- Email: [email protected]
In situ radiotracer method for study of adsorption on semiconductor single crystal surfaces
surface science ELSEVIER
Applied Surface Science 92 (1996) 431-435
In situ radiotracer method for study of adsorption on semiconductor single crystal surfaces Marek Szldarczyk * Department of Chemistry, Warsaw University, AL Zwirki i Wigury 101, 02-089 Warsaw,Poland Received 16 December 1994; accepted for publication 4 March 1995
Abstract The in situ radiotracer method has been applied to the studies of adsorption on single crystal semiconductor surfaces. On the basis of studies of formic acid and thiourea adsorption on n-Si (100) surface it was shown that the radiotracer method yields a qualitative and quantitative characterization of surface processes taking place on the surface of the semiconductor materials. Surface densities of adsorbed species as low as 5 x 1012 and as high as 2 × 1015 molecules per cm 2 were detected. It has been found that during adsorption of formic acid a two-layer adsorbate is formed. The first layer is formed of COOH radicals while the second layer is formed of HCOOH molecules. Both layers are linked by hydrogen bonds. The adsorption layer formed during adsorption of thiourea consists of two products: thiourea molecules and either sulphur atoms or SH- anions.
1. Introduction Adsorption studies of organic and inorganic species on semiconductor materials have been carded out using a variety of methods; spectroscopic, microscopic and those based on measurement of current-potential dependencies. None of these techniques can give an answer to the question what is the number o f adsorbed species unless a specific calibration procedure based sometimes on uncertain assumption is used. Without knowledge of the surface density of adsorbed species precise determination of the surface
* Tel.: +48 22 224-881; fax: +48 22 225-996; e-mail: [email protected].
structure of adsorbate and of the adsorption mechanism is not readily attained. For these reasons we have attempted to apply the in situ radiotracer technique to the studies of adsorption on monocrystalline semiconductor electrodes. The advantage of the radiotracer technique is its experimental simplicity [1,2]. In this report the results of electrochemical studies of formic acid (HCOOH) and thiourea (TU) adsorption on the n-Si (100) surface are described.
2. Description of radiotracer technique In the present study we used a variant of the radiotracer technique termed, the electrode lowering method [1,2]. The concept of this method is based on the distinguishing between two counting rate signals
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00271-5
432
M. Szklarczyk / Applied Surface Science 92 (1996) 431-435
upcoming from the molecules whose adsorption is studied and which are labelled with a radionuclide emitting soft [3-radiation. The first counting rate signal is due to the labelled molecules present in the bulk phase of either liquid or gas, the second one is due to the species adsorbed on the surface of a sample. In practice the concept described above is realized in the following way. An optically flat glass scintillator, which is used as the nuclear radiation detector, is located at the bottom of the cell with the semiconductor surface staying about 4 mm above its surface. With the semiconductor sample in this position only the counting rate due to the radioactivity of a solution (N~up) is measured. The radiation of adsorbed species is not detected in this position by the glass scintillator because the soft ~-radiation characterized by a low range of energy is absorbed by a few millimeters layer of solution. When the semiconductor is pressed down against the scintillator within a distance of a few microns most of the solution is removed from between the scintillator and the surface. With the sample in the lowered position the detector responds to the radioactive species adsorbed on the surface of a sample (Na), as well as to the radioactive molecules trapped in the thin gap between the two surfaces (Nd). The total of the two signals (Ntd) is measured. The thin gap component of the total signal can be determined separately, e.g. by carrying measurement under the condition when no adsorption occurs [2]. The density of adsorbed species, F, can be expressed in terms of measured counting rates [1,2]: Na
10-3CNAv
F_- m N~"p t~Rfb exp[-/.LsX] where c (mol dm-3) is the bulk concentration of the adsorbate, NAy the Avogadro number (molec cm-2 ), ~s the linear absorption coefficient of the B-radiation in the bulk phase (cm-l), e.g. for water it equals to 300 or 287 cm-i for 14C and 35S nuclides, respectively [3,4], R the roughness factor of the electrode surface, x the distance between the electrode and the glass scintillator surface (cm), and fb the backscattering factor equal to 1.3 [2]. The gap thickness x can be determined from the Nsd/Ns up ratio [1,2]. In
the experiments described in this paper it was 5-6 Ixm depending on the sample used.
3. Experimental The samples were prepared from the phosphorus doped n-Si (100) single crystals and had resistivity of 0.09-0.06 l~. cm. The diameter of each electrode was 12.5 mm. The ohmic contact was provided by a gallium-indium alloy. Before each experiment, the electrode surface was polished with diamond pastes down to 0.25 p~m diameter and, immediately before placing it in the cell, etched with HF: water = 1:10 solution for one minute. After transferring to the experimental cell, the activity of silicon surface was electrochemically checked [2]. A freshly prepared electrode was used in each experiment. The roughness factor of the polished n-Si electrode was assumed to be equal to unity on the base of the scanning tunneling microscopy photographs taken for silicon surfaces prepared in a similar way to that described in this article [5]. The experiments were carried out in 0.1 N HCIO4 solution. 14 The radioactive samples of C labelled HCOOH, and 14C and 35S. labelled TU were supplied by Amersham, England and OPiDI, Poland. The original radioactive material was isotopically diluted to the activity of 10 Ci mol-1.
4. Results and discussion The plots of surface density of adsorbed species versus silicon electrode potential and versus concentration in the bulk of the liquid phase of HCOOH molecules 14C labelled and TU molecules 14C or 35S labelled are shown in Figs. 1 and 2, respectively. The observed sensitivity to the electrode potential and bulk concentration proves that the applied method can be used in the studies of adsorption on the surfaces of semiconductor single crystals. The differences in the interaction of HCOOH and TU with Si surface are easily noted. The number of adsorbed " T U " species at any electrode potential is much higher than that observed for " H C O O H " ones (Fig. 1). Furthermore, there is a difference in the surface density of 35S labelled species (S-species)
433
M. Szklarczyk/ Applied Surface Science 92 (1996) 431-435
and ~4C labelled species (C-species) formed during adsorption of TU (Figs. 1 and 2). The isotherms presented in Fig. 2 show that the increase in F with the hulk concentration of studied compounds is much faster for HCOOH than TU (Fig. 2).
4•1. Adsorption of HCOOH on n-Si (100) surface The values of density of adsorbed species combined with the electrochemically observed flow of charge through the semiconductor/liquid interface (96 p,C cm -2 [2]) yields the number of electrons taking part in the reduction of a single HCOOH molecule (epm) and the number of electrons transferred per adsorption site (eps). For an electrode potential of - 0 . 3 9 V and a bulk concentration of HCOOH of 2.5 × 10-3M the determined surface density of adsorbate is 1.4 × l0 ts molecules cm -2 (Fig. 3). The epm and eps numbers are respectively 0.4 and 0.9 for the Si surface atoms density of 6.8 × 10 ~4 atoms cm -2 [6,7]. The eps = 0.9 value suggests that one electron reduction of adsorbate occurs on each surface atom of silicon. On the other hand the epm = 0.4 value is
20-
O
\
-
I
-05
TUIC-~ ~aet~dl
I
0
-~
i
05
I
1
I
1.5
EIV Fig. 1. Dependence of the surface density of adsorbed species, F, on potential for n-Si (100) electrode. The dependencies marked with ([3), ( X ) and (C)) were determined for 14C labelled formic acid, and 14C and 35S labelled thiourea, respectively. Arrows indicate the direction of change of the electrode potential. The bulk concentrations of the studied compounds were CHOOOH 1.3 × 10-4M and Cthio.rea= 10-4M• =
200
!
=
150
,oo r== 5o
•
o
1
l
2
I
I
3
4
I
5
c i10"a tool. dm-3 Fig. 2. Dependence of the surface density of adsorbed species, F, on the analytical concentration into the bulk of the solution• The dependencies marked with ([]), ( X ) and ( O ) were determined for 14C labelled formic acid, and 14C and 35S labelled thiourea, respectively. Adsorption potentials of HCOOH and of thiourea were -0•34 and -0•28 V, respectively.
half the eps value so there are two adsorbed species for each silicon atom. One of these species may be the COOH radical formed in one electron reduction process and the second one may be the unreduced HCOOH molecule. The comparison of the dimensions of the HCOOH molecule [8,9] with the available space on the Si(100) [7], and the surface density of adsorbed species (1.4 × 10 t5 molecules cm -2) leads to the conclusion that the species are adsorbed in two layers. The species in the first layer, COOH radicals, are directly bounded to the surface atoms. The second layer, HCOOH molecules, is linked via hydrogen bonds with the COOH radicals. As a result the surface associates are created on the n-Si electrode (Fig. 3A). The postulate of formation of HCOOH associates on the electrode surface is in accordance with the high enthalpy of formation of cyclic HCOOH dimers (62.5 kJ mol =l [I0,II]). This energy is higher than the energy of the hydrogen bond between the carboxylic group and water molecule (15.1 Id m o l - I [12]) which should be broken before (HCOOH) 2 dimer is formed. These dimers exist even in dilute HCOOH water solution [12]. The concentration of
M. Szldarczyk / Applied Surface Science 92 (1996) 431 --435
434
A.
adsorbed species are not removed from the surface in the form of carbon dioxide contrary to the behaviour observed for HCOOH adsorbed on other materials [13]. Oxidation of the silicon surface starts at the potential more positive than 0.2 V [2] which is negative with respect to the potential of the CO 2 formation (0.5 V [13]). As a result, a layer of the growing oxide is quite likely to enclose the absorbed HCOOH adsorbate to preventing it from being removed as CO 2 from the surface at more positive potentials.
@ Bulk Si Surface Si ads. HCOOH dimers atom atom
B.
BulkSi SurfaceSi Thiourea atom atom molecule Fig. 3. Scheme of proposed orientation of adsorbed species on the n-Si (100) surface: (A) the adsorbate derived from HCOOH solution and (B) the adsorbate derived from TU solution.
the HCOOH species near the electrode surface is higher than in the bulk of the solution which further increases the probability of dimer formation. The radiotracer results show that when the silicon electrode potential is changed into positive direction, the surface quantities decreases at first and then, above 0.1 V, level off (Fig. 1). It means that the adsorption of HCOOH on n-Si electrode is independent of the oxidation of silicon surface and that
4.2. Adsorption of thiourea on n-Si (100) surface The radiotracer results indicate that the process of thiourea adsorption at silicon electrode surface is a complex one. The different amount of adsorbed species containing 14C (C-species) and 35S (S-species) (Fig. 1) is the evidence that products of different composition are formed on the electrode surface. Their behaviour at the electrode surface is different. The surface concentration of C-species only slightly depends on the electrode potential in the range from 0.06 to - 0 . 4 0 V while the surface concentration of the S-species is strongly potential dependent (Fig. 1). This means that when the quantity of C-species after adsorption remains almost constant, the layer of S-species continuously builds up. When the electrode potential is changed in the anodic direction both types of species are removed from the electrode surface to a similar extent (Fig. 1). Moreover, the results presented in Fig. 2 show that under experimental conditions saturation of silicon surface with the C-species can be obtained whereas it is not the case for the S-species. The electrochemical studies [14] showed that one of this products is physically bonded to electrode surface while the other one (reduced form of TU) is chemically bonded to the surface. The chemical composition of both forms can be suggested on the basis of observed maximum surface concentration of adsorbed species (Figs. 1 and 2), the size of thiourea molecule, silicon (100) lattice distances, and assumed type of interaction of both forms of adsorbate with the silicon electrode surface. The maximum surface concentration of C-species is ,-, 1.5 × 1014 molecules cm -2 for the bulk con-
M. Szldarczyk/ Applied Surface Science 92 (1996) 431-435
centration equal to 2 × 10-3M (Fig. 2). An attempt of correlating this value with the number of outermost silicon atoms [6,7] shows that one C-species is adsorbed on four silicon atoms, i.e. on two surface dimers. The silicon atom diameter is 2.34 ,~ and the distance is either 3.2 or 3.8/~ in dependence on the crystallographic direction [6]. This means that the diameter of C-species has to be equal to about 4 - 6 ,~. The largest carbon containing species in the studied system is the thiourea molecule itself. The diameter of the thiourea molecule, estimated on the basis of free rotation of a molecule about the carbon atom is ~ 5.1 ,~. (The dimensions of the TU molecule wee taken from Refs. [15,16].) An additional argument for the verification of the C-species as the thiourea molecules is their known weak, electrostatic interaction with the electrode surface. For some time the thiourea molecule has been considered as the surface probe dipole acting like an anion [17]. A possible perpendicular orientation of the thiourea molecule is shown in Fig. 3B. With the increase the negative potential this orientation will become flat
[181. The maximum surface concentration of the Sspecies is higher than the concentration of the Cspecies (Fig. 2). It has to be pointed out here that a part of the "sulphur" counting rate is due to the thiourea molecule which contains sulphur atoms. Hence, the actual concentration of species different than thiourea and not-containing carbon atoms is equal to the difference between the surface concentration of the S-species and C-species. This difference is equal to ~ 2.5 X 1014 molecules cm -2 at the maximum surface concentration of thiourea. To fit this number of species between thiourea molecules adsorbed on the silicon surface it is necessary to assume their small dimensions. It is proposed that the S-species that do not contain carbon atoms are either S atoms or SH- anions with a diameter of 2.1-2.5 ,~ [19]. The application of the radiotracer technique yielded information on the adsorbed " H C O O H " and " T U " layers. Furthermore, this technique enables for the determination of the surface density of adsorbed species as low as 5 x 1012 molecules c m -2 (Fig. 1). Such a density is 50 times lower than the detection limit attainable with the mass spectrometry technique previously applied in the studies of adsorp-
435
tion processes on semiconductors [20,21]. The highest surface density detected in this work was 2 × 10 t5 molecules cm- 2.
Acknowledgements This work was supported by the Committee for Scientific Research under grant No. 2 P303 058 07.
References [1] P. Zelanay and A. Wieckowski, Radioactive labeling: toward characterization of well defined electrodes, in: Electrochemical Interfaces: Modem Techniques For In-Situ Interface Characterization, Ed. H.D. Abruna (VCH, New York, 1991) ch. 9, p. 479. [2] M. Szklarczyk, Sl. Smolitiski and J. Sobkowski, Electrochim. Acta 39 (1994) 1903. [3] P. Zelenay, L.M. Rice-Jackson and A. Wi~ckowski, Langmuir 6 (1990) 974. [4] W.F. Libby, Phys. Rev. 103 (1956) 1900. [5] E. Tomita, A. Matsuda and K. Itaya, J. Vac. Technol. A 8 (1990) 534. [6] A. Brodde, Th. Berlrams and H. Neddermeyer, Phys. Rev. B 47 (1993) 4508. [7] R.E. Schlier and H.E. Famsworth, J. Chem. Phys. 30 (1959) 917. [8] C.C. Cheng and J.T. Yates, Jr., Phys. Rev. B 43 (1991) 4041. [9] T. Nahringbauer, Acta Cryst. B 34 (1978) 315. [10] P.A. Kolhnan and L.C. Allen, Chem. Rev. 72 (1972) 283. [11] S. Hashimoto, S. lkuta and M. Imamura, Chem. Phys. Lett. 62 (1979) 567. [12] T. K. Mukherjee, J. Phys. Chem. 71 (1969) 2277. [13] P. Zelenay and J. Sobkowski, J. Electroanal. Chem. 176 (1984) 209. [14] M. Szklarezyk and SI. Smolifiski, J. Electroanal. Chem., in press. [15] D. Mullen and E. HelIner, Acta Cryst. B 34 (1978) 2789. [16] I. Takahashi, A. Onodera and Y. Shiozaki, Acta Cryst. B 46 (1990) 661. [17] A.N. Frumkin, Ergeb. Exacten. Naturwiss. 7 (1928) 258. [18] J. Bukowska and J. Jackowska, J. Electroanal. Chem. 367
(1994) 41. [19] Handbook of Physics and Chemistry, 2rid ed., Ed. Wydawnictwa (Naukowo-Techniezne, Warsaw, 1974) [in Polish]. [20] C.C. Cheng, R.M. Wallace, P.A. Taylor, W.J. Choyke and J.T. Yates, Jr., J. Appl. Phys. 67 (1990) 3698. [21] C.C. Cheng, P.A. Taylor, R.M. Wallace, H. Gutleben, L. Clemen, M.L. Colalanni, P.J. Chen, W.H. Weinberg, W.J. Choyke and J.T. Yates, Jr., Thin Solids Films 225 (1993) 196.
Applied Surface Science 92 (1996) 431-435
In situ radiotracer method for study of adsorption on semiconductor single crystal surfaces Marek Szldarczyk * Department of Chemistry, Warsaw University, AL Zwirki i Wigury 101, 02-089 Warsaw,Poland Received 16 December 1994; accepted for publication 4 March 1995
Abstract The in situ radiotracer method has been applied to the studies of adsorption on single crystal semiconductor surfaces. On the basis of studies of formic acid and thiourea adsorption on n-Si (100) surface it was shown that the radiotracer method yields a qualitative and quantitative characterization of surface processes taking place on the surface of the semiconductor materials. Surface densities of adsorbed species as low as 5 x 1012 and as high as 2 × 1015 molecules per cm 2 were detected. It has been found that during adsorption of formic acid a two-layer adsorbate is formed. The first layer is formed of COOH radicals while the second layer is formed of HCOOH molecules. Both layers are linked by hydrogen bonds. The adsorption layer formed during adsorption of thiourea consists of two products: thiourea molecules and either sulphur atoms or SH- anions.
1. Introduction Adsorption studies of organic and inorganic species on semiconductor materials have been carded out using a variety of methods; spectroscopic, microscopic and those based on measurement of current-potential dependencies. None of these techniques can give an answer to the question what is the number o f adsorbed species unless a specific calibration procedure based sometimes on uncertain assumption is used. Without knowledge of the surface density of adsorbed species precise determination of the surface
* Tel.: +48 22 224-881; fax: +48 22 225-996; e-mail: [email protected].
structure of adsorbate and of the adsorption mechanism is not readily attained. For these reasons we have attempted to apply the in situ radiotracer technique to the studies of adsorption on monocrystalline semiconductor electrodes. The advantage of the radiotracer technique is its experimental simplicity [1,2]. In this report the results of electrochemical studies of formic acid (HCOOH) and thiourea (TU) adsorption on the n-Si (100) surface are described.
2. Description of radiotracer technique In the present study we used a variant of the radiotracer technique termed, the electrode lowering method [1,2]. The concept of this method is based on the distinguishing between two counting rate signals
0169-4332/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00271-5
432
M. Szklarczyk / Applied Surface Science 92 (1996) 431-435
upcoming from the molecules whose adsorption is studied and which are labelled with a radionuclide emitting soft [3-radiation. The first counting rate signal is due to the labelled molecules present in the bulk phase of either liquid or gas, the second one is due to the species adsorbed on the surface of a sample. In practice the concept described above is realized in the following way. An optically flat glass scintillator, which is used as the nuclear radiation detector, is located at the bottom of the cell with the semiconductor surface staying about 4 mm above its surface. With the semiconductor sample in this position only the counting rate due to the radioactivity of a solution (N~up) is measured. The radiation of adsorbed species is not detected in this position by the glass scintillator because the soft ~-radiation characterized by a low range of energy is absorbed by a few millimeters layer of solution. When the semiconductor is pressed down against the scintillator within a distance of a few microns most of the solution is removed from between the scintillator and the surface. With the sample in the lowered position the detector responds to the radioactive species adsorbed on the surface of a sample (Na), as well as to the radioactive molecules trapped in the thin gap between the two surfaces (Nd). The total of the two signals (Ntd) is measured. The thin gap component of the total signal can be determined separately, e.g. by carrying measurement under the condition when no adsorption occurs [2]. The density of adsorbed species, F, can be expressed in terms of measured counting rates [1,2]: Na
10-3CNAv
F_- m N~"p t~Rfb exp[-/.LsX] where c (mol dm-3) is the bulk concentration of the adsorbate, NAy the Avogadro number (molec cm-2 ), ~s the linear absorption coefficient of the B-radiation in the bulk phase (cm-l), e.g. for water it equals to 300 or 287 cm-i for 14C and 35S nuclides, respectively [3,4], R the roughness factor of the electrode surface, x the distance between the electrode and the glass scintillator surface (cm), and fb the backscattering factor equal to 1.3 [2]. The gap thickness x can be determined from the Nsd/Ns up ratio [1,2]. In
the experiments described in this paper it was 5-6 Ixm depending on the sample used.
3. Experimental The samples were prepared from the phosphorus doped n-Si (100) single crystals and had resistivity of 0.09-0.06 l~. cm. The diameter of each electrode was 12.5 mm. The ohmic contact was provided by a gallium-indium alloy. Before each experiment, the electrode surface was polished with diamond pastes down to 0.25 p~m diameter and, immediately before placing it in the cell, etched with HF: water = 1:10 solution for one minute. After transferring to the experimental cell, the activity of silicon surface was electrochemically checked [2]. A freshly prepared electrode was used in each experiment. The roughness factor of the polished n-Si electrode was assumed to be equal to unity on the base of the scanning tunneling microscopy photographs taken for silicon surfaces prepared in a similar way to that described in this article [5]. The experiments were carried out in 0.1 N HCIO4 solution. 14 The radioactive samples of C labelled HCOOH, and 14C and 35S. labelled TU were supplied by Amersham, England and OPiDI, Poland. The original radioactive material was isotopically diluted to the activity of 10 Ci mol-1.
4. Results and discussion The plots of surface density of adsorbed species versus silicon electrode potential and versus concentration in the bulk of the liquid phase of HCOOH molecules 14C labelled and TU molecules 14C or 35S labelled are shown in Figs. 1 and 2, respectively. The observed sensitivity to the electrode potential and bulk concentration proves that the applied method can be used in the studies of adsorption on the surfaces of semiconductor single crystals. The differences in the interaction of HCOOH and TU with Si surface are easily noted. The number of adsorbed " T U " species at any electrode potential is much higher than that observed for " H C O O H " ones (Fig. 1). Furthermore, there is a difference in the surface density of 35S labelled species (S-species)
433
M. Szklarczyk/ Applied Surface Science 92 (1996) 431-435
and ~4C labelled species (C-species) formed during adsorption of TU (Figs. 1 and 2). The isotherms presented in Fig. 2 show that the increase in F with the hulk concentration of studied compounds is much faster for HCOOH than TU (Fig. 2).
4•1. Adsorption of HCOOH on n-Si (100) surface The values of density of adsorbed species combined with the electrochemically observed flow of charge through the semiconductor/liquid interface (96 p,C cm -2 [2]) yields the number of electrons taking part in the reduction of a single HCOOH molecule (epm) and the number of electrons transferred per adsorption site (eps). For an electrode potential of - 0 . 3 9 V and a bulk concentration of HCOOH of 2.5 × 10-3M the determined surface density of adsorbate is 1.4 × l0 ts molecules cm -2 (Fig. 3). The epm and eps numbers are respectively 0.4 and 0.9 for the Si surface atoms density of 6.8 × 10 ~4 atoms cm -2 [6,7]. The eps = 0.9 value suggests that one electron reduction of adsorbate occurs on each surface atom of silicon. On the other hand the epm = 0.4 value is
20-
O
\
-
I
-05
TUIC-~ ~aet~dl
I
0
-~
i
05
I
1
I
1.5
EIV Fig. 1. Dependence of the surface density of adsorbed species, F, on potential for n-Si (100) electrode. The dependencies marked with ([3), ( X ) and (C)) were determined for 14C labelled formic acid, and 14C and 35S labelled thiourea, respectively. Arrows indicate the direction of change of the electrode potential. The bulk concentrations of the studied compounds were CHOOOH 1.3 × 10-4M and Cthio.rea= 10-4M• =
200
!
=
150
,oo r== 5o
•
o
1
l
2
I
I
3
4
I
5
c i10"a tool. dm-3 Fig. 2. Dependence of the surface density of adsorbed species, F, on the analytical concentration into the bulk of the solution• The dependencies marked with ([]), ( X ) and ( O ) were determined for 14C labelled formic acid, and 14C and 35S labelled thiourea, respectively. Adsorption potentials of HCOOH and of thiourea were -0•34 and -0•28 V, respectively.
half the eps value so there are two adsorbed species for each silicon atom. One of these species may be the COOH radical formed in one electron reduction process and the second one may be the unreduced HCOOH molecule. The comparison of the dimensions of the HCOOH molecule [8,9] with the available space on the Si(100) [7], and the surface density of adsorbed species (1.4 × 10 t5 molecules cm -2) leads to the conclusion that the species are adsorbed in two layers. The species in the first layer, COOH radicals, are directly bounded to the surface atoms. The second layer, HCOOH molecules, is linked via hydrogen bonds with the COOH radicals. As a result the surface associates are created on the n-Si electrode (Fig. 3A). The postulate of formation of HCOOH associates on the electrode surface is in accordance with the high enthalpy of formation of cyclic HCOOH dimers (62.5 kJ mol =l [I0,II]). This energy is higher than the energy of the hydrogen bond between the carboxylic group and water molecule (15.1 Id m o l - I [12]) which should be broken before (HCOOH) 2 dimer is formed. These dimers exist even in dilute HCOOH water solution [12]. The concentration of
M. Szldarczyk / Applied Surface Science 92 (1996) 431 --435
434
A.
adsorbed species are not removed from the surface in the form of carbon dioxide contrary to the behaviour observed for HCOOH adsorbed on other materials [13]. Oxidation of the silicon surface starts at the potential more positive than 0.2 V [2] which is negative with respect to the potential of the CO 2 formation (0.5 V [13]). As a result, a layer of the growing oxide is quite likely to enclose the absorbed HCOOH adsorbate to preventing it from being removed as CO 2 from the surface at more positive potentials.
@ Bulk Si Surface Si ads. HCOOH dimers atom atom
B.
BulkSi SurfaceSi Thiourea atom atom molecule Fig. 3. Scheme of proposed orientation of adsorbed species on the n-Si (100) surface: (A) the adsorbate derived from HCOOH solution and (B) the adsorbate derived from TU solution.
the HCOOH species near the electrode surface is higher than in the bulk of the solution which further increases the probability of dimer formation. The radiotracer results show that when the silicon electrode potential is changed into positive direction, the surface quantities decreases at first and then, above 0.1 V, level off (Fig. 1). It means that the adsorption of HCOOH on n-Si electrode is independent of the oxidation of silicon surface and that
4.2. Adsorption of thiourea on n-Si (100) surface The radiotracer results indicate that the process of thiourea adsorption at silicon electrode surface is a complex one. The different amount of adsorbed species containing 14C (C-species) and 35S (S-species) (Fig. 1) is the evidence that products of different composition are formed on the electrode surface. Their behaviour at the electrode surface is different. The surface concentration of C-species only slightly depends on the electrode potential in the range from 0.06 to - 0 . 4 0 V while the surface concentration of the S-species is strongly potential dependent (Fig. 1). This means that when the quantity of C-species after adsorption remains almost constant, the layer of S-species continuously builds up. When the electrode potential is changed in the anodic direction both types of species are removed from the electrode surface to a similar extent (Fig. 1). Moreover, the results presented in Fig. 2 show that under experimental conditions saturation of silicon surface with the C-species can be obtained whereas it is not the case for the S-species. The electrochemical studies [14] showed that one of this products is physically bonded to electrode surface while the other one (reduced form of TU) is chemically bonded to the surface. The chemical composition of both forms can be suggested on the basis of observed maximum surface concentration of adsorbed species (Figs. 1 and 2), the size of thiourea molecule, silicon (100) lattice distances, and assumed type of interaction of both forms of adsorbate with the silicon electrode surface. The maximum surface concentration of C-species is ,-, 1.5 × 1014 molecules cm -2 for the bulk con-
M. Szldarczyk/ Applied Surface Science 92 (1996) 431-435
centration equal to 2 × 10-3M (Fig. 2). An attempt of correlating this value with the number of outermost silicon atoms [6,7] shows that one C-species is adsorbed on four silicon atoms, i.e. on two surface dimers. The silicon atom diameter is 2.34 ,~ and the distance is either 3.2 or 3.8/~ in dependence on the crystallographic direction [6]. This means that the diameter of C-species has to be equal to about 4 - 6 ,~. The largest carbon containing species in the studied system is the thiourea molecule itself. The diameter of the thiourea molecule, estimated on the basis of free rotation of a molecule about the carbon atom is ~ 5.1 ,~. (The dimensions of the TU molecule wee taken from Refs. [15,16].) An additional argument for the verification of the C-species as the thiourea molecules is their known weak, electrostatic interaction with the electrode surface. For some time the thiourea molecule has been considered as the surface probe dipole acting like an anion [17]. A possible perpendicular orientation of the thiourea molecule is shown in Fig. 3B. With the increase the negative potential this orientation will become flat
[181. The maximum surface concentration of the Sspecies is higher than the concentration of the Cspecies (Fig. 2). It has to be pointed out here that a part of the "sulphur" counting rate is due to the thiourea molecule which contains sulphur atoms. Hence, the actual concentration of species different than thiourea and not-containing carbon atoms is equal to the difference between the surface concentration of the S-species and C-species. This difference is equal to ~ 2.5 X 1014 molecules cm -2 at the maximum surface concentration of thiourea. To fit this number of species between thiourea molecules adsorbed on the silicon surface it is necessary to assume their small dimensions. It is proposed that the S-species that do not contain carbon atoms are either S atoms or SH- anions with a diameter of 2.1-2.5 ,~ [19]. The application of the radiotracer technique yielded information on the adsorbed " H C O O H " and " T U " layers. Furthermore, this technique enables for the determination of the surface density of adsorbed species as low as 5 x 1012 molecules c m -2 (Fig. 1). Such a density is 50 times lower than the detection limit attainable with the mass spectrometry technique previously applied in the studies of adsorp-
435
tion processes on semiconductors [20,21]. The highest surface density detected in this work was 2 × 10 t5 molecules cm- 2.
Acknowledgements This work was supported by the Committee for Scientific Research under grant No. 2 P303 058 07.
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