Si(111) “γ-phase”

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surface

science “. j.

Surface Science 316 (1994) L1034-L1038

Surface

Science Letters

LEED-AES reexamination of the AI/Si ( 111) “y-phase” A.V. Zotov *, E.A. ~ra~tsova,

S.V. Ryzhkov, A-A. Saranin, A.B. Chub, V.G. Lifshits

Institute of Automation and Control Processes, Far Eurtern Branch of the Russian Academy of Sciences, 5 Radio Street, 690041 Vladivostok, Russian Federation Received 31 May 1994; accepted for publication 21 June 1994

Abstract The saturating Al adlayer grown on Si( 111) at 500-700’~ (so-called Al/Si ( 111) “y-phase”) was characterized by means of low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The formation of the “y-phase” was found to complete upon adsorption of 0.6 monolayers of Al after which the sticking coefficient of Al drops abruptly and no additional Al can be adsorbed at temperatures above N 500°C. The precise examination of the LEED data revealed that the Al/Si( 111) “y-phase” has an 8 x 8 periodicity.

1. Introduction Surface reconstructions induced by Al on Si ( 111) were studied first as early as in 1964 by Lander and Morrison [ I 1. However, the interest in the system has remained over the years. It is commonly adopted now that aluminum forms several ordered surface phases on the Si ( 111) surface depending on substrate temperature and Al coverage [ l-91. These are the a-7 x 7, & x a, fi x fi and the so-called “y-phase” (or y-7 x 7 according to Ref. [ 1 ] ) . In contrast to the other phases of which the structure and composition have been elucidated sufficiently well, the “y-phase” remains mysterious to a great extent. There is a lack of consensus even about the periodicity of this phase: In the pioneering low-energy electron diffracv Corresponding author. Fax: +7 423 2 310452; E-mail: sm~iapu.ma~ne.su.

tion (LEED) study of Lander and Morrison [ 1 ] and in the later scanning tunneling microscopy (STM) study of Hamers (61, the 7 x 7 periodicity was reported. However, more recent papers have cast some doubt on the periodicity of the “y-phase”. According to the LEED data of Nishikata et al. [ 71, this phase is incommensurate. In contrast, Yoshimura et al. [8,9] concluded from the STM data that the phase is commensurate with a periodicity of 9 x 9. The high-resolution STM images of Refs. [8,9] have revealed that characteristic triangles with about 9 periodicity (and in a few cases with 5 periodicity) cover the surface, each of which is separated by misfit dislocations. As for the composition of the “y-phase” (i.e. the Al coverage corresponding to this phase), it is agreed conventionally that the phase is observed starting from about 0.5 ML up to a saturating coverage of N 1 ML (1 ML = 7.8 x lOI atoms/cm2). It should be outlined, how-

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A. Vi Xotov et al. /Surface Science 316 (1994) L1034-L1038

ever, that, in previously reported works [ 4,6,7 ] ) the Al coverage was controlled by means of a quartz-crystal thickness monitor under the assumptions that sticking coefficient equals unity and that it is independent of substrate temperature and Al coverage in the submono~ayer range. These assumptions seem to be rather hazardous. Say, they do not hold for Al/Si ( 100) [ IO] as well as for Al/%( 110) [ 111 submonolayer systems: In both cases, no more than about OS ML of Al adsorbs on Si surfaces at temperatures above 600°C indicating that the sticking coefficient decreases abruptly beyond a 0.5 ML Al coverage. The remains of the above-listed uncertainties regarding the structure and composition of the AX/Si( 1 f 1) “y-phase” have stimulated us to undertake a thorough analysis of this phase by means of LEED and Auger electron spectroscopy (AES). The obtained results have shown that it has a periodicity of 8 x 8 and a saturating Al coverage of 0.6 ML.

Experiments were carried out in an ultrahigh vacuum chamber with a base pressure of 2 x IO-to Tot-r equipped with a cylindricalmirror AES analyzer and LEED optics. The substrates used in this study were B-doped 10 f&cm Si ( 111) single-crystalline wafers. An atomicallyclean Si ( 111) surfaces were prepared in situ by direct Joule heating to 1200-1250°C. After this treatment, no impurity was detected by AES and a sharp 7 x 7 LEED pattern was displayed by the samples. Al was deposited from an Alcoated tungsten filament at a desired rate from 0.05 to 0.5 ML/mm. fn most experiments, the deposition of Al was carried out onto a Si( 11 I ) sample heated to the desired temperature. However, in some cases, the formation of the ordered Si ( 111 )-Al surface phases was studied upon annealing of an aluminum layer deposited at room temperature (RT 1.

f::k, 0

)

,

,

,

4 6 8 10 Al depositian time f min. f

2

Fig. 1. Ai LVV (68 eV) to Si LVV (92 eV) Auger peak height ratio as a function of Al deposition time and substrate temperature. The deposition rate is 0.35 ML/min.

3. Resnfts and discussion To characterize the Al adsorption onto the Si ( 111) surface the Auger peak ratios of Al LVV (68 eV) to Si LW (92 eV) were measured as a function of Al deposition time and substrate temperature. The results of these measurements are summarized in Fig. 1, showing adsorption curves for the Al/Sit 111) system at RT, 400 and 600°C At RT deposition of Al, the sticking coefficient equals unity and the growth proceeds in a nearly layer-by-layer fashion. The RT growth of Al seems to be accompanied, however, by a slight segregation of Si on top of the Al layer as evidenced by the remains of a small Si LVV Auger peak even after prolonged Al deposition (up to 50 ML) onto Si( 111). For the Al adsorption at the higher temperature of 4OO”C,the growth mode changes to the Stranski-Krastanov type: At the initial stages of Al deposition (up to 2 min deposition time), the two-dimensional (2D) layer forms and the ad= sorption curve for 400°C coincides initially with that for RT deposition. Upon completion of the 2D layer, the formation of the bulk Al islands starts as indicated by the break in the adsorption curves and the appearance of the diffraction spots co~esponding to en&axial Al( 111) domains at the LEED patterns.

A.V. Zotov et al. /Surface Science 316 (1994) LlO34-L1038

As the temperature is further increased, the adsorption curves show apparent saturation. This is illustrated by the curve for 600°C. The curves for the adsorption at 500 and 700°C are essentially similar. This observation is interpreted as a sign that, upon completion of the 2D layer, the sticking coefficient drops rapidly to zero and no further Al adsorption occurs at these temperatures. To prove this suggestion, the time dependence for the Al KLL ( 1396 eV) Auger peak, was recorded since the variation of the Al IUL peak is much more sensitive to both island growth and Al diffusion to the bulk compared to the variation of the Al LVV peak due to the greater electron inelastic mean free path (26.5 and 3.6 A [ 121, respectively). This is clearly seen in Fig. 2a which shows the variation of the Al LVV and Al KLL Auger peak amplitudes upon Al deposition at 400°C: The increase of the Al IUL peak amplitude at the island formation stage is essentially more pronounced in comparison with the slight enhancement of the Al LVV peak amplitude. In contrast, for the Al adsorption at 600°C both Al LVV and Al KLL peak amplitudes reach the saturation values and the Al KLL to Al LVV peak ratio remains essentially constant during the entire time of Al deposition. This observation is consistent with the formation of a saturating 2D Al layer without any bulk Al island growth or intermixing between Al and Si. The Al coverage at saturation was found to be about 0.6 ML (see Fig. 3). The Al coverage was determined from the AES Al LVV (68 eV) to Si LVV (92 eV) peak height ratio within the framework of the model of a thin homogeneous Al layer on a Si substrate [ 131. The inelastic mean free0 paths of Auger electroas (2, (68eV) = 3.6 A and ;Isi (92eV) = 4.0 A) were determined using the TPP-2 (Tanuma, Powell and Penn) formula [ 121. The values of the Auger peak heights for semi-infinite Si and Al samples were obtained in the separate calibration expe~ments. Fig. 3 presents also the data of LEED observations which revealed the sequential formation of the v? x 6, fl x fi and “y-phase”. We would like to outline that the Al coverages at which the fi x fi and v’? x fi structures were observed in the present work

Al-KLL I Si-LVV x IO

Al deposition time

03

( min. )

AI-LVV / Si-LVV

0

AI-KLL f Si-LVV x IO n 0 0

2 Al deposition time

( min. I

Fig. 2. Variation of the Al KLL/Si LVV Al LVV/Si LVV (closed circles) Auger with deposition of Al conducted (a) at 600°C. For convenience, the Al KLL/Si height ratio is multiplied by a factor of

(open circles) and peak height ratios 400°C and Cb) at LVV Auger peak 10.

are in good agreement with known values of i ML for Si(lll)fi x &Al [I,61 and $ ML for Si(l1 I)+‘? x $?-A1 [1,6,14]. This coincidence is believed to prove the correctness of the quantitative AES analysis of the present study. Another question under consideration in this work was the characterization of the periodicity of the “‘y-phase”. Fig. 4a shows the LEED pattern of the “y-phase” and Fig. 4b shows the LEED pattern of the atomically-clean Si ( 111) 7 x 7 surface. Both LEED patterns were recorded at the same electron energy of 65 eV and at the same position of the sample with respect to the LEED

A. VI Zotov et al. /Surface Science 316 (1994) L1034-L1038

0.6 f p 0.4 F I: a

0.2

0.0

,

I

2

4

,

I

6

8 10 AI dapasition time ( min. ) Fig. 3. Adsorption curve
optics to provide the possibility for direct comparison of the two structures. The direct comparison of Figs. 4a and 4b yields a periodicity of the “y-phase” of 8 x 8 rather than 7 x 7 of Refs. [ 1,6 ] or 9 x 9 of Refs. [8,9]. Though, in the LEED pattern ofthe “y-phase”, the extra-reflections are brighter in the vicinity of the normal spots, the weaker extra-reflections distant from the normal spots can be resolved also. This is well seen in Fig. Sa showing the LEED intensity Iine profile of this structure along the line connecting (aif and ( 10) spots in the bottom of the LEED pattern of Fig. 4a. The arrows show the expected positions for the 4th order reflections. The good coincidence is apparent. The similar intensity line profile for the Sit 111) 7 x 7 LEED pattern with arrows indicating the positions of the 4th order reflections is presented to show that the possible distortions in the recorded LEED patterns (e.g. due to the un~ntroI1~ presence of magnetic and electrical fields or due to recording of the pattern from the spherical screen to the plane photo film) are not sufftcient to cause the erroneous dete~ination of the periodicity. It should be pointed out that the 8 x 8 periodicity and the N 0,6 ML Ai saturating coverage were found to be the characteristic features of the “y-phase” independent of the growth temperature in the SO0 to 700°C range and whether the phase was formed by deposition onto the heated

Fig. 4. LEED patterns of fa) AIfSi( 1If ) “B-phase” and (bf atomically-clean Si ( 1I 1) 7 x 7 surfaces. Electran energy is 65 eV.

Si ( 111) sample or by RT AI deposition followed by annealing. The presented LEED-AES data are obviously insufficient to propose the non-speculative atomic structure model of the Al/%( 111) %yphase”. However, these data should not be ignored in the construction of such a model. Say,

A. If. Zotov et al. /Surface Science 316 (1994) L1034-L1038

suggests the existence of sufficient areas with deficiency of Al at the domain boundaries and/or incomplete substitution of Si by Al atoms within domains. 4. Conclusion

(a) Al/Si(?ll)“y-phase”

u

The periodicity and composition of the Al/Si ( 111) “y-phase” forming upon saturating Al adso~tion at 500-700’~ were studied by LEED and AES. The AES data showed that the “y-phase” is essentially submonolayer Al/Si ( 111) phase which exists at Al coverage of 0.5 to 0.6 ML. The LEED observations revealed the 8 x 8 periodicity of the phase. References r11 J.J. Lander and J. Mo~son,

Surf. Sci. 2 (1964) 553. Hansson, R.Z. Bachrach, R.S. Batter and P. Chiaradia, J. Vat. Sci. Technol. 18 ( 198 1) 550. [31 T. Kinoshita, S. Kono and T. Sagawa, Phys. Rev. B 32 (1985) 2714. 141 G.V. Hansson, J.M. Nicholls, P. Martensson and R.I.G. Uhrberg, Surf. Sci. 168 (1986) 105. J.A. Anderson, 151 M.K. Kelly, G. Margaritondo, D.J. Frankel and G.J. Lapeyre, J. Vat. Sci. Technol. A 4 (1986) 1396. I61 R.J. Hamers, Phys. Rev. B 40 (1989) 1657. I71 K. Nishikata, K. Murakami, M. Yoshimura and A. Kawazu, Surf. Sci. 269-270 ( 1992) 995. T. Takaoka, T. Yao, T. Sato, [81 M. Yoshimura, T. Sueyoshi and M. Twatsuki, Mat. Res. Sot. Symp. Proc. 259 (1993) 157. 191 M. Yoshimura, K. Takaoka and T. Yao, in: Extended Abstracts of the 1993 International Conference on Solid State Devices and Materials (Makuhari, 1993) 98 [lOI T. Ide, T. Nishimori and T. Ichinokawa, Surf. Sci. 209 (1989) 335. E.A. Khramtsova, V.G. Lifshits, [ill A.V. Zotov, A.T. Kharchenko, S.V. Ryzhkov and A.N. Demidchik, Surf. Sci. 271 (1992) L71. [121 S. Tanuma, C.J. Powell and D.R. Penn, Surf Interface Anal. 17 (1991) 911. 1131 M.P. Seach, in: Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Eds. D. Briggs and M.P. Seach (Wiley, New York, 1983). R.Z. Bachrach, R.S. Bauer and [141 G.V. Hansson, P. Chiaradia, Phys. Rev. Lett. 46 ( 1981) 1033.

121G.V.

(b) Si(il1)

7x7

Fig. 5. Densitometry scanning along the line connecting (Oi) and (IO) spots of the LEED patterns of (a) AI/B ( 1 I 1) “y-phase” and (b) Si (111) 7 x 7 surface. The arrows indicate the positions of $th order reflections for (a) and 4th order reflections

for (b).

the structural model of the “y-phase” proposed by Yoshimura et al. [9] is not consistent with our data either with respect to Al coverage or with respect to the periodicity of the phase (the proposed structure is based on the 9 x 9 dimeradatom-stacking-fault model in which adsorbed Al atoms replace Si atoms in the second layer (“rest atoms”) and terminates on the surface and, thus, it has a periodicity of 9 x 9 and coverage of $ ML). Further, the LEED pattern and especially STM images [ 6,8,9] of the “y-phase” suggest that it does not have a well-ordered commensurate structure. Instead, the found 8 x 8 periodicity is rather an average size of the triangle-shaped 2D domains in which Al atoms most likely substitute for Si atoms in the outermost atomic layer of the Si( 111) lattice [6]. The found relat;feil !ow Al coverage of 0.6 ML