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A comparison of cathodoluminescence and photoluminescence of porous silicon and the influence of aging and electron irradiation of these properties
Solid State Communications 143 (2007) 197–201 www.elsevier.com/locate/ssc
A comparison of cathodoluminescence and photoluminescence of porous silicon and the influence of aging and electron irradiation of these properties Yue Zhao a,b,∗ , Dongsheng Li b , Shuoxiang Xing c , Wenbin Sang a , Deren Yang b , Minhua Jiang b,d a Department of Electronic Information Materials, Shanghai University, Shanghai 200072, PR China b State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China c Clean Energy and Environment Engineering Key Laboratory of Ministry of Education, Zhejiang University, Hangzhou 310027, PR China d State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
Received 9 February 2007; received in revised form 7 April 2007; accepted 23 May 2007 by C.H.R. Thomsen Available online 2 June 2007
Abstract In this paper, Cathodoluminescence and Photoluminescence in porous silicon prepared by different etching times were reported. The Cathodoluminescence emission may originate from the presence of hydrogen atoms in oxide phase, which is reduced by electron beam irradiation, but the Photoluminescence emission may be associated with the defects in dioxide in porous silicon surface, which can be proved by scanning electron microscopy images, Fourier transform infrared spectra and Raman spectra. Under electron beam irradiation, the degradation of Cathodoluminescence intensity from as-prepared sample is attributed to hydrogen-atom desorption from porous silicon surface or destruction of irradiative center on porous silicon surface. During electron-beam radiation, the stabilization of Cathodoluminescence emission of the samples, which are post treated by electron beam exposure for 60 s and then aged for 2 min, is due to the protection of surface oxidized layer generated from electron beam heating. c 2007 Elsevier Ltd. All rights reserved.
PACS: 78.20.-e Keywords: A. Porous silicon; E. Cathodoluminescence; E. Photoluminescence
1. Introduction Due to potential application in silicon-based opto-electronic devices, many investigations have been carried out on optical properties of porous silicon (PS) since the visible light emission was observed at room temperature [1]. Up to now, many studies have been carried out in terms of structure [2], photoluminescence (PL) [3–5] and electroluminescence (EL) [6,7]. Cathodoluminescence (CL) imaging has emerged as an important technique to study semiconductor materials. The CL technique has a big advantage for its non-contact and high spatial resolution features. But due to weak CL signal and its poor stability often found during the electron radiation of PS, the CL technique has been
∗ Corresponding author at: Department of Electronic Information Materials, Shanghai University, Shanghai 200072, PR China. E-mail address: [email protected] (Y. Zhao).
c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.05.030
less frequently applied to PS study. A few studies have been reported using CL technique [8–12]. Jardin et al. discovered strong local intensity variation with a focused electron beam, particularly in oxidized sample, which is attributed to localized electron beam heating, corresponding to the temperature rise of Si crystallites embedded in insulating SiO2 known for its low thermal conductivity. Cullis et al. reported that the CL emission at 520 and 650–670 nm are clearly linked with the oxide phase, but 750 nm emission correlates well with the observation of dispersed Si nano-crystallites. Suzuki et al. concluded that the electron beam excitation mainly occurs in amorphous SiO2 in PS, and two bands at 460 nm and 650 nm, respectively, are caused from different defects in amorphous SiO2 covering on PS. Bruska et al. reported about the efforts of a highly localized PS luminescence. The reduced CL brightness is probably due to electronic damage of the luminescent structure as a formation of contamination films. The electron beam radiation on PS involved the changes of the surface or interface states, and the
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Y. Zhao et al. / Solid State Communications 143 (2007) 197–201
changes of point defects in the surface or in the crystalline. J. Rams et al. studied the crystalline-related CL emission on a clean PS surface. They found that the CL emission of fresh exposed surfaces is due to the presence of nano-crystalline silicon, extended in 600–800 nm range. In addition, they also found that most of the light is generated in the top surface of PS. In our paper, we have concentrated not only on the CL stability, but also on the relationship between CL and PL. We studied CL property of PS under different charge time or posttreat process for an insight into the CL nature. Furthermore, we also researched PL property of the corresponding samples and discussed the relationship between PL and CL by Fourier transform infrared (FTIR) spectra, Raman spectra and scanning electron microscopy (SEM) observation. 2. Experiment The PS samples were prepared from h100i oriented 500 µm thick P-type silicon wafers with resistivity of 1–10 cm. An aluminum contact layer was deposited by thermal evaporation on the back side of the wafers. The PS layers were formed by electrochemical etching in a HF:C2 H5 OH (1:2 in volume) solution for the etching time from 30 to 60 min at a current density of 30 mA/cm2 . The etching was performed under back illumination (50 W). Then the samples were dried in air at room temperature. The samples were characterized by PL spectroscopy, CL spectroscopy, FTIR spectroscopy, Raman spectroscopy and SEM observation. The PL spectra excited by a 360 nm wavelength laser were measured using a HITACHI F4500 fluorescence spectrophotometer. The infrared absorption spectra were taken using a Bruker IFS 66 v/S FTIR spectroscope. The Raman spectra excited with 532 nm line were measured using a Nicolet Almege Raman Spectrometer. The appearance observations were carried out using FEI SIRION (Fig. 1(a)) under acceleration voltage 5 kV and FEI Hitachi S4200 (Fig. 1(b)) under acceleration voltage 10 kV. The CL spectra in the spectral range from 168 to 1024 nm with the resolution of 8 nm were performed in an FEI Hitachi S4200 with a CL system under acceleration voltage 10 keV and beam current density 1nA (the measurement of CL spectra were made in Nanomaterials Laboratory, National Institute for Materials Science, Tsukuba 305-0047, Japan). The optical system for light collection was designed to realize a high collection efficiency of luminescence photons and to achieve uniformity of collection efficiency. An ellipsoidal mirror of low magnification and an optical fiber were adopted. The focal length of the ellipsoid was 35 mm and its principal axis was tilted by 12◦ from horizontal. The light was led to the monochromator (Jobin Yvon, Triax 320), which has 3 gratings and two exits of light. For the spectral work, a liquid N2 cooling CCD (Jobin Yvon, Spectrum One) was adopted. A parallel detection of CL photons remarkably reduced the acquisition time of one spectrum, not only reducing damage and/or contamination but also avoiding drift of the specimen. The interval time between CL spectra is 2 s. All the measurements were carried out at room temperature.
Fig. 1. SEM images of PS prepared at 60 min before (a) and after (b) electron beam radiation.
3. Results and discussion The Fig. 1 shows the morphologies of PS sample prepared at 60 min before and after electron beam radiation. The sample has the columnar structure and the diameter was several micrometers. It also can be seen from Fig. 1(a) and (b) that after electron beam radiation the fine structure of as-prepared sample was lost, which led to the continuous decrease of CL intensity, as shown in Figs. 2(a) and 3(a). It may be due to an electronic damage of the surface structure as a formation of new morphology and chemical bonds on PS surface. Figs. 2(a) and 3(a) gave continuously changed threedimensional CL spectra and two-dimensional CL spectra of PS sample during charging for 60 s. During charging for 60 s, continuously changed three-dimensional CL spectra and twodimensional CL spectra of similar PS sample after electron beam radiation for 60 s and then aged for 2 min were shown in Fig. 2(b) and Fig. 3(b), respectively. The Fig. 3 is converted from Fig. 2 in order to intuitively show the change of CL intensity, respectively. The color of the various curves in Fig. 3(a) and (b) was used to differentiate the curves tested in different times. The nonlinear variation of CL intensity of as-prepared sample was showed with charging time, as shown in Figs. 2(a) and 3(a) and CL emission band simultaneous shifted from 577 to 450 nm. When the sample aged for 2 min after electron beam radiation for 60 s, the CL intensity of PS
Y. Zhao et al. / Solid State Communications 143 (2007) 197–201
199
Fig. 3. Two-dimensional CL spectra converted from Fig. 2, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Three-dimensional CL spectra of PS samples without (a) or with (b) electron beam radiation for 60 s under the situation for CL measurements and then stop electron beam radiation about 2 min for aging before CL measurements.
sample showed little change during charging for 60 s. A blue shift of CL spectra from 464 to 457 nm is also simultaneously observed. The CL peaks at 520 and 650–670 nm are considered to be linked with the oxide phase presented in PS and its origin is associated with the presence of hydrogen atoms in the oxide phase [9]. But in other papers, the observed CL spectrum is linked with the amorphous SiO2 content on PS surface [10]. In addition, the degradation of CL signal observed during electron-beam radiation has been reported. Maurice et al. [13] considered that the CL degradation is probably related to removing hydrogen atoms. Cullis et al. [9] suggested the variation of CL intensity is related to the removing or the reintroduction of the hydrogen atoms on the surface of the PS samples. But other authors [11] reported that the reduced CL intensity is probably due to the electronic damage of the luminescent structure as the formation of contamination films. In our experiment, it can be seen that the CL peak of asprepared PS exhibited a continuous shift from 577 to 450 nm and the CL intensity also showed a continuous decrease with the charging time, so the CL emission may be attributed to the oxide phase presented in PS and its origin is associated with the presence of hydrogen in the oxide phase, as illustrated in
Refs. [9,10]. Similar to the results in Ref. [11], the surface fine structure of porous silicon was lost after electron beam radiation, as shown in Fig. 1, which may lead to the decrease of CL intensity, as shown in Fig. 3(a). The use of electron beam radiation permitted us to modify the density of irradiative centers as point defects or destroy luminescent centers, which might be possible through the damage of PS delicate structure or the variation of the number of hydrogen-containing groups. In addition, similar to the laser-beam radiation [14,15], the thermal effect of electron-beam radiation on PS sample may lead to the oxidation of PS surface, which may lead to stabilize CL emission of PS in further electron-beam radiation process and decrease CL intensity slightly. The Fig. 4 gave CL spectra and PL spectra of PS samples prepared at different etching times. A peak centered at 570 nm was shown in PL spectra of PS prepared in different etching time, but CL spectra of corresponding samples showed a peak at 600 nm. While the etching time increases, CL intensity increases and PL intensity decreases simultaneously, but we cannot see any shift of these peaks, as shown in Fig. 4. Fig. 5(a) represents the FTIR spectra of PS samples prepared in different times. It is reported that the adsorption peaks at about 1058 and 459 cm−1 are related to Si–O–Si stretching mode [16], and one peak around 840 cm−1 is related to Si–O–H stretching mode [17]. Further, the peak at 619 cm−1 presents the binding states of Si–Si vibration in bulk silicon [18], while the peaks at around 2086 and 2243 cm−1 are attributed to the Si–H stretching mode.
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surface [23–25]. In our experiment, from the frequencies of the Raman peaks in Fig. 5(b), the quantum size effects cannot be observed in the PS film, so it shows that the visible PL emission may be related to the defects of dioxide or the radiative center on Si-wire surface. Furthermore, CL intensity of the samples prepared from different times was enhanced when the peak’s intensity related to the Si–H stretching mode in oxide phase was increased. Furthermore, it was well known that the quantum of Si–O vibration mode increased with the decrease of the Si–H vibration mode. It was indicated that the PL emission of PS samples is related to the defects of dioxide because the PL intensity decreased with the decrease of the Si–O vibration mode. Furthermore, the same results were received from the PS samples prepared by N-type silicon wafers under the same experimental parameters. 4. Summary
Fig. 4. CL spectra (a) and PL spectra (b) of as-prepared PS prepared under different etched time.
CL emission and PL emission of PS prepared from different etching times were investigated. Strong CL emission may be attributed to the presence of hydrogen in the oxide phase, but PL emission may be associated with the defects of the dioxide. The degradation of CL intensity of as-prepared samples under electron beam radiation is attributed to removing of the hydrogen atoms or destruction of radiative centers on PS surface. During electron-beam radiation, the stabilization of CL emission of the sample after electron beam radiation for 60 s and then aged for 2 min is due to the oxidation of PS surface. References
Fig. 5. FTIR spectra (a) and Raman spectra (b) of as-prepared PS prepared under different etched time.
It was indicated that the visible PL emission might come from the defects in silicon dioxide [19–21] or from crystalline quantum dots [22], which can lead to enhancement of energy gap, or from different radiative centers on Si-wire
[1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] A. Parisini, R. Angelucci, L. Dori, A. Poggi, P. Maccagnani, G.C. Cardinali, G. Amato, G. Lerondel, D. Midellino, Micron 31 (2000) 223. [3] N. Rigakis, J. Hilliard, L. AbuHassan, J.M. Hetrick, D. Andsager, M.H. Nayfeh, J. Appl. Phys. 81 (1) (1997) 440. [4] H. Koyama, Y. Matsushita, N. Koshida, J. Appl. Phys. 83 (3) (1998) 1776. [5] Gubo Li, Xiaoyuan Hou, Shuai Yuan, Huajie Chen, Fulong Zhang, Honglei Fan, Xun Wang, J. Appl. Phys. 80 (10) (1996) 5967. [6] K. Molnar, T. Mohacsy, P. Varga, E. Vazsonyi, I. Barsony, J. Lumin. 80 (1999) 91. [7] Y.E. Babanov, A.V. Prokaznikov, N.A. Rud, V.B. Svetovoy, Phys. Status Solidi A 162 (1997) R7. [8] C. Jardin, B. Gruzza, E. Vazsonyi, G. Gergely, Vacuum 46 (5) (1995) 497. [9] A.G. Cullis, L.T. Canham, G.M. Williams, P.W. Smith, O.D. Dosser, J. Appl. Phys. 75 (1) (1994) 493. [10] T. Suzuki, T. Sakai, L. Zhang, Y. Nishiyama, Appl. Phys. Lett. 66 (2) (1995) 215. [11] A. Bruska, A. Chernook, S. Schulze, M. Hietschold, Appl. Phys. Lett. 68 (17) (1996) 2378. [12] J. Rams, B. Mendez, G. Craciun, R. Plugaru, J. Piqueras, Appl. Phys. Lett. 74 (12) (1999) 1728. [13] J.L. Maurice, A. Riviere, A. Alapini, C.L. Clement, Appl. Phys. Lett. 66 (13) (1994) 1665. [14] S. Manotas, F. Rueda, J.D. Moreno, F.B. Hander, R.G. Lemus, J.M.M. Duart, Phys. Status Solidi A 182 (2000) 331. [15] W.J. Salcedo, F.J.R. Fernandez, J.C. Rubim, J. Raman, Spectrosec. 32 (2001) 151.
Y. Zhao et al. / Solid State Communications 143 (2007) 197–201 [16] T. Maruyama, S. Ohtani, Appl. Phys. Lett. 65 (11) (1994) 1346. [17] G. Tochitani, M. Shimozuma, H. Tagashira, J. Appl. Phys. 72 (1) (1992) 234. [18] T.F. Young, C.P. Chen, J.F. Fiou, J. Porous Mater. 7 (2000) 339. [19] A.J. Kontkiewicz, A.M. Kontkiewicz, J. Siejka, S. Sen, G. Nowak, A.M. Hoff, P. Sakthivel, K. Ahmed, P. Mukherjee, S. Witanachchi, J. Lagowski, Appl. Phys. Lett. 65 (11) (1994) 1436. [20] Yue Zhao, Dongsheng Li, Wenbin Sang, Deren Yang, Solid-State
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Electron. 50 (2006) 1529–1531. [21] Yue Zhao, Deren Yang, Dongsheng Li, Minghua Jiang, Mater. Sci. Eng. B 116 (2005) 95–98. [22] M.K. Lee, K.R. Peng, Appl. Phys. Lett. 62 (24) (1993) 3159. [23] J.L. Gole, F.P. Dudel, D. Grantier, Phys. Rev. B 56 (4) (1997) 2137. [24] F.P. Dudel, M.M. Rieger, J.P. Pickering, J.L. Gole, P.A. Kohl, L.A. Bottomley, J. Electrochem. Soc. 143 (8) (1996) L164. [25] F.P. Dudel, J.L. Gole, J. Appl. Phys. 82 (1) (1997) 402.
A comparison of cathodoluminescence and photoluminescence of porous silicon and the influence of aging and electron irradiation of these properties Yue Zhao a,b,∗ , Dongsheng Li b , Shuoxiang Xing c , Wenbin Sang a , Deren Yang b , Minhua Jiang b,d a Department of Electronic Information Materials, Shanghai University, Shanghai 200072, PR China b State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China c Clean Energy and Environment Engineering Key Laboratory of Ministry of Education, Zhejiang University, Hangzhou 310027, PR China d State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
Received 9 February 2007; received in revised form 7 April 2007; accepted 23 May 2007 by C.H.R. Thomsen Available online 2 June 2007
Abstract In this paper, Cathodoluminescence and Photoluminescence in porous silicon prepared by different etching times were reported. The Cathodoluminescence emission may originate from the presence of hydrogen atoms in oxide phase, which is reduced by electron beam irradiation, but the Photoluminescence emission may be associated with the defects in dioxide in porous silicon surface, which can be proved by scanning electron microscopy images, Fourier transform infrared spectra and Raman spectra. Under electron beam irradiation, the degradation of Cathodoluminescence intensity from as-prepared sample is attributed to hydrogen-atom desorption from porous silicon surface or destruction of irradiative center on porous silicon surface. During electron-beam radiation, the stabilization of Cathodoluminescence emission of the samples, which are post treated by electron beam exposure for 60 s and then aged for 2 min, is due to the protection of surface oxidized layer generated from electron beam heating. c 2007 Elsevier Ltd. All rights reserved.
PACS: 78.20.-e Keywords: A. Porous silicon; E. Cathodoluminescence; E. Photoluminescence
1. Introduction Due to potential application in silicon-based opto-electronic devices, many investigations have been carried out on optical properties of porous silicon (PS) since the visible light emission was observed at room temperature [1]. Up to now, many studies have been carried out in terms of structure [2], photoluminescence (PL) [3–5] and electroluminescence (EL) [6,7]. Cathodoluminescence (CL) imaging has emerged as an important technique to study semiconductor materials. The CL technique has a big advantage for its non-contact and high spatial resolution features. But due to weak CL signal and its poor stability often found during the electron radiation of PS, the CL technique has been
∗ Corresponding author at: Department of Electronic Information Materials, Shanghai University, Shanghai 200072, PR China. E-mail address: [email protected] (Y. Zhao).
c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.05.030
less frequently applied to PS study. A few studies have been reported using CL technique [8–12]. Jardin et al. discovered strong local intensity variation with a focused electron beam, particularly in oxidized sample, which is attributed to localized electron beam heating, corresponding to the temperature rise of Si crystallites embedded in insulating SiO2 known for its low thermal conductivity. Cullis et al. reported that the CL emission at 520 and 650–670 nm are clearly linked with the oxide phase, but 750 nm emission correlates well with the observation of dispersed Si nano-crystallites. Suzuki et al. concluded that the electron beam excitation mainly occurs in amorphous SiO2 in PS, and two bands at 460 nm and 650 nm, respectively, are caused from different defects in amorphous SiO2 covering on PS. Bruska et al. reported about the efforts of a highly localized PS luminescence. The reduced CL brightness is probably due to electronic damage of the luminescent structure as a formation of contamination films. The electron beam radiation on PS involved the changes of the surface or interface states, and the
198
Y. Zhao et al. / Solid State Communications 143 (2007) 197–201
changes of point defects in the surface or in the crystalline. J. Rams et al. studied the crystalline-related CL emission on a clean PS surface. They found that the CL emission of fresh exposed surfaces is due to the presence of nano-crystalline silicon, extended in 600–800 nm range. In addition, they also found that most of the light is generated in the top surface of PS. In our paper, we have concentrated not only on the CL stability, but also on the relationship between CL and PL. We studied CL property of PS under different charge time or posttreat process for an insight into the CL nature. Furthermore, we also researched PL property of the corresponding samples and discussed the relationship between PL and CL by Fourier transform infrared (FTIR) spectra, Raman spectra and scanning electron microscopy (SEM) observation. 2. Experiment The PS samples were prepared from h100i oriented 500 µm thick P-type silicon wafers with resistivity of 1–10 cm. An aluminum contact layer was deposited by thermal evaporation on the back side of the wafers. The PS layers were formed by electrochemical etching in a HF:C2 H5 OH (1:2 in volume) solution for the etching time from 30 to 60 min at a current density of 30 mA/cm2 . The etching was performed under back illumination (50 W). Then the samples were dried in air at room temperature. The samples were characterized by PL spectroscopy, CL spectroscopy, FTIR spectroscopy, Raman spectroscopy and SEM observation. The PL spectra excited by a 360 nm wavelength laser were measured using a HITACHI F4500 fluorescence spectrophotometer. The infrared absorption spectra were taken using a Bruker IFS 66 v/S FTIR spectroscope. The Raman spectra excited with 532 nm line were measured using a Nicolet Almege Raman Spectrometer. The appearance observations were carried out using FEI SIRION (Fig. 1(a)) under acceleration voltage 5 kV and FEI Hitachi S4200 (Fig. 1(b)) under acceleration voltage 10 kV. The CL spectra in the spectral range from 168 to 1024 nm with the resolution of 8 nm were performed in an FEI Hitachi S4200 with a CL system under acceleration voltage 10 keV and beam current density 1nA (the measurement of CL spectra were made in Nanomaterials Laboratory, National Institute for Materials Science, Tsukuba 305-0047, Japan). The optical system for light collection was designed to realize a high collection efficiency of luminescence photons and to achieve uniformity of collection efficiency. An ellipsoidal mirror of low magnification and an optical fiber were adopted. The focal length of the ellipsoid was 35 mm and its principal axis was tilted by 12◦ from horizontal. The light was led to the monochromator (Jobin Yvon, Triax 320), which has 3 gratings and two exits of light. For the spectral work, a liquid N2 cooling CCD (Jobin Yvon, Spectrum One) was adopted. A parallel detection of CL photons remarkably reduced the acquisition time of one spectrum, not only reducing damage and/or contamination but also avoiding drift of the specimen. The interval time between CL spectra is 2 s. All the measurements were carried out at room temperature.
Fig. 1. SEM images of PS prepared at 60 min before (a) and after (b) electron beam radiation.
3. Results and discussion The Fig. 1 shows the morphologies of PS sample prepared at 60 min before and after electron beam radiation. The sample has the columnar structure and the diameter was several micrometers. It also can be seen from Fig. 1(a) and (b) that after electron beam radiation the fine structure of as-prepared sample was lost, which led to the continuous decrease of CL intensity, as shown in Figs. 2(a) and 3(a). It may be due to an electronic damage of the surface structure as a formation of new morphology and chemical bonds on PS surface. Figs. 2(a) and 3(a) gave continuously changed threedimensional CL spectra and two-dimensional CL spectra of PS sample during charging for 60 s. During charging for 60 s, continuously changed three-dimensional CL spectra and twodimensional CL spectra of similar PS sample after electron beam radiation for 60 s and then aged for 2 min were shown in Fig. 2(b) and Fig. 3(b), respectively. The Fig. 3 is converted from Fig. 2 in order to intuitively show the change of CL intensity, respectively. The color of the various curves in Fig. 3(a) and (b) was used to differentiate the curves tested in different times. The nonlinear variation of CL intensity of as-prepared sample was showed with charging time, as shown in Figs. 2(a) and 3(a) and CL emission band simultaneous shifted from 577 to 450 nm. When the sample aged for 2 min after electron beam radiation for 60 s, the CL intensity of PS
Y. Zhao et al. / Solid State Communications 143 (2007) 197–201
199
Fig. 3. Two-dimensional CL spectra converted from Fig. 2, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2. Three-dimensional CL spectra of PS samples without (a) or with (b) electron beam radiation for 60 s under the situation for CL measurements and then stop electron beam radiation about 2 min for aging before CL measurements.
sample showed little change during charging for 60 s. A blue shift of CL spectra from 464 to 457 nm is also simultaneously observed. The CL peaks at 520 and 650–670 nm are considered to be linked with the oxide phase presented in PS and its origin is associated with the presence of hydrogen atoms in the oxide phase [9]. But in other papers, the observed CL spectrum is linked with the amorphous SiO2 content on PS surface [10]. In addition, the degradation of CL signal observed during electron-beam radiation has been reported. Maurice et al. [13] considered that the CL degradation is probably related to removing hydrogen atoms. Cullis et al. [9] suggested the variation of CL intensity is related to the removing or the reintroduction of the hydrogen atoms on the surface of the PS samples. But other authors [11] reported that the reduced CL intensity is probably due to the electronic damage of the luminescent structure as the formation of contamination films. In our experiment, it can be seen that the CL peak of asprepared PS exhibited a continuous shift from 577 to 450 nm and the CL intensity also showed a continuous decrease with the charging time, so the CL emission may be attributed to the oxide phase presented in PS and its origin is associated with the presence of hydrogen in the oxide phase, as illustrated in
Refs. [9,10]. Similar to the results in Ref. [11], the surface fine structure of porous silicon was lost after electron beam radiation, as shown in Fig. 1, which may lead to the decrease of CL intensity, as shown in Fig. 3(a). The use of electron beam radiation permitted us to modify the density of irradiative centers as point defects or destroy luminescent centers, which might be possible through the damage of PS delicate structure or the variation of the number of hydrogen-containing groups. In addition, similar to the laser-beam radiation [14,15], the thermal effect of electron-beam radiation on PS sample may lead to the oxidation of PS surface, which may lead to stabilize CL emission of PS in further electron-beam radiation process and decrease CL intensity slightly. The Fig. 4 gave CL spectra and PL spectra of PS samples prepared at different etching times. A peak centered at 570 nm was shown in PL spectra of PS prepared in different etching time, but CL spectra of corresponding samples showed a peak at 600 nm. While the etching time increases, CL intensity increases and PL intensity decreases simultaneously, but we cannot see any shift of these peaks, as shown in Fig. 4. Fig. 5(a) represents the FTIR spectra of PS samples prepared in different times. It is reported that the adsorption peaks at about 1058 and 459 cm−1 are related to Si–O–Si stretching mode [16], and one peak around 840 cm−1 is related to Si–O–H stretching mode [17]. Further, the peak at 619 cm−1 presents the binding states of Si–Si vibration in bulk silicon [18], while the peaks at around 2086 and 2243 cm−1 are attributed to the Si–H stretching mode.
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Y. Zhao et al. / Solid State Communications 143 (2007) 197–201
surface [23–25]. In our experiment, from the frequencies of the Raman peaks in Fig. 5(b), the quantum size effects cannot be observed in the PS film, so it shows that the visible PL emission may be related to the defects of dioxide or the radiative center on Si-wire surface. Furthermore, CL intensity of the samples prepared from different times was enhanced when the peak’s intensity related to the Si–H stretching mode in oxide phase was increased. Furthermore, it was well known that the quantum of Si–O vibration mode increased with the decrease of the Si–H vibration mode. It was indicated that the PL emission of PS samples is related to the defects of dioxide because the PL intensity decreased with the decrease of the Si–O vibration mode. Furthermore, the same results were received from the PS samples prepared by N-type silicon wafers under the same experimental parameters. 4. Summary
Fig. 4. CL spectra (a) and PL spectra (b) of as-prepared PS prepared under different etched time.
CL emission and PL emission of PS prepared from different etching times were investigated. Strong CL emission may be attributed to the presence of hydrogen in the oxide phase, but PL emission may be associated with the defects of the dioxide. The degradation of CL intensity of as-prepared samples under electron beam radiation is attributed to removing of the hydrogen atoms or destruction of radiative centers on PS surface. During electron-beam radiation, the stabilization of CL emission of the sample after electron beam radiation for 60 s and then aged for 2 min is due to the oxidation of PS surface. References
Fig. 5. FTIR spectra (a) and Raman spectra (b) of as-prepared PS prepared under different etched time.
It was indicated that the visible PL emission might come from the defects in silicon dioxide [19–21] or from crystalline quantum dots [22], which can lead to enhancement of energy gap, or from different radiative centers on Si-wire
[1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] A. Parisini, R. Angelucci, L. Dori, A. Poggi, P. Maccagnani, G.C. Cardinali, G. Amato, G. Lerondel, D. Midellino, Micron 31 (2000) 223. [3] N. Rigakis, J. Hilliard, L. AbuHassan, J.M. Hetrick, D. Andsager, M.H. Nayfeh, J. Appl. Phys. 81 (1) (1997) 440. [4] H. Koyama, Y. Matsushita, N. Koshida, J. Appl. Phys. 83 (3) (1998) 1776. [5] Gubo Li, Xiaoyuan Hou, Shuai Yuan, Huajie Chen, Fulong Zhang, Honglei Fan, Xun Wang, J. Appl. Phys. 80 (10) (1996) 5967. [6] K. Molnar, T. Mohacsy, P. Varga, E. Vazsonyi, I. Barsony, J. Lumin. 80 (1999) 91. [7] Y.E. Babanov, A.V. Prokaznikov, N.A. Rud, V.B. Svetovoy, Phys. Status Solidi A 162 (1997) R7. [8] C. Jardin, B. Gruzza, E. Vazsonyi, G. Gergely, Vacuum 46 (5) (1995) 497. [9] A.G. Cullis, L.T. Canham, G.M. Williams, P.W. Smith, O.D. Dosser, J. Appl. Phys. 75 (1) (1994) 493. [10] T. Suzuki, T. Sakai, L. Zhang, Y. Nishiyama, Appl. Phys. Lett. 66 (2) (1995) 215. [11] A. Bruska, A. Chernook, S. Schulze, M. Hietschold, Appl. Phys. Lett. 68 (17) (1996) 2378. [12] J. Rams, B. Mendez, G. Craciun, R. Plugaru, J. Piqueras, Appl. Phys. Lett. 74 (12) (1999) 1728. [13] J.L. Maurice, A. Riviere, A. Alapini, C.L. Clement, Appl. Phys. Lett. 66 (13) (1994) 1665. [14] S. Manotas, F. Rueda, J.D. Moreno, F.B. Hander, R.G. Lemus, J.M.M. Duart, Phys. Status Solidi A 182 (2000) 331. [15] W.J. Salcedo, F.J.R. Fernandez, J.C. Rubim, J. Raman, Spectrosec. 32 (2001) 151.
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