Gold-induced one-dimensional ordering on vicinal Si(111)

Vacuum/volume

314Ipages 273 to 27511997 0 1997 Elsevier Science Ltd Printed in Great Britain. All riahts reserved 0042-207X/67 $17.00+.00

Pergamon PII: SOO42-207X(96)00272-2

Gold-induced Si(ll1)

one-dimensional

48/number

ordering on vicinal

M Jalochowski, M Str6iak and R Zdyb, Insrirure of Physics, Maria Curie-Sklodowska Sklodowskiej

University,

Pi. M. Curie-

1, PL-20031, Lublin Poland

Behaviour of a sub-monolayer Au-coverage on the vicinal SitI 1 I) has been studied by RHEED. For the sample misoriented by 8.46” towards the Li i21 azimuthal direction at the coverage equal to 0.20 ML of Au, the RHEED pattern shoyed almost perfect distribution of the monoatomic Si(l1 I) steps with the staircase width equal to 18.522 0.66A. Au-induced order was identified with the (755) facet of Si with a one-dimensional chains of Au. Upon further increase of the Au deposition this regular distribution was gradually disturbed and at the coverage of about 0.50 ML of Au onset of an (,/3x J3)R 30” reconstruction was visible. 0 1997 Elsevier Science Lrd. All rights reserved

Introduction

Experimental

The (5 x 2)Au superstructure on the bulk terminated Si( 111) surface possesses 2n/3 symmetry with three equivalent domains oriented in three equivalent directions.’ The presence of atomic steps on vicinal Si( 111) surface reduces this symmetry and induces the single orientation of (5 x 2)Au chains.2 Using STM O’Mahony et al.’ on Si(ll1) tilted 4” off the [l 1 l] axis towards the [112] direction observed flat terraces with (5 x 2) reconstruction separated by regular stepped regions with the average width equal to 21 k 1 A. The average number of Au atoms occupying the (5 x 2) unit cell was not determined in their work but using the coverage reported by Swiech” it could be estimated as being equal to 4.5 if the saturation value for (43 x J3)R30” phase is 2/3 ML (with 1 ML corresponding to 7.84 x 10”’ Si atoms per cm2, the atomic density of the upper atoms in the bulk-terminated Si( 11 I) surface bilayer). Au-covered vicinal Si(ll1) surfaces with an inclination of 2”, 4’: and 9” towards the [l 121 azimuth were recently studied by Seehofer et ccl.’ Apart from the (111) terraces with (5 x 2) reconstruction other facets were also identified. On the samples with a misorientation of 9” with an Au-coverage of - 0.7 ML they observed with STM bright parallel stripes and trenches on areas identified as (775) facet. The periodicity along these stripes corresponded to la, where a is Si(l1 l)(l x 1) surface lattice constant (a = 3.84A), whereas that of the trenches was equal to 2a. In this paper we present an experimental study of the Auinduced one-dimensional reconstruction on vicinal Si( 111) tilted 8.46” off the [I 1 l] axis towards the opposite direction, i.e. [ii21 direction. Using RHEED technique we were able to trace, on the macroscopic scale, the development of one-dimensional ordering, and an onset of the (./3 x ,/3)R30” phase.

The measurements were performed in an ultrahigh vacuum system pumped by ion and Ti sublimation pumps. The molecular beam epitaxy deposition chamber was equipped with a RHEED apparatus, sample heating, gas-flow UHV liquid helium cryostat and a quartz film-thickness monitor. The base pressure was 5 x lo-” mbar. The Si samples with the dimensions 18 x 4 x 0.6 mm3 and with 20 Rem specific resistivity at room temperature used here were fabricated from a p-type boron-doped single-crystal, cut 8.46” off the [ 11 l] axis toward the [ii21 direction. The tilt of the sample was determined with the accuracy better than f0.03” with respect to the (111) plane and within f3” with respect to the azimuth [fi2]. The samples were Syton polished, etched in 1O:l ethanol +HF, rinsed in distilled water and were mounted with cooled MO clamps on UHV sample manipulator. Flashing for a few seconds to about 1500 K resulted in removal of Sic and in the appearance of the (7 x 7) superstructure RHEED pattern. Direct resistive DC heating was used. The RHEED system consisted of an electron gun with magnetic focusing lens and deflection coils, a Faraday cup for collection of the specularly reflected electrons and a CCD camera for recording the RHEED pattern displayed on a fluorescent screen. This system could operate anywhere within the electron energy from IO to 25 kcV. The instrumental limitation (the transfer length) of the RHEED optics, estimated using a flat Si(ll1) surface with (7 x 7) superstructure was 200 A normal to the incident direction and exceeded 0.5 pm in the plane of electron beam incidence. A precise quartz-crystal monitor with frequency-to-voltage converter allowed us to control of Au deposition with an accuracy better than l/SO ML of (111) fee Au. During deposition of Au on 273

M Ja~ochowski et al: Gold-induced

one-dimensional

ordering on vicinal Si(ll1)

the vicinal Si the samples were held at 130 K. They were annealed for OSmin at about 800K and the temperature was gradually lowered over the next 1.5 min. During these processes the pressure stayed below 2 x IO-‘“mbar. All cxperimcntal data reported here were recorded at a sample temperature of 130 K. Results and discussion The contour plot of the RHEED intensity presented in Figure l(a) shows a trace of 0th Lauc zone with hardly marked streaks and spots. The detailed analysis of the RHEED pattern originating from the bare Si does not show the existence of a regular stepped structure. Similar results were also reported by Hibino et ul.” The bunching of the steps was recently confirmed in STM experiments,‘,” with LEEM’ and was discussed theoretically.‘O~” Upon increasing the amount of the deposited Au WC observed the development of regularly spaced streaks as is shown in Figure l(b). The contour lines of Figure I are drawn through the points with the same intensity and are arranged in the logarithmic scale. The plane of incidence was parallel to the [I TO] azimuth and the angle of incidence was I .8 ‘. The plant (11 I) of Si is parallel to the horizontal axis of this figure. The separation of the narrow structures is proportional to the reciprocal of the terrace width. Using the data of Figure l(b), this separation was estimated as equal to 5.58 kO.20 in units of unreconstructed Sijl I I) surface periodicity along azimuth [ii2], i.c. 18.52+0.66A. This value agrees with the theoretical step separation characteristic of ideal (755)

facet

equal

to 18.84A.l’

The development

of this ordering

was accompanied

by diminishing of the RHEED features origthe (7 x 7) reconstruction [not resolved in Figure l(a)]. Further increase of the Au-deposit above 0.20 ML resulted in the development of the (,/3 x :3)R30” phase. In order to establish the optimal amount of the Au-deposit we determined the relative intensity of the RHEED spot produced by the (7 x 7) reconstruction and the regular steps. Firstly, Au was deposited on a well-oriented (flat) Si(ll1) surface and RHEED specular beam intensity oscillations were recorded.13 These are shown in Figure 2 and were subsequently used for the precise calibration of the quartz-crystal monitor. After deposition of a specific amount of Au on the vicinal Si the resulting RHEED pattern was recorded and the deposit was flashed out. This procedure was repeated several times. Figure 3 shows the plots of square roots of the integrated intensities RHEED spots produced by the (7 x 7) reconstruction and the regular steps versus amount of the deposited Au. Although the structures originating from the regularly spaced steps were visible in broad range of Au-coverage from about 0. I2 to 0.51 ML they were best developed at 0.20+_0.03ML. The measured integrated intensity of the diffraction spots produced by the one-dimensional structures increased continuously with increasing amount of the deposited Au. This is shown in Figure 3. Here. two curves display the variation of the RHEED-spot intensity identified as produced by one-dimensional structures with the periodicity I x anI,, (upper curve) and 2 x anlo (lower curve) where u,,~,,, is the surface lattice constant in the azimuth [I iO] and is equal 3.839 A. inating

from

Au on Si(ll1) T= 130K E = 20 keV

0.20

ML Au 100

150

pixels

200

Figure 1. RIIEED pattern from the vicinal Si(1 II) surface misoriented 8.64- toward the [ii21 azimuth. The incident electron beam is directed toward [liO] azimuth, the incident angle is IX, the electron energy is 10 keV. (a) Bare Si, (b) sample with deposited 0.20ML of Au. 1 ML corresponds to 7.84x IO“’ Si atoms per cm’. The temperature of the sample was 130 K. The contour lines arc drawn through the points with the same intensity and are arranged in the logarithmic scale. 274

I

l’l’l’l’l’l’1’1’1’l’~‘~’

0 2 4 6 8 1012141618202224 Au thickness(ML) Figure 2. RHEED specular beam intensity recorded during deposition of Au on well oriented Si( 1I I) surface held at 130 K. The polar angle was equal 0.26’, the electron energy 20 keV.

M Jatochowski

et al: Gold-induced

one-dimensional

ordering

on vicinal

Si(ll1)

In fact these were seen in the RHEED pattern where Si(l1 l)(1 x 1) surface lattice produced additional rods. The value of 0.20ML of Au that saturate the uniform distribution of terraces is half that determined for vicinal Si( 111) with inclination of 2”, 4” and 9” towards [llj]‘. In recently reported studies on the atomic structure of the Si( 11 l)-(5 x 2)Au surface Marks and Plass14 have confirmed previously discussed models of the (5 x 2) surface reconstruction. Using a high resolution electron microscopy they found 4 atoms of Au in the unit cell of double chains running along < ilO> equivalent directions. WC show that for the vicinal surface studied in this work the one-dimensional order can be induced with only 2 atoms of Au occupying the surface unit cell with even slightly larger area. In conclusion, we show that the Si(755) facet covered with 0.20 ML of Au undergoes the surface reconstruction with onedimensional Au-chains matched to the perfectly ordered terraces. Contrary to the previous studies on bare vicinal Si(ll1) surfaces” where stable and uniform distribution of single and double step height terraces were observed only at temperatures higher than 860 K, we could observe uniform single steps at the temperatures as low as 130 K. Acknowledgements This work was supported by Grant No 2 P03B 105 08 of Polish Committee of Scientific Research.

0.0

0.4 0.2 Au-coverage (ML of Si)

Figure 3. Plots of the RHEED pattern integrated spot intensity from the vicinal Si(ll1) surface versus amount of deposited Au. The crosses, open circles, and full circles correspond to the intensities originating from the (7 x 7) reconstruction, the spots produced by 1x alllO pcriodicity, and 2 x a,! ,,,, periodicity of the regular steps, respectively. The incident electron beam is directed towards the [l iO] azimuth, the incident angle is 2”, the electron energy is IO kcV.

Presented data show that at certain coverage of the Au on the vicinal Si( 111) tilted 8.46’ off the [I I I] axis towards the [l i2] directional most perfect ordering of uniform single-step terraces can be produced. This order can be maintained within rather narrow range of the coverage close to 0.20 ML. The separation of the diffraction streaks and their inclination to the plane Si( 111) indicate that the reconstruction is produced on the (755) facet which is oriented in respect to the Si(l1 I) plane at the angle of 9.44”. Comparing this with 8.46 ’ for inclination of the macroscopic surface of the sample we can expect that this gain of the slope has to be compensated by presence of flat Si( 111) terraces.

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