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Electrical, optical and structural properties of a-SiNGe:H films prepared by the r.f. glow-discharged decomposition
ELSEVIER
Thin Solid Films 281-282 (1996) 308-310
Electrical, optical and structural properties of a-SiNGe:H films prepared by the r.f. glow-discharged decomposition I. Nakaaki a, N. Saito b a Hamamatsu b
Technical Training School ofShizuoka Prefecture, Hamamatsu 435, Japan Takamatsu National College a/Technology, Takamatsu 761, Japan
Abstract Hydrogenated amorphous quaternary alloy (a-SiNGe:H) films have been synthesized by r.f glow-discharge (GD) decomposition of gas mixtures ofsilane, germane, nitrogen and helium. The effects ofnitrogen onthe structural, optical, electrical and optoelectronic properties of the films prepared at various nitrogen-silane flow-rates have been investigated. The incorporation of small amounts of nitrogen slightly increases the dark conductivity, while the optical bandgap remains almost unchanged, The large incorporation leads toa decrease ofthe dark and photoconductivity and anincrease ofthe optical bandgap by forming analloy with nitrogen. Activation energies of the dark conductivity show a minimum against nitrogen contents. The results reveal that incorporation ofsmall amounts ofnitrogen inthe films relaxes the structural disorder and lor decreases the density ofdefects, aswell asacting as dopant. Keywords: Amorphous materials; Chemical vapour deposition; electrical properties and measurements; Nitrogen
1. Introduction Hydrogenated amorphous silicon (a-Si:H) and its alloy have been extensively investigated aspromising materials for their application in various electronic and optoelectronic devices [1]. Wehave already reported on wide optical bandgap materials with high photoconductivity and high photosensitivity and the control of their electronic properties by substitutional doping using a co-sputtering (SP) method [2-4] .
Recently, we have found that the effective photoconductivity 1J1J:rof SPand GD a-SiC:H films is improved byintroducing nitrogen during the deposition process {5,6]. Moreover, it is also reported that the same phenomenon appears ina-SiGe:N:H films prepared byd.c. magnetronsputtering [7]. It is therefore interesting to study thebehavior of GDdeposited a-SiNGe:H films, since GD methods are generally considered tobemore important for device applications in contrast to SP techniques.
2. Experimental
The film deposition was carried out in a conventional r.f glow-discharge decomposition system with capacitively coupled parallel-plate electrodes (ANELVA PED-30t). The 0040·6090/96/$15,00 @ 1996 Elsevier Science SA All rights reserved
rnS0040-6090( 96 )08660-9
films were prepared by varying the ratio of the flow-rate of nitrogen to that of silane: F, =F(N2/ F(SiH4)' The measurement of conductivity of the films was carried out using the specimens with aluminum electrodes evaporated in a coplanar interdigital configuration. Photoconductivity was deduced from the difference between the dark and photocurrents, where the photocurrent was measured under illumination of a He-Ne laser (photon energy = 1.96 eV) with the photon flux of 6.3 X 1014 em - 2 S - I at the film surface. The details in the measurements can be found in ourearlier papers.
3. Results and discussion Fig. 1 shows thevariation in the infrared absorption spectrum in the range from 500 to 1300 em - J with the rate Fr. The main absorption bands observed are those for the Si-H, wagging vibration atabout 630em -I and the Ge-Hwagging vibration at about 570 em - I. Some small peaks appear at about 850 and 880 em - J, corresponding to Si-H2 and (SiH2 ) n vibration. With the addition of small amounts of nitrogen, anew broad intensive absorption band, which isassigned to theSi-N stretching vibration, appears at about 850 em-1. With increasing Fr, the intensity of the Si-N band tends to Increase.
I. Nakaoki, N. Saito/Thin Solid Films 28/-282 (1996) 308-3/0
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Fig. I. Variation in the infrared absorption spectrum with the ratio of the flow-rate of nitrogen to that of silane: Fr ( = F(N 2 ) I F(Si~) in therange 500 to 1300 cm",
Fig. 3. Temperature-dependent conductivity of a-SiNGe:H films. Closed symbols and open symbols correspond to dark d.e. conductivity O'd and photoconductivity 0'p' respectively.
The optical absorption spectra expressed asTauc plots shift to higher photon energy with increasing F, with apparently changing their slopes to smaller values. Optical bandgap Eo isdetermined from therelation allv =B(hv - Eo )2, where a, hv and B are the absorption coefficient, the photon energy and aconstant respectively. Eo isshown inFig. 2asafunction of Fr. Intherange F, < 1,Eo remains almost constant atabout 1.6 eV, even though the infrared spectra of a-SiNGe:H films show the intensive absorption band due to Si-N bonding. 'This result indicates that the incorporated nitrogen atoms in this F, range do not contribute to the widening of Eo, which is generally expected forfilms containing Si-N bonds. In the range F;> 1, Eo increases steeply up to about 1.73 eV with an increase of Fr. The variations in the infrared spectra with F, suggest thatthewidening ofEo with increasing F, ismainly due to an increase in the number of nitrogen atoms forming Si-N bonding. Fig. 3 shows the temperature dependence of dark conductivity U d and photoconductivity 0'p' The decrease in (I II at high F, is ascribed to an increase in the concentration of nitrogen forming Si-N bonding, which is generally observed ina-SiN:H films [8]. Thedecrease in (J" p at high F, correlates with the increase in nitrogen incorporation, which results in a widening of theoptical bandgaps and increases the disorder
and/orthe concentration ofdefects intheam orpheus network [9].
Though the semilog plots of thedark conductivities of aSiNGe:H films exhibit a linear dependence at high temperatures, a deviation from this linear relationship appears at low temperatures. This indicates that hopping conduction contribute to theconductivity at low temperatures, as is usually observed intetrahedrally bonded amorphous semiconductors with low hydrogen concentrations [10,II]. The component of hopping conduction is almost unchanged in spite of increasing Fr, thus, it does not contribute significantly to the conductivity even for large-F, samples; i.e. the increase in the disorder and/or the concentration ofdefects in the amorphous network of these films seems to besmall for large Fr. The plot of (Id above room temperature (IOOO1T=2.5) against F, (not shown) reveals that 0'd has a small peak around F, = 0.1. This reflects either adecrease inthe structural disorder and!or the density of defects, or the doping effect due to nitrogen, as observed in a-Si.H films incorporating nitrogen [12,13]. Fig. 4 shows the activation energies Ed and Ep asa function of Fp where Ed and Ep are derived, respectively, from the
s
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:> 1.8....-J'v--------r----.----. .....cu o
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Fig. 2. Optical bandgap Eo as a function of Fr.
10
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FISIH,I
Fig. 4. Activation energies derived from thetemperature-dependent conductivities asa function of Fr. Closed symbols show the activation energy of (1 d andopen symbols thatof 0'p' respectively.
310
I. Nakaaki, N. SaitolTllin Solid Films 281-282 (1996) 308-310
temperature dependence of U d and up in the thermally activated conduction regimes above room temperature. Following the result reported by Spear et a1. [14], Ep means the band tail width. Thus, an increase in Ep implies an increase inthe structural disorder. Ed tends to decrease with an increase of F, up to around Fr==O.l, then increase gradually with an increase of Fr. This variation of Ed appears to show a minimum F,= 0.1. The variation in Ed indicates the shifts of Fermi level [14]. The decrease of Ed for small F, suggests the doping effect of nitrogen as well as the change of the gap states distribution. The increase of Ed in large F, reveals the effect of alloying with nitrogen. However, the increasing rate of Ed is small compared with the widening rate of Eo. The value of Ep remains almost constant for all F, investigated. This implies that the incorporation of nitrogen into the amorphous network composed of Si, N, Ge and H does not alter the disorder ofthe films significantly.
4. Conclusions
Incorporation of small amounts of nitrogen increases slightly the dark. conductivity without noticeably changing the optical bandgap. The large incorporation reduces the conductivity with a widening of the gap. These results indicate that small amounts of nitrogen reduce the structural disorder
andlor the density of defects, as well as acting as dopants, while large a incorporation increases them a little by forming a quaternary alloy composed ofSi, N, Ge and H. References [I] U. Pankov, Hydrogenated AmOlphous Silicon, Semiconductors and Semimeta/s, Vol. 21, A-D, Academic Press, 1984. [2} N. Saito. T. Yamada, T. Yamaguchi, I. Nakaak! and Tanaka, Phi/os. Mag. B.52 (1985) 987. [3} N. Saito, Y. Tomioka, T.Yamaguchi and K.Kawamura, Philos.. Mag. Lett; 59 (1989) 43. [4} N. Saito, Y. Tomioka, H. Senda, T. Yamaguchi and K. Kawamura. Philos. Mag. B. 62 (1990) 527. [5} N. Saito, T. Goto, Y. Tomioka, T. Yamaguchi and M. Shibayama, J. Appl. Phys., 69 (1991) 1518. [6J I. Nakaaki, N. Saito, Y. Inui, S. Yosioka and S. Nakamura, Philos. Mag. B, 68 (1993)55. [7J T. Druesedau, J. Non-crystalline Solids, J35 ( 1991) 204. [8} B. Dunnett, 0.1. Jones and A.D. Stewart, Philos. Mag. B.53 (1986) 159. [9] T. Shimizu, J. Non-crystalline Solids, 59/60 (1983) 117. [10] D.A. Anderson and W.E. Spear, Philos. Mag., 35 (1977) I. [11] R. Dutta, P.K. Banerjee and 8.S. Mitra, Solid State Commun., 42 (1982) 2i9. [12] J. Singh arid R.C. Budhani, Solid State Commun., 64 (1987) 349. [13} T. Noguchi, S. Usui, A. Sawada. Y. Kanoh and M. Kikuchi, Jpn. J. App/. Phys., 21 (1982) lA85. [14] W.E. Spear. RJ. Loveland and A. Al-Sharbaty, J. Non-crystalline Solids, 15 (1974) 410.
Thin Solid Films 281-282 (1996) 308-310
Electrical, optical and structural properties of a-SiNGe:H films prepared by the r.f. glow-discharged decomposition I. Nakaaki a, N. Saito b a Hamamatsu b
Technical Training School ofShizuoka Prefecture, Hamamatsu 435, Japan Takamatsu National College a/Technology, Takamatsu 761, Japan
Abstract Hydrogenated amorphous quaternary alloy (a-SiNGe:H) films have been synthesized by r.f glow-discharge (GD) decomposition of gas mixtures ofsilane, germane, nitrogen and helium. The effects ofnitrogen onthe structural, optical, electrical and optoelectronic properties of the films prepared at various nitrogen-silane flow-rates have been investigated. The incorporation of small amounts of nitrogen slightly increases the dark conductivity, while the optical bandgap remains almost unchanged, The large incorporation leads toa decrease ofthe dark and photoconductivity and anincrease ofthe optical bandgap by forming analloy with nitrogen. Activation energies of the dark conductivity show a minimum against nitrogen contents. The results reveal that incorporation ofsmall amounts ofnitrogen inthe films relaxes the structural disorder and lor decreases the density ofdefects, aswell asacting as dopant. Keywords: Amorphous materials; Chemical vapour deposition; electrical properties and measurements; Nitrogen
1. Introduction Hydrogenated amorphous silicon (a-Si:H) and its alloy have been extensively investigated aspromising materials for their application in various electronic and optoelectronic devices [1]. Wehave already reported on wide optical bandgap materials with high photoconductivity and high photosensitivity and the control of their electronic properties by substitutional doping using a co-sputtering (SP) method [2-4] .
Recently, we have found that the effective photoconductivity 1J1J:rof SPand GD a-SiC:H films is improved byintroducing nitrogen during the deposition process {5,6]. Moreover, it is also reported that the same phenomenon appears ina-SiGe:N:H films prepared byd.c. magnetronsputtering [7]. It is therefore interesting to study thebehavior of GDdeposited a-SiNGe:H films, since GD methods are generally considered tobemore important for device applications in contrast to SP techniques.
2. Experimental
The film deposition was carried out in a conventional r.f glow-discharge decomposition system with capacitively coupled parallel-plate electrodes (ANELVA PED-30t). The 0040·6090/96/$15,00 @ 1996 Elsevier Science SA All rights reserved
rnS0040-6090( 96 )08660-9
films were prepared by varying the ratio of the flow-rate of nitrogen to that of silane: F, =F(N2/ F(SiH4)' The measurement of conductivity of the films was carried out using the specimens with aluminum electrodes evaporated in a coplanar interdigital configuration. Photoconductivity was deduced from the difference between the dark and photocurrents, where the photocurrent was measured under illumination of a He-Ne laser (photon energy = 1.96 eV) with the photon flux of 6.3 X 1014 em - 2 S - I at the film surface. The details in the measurements can be found in ourearlier papers.
3. Results and discussion Fig. 1 shows thevariation in the infrared absorption spectrum in the range from 500 to 1300 em - J with the rate Fr. The main absorption bands observed are those for the Si-H, wagging vibration atabout 630em -I and the Ge-Hwagging vibration at about 570 em - I. Some small peaks appear at about 850 and 880 em - J, corresponding to Si-H2 and (SiH2 ) n vibration. With the addition of small amounts of nitrogen, anew broad intensive absorption band, which isassigned to theSi-N stretching vibration, appears at about 850 em-1. With increasing Fr, the intensity of the Si-N band tends to Increase.
I. Nakaoki, N. Saito/Thin Solid Films 28/-282 (1996) 308-3/0
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c:
c
....c. ~
c
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~
->,>
Itoo Wavenumber BOO
900 1000
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l-
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.
up
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.
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•
-6
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600
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309
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(/(')
Fig. I. Variation in the infrared absorption spectrum with the ratio of the flow-rate of nitrogen to that of silane: Fr ( = F(N 2 ) I F(Si~) in therange 500 to 1300 cm",
Fig. 3. Temperature-dependent conductivity of a-SiNGe:H films. Closed symbols and open symbols correspond to dark d.e. conductivity O'd and photoconductivity 0'p' respectively.
The optical absorption spectra expressed asTauc plots shift to higher photon energy with increasing F, with apparently changing their slopes to smaller values. Optical bandgap Eo isdetermined from therelation allv =B(hv - Eo )2, where a, hv and B are the absorption coefficient, the photon energy and aconstant respectively. Eo isshown inFig. 2asafunction of Fr. Intherange F, < 1,Eo remains almost constant atabout 1.6 eV, even though the infrared spectra of a-SiNGe:H films show the intensive absorption band due to Si-N bonding. 'This result indicates that the incorporated nitrogen atoms in this F, range do not contribute to the widening of Eo, which is generally expected forfilms containing Si-N bonds. In the range F;> 1, Eo increases steeply up to about 1.73 eV with an increase of Fr. The variations in the infrared spectra with F, suggest thatthewidening ofEo with increasing F, ismainly due to an increase in the number of nitrogen atoms forming Si-N bonding. Fig. 3 shows the temperature dependence of dark conductivity U d and photoconductivity 0'p' The decrease in (I II at high F, is ascribed to an increase in the concentration of nitrogen forming Si-N bonding, which is generally observed ina-SiN:H films [8]. Thedecrease in (J" p at high F, correlates with the increase in nitrogen incorporation, which results in a widening of theoptical bandgaps and increases the disorder
and/orthe concentration ofdefects intheam orpheus network [9].
Though the semilog plots of thedark conductivities of aSiNGe:H films exhibit a linear dependence at high temperatures, a deviation from this linear relationship appears at low temperatures. This indicates that hopping conduction contribute to theconductivity at low temperatures, as is usually observed intetrahedrally bonded amorphous semiconductors with low hydrogen concentrations [10,II]. The component of hopping conduction is almost unchanged in spite of increasing Fr, thus, it does not contribute significantly to the conductivity even for large-F, samples; i.e. the increase in the disorder and/or the concentration ofdefects in the amorphous network of these films seems to besmall for large Fr. The plot of (Id above room temperature (IOOO1T=2.5) against F, (not shown) reveals that 0'd has a small peak around F, = 0.1. This reflects either adecrease inthe structural disorder and!or the density of defects, or the doping effect due to nitrogen, as observed in a-Si.H films incorporating nitrogen [12,13]. Fig. 4 shows the activation energies Ed and Ep asa function of Fp where Ed and Ep are derived, respectively, from the
s
1.0
- O.B Cl)
:> 1.8....-J'v--------r----.----. .....cu o
0-
m
-g'1.7 c m
o o
..c
CD 1.6
o
0
0
c 0.4-
• dark o photo
~
Cl)
c
IIJ
.-0 .... 0.2 a:l
>
o
0
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-••
>-
Q'l
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0
0
•
--
0
0
• •
0
0
0
(,)
o. I
1 FIN21/
0.1
1
10
FIN21/ FlSIH41
Fig. 2. Optical bandgap Eo as a function of Fr.
10
100
FISIH,I
Fig. 4. Activation energies derived from thetemperature-dependent conductivities asa function of Fr. Closed symbols show the activation energy of (1 d andopen symbols thatof 0'p' respectively.
310
I. Nakaaki, N. SaitolTllin Solid Films 281-282 (1996) 308-310
temperature dependence of U d and up in the thermally activated conduction regimes above room temperature. Following the result reported by Spear et a1. [14], Ep means the band tail width. Thus, an increase in Ep implies an increase inthe structural disorder. Ed tends to decrease with an increase of F, up to around Fr==O.l, then increase gradually with an increase of Fr. This variation of Ed appears to show a minimum F,= 0.1. The variation in Ed indicates the shifts of Fermi level [14]. The decrease of Ed for small F, suggests the doping effect of nitrogen as well as the change of the gap states distribution. The increase of Ed in large F, reveals the effect of alloying with nitrogen. However, the increasing rate of Ed is small compared with the widening rate of Eo. The value of Ep remains almost constant for all F, investigated. This implies that the incorporation of nitrogen into the amorphous network composed of Si, N, Ge and H does not alter the disorder ofthe films significantly.
4. Conclusions
Incorporation of small amounts of nitrogen increases slightly the dark. conductivity without noticeably changing the optical bandgap. The large incorporation reduces the conductivity with a widening of the gap. These results indicate that small amounts of nitrogen reduce the structural disorder
andlor the density of defects, as well as acting as dopants, while large a incorporation increases them a little by forming a quaternary alloy composed ofSi, N, Ge and H. References [I] U. Pankov, Hydrogenated AmOlphous Silicon, Semiconductors and Semimeta/s, Vol. 21, A-D, Academic Press, 1984. [2} N. Saito. T. Yamada, T. Yamaguchi, I. Nakaak! and Tanaka, Phi/os. Mag. B.52 (1985) 987. [3} N. Saito, Y. Tomioka, T.Yamaguchi and K.Kawamura, Philos.. Mag. Lett; 59 (1989) 43. [4} N. Saito, Y. Tomioka, H. Senda, T. Yamaguchi and K. Kawamura. Philos. Mag. B. 62 (1990) 527. [5} N. Saito, T. Goto, Y. Tomioka, T. Yamaguchi and M. Shibayama, J. Appl. Phys., 69 (1991) 1518. [6J I. Nakaaki, N. Saito, Y. Inui, S. Yosioka and S. Nakamura, Philos. Mag. B, 68 (1993)55. [7J T. Druesedau, J. Non-crystalline Solids, J35 ( 1991) 204. [8} B. Dunnett, 0.1. Jones and A.D. Stewart, Philos. Mag. B.53 (1986) 159. [9] T. Shimizu, J. Non-crystalline Solids, 59/60 (1983) 117. [10] D.A. Anderson and W.E. Spear, Philos. Mag., 35 (1977) I. [11] R. Dutta, P.K. Banerjee and 8.S. Mitra, Solid State Commun., 42 (1982) 2i9. [12] J. Singh arid R.C. Budhani, Solid State Commun., 64 (1987) 349. [13} T. Noguchi, S. Usui, A. Sawada. Y. Kanoh and M. Kikuchi, Jpn. J. App/. Phys., 21 (1982) lA85. [14] W.E. Spear. RJ. Loveland and A. Al-Sharbaty, J. Non-crystalline Solids, 15 (1974) 410.