Off-Stoichiometry Spectroscopic Investigations of Pure Amorphous

Jan 23, 2013 - Department of Molecular Sciences and Nanosystems, Ca' Foscari University of Venice, Dorsoduro 2137, 30123 Venezia, Italy. ‡. Kyoto In...
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Off-Stoichiometry Spectroscopic Investigations of Pure Amorphous Silica and N‑Doped Silica Thin Films M. Boffelli,†,‡ M. Back,† E. Cattaruzza,† F. Gonella,† E. Trave,† A. Leto,§ A. Glisenti,∥ and G. Pezzotti*,‡ †

Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Dorsoduro 2137, 30123 Venezia, Italy Kyoto Institute of Technology and Research Institute for Nanoscience, Sakyo-ku, Matsugasaki 606-8585, Kyoto, Japan § Piezotech Japan Ltd., Sakyo-ku, Ichijouji, Mukaibata-cho 606-8126, Kyoto, Japan ∥ Department of Chemical Sciences, University of Padua, via Marzolo 1, 35138 Padua, Italy ‡

S Supporting Information *

ABSTRACT: Cathodoluminescence spectroscopy and X-ray photoelectron spectroscopy were concurrently used to investigate the local physicochemical nature of the amorphous lattice in pure SiO2 and N-doped SiO2 thin films prepared by radiofrequency magnetron sputtering (the latter samples deposited under a set of different conditions of N2 partial pressure). The main aim of this investigation was twofold: (i) to extend our knowledge of the physical and chemical structure of SiO2 films and (ii) to explore our capacity of manipulating, fine tuning, and measuring their stoichiometry characteristics. The presence of nitrogen atoms in the amorphous host structure was confirmed to significantly affect the formation of oxygen-deficient centers, nonbridging oxygen hole centers, and other kinds of defect complexes. The main challenge here was to relate the variations in type and concentration of these peculiar defects to the processing conditions and to the amount of nitrogen incorporated in the SiO2 amorphous matrix. The evolution of both pure and doped systems was monitored with increasing the temperature of an annealing cycle following film deposition (1 h in air, at temperatures ranging between 50 and 1200 °C, with 50 °C step). Stoichiometry changes could thus be clarified and temperature thresholds found for the annihilation of N sites and for the formation of a pseudoequilibrium stoichiometric structure in silica glass.

1. INTRODUCTION The incorporation of nitrogen in glassy silica (N:SiO2) has attracted much interest in the technological community as one quite promising way to synthesize improved film structures for various advanced applications, in particular, for optoelectronics devices.1,2 From a chemical point of view, doping the SiO2 structure with nitrogen is expected to lead to the local formation of silicon oxynitride (SiOxNy) clusters, whereas, from the physicochemical side, the structure of the SiO2 matrix might undergo local stoichiometry alterations through the formation of peculiar defect populations affiliated to a 3-fold valence of incorporated nitrogen versus the 2-fold one of the (replaced) native oxygen anions. The technological importance of silicon oxynitride films in optoelectronics applications, for example, resides in the quite appealing possibility of tuning their bandgap energy (i.e., from 5 to 9 eV) through straightforward modifications of their structural stoichiometry.3 However, © 2013 American Chemical Society

structural stability issues represent the actual drawback to this approach since a number of functional properties in almost all optoelectronic and photonic materials are negatively affected by local off-stoichiometry fluctuations. Populations of different lattice defects, for example, oxygen vacancies, which might cluster on a mesoscopic length scale and in narrow regions close to heterogeneous interfaces, can lower dramatically the functional performance of the material and cause reliability problems.4 Moreover, in many cases of technological interest, defect populations are not only uncontrollable from the manufacturing side but also hardly detectable because of their local nature and abrupt spatial distribution. In such a technological context, the development of a reliable spectroReceived: November 28, 2012 Revised: January 10, 2013 Published: January 23, 2013 3475

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sample holder during the film growth was kept constant in the range of 30−40 °C, that is, relatively close to room temperature. By changing the relative partial pressure of the gaseous phase while keeping a constant total pressure of 50 × 10−2 Pa, it was possible to obtain films with different nitrogen contents up to about 5 at. % in average, as determined by XPS analyses. In the remainder of this paper, N-containing samples will be henceforth labeled as SiO2-N(x), where x = 5, 10, 15, 20, 25 indicates the partial pressure of N2 in the Ar/N2 mixture (×10−2 Pa). The final thickness of all the deposited films was around 1 μm, as determined by linear profilometry. Table 1

scopic probe for local stoichiometry assessments with high spatial resolution is desirable, which could selectively provide fundamental information regarding both the nature and the local spatial distribution of point defects. The availability of such a probe and its implementation to a quantitative analytical tool could then help in gaining a fundamental understanding of the chemical nature of amorphous silica films and greatly speed up the structural and functional optimization of advanced electronic devices. Research experience has also taught us that such a local probe should be applied to measurement protocols leading to the collection of a statistically meaningful data set and, preferably, also coupled (i.e., for validating its outputs) with an additional probe of a different physical nature, which reliably reads average properties on a larger length scale. With this background in mind, in this paper, we have selected cathodoluminescence (CL) spectroscopy, operated in a scanning electron microscope (SEM), as a local stoichiometry probe, because of its highly resolved spatial (down to the nanometer length scale) and spectral (better than 10−1 nm) characteristics.5 As an additional nonlocal probe, we have adopted the X-ray photoelectron spectroscopy (XPS) emission from film samples, which is a more conventional probe for surface screening on a length scale 3−4 orders of magnitude larger than CL. XPS was proved intrinsically capable of reliable assessments of the nature of atomic bonding in amorphous silica and in more complex amorphous matrixes doped with rare earths,6 whereas the CL probe is yet somewhat lessestablished for silica glass in its quantitative details. The main aim in the present work was to obtain, through a statistically meaningful set of CL/XPS analyses, deeper insight into the physicochemical behavior of the amorphous SiO2 network in its N-modified structure, which could be a guide to tailor stoichiometry in thin-film-based devices. The nature of lattice defect populations was studied here on thin-film samples prepared by radiofrequency magnetron sputtering (RFMS) deposition. The RFMS film deposition has proved to be a suitable synthesis technique to our purpose here because, according to the possibility of strictly controlling deposition parameters, film samples with a reliable stoichiometry control could be obtained, free of contaminations and rather homogeneous in their morphological and chemical characteristics.7 Furthermore, controlled off-stoichiometry characteristics could be obtained by systematically performing postdeposition heat treatments on the samples and by following the evolution of the system after different annealing cycles. The examined thin films can be considered as standard samples for both highpurity (and water-free) and N-doped amorphous silica films.

Table 1. Investigated Film Samples and Their Processing and Morphological Details sample SiO2 SiO2-N(5) SiO2-N(10) SiO2-N(15) SiO2-N(20) SiO2-N(25)

P (Ar) (Pa) 50 45 40 35 30 25

× × × × × ×

10−2 10−2 10−2 10−2 10−2 10−2

P (N2) (Pa) 5 10 15 20 25

× × × × ×

10−2 10−2 10−2 10−2 10−2

thickness (μm)

dep. time (h)

± ± ± ± ± ±

2 2 2 3 3 3.5

1.70 0.82 0.72 1.00 0.95 1.10

0.05 0.02 0.02 0.02 0.03 0.05

summarizes the deposition conditions and the thickness of the studied samples. After deposition, samples were thermally (and isochronally) annealed for 1 h in air at temperatures ranging from T = 50 to 1200 °C, with incremental steps of 50 °C. CL spectroscopy was performed inside the chamber of a field emission gun scanning electron microscope (FEG-SEM), equipped with a Schottky-emission type gun (S-4300SE, Hitachi, Tokyo, Japan) as the excitation source. The acceleration voltage was set at 5 kV throughout all the experiments, and the probe current was also fixed at 180 pA. A high-sensitivity CL detector unit (MP-32FE, Horiba Ltd., Kyoto, Japan) was employed for the collection of light upon reflection into an ellipsoidal mirror and transmission through a bundle of optical fibers, using a high-resolution monochromator (TRIAX 320, Jobin-Ivon-Spex, Kyoto, Japan) equipped with a nitrogen-cooled CCD camera. Integration time was 10 s, and the electron beam spot size was 500 nm. XPS measurements were performed by a Perkin-Elmer Θ 5600ci spectrometer using nonmonochromatic Al Kα radiation (1486.6 eV) in the 10−7 Pa pressure range. Surface charging was corrected for by assuming the binding energy (BE) of the silica O 1s band to be 532.7 eV. In this way, the Si 2p binding energy resulted in equal to 103.6 eV at all depths, as expected for amorphous silica. The chemical state of nitrogen in samples deposited under different conditions was monitored by XPS at the silicon 2p, oxygen 1s, and nitrogen 1s edges. The KLL silicon band was also recorded, for an additional check. In-depth XPS spectra were collected upon eroding by sputtering the sample surface with a 3 keV energy Ar+-ion gun, at an estimated erosion rate of about 1 nm/ min.

2. EXPERIMENTAL METHOD Amorphous N:SiO2 and SiO2 films were deposited by RFMS deposition starting from a high-purity silica target (SiO2, 99.995% purity) on high-grade fused-silica slides, which were preliminarily subjected to a suitable cleaning cycle. Deposition was performed by means of a 13.56 MHz radiofrequency source acting in a pure Ar atmosphere (for pure silica films) or in an atmosphere composed of an Ar/N2 gas mixture (for N:SiO2 thin films). The total pressure during deposition was fixed at 50 × 10−2 Pa, and the radiofrequency power to the silica target was kept constant at 250 W. The magnetron configuration of the source was preliminarily optimized in order to obtain the maximum deposition rate. Before deposition, a 20 min presputtering was performed both on the silica slide substrate and on the silica target. The temperature of the

3. RESULT AND DISCUSSION 3.1. XPS Results. Figure 1a−c shows the nitrogen 1s peakafter 150 min of sputtering timerecorded from the sample with the highest nitrogen content in the deposition atmosphere, that is, the SiO2-N(25) sample, in the following conditions: (a) as-deposited, (b) after annealing at T = 500 °C, and (c) after annealing at T = 1050 °C. A main peak was clearly resolved, centered at around 398.5 eV of BE, which is the characteristic N 1s band position associated with the presence 3476

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Figure 2. Nitrogen concentration depth profile of the SiO2-N(25) sample in its as-deposited and 1050 °C annealed states. Sputtering time of zero in the abscissa corresponds to the free surface of the sample. The film erosion proceeded at a speed of approximately 1 nm per minute.

driving force for the deposited films to partially free themselves of the incorporated nitrogen atoms, presumably through a thermally induced out-diffusion process, taking place mainly below 500 °C. In particular, XPS analyses indicated that a marked degradation of the oxynitride structure, as built up during deposition, takes place starting from temperatures as low as a few hundreds of °C. This might lead to severe functionality fluctuations in those practical applications for which significant device heating up takes place in service. 3.2. CL Results. A classification of point defects in the studied amorphous film samples should generally include both intrinsic defects, namely, the defect populations of the silica host matrix, and extrinsic defects, as directly caused by the presence of foreign nitrogen anions and the related excess/ deficiency of oxygen. Moreover, in oxynitride compounds, a more complex network could develop, in which silicon could be randomly bonded to either nitrogen or oxygen atoms in a different morphological assembly. In the amorphous networks, defects are electrostatically neutral but may become positively or negatively charged by trapping holes or electrons, respectively.9 Amorphous silicon oxynitride (a-SiOxNy) structures with different compositions should be comprehensive of a mixture of Si−N bonds and Si−O bonds, involving five types of different tetrahedral structures, SiOxN4−x (x = 0...4), estimated to obey a random distribution.10 The CL spectrum of pure SiO2 is composed of several overlapping bands, which can be resolved by performing a deconvolutive fitting procedure of the spectrum through a suitable algorithm. According to the published literature,11 different defect complexes generate different CL sub-bands. A fitting procedure could then be selected, which was based on a spectral deconvolution involving several Gaussian sub-bands corresponding to theoretically known defect complexes, whose respective positions were extracted from the previous literature and superimposed in correspondence of the observed (local) maxima of the spectra. In the present study, a spectral deconvolution according to the above criteria showed that the CL spectrum of silica always consisted of seven overlapping bands. We were able to fit all the recorded CL spectra by using values of both sub-band spectral position and full with at half-maximum (fwhm) fixed within

Figure 1. XPS nitrogen 1s peak for the SiO2-N(25) sample, as recorded in the bulk of (a) the as-deposited film, (b) the film after annealing at 500 °C, and (c) the film after annealing at 1050 °C.

of SiOxNy compounds.8 This confirms that the used synthesis route was effective in originating silicon oxynitride thin films. A quite weak additional peak could also be resolved at around 403.5 eV, which is usually attributed to the presence of gaseous species of nitrogen.8 Such a peak was clearly visible only in the as-deposited state. On the other hand, no presence of silicon nitride could be detected in any sample. From a quantitative point of view, XPS showed that the maximum content of nitrogen detected in the investigated samples covered a fraction of about 5 at. % and was detected on the free surface of the asdeposited SiO2-N(25) sample. As shown by the XPS spectra in Figure 1, the intensity of the N 1s peak decreased with increasing annealing temperature, starting from a nitrogen atomic fraction, detected in the bulk of the deposited film, of about 4.4 at. % (as-deposited) to about 1.5 at. % corresponding to a maximum of 2.0 at. % on the free surface (after annealing at 500 °C), and to 0.9 at. % for a maximum of 1.1 at. % on the free surface (after annealing at 1050 °C). Figure 2 shows, for the same SiO2-N(25) sample, the nitrogen content as a function of the sputtering time along the film thickness for the as-deposited and the 1050 °C annealed sample. Besides the clear detection of compositional gradients in the first 1−2 hundreds of nanometers for fixed synthesis and annealing conditions, the overall behavior evidenced a definite 3477

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Table 2. Summary of the Origin and the Spectral Location of the Main Cathodoluminescence Bands Contained in the CL Spectrum of Amorphous SiO2 band (eV) violet (3.1) blue (2.64) green (2.32) yellow (2.15) red (1.93) magenta (1.8) pink (1.62)

λ (nm) 394 468 534 576 639 687 763

± ± ± ± ± ± ±

description

3 3 4 4 2 2 3

2-fold coordinated silicon (ODC(II)) =Si•• neutral oxygen vacancy (ODC(I)) Si−Si contamination band self-trapped exciton (STE) or Si hexamers nonbridging oxygen hole center (NBOHC) O2Si-O• peroxyl radical center (POR) Si-O-O• silicon nanocluster or interstitial O2

(POR) centers. As far as the near-IR region is concerned, the assignment of the band is still unclear. The weak band at 763 nm can be related either to the presence of interstitial molecular oxygen or to silicon cluster aggregates; in the latter case, unlike the former, its presence shall depend on local silica stoichiometry.12,13 It is also possible to consider this quite weak band as a spectral artifact due to asymmetric broadening of the closest peak. Both the 763 nm weak emission and the contamination band at around 530 nm will not be discussed further in this paper. Figure 4 shows a comparison among selected CL emission spectra as collected from pure SiO2 and N:SiO2 films annealed at different temperatures. The recorded CL spectra exhibit quite large variations for different synthesis and annealing conditions. However, as general spectral characteristics, two main sub-bands located at around 460 and 640 nm and a weaker sub-band located at 550 nm were found. Spectra could always be precisely deconvoluted with using the same seven Gaussian curves discussed above, which were always located at about the same respective energy positions and showed, on spectra from different samples, the same respective fwhm. The following main spectroscopic characteristics could be extracted from examining the spectral changes upon N-doping/ annealing: (i) In the as-deposited samples (Figure 4a−d), an increasing amount of nitrogen incorporated in the silica structure had the 2-fold effect of reducing the overall amount of emitting centers in the sample (and thus the overall intensity of the CL spectra) and to ultimately produce an equipartitioned sub-band structure in which no particular defect population prevails on the others (cf. spectrum for the as-deposited SiO2N(25) sample in Figure 4d). (ii) The as-deposited pure SiO2 sample underwent a substantial recombination of structural defect centers upon annealing, with a gradual increase of NBOHC population contained in the initially more densely ODC populated amorphous structure (cf. Figure 4a,e,i). This can easily be regarded as a logical consequence of the abundance of oxygen provided by annealing in air as compared to the initial oxygen deficiency in the vacuum chamber of film deposition (pure Ar atmosphere). The trend showing a gradual increase of CL intensity from NBOHC upon annealing seems to be also common to all N-doped samples, although also the presence of ODC centers results to be enhanced as compared with pure silica at an intermediate annealing temperature (cf. CL spectra in Figure 4f,g,h for SiO2-N(10), (15), and (25) samples, respectively, after annealing at 850 °C). Remarkably, all N-doped samples seem to reach a “standard” stoichiometric structure, the same as that obtained for undoped silica after annealing at 1200 °C (cf. Figure 4i−l). This evolution of the CL spectrum in N-doped films confirms the substantial process of N depletion already observed by XPS experiments.

quite narrow intervals. All of these sub-bands, as labeled in the published literature, are originated by different mechanisms of defect-induced luminescence. Table 2 summarizes the location and origin of the deconvoluted Gaussian sub-bands, labeled according to Skuja and Salh.12,13 A CL spectrum collected from the as-deposited bulk SiO2 sample served as a reference, for which the associated spectral deconvolution is shown in Figure 3. It should be noted that

Figure 3. Deconvoluted CL spectra of the pure SiO2 as-deposited film. Sub-bands are labeled according to the defect classification given in Table 2.

many aspects regarding the nature of CL bands associated with point defects are still controversial and not yet completely understood, especially for the near-IR region. Despite such partial lack of knowledge for the IR region, the defects present in the amorphous silica network can be related either to oxygen deficiency or to oxygen excess. Bands in the blue region (i.e., at around 394 and 468 nm) and in the red region (i.e., at around 639 and 687 nm) are associated with the presence of oxygen deficiency centers (ODCs),14−21 and nonbridging oxygen hole centers (NBOHCs),20,22 respectively. Another band in the green region of the spectrum (i.e., centered at around 530 nm) arises from the presence of either carbon contamination or structural water.23 The accumulation of contamination during low voltage measurement in the vacuum environment is indeed an intrinsic problem of a conventional SEM apparatus and can hardly be eliminated, but could be minimized by accurate vacuum maintenance and care to avoid external contamination. A further luminescence band of detectable intensity could be found in the yellow region around 576 nm, which has been ascribed to a self-trapped exciton (STE)24,25 or to the presence of Si hexamer rings.13 The additional band in the red zone of the spectrum (at 687 nm) has been assigned to peroxyl radical 3478

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Figure 4. CL spectra collected on film samples as-deposited (a−d), annealed at 850 °C (e−h), and annealed at 1200 °C (i−l) for the series SiO2, SiO2-N(10), SiO2-N(15), and SiO2-N(25).

4. DISCUSSION In an attempt to rationalize the complete structural trends developed upon annealing, we have plotted the evolution of the CL intensity of different spectral sub-bands as a function of annealing temperature (Figure 5). According to the defect classification given in Table 2, the plots correspond to CL intensities emitted from ODC II (395 nm), ODC I (468 nm), NBOHC (639 nm), and POR (687 nm), as shown in Figure 5a−d, respectively. Figure 5 gives isochronal plots for 1 h annealing in air, and more completely envisages the general trend followed by N-doped systems with increasing annealing temperature, T, as compared to pure silica. Namely, samples with increasing nitrogen contents exhibited an increasingly lower CL emission in their as-deposited state. However, a comparison of spectral evolution among all the annealing temperature conditions further reveals an initial progressive decrease of luminescence intensity for annealing up to approximately T = 400−600 °C (depending on the initial N content), followed by a steeper intensity increase for larger T. This increase takes place starting from an intermediate annealing temperature comprised within an interval at around 500 °C (cf. shaded areas in Figure 5), and it is more marked in heavily N-doped samples. Both ODC sub-band intensities, however, show a maximum at just above 1000 °C and then decrease the more markedly the higher the N content. On the other hand, the NBOHC population seems to continuously increase without reaching a maximum up to 1200 °C (except for the SiO2-N(25) sample), although the isochronal curves yet preserve a “turning point” at Tc from a mild decrease to a steeply rising behavior at intermediate temperatures. As far as

the pure silica system is concerned, it showed the most marked decrease in the band intensities of both ODC defect types, the highest threshold temperature, Tc, for rising defect population (∼700 °C, commonly for ODC (I), (II), and NBOHC), and the slowest increase up to 1200 °C. Brückner26 suggested that the relaxation of internal structural stresses below what we have here defined as the “turning point” (i.e., in the interval of Tc between 400 and 600 °C for N-doped samples and at ∼700 °C for pure silica) is partly connected with a healing process of “open bonds”, since a thermal treatmenteven at low temperatureallows for occasional linking of Si−O bonds. This could explain the decrease in NBOHC below an intermediate threshold temperature. A decrease in ODC band intensity, when silica is annealed at 500 °C in an oxidizing atmosphere, was noticed by Jones et al.,19 who thus proposed that atmosphere oxygen fills vacancy centers. The rapid increase of NBOHC band intensity to overcome the intensity of the ODC centers toward annealing temperatures of 1000 °C indicates an incremental probability for the Si−O bondbreaking process in basic silica units with formation of 3-fold coordinated centers and an NBOHC according to the following off-stoichiometry reaction (eq 1) ≡Si‐O‐Si ≡→ ≡Si • + •O‐Si≡

(1)

Consequently, two 3-fold silicon centers can eventually recombine to form a Si−Si homobond, thus creating an ODC center or reacting with oxygen molecules of the atmosphere and producing a POR, according to the following eqs 2 and 3, respectively ≡Si •+ •Si ≡→ ≡Si‐Si≡ 3479

(2)

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Figure 5. Isochronal curves of CL intensity for different deconvoluted sub-bands as a function of annealing temperature for all the series of studied films: ODC (I), ODC(II), NBOHC, and POR in (a)−(d), respectively. Data points have been intentionally omitted for the sake of clarity. The range of threshold temperature, Tc, for N-doped samples is shown with a shadowed area, while the (higher) Tc found in the pure silica film is located by a broken line.

≡Si •+ O2 → ≡Si‐O‐O •

formation of a more stable network, decreasing the density of point defects. The preparation method thus plays a key role in the determination of the final structure, since the nitrogen introduced during film deposition may react with the SiO2 sputtered from the target, allowing a 2-fold coordinated silicon and/or a nonbridging oxygen (i.e., both with an unpaired electron) to bond with a nitrogen atom, forming an oxynitride compound and removing either ODC or NBOHC defect centers. The apparent contradiction between the conspicuous lack of nitrogen detected by XPS at around Tc and the quite low CL intensity of the detectable defects at the same temperature could be explained by considering that annealing in air up to Tc promotes a degradation of the structural network, as described above, with consequent out-diffusion of nitrogen, but also with formation of stable Si−O−Si bonds at evacuated N sites, and 3-fold coordinated silicon centers that are undetectable by CL. Such a process should occur according to the following off-stoichiometry reaction (eq 4)

(3)

This combination of reactions can explain the concurrent rising of both ODC and NBOHC band intensities observed with increasing temperature above the intermediate threshold, Tc. However, while ODC sites reach a saturated concentration and eventually annihilate upon interaction with the oxygen from the atmosphere at above 1000 °C, NBOHC (and POR) formation shows no maximum and proves continuously enhanced in the studied interval of temperature. Defect structures in silicon oxynitride glasses have not been so far the subject of extended spectroscopic CL investigations, even if, in the literature, topical reports appeared of some characteristic features.2,27 It is expected that the introduction of nitrogen in the silica matrix gives rise to a rather complex network of defects. For different nitrogen concentrations, CL spectra exhibit important modifications depending on annealing temperature as well as on dopant concentration, showing that the presence of nitrogen in the amorphous network significantly affects the behavior of defects in the matrix. Nitrogen doping actually does not give rise to new bands, but it significantly contributes in changing both absolute and relative intensities of the bands themselves. It was noticed above that, in the as-deposited samples, the overall CL intensity scales down with increasing the amount of nitrogen incorporated in the structure (Figure 4a−d). The nitrogen doping of as-deposited silica may indeed promote the

2(≡Si3N) + O2 → 2(≡Si 2O) + 2(≡Si•) + N2

(4)

N-doped samples exhibited similar trends as compared to pure silica, but the intermediate temperature threshold, Tc, was increasingly shifted toward lower temperatures with increasing N content. According to XPS data, the temperature threshold, Tc, can actually be identified with the annealing temperature at 3480

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structure of CL sub-bands, with the same intensity and width as those collected for the undoped silica film. This observation suggested that a somewhat stable (and “standard”) defect distribution could be realized with the amorphous network tending to reach a metastable equilibrium independent of initial composition. In conclusion, RFMS deposition has showed in this study its potential as a versatile technique for tailoring the stoichiometry of amorphous silica films with or without dopant addition. Spatially resolved CL has been demonstrated capable of detecting a variety of optically active centers in a complex system like amorphous silica, for which the short-range local structure may actually play a major role in functionality issues (e.g., band-gap tuning, enhanced thermal conductivity, relaxation of thermal stresses, etc.). The preliminary findings shown here, in particular, the structure of the isochronal curves shown in Figure 5, might already offer a guide to the industrial entourage in rationalizing and designing silica thin films with respect to their off-stoichiometry characteristics. However, this study also calls for additional efforts in the quantification of the relative intensity of the CL sub-bands in order to finally relate them to the actual stoichiometry figures of the amorphous silica network.

which the main fraction of the nitrogen incorporated into the silica structure leaves the sample and, from which on, a newly formed “pure” amorphous silica structure is restored and starts undergoing off-stoichiometry modifications under the effect of the hot-air annealing atmosphere. This assertion is supported by a strong morphological similarity among the curves belonging to samples with different N contents for both ODC and NBOHC (cf. Figure 5), when artificially translated to (apparently) rise from the same Tc. Compared to pure SiO2, the complex network of bonds induced by nitrogen can lead to the formation of different kinds of defects, where the silicon atoms should be randomly coordinated by a different number of nitrogen and oxygen atoms. The degradation of the SiOxNy structure, caused by annealing in air, leads to the formation of nitrogen-related defects, such as Si2N• and NOSi•, which, in turn, affect both red and blue bands in the CL spectrum.26,27 A possible qualitative explanation for the shift of the temperature threshold, Tc, toward lower temperatures with increasing N content could be that the oxynitride structure formed during film deposition begins to undergo breaking of N-related dangling bonds at an increasingly lower temperature, thus forming a higher density of defects for each fraction of N leaving the system as compared to an undoped structure. Indeed, the NBOHC red band, related to SiO•, is emitted from the Si2N• complex and from other subcoordinated forms of it, which, in turn, originate from the breaking of a Si− N bond. Annihilation of ODC defects at temperatures above 1000 °C was another striking feature systematically observed in all the investigated film samples; the higher the N content, the lower the temperature at which such a process starts. This phenomenon was already reported for a Si/SiO2 interface upon annealing to form silicon oxynitride in a nitrogen atmosphere,28 but the nature of this phenomenon is still unclear. However, we newly find here that, for annealing close to the glass transition temperature, a “standard” defect distribution is built up in the network, which thus tends to a kind of metastable equilibrium in a status similar to that of pure silica, and undergoes the same permanent modifications in differently manufactured samples.

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ASSOCIATED CONTENT

S Supporting Information *

Isochronal curves of CL intensity for (a) ODC (I) and (b) NBOHC defects relating bands as a function of annealing temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/Fax: +81-75-724-7568. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Rand, M. J.; Standley, R. D. Silicon Oxynitride Films on Fused Silica for Optical Waveguides. Appl. Opt. 1972, 11, 2482−2488. (2) Wong, H. Recent Developments in Silicon Optoelectronic Devices. Microelectron. Reliab. 2002, 42, 317−326. (3) Bayliss, S.; Gurman, S. Silicon Oxynitride as a Tunable Optical Material. J. Phys.: Condens. Matter 1994, 6, 4961−4970. (4) Sushko, P. V.; Mukhopadhyay, S.; Stoneham, A. M.; Shluger, A. L. Oxygen Vacancies in Amorphous Silica: Structure and Distribution of Properties. Microelectron. Eng. 2005, 80, 292−295. (5) Pezzotti, G.; Leto, A. Contribution of Spatially and Spectrally Resolved Cathodoluminescence to Study Crack-Tip Phenomena in Silica Glass. Phys. Rev. Lett. 2009, 103, 175501−175505. (6) Almeida, R. M.; Vasconcelos, H. C.; Goncalves, M. C.; Santos, L. F. XPS and NEXAFS Studies of Rare-Earth Doped Amorphous SolGel Films. J. Non-Cryst. Solids 1998, 232, 65−71. (7) Del Giudice, M.; Bruno, F.; Cicinelli, T. Structural and Optical Properties of Silicon Oxynitride on Silicon Planar Waveguides. Appl. Opt. 1990, 29, 3489−3496. (8) Mazzoldi, P.; Carnera, A.; Caccavale, F.; Favaro, M. L.; BoscoloBoscoletto, A.; Granozzi, G.; Bertoncello, R.; Battaglin, G. N and Ar Ion-Implantation Effects in SiO2 Films on Si Single-Crystal Substrates. J. Appl. Phys. 1991, 70, 3528−3536. (9) Gritsenko, V. A.; Xu, J.; Kwok, R.; Ng, Y. H.; Wilson, Y. H. Short Range Order and the Nature of Defects and Traps in Amorphous Silicon Oxynitride Governed by the Mott Rule. Phys. Rev. Lett. 1998, 81, 1054−1057.

5. CONCLUSION In this work, two independent spectroscopic techniques, XPS and CL, were used to investigate the structural defect configurations developed in pure and N-doped SiO2 films prepared by an RFMS deposition process. The effect of nitrogen on the off-stoichiometry characteristics of doped silica and the role played by postsynthesis annealing treatments were investigated in detail. The formation of silicon oxynitride structures was ascertained by XPS, which also served to quantify the average content of N in both as-deposited and annealed samples. Nitrogen introduced in the chamber atmosphere during the deposition reacted with 2-foldcoordinated silicon and nonbridging oxygen sites, conspicuously removing both ODC and NBOHC defects centers, as revealed by CL spectroscopy. However, for annealing cycles performed above an intermediate temperature threshold, Tc, N conspicuously left the system due to a process of thermally induced out-diffusion. As a consequence, increasingly stronger CL bands appeared as a result of an enhanced off-stoichiometry of the amorphous network. Such a process culminated at temperatures around 1200 °C, at which all the film samples, independent of their N concentration, exhibited a common 3481

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(10) Scopel, W.; Fantini, M. C.; Alayo, M.; Pereyra, I. Local Structure and Bonds of Amorphous Silicon Oxynitride Thin Films. Thin Solid Films 2002, 413, 59−64. (11) Leto, A.; Munisso, M. C.; Porporati, A.; Zhu, W.; Pezzotti, G. Stress Dependence of Paramagnetic Point Defects in Amorphous Silicon Oxide. J. Phys. Chem. A 2008, 112, 3927−3934. (12) Skuja, L. Optically Active Oxygen-Deficiency-Related Centers in Amorphous Silicon Dioxide. J. Non-Cryst. Solids 1998, 239, 16−48. (13) Salh, R. Silicon Nanocluster in Silicon Dioxide: Cathodoluminescence, Energy Dispersive X-Ray Analysis, Infrared Spectroscopy Studies. In Crystalline Silicon - Properties and Uses [Online]; Basu, S., Ed.; InTech: New York, 2011; pp 136−172. s/crystalline-siliconproperties-and-uses/silicon-nanocluster-in-silicon-dioxidecathodoluminescence-energy-dispersive-x-ray-analysis-infrared (accessed February 22, 2012). (14) Tohmon, R.; Shimogaichi, Y.; Mizuno, H.; Ohki, Y.; Nagasawa, K.; Hama, Y. 2.7-eV Luminescence in As-Manufactured High-Purity Silica Glass. Phys. Rev. Lett. 1989, 62, 1388−1391. (15) Skuja, L.; Entzian, W. Cathodoluminescence of Intrinsic Defects in Glassy SiO2, Thermal SiO2 Films, and α-Quartz. Phys. Status Solidi A 1986, 96, 191−198. (16) Nishikawa, H.; Shiroyama, T.; Nakamura, R.; Ohki, Y.; Nagasawa, K.; Hama, Y. Photoluminescence From Defect Centers in High-Purity Silica Glasses Observed Under 7.9-eV Excitation. Phys. Rev. B 1992, 45, 586−591. (17) Skuja, L. Comment on ‘‘Luminescence of Fused Silica: Observation of the O2− Emission Band’’. Phys. Rev. B 1989, 39, 3909−3911. (18) Imai, H.; Arai, K.; Imagawa, H.; Hosono, H.; Abe, Y. Two Types of Oxygen-Deficient Centers in Synthetic Silica Glass. Phys. Rev. B 1988, 38, 12772−12775. (19) Jones, C. E.; Embree, D. Correlations of the 4.77−4.28-eV Luminescence Band in Silicon Dioxide with the Oxygen Vacancy. J. Appl. Phys. 1976, 47, 5365−5370. (20) Pacchioni, G.; Ierańo, G. Ab Initio Theory of Optical Transitions of Point Defects in SiO2. Phys. Rev. B 1998, 57, 818−832. (21) Nishikawa, H.; Watanabe, E.; Ito, D.; Ohki, Y. Decay Kinetics of the 4.4-eV Photoluminescence Associated with the Two States of Oxygen-Deficient-Type Defect in Amorphous SiO2. Phys. Rev. Lett. 1994, 72, 2101−2104. (22) Munekuni, S.; Yamanaka, T.; Shimogaichi, Y.; Tohmon, R.; Ohki, Y.; Nagasawa, K.; Hama, Y. Various Types of Nonbridging Oxygen Hole Center in High-Purity Silica Glass. J. Appl. Phys. 1990, 68, 1212−1217. (23) Salh, R.; Vonczarnowski, A.; Fitting, H. Cathodoluminescence of Non-Stoichiometric Silica: The Role of Oxygen. J. Non-Cryst. Solids 2007, 353, 546−549. (24) Fitting, H. Cathodoluminescence of Crystalline and Amorphous SiO2 and GeO2. J. Non-Cryst. Solids 2001, 279, 51−59. (25) Shluger, A. Models of the Self-Trapped Exciton and NearestNeighbor Defect Pair in SiO2. Phys. Rev. B 1990, 42, 9664−9673. (26) Bruckner, R. Properties and Structure of Vitreous Silica. I. J. Non-Cryst. Solids 1970, 5, 123−175. (27) Pezzotti, G.; Hosokawa, K.; Munisso, M. C.; Leto, A.; Zhu, W. Stress Dependence of Optically Active Diamagnetic Point Defects in Silicon Oxynitride. J. Phys. Chem. A 2007, 111, 8367−8373. (28) Gritsenko, V.; Shavalgin, Yu. G.; Pundur, P. A.; Wong, H.; Lau, W. M. Cathodoluminescence and Photoluminescence of Amorphous Silicon Oxynitride. Microelectron. Reliab. 1999, 39, 715−718.

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