Electronic states on Si(1 0 0)2 × 1-Sb: Existence of two semiconducting phases

Solid State Communications, Printed in Great Britain.

Vol. 86, No. 10, pp. 667-670, 1993.

ELECTRONIC

STATES ON Si(lOO)2 x 1-Sb: EXISTENCE SEMICONDUCTING PHASES

0038- 1098/93 $6.00 + .OO Pergamon Press Ltd

OF TWO

A. Cricenti, S. Selci, A.C. Felici, L. Ferrari and G. Chiarotti* Istituto di Struttura della Materia de1 Consiglio Nazionale delle Ricerche, via Enrico Fermi 38, I-00044 Frascati, Italy (Received 17 November 1992; in revisedform

11 January 1993 by E. Molinari)

Two different Sb-induced phases have been found upon annealing on freshly evaporated Si( 10 0)2 x 1 surfaces. The surface electronic structures of the two phases (1 x I-Sb and 2 x 1-Sb) have been studied with surface differential reflectivity (SDR) and angle-resolved photoelectron spectroscopy (ARUPS). Both techniques show the existence of a gap of approximately 1.6 and 1.4eV for the two phases, with the empty state located near the Fermi level.

A GREAT INTEREST for the properties of group V adsorbates, such as Sb and As, on various semiconductors (GaAs, Si, etc.) [l-4] arose in recent years because of the intrinsic interest for the study of a metallic layer on top of a semiconductor as well as for the technological relevance related to the need of improving the quality of epitaxial growth. The structural properties of Sb on Si(lO0) has been studied by various techniques, namely Scanning Tunneling Microscopy (STM) [5,6], SEXAFS [6] and core-level spectroscopy [5]. It was found that upon annealing a 2 x 1 reconstruction, mainly consisting of symmetric dimers, develops [5, 61 and surface Si atoms relax towards ideal bulk termination [5]. SEXAFS allowed the determination of the bond length between Si-Sb and Sb-Sb atoms in the utmost top layer [6]. In this paper we present data on the electronic properties of Sb on Si(lOO)2 x 1 surfaces that show two different phases with 1 x 1 and 2 x 1 structures upon annealing at 350 and 450°C respectively. Both phases show the presence of a gap in the electronic structure at energies of 1.6 and 1.4eV, respectively. The techniques used were Surface Differential Reflectivity (SDR) and Angle Resolved Photoemission (ARUPS) complemented by Low Energy Electron Diffraction (LEED) pattern observation. SDR, ARUPS, and LEED measurements as well as Sb evaporation have been performed in the same chamber with the same sample. The pressure was below 2 x lo-” Torr during measurements and * Dipartimento di Fisica, Universita di Roma “Tor Vergeta”, Rome, Italy.

was maintained into the lo-“Torr during Sb evaporation. The (100)2 x 1 surface of n-doped silicon (p = 10%cm) was obtained through the method developed by Ishzaka et al. [7] that produces a sharp two-domains diffraction pattern with low background as checked by LEED before the experiment. Under the above procedures the level of contamination was undetectable with Auger spectroscopy. Sb was evaporated during 10min with a Knudsen cell at a rate of 1 Amin-’ as monitored with a quartz microbalance. The experiments were done on freshly evaporated (“as grown”) samples as well as on samples annealed at 350 and 450°C. The SDR technique used in the present experiment has been described elsewhere [8]. The only modification introduced to allow a much faster data acquisition was the use of an Optical Multichannel Array (OMA) as a detector. With this system a single spectrum (between 1.3 and 3.0eV) was recorded in approximately 30s so that the optical peaks could be followed during the annealing process. ARUPS experiments were done with unpolarized 21.2 eV radiation from a helium discharge lamp. The estimated total energy resolution, as determined by the analyzer voltages and the width of the He1 line, was 150meV and the angular resolution of the hemispherical analyzer was f 1”. The position of the Fermi level (EF) was determined to an accuracy of f50 meV by photoemission from the titanium sample holder. LEED pattern observations were done at the end of the optical and ARUPS experiments. ARUPS and SDR spectra were recorded for

667

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STATES ON Si( 10 0)2x 1-Sb

several 1 x l- and 2 x 1-Sb surfaces, obtained for somewhat different initial amounts of Sb, and the results show that the electronic structures of these surfaces are well reproduced. The results of SDR are given in terms of AR/R: AR/R

= (Rannea~ed-

%aporated

) /&vaporated

7

(1)

is the reflectivity of the sample after &nnealed annealing for a given time at a given temperature. & vaporated is the reflectivity of the sample freshly evaporated at room temperature. &apor&d presents a broad, structureless spectral behaviour in the range 1.3-3.0 eV. The value of &porated is, however, rather high due to the metallic character of the “as grown” surface. Upon annealing Sb in excess of one monolayer desorbs [5] and the absolute value of the reflectivity decreases. AR/R as defined in equation (1) is therefore negative. For the sake of simplicity the negative offset has been subtracted, assuming it constant in the range of observation and the results are shown in curve (a) of Fig. 1 for a sample annealed 20min at a temperature of 350°C. Both Ranneald and &vaporated were taken at room temperature. The approximation of a constant offset is justified by the following experimental findings: during annealing of the “as grown” surface at 350°C reflectivity spectra have been taken every minute. It is observed that in the first 5min there is a strong structureless reduction of reflectivity related to Sb desorption while in the subsequent time a structured growth of reflectivity is observed, presumably due to rearrangement of atoms at the surface. The curve (b) of Fig. 1 shows where

WR

(2)

= (Ranneated- Ro)lRo,

where Rs is the reflectivity after 5min of annealing. No offset has been subtracted for curve (b). It is seen that curves (a) and (b) are nearly the same, in spite of the fact that Ranneal is taken at room temperature and R,, at 350°C. The substantial identity of curves (a) and (b) justifies the procedure used for plotting results defined in equation (1). The difference between the two curves [9] around 1.6 and 2.5eV is presumably due to the exposure, in the “as grown” surface, of clean regions of Si( 100)2 x 1 which absorbs in this spectral range [lo]. Such an interpretation is supported by results (not shown) [ll] for samples annealed at 550°C that show a growth of a structure in the range 1.5-1.8eV presumably due to Sb desorption or segregation. The island-type growth of Sb in “as grown” samples has been confirmed by Auger spectroscopy in offnormal conditions [l l] and is in qualitative agreement with the STM results [6] that show uncovered and disordered regions. The curve (a) of Fig. 2 shows AR/R as defined in equation (1) with the negative offset subtracted, as in curve (a) of Fig. 1, for a sample annealed 20 min at a temperature of 450°C. In the same way the curve (b) of Fig. 2 shows AR/R as defined in equation (2), where R. is still the reflectivity after 5min of annealing at 350 “C. Also in this case no offset has been subtracted and the substantial identity of curves (a) and (b) confirm the conclusions reached above. LEED patterns taken at room temperature after annealing at 350°C and 450°C show 1 x 1 and 2 x 1 structures, respectively. The 1 x 1 pattern was quite sharp with low background and no traces of halforder spots while in the 2 x 1 pattern weak half-order spots with high background were present, indicating some kind of disorder. S*

Si(lOO):Sb-1x1

1.0

2.0 Photon

3.0

energy (eV)

Fig. 1. Surface differential reflectivity spectrum of a Si(lO0) : Sb-1 x 1 surface in the 1.3-3.0eV energy range. Curve (a) (dashed line) represents AR/R as defined in equation (1) while curve (b) (full line) represents AR/R as defined in equation (2).

Vol. 86, No. 10

I-

Si(lOOkSb-2x1

0.0

3.0 2.0 Photon Energy (eV) Fig. 2. Surface differential reflectivity spectrum of a Si(lO0) : Sb-2 x 1 surface in the 1.3-3.0eV energy range. Curve (a) (dashed line) represents AR/R as defined in equation (1) while curve (b) (full line) represents AR/R as defined in equation (2). 1.0

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Vol. 86, No. 10

STATES ON Si( 10 0)2x 1-Sb

The observation of the curves of Figs. 1 and 2 shows for both surfaces the presence of a gap, evidenced by a well defined peak of AR/R at 1.6 (St) and 1.4 (S,) eV. In fact, for photon energies below the silicon gap, AR/R gives directly the imaginary part of the surface dielectric function [12]. However, it has been shown that the same is approximately true for energies below the direct gap [9], i.e. for Si up to 3 eV. Both the Sb-1 x 1 and -2 x 1 surfaces show then a semiconducting behaviour. The same conclusion can be drawn from the ARUPS results described below. Angle-resolved photoemission spectra, as a function of emission angle, are shown in Fig. 3 (a) for the Si( 10 0) : Sb-1 x 1 surface along the [0 lo] direction. A surface state (A’) is identified close to the r point, 1.6eV below the Fermi level, dispersing downwards towards the J& point with an observed bandwidth of 0.75eV. The notations are defined in the inset of Fig. 4. Figure 3(b) shows ARUPS spectra recorded for a Si(lO0) : Sb-2 x 1 surface. A surface state (A”) is identified close to the r point, 1.4 eV below the Fermi level dispersing downward towards the I’a,b point with an observed bandwidth of 0.9eV. Figure 4 shows the measured initial-energy dispersions Ei($) relative to the Fermi level along

Si(lOO):Sb-1x1 towards [OlO]

(a)

Si(lOO):Sb-2x1 towards [OlO]

(b)

r

x ’

x I! I

A’

X A’ 1 /

0.0

0.5

1.0 %,b

WAVE VECTOR

&‘)

Fig. 4. Experimental energy dispersion of the filled surface states for the Si( 10 0) : Sb- 1 x 1 surface (x) and for the Si( 100) : Sb-2 x 1 surface (M), along the [0 IO] azimuthal direction. In the inset, the two-domain 2 x 1 surface Brillouin zone is shown. the [0 lo] azimuthal direction, for the surface states A’ and A” of the two Sb-induced phases. The surface states A’ and A” show strong p,-character, upon changing the incidence angle to 0” in the ARUPS experiment, and are sensitive to oxygen contamination [13], as expected for dangling-bond type states. Though ARUPS experiments give information only on occupied states, while SDR is related to both occupied and empty states, a qualitative comparison of the two types of results is nevertheless possible and gives rise to the same conclusions. (i) The existence of a gap is clearly evidenced from the dispersion curves of Fig. 4. (ii) Two different occupied surface states for Sb-1 x 1 and -2 x 1 surfaces are clearly resolved. Their energy separation roughly corresponds to the separation between S, and S, of Figs. 1 and 2, showing that excitonic effects in AR/R spectra are negligible. (iii) Energy considerations indicate that the initial state of the optical transitions~ (S, and S2) is located around the point kil = 0.35A-’ along the [OIO] direction of the surface Brillouin zone. In this picture the empty band connected to the optical transitions must reach in its dispersion the Fermi level at about the same kii point.

-6

Energy below E&N)

-5

-4

-3

Energy b&w

-2

-1

0

1

EF(eV)

Fig. 3. Photoemission spectra recorded for various emission angles along the [0 1 O] azimuthal direction with Qj = 45” for a Si(1 OO)-Sb: (a) for Sb-1 x 1 surface. (b) for a Sb-2 x 1 surface.

In conclusion we have shown the existence of two phases (1 x 1 and 2 x 1) obtained by different annealing temperatures of Sb evaporated on Si(lOO)2 x 1 surfaces. The two phases have a semiconducting behaviour as shown by SDR and ARUPS. The two techniques give consistent results that are in qualitative agreement.

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STATES ON Si( 10 0)2x 1-Sb

REFERENCES 1. 2. 3. 4. 5.

W.K. Ford, T. Guo, D.L. Lessor & C.B. Duke, Phys. Rev. B42, 8952 (1990). R.I.G. Uhrberg, R.D. Bringans, R.Z. Bachrach & J.E. Northrup, J. Vat. Sci. Technol. A4, 1259 (1986). H.B. Elswijk, D. Dijkkamp & E.J. van Loenen, Phys. Rev. B44,3802 (1991). R.S. Becker, T. Klitser & J.S. Wickers, J. Microscopy 152, 157 (1988). D.H. Rich, F.M. Leisble, A. Samsavar, E.S. Hirschorn, T. Miller & T.-C. Chiang, Phys. Rev. B39, 12758 (1989).

6. M. Richter, J.C. Woicik, J. Nogami, P. Pianetta, K.E. Miyano, A.A. Baski, T. Kendelewicz, C.E. Bouldin, W.E. Spicer, C.F. Quate & I. Lindau, Phys. Rev. Lett. 65, 3417 (1990).

7. A. Ishizaka & Y. Shiraki, J. Electrochem.

Sot. 133, 666 (1986). 8. S. Selci, F. Ciccacci, G. Chirotti, P. Chiaradia & A. Cricenti, J. Vat. Sci. Technol. 5, 327 (1987).

9.

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It was not possible to perform oxidation of Sbcovered surfaces on the experimental chamber order the usual ER/R = (RCIef)-- RE;/ROx. Thiyze the reason why we are not going to discuss any other structure but Si and S,, which are important for the determination of the surface state gap. Further experimental studies, by using the more usual oxidation procedure and with the help of polarized light (on single-domain surfaces) are needed to fully understand the all AR/R spectrum. lo P.E. Wierenga, M.J. Sparnaay & A. van Silfhout, Surf. Sci. 99, 56 (1980). 11 A. Cricenti et al. (to be published). 12’ J.D.E. McIntyre & D.E. Aspnes, Surf. Sci. 24, ’ 417 (1971). l3 Oxygen contamination was checked by ARUPS by leaving the sample one night in the introduction chamber at a residual oxygen pressure of low8 Torr.