Si multilayered sample

Applied Surface Science 257 (2011) 9578–9582

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Photoluminescence of Si from Si nanocrystal-doped SiO2 /Si multilayered sample Yong Ren, Yong-Bin Chen, Miao Zhang, Jiang Zhu, Xing-Wang Zhang, You-Yuan Zhao, Ming Lu ∗ Department of Optical Science and Engineering, Shanghai 200433, China

a r t i c l e

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Article history: Received 11 January 2011 Received in revised form 2 June 2011 Accepted 13 June 2011 Available online 21 June 2011 PACS: 78.55.−m 78.67.Bf 81.07.Bc

a b s t r a c t A multilayered Si nanocrystal-doped SiO2 /Si (or Si-nc:SiO2 /Si) sample structure is studied to acquire strong photoluminescence (PL) emission of Si via modulating excess Si concentration. The Si-nc:SiO2 results from SiO thin film after thermal annealing. The total thickness of SiO layer remains 150 nm, and is partitioned equally into a number of sublayers (N = 3, 5, 10, or 30) by Si interlayers. For each N-layered sample, a maximal PL intensity of Si can be obtained via optimizing the thickness of Si interlayer (or dSi ). This maximal PL intensity varies with N, but the ratio of Si to O is nearly a constant. The brightest sample is found to be that of N = 10 and dSi = 1 nm, whose PL intensity is ∼5 times that of N = 1 without additional Si doping, and ∼2.5 times that of Si-nc:SiO2 prepared by co-evaporating of SiO and Si at the same optimized ratio of Si to O. Discussions are made based on PL, TEM, EDX and reflectance measurements. © 2011 Elsevier B.V. All rights reserved.

Keywords: Si nanocrystal Photoluminescence Multilayer

1. Introduction Si light emission is vital for the integrated optoelectronics based on modern Si technologies [1]. In the past two decades, a number of approaches have been developed to acquire roomtemperature Si light emission, such as electrochemical etching [2,3], plasma-enhanced chemical vapor deposition [4–6], Si ion implantation [7,8], Si/SiO2 and SiO/SiO2 multilayered structures, and so on [9–16]. All these methods focus on preparation of Si particles with size less than or close to Bohr’s exciton radius of Si of 4.3 nm [17], so that quantum confinement takes effect and strong Si light emission is achieved as compared to bulk Si. The sample of Si nanocrystal doped SiO2 , or briefly Si-nc:SiO2 , has been a promising material for Si light emission due to its robust structure, stableness in light emission and feature of stimulated light emission [4–14]. The approach of evaporation of SiO followed by high-temperature thermal annealing has been frequently adopted for preparation of Si-nc:SiO2 for its ease to operate and compatibility with the modern Si technologies [9–11,13]. However, with this method, the excess Si concentration within the sample cannot be readily changed, which usually causes insufficiency of Si nanocrystals, leading to weak Si light emission. In this work, we report fabrication of a multilayered Si-nc:SiO2 /Si structure resulting from SiO/Si after thermal anneal-

∗ Corresponding author. Tel.: +86 21 65642177; fax: +86 21 65641344. E-mail address: [email protected] (M. Lu). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.06.068

ing, with the purpose to acquire strong photoluminescence (PL) emission of Si via optimizing the excess Si amount. It is found that additional Si nanocrystals are formed in both SiO and Si layers. A maximal PL intensity occurs for each multilayered sample via optimizing the thickness of Si interlayer, which can be even greater than that of Si-nc:SiO2 prepared by co-evaporating of Si and SiO at the same optimized ratio of Si to O by a factor of ∼2.5. This work suggests a proper approach of doping Si to achieve high PL emissions of Si nanocrystals.

2. Experimental Si-nc:SiO2 sample was prepared by evaporating SiO powder via resistance-heating onto a quartz substrate in a vacuum chamber at a pressure less than 8 × 10−5 Pa, followed by ex situ thermal annealing in a tube furnace in nitrogen atmosphere at 1100 ◦ C for 1 h. A phase separation process was induced during thermal annealing [11]. The film thickness was monitored by a calibrated microbalance. To modulate the Si excess amount, SiO/Si multilayered structure was adopted. Pure Si was electron-beam evaporated in situ. The SiO layer was partitioned equally by Si interlayers into N (N = 3, 5, 10, and 30) SiO sublayers. The total SiO thickness is 150 nm throughout this work. After thermal annealing of SiO/Si, multilayered Si-nc:SiO2 /Si samples were formed. The PL spectra were recorded by a photospectrometer (Hitachi, F4500) with a xenon lamp as an excitation source. The wavelength of exciting beam was selected 300 nm. To check the formation of Si nanocrystals,

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Wavelength (nm) Fig. 2. PL spectra with maximal intensity for various N.

Fig. 1. PL intensity versus thickness of Si interlayer for various N. The total thickness of SiO is 150 nm. The inset is the diagram of the Si/SiO sample.

transmission electron microscopy (TEM, model TECNAI G2 F20) was used. Energy-dispersive X-ray spectroscopy (EDX) attached to the TEM spectrometer was applied to obtain the depth profiles of Si and O. Reflectance spectra of the multilayered samples were measured on a photospectrometer (model Shimadzu UV-3101 PC) in order to elucidate more accurately the enhancement of PL intensity. 3. Results and discussion Fig. 1 plots the PL intensity of Si nanocrystal as a function of the thickness of Si interlayer (or dSi ) for four types of Si-nc:SiO2 /Si mutilayers, i.e. N = 3, 5, 10, and 30. The PL intensity is defined as the highest peak height of the PL spectrum. In the inset of Fig. 1 the diagram of a SiO/Si sample is shown. For each N, the PL intensity increases at first with the increasing dSi , and then beyond an optimized dSi , it decreases. The increasing trend of PL intensity can be attributed to the increasing content of excess Si, which helps to increase the density of Si nanocrystals, while the declining trend is due both to the formation of large Si crystallites that contribute a little to the PL emission and the increasing reflectance of the sample. We will discuss this issue in the following based on TEM and reflectance results. Another trend reflected in Fig. 1 is of the maximal PL intensity obtained after dSi optimization versus N. It is seen that the maximal PL intensity increases at first with the increasing N, then beyond N = 10 in this work, it declines. This relation will also be discussed in the following. The PL intensity of sample N = 10, dSi = 1 nm is ∼5 times that of N = 1, i.e. monolayered one. For N = 3, 5, 10 and 30, we find that the maximal PL intensity of Si occurs at dSi = 4, 2, 1 and 0.4 nm, respectively. A ratio of Si to O, or R, is thus defined as (total thickness of Si + equivalent Si thickness in SiO)/equivalent O thickness in SiO, therefore for the sample of N = 3, dSi = 4 nm, one has R = (4 × 3 + 75)/75 = 1.16, and for the other three cases of N (N = 5, 10 and 30), corresponding to their maximal PL intensities, R = 1.13, 1.13 and 1.16, respectively. It is seen that these R’s are very close to one another. In Fig. 2, the PL spectra of Si nanocrystal for five multilayered samples with maximized PL intensity are given. The spectrum changes slightly from N = 1–10 in both the peak position and shape, but for N = 30, an overall blue shift of ∼12 nm is observed and the peak with  > 800 nm is reduced in weight. At N =30, the thickness of SiO sublayer is 5 nm, which is close to the Si Bohr’s exciton radius of 4.3 nm, so quantum confinement takes effect for Si crystallites formed in SiO layers, and the blue shift in peak position is expected [17]. The peak with  >800 nm can be attributed to well-crystalline Si nanoparticles, and that with  <800 nm to poorly-crystalline

ones, considering that the band gap of crystalline Si is smaller than that of the amorphous one [18], and, with the decreasing thicknesses of both SiO and Si, crystallization of Si nanoparticle becomes increasingly hard if the annealing temperature remains the same [19]. Fig. 3(a and b) presents the TEM and high-resolution TEM (or HRTEM) images of the sample of N = 3, dSi = 8 nm, respectively. In Fig. 3(a), the dark stripes stand for Si interlayers, while the bright areas for SiO or Si-nc:SiO2 layers after annealing. Five spots are selected for measurement of atomic concentration profile by EDX and the spot size of EDX on the sample is 15 nm. The measured atomic ratios of Si to O are tabulated in the second line of Table 1. It is seen that the further away the spot is from the Si interlayer, the less the atomic ratio of Si to O. This is in accord with the diffusion behavior of Si. It should be noted that the atomic ratio here is defined as the atomic concentration of Si to that of O as measured by EDX, which is not the same as the Si to O ratio defined above in terms of evaporated thickness. Fig. 3(b) shows an enlarged HRTEM image. It is clear that in both SiO and Si layers, Si nanocrystals are formed as indicated by diffraction spots, some of which are illustrated with white circles. Similarly, Fig. 4(a and b) presents the HRTEM images of the sample of N = 3, dSi = 4 nm, respectively. In Fig. 4(a), also five spots are selected for compositional analysis, and the atomic ratios of Si to O are given in the third line of Table 1. In Fig. 4(b), Si nanocrystals are observed in both SiO and Si layers, too, although the Si layer, or strictly speaking, the Si layer after annealing as indicated by arrows between two parallel white lines, is not easy to see now due to weak contrast. From Table 1, one finds that for the sample of N = 3, dSi = 8 nm, the atomic ratio of Si to O is higher than that of N = 3, dSi = 4 nm at each corresponding spot. This is because the diffusant Si source in the sample of N = 3, dSi = 8 nm is richer than that of N = 3, dSi = 4 nm. In fact, O atoms from SiO also diffuse into Si interlayer as found by EDX measurements. At the Si interlayer, the atomic ratio of Si to O for the sample of N = 3, dSi = 4 nm is significantly less than that of N = 3, dSi = 8 nm. That explains the weak contrast between Si and Si-nc:SiO2 layers of N = 3, dSi = 4 nm (Fig. 4) as compared to that of N = 3, dSi = 8 nm (Fig. 3). By comparing Fig. 3(b) with Fig. 4(b), it is seen that in the Table 1 The atomic ratios of Si to O measured by EDX. Position

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0.57 0.52 0.87

0.61 0.56 0.81

2.31 0.95 0.87

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Fig. 3. TEM (a) and HRTEM (b) of the sample N = 3, dSi = 8 nm. The white circles indicate Si nanocrystals.

sample of N = 3, dSi = 8 nm, large Si crystallites (>5 nm) are formed in the Si interlayer. These large Si crystallites contribute a little to the PL emission of Si as compared to the small ones due to weak confinement effect, and meanwhile they “consume” small Si crystallite via ripening process [20]. There is another factor, in terms of thin film optics [21], it is known that if a light beam impinges from air onto a thin film with greater refractive index, more light would be reflected. Since the refractive index of Si is much greater than SiO2 [22], the effective refractive index of the sample of N = 3, dSi = 8 nm is then greater than that of N = 3, dSi = 4 nm, hence the reflectance of N = 3, dSi = 8 nm is greater, making the light flux for exciting PL emission less. These explain why the PL intensity of Si from the sample of N = 3, dSi = 4 nm is greater than that from N = 3, dSi = 8 nm. In general, with the increasing of dSi , the content of excess Si increases, so the number of Si nanocrystal increases, which is good for PL intensity increase, but meanwhile refractive index increases, which enhances the surface reflectance and reduces the PL emission; on the other hand, further increases in dSi could cause the formation of large Si crystallites, which also tend to reduce the PL emission. These three factors finally determine the trend of PL intensity versus dSi shown in Fig. 1.

Fig. 4. TEM (a) and HRTEM (b) of the sample N = 3, dSi = 4 nm. The white circles indicate Si nanocrystals.

Fig. 5(a and b) presents the TEM and HRTEM images of the sample of N = 10, dSi = 1 nm, respectively. As compared to the samples of N = 3, dSi = 8 nm in Fig. 3 and N = 3, dSi = 4 nm in Fig. 4, the contrast between the Si and SiO layers are rather weak, suggesting that the distribution of Si becomes more uniform with the increasing N and decreasing dSi . This is proved by the EDX measurement. In Fig. 5(a), 5 spots were selected. Since the SiO layer is rather thin and the space resolution of EDX is limited, only regions around Si and SiO layers can be detected by EDX, hence in here, spots 1, 3 and 5 are in the regions of Si layer, while spots 2 and 4 are mainly in SiO layers. The measured atomic ratios of Si to O are given in the fourth line of Table 1. As compared to the samples of N = 3 (lines 2 and 3 in Table 1), the distributions of Si and O are more uniform. It should be pointed out that since the spot size of EDX is 15 nm, the measured elemental compositions for Si layers are actually from both the Si layers and the adjacent parts of SiO layers. Fig. 6 plots the maximal PL intensity versus N according to the results of Fig. 1. Since R remains constant, with the increase of N and decreases of dSi and thickness of SiO, the distributions of Si and O become more uniform, therefore, the number of Si nanocrystals increases, which helps to enhance the PL emission of Si. On

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Wavelength (nm) Fig. 7. PL spectra for the samples of Si/SiO, SiO2 /SiO and co-evaporation of Si and SiO.

Fig. 5. TEM (a) and HRTEM (b) of the sample N = 10, dSi = 1 nm. The white circles indicate Si nanocrystals.

the other hand, surface reflectance also increases with the increasing N, although the ratio of Si to O remains constant. This tends to decrease the PL emission. Therefore, the relationship is a compromise between these two effects. In Fig. 6, the measured reflecting coefficients versus N are given at  =300 nm, the wavelength of the exciting light. Through software simulation [21], it is found that as the Si atoms are more uniformly dispersed within SiO2 , the reflectance becomes stronger, although the Si to O ratio is the same. The reason could be that if Si is more thoroughly dispersed, the total area of the interface between Si and SiO2 is larger, which causes more reflecting events at the interfaces. Finally, in Fig. 7, we depict three PL spectra. Curve 1 is for the sample of N = 10, dSi = 1 nm, with R = 1.13. Curve 2 is for a multilayered sample of SiO/SiO2 , with N = 10 and the thickness of SiO2 being 3 nm; here R = 1 and the total thickness of SiO remains 150 nm. It shows again that with addition of excess Si, PL intensity becomes largely enhanced. Curve 3 is for a monolayered Si-nc:SiO2 sample prepared by co-evaporating of SiO and Si at the same Si to O ratio of R = 1.13 followed by thermal annealing. This sample can in fact be treated as a multilayered SiO/Si after thermal annealing with N → ∞

at R = 1.13. Its intensity has been indicated in Fig. 1 as a solid dot at dSi = 0 nm. The PL intensity of sample N = 10, dSi = 1 nm is ∼2.5 times that of the co-evaporated sample. It is therefore emphasized that the approach of the multilayered SiO/Si structure can even result in a stronger PL intensity of Si than that of co-evaporation of SiO and Si at the same optimized content of excess Si. 4. Summary In this report, a SiO/Si (or Si-nc:SiO2 /Si after thermal annealing) multilayered sample structure is studied with the purpose to modulate the Si excess content so as to increase the density of Si nanocrystals and enhance the PL intensity of Si. It is found that for each multilayered sample, there exists an optimized thickness of Si interlayer, which corresponds to a maximal PL intensity. For a constant total thickness of SiO, the maximal PL intensity increases at first with the increasing partition of SiO, or the increasing number of SiO sublayer N, and then beyond a certain N, the maximal PL intensity declines. These results are caused mainly by three factors relating to variations in the number of Si nanocrystal, the number of large Si crystallite and the surface reflectance of the sample. Furthermore, it is emphasized that the approach of the multilayered SiO/Si structure can even result in a stronger PL intensity of Si than that of co-evaporation of SiO and Si at the same optimized content of excess Si.

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Acknowledgments This work was supported by National Basic Research Program of China (973 Program), grant no. 2010CB933703, and National Science Foundation of China, grant nos. 60638010 and 60878044. References [1] O. Bisi, S.U. Campisano, L. Pavesi, F. Priolo (Eds.), Silicon Based Microphotonics: From Basic to Applications, IOS Press, Amsterdam, 1999. [2] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [3] O. Bisi, S. Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1. [4] L. Dal Negro, M. Cazzanelli, L. Pavesi, S. Ossicini, D. Pacifici, G. Franzo, F. Priolo, F. Iacona, Appl. Phys. Lett. 82 (2003) 4636. [5] G.R. Lin, C.J. Lin, Y.C. Chang, Appl. Phys. Lett. 90 (2007) 151903. [6] G.R. Lin, C.W. Lian, C.L. Wu, Y.H. Lin, Opt. Express 16 (2010) 9213. [7] L. Pavesi, L.D. Negro, C. Mazzoleni, G. Franzo, J.P. Prolo, Nature 408 (2000) 440.

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