- Email: [email protected]
Towards spectroscopy of a few silicon nanocrystals embedded in silica
ARTICLE IN PRESS Physica E 41 (2009) 998–1001
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Towards spectroscopy of a few silicon nanocrystals embedded in silica M. Gru¨n a, P. Miska a,, E. Neu b, D. Steinmetz b, F. Montaigne a, H. Rinnert a, C. Becher b, M. Vergnat a a b
Laboratoire de Physique des Mate´riaux (LPM), Nancy-Universite´, CNRS, Boulevard des Aiguillettes, B.P. 239, 54506 Vandœuvre-le`s-Nancy, France ¨t des Saarlandes, Postfach 151150, 66041 Saarbru ¨ cken, Germany Fachrichtung 7.3 (Technische Physik), Universita
a r t i c l e in f o
a b s t r a c t
Available online 13 August 2008
This work aims at optical isolation of a few silicon nanocrystals (Si-NCs) embedded in silica. We rely on realization of a SiO single layer followed by annealing under vacuum to generate Si-NCs. We first report on optimization of luminescence of single Si-NC layer in SiO2 thin film. An optimum of photoluminescence signal was found for annealing at 1050 1C for 95 min. Then, in order to optically isolate single Si-NCs, lithographic processes such as creation of aluminum masks have been employed. We discuss the challenges and the chances of measuring a photoluminescence signal from few Si-NCs. & 2008 Published by Elsevier B.V.
PACS: 78.55.Ap 61.46.Hk 68.37.Ps 81.07.Bc 81.16.Nd Keywords: Silicon nanocrystals Lithography Photoluminescence spectroscopy Single object analysis
1. Introduction The study of silicon nanocrystals (Si-NCs) is a very active field of research because of their light emission properties in the visible range, which could lead to promising applications for optoelectronic devices. Most of the experiments realized with Si-NCs have focused on investigations of large Si-NC ensembles and very few experiments have been done on a single Si-NC. Such studies can give more precise information on spectral width of Si-NC. Isolating a single silicon quantum dot could also give the possibility of realizing a source of single photons for quantum communication [1]. An original method for spatially localizing Si-NCs is to implant silicon through stencil mask before annealing [2]. This technique has offered the possibility of observing a blue shift of the PL signal near the edge of the pattern where the smallest crystals are situated [3]. Another way was to apply sequential wet-etching/ oxidizing processes on silicon micropillars [4,5]. Such processes led to the formation of one silicon nanostructure at the top of the pillar. The advantage of such a technique is of obtaining a regular arrangement of individual nanostructures easily locatable for spectroscopic analysis. The photoluminescence (PL) analysis of this kind of nanostructure has revealed sharp PL [6] and electroluminescence [7] peaks.
Corresponding author. Tel.: +33 3 83 68 48 07; fax: +33 3 83 68 48 01.
E-mail address: [email protected] (P. Miska). 1386-9477/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.physe.2008.08.005
The goal of this work is the realization and optical characterization of Si-NC ensembles containing a reduced number of nanocrystals as well as exploration of methods to optically isolate single Si-NCs. In order to reach this goal, we follow a stepwise approach. In the first step, we reduced the number of Si-NCs layers and we optimized the luminescence yield from a single SiNCs layer. By investigating continuous-wave (cw) PL, we tuned the processing parameters like annealing temperature and annealing time to obtain the maximum PL signal. In the second step, the lateral isolation of few Si-NCs is explored. To reach this aim, an aluminum mask containing small apertures (from 2 mm down to 100 nm) was produced by metal deposition, electron beam lithography (EBL) and wet etching above the Si-NC layer, in order to allow the optical excitation of a reduced number of Si-NCs through these apertures.
2. Experimental A SiO2/SiO/SiO2 multilayered structure containing a single SiO layer embedded between two SiO2 layers was deposited in an ultrahigh vacuum chamber, onto a silicon substrate maintained at 100 1C. The layers were obtained by successive thermal evaporation of SiO powder and electron beam evaporation of SiO2 powder. The deposition rate of 0.1 nm/s was controlled by quartz microbalances. The thickness of the active SiO layer was 3 nm. The thicknesses of the SiO2 layers were tuned to obtain the maximum PL signal. With annealing treatment above 1000 1C, the SiO layer presents a phase separation inducing the appearance
ARTICLE IN PRESS ¨ n et al. / Physica E 41 (2009) 998–1001 M. Gru
and the crystallization of Si aggregates, the size of which is equal to the thickness of the SiO layer [8]. The multilayers were annealed in a high-vacuum furnace at 1050 1C for different durations. A JEOL 6500f microscope was used in both EBL processing and also the subsequent scanning electron microscopy (SEM) investigation of the lithographed structures. Polymethylmetacrylate positive tone resist was used for wet-etching procedure. The aluminum film was deposited by evaporation with deposition rate 1 A˚/s. The etching steps were performed in diluted phosphoric acid (80:20 H3PO4:H2O). Lithographed structures were controlled by SEM and by atomic force microscopy (AFM) using non-contact mode. For optimization of thermal treatment of the multilayer, PL measurements were performed with a monochromator equipped with a 150 groves/mm grating and a charge-coupled device (CCD) detector cooled at 140 K, with a detection range from 400 to 950 nm. The excitation was obtained with the 313 nm line of a mercury arc lamp. Spectra are corrected for the spectral sensitivity of the apparatus. PL measurements on the lithographed samples were performed using a frequency-doubled cw Nd:YVO4 laser at 532 nm, at a mean set power of 50 mW. The PL was analyzed with a monochromator equipped with a 600 grooves/mm grating and by a liquid nitrogen cooled CCD. Signal-to-noise ratio was improved using a confocal setup suppressing stray light or light from outside the focal plane. Due to beam splitting and optical losses, the mean power arriving at the sample is only about 5% (2.5 mW) of the laser output.
999
annealed at 1050 1C with different durations ta are reported in Fig. 1. The as-deposited sample shows only a small band around 500 nm, which is generally attributed to the presence of defects in the SiOx matrix as, for example, non-bridging oxygen hole centers [9–11]. This peak remains visible with annealing and its intensity increases, as if new defects appear with the decomposition process. Another band appears around 725 nm with intensity increasing with annealing duration. This band, already observed in previous study [8], can be attributed to Si-NCs. After 95 min annealing, the intensity of this peak is comparable to intensity of the peak at 500 nm, which proves the strong PL of Si-NCs since the SiO layer is 50 times thinner than the cumulative thickness of SiO2 layers. Several attempts have been made to optimize PL intensity by varying the dimensions of the multilayer. The thicknesses of the surrounding SiO2 layers have an influence on the PL intensity and the best sample corresponds to SiO2 thicknesses equal to 50 nm. In the case of a thicker lower layer, the PL intensity from the oxygen defects becomes more intense than the PL intensity arising from the Si-NCs. For a thinner upper layer, the PL intensity at around 800 nm is reduced compared with the sample consisting of two 50 nm SiO2 layers surrounding the Si-NCs. In this case, excitons may be trapped by surface defects, reducing thus the radiative emission of the Si-NCs. In summary, with appropriate annealing treatment, it is possible to observe the PL of a single Si-NCs layer. The thicknesses of the surrounding SiO2 layers have influence on PL intensity and the best sample corresponds to SiO2 thicknesses 50 nm.
3.2. Mask realization by lithography techniques 3. Results and discussion 3.1. PL of a Si-NCs layer With annealing treatment, decomposition of the SiO layer into the Si and SiO2 stable phases occurs, therefore inducing the appearance and afterwards the crystallization of Si aggregates if temperature is high enough. Influence of annealing duration on the PL properties was studied first. Thicknesses of layers of the studied SiO2/SiO/SiO2 structure was 50 nm for the upper SiO2 layer, 3 nm for the SiO layer and 100 nm for the lower SiO2 layer in contact with the substrate. The PL spectra of the samples as deposited and
PL intensity (a.u)
Ta = 1050°C
1800
Annealing time As deposited 5 min 35 min 65 min 95 min
900
0 500
600
700 Wavelength (nm)
800
900
Fig. 1. Photoluminescence spectra of the SiO2/SiO/SiO2 structure annealed at 1050 1C for different durations. The thicknesses of the layers are 50, 3 and 100 nm, respectively.
After optimizing the vertical structure of the samples, we attempted to decrease the number of observed Si-NCs in the single layer. The samples were therefore structured by EBL. The idea was to reduce the number of excited Si-NCs by using an opaque mask containing small apertures. Such a technique was already used by Yasin et al. [12] for the observation of the PL of individual InGaN quantum dots through aluminum mask apertures with a 200-nm diameter. The opacity of the aluminum layer has been firstly tested for different layer thicknesses using a multilayer containing 22 layers of Si-NCs providing a much more intense signal than a single layer. For a 30 nm thick layer, there was a residual signal from the Si-NCs. For the 50 nm thick layer, however, no PL signal from the Si-NCs was observed. It was not necessary to use thicker aluminum layers because thin layers are more adapted to structure the apertures. Nano-apertures were then created in aluminum by both EBL and wet etching of the 50 nm thick films using diluted phosphoric acid (80:20 H3PO4:H2O). Fig. 2 shows SEM pictures of the nanostructured aluminum layer. Details of one ensemble of holes can be seen in Fig. 2a. Most of the holes are correctly etched. The tailored structures are arranged as a 100 mm 100 mm wide array containing eight lines of eight identical structures that differ in exposure dose during the EBL. Apertures were created in the resist with size and shape varying from 2 mm 2 mm big squares to 100 nm diameter circles. The pitch between each line and each center of a structure is of about 10 mm in order to have wellseparated structures when exciting with laser. This size has been chosen to allow exposure of a single aperture under the laser spot. In order to easily locate these points, 5 mm long and 1 mm wide alignment marks have been designed between those structures. A SEM picture of the smallest aperture is reported in Fig. 2b. It shows that it is possible to obtain well etched hole with clear border and with sizes about 100 nm.
ARTICLE IN PRESS ¨ n et al. / Physica E 41 (2009) 998–1001 M. Gru
1000
Without mask
PL intensity (a.u)
600
10 µm
400 Square 1 µm2
200
Circle 500 nm
Circle 100 nm
0 600
700
800
900
Wavelength (nm) Fig. 4. PL spectra obtained for a non-lithographed SiO2/SiO/SiO2 sample and through apertures 2 mm, 500 and 100 nm.
100 nm
Fig. 2. (a) SEM global view of the mask and (b) details of one hole with a diameter of 100 nm.
67.32
sample shows a peak centered around 760 nm corresponding to the luminescence of the Si-NCs discussed above. A shoulder appears at around 625 nm. It might be linked to the luminescence of defects in silica. Differences in shapes between this spectrum and those obtained in Fig. 1, especially for luminescence of defects, may be due to different excitation wavelengths. When exciting the sample through a big square aperture (2 mm 2 mm), the peak around 750 nm is still observed. Nevertheless, it is less intense, especially in the high wavelength range, yielding a blue shift of the global PL signal. This phenomenon can be linked to many physical processes. It could, for example, be due to a coupling between the Si-NC emission and plasmons of the aluminum mask that might quench the PL of the Si-NCs. The PL signal of the defects is still present. For a smaller aperture (500 nm 500 nm), the Si-NCs PL peak vanishes. It is of the order of magnitude of the signal located at around 625 nm. For excitation through the smallest apertures, no clear signal of the Si-NCs can be measured. In this case, it could be interesting to use near-field setup to improve the detection and signal-to-noise ratio.
4. Conclusion
0.00
100 nm
Fig. 3. AFM view of a 100-nm diameter aperture. The z scale is in nm.
The masks have been analyzed by AFM to verify the quality of the etching process. Fig. 3 shows an AFM picture of the etched mask centered at around a 100 nm wide circular holes. This picture shows a hole with a well-defined outline. Measurements of depth have confirmed the quality of the etching process. The 50 nm aluminum layer is well etched, yielding an aperture with diameter 100 nm and depth of 50 nm. The surface density evaluated in equivalent systems is estimated between 5 1010 and 6 1011 Si-NCs/cm2. The two values of Si-NCs density give an average number of grains of 30 and 2.5 under an aperture of 100 nm diameter. Then, it is possible, through our small apertures, to excite selectively a reduce number of Si-NCs. 3.3. PL of a Si-NCs layer through small apertures Fig. 4 shows the PL spectra recorded both for lithographed and non-lithographed samples. The PL spectrum of the non-lithographed
In summary, SiO2/SiO/SiO2 multilayered structures have been prepared by evaporation. With annealing treatment at 1050 1C, the decomposition of the 3-nm thick SiO layer leads to the formation of a Si-NCs layer. When this layer is surrounded by two 50-nm thick SiO2 layers, it is possible to observe the PL of this single layer, of the same order of magnitude as the PL of the defects in the SiO2 layers. In order to reduce the number of observed Si-NCs, technological processes have been developed. A 50-nm thick aluminum mask was structured by EBL and wet etching to realize small apertures through which Si-NCs have been excited. The smallest realized size of the holes was 100 nm. PL of Si-NCs through nanoapertures has been detected for apertures with diameter down to 500 nm. However, no reliable luminescence has been obtained from the smallest apertures.
Acknowledgements The authors would like to thank F. Mouginet and J. ArocasGarcia for sample preparation, as well as G. Lengaigne and D. Lacour, for preparing the preliminary tests for wet etching and helping in AFM measurement.
ARTICLE IN PRESS ¨ n et al. / Physica E 41 (2009) 998–1001 M. Gru
References [1] N. Gisin, R. Thew, Nature Photonics 1 (2007) 165. [2] A. Arbouet, M. Carrada, F. Demangeot, V. Paillard, G. BenAssayag, C. Bonafos, A. Claverie, S. Schamm, C. Dumas, J. Grisolia, M.A.F. Van den Boogaart, J. Brugger, L. Doeswijk, J. Lumin. 121 (2006) 340. [3] C. Dumas, J. Grisolia, L. Ressier, A. Arbouet, V. Paillard, G. Ben Assayag, A. Claverie, M.A.F. van den Boogaart, J. Brugger, Phys. Status Solidi A—Appl. Mater. Sci. 204 (2007) 487. [4] I. Sychugov, R. Juhasz, J. Valenta, J. Linnros, Phys. Rev. Lett. 94 (2005) 087405. [5] I. Sychugov, R. Juhasz, J. Valenta, A. Zhang, P. Pirouz, J. Linnros, Appl. Surf. Sci. 252 (2006) 5249.
1001
[6] I. Sychugov, R. Juhasz, A. Galeckas, J. Valenta, J. Linnros, Opt. Mater. 27 (2005) 973. [7] J. Valenta, N. Lalic, J. Linnros, Appl. Phys. Lett. 84 (2004) 1459. [8] O. Jambois, H. Rinnert, X. Devaux, M. Vergnat, J. Appl. Phys. 100 (2006) 123504. [9] T. Shimizu-Iwayama, K. Fujita, S. Nakao, K. Saitoh, T. Fujita, N. Itoh, J. Appl. Phys. 75 (1994) 7779. [10] A.J. Kenyon, P.F. Trwoga, C.W. Pitt, G. Rehm, J. Appl. Phys. 79 (1996) 9291. [11] K.S. Min, K.V. Shcheglov, C.M. Yang, H.A. Atwater, M.L. Brongersma, A. Polman, Appl. Phys. Lett. 69 (1996) 2033. [12] S. Yasin, M.N. Khalig, J.H. Rice, R.A. Taylor, Microelectron. Eng. 73–74 (2004) 762.
Contents lists available at ScienceDirect
Physica E journal homepage: www.elsevier.com/locate/physe
Towards spectroscopy of a few silicon nanocrystals embedded in silica M. Gru¨n a, P. Miska a,, E. Neu b, D. Steinmetz b, F. Montaigne a, H. Rinnert a, C. Becher b, M. Vergnat a a b
Laboratoire de Physique des Mate´riaux (LPM), Nancy-Universite´, CNRS, Boulevard des Aiguillettes, B.P. 239, 54506 Vandœuvre-le`s-Nancy, France ¨t des Saarlandes, Postfach 151150, 66041 Saarbru ¨ cken, Germany Fachrichtung 7.3 (Technische Physik), Universita
a r t i c l e in f o
a b s t r a c t
Available online 13 August 2008
This work aims at optical isolation of a few silicon nanocrystals (Si-NCs) embedded in silica. We rely on realization of a SiO single layer followed by annealing under vacuum to generate Si-NCs. We first report on optimization of luminescence of single Si-NC layer in SiO2 thin film. An optimum of photoluminescence signal was found for annealing at 1050 1C for 95 min. Then, in order to optically isolate single Si-NCs, lithographic processes such as creation of aluminum masks have been employed. We discuss the challenges and the chances of measuring a photoluminescence signal from few Si-NCs. & 2008 Published by Elsevier B.V.
PACS: 78.55.Ap 61.46.Hk 68.37.Ps 81.07.Bc 81.16.Nd Keywords: Silicon nanocrystals Lithography Photoluminescence spectroscopy Single object analysis
1. Introduction The study of silicon nanocrystals (Si-NCs) is a very active field of research because of their light emission properties in the visible range, which could lead to promising applications for optoelectronic devices. Most of the experiments realized with Si-NCs have focused on investigations of large Si-NC ensembles and very few experiments have been done on a single Si-NC. Such studies can give more precise information on spectral width of Si-NC. Isolating a single silicon quantum dot could also give the possibility of realizing a source of single photons for quantum communication [1]. An original method for spatially localizing Si-NCs is to implant silicon through stencil mask before annealing [2]. This technique has offered the possibility of observing a blue shift of the PL signal near the edge of the pattern where the smallest crystals are situated [3]. Another way was to apply sequential wet-etching/ oxidizing processes on silicon micropillars [4,5]. Such processes led to the formation of one silicon nanostructure at the top of the pillar. The advantage of such a technique is of obtaining a regular arrangement of individual nanostructures easily locatable for spectroscopic analysis. The photoluminescence (PL) analysis of this kind of nanostructure has revealed sharp PL [6] and electroluminescence [7] peaks.
Corresponding author. Tel.: +33 3 83 68 48 07; fax: +33 3 83 68 48 01.
E-mail address: [email protected] (P. Miska). 1386-9477/$ - see front matter & 2008 Published by Elsevier B.V. doi:10.1016/j.physe.2008.08.005
The goal of this work is the realization and optical characterization of Si-NC ensembles containing a reduced number of nanocrystals as well as exploration of methods to optically isolate single Si-NCs. In order to reach this goal, we follow a stepwise approach. In the first step, we reduced the number of Si-NCs layers and we optimized the luminescence yield from a single SiNCs layer. By investigating continuous-wave (cw) PL, we tuned the processing parameters like annealing temperature and annealing time to obtain the maximum PL signal. In the second step, the lateral isolation of few Si-NCs is explored. To reach this aim, an aluminum mask containing small apertures (from 2 mm down to 100 nm) was produced by metal deposition, electron beam lithography (EBL) and wet etching above the Si-NC layer, in order to allow the optical excitation of a reduced number of Si-NCs through these apertures.
2. Experimental A SiO2/SiO/SiO2 multilayered structure containing a single SiO layer embedded between two SiO2 layers was deposited in an ultrahigh vacuum chamber, onto a silicon substrate maintained at 100 1C. The layers were obtained by successive thermal evaporation of SiO powder and electron beam evaporation of SiO2 powder. The deposition rate of 0.1 nm/s was controlled by quartz microbalances. The thickness of the active SiO layer was 3 nm. The thicknesses of the SiO2 layers were tuned to obtain the maximum PL signal. With annealing treatment above 1000 1C, the SiO layer presents a phase separation inducing the appearance
ARTICLE IN PRESS ¨ n et al. / Physica E 41 (2009) 998–1001 M. Gru
and the crystallization of Si aggregates, the size of which is equal to the thickness of the SiO layer [8]. The multilayers were annealed in a high-vacuum furnace at 1050 1C for different durations. A JEOL 6500f microscope was used in both EBL processing and also the subsequent scanning electron microscopy (SEM) investigation of the lithographed structures. Polymethylmetacrylate positive tone resist was used for wet-etching procedure. The aluminum film was deposited by evaporation with deposition rate 1 A˚/s. The etching steps were performed in diluted phosphoric acid (80:20 H3PO4:H2O). Lithographed structures were controlled by SEM and by atomic force microscopy (AFM) using non-contact mode. For optimization of thermal treatment of the multilayer, PL measurements were performed with a monochromator equipped with a 150 groves/mm grating and a charge-coupled device (CCD) detector cooled at 140 K, with a detection range from 400 to 950 nm. The excitation was obtained with the 313 nm line of a mercury arc lamp. Spectra are corrected for the spectral sensitivity of the apparatus. PL measurements on the lithographed samples were performed using a frequency-doubled cw Nd:YVO4 laser at 532 nm, at a mean set power of 50 mW. The PL was analyzed with a monochromator equipped with a 600 grooves/mm grating and by a liquid nitrogen cooled CCD. Signal-to-noise ratio was improved using a confocal setup suppressing stray light or light from outside the focal plane. Due to beam splitting and optical losses, the mean power arriving at the sample is only about 5% (2.5 mW) of the laser output.
999
annealed at 1050 1C with different durations ta are reported in Fig. 1. The as-deposited sample shows only a small band around 500 nm, which is generally attributed to the presence of defects in the SiOx matrix as, for example, non-bridging oxygen hole centers [9–11]. This peak remains visible with annealing and its intensity increases, as if new defects appear with the decomposition process. Another band appears around 725 nm with intensity increasing with annealing duration. This band, already observed in previous study [8], can be attributed to Si-NCs. After 95 min annealing, the intensity of this peak is comparable to intensity of the peak at 500 nm, which proves the strong PL of Si-NCs since the SiO layer is 50 times thinner than the cumulative thickness of SiO2 layers. Several attempts have been made to optimize PL intensity by varying the dimensions of the multilayer. The thicknesses of the surrounding SiO2 layers have an influence on the PL intensity and the best sample corresponds to SiO2 thicknesses equal to 50 nm. In the case of a thicker lower layer, the PL intensity from the oxygen defects becomes more intense than the PL intensity arising from the Si-NCs. For a thinner upper layer, the PL intensity at around 800 nm is reduced compared with the sample consisting of two 50 nm SiO2 layers surrounding the Si-NCs. In this case, excitons may be trapped by surface defects, reducing thus the radiative emission of the Si-NCs. In summary, with appropriate annealing treatment, it is possible to observe the PL of a single Si-NCs layer. The thicknesses of the surrounding SiO2 layers have influence on PL intensity and the best sample corresponds to SiO2 thicknesses 50 nm.
3.2. Mask realization by lithography techniques 3. Results and discussion 3.1. PL of a Si-NCs layer With annealing treatment, decomposition of the SiO layer into the Si and SiO2 stable phases occurs, therefore inducing the appearance and afterwards the crystallization of Si aggregates if temperature is high enough. Influence of annealing duration on the PL properties was studied first. Thicknesses of layers of the studied SiO2/SiO/SiO2 structure was 50 nm for the upper SiO2 layer, 3 nm for the SiO layer and 100 nm for the lower SiO2 layer in contact with the substrate. The PL spectra of the samples as deposited and
PL intensity (a.u)
Ta = 1050°C
1800
Annealing time As deposited 5 min 35 min 65 min 95 min
900
0 500
600
700 Wavelength (nm)
800
900
Fig. 1. Photoluminescence spectra of the SiO2/SiO/SiO2 structure annealed at 1050 1C for different durations. The thicknesses of the layers are 50, 3 and 100 nm, respectively.
After optimizing the vertical structure of the samples, we attempted to decrease the number of observed Si-NCs in the single layer. The samples were therefore structured by EBL. The idea was to reduce the number of excited Si-NCs by using an opaque mask containing small apertures. Such a technique was already used by Yasin et al. [12] for the observation of the PL of individual InGaN quantum dots through aluminum mask apertures with a 200-nm diameter. The opacity of the aluminum layer has been firstly tested for different layer thicknesses using a multilayer containing 22 layers of Si-NCs providing a much more intense signal than a single layer. For a 30 nm thick layer, there was a residual signal from the Si-NCs. For the 50 nm thick layer, however, no PL signal from the Si-NCs was observed. It was not necessary to use thicker aluminum layers because thin layers are more adapted to structure the apertures. Nano-apertures were then created in aluminum by both EBL and wet etching of the 50 nm thick films using diluted phosphoric acid (80:20 H3PO4:H2O). Fig. 2 shows SEM pictures of the nanostructured aluminum layer. Details of one ensemble of holes can be seen in Fig. 2a. Most of the holes are correctly etched. The tailored structures are arranged as a 100 mm 100 mm wide array containing eight lines of eight identical structures that differ in exposure dose during the EBL. Apertures were created in the resist with size and shape varying from 2 mm 2 mm big squares to 100 nm diameter circles. The pitch between each line and each center of a structure is of about 10 mm in order to have wellseparated structures when exciting with laser. This size has been chosen to allow exposure of a single aperture under the laser spot. In order to easily locate these points, 5 mm long and 1 mm wide alignment marks have been designed between those structures. A SEM picture of the smallest aperture is reported in Fig. 2b. It shows that it is possible to obtain well etched hole with clear border and with sizes about 100 nm.
ARTICLE IN PRESS ¨ n et al. / Physica E 41 (2009) 998–1001 M. Gru
1000
Without mask
PL intensity (a.u)
600
10 µm
400 Square 1 µm2
200
Circle 500 nm
Circle 100 nm
0 600
700
800
900
Wavelength (nm) Fig. 4. PL spectra obtained for a non-lithographed SiO2/SiO/SiO2 sample and through apertures 2 mm, 500 and 100 nm.
100 nm
Fig. 2. (a) SEM global view of the mask and (b) details of one hole with a diameter of 100 nm.
67.32
sample shows a peak centered around 760 nm corresponding to the luminescence of the Si-NCs discussed above. A shoulder appears at around 625 nm. It might be linked to the luminescence of defects in silica. Differences in shapes between this spectrum and those obtained in Fig. 1, especially for luminescence of defects, may be due to different excitation wavelengths. When exciting the sample through a big square aperture (2 mm 2 mm), the peak around 750 nm is still observed. Nevertheless, it is less intense, especially in the high wavelength range, yielding a blue shift of the global PL signal. This phenomenon can be linked to many physical processes. It could, for example, be due to a coupling between the Si-NC emission and plasmons of the aluminum mask that might quench the PL of the Si-NCs. The PL signal of the defects is still present. For a smaller aperture (500 nm 500 nm), the Si-NCs PL peak vanishes. It is of the order of magnitude of the signal located at around 625 nm. For excitation through the smallest apertures, no clear signal of the Si-NCs can be measured. In this case, it could be interesting to use near-field setup to improve the detection and signal-to-noise ratio.
4. Conclusion
0.00
100 nm
Fig. 3. AFM view of a 100-nm diameter aperture. The z scale is in nm.
The masks have been analyzed by AFM to verify the quality of the etching process. Fig. 3 shows an AFM picture of the etched mask centered at around a 100 nm wide circular holes. This picture shows a hole with a well-defined outline. Measurements of depth have confirmed the quality of the etching process. The 50 nm aluminum layer is well etched, yielding an aperture with diameter 100 nm and depth of 50 nm. The surface density evaluated in equivalent systems is estimated between 5 1010 and 6 1011 Si-NCs/cm2. The two values of Si-NCs density give an average number of grains of 30 and 2.5 under an aperture of 100 nm diameter. Then, it is possible, through our small apertures, to excite selectively a reduce number of Si-NCs. 3.3. PL of a Si-NCs layer through small apertures Fig. 4 shows the PL spectra recorded both for lithographed and non-lithographed samples. The PL spectrum of the non-lithographed
In summary, SiO2/SiO/SiO2 multilayered structures have been prepared by evaporation. With annealing treatment at 1050 1C, the decomposition of the 3-nm thick SiO layer leads to the formation of a Si-NCs layer. When this layer is surrounded by two 50-nm thick SiO2 layers, it is possible to observe the PL of this single layer, of the same order of magnitude as the PL of the defects in the SiO2 layers. In order to reduce the number of observed Si-NCs, technological processes have been developed. A 50-nm thick aluminum mask was structured by EBL and wet etching to realize small apertures through which Si-NCs have been excited. The smallest realized size of the holes was 100 nm. PL of Si-NCs through nanoapertures has been detected for apertures with diameter down to 500 nm. However, no reliable luminescence has been obtained from the smallest apertures.
Acknowledgements The authors would like to thank F. Mouginet and J. ArocasGarcia for sample preparation, as well as G. Lengaigne and D. Lacour, for preparing the preliminary tests for wet etching and helping in AFM measurement.
ARTICLE IN PRESS ¨ n et al. / Physica E 41 (2009) 998–1001 M. Gru
References [1] N. Gisin, R. Thew, Nature Photonics 1 (2007) 165. [2] A. Arbouet, M. Carrada, F. Demangeot, V. Paillard, G. BenAssayag, C. Bonafos, A. Claverie, S. Schamm, C. Dumas, J. Grisolia, M.A.F. Van den Boogaart, J. Brugger, L. Doeswijk, J. Lumin. 121 (2006) 340. [3] C. Dumas, J. Grisolia, L. Ressier, A. Arbouet, V. Paillard, G. Ben Assayag, A. Claverie, M.A.F. van den Boogaart, J. Brugger, Phys. Status Solidi A—Appl. Mater. Sci. 204 (2007) 487. [4] I. Sychugov, R. Juhasz, J. Valenta, J. Linnros, Phys. Rev. Lett. 94 (2005) 087405. [5] I. Sychugov, R. Juhasz, J. Valenta, A. Zhang, P. Pirouz, J. Linnros, Appl. Surf. Sci. 252 (2006) 5249.
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