1. Introduction

Since the discovery of the quantum confinement (QC) effect in porous silicon leading to an intense photoluminescence (PL) in the visible range at room temperature in 1990, photo and electro-luminescence from various Si quantum structures have been widely studied in recent years in order to realize the Si-based micro/nano optoelectronic integration [1–6]. Among the many kinds of Si quantum structures, Si quantum dots (Si QDs)-based multilayer, such as Si QDs/SiO₂ and Si QDs/SiN_x multilayer, has been considered as one of the promising candidates in the Si-based light emitting devices as well as in the next generation Si solar cells because the dot size can be well controlled in the multilayered structures [5–7].

Despite of the huge progress in the fabrication and luminescence properties of Si QDs-based system, Si-based light source is still the unique bottleneck in Si photonics. One of the key points is to improve the light emission efficiency for Si QDs-based materials. Due to the large difference of the refractive index between the Si and SiO₂ materials, the emission light will be internal reflected in the interface as a consequence of Snell’s law. It is implied that only a small portion of emission light can be escaped from the film front surface which results in the very low external quantum efficiency. The situation becomes much more serious in the Si QDs-based multilayered structures. So far, several approaches have been proposed to improve the external luminescence efficiency by enhancing the light extraction ratio, such as using surface plasmon polaritons (SPP), constructing photonic crystal structures [8–10], or using semiconductor nano-wires [11].

Surface roughening is an effective way to reduce the surface reflection and to improve the light extraction ratio as reported previously [12–16]. Min-Yann Hsieh et al. fabricated textured GaN-based light-emitting diodes (LEDs) by nano-sphere lithography technique which obviously improved the device performance [12]. Corrugated organic light emitting diodes were also fabricated on patterned substrates using UV nano-imprint lithography process; light extraction has been improved and power efficiencies of the corrugated OLEDs has been enhanced by 93% [13]. Philippe Lalanne et al. fabricated subwavelength periodic structures on the Si wafers by holographically recording a crossed-grating in a photoresist mask followed by reactive-ion etching process which exhibited the good antireflection characteristics [16]. Recently, SiO₂ nano-spheres were used to obtain the Si-based nano-cone or nano-pillar structures and the antireflection and enhanced optical absorption was achieved [17]. In our previous work, we prepared the nano-patterned Si substrates by polystyrene (PS) nano-sphere lithography technique, which is an efficient and inexpensive method for fabrication of periodic nanostructures with feature size in the sub-micron region [18]. The improved field emission characteristics from amorphous Si layer and electroluminescence from Si quantum dots were found by using nano-patterned Si substrates [4,19]. In the present work, we fabricated the nano-patterned Si substrates with various depths by controlling the etching time in order to improve the light emission efficiency of Si QDs-based multilayer. It was found that the surface reflection of Si wafers can be obviously suppressed in a wide spectral range and depth-dependent anti-reflection was confirmed. By depositing Si QDs/SiO₂ multilayer on the nano-patterned substrates, the surface reflection can be as low as 5% in visible light range. The depth-dependent photo-luminescence enhancement for Si QDs was observed and the emission intensity can be enhanced by over one order of magnitude.

2. Experimental

The nano-patterned Si substrates were fabricated in three steps. First, a monolayer of PS nano-spheres with diameter of 220nm were coated on the p-type Si (1.5~3Ω·cm) substrates by using the self-assembly technique. Then, the coated substrates with a mask made of PS nano-spheres were etched in the reaction ion etching (RIE) system by using 30sccm CF₄ gas under RF power of 20W. Finally, the PS nano-spheres were removed in tetrahydrofuran (THF). The etching time in the second step was varied from 8 min to 16 min in order to acquire various etching depths.

Both the flat and the nano-patterned p-Si substrates were cleaned and placed in the conventional plasma enhanced chemical vapor deposition (PECVD) system, hydrogenated amorphous silicon (a-Si:H)/SiO₂ multilayer samples were deposited by alternatively changing the hydrogenated amorphous silicon (a-Si:H) film deposition and in situ plasma oxidation processes [20]. The substrate temperature and the r.f. power were maintained at 250°C and 50W, respectively. The a-Si:H/SiO₂ multilayer samples were dehydrogenated at 450°C for 1 hr and then thermally annealed at 1000°C for 1 hr to obtain Si QDs/SiO₂ multilayer. The surface morphology was characterized by Atomic Force Microscopy (AFM) after deposition of Si QDs/SiO₂ multilayer on nano-patterned substrate as shown in Fig. 1(a) ; it exhibits the periodic nano-cone structures. The periodicity of formed nano-pattern is about 220 nm which is in agreement with the size of the nano-spheres. Figure 1(b) gives the cross-sectional transmission electron microscopy (TEM) image of the Si QDs/SiO₂ multilayer on the nano-patterned Si substrate. Inset is the high resolution TEM image which shows the formation of Si QDs with size of 2.6 nm after the annealing. The thickness of SiO₂ layer is about 1.5 nm. The reflection spectra of front surface were measured by using Shimadzu UV-3600 spectrophotometer for nearly normal light incidence (5% offset) in the wavelength range from 200nm to 1000nm. Room temperature photoluminescence signals were collected by CCD detector under the excitation of He-Cd laser (325nm).

 

Fig. 1 (a) AFM image of periodic nano-cone arrays with Si QDs/SiO₂ multilayer on the nano-patterned p-Si substrate. Inset is the cross-sectional AFM images for 8min and 16min etched Si substrates. (b) The cross-sectional TEM image of 10min etched nano-patterned substrate with Si QDs/SiO₂ multilayer on it. Inset of figure (b) is the high resolution TEM image and the formation of Si QDs can be clearly identified.

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3. Results and Discussion

As shown in Fig. 1(b), the periodicity of the nano-patterned structures is 220nm which corresponds to the size of the PS nano-sphere and the depth of the nano-cone is about 69nm after 10min etching. Si QDs/SiO₂ multilayer deposited on the nano-patterned substrate keeps the periodically patterned morphology. The depth of nano-cone as a function of RIE etching time is given in Fig. 2 by summarizing the AFM measurement results. The depth is estimated from the top of the nano-cone to the bottom as indicated in the inset of Fig. 2. It is found that the depth can be controlled by changing the etching time, which is increased gradually as the etching process evolves. It is found that the depth estimated from AFM is smaller than that shown in TEM image. However, the depths derived by AFM image do inflect the depth increase with the development of etching time. Figure 3 gives the reflection spectra for flat and nano-patterned Si substrates with different etching time in the spectra range of 200-1000nm. It is shown that the reflectivity from the polished front surface of flat Si wafer is quite high in the whole measurement range and comes up to higher than 40% in the visible light region. It is interesting to find that the light reflection from the front surface is obviously suppressed for all the nano-patterned samples within the whole spectral range, indicating the anti-reflection characteristics for nano-patterned Si substrates. It is also found that the reflection is gradually reduced with increasing etching time. In the visible light region (400-800nm), the reflectivity is reduced from 20% for 8min etched sample to 5% for 16min etched sample, which is as low as the reflection from nano-scale textured Si surface obtained by wet chemical etching technique [21] and close to the antireflection effect reported in Ref. [16].

 

Fig. 2 Etching depths of the nano-patterned substrates versus the etching time.

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Fig. 3 Reflection spectra of the polished and nano-patterned substrates with various depths by changing the etching time.

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The broadband anti-reflection of nano-patterned Si substrates can be understood by considering the front surface morphology of the etched Si wafer as given in Fig. 1, which shows the ordered and densely packed Si nano-structures. It is found that the formed Si nano-structures have the cone-shape which causes the gradually increase of fractional area occupied by Si from top to bottom as shown in Fig. 1(b). It has been reported that the gradually changing reflective index from the surface can effectively eliminate the reflected light across a wide spectrum due to the reflective index matching [11,21,22]. The gradually changing of Si filling factor can result in the formation of gradual refraction index from front surface to bottom as in our case. Therefore, the reflection of the front surface can be significantly reduced in a wide spectral range compared with the flat Si wafer [22]. As increasing the etching time, the cone depth became larger and larger; the high aspect ratio can further improve the anti-reflection in a wide spectral range as demonstrated in our experiments. It is worth noting that both the vertical and lateral sizes of present nano-patterned structures are smaller than the wavelength of the incident light, which is more suitable for the nano-optoelectronic devices with the sub-micrometer-thick active layers.

Figure 4 is the reflection spectra for Si QDs/SiO₂ multilayer deposited on nano-patterned Si substrates with various etching time. It is found that the anti-reflection effect is further improved with the multilayer structures deposited on. The reflectivity is almost less than 10% for all samples in the whole spectra. For 16min etched Si substrate, the reflectivity decreases down to the lowest level (less than 2%) in 400-900nm. This is partly because the top SiO₂ layer has a low reflection index than Si substrates. More importantly, the deposited Si QDs/SiO₂ multilayer keeps the nano-patterned morphology due to the good conformal deposition process in PECVD systems as shown in Fig. 1. The nano-textured structures effectively eliminate the reflection of light in the periodic interfaces between Si QDs and SiO₂ layers in multilayered structures.

 

Fig. 4 Reflection spectra of the Si QDs/SiO₂ multilayer on nano-patterned substrates with various depths.

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In order to study the influence of nano-patterned structures on the luminescence behaviors of Si QDs-based materials, room temperature photoluminescence (PL) was carried out for the Si QDs/SiO₂ multilayer deposited on nano-patterned Si wafers and compared with that on flat substrate. The PL from Si QDs/SiO₂ multilayer on flat Si wafer is the weakest among all the measured PL spectra as given in Fig. 5 . The PL intensities of samples on nano-patterned substrates are significantly increased compared to that of sample deposited on flat substrate and the depth-dependent luminescence enhancement can be observed. The luminescent intensities are gradually enhanced by increasing the etching time. We plot the integrated PL intensities as a function of etching time in Fig. 6 . It is found that the luminescence intensity is increased proportionally to the etching time. The integrated PL intensity is enhanced by more than one order of magnitude for sample on 16min etched substrate compared to that on flat substrate.

 

Fig. 5 Room temperature photo-luminescence from Si QDs/SiO₂ multilayer on flat and nano-patterned substrates under the excitation of He-Cd laser (325nm).

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Fig. 6 Integrated photo-luminescence intensity of Si QDs/SiO₂ multilayer as a function of the etching time.

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Because the internal reflection between the sub-layers in Si QDs/SiO₂ multilayer can be significantly suppressed by using nano-patterned structures, the light extraction efficiency is effectively increased due to the anti-reflection in a wide spectral range which causes the light emission enhancement as revealed by our experimental results. On the other hand, the anti-reflection property in the PL measurement is bidirectional. The excitation light (He-Cd Laser, 325nm) is also benefited from the anti-reflection behavior. Since the reflection of incident light at UV region is also reduced as indicated in Fig. 2 and Fig. 3, the more incident light can penetrate into the Si QDs/SiO₂ multilayer and be absorbed by Si QDs to excite much more electron-hole pairs to generate the strong emission. Therefore, both the increase of absorption of incident excitation light and extraction efficiency of emission light contribute to the significant enhancement of photoluminescence intensity from Si QDs/SiO₂ multilayer on nano-patterned substrates. It has been reported that the amount of light extracted from GaN-based materials can be increased by forming the textured structures on the deposited luminescent layers via the surface roughening process [4,8]. In our case, we developed the nano-sphere lithography technique to get nano-patterned substrates and subsequently deposited the Si QDs-based luminescent films on them. Therefore, the surface textures can be naturally formed due to the conformal deposition process in CVD system, which avoids the damages to the formed device structures, especially for Si QDs/SiO₂ multilayer sample, in which the thicknesses of both sub-layers are in the nano-meter scale.

4. Conclusion

In summary, depth-dependent antireflection and luminescence enhancement were observed for Si QDs/SiO₂ multilayer samples deposited on nano-patterned Si substrates, which were fabricated by using nano-sphere lithography technique. By using nano-patterned p-Si substrates instead of the flat one, the PL intensity of Si QDs/SiO₂ multilayer is significantly enhanced by over one order of magnitude due to both the improved incident light absorption and the light extract ratio of emission light from the front surface. Our experimental results demonstrate that the light emission efficiency of Si-based light resource can be improved by forming nano-patterned structures. The present approach can also be applied for next generation Si QDs-based solar cells for improving the light absorption efficiency.