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Characterization of MOCVD-grown non-stoichiometric SiNx
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Surface & Coatings Technology 202 (2008) 4198 – 4203 www.elsevier.com/locate/surfcoat
Characterization of MOCVD-grown non-stoichiometric SiNx Zhilai Fang ⁎ Semiconductor Photonics Research Center and Department of Physics, Xiamen University, Xiamen 361005, People's Republic of China Received 3 November 2007; accepted in revised form 9 March 2008 Available online 15 March 2008
Abstract The surface morphologies and X-ray photoelectron spectra of MOCVD-grown SiNx were investigated. Highly Si-rich SiNx nanoislands not fully covering the sapphire surface were observed for SiNx deposition at low temperature (545 °C) with NH3/SiH4 flow rate of 2500/40 sccm. The surface roughness decreased from 0.91 nm to 0.23 nm with the reduction of SiH4 flow rate from 40 sccm to 3 sccm. The reduction of the SiH4 flow rate did not cause a linear decrease of Si/N ratio, which indicated that the SiH4 supply was saturated when the NH3 supply was 2500 sccm and deposition temperature was fixed at 545 °C. Relatively “thick” SiNx layers with stoichiometry close to 1 were formed for SiNx deposition at high temperature due to high decomposition rate of ammonia and high reaction rate between silane and ammonia. The SiNx layers almost fully covered the sapphire surface and showed surface structures of both nanoislands and nanoholes. By employing the same NH3/SiH4 flow rate of 2500/40 sccm the surface roughness of SiNx layers decreased from 0.91 nm to 0.17 nm with the increase of deposition temperature from 545 °C to 1035 °C. Saturated pre-nitridation would likely cause surface roughening. © 2008 Elsevier B.V. All rights reserved. PACS: 81.15.-z; 81.15.Gh; 77.84.-s; 68.37.Ps; 79.60.-I Keywords: Si-rich silicon nitride; Organometallic CVD; Photoelectron spectroscopy; AFM; Stoichiometry
1. Introduction Silicon nitride (SiNx) thin films have been widely used in the fabrication of semiconductor devices as electrical insulators [1], dielectric masks [2,3], passivation layers [4], diffusion barriers [5], charge storage layers [6], gate insulator films of thin-film transistors [7], and antireflection coating for silicon solar cells [8]. The chemical compositions and properties of SiNx films are strongly dependent on growth conditions and deposition methods such as low-pressure chemical vapor deposition (LPCVD) [9], plasma-enhanced chemical vapor deposition (PECVD) [10], reactive sputtering [11], pulsed laser deposition (PLD) [12] etc. Studies on the relationship between the chemical compositions and properties of the SiNx films deposited by PLD and LPCVD have been reported [12,13]. By precisely manipulating the growth conditions for specific growth techniques non-stoichiometric SiNx thin films of desirable properties could be achieved. Special
⁎ Tel.: +86 592 2184220; fax: +86 592 214220. E-mail address: [email protected]. 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.03.007
and increasing research attention has been paid to Si-rich SiNx due to the visible luminescence properties of the silicon nanocrystals (nc-Si) embedded in the silicon nitride [14–18]. In epitaxial lateral overgrowth (ELO) of high-quality GaN films, SiNx layers of ~ 100 nm are often used as the dielectric masks on GaN templates by use of CVD or PECVD followed by removal of partial SiNx coverage by means of standard photolithographic techniques [2,19]. Recently in situ SiNx pretreatment of sapphire substrates has also been employed to improve the crystalline qualities and optical properties of GaN films [20–26]. Studies of the SiNx treatment chemistry, which influences the growth behaviour, crystalline qualities, and optical properties of subsequent GaN epilayers, are certainly very important for further understanding and more precise control of the GaN growth processes. As the deposition conditions and stoichiometry of SiNx are strongly dependent on the deposition techniques and presently reports on metal-organic chemical vapor deposition (MOCVD) growth of very thin SiNx are scarce, it is essential and important to study the MOCVDgrowth of nanoscale SiNx and the effects on the growth of GaN nucleation layers (NLs). In this paper, we present in detail the
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surface morphologies and X-ray photoelectron spectroscopy (XPS) studies of MOCVD-grown SiNx on sapphire substrates. 2. Experimental The experiments were carried out in a close coupled showerhead (CCS) planetary reactor of the Thomas Swan 3 × 2″ MOCVD system. The (0001) sapphire substrates were cleaned at 1060 °C by flowing high-purity hydrogen gas onto the substrate surface for 8 min. The sapphire surface was then nitridated for 5 min by flowing 2500 sccm NH3 and using 5500 sccm H2 as the carrier gas. The in situ SiNx deposition was performed by simultaneously introducing SiH4 (100 ppm) and NH3 into the reactor with H2 as the carrier gas. The flow rate of SiH4 was varied from 3 to 40 sccm whereas the flow rate of NH3 was always fixed at 2500 sccm. The SiNx deposition was carried out at 545 °C, 700 °C, 850 °C, 950 °C, and 1035 °C, respectively. The surface chemical compositions of the SiNx layers prepared with different SiH4 flow rates and at different temperatures were analyzed by XPS (PHI Quantum2000) with an Al Kα X-ray excitation source (hν = 1486.6 eV). The surface morphologies of the SiNx layers and GaN NLs were investigated by atomic force microscope (AFM, SPA400, Seiko Instruments Inc.). 3. Results and discussion As described in Table 1, different SiNx layers were prepared under different deposition conditions by variation of growth temperature, SiH4 flow rate, and pre-nitridation status. The stoichiometry and morphologies were investigated by use of XPS and AFM, respectively. In the XPS experiments all the XPS spectra were referenced to the XPS data book [27] and the C1s peak of fixed peak position at 284.8 eV. 3.1. The surface stoichiometry and morphologies of Sample “A01” Fig. 1a shows the surface morphology of Sample “A01”, which was deposited at low temperature (545 °C) with a high SiH4 flow rate of 40 sccm. The as-deposited SiNx layers exhibited islandlike surface structure with a RMS roughness of ~0.91 nm because of the island growth mode at low temperature. By doing
Table 1 Sample reference numbers and the deposition parameters Sample name
Nitridation temperature (°C)
Deposition temperature (°C)
Flow rate of iH4 (sccm)
Deposition time (s)
A01 A02 A03 A04 A05 A11 A21 A31 A41 A10
545 545 545 545 545 700 850 950 1035 No nitridation
545 545 545 545 545 700 850 950 1035 545
40 16 10 5 3 40 40 40 40 40
400 1000 1600 3200 5333 400 400 400 400 400
Fig. 1. (a) The surface morphology and (b) grain analysis for calculation of the island density of Sample “A01”. The RMS roughness is 0.91 nm as derived by surface analysis of the AFM image. SEM-EDX spectra of the SiNx layers on sapphire (“A01”) detected at (c) non-island sites and (d) island sites show that the sapphire surface was likely not fully covered by the as-deposited SiNx.
grain analysis in Fig. 1b we found that the island density was ~2.0 × 109 cm− 2 with an average island size of ~53 nm. SEMEDX analysis (see Fig. 1c and d) revealed that at the island sites silicon was detected whereas at the non-island sites silicon was not detectable. This has indicated that the sapphire surface was not fully covered by the SiNx layers. By doing surface analysis we found that more than 80% surface area of the sapphire substrate was not covered by the deposited SiNx nanoislands. This has made it possible for GaN epitaxy on sapphire, as the SiNx mask would block the GaN growth. The chemical compositions of the as-deposited SiNx layers have been investigated by XPS measurements. As shown in Fig. 2a the Si2p photoelectron peak can be fitted into 3 peaks corresponding to SiNx (102.2 eV), Si (99.3 eV), and SiAl (96.5 eV), respectively, where the SiNx could be composed of Si3N, Si3N2, SiN, and Si3N4. In Fig. 2b the N1s photoelectron peak can be deconvoluted into 3 peaks: N1s (surface N, 399.5 eV), N1s (SiNx, 397.5 eV), and N1s (AlN, 396.2 eV). The peak broadening of the X-ray photoelectron peaks is possible due to the weak signals from the as-investigated SiNx nanoislands not fully covering the sapphire substrates. The emergence of the N1s (AlN) peak indicates the initial nitridation effect— formation of AlN thin layers at the outermost surface. By doing Ar+ ion sputtering the N1s (surface N, e.g. NO) photoelectron peak could be easily removed. Taking into account the sensitivity factors of the Si2p and N1s, the fitting peak area of the N1s (SiNx) is consistent with that of the Si2p (SiNx). A A A We define here the Si/N ratio as RSi/N = SSi /SN, where A is the
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factor of Si2p (0.368) and FN1s the sensitivity factor of N1s A 1 (0.499). For Sample “A01”, we get a Si/N ratio of RSi/N = 3.21. Apparently the as-deposited SiNx layers were highly Si-rich. 3.2. Influences of the SiH4 flow rates
Fig. 2. The X-ray photoelectron peaks (a) Si2p and (b) N1s of Sample “A01”. The Si2p photoelectron peak can be fitted into 3 sub-peaks “SiAl”, “Si”, and “SiNx” where SiNx is composed of Si3N, Si3N2, SiN, and Si3N4. The N1s peak can be fitted into 3 sub-peaks “AlN”, “SiNx”, and “surface N”.
sample reference number and S is the intensity of the fitted A A peak calibrated by the sensitivity factor, i.e. SSi = [SSi2p (SiNx) + A A A SSi2p(Si)]/FSi2p, and SN = SN1s(SiNx)/FN1s. FSi2p is the sensitivity
To study the influences of the SiH4 flow rate, we prepared several samples “A02”, “A03”, “A04”, and “A05” (as described in Table 1) with different SiH4 flow rate of 16 sccm, 10 sccm, 5 sccm, and 3 sccm, respectively. The other deposition conditions such as deposition temperature, growth pressure, flow rates of ammonia and H2 carrier gas remained the same as that of Sample “A01”. In Fig. 3 we show the surface morphology of the samples “A02”, “A03”, “A04”, and “A05”. With the decrease of the SiH4 flow rate from 40 sccm to 16/10/5/3 sccm, the RMS roughness of the SiNx layers decreased from 0.91 nm to 0.51/0.33/0.25/ 0.23 nm as derived from the surface analysis of the AFM images; the visible island height and density also decreased with the SiH4 flow rate; terrace surface structure with terrace width of about 250 nm was observed for SiH4 flow rate below 5 sccm (see Samples “A04” and “A05”). This indicates that the growth mode of SiNx has changed from island growth to step-flow growth when the SiH4/NH3 ratio was substantially decreased.
Fig. 3. The surface morphology of the samples (a) “A02”, (b) “A03”, (c) “A04”, and (d) “A05” with a RMS roughness of 0.51 nm, 0.33 nm, 0.25 nm, and 0.23 nm, respectively.
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effect) when the SiH4/NH3 ratio is high. It seems that, for the flow rate ranges of the silane and ammonia gases in our experiments, the change of the SiH4 flow rate was not very effective for modification of the Si/N ratio. At such low temperature the bottleneck of the reaction rate of SiH4 and NH3 is the decomposition rate of ammonia. An efficient way to increase the decomposition rate of ammonia is to deposit the SiNx layers at high temperature. 3.3. Effects of the SiNx deposition temperature
Fig. 4. The X-ray photoelectron peaks (a) Si2p and (b) N1 s of Sample “A03”.
In Fig. 4 we show the Si2p and N1s photoelectron peaks of Sample “A03”. Only a small decrease of the Si2p (Si) peak area (Fig. 4a) and increase of the N1s (SiNx) peak area (Fig. 4b) were observed. Based on the same peak-fitting rules and calA2 culations we get RSi/N = 2.46. This has indicated that the Si/N ratio is not linearly dependent the SiH4/NH3 ratio (saturation
In Fig. 5 we show the surface morphology of the samples “A41”, “A31”, “A21”, and “A11” with different deposition temperatures of 1035 °C, 950 °C, 850 °C, 700 °C, respectively. With increase of deposition temperature from 545 °C to 700/850/950/ 1035 °C, the surface RMS roughness decreased from 0.91 nm to 0.37/0.32/0/17/0.15 nm. Terrace surface structure was observed at middle deposition temperature (700 °C, 850 °C, and 950 °C). Disappearance of terrace structure at high temperature (1035 °C) is possibly due to the high reaction rate between silane and ammonia and thus formation of thick SiNx layers. It seems that by gradual increase of deposition temperature from 545 °C to 1035 °C the growth mode has changed from island growth to stepflow and then Stransky–Krastanov growth.
Fig. 5. The surface morphology of the samples (a) “A41”, (b) “A31”, (c) “A21”, and (d) “A11” with a RMS roughness of 0.17 nm, 0.17 nm, 0.32 nm, and 0.37 nm, respectively.
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3.4. Nitridation effects
Fig. 6. The X-ray photoelectron peaks (a) Si2p and (b) N1s of Sample “A41”.
In Fig. 6 we show the XPS spectra of the Si2p and N1s photoelectron peaks of Sample “A41”. Drastic changes of the photoelectron peak shapes for the Si2p (Fig. 6a) and the N1s peaks (Fig. 6b) were observed. Obviously the Si2p (SiNx) peak became very sharp with strong intensity whereas the Si2p (Si) and Si2p (SiAl) broad peaks were comparatively weakened. The observation of enhanced intensity of Si2p (SiNx) (about 5 times that of Sample “A01”) has supported our previous judgment that “thicker” films were deposited. Considering the island height (and coverage) we conclude that the sapphire substrate surface was almost fully covered by SiNx (at the island sites and flat areas) with only partial regions (nanoholes) exposed without SiNx covering. By calculating the Si/N ratio of Sample “A41” A 3 we get RSi/N = 1.11. For the five principally chemically shifted components (Si0, Si1+, Si2+, Si3+, and Si4+) of Si2p corresponding to Si, Si3N, Si3N2, SiN, and Si3N4, respectively, there is a lowest binding energy for Si2p (Si) and a highest binding energy for Si2p (Si3N4). As the Si2p (SiNx) peak shifted to higher binding energy and the full width at half maximum of the peak became sharper, we judge that SiN and Si3N4 were likely the dominating chemical components of SiNx, which is consistent with a Si/N ratio close to 1.
In our experiments the nitridation processes of sapphire substrate surface prior to the SiNx deposition have been carried out at different temperatures (see Table 1). As shown in Figs. 2b and 4b for the nitridation process performed at low temperature, the peak shapes of the N1s (AlN) peaks were similar and also the intensities were very close. In general, the formation of AlN at high temperature would be easier than that at low temperature. As a result thicker AlN layers should be formed. However, we did not observe an obvious increase of the N1s (AlN) peak intensity. Instead, in our XPS measurements (Fig. 6b) the N1s (AlN) peak was relatively reduced compared with the N1s (SiNx) peak. This is possibly due to the saturation effect of sapphire surface nitridation [28] and the enhanced SiNx deposition at high temperature. Considering the saturation of sapphire surface nitridation at both low temperature (545 °C) and high temperature (1035 °C), for further comparisons we prepared Sample “A10” without the pre-nitridation process. The other deposition conditions of Sample “A10” are the same as that of Sample “A01”. The surface morphology of Sample “A10” and the XPS spectra of the Si2p and N1s photoelectron peaks are shown in Fig. 7. The surface structure is islandlike similar to that of other samples. The island density (~1.0 × 109 cm− 2) and height are much less than that of Sample “A01”. Especially, the surface RMS roughness decreased to 0.31 nm. The sample surface with saturated nitridation rougher than that of the sample without pre-nitridation was likely due to saturated nitridation caused surface roughening [28]. Although the SiNx deposition conditions for both samples “A01” and “A10” were the same except the pre-nitridation process, by doing XPS analysis of the surface chemical compositions A 4 we found that Sample “A10” has a higher Si/N ratio RSi/N = 5.49 than that of Sample “A01” (see Fig. 7b). During sapphire substrate surface cleaning by flowing high volume of H2 over the surface at high temperature (1060 °C), the outermost surface oxygen would be removed and thus high density of Al dangling bonds would be formed. During the subsequent SiNx deposition processes, the partial NH3 would react with sapphire and form thin AlN layers, which has been confirmed by the observation of similar N1s (AlN) photoelectron peak (see Fig. 7c). The consumption
Fig. 7. (a) The surface morphology and the X-ray photoelectron peaks (b) Si2p and (c) N1s of Sample “A10”. The RMS roughness is 0.31 nm.
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of partial NH3 for formation of AlN further reduced the actual NH3/SiH4 ratio for formation of SiNx and thus SiNx films of A4 higher Si/N ratio RSi/N = 5.49 were obtained, i.e. the so-called “post-nitridation effect”.
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increased from 3.21 to 5.49. By means of Si-rich SiNx pretreatment to sapphire the crystalline qualities and optical properties of subsequently grown GaN films can be improved. Acknowledgments
3.5. Brief discussions on applications of SiNx coating in MOCVD growth of high-quality GaN films In our previous studies [21,26], in situ SiNx treatment prior to the growth of low temperature GaN NLs has been used to improve the crystalline qualities and optical properties of GaN films. Generally, without the SiNx treatment to the sapphire substrates, non-distinct GaN islands were observed after annealing of the GaN NLs. In comparison, with the Si-rich SiNx nanoislands patterning, distinct GaN nanoislands of triangular (0001) base and smooth sidewall faceting were observed due to the enhanced diffusion and regrowth anisotropy [21]. With Si-rich SiNx pretreatment of sapphire substrate, we found that the crystalline quality of GaN films was substantially improved. The threading dislocation density was reduced from mid 109 cm− 2 to about 1 ×108 cm− 2 [26]. Besides the effects on GaN island shaping and subsequent growth behavior of GaN films, the SiNx coating has also improved the optical properties of GaN NLs and films. For GaN NLs on treated sapphire the near bandedge emission became narrower with a lower “background” (e.g., ultraviolet, blue, and yellow band emission), which suggested an improvement of the crystalline qualities [21]. The photoluminescence measurements showed that the yellowband emission was clearly visible for GaN films without SiNx pretreatment whereas became invisible for GaN films with SiNx pretreatment [26]. For low temperature (77 K) photoluminescence of GaN films with SiNx pretreatment the full width at half maximum of GaN donor bound exciton peak is only 5 meV. The observation of free excitons for GaN films prepared with Si-rich SiNx treatment also indicated high quality GaN films of good optical properties. In summary, by controlling the growth conditions of SiNx layers coated on sapphire and thus the surface morphologies and stoichiometry of SiNx, the growth behaviour, crystalline qualities, and optical properties of GaN layers can be modified. 4. Conclusion We have presented studies on the surface morphologies and XPS of MOCVD-grown SiNx prepared under different conditions by variations of deposition temperature, flow rate of SiH4 source, and pre-nitridation conditions. We found that 1) the surface roughness decreased either by increase of NH3/SiH4 ratio or deposition temperature. 2) Saturated pre-nitridation would increase the surface roughness. 3) Growth at high temperature enhanced the SiNx deposition with most of the sapphire surface covered by SiNx. In comparison, low temperature SiNx deposition resulted in low reaction rate of NH3 and SiH4 and low decomposition rate of NH3. As a result, only a small fraction of the sapphire surface was covered by Si-rich SiNx nanoislands. 4) The Si/N ratio decreased from 3.21 to 1.11 when the deposition temperature increased from 545 °C to 1035 °C. 5) Due to the “post-nitridation effect” the volume of ammonia participating in the SiNx formation was reduced and thus the Si/N ratio was
This work was partially supported by the National Natural Science Foundation of China. The author would like to thank S. J. Wang and H. Z. Sun for XPS measurements and D. Q. Lin for AFM measurements. References [1] R.D. Gould, S.A. Awan, Thin Solid Films 515 (2006) 509. [2] P. Gibart, Rep. Prog. Phys. 67 (2004) 667. [3] Z.T. Jiang, T. Yamaguchi, K. Ohshimo, M. Aoyama, L. Asinovsky, Jpn. J. Appl. Phys. 37 (1998) 571. [4] C.H.M. van der Werf, H.D. Goldbach, J. Loffler, A. Scarfo, A.M.C. Kylner, B. Stannowski, W.M. Arnoldbik, A. Weeber, H. Rieffe, W.J. Soppe, J.K. Rath, R.E.I. Schropp, Thin Solid Films 501 (2006) 51. [5] G. Bilger, T. Voss, T. Schlenker, A. Strohm, Surf. Interface Anal. 38 (2006) 1687. [6] H. Amjadi, IEEE Trans. Dielectr. Electr. Insul. 6 (1999) 852. [7] I. Mártil, A. del Prado, E.S. Andrés, G. González Díaz, F.L. Martínez, J. Appl. Phys. 94 (2003) 2642. [8] F.W. Chen, T.T.A. Li, J.E. Cotter, Appl. Phys. Lett. 88 (2006) 263514. [9] V.M. Bermudez, F.K. Perkins, Appl. Surf. Sci. 235 (2004) 406. [10] M. Lattemann, E. Nold, S. Ulrich, H. Leiste, H. Holleck, Surf. Coat. Technol. 174 (2003) 365. [11] M. Vetter, Thin Solid Films 337 (1999) 118. [12] E. Dehan, P. Temple-Boyer, R. Henda, J.J. Pedroviejo, E. Scheid, Thin Solid Films 266 (1995) 14. [13] J.M. Lackner, W. Waldhauser, R. Berghauser, R. Ebner, M. Beutl, G. Jakopic, G. Leising, H. Hutter, M. Rosner, T. Schoberl, Surf. Coat. Technol. 192 (2005) 225. [14] Minghua Wang, Dongsheng Li, Zhizhong Yuan, Deren Yang, Duanlin Que, Appl. Phys. Lett. 90 (2007) 131903. [15] A. Benami, G. Santana, A. Ortiz, A. Ponce, D. Romeu, J. Aguilar-Hernandez, G. Contreras-Puente, J.C. Alonso, Nanotechnology 18 (2007) 155704. [16] V. Em. Vamvakas, S. Gardelis, Surf. Coat. Technol. 201 (2007) 9359. [17] Tai-Cheng Tsai, Li-Zhen Yu , Ching-Ting Lee, Nanotechnology 18 (2007) 275707. [18] E. Reimhult, K. Kumar, W. Knoll, Nanotechnology 18 (2007) 275303. [19] B. Beaumont, Ph. Vennéguès, P. Gibart, Phys. Status Solidi B 227 (2001) 1. [20] E. Frayssinet, B. Beaumont, J.P. Faurie, P. Gibart, Zs. Makkai, B. Pécz, P. Lefebvre, P. Valvin, MRS Internet J. Nitride Semicond. Res. 7 (2002) 8. [21] Z.L. Fang, J.Y. Kang, J. Phys. Chem. C 111 (2007) 7889; Z.L. Fang, J.Y. Kang, W.J. Huang, H.T. Sun, M. Lu, J.F. Kong, W.Z. Shen, J. Phys. Chem. C 112 (2008) 4925. [22] S. Haffouz, H. Lahreche, P. Vennegues, P. de Mierry, B. Beaumont, F. Omnes, P. Gibart, Appl. Phys. Lett. 73 (1998) 1278. [23] T. Wang, Y. Morishima, N. Naoi, S. Sakai, J. Cryst. Growth 213 (2000) 188. [24] C.H. Liu, R.W. Chuang, S.J. Chang, Y.K. Su, C.H. Kuo, J.M. Tsai, C.C. Lin, Mater. Sci. Eng. B 111 (2004) 214. [25] Y.T. Moon, J. Xie, C. Liu, Y. Fu, X. Ni, N. Biyikli, K. Zhu, F. Yun, H. Morkoç, A. Sagar, R.M. Feenstra, J. Cryst. Growth 291 (2006) 301. [26] Z. L Fang, S. P. Li, J. C. Li, H. Z. Sun, S. J. Wang, J. Y. Kang, Thin Solid Films (in press), doi:10.1016/j.tsf.2007.12.132. bhttp://dx.doi.org/10.1016/j. tsf.2007.12.132N. [27] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data, Physical Electronics, Minnesota, 1995. [28] K. Uchida, A. Watanabe, F. Yano, M. Kouguchi, T. Tanaka, S. Minagawa, J. Appl. Phys. 79 (1996) 3487.
Surface & Coatings Technology 202 (2008) 4198 – 4203 www.elsevier.com/locate/surfcoat
Characterization of MOCVD-grown non-stoichiometric SiNx Zhilai Fang ⁎ Semiconductor Photonics Research Center and Department of Physics, Xiamen University, Xiamen 361005, People's Republic of China Received 3 November 2007; accepted in revised form 9 March 2008 Available online 15 March 2008
Abstract The surface morphologies and X-ray photoelectron spectra of MOCVD-grown SiNx were investigated. Highly Si-rich SiNx nanoislands not fully covering the sapphire surface were observed for SiNx deposition at low temperature (545 °C) with NH3/SiH4 flow rate of 2500/40 sccm. The surface roughness decreased from 0.91 nm to 0.23 nm with the reduction of SiH4 flow rate from 40 sccm to 3 sccm. The reduction of the SiH4 flow rate did not cause a linear decrease of Si/N ratio, which indicated that the SiH4 supply was saturated when the NH3 supply was 2500 sccm and deposition temperature was fixed at 545 °C. Relatively “thick” SiNx layers with stoichiometry close to 1 were formed for SiNx deposition at high temperature due to high decomposition rate of ammonia and high reaction rate between silane and ammonia. The SiNx layers almost fully covered the sapphire surface and showed surface structures of both nanoislands and nanoholes. By employing the same NH3/SiH4 flow rate of 2500/40 sccm the surface roughness of SiNx layers decreased from 0.91 nm to 0.17 nm with the increase of deposition temperature from 545 °C to 1035 °C. Saturated pre-nitridation would likely cause surface roughening. © 2008 Elsevier B.V. All rights reserved. PACS: 81.15.-z; 81.15.Gh; 77.84.-s; 68.37.Ps; 79.60.-I Keywords: Si-rich silicon nitride; Organometallic CVD; Photoelectron spectroscopy; AFM; Stoichiometry
1. Introduction Silicon nitride (SiNx) thin films have been widely used in the fabrication of semiconductor devices as electrical insulators [1], dielectric masks [2,3], passivation layers [4], diffusion barriers [5], charge storage layers [6], gate insulator films of thin-film transistors [7], and antireflection coating for silicon solar cells [8]. The chemical compositions and properties of SiNx films are strongly dependent on growth conditions and deposition methods such as low-pressure chemical vapor deposition (LPCVD) [9], plasma-enhanced chemical vapor deposition (PECVD) [10], reactive sputtering [11], pulsed laser deposition (PLD) [12] etc. Studies on the relationship between the chemical compositions and properties of the SiNx films deposited by PLD and LPCVD have been reported [12,13]. By precisely manipulating the growth conditions for specific growth techniques non-stoichiometric SiNx thin films of desirable properties could be achieved. Special
⁎ Tel.: +86 592 2184220; fax: +86 592 214220. E-mail address: [email protected]. 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.03.007
and increasing research attention has been paid to Si-rich SiNx due to the visible luminescence properties of the silicon nanocrystals (nc-Si) embedded in the silicon nitride [14–18]. In epitaxial lateral overgrowth (ELO) of high-quality GaN films, SiNx layers of ~ 100 nm are often used as the dielectric masks on GaN templates by use of CVD or PECVD followed by removal of partial SiNx coverage by means of standard photolithographic techniques [2,19]. Recently in situ SiNx pretreatment of sapphire substrates has also been employed to improve the crystalline qualities and optical properties of GaN films [20–26]. Studies of the SiNx treatment chemistry, which influences the growth behaviour, crystalline qualities, and optical properties of subsequent GaN epilayers, are certainly very important for further understanding and more precise control of the GaN growth processes. As the deposition conditions and stoichiometry of SiNx are strongly dependent on the deposition techniques and presently reports on metal-organic chemical vapor deposition (MOCVD) growth of very thin SiNx are scarce, it is essential and important to study the MOCVDgrowth of nanoscale SiNx and the effects on the growth of GaN nucleation layers (NLs). In this paper, we present in detail the
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surface morphologies and X-ray photoelectron spectroscopy (XPS) studies of MOCVD-grown SiNx on sapphire substrates. 2. Experimental The experiments were carried out in a close coupled showerhead (CCS) planetary reactor of the Thomas Swan 3 × 2″ MOCVD system. The (0001) sapphire substrates were cleaned at 1060 °C by flowing high-purity hydrogen gas onto the substrate surface for 8 min. The sapphire surface was then nitridated for 5 min by flowing 2500 sccm NH3 and using 5500 sccm H2 as the carrier gas. The in situ SiNx deposition was performed by simultaneously introducing SiH4 (100 ppm) and NH3 into the reactor with H2 as the carrier gas. The flow rate of SiH4 was varied from 3 to 40 sccm whereas the flow rate of NH3 was always fixed at 2500 sccm. The SiNx deposition was carried out at 545 °C, 700 °C, 850 °C, 950 °C, and 1035 °C, respectively. The surface chemical compositions of the SiNx layers prepared with different SiH4 flow rates and at different temperatures were analyzed by XPS (PHI Quantum2000) with an Al Kα X-ray excitation source (hν = 1486.6 eV). The surface morphologies of the SiNx layers and GaN NLs were investigated by atomic force microscope (AFM, SPA400, Seiko Instruments Inc.). 3. Results and discussion As described in Table 1, different SiNx layers were prepared under different deposition conditions by variation of growth temperature, SiH4 flow rate, and pre-nitridation status. The stoichiometry and morphologies were investigated by use of XPS and AFM, respectively. In the XPS experiments all the XPS spectra were referenced to the XPS data book [27] and the C1s peak of fixed peak position at 284.8 eV. 3.1. The surface stoichiometry and morphologies of Sample “A01” Fig. 1a shows the surface morphology of Sample “A01”, which was deposited at low temperature (545 °C) with a high SiH4 flow rate of 40 sccm. The as-deposited SiNx layers exhibited islandlike surface structure with a RMS roughness of ~0.91 nm because of the island growth mode at low temperature. By doing
Table 1 Sample reference numbers and the deposition parameters Sample name
Nitridation temperature (°C)
Deposition temperature (°C)
Flow rate of iH4 (sccm)
Deposition time (s)
A01 A02 A03 A04 A05 A11 A21 A31 A41 A10
545 545 545 545 545 700 850 950 1035 No nitridation
545 545 545 545 545 700 850 950 1035 545
40 16 10 5 3 40 40 40 40 40
400 1000 1600 3200 5333 400 400 400 400 400
Fig. 1. (a) The surface morphology and (b) grain analysis for calculation of the island density of Sample “A01”. The RMS roughness is 0.91 nm as derived by surface analysis of the AFM image. SEM-EDX spectra of the SiNx layers on sapphire (“A01”) detected at (c) non-island sites and (d) island sites show that the sapphire surface was likely not fully covered by the as-deposited SiNx.
grain analysis in Fig. 1b we found that the island density was ~2.0 × 109 cm− 2 with an average island size of ~53 nm. SEMEDX analysis (see Fig. 1c and d) revealed that at the island sites silicon was detected whereas at the non-island sites silicon was not detectable. This has indicated that the sapphire surface was not fully covered by the SiNx layers. By doing surface analysis we found that more than 80% surface area of the sapphire substrate was not covered by the deposited SiNx nanoislands. This has made it possible for GaN epitaxy on sapphire, as the SiNx mask would block the GaN growth. The chemical compositions of the as-deposited SiNx layers have been investigated by XPS measurements. As shown in Fig. 2a the Si2p photoelectron peak can be fitted into 3 peaks corresponding to SiNx (102.2 eV), Si (99.3 eV), and SiAl (96.5 eV), respectively, where the SiNx could be composed of Si3N, Si3N2, SiN, and Si3N4. In Fig. 2b the N1s photoelectron peak can be deconvoluted into 3 peaks: N1s (surface N, 399.5 eV), N1s (SiNx, 397.5 eV), and N1s (AlN, 396.2 eV). The peak broadening of the X-ray photoelectron peaks is possible due to the weak signals from the as-investigated SiNx nanoislands not fully covering the sapphire substrates. The emergence of the N1s (AlN) peak indicates the initial nitridation effect— formation of AlN thin layers at the outermost surface. By doing Ar+ ion sputtering the N1s (surface N, e.g. NO) photoelectron peak could be easily removed. Taking into account the sensitivity factors of the Si2p and N1s, the fitting peak area of the N1s (SiNx) is consistent with that of the Si2p (SiNx). A A A We define here the Si/N ratio as RSi/N = SSi /SN, where A is the
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factor of Si2p (0.368) and FN1s the sensitivity factor of N1s A 1 (0.499). For Sample “A01”, we get a Si/N ratio of RSi/N = 3.21. Apparently the as-deposited SiNx layers were highly Si-rich. 3.2. Influences of the SiH4 flow rates
Fig. 2. The X-ray photoelectron peaks (a) Si2p and (b) N1s of Sample “A01”. The Si2p photoelectron peak can be fitted into 3 sub-peaks “SiAl”, “Si”, and “SiNx” where SiNx is composed of Si3N, Si3N2, SiN, and Si3N4. The N1s peak can be fitted into 3 sub-peaks “AlN”, “SiNx”, and “surface N”.
sample reference number and S is the intensity of the fitted A A peak calibrated by the sensitivity factor, i.e. SSi = [SSi2p (SiNx) + A A A SSi2p(Si)]/FSi2p, and SN = SN1s(SiNx)/FN1s. FSi2p is the sensitivity
To study the influences of the SiH4 flow rate, we prepared several samples “A02”, “A03”, “A04”, and “A05” (as described in Table 1) with different SiH4 flow rate of 16 sccm, 10 sccm, 5 sccm, and 3 sccm, respectively. The other deposition conditions such as deposition temperature, growth pressure, flow rates of ammonia and H2 carrier gas remained the same as that of Sample “A01”. In Fig. 3 we show the surface morphology of the samples “A02”, “A03”, “A04”, and “A05”. With the decrease of the SiH4 flow rate from 40 sccm to 16/10/5/3 sccm, the RMS roughness of the SiNx layers decreased from 0.91 nm to 0.51/0.33/0.25/ 0.23 nm as derived from the surface analysis of the AFM images; the visible island height and density also decreased with the SiH4 flow rate; terrace surface structure with terrace width of about 250 nm was observed for SiH4 flow rate below 5 sccm (see Samples “A04” and “A05”). This indicates that the growth mode of SiNx has changed from island growth to step-flow growth when the SiH4/NH3 ratio was substantially decreased.
Fig. 3. The surface morphology of the samples (a) “A02”, (b) “A03”, (c) “A04”, and (d) “A05” with a RMS roughness of 0.51 nm, 0.33 nm, 0.25 nm, and 0.23 nm, respectively.
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effect) when the SiH4/NH3 ratio is high. It seems that, for the flow rate ranges of the silane and ammonia gases in our experiments, the change of the SiH4 flow rate was not very effective for modification of the Si/N ratio. At such low temperature the bottleneck of the reaction rate of SiH4 and NH3 is the decomposition rate of ammonia. An efficient way to increase the decomposition rate of ammonia is to deposit the SiNx layers at high temperature. 3.3. Effects of the SiNx deposition temperature
Fig. 4. The X-ray photoelectron peaks (a) Si2p and (b) N1 s of Sample “A03”.
In Fig. 4 we show the Si2p and N1s photoelectron peaks of Sample “A03”. Only a small decrease of the Si2p (Si) peak area (Fig. 4a) and increase of the N1s (SiNx) peak area (Fig. 4b) were observed. Based on the same peak-fitting rules and calA2 culations we get RSi/N = 2.46. This has indicated that the Si/N ratio is not linearly dependent the SiH4/NH3 ratio (saturation
In Fig. 5 we show the surface morphology of the samples “A41”, “A31”, “A21”, and “A11” with different deposition temperatures of 1035 °C, 950 °C, 850 °C, 700 °C, respectively. With increase of deposition temperature from 545 °C to 700/850/950/ 1035 °C, the surface RMS roughness decreased from 0.91 nm to 0.37/0.32/0/17/0.15 nm. Terrace surface structure was observed at middle deposition temperature (700 °C, 850 °C, and 950 °C). Disappearance of terrace structure at high temperature (1035 °C) is possibly due to the high reaction rate between silane and ammonia and thus formation of thick SiNx layers. It seems that by gradual increase of deposition temperature from 545 °C to 1035 °C the growth mode has changed from island growth to stepflow and then Stransky–Krastanov growth.
Fig. 5. The surface morphology of the samples (a) “A41”, (b) “A31”, (c) “A21”, and (d) “A11” with a RMS roughness of 0.17 nm, 0.17 nm, 0.32 nm, and 0.37 nm, respectively.
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3.4. Nitridation effects
Fig. 6. The X-ray photoelectron peaks (a) Si2p and (b) N1s of Sample “A41”.
In Fig. 6 we show the XPS spectra of the Si2p and N1s photoelectron peaks of Sample “A41”. Drastic changes of the photoelectron peak shapes for the Si2p (Fig. 6a) and the N1s peaks (Fig. 6b) were observed. Obviously the Si2p (SiNx) peak became very sharp with strong intensity whereas the Si2p (Si) and Si2p (SiAl) broad peaks were comparatively weakened. The observation of enhanced intensity of Si2p (SiNx) (about 5 times that of Sample “A01”) has supported our previous judgment that “thicker” films were deposited. Considering the island height (and coverage) we conclude that the sapphire substrate surface was almost fully covered by SiNx (at the island sites and flat areas) with only partial regions (nanoholes) exposed without SiNx covering. By calculating the Si/N ratio of Sample “A41” A 3 we get RSi/N = 1.11. For the five principally chemically shifted components (Si0, Si1+, Si2+, Si3+, and Si4+) of Si2p corresponding to Si, Si3N, Si3N2, SiN, and Si3N4, respectively, there is a lowest binding energy for Si2p (Si) and a highest binding energy for Si2p (Si3N4). As the Si2p (SiNx) peak shifted to higher binding energy and the full width at half maximum of the peak became sharper, we judge that SiN and Si3N4 were likely the dominating chemical components of SiNx, which is consistent with a Si/N ratio close to 1.
In our experiments the nitridation processes of sapphire substrate surface prior to the SiNx deposition have been carried out at different temperatures (see Table 1). As shown in Figs. 2b and 4b for the nitridation process performed at low temperature, the peak shapes of the N1s (AlN) peaks were similar and also the intensities were very close. In general, the formation of AlN at high temperature would be easier than that at low temperature. As a result thicker AlN layers should be formed. However, we did not observe an obvious increase of the N1s (AlN) peak intensity. Instead, in our XPS measurements (Fig. 6b) the N1s (AlN) peak was relatively reduced compared with the N1s (SiNx) peak. This is possibly due to the saturation effect of sapphire surface nitridation [28] and the enhanced SiNx deposition at high temperature. Considering the saturation of sapphire surface nitridation at both low temperature (545 °C) and high temperature (1035 °C), for further comparisons we prepared Sample “A10” without the pre-nitridation process. The other deposition conditions of Sample “A10” are the same as that of Sample “A01”. The surface morphology of Sample “A10” and the XPS spectra of the Si2p and N1s photoelectron peaks are shown in Fig. 7. The surface structure is islandlike similar to that of other samples. The island density (~1.0 × 109 cm− 2) and height are much less than that of Sample “A01”. Especially, the surface RMS roughness decreased to 0.31 nm. The sample surface with saturated nitridation rougher than that of the sample without pre-nitridation was likely due to saturated nitridation caused surface roughening [28]. Although the SiNx deposition conditions for both samples “A01” and “A10” were the same except the pre-nitridation process, by doing XPS analysis of the surface chemical compositions A 4 we found that Sample “A10” has a higher Si/N ratio RSi/N = 5.49 than that of Sample “A01” (see Fig. 7b). During sapphire substrate surface cleaning by flowing high volume of H2 over the surface at high temperature (1060 °C), the outermost surface oxygen would be removed and thus high density of Al dangling bonds would be formed. During the subsequent SiNx deposition processes, the partial NH3 would react with sapphire and form thin AlN layers, which has been confirmed by the observation of similar N1s (AlN) photoelectron peak (see Fig. 7c). The consumption
Fig. 7. (a) The surface morphology and the X-ray photoelectron peaks (b) Si2p and (c) N1s of Sample “A10”. The RMS roughness is 0.31 nm.
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of partial NH3 for formation of AlN further reduced the actual NH3/SiH4 ratio for formation of SiNx and thus SiNx films of A4 higher Si/N ratio RSi/N = 5.49 were obtained, i.e. the so-called “post-nitridation effect”.
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increased from 3.21 to 5.49. By means of Si-rich SiNx pretreatment to sapphire the crystalline qualities and optical properties of subsequently grown GaN films can be improved. Acknowledgments
3.5. Brief discussions on applications of SiNx coating in MOCVD growth of high-quality GaN films In our previous studies [21,26], in situ SiNx treatment prior to the growth of low temperature GaN NLs has been used to improve the crystalline qualities and optical properties of GaN films. Generally, without the SiNx treatment to the sapphire substrates, non-distinct GaN islands were observed after annealing of the GaN NLs. In comparison, with the Si-rich SiNx nanoislands patterning, distinct GaN nanoislands of triangular (0001) base and smooth sidewall faceting were observed due to the enhanced diffusion and regrowth anisotropy [21]. With Si-rich SiNx pretreatment of sapphire substrate, we found that the crystalline quality of GaN films was substantially improved. The threading dislocation density was reduced from mid 109 cm− 2 to about 1 ×108 cm− 2 [26]. Besides the effects on GaN island shaping and subsequent growth behavior of GaN films, the SiNx coating has also improved the optical properties of GaN NLs and films. For GaN NLs on treated sapphire the near bandedge emission became narrower with a lower “background” (e.g., ultraviolet, blue, and yellow band emission), which suggested an improvement of the crystalline qualities [21]. The photoluminescence measurements showed that the yellowband emission was clearly visible for GaN films without SiNx pretreatment whereas became invisible for GaN films with SiNx pretreatment [26]. For low temperature (77 K) photoluminescence of GaN films with SiNx pretreatment the full width at half maximum of GaN donor bound exciton peak is only 5 meV. The observation of free excitons for GaN films prepared with Si-rich SiNx treatment also indicated high quality GaN films of good optical properties. In summary, by controlling the growth conditions of SiNx layers coated on sapphire and thus the surface morphologies and stoichiometry of SiNx, the growth behaviour, crystalline qualities, and optical properties of GaN layers can be modified. 4. Conclusion We have presented studies on the surface morphologies and XPS of MOCVD-grown SiNx prepared under different conditions by variations of deposition temperature, flow rate of SiH4 source, and pre-nitridation conditions. We found that 1) the surface roughness decreased either by increase of NH3/SiH4 ratio or deposition temperature. 2) Saturated pre-nitridation would increase the surface roughness. 3) Growth at high temperature enhanced the SiNx deposition with most of the sapphire surface covered by SiNx. In comparison, low temperature SiNx deposition resulted in low reaction rate of NH3 and SiH4 and low decomposition rate of NH3. As a result, only a small fraction of the sapphire surface was covered by Si-rich SiNx nanoislands. 4) The Si/N ratio decreased from 3.21 to 1.11 when the deposition temperature increased from 545 °C to 1035 °C. 5) Due to the “post-nitridation effect” the volume of ammonia participating in the SiNx formation was reduced and thus the Si/N ratio was
This work was partially supported by the National Natural Science Foundation of China. The author would like to thank S. J. Wang and H. Z. Sun for XPS measurements and D. Q. Lin for AFM measurements. References [1] R.D. Gould, S.A. Awan, Thin Solid Films 515 (2006) 509. [2] P. Gibart, Rep. Prog. Phys. 67 (2004) 667. [3] Z.T. Jiang, T. Yamaguchi, K. Ohshimo, M. Aoyama, L. Asinovsky, Jpn. J. Appl. Phys. 37 (1998) 571. [4] C.H.M. van der Werf, H.D. Goldbach, J. Loffler, A. Scarfo, A.M.C. Kylner, B. Stannowski, W.M. Arnoldbik, A. Weeber, H. Rieffe, W.J. Soppe, J.K. Rath, R.E.I. Schropp, Thin Solid Films 501 (2006) 51. [5] G. Bilger, T. Voss, T. Schlenker, A. Strohm, Surf. Interface Anal. 38 (2006) 1687. [6] H. Amjadi, IEEE Trans. Dielectr. Electr. Insul. 6 (1999) 852. [7] I. Mártil, A. del Prado, E.S. Andrés, G. González Díaz, F.L. Martínez, J. Appl. Phys. 94 (2003) 2642. [8] F.W. Chen, T.T.A. Li, J.E. Cotter, Appl. Phys. Lett. 88 (2006) 263514. [9] V.M. Bermudez, F.K. Perkins, Appl. Surf. Sci. 235 (2004) 406. [10] M. Lattemann, E. Nold, S. Ulrich, H. Leiste, H. Holleck, Surf. Coat. Technol. 174 (2003) 365. [11] M. Vetter, Thin Solid Films 337 (1999) 118. [12] E. Dehan, P. Temple-Boyer, R. Henda, J.J. Pedroviejo, E. Scheid, Thin Solid Films 266 (1995) 14. [13] J.M. Lackner, W. Waldhauser, R. Berghauser, R. Ebner, M. Beutl, G. Jakopic, G. Leising, H. Hutter, M. Rosner, T. Schoberl, Surf. Coat. Technol. 192 (2005) 225. [14] Minghua Wang, Dongsheng Li, Zhizhong Yuan, Deren Yang, Duanlin Que, Appl. Phys. Lett. 90 (2007) 131903. [15] A. Benami, G. Santana, A. Ortiz, A. Ponce, D. Romeu, J. Aguilar-Hernandez, G. Contreras-Puente, J.C. Alonso, Nanotechnology 18 (2007) 155704. [16] V. Em. Vamvakas, S. Gardelis, Surf. Coat. Technol. 201 (2007) 9359. [17] Tai-Cheng Tsai, Li-Zhen Yu , Ching-Ting Lee, Nanotechnology 18 (2007) 275707. [18] E. Reimhult, K. Kumar, W. Knoll, Nanotechnology 18 (2007) 275303. [19] B. Beaumont, Ph. Vennéguès, P. Gibart, Phys. Status Solidi B 227 (2001) 1. [20] E. Frayssinet, B. Beaumont, J.P. Faurie, P. Gibart, Zs. Makkai, B. Pécz, P. Lefebvre, P. Valvin, MRS Internet J. Nitride Semicond. Res. 7 (2002) 8. [21] Z.L. Fang, J.Y. Kang, J. Phys. Chem. C 111 (2007) 7889; Z.L. Fang, J.Y. Kang, W.J. Huang, H.T. Sun, M. Lu, J.F. Kong, W.Z. Shen, J. Phys. Chem. C 112 (2008) 4925. [22] S. Haffouz, H. Lahreche, P. Vennegues, P. de Mierry, B. Beaumont, F. Omnes, P. Gibart, Appl. Phys. Lett. 73 (1998) 1278. [23] T. Wang, Y. Morishima, N. Naoi, S. Sakai, J. Cryst. Growth 213 (2000) 188. [24] C.H. Liu, R.W. Chuang, S.J. Chang, Y.K. Su, C.H. Kuo, J.M. Tsai, C.C. Lin, Mater. Sci. Eng. B 111 (2004) 214. [25] Y.T. Moon, J. Xie, C. Liu, Y. Fu, X. Ni, N. Biyikli, K. Zhu, F. Yun, H. Morkoç, A. Sagar, R.M. Feenstra, J. Cryst. Growth 291 (2006) 301. [26] Z. L Fang, S. P. Li, J. C. Li, H. Z. Sun, S. J. Wang, J. Y. Kang, Thin Solid Films (in press), doi:10.1016/j.tsf.2007.12.132. bhttp://dx.doi.org/10.1016/j. tsf.2007.12.132N. [27] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Spectra for Identification and Interpretation of XPS Data, Physical Electronics, Minnesota, 1995. [28] K. Uchida, A. Watanabe, F. Yano, M. Kouguchi, T. Tanaka, S. Minagawa, J. Appl. Phys. 79 (1996) 3487.