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Development and characterization of porous silicon based photodiodes
Materials Science and Engineering B69 – 70 (2000) 87 – 91 www.elsevier.com/locate/mseb
Development and characterization of porous silicon based photodiodes R.J. Martı´n-Palma a,*, R. Guerrero-Lemus a, J.D. Moreno a, J.M. Martı´nez-Duart a, A. Gras b, D. Levy b b
a Departamento de Fı´sica Aplicada, C-12, Uni6ersidad Auto´noma de Madrid, 28049 Cantoblanco, Madrid, Spain SPASOLAB, Di6isio´n de Sistemas y Equipos, Laboratorio de Instrumentacio´n Espacial LINES, Di6isio´n de Ciencias de Espacio, Instituto Nacional de Te´cnica Aeroespacial INTA, Carretera de Ajal6ir km. 4, 28850 Torrejo´n de Ardoz, Madrid, Spain
Abstract Porous silicon (PS) based photodiodes have been developed from multicrystalline n+/p junctions, since this material can efficiently be employed to reduce optical reflection and to enhance absorption. These devices show a strong rectifying characteristic, in which the reverse and short-circuit current strongly depends on the presence of light. According to this, their use as solar sensors is considered as a practical application of PS-based photodiodes. The electrical performance (forward and reversed biased) of the PS-based devices under standard conditions of illumination and temperature has been determined. The angular dependence of the electrical response as a function of incidence light has also been established showing a cosine-type behavior. The degradation of these devices after extended photon irradiation as well as after long periods of atmospheric exposure has been found to be small. Finally, the variation of the electrical response under standard illumination conditions of PS-based photodetectors has also been estimated by varying the device working temperature from 10 up to 80°C. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Porous silicon; Photodiodes; Solar cells; Electrical performance
1. Introduction The possibility of producing optoelectronic devices has attracted a great deal of attention towards porous silicon (PS) since the discovery of its room temperature photoluminescence [1] and electroluminescence [2]. Although the research has been mainly focused on the photo- and electroluminescent properties, it was found a few years ago that PS can be efficiently employed in the development of photodetectors and solar cells [3]. Advanced optical detection devices incorporate surface texturing to reduce optical reflection and enhance absorption [4]. This objective can be achieved by employing PS layers grown on silicon substrates (mono- and multicrystalline), since the structure of PS can modulate the optical absorption and reflectance properties. Depending on the characteristic dimensions of the pores, a PS layer can be employed either for enhancing optical confinement, or as antireflective coating in silicon * Corresponding author. Tel.: +34-91-397-4919; fax: + 34-91-3973969. E-mail address: [email protected] (R.J. Martı´n-Palma)
photodiodes or solar cells [5]. Other important advantages of using PS in optical detection devices are that PS-based interference filters can be developed to match the desired optical properties which avoid the use of extra anti-reflection coatings [6,7], and that there exists the possibility of employing the photoluminescent properties of PS to convert ultraviolet and blue light into longer wavelength light with better quantum efficiency in silicon solar cells and photodiodes. In the case of the development of solar sensors and cells, other important advantage of using PS is that the bandgap of PS may be adjusted for optimum sun light absorption [8]. In the present work, PS based photodiodes were elaborated from multicrystalline n+/p junctions to be employed as solar sensors. Their use for space applications as part of solar panel tracking system or attitude control was evaluated. Accordingly, the current-voltage behavior of these devices under AM0 illumination as well as their angular response has been determined. The determination of the angular response is of great significance in the development of solar sensors when employed to determine the relative position with respect to the sun. Apart from light trapping, long-term stability
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and surface passivation are two of the major drawbacks in the use of PS. Thus, in this work, the photodiodes were exposed to the atmosphere for prolonged time and their electrical response periodically checked. In order to determine any possible degradation due to extended illumination exposures, variations of the electrical performance under AM0 light conditions has also been studied after photon irradiation periods of 48 h. Finally, due to the temperature dependence of the electrical performance of these devices, it is important for practical applications to evaluate the behavior of relevant electrical parameters at various working temperatures. For this purpose, the values of the open-circuit voltage, short-circuit current and maximum power (AM0 illumination) were measured at different temperatures ranging from 10 to 80°C.
In order to evaluate the electrical performance of these PS-based photodiodes for practical applications, current-voltage characteristics (forward and reverse biased) under AM0 illumination conditions and controlled temperature were obtained. For this purpose a Xe-lamp solar simulator was used and the devices were placed in a special holder that provided both electrical contact (four points probe) to an electronic load (used as variable automatic power supply that permits biasing of the device in the forward and reverse direction), and temperature control by means of Peltier elements. For long-term photon irradiation exposures, the devices were placed in a chamber at 25°C and illuminated to standard conditions by means of halogen lamps for 48 h.
3. Results and discussion 2. Experimental PS based photodiodes were elaborated from p-type, boron doped multicrystalline silicon wafers, with a resistivity of about 1 Vcm. The n+ emitter was formed by the thermal diffusion of phosphor. Aluminium was employed for the back contact and silver for the front contact. The front contact consists of a square-shaped grid, with external area of 36 mm2 and internal area of 16 mm2 [9]. The resulting devices were cut into 6 × 6 mm2 pieces, which were mounted into a sample holder. The PS layers were formed by chemically etching the emitter of the n+/p junctions, after the deposition of the front and back contacts. A HF/HNO3-based solution was employed at room temperature in the dark. The samples were rinsed in ethanol after the formation of the porous structure. Since PS is a highly resistive material and shows poor electrical transport properties [10], this technique allows top contacts to be made directly onto the emitter, thus avoiding electrical conduction through PS.
Fig. 1. I-V behavior of the PS-based photodiodes under AM0 illumination.
A non-linear current-voltage (I-V) rectifying behavior has been measured in the dark in the PS-based devices developed. I-V curves were determined for the different samples in the dark and under room light, from which it was determined that the current across the photodiode (under reverse bias and low forward voltages) strongly depends on the presence of light. Also, the I-V behavior of the devices under AM0 illumination (Fig. 1) has been determined. From Fig. 1 it can be observed that these devices can be employed as photodetectors, if they are reverse-biased or operated at low forward voltages, and as solar cells, if they are working in the fourth quadrant. It has to be noted that the photo-generated current, i.e. the difference between the current under illumination and the dark current at a constant voltage, increases slowly with reverse bias and shows a linear behavior from 0 V to approximately −2 V. Apart from this, the angular response of porous silicon photodiodes under AM0 illumination has been measured. The results are represented in Fig. 2, from which it can be observed that the ratio between the short circuit current at a given angle, Isc, and that at 0°, Isc(0), follows, almost exactly, a cosine-type law. However, for angles above 55°, the ratio Isc/Isc(0) shows lower values that those corresponding to the cosine law. From these results it is considered that PS-based photodiodes can be employed to determine the relative orientation of these devices with respect to a source of light. Thus, they may be used in applications for solar panel tracking systems, orientation of artificial satellites, etc., by using the sun as a reference. Fig. 3 shows the I-V characteristic of the as-formed photodiodes and its electrical response after 5, 10 and 40 days of atmospheric exposure. From this figure, it can be observed that there is no appreciable variation of the I-V characteristics of the photodiodes after prolonged time of exposition to the atmosphere. However,
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Fig. 3. Reverse current measurements after different times of exposition to the atmosphere. Fig. 2. Angular response of the PS-based photodiodes.
Fig. 4. Variation of the electrical response after long periods of irradiation.
Fig. 5. Variation of the short-circuit current density as a function of temperature. .
a slight rise of the values of the open-circuit voltage has been found, whereas short-circuit current variations have not been detected. Fig. 4 shows the variation of the behavior of the photodiodes after 48 h of continued AM0 illumination. As in the previous case, there exists a small rise of the value of the open-circuit voltage. Finally, it has been determined the variation of the open-circuit voltage, short-circuit current and maximum power, by varying the working temperature from 10 to 80°C (Figs. 5–7). Temperature behavior of these parameters was as expected. The variation of the opencircuit voltage is found to decrease with temperature at a rate of 2.0 mV °C − 1. However, the values of shortcircuit current rise with increasing temperature. In our case, a variation of the short-circuit current density of 0.031 (mA/cm2)/°C is detected. In the case of the maxi-
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Fig. 6. Variation of the open-circuit voltage as a function of temperature.
Fig. 7. Variation of the maximum power as a function of temperature.
mum power density, its variation shows a similar behavior that the open-circuit voltage but less significant, and the corresponding obtained value is of 0.046 (mW/ cm2)/°C.
4. Conclusions From the previous work, it can be concluded that .
porous silicon based diodes show rectifying characteristics. Under illumination they exhibit a relatively strong photo-generated current, and therefore it has been demonstrated that they can be employed as photodetectors and solar cells. It has been studied the angular response of the devices with respect to a source of light, approximately following a cosine-type law. The variation of the electrical response of the photodiodes as a consequence of their extended exposure to the atmo-
R.J. Martı´n-Palma et al. / Materials Science and Engineering B69–70 (2000) 87–91
sphere and to continued photonic irradiation has been also determined. It has been observed that, in both cases, the short-circuit current remains almost constant while there exists a small rise in the values of the open-circuit voltage. The dependence of the open-circuit voltage, short-circuit current and maximum power as a function of the working temperature were also studied. It was found, as expected, that an increase in temperature results in a slight increase of the short-circuit current and a decrease of the opencircuit voltage and the maximum power.
Acknowledgements The authors would like to acknowledge the Spanish Research Office (CICyT) for financial support under Contract No. MAT96-0602, and to SPASOLAB (SPAce Solar cell LABoratory operated by INTA) for the testing tasks.
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91
References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] N. Koshida, H. Koyama, Appl. Phys. Lett. 60 (1992) 347. [3] J.P. Zheng, K.L. Jiao, W.P. Shen, W.A. Anderson, H.S. Kwok, Appl. Phys. Lett. 61 (1992) 459. [4] V.M. Bright, E.S. Kolesar, D.M. Sowders, Opt. Eng. 36 (4) (1997) 1088. [5] A. Krotkus, K. Grigoras, V. Paebutas, I. Barsony, E. Vazsonyi, M. Fried, J. Szlufcik, J. Nijs, C. Levy-Clement, Sol. Ener. Mater. Sol. Cells 45 (1997) 267. [6] M.G. Berger, C. Dieker, M. Tho¨nissen, L. Vescan, H. Lu¨th, H. Mu¨nder, W. Theiß, M. Wernke, P. Grosse, J. Phys. D Appl. Phys. 27 (1994) 1333. [7] R.J. Martı´n-Palma, P. Herrero, R. Guerrero-Lemus, J.D. Moreno, J.M. Martı´nez-Duart, J. Mat. Sci. Lett. 17 (1998) 845. [8] Y.S. Tsuo, Y. Xiao, M.J. Heben, X. Wu, F.J. Pern, S.K. Deb, Proc. 23rd IEEE Photovoltaic Specialist Conference, IEEE 287, 1993. [9] R.J. Martı´n-Palma, R. Guerrero-Lemus, J.D. Moreno, J.M. Martı´nez-Duart, Sol. State Electron. 43 (1999) 1153. [10] R.J. Martı´n-Palma, J. Pe´rez-Rigueiro, R. Guerrero-Lemus, J.D. Moreno, J.M. Martı´nez-Duart, J. Appl. Phys. 85 (1) (1999) 583.
Development and characterization of porous silicon based photodiodes R.J. Martı´n-Palma a,*, R. Guerrero-Lemus a, J.D. Moreno a, J.M. Martı´nez-Duart a, A. Gras b, D. Levy b b
a Departamento de Fı´sica Aplicada, C-12, Uni6ersidad Auto´noma de Madrid, 28049 Cantoblanco, Madrid, Spain SPASOLAB, Di6isio´n de Sistemas y Equipos, Laboratorio de Instrumentacio´n Espacial LINES, Di6isio´n de Ciencias de Espacio, Instituto Nacional de Te´cnica Aeroespacial INTA, Carretera de Ajal6ir km. 4, 28850 Torrejo´n de Ardoz, Madrid, Spain
Abstract Porous silicon (PS) based photodiodes have been developed from multicrystalline n+/p junctions, since this material can efficiently be employed to reduce optical reflection and to enhance absorption. These devices show a strong rectifying characteristic, in which the reverse and short-circuit current strongly depends on the presence of light. According to this, their use as solar sensors is considered as a practical application of PS-based photodiodes. The electrical performance (forward and reversed biased) of the PS-based devices under standard conditions of illumination and temperature has been determined. The angular dependence of the electrical response as a function of incidence light has also been established showing a cosine-type behavior. The degradation of these devices after extended photon irradiation as well as after long periods of atmospheric exposure has been found to be small. Finally, the variation of the electrical response under standard illumination conditions of PS-based photodetectors has also been estimated by varying the device working temperature from 10 up to 80°C. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Porous silicon; Photodiodes; Solar cells; Electrical performance
1. Introduction The possibility of producing optoelectronic devices has attracted a great deal of attention towards porous silicon (PS) since the discovery of its room temperature photoluminescence [1] and electroluminescence [2]. Although the research has been mainly focused on the photo- and electroluminescent properties, it was found a few years ago that PS can be efficiently employed in the development of photodetectors and solar cells [3]. Advanced optical detection devices incorporate surface texturing to reduce optical reflection and enhance absorption [4]. This objective can be achieved by employing PS layers grown on silicon substrates (mono- and multicrystalline), since the structure of PS can modulate the optical absorption and reflectance properties. Depending on the characteristic dimensions of the pores, a PS layer can be employed either for enhancing optical confinement, or as antireflective coating in silicon * Corresponding author. Tel.: +34-91-397-4919; fax: + 34-91-3973969. E-mail address: [email protected] (R.J. Martı´n-Palma)
photodiodes or solar cells [5]. Other important advantages of using PS in optical detection devices are that PS-based interference filters can be developed to match the desired optical properties which avoid the use of extra anti-reflection coatings [6,7], and that there exists the possibility of employing the photoluminescent properties of PS to convert ultraviolet and blue light into longer wavelength light with better quantum efficiency in silicon solar cells and photodiodes. In the case of the development of solar sensors and cells, other important advantage of using PS is that the bandgap of PS may be adjusted for optimum sun light absorption [8]. In the present work, PS based photodiodes were elaborated from multicrystalline n+/p junctions to be employed as solar sensors. Their use for space applications as part of solar panel tracking system or attitude control was evaluated. Accordingly, the current-voltage behavior of these devices under AM0 illumination as well as their angular response has been determined. The determination of the angular response is of great significance in the development of solar sensors when employed to determine the relative position with respect to the sun. Apart from light trapping, long-term stability
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R.J. Martı´n-Palma et al. / Materials Science and Engineering B69–70 (2000) 87–91
and surface passivation are two of the major drawbacks in the use of PS. Thus, in this work, the photodiodes were exposed to the atmosphere for prolonged time and their electrical response periodically checked. In order to determine any possible degradation due to extended illumination exposures, variations of the electrical performance under AM0 light conditions has also been studied after photon irradiation periods of 48 h. Finally, due to the temperature dependence of the electrical performance of these devices, it is important for practical applications to evaluate the behavior of relevant electrical parameters at various working temperatures. For this purpose, the values of the open-circuit voltage, short-circuit current and maximum power (AM0 illumination) were measured at different temperatures ranging from 10 to 80°C.
In order to evaluate the electrical performance of these PS-based photodiodes for practical applications, current-voltage characteristics (forward and reverse biased) under AM0 illumination conditions and controlled temperature were obtained. For this purpose a Xe-lamp solar simulator was used and the devices were placed in a special holder that provided both electrical contact (four points probe) to an electronic load (used as variable automatic power supply that permits biasing of the device in the forward and reverse direction), and temperature control by means of Peltier elements. For long-term photon irradiation exposures, the devices were placed in a chamber at 25°C and illuminated to standard conditions by means of halogen lamps for 48 h.
3. Results and discussion 2. Experimental PS based photodiodes were elaborated from p-type, boron doped multicrystalline silicon wafers, with a resistivity of about 1 Vcm. The n+ emitter was formed by the thermal diffusion of phosphor. Aluminium was employed for the back contact and silver for the front contact. The front contact consists of a square-shaped grid, with external area of 36 mm2 and internal area of 16 mm2 [9]. The resulting devices were cut into 6 × 6 mm2 pieces, which were mounted into a sample holder. The PS layers were formed by chemically etching the emitter of the n+/p junctions, after the deposition of the front and back contacts. A HF/HNO3-based solution was employed at room temperature in the dark. The samples were rinsed in ethanol after the formation of the porous structure. Since PS is a highly resistive material and shows poor electrical transport properties [10], this technique allows top contacts to be made directly onto the emitter, thus avoiding electrical conduction through PS.
Fig. 1. I-V behavior of the PS-based photodiodes under AM0 illumination.
A non-linear current-voltage (I-V) rectifying behavior has been measured in the dark in the PS-based devices developed. I-V curves were determined for the different samples in the dark and under room light, from which it was determined that the current across the photodiode (under reverse bias and low forward voltages) strongly depends on the presence of light. Also, the I-V behavior of the devices under AM0 illumination (Fig. 1) has been determined. From Fig. 1 it can be observed that these devices can be employed as photodetectors, if they are reverse-biased or operated at low forward voltages, and as solar cells, if they are working in the fourth quadrant. It has to be noted that the photo-generated current, i.e. the difference between the current under illumination and the dark current at a constant voltage, increases slowly with reverse bias and shows a linear behavior from 0 V to approximately −2 V. Apart from this, the angular response of porous silicon photodiodes under AM0 illumination has been measured. The results are represented in Fig. 2, from which it can be observed that the ratio between the short circuit current at a given angle, Isc, and that at 0°, Isc(0), follows, almost exactly, a cosine-type law. However, for angles above 55°, the ratio Isc/Isc(0) shows lower values that those corresponding to the cosine law. From these results it is considered that PS-based photodiodes can be employed to determine the relative orientation of these devices with respect to a source of light. Thus, they may be used in applications for solar panel tracking systems, orientation of artificial satellites, etc., by using the sun as a reference. Fig. 3 shows the I-V characteristic of the as-formed photodiodes and its electrical response after 5, 10 and 40 days of atmospheric exposure. From this figure, it can be observed that there is no appreciable variation of the I-V characteristics of the photodiodes after prolonged time of exposition to the atmosphere. However,
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Fig. 3. Reverse current measurements after different times of exposition to the atmosphere. Fig. 2. Angular response of the PS-based photodiodes.
Fig. 4. Variation of the electrical response after long periods of irradiation.
Fig. 5. Variation of the short-circuit current density as a function of temperature. .
a slight rise of the values of the open-circuit voltage has been found, whereas short-circuit current variations have not been detected. Fig. 4 shows the variation of the behavior of the photodiodes after 48 h of continued AM0 illumination. As in the previous case, there exists a small rise of the value of the open-circuit voltage. Finally, it has been determined the variation of the open-circuit voltage, short-circuit current and maximum power, by varying the working temperature from 10 to 80°C (Figs. 5–7). Temperature behavior of these parameters was as expected. The variation of the opencircuit voltage is found to decrease with temperature at a rate of 2.0 mV °C − 1. However, the values of shortcircuit current rise with increasing temperature. In our case, a variation of the short-circuit current density of 0.031 (mA/cm2)/°C is detected. In the case of the maxi-
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Fig. 6. Variation of the open-circuit voltage as a function of temperature.
Fig. 7. Variation of the maximum power as a function of temperature.
mum power density, its variation shows a similar behavior that the open-circuit voltage but less significant, and the corresponding obtained value is of 0.046 (mW/ cm2)/°C.
4. Conclusions From the previous work, it can be concluded that .
porous silicon based diodes show rectifying characteristics. Under illumination they exhibit a relatively strong photo-generated current, and therefore it has been demonstrated that they can be employed as photodetectors and solar cells. It has been studied the angular response of the devices with respect to a source of light, approximately following a cosine-type law. The variation of the electrical response of the photodiodes as a consequence of their extended exposure to the atmo-
R.J. Martı´n-Palma et al. / Materials Science and Engineering B69–70 (2000) 87–91
sphere and to continued photonic irradiation has been also determined. It has been observed that, in both cases, the short-circuit current remains almost constant while there exists a small rise in the values of the open-circuit voltage. The dependence of the open-circuit voltage, short-circuit current and maximum power as a function of the working temperature were also studied. It was found, as expected, that an increase in temperature results in a slight increase of the short-circuit current and a decrease of the opencircuit voltage and the maximum power.
Acknowledgements The authors would like to acknowledge the Spanish Research Office (CICyT) for financial support under Contract No. MAT96-0602, and to SPASOLAB (SPAce Solar cell LABoratory operated by INTA) for the testing tasks.
.
91
References [1] L.T. Canham, Appl. Phys. Lett. 57 (1990) 1046. [2] N. Koshida, H. Koyama, Appl. Phys. Lett. 60 (1992) 347. [3] J.P. Zheng, K.L. Jiao, W.P. Shen, W.A. Anderson, H.S. Kwok, Appl. Phys. Lett. 61 (1992) 459. [4] V.M. Bright, E.S. Kolesar, D.M. Sowders, Opt. Eng. 36 (4) (1997) 1088. [5] A. Krotkus, K. Grigoras, V. Paebutas, I. Barsony, E. Vazsonyi, M. Fried, J. Szlufcik, J. Nijs, C. Levy-Clement, Sol. Ener. Mater. Sol. Cells 45 (1997) 267. [6] M.G. Berger, C. Dieker, M. Tho¨nissen, L. Vescan, H. Lu¨th, H. Mu¨nder, W. Theiß, M. Wernke, P. Grosse, J. Phys. D Appl. Phys. 27 (1994) 1333. [7] R.J. Martı´n-Palma, P. Herrero, R. Guerrero-Lemus, J.D. Moreno, J.M. Martı´nez-Duart, J. Mat. Sci. Lett. 17 (1998) 845. [8] Y.S. Tsuo, Y. Xiao, M.J. Heben, X. Wu, F.J. Pern, S.K. Deb, Proc. 23rd IEEE Photovoltaic Specialist Conference, IEEE 287, 1993. [9] R.J. Martı´n-Palma, R. Guerrero-Lemus, J.D. Moreno, J.M. Martı´nez-Duart, Sol. State Electron. 43 (1999) 1153. [10] R.J. Martı´n-Palma, J. Pe´rez-Rigueiro, R. Guerrero-Lemus, J.D. Moreno, J.M. Martı´nez-Duart, J. Appl. Phys. 85 (1) (1999) 583.