Si(111) surfaces by LEED

Surface Science 269/270 (1992) 995-999 North-Holland

'surface science

Structural studies of AI/Si(111) surfaces by LEED Kazuaki Nishikata, Ken-ichi Murakami, Masamichi Yoshimura and Akira Kawazu Department of Apphed Phy~ws, The Utm erslty of Tokyo. 7-3-1, Hongo, Bunkyo-ku, Tol
Al-adsorbed Sl(lll) surface,, were studied by observing patterns, line profiles and mtenMty versus energy (i-V) curves of low-energy electron diffraction (LEED). Four different surface structures, i e., ~ × x{33,f7 x qr~- incommensurate and a-phase 7 × 7 structures were observed as a function of AI coverage and the substrate temperature. Dynamical LEED analysts of a Si(l 11)V~- × qr3R30° surface, appearing after the deposition of A! of about 1/3 monolayer at the substrate temperature o1 650°C, shows adsorption of AI atoms above the second layer Si atoms (T4 site). The ~ × V~- structure was observed in the narrow range u,~ c,~verage of abot,t 0 5 monolayer at 650°C. Below 600°C, the structure with a 7 × 7 pattern (~-7 :< 7) whose line profile and I-V cul~,es were different from those of the clean SI(I I 1) surface, was formed. Above the 0 65 monolayer, an incommensurate phase, which Is often referred to as a "7 x 7" structure, was observed. The phase transition from the ct-7 × 7 phase to this incommensurate phase is considered to accompany large reconstructions and misfits

1. Introduction T h e atomic structures of metal overlayers on s e m i c o n d u c t o r surfaces have been widely studied to b e t t e r understand the processes of initial growth of metal films on semiconductors and of the formation of m e t a l - s e m i c o n d u c t o r interfaces. T h e A I / S i ( 1 1 1 ) system is particularly interesting since several different o r d e r e d two-dimensional (2D) phases can be f o r m e d depending on the coverage of aluminum a t o m s and substrate temp e r a t u r e during deposition. This system has b e e n studied with a variety of techniques, such as angle-resolved photoemission spectroscopy [1-4] and scanning tunneling microscopy (STM) [5]. H o w e v e r , the detailed formation conditions of 2D p h a s e s and precise atomic geometries o f t h e s e structures are unknown except for that of the •f 3 × V'3- structure. L a n d e r and Morrison [6] have observed several differen," phases; ct, 13, ~,-7 × 7, oL, 13-vr3- x v ~ structures and an AI thin epitaxial layer of the A! film. T h e structure of the v ~ × V~- phase has b e e n studied by dynamical L E E D [7], S T M [5] and band-structure-total-energy-minimization calculations [8,9]. H o w e v e r , systematic studies o f

different structures observed u n d e r a definite condition have not been carried out. In this paper, surface structures and the condition of their formation were studied by means of L E E D when a specified amount of AI was deposited on the clean S l ( l l 1)7 × 7 surfaces kept at constant temperature. Starting with clean S i ( l l 1)7 × 7, AI-induced overlaycrs having c~-7 x 7, f 3 × v ~ R 3 0 °, x/ff x ¢ffR19.1 ° and incommensurate structures, and an epitaxial layer of the A! film have been observed. The results of dynamical L E E D intensity analysis of the V~- x v~ structure [10] and comparison b e t w e e n the result here and that reported by H u a n g et al. [7] are presented. Structural study of the o t h e r ,~hases are also reported.

2. Experiments The experiments were carried out in a standard ultrahigh-vacuum ( U H V ) c h a m b e r equipped with four-grid L E E D optics. T h e base pressure of this c h a m b e r was less than 1 x 1 0 - ' " Torr. The results on all overlayers were obtained on n-t-vpe silicon wafers of high resistivity ( > 10 kl) cm),

0039-6028/92/$05 00 i-~ 1992 - Elsevier Science Pubhshers B V All rights reserved

996

K Ntstukata et al / Structural studies o r A l ~ S t ( I l l ) surfaces bv LEED

cut to within 1' parallel to the (111) plane. Prior to the deposition of aluminum, these wafers were carefully cleaned by repeated heating up to 1200°C and by slow cooling below 800°C. After such treatment, these samples show a sharp 7 x 7 LEED pattern. Aluminum atoms from a pyroritic boron nitride crucible were impinged on the Si(lll)7 x 7 surfaces kept at constant temperatree at a deposition rate of 1/240 M L / s e c (1 M L = 7.8 x 10 ~4 atoms/cm2), which was controlled by means of a quartz-crystal thickness monitor. The LEED observations were performed at room temperature. A fast L E E D system with a highly sensitive TV camera, a microcomputer, a large-volume memory and a magnetic optical disk were used to measure the I - V curves [11]. The data acquisition rate of this system is nearly equal to the TV rate. The intensity data of thirty frames were averaged at each energy to increase the reliability of data. The I - V curves were normalized to incident electron current after the background subtraction. The correction for the variation of the optical transparencies of four grids of LEED optics and lenses of the TV camera, with the direction of observation of L E E D spots, were performed.

3. Results and discussions

When the substrate temperature T~ was kept at 650°C during the adsorption of AI atoms, LEED patterns showed the 7 x 7, v~- x x/3-, ~ x x/ff and incommensurate structures with increasing amount of deposited Al atoms, as shown in fig. 1. The observed 7 x 7 pattern at this temperature comes from the clean S i ( l l l ) surface, because I - V spectra of this pattern are identical with those from the clean surface. The v~ x v~ phase and the incommensmate phase may correspond to the [3 and ~-7 × 7 phases observed by Lander and Morrison, considering the condition of the formation of these phases. At 300 < T, < 600°C, 7 × 7 patterns from a clean surface changed to those from an Al-induced 7 x 7 surface, whose I - V spectra is quite different from the clean 7 x 7 surface. This phase corresponds

O" 800 ; .... :~'~:~ ....... i" "'~'7"~" ........ i .............................

600 i

< Ill

• %'0

•

•

•

•

•

incommensurate

400

200

; ' " " ' i ............ • ...................

'x7

Aq111) ec-7x7

oO 03

I

I

I

I

I

05 COVERAGE

I

I

I, 10

[ML]

Fig. 1. Formation diagram of surface structures as a function of coverage and the substrate temperature• Solid circles are measuring points determined by LEED.

to the so-called a-7 X 7 structure observed by Lander and Morrison. With increasing amount of AI, the incommensurate phase appeared. At T, < 300°C, the a-7 × 7 phase changed to a A l ( l l l ) epitaxial growth phase with increased A! coverage. A very clear V~- × ~ phase can be observed when 1/3 ML of Al atoms are deposited at T, = 650°C. The I - [ curves of eleven different nonsymmetrical beams were measured for this structure. The normal incidence of the electron beams was confirmed by the agreement of l - V curves of three symmetrical beams. The LEED programs developed by Pendry [12], and Tong and Van Hove [13,14] were modified for a vector processor [15]. The dynamical L E E D calculations were performed for structural models in which 1/3 ML of Al atoms adsorbed on the Si surface. When 1/3 monolayer of Al atoms are deposited, two different types of three-fold symmetrical sites on S i ( l l l ) can be considered; sites above the Si atoms in the fourth layer (H 3 ~ites) and sites above the Si atoms in the second layer (T a sites). The best fit to the experiment was achieved with an optimized "1"4structure. Fig. 2 shows the corresponding unit cell of the T 4 structural model with eight structure parameters used to adjust the geometry in this study. Deep-lying Si atoms were held at their bulk positions. Fig. 3 shows the comparison between theoletical and experimental I - V spectra for the optimal structure. The reliability, factor (R factor) proposed by Zannazi

K Nlshlkata et aL / Structural studws o f A I / & ( l l l ) surface~ by LEED

[1To]

997

oO~ 0

[1121

Fig 4. The optimized Sl(lll)-(,,~ × V ~ ) R 3 0 ° - A I structure with AI {solid circle) on the T4 site.

h'

Fig. 2. Top and side viewsof the surface structuralmodel with Ai atoms (black circles)occupyingthe T~ site

and Jona [16] was calculated as 0.178 for this geometry. The values of the geometrical parameters of the optimal structure are shown in fig. 4. The obtained geometry of this surface is almost

....

/'~ /

' " " ' ....

' " "(2/'3'2i3) theor

\ A / \

( 2 / 3 2•3) expt

/~

( 1 / 3 1/3)

~

theor. (113 1/3)' expt.

//~

i. t¢ v

>I--

/~~..

z t.kl I.-

(0 1 ) theor.

(o ~)

/X

m

~k

expt.

.._~./~

/ ( 1 0 theor

!

iWL 0 Fig

50

,

I

J\_o, '

100 150 ENERGY (eV)

200

3. Comparison between theoretical and experimental

I- V curves for the optimal geometry

the same as the results reported by Huang et ai. [7]. In this case, however, the displacement of the second Si layer is rather small and is closer to the value predicated by Northrup [8]. The I - V curves of integral order spots of this reconstructed structure induced by the adsorption of Al atoms are similar to those for the Si(lll)v~- x v ~ - G a surface [15]. In such kinds of surfaces, the 1 - V curves of integral order spots stropgly depend on the geometry of the second and third Si layers; the I - V curves of the reconstructed layers without the adsorbed atoms, in which the geometric structure is the same as those induced by adsorbed atoms, are almost the same as those with the adsorbed atoms [17]. The main motive force forming these structures when the third column atoms are adsorbed on the Si(111) surface is the saturation of three dangling bonds of the topmost Si atoms. So the v~ structure of Al is considered to have the same geometries, especially the same substrate layers, as those of Ga. The resemblance of I - V curves of both the structures reflects this resuit. Fig. 5 shows comparisons of I - V curves of integral order beams between ~he different surface structures which appeared at the AI coverage of 0 < 0 < 1 ML at T, = 650°C. The resemblance of the I - V curves between the v~ x v~ and the ~ x ~/Y structures can be observed. This indicates that atomic coordination of atoms in the ~ - x ¢'7 structure is not so different from that in the v~- × vr3 structure. The results of the

998

K. Ntshlkata et al / Smlctural studies orAl~St(Ill) sur]aces by LEED

accurate analysis of the ¢ff x ¢ 7 structure will be described elsewhere. The incommensurate phase whose periodicities of the diffraction pattern are apparently similar to that of 7 x 7 was observed for the wide range of AI coverage, as shown in fig. 1. This structure was referred to as the 7 x 7 phase in the previous work [6]. Fig. 6 shows a LEED pattern and a LEED intensity line profile of this structure along the line indicated by arrows A and B in the L E E D pattern at 58 eV. The satellite beams about the integral order beams are shown. The direction of these beams is parallel to that of the S i ( l l l ) l x 1 surface. The satellite beams are produced by thc overlayer whose lattice vectors have similar length to those of the substrate and are related to those of the substrate by irrational numbers [14]. If these diffraction spots are caused by the A I ( l l l ) overlayer, whose lattice constant is smaller than +hose of the Si(111) surface, the (1 0) spot of this overlayer should

x L ' A

VV (0 1

Y"--, / ' \/W//,

1) /~ /--

.,/~ ,.i

I

r

iI

",,

' '

'

f'%;\ t' ',

,.oom, F 0 50ML] V~Xs/71

' I

(o l)

(3 3,3r,lL , ~/3 X',,/3 I

/ (0 1) / / " ~

-'

[1 O)

""

/

I-

clean 7X7 '\,

Z HJ

"

Z

(~ O) /

r-

.

.

.

.

f "\ ' i'

(1 O)

/ , ,r, /

0

0 50ML ",/7X~/7

~

0 33ML •d 3 X ' , / 3

i ,

l

.

(1 O)

,

.~ ,' \

; ,

,.___

,,/' ,, ~j/

.

tJ

0 83ML Incom

t

/ ..' 50

clean 7X7

,, . . . / ' , , ~ - , _ ~ 100 ENERGY (eV)

.... r~ 150

..... 200

Fig 5. Comparison of I - V curves of the mtegral order beams

between different structures appearing at 7-, = 650"C

/B

A/ (1 0 )

•~

a

A

(TO)

a

relative scattering vector

B

Fig 6 L E E D pattern a n d intensity line profile o f the incommensurate phase. Arrows in the upper part indicate a direc-

tion of the line AB taking the intensity profile. Arrow in the lower part refers to the shadowof a specimen holder.

appear at the outside of the (1 0) spot of the S i ( l l l ) surface. In this case, the beams (a) indicated in fig. 6 are considered to be the single diffraction beams from this overlayer The length bctween this beam (a) and the specular beam is about 1.11 times as long as those between the (1 0) beam of Si(111) and the specular beam. This does not correspond to those of the AI epitaxial growth on the S i ( l l l ) surface, whose length is 1.34 times as long. T h e diffraction pattern of the AI epitaxial growth can be obsewed at T, < 300°C when more than 0.8 ML of A1 atoms are deposited, as shown in fig. 1. So, the diffraction . . . . . . . I"..... ;'~ fig. ~, i,. nc~t caused bv the simple A I ( l l l ) epitaxial layer, but by the incommensurate layer whose structure is rather complex. The e~-7 x 7 phase has the periodicity of exactly seven times as large as that of a bulk 1 x 1 structure, but the intensity of each corresponding beam is quite different from that of the clean 7 x 7 surface. The examples of intensity line profiles at 43 eV are shown in fig. 7. Different features of line profiles can be observed, showing

K. Ntshtkata et al / Structural ~tud~es ofAI / S,( l l l ) surfaces by LEED

T)

(¥1) e

t

t

A

r"

...J

(1:1-)

o¢-7x7

(1"1)

m >.,

7x7

Of) r"

999

of integral order beams between the ~ x v~and the v/ff x vcff structures indicates that the atomic geometries of the substrate layers ot these structures are almost idenucal to each other. An incommensurate structure has large reconstructions and misfits. A dynamical LEED intensity. analysis of a v ~ x ¢3 structure was carried out. AI atoms were adsorbed on the threefold-symmetrical sites above Si atoms in the second layer. The resemblance of I - V curves of integral order beams between the ¢3 x vr3--Al and V~ x v~-Ga indicates the similarity of these geometries.

c_

References

relativescatteringvector

Fig. 7 Compansnn of intensity line profile~ between the Si(111)7x7 structure and the AI-induced (x-Tx7 ~tructure. Arrows refer to the shadow of a specimen holder

the difference of intensities of the fractional order beams, especially of (v4-2r v). At the coverage between 0.25 and 0.8 ML, and at a temperature between 400 and 600°C, the a - " × 7 phase was changed to the incommensurate V,'~.,~e. This phase transition occurs at lower t e m p - ature when the coverage is higher. If desorption of AI atoms occurs, this transition occurs at higher temperalure when the coverage is higher. The tendency of occurrence of this transition ~s not caused by desorption of AI atoms but by the necessity of the high transition activation energy. Th~s ':nd;.cates that this phase transition accompanies large reconstructions and misfits.

4. Conclusion Al-adsorbed Si(111) surfaces have been studied by observing I - V curves and intensity line profiles of LEED. The resemblance of I - V curves

[1] T. KmosMta, S Kono and T. Sagawa, Phys Rev B 32 (1985) 2714. [2] T. Kmoshita, S. Kono and T Sagawa, Sohd State Commun. 56 (1985) 681. [3] G.V. Hansson, R.Z Bachrach R.S Bauer and P. Chlaradia, Phys. Rev. Lett. 46 (1981j 1033 [4] R I G Uhrberg, G V Hans,,on, J M Nicholls, P E S Persson and S.A Flodstr6m, Phys. Rev B 31 (1985)3805 [5] R J. Hamers, Phys. Rev B 40 (19,',9) 1657 [6] J.J. Lander and J Mornson, Surf. Sc= 2 (1964) 553 [7] H. Huang, S Y. Tong, W S Yang, it D Shin and F Jona, Phys Rev B 42 (1990) 7483 [8] J.E Northrup. Phy,, Rev Lett 53(1984) 683 [9] J.S. Nelson and I P Batra. Metalllzatmn and MetalSemmonductor Interface,,, Ed 1 P Batra, Vul 195, NATO ,aC,l ',cNc,, B t'Plenum Ncw Yerk, lU,~9) p 5~ [10] A Kawazu, K NIsh&ata, M M,dsumoto, M Yoshtmura and H. Sh]gekawa, read at the 441h annual meeting of the Phys. Soc Jpn, Kago~hlma, Oct 198% 5p-T 6 (advance abstract, Vol 2, p 414). [!1] K. Nv,htkata, M Matsumoto, Y Endo, H. Shlgekawa and A. Kawazu, 1 Surf Scl Jpn. 10 (1989) 47 [12] J B. Pendry, Low Energy Electron Dfffractmn (Academic, London, 1974) 1131 M A. Van Ho~c and S Y Tong, Surface Crystallography by LEED (SpT, ~cr, Berhn, 1979) W.H. Wemberg and C E Chan, Low [141 M.A Van ttv Energy Electn D]ffrachon (Springer, Berhn, 1986)

[:5]

A

KaWaZI.I

,~,~"

[161 E Zannazl a• [171 A Osakl, K verstt~, of ToM

otto(tiller,

i ~ v

J

t

i

.o,~t

Jona, Surf Scl 62 (1977) 61 ~uchl, undergraduate thesis, The Urn; 982 )