Open Access
Add to BibSonomyAbstract
ZnO and Zn2SiO4 nanoparticles embedded SiO2 waveguides, a new candidate for fabrication of low-loss glass-ceramic active-waveguides for integrated optic applications, were fabricated by the sol-gel technique using dip-coating process. The waveguides fabricated from the sol-gel solution composed of (100-x) SiO2–x ZnO (x = 25, 30, and 35 mol %) exhibited uniform thickness (1.5 ± 0.1 µm), and refractive index of 1.529 ± 0.005 (for x = 35 mol %) at 632.8 nm. Propagation loss of 1.4 ± 0.2 dB/cm at 632.8 nm was observed in the transparent glass-ceramic waveguides. The as-prepared waveguides contained nanoparticles of average size ~15 nm uniformly dispersed in the SiO2 matrix. Formation of Zn2SiO4 along with ZnO nanoparticles in the waveguides is confirmed from the X-ray diffraction patterns and photoluminescence spectra. The tuning of the optical and spectroscopic properties by controlled heat-treatment of the as-prepared active-waveguides has been demonstrated.
© 2013 Optical Society of America
1. Introduction
Optical functional materials play a pivotal role in the design and fabrication of components for integrated optical circuits. Among many functional materials being studied for the development of active photonic systems, ZnO holds great promise [1–4]. The growing interest in the fabrication of ZnO based optoelectronic devices such as light-emitting diodes (LEDs), semiconductor micro-lasers, flat-panel displays, photovoltaic cells and active waveguides is due to its direct bandgap of 3.3 eV at room temperature and large exciton binding energy of 60 meV [5,6]. The luminescence efficiency of ZnO is further enhanced in nanocomposite systems which exhibit unique physical properties arising due to the quantum confinement effect, and can lead to the fabrication of more efficient photonic devices [7].
From the perspective of photonic applications, ZnO is transparent in the visible region of the electromagnetic spectrum and exhibits a refractive index of ~2.00 at 632.8 nm. These properties make ZnO a material of great importance for the development of active optical waveguides. Furthermore, interesting emission properties have been found in coexisting Zn2SiO4 and ZnO nanocomposite films [8]. However, limited success has been achieved so far in fabricating ZnO and Zn2SiO4 based waveguides with optical and spectroscopic properties acceptable for device applications. Ibanga et al., reported the fabrication of ZnO waveguides by thermal oxidation of ZnS thin films [9] and waveguide effect was also observed in He+ ion implanted ZnO crystal by Ming et al. [10]. But, the large values of propagation loss reported (3.0 ± 0.5 dB cm−1 at λ = 632.8 nm) for ZnO waveguides fabricated by these techniques [9,10] has been detrimental in the development of ZnO waveguide based active devices.
In the literature other competing technologies are studied for the development of active photonic systems based on III-Nitride materials [11,12]. However, the III-Nitride materials are generally fabricated via expensive vacuum based deposition techniques. In the present work, we demonstrate a facile technique for the development of waveguides based on ZnO and Zn2SiO4 nanoparticles embedded in silica matrix, which is a new candidate for fabrication of low-loss glass-ceramic active-waveguides for integrated optic applications. A well controlled, low-cost sol-gel technique has been developed for the synthesis of the required solutions and the glass-ceramic waveguides are then fabricated by dip-coating technique. Glass-ceramics are composite systems which involve amorphous glasses doped with nanoparticles either amorphous or crystalline in nature. Luminescent rare-earth ions doped glass-ceramic waveguides, viz., Er3+ doped SiO2 – HfO2 [13], and Eu3+ doped SiO2 – SnO2 [14], are known to be attractive candidates for active waveguiding photonic systems due to their interesting mechanical and optical properties. Here, we optimize the parameters for the sol-gel fabrication of low-loss glass-ceramic waveguides, exploit the passive and active photonic characteristics of ZnO and Zn2SiO4, and investigate its structural and optical properties for photonic applications. Finally, we demonstrate the utility of the glass-ceramic waveguide as a possible on-chip white-light source for integrated optical devices.
2. Experiments
The glass-ceramic planar-waveguides fabricated from the sol-gel solution composed of (100-x) SiO2 – x ZnO (x = 25, 30, and 35 mol %) were coated by using dip-coating process. The molar content of ZnO was chosen so as to have good optical and spectroscopic characteristics required for the development of a low-loss waveguide. The starting solution was obtained by mixing tetraethylorthosilicate (TEOS), ethanol (EtOH), deionised water, and hydrochloric acid (HCl) as a catalyst and was prehydrolyzed for 1 hour at 65 °C. The TEOS:HCl:EtOH:H2O molar ratio was 1:0.01:63.26:2. An ethanolic colloidal suspension prepared using Zn(CH3COO)2 · 2H2O as precursor, was added to the solution containing TEOS. The final mixture was continuously stirred at room temperature for 16 hours. The resulting sol was filtered with the help of 0.2 µm Whatman Puradisc syringe filters and then deposited on pure vitreous SiO2 (v-SiO2) and silicon substrates by dip coating technique, at a dipping rate of 25 mm/minute. Each layer was annealed at 800 °C for 4 min in air, prior to the application of the next coat. The films resulting from 25 coatings were stabilized by a final thermal treatment at 800 °C for 1 hour in air, thus yielding crack-free and low-loss waveguides. The samples deposited on v-SiO2 were used for the optical characterizations of the waveguide and the thin films deposited on Si were used for structural characterizations.
An additional heat-treatment in air at temperatures ranging from 900 to 1000 °C for 1 hour was performed on the as-prepared waveguides to study the growth of nanoparticles in the waveguides as a function of the heat-treatment temperature. In order to avoid surface cracking, the parent waveguides were introduced into the furnace at ambient temperature and then heated with a ramp of ~15 °C / minute.
The thin films deposited on silicon substrates were employed for transmission electron microscope (TEM) imaging, and selected area electron diffraction (SAED) studies using the Philips CM200 FEG/ST microscope working at an acceleration voltage of 200 kV. The SiO2 - ZnO composite system was imaged in plan-view configuration using diffraction contrast imaging. The standard TEM sample preparation techniques were used which included grinding, polishing to a thickness of about 10–20 μm, dimpling, and finally Ar+ ion milling. X-ray diffraction (XRD) measurements were performed on the thin films deposited on Si substrate to study the structural evolution of the nanoparticles upon heat-treatment. High Resolution grazing incidence XRD was performed on a Philips X'Pert PRO MRD system, PW 3050/60 equipped with Cu Kα source (λ1 = 1.540598 Å, λ2 = 1.544426 Å, with the ratio Kα1 / Kα2 = 0.5). The angle of the incident X-ray beam was fixed at 2°. The scanning range (2θ) was 10–80° with a step size of 0.02° and a step time of 0.4 seconds. UV-visible absorption studies of the glass-ceramic waveguides fabricated on v-SiO2 substrates were performed by using an Avantes AvaSpec-ULS3648-USB2 fiber-coupled CCD based spectrometer with a spectral resolution of 0.4 nm. The thickness (d) and the refractive index (n) of the glass-ceramic planar waveguides deposited on v-SiO2 substrates were determined using a home built m-line setup which primarily consisted of a 632.8 nm TE polarized He-Ne laser, a Gadolinium Gallium Garnet (GGG) prism (of refractive index 1.965 at 632.8 nm) mounted on a high-precision motorized rotation stage of resolution 0.007°, and a large area Si photodiode. The propagation losses in the glass-ceramic waveguides at 632.8 nm for the TE0 mode were evaluated by photometric detection of the light intensity scattered out of the waveguide plane [15,16]. A He-Ne laser beam at 632.8 nm was coupled into the waveguide using the prism coupler and a CCD (Stingray F-146B) camera was used to capture the image of the propagating streak of light in the waveguide. The photoluminescence (PL) spectra were recorded by exciting the samples using a CW He-Cd laser operating at a wavelength of 325 nm. The photoluminescence from the samples were dispersed using a Triax 320 monochromator and collected using a photomultiplier tube.
3. Results and discussion
The bright-field TEM images for x = 35 mol% glass-ceramic samples heat treated at different temperatures are shown in Figs. 1(a)-1(c). ZnO and Zn2SiO4 nanoparticles of random shapes dispersed in SiO2 matrix can be observed for the sample heat-treated at 800 °C in Fig. 1(a). The particles become more symmetric in shape tending toward spherical nanoparticles for the samples heat-treated at 900 °C and 1000 °C as seen in Figs. 1(b) and 1(c) respectively. As the temperature of heat-treatment increased, the average size of the nanoparticles increased from 15 nm at 800 °C to 25 nm at 1000 °C. The SAED pattern for the sample heat-treated at 1000 °C is shown in Fig. 1(d) which confirms the presence of crystalline nanoparticles.
Fig. 1 TEM images of 65 SiO2 – 35 ZnO samples heat-treated for 1 hour at (a) 800 °C, (b) 900 °C, and (c) 1000 °C, showing the growth of ZnO and Zn2SiO4 nanoparticles in amorphous silica matrix as a function of heat-treatment temperature. The SAED pattern for the sample heat-treated at 1000 °C is shown in (d).
High resolution grazing incidence XRD patterns are shown in Fig. 2 for the thin-films deposited on Si substrate and heat-treated at different temperatures. The XRD spectrum of 1 μm thick silica thin-film deposited on Si substrate measured under similar circumstances has been plotted for reference. The three spectra for x = 35 mol% glass-ceramic samples heat-treated at 800 °C, 900 °C and 1000 °C for 1 hour exhibit sharp peaks corresponding to ZnO and Zn2SiO4 crystal planes [8,17]. The peaks arising from ZnO and Zn2SiO4 crystals are identified in Fig. 2. The heat-treatment process does not alter the structure of the nanocrystals and is evident from the similar XRD spectra obtained for the three samples. The inset of Fig. 2 shows the XRD spectra for x = 25, 30 and 35 mol % samples heat-treated for 1 hour at 800 °C, wherein the effect of concentration of ZnO on its structure is studied. The peak-intensities are relatively weak for samples with lower concentrations of ZnO. However, the peak positions in the spectra do not change with concentration, signifying no alteration of the structure of the nanocrystals as a function of ZnO concentration. The broad peak in the background around 20 degrees is attributed to the amorphous SiO2-ZnO nanocomposite matrix [18]. The observation of the crystalline XRD peaks is consistent with the SAED pattern observed in Fig. 1 (d). This confirms that nanoparticles of ZnO and Zn2SiO4 coexist in the nanocomposite thin-films.
Fig. 2 X-ray diffractograms for 65 SiO2 – 35 ZnO thin film samples heat-treated for 1 hour at 800 °C, 900 °C, and 1000°C. The spectrum of silica thin film on Si substrate is the reference spectrum. The inset shows the spectra for x = 25, 30 and 35 mol % samples heat-treated for 1 hour at 800 °C. (The diffractograms have been offset along the vertical axis for clarity.)
The absorption spectra of 65 SiO2 – 35 ZnO waveguide samples heat-treated for 1 hour at 800 °C, 900 °C, and 1000°C is presented in Fig. 3.The increase in the absorption intensity in the range of 220 nm - 450 nm with increasing heat-treatments can be assigned to the precipitation of larger number of ZnO and Zn2SiO4 nanoparticles. Furthermore, the red-shift of the absorption edge with progressive heat-treatments can be attributed to the precipitation of larger sized nanoparticles in the waveguides [19]. Importantly, the transparency of the glass-ceramic samples across the entire visible region of the electromagnetic spectrum is shown in the inset spectra of Fig. 3. The modulations observed in the absorption spectra are well understood to be due to the thin-film interference effects.
Fig. 3 UV-Visible absorption spectra of 65 SiO2 – 35 ZnO waveguide samples heat-treated for 1 hour at 800 °C, 900 °C, and 1000°C. The inset shows the transparency of the waveguides in the visible region.
The n and d values of waveguides with different ZnO content and various heat-treatments are tabulated in Table 1.The increase in ZnO content in the as-prepared waveguide at 800 °C increases its refractive index from 1.510 ± 0.005 at x = 25 mol % to 1.529 ± 0.005 at x = 35 mol %. The heat-treatments for 1 hour at 900 °C and 1000 °C lead to further densification of the as-prepared waveguides and consequently increase their refractive indices. In the case of x = 30 mol % waveguides, the refractive index increases from 1.519 ± 0.005 at 800 °C to 1.532 ± 0.005 at 1000°C. The thicknesses of the waveguides are in the range of 1.5 ± 0.1 to 1.9 ± 0.1 µm, which can be controlled by the number of dips during the dipping process. The thickness value of ~1.5 µm for the waveguides has been chosen so as to confine and support two modes at 632.8 nm.
Table 1. Refractive index (n), thickness (d), and propagation losses of (100 - x) SiO2 – x ZnO (x = 25, 30, and 35 mol %) waveguides heat-treated at different temperatures.
The propagation losses for the samples are determined by the photometric detection of scattered light from the waveguides. A CCD image of light at 632.8 nm propagating in the x = 35 mol% waveguide, heat-treated at 800 °C is shown in Fig. 4. The light is evanescently coupled into the waveguide using a GGG prism which is seen at the left-edge of the propagating streak in the Fig. 4. The values of the losses for different waveguides are tabulated in Table 1. The as-prepared glass-ceramic waveguide containing 35 mol % ZnO exhibited a lowest loss value of 1.4 ± 0.2 dB/cm at 632.8 nm, comparable to the values reported for glass-ceramic waveguides earlier [13,14,20]. In the literature, attenuation coefficients of about 1 dB/cm have been reported by Jestin et al. [13], and Peron et al. [20], for silica-hafnia and fluoride glass-ceramic waveguides respectively. The relatively higher attenuation coefficients in the SiO2 - ZnO glass-ceramic waveguides heat-treated at 900 °C and 1000 °C, can be attributed to the presence of larger nanoparticles which leads to increased scattering losses in the fabricated composite waveguides. The propagation losses for waveguides heat-treated at 1000 °C were high and are not reported here.
Fig. 4 A CCD image of light at 632.8 nm propagating in the x = 35 mol% waveguide, heat-treated at 800 °C. The light is evanescently coupled into the waveguide using a GGG prism which can be seen at the left-edge of the propagating streak in the image.
Though the propagation losses in the SiO2 - ZnO glass-ceramic waveguides are relatively high in comparison to glass waveguides reported in literature [21,22], they are of great interest considering the novelty of the functional ZnO and Zn2SiO4 embedded SiO2 system, and its PL properties. The room-temperature PL of the 65 SiO2 – 35 ZnO (mol %) nanocomposite thin-films deposited on Si substrate, heat-treated at different temperatures, was recorded under excitation with 325 nm laser source. The broad-band emission spans the visible region of blue to red between 360 to 600 nm as shown in Fig. 5. In general, the room-temperature PL spectrum for ZnO comprises of a UV near-band-edge emission peak around 380 nm, and a visible broad-band emission around 505 nm attributed to deep-level defects in ZnO crystal, such as vacancies and interstitials of zinc and oxygen [18]. Liu et al., observed suppression of the 380 nm peak and presence of blue-red emission in oxygen-rich ZnO films, which is attributed to oxygen interstitial defects [23]. During the fabrication process, as the waveguides have been annealed in air, in oxidizing atmosphere, the near-band-edge emission peak is completely suppressed in our case. This implies that when ZnO and Zn2SiO4 embedded SiO2 nanocomposite films were annealed in air at high temperatures, new defects of oxygen interstitials were created. The spectral broadening of the blue-red emission for the sample heat-treated at 1000 °C is also attributed to the influence of Zn2SiO4 nanoparticles [8]. Zn2SiO4 has a strong emission band centered at ~525 nm and ZnO has a green emission band at ~505 nm. The overlapping of these two green bands should be the source of the broad band centered at ~520 nm in Fig. 5. Additional heat-treatments to the waveguides at higher temperatures leads to higher PL intensities as shown in Fig. 5, which can be attributed to the precipitation of larger number of ZnO and Zn2SiO4 particles at higher temperatures of heat-treatment as was confirmed from TEM studies.
Fig. 5 Room temperature photoluminescence spectrum of the 65 SiO2 – 35 ZnO (mol %) nanocomposite waveguides heat-treated at different temperatures.
4. Conclusions
We have successfully fabricated low-loss (1.4 ± 0.2 dB/cm at 632.8 nm), glass-ceramic planar waveguides of (100-x) SiO2 – x ZnO (x = 25, 30, and 35 mol %) following the sol-gel technique and dip-coating process. The optical parameters, refractive index and thickness, were determined to be 1.529 ± 0.005 at 632.8 nm and 1.5 ± 0.1 µm respectively, for x = 35 mol % as-prepared waveguide, using the m-line setup. High resolution grazing incidence XRD studies and TEM micrographs showed the formation and growth of ZnO and Zn2SiO4 nanoparticles in SiO2 nanocomposite thin-films on extended heat-treatments. A uniform distribution of particles throughout the film was confirmed by TEM measurements, and particles with an average size of 25 nm were grown upon heat-treatment at 1000 °C for 1 hour. Photoluminescence spectra recorded on excitation with λ = 325 nm source exhibit visible broad-band emission attributed to deep-level defects in ZnO crystal, such as vacancies and interstitials of zinc and oxygen. The extended emission till 600 nm is attributed to the presence of Zn2SiO4 nanoparticles in the glass-ceramic samples. In conclusion, SiO2 - ZnO glass-ceramic waveguides fabricated by sol-gel technique appear to be a viable system for developing functional devices for integrated optic applications.
Acknowledgments
This work was supported in part by IIT Kharagpur-ISIRD, and DST Fast Track Scheme for Young Scientists project funds. The authors acknowledge the support from CRF, IIT Kharagpur for TEM measurements. The authors also acknowledge Tamita Rakshit, Sudipta Bhaumik and Prof. S.K. Ray for their help in PL and XRD measurements.
References and links
1. H. Cao, J. Y. Xu, E. W. Seeling, and R. P. H. Chang, “Microlaser made of disordered media,” Appl. Phys. Lett. 76(21), 2997–2999 (2000). [CrossRef]
2. D. I. Son, B. W. Kwon, D. H. Park, W.-S. Seo, Y. Yi, B. Angadi, C.-L. Lee, and W. K. Choi, “Emissive ZnO-graphene quantum dots for white-light-emitting diodes,” Nat. Nanotechnol. 7(7), 465–471 (2012). [PubMed]
3. K.-K. Kim, J. H. Song, H. J. Jung, W. K. Choi, S.-J. Park, and J. H. Song, “The grain size effects on the photoluminescence of ZnO/α-Al2O3 grown by radio-frequency magnetron sputtering,” J. Appl. Phys. 87(7), 3573–3575 (2000). [CrossRef]
4. L. Wang, Y. Kang, X. Liu, S. Zhang, W. Huang, and S. Wang, “ZnO nanorod gas sensor for ethanol detection,” Sens. Actuators B Chem. 162(1), 237–243 (2012). [CrossRef]
5. X. W. Sun, J. Z. Huang, J. X. Wang, and Z. Xu, “A ZnO nanorod inorganic/organic heterostructure light-emitting diode emitting at 342 nm,” Nano Lett. 8(4), 1219–1223 (2008). [CrossRef] [PubMed]
6. J. J. Cole, X. Wang, R. J. Knuesel, and H. O. Jacobs, “Integration of ZnO microcrystals with tailored dimensions forming light emitting diodes and UV photovoltaic cells,” Nano Lett. 8(5), 1477–1481 (2008). [CrossRef] [PubMed]
7. P. O. Anikeeva, J. E. Halpert, M. G. Bawendi, and V. Bulović, “Electroluminescence from a mixed red-green-blue colloidal quantum dot monolayer,” Nano Lett. 7(8), 2196–2200 (2007). [CrossRef] [PubMed]
8. X. Xu, C. Guo, Z. Qi, H. Liu, J. Xu, C. Shi, C. Chong, W. Huang, Y. Zhou, and C. Xu, “Annealing effect for surface morphology and luminescence of ZnO film on silicon,” Chem. Phys. Lett. 364(1-2), 57–63 (2002). [CrossRef]
9. E. J. Ibanga, C. L. Luyer, and J. Mugnier, “Zinc oxide waveguide produced by thermal oxidation of chemical bath deposited zinc sulphide thin films,” Mater. Chem. Phys. 80(2), 490–495 (2003). [CrossRef]
10. X. Ming, F. Lu, J. Yin, M. Chen, S. Zhang, J. Zhao, X. Liu, Y. Ma, and X. Liu, “Waveguide effect in ZnO crystal by He+ ions implantation: Analysis of optical confinement from implant-induced lattice damage,” Opt. Commun. 285(6), 1225–1228 (2012). [CrossRef]
11. M. R. Krames, O. B. Shchekin, R. Mueller-Mach, G. O. Mueller, L. Zhou, G. Harbers, and M. G. Craford, “Status and future of high-power light-emitting diodes for solid-state lighting,” J. Disp. Technol. 3(2), 160–175 (2007). [CrossRef]
12. M. H. Crawford, “LEDs for solid-state lighting: performance challenges and recent advances,” IEEE J. Sel. Top. Quantum Electron. 15(4), 1028–1040 (2009). [CrossRef]
13. Y. Jestin, C. Armellini, A. Chiasera, A. Chiappini, M. Ferrari, E. Moser, R. Retoux, and G. C. Righini, “Low-loss optical Er3+-activated glass-ceramics planar waveguides fabricated by bottom-up approach,” Appl. Phys. Lett . 91(7), 071909 (2007).
14. S. N. B. Bhaktha, F. Beclin, M. Bouazaoui, B. Capoen, A. Chiasera, M. Ferrari, C. Kinowski, G. C. Righini, O. Robbe, and S. Turrell, “Enhanced fluorescence from Eu3+ in low-loss silica glass-ceramic waveguides with high SnO2 content,” Appl. Phys. Lett . 93(21), 211904 (2008).
15. M. Nogami, T. Enomoto, and T. Hayakawa, “Enhanced fluorescence of Eu3+ induced by energy transfer from nano sized SnO2 crystals in glass,” J. Lumin. 97(3-4), 147–152 (2002). [CrossRef]
16. S. J. L. Ribeiro, Y. Messaddeq, R. R. Gonçalves, M. Ferrari, M. Montagna, and M. A. Aegerter, “Low optical loss planar waveguides prepared in an organic–inorganic hybrid system,” Appl. Phys. Lett. 77(22), 3502–3504 (2000). [CrossRef]
17. J. El Ghoul, K. Omri, L. El Mir, C. Barthou, and S. Alaya, “Sol–gel synthesis and luminescent properties of SiO2/Zn2SiO4 and SiO2/Zn2SiO4:V composite materials,” J. Lumin. 132(9), 2288–2292 (2012). [CrossRef]
18. Z. Fu, B. Yang, L. Li, W. Dong, C. Jia, and W. Wu, “An intense ultraviolet photoluminescence in sol–gel ZnO–SiO2 nanocomposites,” J. Phys. Condens. Matter 15(17), 2867–2873 (2003). [CrossRef]
19. S. Chakrabarti, D. Ganguli, and S. Chaudhuri, “Excitonic and defect related transitions in ZnO–SiO2 nanocomposites synthesized by sol-gel technique,” Phys. Status Solidi 201(9), 2134–2142 (2004). [CrossRef]
20. O. Péron, B. Boulard, Y. Jestin, M. Ferrari, C. Duverger-Arfuso, S. Kodjikian, and Y. Gao, “Erbium doped fluoride glass–ceramics waveguides fabricated by PVD,” J. Non-Cryst. Solids 354(30), 3586–3591 (2008). [CrossRef]
21. A. Chiasera, M. Montagna, C. Tosello, S. Pelli, G. C. Righini, M. Ferrari, L. Zampedri, A. Monteil, and P. Lazzeri, “Enhanced spectroscopic properties at 1.5 μm in Er3+/Yb3+ -activated silica–titania planar waveguides fabricated by rf-sputtering,” Opt. Mater. 25(2), 117–122 (2004). [CrossRef]
22. R. R. Gonçalves, G. Carturan, M. Montagna, A. Ferrari, L. Zampedri, S. Pelli, G. C. Righini, S. J. L. Ribeiro, and Y. Messaddeq, “Erbium-activated HfO2-based waveguides for photonics,” Opt. Mater. 25(2), 131–139 (2004). [CrossRef]
23. X. Liu, X. Wu, H. Cao, and R. P. H. Chang, “Growth mechanism and properties of ZnO nanorods synthesized by plasma-enhanced chemical vapor deposition,” J. Appl. Phys. 95(6), 3141–3147 (2004). [CrossRef]
