Gas-phase diagnosis and high-rate growth of stable a-Si:H

T. Takagia,*, R. Hayashia, G. Gangulyb, M. Kondoa, A. Matsudaa

aThin Film Silicon Solar Cells Super Lab., Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305-8568, JapanbSolarex, 3601 LaGrange Parkway, Toano, VA 23168, USA

Abstract

Gas-phase molecular composition in silane (pure and H2-diluted) plasma was studied using a mass spectrometry under various deposition

conditions of hydrogenated amorphous silicon (a-Si:H). Our aim was to correlate the gas-phase species with the ®lm property after light-

induced degradation and to ®nd the ideal deposition conditions to achieve high-rate growth of stable a-Si:H ®lms. A parallel-plate plasma

enhanced chemical vapour deposition (PECVD) system with an excitation frequency of 13.56 MHz (RF) was used. Both the ®lm deposition

and mass spectrometry were performed under deposition conditions of a-Si:H giving growth rates ranging from 2 to 20 AÊ /s. We observed the

signal intensities of SiH21 (m=e � 30) and Si2H4

1 (m=e � 60) as the most abundant ions in the mass fragmentation related to monosilane and

disilane molecules, respectively. The ®lm property of a-Si:H ®lms after light-induced degradation, monitored by the ®ll factor (FF) of photo

current - voltage characteristics in n 1 crystalline Si/a-Si:H/Ni Schottky cells after light soaking, deteriorated with the increase in the growth

rate. The contribution of gas-phase disilane related radicals to the ®lm growth, represented as the disilane fraction ([m=e � 60�=�m=e � 30�),showed a good correspondence with the FF after light soaking. It is suggested that the suppression of gas-phase higher-order silane-related

radicals as well as short-lifetime radicals is a clue for obtaining stable a-Si:H solar cells at high growth rate. q 1999 Elsevier Science S.A. All

rights reserved.

Keywords: a-Si:H; Mass spectrometry; Higher-silane

1. Introduction

A-Si:H ®lm is an important material in solar cell industry.

It is essential to achieve high rate and uniform deposition on

large area substrates in order to reduce the production cost,

but the ®lm property, initial and more importantly after

light-induced degradation, is known to deteriorate with

increasing growth rate. The increase in the growth rate is

conventionally obtained by increasing the silane partial

pressure and the RF power, however, the amount of gas-

phase higher-order silanes is known to increase with silane

pressure, while short lifetime radicals such as Si, SiH and

SiH2 contribute to the ®lm growth under a silane depletion

condition. These changes of ®lm-growth precursors in the

gas-phase and/or their subsequent interactions on the growth

surface are considered to be the cause of the inferior ®lm

property.

There have been many works on gas-phase analysis using

mass spectrometry in order to analyze the ionic and neutral

species in the silane RF glow-discharge plasma [1±4], but

no work touched upon the relationship between the growth

kinetics and the light-induced degradation property of the

resulting ®lms.

In this work, we studied the gas-phase species in pure and

H2-diluted silane plasma under various deposition condi-

tions of a-Si:H for the growth rates up to 20 AÊ /s. Our aim

was to correlate the gas-phase species with the ®lm property

after light-induced degradation, in order to identify the

mechanism of deterioration, and to ®nd the ideal deposition

conditions to achieve high growth rate of stable ®lms.

2. Experimental

Fig. 1 shows the experimental setup. A parallel-plate

plasma enhanced chemical vapour deposition (PECVD)

system with electrodes of 100 mm in diameter was used

for deposition of a-Si:H ®lms and mass spectrometry. Either

pure silane or H2-diluted silane plasma was generated by an

excitation frequency of 13.56 MHz. The electrode distance

was kept constant at 40 mm.

Mass spectrometry was carried out using differentially

evacuated quadrupole mass spectrometer system mounted

underneath the anode (grounded electrode). The ionic and

neutral species, entering the mass-analyzing system through

a ®ne ori®ce, are dissociated and ionized by the ion source

Thin Solid Films 345 (1999) 75±79

0040-6090/99/$ - see front matter q 1999 Elsevier Science S.A. All rights reserved.

PII: S0040-6090(99)00067-X

* Corresponding author.

E-mail address: ttakagi@etl.go.jp (T. Takagi)

with an ionization energy of 70 eV. As the density of ionic

species in the plasma is four to ®ve orders of magnitude

lower than that of the total intensity of all the species

involved in the plasma, the observed signal intensities

must be regarded as the fragments of neutral molecules

[1]. In this work, the signal intensities of SiH21 (m=e � 30)

and Si2H41 (m=e � 60) were observed as the most abundant

ions in the mass fragmentation related to monosilane and

disilane molecules, respectively.

Both mass spectrometry and ®lm deposition were carried

out under various discharge conditions giving a-Si:H growth

rates ranging from 2 to 20 AÊ /s. The ¯ow rate of SiH4 was

kept constant at 30 sccm, and H2 of 120 sccm was added for

H2-dilution condition. The discharge pressures were 20 and

100 mTorr for pure silane plasma, and 100 mTorr for H2-

diluted silane plasma where the silane partial pressure was

set at 20 mTorr. The substrate temperature was kept

constant at 2508C and the power was changed from 5 to

100 W.

The hydrogen bonding con®gurations were investigated

by FT-IR absorption spectroscopy for the ®lms deposited on

silicon wafers. The Si±H and Si±H2 stretching modes were

observed at wavelengths of 2000 and 2090 cm21, respec-

tively [5].

The ®lm property of a-Si:H ®lms after light-induced

degradation was monitored using the ®ll factor (FF) of

n 1 crystalline Si/a-Si:H/Ni Schottky cells having a-Si:H

thickness of 7000±8000 AÊ . The Schottky cell performance

was measured under AM 1.5 (100 mW/cm2) light after 6

hours light soaking at 608C with 3-sun (300 mW/cm2) illu-

mination. The advantage of using a Schottky cell con®gura-

tion is that the ®lm preparation conditions can be extended

to a variety of growth conditions including high substrate

temperature.

3. Results and discussion

3.1. Light-induced degradation property

It is well known that the ®lm property of a-Si:H, initial

and after light-induced degradation, deteriorate with the

increase in growth rate. Fig. 2 shows the relationship

between the FF, initial and after light soaking, and the

growth rate of a-Si:H deposited in pure silane plasma

under various growth conditions. The initial FF shows a

slight decrease, and the FF after light soaking shows a

clear deterioration with the growth rate.

The cause of light-induced degradation is not clear yet,

but the correlation between the hydrogen bonding con®g-

uration and the degradation property of solar cells has been

pointed out [6,7]. In our study, we observed a correlation

T. Takagi et al. / Thin Solid Films 345 (1999) 75±7976

Fig. 1. Schematic diagram of the PECVD system with mass spectrometer.

Fig. 2. The ®ll factor of Schottky cells (FF) as a function of growth rate;

initial and after 6 h of light soaking under 3-sun (300 mW/cm2) illumination

at 608C.

Fig. 3. Relationship between the FF after light soaking and the fraction of

Si±H2 con®guration ([Si±H2]/([Si±H] 1 [Si±H2])) for a-Si:H ®lms

prepared from pure silane at a substrate temperature of 2508C.

between the FF after light soaking and the fraction of Si±H2

con®guration observed from the FT-IR spectra represented

as [Si±H2]/([Si±H] 1 [Si±H2]) for a-Si:H ®lms prepared

from various deposition conditions using pure silane as a

source gas at a substrate temperature of 2508C (Fig. 3).

The origin of Si±H2 con®guration in the ®lm is suggested

as the contribution of SiH31 or H1 ions, or higher-order

silane related radicals to the ®lm growth. As the Si±H2

con®guration can be suppressed by increasing the substrate

temperature [5], we took notice of higher-order silanes,

which contribution is suggested to be relaxed by increasing

the temperature.

3.2. Mass spectrometry

In our system, the mass signal intensities related to disi-

lane (Si2Hx1) and trisilane (Si3Hx

1) molecules were two and

three orders of magnitude lower than that of monosilane

(SiHx1) molecule, respectively. Fig. 4 shows the signal

intensity of monosilane (m=e � 30) and disilane

(m=e � 60) related fragments as a function of RF power.

The discharge conditions are pure silane plasma at the pres-

sures of 20 and 100 mTorr (hereafter simply pure 20 and

pure 100 mTorr, respectively), and H2-diluted silane plasma

at the pressure of 100 mTorr with silane partial pressure of

20 mTorr (H2-dilution). Monosilane related signal intensity

decreases with the power, indicating the enhancement of

silane dissociation with an increase in the RF power [1].

The monosilane intensity is almost the same for the equiva-

lent silane partial pressure conditions, pure 20 mTorr and

H2-dilution, and much higher for higher partial pressure at

pure 100 mTorr. The disilane intensity at high partial pres-

sure shows a maximum at low power region, while it

increases with power for low partial pressure cases,

approaching the value of pure 100 mTorr at high power

region. For the whole power range, the disilane intensity

for H2-dilution is lower than that for 20 mTorr. This lower-

ing of disilane intensity is recognized as an effect of H2-

dilution.

We made the following assumption to estimate the contri-

bution of higher-order silane related radicals to the ®lm

growth. The main ®lm growth precursor in SiH4 plasma,

SiH3, is produced from a single collision of SiH4 and elec-

tron. Then, the change in the density of SiH3 in SiH4 plasma

may be represented as the difference between the generation

rate of SiH3 by the dissociation of SiH4 and the diffusion

(annihilation rate) of SiH3 as

d SiH3

� �dt

� Neves3 SiH4

� �2

SiH3

� �t3

: �1�

Here, Ne is the electron density, ve is the thermal velocity

of electron, s 3 is the dissociation cross-section of SiH3 from

SiH4, t3 is the characteristic residence time of SiH3, and

[SiH4] is the partial pressure of SiH4. The density of SiH3

in the steady state plasma is lead from d�SiH3�=dt � 0 to be

SiH3

� � � t3Neves3 SiH4

� �: �2�

Similarly, the density of higher-order silane related radi-

cals (SixH2x11) in the steady state plasma is expressed as

SixH2x11

� � � t2x11Neves2x11 SixH2x12

� �; �3�

where [SixH2x12] is the partial pressure of higher-order

silane molecules.

The contribution rate of higher-order silane related radi-

cals to the ®lm growth is represented from the ratio of

T. Takagi et al. / Thin Solid Films 345 (1999) 75±79 77

Fig. 4. Mass signal intensity of monosilane (top) and disilane (bottom)

related fragments as a function of RF power.

Fig. 5. Disilane fraction (�m=e � 60�=�m=e � 30�) as a function of RF

power.

Eq. (3)/Eq. (2), i.e.

SixH2x11

� �= SiH3

� � / SixH2x12

� �= SiH4

� �: �4�

Therefore, the contribution of higher-order silane related

radicals, such as Si2H5, to the ®lm growth is considered to be

represented as the higher-order silane fraction, which is

expressed as the ratio of the mass signal intensities from

higher-order silane molecules to the remaining monosilane

molecule (e.g. �m=e � 60�=�m=e � 30� for disilane fraction).

Fig. 5 shows the relationship between the disilane frac-

tion and the RF power. The disilane fraction for pure 20

mTorr and H2-dilution increases linearly with power, while

it saturates over 10 W for pure 100 mTorr. It is suggested

that the trend of power dependent disilane fraction is related

to the silane partial pressure. Within the same silane partial

pressure, low disilane fraction is obtained in the case of H2-

dilution for the whole power range, suggesting less contri-

bution of disilane related radicals to the ®lm growth.

Fig. 6 shows the relationship between the disilane frac-

tion and the FF after light soaking. Although there is a

scattering in FF among three conditions, higher FF is

obtained for lower disilane fraction. Thus, it is clear that

suppressing the disilane fraction in the gas phase leads to

the growth of more stable ®lm. There may be other causes

leading to the difference in ®lm property after light soaking

under different pressures, such as the contribution of short

lifetime radicals and ionic species.

Fig. 7 shows the relationship between FF and the growth

rate of a-Si:H ®lms for pure 20 mTorr, pure 100 mTorr and

H2-dilution cases. The power needed to realize the growth

rate of 20 AÊ /s was 20 W for pure 100 mTorr, and 100 W for

pure 20 mTorr and H2-dilution, thus much less power was

needed for higher silane partial pressure case. FF decreases

with growth rate for pure 20 mTorr and H2-dilution, where

higher FF is obtained for H2-dilution, while no change is

observed for pure 100 mTorr, showing the minimum value

even at the lowest growth rate. These trends correlate with

the disilane fraction, i.e., less disilane fraction resulting in

higher FF. There might be other factors resulting in the low

FF, such as the presence of much higher-order silanes,

which is omitted in this work.

The substrate temperature was increased up to 300 8Cunder the similar growth conditions as mentioned above.

The growth rates were 20 AÊ /s for pure 20 mTorr and pure

100 mTorr, and 14 AÊ /s for H2-dilution. The improvement in

FF was observed for the ®lms deposited at pure 100 mTorr

(indicated as O in Fig. 7) and H2-dilution, but no change was

seen for the ®lms deposited at pure 20 mTorr. This improve-

ment in FF with the increase of substrate temperature at

high pressure can be ascribed as the thermal relaxation of

the network structure constructed by the contribution of

higher-order silane radicals. On the other hand, at low pres-

sure, the contribution of short-lifetime radicals whose

effects on the network structure due to lack of surface diffu-

sion may not be overcome by thermal energy. We attempted

to ®nd an ideal condition to achieve the growth of stable a-

Si:H at high growth rate. By increasing the total gas ¯ow

rate and the substrate temperature up to 3008C, FF of 0.45

was achieved at a growth rate of 14 AÊ /s under H2-dilution

condition (indicated as K in Fig. 7.)

Further investigation of the gas phase species, searching

for the discharge conditions with low amount of higher-

order silanes, will lead to the observation of much more

stable a-Si:H ®lms at high growth rate.

4. Conclusions

We identi®ed the contribution of higher-order silane-

related radicals to the ®lm growth as a major cause of the

degradation of a-Si:H ®lm property when increasing the

growth rate. It is suggested from the results mentioned

T. Takagi et al. / Thin Solid Films 345 (1999) 75±7978

Fig. 6. The relationship between the FF after light soaking and disilane

fraction.

Fig. 7. The relationship between the FF after light soaking and the growth

rate of a-Si:H. The plot includes improved FF by increasing substrate

temperature to 3008C for pure 100 mTorr (O) and H2-dilution at high

¯ow rate (K).

above that the suppression of gas-phase higher-order silane-

related radicals as well as short-lifetime radicals is a clue for

obtaining stable a-Si:H solar cells at high growth rate.

As shown in our work, monitoring of the higher-order

silane molecules in the gas-phase is a useful way to predict

the property of the ®lm. We are considering to further inves-

tigate the much higher-order silanes (SinHx, n $ 3) to ®nd a

much clearer correlation between gas-phase species and

®lm property.

Acknowledgements

This work is supported by NEDO under the New

Sunshine Project of the Agency of Industrial Science and

Technology.

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