Deposition of nanocrystalline silicon films (nc-Si:H) from a pure ECWR-SiH4 plasma

J O U R N A L OF

NONRYmLI, SOLI ELSEVIER

Journal of Non-Crystalline Solids 198-200 (1996) 895-898

Deposition of nanocrystalline silicon films (nc-Si:H) from a pure ECWR-SiH4 plasma M. S c h e i b a,b B. Sch65der a,* H. O e c h s n e r a,b a Fachbereich Physik und Forschungsschwerpunkt Materialwissenschaften, Unit'ersit~t Kaiserslautern, Erwin-SchriJdinger-StraJ3e, 67663 Kaiserslautern, Germany b InstitutJ~r Ober.fliJchen- und Schichtanal~,tik (IFOS) Universit~itKaiserslautern, Erwin-Schrikdinger-StraJ3e, 67663 Kaiserslautern, Germany

Abstract A novel plasma enhanced chemical vapour deposition (PECVD) technique employing electron cyclotron wave resonance (ECWR) for plasma excitation was used for the deposition of hydrogenated nanocrystalline silicon (nc-Si:H) films. nc-Si:H-films could be produced with large deposition rates up to 6.5 A / s with pure Sill 4 as process gas in contrast to conventional glow-discharge technique where the high hydrogen dilution needed for the formation of the crystalline phase leads to considerably lower deposition rates. The basic dependence of the deposition parameters on the nature of the condensing phase was investigated. Besides the substrate temperature dependence the resulting phase deposited from a pure silane plasma is mainly determined by the dissociation degree of the plasma and the generation of atomic hydrogen which can be varied by the high frequency input power and the SiH4-flow.

1. Introduction Hydrogenated nanocrystalline silicon films have received an increasing interest in the past few years. They have been used as the n-doped layer of amorphous silicon based solar cells and the s o u r c e / d r a i n contact of amorphous silicon thin film transistors (TFT) [1]. Recently entire nanocrystalline solar cells with considerable efficiencies were prepared [2] showing an enhanced absorption in the near-infrared and a possible absence of light induced degradation. The plasma deposition of nanocrystalline silicon thin films is usually performed under a high flux of

* Corresponding author. Tel: +49-631 205 2377; fax: +49-631 205 2996; e-mail: [email protected].

atomic hydrogen. For that reason a high dilution of silane in hydrogen is normally considered to be necessary for the transition from the amorphous to the nanocrystalline state in the glow-discharge PECVD-technique [3]. In the present paper a novel method, the E C W R (electron cyclotron wave resonance) PECVD, for the deposition of nanocrystalline silicon in a pure silane plasma is introduced. Their principle is based on a resonant excitation of a high frequency low pressure plasma [4]. To maintain the resonant condition essential for ECWR, a weak magnetic field is superimposed with proper magnitude and direction. Noble gas plasmas excited by that principle are commonly used as effective postionizing media in secondary neutral mass spectrometry ( S N M S ) [5]. Therefore with a chemical reactive species such as silane, a

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M. Scheib et al. / Journal of Non-Crystalline Solids 198-200 (1996) 895-898

high dissociation degree resulting in a large atomic hydrogen flux during deposition could be expected. We also investigated the deposition of nc-Si:H with variable hydrogen dilution of the process gas. However, recognizing the possibility of crystallization in a pure silane plasma we concentrate on these investigations.

2. Experimental Deposition of nc-Si:H was carried out in a reactor where the ECWR-plasma is confined by a Duran glass cylinder with an outer diameter of 250 mm, a height of 160 mm and two stainless steel plates. The upper one includes pressure measurement and gas inlet, the lower one including a net of diameter 125 mm with a initial transmission of 80%. The transmission decreases with increasing number of deposition processes. The net was replaced when the transmission was less than 40%. It also enables the extraction of a low energy plasma beam. However, it is not required for the experiments reported in this paper. High frequency power is coupled inductively by a single turn coil surrounding the plasma cylinder. The magnetic field maintaining the ECWR-condition is superimposed perpendicular to the cylinder axis. Substrates are transferred via a load lock into the deposition chamber onto a substrate holder located outside the plasma in a distance of 1 cm. The films were deposited onto monosilicon substrates and Corning 7039 glass. The structure of the deposited films was characterized by grazing incidence X-ray diffraction and Raman spectroscopy. To determine the type of hydrogen bonding and hydrogen content, Fourier transformed infrared spectroscopy (FTIR) was performed. Optical emission spectroscopy (OES) was employed over the wavelength range 250-700 nm in order to investigate the relative change of the excited reactive species in the plasma.

3. Results 3. l. Substrate temperature dependence Fig. 1 shows a set of Raman spectra for samples deposited in a pure silane plasma at various substrate

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300 350 460'4gO'5dO'5iO 660 650 relative wavenumber [cm -1] Fig. 1. Raman spectra for nc-Si:H-films deposited in a pure silane plasma at different substrate temperatures.

temperatures Ts, with other deposition conditions being kept constant. At the minimum Ts of 90°C (without substrate heating), the samples exhibit the broad Raman peak at 480 cm -1 attributed to the amorphous structure. In the case of the deposition parameters used crystallization takes place in a narrow temperature interval between 130 and 140°C. This is obvious from an almost sudden appearance of the asymmetrical TO mode at 520 c m - 1, characteristic for the nanocrystalline phase. A deconvolution procedure for the integrated Raman intensities, assuming three different contributions corresponding to a amorphous, a surface or smaller crystallite and a crystalline part [6], leads to a constant value of 65% for the crystalline fraction. The structural change is corroborated by the results of X-ray diffraction, not shown here, typical for the nanocrystalline structure of statistically oriented grains. Using the Scherrer formula the crystallite size estimated from the width of the (111)-diffraction peak is in the range of 20 to 30 nm. The phase transition is additionally accompanied by a drastic reduction of the hydrogen content from 25 to 8 at%, which implies that the hydrogen atoms are restricted to the grain boundaries of the nanocrystallites. The deposition rate during this series remains independent of substrate Ts and phase at ~ 3 ,~/s. Thus no significant etching effect upon the transition was observed. An increase in dark conductivity from 10 r0 to 10 7 ( ~ cm)-I is also caused by the structural change.

897

M. Scheib et al. / Journal of Non-Crystalline Solids 198-200 (1996) 895-898 •

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Fig. 2. X-ray diffraction patterns for nc-Si:H-filmsdeposited in a pure silane plasma at different ECWR power levels.

3.2. P o w e r variation

A second factor determining the structure of the deposited films in a pure E C W R silane plasma is the degree of silane dissociation. It can be varied by the amount of high-frequency input power coupled into the plasma. The X-ray diffraction diagrams (Fig. 2) for a power variation series in a pure silane plasma show that films deposited at a low power are amorphous, but above a certain power level the films grow crystalline. A small amount of crystalline phase is already present at 175 W, visible by the small diffraction peak at 20 = 28.4 ° over the amorphous diffraction pattern. The films seem to be almost nanocrystalline at 225 W. No significant change in crystallite size ( ~ 20 nm) up to 300 W could be observed. The deposition rate increases almost linearly with E C W R power in the amorphous region. For the crystalline films the deposition rate saturates at a value of 3 A / s . The amorphous to crystalline phase transition is accompanied by a change not only in the hydrogen content but also in the hydrogen bonding configuration. A detailed picture of the last is given by an inspection of the infrared absorption region around 2000 c m - I (Fig. 3). At a low power level (80 W) mainly the isolated silicon-hydrogen bond of the amorphous phase at 2000 cm-~ is present. With increasing input power the Sill and SiH2-modes at 2100 cm -j due to inner surfaces of the amorphous material appeared. In the crystalline

[cm-11

wavenumber

Fig. 3. Infrared spectra for nc-Si:H-films deposited in a pure silane plasma at different power levels.

region the corresponding peak splits in two sharp modes at 2085 and 2100 c m - t characteristic for Sill and Sill 2 bonded at the surfaces of the nanocrystallites [7]. As a monitor for a change in plasma species we performed optical emission spectroscopy at the same deposition conditions. In Fig. 4 the ratio of the Sill-band (414 nm) to the Ha-line (656 nm) is shown. The strong decrease of the ratio in the amorphous regime ends up at an almost saturated value for the crystalline deposition conditions.

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Fig. 4. Intensity ratio of the optical emission intensities Sill-band (414 nm) and Ha line (656 nm) in a pure silane plasma as a function of power.

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M. Scheib et al. / Journal of Non-Crystalline Solids 198-200 (1996) 895-898

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sis since the electron temperature and therefore the corresponding rate coefficients remain constant over the varied power range. In addition the electron density increases by one order in magnitude up to 2 X 10 ~° cm -3 at 300 W. Thus we can conclude that in our effective E C W R pure silane plasma the necessary amount of atomic hydrogen flux responsible for nanocrystalline formation is delivered by a high dissociation degree of the S i l l 4 molecules.

,

40

5. Conclusion

[sccm]

Fig. 5. Deposition rate for nc-Si:H films as a function of silane f l o w .

3.3. F l o w variation

A second way for an increase of dissociation in a pure ECWR-silane plasma is a reduction of silane flow at constant hf-power input. The X-ray diffraction analyses showed again a relatively sharp phase transition lowering the flow from 30 to 25 sccm. In the crystalline regime deposition rate increases linearly with the silane flow up to a rate of 5.4 , ~ / s (Fig. 5). The observed decrease in the amorphous region could be due to an incomplete dissociation at a high silane flow. The hydrogen content decreases from 11.5% for 35 sccm in the amorphous phase to a constant value of 6% in the crystalline regime. The maximum deposition rate of the crystalline phase we obtained was 6.5 A / s (320 W, 200°C, 38 sccm 100% Sill4, 2.2 X 10 -2 mbar). Taking into account the transmission of the net (see Section 2), a real deposition rate of at least 8 , ~ / s is expected.

Using a new method, the E C W R - P E C V D , it has been shown that hydrogen dilution is not a necessary condition for the deposition of nanocrystalline silicon from a silane plasma. Nc-Si:H films could be obtained at low substrate temperatures (T s < 200°C) and high deposition rates up to 8 A / s in a pure silane plasma. The principle dependence of the phase transition and the resulting structure on deposition parameters was investigated. The formation of the crystalline phase in the E C W R plasma is attributed to a high dissociation degree, resulting in a high hydrogen flux. This technique offers the possibility for a scale up and effective deposition of nc-Si:H in industrial applications.

Acknowledgements

Support by the B M B F is gratefully acknowledged.

References 4. Discussion

From the saturation behaviour of the deposition rate at high power levels for the power variation series we suppose that all the incoming silane gas is consumed by the deposition process. The corresponding strong decrease of the OES intensity ratio has to be interpretated as an increasing amount of excess hydrogen present in the discharge responsible for the high H-line intensity. Recently performed single probe measurements corroborate this diagno-

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