- Email: [email protected]
Enhanced optical and electrical stability of thermally carbonized porous silicon
Materials Science in Semiconductor Processing 16 (2013) 542–546
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Short communication
Enhanced optical and electrical stability of thermally carbonized porous silicon N. Naderi, M.R. Hashim n, J. Rouhi, H. Mahmodi Nano-optoelectronics Research Laboratory, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
a r t i c l e in f o
abstract
Available online 25 September 2012
In the current paper, the intrinsic instability of physical properties of porous silicon (PS) was minimized using a thermal carbonization (TC) method. A typical PS showed an obvious quenching of its photoluminescence (PL) properties under long-term laser radiation. To resolve this problem, an ultra-thin stabilizing layer was grown on the porous structure under an acetylene gas flow. During the TC process at high temperatures, carbon atoms were detached from the acetylene molecules and dissolved in the porous structure. Thermally carbonized PS (TC-PS) layer changed the PL peak position of PS slightly due to the formation of Si–C species on porous surface after acetylene exposure. The effects of TC-PS layer in electrical properties of fabricated photodetectors were studied. The photocurrent of freshly prepared PS decreased under prolonged green laser radiation (532 nm, 5 mW), but the TC-PS sample showed more stable electrical characteristics under the same conditions. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Thermal carbonization Porous silicon Photoluminescence Photodetectors
1. Introduction Porous silicon (PS) has recently attracted considerable interest in sensing applications because of its extremely large surface-to-volume ratio and compatibility with silicon-based technologies [1]. However, its optical properties are unstable because of the formation of bonds between silicon and hydrogen atoms (Si–H) at the PS surface during the preparation process [2]. Several studies have reported on the effect of prolonged laser radiation in the photoluminescence (PL) quenching of PS [3,4]. This change in the PL peak is a consequence of the intrinsic instability of PS. Based on the literature, the optical properties of PS can be changed through exposure to different ambient [5–7]. Therefore, the PL quenching caused by low-power laser radiation is a good indicator of the variation in surface bond structures. High-power lasers ( 40 W/cm2) cause structural damage
n
Corresponding author. Tel.: þ 60 4 6533677; fax: þ 60 4 6579150. E-mail address: [email protected] (M.R. Hashim).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.09.010
by melting the silicon walls in the porous structure, whereas lower-power ones ( o1 W/cm2) result in a gradual quenching of the PL intensity [8]. The initial metastable bond configurations are affected by low-power radiation, which leads to a change in the PL spectra [9]. As a solution, fabrication of PS-based devices which have the ability to work under harsh environments such as high temperatures [10] and high corrosive chemicals [11], have been reported by thermal oxidation [12] or carbonization [13] of PS samples. The common aim in all of these methods is replacing hydrogen terminations with other more stable species. Thermal carbonization (TC), which is based on the replacement of the existing Si–H terminations with Si–C species, can be considered a breakthrough in enhancing the stability of the PS structures [14]. The unique characteristics of silicon carbide (SiC), such as chemical inertness and thermal stability [15,16] are the main motivations for PS stabilization using the TC method. The formation of a SiC layer on PS is possible through exposure to acetylene gas at high temperatures [17]. In the current study, desorption of the C2H2, C2H, and C2 species occurs at a temperature of
N. Naderi et al. / Materials Science in Semiconductor Processing 16 (2013) 542–546
750 1C. At temperatures above 700 1C, carbon atoms penetrate into the Si lattice, thereby forming a thin SiC layer [18]. Here, the instability of the PL properties of PS is resolved using the TC process under an acetylene gas flow. To investigate the optical behavior of the fabricated porous structures, the PL of the samples is studied under a continuous laser illumination. The stability of the PL properties of PS is enhanced and PL quenching is minimized under a low-power laser illumination. An application of TC process for enhancing the life-time of PS photodetectors is studied. The electrical characteristics of PS and thermally carbonized PS (TC-PS) substrates are investigated under continuous low power laser illumination. The intrinsic instability in electrical properties of PS is arrested by the TC process. The novelty of this research is in establishment of a correlation between PL stability and electrical properties of PS photodetectors.
2. Experiment All experiments in the present research were performed on 10 mm 10 mm square samples cut from a single crystalline n-type (100) silicon wafer with a resistivity of 16 mO cm measured by the four point probe technique. For the chemical cleaning procedures, the silicon substrates were dipped in 1:1:5 (by volume) NH4OH:H2O2:H2O for 10 min, in 1:50 HF:H2O for 20 s, and then in 1:1:6 HCl:H2O2:H2O for another 10 min. Afterward, the samples were washed with deionized water and dried under an ambient nitrogen flow. The electrical resistivity of silicon wafers reduced to 14 mO cm after cleaning process. Aluminum ( 264 nm thick) was deposited through vacuum evaporation on the backside of the samples and annealed at 450 1C for 10 min under a nitrogen gas flow to facilitate anodization by decreasing the electrical resistivity to 0.06 mO cm. The PS layers were created via etching of crystalline silicon (c-Si) in a hydrofluoric acid-based electrochemical bath at room temperature (RT) under a 100 W tungsten lamp placed 30 cm above the samples. Fig. 1 shows a schematic diagram of the photo-electrochemical cell, which is a Teflon container 10 mm in diameter and 25 mm in height. The solution contained a mixture of hydrofluoric acid (49%), ethanol (95%), and hydrogen peroxide at a volume ratio of 1:2:2. A two-electrode setup was used for the photo-electrochemical etching of silicon substrates. The silicon substrate acted as the anode electrode, whereas an inert metal wire (Pt) was used as the cathode. A pulsed current [19] with a period time (T) of 14 ms and a pause
Fig. 1. Schematic image of the photo-electrochemical etching cell.
543
time (Toff) of 4 ms, with current density of 20 mA/cm2 supplied by a 2400 SourceMeter (Keithley, USA), was used to prepare porous samples. After 30 min etching, asprepared PS samples were dried at RT under a nitrogen flow for 1 h to ensure that all solutions have evaporated. The carbonization was conducted in a quartz tube furnace for 10 min under a continuous nitrogen and acetylene flush at flow rates of 30 L/min and 0.48 L/min, respectively. Thermal treatment followed in the furnace at 750 1C for 15 min. During carbonization, the temperature was monitored using a calibrated thermocouple. The samples were then cooled to RT under a nitrogen flow prior to the contact with ambient air. To study a baseline data, the PL for both the PS and TCPS samples was immediately recorded using a PL spectroscopy system equipped with an argon ion laser (514.5 nm, 20 mW) for excitation. To test the stability of the porous layers, the samples were continuously exposed to laser radiation and PL was measured every 20 min. For metallization of samples in order to fabricate back to back metal-semiconductor Schottky contacts and to study the current–voltage (I–V) characteristics of the metal-semiconductor-metal (MSM) photodetectors, a coplanar finger-shaped structure of Ni with the thickness of 204 nm, and dimension of 3300 3950 mm2 containing finger spacing of 400 mm [20,21] was deposited onto all porous substrates by thermal evaporation of pure nickel in vacuum chamber at pressure of 3.8 10 5 mbar. In order to reduce the atomic mismatch, for a high quality Schottky contact, samples were annealed in tube furnace at temperature of 450 1C under nitrogen gas flow for 10 min. The dark currents (Id) for PS and TC-PS samples were measured in a sealed chamber at RT by sweeping the voltage and measuring the current. For studying the stability of photocurrent (IPh), samples were kept under illumination of low power green laser (532 nm, 5 mW) for 120 min and the I–V measurements were carried out every 30 min. The PL spectra of the samples were obtained at RT using a Jobin-Yvon (HR 800 UV) spectrometer with an argon ion laser for excitation. The electrical measurements were carried out at RT with a computer-controlled integrated SourceMeter (Keithley 2400).
3. Results and discussion The percentages of porosity before and after carbonization were determined using gravimetric measurements [22]. The specific surface area of the as-prepared PS sample was 270 m2/cm3 whereas that of TC-PS decreased to 215 m2/cm3. A sample weight increase of 5 mg was observed for an area of 0.5 cm2 and a thickness of 20 mm after the TC process; these results are related to the porosity of PS substrate. If the carbon atoms are considered to form a uniform Si–C layer over the entire surface of the porous structure, the thickness of the stabilizing layer can be estimated at 5.7 nm. This insignificant thickness of the stabilizing layer slightly reduced the specific surface area of the porous substrate, which is not reflected in the morphology of PS.
544
N. Naderi et al. / Materials Science in Semiconductor Processing 16 (2013) 542–546
The high specific surface area of the freshly etched PS implies a large surface energy and suggests that upon high-temperature annealing, the surface energy can be reduced by the surface diffusion of silicon atoms, which leads to pore coalescence. The presence of an ultrathin Si–C layer on the surface hinders the surface diffusion of silicon atoms even though the surface area does not change significantly. Therefore, this Si–C structure can act as a protective layer for the silicon walls during the sputtering and annealing processes and prevent the occurrence of major changes in the PS structure. The morphology of samples which was shown in our last paper [18] indicated that the specific surface area of PS was slightly reduced due to the formation of TC-PS ultrathin layer which was not reflected in the planar and cross-sectional scanning electron micrographs. The optical behavior of TC-PS under low-power laser radiation at specific intervals was investigated. Fig. 2 shows the PL spectra of the PS and TC-PS samples. The PS spectrum shows a noticeable peak at 629.8 nm. This intensive PL behavior of the as-prepared porous sample results from the quantum confinement of charge carriers in the nanocrystallites [23]. However, a gradual decay ( 35%) was observed in the PL spectrum of the PS sample under a continuous laser radiation, which results from the existence of initial metastable bonds on the surface of the bare PS. These structures are affected by low-power radiation and leads to a change in the PL spectra. The PL spectrum of TC-PS shows a small blue shift ( 8 nm) in the position of PL peak compared to the one for PS sample. This slight change in the peak position is due to the formation of the ultra-thin stabilizing layer, which changes the energy bandgap of PS based on the quantum confinement theory [24]. Compared with the PS sample, a reduction ( 20%) in the intensity of PL peak is observed for TC-PS because of the exposure to acetylene. This phenomenon can be explained by a decrease in the specific surface area after carbonization due to the attachment of the carbon atoms. The effect of TC process as a stabilizing treatment on PS substrate can be seen in the spectrum of the TC-PS sample; PL quenching was arrested and reduced to a
small amount even after prolonged exposure to laser illumination. This behavior is a good indicator of the formation of a practically stable PS surface. Therefore, the thermally carbonized stabilizing layer enhances the optical properties of PS by reducing the PL quenching effect. The current–voltage (I–V) characteristics of fabricated MSM photodetectors based on PS and TC-PS samples are shown in Fig. 3. The dark current (Id) is plotted as a base line. The two sensors were found to show good Schottky behavior. Without applying light, the electrical conductivity of TC-PS is lower than that of PS, indicating that the carbonized sample exhibits a more resistive nature. The pioneer photocurrent which was measured immediately after applying light is shown by a solid line in both diagrams. Upon exposure to photon, PS sample showed a tremendous response to produce significant free carriers for enhanced current conduction. It seems that PS sample is more sensitive to the initial photons received from laser radiation compared to the carbonized one. However, the photocurrent of PS decreased due to the long-term impact of radiation, but this rate of reduction was much lower for carbonized sample even after prolonged exposure to laser beam (120 min). Fig. 4 which summarizes the current gain (ratio of light current to dark current [25]), indicates the stability of electrical properties of TC-PS sample while the initially high gain of PS photodetector was diminished exponentially by continuous laser exposure. Because of limitations in the accuracy of our measurement apparatus, there was an expected average amount of uncertainty for measuring the current gain ( 3.5%). The higher surface resistivity of TC-PS compared to PS can be explained by 2-dimentional model of porous silicon which was proposed by Stievenard and Deresmas [26]. In this model, by increase in chemical species and surface couple bonds on bare silicon, a depletion layer (w) will be grown at silicon walls (d) which are located between two adjacent pores. This will decrease the central channel (d-2w) inside the walls which the carriers can move through when a potential is applied. From this model, after carbonization, the central channel in PS walls
Fig. 2. PL spectra of porous silicon prior carbonization (PS) and after that (TC-PS), under continuous exposure of low power laser radiation for 0, 20 and 40 min.
N. Naderi et al. / Materials Science in Semiconductor Processing 16 (2013) 542–546
545
Fig. 3. I–V characteristics of MSM photodetectors based on PS and TC-PS under continuous exposure of low power laser radiation at regular intervals of 30 min.
4. Conclusions
Fig. 4. Ratio of photocurrent to dark current (gain) under continuous radiation of laser at regular intervals for freshly prepared porous silicon (PS) and thermally carbonized porous silicon (TC-PS) samples.
becomes narrow and the dark current decreases compared to PS sample. The lesser initial photocurrent of TC-PS compared to PS sample can be explained by the reduction in specific surface area after carbonization. The photocurrent of PS lacks the stability due to the hydrogen termination on the surface. These chemical species are altered easily by changing ambient temperature, under prolonged laser radiation. The evolution of hydrogen concentration on PS surface alters the Schottky barrier height of Ni/PS photodetectors [27] and changes the photocurrent. Therefore, the stability of PS photodetectors decreases. The TC-PS photodetectors show less variation in detection of laser beam. This is because of disruption of Si–H bonds and formation of stable Si–C terminations on the surface of PS during TC process. Therefore, thermal carbonization process is an outstanding method to enhance the optical and electrical stability of porous silicon samples.
Thermal carbonization of freshly prepared PS was conducted under an acetylene flow at a high temperature. The results show that an ultra-thin layer of Si–C species was coated on the PS substrate, which slightly reduced the specific surface area. The quenching effect on the PL maximum caused by the low-power laser radiation, which is the main indicator of the instability of the optical properties of PS, was arrested and reduced to a small amount in the thermally carbonized samples. A small PL blue shift was observed after acetylene exposure, which can be considered proof of the changing bandgap of PS because of the formation of the thermally carbonized porous silicon. The TC-PS sample showed a relatively stable optical nature under prolonged exposure to laser illumination because of the replacement of the metastable hydrogen terminations with the stable Si–C bonds on the porous surface. The electrical characteristics of PS prior and after TC process were studied by fabrication of MSM photodetectors on PS and TC-PS substrates. The freshly prepared porous sample showed a tremendous response to the pioneer photons from laser beam. However the photocurrent decreased gradually under prolonged illumination of incident photons. The TC-PS devices showed a relatively stable sensitivity to light.
Acknowledgments The provision of financial support from the Institute of Postgraduate Studies (IPS) Universiti Sains Malaysia (USM) Fellowship and RU Grant 1001/PFIZIK/811175 are gratefully acknowledged. References [1] Y. Zhao, D. Yang, D. Li, M. Jiang, Applied Surface Science 252 (2005) 1065–1069.
546
N. Naderi et al. / Materials Science in Semiconductor Processing 16 (2013) 542–546
[2] B. Huy, P.H. Binh, B.Q. Diep, P.V. Luong, Physica E: Low-dimensional Systems and Nanostructures 17 (2003) 134–136. [3] B. Mahmoudi, N. Gabouze, M. Haddadi, B. Mahmoudi, H. Cheraga, K. Beldjilali, D. Dahmane, Sensors and Actuators B: Chemical 123 (2007) 680–684. [4] B. Mahmoudi, N. Gabouze, L. Guerbous, M. Haddadi, H. Cheraga, K. Beldjilali, Materials Science and Engineering B: Solid-State Materials for Advanced Technology 138 (2007) 293–297. [5] L. Tsybeskov, S.P. Duttagupta, P.M. Fauchet, Solid State Communications 95 (1995) 429–433. [6] Y.M. Huang, Applied Physics Letters 69 (1996) 2855–2857. [7] J.X. Shi, X.X. Zhang, M.L. Gong, J.Y. Zhou, K.W. Cheah, W.K. Wong, Physica Status Solidi (A) Applied Research 182 (2000) 353–357. [8] B. Gelloz, A. Bsiesy, R. Herino, Journal of Luminescence 82 (1999) 205–211. [9] H.J. Lewerenz, J. Jakubowicz, H. Jungblut, Electrochemistry Communications 6 (2004) 838–842. [10] R.N. Ghosh, R. Loloee, T. Isaacs-Smith, J.R. Williams, Materials Science Forum 600–603 (2009) 739–742. [11] S. Faraji, A.A. Rahim, N. Mohamed, C.S. Sipaut, B. Raja, Arabian Journal of Chemistry (2010). [12] T. Nakamura, T. Ogawa, N. Hosoya, S. Adachi, Journal of Luminescence 130 (2010) 682–687. ¨ ¨ Physica [13] J. Salonen, V.P. Lehto, M. Bjorkqvist, E. Laine, L. Niinisto, Status Solidi (A) Applied Research 182 (2000) 123–126.
[14] S.T. Lakshmikumar, P.K. Singh, Journal of Applied Physics 92 (2002) 3413–3415. [15] A. Keffous, K. Bourenane, M. Kechouane, N. Gabouze, T. Kerdja, L. Guerbous, S. Lafane, Journal of Luminescence 126 (2007) 561–565. [16] T. Rittenhouse, Solid State Communications 126 (2003) 245–250. [17] V. Torres-Costa, R.J. Martı´n-Palma, J.M. Martı´nez-Duart, J. Salonen, V.P. Lehto, Journal of Applied Physics 103 (2008) 083124-1–083124-4. [18] N. Naderi, M.R. Hashim, Materials Science Forum 717–720 (2012) 1283–1286. [19] N. Naderi, M.R. Hashim, T.S.T. Amran, Superlattices and Microstructures 51 (2012) 626–634. [20] A. Baharin, M.R. Hashim, Semiconductor Science and Technology 22 (2007) 905–910. [21] N. Naderi, G. Nabiyouni, I. Kazeminezhad, International Journal of Nanomanufacturing 5 (2010) 163–168. [22] K.A. Salman, K. Omar, Z. Hassan, Solar Energy 86 (2012) 541–547. [23] N. Naderi, M.R. Hashim, Materials Letters 88 (2012) 65–67. [24] H. Diesinger, A. Bsiesy, R. He´rino, B. Gelloz, Materials Science and Engineering B 69–70 (2000) 167–170. [25] N. Naderi, M.R. Hashim, Applied Surface Science 258 (2012) 6436–6440. [26] D. Stievenard, D. Deresmes, Applied Physics Letters 67 (1995) 1570–1572. [27] S. Basu, J. Kanungo, Nanocrystalline porous silicon, in: S. Basu (Ed.), Crystalline Silicon—Properties and Uses, InTech, 2011.
Contents lists available at SciVerse ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Short communication
Enhanced optical and electrical stability of thermally carbonized porous silicon N. Naderi, M.R. Hashim n, J. Rouhi, H. Mahmodi Nano-optoelectronics Research Laboratory, School of Physics, Universiti Sains Malaysia, 11800 USM, Penang, Malaysia
a r t i c l e in f o
abstract
Available online 25 September 2012
In the current paper, the intrinsic instability of physical properties of porous silicon (PS) was minimized using a thermal carbonization (TC) method. A typical PS showed an obvious quenching of its photoluminescence (PL) properties under long-term laser radiation. To resolve this problem, an ultra-thin stabilizing layer was grown on the porous structure under an acetylene gas flow. During the TC process at high temperatures, carbon atoms were detached from the acetylene molecules and dissolved in the porous structure. Thermally carbonized PS (TC-PS) layer changed the PL peak position of PS slightly due to the formation of Si–C species on porous surface after acetylene exposure. The effects of TC-PS layer in electrical properties of fabricated photodetectors were studied. The photocurrent of freshly prepared PS decreased under prolonged green laser radiation (532 nm, 5 mW), but the TC-PS sample showed more stable electrical characteristics under the same conditions. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Thermal carbonization Porous silicon Photoluminescence Photodetectors
1. Introduction Porous silicon (PS) has recently attracted considerable interest in sensing applications because of its extremely large surface-to-volume ratio and compatibility with silicon-based technologies [1]. However, its optical properties are unstable because of the formation of bonds between silicon and hydrogen atoms (Si–H) at the PS surface during the preparation process [2]. Several studies have reported on the effect of prolonged laser radiation in the photoluminescence (PL) quenching of PS [3,4]. This change in the PL peak is a consequence of the intrinsic instability of PS. Based on the literature, the optical properties of PS can be changed through exposure to different ambient [5–7]. Therefore, the PL quenching caused by low-power laser radiation is a good indicator of the variation in surface bond structures. High-power lasers ( 40 W/cm2) cause structural damage
n
Corresponding author. Tel.: þ 60 4 6533677; fax: þ 60 4 6579150. E-mail address: [email protected] (M.R. Hashim).
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.09.010
by melting the silicon walls in the porous structure, whereas lower-power ones ( o1 W/cm2) result in a gradual quenching of the PL intensity [8]. The initial metastable bond configurations are affected by low-power radiation, which leads to a change in the PL spectra [9]. As a solution, fabrication of PS-based devices which have the ability to work under harsh environments such as high temperatures [10] and high corrosive chemicals [11], have been reported by thermal oxidation [12] or carbonization [13] of PS samples. The common aim in all of these methods is replacing hydrogen terminations with other more stable species. Thermal carbonization (TC), which is based on the replacement of the existing Si–H terminations with Si–C species, can be considered a breakthrough in enhancing the stability of the PS structures [14]. The unique characteristics of silicon carbide (SiC), such as chemical inertness and thermal stability [15,16] are the main motivations for PS stabilization using the TC method. The formation of a SiC layer on PS is possible through exposure to acetylene gas at high temperatures [17]. In the current study, desorption of the C2H2, C2H, and C2 species occurs at a temperature of
N. Naderi et al. / Materials Science in Semiconductor Processing 16 (2013) 542–546
750 1C. At temperatures above 700 1C, carbon atoms penetrate into the Si lattice, thereby forming a thin SiC layer [18]. Here, the instability of the PL properties of PS is resolved using the TC process under an acetylene gas flow. To investigate the optical behavior of the fabricated porous structures, the PL of the samples is studied under a continuous laser illumination. The stability of the PL properties of PS is enhanced and PL quenching is minimized under a low-power laser illumination. An application of TC process for enhancing the life-time of PS photodetectors is studied. The electrical characteristics of PS and thermally carbonized PS (TC-PS) substrates are investigated under continuous low power laser illumination. The intrinsic instability in electrical properties of PS is arrested by the TC process. The novelty of this research is in establishment of a correlation between PL stability and electrical properties of PS photodetectors.
2. Experiment All experiments in the present research were performed on 10 mm 10 mm square samples cut from a single crystalline n-type (100) silicon wafer with a resistivity of 16 mO cm measured by the four point probe technique. For the chemical cleaning procedures, the silicon substrates were dipped in 1:1:5 (by volume) NH4OH:H2O2:H2O for 10 min, in 1:50 HF:H2O for 20 s, and then in 1:1:6 HCl:H2O2:H2O for another 10 min. Afterward, the samples were washed with deionized water and dried under an ambient nitrogen flow. The electrical resistivity of silicon wafers reduced to 14 mO cm after cleaning process. Aluminum ( 264 nm thick) was deposited through vacuum evaporation on the backside of the samples and annealed at 450 1C for 10 min under a nitrogen gas flow to facilitate anodization by decreasing the electrical resistivity to 0.06 mO cm. The PS layers were created via etching of crystalline silicon (c-Si) in a hydrofluoric acid-based electrochemical bath at room temperature (RT) under a 100 W tungsten lamp placed 30 cm above the samples. Fig. 1 shows a schematic diagram of the photo-electrochemical cell, which is a Teflon container 10 mm in diameter and 25 mm in height. The solution contained a mixture of hydrofluoric acid (49%), ethanol (95%), and hydrogen peroxide at a volume ratio of 1:2:2. A two-electrode setup was used for the photo-electrochemical etching of silicon substrates. The silicon substrate acted as the anode electrode, whereas an inert metal wire (Pt) was used as the cathode. A pulsed current [19] with a period time (T) of 14 ms and a pause
Fig. 1. Schematic image of the photo-electrochemical etching cell.
543
time (Toff) of 4 ms, with current density of 20 mA/cm2 supplied by a 2400 SourceMeter (Keithley, USA), was used to prepare porous samples. After 30 min etching, asprepared PS samples were dried at RT under a nitrogen flow for 1 h to ensure that all solutions have evaporated. The carbonization was conducted in a quartz tube furnace for 10 min under a continuous nitrogen and acetylene flush at flow rates of 30 L/min and 0.48 L/min, respectively. Thermal treatment followed in the furnace at 750 1C for 15 min. During carbonization, the temperature was monitored using a calibrated thermocouple. The samples were then cooled to RT under a nitrogen flow prior to the contact with ambient air. To study a baseline data, the PL for both the PS and TCPS samples was immediately recorded using a PL spectroscopy system equipped with an argon ion laser (514.5 nm, 20 mW) for excitation. To test the stability of the porous layers, the samples were continuously exposed to laser radiation and PL was measured every 20 min. For metallization of samples in order to fabricate back to back metal-semiconductor Schottky contacts and to study the current–voltage (I–V) characteristics of the metal-semiconductor-metal (MSM) photodetectors, a coplanar finger-shaped structure of Ni with the thickness of 204 nm, and dimension of 3300 3950 mm2 containing finger spacing of 400 mm [20,21] was deposited onto all porous substrates by thermal evaporation of pure nickel in vacuum chamber at pressure of 3.8 10 5 mbar. In order to reduce the atomic mismatch, for a high quality Schottky contact, samples were annealed in tube furnace at temperature of 450 1C under nitrogen gas flow for 10 min. The dark currents (Id) for PS and TC-PS samples were measured in a sealed chamber at RT by sweeping the voltage and measuring the current. For studying the stability of photocurrent (IPh), samples were kept under illumination of low power green laser (532 nm, 5 mW) for 120 min and the I–V measurements were carried out every 30 min. The PL spectra of the samples were obtained at RT using a Jobin-Yvon (HR 800 UV) spectrometer with an argon ion laser for excitation. The electrical measurements were carried out at RT with a computer-controlled integrated SourceMeter (Keithley 2400).
3. Results and discussion The percentages of porosity before and after carbonization were determined using gravimetric measurements [22]. The specific surface area of the as-prepared PS sample was 270 m2/cm3 whereas that of TC-PS decreased to 215 m2/cm3. A sample weight increase of 5 mg was observed for an area of 0.5 cm2 and a thickness of 20 mm after the TC process; these results are related to the porosity of PS substrate. If the carbon atoms are considered to form a uniform Si–C layer over the entire surface of the porous structure, the thickness of the stabilizing layer can be estimated at 5.7 nm. This insignificant thickness of the stabilizing layer slightly reduced the specific surface area of the porous substrate, which is not reflected in the morphology of PS.
544
N. Naderi et al. / Materials Science in Semiconductor Processing 16 (2013) 542–546
The high specific surface area of the freshly etched PS implies a large surface energy and suggests that upon high-temperature annealing, the surface energy can be reduced by the surface diffusion of silicon atoms, which leads to pore coalescence. The presence of an ultrathin Si–C layer on the surface hinders the surface diffusion of silicon atoms even though the surface area does not change significantly. Therefore, this Si–C structure can act as a protective layer for the silicon walls during the sputtering and annealing processes and prevent the occurrence of major changes in the PS structure. The morphology of samples which was shown in our last paper [18] indicated that the specific surface area of PS was slightly reduced due to the formation of TC-PS ultrathin layer which was not reflected in the planar and cross-sectional scanning electron micrographs. The optical behavior of TC-PS under low-power laser radiation at specific intervals was investigated. Fig. 2 shows the PL spectra of the PS and TC-PS samples. The PS spectrum shows a noticeable peak at 629.8 nm. This intensive PL behavior of the as-prepared porous sample results from the quantum confinement of charge carriers in the nanocrystallites [23]. However, a gradual decay ( 35%) was observed in the PL spectrum of the PS sample under a continuous laser radiation, which results from the existence of initial metastable bonds on the surface of the bare PS. These structures are affected by low-power radiation and leads to a change in the PL spectra. The PL spectrum of TC-PS shows a small blue shift ( 8 nm) in the position of PL peak compared to the one for PS sample. This slight change in the peak position is due to the formation of the ultra-thin stabilizing layer, which changes the energy bandgap of PS based on the quantum confinement theory [24]. Compared with the PS sample, a reduction ( 20%) in the intensity of PL peak is observed for TC-PS because of the exposure to acetylene. This phenomenon can be explained by a decrease in the specific surface area after carbonization due to the attachment of the carbon atoms. The effect of TC process as a stabilizing treatment on PS substrate can be seen in the spectrum of the TC-PS sample; PL quenching was arrested and reduced to a
small amount even after prolonged exposure to laser illumination. This behavior is a good indicator of the formation of a practically stable PS surface. Therefore, the thermally carbonized stabilizing layer enhances the optical properties of PS by reducing the PL quenching effect. The current–voltage (I–V) characteristics of fabricated MSM photodetectors based on PS and TC-PS samples are shown in Fig. 3. The dark current (Id) is plotted as a base line. The two sensors were found to show good Schottky behavior. Without applying light, the electrical conductivity of TC-PS is lower than that of PS, indicating that the carbonized sample exhibits a more resistive nature. The pioneer photocurrent which was measured immediately after applying light is shown by a solid line in both diagrams. Upon exposure to photon, PS sample showed a tremendous response to produce significant free carriers for enhanced current conduction. It seems that PS sample is more sensitive to the initial photons received from laser radiation compared to the carbonized one. However, the photocurrent of PS decreased due to the long-term impact of radiation, but this rate of reduction was much lower for carbonized sample even after prolonged exposure to laser beam (120 min). Fig. 4 which summarizes the current gain (ratio of light current to dark current [25]), indicates the stability of electrical properties of TC-PS sample while the initially high gain of PS photodetector was diminished exponentially by continuous laser exposure. Because of limitations in the accuracy of our measurement apparatus, there was an expected average amount of uncertainty for measuring the current gain ( 3.5%). The higher surface resistivity of TC-PS compared to PS can be explained by 2-dimentional model of porous silicon which was proposed by Stievenard and Deresmas [26]. In this model, by increase in chemical species and surface couple bonds on bare silicon, a depletion layer (w) will be grown at silicon walls (d) which are located between two adjacent pores. This will decrease the central channel (d-2w) inside the walls which the carriers can move through when a potential is applied. From this model, after carbonization, the central channel in PS walls
Fig. 2. PL spectra of porous silicon prior carbonization (PS) and after that (TC-PS), under continuous exposure of low power laser radiation for 0, 20 and 40 min.
N. Naderi et al. / Materials Science in Semiconductor Processing 16 (2013) 542–546
545
Fig. 3. I–V characteristics of MSM photodetectors based on PS and TC-PS under continuous exposure of low power laser radiation at regular intervals of 30 min.
4. Conclusions
Fig. 4. Ratio of photocurrent to dark current (gain) under continuous radiation of laser at regular intervals for freshly prepared porous silicon (PS) and thermally carbonized porous silicon (TC-PS) samples.
becomes narrow and the dark current decreases compared to PS sample. The lesser initial photocurrent of TC-PS compared to PS sample can be explained by the reduction in specific surface area after carbonization. The photocurrent of PS lacks the stability due to the hydrogen termination on the surface. These chemical species are altered easily by changing ambient temperature, under prolonged laser radiation. The evolution of hydrogen concentration on PS surface alters the Schottky barrier height of Ni/PS photodetectors [27] and changes the photocurrent. Therefore, the stability of PS photodetectors decreases. The TC-PS photodetectors show less variation in detection of laser beam. This is because of disruption of Si–H bonds and formation of stable Si–C terminations on the surface of PS during TC process. Therefore, thermal carbonization process is an outstanding method to enhance the optical and electrical stability of porous silicon samples.
Thermal carbonization of freshly prepared PS was conducted under an acetylene flow at a high temperature. The results show that an ultra-thin layer of Si–C species was coated on the PS substrate, which slightly reduced the specific surface area. The quenching effect on the PL maximum caused by the low-power laser radiation, which is the main indicator of the instability of the optical properties of PS, was arrested and reduced to a small amount in the thermally carbonized samples. A small PL blue shift was observed after acetylene exposure, which can be considered proof of the changing bandgap of PS because of the formation of the thermally carbonized porous silicon. The TC-PS sample showed a relatively stable optical nature under prolonged exposure to laser illumination because of the replacement of the metastable hydrogen terminations with the stable Si–C bonds on the porous surface. The electrical characteristics of PS prior and after TC process were studied by fabrication of MSM photodetectors on PS and TC-PS substrates. The freshly prepared porous sample showed a tremendous response to the pioneer photons from laser beam. However the photocurrent decreased gradually under prolonged illumination of incident photons. The TC-PS devices showed a relatively stable sensitivity to light.
Acknowledgments The provision of financial support from the Institute of Postgraduate Studies (IPS) Universiti Sains Malaysia (USM) Fellowship and RU Grant 1001/PFIZIK/811175 are gratefully acknowledged. References [1] Y. Zhao, D. Yang, D. Li, M. Jiang, Applied Surface Science 252 (2005) 1065–1069.
546
N. Naderi et al. / Materials Science in Semiconductor Processing 16 (2013) 542–546
[2] B. Huy, P.H. Binh, B.Q. Diep, P.V. Luong, Physica E: Low-dimensional Systems and Nanostructures 17 (2003) 134–136. [3] B. Mahmoudi, N. Gabouze, M. Haddadi, B. Mahmoudi, H. Cheraga, K. Beldjilali, D. Dahmane, Sensors and Actuators B: Chemical 123 (2007) 680–684. [4] B. Mahmoudi, N. Gabouze, L. Guerbous, M. Haddadi, H. Cheraga, K. Beldjilali, Materials Science and Engineering B: Solid-State Materials for Advanced Technology 138 (2007) 293–297. [5] L. Tsybeskov, S.P. Duttagupta, P.M. Fauchet, Solid State Communications 95 (1995) 429–433. [6] Y.M. Huang, Applied Physics Letters 69 (1996) 2855–2857. [7] J.X. Shi, X.X. Zhang, M.L. Gong, J.Y. Zhou, K.W. Cheah, W.K. Wong, Physica Status Solidi (A) Applied Research 182 (2000) 353–357. [8] B. Gelloz, A. Bsiesy, R. Herino, Journal of Luminescence 82 (1999) 205–211. [9] H.J. Lewerenz, J. Jakubowicz, H. Jungblut, Electrochemistry Communications 6 (2004) 838–842. [10] R.N. Ghosh, R. Loloee, T. Isaacs-Smith, J.R. Williams, Materials Science Forum 600–603 (2009) 739–742. [11] S. Faraji, A.A. Rahim, N. Mohamed, C.S. Sipaut, B. Raja, Arabian Journal of Chemistry (2010). [12] T. Nakamura, T. Ogawa, N. Hosoya, S. Adachi, Journal of Luminescence 130 (2010) 682–687. ¨ ¨ Physica [13] J. Salonen, V.P. Lehto, M. Bjorkqvist, E. Laine, L. Niinisto, Status Solidi (A) Applied Research 182 (2000) 123–126.
[14] S.T. Lakshmikumar, P.K. Singh, Journal of Applied Physics 92 (2002) 3413–3415. [15] A. Keffous, K. Bourenane, M. Kechouane, N. Gabouze, T. Kerdja, L. Guerbous, S. Lafane, Journal of Luminescence 126 (2007) 561–565. [16] T. Rittenhouse, Solid State Communications 126 (2003) 245–250. [17] V. Torres-Costa, R.J. Martı´n-Palma, J.M. Martı´nez-Duart, J. Salonen, V.P. Lehto, Journal of Applied Physics 103 (2008) 083124-1–083124-4. [18] N. Naderi, M.R. Hashim, Materials Science Forum 717–720 (2012) 1283–1286. [19] N. Naderi, M.R. Hashim, T.S.T. Amran, Superlattices and Microstructures 51 (2012) 626–634. [20] A. Baharin, M.R. Hashim, Semiconductor Science and Technology 22 (2007) 905–910. [21] N. Naderi, G. Nabiyouni, I. Kazeminezhad, International Journal of Nanomanufacturing 5 (2010) 163–168. [22] K.A. Salman, K. Omar, Z. Hassan, Solar Energy 86 (2012) 541–547. [23] N. Naderi, M.R. Hashim, Materials Letters 88 (2012) 65–67. [24] H. Diesinger, A. Bsiesy, R. He´rino, B. Gelloz, Materials Science and Engineering B 69–70 (2000) 167–170. [25] N. Naderi, M.R. Hashim, Applied Surface Science 258 (2012) 6436–6440. [26] D. Stievenard, D. Deresmes, Applied Physics Letters 67 (1995) 1570–1572. [27] S. Basu, J. Kanungo, Nanocrystalline porous silicon, in: S. Basu (Ed.), Crystalline Silicon—Properties and Uses, InTech, 2011.