Nanocrystalline Si/SiO2 multilayers-based electroluminescent devices were prepared on nano-patterned p-Si substrates which were fabricated by nano-sphere lithography technique. The formed nano-patterned substrate contains periodic Si nano-cone arrays with the height of 80~95 nm and the diameter around 220 nm. The turn-on voltage of the luminescent device prepared on nano-patterned substrate is 3 V while the electroluminescence intensity is increased by over one order of magnitude compared to that of device prepared on flat substrate. The enhancement of the light emission can be attributed to the improved extraction efficiency of emission light as well as the high carrier-injection efficiency.
©2010 Optical Society of America
Nano-crystalline Si (nc-Si) has attracted much attention nowadays because of its novel properties compared with its crystal counterpart. One of the potential applications of nc-Si films is to develop efficient light emitting devices (LEDs) for realizing monolithic Si-based optoelectronics and photonics [1-3]. It has been reported that the radiative recombination efficiency can be significantly improved in nc-Si materials due to the quantum confinement effect of electron-hole pairs. However, in spite of the extensive studies on the light emission from nc-Si materials, the luminescent efficiency is still low which hampered their actual applications.
As one of the promising candidates, nc-Si/SiO2 multilayers were used as the emissive medium [3-7] which can get the well controllability of nc-Si size and SiO2 thickness. However, considering the large difference of the refractive index between Si (~3.4) and SiO2 (1.5). The fraction of external light is very small since most of the emission light is either absorbed by the various layers or confined in SiO2 layers as guided modes. It was reported that in Al2O3/ZnS:Mn system, though the index difference is relatively small (1.6 vs 2.5), only 7.4% of the emission light can be extracted from the top surface . In fact, we have measured the side emission of nc-Si/SiO2 multilayers which is over one order of magnitude stronger than that from the top surface. More recently, several methods, such as introducing surface plasmon , surface roughing , etc., have been applied to circumvent this problem. In the present work, we fabricated electroluminescent (EL) device containing nc-Si/SiO2 multilayers on nano-patterned p-type substrates in order to reduce the internal light reflection. The process we used here can get the appropriate surface morphology for better effective light extraction without damaging the electrical or optical properties of the films. The obviously enhanced external efficiency was demonstrated experimentally.
The nano-patterned p-Si substrates were fabricated by using nano-sphere lithograph technique. The single layer consisting of self-assembling polystyrene (PS) nano-spheres with diameter of 220 nm was covered on the p-Si (1.5~3Ω∙cm) wafers as a mask. Then the substrates were dry-etched in reactive ion etching system by using CHF3/O2 gas mixtures with the pressure of 40 Pa under the r.f. power of 40 W. The etched substrates were then dipped in tetrahydrofuran (THF) to remove the PS nano-spheres and the nano-patterned p-Si substrates were finally formed. Figure 1(a) shows the surface image measured by atomic force microscopy (AFM). It is found that the formed nano-patterned substrate contains periodic Si nano-cone arrays and the area can be as large as 1x1 cm2. The nano-cone has the height of 80~95 nm with the diameter of 220 nm which can be controlled by the diameter of PS nano-sphere. The detailed preparation process and parameters can be found elsewhere .
The nc-Si (~3 nm) and SiO2 (~2.5 nm) multilayers were deposited in conventional plasma enhanced chemical vapor deposition (PECVD) system by alternatively changing the amorphous Si (a-Si) film deposition and plasma oxidation process . During the fabrication process, the substrate temperature is kept at 250°C and the r.f. power is about 40 W. The multilayered samples were deposited both on the nano-patterned p-Si substrates and the flat p-Si substrates for comparison. After deposition, a-Si/SiO2 multilayers were subjected to the thermal annealing (dehydrogenation at 400 °C for 40 min; rapid thermal annealing at 1100 °C for 50 s; furnace annealing at 1000 °C for 40 min) to obtain nc-Si/SiO2 multilayers. Figure 1(b) gives the surface morphology of nc-Si/SiO2 multilayers with 9 periods deposited on nano-patterned p-Si substrate. It is found that the surface keeps the morphology of the nano-patterned substrate indicating the conformal deposition mechanism of the multilayered films. Finally, Al electrodes were evaporated on both sides to get electroluminescent devices as shown in Fig. 1(c). The diameter of top Al electrode is about 1.5mm and the thickness is about 40 nm (semi-transparent) for light output.
3. Results and discussion
Figure 2 shows the EL spectra for nc-Si/SiO2 multilayer-based luminescent device deposited on flat and nano-patterned p-Si substrate, respectively. Electroluminescence (EL) signals of nc-Si/SiO2 multilayers were collected in Fluoromax-2 spectroscopy (Jobin-Yvon) system by applying DC voltage. The EL intensities for both samples are gradually increased by increasing the applied voltage and the EL spectra shape is independent of the substrate morphology. It looks like that the EL spectra contain at least two sub-bands located at 520 nm and 650 nm, respectively due to the different luminescence routes as suggested before [6, 12-13]. In our previous work, room temperature photoluminescence (PL) was also detected under Ar+ laser (488nm) or He-Cd laser (325nm) excitation. The emission bands are changed in the visible light range with the annealing temperature due to the co-existence of the different recombination routes .
It is found that the EL signals can only be detected under the forward biased conditions (the top electrode negatively biased compared with the grounded p-Si substrate), which indicates that the luminescence is caused by the radiative recombination of injected electron and hole within nc-Si or the nc-Si/SiO2 interfacial regions. However, compared with that of device on flat substrate, the turn-on voltage, at which the EL signal starts to be detected, is significantly reduced for device on nano-patterned substrate. The turn-on voltage is as low as 3 V, which is compatible with the requirement of CMOS technology.
It is found that the EL intensity is obviously enhanced for device deposited on nano-patterned substrate (nano-patterned device) compared to that on flat substrate (flat device). Figure 3(a) gives the integrated EL intensities for both devices as a function of applied voltage. It is shown that the output intensity of nano-patterned device is increased much faster than that of flat device. At the applied voltage of 8 V, the integrated EL intensity of nano-patterned device is about 50 times stronger than that of flat device. Since the output power from sample deposited on the flat substrate was measured around 10nW by using an optical power meter , it can be roughly estimated that the power efficiency at applied voltage of 8V is in the order of 10−6~10−5 for nano-patterned devices. Figure 3(b) shows the ratio of integrated EL intensity to the injection current as a function of applied voltage for both devices. The ratio of integrated EL intensity to the injection current directly reflects the external quantum efficiency. It is clearly shown that the external quantum efficiency is obviously enhanced for devices on nano-patterned substrate compared with that on flat one. The EL efficiency can be improved by almost two orders of magnitude as shown in Fig. 3 (b).
The reduction of turn-on voltage and enhanced emission intensity and efficiency indicates that the EL characteristics can be significantly improved by using nano-patterned substrate. It was reported that the carrier injection efficiency can be improved due to the enhanced Fowler-Nordheim (F-N) tunneling process by introducing Si interfacial nano-pyramids into the Si light emitting diode . The similar results are obtained in our case as shown in Fig. 4 (a) and (b) , which are the plots of ln (I/V2) vs 1/V according to F-N tunneling mechanism. It is found that the threshold voltage to initiate F-N tunneling is reduced obviously from 7 V for flat sample to 0.2 V for nano-patterned one. Meanwhile, from the slopes of F-N plots in Fig. 4, one can deduce the F-N enhancement factor for two samples. It is found that the enhancement factor for nano-patterned sample is 5 times larger than that of flat one which suggests that the effective barrier becomes smaller due to the nano-cone arrays formed on p-Si substrates. The lowering of the effective barriers makes the carrier injection into the nc-Si/SiO2 system easier and the EL intensity can be increased consequently.
However, the enhanced F-N tunneling only cannot fully explain the large improvement of EL intensity as shown in Fig. 3. Another important factor to improve the EL efficiency is related to the enhancement of the light extract efficiency due to the patterned morphologies for device on nano-patterned substrate. As we mentioned before, the emitted light would suffer from the internal reflection at the interfaces of Si/SiO2 and air/SiO2. It was reported that only 1/2n2 (n is the effective refraction index of the materials) of light can be extracted from the top of a planar structures . By considering the index of nc-Si and SiO2 is about 3.4 and 1.5, respectively, it can be estimated that only 5% of light can be escaped from Si/SiO2 system. The light extraction efficiency can be significantly improved by almost one order of magnitude via surface roughing and patterning because it can reduce light trapping caused by differences of refraction index. Recently, it was reported that the light output power of GaN-based LEDs can be enhanced by using micro-hole array patterned substrate . Figure 5 gives the reflectance spectra from samples on patterned Si substrate and on flat Si substrate. The reflectance from sample on patterned substrate is clearly reduced in the whole wavelength region (300-800 nm) compared with that on the flat one. In the short wavelength region, the reflectance is almost reduced by 5-6 times which confirms that the patterned surface can increase the escape opportunity of light.
It is interesting to see the EL enhancement ratio spectrum, which is the EL intensity ratio of patterned sample to flat sample at each emission wavelength. As shown in Fig. 6 , it is found that the EL intensity is enhanced by about two orders of magnitude in the whole spectral region by using patterned substrate. The EL enhancement ratio shows a strong peak at the wavelength of 360 nm, which corresponds to the minimum reflectance given in Fig. 5. The appearance of EL enhancement ratio peak may be related to the periodic patterned structures which can produce the resonant enhancement of the emitted light. The research concerning with this issue will be further addressed.
In summary, the ordered nano-patterned Si substrates were prepared by using nano-sphere lithography technique. By deposition of nc-Si/SiO2 multilayers on the patterned substrates to obtain Si-based LEDs, it is found that the electric field enhancement factor for nano-patterned sample is 5 times larger than that of flat one. Meanwhile, the light extraction efficiency may improved by almost one order of magnitude due to the surface roughing. As a consequence, the EL intensity is obviously increased by 50 times compared with the conventional devices on flat substrates and the turn-on voltage is reduced to 3V. Our results demonstrate the possible route to improve the external quantum efficiency of Si-based LEDs by using a cheap and simple technique.
This work was partly supported by NSF of China (10874070, 60721063 and 50872051), “973” Project (2007CB613401) and Jiangsu Province Foundation for youths (09KJB510008).
References and links
1. L. Pavesi, “Silicon-based light sources for Silicon integrated circuits,” Advances in Optical Technologies 2008, 1–13 (2008). [CrossRef]
2. R. Soref, “The past, present, and future of Silicon photonics,” IEEE J. Sel. Top. Quantum Electron. 12(6), 1678–1687 (2006). [CrossRef]
3. N. Daldosso and L. Pavesi, “Nanosilicon photonics,” Laser & Photon. Rev. 3(6), 508–534 (2009). [CrossRef]
4. J. Heitmann, F. Muller, M. Zacharias, and U. Gosele, “Silicon Nanocrystals: Size Matters,” Adv. Mater. 17(7), 795–803 (2005). [CrossRef]
5. J. Zhou, G. R. Chen, Y. Liu, J. Xu, T. Wang, N. Wan, Z. Y. Ma, W. Li, C. Song, and K. J. Chen, “Electroluminescent devices based on amorphous SiN/Si quantum dots/amorphous SiN sandwiched structures,” Opt. Express 17(1), 156–162 (2009). [CrossRef] [PubMed]
6. D. Y. Chen, D. Y. Wei, J. Xu, P. G. Han, X. Wang, Z. Y. Ma, K. J. Chen, W. H. Shi, and Q. M. Wang, “Enhancement of electroluminescence in p–i–n structures with nano-crystalline Si/SiO2 multilayers,” Semicond. Sci. Technol. 23(1), 015013 (2008). [CrossRef]
7. A. Marconi, A. Anopchenko, M. Wang, G. Pucker, P. Bellutti, and L. Pavesi, “High power efficiency in Si-nc/SiO2 multilayer light emitting devices by bipolar direct tunneling,” Appl. Phys. Lett. 94(22), 221110 (2009). [CrossRef]
8. Y. R. Do, Y.-C. Kim, S.-H. Cho, J.-H. Ahn, and J.-G. Lee, “Improved output coupling efficiency of a ZnS:Mn thin-film electroluminescent device with addition of a two-dimensional SiO2 corrugated substrate,” Appl. Phys. Lett. 82(23), 4172 (2003). [CrossRef]
9. B. H. Kim, C. H. Cho, J. S. Mun, M. K. Kwon, T.-Y. Park, J. S. Kim, C. C. Byeon, J. Lee, and S.-J. Park, “Enhancement of the External Quantum Efficiency of a Silicon Quantum Dot Light-Emitting Diode by Localized Surface Plasmons,” Adv. Mater. 20(16), 3100–3104 (2008). [CrossRef]
10. Z. T. Kang, B. K. Wagner, J. Parrish, D. Schiff, and C. J. Summers, “Enhancement of white luminescence from SiNx films by surface roughening,” Nanotechnology 18(41), 415709 (2007). [CrossRef]
11. W. Li, J. Zhou, X. G. Zhang, J. Xu, L. Xu, W. Zhao, P. Sun, F. Song, J. Wan, and K. Chen, “Field emission from a periodic amorphous silicon pillar array fabricated by modified nanosphere lithography,” Nanotechnology 19(13), 135308 (2008). [CrossRef] [PubMed]
12. G. Pucker, P. Bellutti, M. Cazzanelli, Z. Gaburro, and L. Pavesi, “(Si/SiO2)n multilayers and microcavities for LED applications,” Opt. Mater. 17(1-2), 27–30 (2001). [CrossRef]
13. G. R. Lin, C. J. Lin, C. K. Lin, L.-J. Chou, and Y.-L. Chueh, “Oxygen defect and Si nanocrystal dependent white-light and near-infrared electroluminescence of Si-implanted and plasma-enhanced chemical-vapor deposition-grown Si-rich SiO2,” J. Appl. Phys. 97(9), 094306 (2005). [CrossRef]
14. J. Mei, Y. Rui, Z. Ma, J. Xu, D. Zhu, L. Yang, X. Li, W. Li, X. F. Huang, and K. J. Chen, “Contribution of multiple emitting centers to luminescence from Si/SiO2 multilayers with step by step thermal annealing,” Solid State Commun. 131(11), 701–705 (2004). [CrossRef]
15. G. R. Lin, C. J. Lin, and C. K. Lin, “Enhanced Fowler-Nordheim tunneling effect in nanocrystallite Si based LED with interfacial Si nano-pyramids,” Opt. Express 15(5), 2555–2563 (2007). [CrossRef] [PubMed]
16. A. A. Erchak, D. J. Ripin, S. Fan, P. Rakich, J. D. Joannopoulos, E. P. Ippen, G. S. Petrich, and L. A. Kolodziejski, “Enhanced coupling to vertical radiation using a two-dimensional photonic crystal in a semiconductor light-emitting diode,” Appl. Phys. Lett. 78(5), 563 (2001). [CrossRef]
17. F. Lai, S. C. Ling, C. E. Hsieh, T. H. Hsueh, H. C. Kuo, and T. C. Lu, “Extraction Efficiency Enhancement of GaN-Based Light-Emitting Diodes by Microhole Array and Roughened Surface Oxide,” IEEE Electron Device Lett. 30(5), 496–498 (2009). [CrossRef]