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A significant enhancement of blue light emission from amorphous oxidized silicon nitride (a-SiNx:O) films is achieved by introduction of ordered and size-controllable arrays of Ag nanoparticles between the silicon substrate and a-SiNx:O films. Using hexagonal arrays of Ag nanoparticles fabricated by nanosphere lithography, the localized surface plasmons (LSPs) resonance can effectively increase the internal quantum efficiency from 3.9% to 13.3%. Theoretical calculation confirms that the electromagnetic field-intensity enhancement is through the dipole surface plasma coupling with the excitons of a-SiNx:O films, which demonstrates a-SiNx:O films with enhanced blue emission are promising for silicon-based light-emitting applications by patterned Ag arrays.
© 2014 Optical Society of America
1. Introduction
Improvement of the efficiency of light emission from silicon-based materials is a great challenge in silicon optoelectronic integration [1–10]. Enhancement strategies for efficient light emission have been intensively explored. One effective approach to develop highly efficient light emitters is based on the incorporation of localized surface plasmons (LSPs) of metal nanoparticles to obtain efficient extraction of light [11–15]. Amorphous oxidized silicon nitride (a-SiNx:O) is a versatile silicon-based material with a tunable band gap that has attracted attention because of its excellent performances in photonic devices and fabrication compatibility with modern CMOS integration [8,9]. To further enhance luminescence efficiency, ordered and size-controllable metal nanoparticles are needed because variation in nanoparticle size and shape can produce vastly different plasmon modes [16,17], which influences the coupling between LSPs and emitters. Recently nanosphere lithography has been demonstrated to be a high-throughput, materials-general, and inexpensive nanofabrication technique capable of producing well-ordered 2D periodic particles [16]. By choosing an appropriate diameter of nanosphere and thickness of metal deposited, metal nanoparticles with uniform size and controllable position can be obtained. The energy of LSPs can be tuned by varying the sizes of the metal nanoparticles [17]. In our previous work, we used Ag nanoparticles to enhance the internal quantum efficiency of a-SiNx:O films based on LSPs [14]. The energy of LSPs from Ag nanoparticles was matched with that of excitons in a-SiNx:O emitters by tuning the ratio of Si to N. However, the energy of LSPs can also be affected by the sizes of the metal nanoparticles, so a nanofabrication technique to produce Ag nanoparticles with controllable and uniform size is needed.
In this letter we focus on increasing the efficiency of blue light emission from a-SiNx:O using hexagonal arrays of Ag nanoparticles fabricated by nanosphere lithography. Single-layer and double-layer nanospheres are used as deposition masks to obtain various hexagonal arrays of Ag nanoparticles. The size of hexagonal arrays of Ag nanoparticles is tuned to improve the matching between LSPs wavelength and the emission wavelength of a-SiNx:O films. A significant, stable enhancement of blue photoluminescence (PL) at room temperature can be observed from the resulting structure by the naked eye. The PL enhancement factor of a-SiNx:O films with hexagonal arrays of Ag nanoparticles is greater than 4-fold, compared with that of a-SiNx:O films on bare silicon substrates. The coupling between the LSPs and the excitons of a-SiNx:O results in the increase of the internal quantum efficiency. These a-SiNx:O films showing size-controlled, patterned Ag nanoparticles-enhanced blue emission are promising for silicon-based light-emitting devices.
2. Experimental methods
The patterned Ag arrays/a-SiNx:O structures were fabricated by the nanosphere lithography self-assembly process and a subsequent plasma-enhanced chemical vapor deposition (PECVD). A schematic illustration of this process is displayed in Fig. 1. Monodisperse polystyrene (PS) nanospheres (170 nm and 220 nm in diameter) were transferred into water using the technique reported by Jensen et al. [17]. A close-packed single layer of PS spheres was self-assembled on the surface of water. This layer was then lifted off from the surface of water to the silicon and quartz substrate to be used as a deposition mask. Depending on the concentration of PS spheres, mono- and bilayers of spheres were formed. Double layers of PS spheres (220 nm in diameter) were also self-assembled using the technique described by Hulteen et al. [16]. Then, thin films of Ag with a thickness of 110 nm were deposited over the nanosphere masks in an electron beam evaporation system under a vacuum of 5 × 10−4 Pa at room temperature. For the masks composed of a monolayer of PS spheres, the deposited metal particles were arranged in a hexagonal 2D lattice with a two-point basis, which is known as honeycomb structure. In contrast, true hexagonal arrays of the Ag nanoparticles, which contain Ag nanoparticles both in the corners and in the centers, were obtained using the PS bilayers. After the deposition of Ag, the PS nanospheres were removed from the substrate by CH2Cl2 in an ultrasonic bath and processed by sequential rinsing in acetone, ethanol and deionized water to give periodic Ag nanoparticle arrays [18]. Then amorphous silicon nitride films were deposited on the Ag nanoparticle interlayers and bare silicon wafers by PECVD at room temperature by decomposing SiH4 mixed with NH3 under an rf power of 30 W. The ratio of SiH4 to NH3 was 1:6. The thickness of the amorphous silicon nitride films was 90 nm. Finally, the films were post-treated by in situ plasma oxidation using pure oxygen gas at a pressure of 280 mTorr with an rf power of 30 W for 30 min to obtain a-SiNx:O films.
Fig. 1 Schematic diagrams of the patterned Ag arrays/a-SiNx:O structures fabricated using nanosphere lithography and PECVD technology.
Atomic force microscopy (AFM) was used to investigate the distribution and morphology of Ag nanoparticles. The extinction spectra measurement has been performed in reflection and transmission modes. Transmission mode can measure highly transparent material like Ag nanoparticles coating on quartz. The sample is illuminated with collimated broadband white light. The light is scattered and absorbed by the Ag nanoparticles and the transmitted light is collected by fiber to a spectrometer. In contrast, mirrorlike substrates such as silicon need to be measured by reflection mode. The white light is guided from one branch of a multimode fiber optical coupler to the combined port. The sample is placed very close to the fiber and the reflected light is collected by the same fiber and delivered to the other branch which is connected to the spectrometer (Shimadzu UV-3600). The measurements of the transmission spectrum of a bare quartz substrate and the reflection spectrum of a bare Si substrate have been performed. The intensity of signal from the bare quartz substrate and the bare silicon substrate has been deducted from the extinction spectrum of the samples on quartz substrate in transmission and on silicon substrate in reflection. Finally the extinction spectra were obtained by the superposition of the absorption spectrum of the sample on a quartz substrate and the reflectivity spectrum of the sample on a silicon substrate. PL signals were detected from the samples on silicon substrate at room and low temperature on a Perkin-Elmer LS50B fluorescence spectrophotometer using a Xe lamp with a wavelength of 325 nm as an excitation source. A specific excitation wavelength can be selected by focusing and separating the light from the continuous-source Xe lamp using a grating monochromator.
3. Results and discussion
Figure 2(a) shows an AFM image of the Ag nanoparticles arrays fabricated on a silicon substrate with a single-layer mask of PS spheres with a diameter of 170 nm. Hexagonal arrays of Ag nanoparticles were observed on the silicon substrate. The structural parameters of the Ag nanoparticles arrays were derived from section analysis and are presented in the right inset. The distance between Ag nanoparticles is 95 nm. The right inset schematically illustrates the formation process of the hexagonal arrays of Ag nanoparticles using a single-layer PS masks. The calculated distance between Ag nanoparticles is 98.2 nm, which is close to the experimental value of 95 nm. The in-plane size and height of Ag nanoparticles depend on both the size of the mask spheres and the thickness of Ag deposited. The interstices of mask spheres with a diameter of 170 nm are partially filled with Ag with a thickness of 20 nm. The in-plane size and height of Ag nanoparticles are 42 and 12 nm, respectively, which are both less than the diameter of the mask spheres and the thickness of Ag deposited. Because void space among the PS spheres is expanded after the deposition. And the Ag particles at the tips and edges are delaminated during the removal of PS mask in absolute ethanol sonication. It results in the array units with smaller diameter of the denuded zone and the lower value of Ag nanoparticle height.
Fig. 2 (a) AFM images of hexagonal arrays of Ag nanoparticles fabricated using a single-layer PS mask. (b) AFM images of hexagonal arrays of Ag nanoparticle containing center fabricated using a double-layer PS mask. The geometry and section analysis of Ag nanoparticles are for both single and double layer PS mask.
For comparison, an AFM images of Ag nanoparticles fabricated on a silicon substrate with a double-layer mask of PS sphere with a diameter of 220 nm is displayed in Fig. 2(b). Hexagonal arrays with an Ag nanoparticle in the center of each hexagon were observed, which is different from that fabricated with a single-layer mask of PS spheres with a diameter of 170 nm. The pattern fabricated with a single-layer mask of PS spheres does not contain an Ag nanoparticle in the center of each hexagon. In addition, the interparticle distance of Ag increases from 98.5 to 220 nm using the double-layer mask instead of the single. The formation of hexagonal arrays with central Ag nanoparticles using double-layers PS masks is illuminated in the right inset. The interparticle distance of Ag is equal to the diameter of PS, so the interparticle distance of Ag of 220 nm is in agreement with the theoretical value. The in-plane size and height of Ag nanoparticle are 53 and 6.2 nm, respectively. Overall, the size and shape of nanoparticles arrays can be tuned by variation of the size and layer-number of PS spheres. The statistics over a 5 umx5 um area of hexagonal Ag nanoarrays show that there is no defect existing in it. The deviation of average lateral size and vertical height distribution is 5% and 8%. The surface coverage reaches 14.4%. As for hexagonal arrays of Ag nanoparticles containing central nanoparticles, clear periodic particle arrays with defect-free areas of 3umx3um can be observed. The deviation of average lateral size and vertical height distribution is 7% and 9%. The surface coverage is 4%.
The extinction spectra of the a-SiNx:O films on a bare silicon substrate and the substrates with different hexagonal arrays of Ag nanoparticles are presented in Fig. 3. For the a-SiNx:O films on a bare substrate (sample A), the curve shows a low extinction coefficient over the wavelength range from 350 to 2000 nm with a moderate extinction peak at 580 nm. After introduction of a hexagonal arrays of Ag nanoparticles (sample B), the extinction peak shifts from 580 to 465 nm because of the LSPs produced in hexagonal arrays of Ag nanoparticles under incident light of 465 nm. For sample C with hexagonal arrays of Ag nanoparticles containing a central nanoparticle, the extinction peak shift from 465 to 503 nm because of the smaller height and larger in-plane size of the array compared with those of sample B. The extinction peak of LSPs exhibits a red shift when the height of Ag nanoparticles decreases or the in-plane size of Ag nanoparticles increases [17]. The shift of extinction peak reveals that the LSPs wavelength can be tuned by variation of the size of Ag nanoparticles. It can be seen that another resonance peak located near 357 nm emerges besides the dipole mode LSPs, which should come from the quadrupole mode of LSPs [19]. We found that the interparticle separation of Ag nanoparticles for sample C is nearly 2 times larger than that of sample B. According to the report of Burrows et al., the interparticle coupling between the Ag nanoparticles in arrays will significantly influence the resultant extinction pattern depending upon the interparticle separation [20–23], which explains the origin of different resonance mode from the extinction spectra for various patterned Ag arrays.
Fig. 3 Extinction spectra of a-SiNx:O films on a bare Si substrate and the Si substrates with different hexagonal arrays of Ag nanoparticles.
Strong and stable blue PL at room temperature was observed from the a-SiNx:O films with hexagonal arrays of Ag nanoparticles inserted between the active layer and silicon substrate by the naked eye in a bright room. A photograph of the blue PL from sample B is illustrated in Fig. 4(a). The sample is fixed on a planar holder, which is positioned vertical to the light path of the excitation source. The surface of the sample is parallel to the holder. An intense region of blue PL can be seen from the sample illuminated by a Xe lamp with an excitation wavelength of 325 nm. The brightness of this blue emission spot was maintained after continuous operation for more than 18 months.
Fig. 4 (a) Photograph of blue PL from a-SiNx:O films on the Si substrate with hexagonal arrays of Ag nanoparticles at room temperature. (b) PL spectra of a-SiNx:O films on different substrates measured at room temperature. The inset shows PL intensity as a function of temperature from 120 to 9.3 K. (c) PL enhancement factors as a function of LSPs wavelength for a-SiNx:O films on the Si substrate with different patterned Ag arrays of nanoparticles.
Enhanced blue light emission was demonstrated from a-SiNx:O films on different silicon substrates as illustrated in Fig. 4(b). The main PL peak from an a-SiNx:O film on a bare silicon substrate was located at 467 nm. An obvious enhancement of this PL band at 467 nm was observed from an a-SiNx:O film with hexagonal arrays of Ag nanoparticles inserted between the a-SiNx:O film and silicon substrate. For comparison the PL enhancement factor as a function of LSPs wavelength for sample B and sample C is depicted in Fig. 4(c).The PL enhancement factor for sample B with hexagonal arrays of Ag nanoparticles reaches 4.1. While for sample C with hexagonal arrays of Ag nanoparticles containing centers the PL enhancement factor is about 1.8. The relative intensity ratio is 2.2. As shown in Fig. 3, the extinction peak for sample B is at 465 nm, which is closer to the emission peak of a-SiNx:O films at 467 nm. This indicates that the PL enhancement is related to the coupling between the emissions of the a-SiNx:O film and the LSPs of the hexagonal arrays of Ag nanoparticles. It is noticed that the main extinction peak of sample C appears at 503 nm, accompanied with a shoulder at 357 nm. The mismatch between the extinction peak and PL peak induces the relatively weaker PL enhancement in this case.
To obtain a more detailed insight into the PL enhancement induced by the coupling between a-SiNx:O and Ag nanoparticles, we measured the temperature dependence of the PL intensity of a-SiNx:O films with hexagonal Ag particles. The inset in Fig. 4(b) presents an Arrhenius plot of the integrated PL intensity between 9.3 and 300 K for a-SiNx:O film on a bare Si substrate and a Si substrate with a hexagonal arrays of Ag nanoparticles. The internal quantum efficiency of an active layer can be calculated from the ratio of the measured PL signal at room temperature and near 10 K. We found that the internal quantum efficiency of the a-SiNx:O film increases from 3.9% to 13.3% when a hexagonal array of Ag nanoparticles was introduced between the a-SiNx:O film and silicon substrate.
To gain further understanding the influence of the LSPs of the different silver particle arrays on the PL of the a-SiNx:O film, the overall extinction spectrum of hexagonal Ag arrays/a-SiNx:O system was calculated by using Finite Difference Time Domain (FDTD) method to solve Maxwell's curl equations of electromagnetism according to the dipole model of Ag nanosphere [24,25]. Here, we use Lumerical FDTD Solutions, software specially designed for computing optical properties of multilayered spheres. Users should provide initial parameters, including number of layers, size distribution of each layer, complex refractive index tables of each layer, and refractive index of ambient medium. Perfectly matched layer boundary conditions were adopted in the incident direction to prevent nonphysical scattering at the boundaries. Optical constants for Ag were taken from Palik and fitted using the combined Drude and Lorentz model [26]. A detailed description of the FDTD method can be found elsewhere [24,25]. The inset in Fig. 5(a) is the 3D diagram of the sample. The profile of the calculated extinction spectrum is consistent with the experimental one except for the intensity difference. It is suggested that the experimental extinction spectrum reflects the collective effect of many Ag nanoparticles. The calculation is based on the model of isolated Ag nanoparticle. As reported by T.R. Jensen et al. [16], the extinction spectra are quite intense when considering the number of nanoparticles sampled in the macroscopic probe beam. To assess the strength of the extinction, T.R. Jensen et al. have calculated the molar extinction coefficient and the oscillator strength, which further prove our point of view.
Fig. 5 (a) FDTD theoretical calculation of extinction spectrum from the a-SiNx:O films on the substrate with hexagonal Ag nanoparticles arrays, the inset shows the 3D diagram of Si substrate/hexagonal Ag nanoparticles arrays/a-SiNx:O system. (b) and (c) The near-field intensity enhancement of LSPs along the x-y plane for different Ag arrays/a-SiNx:O structure under 465 nm light illumination through FDTD calculation.
As the electric field intensity generated by plane wave illumination depends on polarization, FDTD simulations of the electric field intensity generated by plane wave illumination was used to obtain the near-field intensity distribution of LSPs for different Ag arrays/a-SiNx:O structure under 465 nm light illumination. The electric field intensity enhancement along the x-y plane for samples with different hexagonal arrays of Ag nanoparticles is shown in Fig. 5(b) and (c), which is parallel to the Si substrate. The two kinds of representative pattern include the nearest neighbor hexagonal pattern units. The dipole-dipole longitudinal mode can be distinguished from the near-field distribution pattern between adjacent Ag nanoparticles in hexagonal Ag arrays [27]. As for hexagonal Ag arrays containing central particles, the dipole mode are observed. It is interesting found that the enhancement of electric field intensity of sample B is stronger than that of sample C. The electric field intensity enhancement for sample B and sample C is close to that of their PL enhancement factor as displayed in Fig. 4(c), which means that the field intensity increase is equal to the PL intensity enhancement. According the formalism suggested by Ku¨mmerlen et al. [28], the enhancement of the field intensity, is directly related to the photoluminescence radiative decay rate enhancement, Γrad,enh, that is
A measured enhancement of the PL intensity reflects an increase in the radiative decay rate, regardless of the existence of nonradiative decay paths. The measured PL intensity enhancement should therefore be equal to the field intensity enhancement computed in our work. J. S. Biteen et al. have found that an optimum PL enhancement can be realized by optimizing the field enhancement and the density of the array. Optimization of the PL enhancement involves optimization of the metal particle diameter and the array pitch changes [29]. The largest field enhancement in their simulations is a factor 6 for 50 nm diameter particles. The surface coverage for their sample is 6%. The diameter of Ag nanparticles in our sample is 46 nm, which is close to their value. Note that our surface coverage reaches 14.4%. The hexagonal Ag nanoarrays have an areal density of 1.04x1010/cm2, which is 3.72 times higher than that areal density of 2.79x109/cm2 for the hexagonal Ag nanoarrays containing central particles. Thus surface coverage of Ag nanoarrays plays important role for the enhancement factor.
The relation between the increase of the internal quantum efficiency and the electromagnetic field-intensity enhancement can be analyzed from the formula of internal quantum efficiency expressed as [30]:
Γrad and Γnr are the radiative and nonradiative recombination rates. The effective Purcell factor F represents the average electromagnetic field-intensity enhancement. Its definition is , ΓLSPs is the recombination rate due to emission of LSPs. Both Γrad and ΓLSPs can make contribution to the increase of ŋint. To testify the extend of the electric field of LSP into the a-SiNx:O layer, the field distribution in the x-z and y-z plane along the polarization direction passing over an Ag nanoparticle in the two kinds of Ag nanoarrays is calculated by FDTD method as displayed in Fig. 6(a)-(d).The electric field intensity along x-z and x-y direction for the Ag nanoparticle in hexagonal nanoarrays is stronger than that of the Ag nanoparticle in hexagonal nanoarrays containing central particles. When the a-SiNx:O thickness is increased from d = 0, an increase in the electric field intensity is observed. With further increase in a-SiNx:O thickness, the electric field intensity reduces. The change trend of electric field intensity is similar to that of luminescence intensity reported by A. Wocaun et al. [31]. Therefore the internal quantum efficiency increase from 3.9% to 13.3% is due to the higher spontaneous emission rate induced by LSPs [30, 32].Fig. 6 FDTD theoretical calculation of the near-field intensity enhancement of LSPs along the x-z and y-z plane (a)-(b) for the single Ag nanoparticle in hexagonal Ag nanoarrays/a-SiNx:O structure under 465 nm light illumination and (c)-(d)) for the single Ag nanoparticle in hexagonal Ag nanoarrays containing central particles/a-SiNx:O structure under 465 nm light illumination.
4. Conclusion
In summary, we demonstrated sharp enhancement of the blue PL from a-SiNx:O films at room temperature by coupling with the LSPs from hexagonal arrays of Ag nanoparticles. The coupling between the LSPs and excitons of a-SiNx:O increases the internal quantum efficiency of the a-SiNx:O film from 3.9% to 13.3%. Highly efficient, stable light emission from a-SiNx:O films with size-controlled and ordered Ag arrays can provide alternative routes towards the fabrication of efficient silicon-based optical devices.
Acknowledgments
The authors would like to thank the supports of the State Key Development Program for Basic Research of China (Grant No. 2010CB934402, 2013CB632101), National Nature Science Foundation of China (Grant No. 61071008, 61376004, 11374153), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20130091110024), the Fundamental Research Funds for the Central Universities (Grant No. 1095021030, 1116021004, 1114021005) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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