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Optoelectronic properties of microcrystalline silicon films
Thin Solid Films 403 – 404 (2002) 526–529
Optoelectronic properties of microcrystalline silicon films ¨ F. Wunsch, G. Citarella, M. Kunst* Section Solare Energetik, Hahn Meitner Institut, Glienicker Strasse 100, 14109 Berlin, Germany
Abstract The optoelectronic properties of microcrystalline silicon (mc-Si) films were determined by contactless transient photoconductivity measurements in the microwave frequency range. High mobilities were observed in mc-Si produced by laser annealing although the material contains impurities diffused from the substrate and is doped. Hot wire and plasma enhanced chemical vapor deposited (PECVD) mc-Si films are characterized in the best case by mobilities in the order of magnitude of the mobility in a-Si:H. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Microcrystalline silicon; Transient photoconductivity; Mobility
1. Introduction Silicon films are promising candidates for thin film photovoltaics. The optoelectronic properties are of decisive importance, but still unsatisfactorily known. In this work, a contactless determination of these properties is proposed avoiding several problems adherent to the presence of contacts. In this work, mc-Si films produced by laser annealing of a-Si:H films by plasma enhanced chemical vapor deposition (PECVD) and by hot wire (HW) deposition will be investigated. 2. Experiment Hydrogenated amorphous silicon (a-Si:H) films have been produced by plasma enhanced chemical vapor deposition (PECVD) of silane on (Corning) glass substrates. Microcrystalline silicon (mc-Si) has been produced by PECVD, hot wire (HW) deposition and by laser-induced crystallization of a-Si:H films. Contactless transient photoconductivity measurements in the microwave frequency range have been performed at 10 GHz with the time resolved microwave conductiv* Corresponding author. Tel.: q49-30-80622923; fax: q49-3080622434. E-mail address: [email protected] (M. Kunst).
ity (TRMC) method as described previously w1x. TRMC signals were induced by 532-nm laser pulses of 10 ns (FWHM). The TRMC signal wDP(t)yPx is proportional to the photoconductance DS(t) induced by the excitation pulse: DPŽt. P
s A DSŽt.
(1)
where A is a proportionality factor (the sensitivity factor), and DS(t) given by: DSŽt.sDnŽt.mneqDpŽt.mpe
(2)
where Dn(t) and (Dp(t)) is the number (in cmy2) of excess electrons (holes) per unit area (cm2) distributed between the front and the back face of the sample at time t after the start of the excitation, characterized by the mobilities mn and mp. TRMC measurements yield two kinds of information: ● The decay behavior, and ● The signal height. For a quantitative use of the signal height it is convenient to take the TRMC amplitude (i.e. the maximum of the TRMC signal at approx. 10 ns for the present measurements) representing the end of pulse photoconductance DSend. This property can be normalized to yield the microwave mobility mm, defined as w1x:
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 6 6 5 - 0
¨ F. Wunsch et al. / Thin Solid Films 403 – 404 (2002) 526–529
Fig. 1. TRMC transients induced by 532-nm laser pulses in a stateof-the-art a-Si:H film and in a mc-Si film produced by laser annealing.
m ms
DSend Dnend Dpend s mnq mp eDn0 Dn0 Dn0
(3)
where Dnend(Dpend) is the number of excess electrons (holes) per unit area at the end of the pulse (at 10 ns). The parameter Dn0 is the number of excess charge carrier pairs induced by the pulse — i.e. the photon flux absorbed, the number of photons per unit time and unit area that are neither reflected nor transmitted through the sample, determined by reflection, transmission measurements (generally, the layers are thick enough to absorb all the light). In imperfect semiconductors as thin silicon films, mm is often exclusively due to the majority carrier w2x. The parameter mm must be determined in the range where the amplitude is proportional to the excitation density. For simple comparison, it is sufficient to compare the TRMC amplitudes determined at the same excitation density.
527
a-Si:H, but in addition to trapping, in mc-Si the situation is of another quality than in a-Si:H as at the grain boundaries, charge carriers can be immobilized by trapping in grain boundary gap states, but also a lot of mobile charge can be stored in the grain boundary regions if a sufficient intergrain space charge region is present. This mobile charge contributes to the microwave reflection signal, resulting in microwave signal decaying as slow as the decay of the space charge layer capacitance. For the microwave mobility of the sample, mms 80 cm2 Vy1 sy1 was derived. The TRMC amplitude of this sample depends linearly on the excitation density up to approximately 1 mJ cmy2, whereas at higher excitation densities the amplitude depends sublinearly on the excitation density (Fig. 2). In a-Si:H film this transition occurs at somewhat higher excitation densities (Fig. 2). This is remarkable because the absorption coefficient at 532 nm of mc-Si is approximately one order of magnitude smaller than that of a-Si:H. Consequently, a higher order (probably second order) decay process becomes active during the (10 ns) excitation at much lower carrier concentration in mc-Si than in a-Si:H. It must be pointed out that the two samples exhibit the same characteristics of light absorption within the experimental error, which is much smaller than the order of magnitude separating the excitation densities at which the transition between the linear and the non-linear happens in the two samples (Fig. 2). Other measurements on this kind of samples w3x showed a pronounced p-type conductivity due to diffusion of boron from the glass substrate in the sample. The resulting boron gradient may explain the slow decay (Fig. 1). In this case, mm probably refers to hole mobility.
3. Results and discussion 3.1. Microcrystalline silicon (mc-Si) films produced by laser annealing A TRMC transient measured for a sample of mc-Si produced by laser crystallization of a-Si:H is compared with that of a state-of-the-art a-Si:H film in Fig. 1. It is obvious that the signal in mc-Si film is higher and decays slower. As the initial decay of the mc-Si film disappears at lower excitation density, this initial decay is due to a high order decay processes. The slow decay is also remarkable since surface recombination does not play a role. Trapping phenomena occur both in mc-Si and in
Fig. 2. The TRMC amplitudes induced by 532-nm laser pulses as a function of the excitation density in a state-of-the-art a-Si:H film and in a mc-Si film produced by laser annealing.
528
¨ F. Wunsch et al. / Thin Solid Films 403 – 404 (2002) 526–529
Fig. 3. The TRMC amplitudes induced by 532-nm laser pulses in a state-of-the-art a-Si:H film and in a PECVD mc-Si film as a function of the excitation density.
3.2. Microcrystalline silicon (mc-Si) films produced by PECVD In PECVD films, the TRMC amplitude is proportional to the excitation density up to approximately 100 mJ cmy2 (Fig. 3). In the PECVD mc-Si film, the transition to a sublinear dependence lies at a much higher excitation density than for the a-Si:H (Fig. 3). Besides, the deviation from linearity in the high excitation density range seems to be much smaller in this kind of mc-Si than in a-Si:H. As it is argued in Section 2, a higher transition concentration for the onset of higher order recombination is expected in mc-Si. However, it is implausible that the non-linear decay process active at high excitation densities is the same in this type of mc-Si as in a-Si:H. For the microwave mobility of this sample, a value mms0.3 cm2 Vy1 cmy1 is derived from the TRMC amplitude in the low excitation density range, smaller than the value mms0.5 cm2 Vy1 sy1 determined for a-Si:H. The decay behavior of this kind of mc-Si film is characterized by a very fast initial decay giving way to a power law decay (Fig. 4). In principle this is very much alike the decay behavior in a-Si:H films (Fig. 1). However, the initial decay is even faster in this kind of m c-Si than in a-Si:H. This might be due to fast charge carrier trapping probably by traps related to the grain boundaries.
Fig. 4. TRMC transients induced by 532-nm laser pulses in a PECVD mc-Si film.
hot wire (HW) method w4x. This method is characterized by a large number of process parameters as, e.g. the substrate temperature, the wire temperature and the distance between wire and substrate. Therefore, it seems interesting to have the disposal of a method permitting the determination of optoelectronic quality mc-Si film in a quick and easy way. Here, the determination of the microwave mobility seems a very appropriate tool. As an example, the microwave mobilities determined for different mc-Si films produced by hot wire deposition are shown in Fig. 5. These films were deposited under the same process conditions where only the substrate temperature varied. The largest microwave mobility is found in the substrate temperature range of 250 8C. This result implies that the electric transport properties of the majority carrier (for these films the electron) are optimal in this temper-
3.3. Microcrystalline silicon (mc-Si) films produced by hot wire deposition A promising way to produce mc-Si films is by the
Fig. 5. The microwave mobility of mc-Si films produced by hot wire deposition as a function of the substrate temperature.
¨ F. Wunsch et al. / Thin Solid Films 403 – 404 (2002) 526–529
ature range. This seems a rather unambiguous criterion for the selection of the samples. So the optimal substrate temperature range for the production of hot wire mc-Si films is approximately 250 8C. In the films with lower mobilities, the initial decay is also faster than in films deposited in the optimal temperature range. This points to stronger trapping in the films of lower quality. Consequently, the decrease of the microwave mobility is probably not only due to a decrease in mobility, but also to increased trapping during the excitation (compare the TRMC results obtained on a-Si:H deposited at different temperatures w5x). It is interesting to separate the influence of these two effects on the microwave mobility, but this is not necessary for a comparison of the quality; both effects, i.e. increased trapping and a decrease in mobility, are characteristic for a deterioration of optoelectronic material properties and both lead to a decrease in effective microwave mobility, which is important for device performance.
529
4. Conclusions ● Microcrystalline silicon (mc-Si) films produced by laser annealing of a-Si:H films show good majority carrier transport properties, but diffusion of species from the substrate may be a drawback. ● PECVD and hot wire mc-Si films have relatively low mobilities (approx. 1 cm2 Vy1 cmy1) and complicated charge carrier kinetics. ● The production conditions of mc-Si films can be optimized by microwave mobility measurements. References w1x C. Swiatkowski, M. Kunst, Appl. Phys. A 61 (1995) 623. w2x D. Herm, S. von Aichberger, M. Kunst, Sol. Energy Mater. Sol. Cells 66 (2001) 195. w3x G. Andra, ¨ J. Bergmann, F. Falk, E. Ose, N.D. Sinh, Solid State Phenom. 67–68 (1999) 187. w4x A.H. Mahan, J. Carapella, B.P. Nelson, R.S. Crandall, I. Balberg, J. Appl. Phys. 69 (1991) 6728. w5x M. Kunst, H.-C. Neitzert, J. Appl. Phys. 69 (1991) 8320.
Optoelectronic properties of microcrystalline silicon films ¨ F. Wunsch, G. Citarella, M. Kunst* Section Solare Energetik, Hahn Meitner Institut, Glienicker Strasse 100, 14109 Berlin, Germany
Abstract The optoelectronic properties of microcrystalline silicon (mc-Si) films were determined by contactless transient photoconductivity measurements in the microwave frequency range. High mobilities were observed in mc-Si produced by laser annealing although the material contains impurities diffused from the substrate and is doped. Hot wire and plasma enhanced chemical vapor deposited (PECVD) mc-Si films are characterized in the best case by mobilities in the order of magnitude of the mobility in a-Si:H. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Microcrystalline silicon; Transient photoconductivity; Mobility
1. Introduction Silicon films are promising candidates for thin film photovoltaics. The optoelectronic properties are of decisive importance, but still unsatisfactorily known. In this work, a contactless determination of these properties is proposed avoiding several problems adherent to the presence of contacts. In this work, mc-Si films produced by laser annealing of a-Si:H films by plasma enhanced chemical vapor deposition (PECVD) and by hot wire (HW) deposition will be investigated. 2. Experiment Hydrogenated amorphous silicon (a-Si:H) films have been produced by plasma enhanced chemical vapor deposition (PECVD) of silane on (Corning) glass substrates. Microcrystalline silicon (mc-Si) has been produced by PECVD, hot wire (HW) deposition and by laser-induced crystallization of a-Si:H films. Contactless transient photoconductivity measurements in the microwave frequency range have been performed at 10 GHz with the time resolved microwave conductiv* Corresponding author. Tel.: q49-30-80622923; fax: q49-3080622434. E-mail address: [email protected] (M. Kunst).
ity (TRMC) method as described previously w1x. TRMC signals were induced by 532-nm laser pulses of 10 ns (FWHM). The TRMC signal wDP(t)yPx is proportional to the photoconductance DS(t) induced by the excitation pulse: DPŽt. P
s A DSŽt.
(1)
where A is a proportionality factor (the sensitivity factor), and DS(t) given by: DSŽt.sDnŽt.mneqDpŽt.mpe
(2)
where Dn(t) and (Dp(t)) is the number (in cmy2) of excess electrons (holes) per unit area (cm2) distributed between the front and the back face of the sample at time t after the start of the excitation, characterized by the mobilities mn and mp. TRMC measurements yield two kinds of information: ● The decay behavior, and ● The signal height. For a quantitative use of the signal height it is convenient to take the TRMC amplitude (i.e. the maximum of the TRMC signal at approx. 10 ns for the present measurements) representing the end of pulse photoconductance DSend. This property can be normalized to yield the microwave mobility mm, defined as w1x:
0040-6090/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 6 6 5 - 0
¨ F. Wunsch et al. / Thin Solid Films 403 – 404 (2002) 526–529
Fig. 1. TRMC transients induced by 532-nm laser pulses in a stateof-the-art a-Si:H film and in a mc-Si film produced by laser annealing.
m ms
DSend Dnend Dpend s mnq mp eDn0 Dn0 Dn0
(3)
where Dnend(Dpend) is the number of excess electrons (holes) per unit area at the end of the pulse (at 10 ns). The parameter Dn0 is the number of excess charge carrier pairs induced by the pulse — i.e. the photon flux absorbed, the number of photons per unit time and unit area that are neither reflected nor transmitted through the sample, determined by reflection, transmission measurements (generally, the layers are thick enough to absorb all the light). In imperfect semiconductors as thin silicon films, mm is often exclusively due to the majority carrier w2x. The parameter mm must be determined in the range where the amplitude is proportional to the excitation density. For simple comparison, it is sufficient to compare the TRMC amplitudes determined at the same excitation density.
527
a-Si:H, but in addition to trapping, in mc-Si the situation is of another quality than in a-Si:H as at the grain boundaries, charge carriers can be immobilized by trapping in grain boundary gap states, but also a lot of mobile charge can be stored in the grain boundary regions if a sufficient intergrain space charge region is present. This mobile charge contributes to the microwave reflection signal, resulting in microwave signal decaying as slow as the decay of the space charge layer capacitance. For the microwave mobility of the sample, mms 80 cm2 Vy1 sy1 was derived. The TRMC amplitude of this sample depends linearly on the excitation density up to approximately 1 mJ cmy2, whereas at higher excitation densities the amplitude depends sublinearly on the excitation density (Fig. 2). In a-Si:H film this transition occurs at somewhat higher excitation densities (Fig. 2). This is remarkable because the absorption coefficient at 532 nm of mc-Si is approximately one order of magnitude smaller than that of a-Si:H. Consequently, a higher order (probably second order) decay process becomes active during the (10 ns) excitation at much lower carrier concentration in mc-Si than in a-Si:H. It must be pointed out that the two samples exhibit the same characteristics of light absorption within the experimental error, which is much smaller than the order of magnitude separating the excitation densities at which the transition between the linear and the non-linear happens in the two samples (Fig. 2). Other measurements on this kind of samples w3x showed a pronounced p-type conductivity due to diffusion of boron from the glass substrate in the sample. The resulting boron gradient may explain the slow decay (Fig. 1). In this case, mm probably refers to hole mobility.
3. Results and discussion 3.1. Microcrystalline silicon (mc-Si) films produced by laser annealing A TRMC transient measured for a sample of mc-Si produced by laser crystallization of a-Si:H is compared with that of a state-of-the-art a-Si:H film in Fig. 1. It is obvious that the signal in mc-Si film is higher and decays slower. As the initial decay of the mc-Si film disappears at lower excitation density, this initial decay is due to a high order decay processes. The slow decay is also remarkable since surface recombination does not play a role. Trapping phenomena occur both in mc-Si and in
Fig. 2. The TRMC amplitudes induced by 532-nm laser pulses as a function of the excitation density in a state-of-the-art a-Si:H film and in a mc-Si film produced by laser annealing.
528
¨ F. Wunsch et al. / Thin Solid Films 403 – 404 (2002) 526–529
Fig. 3. The TRMC amplitudes induced by 532-nm laser pulses in a state-of-the-art a-Si:H film and in a PECVD mc-Si film as a function of the excitation density.
3.2. Microcrystalline silicon (mc-Si) films produced by PECVD In PECVD films, the TRMC amplitude is proportional to the excitation density up to approximately 100 mJ cmy2 (Fig. 3). In the PECVD mc-Si film, the transition to a sublinear dependence lies at a much higher excitation density than for the a-Si:H (Fig. 3). Besides, the deviation from linearity in the high excitation density range seems to be much smaller in this kind of mc-Si than in a-Si:H. As it is argued in Section 2, a higher transition concentration for the onset of higher order recombination is expected in mc-Si. However, it is implausible that the non-linear decay process active at high excitation densities is the same in this type of mc-Si as in a-Si:H. For the microwave mobility of this sample, a value mms0.3 cm2 Vy1 cmy1 is derived from the TRMC amplitude in the low excitation density range, smaller than the value mms0.5 cm2 Vy1 sy1 determined for a-Si:H. The decay behavior of this kind of mc-Si film is characterized by a very fast initial decay giving way to a power law decay (Fig. 4). In principle this is very much alike the decay behavior in a-Si:H films (Fig. 1). However, the initial decay is even faster in this kind of m c-Si than in a-Si:H. This might be due to fast charge carrier trapping probably by traps related to the grain boundaries.
Fig. 4. TRMC transients induced by 532-nm laser pulses in a PECVD mc-Si film.
hot wire (HW) method w4x. This method is characterized by a large number of process parameters as, e.g. the substrate temperature, the wire temperature and the distance between wire and substrate. Therefore, it seems interesting to have the disposal of a method permitting the determination of optoelectronic quality mc-Si film in a quick and easy way. Here, the determination of the microwave mobility seems a very appropriate tool. As an example, the microwave mobilities determined for different mc-Si films produced by hot wire deposition are shown in Fig. 5. These films were deposited under the same process conditions where only the substrate temperature varied. The largest microwave mobility is found in the substrate temperature range of 250 8C. This result implies that the electric transport properties of the majority carrier (for these films the electron) are optimal in this temper-
3.3. Microcrystalline silicon (mc-Si) films produced by hot wire deposition A promising way to produce mc-Si films is by the
Fig. 5. The microwave mobility of mc-Si films produced by hot wire deposition as a function of the substrate temperature.
¨ F. Wunsch et al. / Thin Solid Films 403 – 404 (2002) 526–529
ature range. This seems a rather unambiguous criterion for the selection of the samples. So the optimal substrate temperature range for the production of hot wire mc-Si films is approximately 250 8C. In the films with lower mobilities, the initial decay is also faster than in films deposited in the optimal temperature range. This points to stronger trapping in the films of lower quality. Consequently, the decrease of the microwave mobility is probably not only due to a decrease in mobility, but also to increased trapping during the excitation (compare the TRMC results obtained on a-Si:H deposited at different temperatures w5x). It is interesting to separate the influence of these two effects on the microwave mobility, but this is not necessary for a comparison of the quality; both effects, i.e. increased trapping and a decrease in mobility, are characteristic for a deterioration of optoelectronic material properties and both lead to a decrease in effective microwave mobility, which is important for device performance.
529
4. Conclusions ● Microcrystalline silicon (mc-Si) films produced by laser annealing of a-Si:H films show good majority carrier transport properties, but diffusion of species from the substrate may be a drawback. ● PECVD and hot wire mc-Si films have relatively low mobilities (approx. 1 cm2 Vy1 cmy1) and complicated charge carrier kinetics. ● The production conditions of mc-Si films can be optimized by microwave mobility measurements. References w1x C. Swiatkowski, M. Kunst, Appl. Phys. A 61 (1995) 623. w2x D. Herm, S. von Aichberger, M. Kunst, Sol. Energy Mater. Sol. Cells 66 (2001) 195. w3x G. Andra, ¨ J. Bergmann, F. Falk, E. Ose, N.D. Sinh, Solid State Phenom. 67–68 (1999) 187. w4x A.H. Mahan, J. Carapella, B.P. Nelson, R.S. Crandall, I. Balberg, J. Appl. Phys. 69 (1991) 6728. w5x M. Kunst, H.-C. Neitzert, J. Appl. Phys. 69 (1991) 8320.