- Email: [email protected]
ISS and AES studies of the initial oxidation of Dy, Tb and their silicides
Materials Chemistry and Physics 58 (1999) 26±30
ISS and AES studies of the initial oxidation of Dy, Tb and their silicides G.L.P. Berninga,*, H.C. Swarta, W.D. Roosa, B. de Wittb a
University of the Orange Free State, P.O. Box 339, ZA-9300, Bloemfontein, South Africa b Technikon OFS, Private Bag X20539, Za-9300, Bloemfontein, South Africa
Received 11 May 1998; received in revised form 2 October 1998; accepted 4 October 1998
Abstract Samples of Tb and Dy as well as their silicides were oxidized in high vacuum under controlled conditions for oxygen exposures ranging up to 50 L (1 L 1 10ÿ6 Torrs). The oxides were studied by Auger electron spectroscopy (AES) and ion scattering spectroscopy (ISS). Distinct low energy Auger peak shape changes occurred during the oxidation of the metals as well as during the oxidation of the metal silicides. These peak shape changes are very similar for Tb and Dy as expected due to the similarities in chemical properties of the lanthanide elements. When the silicides were exposed to oxygen, both the metal and the Si were oxidized. In both cases the oxide layers were enriched by the metal. The depletion of the Dy just below the surface suggested oxygen induced surface segregation of Dy. A model is proposed in which the low energy region of the ISS spectra were used to determine the Si peak height. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Oxidation; Dy; Tb; Rare earths; ISS; AES
1. Introduction The rare-earth (RE) metal silicides form the highest Schottky barrier heights on P-type Si and the lowest on N-type Si, approaching ohmic contacts [1]. RE metals have a potential application in the development of solar cells [2], and have been investigated as potential gate materials in metal-insulator-semiconductor (MIS) devices [3]. It is almost impossible to isolate these metals chemically as the number of electrons in the outermost shells are the same. The remarkable similarity in chemical properties of the lanthanide elements can be ascribed to the fact that their ultimate and most of their penultimate electron orbitals are identical [4]. A disadvantage of the higher reactivity of RE metals is that they are more prone to unwanted oxidation. Oxidation is of major concern during processing at elevated temperatures. In order to use RE metals and their silicides in these types of applications it is necessary to require a knowledge of their surface oxidation. It would thus be advantageous to study the initial oxidation of Dy, Tb and their silicides, as examples of the RE metals, and at the same time point out the similarities and differences of the Auger spectra of the two silicides. Room temperature oxidation of
*Corresponding author. Fax: +51-4306490; e-mail: [email protected]
RE metal ± Si interfaces shows an enhancement of Si oxidation [5±8]. It is well known that Auger electron spectroscopy (AES) peak changes can be used as an indication of surface oxidation [9]. This technique as well as ISS was used to investigate the oxidation of terbium silicide and dysprosium silicide. Ion scattering spectroscopy is a technique by which an intensity analysis is made from the energy distribution of elastically backscattered ions. One of this technique's most important features, when using noble gas ions, is its exclusive ®rst layer sensitivity. This feature was utilized to determine which elements enriched the surface during the exposure of terbium silicide and dysprosium silicide to oxygen. 2. Experimental details Terbium silicide samples were prepared as follows: A Ê layer of Si followed by a 2000 A Ê layer of Tb was 3000 A deposited onto a previously oxidized Si(100) substrate. Electron beam evaporation of pure Tb and Si was used at a base pressure of 1 10ÿ7 Torr. Silicide formation was accomplished by annealing the samples at a temperature of 3208C for 45 min in a high vacuum (7 10ÿ7 Torr) quartz furnace. The same procedure as for Tb was used to form dysprosium silicide.
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S0254-0584(98)00242-9
G.L.P. Berning et al. / Materials Chemistry and Physics 58 (1999) 26±30
The following oxidation procedure was followed: the unannealed Tb and Dy samples were ®rst sputtered clean until the Tb and Dy were exposed with no detectable impurities. The system was then pumped down to a pressure of <2 10ÿ9 Torr before oxidation (the base pressure of the vacuum system was <8 10ÿ10 Torr). During oxidation the valve between the ion pump and the work chamber was partially closed while admitting high purity oxygen through a leak valve in order to maintain the required oxygen pressure (5 10ÿ9 to 5 10ÿ8 Torr) for the desired exposure. The same procedure was followed for the silicide samples except that they were ®rst sputtered clean until the silicide ®lms were exposed. The AES measurements were done using a Physical Electronics Model 545 equipped with a double pass cylindrical mirror analyzer and coaxial electron gun. An electron beam energy of 3 keVand beam current of 20 mA were used. Detail Auger spectra were recorded using a modulation of 1 eV. The Ar ion sputtering for AES was done using a differentially pumped ion gun with 2 keV ions. The ISS system used was equipped with a 908 spherical sector energy analyzer capable of measuring scattered ions with energies 0±2000 eV. The ISS measurements were done by using 2 keV Ne and Ar ions. The angle of incidence of the probing ions was 458 and the ion current was 10 nA. The gas pressure in the differentially pumped ion gun was 2 10ÿ6 Torr. The energy analyzer, which is mounted on a rotatable disk, was positioned so that ions, scattered at an angle of 858, were counted. The energy range investigated was 0±2000 eV. The number of channels used in the complete energy range was 200 and the dwell time per channel was 500 ms. In order to create a depth pro®le Ar ions were used to sputter the samples. At the same time Ar ions served as probing ions for the analyses. The depth pro®les of the initially oxidized dysprosium silicide were done by measuring the ISS peak heights of the elements at the following energies: oxygen at 89 eV, Si at 160 eV and Dy at 1210 eV. The various peaks instead of the areas were measured in order to reduce the data acquisition time so that the oxygen and newly exposed dysprosium would not be sputtered unduly.
27
Fig. 1. Full Auger spectrum of (a) Tb as deposited, and (b) TbSi1.7 after annealing of Tb on Si.
those of Tb and Dy after oxygen exposure of 10 L (1 L 1 10ÿ6 Torrs) and 7 L respectively. No further signi®cant peak shape changes were measured for exposures higher than about 10 L. All the peak positions of Dy are a few eV higher than those for Tb. One would expect this because Dy has a larger atomic number than Tb. There are distinct changes in the Auger spectra as a result of the oxidation. Although there are differences in the peak shapes between the unoxidized Tb and Dy, the peak shapes of Tb and Dy show a remarkable similarity after oxidation. These results are in agreement with the general similarity in chemical properties of the lanthanide elements. The peak shapes of Tb oxidized at 6008C for 30 min [12] are the same as for pure Tb after 7 L of oxygen exposure at room temperature. The X-ray diffraction (XRD) results showed that TbO1.81 forms at 6008C after 30 min oxidation. The Auger peak shapes of pure Tb and Dy and their peak shapes after oxidation are used in the following section
3. Results and discussion 3.1. Oxidation of Tb and Dy Full Auger spectra of the deposited Tb, see Fig. 1 (a), and Dy (not shown here) ®lms, on Si substrates, show impurities of less than 2 at.% in the metal ®lms. The oxygen to metal ratio for Tb in Fig. 1 is even better than for the scraped surface reported by RivieÁre et al. [10]. Auger and autoionisation spectra of clean and oxidised RE metals samarium and erbium are extensively discussed by Netzer et al. [11]. Consider the detailed Auger spectra of Tb and Dy, Fig. 2, in the deposited ®lms after sputter cleaning as well as
Fig. 2. Differentiated Auger spectra of Tb and Dy after the indicated oxygen exposures.
28
G.L.P. Berning et al. / Materials Chemistry and Physics 58 (1999) 26±30
to determine whether the metal in the silicides is oxidized or not. 3.2. Oxidation of the silicides of Tb and Dy
Tb2SiO5. One may therefore assume that these peaks are due to Si in a metal (Tb or Dy) silicate. The formation of a silicate implies an increase in the metal to Si concentration ratio. Rare-earth silicate formation was ®rst observed by Hillebrecht et al. [7]. For room temperature oxidation it is not self-evident that both the metal and the Si will oxidize. The room temperature oxidation of CoSi2 [15], for example, shows that only the Si oxidizes and that the outermost layer contains only Si. From the spectra in Fig. 3, and also from N(E) spectra for Tb [16] it can be concluded that the ratio of the metal concentration to the Si concentration has increased signi®cantly during oxidation.
An Auger depth pro®le showed that the reaction between Tb and Si was completed after 45 min annealing at 3208C. According to Knapp et al. [13] the RE metals form silicides having a hexagonal phase based on the structure AlB2 but with 15±20% vacancies on the Si sublattice which results in the formation of RESi1.7. It has been reported however that TbSi1.7 has an orthorhombic phase [12] and that DySi1.7 exists in both the orthorhombic and tetragonal phase [14]. The formation of TbSi1.7 has been con®rmed by our RBS data [12]. Fig. 1 (b) shows the full Auger spectrum of TbSi1.7. The concentration of oxygen is below 2 at.%. During sputter cleaning of TbSi1.7, the surface was enriched by Si [16]. Fig. 3 shows the low energy Auger spectra of TbSi1.7 and DySi1.7 before and after 10 L and 17 L oxygen exposures, respectively. The Auger spectrum of Tb and TbSi1.7 between 110 eV and 180 eV are almost identical. In this energy range there appears to be more differences between Dy and DySi1.7. These differences are minor compared to the changes in the spectra when the metals are oxidized, see Fig. 2. The peaks of Tb and Dy of the oxidized TbSi1.7 and DySi1.7 in Fig. 3 are almost identical to the peaks of the oxidized metals see Fig. 2. From this resemblance, it can be inferred that the metals in the silicides were oxidized. Furthermore, Si in the silicides is also oxidized after oxygen exposure. This can be seen from the decrease in the Si peak at 92 eV and the appearance of the peaks between about 60 and 90 eV. The peak positions and the shape of the peaks between 60 and 90 eV are, however, very different from that of SiO2 [9]. TbSi1.7 oxidized at 6008C for 30 min shows similar peaks but with a more prominent Si peak at 92 eV [12]. The oxide layer was characterized by XRD as
Fig. 4 shows the ISS spectra (using Ne ions) of DySi1.7 in its clean (sputtered) as well as its oxidized state (50 L). In the unexposed spectrum, Si and Dy peaks are visible. After oxygen exposure there is a broad peak, at the low energy end, indicated by O in Fig. 4 and the Dy peak has increased. A very small Si peak could be detected which indicates that almost no Si atoms are present in the outermost layer. Normally the detectability of low mass surface impurities is severely limited when Ne or heavier ions are used as a probe because a broad peak due to sputtered lattice particles appears at the low energy end [17]. The broad peak can thus be ascribed to sputtered oxygen ions entering the analyzer. The increase of the Dy peak in the spectrum of Fig. 3 indicates an oxygen-induced segregation of Dy to the outermost layer. Similar results were obtained for TbSi1.7 [18]. Due to the af®nity of Dy for oxygen and the low sputtering rate of Ne it was decided to use Ar for depth pro®ling. Fig. 5 shows ISS spectra for clean Dy, DySi1.7 and oxidized DySi1.7. The spectrum of clean Dy shows no peak in the low energy region. There are, however, broad peaks at low energies for both the sputtered clean DySi1.7 and the
Fig. 3. Differentiated Auger spectra of TbSi1.7 and DySi1.7 after the indicated oxygen exposures.
Fig. 4. Ne ISS spectra of DySi1.7 before and after 50 L oxygen exposure.
3.3. ISS measurements
G.L.P. Berning et al. / Materials Chemistry and Physics 58 (1999) 26±30
29
Fig. 6. ISS depth profile of DySi1.7 after an oxygen exposure of 50 L. Fig. 5. Ar ISS spectra of Dy, and DySi1.7 before and after 50 L oxygen exposure.
oxidized DySi1.7. The energy positions (EB, EA) where these peaks have maximum intensities are however different. These peaks are therefore attributed to sputtered silicon (EB) and oxygen (EA) ions. (Note: A distinct Si peak due to scattered Ar ions is not possible. In Fig. 4 we have, however, shown the presence of the Si peak for a clean DySi1.7 surface due to scattered Ne ions.). During Ar sputtering the broad peak at low energies might contain sputtered oxygen as well as silicon ions. In order to get the intensity of the sputtered Si, which should be proportional to the concentration of the Si, the following procedure was followed: After oxidation the intensities t, at EB 160 eV and IEOA t, at EA 89 eV, IESiO B
IEDy t, at EC 1210 eV were monitored as a function of tis the intensity due to oxygen and sputtering time. IESiO B silicon during sputtering. However, before sputtering the silicon concentration was zero due to the absence of a shoulder at EB on the broad peak of the oxidized DySi1.7. The pro®le for the oxygen sputtered ions just after oxidation (Fig. 5) was used to determine the peak heights IEOA 0 and 0. The Si peak height, IESiB t, as function of time was IESiO B calculated as: t ÿ IESiB t IESiO B
face segregation. The Dy to Si intensity ratio remains larger than the bulk value up to a sputtering time of about 600 s. 4. Conclusions During the oxidation of Tb and Dy remarkable changes in the Auger peak shapes occur. These changes are very similar for the two metals. When TbSi1.7 and DySi1.7 were exposed to oxygen the metal as well as the Si were oxidized suggesting the formation of a metal silicate. The peak shapes of the oxidized silicides of Tb and Dy are also very similar. An enrichment of the metal in the oxide layer was observed indicating oxygen-induced segregation. Acknowledgements Financial support by the Technikon OFS is gratefully acknowledged by B. de Witt.
IESiO 0 O B I t O IEA 0 EA
This Si peak height, IESiB t, was used in Fig. 6 which shows the ISS depth pro®le of dysprosium silicide which was initially exposed to oxygen (50 L). During the initial sputtering both the Dy and Si signals increase as the oxygen signal gets smaller. After about 600 s there is no change in the intensities of the different elements which indicates the intensities of the bulk. Fig. 7 shows the ratio of the intensities of Dy to Si as a function of sputtering time. After about 25 s, at A, this ratio shows a minimum. This is a typical segregation pro®le [19,20]. It indicates a Dy depleted region just below the outermost layer due to oxygen-induced sur-
Fig. 7. The peak heights ratio of Dy to Si as calculated from the curves in Fig. 6 as a function of sputtering time.
30
G.L.P. Berning et al. / Materials Chemistry and Physics 58 (1999) 26±30
References [1] V.M. Kolesko, V.F. Belitskii, A.A. Khodin, Phys. Stat. Sol. (a) 93 (1986) 29. È fner, M.G. Ramsey, F.P. Netzer, W. [2] R. Hofmann, W.A. Henle, H. O Braun, K. Horn, Phys. Rev. B 47 (1993) 10407. [3] P. GroÈnig, T. Greber, J. Osterwalder, L. Schlapbach, Vacuum 41 (1990) 1439. [4] G.W. Mellors, Mellors Modern Inorganic Chemistry, G.D. Parkes (Ed.), Longmans, 1951. [5] W.A. Henle, M.G. Ramsey, F.P. Netzer, R. Cimino, W. Braun, S. Witzel, Phys. Rev. B 42 (1990) 11073. [6] W.A. Henle, M.G. Ramsey, F.P. Netzer, Vacuum 41 (1990) 814. [7] F.U. Hillebrecht, Ronay Maria, D. Rieger, F.J. Himpsel, Phys. Rev. B 34 (1986) 5377. [8] F.P. Netzer, J. Phys. Condensed Matter, 7 (1995) 991. [9] M.P. Seah, Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, D. Briggs, M.P. Seah (Eds.), Wiley, New York, 1983.
[10] J.C. RivieÁre, F.P. Netzer, G. Rosina, G. Strasser, J.A.D. Matthew, J. Electron Spectrosc. 36 (1985) 331. [11] F.P. Netzer, E. Bertel, J.A.D. Matthew, J. Phys. C: Solid State Phys. 14 (1981) 1891. [12] G.L.P. Berning, H.C. Swart, Appl. Surf. Sci. 78 (1994) 339. [13] J.A. Knapp, S.T. Picraux, Appl. Phys. Lett. 48 (1986) 466. [14] J.E. Baglin, F.M. d'Heurle, C.S. Petersson, J. Appl. Phys. 52 (1981) 2841. [15] G. Castro, J.E. Hulse, J.K. KuÈppers, A. Rodriguez Gonzalez-Elipe, Surf. Sci. 117 (1982) 621. [16] G.L.P. Berning, H.C. Swart, B. de Witt, Appl. Surf. Sci. 64 (1993) 1. [17] D.G. Armour, Methods of Surface Analysis, in: J.M. Walls (Ed.), Cambrige University Press, 1989. [18] G.L.P. Berning, H.C. Swart, W.D. Roos, Appl. Surf. Sci. 73 (1993) 305. [19] J. Crank, The Mathematics of Diffusion, Oxford University Press, Oxford, 1975. [20] J. du Plessis, Solid State Phenomena 11 (1990) 1.
ISS and AES studies of the initial oxidation of Dy, Tb and their silicides G.L.P. Berninga,*, H.C. Swarta, W.D. Roosa, B. de Wittb a
University of the Orange Free State, P.O. Box 339, ZA-9300, Bloemfontein, South Africa b Technikon OFS, Private Bag X20539, Za-9300, Bloemfontein, South Africa
Received 11 May 1998; received in revised form 2 October 1998; accepted 4 October 1998
Abstract Samples of Tb and Dy as well as their silicides were oxidized in high vacuum under controlled conditions for oxygen exposures ranging up to 50 L (1 L 1 10ÿ6 Torrs). The oxides were studied by Auger electron spectroscopy (AES) and ion scattering spectroscopy (ISS). Distinct low energy Auger peak shape changes occurred during the oxidation of the metals as well as during the oxidation of the metal silicides. These peak shape changes are very similar for Tb and Dy as expected due to the similarities in chemical properties of the lanthanide elements. When the silicides were exposed to oxygen, both the metal and the Si were oxidized. In both cases the oxide layers were enriched by the metal. The depletion of the Dy just below the surface suggested oxygen induced surface segregation of Dy. A model is proposed in which the low energy region of the ISS spectra were used to determine the Si peak height. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Oxidation; Dy; Tb; Rare earths; ISS; AES
1. Introduction The rare-earth (RE) metal silicides form the highest Schottky barrier heights on P-type Si and the lowest on N-type Si, approaching ohmic contacts [1]. RE metals have a potential application in the development of solar cells [2], and have been investigated as potential gate materials in metal-insulator-semiconductor (MIS) devices [3]. It is almost impossible to isolate these metals chemically as the number of electrons in the outermost shells are the same. The remarkable similarity in chemical properties of the lanthanide elements can be ascribed to the fact that their ultimate and most of their penultimate electron orbitals are identical [4]. A disadvantage of the higher reactivity of RE metals is that they are more prone to unwanted oxidation. Oxidation is of major concern during processing at elevated temperatures. In order to use RE metals and their silicides in these types of applications it is necessary to require a knowledge of their surface oxidation. It would thus be advantageous to study the initial oxidation of Dy, Tb and their silicides, as examples of the RE metals, and at the same time point out the similarities and differences of the Auger spectra of the two silicides. Room temperature oxidation of
*Corresponding author. Fax: +51-4306490; e-mail: [email protected]
RE metal ± Si interfaces shows an enhancement of Si oxidation [5±8]. It is well known that Auger electron spectroscopy (AES) peak changes can be used as an indication of surface oxidation [9]. This technique as well as ISS was used to investigate the oxidation of terbium silicide and dysprosium silicide. Ion scattering spectroscopy is a technique by which an intensity analysis is made from the energy distribution of elastically backscattered ions. One of this technique's most important features, when using noble gas ions, is its exclusive ®rst layer sensitivity. This feature was utilized to determine which elements enriched the surface during the exposure of terbium silicide and dysprosium silicide to oxygen. 2. Experimental details Terbium silicide samples were prepared as follows: A Ê layer of Si followed by a 2000 A Ê layer of Tb was 3000 A deposited onto a previously oxidized Si(100) substrate. Electron beam evaporation of pure Tb and Si was used at a base pressure of 1 10ÿ7 Torr. Silicide formation was accomplished by annealing the samples at a temperature of 3208C for 45 min in a high vacuum (7 10ÿ7 Torr) quartz furnace. The same procedure as for Tb was used to form dysprosium silicide.
0254-0584/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S0254-0584(98)00242-9
G.L.P. Berning et al. / Materials Chemistry and Physics 58 (1999) 26±30
The following oxidation procedure was followed: the unannealed Tb and Dy samples were ®rst sputtered clean until the Tb and Dy were exposed with no detectable impurities. The system was then pumped down to a pressure of <2 10ÿ9 Torr before oxidation (the base pressure of the vacuum system was <8 10ÿ10 Torr). During oxidation the valve between the ion pump and the work chamber was partially closed while admitting high purity oxygen through a leak valve in order to maintain the required oxygen pressure (5 10ÿ9 to 5 10ÿ8 Torr) for the desired exposure. The same procedure was followed for the silicide samples except that they were ®rst sputtered clean until the silicide ®lms were exposed. The AES measurements were done using a Physical Electronics Model 545 equipped with a double pass cylindrical mirror analyzer and coaxial electron gun. An electron beam energy of 3 keVand beam current of 20 mA were used. Detail Auger spectra were recorded using a modulation of 1 eV. The Ar ion sputtering for AES was done using a differentially pumped ion gun with 2 keV ions. The ISS system used was equipped with a 908 spherical sector energy analyzer capable of measuring scattered ions with energies 0±2000 eV. The ISS measurements were done by using 2 keV Ne and Ar ions. The angle of incidence of the probing ions was 458 and the ion current was 10 nA. The gas pressure in the differentially pumped ion gun was 2 10ÿ6 Torr. The energy analyzer, which is mounted on a rotatable disk, was positioned so that ions, scattered at an angle of 858, were counted. The energy range investigated was 0±2000 eV. The number of channels used in the complete energy range was 200 and the dwell time per channel was 500 ms. In order to create a depth pro®le Ar ions were used to sputter the samples. At the same time Ar ions served as probing ions for the analyses. The depth pro®les of the initially oxidized dysprosium silicide were done by measuring the ISS peak heights of the elements at the following energies: oxygen at 89 eV, Si at 160 eV and Dy at 1210 eV. The various peaks instead of the areas were measured in order to reduce the data acquisition time so that the oxygen and newly exposed dysprosium would not be sputtered unduly.
27
Fig. 1. Full Auger spectrum of (a) Tb as deposited, and (b) TbSi1.7 after annealing of Tb on Si.
those of Tb and Dy after oxygen exposure of 10 L (1 L 1 10ÿ6 Torrs) and 7 L respectively. No further signi®cant peak shape changes were measured for exposures higher than about 10 L. All the peak positions of Dy are a few eV higher than those for Tb. One would expect this because Dy has a larger atomic number than Tb. There are distinct changes in the Auger spectra as a result of the oxidation. Although there are differences in the peak shapes between the unoxidized Tb and Dy, the peak shapes of Tb and Dy show a remarkable similarity after oxidation. These results are in agreement with the general similarity in chemical properties of the lanthanide elements. The peak shapes of Tb oxidized at 6008C for 30 min [12] are the same as for pure Tb after 7 L of oxygen exposure at room temperature. The X-ray diffraction (XRD) results showed that TbO1.81 forms at 6008C after 30 min oxidation. The Auger peak shapes of pure Tb and Dy and their peak shapes after oxidation are used in the following section
3. Results and discussion 3.1. Oxidation of Tb and Dy Full Auger spectra of the deposited Tb, see Fig. 1 (a), and Dy (not shown here) ®lms, on Si substrates, show impurities of less than 2 at.% in the metal ®lms. The oxygen to metal ratio for Tb in Fig. 1 is even better than for the scraped surface reported by RivieÁre et al. [10]. Auger and autoionisation spectra of clean and oxidised RE metals samarium and erbium are extensively discussed by Netzer et al. [11]. Consider the detailed Auger spectra of Tb and Dy, Fig. 2, in the deposited ®lms after sputter cleaning as well as
Fig. 2. Differentiated Auger spectra of Tb and Dy after the indicated oxygen exposures.
28
G.L.P. Berning et al. / Materials Chemistry and Physics 58 (1999) 26±30
to determine whether the metal in the silicides is oxidized or not. 3.2. Oxidation of the silicides of Tb and Dy
Tb2SiO5. One may therefore assume that these peaks are due to Si in a metal (Tb or Dy) silicate. The formation of a silicate implies an increase in the metal to Si concentration ratio. Rare-earth silicate formation was ®rst observed by Hillebrecht et al. [7]. For room temperature oxidation it is not self-evident that both the metal and the Si will oxidize. The room temperature oxidation of CoSi2 [15], for example, shows that only the Si oxidizes and that the outermost layer contains only Si. From the spectra in Fig. 3, and also from N(E) spectra for Tb [16] it can be concluded that the ratio of the metal concentration to the Si concentration has increased signi®cantly during oxidation.
An Auger depth pro®le showed that the reaction between Tb and Si was completed after 45 min annealing at 3208C. According to Knapp et al. [13] the RE metals form silicides having a hexagonal phase based on the structure AlB2 but with 15±20% vacancies on the Si sublattice which results in the formation of RESi1.7. It has been reported however that TbSi1.7 has an orthorhombic phase [12] and that DySi1.7 exists in both the orthorhombic and tetragonal phase [14]. The formation of TbSi1.7 has been con®rmed by our RBS data [12]. Fig. 1 (b) shows the full Auger spectrum of TbSi1.7. The concentration of oxygen is below 2 at.%. During sputter cleaning of TbSi1.7, the surface was enriched by Si [16]. Fig. 3 shows the low energy Auger spectra of TbSi1.7 and DySi1.7 before and after 10 L and 17 L oxygen exposures, respectively. The Auger spectrum of Tb and TbSi1.7 between 110 eV and 180 eV are almost identical. In this energy range there appears to be more differences between Dy and DySi1.7. These differences are minor compared to the changes in the spectra when the metals are oxidized, see Fig. 2. The peaks of Tb and Dy of the oxidized TbSi1.7 and DySi1.7 in Fig. 3 are almost identical to the peaks of the oxidized metals see Fig. 2. From this resemblance, it can be inferred that the metals in the silicides were oxidized. Furthermore, Si in the silicides is also oxidized after oxygen exposure. This can be seen from the decrease in the Si peak at 92 eV and the appearance of the peaks between about 60 and 90 eV. The peak positions and the shape of the peaks between 60 and 90 eV are, however, very different from that of SiO2 [9]. TbSi1.7 oxidized at 6008C for 30 min shows similar peaks but with a more prominent Si peak at 92 eV [12]. The oxide layer was characterized by XRD as
Fig. 4 shows the ISS spectra (using Ne ions) of DySi1.7 in its clean (sputtered) as well as its oxidized state (50 L). In the unexposed spectrum, Si and Dy peaks are visible. After oxygen exposure there is a broad peak, at the low energy end, indicated by O in Fig. 4 and the Dy peak has increased. A very small Si peak could be detected which indicates that almost no Si atoms are present in the outermost layer. Normally the detectability of low mass surface impurities is severely limited when Ne or heavier ions are used as a probe because a broad peak due to sputtered lattice particles appears at the low energy end [17]. The broad peak can thus be ascribed to sputtered oxygen ions entering the analyzer. The increase of the Dy peak in the spectrum of Fig. 3 indicates an oxygen-induced segregation of Dy to the outermost layer. Similar results were obtained for TbSi1.7 [18]. Due to the af®nity of Dy for oxygen and the low sputtering rate of Ne it was decided to use Ar for depth pro®ling. Fig. 5 shows ISS spectra for clean Dy, DySi1.7 and oxidized DySi1.7. The spectrum of clean Dy shows no peak in the low energy region. There are, however, broad peaks at low energies for both the sputtered clean DySi1.7 and the
Fig. 3. Differentiated Auger spectra of TbSi1.7 and DySi1.7 after the indicated oxygen exposures.
Fig. 4. Ne ISS spectra of DySi1.7 before and after 50 L oxygen exposure.
3.3. ISS measurements
G.L.P. Berning et al. / Materials Chemistry and Physics 58 (1999) 26±30
29
Fig. 6. ISS depth profile of DySi1.7 after an oxygen exposure of 50 L. Fig. 5. Ar ISS spectra of Dy, and DySi1.7 before and after 50 L oxygen exposure.
oxidized DySi1.7. The energy positions (EB, EA) where these peaks have maximum intensities are however different. These peaks are therefore attributed to sputtered silicon (EB) and oxygen (EA) ions. (Note: A distinct Si peak due to scattered Ar ions is not possible. In Fig. 4 we have, however, shown the presence of the Si peak for a clean DySi1.7 surface due to scattered Ne ions.). During Ar sputtering the broad peak at low energies might contain sputtered oxygen as well as silicon ions. In order to get the intensity of the sputtered Si, which should be proportional to the concentration of the Si, the following procedure was followed: After oxidation the intensities t, at EB 160 eV and IEOA t, at EA 89 eV, IESiO B
IEDy t, at EC 1210 eV were monitored as a function of tis the intensity due to oxygen and sputtering time. IESiO B silicon during sputtering. However, before sputtering the silicon concentration was zero due to the absence of a shoulder at EB on the broad peak of the oxidized DySi1.7. The pro®le for the oxygen sputtered ions just after oxidation (Fig. 5) was used to determine the peak heights IEOA 0 and 0. The Si peak height, IESiB t, as function of time was IESiO B calculated as: t ÿ IESiB t IESiO B
face segregation. The Dy to Si intensity ratio remains larger than the bulk value up to a sputtering time of about 600 s. 4. Conclusions During the oxidation of Tb and Dy remarkable changes in the Auger peak shapes occur. These changes are very similar for the two metals. When TbSi1.7 and DySi1.7 were exposed to oxygen the metal as well as the Si were oxidized suggesting the formation of a metal silicate. The peak shapes of the oxidized silicides of Tb and Dy are also very similar. An enrichment of the metal in the oxide layer was observed indicating oxygen-induced segregation. Acknowledgements Financial support by the Technikon OFS is gratefully acknowledged by B. de Witt.
IESiO 0 O B I t O IEA 0 EA
This Si peak height, IESiB t, was used in Fig. 6 which shows the ISS depth pro®le of dysprosium silicide which was initially exposed to oxygen (50 L). During the initial sputtering both the Dy and Si signals increase as the oxygen signal gets smaller. After about 600 s there is no change in the intensities of the different elements which indicates the intensities of the bulk. Fig. 7 shows the ratio of the intensities of Dy to Si as a function of sputtering time. After about 25 s, at A, this ratio shows a minimum. This is a typical segregation pro®le [19,20]. It indicates a Dy depleted region just below the outermost layer due to oxygen-induced sur-
Fig. 7. The peak heights ratio of Dy to Si as calculated from the curves in Fig. 6 as a function of sputtering time.
30
G.L.P. Berning et al. / Materials Chemistry and Physics 58 (1999) 26±30
References [1] V.M. Kolesko, V.F. Belitskii, A.A. Khodin, Phys. Stat. Sol. (a) 93 (1986) 29. È fner, M.G. Ramsey, F.P. Netzer, W. [2] R. Hofmann, W.A. Henle, H. O Braun, K. Horn, Phys. Rev. B 47 (1993) 10407. [3] P. GroÈnig, T. Greber, J. Osterwalder, L. Schlapbach, Vacuum 41 (1990) 1439. [4] G.W. Mellors, Mellors Modern Inorganic Chemistry, G.D. Parkes (Ed.), Longmans, 1951. [5] W.A. Henle, M.G. Ramsey, F.P. Netzer, R. Cimino, W. Braun, S. Witzel, Phys. Rev. B 42 (1990) 11073. [6] W.A. Henle, M.G. Ramsey, F.P. Netzer, Vacuum 41 (1990) 814. [7] F.U. Hillebrecht, Ronay Maria, D. Rieger, F.J. Himpsel, Phys. Rev. B 34 (1986) 5377. [8] F.P. Netzer, J. Phys. Condensed Matter, 7 (1995) 991. [9] M.P. Seah, Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, D. Briggs, M.P. Seah (Eds.), Wiley, New York, 1983.
[10] J.C. RivieÁre, F.P. Netzer, G. Rosina, G. Strasser, J.A.D. Matthew, J. Electron Spectrosc. 36 (1985) 331. [11] F.P. Netzer, E. Bertel, J.A.D. Matthew, J. Phys. C: Solid State Phys. 14 (1981) 1891. [12] G.L.P. Berning, H.C. Swart, Appl. Surf. Sci. 78 (1994) 339. [13] J.A. Knapp, S.T. Picraux, Appl. Phys. Lett. 48 (1986) 466. [14] J.E. Baglin, F.M. d'Heurle, C.S. Petersson, J. Appl. Phys. 52 (1981) 2841. [15] G. Castro, J.E. Hulse, J.K. KuÈppers, A. Rodriguez Gonzalez-Elipe, Surf. Sci. 117 (1982) 621. [16] G.L.P. Berning, H.C. Swart, B. de Witt, Appl. Surf. Sci. 64 (1993) 1. [17] D.G. Armour, Methods of Surface Analysis, in: J.M. Walls (Ed.), Cambrige University Press, 1989. [18] G.L.P. Berning, H.C. Swart, W.D. Roos, Appl. Surf. Sci. 73 (1993) 305. [19] J. Crank, The Mathematics of Diffusion, Oxford University Press, Oxford, 1975. [20] J. du Plessis, Solid State Phenomena 11 (1990) 1.