content as measured by elastic recoil detection (ERD) is also strongly dependent on the oxygen flow rate, with more erbium being incorporated in the more ...
Mater. Res. Soc. Symp. Proc. Vol. 866 © 2005 Materials Research Society
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The Impact of Deposition Parameters on the Optical and Compositional Properties of Er Doped SRSO Thin Films Deposited by ECR-PECVD Michael Flynn1, Jacek Wojcik1, Subhash Gujrathi2, Edward Irving1 and Peter Mascher1 1) Centre for Electrophotonic Materials and Devices, Department of Engineering Physics, McMaster University, Hamilton, Canada 2) ) Groupe de recherche en physique et technologie des couches minces, Lab. René -J.-A. Lévesque, Département de Physique, Université de Montréal, Montréal, Canada Abstract Silicon rich silicon oxide films were fabricated by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD). The films were doped in situ using an organometallic precursor. Optical measurements show that the refractive indices of the films are determined by the silane to oxygen flow rate ratio used during the depositions. The erbium content as measured by elastic recoil detection (ERD) is also strongly dependent on the oxygen flow rate, with more erbium being incorporated in the more highly oxygenated films. The erbium content was also found to vary inversely with plasma power, which did not have a significant effect on the silicon to oxygen ratio. This allows the erbium and excess silicon levels of the films to be controlled independently. Introduction Silicon rich silicon oxide (SRSO) has attracted considerable attention in recent years due to its interesting optical properties. When this material is annealed the excess Si forms nanocrystalline precipitates which, unlike bulk silicon, emit light when excited either optically or electrically [1,2]. Furthermore, when doped with erbium, there is a highly efficient energy transfer from the nanocrystals to the rare earth ion, leading to strong 1.5µm emissions. The development of a silicon-based emitter, which is compatible with standard CMOS processing would allow for the development of highly integrated monolithic photonic circuits. Erbium can be introduced into thin films of SRSO either post-deposition by ion implantation [3] or during the SRSO deposition by sputtering a metallic erbium source [4]. A third means of doping is to use an organometallic erbium precursor which has been used to produce erbium doped Si films [5]. In this paper we discuss the characterization of SRSO films deposited by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) doped during deposition using Er(TMHD)3. The relationship between the process parameters and the film composition is explored. Experimental Details The films used in this study were deposited using an ECR-PECVD system that has been described elsewhere [6]. The erbium source, Er(TMHD)3 (Alfa Aesar), was vaporized in a stainless steel cell and supplied to the deposition chamber by an argon carrier gas. The erbium cell temperature was varied between 100 and 140 ºC. The oxygen flow rate was varied between 40 and 64sccm, and the microwave power was adjusted from 500 to 800W. Silane (SiH4) was
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used as the silicon source and O2 as the oxygen source. Films were deposited onto high resistivity double sided polished silicon wafers and glassy carbon substrates. The optical properties of the films were measured with a Jobin Yvon spectroscopic ellipsometer in the wavelength range from 350 to 1600nm. The refractive index and extinction coefficient were determined using a Cauchy model [7]. Infrared absorption was measure using an ABB Bomem WorkIR 100 Fourier transform infrared (FTIR) spectrometer in the range from 650cm-1 to 3000cm-1. Depth profiles of the various elements in the films were measured using Rutherford backscattering spectroscopy (RBS) and elastic recoil detection (ERD) [8]. The RBS measurements were performed on films deposited on glassy carbon substrates using a 1.5MeV He4 beam. Carbon rather than silicon substrates were used since the RBS data is essentially a mass spectrum of the film and substrate. Elements of lower atomic number than the substrate are very difficult, if not impossible to detect. ERD as well as simultaneous RBS was performed on films deposited on Si using a 40MeV Cu8+ beam. The erbium content of these films was determined by forward scattering RBS using the same Cu beam. Discussion The refractive indices of the films as a function of wavelength are shown in Figure 1. The high refractive index of the film deposited with 40sccm oxygen flow is the result of the large excess silicon content of this film. The 64sccm oxygen flow rate results in films with lower levels of excess Si and hence the index of these films lies closer to that of stoichiometric SiO2. The extinction coefficient was zero for all three films. Because the refractive index varies significantly with excess silicon content this is a useful metric for monitoring film composition. The large range of possible refractive indices coupled with the low extinction coefficients also makes these films useful for optical coatings [9]. The variation in Si:O of the films was confirmed by RBS of films on glassy carbon substrates. Elements of lower atomic number than the substrate are very difficult, if not impossible to detect.
Figure 1) Refractive index as a function of wavelength showing the influence of deposition parameters on the optical properties of the films.
Figure 2) FTIR absorbance of Er doped SRSO films showing Si-H bonds due to excess Si.
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The FTIR absorbance spectra are shown in Figure 2. The main peak at 1025cm-1 is the Si-O stretching mode [10]. The presence of hydrogen in the films is also indicated by the small peak at approximately 830cm-1 [11]. Hydrogen levels in the films were measured via ERD, and shown in figure 3. The presence of hydrogen in the films has been observed previously and is an artifact of the deposition process. While hydrogen in the films is masked by the substrates in RBS measurements, it is clearly visible when the films are examined by ERD and follows the silicon deposition profile. The somewhat gradual rise in silicon up to the level of the substrate is an artifact of the ERD measurement technique, and is a result of a smearing of the Cu8+ energy profile as the ions penetrate the film [12]. In all the films studied the hydrogen concentration was approximately 25% that of the silicon in the films. From previous studies on undoped SRSO we know that the hydrogen is driven off if the films are annealed [13]. Hydrogen levels after the nanocrystal forming anneals will be examined in future studies. ERD was also used to measure the Si/(Si+O) ratios of the films. This data is shown in Figure 4. The ratio rises sharply to a value of one at the depth of the film-substrate interface. The oxygen flow rates used in this study resulted in films ranging from approximately 37 to 50% Si. The films with high silicon concentration were deposited with an oxygen flow rate of 40sccm. This increased silicon to oxygen ratio is the cause of the refractive index shift shown in Figure 1. Low excess silicon levels in the films results in optical properties closer to those of SiO2. The oxygen flow rate also dramatically impacts the erbium content of the films, as shown by the RBS data in Figure 5. Films deposited at 64sccm oxygen have peak erbium concentrations roughly 4 times that of films deposited at 40sccm. The erbium levels in the films correspond to volume densities of the order of 1020-1021 erbium atoms/cm3. The increased erbium concentrations at higher oxygen flow rates are due to the lower vapor pressure of erbium oxide relative to erbium. This will increase the desorption of erbium from the depositing film at lower oxygen flow rates since the fraction of unoxidized erbium arriving at the substrate will be larger. The silicon and erbium levels in the film can be decoupled by adjusting the microwave power. Figure 6 shows the silicon and erbium depth profile of films deposited at 500 and 800W. The erbium content of the films is dramatically influenced by the change in microwave power, with the peak erbium concentration increases by more than a factor of 3, despite no change being
Figure 3) ERD depth profiles showing the presence of hydrogen following that of the silicon in the films.
Figure 4) ERD depth profiles showing the influence of oxygen flow rate on the concentration of erbium in the films.
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Figure 5) ERD depth profiles showing the influence of oxygen flow rate on the concentration of erbium in the films.
Figure 6) ERD depth profiles showing the influence of microwave power on the concentration of erbium and silicon in the films.
observed in the silicon levels. The exact reason for this trend is not yet clear; however the ability to decouple the erbium and silicon levels in the films is of critical importance. By controlling both the oxygen flow rate and the microwave power both silicon and erbium levels can be controlled independently. Conclusions ECR-PECVD has been used to deposit Er-doped SRSO films, the compositions of which were subsequently measured by ERD. Ellipsometry and FTIR were used to characterize the films optical properties. By varying both the oxygen flow rate and microwave power during the deposition the Er and Si concentrations could be controlled independently. This allows the composition to be tuned over a wide range. These materials have potential applications in a variety of areas, including as light sources operating in the 1.5µm telecommunications window. Further research will concentrate on the effects of different annealing conditions on the formation of silicon nanocrystals and their optical properties. The erbium and silicon levels in these films will be adjusted in order to optimize the optical performance. Acknowledgements This work was funded by the Ontario Photonics Consortium (OPC), Photonics Research Ontario and Materials and Manufacturing Ontario. The latter two organizations are divisions of the Ontario Centres of Excellence (OCE) Inc. References 1) S. Ossicini, L. Pavesi and F. Priolo, Light Emitting Silicon for Microphotonics, Springer, 2003. 2) R. J. Walters, G.I. Bourianoff and H.A. Atwater, Nature Materials, 4, 143 (2005). 3) D. Kuritsyn, A. Kozanecki, H. Przybylińska and W. Jantsch, Phys. Stat. Sol. (c), 1, 229 (2004). 4) H.-S. Han, S.-Y. Seo and J.H. Shin, Appl. Phys. Lett. 79, 4568 (2001).
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5) P.S. Andry, W.J. Varhue, F. Ladipo, K. Ahmed, E. Adams, M. Lavoie, P.B. Klein, R. Hengehold and J. Hunter, J. Appl. Phys., 80, 551 (1996). 6) M. Boudreau, M. Boumerzoug, P. Mascher, and P. Jessop, Applied Physics Letters, 63, 3014 (1993). 7) D.J. Griffiths, Introduction to Electrodynamics, Prentice Hall, p. 404 (1999). 8) S.C. Gujrathi, S. Rooda, J.G. D’Arcy, Randall, J. Pflueger, P. Desjardians ,I. Petrov, J.E. Greene, Nuc. Inst. and Meth. B 136-138 (1998). 9) M. Flynn, E. Irving, T. Roschuk, J. Wojcik and P. Mascher, Compositional and Optical Characterization of SiOx Films Deposited by ECR-PECVD for Photonic Applications, 1st International Conference on Group IV Photonics, Hong Kong (2004). 10) A. Sassella, A. Borghesi, F. Corni, A. Monelli, G. Ottaviani, R. Tonini, B. Pivac, M. Bacchetta and L. Zanotti, J. Vac. Sci. Technol. A, 15, 377 (1997). 11) V. Chu, J.P. Conde, J. Jarego, P. Brogueira, J. Rodriguez, N. Barradas and J.C. Soares, J. Appl. Phys., 78, 3164 (1995). 12) J.C. Barbour and B.L. Doyle in Handbook of Modern Ion Beam Materials Analysis, J.R. Tesmer and M. Natasi eds., Materials Research Society (1995). 13) T. Roschuk, J. Wojcik and P. Mascher, Proceedings of the SVC 47th Annual Technical Conference, 362 (2004).
