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Electron transport of amorphous hydrogenated silicon-carbon alloy films prepared by glow discharge decomposition
Solar Energy Materials 23 (1991) 340-346 North-Hblland
Solar Energy Materials
Electron transport of amorphous hydrogenated silicon-carbon alloy films prepared by glow discharge decomposition T. Enomoto, R. Hattori and J. Shirafuji Department of Electrical Engineering, Faculty of Engineering, Osaka Unicersity, Yamada-Oka, Suita, Osaka 565, Japan Electron transport of glow-discharged C2H 2- and CH4-based SiCx:H films has been studied by time-of-flight method. The electron mobility at room temperature decreases nearly exponentially with increasing carbon content in both C2H 2- and CH4-based films. This decrease is mainly due to a broadening of the tail states width in SiCx:H films, as implied from the carbon content dependence of the activation energy of the electron mobility in CH4-based films.
1. Introduction
Amorphous hydrogenated silicon-based alloys, s'ach as a-SiCx:H, a-SiGex:H and a-SiN,.:H have been widely applied to solar cells, photodetectors, thin film transistors and superlattice structures, because their optical gaps can be continuously controlled by changing the composition. Especially, a-SiCx:H films have been attractive in applications to near-visible eiectroluminescent devices [1], window layers of solar cells [2-4], phototransistors [5] and color sensors [6]_ However, the photoconductivity of undoped a-SiC.,.:H films is inferior to that of a-Si:H even when a-SiCx:H films are sufficiently doped. This has been thought to arise from higher density of dangling bonds in a-SiCx:H films which behave as recombination centers for photoexcited carriers. Much attention has been paid to the realization of a-SiCx:H with a high photoconductivit~. One approach is to examine various source gas species. It has been reported that the dark conductivity and the photoconductivity of a-SiC,:H films are incdependent of source gas species (CH4/SiH 4, C2H4/SiH4 and SiH3CH3/SiH ~) [7]. Although many works have been carried out to improve the photoconductivity of a-SiCx:H films, no attempt has been reported so far to measure separately the drift mobility P~d and the lifetime "r in order to gain insight into the origin of the degraded photoconductivity. In this paper we study the electron transport in a-SiCx:H films by means of time-of-flight method, which allows us to estimate the drift mobility and the lifetime separately. This method is also useful to obtain information on tail states from the temperature dependence of the drift mobility, because the distribution of the tail states determines the carrier mobility. Two kinds of carbon source gases, . . I . I ' h ers B.V. All rights reserved 0165-1633/91/$03.50 © 1991 - Elsevier Science Puo~.~:,
T. Enomoto et al. / Electron transport of a-SiC:H alloy films
341
C H 4 and C 2 H 2 , a r e employed in the present experiment to study the influence of the source gas species. The bonding of CH 4 molecules is tetrahedral sp 3 configuration which is preferable in Si-C three-dimensional network, while C E H 2 molecules
have the structure governed by sp ~ bonding configuration which may be not favorable to SiC x network.
2. Experimental procedures a-SiCx:H films were deposited by the conventional RF glow discharge decomposition method using undiluted gas mixtures of Sill 4 and CH 4 or C 2 H 2. The substrate temperature was held at 230 ° C. The RF input power at 13.56 MHz was as low as 10 W corresponding to a power density of 0.35 W / c m 2. Samples for time-of-flight (TOF) measurements had a laminated structure of P d / u n d o p e d a - S i : H / n + a - S i : H / I T O / g l a s s . P-doped n + a-Si:H layer was a good ohmic contact to the ITO electrode, and could suppress hole injection from the ITO electrode into undoped a-SiCx:H layer. On the other hand, the evaporated Pd electrode served as a good Schottky barrier junction which suppressed the electron injection from the Pd electrode. In order to avoid space charge effects in the high resistivity a-SiCx:H layer, a pulsed bias voltage was applied in the reverse direction of the Pd Schottky barrier junction. A pulsed light (300 ps duration) at 337.1 nm from a nitrogen laser synchronized with the pulsed bias voltage was used as an excitation source. The transit time of photoexcited electrons was determined by the intersection of two straight line~ drawn on the loglog plot of the trans:,ent photocurrent, even though the photocurrent showed a non-dispersive waveform. The carbon content is estimated from XPS measurement.
3. Results and discussion
3.1. Comparison between C2H 2- and CH4-based a-SiCx:H Fig. 1 shows the optical band gap of undoped a-SiC,:H films prepared by decomposition of Sill 4 and C 2 H 2 or C H 4 gas mixtures as a function of gas flow ratio which is defined as [CH4]/[SiH 4 + CH 4] or [C2H2]/[2SiH4 + C2H2]. The optical band gap was determined in the usual way from (ahv)i/2-hv plots. The optical gap of the CH4-based films increases gradually from 1.70 to 2.04 eV with increasing flow rate ratio from 0 to 0.8. In the case of C2H2-based a-SiCx:H, on the other hand, the increase in the optical gap in the same range of flow rate ratio is more remarkable than CH 4. This result indicates that carbon can be incorporated easily into a-SiC.,.:H because of easier decomposition of C 2 H 2 molecules than CH~. Similar effects have been observed in the ca~e of C2H 4 gas against CH 4 gas [4]. Fig. 2 shows the room temperature electron drift mobility i.% of a-SiC.,.:H films prepared by different source gases as a functioa of optical band gap. The drift
342
T. Enomoto et al. / Electron transport of a-SiC:H alloy films
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Optical gap of undoped a-SiC.~.:H films as a function of gas flow rate ratio.
mobility is usually determined from the relation of /~d =L2//VtT (V: applied voltage, tr: transit time, L: sample thickness). However, TOF samples may have a built-in field due to the Schottky barrier, which may introduce an error in the estimation of the mobility. In the present experime,]t the mobility was determined from the slope of the straight portion of 1/tT-E plot (E = V/L). When the optical band gap is increased, ~d decreases exponentially with little difference between C H 4- and C2H2-based films as seen in fig. 2. However, the drift mobility of CHa-based samples with an optical gap larger than 1.8 eV was impossible to be measured because of too small mobility-lifetime product as will be shown in the next figure. The result of fig. 2 indicates that the difference in the intermolecular bonding of gas molecules ( s p 3 for C H 4 , s p ! for C2H 2) does not directly influence
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T. Enomoio et aL / Electron transport of a-SiC:H alloy fihns
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CARBON CONTENT (%) 5 10
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the atomic configuration in SiC, alloys possibly because of sufficient decomposition of gas molecules in glow discharge plasma. Fig. 3 shows the relationship between the optical gap and the mobility-lifetime product ~d z. The value of ~d ~" was determined from the applied voltage dependence of the collected charge by fitting to Hecht expression [8]. The value of /~dr of C2Hz-based films is a little larger than that of CH~-based films especially in the wide gap range. In combination with the result of fig. 2, C aH2-based films have a little higher carrier lifetime than CH4-based films. This result is in contrast with a previous report in which no difference was observed in the photoconductivity between CH,- and C2H4-based films [4]. 3.2. Influence of carbon content on lifetime and tail states width
Fig. 4 shows the carrier lifetime of CH z-based films estimated from figs. 2 and 3 as a function of optical band gap. The dangling bond density was estimated from ESR measurement. The ESR spin dellsity of CH4-based films was also plotted in fig. 4 for comparison. Regardless of the increase i,i the IEctimc, the spin density increases with the optical band gap. The apparent contradiction between the TOF lifetime and the ESR spin density is mostly associated with the difference in the experimental sit~atio:~s. The, TOF lifcthae i,~ actually relating to deep level trapping event which can be much influenced by the spread of tail states. When the tail states width becomes broad, the transient photocurrent can show a longer tail; this may cause an apparent large TOF lifetime. On the other hand. the ESR spin density is, as widely accepted, inversely proportional to the steady state photoconductivity in which the lifetime is determined to actual recombination process through dangling bond states.
344
T. Enomoto et aL / Electron transport of a-SiC'H alloy fihns
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In order to obtain information about the tall states, we measured the temperature dependence of the electron drift mobility in CH4-based samples with the optical gap below 1.8 eV. The linear relation between t T' and E was observed in the high temperature region above 247 K indicating non-dispersive transport. We estimated the mobility from the slope of the straight line in this case. On the other hand, in the !ow temperature region below 210 K, t:r ] versus E curves show a slight deviation from the straight line at higher applied fields. This non-linearity is due to the appearance of dispersive character in the transport. Nevertheless, we approximated the tT~-E curve with a straight line to elucidate the electron mobility at low temperatures, so that we can obtain the activation energy of the electron mobility for various carbon contents. Fig. 5 shows the activation energy of the electron drift mobility of CH4-based a-SiC.,.:H as a function of carbon content. The activation energy was estimated 0.2
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Fig. 5. Activation energy of electron drift mobility as a function of carbon content.
7". Enomoto et al. / Electron transport of a-SiC.'H alloy fihns
345
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CARBON .CONTENT ( % ) Fig. 6. Experimental and caluclated results of electron drift mobility of CH4-based a-SiC,:H films against optical gap.
from Arrhenius plot. The activation energy increases gradually as the carbon content is increased. This increase in the activation energy has a direct relation to the decrease in the drift mobility with carbon content. Fig. 6 shows the comparison of the experimental electron mobility with the calculated value based on a simple consideration. The open circles are the experimental results of CH4-based aSiC,:H. In order to estimate the contribution of the activation energy to the mobility, the drift mobility was roughly calculated using the following simple relation: tz d = t z , , N c / N T e x p ( - E , , / k T ) ,
(1)
where/z,,~ is the mobility at the conduction band edge, NT is .'.he density of state of traps, N c is that of the conduction band, E,, is the activation energy of the drift mobility, k is the Boltzmann constant and T is the temperature. The calculated values are plotted in fig. 6 by the solid triangles. Since the value of t t o N c / N x is not known a priori, it was chosen to fit the experimental value at zero carbon content and assumed constant. Both experimental and calculated mobilities reasonably coincide with each other, indicating clearly that the exponential decrease in the electron mobility with increasing carbon content is due to a direct consequence of the broadening of the tail states with increasing carbon content. We turn here back to the issue on the slightly non-linear relationship between t~r ~ and E at temperatures below 210 K, whic): has been intentionally ignored in the discussion on the mobility activation energy. We stand tentatively on the model by Tiedje and Rose [9], although it has not been concretely accepted. According to Tiedje and Rose [9] the following relation holds: 1/tTOt ( E / L )
'/",
(2)
346
T. Enomoto et aL / Eh, ctron transport
of a-SiC:H alloy films
where ~ is the dispersion parameter given as a = T/T~ for multiple trapping transport at art exponential tail state [9]. T~ is a parameter caracterizing the decay of the e.~¢ponential tail. From the temperature dependence of a which was obtained from the slope of the Iog(1/tT)-Iog E c,irve at low temperatures, we estimated rough values of T~. When the carbon content was increased from 0% to 5.4%, T~ increased from 240 to 330 K indicating that the extent of the tail states below the conduction band becomes larger with increasing carbon content. This is consistent with the carbon content dependence of the activation energy of the electron mobility.
4. Conclusion The cxperimcntal findings can bc summarized as follows: (1) Carbon is more efficiently incorporated into a-SiC~:H from C , H , than from CH~. (2) The room temperature electron mobility decreases exponentially with increasing carbon content in both C2H 2- and CH4-based films. (3) CH4-based a-SiCx:H shows a somewhat rapid reduction in tZu~- product with carbon content in comparison with C , H ,-based films. (4) It is confirmed that the exponential decrease in the electron drift mobility of a-SiC~:H with increasing carbon content is mainly associated with the broadening of the tail states.
Acknowledgement The authors arc much indebted to T. Tomikawa and N. Fujita, ltami Research Laboratory of Sumitomo Electric Industries for XPS analysis.
References [1] [2] 13] [4] [5] [6] [7]
it. Munekala and ii. Kukimoto, Appl. Phys. Lcti. 42 (1983) 432. K.S. Linl, M. Konagai and K. Takahashi, J. Appl. Phys. 56 (1q84) 538. W,T. Kim. H. TasakL M. Konagai and K. Takahashi. J. Appl. Phys. 61 (1987) 3~J71. Y. Tawada, K. Tsugc, M. Kondo, I1. Okamoto and Y. Ilamakawa. J. Appl. Phys. 53 (1982) 5273 K.C, ('hang, C.Y. Chang, Y.K. Fang and S.C. Jwo, IEEE Electron Device Left. EDL-8 (1987) 04 tI.K. Tsai, S.('. Lee and W . L Lin. IEEE Eleclron Device Lctt. EDL-8 (Iq87) 365. A. Malsuda. T. Yamaoka, S. Wolff. M. Koyama, Y. Imanishi, H. Kataoka, H. Matsuura and K Tanaka, J. Appl. Phys. 60 (Iq86) 4025. [8] J. Shirafuji. II. Matsui, Y. lnuishi and Y. Itamakawa, Jpn. J. Appl. Phys. 22 (1983) 775. [q] T. Ticdjc and A. Rose. Solid State Commun. 37 (108(1)49.
Solar Energy Materials
Electron transport of amorphous hydrogenated silicon-carbon alloy films prepared by glow discharge decomposition T. Enomoto, R. Hattori and J. Shirafuji Department of Electrical Engineering, Faculty of Engineering, Osaka Unicersity, Yamada-Oka, Suita, Osaka 565, Japan Electron transport of glow-discharged C2H 2- and CH4-based SiCx:H films has been studied by time-of-flight method. The electron mobility at room temperature decreases nearly exponentially with increasing carbon content in both C2H 2- and CH4-based films. This decrease is mainly due to a broadening of the tail states width in SiCx:H films, as implied from the carbon content dependence of the activation energy of the electron mobility in CH4-based films.
1. Introduction
Amorphous hydrogenated silicon-based alloys, s'ach as a-SiCx:H, a-SiGex:H and a-SiN,.:H have been widely applied to solar cells, photodetectors, thin film transistors and superlattice structures, because their optical gaps can be continuously controlled by changing the composition. Especially, a-SiCx:H films have been attractive in applications to near-visible eiectroluminescent devices [1], window layers of solar cells [2-4], phototransistors [5] and color sensors [6]_ However, the photoconductivity of undoped a-SiC.,.:H films is inferior to that of a-Si:H even when a-SiCx:H films are sufficiently doped. This has been thought to arise from higher density of dangling bonds in a-SiCx:H films which behave as recombination centers for photoexcited carriers. Much attention has been paid to the realization of a-SiCx:H with a high photoconductivit~. One approach is to examine various source gas species. It has been reported that the dark conductivity and the photoconductivity of a-SiC,:H films are incdependent of source gas species (CH4/SiH 4, C2H4/SiH4 and SiH3CH3/SiH ~) [7]. Although many works have been carried out to improve the photoconductivity of a-SiCx:H films, no attempt has been reported so far to measure separately the drift mobility P~d and the lifetime "r in order to gain insight into the origin of the degraded photoconductivity. In this paper we study the electron transport in a-SiCx:H films by means of time-of-flight method, which allows us to estimate the drift mobility and the lifetime separately. This method is also useful to obtain information on tail states from the temperature dependence of the drift mobility, because the distribution of the tail states determines the carrier mobility. Two kinds of carbon source gases, . . I . I ' h ers B.V. All rights reserved 0165-1633/91/$03.50 © 1991 - Elsevier Science Puo~.~:,
T. Enomoto et al. / Electron transport of a-SiC:H alloy films
341
C H 4 and C 2 H 2 , a r e employed in the present experiment to study the influence of the source gas species. The bonding of CH 4 molecules is tetrahedral sp 3 configuration which is preferable in Si-C three-dimensional network, while C E H 2 molecules
have the structure governed by sp ~ bonding configuration which may be not favorable to SiC x network.
2. Experimental procedures a-SiCx:H films were deposited by the conventional RF glow discharge decomposition method using undiluted gas mixtures of Sill 4 and CH 4 or C 2 H 2. The substrate temperature was held at 230 ° C. The RF input power at 13.56 MHz was as low as 10 W corresponding to a power density of 0.35 W / c m 2. Samples for time-of-flight (TOF) measurements had a laminated structure of P d / u n d o p e d a - S i : H / n + a - S i : H / I T O / g l a s s . P-doped n + a-Si:H layer was a good ohmic contact to the ITO electrode, and could suppress hole injection from the ITO electrode into undoped a-SiCx:H layer. On the other hand, the evaporated Pd electrode served as a good Schottky barrier junction which suppressed the electron injection from the Pd electrode. In order to avoid space charge effects in the high resistivity a-SiCx:H layer, a pulsed bias voltage was applied in the reverse direction of the Pd Schottky barrier junction. A pulsed light (300 ps duration) at 337.1 nm from a nitrogen laser synchronized with the pulsed bias voltage was used as an excitation source. The transit time of photoexcited electrons was determined by the intersection of two straight line~ drawn on the loglog plot of the trans:,ent photocurrent, even though the photocurrent showed a non-dispersive waveform. The carbon content is estimated from XPS measurement.
3. Results and discussion
3.1. Comparison between C2H 2- and CH4-based a-SiCx:H Fig. 1 shows the optical band gap of undoped a-SiC,:H films prepared by decomposition of Sill 4 and C 2 H 2 or C H 4 gas mixtures as a function of gas flow ratio which is defined as [CH4]/[SiH 4 + CH 4] or [C2H2]/[2SiH4 + C2H2]. The optical band gap was determined in the usual way from (ahv)i/2-hv plots. The optical gap of the CH4-based films increases gradually from 1.70 to 2.04 eV with increasing flow rate ratio from 0 to 0.8. In the case of C2H2-based a-SiCx:H, on the other hand, the increase in the optical gap in the same range of flow rate ratio is more remarkable than CH 4. This result indicates that carbon can be incorporated easily into a-SiC.,.:H because of easier decomposition of C 2 H 2 molecules than CH~. Similar effects have been observed in the ca~e of C2H 4 gas against CH 4 gas [4]. Fig. 2 shows the room temperature electron drift mobility i.% of a-SiC.,.:H films prepared by different source gases as a functioa of optical band gap. The drift
342
T. Enomoto et al. / Electron transport of a-SiC:H alloy films
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Optical gap of undoped a-SiC.~.:H films as a function of gas flow rate ratio.
mobility is usually determined from the relation of /~d =L2//VtT (V: applied voltage, tr: transit time, L: sample thickness). However, TOF samples may have a built-in field due to the Schottky barrier, which may introduce an error in the estimation of the mobility. In the present experime,]t the mobility was determined from the slope of the straight portion of 1/tT-E plot (E = V/L). When the optical band gap is increased, ~d decreases exponentially with little difference between C H 4- and C2H2-based films as seen in fig. 2. However, the drift mobility of CHa-based samples with an optical gap larger than 1.8 eV was impossible to be measured because of too small mobility-lifetime product as will be shown in the next figure. The result of fig. 2 indicates that the difference in the intermolecular bonding of gas molecules ( s p 3 for C H 4 , s p ! for C2H 2) does not directly influence
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Fig. 2. Electron
I 1.7 drift mobility
I
I _ , 1.8 OPTICAL GAP ( e V ) of a-SiC.,.:H
films against
1
1.9
optical
gap.
T. Enomoio et aL / Electron transport of a-SiC:H alloy fihns
10-7 0
"-'...> % t.) >"E3
343
CARBON CONTENT (%) 5 10
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1.8 1.9 OPTICAL GAP (eV ) Fig. 3. Electron mobility-lifetime product plotted against optical gap,
the atomic configuration in SiC, alloys possibly because of sufficient decomposition of gas molecules in glow discharge plasma. Fig. 3 shows the relationship between the optical gap and the mobility-lifetime product ~d z. The value of ~d ~" was determined from the applied voltage dependence of the collected charge by fitting to Hecht expression [8]. The value of /~dr of C2Hz-based films is a little larger than that of CH~-based films especially in the wide gap range. In combination with the result of fig. 2, C aH2-based films have a little higher carrier lifetime than CH4-based films. This result is in contrast with a previous report in which no difference was observed in the photoconductivity between CH,- and C2H4-based films [4]. 3.2. Influence of carbon content on lifetime and tail states width
Fig. 4 shows the carrier lifetime of CH z-based films estimated from figs. 2 and 3 as a function of optical band gap. The dangling bond density was estimated from ESR measurement. The ESR spin dellsity of CH4-based films was also plotted in fig. 4 for comparison. Regardless of the increase i,i the IEctimc, the spin density increases with the optical band gap. The apparent contradiction between the TOF lifetime and the ESR spin density is mostly associated with the difference in the experimental sit~atio:~s. The, TOF lifcthae i,~ actually relating to deep level trapping event which can be much influenced by the spread of tail states. When the tail states width becomes broad, the transient photocurrent can show a longer tail; this may cause an apparent large TOF lifetime. On the other hand. the ESR spin density is, as widely accepted, inversely proportional to the steady state photoconductivity in which the lifetime is determined to actual recombination process through dangling bond states.
344
T. Enomoto et aL / Electron transport of a-SiC'H alloy fihns
CARBON 1 r'-5v
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I I 1015 I..~ 1.9 OPTICAL GAP ( eV ) Fig. 4. Lifetime and spin density of CH4-based a-SiC~:H film agains* optical gap.
In order to obtain information about the tall states, we measured the temperature dependence of the electron drift mobility in CH4-based samples with the optical gap below 1.8 eV. The linear relation between t T' and E was observed in the high temperature region above 247 K indicating non-dispersive transport. We estimated the mobility from the slope of the straight line in this case. On the other hand, in the !ow temperature region below 210 K, t:r ] versus E curves show a slight deviation from the straight line at higher applied fields. This non-linearity is due to the appearance of dispersive character in the transport. Nevertheless, we approximated the tT~-E curve with a straight line to elucidate the electron mobility at low temperatures, so that we can obtain the activation energy of the electron mobility for various carbon contents. Fig. 5 shows the activation energy of the electron drift mobility of CH4-based a-SiC.,.:H as a function of carbon content. The activation energy was estimated 0.2
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7". Enomoto et al. / Electron transport of a-SiC.'H alloy fihns
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from Arrhenius plot. The activation energy increases gradually as the carbon content is increased. This increase in the activation energy has a direct relation to the decrease in the drift mobility with carbon content. Fig. 6 shows the comparison of the experimental electron mobility with the calculated value based on a simple consideration. The open circles are the experimental results of CH4-based aSiC,:H. In order to estimate the contribution of the activation energy to the mobility, the drift mobility was roughly calculated using the following simple relation: tz d = t z , , N c / N T e x p ( - E , , / k T ) ,
(1)
where/z,,~ is the mobility at the conduction band edge, NT is .'.he density of state of traps, N c is that of the conduction band, E,, is the activation energy of the drift mobility, k is the Boltzmann constant and T is the temperature. The calculated values are plotted in fig. 6 by the solid triangles. Since the value of t t o N c / N x is not known a priori, it was chosen to fit the experimental value at zero carbon content and assumed constant. Both experimental and calculated mobilities reasonably coincide with each other, indicating clearly that the exponential decrease in the electron mobility with increasing carbon content is due to a direct consequence of the broadening of the tail states with increasing carbon content. We turn here back to the issue on the slightly non-linear relationship between t~r ~ and E at temperatures below 210 K, whic): has been intentionally ignored in the discussion on the mobility activation energy. We stand tentatively on the model by Tiedje and Rose [9], although it has not been concretely accepted. According to Tiedje and Rose [9] the following relation holds: 1/tTOt ( E / L )
'/",
(2)
346
T. Enomoto et aL / Eh, ctron transport
of a-SiC:H alloy films
where ~ is the dispersion parameter given as a = T/T~ for multiple trapping transport at art exponential tail state [9]. T~ is a parameter caracterizing the decay of the e.~¢ponential tail. From the temperature dependence of a which was obtained from the slope of the Iog(1/tT)-Iog E c,irve at low temperatures, we estimated rough values of T~. When the carbon content was increased from 0% to 5.4%, T~ increased from 240 to 330 K indicating that the extent of the tail states below the conduction band becomes larger with increasing carbon content. This is consistent with the carbon content dependence of the activation energy of the electron mobility.
4. Conclusion The cxperimcntal findings can bc summarized as follows: (1) Carbon is more efficiently incorporated into a-SiC~:H from C , H , than from CH~. (2) The room temperature electron mobility decreases exponentially with increasing carbon content in both C2H 2- and CH4-based films. (3) CH4-based a-SiCx:H shows a somewhat rapid reduction in tZu~- product with carbon content in comparison with C , H ,-based films. (4) It is confirmed that the exponential decrease in the electron drift mobility of a-SiC~:H with increasing carbon content is mainly associated with the broadening of the tail states.
Acknowledgement The authors arc much indebted to T. Tomikawa and N. Fujita, ltami Research Laboratory of Sumitomo Electric Industries for XPS analysis.
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