- Email: [email protected]
The DC conductivity and structural ordering of thin silicon films at the amorphous to nano-crystalline phase transition
ARTICLE IN PRESS
Vacuum 82 (2008) 205–208 www.elsevier.com/locate/vacuum
The DC conductivity and structural ordering of thin silicon films at the amorphous to nano-crystalline phase transition D. Gracina, B. Etlingera,, K. Juraic´a, A. Gajovic´a, P. Dubcˇeka, S. Bernstorffb a
RuXer Bosˇkovic´ Institute, 10000 Zagreb, Bijenicˇka 54, Croatia b Sincrotrone Trieste, SS 14, km 163.5, Basovizza (TS), Italy
Abstract Thin silicon films were deposited by the plasma-enhanced chemical vapor deposition using standard 13.6 MHz radiofrequency gas discharge in silane diluted by hydrogen. The deposition conditions were kept constant for all samples, with the exception of only one parameter: the degree of dilution was varied from low values that produce amorphous layers up to the high dilution that resulted in a high degree of crystalline fraction. The structural properties of the samples were analyzed by Raman and grazing incidence small angle X-ray scattering (GISAXS) while direct current (DC) dark conductivity was measured by standard methods. The ratio of the areas under corresponding transversal optical (TO) phonon peaks in Raman was taken as the ratio between crystal and amorphous volume fraction while the shift of the TO peak position was used for the estimation of the crystal size. By increasing the working gas dilution, the crystalline fraction grew from 0% to 40% and the average individual crystal size increased from 2 to 10 nm. The size of the ‘‘particles’’, estimated by GISAXS using the Guinier approximation, varied from 2 to 4 nm. For a lower working gas dilution, the ‘‘particles’’ detected by GISAXS were spherically symmetric and showed no difference between near surface and ‘‘bulk’’ of the film. For a higher dilution, the particles became asymmetric and larger closer to the surface, which indicates columnar growth. The DC dark conductivity increased exponentially with the crystalline fraction, except for a very low crystal to amorphous volume ratio where the conductivity was larger, probably due to a better ordering of the amorphous phase in the vicinity of the amorphous to crystalline transition. r 2007 Elsevier Ltd. All rights reserved. PACS: 81.07.–b; 61.10.Eq; 78.30.–j; 73.63.Bd Keywords: Nano-crystalline silicon; GISAXS; Raman; DC conductivity
1. Introduction Amorphous thin silicon films are widely used as optoelectronic materials in particularly as active part of thin film photovoltaic solar cells. However, the material is not stable when exposed to the solar radiation and thus in the last decade the mixture between the amorphous and the nano-crystalline structural form has been extensively investigated as a possible candidate for replacing the fully amorphous material. The technological and scientific interest for silicon nano-structures appeared due to the influence of the small crystal dimensions on the distribution Corresponding author. Tel.: +385 1 4561137; fax: +385 1 4680114.
E-mail address: [email protected] (B. Etlinger). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.07.039
of phonon and electron states [1,2]. Generally, material contains the amorphous phase, nano-crystals with different individual sizes and voids, and that is why the transport phenomena should be dependent on the detailed distribution of the different structural elements. In this work, we demonstrate a possible way of estimating what are the main structural properties that influence the direct current (DC) dark conductivity of amorphous material on the transition to the nano-crystalline phase. 2. Experimental The deposition of the samples was performed by using a 13.6 MHz radiofrequency discharge and plasma-enhanced chemical vapor deposition (RFPECVD) in a capacitively
ARTICLE IN PRESS 206
D. Gracin et al. / Vacuum 82 (2008) 205–208
coupled planar diode source. The deposition of amorphous samples was performed in pure silane gas while for the deposition of crystalline samples the silane was highly diluted by hydrogen. The first crystals visible by Raman spectroscopy appeared in the samples deposited with a working gas mixture consisting of 7% silane and 93% hydrogen. Further dilution, down to 5% of silane, resulted in an increase of both the crystal fraction and the individual crystal sizes. Raman spectra were recorded by using a computerized DILOR Z24 triple monochromator with a Coherent INNOVA 100 argon ion laser operating at 514.5 nm wavelength for the excitation. The typical resolution was between 1 and 2 cm1. In the analysis of the Raman spectra, the ratio of the areas under corresponding transversal optical (TO) phonon peaks in Raman was taken as a measure of the crystal to amorphous fraction. In the fitting procedure the contribution of the amorphous phase to the Raman signal was estimated using the four Gaussian-like peaks for the amorphous and the two Voight-like for the crystalline phase. The peak widths and frequency shifts were allowed to vary. This approach resulted in a variation of the peak positions depending on the degree of crystallinity. The crystalline TO peak position, oTO ( measured in cm1) was taken as an estimation of the crystal size, dRaman (in nm), using the semi-empirical formula [3] d Raman ¼ 2pð2=ð522 oTO ÞÞ1=2 .
(1)
The grazing incidence small angle X-ray scattering (GISAXS) measurements were performed at the synchrotron ELETTRA, Trieste (Italy), at the SAXS beam-line [4], using an X-ray beam energy of 8 keV (l ¼ 0.154 nm). Details are given in Ref. [5]. 3. Results and discussion 3.1. Raman spectroscopy A typical set of Raman spectra is plotted in Fig. 1. The amorphous Si has a TO related peak around 480 cm1 while the TO related peak of mono-crystalline Si is at 520 cm1. For nano-crystalline material, due to quantum effects and stress, the distribution of phonon states changes and the TO related peak is shifted towards lower wave numbers [6]. In particularly, samples with a very low concentration of crystalline fraction and small crystal dimensions are difficult to analyze quantitatively. The peak position of 2 nm sized particles is close to 500 cm1 and it is overlapping with the amorphous TO related peak around 480 cm1, as it is seen in Fig. 1. That is why in many cases this type of samples is considered amorphous. The volume contribution of the nano-crystalline fraction was estimated from the difference of the spectra from the mixed amorphous and nano-crystalline, and the pure amorphous samples (plotted in Fig. 1 with a thinner line). For all studied samples, the nano-crystal sizes ranged
Fig. 1. Raman spectra of pure amorphous samples and samples containing amorphous and nano-crystalline phases. The amorphous silicon TO related peak and the corresponding nano-crystalline TO related peaks obtained by mathematical fit are also plotted and denoted by arrows.
between 2 and 10 nm for samples with a crystalline fraction of only a few %, or 40%, respectively. 3.2. GISAXS A typical result of the GISAXS data analysis is plotted in Fig. 2 for a sample with 35% of nano-crystalline phase. The individual sizes of the detected ‘‘particles’’, most probably voids, varied from 2 to 4 nm and it was found that their shape is not spherical. The particles are larger for lower GISAXS angles, e.g. closer to the surface of the sample. The dimensions of the particles are larger in the direction parallel to the surface than in the direction perpendicular to it. This ‘‘elliptical’’ shape of the particles, and the difference in sizes across the depth of the sample, were less pronounced in the case of a lower level of crystallinity, just revealing the transition from transport limiting growth to a growth strongly influenced by plasma–surface interactions which is typical for the deposition of fully crystalline samples. 3.3. DC resistivity The results of the measurement of the DC dark conductivity as a function of the crystalline fraction are plotted in Fig. 3 as full circles. In the same graph are plotted also results taken from Ref. [7], obtained for samples deposited by microwave plasma-enhanced chemical vapor deposition (MWPECVD) [8], but with a similar structure, e.g. for samples that contained a mixture of amorphous and nano-crystalline phases, where nanocrystals are embedded in an amorphous matrix. The initial drop in resistivity at very low volume contribution of nanocrystals is most probably related to the ordering of the amorphous silicon matrix, which is present at the
ARTICLE IN PRESS D. Gracin et al. / Vacuum 82 (2008) 205–208
Fig. 2. The radius of gyration of a sample containing 35% of nanocrystalline phase, as a function of the difference between GISAXS angle and critical angle. The ‘‘H’’ denotes the values obtained for the direction parallel to the sample surface while ‘‘V’’ corresponds to the direction perpendicular to the surface.
207
Fig. 4. Comparison of the Raman spectra of two samples having a structure closer to the amorphous to crystalline transition (2% nanocrystalline phase) and further from this border (5% nano-crystalline phase).
the ratio between the transversal acoustic (TA) related peak (around 150 cm1) and the TO related peak is a measure of the topological ordering—the lower TA is in comparison with TO, the higher is the degree of ordering. The analysis of the data plotted in Fig. 4 shows that the sample with a lower nano-crystal fraction has a TO related peak with a slightly lower FWHM and obviously a lower ITA/ITO ratio, e.g. a better topological ordering. This fact explains the lower resistivity of this sample (2 107 S1 cm) as compared to the other one (1 107 S1 cm), although the latter has a higher crystalline fraction (2% in comparison with 5%).
4. Conclusion
Fig. 3. The DC dark resistivity of samples with different crystal fraction for RFPECVD samples (full circles, this work) and for MWPECVD samples (open circles, Ref. [7]).
amorphous to nano-crystalline transition. The main condition for the deposition of crystalline samples is a high dilution of silane gas with hydrogen that should have beneficial effects on the ordering of the amorphous silicon matrix, similar to the effects of working gas dilution on magnetron sputtered samples [5]. In order to support this presumption, the Raman spectra of samples with 2% and 5% of nano-crystalline phase are plotted in Fig. 4. In the theory of Raman spectra from amorphous samples, the full-width at half-maximum (FWHM) of the TO related peak is proportional to the mean square deviation of the dihedral angle between the silicon atoms. Furthermore,
A series of samples with structures between pure amorphous, and mixtures of amorphous and up to 40% of nano-crystalline phase, deposited by RFPECVD, was examined by Raman spectroscopy, GISAXS and DC dark conductivity. The samples contained from 0 to 40 vol% of nano-cystalline phase, with individual nanocrystalline sizes between 2 and 8 nm and voids with individual sizes of 2–4 nm which are larger for higher crystalline fractions. The individual sizes of voids and nano-crystals increase with the volume contribution of the crystal phase. It was shown that the resistivity decreased exponentially with the increase of the nanocrystalline fraction, very similar as in samples obtained with the MWPECVD. The complex structural changes accompanied with an increase of the crystalline volume fraction makes the exact modeling of the DC dark conductivity rather demanding. However, the faster drop of the resistivity for the samples with only a few % of nanocrystalline phase has been explained with a better
ARTICLE IN PRESS 208
D. Gracin et al. / Vacuum 82 (2008) 205–208
topological ordering of the amorphous matrix at the amorphous to crystalline transition than in the pure amorphous matrix.
Acknowledgments This work has been financially supported by the Croatian Ministry of Science, Education and Sport (grants ‘‘Physics and applications of nanostructures and bulk matter’’, 098-0982904-2898 and ‘‘The thin film silicon on the amorphous to crystalline transition’’, 098-09828862894) and by the European Commission under the Contract no. INCO-CT-2004-509178.
References [1] Green M. Mater Sci Eng B 2000;B74:118–24. [2] Bukowski TJ, Simmons JH. Crit Rev Solid State Mater Sci 2002;27:119–42. [3] Jadkar SR, Sali JV, Takwale MG, Musale DV, Kshirsagar ST. Sol Energy Mater Sol Cells 2000;64:333–46. [4] Amenitsch H, Bernstorff S, Laggner P. Rev Sci Instrum 1995;66:1624–6. [5] Gracin D, Juraic K, Dubcek P, Gajovic A, Bernstorff S. Appl Surf Sci 2006;252:5598–601. [6] Zi J, Buescher H, Faler C, Ludwig W, Zhang K, Xie X. Appl Phys Lett 1996;69:200–2. [7] Devilee C, Muffler HJ, Soppe WJ. Private communication. [8] Soppe WJ, Biebericher ACW, Devilee C, Donker H, Schlemm H. In: Proceedings of third world conference on photovoltaic energy conversion (IEEE Cat. no. 03CH37497), vol. 2. Arisumi Printing Inc.; 2003. p. 1655–8.
Vacuum 82 (2008) 205–208 www.elsevier.com/locate/vacuum
The DC conductivity and structural ordering of thin silicon films at the amorphous to nano-crystalline phase transition D. Gracina, B. Etlingera,, K. Juraic´a, A. Gajovic´a, P. Dubcˇeka, S. Bernstorffb a
RuXer Bosˇkovic´ Institute, 10000 Zagreb, Bijenicˇka 54, Croatia b Sincrotrone Trieste, SS 14, km 163.5, Basovizza (TS), Italy
Abstract Thin silicon films were deposited by the plasma-enhanced chemical vapor deposition using standard 13.6 MHz radiofrequency gas discharge in silane diluted by hydrogen. The deposition conditions were kept constant for all samples, with the exception of only one parameter: the degree of dilution was varied from low values that produce amorphous layers up to the high dilution that resulted in a high degree of crystalline fraction. The structural properties of the samples were analyzed by Raman and grazing incidence small angle X-ray scattering (GISAXS) while direct current (DC) dark conductivity was measured by standard methods. The ratio of the areas under corresponding transversal optical (TO) phonon peaks in Raman was taken as the ratio between crystal and amorphous volume fraction while the shift of the TO peak position was used for the estimation of the crystal size. By increasing the working gas dilution, the crystalline fraction grew from 0% to 40% and the average individual crystal size increased from 2 to 10 nm. The size of the ‘‘particles’’, estimated by GISAXS using the Guinier approximation, varied from 2 to 4 nm. For a lower working gas dilution, the ‘‘particles’’ detected by GISAXS were spherically symmetric and showed no difference between near surface and ‘‘bulk’’ of the film. For a higher dilution, the particles became asymmetric and larger closer to the surface, which indicates columnar growth. The DC dark conductivity increased exponentially with the crystalline fraction, except for a very low crystal to amorphous volume ratio where the conductivity was larger, probably due to a better ordering of the amorphous phase in the vicinity of the amorphous to crystalline transition. r 2007 Elsevier Ltd. All rights reserved. PACS: 81.07.–b; 61.10.Eq; 78.30.–j; 73.63.Bd Keywords: Nano-crystalline silicon; GISAXS; Raman; DC conductivity
1. Introduction Amorphous thin silicon films are widely used as optoelectronic materials in particularly as active part of thin film photovoltaic solar cells. However, the material is not stable when exposed to the solar radiation and thus in the last decade the mixture between the amorphous and the nano-crystalline structural form has been extensively investigated as a possible candidate for replacing the fully amorphous material. The technological and scientific interest for silicon nano-structures appeared due to the influence of the small crystal dimensions on the distribution Corresponding author. Tel.: +385 1 4561137; fax: +385 1 4680114.
E-mail address: [email protected] (B. Etlinger). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.07.039
of phonon and electron states [1,2]. Generally, material contains the amorphous phase, nano-crystals with different individual sizes and voids, and that is why the transport phenomena should be dependent on the detailed distribution of the different structural elements. In this work, we demonstrate a possible way of estimating what are the main structural properties that influence the direct current (DC) dark conductivity of amorphous material on the transition to the nano-crystalline phase. 2. Experimental The deposition of the samples was performed by using a 13.6 MHz radiofrequency discharge and plasma-enhanced chemical vapor deposition (RFPECVD) in a capacitively
ARTICLE IN PRESS 206
D. Gracin et al. / Vacuum 82 (2008) 205–208
coupled planar diode source. The deposition of amorphous samples was performed in pure silane gas while for the deposition of crystalline samples the silane was highly diluted by hydrogen. The first crystals visible by Raman spectroscopy appeared in the samples deposited with a working gas mixture consisting of 7% silane and 93% hydrogen. Further dilution, down to 5% of silane, resulted in an increase of both the crystal fraction and the individual crystal sizes. Raman spectra were recorded by using a computerized DILOR Z24 triple monochromator with a Coherent INNOVA 100 argon ion laser operating at 514.5 nm wavelength for the excitation. The typical resolution was between 1 and 2 cm1. In the analysis of the Raman spectra, the ratio of the areas under corresponding transversal optical (TO) phonon peaks in Raman was taken as a measure of the crystal to amorphous fraction. In the fitting procedure the contribution of the amorphous phase to the Raman signal was estimated using the four Gaussian-like peaks for the amorphous and the two Voight-like for the crystalline phase. The peak widths and frequency shifts were allowed to vary. This approach resulted in a variation of the peak positions depending on the degree of crystallinity. The crystalline TO peak position, oTO ( measured in cm1) was taken as an estimation of the crystal size, dRaman (in nm), using the semi-empirical formula [3] d Raman ¼ 2pð2=ð522 oTO ÞÞ1=2 .
(1)
The grazing incidence small angle X-ray scattering (GISAXS) measurements were performed at the synchrotron ELETTRA, Trieste (Italy), at the SAXS beam-line [4], using an X-ray beam energy of 8 keV (l ¼ 0.154 nm). Details are given in Ref. [5]. 3. Results and discussion 3.1. Raman spectroscopy A typical set of Raman spectra is plotted in Fig. 1. The amorphous Si has a TO related peak around 480 cm1 while the TO related peak of mono-crystalline Si is at 520 cm1. For nano-crystalline material, due to quantum effects and stress, the distribution of phonon states changes and the TO related peak is shifted towards lower wave numbers [6]. In particularly, samples with a very low concentration of crystalline fraction and small crystal dimensions are difficult to analyze quantitatively. The peak position of 2 nm sized particles is close to 500 cm1 and it is overlapping with the amorphous TO related peak around 480 cm1, as it is seen in Fig. 1. That is why in many cases this type of samples is considered amorphous. The volume contribution of the nano-crystalline fraction was estimated from the difference of the spectra from the mixed amorphous and nano-crystalline, and the pure amorphous samples (plotted in Fig. 1 with a thinner line). For all studied samples, the nano-crystal sizes ranged
Fig. 1. Raman spectra of pure amorphous samples and samples containing amorphous and nano-crystalline phases. The amorphous silicon TO related peak and the corresponding nano-crystalline TO related peaks obtained by mathematical fit are also plotted and denoted by arrows.
between 2 and 10 nm for samples with a crystalline fraction of only a few %, or 40%, respectively. 3.2. GISAXS A typical result of the GISAXS data analysis is plotted in Fig. 2 for a sample with 35% of nano-crystalline phase. The individual sizes of the detected ‘‘particles’’, most probably voids, varied from 2 to 4 nm and it was found that their shape is not spherical. The particles are larger for lower GISAXS angles, e.g. closer to the surface of the sample. The dimensions of the particles are larger in the direction parallel to the surface than in the direction perpendicular to it. This ‘‘elliptical’’ shape of the particles, and the difference in sizes across the depth of the sample, were less pronounced in the case of a lower level of crystallinity, just revealing the transition from transport limiting growth to a growth strongly influenced by plasma–surface interactions which is typical for the deposition of fully crystalline samples. 3.3. DC resistivity The results of the measurement of the DC dark conductivity as a function of the crystalline fraction are plotted in Fig. 3 as full circles. In the same graph are plotted also results taken from Ref. [7], obtained for samples deposited by microwave plasma-enhanced chemical vapor deposition (MWPECVD) [8], but with a similar structure, e.g. for samples that contained a mixture of amorphous and nano-crystalline phases, where nanocrystals are embedded in an amorphous matrix. The initial drop in resistivity at very low volume contribution of nanocrystals is most probably related to the ordering of the amorphous silicon matrix, which is present at the
ARTICLE IN PRESS D. Gracin et al. / Vacuum 82 (2008) 205–208
Fig. 2. The radius of gyration of a sample containing 35% of nanocrystalline phase, as a function of the difference between GISAXS angle and critical angle. The ‘‘H’’ denotes the values obtained for the direction parallel to the sample surface while ‘‘V’’ corresponds to the direction perpendicular to the surface.
207
Fig. 4. Comparison of the Raman spectra of two samples having a structure closer to the amorphous to crystalline transition (2% nanocrystalline phase) and further from this border (5% nano-crystalline phase).
the ratio between the transversal acoustic (TA) related peak (around 150 cm1) and the TO related peak is a measure of the topological ordering—the lower TA is in comparison with TO, the higher is the degree of ordering. The analysis of the data plotted in Fig. 4 shows that the sample with a lower nano-crystal fraction has a TO related peak with a slightly lower FWHM and obviously a lower ITA/ITO ratio, e.g. a better topological ordering. This fact explains the lower resistivity of this sample (2 107 S1 cm) as compared to the other one (1 107 S1 cm), although the latter has a higher crystalline fraction (2% in comparison with 5%).
4. Conclusion
Fig. 3. The DC dark resistivity of samples with different crystal fraction for RFPECVD samples (full circles, this work) and for MWPECVD samples (open circles, Ref. [7]).
amorphous to nano-crystalline transition. The main condition for the deposition of crystalline samples is a high dilution of silane gas with hydrogen that should have beneficial effects on the ordering of the amorphous silicon matrix, similar to the effects of working gas dilution on magnetron sputtered samples [5]. In order to support this presumption, the Raman spectra of samples with 2% and 5% of nano-crystalline phase are plotted in Fig. 4. In the theory of Raman spectra from amorphous samples, the full-width at half-maximum (FWHM) of the TO related peak is proportional to the mean square deviation of the dihedral angle between the silicon atoms. Furthermore,
A series of samples with structures between pure amorphous, and mixtures of amorphous and up to 40% of nano-crystalline phase, deposited by RFPECVD, was examined by Raman spectroscopy, GISAXS and DC dark conductivity. The samples contained from 0 to 40 vol% of nano-cystalline phase, with individual nanocrystalline sizes between 2 and 8 nm and voids with individual sizes of 2–4 nm which are larger for higher crystalline fractions. The individual sizes of voids and nano-crystals increase with the volume contribution of the crystal phase. It was shown that the resistivity decreased exponentially with the increase of the nanocrystalline fraction, very similar as in samples obtained with the MWPECVD. The complex structural changes accompanied with an increase of the crystalline volume fraction makes the exact modeling of the DC dark conductivity rather demanding. However, the faster drop of the resistivity for the samples with only a few % of nanocrystalline phase has been explained with a better
ARTICLE IN PRESS 208
D. Gracin et al. / Vacuum 82 (2008) 205–208
topological ordering of the amorphous matrix at the amorphous to crystalline transition than in the pure amorphous matrix.
Acknowledgments This work has been financially supported by the Croatian Ministry of Science, Education and Sport (grants ‘‘Physics and applications of nanostructures and bulk matter’’, 098-0982904-2898 and ‘‘The thin film silicon on the amorphous to crystalline transition’’, 098-09828862894) and by the European Commission under the Contract no. INCO-CT-2004-509178.
References [1] Green M. Mater Sci Eng B 2000;B74:118–24. [2] Bukowski TJ, Simmons JH. Crit Rev Solid State Mater Sci 2002;27:119–42. [3] Jadkar SR, Sali JV, Takwale MG, Musale DV, Kshirsagar ST. Sol Energy Mater Sol Cells 2000;64:333–46. [4] Amenitsch H, Bernstorff S, Laggner P. Rev Sci Instrum 1995;66:1624–6. [5] Gracin D, Juraic K, Dubcek P, Gajovic A, Bernstorff S. Appl Surf Sci 2006;252:5598–601. [6] Zi J, Buescher H, Faler C, Ludwig W, Zhang K, Xie X. Appl Phys Lett 1996;69:200–2. [7] Devilee C, Muffler HJ, Soppe WJ. Private communication. [8] Soppe WJ, Biebericher ACW, Devilee C, Donker H, Schlemm H. In: Proceedings of third world conference on photovoltaic energy conversion (IEEE Cat. no. 03CH37497), vol. 2. Arisumi Printing Inc.; 2003. p. 1655–8.