Thermal decomposition and new luminescence bands in wet, dry, and additional oxygen implanted silica layers

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Journal of Non-Crystalline Solids 354 (2008) 1697–1702 www.elsevier.com/locate/jnoncrysol

Thermal decomposition and new luminescence bands in wet, dry, and additional oxygen implanted silica layers H.-J. Fitting

a,*

, Roushdey Salh b, B. Schmidt

c

a Institute of Physics, University of Rostock, Universita¨tsplatz 3, D-18051 Rostock, Germany German Institute for Polymers (DKI), Department of Physics, Schlossgartenstrasse 6, D-64289 Darmstadt, Germany Research Center Rossendorf, Institute of Ion Beam Physics and Material Research, P.O. Box 510119, D-01314 Dresden, Germany b

c

Received 26 June 2007; received in revised form 21 August 2007 Available online 25 October 2007

Abstract Wet and dry silica oxide layers have been treated thermally up to Ta = 1300 C and were investigated by cathodoluminescence (CL) spectroscopy. Whereas the dry oxides after high temperature treatment show an increase of the yellow–red spectra region, contrary, in wet oxides the UV–blue region is enhanced. Even a new strong band in the near-UV region (NV) at 330 nm (3.76 eV) is found for wet oxides at liquid nitrogen temperature (LNT), but much broader and with lower intensity for room temperature (RT) in a triple band structure UV: 290 nm, NV: 330 nm, and V: 400 nm. These violet bands should be associated with a thermally decomposed and rapidly cooled-down silica network in presence of OH groups or even dissociated oxygen. Additional oxygen implantation into dry silica with high doses up to 1017 ions/cm2 and high thermal treatment T > 1100 C leads as well to enhanced UV–NV–V luminescence emission bands supporting the fact that oxygen and structural decomposition play a decisive role in formation of near-UV luminescent defects in silica.  2007 Elsevier B.V. All rights reserved. PACS: 68.37.Hk; 68.37.Lp; 71.55.Ht; 78.30.Hv; 78.60.Hk Keywords: Glass formation; Glass transition; Hydrogen in glass; Radiation effects; SEM S100; Optical properties; Luminescence; Silica; Defects; Short-range order; Thermal properties; Viscosity and relaxation; Structural relaxation

1. Introduction Silicon dioxide has revealed as an important material for microelectronics, optics, and even photonics. The performance of electronic and optical devices depends in a high degree on the presence of extrinsic as well as intrinsic defects in the atomic SiO2 network. Photoluminescence (PL), electron spin resonance (ESR), photoabsorption (PA), photoluminescence excitation spectroscopy (PLE) as well as cathodoluminescence (CL) are commonly used to study the structural and luminescent defects in silica materials [1]. A very special attention has *

Corresponding author. Tel.: +49 381 498 6760; fax: +49 381 498 6802. E-mail address: hans-joachim.fi[email protected] (H.-J. Fitting).

0022-3093/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.003

been paid to the PL and CL emission from the oxygen deficiency centers ODCs (like the oxygen vacancy) [2–4] and the non-bridging oxygen hole center (NBOHC) Si–O with an unpaired electron [5–9]. Two kinds of ODC species have been distinguished by their optical absorption and luminescence features; ODC(I): Si–Si and ODC(II) which also has two proposed alternative models: the unrelaxed oxygen vacancy (Si    Si) and the twofold co-ordinated silicon (=Si:) possessing a lone pair of non-bonding electrons [10,11]. A link between these ODC variants was put forward by the observation that under excitation of ODC(I), the UV emission band at 4.3 eV can be excited too which is attributed to ODC(II), this proposes the effectiveness of a conversion process between these two centers [12,13].

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According to the Skuja model of ODC(II) the twofold co-ordinated silicon =Si: possesses a fast singlet–singlet transition with the UV (4.3 eV) luminescence and the slow triplet–singlet transition of the blue B (2.7 eV) luminescence [12]. However, Meinhardi and Paleari [14] reported photoluminescence (PL) data on amorphous SiO2 excited with synchrotron radiation, and their data indicate that the blue B band at 2.7 eV is independent of the UV band at 4.3 eV. These results are also reviewed by Leone et al. [15]. Morimoto et al. [16] have shown that the implantation of H+ ions reduces the magnitude of the B 2.7 eV blue emission whereas the UV emission is not affected. Even in a very recent work [17] X-ray excited luminescence (XRL) and successive thermally stimulated luminescence (TSL) show identical spectra but quite different in comparison to photoluminescence (PL) regarding the blue B (2.7 eV) and the UV (4.4 eV) luminescence bands. Moreover, the intensity of the blue emission B increases during electron beam irradiation whereas the UV emission remains nearly constant as we have observed in many CL dose experiments [18]. From this dose behavior the latter authors conclude that both centers of the blue and UV luminescence bands are not attributed to the same defect. The electron beam irradiation produces competitive reactions between luminescence center creation and destruction processes. During this competition reaction, the glassy network of SiO2 changes also due to loss of oxygen [19] producing ODC’s and even Si nanoclusters [20]. To investigate whether the different luminescent centers are related to oxygen or to silicon, we have compared nonstoichiometric SiO2 layers produced by direct ion implantation. Based on these findings we have proposed a model of defect creation by stressing the role of mobile oxygen in defect transformation [18]. Similar results were obtained after Si+ and O+ implantation by photoabsorption measurements [21]. In the present paper we want to investigate how strong thermal treatment with beginning dissociation of wet and dry silica acts to the luminescence behavior of those and what role plays additionally implanted oxygen. 2. Experimental The CL spectra and related excitation dose measurements were performed in a digital scanning electron microscope via a parabolic mirror collector, a spectrograph and a CCD camera, see e.g. [18,20]. CL spectra ranging from 1.5 to 6.5 eV were accumulated in single shot mode within a time of 1 s and with a spectral resolution of 4 nm. A temperature controlled sample stage provides sample temperatures between 80 K (liquid nitrogen temperature, LNT) via room temperature (RT) up to 380 C. In general, the CL excitation was performed with an electron energy of 10 keV and a beam current of I0 = 500 nA. This beam is scanned over an area of (106 · 110) lm2 providing a current density of j0  5 mA/cm2. The samples under investigation were ‘wet’ and ‘dry’ oxidized SiO2layers on Si h 1 1 1i substrate. The dry oxida-

tion has been performed at a temperature 1100 C in O2 ambient with a residual water content of 4 ppm by weight. For wet oxidation the oxygen flow was passed through de-ionized water at 95 C. The thicknesses of dry and wet oxide layers were 200 and 250 nm, respectively. The mass density of wet oxidized layers is q = 2.18– 2.20 g/cm3, of dry oxidized ones q = 2.25–2.27 g/cm3. Both types of layers are of microelectronic quality prepared in the Research Center Rossendorf in Dresden. Further on, dry oxidized layers have been implanted additionally by oxygen ion O+ with doses D = 1; 5; 10 · 1016 ions/cm2. This ion implantation technique is described more detailed in [18]. Afterwards, the samples have been thermally treated (in case of ion implantation one may say as well postannealed) for 1 h in vacuum at temperatures Ta = 700, 800, 900, 1000, 1100, 1200, and 1300 C. After switchingoff the heating the rapid cooling-down process to room temperature RT can be described by an exponential law T(t)  (Ta  RT) exp (t/s) + RT with a mean chilling time of s  90 s. 3. Results The CL measurements of wet and dry as well as dry silica layers implanted additionally with oxygen ions are presented in Figs. 1–5. 3.1. Spectra of wet and dry oxides The high thermal treatment of dry oxide layers at a temperature of Ta = 1300 C in the present work in Fig. 1(top) shows the same effect of the enhanced red–yellow luminescence as described before in a previous work for SiOx [20]. The yellow band Y is especially developed when measured at room temperature RT. However, here not SiOx but dry full stoichiometric SiO2 layers are investigated. Furthermore, we observe only slightly elevated UV and violet V regions. Looking to the wet oxide in Fig. 1(bottom) we see an increase of the yellow region too, but additionally a huge increase of the near ultra-violet (NV) region forming even a new NV band at 330 nm (3.76 eV) when measured at liquid nitrogen temperature LNT, Fig. 1(bottom, left). Moreover, at RT (right) we see a broad band extended from the UV region 250 nm up to the blue region 470 nm. Thereby it exhibits three subbands: the wellknown UV band at 290 nm, the new near-UV (NV) band at 330 nm and a violet band at about 400 nm. In Fig. 2 we see the different bands on their Ta dependences. In dry oxide the yellow–red bands are increasing for Ta > 1100 C; in wet oxide contrary, the blue–UV bands are arising beyond 1100 C. However, under electron beam irradiation of j0 = 5 mA/cm2 these new bands live only a short time t < 100 s as can be seen in Fig. 3. Afterwards the spectra resemble each other and possess almost the same common shape of silica CL spectra as to be seen in Fig. 4.

H.-J. Fitting et al. / Journal of Non-Crystalline Solids 354 (2008) 1697–1702

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Fig. 1. Initial CL spectra (1 s) of dry and wet silica layers measured at room temperature (RT) and liquid nitrogen temperature (LNT). The samples were thermally treated at temperatures Ta between 700 C up to 1300 C. Above 1100 C the wet silica layers show increasing thermal dissociation with a strong increase of the UV, near ultra-violet (NV), violet (V), and blue (B) luminescence.

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Fig. 3. CL excitation dose behavior of the UV and NV cathodoluminescence bands (see Fig. 1) in thermally treated (Ta = 1300 C) wet silica, measured at LNT and RT.

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annealing temperature Ta (oC) Fig. 2. CL band intensities R, Y, B, NV, and UV (see Fig. 1) of dry and wet silica layers in dependence on the thermal treatment temperature Ta.

3.2. Additional oxygen implanted dry silica In the following the role of oxygen in forming UV spectra should be investigated more detailed. Thus we have

implanted additional oxygen into dry silica layers in order to exclude extensively the presence of hydrogen, see Fig. 5. As we know already from Ref. [18] oxygen will favor the development of NBOHC and the red band R at 660 nm. Indeed, in Fig. 5(top) we recognize a strong increase of this band R already after an O+ implantation dose of D = 1 · 1016 ions/cm2. This band is maximum developed after an annealing temperature of Ta = 800 C. With an O+ implantation dose of D = 5 · 1016 ions/cm2 we observe at annealing temperatures above 900 C arising bands in the UV–violet (V) range, Fig. 5(middle). Further on we see, that the common UV-band (290 nm, 4.3 eV) is raised too and takes part in this broad band formation. Increasing

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the implantation dose to a maximum dose of D = 1017 ions/cm2 where we have to expect already remarkable volume changes, sometimes accompanied with bubble formation, we observe a huge increase of the broad UV–near-UV(NV)–violet (V) band region. With respect to the non-implanted wet silica samples in Fig. 1 with a band maximum at 330 nm (3.76 eV) the NV maximum band position here in dry oxygen-implanted samples is shifted to the violet side and can be fixed to 370 nm (3.35 eV). Because these luminescent defects appear only after very high thermal treatment we can exclude implantation damage.

wavelength (nm) Fig. 4. Saturated CL spectra (30 min) of wet and dry silica layers after thermal treatment at Ta = 1300 C showing under continuous electron irradiation a transition to the same common type of silica spectra as measured, e.g. in [19].

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wavelength (nm) Fig. 5. CL spectra of additional oxygen implanted dry silica layers with different ion implantation doses D and successive thermal treatment (annealing) at temperatures Ta.

4. Discussion In previous work we did investigate ion-implanted SiO2 layers: Si+, O+, Ge+ in [18]; H+ [22], as well as isoelectronic substitutions: S+, Se+ for oxygen and C+, Ge+, Sn+, Pb+ for silicon [23]. There we applied a post-implantation thermal annealing up to Ta = 1100 C and have obtained maximum luminescence for annealing temperatures Ta = 80– 900 C, i.e. an optimum formation and concentration of luminescent centers. This process was associated with agglomeration of implanted species to molecules and low dimensional clusters like dimers (ODC I), trimers, and higher aggregates [24]. Moreover, in Ref. [20] we tried to treat under stoichiometric SiOx thermally up to Ta = 1300 C and did observe thermal phase separation according to the reaction  x x SiOx ! SiO2 þ 1  Si ð1Þ 2 2 connected with a yellow luminescence Y (2.15 eV), very probably associated to higher ODC’s and silicon hexamer ring formation as an initial step of Si nanocrystal formation. The latter ones have been monitored by high resolution transmission electron microscopy HR-TEM, [20,24]. Moreover, a violet luminescence V around k = 400 nm (3.1 eV) is often observed after ion implantation, e.g. after Ge+, Sn+, and S+ implantation as could be shown in [22]. In many papers such a violet band at 400 nm is attributed to the implanted species, e.g. to the twofold co-ordinated =Ge [25]. Hence it appears also in thermally, and partly structural decomposed non-doped silica so one may deduce that it is generally an intrinsic effect of the damaged silica network itself. However, we know that same band positions in spectra do not mean sufficiently that they are due to the same luminescent defects. Other methods as mentioned in Section 1 , e.g. PLE, ESR, or life time measurements should give a more precise picture. Obviously, the H and/or the OH contents play an important role for the UV–V defect formation at high temperatures, maybe, as catalyst or even as a constituent of the defect. However, in [22] we have implanted additional hydrogen H+ and could not observe any influence on the UV emission. Thus oxygen remains as the main reason for this UV–V luminescence.

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In Figs. 3 and 4 we have shown that after a certain electron irradiation dose the near-violet NV band disappears and the ‘saturated’ spectra of wet and dry oxides resemble each other and show almost the same common shape of silica CL spectra. It means, the electron beam irradiation at room temperature RT leads to certain reconstruction and relaxation of the silica network previously disturbed by strong thermal treatment and structural decomposition in beginning viscosity and then rapidly cooled-down to RT in a short mean chilling time of s  90 s, see Section 2. Thus the structural decomposition of the viscous silica layer was frozen in at a relatively high glass temperature Tg > 1100 C. During the CL excitation this structural disorder, far from equilibrium at room temperature RT, is reconstructed by the electron beam dose of j0 Æ t  0.5 As/cm2. After that dose the ODC blue band B and the NBOHC red band R in the CL saturation spectra are developed as in normal prepared oxide layers with moderate thermal annealing Ta < 1100 C, see [20]. Note, that all saturated bands R, B, UV are to a certain step slightly more developed in wet than in dry oxide, probably, due to a higher concentration of precursors for them in wet oxides. In Section 3.2 we have described the effect of additional oxygen ion implantation into dry silica and the appearance of a broad near-violet band NV (330 nm, 3.76 eV) too. Rather these luminescent defects should be connected with oxygen, we observe here in O+ implanted samples as well as in wet non-implanted oxides of Fig. 1 where OH groups or even only dissociated oxygen are responsible for the UV–V luminescent bands. Hydrogen we may exclude because in Ref. [22] we found almost no influence of H+ implantation on the common UV (290 nm) luminescence. So it remains that oxygen surplus in silica plays a decisive role in formation of UV–V luminescence where the common UV band (290 nm, 4.3 eV) seems to be an intrinsic luminescent defect of the silica network produced already during oxidation. Oxygen excess and high thermal treatment, even with Ta = 1300 C exceeding the oxidation temperature of 1100 C, will enhance the concentration of this defect, and the strongly disturbed structural surrounding in the amorphous silica network will broaden the emission band as to be seen in Figs. 1 and 5. 5. Conclusions High thermal treatment above Ta = 1100 C up to a temperature 1300 C produces different luminescence centers in wet and dry silica layers. In dry oxide the yellow– red region with bands Y: 580 nm and R: 660 nm is strongly enhanced whereas in wet oxide new bands appear in the UV–blue region (260–465) nm: the well-known UV band at 290 nm, a hugh band in the near-UV (called NV) at 330 nm, and a violet band V at 400 nm. The latter one often appears in ion-implanted layers, e.g. Ge+, Sn+, and S+ [23] and was attributed to twofold co-ordinated implan-

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tation species, but in the present paper it appears as an intrinsic silica decomposition defect associated most probably by OH groups or even dissociated oxygen. As far as we may exclude hydrogen for UV–V luminescence promotion, see [22], oxygen remains as the decisive species for that short wavelength luminescence. Moreover, additional oxygen O+ implantation and high temperature treatment produce strongly enhanced UV–V luminescence emission too. However, this UV–V luminescence appears after high thermal treatment followed by rapid cooling, thus a structural disorder is frozen in at relatively high glass temperatures Tg > 1100 C and should be associated to these luminescent centers. Successive electron beam CL excitation leads to structural reconstructions and after irradiation with a dose j0 Æ t  0.5 As/cm2 the common CL spectra of silica in typical saturation appear. As a main result we may state: wet or oxygen implanted silica after high temperature treatment up to beginning viscosity and successive rapid chilling exhibit evident short wavelength UV–V luminescence. References [1] D.L. Griscom, J. Ceram. Soc. Jpn. 99 (1991) 923. [2] D.L. Griscom, in: Proceedings Thirty-Third Frequency Control Symposium, Electronics Industries Assn., Washington, DC, 1979. [3] L.N. Skuja, A.N. Streletsky, A.B. Pakovich, Solid State Commun. 50 (1984) 1069. [4] H. Nishikawa, E. Watanabe, D. Ito, Y. Ohki, Phys. Rev. Lett. 72 (1994) 2101. [5] L.N. Skuja, A.R. Silin, Phys. Stat. Sol. A 56 (1979) K11. [6] G.H. Sigel, M.J. Marrone, J. Non-Cryst. Solids 45 (1981) 235. [7] H. Nishikawa, T. Shiroyama, R. Nakamura, Y. Ohki, K. Nagasawa, Y. Hama, Phys. Rev. B 45 (1992) 586. [8] L. Skuja, J. Non-Cryst. Solids 179 (1994) 51. [9] K. Kajihara, L. Skuja, M. Hirano, H. Hosono, Appl. Phys. Lett. 79 (2001) 1757. [10] D.L. Griscom, Rev. Solid State Sci. 4 (1990) 565. [11] H.-J. Fitting, T. Ziems, A. von Czarnowski, B. Schmidt, Radiat. Measur. 38 (2004) 649. [12] L. Skuja, J. Non-Cryst. Solids 167 (1994) 229; L. Skuja, J. Non-Cryst. Solids 239 (1998) 16. [13] A.N. Trukhin, M. Goldberg, J. Jansons, H.-J. Fitting, I.A. Tale, J. Non-Cryst. Solids 223 (1998) 114. [14] F. Meinardi, A. Paleari, Phys. Rev. B 58 (1998) 3511. [15] M. Leone, S. Agnello, R. Boscaino, M. Cannas, F.M. Gelardi, Optical absorption, luminescence, and ESR spectral properties of point defects in silica, in: H.S. Nalwa (Ed.), Silicon-Based Materials and Devices: Properties and Devices, Academic Press, San Diego, San Francisco, Nwe York, Boston, London, Sydney, Tokyo, 2001, p. 1. [16] Y. Morimoto, R.A. Weeks, A.V. Barnes, N.H. Tolk, R.A. Zuhr, J. Non-Cryst. Solids 196 (1996) 106. [17] A.N. Trukhin, J. Troks, D.L. Griscom, J. Non-Cryst. Solids 353 (2007) 1560. [18] H.-J. Fitting, T. Barfels, A.N. Trukhin, B. Schmidt, A. Gulans, A. von Czarnowski, J. Non-Cryst. Solids 303 (2002) 218. [19] R.H. Magruder, R.A. Weeks, R.A. Weller, J. Non-Cryst. Solids 322 (2003) 58. [20] Roushdey Salh, A. von Czarnowski, M.V. Zamoryanskaya, E.V. Kolesnikova, B. Schmidt, H.-J. Fitting, Stat. Sol. A 203 (2006) 2049. [21] R.H. Magruder, R.A. Weeks, R.A. Weller, R.A. Zuhr, J. Non-Cryst. Solids 304 (2002) 224.

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