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K-induced surface structural change of Si(1 1 1)-7 × 7 probed by second-harmonic generation
Applied Surface Science 252 (2006) 5296–5299 www.elsevier.com/locate/apsusc
K-induced surface structural change of Si(1 1 1)-7 7 probed by second-harmonic generation Takanori Suzuki a,*, Youichi Karaki a,b, Dongmei Deng a, Masatoshi Tanaka b a
The Institute of Physical and Chemical Research (Riken), Hirosawa 2-1, Wako, Saitama 351-0198, Japan b Yokohama National University, Tokiwadai 79-5, Hodogaya, Yokohama 240-8501, Japan Available online 18 January 2006
Abstract The growth of thin K films on Si(1 1 1)-7 7 has been investigated by selecting the input and output polarizations of second-harmonic generation (SHG) at room temperature (RT) and at an elevated temperature of 350 8C. The SH intensity at 350 8C showed a monotonic increase with K coverages up to a saturated level, where low energy electron diffraction (LEED) showed a 3 1 reconstructed structure. The additional deposition onto the K-saturated surface at 350 8C showed only a marginal change in the SH intensity. These variations are different from the multicomponent variations up to 1 ML and orders of magnitude increase due to excitation of plasmons in the multilayers at RT. The variations of SHG during desorption of K at 350 8C showed a two-step decay with a marked shoulder which most likely corresponds to the saturation K coverage of the Si(1 1 1)-3 1-K surface. The dominant tensor elements contributing to SHG are also identified for each surface. # 2005 Elsevier B.V. All rights reserved. PACS: 68.35.Rh; 68.43.h; 73.20.Mf; 78.66.D6 Keywords: Second-harmonic generation; SHG; Potassium; K; Si(1 1 1)-7 7; Si(1 1 1)-3 1; Plasmon
1. Introduction Structural and electronic properties of alkali-metal modified silicon surfaces are of great interest for both fundamental and practical point of view [1,2]. Owing to the extensive studies dedicated to the alkali-metal (AM) modification of Si(1 1 1)7 7 (hereafter referred to as 7 7) using rather standard methods of surface science, such as RHEED [3], surface X-ray diffraction and LEED [4], photoelectron spectroscopy [5] or scanning tunneling microscopy [6,7], it has now been accepted that AM atoms initially interact preferentially with the Si adatom dangling bonds, and that room temperature (RT) growth usually saturates when the first monolayer (ML) has been deposited [5]. Metallic multilayers grown at low temperature (LT) have the characteristics of supporting plasmon oscillations [8]. Previous investigations of growth of thin AM overlayers on metal surfaces have revealed orders of magnitude enhancement of second-harmonic generation (SHG) during the growth of
* Corresponding author. Fax: +81 48 467 9329. E-mail address: [email protected] (T. Suzuki). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.12.061
more than one ML of Na, K, and Cs on Rh [9] and Rb on Ag [10] at RT or LT. Liebsch has shown from the density-functional approach that such enhancements can take place for AM on Al when surface plasmons are excited [11]. AM growth on silicon surfaces has also been investigated by SHG [12–16]. Strong increase in SHG during deposition of the second ML has been demonstrated for Na adsorption by Boness et al. [14] and for Cs [15] and K adsorption by Pedersen et al. [16], which are interpreted as due to excitation of plasmons in the thin film on 7 7 at LT. LEED observation shows a 7 7 pattern at saturation coverage for both K and Cs [2]. Photoelectron spectroscopy [2] observed metal-to-semiconductor transition with increasing coverage, and identified the K-saturated surface to be semiconducting. The SH variations up to the formation of the first monolayer are known to show a rather complicated behavior; an initial increase with a shoulder, another increase to a local maxima and then a decrease [9,12,14,16]. Initial increase of the signals with exposure is usually related to the polarizability of the AM/ Si bonds forming surface dipoles as the main role, and the cease of the signal growth is attributed to the interaction among the dipoles that leads to depolarization [9]. It is to be noted, however, that the shoulder observed in the initial increase of the
T. Suzuki et al. / Applied Surface Science 252 (2006) 5296–5299
SH intensity appeared irrespective of AM species both for metal and silicon substrates [9,12,14,16]. Thus, the previous efforts to relate this shoulder either to the adsorbate interband transition [9] or to the destructive interference between the contributions from adsorbate and substrates or to the saturation of the substrate dangling bonds [16] have not yet reached the consensus. A further detailed study is clearly needed to reveal the physics in the alkali metal adsorption onto Si in the submonolayer coverages. On the other hand, the adsorption of AM is known to induce 3 1 reconstructions on Si(1 1 1) surfaces kept at elevated temperatures. Generally, substrate temperatures in the range from 300 to 500 8C suffice for the formation of alkali-induced 3 1 reconstructions on 7 7 [1,3,6,7]. There have been, to our knowledge, no SHG works that follow this surface reconstruction. In this study we follow the growth of thin potassium films on 7 7 in the first few monolayers regime by selecting the input and output polarizations of SHG at RT and at an elevated temperature of 350 8C where the alkali-covered Si(1 1 1) is known to transform to Si(1 1 1)-3 1-K. 2. Experimental details The experimental setup has been described elsewhere [17]. In short, the sample was a p-doped just-angle Si(1 1 1) wafer ¯ with a thickness of 0.33 mm. It was mounted with the ½2 1¯ 1 direction, one of the three mirror planes, lying in the horizontal plane in an ultrahigh vacuum (UHV) chamber equipped with LEED and Auger electron spectroscopy. Sharp 7 7 LEED patterns were obtained by direct resistive heating of the samples to about 11008 C. K was evaporated from a resistively heated, well-outgassed, dispenser oven (SAES Getters). It has not been attempted to directly measure the exposure or to determine the surface coverage of AM. The pressure during evaporation stayed below 3.0 108 Pa. The pump for the SHG was 1064 nm radiation (1.17 eV) from a YAG laser, with a pulse energy of 1 mJ, a repetition rate of 10 Hz and a nominal pulse duration of 10 ns. The reflected SH light was selected by a monochromator and detected by a photomultiplier tube connected to gated electronics. For an in situ study of isothermal adsorption, the laser was directed at an incidence angle of 458. The SHG was detected with the polarization along the ¯ direction of the sample. The input beam was polarized ½2 1¯ 1 ¯ PinPout) or perpendicular (along either parallel (along ½2 1¯ 1, ¯ SinPout) to the output polarization. When the 7 7 sample ½0 1 1, ¯ direction, ~ is placed, as in this experiment, along the ½21¯ 1 x, parallel to the incident plane, the PinPout SHG is contributed by the xxxx, xzxx, xxzx and xzzz tensor elements. While the xxyy and xzyy tensor elements contribute to the SinPout SHG [18]. The relations xxyy = xxxx, xzxx = xzyy hold for the 3m symmetry [18]. The sample temperaturewas estimated from the sample currents using the same calibration method as described in [17]. 3. Results and discussion The variations in SHG during K deposition onto Si(1 1 1)7 7 with an incidence angle of 458 are shown in Fig. 1 for two
5297
Fig. 1. SHG as a function of time for K deposition onto Si(1 1 1)-7 7 at RT: (a) PinPout and (b) SinPout. The arrow and A indicates when the shutter of the dispenser is closed.
different polarization combinations at room temperature. The PinPout SHG initially showed a monotonic increase with K deposition. A shoulder at 4 min deposition time is clearly seen. Continued deposition leads to maxima at 8 min deposition time, followed by a sharp drop in the signals to near zero at 10 min. The stability of the deposited layers has been tested by following the subsequent variations in SHG after closing the dispenser shutter at different times during the deposition process as has been demonstrated by Hansen et al. [15]. Up to the minima at 10 min, SHG was constant in time after closure of the shutter. Further deposition leads to an abrupt increase of SHG as shown in Fig. 1(a) through forming unstable layers, where the signals decayed towards the level comparable to the minima when the shutter was closed at A. A prolonged deposition beyond A leads to a saturation in the SH signal from the unstable layers, which was sensitively dependent on the substrate temperature and the AM flux; getting larger with decreasing temperature and increasing AM flux. Since the absolute calibration of the coverage is difficult, we prefer (like others) to use ‘‘saturation-monolayer coverage’’ and call it ‘‘1 ML’’ [2] for the coverage corresponding to the minima. LEED patterns showed a diffuse 7 7 structure at this coverage, which is in accordance with previous results of remaining of the 7 7 framework beneath the K thin layer [5]. The AM saturated substrate at RT can therefore be considered as the substrate for growth of additional layers. The fast rise of SHG during deposition of the second ML is usually interpreted by excitation of collective electronic modes (plasmon) in the K islands [9,14,16]. While the PinPout signal in (a) showed an over-all increase for the surface kept at RT in the initial stage of K exposure, the variation of SinPout in (b) showed an initial decrease to near zero and then a recovery to the saturation level. It is to be noted that this coverage indicating the saturation in SinPout coincides with the dip in (a) that has been related to saturation in PinPout. This behavior p offfiffiSffi inPp out ffiffiffi is quite similar to what was observed for the 7 7 to 3 3 Ag transformation, where the dip in the SinPout SHG was induced due to the destructive interference between the from the shrinking 7 7 area and the pffiffifficontributions pffiffiffi growing 3 3 Ag area [17]. Thus, we may relate the
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T. Suzuki et al. / Applied Surface Science 252 (2006) 5296–5299
behaviour in (b) to the phase relation-ship of the tensor elements in 7 7 and K-covered 7 7. A destructive interference in SinPout corresponds to that the summed contributions from the xxyy and xzyy tensor elements are out of phase among those of 7 7 and K-covered 7 7. Then, the in-phase interference in PinPout may occur only when the xzzz contribution is larger than the summed contribution of the rest tensor elements, and has the same sign among those of 7 7 and K-covered 7 7. After closing the shutter of the dispenser at A in (b), the signal decreased only a little and stayed at a constant level, which may be related to the saturation coverage. A separate SinPout experiment for a sample azimuthally rotated by 308, consisting of a pure anisotropic contribution of xxyy [18], showed a monotonic decrease to zero (not shown). This and the above SinPout results unambiguously identify xzyy to be the main contribution of SinPout SHG for the K-covered 7 7 surfaces. This is reasonable since the other tensor element of 7 7, xxyy, being resonantly enhanced at this photon energy (1.17 eV) due to the surface-state transition at 1.2 eV [19], can easily be reduced by adsorbates due to quenching. The relatively small SinPout in contrast to the large PinPout in Fig. 1 identifies a small xzyy especially in the saturation range. This in turn shows that the large PinPout for the K-covered 7 7 surfaces (plasmon resonance) is mainly contributed by xzzz. During the experiments, it was also noticed that the SH intensity corresponding to the saturation coverage depended very sensitively on the sample temperature. A detailed and systematic study will be discussed elsewhere [20]. The PinPout SHG at 350 8C in Fig. 2 shows a monotonic increase with coverage to a saturation level at around 7 min. The inset in Fig. 2 is the LEED pattern of the saturated surface after it was cooled down to room temperature, which showed a complete replacement of the 7 7 spots by the 3-domain 3 1 spots. Thus the structural transformation to 3 1 may have completed at certain point during the continuous increase in SHG up to 7 min in Fig. 2. The saturated signal after 7 min showed an increase in response to a two-times increase in the K flux at B. When the shutter of the dispenser was closed at A, the SH intensity
Fig. 2. SHG recorded during K deposition onto Si(1 1 1)-7 7 at 350 8C: (a) PinPout and (b) SinPout. The arrow and A indicates when the shutter of the dispenser was closed, and the arrow and B indicates when the K flux was increased to twice as large as before. The inset is the LEED pattern of the K saturated surface (just before B) taken after the sample is allowed to return to RT. The low brightness of the LEED spots is due to a contamination of the screen. C shows a shoulder observed during evaporation.
showed a two-step decay composed of a fast decay to the shoulder (C) followed by a slower decay. Each part of the twostep decay in PinPout was fitted with a simple exponential function of exp(kt). The fitted curves shown by dotted lines in Fig. 3 are for k = 1.6 102 s1 for the fast decay and k = 7.4 103 s1 for the slow decay in PinPout. The corresponding RT decay rate is obtained for the unstable layers (after A) in Fig. 1 to be 1.1 102 s1. Thus the fast decay in Fig. 2 may be interpreted by a thermally activated desorption of the unstable excess K. The present data do not show any clear indications as to what happens to the excess K, one could suggest desorption, diffusion along the surface to form islands [15]. While, the slower decay with a decay rate smaller than the RT decay rate of the excess K, indicating a more stable layer, could be due to K desorption from the 3 1 structure. The increase of the SH signal in response to the K flux at B in Fig. 2(a) then would correspond to the increased population in the excess K on the 3 1 surface. The SinPout data at 350 8C in (b) show an initial decrease with increasing coverage, which may also be interpreted by quenching of the surface states as in the RT data in Fig. 1. The decaying behavior in the SinPout SHG is reproduced well by the fast decay rate, i.e., k = 1.6 102 s1, as is shown by dotted lines in the inset of Fig. 3. Thus, the small SinPout SHG for the saturated surface is mainly from the excess K, with a negligible contribution from xzyy of 3 1. The signal level at the shoulder in Fig. 2(a) was found to be independent of the K flux. Thus, the shoulder may correspond to the signal level of the full coverage of 3 1. The saturation in the SHG may then be interpreted by the flux-dependent formation of the K islands on the 3 1 surface. With the interpretation of the PinPout signal given above, it may also be expected to identify the completion of 3 1 transformation. A detailed study of these growth and decay behaviours will elucidate the nucleation and growth kinetics as has been demonstrated by the observation of the temperature-dependent critical nuclear size in the 7 7 to H3 H3-Ag transformation [21]. Such study is now under way. In summary, we have demonstrated that the structural changes of Si(1 1 1)-7 7 induced by K adsorption have been sensitively monitored with the polarization selected SHG. The SH variations sensitively depended on the sample temperature;
Fig. 3. Reproduction of the decaying part of SHG at 350 8C in Fig. 2 and the fitting results: (a) PinPout and (b) SinPout. Each part in the two-step decay is fitted with a simple exponential function of exp(kt). Inset: a reproduction of SinPout by using the decay rate of the fast decay (A–C) in PinPout.
T. Suzuki et al. / Applied Surface Science 252 (2006) 5296–5299
multi-components for depositions up to 1 ML at RT were followed by a large enhancement beyond 1 ML which has been characterized by xzzz as a main contribution from the plasmon excitation. The 3 1 transformation has been monitored in the K-covered 7 7 at 350 8C. The desorption showed a two step decay with the fast decay corresponding to the K evaporation from the K islands and a slower decay to that from 3 1. Acknowledgement We thank Daisuke Inoue for his technical assistance. References [1] W. Mo¨nch, Semiconductor Surfaces and Interfaces, 2nd ed., SpringerVerlag, Berlin-Heidelberg, Germany, 1995. [2] B. Reihl, R. Dudde, L.S.O. Johansson, K.O. Magnusson, Appl. Phys. A 55 (1992) 449. [3] H. Daimon, S. Ino, Surf. Sci. 164 (1985) 320. [4] L. Lottermoser, E. Landemark, D.-M. Smilgies, M. Nielsen, R. Feidenhans, G. Falkenberg, R.L. Johnson, M. Gierer, A.P. Seitsonen, H. Kleine, H. Bludau, H. Over, S.K. Kim, F. Jona, Phys. Rev. Lett. 80 (1998) 3980. [5] K.O. Magnusson, S. Wiklund, R. Dudde, B. Reihl, Phys. Rev. B 44 (1991) 5657.
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[6] D. Jeon, T. Hashizume, T. Sakurai, R.F. Willis, Phys. Rev. Lett. 69 (1992) 1419; D. Jeon, T. Hashizume, T. Sakurai, J. Vac. Sci. Technol. B 12 (1994) 2044. [7] A.A. Saranin, A.V. Zotov, V.G. Lifshits, M. Katayama, K. Oura, Surf. Sci 426 (1999) 298. [8] J. Jostell, Surf. Sci. 82 (1979) 333. [9] H.W.K. Tom, C.M. Mate, X.D. Zhu, J.E. Crowell, Y.R. Shen, G.A. Somorjai, Surf. Sci. 172 (1986) 466. [10] K.J. Song, D. Heskett, H.L. Dai, A. Liebsch, E.W. Plummer, Phys. Rev. Lett. 61 (1988) 1380. [11] A. Liebsch, Phys. Rev. B 40 (1989) 3421. [12] S. Arekat, S.D. Kevan, G.L. Richmond, Europhys. Lett. 22 (1993) 377. [13] I.V. Kravetsky, G. Marowsky, H.G. Rubahn, Surf. Sci. 474 (2001) 47; V.G. Bordo, G. Marowsky, J. Zhang, H.-G. Rubahn, Europhys. Lett. 69 (2005) 61. [14] J. Boness, G. Marowsky, J. Braun, G. Witte, H.-G. Rubahn, Surf. Sci. 402– 404 (1998) 513. [15] P.-E. Hansen, K. Pedersen, L. Liu, P. Morgen, Surf. Sci. 391 (1997) 252. [16] K. Pedersen, P.-E. Hansen, P. Morgen, Phys. Status Solidi A 170 (1998) 411. [17] D. Deng, M. Numata, T. Suzuki, J. Phys. Soc. Jpn. 73 (2004) 3384. [18] Y.R. Shen, The Principles of Nonlinear Optics, Wiley, New York, 1984. [19] T. Suzuki, Phys. Rev. B 61 (2000) R5117. [20] Y. Karaki, D. Deng, M. Tanaka, T. Suzuki, unpublished. [21] D. Deng, T. Suzuki, Phys. Rev. B 72 (2005) 085308.
K-induced surface structural change of Si(1 1 1)-7 7 probed by second-harmonic generation Takanori Suzuki a,*, Youichi Karaki a,b, Dongmei Deng a, Masatoshi Tanaka b a
The Institute of Physical and Chemical Research (Riken), Hirosawa 2-1, Wako, Saitama 351-0198, Japan b Yokohama National University, Tokiwadai 79-5, Hodogaya, Yokohama 240-8501, Japan Available online 18 January 2006
Abstract The growth of thin K films on Si(1 1 1)-7 7 has been investigated by selecting the input and output polarizations of second-harmonic generation (SHG) at room temperature (RT) and at an elevated temperature of 350 8C. The SH intensity at 350 8C showed a monotonic increase with K coverages up to a saturated level, where low energy electron diffraction (LEED) showed a 3 1 reconstructed structure. The additional deposition onto the K-saturated surface at 350 8C showed only a marginal change in the SH intensity. These variations are different from the multicomponent variations up to 1 ML and orders of magnitude increase due to excitation of plasmons in the multilayers at RT. The variations of SHG during desorption of K at 350 8C showed a two-step decay with a marked shoulder which most likely corresponds to the saturation K coverage of the Si(1 1 1)-3 1-K surface. The dominant tensor elements contributing to SHG are also identified for each surface. # 2005 Elsevier B.V. All rights reserved. PACS: 68.35.Rh; 68.43.h; 73.20.Mf; 78.66.D6 Keywords: Second-harmonic generation; SHG; Potassium; K; Si(1 1 1)-7 7; Si(1 1 1)-3 1; Plasmon
1. Introduction Structural and electronic properties of alkali-metal modified silicon surfaces are of great interest for both fundamental and practical point of view [1,2]. Owing to the extensive studies dedicated to the alkali-metal (AM) modification of Si(1 1 1)7 7 (hereafter referred to as 7 7) using rather standard methods of surface science, such as RHEED [3], surface X-ray diffraction and LEED [4], photoelectron spectroscopy [5] or scanning tunneling microscopy [6,7], it has now been accepted that AM atoms initially interact preferentially with the Si adatom dangling bonds, and that room temperature (RT) growth usually saturates when the first monolayer (ML) has been deposited [5]. Metallic multilayers grown at low temperature (LT) have the characteristics of supporting plasmon oscillations [8]. Previous investigations of growth of thin AM overlayers on metal surfaces have revealed orders of magnitude enhancement of second-harmonic generation (SHG) during the growth of
* Corresponding author. Fax: +81 48 467 9329. E-mail address: [email protected] (T. Suzuki). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.12.061
more than one ML of Na, K, and Cs on Rh [9] and Rb on Ag [10] at RT or LT. Liebsch has shown from the density-functional approach that such enhancements can take place for AM on Al when surface plasmons are excited [11]. AM growth on silicon surfaces has also been investigated by SHG [12–16]. Strong increase in SHG during deposition of the second ML has been demonstrated for Na adsorption by Boness et al. [14] and for Cs [15] and K adsorption by Pedersen et al. [16], which are interpreted as due to excitation of plasmons in the thin film on 7 7 at LT. LEED observation shows a 7 7 pattern at saturation coverage for both K and Cs [2]. Photoelectron spectroscopy [2] observed metal-to-semiconductor transition with increasing coverage, and identified the K-saturated surface to be semiconducting. The SH variations up to the formation of the first monolayer are known to show a rather complicated behavior; an initial increase with a shoulder, another increase to a local maxima and then a decrease [9,12,14,16]. Initial increase of the signals with exposure is usually related to the polarizability of the AM/ Si bonds forming surface dipoles as the main role, and the cease of the signal growth is attributed to the interaction among the dipoles that leads to depolarization [9]. It is to be noted, however, that the shoulder observed in the initial increase of the
T. Suzuki et al. / Applied Surface Science 252 (2006) 5296–5299
SH intensity appeared irrespective of AM species both for metal and silicon substrates [9,12,14,16]. Thus, the previous efforts to relate this shoulder either to the adsorbate interband transition [9] or to the destructive interference between the contributions from adsorbate and substrates or to the saturation of the substrate dangling bonds [16] have not yet reached the consensus. A further detailed study is clearly needed to reveal the physics in the alkali metal adsorption onto Si in the submonolayer coverages. On the other hand, the adsorption of AM is known to induce 3 1 reconstructions on Si(1 1 1) surfaces kept at elevated temperatures. Generally, substrate temperatures in the range from 300 to 500 8C suffice for the formation of alkali-induced 3 1 reconstructions on 7 7 [1,3,6,7]. There have been, to our knowledge, no SHG works that follow this surface reconstruction. In this study we follow the growth of thin potassium films on 7 7 in the first few monolayers regime by selecting the input and output polarizations of SHG at RT and at an elevated temperature of 350 8C where the alkali-covered Si(1 1 1) is known to transform to Si(1 1 1)-3 1-K. 2. Experimental details The experimental setup has been described elsewhere [17]. In short, the sample was a p-doped just-angle Si(1 1 1) wafer ¯ with a thickness of 0.33 mm. It was mounted with the ½2 1¯ 1 direction, one of the three mirror planes, lying in the horizontal plane in an ultrahigh vacuum (UHV) chamber equipped with LEED and Auger electron spectroscopy. Sharp 7 7 LEED patterns were obtained by direct resistive heating of the samples to about 11008 C. K was evaporated from a resistively heated, well-outgassed, dispenser oven (SAES Getters). It has not been attempted to directly measure the exposure or to determine the surface coverage of AM. The pressure during evaporation stayed below 3.0 108 Pa. The pump for the SHG was 1064 nm radiation (1.17 eV) from a YAG laser, with a pulse energy of 1 mJ, a repetition rate of 10 Hz and a nominal pulse duration of 10 ns. The reflected SH light was selected by a monochromator and detected by a photomultiplier tube connected to gated electronics. For an in situ study of isothermal adsorption, the laser was directed at an incidence angle of 458. The SHG was detected with the polarization along the ¯ direction of the sample. The input beam was polarized ½2 1¯ 1 ¯ PinPout) or perpendicular (along either parallel (along ½2 1¯ 1, ¯ SinPout) to the output polarization. When the 7 7 sample ½0 1 1, ¯ direction, ~ is placed, as in this experiment, along the ½21¯ 1 x, parallel to the incident plane, the PinPout SHG is contributed by the xxxx, xzxx, xxzx and xzzz tensor elements. While the xxyy and xzyy tensor elements contribute to the SinPout SHG [18]. The relations xxyy = xxxx, xzxx = xzyy hold for the 3m symmetry [18]. The sample temperaturewas estimated from the sample currents using the same calibration method as described in [17]. 3. Results and discussion The variations in SHG during K deposition onto Si(1 1 1)7 7 with an incidence angle of 458 are shown in Fig. 1 for two
5297
Fig. 1. SHG as a function of time for K deposition onto Si(1 1 1)-7 7 at RT: (a) PinPout and (b) SinPout. The arrow and A indicates when the shutter of the dispenser is closed.
different polarization combinations at room temperature. The PinPout SHG initially showed a monotonic increase with K deposition. A shoulder at 4 min deposition time is clearly seen. Continued deposition leads to maxima at 8 min deposition time, followed by a sharp drop in the signals to near zero at 10 min. The stability of the deposited layers has been tested by following the subsequent variations in SHG after closing the dispenser shutter at different times during the deposition process as has been demonstrated by Hansen et al. [15]. Up to the minima at 10 min, SHG was constant in time after closure of the shutter. Further deposition leads to an abrupt increase of SHG as shown in Fig. 1(a) through forming unstable layers, where the signals decayed towards the level comparable to the minima when the shutter was closed at A. A prolonged deposition beyond A leads to a saturation in the SH signal from the unstable layers, which was sensitively dependent on the substrate temperature and the AM flux; getting larger with decreasing temperature and increasing AM flux. Since the absolute calibration of the coverage is difficult, we prefer (like others) to use ‘‘saturation-monolayer coverage’’ and call it ‘‘1 ML’’ [2] for the coverage corresponding to the minima. LEED patterns showed a diffuse 7 7 structure at this coverage, which is in accordance with previous results of remaining of the 7 7 framework beneath the K thin layer [5]. The AM saturated substrate at RT can therefore be considered as the substrate for growth of additional layers. The fast rise of SHG during deposition of the second ML is usually interpreted by excitation of collective electronic modes (plasmon) in the K islands [9,14,16]. While the PinPout signal in (a) showed an over-all increase for the surface kept at RT in the initial stage of K exposure, the variation of SinPout in (b) showed an initial decrease to near zero and then a recovery to the saturation level. It is to be noted that this coverage indicating the saturation in SinPout coincides with the dip in (a) that has been related to saturation in PinPout. This behavior p offfiffiSffi inPp out ffiffiffi is quite similar to what was observed for the 7 7 to 3 3 Ag transformation, where the dip in the SinPout SHG was induced due to the destructive interference between the from the shrinking 7 7 area and the pffiffifficontributions pffiffiffi growing 3 3 Ag area [17]. Thus, we may relate the
5298
T. Suzuki et al. / Applied Surface Science 252 (2006) 5296–5299
behaviour in (b) to the phase relation-ship of the tensor elements in 7 7 and K-covered 7 7. A destructive interference in SinPout corresponds to that the summed contributions from the xxyy and xzyy tensor elements are out of phase among those of 7 7 and K-covered 7 7. Then, the in-phase interference in PinPout may occur only when the xzzz contribution is larger than the summed contribution of the rest tensor elements, and has the same sign among those of 7 7 and K-covered 7 7. After closing the shutter of the dispenser at A in (b), the signal decreased only a little and stayed at a constant level, which may be related to the saturation coverage. A separate SinPout experiment for a sample azimuthally rotated by 308, consisting of a pure anisotropic contribution of xxyy [18], showed a monotonic decrease to zero (not shown). This and the above SinPout results unambiguously identify xzyy to be the main contribution of SinPout SHG for the K-covered 7 7 surfaces. This is reasonable since the other tensor element of 7 7, xxyy, being resonantly enhanced at this photon energy (1.17 eV) due to the surface-state transition at 1.2 eV [19], can easily be reduced by adsorbates due to quenching. The relatively small SinPout in contrast to the large PinPout in Fig. 1 identifies a small xzyy especially in the saturation range. This in turn shows that the large PinPout for the K-covered 7 7 surfaces (plasmon resonance) is mainly contributed by xzzz. During the experiments, it was also noticed that the SH intensity corresponding to the saturation coverage depended very sensitively on the sample temperature. A detailed and systematic study will be discussed elsewhere [20]. The PinPout SHG at 350 8C in Fig. 2 shows a monotonic increase with coverage to a saturation level at around 7 min. The inset in Fig. 2 is the LEED pattern of the saturated surface after it was cooled down to room temperature, which showed a complete replacement of the 7 7 spots by the 3-domain 3 1 spots. Thus the structural transformation to 3 1 may have completed at certain point during the continuous increase in SHG up to 7 min in Fig. 2. The saturated signal after 7 min showed an increase in response to a two-times increase in the K flux at B. When the shutter of the dispenser was closed at A, the SH intensity
Fig. 2. SHG recorded during K deposition onto Si(1 1 1)-7 7 at 350 8C: (a) PinPout and (b) SinPout. The arrow and A indicates when the shutter of the dispenser was closed, and the arrow and B indicates when the K flux was increased to twice as large as before. The inset is the LEED pattern of the K saturated surface (just before B) taken after the sample is allowed to return to RT. The low brightness of the LEED spots is due to a contamination of the screen. C shows a shoulder observed during evaporation.
showed a two-step decay composed of a fast decay to the shoulder (C) followed by a slower decay. Each part of the twostep decay in PinPout was fitted with a simple exponential function of exp(kt). The fitted curves shown by dotted lines in Fig. 3 are for k = 1.6 102 s1 for the fast decay and k = 7.4 103 s1 for the slow decay in PinPout. The corresponding RT decay rate is obtained for the unstable layers (after A) in Fig. 1 to be 1.1 102 s1. Thus the fast decay in Fig. 2 may be interpreted by a thermally activated desorption of the unstable excess K. The present data do not show any clear indications as to what happens to the excess K, one could suggest desorption, diffusion along the surface to form islands [15]. While, the slower decay with a decay rate smaller than the RT decay rate of the excess K, indicating a more stable layer, could be due to K desorption from the 3 1 structure. The increase of the SH signal in response to the K flux at B in Fig. 2(a) then would correspond to the increased population in the excess K on the 3 1 surface. The SinPout data at 350 8C in (b) show an initial decrease with increasing coverage, which may also be interpreted by quenching of the surface states as in the RT data in Fig. 1. The decaying behavior in the SinPout SHG is reproduced well by the fast decay rate, i.e., k = 1.6 102 s1, as is shown by dotted lines in the inset of Fig. 3. Thus, the small SinPout SHG for the saturated surface is mainly from the excess K, with a negligible contribution from xzyy of 3 1. The signal level at the shoulder in Fig. 2(a) was found to be independent of the K flux. Thus, the shoulder may correspond to the signal level of the full coverage of 3 1. The saturation in the SHG may then be interpreted by the flux-dependent formation of the K islands on the 3 1 surface. With the interpretation of the PinPout signal given above, it may also be expected to identify the completion of 3 1 transformation. A detailed study of these growth and decay behaviours will elucidate the nucleation and growth kinetics as has been demonstrated by the observation of the temperature-dependent critical nuclear size in the 7 7 to H3 H3-Ag transformation [21]. Such study is now under way. In summary, we have demonstrated that the structural changes of Si(1 1 1)-7 7 induced by K adsorption have been sensitively monitored with the polarization selected SHG. The SH variations sensitively depended on the sample temperature;
Fig. 3. Reproduction of the decaying part of SHG at 350 8C in Fig. 2 and the fitting results: (a) PinPout and (b) SinPout. Each part in the two-step decay is fitted with a simple exponential function of exp(kt). Inset: a reproduction of SinPout by using the decay rate of the fast decay (A–C) in PinPout.
T. Suzuki et al. / Applied Surface Science 252 (2006) 5296–5299
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