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Electron spectroscopy investigation of Te thin films deposited at room temperature on Si(100) 2 × 1
surface science ELSEVIER
Surface Science 331-333 (1995) 569-574
Electron spectroscopy investigation of Te thin films deposited at room temperature on Si(100)2 × 1 S. Di Nardo, L. Lozzi *, M. Passacantando, P. Picozzi, S. Santucci Dipartimento di Fisica, Universita' degli Studi de L 'Aquila, 67010 Coppito (AQ), Italy Received 30 July 1994; accepted for publication 9 December 1994
Abstract Ultraviolet and X-ray photoelectron spectroscopies, Auger electron spectroscopy and low energy electron diffraction were used in order to investigate the electronic properties and the growth mode of very thin films of tellurium with mean thickness between 0.5 and 1000 A deposited at room temperature on a Si(100)2 x 1 surface. The adsorbate-substrate interaction is found to be weak. In the initial stage of the growth the tellurium atoms are chemisorbed on the silicon surface and form a continuous monolayer. The Te 5p valence electrons are involved in order to saturate the Si(100) dangling bonds inducing a 1 × 1 reconstruction of the surface. Tellurium deposited after the formation of the first continuous layer and up to 10 A of average thickness, forms 1 × 1 ordered islands, that is Te deposited at room temperature on Si(100) follows a Stranski-Krastanov growth mode (one layer plus islands). Increasing the amount of Te the islands coalesce and the deposited atoms show a bulk-like behaviour. Keywords: Auger electron spectroscopy; Chalcogens; Growth; Low energy electron diffraction (LEED); Metal-semiconductor interfaces; Photoemission; Single crystal epitaxy
1. Introduction In recent years the properties of silicon surfaces have been one of the most investigated topics in surface science. This is due to the fact that the Si(100) surface is the most widely used material in the realisation of electronic devices. However, many efforts come also from fundamental scientific interest. The dominant structure in the clean Si(100) surface is the 2 × 1 reconstruction due to the formation of dimers arranged in parallel rows [1]. Each Si surface atom has a dangling bond that causes the surface to be reactive. Recent works have shown that
* Corresponding author. Fax: + 39 862 433033; E-mail: [email protected].
the 2 × 1 reconstruction of Si(100) can be removed by the adsorption of different kinds of atoms that leads to different reconstructions of the silicon surface [2-6]. In particular the deposition of tellurium on Si(100) both at room temperature [5] and at 400 K [6] leads the surface atoms to have a 1 × 1 order. In order to study the electronic properties of the T e / S i interface, in this paper we analyze the UPS and XPS spectra collected, using He I and Mg K c~ sources, respectively, from thin tellurium films deposited at room temperature on Si(100)2 × 1. Moreover we deduce the growth mode of the overlayer by plotting the AES weighted peak-to-peak height ratio as a function of the nominal coverage. Observing the changes in the LEED pattern we check the crystalline structure of the surface of the films. The
0039-6028/95//$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSD! 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 0 3 1 9 - 3
570
X Di Nardo et al. / Surface Science 331-333 (1995) 569-574 ,
experiments were performed in ultra-high vacuum varying between 0.5 and 1000 ,~ the nominal thickness, measured with a quartz microbalance, of the tellurium films deposited on silicon.
,
,
,
,
,
,
,
,
,
,
,
,~/~ Te/Si
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2. Experimental The experiments were performed in an ultra-high vacuum (UHV; about 4 × 10 -1° Torr) system consisting of an analysis chamber connected to a sample preparation chamber. Te thin films were prepared by evaporation of high purity Te (99.999%) on Si(100) p-doped, cut from a commercial wafer, by means of an alumina crucible contained in an electrically heated tungsten basket. The substrate was cleaned by ion gun bombardment and successively 2 × 1 reconstructed by heating at about 900°C for 5 min. The substrate was maintained at room temperature during the deposition. The amount of the deposited tellurium (the nominal coverage) was controlled with a quartz microbalance. No traces of impurities were detected from the XPS analysis of the samples. The photoemission spectra were recorded by means of an hemispherical analyzer using He I (hv= 21.2 eV) and M g K a (hv= 1253.6 eV) photon sources. The energy calibration was based on the carbon ls photoline position (BE = 284.8 eV). The AES spectra were recorded with a single-pass cylindrical mirror analyzer equipped with a coaxial electron gun. The first derivative of the Auger signal was recorded applying a 2 V peak-to-peak modulation to the outer cylinder of the analyzer and using the lock-in technique. A four grid reverse view LEED was used and the patterns on the screen were recorded by a PC interfaced camera.
3. Results and discussion Using the XPS technique both core levels and Auger peaks have been followed as a function of the Te film thickness. In particular the Auger parameter of silicon has been carefully checked. This parameter, c~', is defined as the sum of the kinetic energy (KE) of the most prominent peak of an Auger feature involving three core levels and the binding energy (BE) of one of these core levels [7]. For silicon:
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BINDING ENERGY (eV) Fig. 1. XPS spectra of the Te4d core levels from Te/Si(100)2 × 1 films, with different thicknesses, and from bulk tellurium. The dashed and the solid lines indicate the binding energy position of the Te4ds/2 peak in the bulk tellurium and in the 2 A T e / S i film, respectively. The shift of the Te4ds/2 peak is about 0.2 eV.
od(Si) = KE(Si KL2,3L2,3) d- BE(Si2p) = 1716 eV. It has been shown that this parameter is very sensitive to the chemical bonds [7]. In our data no changes in the Si 2p binding energy (99.8 eV) and in the Si Auger parameter have been observed varying the Te film thickness. The same behaviour has been obtained in the Te3d features (3d5/2 = 572.8 eV and 3d3/2 = 583.2 eV). On the contrary a slight difference in the Te4d spectrum of different films has been observed as the Te film thickness increases, as shown in Fig. 1. The 2 .~ T e / S i spectrum and the bulk Te one have the same shape, but the Te4ds/2 peak binding energy, in the 2 A T e / S i film (continuous line), is about 0.2 eV higher than the one in the bulk tellurium (dashed line). The shift towards higher binding energies of the Te levels can be explained by noting that Te is slightly more electronegative than the silicon. Moreover some changes in shape are detectable in films with intermediate thickness. In particular the spectrum of the 3 .~ film, which shows non-resolved 4d3/2 and 4d5/2 peaks, seems to be the result of a superposition between the 4d spectrum of the bulk Te and the shifted one of the 2 A T e / S i film. This superposition effect is less evident but still visible in the 5 A sample and allows us to conclude that the Te 4d XPS spectrum of tellurium films up to 5 .~ is the sum of two slightly shifted components,
.
S. Di Nardo et al. / Surface Science 331-333 (1995) 569-574
one due to the first continuous layer, that interacts with the Si(100)2 × 1 substrate, and another due to the subsequently deposited Te atoms. A similar effect is not detected in the Te 3d peaks, allowing the conclusion that the 3d electrons, that are in an inner shell, are not influenced by the presence of silicon. In Fig. 2 we show the Te MNN Auger transition of several Te/Si(100)2 × 1 thin films and of bulk tellurium. This Auger transition involves the Te 3d and Te 4d levels. Visible changes are detectable in the spectra varying the thickness of the films. The Te MNN spectra of the thinnest tellurium films, 1 and 2 of nominal thickness, have the same shape and energy position, but they show a rigid shift of about 1.5 eV, towards lower kinetic energies, with respect to the bulk Te spectrum. As in the case of Te 4d, when the tellurium film thickness is in the range of the first continuous layer, the Te MNN Auger spectrum is influenced by the interaction between tellurium and silicon. Also in this case the spectra of Te films having a thickness greater than 2 A are found to be a superposition between the spectrum of bulk Te and that of 2 A T e / S i film. In fact the Te MNN
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KINETIC ENERGY (eV) Fig. 2. XPS spectra of the Te MNN Auger transition from Te/Si(100)2 × 1 films, with different thicknesses, and from bulk tellurium. The dashed lines indicate the kinetic energy position of the TeoM4N4,sN4. 5 Auger transition in the bulk tellurium and in the 2 A T e / S i film. The dashed line spectrum has been artfully obtained by adding the 2 A, T e / S i spectrum, multiplied by 0.3, and the bulk Te one, multiplied by 0.7.
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Fig. 3. Plot of the AES weighted peak-to-peak height ratio R between Te and Si as a function of the nominal coverage of Te on the Si(lO0)2× 1 surfacc. In the inset the initial stage of the growth is reported with a larger scale; the solid lines have been obtained by linear interpolation of the data in the 0 - 3 and 3 - 1 0 ,~ range, respectively.
spectrum of the 15 A T e / S i sample is very well fitted by a spectrum, reported with a dashed line in Fig. 2, obtained adding the 2 A T e / S i spectrum, multiplied by 0.3, and the bulk Te one, multiplied by 0.7. Obviously, increasing the film thickness, the bulk Te features dominate with respect to those due to the Te atoms interacting with the silicon substrate. The XPS results suggest the formation of a first layer interacting with the silicon substrate and the following growth of other Te layers which show a bulk-like behaviour. In order to study the Te growth mode, Auger and LEED spectroscopies have been applied. In Fig. 3 we present a plot of the first derivative AES weighted peak- to-peak height ratio R between Te and Si signals as a function of the nominal coverage of Te on the Si(100) surface: R = (ITe//STe)/(Isi//Ssi). The intensities Isi,Xe used for this analysis were taken from Si(92 eV) and Te(483 eV) Auger lines and the sensitivity factors were Ssi = 0.31 and STe = 0.70, respectively [8]. The data in the inset of Fig. 3 clearly indicate that, in the early stage of the growth, tellurium deposited at room temperature on Si(100)2 × 1 follows a StranskiKrastanov (SK: one layer plus islands) growth mode [5,9]. The first continuous layer of Te is completed when the nominal coverage reaches the value of
572
S. Di Nardo et al. / Surface Science 331-333 (1995) 569-574
Fig. 4. LEED patterns of (a) Si(100) surface after 2 × 1 reconstruction of the surface in two orthogonal domains and (b) 2 .~ of tellurium deposited on silicon.
about 2 .~. The deposited tellurium follows the SK growth mode up to about 10 A, then, for thicker films, the exponential increase of R indicates the coalescence of the islands. Fig. 4a shows the LEED pattern obtained from the Si(100) substrate, after 2 X 1 reconstruction of the surface in two orthogonal domains. After deposition of 2 .~ of nominal thickness of tellurium on silicon the sample exhibits a well-defined 1 X 1 LEED °pattern, as shown in Fig. 4b. In the range 2-10 A of T e / S i the intensity of the 1 X 1 LEED pattern does not change appreciably, then, it becomes more and more diffuse and, at about 90 A, it quenches. This result, together with the AES data, indicates the formation of well-ordered islands, in the range 2-10 ~, of Te, on one ordered continuous layer of tellurium. The interaction of the first Te layer with the Si atoms and its 1 X 1 reconstruction, should modify the Te valence states. Fig. 5 shows the UPS spectra of Te thin films, with nominal thickness between 0.5 and 1000 A, deposited at room temperature on Si(100)2 X 1 surface. The silicon substrate UPS spectrum is reported too (bottom curve). The spectrum of Si(100)2 X 1 is composed of two main features, labelled A and/~, located at about 3 and 7 eV, respectively. The former is due to the Si 3p bands and the latter to the Si 3s ones. The bulk silicon has been extensively investigated in the reo
cent years and our UPS spectrum is in agreement with the spectra reported in previous work [10]. In the spectrum of bulk tellurium, reported on the top of Fig. 5, three well-resolved peaks, B °, B ~, B 2, are clearly distinguishable. This sample has been prepared by evaporating about 1000 A of Te on the silicon substrate. No LEED pattern was detectable from the sample, leading to the conclusion that thick
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BINDING ENERGY (eV) Fig. 5. UPS spectra from clean Si(100)2Xl substrate (lower curve), Te/Si films, with different thicknesses, and bulk tellurium (upper curve).
S. Di Nardo et al. / Surface Science 331-333 (1995) 569-574
tellurium films grown at room temperature on Si(100)2 × 1 are amorphous. The B °'1'2 peaks were not observed in previous works [11] where the UPS spectrum of amorphous Te does not show clearly resolved B 1 and B 2 peaks but only a single broad peak [11]. On the contrary our spectrum is more similar to that attributed to crystalline tellurium [11]. The features B °'1'2 are due to the tellurium 5p 4 electrons. The B ° peak is known to be due to the Te lone-pair p-electrons [11]. The lone-pairs are nonbonding orbitals while W': are bonding orbitals. At present a more accurate assignment of the B 1 and B 2 peaks seems to be still questionable. Starting from bulk Te and decreasing the amount of deposited tellurium, the B ° peak decreases wi!h respect to B 1'2 and, in samples with less than 7 A Te/Si, only a single broad peak is present in the same energy interval. The presence of the silicon features do not influence the qualitative observation performed about the ratio between the B ° peak and the B 1'2 one. The reason is that the peak A of silicon lies in an energy range where the tellurium spectrum has a minimum. Moreover the former has a smooth shape while the latter has three distinct peaks that are clearly detectable in samples with more than about 10 A of Te, that is when tellurium does not follow the SK growth mode anymore. We can give a simple explanation of the lowering of the B ° peak with respect to the B 1'2 one as follows: in the initial stage of growth no lone-pair electrons are present because all the 5p electrons participate to the bonds. This may be due to the fact that the tellurium atoms need all the four 5p electrons in order to saturate the Si dangling bonds and to follow the tetrahedral lattice order of the silicon, as shown from LEED results. In fact the crystal structure of bulk tellurium consists of hexagonal arrays of helical chains of atoms. This is a distorted simple cubic structure, where the four Te 5p valence electrons are involved in different kinds of bonds that lead to the formation of the different well-defined B °'L2 peaks in the U P S spectrum of bulk Te [11]. On the contrary in the 2 A T e / S i film, because of the T e - S i interaction (as shown from XPS results), Te atoms are constrained to follow the more symmetric square lattice of the Si(100) surface, so the shape of the UPS spectra up to 5 ,~ show a single broad peak. Subsequently, increasing the thickness of the tellurium film, the number of bulk-
573
like atoms of tellurium, with lone pair electrons, that do not follow the order of the silicon substrate anymore, increases, determining the growth of the B ° peak with respect to the B 1'2 one.
4. Conclusions From this work we deduce that the interaction between Te and Si is weak. In fact only the atoms in the first layer of Te deposited on Si(100) were found to have slight different electronic properties with respect to those of bulk Te. This weak substrate-adsorbate interaction together with the larger lattice parameters of bulk Te, with respect to the one of the silicon substrate, leads to the observed initial SK growth mode of Te on Si(100). When tellurium grows in the SK mode, that is until the nominal coverage reaches the value of about 10 .~, it saturates the dangling bonds of Si(100)2 × 1, that result in a bulk-like arrangement, and the Te atoms follow the lattice order of the substrate. On the contrary, with larger amounts of tellurium, when islands begin to coalesce, the atoms do not grow following the silicon lattice anymore. This growth mode determines important variation in the valence band electronic structure recorded as a function of Te thickness. In the bulk tellurium, two of the four 5p electrons that are present in the outer shell, are coupled in lone pairs in order to form non-bonding orbitals. On the contrary, in the early stage of growth of tellurium deposited at room temperature on Si(100)2 × 1, the above 5p electrons are involved in bonding orbitals in order to saturate the dangling bonds of the Si(100) surface and this leads the atoms to follow the 1 × 1 order of the substrate.
References [1] G. Ertl and J. Kiippers, Low Energy Electrons and Surface Chemistry (VCH, Weinheim, 1985) p. 201. [2] R.I.G. Uhrberg, R.D. Bringans, R.Z. Bachrach and J.E. Northrup, Phys. Rev. Lett. 56 (1986) 520; R.D. Bringans, M.A. Olmstead and R.Z. Bachraeh, Phys. Rev. B 34 (1986) 7447. [3] D.H. Rich, G.E. Franklin, F.M. Leibsle, T. Miller and T.C. Chiang, Phys. Rev. B 40 (1989) 11804. [4] M. Richter, J.C. Woicik, P. Pianetta, K.E. Miyano, T.
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[5] [6] [7] [8]
S. Di Nardo et al. / Surface Science 331-333 (1995) 569-574
Kendelewicz, C.E. Bouldin, W.E. Spicer and I. Lindau, J. Vac. Sci. Technol. A 9 (1991) 1951. S. Di Nardo, L. Lozzi, M. Passacantando, P. Picozzi and S. Santucci, J. Electron Spectrosc. Relat. Phenom. 71 (1995) 39. S. Higuchi and Y. Nakanishi, J. Appl. Phys. 71 (1992) 4277. D. Briggs and M. Seah, Practical Surface Analysis (Wiley, New York, 1983). R. Payling, J. Electron Spectrosc. Relat. Phenom. 36 (1985) 99.
[9] S. Ossicini, R. Memeo and F. Ciccacci C. Mariani, J. Vac. Sci. Technol. A 3 (1985) 387. [10] G. Hollinger and F.J. Himpsel, J. Vac. Sci. Technol. A 1 (1983) 640. [11] L. Ley, M. Cardona and R.A. Pollak, in: Photoemission in Solids II, Eds. L. Ley and M. Cardona (Springer, Berlin, 1979) p. 110,
Surface Science 331-333 (1995) 569-574
Electron spectroscopy investigation of Te thin films deposited at room temperature on Si(100)2 × 1 S. Di Nardo, L. Lozzi *, M. Passacantando, P. Picozzi, S. Santucci Dipartimento di Fisica, Universita' degli Studi de L 'Aquila, 67010 Coppito (AQ), Italy Received 30 July 1994; accepted for publication 9 December 1994
Abstract Ultraviolet and X-ray photoelectron spectroscopies, Auger electron spectroscopy and low energy electron diffraction were used in order to investigate the electronic properties and the growth mode of very thin films of tellurium with mean thickness between 0.5 and 1000 A deposited at room temperature on a Si(100)2 x 1 surface. The adsorbate-substrate interaction is found to be weak. In the initial stage of the growth the tellurium atoms are chemisorbed on the silicon surface and form a continuous monolayer. The Te 5p valence electrons are involved in order to saturate the Si(100) dangling bonds inducing a 1 × 1 reconstruction of the surface. Tellurium deposited after the formation of the first continuous layer and up to 10 A of average thickness, forms 1 × 1 ordered islands, that is Te deposited at room temperature on Si(100) follows a Stranski-Krastanov growth mode (one layer plus islands). Increasing the amount of Te the islands coalesce and the deposited atoms show a bulk-like behaviour. Keywords: Auger electron spectroscopy; Chalcogens; Growth; Low energy electron diffraction (LEED); Metal-semiconductor interfaces; Photoemission; Single crystal epitaxy
1. Introduction In recent years the properties of silicon surfaces have been one of the most investigated topics in surface science. This is due to the fact that the Si(100) surface is the most widely used material in the realisation of electronic devices. However, many efforts come also from fundamental scientific interest. The dominant structure in the clean Si(100) surface is the 2 × 1 reconstruction due to the formation of dimers arranged in parallel rows [1]. Each Si surface atom has a dangling bond that causes the surface to be reactive. Recent works have shown that
* Corresponding author. Fax: + 39 862 433033; E-mail: [email protected].
the 2 × 1 reconstruction of Si(100) can be removed by the adsorption of different kinds of atoms that leads to different reconstructions of the silicon surface [2-6]. In particular the deposition of tellurium on Si(100) both at room temperature [5] and at 400 K [6] leads the surface atoms to have a 1 × 1 order. In order to study the electronic properties of the T e / S i interface, in this paper we analyze the UPS and XPS spectra collected, using He I and Mg K c~ sources, respectively, from thin tellurium films deposited at room temperature on Si(100)2 × 1. Moreover we deduce the growth mode of the overlayer by plotting the AES weighted peak-to-peak height ratio as a function of the nominal coverage. Observing the changes in the LEED pattern we check the crystalline structure of the surface of the films. The
0039-6028/95//$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSD! 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 0 3 1 9 - 3
570
X Di Nardo et al. / Surface Science 331-333 (1995) 569-574 ,
experiments were performed in ultra-high vacuum varying between 0.5 and 1000 ,~ the nominal thickness, measured with a quartz microbalance, of the tellurium films deposited on silicon.
,
,
,
,
,
,
,
,
,
,
,
,~/~ Te/Si
Te 4d
I'\
2. Experimental The experiments were performed in an ultra-high vacuum (UHV; about 4 × 10 -1° Torr) system consisting of an analysis chamber connected to a sample preparation chamber. Te thin films were prepared by evaporation of high purity Te (99.999%) on Si(100) p-doped, cut from a commercial wafer, by means of an alumina crucible contained in an electrically heated tungsten basket. The substrate was cleaned by ion gun bombardment and successively 2 × 1 reconstructed by heating at about 900°C for 5 min. The substrate was maintained at room temperature during the deposition. The amount of the deposited tellurium (the nominal coverage) was controlled with a quartz microbalance. No traces of impurities were detected from the XPS analysis of the samples. The photoemission spectra were recorded by means of an hemispherical analyzer using He I (hv= 21.2 eV) and M g K a (hv= 1253.6 eV) photon sources. The energy calibration was based on the carbon ls photoline position (BE = 284.8 eV). The AES spectra were recorded with a single-pass cylindrical mirror analyzer equipped with a coaxial electron gun. The first derivative of the Auger signal was recorded applying a 2 V peak-to-peak modulation to the outer cylinder of the analyzer and using the lock-in technique. A four grid reverse view LEED was used and the patterns on the screen were recorded by a PC interfaced camera.
3. Results and discussion Using the XPS technique both core levels and Auger peaks have been followed as a function of the Te film thickness. In particular the Auger parameter of silicon has been carefully checked. This parameter, c~', is defined as the sum of the kinetic energy (KE) of the most prominent peak of an Auger feature involving three core levels and the binding energy (BE) of one of these core levels [7]. For silicon:
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/
,
50
i
i
,
t
45
i
i
i
b
,
i
,
40
,
35
BINDING ENERGY (eV) Fig. 1. XPS spectra of the Te4d core levels from Te/Si(100)2 × 1 films, with different thicknesses, and from bulk tellurium. The dashed and the solid lines indicate the binding energy position of the Te4ds/2 peak in the bulk tellurium and in the 2 A T e / S i film, respectively. The shift of the Te4ds/2 peak is about 0.2 eV.
od(Si) = KE(Si KL2,3L2,3) d- BE(Si2p) = 1716 eV. It has been shown that this parameter is very sensitive to the chemical bonds [7]. In our data no changes in the Si 2p binding energy (99.8 eV) and in the Si Auger parameter have been observed varying the Te film thickness. The same behaviour has been obtained in the Te3d features (3d5/2 = 572.8 eV and 3d3/2 = 583.2 eV). On the contrary a slight difference in the Te4d spectrum of different films has been observed as the Te film thickness increases, as shown in Fig. 1. The 2 .~ T e / S i spectrum and the bulk Te one have the same shape, but the Te4ds/2 peak binding energy, in the 2 A T e / S i film (continuous line), is about 0.2 eV higher than the one in the bulk tellurium (dashed line). The shift towards higher binding energies of the Te levels can be explained by noting that Te is slightly more electronegative than the silicon. Moreover some changes in shape are detectable in films with intermediate thickness. In particular the spectrum of the 3 .~ film, which shows non-resolved 4d3/2 and 4d5/2 peaks, seems to be the result of a superposition between the 4d spectrum of the bulk Te and the shifted one of the 2 A T e / S i film. This superposition effect is less evident but still visible in the 5 A sample and allows us to conclude that the Te 4d XPS spectrum of tellurium films up to 5 .~ is the sum of two slightly shifted components,
.
S. Di Nardo et al. / Surface Science 331-333 (1995) 569-574
one due to the first continuous layer, that interacts with the Si(100)2 × 1 substrate, and another due to the subsequently deposited Te atoms. A similar effect is not detected in the Te 3d peaks, allowing the conclusion that the 3d electrons, that are in an inner shell, are not influenced by the presence of silicon. In Fig. 2 we show the Te MNN Auger transition of several Te/Si(100)2 × 1 thin films and of bulk tellurium. This Auger transition involves the Te 3d and Te 4d levels. Visible changes are detectable in the spectra varying the thickness of the films. The Te MNN spectra of the thinnest tellurium films, 1 and 2 of nominal thickness, have the same shape and energy position, but they show a rigid shift of about 1.5 eV, towards lower kinetic energies, with respect to the bulk Te spectrum. As in the case of Te 4d, when the tellurium film thickness is in the range of the first continuous layer, the Te MNN Auger spectrum is influenced by the interaction between tellurium and silicon. Also in this case the spectra of Te films having a thickness greater than 2 A are found to be a superposition between the spectrum of bulk Te and that of 2 A T e / S i film. In fact the Te MNN
+e iNN .
.
.
.
.
.
.
+e/si
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478
482
486
490
494
498
KINETIC ENERGY (eV) Fig. 2. XPS spectra of the Te MNN Auger transition from Te/Si(100)2 × 1 films, with different thicknesses, and from bulk tellurium. The dashed lines indicate the kinetic energy position of the TeoM4N4,sN4. 5 Auger transition in the bulk tellurium and in the 2 A T e / S i film. The dashed line spectrum has been artfully obtained by adding the 2 A, T e / S i spectrum, multiplied by 0.3, and the bulk Te one, multiplied by 0.7.
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Fig. 3. Plot of the AES weighted peak-to-peak height ratio R between Te and Si as a function of the nominal coverage of Te on the Si(lO0)2× 1 surfacc. In the inset the initial stage of the growth is reported with a larger scale; the solid lines have been obtained by linear interpolation of the data in the 0 - 3 and 3 - 1 0 ,~ range, respectively.
spectrum of the 15 A T e / S i sample is very well fitted by a spectrum, reported with a dashed line in Fig. 2, obtained adding the 2 A T e / S i spectrum, multiplied by 0.3, and the bulk Te one, multiplied by 0.7. Obviously, increasing the film thickness, the bulk Te features dominate with respect to those due to the Te atoms interacting with the silicon substrate. The XPS results suggest the formation of a first layer interacting with the silicon substrate and the following growth of other Te layers which show a bulk-like behaviour. In order to study the Te growth mode, Auger and LEED spectroscopies have been applied. In Fig. 3 we present a plot of the first derivative AES weighted peak- to-peak height ratio R between Te and Si signals as a function of the nominal coverage of Te on the Si(100) surface: R = (ITe//STe)/(Isi//Ssi). The intensities Isi,Xe used for this analysis were taken from Si(92 eV) and Te(483 eV) Auger lines and the sensitivity factors were Ssi = 0.31 and STe = 0.70, respectively [8]. The data in the inset of Fig. 3 clearly indicate that, in the early stage of the growth, tellurium deposited at room temperature on Si(100)2 × 1 follows a StranskiKrastanov (SK: one layer plus islands) growth mode [5,9]. The first continuous layer of Te is completed when the nominal coverage reaches the value of
572
S. Di Nardo et al. / Surface Science 331-333 (1995) 569-574
Fig. 4. LEED patterns of (a) Si(100) surface after 2 × 1 reconstruction of the surface in two orthogonal domains and (b) 2 .~ of tellurium deposited on silicon.
about 2 .~. The deposited tellurium follows the SK growth mode up to about 10 A, then, for thicker films, the exponential increase of R indicates the coalescence of the islands. Fig. 4a shows the LEED pattern obtained from the Si(100) substrate, after 2 X 1 reconstruction of the surface in two orthogonal domains. After deposition of 2 .~ of nominal thickness of tellurium on silicon the sample exhibits a well-defined 1 X 1 LEED °pattern, as shown in Fig. 4b. In the range 2-10 A of T e / S i the intensity of the 1 X 1 LEED pattern does not change appreciably, then, it becomes more and more diffuse and, at about 90 A, it quenches. This result, together with the AES data, indicates the formation of well-ordered islands, in the range 2-10 ~, of Te, on one ordered continuous layer of tellurium. The interaction of the first Te layer with the Si atoms and its 1 X 1 reconstruction, should modify the Te valence states. Fig. 5 shows the UPS spectra of Te thin films, with nominal thickness between 0.5 and 1000 A, deposited at room temperature on Si(100)2 X 1 surface. The silicon substrate UPS spectrum is reported too (bottom curve). The spectrum of Si(100)2 X 1 is composed of two main features, labelled A and/~, located at about 3 and 7 eV, respectively. The former is due to the Si 3p bands and the latter to the Si 3s ones. The bulk silicon has been extensively investigated in the reo
cent years and our UPS spectrum is in agreement with the spectra reported in previous work [10]. In the spectrum of bulk tellurium, reported on the top of Fig. 5, three well-resolved peaks, B °, B ~, B 2, are clearly distinguishable. This sample has been prepared by evaporating about 1000 A of Te on the silicon substrate. No LEED pattern was detectable from the sample, leading to the conclusion that thick
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BINDING ENERGY (eV) Fig. 5. UPS spectra from clean Si(100)2Xl substrate (lower curve), Te/Si films, with different thicknesses, and bulk tellurium (upper curve).
S. Di Nardo et al. / Surface Science 331-333 (1995) 569-574
tellurium films grown at room temperature on Si(100)2 × 1 are amorphous. The B °'1'2 peaks were not observed in previous works [11] where the UPS spectrum of amorphous Te does not show clearly resolved B 1 and B 2 peaks but only a single broad peak [11]. On the contrary our spectrum is more similar to that attributed to crystalline tellurium [11]. The features B °'1'2 are due to the tellurium 5p 4 electrons. The B ° peak is known to be due to the Te lone-pair p-electrons [11]. The lone-pairs are nonbonding orbitals while W': are bonding orbitals. At present a more accurate assignment of the B 1 and B 2 peaks seems to be still questionable. Starting from bulk Te and decreasing the amount of deposited tellurium, the B ° peak decreases wi!h respect to B 1'2 and, in samples with less than 7 A Te/Si, only a single broad peak is present in the same energy interval. The presence of the silicon features do not influence the qualitative observation performed about the ratio between the B ° peak and the B 1'2 one. The reason is that the peak A of silicon lies in an energy range where the tellurium spectrum has a minimum. Moreover the former has a smooth shape while the latter has three distinct peaks that are clearly detectable in samples with more than about 10 A of Te, that is when tellurium does not follow the SK growth mode anymore. We can give a simple explanation of the lowering of the B ° peak with respect to the B 1'2 one as follows: in the initial stage of growth no lone-pair electrons are present because all the 5p electrons participate to the bonds. This may be due to the fact that the tellurium atoms need all the four 5p electrons in order to saturate the Si dangling bonds and to follow the tetrahedral lattice order of the silicon, as shown from LEED results. In fact the crystal structure of bulk tellurium consists of hexagonal arrays of helical chains of atoms. This is a distorted simple cubic structure, where the four Te 5p valence electrons are involved in different kinds of bonds that lead to the formation of the different well-defined B °'L2 peaks in the U P S spectrum of bulk Te [11]. On the contrary in the 2 A T e / S i film, because of the T e - S i interaction (as shown from XPS results), Te atoms are constrained to follow the more symmetric square lattice of the Si(100) surface, so the shape of the UPS spectra up to 5 ,~ show a single broad peak. Subsequently, increasing the thickness of the tellurium film, the number of bulk-
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like atoms of tellurium, with lone pair electrons, that do not follow the order of the silicon substrate anymore, increases, determining the growth of the B ° peak with respect to the B 1'2 one.
4. Conclusions From this work we deduce that the interaction between Te and Si is weak. In fact only the atoms in the first layer of Te deposited on Si(100) were found to have slight different electronic properties with respect to those of bulk Te. This weak substrate-adsorbate interaction together with the larger lattice parameters of bulk Te, with respect to the one of the silicon substrate, leads to the observed initial SK growth mode of Te on Si(100). When tellurium grows in the SK mode, that is until the nominal coverage reaches the value of about 10 .~, it saturates the dangling bonds of Si(100)2 × 1, that result in a bulk-like arrangement, and the Te atoms follow the lattice order of the substrate. On the contrary, with larger amounts of tellurium, when islands begin to coalesce, the atoms do not grow following the silicon lattice anymore. This growth mode determines important variation in the valence band electronic structure recorded as a function of Te thickness. In the bulk tellurium, two of the four 5p electrons that are present in the outer shell, are coupled in lone pairs in order to form non-bonding orbitals. On the contrary, in the early stage of growth of tellurium deposited at room temperature on Si(100)2 × 1, the above 5p electrons are involved in bonding orbitals in order to saturate the dangling bonds of the Si(100) surface and this leads the atoms to follow the 1 × 1 order of the substrate.
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