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Silicon nanocrystals as fast and efficient light emitters for optical gain
Journal of Non-Crystalline Solids 358 (2012) 2130–2133
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Silicon nanocrystals as fast and efficient light emitters for optical gain Kateřina Kůsová Institute of Physics of the Academy of Sciences of the Czech Republic, Cukrovarnická 10, 162 00 Prague 6, Czech Republic
a r t i c l e
i n f o
Article history: Received 15 August 2011 Received in revised form 24 October 2011 Available online 6 December 2011 Keywords: Silicon nanocrystals; Photoluminescence; Surface capping; Colloidal dispersion
a b s t r a c t Light-emission and optical gain in a silicon-based material is an intensely studied scientific topic due to its possible application impact. In this article, we study colloidal dispersion of silicon nanocrystals with methyl-based capping. Excellent optical properties of this material include reasonably high luminescence quantum yield (20%) and fast radiative lifetime (10 ns), comparable to direct-bandgap semiconductors. Moreover, luminescence maximum situated in the yellow spectral region (570 nm) indicates lowered freecarrier absorption. Photoluminescence and FTIR measurements combined with ultrafine filtration show that the nanocrystals are dispersed in the solvent as single particles or small clusters, implying that scattering-related losses are also nearly eliminated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Photonics has numerous advantages over electronics when it comes to information transfer, most notably higher bandwidth and much lower energy dissipation [1-4]. Up to now, photonics has been used for the transfer of information only over long distances, connecting separate computers into networks. If this photonics concept were pushed further, the benefits of photonics could be exploited for information transfer also on a shorter-distance level, such as connecting computer peripherals, inter- or, eventually, intra-board connections. However, in order to cross the integration borderline and make photonic components cost-effective and thus available for every-day use, photonic circuitry needs to be integrated onto the silicon platform. The idea of CMOS photonics, or more precisely speaking optoelectronics, is being explored by large technology corporations such as Intel [4], IBM or Imec for quite some time. Moreover, smaller companies focused specially on integrated silicon photonics start to emerge: e.g. Luxtera, a Caltech spin-off, has already launched a world's first integrated opto-electronic transceiver. When integrating photonics onto the silicon platform, several problems need to be tackled, such as fast modulation in silicon [5], i.e. a centrosymmetric material; the greatest challenge so far, however, remains to be an electrically driven silicon-integrated light source, preferably a laser [6]. As bulk silicon itself is a very poor light emitter (quantum efficiency as low as 10 − 6), alternative solutions are sought after. It is either possible to find a way to integrate existing III–V lasers onto silicon [6], or one has to investigate new, silicon-based lightemitting materials [7]. However, a silicon-based laser would still be advantageous, allowing for smaller sizes and true integration.
E-mail address: [email protected]. 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.11.027
The problem of lasing in silicon is still addressed in fundamental research since true lasing has never been achieved in silicon [6]. A device probably the closest to laser action in silicon was the so-called Raman laser [8] by Intel; however, as it is based on stimulated Raman scattering and not on stimulated emission, it can never be electrically driven and requires external optical pumping. Various approaches towards making silicon luminesce and lase have been undertaken [7], including luminescence from optically active impurities (most notably erbium) and defects (e.g. dislocation loops [9]). Nevertheless, in our opinion, the most promising candidate for lasing in a silicon-based material are silicon nanocrystals [10] (SiNcs), among other reasons because both luminescence with high quantum yield [11] and optical gain [12-14] (single-passage light amplification, a prerequisite for lasing) have been achieved in SiNcs. (Nevertheless, it is noteworthy to say that the accurate identification of optical gain, especially for low gain values, and sometimes also of lasing [15], can be tricky and must be carefully assessed to avoid false positive results [16].) Moreover, electrical injection into SiNcs is possible [17,18], although tricky, and efficient SiNc-based lightemitting diodes have already been fabricated. The requirements for the maximization of optical gain to eventually attain lasing naturally consist in the trade-off between maximizing the emission and minimizing losses at the same time. Thus, these requirements can be summarized into several items that should be addressed when considering optical gain in SiNcs. (i) High luminescence quantum yield. Typically, SiNcs have luminescence quantum yields of several per cent. Much higher values have been achieved [11], but are often unstable with time. Higher efficiencies are expected for shorter-wavelength emission. (ii) Minimization of free-carrier absorption [10], an important competing channel for optical gain in silicon; it decreases at
K. Kůsová / Journal of Non-Crystalline Solids 358 (2012) 2130–2133 Table 1 Comparison of gain-related properties of SiNc:O and SiNc:C. The values of concentration of nanocrystals in the last column are order-of-magnitude estimates.
SiNc:O SiNc:C
PL
τrad
QE
nc/cm3
red yellow
300 μs 10 ns
2–3% 20%
1019 1017
shorter emission wavelengths (αFCA ∼ λ 2). Interestingly, in accordance with (i) and (ii), higher optical gain with shorter emission wavelength in SiNcs has been observed ( [4], p. 20). (iii) Sufficiently high concentration of SiNcs. (iv) Minimization of scattering-related losses. An important issue, since SiNcs often heavily agglomerate (especially with a native-oxide surface). Naturally, a trade-off between (iii) and (iv) needs to be found. In the following, we address the above issues of light emission in SiNcs. We study a model sample of a colloidal dispersion of SiNcs with methyl-based capping and reasonably high quantum efficiency of 20% stable in time. Short radiative lifetime (10 ns) characterizing these nanocrystals is comparable to that of efficient direct-bandgap semiconductors. Luminescence situated in the yellow spectral region (570 nm) indicates lower free-carrier absorption in comparison with traditionally studied oxidized SiNcs (600 nm and more). Moreover, we show, using photoluminescence and FTIR measurements combined with ultrafine filtration, that the nanocrystals are dispersed in the solvent as single particles or small clusters, which implies that scattering-related losses are nearly eliminated. 2. Experimental Oxidized SiNcs (SiNcs:O) were prepared by electrochemical etching of monocrystalline Si wafers in a solution of HF, ethanol and hydrogen peroxide and subsequent mechanical pulverization. Details can be found in [20]. The photo-chemical procedure for the preparation of SiNcs with methyl-based capping (SiNcs:C) is based on dispersing the SiNc:O powder in a mixture of aromatic hydrocarbons. This suspension undergoes irradiation with a 325-nm HeCd laser, which triggers the surface modification. Details on the preparation can be found in [19]. The filtration is carried out using filtration units. In all cases, prefiltration using Millipore Ultrafree MC centrifugal filters (pore sizes 650 nm, 3 200 g, 90 s) was applied. Other filtration units and procedures are as follows: 220 nm (Millipore Millex-GV syringe filter), 100 nm (Millipore Ultrafree MC centrifugal filters, 7800 g, 2 min), 35 nm (Pall Nanosep, 300 kDa, 7800 g, 3 min) and 10 nm (Pall Nanosep, 100 kDa, 7800 g, 10 min). Nominal pore sizes are stated by the
a
b
2131
manufacturer, durations of centrifugation apply for 0.5 ml of the sample. These filtration units were chosen because they are compatible with our solvent. The PL decay of the SiNc:O was excited using 355-nm line from a pulsed Nd:YAG laser (Ekspla, 3 rd harmonics, 8 ns pulse duration, 10 Hz repetition rate) and detected with Andor iCCD camera (minimal optical gate width 3.81 ns, irising time 0.20 ns) coupled with Andor Shamrock SR163i imaging spectrograph. The standard error of the stretched-exponential fit amounts to ∼ 2.5%. The fast PL decay of SiNcs:C was acquired using EasyLife X fluorescence lifetime system (excitation LED 450 nm, detection 550 nm). The fit was performed using the original EasyLife software, the error is the standard deviation as evaluated by the software and amounts to ∼ 3.5% of the measured value. The presented cw PL spectra of SiNcs:c were excited using 442-nm line of HeCd laser (325 nm for the oxidized aggregates, respectively) and detected using a grating spectrometer coupled with Andor CCD camera. All the presented PL spectra are corrected for the spectral response of the whole collection system. Wavelength axis was calibrated using a pen-ray Hg:Ar light source. The difference between tabulated and calibrated line positions was b 0.1 nm for 47 lines, wavelength step in the spectrum is b 0.2nm. Quantum efficiency measurements of the SiNc:O were carried out using absolute method and integration sphere. Quantum efficiency of SiNc:C was measured by comparing the PL of the sample with that of rhodamine 6 G standard [21]. The overall error can be estimated to 10%. Thus, the maximum estimated of the error of the computed radiative lifetimes values in Table 1 is about 10%. The concentration of nanocrystals in Table 1 was estimated from the amount of the SiNc powder directly used for the preparation of the sample and the measured thickness of the resulting Si:Nc layer in the case of SiNc:O and from absorption measurements in the case of SiNc:C. Both values are order-of-magnitude estimates. The FTIR spectra were collected using FTIR spectrometer Nicolet Nexus 860 equipped with IR light source and 20 W halogen lamp, KbR and CaF2 beamsplitters, standard DTGS pyrodetector and highsensitivity liquid-nitrogen-cooled MCT photodiode as detectors. Wavenumber step of 2 cm − 1 was applied. To reduce H2O and CO2, the spectrometer is continuously purged with nitrogen blow. The ATR accessory Specac Gateway 6 Reflection Horizontal ATR with ZnSe prism was employed.
3. Results 3.1. Silicon nanocrystals with oxidized surface The prepared silicon nanocrystals with oxidized surface (SiNc:O) have the size of the crystalline core of about 2.5–3 nm [20]. Their
c
Fig. 1. Photoluminescence of SiNcs with oxidized surface (red curve) and metyl-based capping (yellow curve). Panel (a) shows a comparison of PL spectra whereas panel (b) compares PL decays (note the different time scales). (c) PL spectra of methyl-capped SiNcs (black curve: as-prepared sample, red curve: filtered sample, 220 nm) are compared to the photoluminescence of the solvent (green curve). The detected scattered excitation laser line (442 nm) serves as a measure of scattering present in the sample. Panels (a) and (b) reprinted with permission from [19]. Copyright 2010 American Chemical Society.
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K. Kůsová / Journal of Non-Crystalline Solids 358 (2012) 2130–2133
Fig. 2. Comparison of FTIR-ATR spectra of oxidized (black curve) and methyl-capped (red curve) SiNcs. The as-prepared SiNcs:C were dried out and redispersed in ethanol to get rid of the parasitic signal of the solvent. The ethanol-dispersed SiNcs:Cwere subject to further filtration (pore sizes 100 nm). Reprinted with permission from [19]. Copyright 2010 American Chemical Society.
photoluminescence (PL) is quite typical for SiNcs as shown in Fig. 1(a) and (b), characterized by slow-decaying (τ∼ 10 µs) band with PL maximum between 600 and 670 nm (depending on the exact preparation procedure). This band is generally assumed to be due to an interplay of quantum confinement and (oxide-related) surface states [22, 23]. Positive optical gain has been repeatedly measured on samples with SiNc:O embedded in SiO2 matrix [24-26]. Nevertheless, even though we tried to combine these SiNcs with a resonator, no real lasing was observed [24]. The absence of lasing was attributed to the presence of large aggregates of SiNcs in the sample and resulting considerable scatteringrelated losses. The ensemble quantum efficiency of the emission at the slow orange/red band reaches 2–3%. As both decay lifetime τ and PL quantum efficiency η have been measured, we can deduce the radiative lifetime τrad (τrad = τ/η). In SiNcs:O, it amounts to τrad ≈ 300 µs, which is still far from the radiative lifetime of direct-bandgap semiconductors (the range of nanoseconds), where both optical gain and lasing are routinely observed. 3.2. Silicon nanocrystals with methyl-based capping It is possible to significantly improve the PL properties of SiNcs:O by applying a photo-chemical treatment resulting in the replacement of the oxide surface capping with passivation based on methyl groups (–CH3), as evidenced by FTIR spectra. Whereas the FTIR spectra of SiNc:O are dominated by wide Si–O–Si bridge-bond bands (975– 1130 cm − 1 and 795 cm − 1) with some silanol Si–OH (940 and
a
b
c
Fig. 3. Photographs of luminescence of: (a) oxidized SiNcs in ethanol (reprinted with permission of Elsevier [27]) and (b) the colloid of SiNcs with methyl-based capping. Substantially suppressed light scatteringin the colloid is apparent. (c) The colloid is shown also in ambient light.
875 cm − 1) and oxidized hydride groups OxSiHy (833 cm − 1), a characteristic Si–CH3 vibration (852 and 677 cm − 1) appears in the SiNcs:C, see Fig. 2. (The same conclusion is also strongly backed up by nuclear magnetic resonance measurements explained in more details elsewhere [19]). The significantly altered PL properties include a blue-shift in the PL spectrum (down to 570 nm) as well as a dramatic change in PL lifetime down to as little as 2 ns (see Fig. 1(a) and (b)). Moreover, the measured PL quantum efficiency (20%) allows us to directly estimate the radiative lifetime (10 ns), which indicates that luminescence performance of these SiNcs:C is comparable to that of direct-bandgap semiconductor nanocrystals. It is important to point out that this luminescence performance is stable in time and does not spontaneously deteriorate. Apart from enhanced luminescence, the colloidal dispersion of nanometer-sized particles was meant as a parallel to laser dyes, which exhibit very low light-scattering losses. When inspected with a naked eye, our SiNc:C samples appear optically clear (see Fig. 3 for comparison of turbidity). A possible simple measure of turbidity of a sample can be based on a simultaneous collection of a PL spectrum and the scattered excitation laser line. Fig. 1(c) shows such PL spectra of SiNcs:C (needless to say that the mere fact that the detection of the scattered excitation laser and a PL spectrum at the same time is possible, implies high optical quality). In the as-prepared sample (black curve; sample was let to settle down for about 20 min), some scattering is still present. However, if the sample undergoes filtration (filter pore size 220 nm), the scattering starts to be much lower (red curve in Fig. 1(c)). At the filtered sample, the amplitude of the collected intensity at 442 nm nearly reaches that of the solvent (which is most probably the result of Raman scattering in the solvent). Clearly, the PL intensity is nearly unaffected by the filtration procedure (although unfiltered samples tend to have slightly lower PL intensity than the filtered ones, Fig. 1(c)). We verified that the same applies also for filtrations using smaller filter pore sizes of 100 and 35 nm; only when the nominal pore size reached 10 nm, some decrease in PL intensity (∼ 8%) was observed. This indicates (in agreement with dynamic-light-scattering measurements published in [19]) that the SiNcs are dispersed in the solvent as single particles or small cluster of just a few particles, implying high optical quality. 4. Discussion We would like to point out here that our methyl-capped SiNcs exhibit quite unique PL properties: the yellow luminescence (570 nm) is situated in the spectral region which is generally difficult to reach in SiNcs and the radiative lifetime is fast (10 ns). To the best of our knowledge, the only SiNcs with comparable PL properties were octanethiol-capped SiNcs by English et al. [28]. The fast yellow luminescence can be qualitatively understood assuming the traditional model that the slow orange/red PL coming from SiNcs:O is due to oxide surface states [22]. If the surface oxide is removed, other recombination channels are made possible. We can tentatively expect that this luminescence is connected directly with the core of the nanocrystal, but further investigations are necessary to support this conclusion. Regarding the optical-gain-related properties, the SiNc:C samples show clear improvement, see Table 1. First of all, the blueshift of the PL spectrum implies lowered free-carrier absorption and very probably participates also in the increase of quantum efficiency (from 2–3% to 20%). Secondly, the significantly shorter radiative lifetime (10 ns versus 300 µs) indicates enhanced radiative recombination. Nevertheless, probably the most important property here is the very high optical quality of the resulting SiNc:C sample as it is the key obstacle towards achieving lasing in SiNcs [24]. The change is, however, quite difficult to quantify. The difference is clear in naked-eye observation (compare Fig. 3(a) and (b)) and, moreover, what very well illustrates the improvement is the fact that whereas the signal of the scattered
K. Kůsová / Journal of Non-Crystalline Solids 358 (2012) 2130–2133
light from the exciting laser line can be measured together with a PL spectrum in the case of the SiNc:C sample (Fig. 1(c)), the same measurement of the SiNc:O sample saturates the CCD detector. It is also important to point out here that the problem of high scatteringrelated losses in connection with SiNcs is universal because SiNcs are generally highly prone to oxidation of the surface the formation of oxide bridge bonds Si–O–Si is the main cause of agglomeration of SiNcs into large, light-scattering aggregates [29]. Therefore, finding a suitable type of surface passivation that can prevent oxidation and agglomeration even over longer-time storage is of immense importance and the proposed methyl-based capping is a prime example of such stable passivation. What remains a problem here, however, is the sufficiently high concentration of nanocrystals, which is a bit more difficult to achieve in our SiNcs:C because our preparation procedure is time-consuming. In order to solve this problem, we want to prepare a larger number of batches of the sample and concentrate them into a single colloid. 5. Conclusions Our silicon nanocrystals with methyl-based capping successfully address several issues that need to be tackled if high optical gain is to be achieved in silicon nanocrystals. Reasonably high quantum yield (20%) and fast radiative lifetime, comparable to directbandgap semiconductors, are achieved. Luminescence maximum is blue-shifted to the yellow spectral region (570 nm), implying lowered free-carrier absorption. Moreover, the nanocrystals are dispersed in the solvent as single particles or small clusters, ensuring high optical quality of the colloid. Acknowledgements This work was supported by the projects IAA101120804 and KJB100100903 of the Grant Agency of the Academy of Sciences, by the Czech Ministry of Education, Youth and Sports through the research center LC510, by the Institutional Research Plan AV0Z10100521, by the Academy of Sciences of the Czech Republic KAN400100701 and by the Technology Agency of the Czech Republic through the project TA01020972. Zdeněk Remeš is acknowledged for experimental assistance with FTIR measurements.
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References [1] N. Daldosso, L. Pavesi, Laser Photonics Rev. 3 (2009) 508–534. [2] L. Khriachtchev (Ed.), Silicon Nanophotonics: Basic Principles, Present Status and Perspectives, World Scientific, Singapore, 2009. [3] L. Pavesi, J. Phys. Condens. Matter 15 (2003) R1169–R1196. [4] L. Pavesi, D. Lockwood (Eds.), Silicon Photonics, volume 94 of Topics in Applied Physics, Springer-Verlag, Hiedelberg, 2004. [5] A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, M. Paniccia, Nature 427 (2004) 615–618. [6] G. Roelkens, L. Liu, D. Liang, R. Jones, A. Fang, B. Koch, J. Bowers, Laser Photonics Rev. 4 (2010) 751–779. [7] J.M. Shainline, J. Xu, Laser Photonics Rev. 1 (2007) 334–348. [8] O. Ozdal, B. Jalali, Opt. Express 12 (2004) 5269–5273. [9] W. Ng, M. Lourenco, R. Gwilliam, S. Ledain, G. Shao, K. Homewood, Nature 410 (2001) 192. [10] I. Pelant, Phys. Status Solidi A 208 (2011) 625–630. [11] D. Jurbergs, E. Rogojina, L. Mangolini, U. Kortshagen, Appl. Phys. Lett. 88 (2006) 233116. [12] L. Pavesi, L.D. Negro, L. Mazzoleni, G. Franzò, F. Priolo, Nature 408 (2000) 440–444. [13] K. Luterová, K. Dohnalová, F. Trojánek, K. Neudert, P. Gilliot, B. Hönerlage, P. Malý, I. Pelant, J. Non-Cryst. Solids 352 (2006) 3041–3046. [14] J. Ruan, P. Fauchet, L.D. Negro, M. Cazzaneli, L. Pavesi, Appl. Phys. Lett. 83 (2003) 5479. [15] J. Valenta, I. Pelant, Appl. Phys. Lett. 93 (2008) 066101. [16] J. Valenta, I. Pelant, J. Linnros, Appl. Phys. Lett. 81 (2002) 1396. [17] R. Walters, H. Atwater, G. Bourianoff, Nat. Mater. 4 (2005) 143–146. [18] K.-Y. Cheng, R. Anthony, U.R. Kortshagen, R.J. Holmes, Nano Lett. 11 (2011) 1952–1956. [19] K. Kůsová, O. Cibulka, K. Dohnalová, I. Pelant, J. Valenta, A. Fučíková, K. Žídek, J. Lang, J. Englich, P. Matějka, P. Štěpánek, S. Bakardjieva, ACS Nano 4 (2010) 4495–4504. [20] K. Dohnalová, L. Ondič, K. Kůsová, I. Pelant, J.L. Rehspringer, R.-R. Mafouana, J. Appl. Phys. 107 (2010) 053102. [21] K. Kůsová, O. Cibulka, K. Dohnalová, I. Pelant, P. Matějka, K. Žídek, J. Valenta, F. Trojánek, Mater. Res. Soc. Symp. Proc. 1145E (2009) 1145–MM04–13; T. van Buuren, L. Tsybeskov, S. Fukatsu, L. Dal Negro, F. Gourbilleau (Eds.), Applications of Group IV Semiconductor Nanostructures, 2009, Warrendale, PA. [22] M.V. Wolkin, J. Jorne, P.M. Fauchet, G. Allan, C. Delerue, Phys. Rev. Lett. 82 (1999) 197–200. [23] K. Dohnalová, K. Kůsová, I. Pelant, Appl. Phys. Lett. 94 (2009) 211903. [24] K. Dohnalová, I. Pelant, K. Kůsová, P. Gilliot, M. Galart, O. Crégut, J.-L. Rehspringer, B. Hőnerlage, T. Ostatnický, S. Bakardjieva, New J. Phys. 10 (2008) 063014. [25] K. Dohnalová, K. Žídek, L. Ondič, K. Kůsová, O. Cibulka, I. Pelant, J. Phys. D Appl. Phys. 42 (2009) 135102. [26] K. Dohnalová, K. Kůsová, O. Cibulka, L. Ondič, I. Pelant, Phys. Scr. T141 (2010) 014011. [27] K. Kůsová, O. Cibulka, K. Dohnalová, I. Pelant, A. Fučíková, J. Valenta, Physica E 41 (2009) 982–985. [28] D. English, L. Pell, Z. Yu, P. Barbara, B. Korgel, Nano Lett. 2 (2002) 681–685. [29] E. Rogozhina, D. Eckhoff, E. Gratton, P. Braun, J. Mater. Chem. 16 (2006) 1421–1430.
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
Silicon nanocrystals as fast and efficient light emitters for optical gain Kateřina Kůsová Institute of Physics of the Academy of Sciences of the Czech Republic, Cukrovarnická 10, 162 00 Prague 6, Czech Republic
a r t i c l e
i n f o
Article history: Received 15 August 2011 Received in revised form 24 October 2011 Available online 6 December 2011 Keywords: Silicon nanocrystals; Photoluminescence; Surface capping; Colloidal dispersion
a b s t r a c t Light-emission and optical gain in a silicon-based material is an intensely studied scientific topic due to its possible application impact. In this article, we study colloidal dispersion of silicon nanocrystals with methyl-based capping. Excellent optical properties of this material include reasonably high luminescence quantum yield (20%) and fast radiative lifetime (10 ns), comparable to direct-bandgap semiconductors. Moreover, luminescence maximum situated in the yellow spectral region (570 nm) indicates lowered freecarrier absorption. Photoluminescence and FTIR measurements combined with ultrafine filtration show that the nanocrystals are dispersed in the solvent as single particles or small clusters, implying that scattering-related losses are also nearly eliminated. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Photonics has numerous advantages over electronics when it comes to information transfer, most notably higher bandwidth and much lower energy dissipation [1-4]. Up to now, photonics has been used for the transfer of information only over long distances, connecting separate computers into networks. If this photonics concept were pushed further, the benefits of photonics could be exploited for information transfer also on a shorter-distance level, such as connecting computer peripherals, inter- or, eventually, intra-board connections. However, in order to cross the integration borderline and make photonic components cost-effective and thus available for every-day use, photonic circuitry needs to be integrated onto the silicon platform. The idea of CMOS photonics, or more precisely speaking optoelectronics, is being explored by large technology corporations such as Intel [4], IBM or Imec for quite some time. Moreover, smaller companies focused specially on integrated silicon photonics start to emerge: e.g. Luxtera, a Caltech spin-off, has already launched a world's first integrated opto-electronic transceiver. When integrating photonics onto the silicon platform, several problems need to be tackled, such as fast modulation in silicon [5], i.e. a centrosymmetric material; the greatest challenge so far, however, remains to be an electrically driven silicon-integrated light source, preferably a laser [6]. As bulk silicon itself is a very poor light emitter (quantum efficiency as low as 10 − 6), alternative solutions are sought after. It is either possible to find a way to integrate existing III–V lasers onto silicon [6], or one has to investigate new, silicon-based lightemitting materials [7]. However, a silicon-based laser would still be advantageous, allowing for smaller sizes and true integration.
E-mail address: [email protected]. 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.11.027
The problem of lasing in silicon is still addressed in fundamental research since true lasing has never been achieved in silicon [6]. A device probably the closest to laser action in silicon was the so-called Raman laser [8] by Intel; however, as it is based on stimulated Raman scattering and not on stimulated emission, it can never be electrically driven and requires external optical pumping. Various approaches towards making silicon luminesce and lase have been undertaken [7], including luminescence from optically active impurities (most notably erbium) and defects (e.g. dislocation loops [9]). Nevertheless, in our opinion, the most promising candidate for lasing in a silicon-based material are silicon nanocrystals [10] (SiNcs), among other reasons because both luminescence with high quantum yield [11] and optical gain [12-14] (single-passage light amplification, a prerequisite for lasing) have been achieved in SiNcs. (Nevertheless, it is noteworthy to say that the accurate identification of optical gain, especially for low gain values, and sometimes also of lasing [15], can be tricky and must be carefully assessed to avoid false positive results [16].) Moreover, electrical injection into SiNcs is possible [17,18], although tricky, and efficient SiNc-based lightemitting diodes have already been fabricated. The requirements for the maximization of optical gain to eventually attain lasing naturally consist in the trade-off between maximizing the emission and minimizing losses at the same time. Thus, these requirements can be summarized into several items that should be addressed when considering optical gain in SiNcs. (i) High luminescence quantum yield. Typically, SiNcs have luminescence quantum yields of several per cent. Much higher values have been achieved [11], but are often unstable with time. Higher efficiencies are expected for shorter-wavelength emission. (ii) Minimization of free-carrier absorption [10], an important competing channel for optical gain in silicon; it decreases at
K. Kůsová / Journal of Non-Crystalline Solids 358 (2012) 2130–2133 Table 1 Comparison of gain-related properties of SiNc:O and SiNc:C. The values of concentration of nanocrystals in the last column are order-of-magnitude estimates.
SiNc:O SiNc:C
PL
τrad
QE
nc/cm3
red yellow
300 μs 10 ns
2–3% 20%
1019 1017
shorter emission wavelengths (αFCA ∼ λ 2). Interestingly, in accordance with (i) and (ii), higher optical gain with shorter emission wavelength in SiNcs has been observed ( [4], p. 20). (iii) Sufficiently high concentration of SiNcs. (iv) Minimization of scattering-related losses. An important issue, since SiNcs often heavily agglomerate (especially with a native-oxide surface). Naturally, a trade-off between (iii) and (iv) needs to be found. In the following, we address the above issues of light emission in SiNcs. We study a model sample of a colloidal dispersion of SiNcs with methyl-based capping and reasonably high quantum efficiency of 20% stable in time. Short radiative lifetime (10 ns) characterizing these nanocrystals is comparable to that of efficient direct-bandgap semiconductors. Luminescence situated in the yellow spectral region (570 nm) indicates lower free-carrier absorption in comparison with traditionally studied oxidized SiNcs (600 nm and more). Moreover, we show, using photoluminescence and FTIR measurements combined with ultrafine filtration, that the nanocrystals are dispersed in the solvent as single particles or small clusters, which implies that scattering-related losses are nearly eliminated. 2. Experimental Oxidized SiNcs (SiNcs:O) were prepared by electrochemical etching of monocrystalline Si wafers in a solution of HF, ethanol and hydrogen peroxide and subsequent mechanical pulverization. Details can be found in [20]. The photo-chemical procedure for the preparation of SiNcs with methyl-based capping (SiNcs:C) is based on dispersing the SiNc:O powder in a mixture of aromatic hydrocarbons. This suspension undergoes irradiation with a 325-nm HeCd laser, which triggers the surface modification. Details on the preparation can be found in [19]. The filtration is carried out using filtration units. In all cases, prefiltration using Millipore Ultrafree MC centrifugal filters (pore sizes 650 nm, 3 200 g, 90 s) was applied. Other filtration units and procedures are as follows: 220 nm (Millipore Millex-GV syringe filter), 100 nm (Millipore Ultrafree MC centrifugal filters, 7800 g, 2 min), 35 nm (Pall Nanosep, 300 kDa, 7800 g, 3 min) and 10 nm (Pall Nanosep, 100 kDa, 7800 g, 10 min). Nominal pore sizes are stated by the
a
b
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manufacturer, durations of centrifugation apply for 0.5 ml of the sample. These filtration units were chosen because they are compatible with our solvent. The PL decay of the SiNc:O was excited using 355-nm line from a pulsed Nd:YAG laser (Ekspla, 3 rd harmonics, 8 ns pulse duration, 10 Hz repetition rate) and detected with Andor iCCD camera (minimal optical gate width 3.81 ns, irising time 0.20 ns) coupled with Andor Shamrock SR163i imaging spectrograph. The standard error of the stretched-exponential fit amounts to ∼ 2.5%. The fast PL decay of SiNcs:C was acquired using EasyLife X fluorescence lifetime system (excitation LED 450 nm, detection 550 nm). The fit was performed using the original EasyLife software, the error is the standard deviation as evaluated by the software and amounts to ∼ 3.5% of the measured value. The presented cw PL spectra of SiNcs:c were excited using 442-nm line of HeCd laser (325 nm for the oxidized aggregates, respectively) and detected using a grating spectrometer coupled with Andor CCD camera. All the presented PL spectra are corrected for the spectral response of the whole collection system. Wavelength axis was calibrated using a pen-ray Hg:Ar light source. The difference between tabulated and calibrated line positions was b 0.1 nm for 47 lines, wavelength step in the spectrum is b 0.2nm. Quantum efficiency measurements of the SiNc:O were carried out using absolute method and integration sphere. Quantum efficiency of SiNc:C was measured by comparing the PL of the sample with that of rhodamine 6 G standard [21]. The overall error can be estimated to 10%. Thus, the maximum estimated of the error of the computed radiative lifetimes values in Table 1 is about 10%. The concentration of nanocrystals in Table 1 was estimated from the amount of the SiNc powder directly used for the preparation of the sample and the measured thickness of the resulting Si:Nc layer in the case of SiNc:O and from absorption measurements in the case of SiNc:C. Both values are order-of-magnitude estimates. The FTIR spectra were collected using FTIR spectrometer Nicolet Nexus 860 equipped with IR light source and 20 W halogen lamp, KbR and CaF2 beamsplitters, standard DTGS pyrodetector and highsensitivity liquid-nitrogen-cooled MCT photodiode as detectors. Wavenumber step of 2 cm − 1 was applied. To reduce H2O and CO2, the spectrometer is continuously purged with nitrogen blow. The ATR accessory Specac Gateway 6 Reflection Horizontal ATR with ZnSe prism was employed.
3. Results 3.1. Silicon nanocrystals with oxidized surface The prepared silicon nanocrystals with oxidized surface (SiNc:O) have the size of the crystalline core of about 2.5–3 nm [20]. Their
c
Fig. 1. Photoluminescence of SiNcs with oxidized surface (red curve) and metyl-based capping (yellow curve). Panel (a) shows a comparison of PL spectra whereas panel (b) compares PL decays (note the different time scales). (c) PL spectra of methyl-capped SiNcs (black curve: as-prepared sample, red curve: filtered sample, 220 nm) are compared to the photoluminescence of the solvent (green curve). The detected scattered excitation laser line (442 nm) serves as a measure of scattering present in the sample. Panels (a) and (b) reprinted with permission from [19]. Copyright 2010 American Chemical Society.
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Fig. 2. Comparison of FTIR-ATR spectra of oxidized (black curve) and methyl-capped (red curve) SiNcs. The as-prepared SiNcs:C were dried out and redispersed in ethanol to get rid of the parasitic signal of the solvent. The ethanol-dispersed SiNcs:Cwere subject to further filtration (pore sizes 100 nm). Reprinted with permission from [19]. Copyright 2010 American Chemical Society.
photoluminescence (PL) is quite typical for SiNcs as shown in Fig. 1(a) and (b), characterized by slow-decaying (τ∼ 10 µs) band with PL maximum between 600 and 670 nm (depending on the exact preparation procedure). This band is generally assumed to be due to an interplay of quantum confinement and (oxide-related) surface states [22, 23]. Positive optical gain has been repeatedly measured on samples with SiNc:O embedded in SiO2 matrix [24-26]. Nevertheless, even though we tried to combine these SiNcs with a resonator, no real lasing was observed [24]. The absence of lasing was attributed to the presence of large aggregates of SiNcs in the sample and resulting considerable scatteringrelated losses. The ensemble quantum efficiency of the emission at the slow orange/red band reaches 2–3%. As both decay lifetime τ and PL quantum efficiency η have been measured, we can deduce the radiative lifetime τrad (τrad = τ/η). In SiNcs:O, it amounts to τrad ≈ 300 µs, which is still far from the radiative lifetime of direct-bandgap semiconductors (the range of nanoseconds), where both optical gain and lasing are routinely observed. 3.2. Silicon nanocrystals with methyl-based capping It is possible to significantly improve the PL properties of SiNcs:O by applying a photo-chemical treatment resulting in the replacement of the oxide surface capping with passivation based on methyl groups (–CH3), as evidenced by FTIR spectra. Whereas the FTIR spectra of SiNc:O are dominated by wide Si–O–Si bridge-bond bands (975– 1130 cm − 1 and 795 cm − 1) with some silanol Si–OH (940 and
a
b
c
Fig. 3. Photographs of luminescence of: (a) oxidized SiNcs in ethanol (reprinted with permission of Elsevier [27]) and (b) the colloid of SiNcs with methyl-based capping. Substantially suppressed light scatteringin the colloid is apparent. (c) The colloid is shown also in ambient light.
875 cm − 1) and oxidized hydride groups OxSiHy (833 cm − 1), a characteristic Si–CH3 vibration (852 and 677 cm − 1) appears in the SiNcs:C, see Fig. 2. (The same conclusion is also strongly backed up by nuclear magnetic resonance measurements explained in more details elsewhere [19]). The significantly altered PL properties include a blue-shift in the PL spectrum (down to 570 nm) as well as a dramatic change in PL lifetime down to as little as 2 ns (see Fig. 1(a) and (b)). Moreover, the measured PL quantum efficiency (20%) allows us to directly estimate the radiative lifetime (10 ns), which indicates that luminescence performance of these SiNcs:C is comparable to that of direct-bandgap semiconductor nanocrystals. It is important to point out that this luminescence performance is stable in time and does not spontaneously deteriorate. Apart from enhanced luminescence, the colloidal dispersion of nanometer-sized particles was meant as a parallel to laser dyes, which exhibit very low light-scattering losses. When inspected with a naked eye, our SiNc:C samples appear optically clear (see Fig. 3 for comparison of turbidity). A possible simple measure of turbidity of a sample can be based on a simultaneous collection of a PL spectrum and the scattered excitation laser line. Fig. 1(c) shows such PL spectra of SiNcs:C (needless to say that the mere fact that the detection of the scattered excitation laser and a PL spectrum at the same time is possible, implies high optical quality). In the as-prepared sample (black curve; sample was let to settle down for about 20 min), some scattering is still present. However, if the sample undergoes filtration (filter pore size 220 nm), the scattering starts to be much lower (red curve in Fig. 1(c)). At the filtered sample, the amplitude of the collected intensity at 442 nm nearly reaches that of the solvent (which is most probably the result of Raman scattering in the solvent). Clearly, the PL intensity is nearly unaffected by the filtration procedure (although unfiltered samples tend to have slightly lower PL intensity than the filtered ones, Fig. 1(c)). We verified that the same applies also for filtrations using smaller filter pore sizes of 100 and 35 nm; only when the nominal pore size reached 10 nm, some decrease in PL intensity (∼ 8%) was observed. This indicates (in agreement with dynamic-light-scattering measurements published in [19]) that the SiNcs are dispersed in the solvent as single particles or small cluster of just a few particles, implying high optical quality. 4. Discussion We would like to point out here that our methyl-capped SiNcs exhibit quite unique PL properties: the yellow luminescence (570 nm) is situated in the spectral region which is generally difficult to reach in SiNcs and the radiative lifetime is fast (10 ns). To the best of our knowledge, the only SiNcs with comparable PL properties were octanethiol-capped SiNcs by English et al. [28]. The fast yellow luminescence can be qualitatively understood assuming the traditional model that the slow orange/red PL coming from SiNcs:O is due to oxide surface states [22]. If the surface oxide is removed, other recombination channels are made possible. We can tentatively expect that this luminescence is connected directly with the core of the nanocrystal, but further investigations are necessary to support this conclusion. Regarding the optical-gain-related properties, the SiNc:C samples show clear improvement, see Table 1. First of all, the blueshift of the PL spectrum implies lowered free-carrier absorption and very probably participates also in the increase of quantum efficiency (from 2–3% to 20%). Secondly, the significantly shorter radiative lifetime (10 ns versus 300 µs) indicates enhanced radiative recombination. Nevertheless, probably the most important property here is the very high optical quality of the resulting SiNc:C sample as it is the key obstacle towards achieving lasing in SiNcs [24]. The change is, however, quite difficult to quantify. The difference is clear in naked-eye observation (compare Fig. 3(a) and (b)) and, moreover, what very well illustrates the improvement is the fact that whereas the signal of the scattered
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light from the exciting laser line can be measured together with a PL spectrum in the case of the SiNc:C sample (Fig. 1(c)), the same measurement of the SiNc:O sample saturates the CCD detector. It is also important to point out here that the problem of high scatteringrelated losses in connection with SiNcs is universal because SiNcs are generally highly prone to oxidation of the surface the formation of oxide bridge bonds Si–O–Si is the main cause of agglomeration of SiNcs into large, light-scattering aggregates [29]. Therefore, finding a suitable type of surface passivation that can prevent oxidation and agglomeration even over longer-time storage is of immense importance and the proposed methyl-based capping is a prime example of such stable passivation. What remains a problem here, however, is the sufficiently high concentration of nanocrystals, which is a bit more difficult to achieve in our SiNcs:C because our preparation procedure is time-consuming. In order to solve this problem, we want to prepare a larger number of batches of the sample and concentrate them into a single colloid. 5. Conclusions Our silicon nanocrystals with methyl-based capping successfully address several issues that need to be tackled if high optical gain is to be achieved in silicon nanocrystals. Reasonably high quantum yield (20%) and fast radiative lifetime, comparable to directbandgap semiconductors, are achieved. Luminescence maximum is blue-shifted to the yellow spectral region (570 nm), implying lowered free-carrier absorption. Moreover, the nanocrystals are dispersed in the solvent as single particles or small clusters, ensuring high optical quality of the colloid. Acknowledgements This work was supported by the projects IAA101120804 and KJB100100903 of the Grant Agency of the Academy of Sciences, by the Czech Ministry of Education, Youth and Sports through the research center LC510, by the Institutional Research Plan AV0Z10100521, by the Academy of Sciences of the Czech Republic KAN400100701 and by the Technology Agency of the Czech Republic through the project TA01020972. Zdeněk Remeš is acknowledged for experimental assistance with FTIR measurements.
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