Si(100) by LEED

bL-L Murakaod el at / S/rutTtlral ~tullie~oral/SitlO0) b) LEED

147

(

RELATIVE SCATTERING VECTOR Fig, 4. Tile streaky 2 ~<3 LEED I~~aern at a primary energy tlf 64 eV (a) and ils inlensL line profile from ( - I ~) to ( I ~) spills (b), Arrows in [a) ~ihowits diieclilm. Three-times periodicity peaks are shown 119 a r r o w s in (bL

culty in making a structural model of an AI-adsorbed 2 x 1 surface. A unit cell of the clean 2 × l surlace has two dangling bonds, and an Al atom has three bonds. Thus, it is difficult to make au AI-induced 2 x 1 structural model below about 0.3 M L deposition of At. 3.2. 2 x 3 phase Fig. 4 shows (a) the L E E D pattern of the Sill00)-2 × 3 structure with the streaky spots and (b) its intensity tine profile. In this L E E D pattern and line profile, the periodicity of three times that of the I × I structure can be observed along [011] and [01 - I ] directions, lde et al. [2] have suggested structural models of these 2 x 2 and 2 × 3 surlhces. In their 2 X 2 structural model, one AI-dimer exists on the top of the surface within a unit mesh of the 2 x 2. In this model, each AI atom of the dimer combines with two Si atoms in the underlayer, thus saturating the dangling bonds of these Si atoms which form the dimer in the 2 × I surface (fig. 5a), In this m o d e l AI-dimer rows are perpendicular to Si-dimer rows, and this is consistent with the result of the STM observation [8]. lit the 2 x 3 structural model, the Al-dimer rows are arranged in every third Si-dimer (fig. 5b). In the 2 × 3 L E E D pattern of our experiment, the streaks are in the three-times periodicity direction. This suggests that Abdimer rows arc not arranged perfectly in every third Si-dimer at the coverage shown in fig. 5a.

3,3. 4 x I. 7 x I p[iase At 0 = 0.33 M L and 7~ = 350°C, at which the existence of the 4 x 5 L E E D structure had been reporte~l in rof [2], we c,si~]d ~nly observe (0 ~' (0 ¼) spots, but not iO n / 5 ) spots, showing the existence of only the 4 >¢ 1 structure. At 0 = 0,33 M L and T, = 450°C, at which the 7 × 1 L E E D pattern had been observed [2], we could also observe spots with seven-times periodicity of the bulk surface unit cell on a line between the (0 - 1) and (0 1) spots.

Fig. 5. Structural mt]de]s tff 2 × 2 (at and 2 X 3 (b) sur f~ces. AI

adaloms arc shown in black.

148

K..L Murakami el at. / Structllrtd stu,lit,s orAl/Si(lOOt br LEED

3.4. h i - 2 X 2 p h a s e

We have also studied ~he St(100)2 x 2-In surface, which is suggested to have the same structure as that of AI-2 x 2 surfaces [2,3]. Fig. 6 shows a comparison of the I - V curves of the integral- and fractional-order spots between AIand In-2 x 2 phases. For the I - V curves of the corresponding integral-order spots, which are considered mainly to reflect the Si substrate structures, the peaks are observed almost at the same positions except for the (1 2) spot, though the peak heights differ somewhat between them. The corresponding I - V curves of the fractionalorder spots, which mainly reflect the structure of the topmost layer, resemble each other closely. The 1-11 curves of the (½ ½) spot, which appear to be very sensitive to the position of the adatom dimers, are very much alike. These results show ~.nat the positions of the AI- and ln-dimers arc o3nsidercd to be almost the same. T h e resemblance of I - V curves of these two phases sup-

°l ~.~

(o i/2)

AI-2x2 6S eV

10 112)

In.212

....

RELATIVE SCA'FrERING VECTOR

Fig. 7. The inten~ilyllne profiles fnlm ( - l ~) to ( I ~) spols of the AI- and In-2× 2 LEED patterns. ports the proposal that the structure Ior the AIand in-2 x 2 are considered to be identical [2]. Fig. 7 shows the intensity line profiles across the ( - 1 t2) and (1 ½) spots of the AI- and In-2 x 2 L E E D patterns, Since the F W H M of the ( t ~) spot of the AI-2 × 2 is iarger than that of'tile In-2 x 2, the main size of the AI-2 x 2 domains is considered to be smaller lhan that of the ln-2 x 2. This tendency was observed over the entire region of the 2 x 2 structure investigated. This shows that the lower diffusion coefficient or higher reactivity of AI atoms with silicon surface than In atoms, which results in a formation of a two-dimensional nucleus with higher density and smaller size of each domain.

4. Concgusion

o

so

1oo

15o

200

E N E R G Y (eV) Fig. 6. Comparison of the I - V cuwcs of the integral- and fractional-order spots between the AI- and ]n-2×3 structures. Solid lines and dashed lines represent the [ - V curves of the

AI-2 x2 and [n-2x ~- respeclivels.

We have studied surface structures of the AISt(100) system by observing I - V curves and intensity line profiles of LEED. " T h e Al-adsorbed 2 x I structure", whose I - V curves are different from those of the Si(100)2x 1 structure, is not considered to actually exist. For the 2 x 3 phase the streaky L E E D pattern was observed, and it is probably because Al-dimcr rows arc not arranged perfectly in every other Si-dimer. The resemblance of I - V curves between the AI-2 × 2 and

K-L Murakami el aL / Stmcnlra/ studies ofAI / Si(lO0) by LEED

with the increase in the amount of A1 atoms. We investigated these structures by considering I - V curves of L E E D spots.

~eoo F 5~o ~ tu gl~ 400 I,U;~

2. Experimental The experim-nts were carried out in a standard ultrahlgh-vacuum ( U H V ) chamber equipped with four-grid L E E D optics. T h e base pressure of the chamber was less then 1 x 10- "~ Tort. The pressure during the deposition was kept below 3 × 10 ]'~ Torr. The results ou all overlayers were obtained on nqype silicon wafers of high resistivity ( > l0 k [ l ' cm), cut to within 1' parallel to the (100) plain. Prior to the deposition of AI, these wafers were carefully cleaned by repeated heating up to 12fi0=C and by slow cooling below 800=C. After such treatment, these salaples show a sharp 2 x 1 L E E D pattern. AI atoms from a pyroritic boron nitride (PBN) crucible were impinged on the Si(100)2 x 1 surfaces kept tit constant temperature at a deposition rate of 1/360 M L / s (l M L = 6.8 x 10 TM atoms/cm2). After the shutter was turned off, these samples were kept at the same tempcrature for 30 s. T h e L E E D observations were performed at room temperatufa. A L E E D system with a highly sensitive T V camera, a microcomputer, a large-volume memory and a magnetic optical (MO) disk was used to measure the I - V carves [12]. The data-acquisition rate of this system is nearly equal to the T V rate, T h e intensity data of thirty frames were averaged at each energy to incr¢ase the reliability. of data. The I - I / curves were normalized to incident electron current, and correction for optical transparencies of the four grids of the L E E D optics and lenses of the T V camera was performed after the background subtraction. The normal incidence of the electron beam to the Si substrata surfaces was examined by observing the identification of the I - V curves of symmetrical integral-order beams. 3. Results and d i s c u s s i o n Several different surface structures have been observed for the Al/Si(100) system depending of

diffuse 2xl * *

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o o.1 0.2 0.3 o,4 o.~ 06 COVERAGE (ML) F i g I. Formation diggram tit surface structures as a functinn el coverage and the substrata temperature. Stflid circle~ are measuring poi•ts determined by L E E D .

thc coverage (0) and substratc temperature (T,), as shown in fig. I. The AI coverages were calibrated, at which most clearly an AI-2 × 2 L E E D pattern appeared, to be 0.5 ML [2]. At 7..,< 200°C, a two-domain 2 x l L E E D pattern from the Si(100) surface changed into a two-domaln AI-adsorbed 2 x l L E E D pattern with increasing Al coverage, which resulted in a t~'o-domain 2 x 2 L E E D pattern at higher coverages of AL A t a temperature of 200 < T, < 300°C and a coverage of 0.2 < 0 < 0.5 ML, thc two-domain strealey 2 x 3 L E E D pattern was observed, which was also confinned by taking its line profiles from the ( - l ½) to the (1 ½) spot. At 7~ = 350°C and 0 - 0.33 M E at which the existence of a 4 x 5 pattern has been reported [2]. we observed only a poorly ordered 4 x l L E E D pattern. At 7~, = 450°C and O = 0.33 ML. a poorly ordered 7 × 1 L E E D pattern was observed. 3.1. 2 × 2 p h a s e

Fig. 2 shows the L E E D patterns (a) from the two-domain clean 5i(100)2 × 1 surface, (b) the two-domain Si(100)2 x 2-AI surface, and (c) the intensity line profile of the 2 X 2 pattern along the line A - B indicated in (b). As can be seen in fig. 2c, the full width at half-maximum ( F W H M ) of the (½ ½) spot is larger than those of the (1 ½) and (0 ½) spots. This can be explained by considering the domain structure of the AI top layer on the well-ordered Si substrate layer with 2 x 1

150

K-L ilhlrl~kaltli a =/,i. / SlrLl(tetralsttuties oral~ Sd I1~0)hv LEED

observed lhat I - V curves of the clean 2 X 1 structure change into those of "the AI-iuduced 2 x 1 structure" with different features, and finalh., turn into the 2 × 2 structure. This continuous change of the I - V curves with coverage leads to two different explauations. One is that "the A[-induced 2 X I structure" exists, but the continuous change is due to the amount of the two domains perpendicular to each other, l'his means that the rate of each domain of "the AI-induced 2 × 1 structure" changes in accordance with the increase of AI coverage. Thus, the I - V curves, whose features are rather simular to those of the St(101))2× I surface, change continuously. The other explanation is that "the AI-indueed 2 x 1 structure" does not actually exist. We support the latter onc because of the following evidence, e.g., the result of the STM observation by Nogami et al. [8], who reported that AI adatoms form dimer rows perpendicular to Si dimer rows. And furthermore, there is the diffi-

(a)

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RELATIVE SCATTERING VECTOR Fig. 2. LEI:.D pattcrn~ and the inlen,~ityllnc profile at a primary energy of 69 eV; (a) the clean 2× I LEED pattern. (b) the 2×2 LEED pattern, (c) the intensityline profile from ( - I ~) to ( 1 ~) spots of the 2 >:2 LEED pattern. The arrows !,tm',~its direction.

structure. For such a surface, the integral-order spots, and fractional-order (n i ) and (½ n) spo(s, where n denotes the integer, mainly reflect the geometry of the well-ordered substrate layers including the Si-dimer layer, whereas the ((2n + I ) / 2 (2n + 1)/2) order spots reflect the geometry of the Al layer with less-ordered or antiphase 2 × 2 domain structure. Hence the observed result of the larger value of the F W H M of ( i ~) beam can he realized. Fi[ 3 shows the change of the I - V curves of (1 1) and (½ 1) spots with a coverage of AI at 10tl°C of substrate temperature. It can be clearly

j

0

50

:

:

100 150 ENERGY (eV)

0 17ML

200

Fig. 3. Comparisonof the r _ V curvesof the ( I ] ) and ( ~ 1) spots for various coverages. The sub~tral¢temperature during the dcpnsititm is I~}°C.AI cl+vcragcsare I). 11.17.0,28 and 0,5 ML.

K.-£ Murukunti et aL / Structural studies of ,41/ Si( lO01 by LEED

t h e I n - 2 x 2 p h a s e s i n d i c a t e s t h e similarity o f t h e a t o m i c g e o m e t r y o f t h e s e surfaces.

Acknow[~dgemen|s T h e a u t h o r s wish to t h a n k DI". N. A k i y a m a of K o m a t s u E l e c t r o n i c M e t a l s Co. for s u p p l y i n g o f t h e siticon wafers.

References [I] J.J. Lander and J. Morrison, Sure Sci. 2 (!gr4) 553. [2] T, Ide. T. Hi~himeri and T. tchinokawa, Sttrf. Sci. 289 (19891 335.

151

[3] J, Kn:dl. J.-E. Sundgrcn. G.V, Hansson and J.E. Greene, Surf. Sci. 166 (1986) 512. i4] T. Sakamoto and H. Kawanaml. Surf, Sci. 111 (198!) 177. [SJ B. Bourguigaon. K.L. Carlcton and S,R. L~anc, Surf, Sci, ~}4 (1988) 455. 16] B, Btmrsuignon, R.V, Smilgys and S,R. Leone. Surf. ScL 2(}4 (19861 473. [7] S. Shimizu and 2, Komlya. J. Vac. 2cl, TechnoL 18 (1981 765, [2[ J. Nogami, A.A, Baski and C,F, Ouate. Phys. Rev. B 44 (1991) 1415. [9l J. Nogami, S.-I. Park and C.F, Quate, AoPl. Phys, Left, 53 (19281 2086. Illll A.A. Baski. J. Nngami and C,F. Ouate J. Vac, Sci, Tcchnol. A 2 (!9901 245 fill A.A, Baski, J. Nogami atad C.F Ouale. Phys. Rev. B 43 ( 199D} 9316,

[121 K. Ni~hikala. M. Malsumoto. Y. Ertdo, H. Shisekawa and A. Kawazu, J. 2ur£ S¢i, Soc, Jpn. 10 (1989) 47.