Simulation and Experimental Study on Anti-reflection Characteristics of Nano-patterned Si Structures for Si Quantum Dot-Based Light-Emitting Devices
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Surface-textured structure is currently an interesting topic since it can efficiently reduce the optical losses in advanced optoelectronic devices via light management. In this work, we built a model in finite-difference time-domain (FDTD) solutions by setting the simulation parameters based on the morphology of the Si nanostructures and compared with the experimental results in order to study the anti-reflection behaviors of the present nano-patterned structures. It is found that the reflectance is gradually reduced by increasing the depth of Si nanostructures which is in well agreement with the experimental observations. The reflectance can be lower than 10 % in the light range from 400 to 850 nm for Si nano-patterned structures with a depth of 150 nm despite the quite low aspect ratio, which can be understood as the formation of gradually changed index layer and the scattering effect of Si nano-patterned structures. By depositing the Si quantum dots/SiO2 multilayers on nano-patterned Si substrate, the reflectance can be further suppressed and the luminescence intensity centered at 820 nm from Si quantum dots is enhanced by 6.6-fold compared with that of flat one, which can be attributed to the improved light extraction efficiency. However, the further etch time causes the reduction of luminescence intensity from Si quantum dots which may ascribe to the serious surface recombination of carriers.
KeywordsSi nano-patterned structures Anti-reflection Photoluminescence
Crystalline Si plays a crucial role in today’s microelectronics industry, but it is hard to be utilized in optoelectronic devices because of its indirect band gap. The indirect band gap can cause low light-emitting efficiency and low absorption coefficient, which impede the improvement of device properties [1, 2]. Compared with the bulk Si materials, Si quantum dot (Si QD)-based materials exhibit the efficient light emission ability due to the enhanced recombination efficiency of electron-hole pairs in quantum-confined system . So far, Si QD-based materials such as Si QDs/SiO2, Si QDs/SiNx, and Si QDs/SiCx structures have been fabricated, and the photoluminescence and photovoltaic properties have been studied extensively [4, 5, 6]. However, the large index difference between Si and Si-based dielectric materials (SiO2, SiNx, or SiCx) causes strong light reflection which generates the optical losses in Si-based photo-electronic devices. For example, it was reported that in the Al2O3/ZnS:Mn system, though the index difference is relatively small (1.6 vs. 2.5), only 7.4 % light can be extracted from the front surface of the device . It is reasonable to imagine that the situation will become more serious in Si/SiO2 system.
Recently, surface-textured structure using the nano-sphere lithography technique is currently an interesting topic since it can efficiently reduce the optical losses in advanced optoelectronic devices such as light-emitting diodes and solar cells via light management [8, 9, 10, 11]. For example, Hsieh et al. fabricated textured GaN-based light-emitting diodes (LEDs) by nano-sphere lithography technique which obviously improved the device performance . They have achieved an average of 38.5 % efficiency improvement of the textured p-GaN LED over the conventional device at a bias current of 20 mA. In our previous work, nano-sphere lithography technique was used to prepare nano-patterned Si structures. It was demonstrated that nano-patterned structures can improve the property of light-emitting devices as well as the heterojunction Si thin film solar cells [12, 13, 14]. In order to further understand the light management behaviors of Si-based nanostructures and to improve the device performance, it is necessary to systematically study the anti-reflection characteristics as a function of the parameters of the formed nanostructures. In the present work, both the simulation and experimental characterization is used to study the anti-reflection characteristics of periodically Si nano-patterned structures prepared by nano-sphere lithography technique. Based on the structural parameters extracted from atomic force microscopy (AFM) results, we built a model in finite-difference time-domain (FDTD) solutions. The influences of structural parameters on the optical properties of Si-nano-patterned structures were simulated and compared with the experimental results. Furthermore, we deposited Si quantum dots/SiO2 multilayers on the Si nano-patterned structures, and the luminescence intensity is enhanced by 6.6-folds.
Furthermore, Si QDs/SiO2 multilayers (MLs) are fabricated on the Si nano-patterned structures by annealing amorphous Si/SiO2 stacked layers at 1000 °C for 1 h. The average size of Si QDs is about 2 nm, and the thickness of SiO2 layers is 2 nm too. The detailed fabrication process and conditions of Si QDs/SiO2 multilayers and TEM images can be found elsewhere in . Room temperature photoluminescence measurements are performed by excited He–Cd laser with wavelength of 325 nm. As a reference, the Si QDs/SiO2 MLs are also prepared on a flat Si substrate for comparison.
Results and Discussion
The etched deep hole between the Si nano-patterned structures may also contributed to the anti-reflection behaviors. Usually, chemical etching methods were used to fabricate nano-hole structures . The etching rate is very fast, and the depth of nano-holes usually exceeds 200 nm and the aspect ratio of nano-holes is much larger than 1. In our case, the aspect ratio of height/diameter is no more than 0.5 and the hole is not so deep to influence the observed anti-reflection characteristics strongly, though we cannot completely rule out of the contribution from nano-hole structures.
The anti-reflection behavior of Si-nano-patterned structures can be understood as below. First, the Si nano-patterned structures can be considered as a graded-index layer between the Si substrates and air due to the gradually changed volume ratio. The graded index layer can effectively eliminate the light reflection as revealed by Fresnel theory . Second, the suppression of light reflection can also be attributed to the strong scattering between the incident light and formed nano-patterned structures because the light wavelength is comparable with the geometric size of Si nanostructures. According to Mie’s scattering theory, the strong forward scattering results in the enhanced optical absorption of incident light which can significantly reduce the surface reflection . With increasing the depth of Si nano-patterned structures, the incident light can be trapped more effectively which improves the anti-reflection behaviors as shown in our simulation results. If the depth remains constant, which indicates the thickness of the graded index layer does not change. However, if the diameter of Si nano-patterned structures becomes small, the scattering between the incident light and nano-patterned structures is enhanced. Therefore, the reflectance is decreased in the whole spectral range as shown in Fig. 5. Usually, the significantly anti-reflection behaviors were usually realized in the nanostructures with high aspect ratio. Our simulation results suggest that the good anti-reflection characteristics can also be achieved by using Si nanostructures with lower aspect ratio, which is in well agreement with the previous work , in which, they found that the optical absorption can be obviously enhanced by forming low aspect ratio hemispherical nanopit structures. It is worth noticing that the reflectance will be strongly suppressed when the wavelength is comparable with diameter (Fig. 5 D = 400 nm, 500 nm), and the possible reason may due to the strongest resonant scattering effect when the size is equal to the wavelength of the incident light and it is needed to be further clarified in our future work.
Figure 6a shows the room temperature photoluminescence (PL) spectra from Si QDs/SiO2 multilayers on flat and nano-patterned structures excited by Hd–Cd laser with wavelength of 325 nm. The PL peak is centered at around 820 nm. The origin of 750–850 nm emission is still under the debate. Some groups attributed it to the quantum confinement effect, and the others believed it is due to the radiative recombination of photo-excited electron-hole pairs via interface states [22, 23, 24, 25, 26]. For example, Franzò et al. reported that their EL peak is due to electron-hole recombination in the Si nanocrystals and not to defects , but N. Daldosso et al. concluded that the PL of Si/SiO2 system is surface related . Since we have measured the PL spectra from Si QDs-based multilayers with various dot sizes and we found that the peaks position is size independent, which is contrary to the quantum confinement expect. We attribute the 820 nm emission in our case mainly to the recombination of electron-hole pairs via the interface states on the Si nanocrystal, though we cannot completely rule out of the contribution from the band-to-band recombination within the Si QDs due to the quite broad PL band. Qin et al. also attributed the observed visible light emission to the nc-Si/SiO2 interfacial states based on their experimental results although the excitation of electrons and holes occurred within the Si QDs .
It is clearly shown that the PL intensity is obviously enhanced by using nano-patterned structures. With increasing the etching depth, the PL intensity is first enhanced and then decreased. The results are shown in Fig. 6b, and the maximum PL intensity can be obtained by using nanostructures with depth of 91 nm, which is improved by 6.6-fold compared with that of flat one. The enhanced PL intensity can be explained in terms of the improved light-extraction efficiency due to the reduction of the optical reflection on the surface of samples and interfaces between Si QDs and SiO2 layers by using nanostructures. As in the flat sample, the light is emitted under the all directions and only a part of light whose incident angle is less than angle of total reflection. However, the nanostructure can approximately be seen as a graded-index layer. More light can travel through a graded-index layer than that of the flat samples. The greater the H is, the index of this layer changes more slowly. The reflection spectra are measured and reflectance at 820 nm are given in Fig. 6c as a function of etching depth. It is found that the reflectance of Si QDs/SiO2 multilayers is decreased from 38.8 % for flat one to 3.6 % for sample on nano-patterned structures with depth of 150 nm. However, further increasing the etching depth causes the reduction of PL intensity, though the surface reflection becomes much lower. The possible reason is that the surface recombination will become more serious which may influence the luminescence efficiency as seen in our case. Since the thickness of SiO2 in our present work is about 2 nm and the total thickness of Si QDs/SiO2 multilayers is about 40 nm, it is possible that the part of the photo-excited carriers can tunnel through the ultrathin SiO2 layer to the surface of Si substrates, where they recombine via the surface states and result in the reduced photoluminescence intensity as we observed experimentally. It looks like that the improved device performance will be limited by the surface states by using Si nano-patterned structures which determines the optimum size parameters of nanostructures.
In summary, the anti-reflection behaviors of Si nano-patterned structures prepared by nano-sphere lithography technique are studied via FDTD solutions as well as experimental observations. It is found that a good anti-reflection characteristic can be achieved by using Si nano-patterned structures even if they have the low aspect ratio which can be attributed to the graded-index as well as the strong scattering effect. With increasing the etching depth from 0 to 150 nm and reducing the diameter of nanostructures from 500 to 200 nm, the surface reflection can be obviously suppressed. In the experiment, we fabricated substrates with diameter of 300 nm and depths of 51, 91, 115, and 150 nm, and the reflectance is lower than 10 % in the measurement spectral range which is consistent with the simulation results. By depositing Si QDs/SiO2 multilayers on the nano-patterned structures, the reflectance can be further reduced and the reflectance at 820 nm can be as low as 3.6 %. The maximum PL intensity of quantum dots can be achieved and is improved by 6.6-folds for samples deposited on the nano-patterned structures with depth of 91 nm compared with that of flat one. However, further increasing the etching depth results in the reduction of PL intensity although the surface reflection is still reduced, which can be ascribed to the strong surface recombination of carriers. Our results demonstrate that the performance of Si-based photonic devices can be improved by optimizing the Si-based light trapping structures.
This work is supported by the “973” project (2007CB613401) and PAPD.
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