poverkhnost, 1987 (in press). 14.

Le Lay G., 1976, 35,

Quentel

G.,

Faurie J.P., Masson A. Thin Sol.Films,

273.

15. V.Yu.Arlstov, Bolotin I.L., Grazhulis V.A.: Pisma Zh.Eksperim.Teor. Fiz., 45, N 16

1

(1987)

49.

Arlstov V. Y'u, , Bolotin I.L., 14-th Annual

Grazhulis V.A, :

Proceedings of the

Conference on Physics & Chemistry of Semiconductor

Interfaces, Salt Lake City, USA, February. 191il7.