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Spatially-resolved spectroscopic investigations on the formation of PtSi by annealing on a silicon surface
Chemical Physics Letters 608 (2014) 324–327
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Spatially-resolved spectroscopic investigations on the formation of PtSi by annealing on a silicon surface Yogesh Kumar a,b,1, Mihyun Yang a,c,1, Kyuwook Ihm a,⇑, Kyoung-Jae Lee d, Chan Cuk Hwang a,⇑ a
Pohang Accelerator Laboratory, Pohang, Kyungbuk 790784, Republic of Korea Division of Superconductivity and Magnetism, Faculty of Physics and Geosciences, University of Leipzig, D-04103 Leipzig, Germany c Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea d Department of Physics, POSTECH, Pohang, Kyungbuk 790784, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 10 April 2014 In final form 6 June 2014 Available online 14 June 2014
a b s t r a c t We report spatially-resolved spectroscopic investigations on the effect of annealing a Pt (100 Å)/Si interface in the presence of a native silicon oxide layer. Scanning photoelectron microscopy showed a spatially non-uniform growth pattern of PtSi depending on the annealing temperature and annealing time. Pt 4f and Si 2p core-level spectra measured using different photon energies and at different points suggest both in-plane and out-of-plane inhomogeneous growth of PtSi at 773 K. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Metal silicides are one of the crucial components of siliconbased microelectronic and optoelectronic devices, e.g., they are used for ohmic and Schottky contacts in the devices [1–4]. Besides compatibility with Si-planar processing, various other properties of silicides such as thermal stability and high conductivity make them good candidates for use in sensors and photodetectors [5– 8]. Noble-metal silicides are important as they do not easily form an insulating oxide layer when used as an electrode material in semiconductor devices [9]. Compared to other silicides, platinum silicide (PtSi) is peculiar in the sense that its structural and electronic properties are less sensitive to modification in its dimensions [10,11]. The structural and electronic stability of PtSi make it a strong candidate as a buffer layer to overcome the junction spiking problems of Al–Si contacts, and for applications that require both ohmic and Schottky contacts [12]. Relatively high work function of platinum silicide enables it to be employed in the infrared detectors by making use of Schottky barrier junctions. Even though PtSi has low quantum yield still it offers considerable potential for application in devices due to compatibility with Si-planar processing [13–15]. Generally, PtSi is formed by depositing Pt on the Si surface and then annealing. Depending on annealing temperature Ta, various phases of platinum silicide (e.g., Pt2Si, PtSi) can form [16]. The native oxide (SiOx) layer, normally unavoidably present on the Si ⇑ Corresponding authors. E-mail addresses: [email protected] (K. Ihm), [email protected] (C.C. Hwang). 1 Y. Kumar and M. Yang contributed equally to this study. http://dx.doi.org/10.1016/j.cplett.2014.06.014 0009-2614/Ó 2014 Elsevier B.V. All rights reserved.
surface, is known to inhibit the growth of PtSi. It is particularly due to the high dissociation temperature of SiOx (>1000 K), under ultra-high vacuum condition. However, for the present work, we used core-level spectroscopy to demonstrate the formation of PtSi in the presence of native oxide layer at relatively low temperatures Ta (773 K). Scanning photoelectron microscopy (SPEM), which is a spatially-resolved photoelectron spectroscopy with a resolution of 500 nm, was used to study the spatial growth pattern of PtSi. We used SPEM to measure surface homogeneity in the growth by contrasting the SPEM images with respect to different core level energies and also used different photon energies to obtain the corelevel spectra having different depth ranges for probing the outof-plane growth pattern.
2. Experimental All experiments were conducted in situ at the 8A1 (base pressure: 5 10 10 Torr) and 10D beamlines (base pressure: 2 10 10 Torr) in the Pohang Accelerator Laboratory (PAL). Before Pt deposition, Si(1 0 0) substrate was heated at 373 K for 12 h by flowing direct current to remove the physisorbed contaminants and to release the stress of the substrates. The sample with a 100 Å platinum (Pt) layer deposited on the Si substrate that had a native SiOx layer, (Pt (100 Å)/SiOx (6 ± 2 Å)/Si) were prepared for the study. The sample was annealed up to 773 K in the stepwise manner and directly characterized using SPEM and X-ray photon spectroscopy (XPS) at each step. Final annealing temperature was limited to 773 K in order to avoid any possibility of thermal decomposition of SiOx. For the present work, two different photon energies of 650 and 400 eV were used. Area of photon beam for the XPS spec-
Y. Kumar et al. / Chemical Physics Letters 608 (2014) 324–327
325
tra collected using an energy of 400 eV was 1 3 mm2 and for the SPEM images and XPS spectra obtained with 650 eV photon beam was focused to a diameter of 15 lm using zone plate where the whole scanned area for a SPEM image was 600 600 lm2. The acquired spectra were normalized to the incident photon flux and calibrated using Au 4f and C 1s core-level peaks.
3. Results and discussion
Figure 1. Pt 4f core level spectra of pristine sample and after several annealing steps at 773 K for different annealing times taken with 400 eV incident photon energy.
In Pt 4f core level spectra, obtained as a function of annealing time at Ta = 773 K and using 400 eV of incident photon energy, the pristine sample (Figure 1, bottom spectrum) showed spin–orbit split Pt 4f7/2 and 4f5/2 peaks; these peaks corresponds to the metallic Pt [17]. Spectra were also recorded after annealing at Ta < 773 K, but, only peaks corresponding to metallic Pt were observed during all these steps. This construction was maintained on the surface within the probing depth of XPS even after annealing the sample at 773 K for 10 min; after further annealing, a new feature in the core level spectra started to appear at 72.8 eV for 4f7/2. This new feature originated from the PtASi bond formation wherein Si is supplied by diffusion of dissociated Si atoms from the native SiOx layer during annealing [16,18,19]. Metallic Pt was completely transferred to the PtSi after annealing the sample for 1 h (Figure 1). The temperature 773 K is that required for the dissociation of SiOx underneath the Au layer [20]. The native SiOx layer was expected to restrict the formation of PtSi even after annealing, but our spectra (Figure 1) demonstrate the formation of PtSi after annealing at 773 K. This observation suggests that a localized diffusion path for Si atoms occurs on the native oxide layer. To test this possibility, SPEM was used to probe spatially non-uniform surface kinetics (Figure 2a–d). Figure 2 shows the SPEM images corresponding to Pt 4f at room
Figure 2. SPEM images (600 600 lm2), contrasted by the intensity of Pt 4f peak for metallic Pt (a and c) and Pt–Si (b and d), taken for pristine sample (a and b) and after annealing at 773 K for 1 h (c and d) with incident photon energy of 650 eV. In (b) the pixel values are multiplied by 10.
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Figure 3. Experimental and fitted Pt 4f core-level spectra of pristine sample (a) and after annealing at 773 K for 1 h (b and c) and 3 h (d and e). P1 and P2 correspond to different regions of interest in the SPEM images (Figure 2) and are related to high and low metallic Pt concentrations, respectively.
temperature (a and b) and after annealing at 773 K for 1 h. (c and d). Here, images a and c are contrasted by the intensity of Pt 4f level at binding energy, Eb = 71.2 eV (metallic Pt), while images b and d are contrasted by the intensity of Pt 4f level at binding energy, Eb = 72.8 eV (Pt–Si). In each figure, darker region represents the relative deficiency of that particular entity. The non-uniform pattern observed after annealing of the sample (Figure 2c and d) indicates that the growth of PtSi was non-uniform. Here, it is important to mention that various SPEM images at the different spots of the sample were collected during the experiments where similar kind of concentration fluctuation for PtSi was observed. SPEM images presented here (Figure 2) are thus a representative of the whole sample. To examine the silicide process in more detail we measured core level spectra (Figure 3) at two localized points: one where concentration of metallic Pt was higher than that of PtSi (P1, Figure 2c and d); and another one where concentration of metallic Pt was lower than that of PtSi (P2, Figure 2c and d). We fitted all spectra using asymmetric Doniach-Sunjic curves [22] after using the Shirley-type method [23] to subtract the background. The asymmetric line shape of spectra for pristine sample (Figure 3a) is due to inelastic scattering of photoelectrons with the electron gas near the Fermi level [21], and is a characteristic feature of a metallic surface. The pristine sample showed only one component from curve fitting with spin–orbit split lines wherein the Pt 4f7/2 line was at 71.2 eV. In contrast, all other spectra obtained after annealing had lines corresponding to two components, where one from metallic Pt and one shifted 1.6 eV to higher binding energy than the metallic Pt; this shift closely matches the reported shift of PtSi [16]. After annealing for 1 h, the relative concentration
Figure 4. Experimental and fitted Si 2p core-level spectra of sample after annealing at 773 K for 1 h. P1 and P2 are represented in the SPEM (Figure 2) image.
of PtSi to metallic Pt was much larger at point P2 than that at P1 (Figure 3b and c); this inhomogeneous formation of PtSi concurs with the SPEM images. The intensity of spectra that correspond to PtSi in the Pt core level spectra increased after annealing for 3 h (Figure 3d and e). Incomplete transformation to PtSi in the annealed sample appears to contrast with the results of Figure 1 in which the PtSi peak completely replaced the Pt peak; this difference is due to the higher incident photon energy (650 eV) employed to obtain spectromicroscopic data of Figure 3, which has longer probing depth than that of 400 eV photons used to record spectra of Figure 1. Hence, similar to inhomogeneous in-planar growth of PtSi, out-of-plane growth is also inhomogeneous. Si 2p core-level spectra for quantitative analysis (Figure 4) were obtained at P1 and P2 with the incident photon energy of 650 eV after annealing at 773 K for 1 h. The pristine sample did not show any Si 2p related peak (data not presented), thereby ruling out the presence of Si atoms within the probing depth of the incident beam. The Si 2p core-level spectrum shows a complex structure due to the close overlap of various Si 2p components related to spin–orbit split peaks from diffused Si atoms with and without bonding to Pt [16,24]. The spectra were deconvoluted using the Voigt functions after baseline subtraction using the Shirley-type background method. The spectra confirm that Si diffused into the surface after annealing. The intensity difference shows that the concentration of Pt–Si with respect to elemental Si was higher at point P2 than at P1. These results concur well with Pt 4f data.
Y. Kumar et al. / Chemical Physics Letters 608 (2014) 324–327
For both Pt 4f and Si 2p core levels, the peak corresponding to the PtASi bonding appear towards high binding energy (Figures 3 and 4); this is known as a double-shift in the core level [16]. If only simple transfer of charge between two atoms as a result of the relative electronegativity of Pt and Si is considered, the peak of PtSi in Pt 4f core-level should be located at lower binding energy side of corresponding metallic Pt peak. However, this double shift and corresponding energy difference is consistent with various reports that confirm formation of only the PtSi phase and not any other phases (e.g., PtSi2, PtO, PtO2) [25]. Based on theoretical partial density of state calculations, Franco et al. [16] explained this complex redistribution in terms of a charge compensation mechanism whereby Pt in PtSi loses 5d electrons but this loss is overcompensated by gain of 6p electrons, resulting in overall flow of charge from Si to Pt. Core-level spectra show the formation of PtSi even in the presence of a native SiOx layer on the Si surface; this process is unusual in the sense that PtSi formed suddenly without appearance of any other intermediate phase after annealing the sample at 773 K, which is much below the SiOx dissociation temperature. Also, as observed by SPEM and core-level spectra taken at different photon energies, formation of PtSi was very inhomogeneous in both inplane and out-of-plane directions. These observed results suggests that (i) Pt clusters induced by defect centers act as catalysts to dissociate the native SiOx layer, and (ii) dissociated Si atoms diffuse into the Pt layer and are bonded with Pt to form PtSi without any intermediate phase. We also recorded the O 1s spectra of pristine and annealed sample at positions P1 and P2 (data not presented). In the spectra, a very small amount of oxygen was observed in the pristine sample; these may be physisorbed oxygen on the surface. After annealing, the spectrum’s shape and position did not change noticeably, but the signal intensity increased; this change corroborates the idea of dissociation of SiOx at a relatively low temperature of 773 K. 4. Conclusions We characterized PtSi formation by annealing the Pt/SiOx/Si samples. Dissociation of SiOx and formation of PtSi was observed in samples after annealing at 773 K, which is much lower than the normal dissociation temperature of a native SiOx layer; this reduction in dissociation temperature implies that Pt acted as a catalyst to lower the activation energy for dissociation of the native
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SiOx. Spatially-resolved photoelectron spectroscopy and core-level spectra recorded at different photon energies revealed that the formation of PtSi was inhomogeneous in both out-of-plane and inplane directions. Acknowledgements This work was supported by National Research Foundation of Korea (NRF) (Grant Nos. 2010-0021068, 2012R1A1B3002785, 2011-0009427, 2009-0083380, 2013K000306 and 2011-0030787) through the Ministry of Science, ICT and Future Planning, Korea. Experiments at PLS-II were supported by MSIP-R. O. Korea. References [1] M.P. Lepselter, J.M. Andrews, in: B. Schwartz (Ed.), Ohmic Contacts to Semiconductors, Electrochemical Society, New York, 1969. [2] E.H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts, Oxford University Press, Oxford, 1988. [3] E.F. Fabrizio, T.M. McEvoy, A.J. Bard, J. Electroanal. Chem. 554–555 (2003) 99. [4] J.H. Wu, R.S. Chang, G.J. Horng, Thin Solid Films 466 (2004) 314. [5] S.P. Murarka, Metalization: Theory and Practice for VLSI and ULSI, Butterworth-Heinemann, Boston, 1993. [6] P.W. Pellegrini, C.E. Ludington, M.M. Weeks, J. Appl. Phys. 67 (1990) 1417. [7] M. Wittmer, Phys. Rev. B 43 (1991) 4385. [8] M. Razeghi, A. Rogalski, J. Appl. Phys. 79 (1996) 7433. [9] K.L. Saenger, A. Grill, C. Cabral Jr., J. Mater. Res. 13 (1998) 462. [10] C. Blair, E. Demirlioglu, E. Yoon, J. Pierce, in: R.W. Fathauer, L. Schowalter, S. Mantl, K.N. Tu (Eds.), In: Silicides, Germanides and their Interfaces, MRS Symposia Proceedings No. 320, Materials Research Society, Pittsburgh, 1994. [11] D.-X. Xu, J.P. McCaffrey, S.R. Das, G.C. Aers, L.E. Erickson, Appl. Phys. Lett. 68 (1996) 3588. [12] S. Wolf, Silicon Processing for the VLSI Era, Vol. 2: Process Integration, Lattice Press, Sunset Beach, California, 1990. [13] K. Solt et al., Appl. Phys. Lett. 69 (1996) 3662. [14] L.P. Wang, J.R. Yang, J. Hwang, J. Appl. Phys. 74 (1993) 6251. [15] D. Drouin, J. Beauvais, R. Lemire, E. Lavallée, R. Gauvin, M. Caron, Appl. Phys. Lett. 70 (1997) 3020. [16] N. Franco et al., Phys. Rev. B 68 (2003) 045116. [17] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp, Eden Prairie, MN, 1992. [18] P.J. Grunthaner, F.J. Grunthaner, J. Vac. Sci. Technol. 21 (1982) 637. [19] L. Ley, Y. Wang, V. Nguyen, J. Rivory Thin Solid Films 270 (1995) 561. [20] K.-J. Lee et al., Nanoscale 6 (2014) 1474. [21] J. Joyce, M. del Giudice, J.H. Weaver, J. Electron. Spectrosc. Relat. Phenom. 49 (1989) 31. [22] S. Doniach, M. Sunjic, J. Phys. C: Solid State Phys. 3 (1970) 285. [23] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [24] E. Landemark, C.J. Karlsson, Y.-C. Chao, R.I.G. Uhrberg, Phys. Rev. Lett. 69 (1992) 1588. [25] L.K. Ono, J.R. Croy, H. Heinrich, B.R. Cuenya, J. Phys. Chem. C 115 (2011) 16856.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Spatially-resolved spectroscopic investigations on the formation of PtSi by annealing on a silicon surface Yogesh Kumar a,b,1, Mihyun Yang a,c,1, Kyuwook Ihm a,⇑, Kyoung-Jae Lee d, Chan Cuk Hwang a,⇑ a
Pohang Accelerator Laboratory, Pohang, Kyungbuk 790784, Republic of Korea Division of Superconductivity and Magnetism, Faculty of Physics and Geosciences, University of Leipzig, D-04103 Leipzig, Germany c Department of Physics, Sungkyunkwan University, Suwon 440-746, Republic of Korea d Department of Physics, POSTECH, Pohang, Kyungbuk 790784, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 10 April 2014 In final form 6 June 2014 Available online 14 June 2014
a b s t r a c t We report spatially-resolved spectroscopic investigations on the effect of annealing a Pt (100 Å)/Si interface in the presence of a native silicon oxide layer. Scanning photoelectron microscopy showed a spatially non-uniform growth pattern of PtSi depending on the annealing temperature and annealing time. Pt 4f and Si 2p core-level spectra measured using different photon energies and at different points suggest both in-plane and out-of-plane inhomogeneous growth of PtSi at 773 K. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Metal silicides are one of the crucial components of siliconbased microelectronic and optoelectronic devices, e.g., they are used for ohmic and Schottky contacts in the devices [1–4]. Besides compatibility with Si-planar processing, various other properties of silicides such as thermal stability and high conductivity make them good candidates for use in sensors and photodetectors [5– 8]. Noble-metal silicides are important as they do not easily form an insulating oxide layer when used as an electrode material in semiconductor devices [9]. Compared to other silicides, platinum silicide (PtSi) is peculiar in the sense that its structural and electronic properties are less sensitive to modification in its dimensions [10,11]. The structural and electronic stability of PtSi make it a strong candidate as a buffer layer to overcome the junction spiking problems of Al–Si contacts, and for applications that require both ohmic and Schottky contacts [12]. Relatively high work function of platinum silicide enables it to be employed in the infrared detectors by making use of Schottky barrier junctions. Even though PtSi has low quantum yield still it offers considerable potential for application in devices due to compatibility with Si-planar processing [13–15]. Generally, PtSi is formed by depositing Pt on the Si surface and then annealing. Depending on annealing temperature Ta, various phases of platinum silicide (e.g., Pt2Si, PtSi) can form [16]. The native oxide (SiOx) layer, normally unavoidably present on the Si ⇑ Corresponding authors. E-mail addresses: [email protected] (K. Ihm), [email protected] (C.C. Hwang). 1 Y. Kumar and M. Yang contributed equally to this study. http://dx.doi.org/10.1016/j.cplett.2014.06.014 0009-2614/Ó 2014 Elsevier B.V. All rights reserved.
surface, is known to inhibit the growth of PtSi. It is particularly due to the high dissociation temperature of SiOx (>1000 K), under ultra-high vacuum condition. However, for the present work, we used core-level spectroscopy to demonstrate the formation of PtSi in the presence of native oxide layer at relatively low temperatures Ta (773 K). Scanning photoelectron microscopy (SPEM), which is a spatially-resolved photoelectron spectroscopy with a resolution of 500 nm, was used to study the spatial growth pattern of PtSi. We used SPEM to measure surface homogeneity in the growth by contrasting the SPEM images with respect to different core level energies and also used different photon energies to obtain the corelevel spectra having different depth ranges for probing the outof-plane growth pattern.
2. Experimental All experiments were conducted in situ at the 8A1 (base pressure: 5 10 10 Torr) and 10D beamlines (base pressure: 2 10 10 Torr) in the Pohang Accelerator Laboratory (PAL). Before Pt deposition, Si(1 0 0) substrate was heated at 373 K for 12 h by flowing direct current to remove the physisorbed contaminants and to release the stress of the substrates. The sample with a 100 Å platinum (Pt) layer deposited on the Si substrate that had a native SiOx layer, (Pt (100 Å)/SiOx (6 ± 2 Å)/Si) were prepared for the study. The sample was annealed up to 773 K in the stepwise manner and directly characterized using SPEM and X-ray photon spectroscopy (XPS) at each step. Final annealing temperature was limited to 773 K in order to avoid any possibility of thermal decomposition of SiOx. For the present work, two different photon energies of 650 and 400 eV were used. Area of photon beam for the XPS spec-
Y. Kumar et al. / Chemical Physics Letters 608 (2014) 324–327
325
tra collected using an energy of 400 eV was 1 3 mm2 and for the SPEM images and XPS spectra obtained with 650 eV photon beam was focused to a diameter of 15 lm using zone plate where the whole scanned area for a SPEM image was 600 600 lm2. The acquired spectra were normalized to the incident photon flux and calibrated using Au 4f and C 1s core-level peaks.
3. Results and discussion
Figure 1. Pt 4f core level spectra of pristine sample and after several annealing steps at 773 K for different annealing times taken with 400 eV incident photon energy.
In Pt 4f core level spectra, obtained as a function of annealing time at Ta = 773 K and using 400 eV of incident photon energy, the pristine sample (Figure 1, bottom spectrum) showed spin–orbit split Pt 4f7/2 and 4f5/2 peaks; these peaks corresponds to the metallic Pt [17]. Spectra were also recorded after annealing at Ta < 773 K, but, only peaks corresponding to metallic Pt were observed during all these steps. This construction was maintained on the surface within the probing depth of XPS even after annealing the sample at 773 K for 10 min; after further annealing, a new feature in the core level spectra started to appear at 72.8 eV for 4f7/2. This new feature originated from the PtASi bond formation wherein Si is supplied by diffusion of dissociated Si atoms from the native SiOx layer during annealing [16,18,19]. Metallic Pt was completely transferred to the PtSi after annealing the sample for 1 h (Figure 1). The temperature 773 K is that required for the dissociation of SiOx underneath the Au layer [20]. The native SiOx layer was expected to restrict the formation of PtSi even after annealing, but our spectra (Figure 1) demonstrate the formation of PtSi after annealing at 773 K. This observation suggests that a localized diffusion path for Si atoms occurs on the native oxide layer. To test this possibility, SPEM was used to probe spatially non-uniform surface kinetics (Figure 2a–d). Figure 2 shows the SPEM images corresponding to Pt 4f at room
Figure 2. SPEM images (600 600 lm2), contrasted by the intensity of Pt 4f peak for metallic Pt (a and c) and Pt–Si (b and d), taken for pristine sample (a and b) and after annealing at 773 K for 1 h (c and d) with incident photon energy of 650 eV. In (b) the pixel values are multiplied by 10.
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Figure 3. Experimental and fitted Pt 4f core-level spectra of pristine sample (a) and after annealing at 773 K for 1 h (b and c) and 3 h (d and e). P1 and P2 correspond to different regions of interest in the SPEM images (Figure 2) and are related to high and low metallic Pt concentrations, respectively.
temperature (a and b) and after annealing at 773 K for 1 h. (c and d). Here, images a and c are contrasted by the intensity of Pt 4f level at binding energy, Eb = 71.2 eV (metallic Pt), while images b and d are contrasted by the intensity of Pt 4f level at binding energy, Eb = 72.8 eV (Pt–Si). In each figure, darker region represents the relative deficiency of that particular entity. The non-uniform pattern observed after annealing of the sample (Figure 2c and d) indicates that the growth of PtSi was non-uniform. Here, it is important to mention that various SPEM images at the different spots of the sample were collected during the experiments where similar kind of concentration fluctuation for PtSi was observed. SPEM images presented here (Figure 2) are thus a representative of the whole sample. To examine the silicide process in more detail we measured core level spectra (Figure 3) at two localized points: one where concentration of metallic Pt was higher than that of PtSi (P1, Figure 2c and d); and another one where concentration of metallic Pt was lower than that of PtSi (P2, Figure 2c and d). We fitted all spectra using asymmetric Doniach-Sunjic curves [22] after using the Shirley-type method [23] to subtract the background. The asymmetric line shape of spectra for pristine sample (Figure 3a) is due to inelastic scattering of photoelectrons with the electron gas near the Fermi level [21], and is a characteristic feature of a metallic surface. The pristine sample showed only one component from curve fitting with spin–orbit split lines wherein the Pt 4f7/2 line was at 71.2 eV. In contrast, all other spectra obtained after annealing had lines corresponding to two components, where one from metallic Pt and one shifted 1.6 eV to higher binding energy than the metallic Pt; this shift closely matches the reported shift of PtSi [16]. After annealing for 1 h, the relative concentration
Figure 4. Experimental and fitted Si 2p core-level spectra of sample after annealing at 773 K for 1 h. P1 and P2 are represented in the SPEM (Figure 2) image.
of PtSi to metallic Pt was much larger at point P2 than that at P1 (Figure 3b and c); this inhomogeneous formation of PtSi concurs with the SPEM images. The intensity of spectra that correspond to PtSi in the Pt core level spectra increased after annealing for 3 h (Figure 3d and e). Incomplete transformation to PtSi in the annealed sample appears to contrast with the results of Figure 1 in which the PtSi peak completely replaced the Pt peak; this difference is due to the higher incident photon energy (650 eV) employed to obtain spectromicroscopic data of Figure 3, which has longer probing depth than that of 400 eV photons used to record spectra of Figure 1. Hence, similar to inhomogeneous in-planar growth of PtSi, out-of-plane growth is also inhomogeneous. Si 2p core-level spectra for quantitative analysis (Figure 4) were obtained at P1 and P2 with the incident photon energy of 650 eV after annealing at 773 K for 1 h. The pristine sample did not show any Si 2p related peak (data not presented), thereby ruling out the presence of Si atoms within the probing depth of the incident beam. The Si 2p core-level spectrum shows a complex structure due to the close overlap of various Si 2p components related to spin–orbit split peaks from diffused Si atoms with and without bonding to Pt [16,24]. The spectra were deconvoluted using the Voigt functions after baseline subtraction using the Shirley-type background method. The spectra confirm that Si diffused into the surface after annealing. The intensity difference shows that the concentration of Pt–Si with respect to elemental Si was higher at point P2 than at P1. These results concur well with Pt 4f data.
Y. Kumar et al. / Chemical Physics Letters 608 (2014) 324–327
For both Pt 4f and Si 2p core levels, the peak corresponding to the PtASi bonding appear towards high binding energy (Figures 3 and 4); this is known as a double-shift in the core level [16]. If only simple transfer of charge between two atoms as a result of the relative electronegativity of Pt and Si is considered, the peak of PtSi in Pt 4f core-level should be located at lower binding energy side of corresponding metallic Pt peak. However, this double shift and corresponding energy difference is consistent with various reports that confirm formation of only the PtSi phase and not any other phases (e.g., PtSi2, PtO, PtO2) [25]. Based on theoretical partial density of state calculations, Franco et al. [16] explained this complex redistribution in terms of a charge compensation mechanism whereby Pt in PtSi loses 5d electrons but this loss is overcompensated by gain of 6p electrons, resulting in overall flow of charge from Si to Pt. Core-level spectra show the formation of PtSi even in the presence of a native SiOx layer on the Si surface; this process is unusual in the sense that PtSi formed suddenly without appearance of any other intermediate phase after annealing the sample at 773 K, which is much below the SiOx dissociation temperature. Also, as observed by SPEM and core-level spectra taken at different photon energies, formation of PtSi was very inhomogeneous in both inplane and out-of-plane directions. These observed results suggests that (i) Pt clusters induced by defect centers act as catalysts to dissociate the native SiOx layer, and (ii) dissociated Si atoms diffuse into the Pt layer and are bonded with Pt to form PtSi without any intermediate phase. We also recorded the O 1s spectra of pristine and annealed sample at positions P1 and P2 (data not presented). In the spectra, a very small amount of oxygen was observed in the pristine sample; these may be physisorbed oxygen on the surface. After annealing, the spectrum’s shape and position did not change noticeably, but the signal intensity increased; this change corroborates the idea of dissociation of SiOx at a relatively low temperature of 773 K. 4. Conclusions We characterized PtSi formation by annealing the Pt/SiOx/Si samples. Dissociation of SiOx and formation of PtSi was observed in samples after annealing at 773 K, which is much lower than the normal dissociation temperature of a native SiOx layer; this reduction in dissociation temperature implies that Pt acted as a catalyst to lower the activation energy for dissociation of the native
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SiOx. Spatially-resolved photoelectron spectroscopy and core-level spectra recorded at different photon energies revealed that the formation of PtSi was inhomogeneous in both out-of-plane and inplane directions. Acknowledgements This work was supported by National Research Foundation of Korea (NRF) (Grant Nos. 2010-0021068, 2012R1A1B3002785, 2011-0009427, 2009-0083380, 2013K000306 and 2011-0030787) through the Ministry of Science, ICT and Future Planning, Korea. Experiments at PLS-II were supported by MSIP-R. O. Korea. References [1] M.P. Lepselter, J.M. Andrews, in: B. Schwartz (Ed.), Ohmic Contacts to Semiconductors, Electrochemical Society, New York, 1969. [2] E.H. Rhoderick, R.H. Williams, Metal-Semiconductor Contacts, Oxford University Press, Oxford, 1988. [3] E.F. Fabrizio, T.M. McEvoy, A.J. Bard, J. Electroanal. Chem. 554–555 (2003) 99. [4] J.H. Wu, R.S. Chang, G.J. Horng, Thin Solid Films 466 (2004) 314. [5] S.P. Murarka, Metalization: Theory and Practice for VLSI and ULSI, Butterworth-Heinemann, Boston, 1993. [6] P.W. Pellegrini, C.E. Ludington, M.M. Weeks, J. Appl. Phys. 67 (1990) 1417. [7] M. Wittmer, Phys. Rev. B 43 (1991) 4385. [8] M. Razeghi, A. Rogalski, J. Appl. Phys. 79 (1996) 7433. [9] K.L. Saenger, A. Grill, C. Cabral Jr., J. Mater. Res. 13 (1998) 462. [10] C. Blair, E. Demirlioglu, E. Yoon, J. Pierce, in: R.W. Fathauer, L. Schowalter, S. Mantl, K.N. Tu (Eds.), In: Silicides, Germanides and their Interfaces, MRS Symposia Proceedings No. 320, Materials Research Society, Pittsburgh, 1994. [11] D.-X. Xu, J.P. McCaffrey, S.R. Das, G.C. Aers, L.E. Erickson, Appl. Phys. Lett. 68 (1996) 3588. [12] S. Wolf, Silicon Processing for the VLSI Era, Vol. 2: Process Integration, Lattice Press, Sunset Beach, California, 1990. [13] K. Solt et al., Appl. Phys. Lett. 69 (1996) 3662. [14] L.P. Wang, J.R. Yang, J. Hwang, J. Appl. Phys. 74 (1993) 6251. [15] D. Drouin, J. Beauvais, R. Lemire, E. Lavallée, R. Gauvin, M. Caron, Appl. Phys. Lett. 70 (1997) 3020. [16] N. Franco et al., Phys. Rev. B 68 (2003) 045116. [17] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp, Eden Prairie, MN, 1992. [18] P.J. Grunthaner, F.J. Grunthaner, J. Vac. Sci. Technol. 21 (1982) 637. [19] L. Ley, Y. Wang, V. Nguyen, J. Rivory Thin Solid Films 270 (1995) 561. [20] K.-J. Lee et al., Nanoscale 6 (2014) 1474. [21] J. Joyce, M. del Giudice, J.H. Weaver, J. Electron. Spectrosc. Relat. Phenom. 49 (1989) 31. [22] S. Doniach, M. Sunjic, J. Phys. C: Solid State Phys. 3 (1970) 285. [23] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [24] E. Landemark, C.J. Karlsson, Y.-C. Chao, R.I.G. Uhrberg, Phys. Rev. Lett. 69 (1992) 1588. [25] L.K. Ono, J.R. Croy, H. Heinrich, B.R. Cuenya, J. Phys. Chem. C 115 (2011) 16856.