Growth of Si on Au deposited Si(111) surfaces studied by UHV-REM

Applied Surface Science 6(I/61 ( 1992t 107-11 I North-Holland

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Science

Growth of Si on Au deposited S i ( l l l ) surfaces studied by UHV-REM H . M i n o d a , Y. T a n i s h i r o , N. Y a m a m o t o a n d K. Y a g i Ph~ics Department, Tokyo ltlstiotte Of TechnologY. Oh.oktz~ao!a, Megltro. Tokyo 152. ]anu~l Received 211 November 1991; accepted for publication 30 January 1092

Growth of Si on Au-deposited St(Ill 15 × 2 surfaces ~,as studied by ultra-high-vacuum reflection eleclron microscopy, Above 40ICC RHEED sho~ed the 5 x 2 patlcrn during the Si deposition indicaling Ihal the AU deposits stayed at the top surface during depositinn, Suppression of t~o-dlmensional nucleation of Si to some extent was noted, preferential nuelealion of 2D island~;at orientalioual domain boundaries of the 5 × 2 structure ~ , obse~ed.

1. Introduction A recent interesting problem in studies of epitaxial growth is surfactant-medlat•d epitaxy [1 3]. In this case growth is on a surface which has been pro-covered by a so-called surfactant material. During the deposition of the other materials the surfactant always stays at the top surface, which indicates that the growth proceeds "'below" the surfactant. This type of epitaxial growth is technically very important, because it may reduce the transition temperature between 2D growth and step flow growth modes and may suppress the 3D island formation which occurs on the surface without the surfactant. It is believed that the surfaetant reduces surface energies of thc substrate and the overgrown material (which may be the same material in the case of homoepitaxy as in the present study). Another important point is that during growth the substratc temperature must be high enough so that bond breaking between the surfaetant and the substrate (or the overgrowth) and rebonding between the surfactant and the overgrowth can take place. The atomic processes of this type of growth, however, have not been made clear. We have observed growth of Si and Ge on a Si(I 1 l)5 × 2-Au surface by reflection electron microscopy (REM). The 5 × 2 structure transforms to the disordered 1 x 1 structure at about 750°C

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[4,5] and adsorbate Au atoms disappear from the surface above 800°C. This indicates that the energy for the bond breaking between a Si and a Au atom is rather large. Thus, Au may not be a good candidate of the surfactant on Si. However, there are several reasons for the choice of Au: ( l ) a small amount of Au deposit does not diffuse into the bulk even at high temperatures; (2) a Au deposit transforms the complicated 7 x 7 structure to the rather simple 5 × 2 structure with a low Au coverage of about 0.5 monolaycr (ML) [6]; (3) the 5 × 2 structure introduces new surface defects such as domain boundaries, which may act as preferential nucleation sites of 2D islands; and (4) orientational relations between the 5 × 2 domains on the 2D islands and the 5 × 2 domains of the substrate in the case of Si deposition are interesting problems and such restructuring processes are important in surfactant-mediated growth. The present R E M study shows some details of these growth processes. Here, the case of Si growth is reported. T h e growth of Ge will be reported elsewhere [7].

2. Experimental An ultra-high-vacuum electron microscope equipped with an evaporator for metals and an

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H. Mmoda e~ at, / Growlfi of Si ~mAo dcl~iwd SiO II)

Fig. 1, R H E E D p a l t e m s taken (a) h¢l'nre and (h, c) during deposition of $i u n it Si(I I I}.~ x 2-Au surface at 500°C: (b) 1.5 ML, (c) 3.0 ML. Note that the SpOls and streaks pattern from the 5 x 2 structure are iliWily~. S¢¢ll.

Fig. 2. R E M images of a S i ( l l l l surface taken lal before and (b} after depositiol~ of Si on Ihc 5 x 2 structure at 730°C. NOle changes ol" step configurations and a wide terrace without 2 D nucleus.

H. Minoda et a£ / Growth of Si on Au deposited Si( I I D

e-gun type evaporator for Si [8] was used. Sample crystals (7 x I × 0.35 mm 3) were cut from a p-type (B-doped, a few ft- cm) wafer. They were chemically cleaned, rinsed, clamped between the two electrodes of a specimen holder of the microscope and cleaned by D C current heating in the microscope, Au was oepogited on the 7 × 7 surfaces at around 700°C and the 5 × 2 adsorbate structure was formed. Then, the specimen temperature was changed to a desired one and Si was deposited with a deposition speed of about 0.10.5 M L / m i n .

3. Results

and

discussion

Fig. I shows R H E E D patterns taken during the deposition of Si at 500°C on a Au-depositcd

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S i ( l l l ) surface. The incident electron beam direction is [112]. In fig. la taken before the deposition, spots from the 5 × I Au-adsorbed structure indicated by arrows are seen. Diffuse lines indicated by arrow heads indicate that the adsorbgtc structure is the disordered 5 x 2 structure. Weak spots from 7 x 7 structure regions not covered by Au deposit are also seen. Figs, lb and lc were taken after deposition of 1.5 and 3.0 ML of Si, respectively. It is noted that soots and the diffuse lines from the 5 x 2 structures are seen in figs. lb and le. This indicates that the Au atoms were always on top of the surface, indicating that growth of Si took place below the adsorbate structure. It was reported that the atomic density of Au in the 5 × 2 structure is about 0.5 ML [6]. Therefore, the word "below" does not really mean below.

Fl~. 3 I~E M images of ta) a Si( 111 )7 × 7 surface and (b) a Si( I I I )5 × 2-Au surface, on which Si was deposited al fl00~C with the

sa~,~edeposition rate. Notlcc the difference in the distances of the nuclei from the glens in the two images tsee the text).

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It..1farads et aL / Grotl lh o[ Si an Au deposited Sill III

I[ should be noted that the 5 × 2 R H E E D pattern was always seen during the Si deposition at temperatures between 400 and 730"C. Fig. 2a shows a R E M image of a Si(111)5 × 2 Au surface. Steps on the surface and surface structure domains (bright and dark contrast) arc seen, In fig. 2b taken after the deposition of about 3 ML of Si at 73tPC, changes of the slep configuration arc seen, indicating the step fiow growth mode. The bright contrast domains disappeared from a wide terrace, This means that one of the 5 × 2 structure domains (dark contrast) covered the surface, which accompanied the step flow. Black dots seen on tile terraces and along the steps are contaminations and are not 2D nuclei of Si. A wide terrace in the image is as wide as 2 x 8 p-m-' and a notable point is that no 2D nucleus of Si is formed, This indicates either that the diffusion distance of Si on the 5 x 2 structure is large or that nucleation e,f 2D islands is reduced on the 5 x 2 structure (large critical nucleus). Fig. 3 compares the nucleation behavior on the 7 X 7 structure with that on the 5 x 2 structure at 611O°C under the deposition of about 0.1 M L / m i n . In fig. 3a the nucleation of 2D islands is seen on terraces, which is indicated by arrows. These sites are about 0 . 5 / t i n from the nearest steps. In fig. 3b nucleated islands are seen at places more than 1 / t i n away from the nearest steps as indicated by arrows. No nucleus is seen on a terrace indicated by A. which is as wide as 2 × 5 tzm 2. The sticking probability of Si estimated from the step flow speed was found to be almost Ihe same in both cases. These facts, again, indicate either a longer diffusion distance of Si atoms on the 5 × 2 structure, or a reduced nucleation on the 5 x 2 structure compared to the 7 × 7 structure. It should be noted that such a difference in nucleation behavior on the two surfaces was not noticed clearly below 500°C. Fig. 4 shows a R E M image taken during Si deposition on the 5 × 2 surface at 6O0°C. Nuclei of 2D islands, indicated by arrows, are seen. It is noted that the nuclei A and B are formed on a boundary of slightly different contrast regions on the same terrace. It is a domain boundary of the 5 × 2 structure which can take three equivalent

Fig. 4. A REM image of a $i( I II )5 × 2-Au surface tm which Si was deposited at 61~l~C.Notice the nucl~ntiun of 2D islands indicated by arrmss at Ihu domain boundary on a telrace.

orientations on the S i ( l l 1) surface. The diffusion distance under the present deposition conditions is about 1.0 /,tin and is larger than the distance between A and B. This means that the domain boundaries act as preferential nucleation sites.

4. Discussion Si was deposited on S i ( | l l ) 5 × 2-Au surfaces. D i n i n g the deposition R H E E D always showed patterns from the 5 × 2 structure, which indicates that Au atoms are always on the top surface forming the 5 x 2 adsorbate structure. When Ge was deposited on the 5 x 2 structure, the segregation of Au to the surface was also observed as in the case of Si deposition [7]. R E M images showed that above 500°C the nucleation behavior of the 2D islands on the terraces depends on the surface structure; distances of the nuclei from the nearest steps are larger on the 5 X 2 structure than on the 7 x 7 structure. This indicates either a longer diffusion distance on the 5 × 2 structure or a reduced nucleation probability on it than on the 7 × 7 structure. The reduction may be due to larger critical nucleus on the 5 x 2 structure (or "'below" the 5 x 2 structure). Another possible explanation is the lower sticking probability of Si on the 5 × 2 structure than on the 7 × 7 structure. However, from the measurements of the step flow speed the sticking probability was found to be almost the same on two surfaces.

1t. Minoda et al, / (;rowlh o f Si on A u deposited Sit I I D

The nucleation of 2D islands at domain boundaries of the 5 × 2 structure wag noticed. The 5 × 2 structure should have out-of-phase boundaries (OPg's) [9] as well as domain boundaries between different oricntational domains. Preferential nucleation of the 5 x 2 adsorbate at the OPB's of the 7 × 7 structure was reported [10]. Preferential nucleation of Si 2D islands at OPg"s of the 7 X 7 structure was also reported [8]. At present we do not have clear evidence of the nucleation of the 2D islands of Si at the OPB's of the 5 × 2 structure. It should I~e noted here that the adsorbate forming lots of defects on the surface such as domain boundaries and OPB's which act as preferential nucleation positions may not be a good surfaetant because preferential nucleation at these positions de-accelerates the step flow growth, it is interesting to know whether the nucleation is on or :'below" the 5 × 2 structure. ]t was observed that surfaces of the 2D islands are of the 5 × 2 structttre. Details of changes of domain structure during Si depositions and domains on tile 2D islands will be reported elsewhere [7]. As mentioned in section 1, Au may not be a good surfactant probably due to the relatively strong bonding between Au and Si. This may be a reason why the nucleation behavior is similar on the 7 × 7 and 5 × 2 structures below 500°C. Studies using other metals are needed.

Aelmowledgements Thc present work was supported by a Grantin-Aid from the Ministry of Education, Science and Culture of Japan (No. 036{19501).

RefePenees [I] M. CopeL M,C Reuter. E. Ka~;irasand R,M. Tromp. Phys. Rcv. D:';. 63 II991t 635. [21 K, Fukulani. H. Daimon and S. lop, i.: The Snuclur¢ of Surfaces Ill. Ed,. S,Y, "Conget al. (Springer. New York, 1991) p. 615.

[3] S. Iwanari. Y. Kimura. K. T~,hayun~gi, Presenled at the Spring M¢~llngof Ih¢ PhysicalSK~:i~:;yof Japan (1991). 14l S, lno. Jpn. J, Appl. Phys. i6 (1977)8Y1. I.sI N. Osakab~:, "/. Tani~hiro. K. "Yagland G, Honitl, Surf, Sci. 97 (1980}393. 16l H. Dailnon. C, Chung, S. ]no and Y. Watanabe,Surf. Sci. 235 ill)gO) 142. [7] H. Minoda. Y, Tanishiro. N, Yamamotl~arid K. Yagl,in preparation. [8l M. Shima. K, Kobayashi. Y. Tanishim a,0 K. Yagi, L Cryst. Gro,,~'th 115 ( 1091) 35(L [9l K. Yiigi. IN. Osakab¢. Y. Tanishlro and G. Honio. in: Proc. 4th ICSS. Vol. 2 (198(0 p. 1007. [1O] Y. Tanlshiro and K. Taka~.'anagL Uaramicro~c~py 31 (1989) ~L