The transition state of disilane plasma

Solar Energy Materials and Solar Cells 29 (1993) 233-241 North-Holland

Solar Energy Materials and Solar Cells

The transition state of disilane plasma K. A z u m a

a M. Tanaka a, M. Nakatani b and T. Shimada c

a Production Engineering Research Laboratory, Hitachi, Ltd., Yokohama, Japan b Mobara Works, Hitachi, Ltd., Chiba, Japan c Central Research Laboratory, Hitachi, Ltd., Tokyo, Japan Received 31 July 1992; in revised form 7 December 1992 The initial transition state in the plasma decomposition of disilane was investigated by measuring the electron energy in a disilane plasma, and by studying both optical emission spectroscopy and film properties. At the beginning of the discharge, the emission of Sill * was not observed, and the electron energy in the plasma was about 10 times lower than in the steady state. These phenomena were caused by an abrupt increase of the pressure just after the ignition, followed by disilane decomposition. Under insufficient energy conditions, production of silylene is highly favoured by the disilane decomposition which gives porous hydrogenated amorphous silicon (a-Si:H) films. We shall prove the importance to shorten the initial transition period in which the plasma energy is insufficient for disilane decomposition. Keeping the gas pressure constant at the beginning of the reaction enables to avoid the depression of the plasma energy and to obtain a-Si:H films with good optoelectronic properties.

1. Introduction The deposition of hydrogenated amorphous silicon (a-Si:H) films having high photoconductive properties with deposition rate faster than 0.3 nm/s, is an important issue for the mass production of a-Si : H thin film devices. Especially, in conventional p - i - n junction type a-Si:H solar cells, the deposition of the thick intrinsic a-Si:H layer is one of the steps limiting speed in mass production processing. Lots of methods were tried to obtain a higher deposition rate using monosilane (Sill 4) [1-4] or disilane (Si2H 6) [5,6]. However, the optoelectronic properties of a-Si:H films deposited from monosilane degrade with an increasing deposition rate. Films fabricated at high rates ( > 1.5 nm/s) using disilane are also not perfect. High RF power is needed to get highly photoconductive a-Si:H films at high deposition rate by decomposition of disilane [7]. In case of fabricating p - i - n junction type a-Si:H solar cells, however, high RF power leads to plasma damage on the interface between the doped layer and the intrinsic layer. In order to avoid Correspondence to: K. Azuma, Production Engineering Research Laboratory, Hitachi, Ltd., Yokohama, Japan. 0927-0248/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

I(. Azuma et al. / The transition state o f disilane plasma

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this plasma damage, lower RF power should be introduced, which leads to a deterioration of film properties, i.e. their optical energy gaps become wider, the hydrogen content becomes higher, and they have poor photoconductivities. It was proved that high quality a-Si:H films could be obtained from disilane, without plasma damages to the interface, by controlling both plasma parameters and disilane flow rate [7]. The mechanism of film formation is quite different for the two cases of monosilane and disilane [8,9]. In this paper, we shall study the initial transition state of disilane plasma which has a large effect on film properties in the early stage.

2. Experimental a-Si:H films were prepared using a conventional capacitively coupled plasma chemical vapor deposition system. The reactor had two parallel plate electrodes, having 30 cm in diameter, spaced 4 cm apart. A 13.56 MHz RF generator was used. Pure Si2H 6 was introduced into the reactor through multiple holes on the cathode surface. The typical deposition conditions were as follows: the flow rate of Si2H 6 gas was 40 sccm, deposition pressure was 113 Pa, applied RF power was 100-500 W, the substrate was placed on a grounded and heated anode, the temperature of the substrate was maintained at 2200C. This system was equipped with a microcomputer and an automatic pressure controller. Corning 7059 or FZ silicon wafers were used for the substrate to examine the quality of a-Si:H films. Repeatedly deposited thin films were prepared in order to evaluate the properties of the initial part of a-Si:H layers. They were prepared as follows: After source gas introduction and plasma ignition, disilane was decomposed for 100 s. The plasma was turned off and the source gas was eliminated by evacuating the reactor up to 6 x 10 -5 Pa. This operation was repeated 10 times. Electron energy in the plasma was measured by a single probe method as shown in fig. 1. Measurements were taken every 25 s. The probe used in this report was made of fine wire, guided by coaxial cable, the area of the ends was negligible compared with the barrel area of the cylinder. Electron energy was measured by

rf-Generator

CVD Fleactor Fig. 1. Schematic illustration of electron temperature measurement.

K. Azuma et al. / The transition state of disilane plasma

235

the probe I-V curve. Electron should go over the barrier equivalent for the difference between plasma potential (Vp) and the probe surface potential (is). Assuming that electron energy is distributed isotropically and described as Maxwell distribution, the probe c u r r e n t (ip) is given by the following equation: Ip =I0 e x p ( - q ~V/E),

(1)

where Ip is the probe current (A), 8V= V~- Vp (V), I 0 is the saturated current (A), E the electron energy (eV), and q the electron charge (C). When plotting the log of eq. (1), the electron energy was calculated by the slope of the following equation: In Ip = (q/E)Vp

-

(q/E)V~ + In I 0.

(2)

In this report, the measurement was carried out only when there was no deposition on the probe just after the beginning of the reaction.

3. Results and discussion

Fig. 2 shows the variation of deposition rate of the a-Si: H films as a function of the supplied energy normalized by the disilane flow rate [10]. The supplied energy was defined as follows: P ( W ) Csi2H6

supplied energy = [F(sccm)/60(s)] [Mw(g)/22400(cm3)] ' where P(W) is the applied RF power (W), CSi2H 6 the concentration of Si2H6, F(sccm) the gas flow rate (sccm), and M w the molecular weight (g). The deposition

2.5

...,...,...,...,...,...,...

~" 2.0 E '~ 1.5

g cn 1.0 O

a 0.5

0.0 20

40

60

80

100

120

140

Supplied enemy (~/g-Si2H ~ Fig. 2. Deposition rate of a-Si:H film prepared from disilane as a function of supplied energy normalized by disilane flow rate.

236

K. Azuma et aL / The transition state o f disilane plasma

Table 1 Enthalpy of disilane decomposition Primary step

AH (eV)

Si2H 6 ~ :Sill 2 + Sill 4 SiEH 6 --~ 2 "Sill3 Si2H 6 ~ 'SiEH 5 + "H

2.15 3.17 4.0

rate increases with decreasing supplied energy when low supplied energy is provided. The film prepared from disilane in the low power region is porous and has usually a high hydrogen content. Table 1 shows the enthalpy of the disilane decomposition. Silylene (: Sill 2) seems to be produced when the supplied energy is low, and tends to polymerize via an insertion reaction without hydrogen elimination. On the other hand, when the supplied energy is relatively high, disilanyl radical (. Si2H 5) is mainly produced. It was reported that Si-H bonds of disilane are preferentially broken compared with the Si-Si bond by a factor of 10 when the disilane is decomposed by high energy electrons [11]. The differences in disilane decomposition caused by the supplied energy affects the photoconductive property of a-Si : H film, as shown in fig. 3. This figure shows that a supplied energy of about 100 kJ/g-Si2H 6 is needed in our deposition system to prevent deterioration of the photoconductive property. We found that the quality of the initial part of the a-Si : H, which is deposited during the transition state, is inferior to the usual thick a-Si:H film as shown in table 2. Deposition conditions such as disilane flow rate, RF power, pressure, and substrate temperature were identical. The stacked film was prepared by 10 repeated depositions for 100 s after plasma ignition. The photoconductive property of the stacked film is poor. The film contains more hydrogen than the consecu-

10-4

,

.

,

,

. . . .

,

. . . .

,

. . . .

,

. . . .

,

. . . .

o 10 5

(5m

tJ 10 6

10-7

8

1 0 .8

1 0 .9

....

10-10 60

i .... 70

a .... 80

i .... 90

, .... 100

J .... 110

120

Supplied energy (kJ/g-Si2H6)

Fig. 3. Supplied energy dependence of photo- (Orph) a n d dark (~ra) conductivity of a-Si:H film prepared from disilane.

K. Azuma et al. / The transition state of disilane plasma

237

Table 2 Characteristics of stacked film of initial part of a-Si:H layer and usual thick a-Si:film prepared from disilane

Si2H 6 flow (sccm) Substrate temp. (°C) Pressure (Pa) rf-power (W/cm 2) Deposition time (s) Eopt (eV) O.ph a) (S c m - i) crd b) (S cm -1) CH c) (at%)

Repeatedly deposited thin films

Thick film

40 220 113 0.357 100 × 10 times

40 220 113 0.357 500

1.81 7.8 x 10- 6 1.4×10 -9 17.1

1.81 3.0 × 10- 5 8.0× 10 - l ° 13.8

a) %h: photoconductivity. b) ~rd: dark conductivity. c) CH: hydrogen content.

tively prepared thick film, while the optical gap is the same. From the IR spectra, it was difficult to compare the difference of oxygen content between the repeatedly stacked films and consecutively deposited thick films. However, it is supposed that the surface oxidation did not dominantly affect to the deterioration of the repeatedly stacked films, because these depositions were carried out in the dosed system, i.e. the interval time for preparation between neighboring thin layers was several tens of seconds, and the reactor was evacuated up to 6 × 10 -5 Pa before the next introduction of the source gas. Therefore, these results are supposed to relate to the fact that the glow discharge does not stabilize promptly after plasma ignition. Fig. 4 shows the temporal change in intensity of optical emission spectra due to Sill* and H* after plasma ignition. In this case, as shown in fig. 5, pressure increased at the beginning of ignition while the conductance was kept constant. The increase in pressure was due to the decomposition of the disilane gas which presented in the reactor. The emission intensity of Sill* (413 nm) began to rise at the point that the pressure begins to fall, and it settles down as the pressure becomes constant. The intensity of H* (657 nm) rises a little later than that of Sill *. Thus, the pressure and gas composition undergo changes in disilane plasma. In our deposition system, it takes about 7 min to become stable. The rising pressure lowers the electron energy in the plasma which delays the rate of reactions. Then we tried to keep the reaction pressure constant by using the autoconductance control valve. Fig. 6 shows the temporal change in emission after plasma ignition. This time, pressure fluctuations were kept within 10%. However, it took about 2 minutes to stabilize the emission. As reported, the emission of Sill* is closely related to the deposition rate [9]). In this case also no deposition occurred until these emissions appeared. At the beginning of the discharge, the molar

238

1~ Azuma et al. / The transition state of disilane plasma

Si2H6; 40sccm

power:3OOW

H*(657nm)

'5

I

~

2

3

I

I

I

I

4

5

6

7

8

Time (min) Fig. 4. Change in intensity of the optical emission spectra of H * (657 nm) and Sill* (413 nm) under the condition of fixed-conductance evacuation system.

amount of disilane stored in the reactor was larger than in the case of the steady state. The supplied energy normalized by the disilane flow rate should be corrected. The calculated value of supplied energy for the case of the transient state becomes smaller than that of the steady state. Although the deposition conditions in fig. 6 corresponds to a supplied energy of 140 kJ/g-Si2H6, the value approximately calculated with the correction becomes 50 kJ/g-Si2H 6 during the first 2 rain. The value of 50 kJ/g-SiEH 6 for the supplied energy is consistent with the result shown in fig. 2. Under the supplied energy of about 50 kJ/g-SiEH6, no

200

150

g loo 13-

50

0

I

I

I

I

I

I

I

1

2

3

4

5

6

7

8

Time (min) Fig. 5. Change in pressure under the condition of fixed-conductance evacuation system.

K. Azuma et aL / The transition state of disilane plasma

239

Si2H6: 40sccm =t power: 300 W Pressure: 113pa

H* (657nm)

t~

"5 Sill* (413nm)

/1"/

I

I

,I

I

I

I

1

2

3

4

5

6

7

0

Time (min)

Fig. 6. Change in intensity of optical emission spectra under constant pressure•

deposition occurred in fig. 2. The pressure increase at the plasma ignition is due to the decomposition of disilane, which produces Sill, (n = 1, 2, 3, 4). To observe Sill* emissions, there should exist Sill,, H, H E, with enough energy to excite the above species. So, the reason why Sill * emission is not observed is lack of species with sufficient energy. From this point of view, we measured the electron energy by the probe method. Fig. 7 shows the temporal change in the electron energy in the plasma after plasma ignition. The chamber pressure is now kept constant. The rise of the electron energy in the disilane plasma at the beginning of glow discharge is very slow. It takes about 2 min to stabilize the electron energy. This agrees with the result of fig. 6. In spite of the constant pressure, electron energy undergoes large

25 0

5"

20

==

15

~D

0

0

0

0

0

0

0

0

0

10 0

LU 5 O

0

1

2

3

4

5

Times (min)

Fig. 7. Change in electron energy after disilane plasma ignition under constant pressure.

240

IK Azuma et al. / The transition state of disilane plasma

changes, because the gas composition is modified. Electron energy in the glow discharge is determined by the balance between loss by inelastic collisions and gain by electric field. These inelastic collisions usually accompany ionization, excitation, and dissociation of gas molecules. It seems that the rise in electron energy is suppressed by the disilane, while the partial pressure of disilane is high. This is due to the fact that the dissociation energy of disilane is lower (2.15 eV), than the one of monosilane (4.1 eV) or hydrogen (4.5 eV) [12]. Moreover, the collisional cross section of disilane is large. As the decomposition proceeds, the partial pressure of the disilane decreases and the partial pressure of the hydrogen increases. The electron energy increases reflecting the higher ionization energy of the hydrogen. The results in table 2 agrees with the above explanation. Each layer of the repeatedly deposited thin films was deposited for 100 s. It corresponds to an electron energy in transition state lower than 15 eV in fig. 7. As mentioned in fig. 3, sufficient supplied energy (>/100 k J / g - S i 2 H 6) is needed to obtain high-photoconductive a - S i : H films. With low energy, Si2H 6 decomposition leads to porous, high hydrogen content films with low photoconductivity. The initial part of a-Si : H layer was p r e p a r e d under unstable, transition plasma circumstances, i.e. insufficient supplied energy, which is supposed to correspond to lower supplied energy area ( < 100 k J / g - S i 2 H 6) in fig. 3.

4. Conclusion We proved the existence of a threshold value of the supplied energy to obtain high photoconductive a - S i : H films from Si2H 6. In our deposition system, these value is about 100 k J / g - S i 2 H 6. Due to the existence of a transition state, the electron energy does not reach a saturated level, at the beginning of Si2H 6 plasma ignition, an a-Si : H film prepared from Si2H 6 during this initial transition state has low photoconductivity with large hydrogen content. This result seems to correspond to the :Sill 2 insertion reaction at low supplied energy area. It is important to control the disilane flow rate and to keep the plasma energy high in order to avoid the deteriorated layer of initial transition state.

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[8] J. Perrin, P.R. Cabarrocas, B. Allain, and J. Friedt, Jpn. J. Appl. Phys. 27 (1988) 2041. [9] A. Matsuda, T. Takao, H. Tanaka, L. Malhotra and K. Tanaka, Jpn. J. Appl. Phys. 22 (1983) Ll15. [10] N. Fukuda, S. Ogawa, K. Abe, Y. Ohashi and S. Kobayashi, 1st Int. Photovoltaic Science and Engineering Conf., Kobe, Japan, 1984,.p. 107. [11] R.C. Ross and J. Jaklik Jr., J. Appl. Phys. 55 (1984) 3785. [12] G. Turban, Y. Catherine and B. Grollear, J, Appl. Phys. 77 (1981) 287.