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Emission behavior of pure and lithium intercalated porous silicon
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 3 (2016) 1707–1711
www.materialstoday.com/proceedings
Recent Advances In Nano Science And Technology 2015 (RAINSAT2015)
Emission behavior of pure and lithium intercalated porous silicon R. Mohandoss, C. Lakshmanan, K. K. Madapu, R. N. Viswanath*,S. Dash, A. K. Tyagi Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam – 603 102, TamilNadu, India.
Abstract Photoluminescence studies were performed on the etched porous silicon using a micro-Raman Spectrometer. Three luminescence peaks, two sharp peaks at 1.11 eV (1117 nm) and at 1.20 eV (1033 nm) in the infra-red region and a broad peak at 1.83 eV (678 nm) in the visible region were observed. To understand the significance of these luminescence features found in porous silicon, we compared the results from porous silicon with that of bulk silicon and Li intercalated porous silicon. © 2015Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.]. Keywords: porous silicon; photoluminescence effect; electrochemical etching.
1. Introduction Porous silicon is an important device material and it has been much studied since 1990 after the discovery of efficient visible light emission from its pore surfaces at room temperature [1]. Leigh Canham in his demonstration experiment showed a strong luminescence radiation emitted from silicon at wavelengths spanning from near infrared to visible blue on varying its surface treatment. Followed by Canham’s novel findings, there have been tremendous research activity into the physical and chemical characteristics of porous silicon and the underlying quantum
* Corresponding author. Tel.: +91 44 2748 0500 22514; fax: +91 44 2748 0081. E-mail address: [email protected]
2214-7853© 2015 Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015. ].
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R. Mohandoss, et al./ Materials Today: Proceedings 3 (2016) 1707–1711
confinement effect (QCE) which is to explain the visible luminescence flux observed [2-6]. Takagi et al. found that below the size limit of ~ 5 nm for manifestation of quantum confinement in silicon and at energy where photoluminescence occurs, the variation of luminescence peak energy scales with structure size as 1/d2, where d is the crystallites or pore diameter in porous silicon [7]. However, as evident from reliable reports, the variation in photoluminescence peak energy in silicon depends also on chemical modifications in the porous region along with the presence of defects at the porous-planar interface. Therefore, understanding the relative importance of surface chemical and quantum size effects towards luminescence emission in silicon remains quite challenging and its origin is still under debate and discussions. Nevertheless, the engineering of luminescence peak energy towards blue wavelength region creates opportunity for developing porous silicon based devices to use as cantilever based biosensors, anode material for Li-ion batteries, light emitting diodes, substrate as well as integration of semiconducting devices [8-11]. Our current research interest is mainly focused on fabrication of porous silicon template for growing organic/inorganic nanostructures in the porous region for obtaining three-dimensional functional hybrid materials with requisite optical characteristics. To start with, we have fabricated porous silicon using a metal assisted electrochemical etching process. Besides the fabrication of porous silicon, we have also studied the luminescence behaviour in porous silicon under different treatment achieved by varying etching durations and lithium doping. 2. Experimental Procedure P-type single side polished silicon (100) wafers were ultrasonically degreased with acetone and ethanol for 10 minutes each followed by copious rinsing with 18.2 MΩ Millipore water. The organic residue on the silicon wafers was cleaned with piranha solution (3:1 ratio of con. H2SO4 and 30 % H2O2). Since the piranha solution is a strong oxidizer, it removed most of the organic contaminants and rendered the surface of silicon wafer hydrophilic. Before onset of etching process, the wafers were dipped in 5% HF solution for 5 minutes to remove the native oxide layers. An eutectic InGa alloy was used to establish an ohmic contact between silicon wafers and copper electrical terminal. The low work function of the InGa eutectic makes it a good electrical contact for silicon wafer. Electrochemical etching experiments were performed in a teflon electrochemical cell in two-electrode configuration with Ag deposited silicon used as a counter electrode. A graphite rod served as a working electrode, 10% HF solution was used as an etchant. A commercial AutoLab PG STAT302N potentiostat/galvanostat was connected to the silicon and the carbon electrodes. A constant current of 5 mA was transmitted through the Ag deposited silicon wafer during the etching process. In addition to these studies, the pore surfaces of silicon were intercalated with lithium using an electrochemical route. This intercalation was carried out at room temperature in a three-electrode teflon cell. Porous silicon, carbon cuboids and Ag/AgCl pseudo wire served as working, counter and reference electrodes, respectively. The electrolyte used was 1 M LiClO4/acetonitrile. A Carl Zeiss scanning electron microscope was used to study the surface morphology of porous silicon. The photoluminescence study was performed using a micro-Raman Spectrometer (Renishaw inVia). Excitation light of wavelengths 514 nm and 785 nm from Ar+ ion and diode lasers respectively, were focussed independently on the porous silicon surface in a backscattered configuration. The scattered luminescence radiation signals from the samples were collected and analyzed. The laser spot size fixed in the present study was 0.69 µm for Ar+ laser and 1.06 µm for diode laser. The luminescence experiments were carried out only on the porous silicon samples prepared from the silicon wafers after their surface treatment with 5% HF. 3. Results and Discussion Mechanism involved in synthesis of porous silicon from Ag coated silicon wafer is described here. The Ag coated on the silicon wafers acts as a catalyst for the evolution of porosity. Figure 1 shows a schematic view of twoelectrode electrochemical cell used for the etching experiment. The etching solution used was aerated 10% HF solution. The chemical reactions that occur at the silicon counter electrode and carbon working electrode are Si + 6 HF → H2SiF6 + 4 H+ + 4 e- and O2 + H+ → H2O, respectively [12]. Since the reaction Si + 6 HF → H2SiF6 + 4 H+ + 4 e- releases four electrons, spurious dissolution may occur at the silicon surfaces. A constant current of 5 mA was passed between the silicon and the carbon electrodes to control the etching process. Silver present at the silicon
R. Mohandoss, et al./ Materials Today: Proceedings 3 (2016) 1707–1711
1709
surface facilitates silicon oxidation and formation of porous architecture. Silver attracts the electrons from the reaction Si + 6 HF → H2SiF6 + 4 H+ + 4 e- and transfers to carbon electrode through the solution. Due to this local charge transfer event, silicon oxidation and dissolution occur favourably underneath the Ag sites. In addition to this, charge transfer also occurs between electrodes through the electrical terminals. All these charge transfer events maintain charge neutrality in the cell to facilitate smooth dissolution of silicon.
Fig. 1. Schematic of electrochemical set-up used for the etching experiments. The chemical reaction that occurred at the silicon-solution and carbon-solution interfaces are listed in the figure. The arrows in the figure denote current flow directions in the cell.
Figure 2 shows scanning electron microscopy images taken on the surface of three porous silicon samples. Figures 2a) and 2 b) show the microscopy images of the porous silicon obtained after electrochemical etching in 10 % HF solution for 4 h and 10 h, respectively. It is worth noting that these wafers were not subjected initially to 5% HF treatment to remove the native oxide layer. The pore diameter estimated from the analysis of the microscopy images using the image J software is in the range of 40 - 60 nm for both 4h and 10 h etched samples. Figure 2 c) shows the microscopy image of 4 h etched porous silicon in 10% HF solution. The micrograph as shown in Fig. 2c) was obtained from the native oxide removed silicon wafers. The estimated mean pore diameter value is 20 nm. As expected, pore size in case of 5% HF treated silicon wafer is smaller than that obtained from the untreated silicon wafer. This suggests that the prolonged longitudinal etching is quite favourable for silicon wafers free of native oxide layers. Figure 3 summarizes photoluminescence results of porous silicon samples laser excited at two wavelengths λexc= 514 nm and 785 nm. The figure also illustrates the luminescence spectra obtained from bulk silicon. It is seen that the bulk silicon (grey curve) does not show any luminescence peak in the visible light emitting region because of its indirect band gap. The luminescence spectra of bulk silicon show two strong emission peaks at 1.11 eV and at 1.20 eV. These peak energies are close to the silicon band gap energy of 1.13 eV that emits infrared radiation [13]. This implicates that the origin of the emission peaks at 1.11 eV and at 1.20 eV might be due to the momentum conserving transverse-optical phonon mode (1117 nm) and transverse-acoustical phonons assisted absorption edge (1033 nm) present in silicon. The light emitting behaviour in bulk silicon varies when the silicon surface become porous. As shown in the Fig. 3, porous silicon samples etched for 4 h and 10 h showed luminescence emission peak at 678 nm under the illumination from Ar+ laser. In addition, the porous silicon emits characteristic silicon radiations at 1.11 eV and at 1.20 eV. These intrinsic radiations might be emitted from planar silicon surfaces present underneath to the porous region. It is seen that the features present in luminescence spectra for 4 h and 10 h etched porous silicon are quite similar, except that there is a variation in their luminescence intensities. This implies that the evolved structures in the porous region that are responsible for the emission of visible radiation increasing with etching period. This finding supports the previous works on luminescence behaviour in porous silicon where it has been reported that the luminescence yield depends strongly on surface atomic site and chemical nature of the silicon surfaces [14, 15].
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R. Mohandoss, et al./ Materials Tod oday: Proceedings 3 (2016) 1707–1711
Fig. 2: Scanning electron microscopy images of porous silicon samples es oobtained from three different porous silicon samples. a) and b) microscopy images of porous silicon obtained from untreated silicon wa wafers. c) microscopy image of porous silicon obtained from native oxide removed silicon wafer.
Intensity [a.u.]
3000 2500
1117 nm 103 033 nm
2000
678 nm
1500 1000 500 0 1.0
1.2
1.4
1.6
En Energy
1.8
2.0
2.2
[eV]
Fig. 3. Photoluminescence results for four different silicon samples:: pporous silicon (black - 4h of etching, red - 10 h of etching), bulk silicon (Grey), Li implanted porous silicon (blue). The results depicted in the figure are obtained on illuminating the sample surface with laser excitations at two wavelengths λexc= 514 nm and 785 nm. The peaks ks at 1.11 eV and at 1.20 eV are resolved from the original luminescence emission peak pronounced at 1.13 eV.
Based on the preliminary results as discussed above,, w we attempt to explain whether the emission peak appearing in the visible region can be attributed to the quantum con confinement effect or due to any chemical modifications that occur on the pore surfaces during the etching process. Ass discussed in the introduction, there are a number of reports pointing out that by varying the externally controlled etch tching parameters, for example, concentration of HF solution, current density, degree of impurity doping, temperatur ture, the luminescence properties in porous silicon can be tailored. The following is an exemplary result that we ex explored to explain the role of external process parameters which influences luminescence property of porous silic ilicon. Porous silicon treated electrochemically in lithium perchlorate solution did not produce any luminescence ra radiation both in the visible and in the infrared regions (blue curve). In this case, the over deposited lithium on the po porous region suppresses the luminescence behaviour of the silicon by the fluorescence quenching effect [16, 17].. T This reveals that the diffused lithium decouple the optical carrier population in porous silicon and enables to prevent ent the emission of luminescent radiations. It is originally predicted by Cullis and Canham thatt vi visible emission might be expected from structures of sizes down below 5 nm in porous silicon. This prediction iss vvaluable irrespective of the delocalised confinement region present in the porous silicon, whether the shape of the reg region is a plate or a wire or a dot. The analysis of scanning electron microscopy images shows that the mean pore dia diameter and the inter-porous region present is ≥ 20 nm. This value is much larger than the quantum confinement region ion that governs silicon energy band structure and contribute to visible luminescence. This suggests that the prese esent porous silicon samples produce luminescence from crystalline regions of sizes comparable or smaller than the their quantum confinement size limit. This is consistent with the previous report by Sham et al. that porous silicon is cr crystalline locally and supports the wave length dependence
R. Mohandoss, et al./ Materials Today: Proceedings 3 (2016) 1707–1711
1711
of the luminescence [18].Since the structure size being discussed for the electronic confinement is comparable or below the resolution of scanning electron microscopy, further studies are being carried out to determine accurately the size of the delocalized crystallites and the type of defects present in the porous silicon. 4. Conclusions We have synthesized porous silicon samples by electrochemical etching of silicon (100) wafers in 10% HF solution by passing a constant current of 5 mA. The analysis of the scanning electron microscopy images reveals that the pore diameter and the width of the inter-porous region are ≥ 20 nm. This is much larger than the confinement size that governs the energy band structure which contributes to visible luminescence. We infer from the analysis of the luminescence results that the dominant source for the visible luminescence peak appearing in the porous silicon is due to quantum confinement effect. The chemical modifications on the porous silicon deteriorate the photon flux density as confirmed from the absence of emission radiations in the lithium over doped or precipitated porous silicon. Acknowledgments RNV acknowledges financial support from DST, Government of India through Ramanujan Fellow award ship. The authors RM and RNV acknowledge the DST for the financial support under sponsored project SR/S2/CMP0125/2012. CL acknowledges the Department of Atomic Energy for the JRF fellowship granted. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
L. T. Canham, Appl. Phys. Letts. 57 (1990) 1046 - 1048. M. Kittler, W. Seifert, T. Arguirov, I. Tarasov, S. Ostapenko, Solar Energy Materials & Solar Cells 72 (2002) 465 - 472. S. S. Walavalkar, C. E. Hofmann, A. P. Homyk, M. D. Henry, H. A. Atwater, A. Scherer, Nano Letts. 10 (2010) 4423 - 4428. Wang, S. Schönherr, R. Ji, A. Herz, C. Ronning, P. Schaaf, J. Micromech. Microeng., 23 (2013) 074004 p 1- 6. P. Granitzer, K. Rumpf, Materials, 3 (2010) 943 - 998. A. G. Cullis, L. T. Canham, Nature 353 (1991) 335 - 338. H. Takagi, H. Owada, Y. Yamazaki, A. Ishizaky, T. Nakagiri, Appl. Phys. Letts. 56 (1990) 2379 - 2380. H. Föll, J. Carstensen, S. Frey, J. Nanomaterials, Hindawi Publishing Corporation (2006) Article ID: 91635, pp. 1-10 H. Föll, M. Christophersen, J. Carstensen, G. Hasse, Mater. Sci. and Eng. R 280 (2002) 1- 49. G. Gautier, P. Leduc, Applied Physics Reviews 1 (2014) 011101 p 1-18. J-K. Kim, J-K. Park, H-K. Kim, Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 113 - 117. L. Liu, K-Q. Peng, Y. Hu, X-L. Wu, S-T. Lee, Adv. Mater. 26 (2014) 1410 - 1413. Wagner, Phys. Rev. B, 29 (1984) 2002 - 2009. X. Y. Hou, G. Shi, W. Wang, F. L. Zhang, P. H. Hao, D. M. Huang, X. Wang, Appl. Phys. Lett. 62 (1993) 1097 - 1098. H. Kim, N. Cho, Nanoscale Research Lett. 7 (2012) 1-8. M. Mac, B. Tokarczyk, Chem. Phys. Lett. 304 (1999) 309 - 316 M. Mac, J. Photochem. Photobiol. A: Chem. 107 (1997) 107- 113. T. K. Sham, D. T. Jiang, I. Coulthard, J. W. Lorimer, X. H. Feng, K. H. Tan, S. P. Frigo, R. A. Rosenberg, D. C.
ScienceDirect Materials Today: Proceedings 3 (2016) 1707–1711
www.materialstoday.com/proceedings
Recent Advances In Nano Science And Technology 2015 (RAINSAT2015)
Emission behavior of pure and lithium intercalated porous silicon R. Mohandoss, C. Lakshmanan, K. K. Madapu, R. N. Viswanath*,S. Dash, A. K. Tyagi Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam – 603 102, TamilNadu, India.
Abstract Photoluminescence studies were performed on the etched porous silicon using a micro-Raman Spectrometer. Three luminescence peaks, two sharp peaks at 1.11 eV (1117 nm) and at 1.20 eV (1033 nm) in the infra-red region and a broad peak at 1.83 eV (678 nm) in the visible region were observed. To understand the significance of these luminescence features found in porous silicon, we compared the results from porous silicon with that of bulk silicon and Li intercalated porous silicon. © 2015Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.]. Keywords: porous silicon; photoluminescence effect; electrochemical etching.
1. Introduction Porous silicon is an important device material and it has been much studied since 1990 after the discovery of efficient visible light emission from its pore surfaces at room temperature [1]. Leigh Canham in his demonstration experiment showed a strong luminescence radiation emitted from silicon at wavelengths spanning from near infrared to visible blue on varying its surface treatment. Followed by Canham’s novel findings, there have been tremendous research activity into the physical and chemical characteristics of porous silicon and the underlying quantum
* Corresponding author. Tel.: +91 44 2748 0500 22514; fax: +91 44 2748 0081. E-mail address: [email protected]
2214-7853© 2015 Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015. ].
1708
R. Mohandoss, et al./ Materials Today: Proceedings 3 (2016) 1707–1711
confinement effect (QCE) which is to explain the visible luminescence flux observed [2-6]. Takagi et al. found that below the size limit of ~ 5 nm for manifestation of quantum confinement in silicon and at energy where photoluminescence occurs, the variation of luminescence peak energy scales with structure size as 1/d2, where d is the crystallites or pore diameter in porous silicon [7]. However, as evident from reliable reports, the variation in photoluminescence peak energy in silicon depends also on chemical modifications in the porous region along with the presence of defects at the porous-planar interface. Therefore, understanding the relative importance of surface chemical and quantum size effects towards luminescence emission in silicon remains quite challenging and its origin is still under debate and discussions. Nevertheless, the engineering of luminescence peak energy towards blue wavelength region creates opportunity for developing porous silicon based devices to use as cantilever based biosensors, anode material for Li-ion batteries, light emitting diodes, substrate as well as integration of semiconducting devices [8-11]. Our current research interest is mainly focused on fabrication of porous silicon template for growing organic/inorganic nanostructures in the porous region for obtaining three-dimensional functional hybrid materials with requisite optical characteristics. To start with, we have fabricated porous silicon using a metal assisted electrochemical etching process. Besides the fabrication of porous silicon, we have also studied the luminescence behaviour in porous silicon under different treatment achieved by varying etching durations and lithium doping. 2. Experimental Procedure P-type single side polished silicon (100) wafers were ultrasonically degreased with acetone and ethanol for 10 minutes each followed by copious rinsing with 18.2 MΩ Millipore water. The organic residue on the silicon wafers was cleaned with piranha solution (3:1 ratio of con. H2SO4 and 30 % H2O2). Since the piranha solution is a strong oxidizer, it removed most of the organic contaminants and rendered the surface of silicon wafer hydrophilic. Before onset of etching process, the wafers were dipped in 5% HF solution for 5 minutes to remove the native oxide layers. An eutectic InGa alloy was used to establish an ohmic contact between silicon wafers and copper electrical terminal. The low work function of the InGa eutectic makes it a good electrical contact for silicon wafer. Electrochemical etching experiments were performed in a teflon electrochemical cell in two-electrode configuration with Ag deposited silicon used as a counter electrode. A graphite rod served as a working electrode, 10% HF solution was used as an etchant. A commercial AutoLab PG STAT302N potentiostat/galvanostat was connected to the silicon and the carbon electrodes. A constant current of 5 mA was transmitted through the Ag deposited silicon wafer during the etching process. In addition to these studies, the pore surfaces of silicon were intercalated with lithium using an electrochemical route. This intercalation was carried out at room temperature in a three-electrode teflon cell. Porous silicon, carbon cuboids and Ag/AgCl pseudo wire served as working, counter and reference electrodes, respectively. The electrolyte used was 1 M LiClO4/acetonitrile. A Carl Zeiss scanning electron microscope was used to study the surface morphology of porous silicon. The photoluminescence study was performed using a micro-Raman Spectrometer (Renishaw inVia). Excitation light of wavelengths 514 nm and 785 nm from Ar+ ion and diode lasers respectively, were focussed independently on the porous silicon surface in a backscattered configuration. The scattered luminescence radiation signals from the samples were collected and analyzed. The laser spot size fixed in the present study was 0.69 µm for Ar+ laser and 1.06 µm for diode laser. The luminescence experiments were carried out only on the porous silicon samples prepared from the silicon wafers after their surface treatment with 5% HF. 3. Results and Discussion Mechanism involved in synthesis of porous silicon from Ag coated silicon wafer is described here. The Ag coated on the silicon wafers acts as a catalyst for the evolution of porosity. Figure 1 shows a schematic view of twoelectrode electrochemical cell used for the etching experiment. The etching solution used was aerated 10% HF solution. The chemical reactions that occur at the silicon counter electrode and carbon working electrode are Si + 6 HF → H2SiF6 + 4 H+ + 4 e- and O2 + H+ → H2O, respectively [12]. Since the reaction Si + 6 HF → H2SiF6 + 4 H+ + 4 e- releases four electrons, spurious dissolution may occur at the silicon surfaces. A constant current of 5 mA was passed between the silicon and the carbon electrodes to control the etching process. Silver present at the silicon
R. Mohandoss, et al./ Materials Today: Proceedings 3 (2016) 1707–1711
1709
surface facilitates silicon oxidation and formation of porous architecture. Silver attracts the electrons from the reaction Si + 6 HF → H2SiF6 + 4 H+ + 4 e- and transfers to carbon electrode through the solution. Due to this local charge transfer event, silicon oxidation and dissolution occur favourably underneath the Ag sites. In addition to this, charge transfer also occurs between electrodes through the electrical terminals. All these charge transfer events maintain charge neutrality in the cell to facilitate smooth dissolution of silicon.
Fig. 1. Schematic of electrochemical set-up used for the etching experiments. The chemical reaction that occurred at the silicon-solution and carbon-solution interfaces are listed in the figure. The arrows in the figure denote current flow directions in the cell.
Figure 2 shows scanning electron microscopy images taken on the surface of three porous silicon samples. Figures 2a) and 2 b) show the microscopy images of the porous silicon obtained after electrochemical etching in 10 % HF solution for 4 h and 10 h, respectively. It is worth noting that these wafers were not subjected initially to 5% HF treatment to remove the native oxide layer. The pore diameter estimated from the analysis of the microscopy images using the image J software is in the range of 40 - 60 nm for both 4h and 10 h etched samples. Figure 2 c) shows the microscopy image of 4 h etched porous silicon in 10% HF solution. The micrograph as shown in Fig. 2c) was obtained from the native oxide removed silicon wafers. The estimated mean pore diameter value is 20 nm. As expected, pore size in case of 5% HF treated silicon wafer is smaller than that obtained from the untreated silicon wafer. This suggests that the prolonged longitudinal etching is quite favourable for silicon wafers free of native oxide layers. Figure 3 summarizes photoluminescence results of porous silicon samples laser excited at two wavelengths λexc= 514 nm and 785 nm. The figure also illustrates the luminescence spectra obtained from bulk silicon. It is seen that the bulk silicon (grey curve) does not show any luminescence peak in the visible light emitting region because of its indirect band gap. The luminescence spectra of bulk silicon show two strong emission peaks at 1.11 eV and at 1.20 eV. These peak energies are close to the silicon band gap energy of 1.13 eV that emits infrared radiation [13]. This implicates that the origin of the emission peaks at 1.11 eV and at 1.20 eV might be due to the momentum conserving transverse-optical phonon mode (1117 nm) and transverse-acoustical phonons assisted absorption edge (1033 nm) present in silicon. The light emitting behaviour in bulk silicon varies when the silicon surface become porous. As shown in the Fig. 3, porous silicon samples etched for 4 h and 10 h showed luminescence emission peak at 678 nm under the illumination from Ar+ laser. In addition, the porous silicon emits characteristic silicon radiations at 1.11 eV and at 1.20 eV. These intrinsic radiations might be emitted from planar silicon surfaces present underneath to the porous region. It is seen that the features present in luminescence spectra for 4 h and 10 h etched porous silicon are quite similar, except that there is a variation in their luminescence intensities. This implies that the evolved structures in the porous region that are responsible for the emission of visible radiation increasing with etching period. This finding supports the previous works on luminescence behaviour in porous silicon where it has been reported that the luminescence yield depends strongly on surface atomic site and chemical nature of the silicon surfaces [14, 15].
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Fig. 2: Scanning electron microscopy images of porous silicon samples es oobtained from three different porous silicon samples. a) and b) microscopy images of porous silicon obtained from untreated silicon wa wafers. c) microscopy image of porous silicon obtained from native oxide removed silicon wafer.
Intensity [a.u.]
3000 2500
1117 nm 103 033 nm
2000
678 nm
1500 1000 500 0 1.0
1.2
1.4
1.6
En Energy
1.8
2.0
2.2
[eV]
Fig. 3. Photoluminescence results for four different silicon samples:: pporous silicon (black - 4h of etching, red - 10 h of etching), bulk silicon (Grey), Li implanted porous silicon (blue). The results depicted in the figure are obtained on illuminating the sample surface with laser excitations at two wavelengths λexc= 514 nm and 785 nm. The peaks ks at 1.11 eV and at 1.20 eV are resolved from the original luminescence emission peak pronounced at 1.13 eV.
Based on the preliminary results as discussed above,, w we attempt to explain whether the emission peak appearing in the visible region can be attributed to the quantum con confinement effect or due to any chemical modifications that occur on the pore surfaces during the etching process. Ass discussed in the introduction, there are a number of reports pointing out that by varying the externally controlled etch tching parameters, for example, concentration of HF solution, current density, degree of impurity doping, temperatur ture, the luminescence properties in porous silicon can be tailored. The following is an exemplary result that we ex explored to explain the role of external process parameters which influences luminescence property of porous silic ilicon. Porous silicon treated electrochemically in lithium perchlorate solution did not produce any luminescence ra radiation both in the visible and in the infrared regions (blue curve). In this case, the over deposited lithium on the po porous region suppresses the luminescence behaviour of the silicon by the fluorescence quenching effect [16, 17].. T This reveals that the diffused lithium decouple the optical carrier population in porous silicon and enables to prevent ent the emission of luminescent radiations. It is originally predicted by Cullis and Canham thatt vi visible emission might be expected from structures of sizes down below 5 nm in porous silicon. This prediction iss vvaluable irrespective of the delocalised confinement region present in the porous silicon, whether the shape of the reg region is a plate or a wire or a dot. The analysis of scanning electron microscopy images shows that the mean pore dia diameter and the inter-porous region present is ≥ 20 nm. This value is much larger than the quantum confinement region ion that governs silicon energy band structure and contribute to visible luminescence. This suggests that the prese esent porous silicon samples produce luminescence from crystalline regions of sizes comparable or smaller than the their quantum confinement size limit. This is consistent with the previous report by Sham et al. that porous silicon is cr crystalline locally and supports the wave length dependence
R. Mohandoss, et al./ Materials Today: Proceedings 3 (2016) 1707–1711
1711
of the luminescence [18].Since the structure size being discussed for the electronic confinement is comparable or below the resolution of scanning electron microscopy, further studies are being carried out to determine accurately the size of the delocalized crystallites and the type of defects present in the porous silicon. 4. Conclusions We have synthesized porous silicon samples by electrochemical etching of silicon (100) wafers in 10% HF solution by passing a constant current of 5 mA. The analysis of the scanning electron microscopy images reveals that the pore diameter and the width of the inter-porous region are ≥ 20 nm. This is much larger than the confinement size that governs the energy band structure which contributes to visible luminescence. We infer from the analysis of the luminescence results that the dominant source for the visible luminescence peak appearing in the porous silicon is due to quantum confinement effect. The chemical modifications on the porous silicon deteriorate the photon flux density as confirmed from the absence of emission radiations in the lithium over doped or precipitated porous silicon. Acknowledgments RNV acknowledges financial support from DST, Government of India through Ramanujan Fellow award ship. The authors RM and RNV acknowledge the DST for the financial support under sponsored project SR/S2/CMP0125/2012. CL acknowledges the Department of Atomic Energy for the JRF fellowship granted. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
L. T. Canham, Appl. Phys. Letts. 57 (1990) 1046 - 1048. M. Kittler, W. Seifert, T. Arguirov, I. Tarasov, S. Ostapenko, Solar Energy Materials & Solar Cells 72 (2002) 465 - 472. S. S. Walavalkar, C. E. Hofmann, A. P. Homyk, M. D. Henry, H. A. Atwater, A. Scherer, Nano Letts. 10 (2010) 4423 - 4428. Wang, S. Schönherr, R. Ji, A. Herz, C. Ronning, P. Schaaf, J. Micromech. Microeng., 23 (2013) 074004 p 1- 6. P. Granitzer, K. Rumpf, Materials, 3 (2010) 943 - 998. A. G. Cullis, L. T. Canham, Nature 353 (1991) 335 - 338. H. Takagi, H. Owada, Y. Yamazaki, A. Ishizaky, T. Nakagiri, Appl. Phys. Letts. 56 (1990) 2379 - 2380. H. Föll, J. Carstensen, S. Frey, J. Nanomaterials, Hindawi Publishing Corporation (2006) Article ID: 91635, pp. 1-10 H. Föll, M. Christophersen, J. Carstensen, G. Hasse, Mater. Sci. and Eng. R 280 (2002) 1- 49. G. Gautier, P. Leduc, Applied Physics Reviews 1 (2014) 011101 p 1-18. J-K. Kim, J-K. Park, H-K. Kim, Colloids and Surfaces A: Physicochem. Eng. Aspects 241 (2004) 113 - 117. L. Liu, K-Q. Peng, Y. Hu, X-L. Wu, S-T. Lee, Adv. Mater. 26 (2014) 1410 - 1413. Wagner, Phys. Rev. B, 29 (1984) 2002 - 2009. X. Y. Hou, G. Shi, W. Wang, F. L. Zhang, P. H. Hao, D. M. Huang, X. Wang, Appl. Phys. Lett. 62 (1993) 1097 - 1098. H. Kim, N. Cho, Nanoscale Research Lett. 7 (2012) 1-8. M. Mac, B. Tokarczyk, Chem. Phys. Lett. 304 (1999) 309 - 316 M. Mac, J. Photochem. Photobiol. A: Chem. 107 (1997) 107- 113. T. K. Sham, D. T. Jiang, I. Coulthard, J. W. Lorimer, X. H. Feng, K. H. Tan, S. P. Frigo, R. A. Rosenberg, D. C.