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Activation energy study of phosphorus-doped microcrystalline silicon thin films
Optik 127 (2016) 10437–10441
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Activation energy study of phosphorus-doped microcrystalline silicon thin films Qingdong Chen a,∗ , Junping Wang a , Yuxiang Zhang b a
Faculty of Aerospace Engineering, Binzhou University, Binzhou 256603, China Key Laboratory of Material Physics of Ministry of Education, Physical Science & Technology College Zhengzhou University, Zhengzhou, 450052, China b
a r t i c l e
i n f o
Article history: Received 21 June 2016 Accepted 24 August 2016 Keywords: PECVD Microcrystalline silicon Conductivity Activation energy
a b s t r a c t The phosphorus-doped microcrystalline silicon thin films were deposited by PECVD (Plasma-enhanced chemical vapor deposition), the activation energy of thin films were measured by activation energy testing equipment. The activation energy of samples with different doping concentration and different depositing temperature were studied. The results showed that: the activation energy of phosphorus-doped microcrystalline silicon thin films lesser than intrinsic films. The unit of activation energy is meV. Crystalline volume fractions have little influence on activation energy when thin films have a good crystalline volume fraction, impurity effect plays an important role on the conductivity. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction Hydrogenated microcrystalline silicon (c-Si:H) is an interesting material as an absorber layer for solar cells. The low absorption coefficient for light with k > 700 nm (␣ = 102 –103 cm−1 ) of this material requires thick layers (1–3 m) to collect the radiation efficiently [1]. Many studies based on plasma-enhanced chemical vapor deposition (PECVD)have been made to improve the growth rate and the film quality, especially for the crystallinity [2,3]. In Si thin-film p-i-n photodiodes, thin p-type hydrogenated microcrystalline has advantages over traditional amorphous p type window layers [4]. In order to prepare microcrystalline silicon thin film solar cells, it is necessary to carry out gas phase doping of c-Si:H thin films. H. Richter found that: under the same doping concentration, the conductivity of microcrystalline silicon thin film is enhanced by two orders of magnitude than the amorphous silicon [5]. In order to study the electrical conductivity of doped microcrystalline silicon thin films, the activation energy of doped microcrystalline silicon thin film prepared by PECVD were studied, the samples were studied by The United States KEITHLEY6517A multimeter/high resistance test instrument. 2. Experiment 2.1. Testing equipment Fig. 1 shows the testing equipment of activation energy, it is a sealed vacuum device made of stainless steel, there is a quartz window on top of the equipment, a light source can add above it, so the photoconductivity of silicon film can be
∗ Corresponding author. E-mail address: [email protected] (Q. Chen). http://dx.doi.org/10.1016/j.ijleo.2016.08.067 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
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Q. Chen et al. / Optik 127 (2016) 10437–10441
Fig. 1. The testing equipment of activation energy.
measured. On the upper flange plate is equipped with air bleed valve. The sample is measured on the copper plate, copper electrode can change measuring distance by adjusting the electrode pillars. The heating pipe is arranged in two sides of copper plate, Electric wire is wound on heating pipe. The cooling block is located below copper plate with cooling water for cooling. The copper plate is located on the pillar of PTFE, one is for the electric insulation, and the second is for heat insulation. Steel walls and lower flange plate is sealed by O ring. There are pumping holes, wire outlet and cooling water pipe outlet under the lower flange plate. The equipment is vacuum pumped by mechanical pump, the temperature is controlled by temperature control instrument, the range from room temperature to 200 ◦ C. The data acquisition system of the equipment is the United States KEITHLEY6517A multimeter/high resistance test instrument, testing voltage range from 0 V to 400 V, testing current range from 20pA to 200 A, testing resistance can reach up to 200T. 2.2. The depositing condition of samples All the samples were prepared by PECVD, depositing frequency: 13.56 MHz. The samples were analyzed by Raman spectroscopy with a Renishaw MR-2000 Raman spectrometer. The depositing condition of samples at different doping concentration: SiH4 concentration:1% temperature:150 ◦ C pressure:133.3pa power:50W depositing time:2h, PH3 concentration:0.5‰ 1‰ 3‰ 5‰ 8‰ and 10‰. The depositing condition of samples at different depositing temperature: SiH4 concentration:1% PH3 concentration:3‰ pressure:133.3pa power:50W depositing time:2 h, temperature:100 ◦ C 150 ◦ C 200 ◦ C 250 ◦ C 300 ◦ C and 350 ◦ C. 3. Results and discussion Fig. 2 shows Raman spectrum of samples at different doping concentration, it shows that: the crystallization of the sample have little different, and the crystallization condition is very good. Fig. 3 shows Raman spectrum of samples at
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Relative intensity
Q. Chen et al. / Optik 127 (2016) 10437–10441
200
300
400
500
600
700
800
-1
Raman Shift(cm ) Fig. 2. Raman spectra of samples at different doping concentration.
Relative intensity
250
200 150
100
200
300
400
500
600
700
800
-1
Raman shift(cm ) Fig. 3. Raman spectra of samples at different depositing temperature.
Fig. 4. The conductivity varied with temperature of different deposited samples(a:different doping concentration b: different depositing temperature).
different depositing temperature, it shows that: the crystallization condition of the sample is good, and the crystalline volume fraction is higher. Fig. 4 shows the curve of − 1000/T of samples, it can be concluded that: the change of the doped sample conductivity is very small with the change of temperature, the curve is almost linear, the slope is very small, so the unit of activation energy is meV. Phosphorus impurity atoms enter into the grain interior to form a substitution type doping. So impurity band is formed between valence band and conduction band, the Fermi level moves toward the bottom of conduction band, the conductivity
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Fig. 5. The conduction and activation energy of different deposited samples(a: different doping concentration b: different depositing temperature).
activation energy of samples is decreased. The concentration of free carrier increases sharply, so the conductivity increases. On the other hand, phosphorus doping affects the crystalline volume fraction of silicon thin films, it is easy to understand the effect of crystalline volume fraction on electrical conductivity, the grain boundary is gathered with carrier traps and impurities, with crystalline volume fraction increases, the crystallization area increases, the probability of impurity atoms entering the grain increases, the concentration of free carrier increases. Meanwhile, with the grain boundary thickness decreases, the probability of electron tunneling increases, and the probability of impurity atoms entering grain interior increases. The above two factors lead to the increase in the conductance of 3–4 orders of magnitude more than the intrinsic microcrystalline silicon thin films, the activation energy decreases to meV unit. Fig. 5 shows the conductivity and activation energy of different deposited samples. With the increase of doping concentration, the conductivity of the samples increased gradually, reached up to 2 s/cm, the activation energy decreased from 57.97 meV to 21.71 meV. With the increase of temperature, the conductivity of the samples increased gradually, and the activation energy decreased, meanwhile, the activation energy of the samples which were deposited at 300 ◦ C and 350 ◦ C were measured, the activation energy were found to be smaller than others. For the microcrystalline silicon thin films which in the amorphous/microcrystalline phase transition zone, the conductivity is affected by crystalline volume fraction and dimensional effect. For the study of the intrinsic film, the conductivity of the sample is determined by crystalline volume fraction and dimensional effect. When the grain size is kept constant, the crystalline volume fraction becomes the main factor that restricts the electrical conductivity, the electrical conductivity can be increased by 2–3 orders of magnitude by increasing crystalline volume fraction [5]. When the crystalline volume fraction have little difference, and samples conductivity have an obvious relationship of grain size [6]. In addition, for the microcrystalline silicon thin film, the silicon grain is in a disordered network structure, there is a wide disorder region between the grains. After doping, a small amount of P atoms can be ionized to form a regular tetrahedral structure, which increases the order degree and dark conductivity of the films. For phosphorus doped hydrogenated microcrystalline silicon thin film [7], because a small amount of phosphorus doping, grain refinement in the film, the order degree of thin film is improved, the crystalline volume fraction is generally higher than intrinsic. The degree of order increases, increases the film conductivity. However, after a large amount of [8] doping, the P atom is in SP3 hybrid mode, which increases the disorder of the network structure of thin film, and makes the film transform to amorphous structure, which leads to the decrease of the dark conductivity. It can be concluded that the conductivity is simultaneously affected by crystalline volume fraction and dimensional effect. In this paper, in the depositing experiment of the different doping concentration and different depositing temperature, the doped microcrystalline silicon film is crystallized better, the crystalline volume fraction have little difference, so crystalline volume fraction and dimensional effect have not obvious effects on conductivity,impurity effects have a major effect on conductivity. With the doping concentration and depositing temperature increases, there will be more P atoms ionized to electrons and formed tetrahedral structure, the order degree of the films is increased, so that P atoms can be more effective doping, thereby dark conductivity increases, activation energy decreases.
4. Conclusions The activation energy of thin films which deposited at different doping concentration and different depositing temperature were measured by activation energy testing equipment. It was found that: with the increase of doping concentration and depositing temperature, conductivity increases, activation energy decreases. For phosphorus doped microcrystalline silicon films, when the crystallization conditions are good,the relationship between activation energy and crystalline volume fraction is not obvious, impurity effects have a major effect on conductivity.
Q. Chen et al. / Optik 127 (2016) 10437–10441
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Acknowledgement The author would like to thank Prof. Zhang for his invaluable discussions and assistance. This work was supported by Chinese National Natural Science Foundation (41401384);The University Science and Technology Project of Shandong Province (J14LJ52); The Science and Technology Development Planning of Binzhou City (2013ZC0107) The Science and Technology Development Planning of Binzhou City (2014ZC0307); The Science and Technology Development Planning of Binzhou City (2015ZC0210); The Doctoral Scientific Research Foundation of Binzhou University (2014Y10); Binzhou University Research Fund Project (BZXYG1513)and Binzhou University Research Fund Project (BZXYG04). References [1] Y.S. Chen, J.H. Wang, J.X. Lu, et al., Microcrystalline silicon grown by VHF PECVD and the fabrication of solar cells, [J] Sol. Energy 82 (11) (2008) 1083–1087. [2] M. Kondo, M. Fukawa, L. Guo, A. Matsuda, High rate growth of microcrystalline silicon at low temperatures, [J] J. Non-Cryst. Solids 266–269 (1) (2000) 84–89. [3] A. Shah, J. Meier, E. Vallat-Sauvain, et al., “Microcrystalline silicon and micromorph tandem solar cells,”, [J] Thin Solid Films (2002) 403–404, 179–187. [4] W.F.L. Tse, I. Khodami, M.M. Adachi, et al., Characterization of low temperature P-type hydrogenated microcrystalline silicon thin films deposited by plasma enhanced chemical vapor deposition, [C] Can. Conf. Electr. Comput. Eng. (2007) 952–955. [5] Richter and Ley, H. Richter, L. Ley, Optical properties and transport in microcrystalline silicon prepared at temperature below 400 ◦ C, [J] Appl. Phys. Lett. 52 (12) (1981) 7281–7286. [6] S.B. Zhang, X.B. Liao, L. An, et al., Micro-Raman study on hydrogenated protocrystalline silicon films, [J] Acta Phys. Sinaca 51 (8) (2002) 1811–1815. [7] Y.S. Chen, S.Z. Wang, S.E. Yang, et al., Microstructures and photo-electric characteristics of phosphorus-doped hydrogenated silicon films, [J] J. Vac. Sci. Technol. 26 (1) (2006) 8–11. [8] Q. Wang, J.N. Ding, Y.L. He, et al., Mesoscopic mechanical characterization of hy-drogenated silicon thin film and the intrinsic relationshipwith the microstructure, [J] Acta Phys. Sinaca 56 (8) (2007) 4835–4840.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Original research article
Activation energy study of phosphorus-doped microcrystalline silicon thin films Qingdong Chen a,∗ , Junping Wang a , Yuxiang Zhang b a
Faculty of Aerospace Engineering, Binzhou University, Binzhou 256603, China Key Laboratory of Material Physics of Ministry of Education, Physical Science & Technology College Zhengzhou University, Zhengzhou, 450052, China b
a r t i c l e
i n f o
Article history: Received 21 June 2016 Accepted 24 August 2016 Keywords: PECVD Microcrystalline silicon Conductivity Activation energy
a b s t r a c t The phosphorus-doped microcrystalline silicon thin films were deposited by PECVD (Plasma-enhanced chemical vapor deposition), the activation energy of thin films were measured by activation energy testing equipment. The activation energy of samples with different doping concentration and different depositing temperature were studied. The results showed that: the activation energy of phosphorus-doped microcrystalline silicon thin films lesser than intrinsic films. The unit of activation energy is meV. Crystalline volume fractions have little influence on activation energy when thin films have a good crystalline volume fraction, impurity effect plays an important role on the conductivity. © 2016 Elsevier GmbH. All rights reserved.
1. Introduction Hydrogenated microcrystalline silicon (c-Si:H) is an interesting material as an absorber layer for solar cells. The low absorption coefficient for light with k > 700 nm (␣ = 102 –103 cm−1 ) of this material requires thick layers (1–3 m) to collect the radiation efficiently [1]. Many studies based on plasma-enhanced chemical vapor deposition (PECVD)have been made to improve the growth rate and the film quality, especially for the crystallinity [2,3]. In Si thin-film p-i-n photodiodes, thin p-type hydrogenated microcrystalline has advantages over traditional amorphous p type window layers [4]. In order to prepare microcrystalline silicon thin film solar cells, it is necessary to carry out gas phase doping of c-Si:H thin films. H. Richter found that: under the same doping concentration, the conductivity of microcrystalline silicon thin film is enhanced by two orders of magnitude than the amorphous silicon [5]. In order to study the electrical conductivity of doped microcrystalline silicon thin films, the activation energy of doped microcrystalline silicon thin film prepared by PECVD were studied, the samples were studied by The United States KEITHLEY6517A multimeter/high resistance test instrument. 2. Experiment 2.1. Testing equipment Fig. 1 shows the testing equipment of activation energy, it is a sealed vacuum device made of stainless steel, there is a quartz window on top of the equipment, a light source can add above it, so the photoconductivity of silicon film can be
∗ Corresponding author. E-mail address: [email protected] (Q. Chen). http://dx.doi.org/10.1016/j.ijleo.2016.08.067 0030-4026/© 2016 Elsevier GmbH. All rights reserved.
10438
Q. Chen et al. / Optik 127 (2016) 10437–10441
Fig. 1. The testing equipment of activation energy.
measured. On the upper flange plate is equipped with air bleed valve. The sample is measured on the copper plate, copper electrode can change measuring distance by adjusting the electrode pillars. The heating pipe is arranged in two sides of copper plate, Electric wire is wound on heating pipe. The cooling block is located below copper plate with cooling water for cooling. The copper plate is located on the pillar of PTFE, one is for the electric insulation, and the second is for heat insulation. Steel walls and lower flange plate is sealed by O ring. There are pumping holes, wire outlet and cooling water pipe outlet under the lower flange plate. The equipment is vacuum pumped by mechanical pump, the temperature is controlled by temperature control instrument, the range from room temperature to 200 ◦ C. The data acquisition system of the equipment is the United States KEITHLEY6517A multimeter/high resistance test instrument, testing voltage range from 0 V to 400 V, testing current range from 20pA to 200 A, testing resistance can reach up to 200T. 2.2. The depositing condition of samples All the samples were prepared by PECVD, depositing frequency: 13.56 MHz. The samples were analyzed by Raman spectroscopy with a Renishaw MR-2000 Raman spectrometer. The depositing condition of samples at different doping concentration: SiH4 concentration:1% temperature:150 ◦ C pressure:133.3pa power:50W depositing time:2h, PH3 concentration:0.5‰ 1‰ 3‰ 5‰ 8‰ and 10‰. The depositing condition of samples at different depositing temperature: SiH4 concentration:1% PH3 concentration:3‰ pressure:133.3pa power:50W depositing time:2 h, temperature:100 ◦ C 150 ◦ C 200 ◦ C 250 ◦ C 300 ◦ C and 350 ◦ C. 3. Results and discussion Fig. 2 shows Raman spectrum of samples at different doping concentration, it shows that: the crystallization of the sample have little different, and the crystallization condition is very good. Fig. 3 shows Raman spectrum of samples at
10439
Relative intensity
Q. Chen et al. / Optik 127 (2016) 10437–10441
200
300
400
500
600
700
800
-1
Raman Shift(cm ) Fig. 2. Raman spectra of samples at different doping concentration.
Relative intensity
250
200 150
100
200
300
400
500
600
700
800
-1
Raman shift(cm ) Fig. 3. Raman spectra of samples at different depositing temperature.
Fig. 4. The conductivity varied with temperature of different deposited samples(a:different doping concentration b: different depositing temperature).
different depositing temperature, it shows that: the crystallization condition of the sample is good, and the crystalline volume fraction is higher. Fig. 4 shows the curve of − 1000/T of samples, it can be concluded that: the change of the doped sample conductivity is very small with the change of temperature, the curve is almost linear, the slope is very small, so the unit of activation energy is meV. Phosphorus impurity atoms enter into the grain interior to form a substitution type doping. So impurity band is formed between valence band and conduction band, the Fermi level moves toward the bottom of conduction band, the conductivity
10440
Q. Chen et al. / Optik 127 (2016) 10437–10441
Fig. 5. The conduction and activation energy of different deposited samples(a: different doping concentration b: different depositing temperature).
activation energy of samples is decreased. The concentration of free carrier increases sharply, so the conductivity increases. On the other hand, phosphorus doping affects the crystalline volume fraction of silicon thin films, it is easy to understand the effect of crystalline volume fraction on electrical conductivity, the grain boundary is gathered with carrier traps and impurities, with crystalline volume fraction increases, the crystallization area increases, the probability of impurity atoms entering the grain increases, the concentration of free carrier increases. Meanwhile, with the grain boundary thickness decreases, the probability of electron tunneling increases, and the probability of impurity atoms entering grain interior increases. The above two factors lead to the increase in the conductance of 3–4 orders of magnitude more than the intrinsic microcrystalline silicon thin films, the activation energy decreases to meV unit. Fig. 5 shows the conductivity and activation energy of different deposited samples. With the increase of doping concentration, the conductivity of the samples increased gradually, reached up to 2 s/cm, the activation energy decreased from 57.97 meV to 21.71 meV. With the increase of temperature, the conductivity of the samples increased gradually, and the activation energy decreased, meanwhile, the activation energy of the samples which were deposited at 300 ◦ C and 350 ◦ C were measured, the activation energy were found to be smaller than others. For the microcrystalline silicon thin films which in the amorphous/microcrystalline phase transition zone, the conductivity is affected by crystalline volume fraction and dimensional effect. For the study of the intrinsic film, the conductivity of the sample is determined by crystalline volume fraction and dimensional effect. When the grain size is kept constant, the crystalline volume fraction becomes the main factor that restricts the electrical conductivity, the electrical conductivity can be increased by 2–3 orders of magnitude by increasing crystalline volume fraction [5]. When the crystalline volume fraction have little difference, and samples conductivity have an obvious relationship of grain size [6]. In addition, for the microcrystalline silicon thin film, the silicon grain is in a disordered network structure, there is a wide disorder region between the grains. After doping, a small amount of P atoms can be ionized to form a regular tetrahedral structure, which increases the order degree and dark conductivity of the films. For phosphorus doped hydrogenated microcrystalline silicon thin film [7], because a small amount of phosphorus doping, grain refinement in the film, the order degree of thin film is improved, the crystalline volume fraction is generally higher than intrinsic. The degree of order increases, increases the film conductivity. However, after a large amount of [8] doping, the P atom is in SP3 hybrid mode, which increases the disorder of the network structure of thin film, and makes the film transform to amorphous structure, which leads to the decrease of the dark conductivity. It can be concluded that the conductivity is simultaneously affected by crystalline volume fraction and dimensional effect. In this paper, in the depositing experiment of the different doping concentration and different depositing temperature, the doped microcrystalline silicon film is crystallized better, the crystalline volume fraction have little difference, so crystalline volume fraction and dimensional effect have not obvious effects on conductivity,impurity effects have a major effect on conductivity. With the doping concentration and depositing temperature increases, there will be more P atoms ionized to electrons and formed tetrahedral structure, the order degree of the films is increased, so that P atoms can be more effective doping, thereby dark conductivity increases, activation energy decreases.
4. Conclusions The activation energy of thin films which deposited at different doping concentration and different depositing temperature were measured by activation energy testing equipment. It was found that: with the increase of doping concentration and depositing temperature, conductivity increases, activation energy decreases. For phosphorus doped microcrystalline silicon films, when the crystallization conditions are good,the relationship between activation energy and crystalline volume fraction is not obvious, impurity effects have a major effect on conductivity.
Q. Chen et al. / Optik 127 (2016) 10437–10441
10441
Acknowledgement The author would like to thank Prof. Zhang for his invaluable discussions and assistance. This work was supported by Chinese National Natural Science Foundation (41401384);The University Science and Technology Project of Shandong Province (J14LJ52); The Science and Technology Development Planning of Binzhou City (2013ZC0107) The Science and Technology Development Planning of Binzhou City (2014ZC0307); The Science and Technology Development Planning of Binzhou City (2015ZC0210); The Doctoral Scientific Research Foundation of Binzhou University (2014Y10); Binzhou University Research Fund Project (BZXYG1513)and Binzhou University Research Fund Project (BZXYG04). References [1] Y.S. Chen, J.H. Wang, J.X. Lu, et al., Microcrystalline silicon grown by VHF PECVD and the fabrication of solar cells, [J] Sol. Energy 82 (11) (2008) 1083–1087. [2] M. Kondo, M. Fukawa, L. Guo, A. Matsuda, High rate growth of microcrystalline silicon at low temperatures, [J] J. Non-Cryst. Solids 266–269 (1) (2000) 84–89. [3] A. Shah, J. Meier, E. Vallat-Sauvain, et al., “Microcrystalline silicon and micromorph tandem solar cells,”, [J] Thin Solid Films (2002) 403–404, 179–187. [4] W.F.L. Tse, I. Khodami, M.M. Adachi, et al., Characterization of low temperature P-type hydrogenated microcrystalline silicon thin films deposited by plasma enhanced chemical vapor deposition, [C] Can. Conf. Electr. Comput. Eng. (2007) 952–955. [5] Richter and Ley, H. Richter, L. Ley, Optical properties and transport in microcrystalline silicon prepared at temperature below 400 ◦ C, [J] Appl. Phys. Lett. 52 (12) (1981) 7281–7286. [6] S.B. Zhang, X.B. Liao, L. An, et al., Micro-Raman study on hydrogenated protocrystalline silicon films, [J] Acta Phys. Sinaca 51 (8) (2002) 1811–1815. [7] Y.S. Chen, S.Z. Wang, S.E. Yang, et al., Microstructures and photo-electric characteristics of phosphorus-doped hydrogenated silicon films, [J] J. Vac. Sci. Technol. 26 (1) (2006) 8–11. [8] Q. Wang, J.N. Ding, Y.L. He, et al., Mesoscopic mechanical characterization of hy-drogenated silicon thin film and the intrinsic relationshipwith the microstructure, [J] Acta Phys. Sinaca 56 (8) (2007) 4835–4840.