Optical properties of nanocrystal-silicon thin films on silicon nanopillar arrays after thermal annealing

Applied Surface Science 265 (2013) 324–328

Contents lists available at SciVerse ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Optical properties of nanocrystal-silicon thin films on silicon nanopillar arrays after thermal annealing Yonglei Li a,b , Bo Qian a,c,∗ , Cong Li c , Jun Xu c , Chunping Jiang a a

Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215125, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China Nanjing National Laboratory of Microstructures and Key Laboratory of Advanced Photonic and Electronic Materials, Department of Physics, Nanjing University, Nanjing 210093, PR China b c

a r t i c l e

i n f o

Article history: Received 4 October 2012 Received in revised form 1 November 2012 Accepted 1 November 2012 Available online 16 November 2012 Keywords: Nanocrystal-silicon Nanopillar array Reflectivity Photoluminescence Solar cell

a b s t r a c t The optical properties of nanocrystal-silicon (nc-Si) thin films on silicon nanopillar arrays after thermal annealing treatments have been investigated in this paper. The structure was prepared by SiO2 nanosphere lithography, reactive-ion etching, conformal deposition method and thermal annealing treatment, successively. The thickness of nc-Si thin film is about 100 nm, while the diameter, period and height of silicon nanopillar arrays are about 250 nm, 500 nm and 1 ␮m, respectively. By reflectance and photoluminescence (PL) spectroscopy measurements in visible to near-infrared wavelength range, a minimum reflectivity of about 0.7% has been demonstrated in such a kind of structure, meanwhile, a large variation (from 12.5 times of suppression to 8.8 times of enhancement compared to the flat referential thin films) of the light emission efficiencies of the samples depending on the thermal annealing temperatures has been observed, which is due to the temperature dependent modification of optical constants of nc-Si thin films. By combining the nc-Si with silicon nanopillar array, the structure will be very promising for the design of radial p–i–n junction solar cell devices. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Silicon nanostructures, including silicon nanocrystals (nc-Si) [1–5] and nanowires (SiNWs) or nanopillars [6], are promising candidates for next generation solar cells with higher energy conversion efficiencies and lower costs. In nc-Si, it has been observed that there is carrier multiplication by generation of two or more electron–hole pairs following the absorption of a single photon [1]. This may lead to improved photovoltaic (PV) efficiencies, which is close to the energy conservation limit [2]. Meanwhile, SiNW or nanopillar arrays are also very attractive for energy conversion for the new generation photovoltaic technology, since the structures’ extraordinary light-trapping path length enhancement factor is above the randomized scattering (Lambertian) limit [7–10] and is superior to other light trapping methods. Furthermore, the design of radial p–n junction structures on silicon nanopillars by conformal deposition method provides short travel distances of photo-excited minority carriers to the collection electrodes, leading to enhanced carrier-collection efficiency and minimum bulk

∗ Corresponding author at: Key Laboratory of Nanodevices and Applications, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou 215125, PR China. Tel.: +86 512 62872727. E-mail address: [email protected] (B. Qian). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.004

recombination [11–13]. Another advantage of the radial geometry is the high tolerance of PVs for material defects, permitting the use of lower-quality Si with shorter minority-carrier diffusion lengths [6]. Therefore, it will be interesting to combining nc-Si and silicon nanopillar arrays to realize solar cell devices to achieve higher PV efficiencies [14], and the radial p–i–n junction geometry will be a suitable design [11]. Different from the previous work [11] on single SiNW, in this paper, a hybrid structure of nc-Si thin films and Si nanopillar arrays were prepared by conformal deposition method. The optical properties including reflectance and photoluminescence spectra of such structures with different thermal annealing temperatures have been investigated. This work will be a promising step to the design of nc-Si based photonics devices. 2. Experimental details The samples were fabricated by five steps as following: (I) self-assembling of a close-packed 500 nm SiO2 nanosphere monolayer was performed on a clean Si wafer by dip-coating method [7,9]. The volume percent (V%) of SiO2 nanospheres in ethyl alcohol is 3–5%. (II) Si nanopillar array was formed by nanosphere lithography technology [15] and an inductively coupled-plasma deep reactive-ion-etching (ICP-DRIE) process. The selective and anisotropic etching process was based on fluorine chemistry

Y. Li et al. / Applied Surface Science 265 (2013) 324–328

325

Fig. 1. Real-time monitoring of our combined structure by SEM images during the whole fabrication process. (a) Closely self-assembled nanosphere monolayer via dipcoating. (b) Nanopillar array with spheres on the tips after ICP-DRIE. (c) Nanopillar array after removal of the residual mask. (d) Final combined structure after depositing a layer of nc-Si film upon pillar array. The inset of each picture is the sectional view. Scale bar of the inset is 1 ␮m.

using a mixture gas of C4 F8 , SF6 and O2 , with the flux ratio of 80 sccm:70 sccm:5 sccm. (III) The remaining nanospheres were selectively wiped off from the tips of the pillars by a buffered HF solution. (IV) 100 nm-thick Si-rich SiOx (x < 2) film was deposited conformably upon the nanopillar array to construct the final combined structure by plasma enhanced chemical vapor deposition (PECVD) [16] with the reaction gas flux ratio here was SiH4 /N2 O/Ar = 35 sccm/710 sccm/180 sccm. Note that the same film was also deposited on flat Si wafer for reference. (V) To form nc-Si, all the samples were annealed at temperatures of 1100 ◦ C, 1150 ◦ C, 1200 ◦ C, and 1250 ◦ C, respectively, for 30 min in N2 protection, which is a usual way to prepare Si nanocrystals [17,18]. The samples’ surface morphologies and sizes were characterized by Hitachi S4800 scanning electron microscope (SEM) in the whole process. High-resolution transmission electron microscopy (HRTEM) was also performed to confirm the existence of nc-Si. To study the optical properties, room-temperature photoluminescence (PL) spectra of our samples, excited by a He–Cd 325 nm laser and received by a CCD detector, were performed and analyzed with a grating monochromator. The reflectance spectra of nc-Si films on flat Si wafers and on nanopillar arrays were also measured. The refractive indices of nc-Si films on flat referential substrates were measured by a J.A. Woollam M-2000 Ellipsometer. 3. Results and discussion Fig. 1 gives the structure details in SEM images for each fabrication step. It can be confirmed in Fig. 1(a) that the large area, close-packed, hexagonal SiO2 nanosphere monolayer has be produced by dip-coating. Although minority line and point defects exist, they are negligible compared to the whole area. After 4 min ICP-DRIE process, the pattern of SiO2 nanosphere monolayer was transformed into silicon nanopillar array as shown in Fig. 1(b). The size of SiO2 nanospheres on nanopillars is reduced after etching. Following the nanosphere mask, the period of nanopillar array is

about 500 nm. The pillar array after wiping off the SiO2 nanospheres is presented in Fig. 1(c). The diameter and height of each nanopillar is about 250 nm and 1 ␮m, respectively. From the inset of Fig. 1(c), the bottom of pillar array is observed relatively larger than the other part, just like a collar around it. This is probably due to the etching gas distribution between nanopillars. Fig. 1(d) shows the final structure after depositing 100 nm SiOx film on pillar array. The height of each pillar remains 1 ␮m while the diameter increases to about 330 nm, from which thickness of nc-Si film along the radial direction can be calculated to be about 40 nm, which means the lateral growth rate of the film is largely reduced. Note that the roughness of pillars is increased because of the conformal growth effect. However, such roughness has been proved to increase the electrical injection efficiency [6]. To study the basic structure and optical properties of nc-Si thin films, the TEM and ellipsometry measurements were performed with the referential nc-Si thin films on flat Si wafers. Fig. 2(a) presents a typical TEM picture of a nc-Si thin film sample, in which nc-Si was obtained after 1200 ◦ C thermal annealing. The average size of the nc-Si is about 3.5 nm. Fig. 2(b) illustrates the evolution of refractive index for SiOx film on flat substrate annealed at different temperature. In the spectrum ranging from 350 nm to 1000 nm, it can be clearly observed that there is a monotonic decreasing of the refractive index when increasing the annealing temperature, which indicates increasingly widen of the optical band gap of the SiOx film [19–21]. This can be due to continuously decrease of the degree of disorder of the thin film, caused by the formation of nc-Si in it [22]. To demonstrate the extraordinary light trapping ability of Si nanopillar arrays [7–10,23], the reflectance spectra of samples with the same nc-Si films on flat Si wafers and on nanopillar arrays at different annealing temperature were measured for comparison. Fig. 3(a) illustrates the results of the samples annealed at 1250 ◦ C. It can be observed that the main band in the reflectance spectrum of the nc-Si film on flat wafer is ranging from 300 nm to 650 nm, the

326

Y. Li et al. / Applied Surface Science 265 (2013) 324–328

a

22 nc-Si film on flat Si wafer nc-Si film on nanopillar array

20 18

Reflectivity(%)

16

375nm 20.5%

14 12 10 8 6

375nm, 1.4%

4

550nm, 0.7%

2 0

300

400

500

600

700

800

900

1000

Wavelength(nm)

b 20

Reflectivity(%)

16

12

nc-Si film on flat wafer, min value nc-Si film on flat wafer, max value nc-Si film on nanopillar array, min value nc-Si film on nanopillar array, max value

8

4

0

1100

1150

1200

1250 O

Fig. 2. (a) High resolution TEM image of the nc-Si film annealed at 1200 ◦ C. The visible silicon nanocrystals are highlighted by circles for clarity with average size about 3.5 nm. (b) Curves of refractive indices of nc-Si thin films deposited on flat Si wafers with different annealing temperatures.

maximum value is about 20.5% at around 375 nm; for the wavelength longer than 650 nm, the reflectivity is lower than 8%. For the sample on nanopillar array, the reflectivity band of flat sample vanishes and the reflectivity at 375 nm is reduced to 1.4%, about 15 times reduction. The minimum reflectivity of the nanopillar sample is about 0.7% at about 550 nm, while the whole reflectance spectrum is lower than 3%. The value is lower than that of black silicon, developed in Eric Mazur’s laboratory by irradiating silicon using femtosecond laser pulse, the reflectivity of which is about 5% [23,24]. Fig. 3(b) shows the maximum and minimum reflectivities in all the reflectance spectra for samples annealed at different temperatures. It can be observed that, for flat samples, the maximum of the reflectivity is increasing from 12.8% to 21.9% with improving annealing temperature, while the minimum of reflectivity is increasing from 2.4% to 4.3%. For nanopillar array samples, the maximum value of reflectivity is decreasing from 2.4% to 1.5%, while the minimum reflectivity is, almost stay the same, lower than 1.0%. The results demonstrate such kind of structure on nanopillar array is very suitable for solar cell applications. Fig. 4(a) shows the schematic of the optical setup for PL measurements. The position of CCD detector is carefully adjusted to avoid the collection of input laser light. Following parts (b–e) of Fig. 4 present the PL spectra of the nc-Si films on nanopillar arrays and flat Si substrates at different annealing temperatures,

Annealing temperature ( C) Fig. 3. (a) Reflectance spectra of the nc-Si thin film on flat Si wafer (represented by squares) and on nanopillar array (represented by circles). Both the samples were annealed at 1250 ◦ C. (b) The maximum and minimum reflectivities of all the reflectance spectra for samples annealed at different temperatures.

respectively. From Fig. 4(b–e), two main emission bands centered around 430 nm and 790 nm can be observed in almost all of the samples, except the flat sample annealed at 1100 ◦ C in Fig. 4(b). The peak at around 430 nm is commonly attributed to the recombination at an oxygen-related defect state (O Si O) [25,26] related to nc-Si in SiO2 matrices [18,27–29]. The peak at around 790 nm in the near-infrared is generally assigned to the excitonic emission [30,31] from Si O state [32,33] on the surface of nc-Si [18]. It should be noted that the peak shift in Fig. 4(b) from flat sample to the related nanopillar structure is probably due to the change of ncSi size on the related combined structure at the lowest annealing temperature of 1100 ◦ C. Interestingly, it can be observed in Fig. 4(b–e) that the combined nanopillar array structure shows a rather large tunability to the emission efficiency of nc-Si by controlling the annealing temperature. When annealed at 1100 ◦ C, as described in Fig. 4(b), the PL intensity of the combined structure shows a big suppression compared with the related flat sample, with the maximum suppression factor of 12.5 at 670 nm. It acts as a role of inhibiting or trapping light emission, just like the anti-reflection surface for solar cells. At 1150 ◦ C in Fig. 4(c), the structure increases a little bit of the emission intensity at around 430 nm, while oppositely decreases that at around 800 nm. When annealing temperature increases to

Y. Li et al. / Applied Surface Science 265 (2013) 324–328

327

Fig. 4. Schematic diagram of PL testing (a) and comparison of PL lineshapes between the same nc-Si film deposited on flat Si wafer and our nanopillar array. Films were annealed at (b) 1100 ◦ C, (c) 1150 ◦ C, (d) 1200 ◦ C and (e) 1250 ◦ C, respectively. (f) The development of spectral-integrated emission intensity (from 390 nm to 950 nm) along with different annealing temperature for the same nc-Si film deposited both on flat wafer and nanopillar array.

1200 ◦ C in Fig. 4(d), the structure slightly reduces the intensity for both of the main emission bands above. At the highest annealing temperature of 1250 ◦ C, comparison between the PL lineshapes of the both reveals that the structure possesses a tremendously large enhancement ability. For the emission band centered around 430 nm, emission intensity of this combined structure is about 8.8 times as tall as that of the flat sample, while for the band at around 800 nm, the enhancement factor is about 5.5. Fig. 4(f) presents the development of spectral-integrated emission intensity (from 390 nm to 950 nm) along with different annealing temperature for the same nc-Si films deposited both on flat wafers and nanopillar arrays. The overall trend of the whole emission intensity for nc-Si on flat wafers decreases as improving annealing temperature, while that of nc-Si on nanopillar arrays increases. To be specific, the curve decreases quickly from 1100 ◦ C to 1150 ◦ C, for nc-Si on flat wafers, while increases little from 1150 ◦ C to 1200 ◦ C, then it goes down again. For nc-Si on nanopillar arrays, the total emission intensity increases slowly when improving annealing temperature from 1100 ◦ C to 1200 ◦ C, until it reaches 1250 ◦ C, the increase trend becomes apparent enough. Such evolutions of integrated emission intensity for both kinds of samples are probably related to the modifications of refractive indices of nc-Si films by thermal annealing, as illustrated in Fig. 2(b). For nc-Si on flat wafers, refractive indices of the films monotonically decrease when increasing annealing temperature, accompanied with the increase of the optical band gap of the nc-Si films [19–22]. This may lead to a continuous weakening of the light excitation efficiency, which could be the main reason for the reduction of the integrated emission intensity. For nc-Si on nanopillar arrays, the effect mentioned above for nc-Si on flat wafer still exists, however, another influence factor, the periodicity of the structure, also plays an important role at the same time. The variation of refractive indices induced optical periodic modification of the combined structure along different annealing temperature is probably the main reason for the trend of spectral-integrated emission intensity in Fig. 4(f). Effects caused by the variation of refractive indices described above could also be the main reason for the tunable PL

efficiency of the combined structure in Fig. 4(b–e). The results are very important and helpful for the design of nc-Si based photonic devices [6,10,34]. 4. Conclusion We have demonstrated a combined structure fabricated by conformally assembling nc-Si film on a periodic arranged silicon nanopillar array. By tuning the thermal annealing temperature, photoluminescence behavior of such kind of structure shows a wide-range tunability of light emission efficiency of nc-Si, from big suppression to large enhancement, compared with the relevant referential flat sample. The maximum emission suppression factor is about 12.5 meanwhile the maximum enhancement factor is 8.8. The reflectance spectra of this combined structure have revealed a remarkable reduction and the minimum reflectivity has been demonstrated to be 0.7%, which shows a great potential application for the design of solar cell devices. Acknowledgments The author is grateful to Nanofabrication Facility of Suzhou Institute of Nano-tech and Nano-bionics (SINANO) for the device fabrication. This work is supported by “973” project (no. 2010CB934100) and NSFC (no. 61107091). References [1] M.C. Beard, K.P. Knutsen, P. Yu, J.M. Luther, Q. Song, W.K. Metzger, R.J. Ellingson, A.J. Nozik, Multiple exciton generation in colloidal silicon nanocrystals, Nano Letters 7 (8) (2007) 2506–2512. [2] D. Timmerman, J. Valenta, K. Dohnalová, W.D.A.M. de Boer, T. Gregorkiewicz, Step-like enhancement of luminescence quantum yield of silicon nanocrystals, Nature Nanotechnology 6 (2011) 710–713. [3] C.-Y. Liu, Z.C. Holman, U.R. Kortshagen, Hybrid solar cells from P3HT and silicon nanocrystals, Nano Letters 9 (1) (2009) 449–452. [4] L. Pavesi, L. Dal Negro, C. Mazzoleni, G. Franzò, F. Priolo, Optical gain in silicon nanocrystals, Nature 408 (2000) 440–444.

328

Y. Li et al. / Applied Surface Science 265 (2013) 324–328

[5] M. Bedjaoui, B. Despax, M. Caumont, C. Bonafos, Si nanocrystal-containing SiOx (x < 2) produced by thermal annealing of PECVD realized thin films, Materials Science and Engineering B 124–125 (2005) 508–512. [6] K.-Q. Peng, S.-T. Lee, Silicon nanowires for photovoltaic solar energy conversion, Advanced Materials 29 (2011) 198–215. [7] E. Garnett, P. Yang, Light trapping in silicon nanowire solar cells, Nano Letters 10 (3) (2010) 1082–1087. [8] J. Zhu, Z. Yu, G.F. Burkhard, C.-M. Hsu, S.T. Connor, Y. Xu, Q. Wang, M. McGehee, S. Fan, Y. Cui, Optical absorption enhancement in amorphous silicon nanowire and nanocone arrays, Nano Letters 9 (1) (2009) 279–282. [9] C.-M. Hsu, S.T. Connor, M.X. Tang, Y. Cui, Wafer-scale silicon nanopillars and nanocones by Langmuir–Blodgett assembly and etching, Applied Physics Letters 93 (2008) 133109. [10] M.D. Kelzenberg, S.W. Boettcher, J.A. Petykiewicz, D.B. Turner-Evans, M.C. Putnam, E.L. Warren, J.M. Spurgeon, R.M. Briggs, N.S. Lewis, H.A. Atwater, Enhanced absorption and carrier collection in Si wire arrays for photovoltaic applications, Nature Materials 9 (2010) 239–244. [11] B. Tian, X. Zheng, T.J. Kempa, Y. Fang, N. Yu, G. Yu, J. Huang, C.M. Lieber, Coaxial silicon nanowires as solar cells and nanoelectronic power sources, Nature 449 (2007) 885–889. [12] L. Tsakalakos, J. Balch, J. Fronheiser, B.A. Korevaar, O. Sulima, J. Rand, Silicon nanowire solar cell, Applied Physics Letters 91 (2007) 233117. [13] E.C. Garnett, P. Yang, Silicon nanowire radial p–n junction solar cells, Journal of the American Chemical Society 130 (29) (2008) 9224–9225. [14] A.J. Nozik, Nanoscience and nanostructures for photovoltaics and solar fuels, Nano Letters 10 (8) (2010) 2735–2741. [15] J.C. Hulteen, R.P. Van Duyne, Nanosphere lithography: a materials general fabrication process for periodic particle array surfaces, Journal of Vacuum Science and Technology A 13 (1995) 1553–1558. [16] G.-R. Lin, C.-J. Lin, C.-T. Lin, Low-plasma and high-temperature PECVD grown silicon-rich SiOx film with enhanced carrier tunneling and light emission, Nanotechnology 18 (2007) 395202. [17] G. Vijaya Prakash, N. Daldosso, E. Degoli, F. Iacona, M. Cazzanelli, F. Rocca, G. Pucker, Z. Gaburro, P. Dalba, E. Ceretta Moreira, D. Pacifici, G. Franzò, F. Priolo, C. Arcangeli, A.B. Filonov, S. Ossicini, L. Pavesi, Structural and optical properties of silicon nanocrystals grown by plasma-enhanced chemical vapor deposition, Journal of Nanoscience and Nanotechnology 1 (2001) 159–168. [18] W.D.A.M. de Boer, D. Timmerman, K. Dohnalová, I.N. Yassievich, H. Zhang, W.J. Buma, T. Gregorkiewicz, Red spectral shift and enhanced quantum efficiency in phonon-free photoluminescence from silicon nanocrystals, Nature Nanotechnology 5 (2010) 878–884. [19] P. Hervé, L.K.J. Vandamme, General relation between refractive index and energy gap in semiconductors, Infrared Physics and Technology 35 (4) (1994) 609–615.

[20] N.M. Ravindra, P. Ganapathy, J. Choi, Energy gap-refractive index relations in semiconductors – an overview, Infrared Physics and Technology 50 (1) (2007) 21–29. [21] T.S. Moss, Relations between the refractive index and energy gap of semiconductors, Physica Status Solidi B 131 (2) (1985) 415–427. [22] R.A. Street, Hydrogenated Amorphous Silicon, Cambridge University Press, New York, 2005. [23] R. Younkin, J.E. Carey, E. Mazur, J.A. Levinson, C.M. Friend, Infrared absorption by conical silicon microstructures made in a variety of background gases using femtosecond-laser pulses, Journal of Applied Physics 93 (2003) 2626–2629. [24] C. Wu, C.H. Crouch, L. Zhao, J.E. Carey, R. Younkin, J.A. Levinson, E. Mazur, R.M. Farrell, P. Gothoskar, A. Karger, Near-unity below-band-gap absorption by microstructured silicon, Applied Physics Letters 78 (2001) 1850–1852. [25] A. Brewer, K. von Haeften, In situ passivation and blue luminescence of silicon clusters using a cluster beam/H2 O codeposition production method, Applied Physics Letters 94 (2009) 261102. ˚ K. Dohnalová, A. Aleknaviˇcius, O. [26] J. Valenta, A. Fucikova, I. Pelant, K. Kusová, Cibulka, A. Fojtík, G. Kada, On the origin of the fast photoluminescence band in small silicon nanoparticles, New Journal of Physics 10 (2008) 073022. [27] Y. Kanemitsu, Luminescence properties of nanometer-sized Si crystallites: core and surface states, Physical Review B 49 (1994) 16845–16848. [28] A. Puzder, A.J. Williamson, J.C. Grossman, G. Galli, Surface chemistry of silicon nanoclusters, Physical Review Letters 88 (2002) 097401. [29] S. Rao, K. Mantey, J. Therrien, A. Smith, M. Nayfeh, Molecular behavior in the vibronic and excitonic properties of hydrogenated silicon nanoparticles, Physical Review B 76 (2007) 155316. [30] G.F. Grom, D.J. Lockwood, J.P. McCaffrey, H.J. Labbé, P.M. Fauchet, B. White Jr., J. Diener, D. Kovalev, F. Koch, L. Tsybeskov, Ordering and self-organization in nanocrystalline silicon, Nature 407 (2000) 358–361. [31] B. Delley, E.F. Steigmeier, Quantum confinement in Si nanocrystals, Physical Review B 47 (1993) 1397–1400. [32] S. Godefroo, M. Hayne, M. Jivanescu, A. Stesmans, M. Zacharias, O.I. Lebedev, G. Van Tendeloo, V.V. Moshchalkov, Classification and control of the origin of photoluminescence from Si nanocrystals, Nature Nanotechnology 3 (2008) 174–178. [33] M.V. Wolkin, J. Jorne, P.M. Fauchet, G. Allan, C. Delerue, Electronic states and luminescence in porous silicon quantum dots: the role of oxygen, Physical Review Letters 82 (1999) 197–200. [34] J.J. Wierer, M.R. Krames, J.E. Epler, N.F. Gardner, M.G. Craford, J.R. Wendt, J.A. Simmons, M.M. Sigalas, InGaN/GaN quantum-well heterostructure lightemitting diodes employing photonic crystal structures, Applied Physics Letters 84 (2004) 3885–3887.