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Effect of ion bombardment during plasma CVD on the film properties of a-Si:H studied by IEC plasma CVD
J O U R N A L OF
NON-C SOI ELSEVIER
Journal of Non-Crystalline Solids 198-200 (1996) 1007-1011
Effect of ion bombardment during plasma CVD on the film properties of a-Si:H studied by IEC plasma CVD T. Sasaki *, Y. Ichikawa Fuji Electric Corporate Research and De~,elopment. Ltd.. Yokosuka City, Japan
Abstract Ion energy controlled (IEC) plasma chemical vapor deposition is developed to control ion energy independently of plasma conditions. The ion energy, ~ , is analyzed and it depends only on the bias voltage, Va, and the electron temperature, Ve. The ion energy V, increased linearly from 20 V to 170 V as Vd decreased from 0 V to - 150 V under normal a-Si:H deposition conditions. The defect density of as-deposited a-Si:H measured by electron spin resonance increased from 2.3 X 1018, to 4.7 × 10 ~s cm 3 for a substrate temperature, T~, of 55°C and from 4.5 X 1016 to 1.8 x 1017 c m - 3 for Ts = 150°C, respectively as V~ increased from 20 V to 170 V. Bonded hydrogen decreased for T, = 55°C as Vs increased, but slightly was increased for T~ = 150°C. 1. Introduction The effect of ion bombardment during plasma chemical vapor deposition (CVD) on the film properties o f hydrogenated amorphous silicon (a-Si:H) or Si based alloys (a-SiX:H; X = C, N, O, Ge, etc.) is still controversial. Because it is difficult to control ion flux or ion energy keeping other plasma conditions constant, only a few quantitative studies have been performed [ 1]. The ion energy is determined by the sheath voltage, ~ , which is the difference between the plasma potential and the substrate potential. In conventional diode type rf plasma CVD, ~ is determined by the self bias voltage depending on the geometry of electrode system and plasma conditions. Thus it is impossible to control ion energy with no change of plasma conditions in such a diode system.
* Corresponding author. 2-2-1, Nagasaka, Yokosuka City, 24001, Japan. Tel.: + 81-468-576730; fax: + 81-468-572791 ; e-mail: toshiaki@lsun I .fujidenki.co.jp.
Triode type rf plasma CVD, with a biased mesh electrode, was developed to control ions or radicals [2]. In order to control V~ at the substrates, the distance between the mesh electrode and the substrate electrode should be less than the sheath thickness, which is approximately equal to the Debye length, and is less than a few m m for typical plasma CVD conditions ( 1 0 - 2 ~ 1 Ton'). Of course it is not realistic to make such triode system because DC discharge would occur between the mesh electrode and the substrate electrode. Moreover the rf electrode and the mesh electrode comprise a discharge circuit, so that the bias voltage significantly affects the plasma conditions between the mesh electrode and the substrate electrode. Therefore, it is impossible to control ion energy in the triode system. We developed 'ion energy controlled plasma CVD (IEC plasma CVD)' to control ion energy independently of plasma conditions. In this paper we present the theory behind the principle of IEC plasma CVD, and the film properties of a-Si:H prepared by the IEC plasma CVD.
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022- 309 3(96)00024-5
1008
7~ Sasaki, E lchikawa / Journal o[ Non-Crystalline Solids 198-200 (1996) 1007 1011
2. Experiment Fig. l(a) is a schematic diagram of IEC plasma CVD. The reactor consists of a conventional diode system, with two planar electrodes inserted into the plasma, parallel with the rf electrode and the grounded electrode. The two electrodes can function as a floating double probe for plasma diagnostics [3], and one of them can also be used as a substrate holder for film deposition with an internal heater. In this configuration, the acceleration voltage for incident ions to the substrate is controlled by the DC bias voltage, Vd, applied between two probes. The current flowing into probes does not exceed the ion saturation current, and is independent of discharge current. Thus the effect of probe insertion on the discharge is negligibly small. Fig. l(b) is a cross sectional diagram of the probe. The probe is made from stainless steel and is 36 mm by 36 mm by 4 mm. The conductive substrate is biased through the metal susceptor, and heated by small insulated heater. The two probes have an identical structure. A 13.56 MHz rf voltage is capaci-
RFElectrode Grounded Electrode \ Reference Probe
Table 1 Deposition condition of a-Si:H; total pressure Pr (Tort), rf power density Pw (mW/cm2), Sill4 flow rate Sill 4 (sccm), H 2 flow rate H 2 (sccm), anode temperature Z~ (°C) Pr
Pw
Sill4
H2
77a
0.05
10
20
20
150
tively coupled to the parallel plate rf and grounded electrodes, which have a diameter of 160 mm. The grounded electrode can be heated. The distance between the rf and the grounded electrodes is 80 mm, and the distance between the probes and the grounded electrode is 45 ram. Powdered a-Si:H for electron spin resonance (ESR) measurements was obtained from a-Si:H film about 1 Ixm thick deposited on the A1 foil. Ag coated Coming 7059 glasses was the substrate for Fourier transform infra-red (FTIR) measurement of the reflection spectra. Infra-red (IR) absorption spectra from 1850 cm -~ to 2200 cm l was divided into Sill stretching mode [SiH] and Sill 2 stretching mode [SiH2], assuming Gaussian distribution centered at 2000 cm l and 2090 cm ~, respectively. Typical deposition conditions are shown in Table 1.
(a) 3. Analysis of ion energy The energy of ion which impacts on substrates is analyzed in IEC plasma CVD. If the mean free path of ions is longer than the Debye length, then the energy of ions, E~, impacting on substrates is,
~ ! i i i t J :h:::::::::::::::::::::::::::~::::::::h: Sheath / // ~L.qlJ~ Seat Substrate/ I~'vd SubstrateProbe
E i = (sheath acceleration energy) + (thermal energy) = eV~ + kTi ,
MetalSusceptor~
(b)
C°nductive Substrate-'~:.:ii ~ ~]
Hea,er
]
where e is the elementary electric charge, k is Boltzmann's constant and Ti is the ion temperature. Under the usual plasma CVD conditions, T~ is nearly equal to the gas temperature, and k T i << e ~ ; for example at Ti = 1000 K, kT~ = 0.086 eV. Then, E i ~eV~.
Bias Port- ~ Fig. 1. (a) Schematic diagram of IEC plasma CVD. (b) Sectional diagram of the probe.
(1)
(2)
Eq. (2) means the ions that impact on the substrates have a constant energy of approximately eV, so that V~ is used as the ion energy in the following discussion.
T. Sasaki, E lchikawa / Journal of'Non-CGstalline Solids 198-200 (1996) 1007-1011
1009
1 0'
In Fig. l(a), the definitions for direction of the substrate probe current, /,, the reference probe current, I~, and the bias voltage, Vd, are indicated. If the plasma space potential is the same for the two probes,
1 0'
'E
V = V~ ln{ l~eo(l + aexp(- Vd/Ve)) } (1 + a) ]si0
z
(3)
,
1 0 '¸
where V~ is the electron temperature in unit eV, a is the area ratio of the reference probe and the substrate probe, I~o0 and I<0 are the saturated electron current and saturated ion current of the substrate probe, respectively [4]. Assuming Maxwellian distribution,
I~o = '4Nev7eS = ~Ne
11
-
-
3Tme
O
1 0 1 6
. . . .
,
0
. . . .
50
,
. . . .
100
,
. . . .
150
200
Vs (v) Fig. 2. Spin density of a-Si:H, %, as a function of ion energy, V~.
eS,
(4)
where N~ is the electron density, ve is the average thermal speed of electrons, Te, is the electron temperature in unit K, rn~ is the mass of electron, S is the area of the substrate probe. From ion sheath theory
and the saturated V~ is almost equal to the floating potential.
[5],
4. Results
/ kro l,m = KN~I/ - - eS,
(5)
~/ m i
where K is the function of Ti/Te; K ~ 0.61 under the normal plasma CVD conditions and m i is the mass of ion. From Eqs. (3), (4), (5), we obtain { 1 V" = v~ In ~
/ 8mi qrmo
l+aexp(-V'l/Ve)} i+~
'
(6) If V~ is constant, V, only depends on Va. The value of V~ is determined by measuring I~-Va characteristics during plasma, using IEC plasma CVD apparatus as floating double probe [3]. V, increases linearly as Va decreases when Vd < 0. In contrast, V~ saturates and keeps constant value when Vd > 0. In this study, we assumed ion species as SIH,, (n = 0 ~ 4). The generation of H + from direct ionization of H , is negligibly small considering ionization cross sections of H 2 and Sill 4 [6,7]. Generation of H,+, from Sill 4 is smaller than that of SIH,"+ because the ionization energy of H is 5.5 eV larger than that of Si [8]. The probes are identical structure so that a = 1, •
Fig. 2 shows the spin density, N~, as a function of Vs. V,~ is changed from -150 V to 150 V, to change V, from 20 v to 170 v. N, increased by a factor of 2 for T~ = 55°C and by a factor of 4 for T, = 150°C, respectively as V~ increased from 20 V to 170 V. N, is scattering about factor of 1.5 ~ 2 at V~ = 20 V, because Vd is changed from 0 V to 150 V at V~ = 20 V and it caused sample to sample error. Fig. 3 shows the intensity of bonded hydrogen spectra, [Sill] + [Sill2], as a function of V,. For = 55°C, [Sill] + [Sill 2] decreased, but it slightly
5O
~ 40
71 E
~30
(3
+
~20 ÷
.:12_10 0
.
.
.
.
.
.
5O
.
.
.
.
.
.
100
Vs (v)
, , i
. . . .
150
200
Fig. 3. The intensity of bonded hydrogen spectra, [Sill] + [Sill 2 ], as a function of V~.
1010
T. Sasaki, Y. lchikawa / Journal of Non -Crystalline Solids 198-200 (1996) 1007- l Ol I 0.4 ~_0.3
atom density of a-Si:H, ( ~ 5 × 1022 cm 3) for [12]. We assumed the precursor radicals, s, as of 2 n m / m i n ,
5
÷
~0.2 0
¢~0.1
o"
-I-
0,0
Fdepo = DR × [ S i ] / 0 . 1 ~ 1.7 × 10 '5 (cm -2 s - ' ) ,
. . . . . . . . . . . . . .
0
[Si], is almost equal to c-Si order estimation of the flux sticking coefficient of the 0.1 [13]. At a deposition rate
50
100 Vs (v)
.... 150 200
(7)
,
Fig. 4. The [Sill2] bonding ratio [SiH2]/([SiH]+[SiH2]) as a function of V,.
where DR is the deposition rate. On the other hand, from the saturated ion current of the probe, I~i0, of 35 i x A / c m 2,
Fion= Isio/e~ Z.2 x lOl4 (cm-Z s-'). increased for ~ = 150°C as V, increased from 20 V to 120 V. Fig. 4 shows the [Sill2] bonding ratio [SiH2]/([SiH] + [SiH2]) as a function of V~. [SiH2]/([SiH] + [SiH2]) decreased 0.29 to 0.19 for = 55°C and it increased from 0.12 to 0.16 for Ts = 150°C as V~ increased from 20 V to 120 V. It is experimentally demonstrated from the above results that the ion energy has an effect on the film properties of a-Si:H.
5. Discussion If the ion energy was transferred to adsorbed precursors on the film surface and enhanced their surface diffusion, Ns would be suppressed [9]. N~, however, increased as VS increased, so this was not the case. The ion energy applied in this experiment is 2 orders of magnitude larger than Si-Si bond strength (2.4 eV) or S i - H bond strength (3.4 eV) [10]. This suggests that impinging ions break bonds, or reconstruct the a-Si:H network. Usually content of hydrogen in a-Si:H decreases as substrate temperature increases. The increase of [Sill] + [SiH 2 ] for TS = 150°C as V~ increased shows the effects of the ion energy are not same as substrate heating effects. In order to confirm whether sufficient ions could attack and effect the film properties, we estimated the order of the flux of film precursor radicals, Faepo, and the flux of ions, F~o.. Weight density of a-Si:H is about 60% of that of c-Si even though hydrogen atomic content is 30% [11], so we assumed the Si
(8)
From the above estimation the F~o. is about 10% of the fdepo and it is not negligible. Thus we conclude that in this experiment the ion energy can effect the film properties of a-Si:H. In this study ion species are assumed as S~H n (n = 0 ~ 4). For later studies, it could be interesting to change ion species and see the effect on film properties. For instance, the plasma of highly diluted Sill 4 with H 2 may include H 2 related ions, or the plasma of highly diluted Sill 4 with inert gas may include inert gas ions. Another interesting experiment is the effects of ion energy on the film properties of a-Si based alloys such as a-SiC, a-SiO, a-SiGe; a-SiGe:H is especially interesting. Highly photosensitive a-SiGe:H films are obtained not only under the conditions where the ion energy is low such as triode method [2,14], but also under conditions where the ion energy is high such as cathode deposition method [15], pulse discharge CVD [16]. Clearly more studies are needed. •
+
Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization as a part of the New Sunshine Program under the Ministry of International Trade and Industry•
References [1] K. Kato and I. Kato, Japan. J. Appl. Phys. 30 (1991) 1245. [2] T. Ichimura, T. Hama, T. Ihara, M. Ohsawa, H. Sakai and Y. Uchida, 18th IEEE Photovoltaic Specialists Conf., Las Vegas (1985) p. 1495.
T. Sasaki, Y. Ichikawa / Journal of Non-Crystalline Solids 198-200 (1996) 1007-1011 [3] E.O. Johnson and L. Malter, Phys. Rev. 80 (1950) 58. [4] T. Sasaki, M. Tanda and Y. Ichikawa, 12th Symp. on Plasma Processing, Sendai, Japan (1995) p. 63. [5] S.-L. Chen and T. Sekiguchi, J. Appl. Phys. 36 (1965) 2363. [6] L.J. Kieffer and GT.H. Dunn, Rev. Mod. Phys. 38 (1966) 1. [7] Y. Ohmori, M. Shimozuma and H. Tagashira, J. Phys. D 19 (1986) 1029. [8] Handbook of Discharge, The Institute of Electrical Engineers of Japan (1955) p. 13. [9] G. Ganguly and A. Matsuda, Phys. Rev. B 47 (1993) 3661. [10] D. Adler, Semiconductors and Semimetals Hydrogenated Amorphous Silicon, Vol. 21, Part A, ed. J.I. Pankove (Academic Press, New York, 1984) p. 295.
1011
[11] M.H. Brodsky, M.A. Frisch, J.F. Ziegler and W.A. Lanford, Appl. Phys. Lett. 30 (1977) 561. [12] S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981)p. 850. [13] A. Matsuda, K. Nomoto, Y. Takeuchi, A. Suzuki, A. Yuuki and J. Perrin, Surf. Sci. 227 (1990) 50. [14] A. Matsuda, K. Yagii, M. Koyama, M. Toyama, Y. Imanishi, N. Ikuchi and K. Tanaka, Appl. Phys. Lett. 47 (1985) 1061. [15] F. Zhong, C.C. Chen, J.D. Chohen, P. Wickboldt and W. Paul, MRS Syrup. Proc. (1995), to be published. [16] T. Sasaki, Y. Ichiakawa, H. Sakai, H. Kito and S. Teii, Plasma Source Sci. Technol. 2 (1993) 30.
NON-C SOI ELSEVIER
Journal of Non-Crystalline Solids 198-200 (1996) 1007-1011
Effect of ion bombardment during plasma CVD on the film properties of a-Si:H studied by IEC plasma CVD T. Sasaki *, Y. Ichikawa Fuji Electric Corporate Research and De~,elopment. Ltd.. Yokosuka City, Japan
Abstract Ion energy controlled (IEC) plasma chemical vapor deposition is developed to control ion energy independently of plasma conditions. The ion energy, ~ , is analyzed and it depends only on the bias voltage, Va, and the electron temperature, Ve. The ion energy V, increased linearly from 20 V to 170 V as Vd decreased from 0 V to - 150 V under normal a-Si:H deposition conditions. The defect density of as-deposited a-Si:H measured by electron spin resonance increased from 2.3 X 1018, to 4.7 × 10 ~s cm 3 for a substrate temperature, T~, of 55°C and from 4.5 X 1016 to 1.8 x 1017 c m - 3 for Ts = 150°C, respectively as V~ increased from 20 V to 170 V. Bonded hydrogen decreased for T, = 55°C as Vs increased, but slightly was increased for T~ = 150°C. 1. Introduction The effect of ion bombardment during plasma chemical vapor deposition (CVD) on the film properties o f hydrogenated amorphous silicon (a-Si:H) or Si based alloys (a-SiX:H; X = C, N, O, Ge, etc.) is still controversial. Because it is difficult to control ion flux or ion energy keeping other plasma conditions constant, only a few quantitative studies have been performed [ 1]. The ion energy is determined by the sheath voltage, ~ , which is the difference between the plasma potential and the substrate potential. In conventional diode type rf plasma CVD, ~ is determined by the self bias voltage depending on the geometry of electrode system and plasma conditions. Thus it is impossible to control ion energy with no change of plasma conditions in such a diode system.
* Corresponding author. 2-2-1, Nagasaka, Yokosuka City, 24001, Japan. Tel.: + 81-468-576730; fax: + 81-468-572791 ; e-mail: toshiaki@lsun I .fujidenki.co.jp.
Triode type rf plasma CVD, with a biased mesh electrode, was developed to control ions or radicals [2]. In order to control V~ at the substrates, the distance between the mesh electrode and the substrate electrode should be less than the sheath thickness, which is approximately equal to the Debye length, and is less than a few m m for typical plasma CVD conditions ( 1 0 - 2 ~ 1 Ton'). Of course it is not realistic to make such triode system because DC discharge would occur between the mesh electrode and the substrate electrode. Moreover the rf electrode and the mesh electrode comprise a discharge circuit, so that the bias voltage significantly affects the plasma conditions between the mesh electrode and the substrate electrode. Therefore, it is impossible to control ion energy in the triode system. We developed 'ion energy controlled plasma CVD (IEC plasma CVD)' to control ion energy independently of plasma conditions. In this paper we present the theory behind the principle of IEC plasma CVD, and the film properties of a-Si:H prepared by the IEC plasma CVD.
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0022- 309 3(96)00024-5
1008
7~ Sasaki, E lchikawa / Journal o[ Non-Crystalline Solids 198-200 (1996) 1007 1011
2. Experiment Fig. l(a) is a schematic diagram of IEC plasma CVD. The reactor consists of a conventional diode system, with two planar electrodes inserted into the plasma, parallel with the rf electrode and the grounded electrode. The two electrodes can function as a floating double probe for plasma diagnostics [3], and one of them can also be used as a substrate holder for film deposition with an internal heater. In this configuration, the acceleration voltage for incident ions to the substrate is controlled by the DC bias voltage, Vd, applied between two probes. The current flowing into probes does not exceed the ion saturation current, and is independent of discharge current. Thus the effect of probe insertion on the discharge is negligibly small. Fig. l(b) is a cross sectional diagram of the probe. The probe is made from stainless steel and is 36 mm by 36 mm by 4 mm. The conductive substrate is biased through the metal susceptor, and heated by small insulated heater. The two probes have an identical structure. A 13.56 MHz rf voltage is capaci-
RFElectrode Grounded Electrode \ Reference Probe
Table 1 Deposition condition of a-Si:H; total pressure Pr (Tort), rf power density Pw (mW/cm2), Sill4 flow rate Sill 4 (sccm), H 2 flow rate H 2 (sccm), anode temperature Z~ (°C) Pr
Pw
Sill4
H2
77a
0.05
10
20
20
150
tively coupled to the parallel plate rf and grounded electrodes, which have a diameter of 160 mm. The grounded electrode can be heated. The distance between the rf and the grounded electrodes is 80 mm, and the distance between the probes and the grounded electrode is 45 ram. Powdered a-Si:H for electron spin resonance (ESR) measurements was obtained from a-Si:H film about 1 Ixm thick deposited on the A1 foil. Ag coated Coming 7059 glasses was the substrate for Fourier transform infra-red (FTIR) measurement of the reflection spectra. Infra-red (IR) absorption spectra from 1850 cm -~ to 2200 cm l was divided into Sill stretching mode [SiH] and Sill 2 stretching mode [SiH2], assuming Gaussian distribution centered at 2000 cm l and 2090 cm ~, respectively. Typical deposition conditions are shown in Table 1.
(a) 3. Analysis of ion energy The energy of ion which impacts on substrates is analyzed in IEC plasma CVD. If the mean free path of ions is longer than the Debye length, then the energy of ions, E~, impacting on substrates is,
~ ! i i i t J :h:::::::::::::::::::::::::::~::::::::h: Sheath / // ~L.qlJ~ Seat Substrate/ I~'vd SubstrateProbe
E i = (sheath acceleration energy) + (thermal energy) = eV~ + kTi ,
MetalSusceptor~
(b)
C°nductive Substrate-'~:.:ii ~ ~]
Hea,er
]
where e is the elementary electric charge, k is Boltzmann's constant and Ti is the ion temperature. Under the usual plasma CVD conditions, T~ is nearly equal to the gas temperature, and k T i << e ~ ; for example at Ti = 1000 K, kT~ = 0.086 eV. Then, E i ~eV~.
Bias Port- ~ Fig. 1. (a) Schematic diagram of IEC plasma CVD. (b) Sectional diagram of the probe.
(1)
(2)
Eq. (2) means the ions that impact on the substrates have a constant energy of approximately eV, so that V~ is used as the ion energy in the following discussion.
T. Sasaki, E lchikawa / Journal of'Non-CGstalline Solids 198-200 (1996) 1007-1011
1009
1 0'
In Fig. l(a), the definitions for direction of the substrate probe current, /,, the reference probe current, I~, and the bias voltage, Vd, are indicated. If the plasma space potential is the same for the two probes,
1 0'
'E
V = V~ ln{ l~eo(l + aexp(- Vd/Ve)) } (1 + a) ]si0
z
(3)
,
1 0 '¸
where V~ is the electron temperature in unit eV, a is the area ratio of the reference probe and the substrate probe, I~o0 and I<0 are the saturated electron current and saturated ion current of the substrate probe, respectively [4]. Assuming Maxwellian distribution,
I~o = '4Nev7eS = ~Ne
11
-
-
3Tme
O
1 0 1 6
. . . .
,
0
. . . .
50
,
. . . .
100
,
. . . .
150
200
Vs (v) Fig. 2. Spin density of a-Si:H, %, as a function of ion energy, V~.
eS,
(4)
where N~ is the electron density, ve is the average thermal speed of electrons, Te, is the electron temperature in unit K, rn~ is the mass of electron, S is the area of the substrate probe. From ion sheath theory
and the saturated V~ is almost equal to the floating potential.
[5],
4. Results
/ kro l,m = KN~I/ - - eS,
(5)
~/ m i
where K is the function of Ti/Te; K ~ 0.61 under the normal plasma CVD conditions and m i is the mass of ion. From Eqs. (3), (4), (5), we obtain { 1 V" = v~ In ~
/ 8mi qrmo
l+aexp(-V'l/Ve)} i+~
'
(6) If V~ is constant, V, only depends on Va. The value of V~ is determined by measuring I~-Va characteristics during plasma, using IEC plasma CVD apparatus as floating double probe [3]. V, increases linearly as Va decreases when Vd < 0. In contrast, V~ saturates and keeps constant value when Vd > 0. In this study, we assumed ion species as SIH,, (n = 0 ~ 4). The generation of H + from direct ionization of H , is negligibly small considering ionization cross sections of H 2 and Sill 4 [6,7]. Generation of H,+, from Sill 4 is smaller than that of SIH,"+ because the ionization energy of H is 5.5 eV larger than that of Si [8]. The probes are identical structure so that a = 1, •
Fig. 2 shows the spin density, N~, as a function of Vs. V,~ is changed from -150 V to 150 V, to change V, from 20 v to 170 v. N, increased by a factor of 2 for T~ = 55°C and by a factor of 4 for T, = 150°C, respectively as V~ increased from 20 V to 170 V. N, is scattering about factor of 1.5 ~ 2 at V~ = 20 V, because Vd is changed from 0 V to 150 V at V~ = 20 V and it caused sample to sample error. Fig. 3 shows the intensity of bonded hydrogen spectra, [Sill] + [Sill2], as a function of V,. For = 55°C, [Sill] + [Sill 2] decreased, but it slightly
5O
~ 40
71 E
~30
(3
+
~20 ÷
.:12_10 0
.
.
.
.
.
.
5O
.
.
.
.
.
.
100
Vs (v)
, , i
. . . .
150
200
Fig. 3. The intensity of bonded hydrogen spectra, [Sill] + [Sill 2 ], as a function of V~.
1010
T. Sasaki, Y. lchikawa / Journal of Non -Crystalline Solids 198-200 (1996) 1007- l Ol I 0.4 ~_0.3
atom density of a-Si:H, ( ~ 5 × 1022 cm 3) for [12]. We assumed the precursor radicals, s, as of 2 n m / m i n ,
5
÷
~0.2 0
¢~0.1
o"
-I-
0,0
Fdepo = DR × [ S i ] / 0 . 1 ~ 1.7 × 10 '5 (cm -2 s - ' ) ,
. . . . . . . . . . . . . .
0
[Si], is almost equal to c-Si order estimation of the flux sticking coefficient of the 0.1 [13]. At a deposition rate
50
100 Vs (v)
.... 150 200
(7)
,
Fig. 4. The [Sill2] bonding ratio [SiH2]/([SiH]+[SiH2]) as a function of V,.
where DR is the deposition rate. On the other hand, from the saturated ion current of the probe, I~i0, of 35 i x A / c m 2,
Fion= Isio/e~ Z.2 x lOl4 (cm-Z s-'). increased for ~ = 150°C as V, increased from 20 V to 120 V. Fig. 4 shows the [Sill2] bonding ratio [SiH2]/([SiH] + [SiH2]) as a function of V~. [SiH2]/([SiH] + [SiH2]) decreased 0.29 to 0.19 for = 55°C and it increased from 0.12 to 0.16 for Ts = 150°C as V~ increased from 20 V to 120 V. It is experimentally demonstrated from the above results that the ion energy has an effect on the film properties of a-Si:H.
5. Discussion If the ion energy was transferred to adsorbed precursors on the film surface and enhanced their surface diffusion, Ns would be suppressed [9]. N~, however, increased as VS increased, so this was not the case. The ion energy applied in this experiment is 2 orders of magnitude larger than Si-Si bond strength (2.4 eV) or S i - H bond strength (3.4 eV) [10]. This suggests that impinging ions break bonds, or reconstruct the a-Si:H network. Usually content of hydrogen in a-Si:H decreases as substrate temperature increases. The increase of [Sill] + [SiH 2 ] for TS = 150°C as V~ increased shows the effects of the ion energy are not same as substrate heating effects. In order to confirm whether sufficient ions could attack and effect the film properties, we estimated the order of the flux of film precursor radicals, Faepo, and the flux of ions, F~o.. Weight density of a-Si:H is about 60% of that of c-Si even though hydrogen atomic content is 30% [11], so we assumed the Si
(8)
From the above estimation the F~o. is about 10% of the fdepo and it is not negligible. Thus we conclude that in this experiment the ion energy can effect the film properties of a-Si:H. In this study ion species are assumed as S~H n (n = 0 ~ 4). For later studies, it could be interesting to change ion species and see the effect on film properties. For instance, the plasma of highly diluted Sill 4 with H 2 may include H 2 related ions, or the plasma of highly diluted Sill 4 with inert gas may include inert gas ions. Another interesting experiment is the effects of ion energy on the film properties of a-Si based alloys such as a-SiC, a-SiO, a-SiGe; a-SiGe:H is especially interesting. Highly photosensitive a-SiGe:H films are obtained not only under the conditions where the ion energy is low such as triode method [2,14], but also under conditions where the ion energy is high such as cathode deposition method [15], pulse discharge CVD [16]. Clearly more studies are needed. •
+
Acknowledgements This work was supported by the New Energy and Industrial Technology Development Organization as a part of the New Sunshine Program under the Ministry of International Trade and Industry•
References [1] K. Kato and I. Kato, Japan. J. Appl. Phys. 30 (1991) 1245. [2] T. Ichimura, T. Hama, T. Ihara, M. Ohsawa, H. Sakai and Y. Uchida, 18th IEEE Photovoltaic Specialists Conf., Las Vegas (1985) p. 1495.
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