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H2 microwave plasma
Thin Solid Films 515 (2007) 6713 – 6720 www.elsevier.com/locate/tsf
High-rate synthesis of microcrystalline silicon films using high-density SiH4/H2 microwave plasma Haijun Jia ⁎, Jhantu K. Saha, Naoyuki Ohse, Hajime Shirai Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura, Saitama 338–8570, Japan Received 9 March 2006; received in revised form 21 December 2006; accepted 29 January 2007 Available online 9 February 2007
Abstract A high electron density (N 1011 cm− 3) and low electron temperature (1–2 eV) plasma is produced by using a microwave plasma source utilizing a spoke antenna, and is applied for the high-rate synthesis of high quality microcrystalline silicon (μc-Si) films. A very fast deposition rate of ∼ 65 Å/s is achieved at a substrate temperature of 150 °C with a high Raman crystallinity and a low defect density of (1–2) × 1016 cm− 3. Optical emission spectroscopy measurements reveal that emission intensity of SiH and intensity ratio of Hα/SiH are good monitors for film deposition rate and film crystallinity, respectively. A high flux of film deposition precursor and atomic hydrogen under a moderate substrate temperature condition is effective for the fast deposition of highly crystallized μc-Si films without creating additional defects as well as for the improvement of film homogeneity. © 2007 Elsevier B.V. All rights reserved. Keywords: Microcrystalline silicon; Microwave plasma; High-rate synthesis; Low defect density
1. Introduction Microcrystalline silicon (μc-Si) was fabricated for the first time in 1968 by Veprek and Maracek using chemical transport in hydrogen plasma [1]. After the development of μc-Si thinfilm solar cells in 1994, much attention has been given to this material for its promising application in photovoltaic industry as a clean and renewable energy source in 21st century, and remarkable progress has been made [2–6]. The excellent performance of μc-Si film results from its wide-range spectrum sensitivities, enhanced carrier mobility and great stability against light exposure. However, in order to realize sufficient sunlight absorption, a large thickness of over 2 μm of the μc-Si film is required due to its indirect band gap nature. In this regard, for mass production of high performance, low-cost thinfilm solar cells, uniform and fast deposition of high quality μcSi is one of the crucial issues in current research efforts. ⁎ Corresponding author. Current address: Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, 305–8568, Japan. E-mail address: [email protected] (H. Jia). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.01.055
As far as the fabrication of high quality μc-Si film is concerned, to date, parallel-plate capacitively coupled radio frequency SiH4/H2 mixture plasma enhanced chemical vapor deposition (rf-PECVD) is widely used and intensively investigated. It is reported that SiH3, which is generated through the inelastic collision between SiH4 and electron with energy above 8.75 eV and below 9.47 eV, is the dominant precursor responsible for film growth [7,8]. Also, a high hydrogen dilution is necessary for the μc-Si formation [9,10]. However, high flux of H2 during film deposition is supposed to reduce concentration of film precursor and to increase electron temperature in plasma [11]. In addition, it seems that possible detrimental factors for the formation process of μc-Si include: 1) strong high-energy ion bombardment [12,13], 2) contribution of short life-time radicals (e.g. SiH2) produced by impact of electrons with high energy above 9.47 eV [7,14], and 3) higher order silane-related reactive species as well as nano-particles formed in plasma originated for short life-time radicals [15,16]. To increase deposition rate while maintaining high quality of deposited μc-Si films for photovoltaic industrial mass production, high electron density and low electron temperature plasma source without using complex components such as magnetic
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parallel cylindrical rods (spokes) arranged between two parallelground plates. The spokes are resonantly coupled by the stray capacitance between adjacent spokes and the inductance of those spokes themselves. In this system, the 2.45 GHz microwave is introduced into chamber through the spoke antenna and a 15 mm-thick quartz plate. Microwave power is supplied from center of the antenna. No magnetic field is needed for the generation of plasma. Ar gas was used to investigate the plasma characteristics. Electron density, ne, and electron temperature, Te, were measured using a Langmuir probe (single probe, W, size: 0.3 mm × 3 mm) as functions of power and pressure. Radial and axial distributions of ne and Te were also evaluated. The μc-Si film depositions were performed at a distance of z = 6 cm between the quartz plate window and substrate holder, wherein highly crystallized Si films were fabricated as demonstrated in our previous work [20]. The plasma was adjusted to the most possible conditions for μc-Si film formation in our previous experiment: a background pressure of lower than 1.3 × 10− 4 Pa, a microwave power of 700 W and a working pressure of ∼ 10 Pa (80 mTorr). Films were deposited on Corning #7059 (3.5 × 1.5 cm2), quartz glass (2.5 × 1 cm2),
Fig. 1. Schematic drawing of (a) the deposition apparatus and (b) the spoke antenna used in this study. A typical Ar plasma picture is also shown in (a). The chamber size is 22 cm in diameter and the antenna's spoke length is fixed at 4 cm. G1 and G2 source gas supply configurations are also illustrated here.
coil is one of the possible candidates. For this purpose, several plasma sources such as very high frequency plasma, inductively coupled plasma, and surface wave plasma have been developed and used to the μc-Si film deposition [17–19]. In this paper, we demonstrate a microwave plasma (MWP, 2.45 GHz) source utilizing a spoke antenna and its application in μc-Si thin-film fabrication. This discharge produces a high electron density of more than 1011 cm− 3 with uniform spacial density distribution and a low electron temperature of 1–2 eV without using external magnetic field. Highly crystallized μc-Si films with low defect density are fabricated at a high deposition rate of 65 Å/s from this high-density microwave plasma of SiH4–H2 mixture. Effects of SiH4 gas supply method on film deposition rate, film crystallinity and defect density are discussed. Guiding principles for the fast deposition of highly crystallized μc-Si films with low defect density are proposed. 2. Experimental details Fig. 1 (a) and (b) illustrate schematically the deposition apparatus used in this study and the spoke antenna, respectively. Chamber size is 22 cm in diameter. Length of each spoke is 4 cm, or about 1/4 of wavelength at 2.45 GHz. Design of the spoke antenna is based on an interdigital filter composed of
Fig. 2. Effect of (a) microwave power and (b) working pressure on plasma density, ne, and electron temperature, Te, in microwave Ar plasma. Measurements were carried out at axial distance of 6 cm from quartz plate and center of the chamber.
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and crystalline Si (100) substrates from a SiH4–H2 mixture plasma. In this research, two different source gas supply configurations were used, as shown in Fig. 1. The first one, termed as G1, was that SiH4 and H2 mixture gas was fed into the chamber through a ring placed just beneath the quartz plate. The other, termed as G2, was that SiH4 was introduced independently using a showerhead placed 2 cm above substrate holder under a steady flow of H2 plasma from the ring. The deposition parameters included SiH4 concentration R = Fr(SiH4) / [Fr (SiH4) + Fr(H2)] (where Fr is the flow rate) and substrate temperature, Ts. R was varied in a range from 5% to 67% by increasing Fr(SiH4) from 3 to 30 sccm with a constant Fr(H2) of 15 sccm. Plasma diagnostics were performed simultaneously with film depositions using optical emission spectroscopy (OES) as a function of R. OES emission intensity of SiH (414 nm) and emission intensity ratio of Hα (656 nm) to SiH, Hα/SiH, were monitored in order to study their correlations with film deposition rate and crystallinity, respectively. The resulting μc-Si films were characterized using profilometry, Raman spectroscopy (Renishaw-1000), and electron spin resonance (ESR, Bruker EMX-6/1) to evaluate deposition rate, Rd, film crystallinity Ic/Ia, and ESR spin density, Ns. The micro-Raman setup is in backscattering configuration at room temperature with the 514.5 nm line from Ar+ laser as the incident light and a power density of 0.1 mW μm2 on sample surface. Here, Ic/Ia is defined as the ratio of integrated intensity of the Raman peak centered at 520 cm− 1 from the crystalline phase, Ic, to the peak centered at 480 cm− 1 from the amorphous phase, Ia. In addition, spectroscopic ellipsometry (SE) technique was employed to study the microstructure of corresponding μc-Si films.
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higher electron density is observed at an axial distance of Z = 4– 6 cm from the quartz plate accompanied by low electron temperature, while relatively high electron temperature is found near the quartz plate. Moreover, strong electric field is observed around the quartz plate/plasma boundary. This suggests that the electron heating occurs along the plasma boundary near dielectric quartz plate window, while the majority of electrons in the bulk plasma region remain relatively cold since no external heating source exists there [21]. Radial distribution of ne shown in Fig. 3 (b) depicts the excellent spacial uniformity of ne with small variation within ± 10% from the center of chamber to a radial position of 6 cm. High electron density of more than
3. Results 3.1. Plasma characteristics To investigate plasma characteristics, the electron density, ne, and electron temperature, Te, were measured as functions of microwave power and pressure in Ar plasma, as illustrated in Fig. 2 (a) and (b). The measurements were performed at axial distance of z = 6 cm from quartz plate and at center of the chamber by inserting the Langmuir probe into the plasma. ne systematically increases with power, whereas Te is almost unchanged at 1–2 eV in whole power region. A high density of more than 1 × 1011 cm− 3 can be obtained over a wide rage of microwave power, which is due to the efficient power absorption, and/or the confinement of electrons in the microwave plasma. In addition, low Te is almost independent of working pressure up to ∼16 Pa (120 mTorr) with an almost constant ne. This is a great advantage of using this high-density plasma for the thin-film processing. Axial and radial distributions of ne and Te were also measured and the results are summarized in Fig. 3 (a) and (b), respectively. The same probe was also used as an antenna to obtain the microwave intensity distribution in the chamber. Inset in Fig. 3 (a) demonstrates microwave electric field intensity picked up by the probe by assuming that it is proportional to the amplitude of the 2.45 GHz signal, as measured by a spectrum analyzer. A
Fig. 3. (a) Radial and (b) axial distributions of ne and Te in microwave Ar plasma. Inset in (a) demonstrates microwave electric field intensity as a function of axial distance from quartz plate.
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temperature microwave plasma is generated over 18 cm in diameter without using magnetic field. 3.2. Fabrication of μc-Si films 3.2.1. Fast deposition of highly crystallized μc-Si films Fig. 4 (a) shows the SiH4 concentration R dependence of OES emission intensity of SiH and intensity ratio of Hα/SiH. The OES plasma diagnostics were performed simultaneously with film depositions in the SiH4–H2 plasma as a function of R for both G1 and G2 gas supply methods. The SiH emission intensity increases with increasing R for both G1 and G2 gas supply methods, while Hα/SiH intensity ratio decreases with R. In Fig. 4 (b) and (c), film deposition rate Rd and Raman crystallinity Ic/Ia are plotted as functions of SiH emission intensity and Hα/SiH intensity ratio, respectively. A continuous increase in Rd with R, namely SiH emission intensity, at a constant pressure of ∼10 Pa (80 mTorr) in both gas supply configurations implies that the SiH4 is almost completely decomposed in the plasma and Rd is limited partially by the amount of SiH4 introduced into chamber. In comparison with Rd in G1 configuration, Rd in G2 configuration significantly increased by a factor of 2 times. The SiH emission intensity in G2 configuration is also almost 2 times larger than that in G1 configuration for same R condition, suggesting that SiH emission intensity is a possible monitor for film deposition rate, and consequently a measurement of film deposition precursor (since film deposition rate is linearly proportional to precursor density). In addition, a good correlation between OES intensity ratio Hα/SiH and Raman crystallinity Ic/Ia is demonstrated, the Ic/Ia increases with a increase in Hα/SiH ratio (a decrease in R). A high deposition rate of 65 Å/s was achieved while maintaining a high Raman crystallinity Ic/Ia above 3.5 despite a high R condition of 67% when using the G2
Fig. 4. (a) The OES emission intensity of SiH (414 nm) and intensity ratio of Hα (656 nm)/SiH plotted as a function of SiH4 concentration R for both G1 and G2 source gas supply configurations. (b) Film deposition rate and Raman crystallinity, Ic/Ia, are shown as functions of SiH emission intensity and Hα/ SiH intensity ratio, respectively. The Fr(H2) and working pressure were constant at 15 sccm and 80 mTorr, respectively, under a substrate temperature Ts of 250 °C.
1011 cm− 3 ranges from the center up to 9 cm in radial direction, indicating that the microwave power can be effectively and uniformly coupled into the discharge by using the spoke antenna. As a consequence, a uniform, high-density and low-
Fig. 5. ESR spin density Ns for corresponding μc-Si films plotted against SiH emission intensity. Insert shows the Ts dependence of Ns for μc-Si films prepared at Fr(SiH4)/Fr(H2) = 5/15 sccm and working pressure of 80 mTorr using this microwave plasma and the reference data from a conventional rf-PECVD.
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Fig. 6. (a) Typical evolution spectra for substrate holder surface temperature and set temperature during plasma process with H2 of 15 sccm, working pressure of 80 mTorr and microwave power of 700 W. (b) Substrate holder surface temperature and set temperature as a function of MW power with H2 of 15 sccm and pressure of 80 mTorr.
method. Size of crystallites in corresponding film calculated using Scherrer formula from the (220) diffraction peak in XRD spectrum was about 100 Å. No saturation trend could be observed for Rd, suggesting further higher deposition rate can be expected. 3.2.2. Reduction of defect density in μc-Si films Fig. 5 shows the ESR spin density, Ns, for corresponding μcSi films fabricated using both G1 and G2 gas supply methods at Ts of 250 °C, plotted as a function of SiH emission intensity (namely the density of deposition precursor). For all the samples, film thickness was ∼2 μm and the ESR measurements were performed directly on those films. It is to be noted that Ns decreases by about one order of magnitude with a increase in SiH emission intensity when the showerhead (in G2 configuration) was used compared with G1 method, despite the other deposition conditions being the same, indicating that highdensity film precursor helps to eliminate defects in deposited films. However, Ns is almost independent of SiH emission intensity on the order of (3–4) × 1016 cm− 3 in G2 method, which is still one order of magnitude larger than that of high quality μc-Si films reported elsewhere [22].
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The inset in Fig. 5 shows Ts dependence of ESR spin density in μc-Si films prepared at Fr(SiH4)/Fr(H2) = 5/15 sccm without the use of the showerhead. The reference data for μc-Si films fabricated using rf-PECVD under optimized conditions are also included [23]. For the MWP case, Ns increases monotonically with Ts, which is contrary to the result of conventional rf plasma where a minimum Ns appears around 250 °C. This suggests that the real temperature of film growing surface is higher than the monitored value because of surface heating effect by plasma [24,25]. To confirm this, temperature of substrate holder surface during plasma process was measured by attaching another thermal couple directly on surface of substrate holder. Temperature measured by the second thermal couple can not give exact value of the surface temperature, but it does provide useful information about the change in temperature. Fig. 6 (a) presents typical temperature evolution spectra for both the bottom thermal couple (set temperature) and the top thermal couple (surface temperature). The measurement was performed using H2 gas of 15 sccm and working pressure of ∼ 10 Pa (80 mTorr) with MW power of 700 W. It is clearly observed that the surface temperature raises rapidly just after the plasma ignition, and after ∼ 100 s it arrives saturation value of ∼ 280 °C while the set temperature keeping at 220 °C. High microwave power made the surface temperature more higher, as shown in Fig. 6 (b). This clarifies that the surface temperature of substrate holder is higher than the set substrate temperature value. This effect results in the promotion of the abstraction of film surface covering H and the creation of dangling bonds during film deposition process even when Ts is below 300 °C. A sufficient supply of SiH3 and atomic H is effective in terminating the generated dangling bonds as in the G2 configuration case. A lower Ts is also one of the probable conditions for further reducing Ns. Consequently, the low Ts of 150 °C in G2 method reduced Ns to as low as (1–2) × 1016 cm− 3, although Raman crystallinity of the film slightly deteriorated with Ic/Ia ∼ 2.5 while maintaining a high deposition rate of 65 Å/s. Dark- and photo-conductivities of the corresponding μc-Si films under 100 mW/cm2 white light exposure were on the order of 10− 7 S/ cm and N 1 × 10− 5 S/cm, respectively. 3.2.3. Microstructure of μc-Si films As has been reported, μc-Si films share common features of growth dynamics, that is a nonequilibrium growth process [26– 28]. It is thought that the formation of μc-Si proceeds through four stages: incubation, nucleation, growth, and steady stage. As a result, a multilayer structure in the direction of growth may result. Spectroscopic ellipsometry has been verified as a powerful tool for the analysis of complex structure of deposited μc-Si films [26,29]. In this study, we also employed the SE technique to discriminate microstructure of μc-Si films fabricated by using the high-density microwave plasma. The imaginary parts of the pseudo-dielectric function bϵ2N spectra of ∼ 2 μm μc-Si films fabricated for different R conditions by using G2 gas supply method are shown in Fig. 7 (a). One can clearly identify the appearance of fine structures at 3.4 eV and 4.2 eV (attributed to the E1 and E2 optical band transitions, respectively), as being a signature of the presence of
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to enhanced reaction between H and a-Si incubation layer. The best fit of volume fractions of c-Si, fc-Si, and voids, fvoid, in the bulk are summarized in Fig. 7 (b) for the corresponding μc-Si samples. No marked decrease in fc-Si in the bulk layer is observed with R up to 67%, which is in the range of 65%–75%. The fvoid in the bulk decreases markedly when R is over 40%. 4. Discussion
Fig. 7. (a) Imaginary parts of pseudo-dielectric function bϵ2N spectra for the resulting μc-Si films fabricated using G2 method as shown in Fig. 4. (b) Changes in fc-Si and fvoid in bulk layer with SiH4 concentration R for some corresponding μc-Si films shown in (a).
crystalline phase [30,31]. This confirmed the formation of μc-Si even for high SiH4 concentrations. Amplitude of the bϵ2N spectrum depends on the crystalline volume fraction, crystallite size, surface roughness and film porosity [27]. In Fig. 7 (a), the magnitude of bϵ2N increases markedly with increasing R, which can be interpreted in terms of improved bulk homogeneity and a lower degree of surface roughness. To better understand film microstructure, some of the bϵ2N spectra were analyzed using a fitting procedure with the Bruggeman effective medium approximation (BEMA) and a four layer model (for detail, see Ref. [32]). This approach, based on the assumption of the linearity of the optical response of a mixed phase material, gives a unique opportunity to define the film structure as expressed in terms of the volume fraction of its different constituents. The fitting results demonstrated that between the initial a-Si incubation layer and the stable bulk layer, there is an interface layer with a high voids volume fraction [32]. Similar report was also given by A. Fontcuberta et al. [33]. In addition, H. Fujiwara et al. reported that an porous amorphous Si:H interface layer with a large amount of SiH2 bonds was formed in μc-Si growth on ZnO substrate [34]. The nucleation process of μc-Si growth is considered to take place in this layer, which is resulted from high concentration and efficient diffusion of atomic H in the initial growth stage leading
A high-electron-density, low-electron-temperature and uniform microwave plasma source utilizing a spoke antenna was demonstrated. It is possible to simply scale up the chamber by modifying the design of the spoke antenna, which shows a great potential for the large-area semiconductor material processing. The fast deposition of highly crystallized μc-Si films with low defect density was realized by using this microwave plasma source in this research. In this study, source gases were introduced by using two different gas supply configurations. When SiH4 was fed independently from the showerhead 2 cm above the substrate holder, Rd was markedly promoted without any degradation in Raman crystallinity. This result is quite different from those of conventional rf-PECVD, in which the μc-Si films are commonly formed only when R is less than 10%. The formation of μc-Si film even for high SiH4 concentration in this experiment is attributed to efficient dissociation of H2 and SiH4 due to the high electron density, providing sufficient atomic hydrogen. Short residence time of SiH4 also suppresses second reaction of H with SiH4 (H + SiH4 → H2 + SiH3), which annihilates atomic hydrogen [32]. On the other hand, the generation rate of film deposition precursors (such as SiH3), d [X]/dt, is determined by the product of electron density and number density of SiH4 as follows [35]: d ½X =dt ¼ rth ½SiH4 Ne
ð1Þ
where σ is the dissociation cross section of silane with electrons having higher energy than the threshold value in the plasma, υth is the thermal velocity of electrons, [SiH4] is the number density of SiH4 molecules, and Ne is the number density of energetic electrons responsible for the reaction. Large SiH3 density is achieved owing to high Ne in this microwave plasma source. However, even if a large number density of SiH3 is generated, the film deposition rate still depends on the position of substrate holder, because it is also determined by the diffusion loss of SiH3 during its transportation to the growth surface. Therefore, supply of SiH4 near substrate holder, wherein higher ne was also observed, is effective in suppressing the diffusion loss of SiH3 and in promoting dissociation efficiency of SiH4. Consequently, as has been shown, the SiH emission intensity (a possible indicator of precursor density) was almost 2 times stronger (G2 configuration) than that for the gas supply from a ring set just beneath the quartz window plate (G1 configuration). The Ns in μc-Si films is one of the most important structural properties, particularly for solar cell applications, because dangling bonds form deeply localized electronic states in the band gap and act as recombination centers for photoexcited
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carriers. Therefore, a lower Ns is one of the crucial demands while maintaining a high crystallinity. As is well known, Ns shows a minimum of (2–3) × 1015 cm− 3 at Ts of ∼ 250 °C and tends to increase at Ts both below and above ∼250 °C in the conventional rf plasma of a SiH4–H2 mixture [23]. However, as shown in the inset of Fig. 5 in the case of high-density microwave plasma, Ns increased with Ts even at Ts below 300 °C due to plasma heating, which indicated that the thermal abstraction of H at the growth surface was promoted for all Ts values. This causes the generation of dangling bonds on the growing surface. According to the surface diffusion model of SiH3 [7], these steady-state surface dangling bonds can be incorporated into the bulk layer, resulting in an increase in number density of defects in resulting films. The steady-state dangling bond density on growing surface is determined by a balance between the generation and saturation rates of dangling bonds. Therefore, it is expected that Nd can be reduced and Rd can be promoted by sufficient introduction of SiH3 (precursorassisted defect suppression) and atomic hydrogen on the film growing surface. On the one hand, generation rate of dangling bonds accelerated by the plasma heating will provide sufficient growth sites (dangling bonds) for the attachment of successive SiH3 to form Si–Si bonds contributing to film growth. Thus, by supplying SiH4 near the growth surface using a showerhead in G2 method, a large number density of SiH3 on the surface saturates the dangling bonds rather effectively, resulting in a significantly enhanced deposition rate and a remarkably reduced defect density in the films than G1 method. In addition, degree of film homogeneity improves with increasing R, which also could be attributed to sufficient termination of dangling bonds on film growing surface by the large amount of film precursor SiH3 arriving on the surface. On the other hand, however, Ns was almost independent of R in G2 method, which was also due to enhanced H desorption caused by the plasma heating. The use of lower Ts could help overcome this difficulty. Lowering Ts and wall temperature also tends to decrease the electron temperature Te, which promotes the preferential growth of (220) orientation beneficial for solar cell performance [36,37]. As a result, Ns was reduced to (1–2) × 1016 cm− 3 while maintaining high deposition rate at a Ts of 150 °C. 5. Conclusions A high electron density of (1–2) × 1011 cm− 3 and a low electron temperature of 1–2 eV (in Ar plasma) were achieved using a microwave plasma source utilizing a spoke antenna. We have applied this plasma to the high-rate synthesis of μc-Si films and proposed guiding principles beneficial for the improvement in film properties. OES measurements revealed that SiH emission intensity and intensity ratio of Hα/SiH were good monitors for the film deposition rate and film crystallinity, respectively. Contrary to the case of a conventional rf plasma, Ns increased with increasing Ts even at Ts below 300 °C in this high-density plasma, suggesting that the real temperature of film growing surface was higher than the monitored value and the abstraction of atomic hydrogen of growing surface was accelerated. A combination of a sufficient supply of SiH3 and
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atomic hydrogen on film growing surface under a lower substrate temperature is effective for fast deposition of highly crystallized μc-Si films without creating additional defects, and for enhancement in film homogeneity and surface smoothness. As a consequence, fast deposition was realized at a very high deposition rate of ∼ 65 Å/s for μc-Si film growth at a Ts of 150 °C with a high Raman crystallinity Ic/Ia N 2.5 and a low defect density of (1–2) × 1016 cm− 3. Acknowledgement The authors thank Dr. T. Kobayashi of the Institute of Physics and Chemical Research (RIKEN) for allowing the use of microRaman spectrometer. References [1] S. Veprek, V. Marecek, Solid-State Electron. 11 (1968) 683. [2] J. Meier, R. Fluckiger, H. Keppner, A. Shah, Appl. Phys. Lett. 65 (1994) 860. [3] R.B. Bergmann, Appl. Phys., A 69 (1999) 155. [4] H. Keppner, J. Meier, P. Torres, D. Fischer, A. Shah, Appl. Phys., A 69 (1999) 169. [5] A. Shah, P. Torres, R. Tscharner, N. Wyrsch, H. Keppner, Science 285 (1999) 692. [6] T. Matsui, M. Kondo, A. Matsuda, Jpn. J. Appl. Phys. 42 (2003) L901. [7] A. Matsuda, J. Non-Cryst. Solids 16 (1998) 365. [8] R. Dewarrat, J. Robertson, Thin Solid Films 427 (1993) 11. [9] A.S. Ferlcuto, R.J. Koval, C.R. Wronski, R.W. Collins, J. Non-Cryst. Solids 299–302 (2002) 68. [10] A.L. Baia Neto, A. Lambert, R. Carius, F. Finger, J. Non-Cryst. Solids 299–302 (2002) 274. [11] A. Matsuda, J. Vac. Sci. Technol., A 16 (1998) 365. [12] M. Kondo, M. Fukawa, L. Guo, A. Matsuda, J. Non-Cryst. Solids 266–269 (2000) 84. [13] E.A.G. Hamers, A. Fontcuberta i Morral, C. Niikura, R. Brenot, P. Roca i Cabarrocas, J. Appl. Phys. 88 (2000) 3674. [14] J.-L. Guizot, K. Nomoto, A. Matsuda, Surf. Sci. 244 (1991) 22. [15] M. Takai, T. Nishimoto, T. Takagi, M. Kondo, A. Matsuda, J. Non-Cryst. Solids 266–269 (2000) 90. [16] M. Shiratani, T. Fukuzama, Y. Watanabe, Jpn. J. Appl. Phys. 38 (1999) 306 (and references therein). [17] A. Shah, E. Vallat-Sauvain, P. Torres, J. Meier, U. Kroll, C. Hof, C. Droz, M. Goerlitzer, N. Wyrsch, M. Vanecek, Mater. Sci. Eng., B 69–70 (2000) 219. [18] J.H. Wu, J.M. Shieh, B.T. Dai, Y.S. Wu, Electrochem. Solid-State Lett. 7 (2004) G128. [19] K. Goshima, H. Toyoda, T. Kojima, M. Nishitani, M. Kitagawa, H. Sugai, Jpn. J. Appl. Phys. 38 (1999) 3655. [20] H. Shirai, Y. Sakuma, Y. Moriya, C. Fukai, H. Ueyama, Jpn. J. Appl. Phys. 38 (1999) 6629. [21] H. Sugai, I. Ghanashev, K. Mizuno, Appl. Phys. Lett. 77 (2000) 3523. [22] G. Ganguly, A. Matsuda, Phys. Rev., B 47 (1993) 3661. [23] A. Matsuda, Jpn. J. Appl. Phys. 43 (2004) 7909. [24] D. Daineka, P. Bulkin, G. Girard, J.-E. Bouree, B. Drevillon, Eur. Phys. J. Appl. Phys. 26 (2004) 3. [25] C. Niikura, M. Kondo, A. Matsuda, J. Non-Cryst. Solids 338–340 (2004) 42. [26] M. Losurdo, R. Rizzoli, C. Summonte, G. Cicala, P. Capezzuto, G. Bruno, J. Appl. Phys. 88 (2000) 2408. [27] M. Fang, B. Drevillon, J. Appl. Phys. 70 (1991) 4894. [28] S. Hanma, P. Roca i Cabarrocas, J. Appl. Phys. 81 (1997) 7282. [29] J. Koh, Y. Lee, H. Fujiwara, C.R. Wronski, R.W. Collins, Appl. Phys. Lett. 73 (1998) 1526.
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High-rate synthesis of microcrystalline silicon films using high-density SiH4/H2 microwave plasma Haijun Jia ⁎, Jhantu K. Saha, Naoyuki Ohse, Hajime Shirai Graduate School of Science and Engineering, Saitama University, 255 Shimo-Okubo, Sakura, Saitama 338–8570, Japan Received 9 March 2006; received in revised form 21 December 2006; accepted 29 January 2007 Available online 9 February 2007
Abstract A high electron density (N 1011 cm− 3) and low electron temperature (1–2 eV) plasma is produced by using a microwave plasma source utilizing a spoke antenna, and is applied for the high-rate synthesis of high quality microcrystalline silicon (μc-Si) films. A very fast deposition rate of ∼ 65 Å/s is achieved at a substrate temperature of 150 °C with a high Raman crystallinity and a low defect density of (1–2) × 1016 cm− 3. Optical emission spectroscopy measurements reveal that emission intensity of SiH and intensity ratio of Hα/SiH are good monitors for film deposition rate and film crystallinity, respectively. A high flux of film deposition precursor and atomic hydrogen under a moderate substrate temperature condition is effective for the fast deposition of highly crystallized μc-Si films without creating additional defects as well as for the improvement of film homogeneity. © 2007 Elsevier B.V. All rights reserved. Keywords: Microcrystalline silicon; Microwave plasma; High-rate synthesis; Low defect density
1. Introduction Microcrystalline silicon (μc-Si) was fabricated for the first time in 1968 by Veprek and Maracek using chemical transport in hydrogen plasma [1]. After the development of μc-Si thinfilm solar cells in 1994, much attention has been given to this material for its promising application in photovoltaic industry as a clean and renewable energy source in 21st century, and remarkable progress has been made [2–6]. The excellent performance of μc-Si film results from its wide-range spectrum sensitivities, enhanced carrier mobility and great stability against light exposure. However, in order to realize sufficient sunlight absorption, a large thickness of over 2 μm of the μc-Si film is required due to its indirect band gap nature. In this regard, for mass production of high performance, low-cost thinfilm solar cells, uniform and fast deposition of high quality μcSi is one of the crucial issues in current research efforts. ⁎ Corresponding author. Current address: Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, 305–8568, Japan. E-mail address: [email protected] (H. Jia). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.01.055
As far as the fabrication of high quality μc-Si film is concerned, to date, parallel-plate capacitively coupled radio frequency SiH4/H2 mixture plasma enhanced chemical vapor deposition (rf-PECVD) is widely used and intensively investigated. It is reported that SiH3, which is generated through the inelastic collision between SiH4 and electron with energy above 8.75 eV and below 9.47 eV, is the dominant precursor responsible for film growth [7,8]. Also, a high hydrogen dilution is necessary for the μc-Si formation [9,10]. However, high flux of H2 during film deposition is supposed to reduce concentration of film precursor and to increase electron temperature in plasma [11]. In addition, it seems that possible detrimental factors for the formation process of μc-Si include: 1) strong high-energy ion bombardment [12,13], 2) contribution of short life-time radicals (e.g. SiH2) produced by impact of electrons with high energy above 9.47 eV [7,14], and 3) higher order silane-related reactive species as well as nano-particles formed in plasma originated for short life-time radicals [15,16]. To increase deposition rate while maintaining high quality of deposited μc-Si films for photovoltaic industrial mass production, high electron density and low electron temperature plasma source without using complex components such as magnetic
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parallel cylindrical rods (spokes) arranged between two parallelground plates. The spokes are resonantly coupled by the stray capacitance between adjacent spokes and the inductance of those spokes themselves. In this system, the 2.45 GHz microwave is introduced into chamber through the spoke antenna and a 15 mm-thick quartz plate. Microwave power is supplied from center of the antenna. No magnetic field is needed for the generation of plasma. Ar gas was used to investigate the plasma characteristics. Electron density, ne, and electron temperature, Te, were measured using a Langmuir probe (single probe, W, size: 0.3 mm × 3 mm) as functions of power and pressure. Radial and axial distributions of ne and Te were also evaluated. The μc-Si film depositions were performed at a distance of z = 6 cm between the quartz plate window and substrate holder, wherein highly crystallized Si films were fabricated as demonstrated in our previous work [20]. The plasma was adjusted to the most possible conditions for μc-Si film formation in our previous experiment: a background pressure of lower than 1.3 × 10− 4 Pa, a microwave power of 700 W and a working pressure of ∼ 10 Pa (80 mTorr). Films were deposited on Corning #7059 (3.5 × 1.5 cm2), quartz glass (2.5 × 1 cm2),
Fig. 1. Schematic drawing of (a) the deposition apparatus and (b) the spoke antenna used in this study. A typical Ar plasma picture is also shown in (a). The chamber size is 22 cm in diameter and the antenna's spoke length is fixed at 4 cm. G1 and G2 source gas supply configurations are also illustrated here.
coil is one of the possible candidates. For this purpose, several plasma sources such as very high frequency plasma, inductively coupled plasma, and surface wave plasma have been developed and used to the μc-Si film deposition [17–19]. In this paper, we demonstrate a microwave plasma (MWP, 2.45 GHz) source utilizing a spoke antenna and its application in μc-Si thin-film fabrication. This discharge produces a high electron density of more than 1011 cm− 3 with uniform spacial density distribution and a low electron temperature of 1–2 eV without using external magnetic field. Highly crystallized μc-Si films with low defect density are fabricated at a high deposition rate of 65 Å/s from this high-density microwave plasma of SiH4–H2 mixture. Effects of SiH4 gas supply method on film deposition rate, film crystallinity and defect density are discussed. Guiding principles for the fast deposition of highly crystallized μc-Si films with low defect density are proposed. 2. Experimental details Fig. 1 (a) and (b) illustrate schematically the deposition apparatus used in this study and the spoke antenna, respectively. Chamber size is 22 cm in diameter. Length of each spoke is 4 cm, or about 1/4 of wavelength at 2.45 GHz. Design of the spoke antenna is based on an interdigital filter composed of
Fig. 2. Effect of (a) microwave power and (b) working pressure on plasma density, ne, and electron temperature, Te, in microwave Ar plasma. Measurements were carried out at axial distance of 6 cm from quartz plate and center of the chamber.
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and crystalline Si (100) substrates from a SiH4–H2 mixture plasma. In this research, two different source gas supply configurations were used, as shown in Fig. 1. The first one, termed as G1, was that SiH4 and H2 mixture gas was fed into the chamber through a ring placed just beneath the quartz plate. The other, termed as G2, was that SiH4 was introduced independently using a showerhead placed 2 cm above substrate holder under a steady flow of H2 plasma from the ring. The deposition parameters included SiH4 concentration R = Fr(SiH4) / [Fr (SiH4) + Fr(H2)] (where Fr is the flow rate) and substrate temperature, Ts. R was varied in a range from 5% to 67% by increasing Fr(SiH4) from 3 to 30 sccm with a constant Fr(H2) of 15 sccm. Plasma diagnostics were performed simultaneously with film depositions using optical emission spectroscopy (OES) as a function of R. OES emission intensity of SiH (414 nm) and emission intensity ratio of Hα (656 nm) to SiH, Hα/SiH, were monitored in order to study their correlations with film deposition rate and crystallinity, respectively. The resulting μc-Si films were characterized using profilometry, Raman spectroscopy (Renishaw-1000), and electron spin resonance (ESR, Bruker EMX-6/1) to evaluate deposition rate, Rd, film crystallinity Ic/Ia, and ESR spin density, Ns. The micro-Raman setup is in backscattering configuration at room temperature with the 514.5 nm line from Ar+ laser as the incident light and a power density of 0.1 mW μm2 on sample surface. Here, Ic/Ia is defined as the ratio of integrated intensity of the Raman peak centered at 520 cm− 1 from the crystalline phase, Ic, to the peak centered at 480 cm− 1 from the amorphous phase, Ia. In addition, spectroscopic ellipsometry (SE) technique was employed to study the microstructure of corresponding μc-Si films.
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higher electron density is observed at an axial distance of Z = 4– 6 cm from the quartz plate accompanied by low electron temperature, while relatively high electron temperature is found near the quartz plate. Moreover, strong electric field is observed around the quartz plate/plasma boundary. This suggests that the electron heating occurs along the plasma boundary near dielectric quartz plate window, while the majority of electrons in the bulk plasma region remain relatively cold since no external heating source exists there [21]. Radial distribution of ne shown in Fig. 3 (b) depicts the excellent spacial uniformity of ne with small variation within ± 10% from the center of chamber to a radial position of 6 cm. High electron density of more than
3. Results 3.1. Plasma characteristics To investigate plasma characteristics, the electron density, ne, and electron temperature, Te, were measured as functions of microwave power and pressure in Ar plasma, as illustrated in Fig. 2 (a) and (b). The measurements were performed at axial distance of z = 6 cm from quartz plate and at center of the chamber by inserting the Langmuir probe into the plasma. ne systematically increases with power, whereas Te is almost unchanged at 1–2 eV in whole power region. A high density of more than 1 × 1011 cm− 3 can be obtained over a wide rage of microwave power, which is due to the efficient power absorption, and/or the confinement of electrons in the microwave plasma. In addition, low Te is almost independent of working pressure up to ∼16 Pa (120 mTorr) with an almost constant ne. This is a great advantage of using this high-density plasma for the thin-film processing. Axial and radial distributions of ne and Te were also measured and the results are summarized in Fig. 3 (a) and (b), respectively. The same probe was also used as an antenna to obtain the microwave intensity distribution in the chamber. Inset in Fig. 3 (a) demonstrates microwave electric field intensity picked up by the probe by assuming that it is proportional to the amplitude of the 2.45 GHz signal, as measured by a spectrum analyzer. A
Fig. 3. (a) Radial and (b) axial distributions of ne and Te in microwave Ar plasma. Inset in (a) demonstrates microwave electric field intensity as a function of axial distance from quartz plate.
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temperature microwave plasma is generated over 18 cm in diameter without using magnetic field. 3.2. Fabrication of μc-Si films 3.2.1. Fast deposition of highly crystallized μc-Si films Fig. 4 (a) shows the SiH4 concentration R dependence of OES emission intensity of SiH and intensity ratio of Hα/SiH. The OES plasma diagnostics were performed simultaneously with film depositions in the SiH4–H2 plasma as a function of R for both G1 and G2 gas supply methods. The SiH emission intensity increases with increasing R for both G1 and G2 gas supply methods, while Hα/SiH intensity ratio decreases with R. In Fig. 4 (b) and (c), film deposition rate Rd and Raman crystallinity Ic/Ia are plotted as functions of SiH emission intensity and Hα/SiH intensity ratio, respectively. A continuous increase in Rd with R, namely SiH emission intensity, at a constant pressure of ∼10 Pa (80 mTorr) in both gas supply configurations implies that the SiH4 is almost completely decomposed in the plasma and Rd is limited partially by the amount of SiH4 introduced into chamber. In comparison with Rd in G1 configuration, Rd in G2 configuration significantly increased by a factor of 2 times. The SiH emission intensity in G2 configuration is also almost 2 times larger than that in G1 configuration for same R condition, suggesting that SiH emission intensity is a possible monitor for film deposition rate, and consequently a measurement of film deposition precursor (since film deposition rate is linearly proportional to precursor density). In addition, a good correlation between OES intensity ratio Hα/SiH and Raman crystallinity Ic/Ia is demonstrated, the Ic/Ia increases with a increase in Hα/SiH ratio (a decrease in R). A high deposition rate of 65 Å/s was achieved while maintaining a high Raman crystallinity Ic/Ia above 3.5 despite a high R condition of 67% when using the G2
Fig. 4. (a) The OES emission intensity of SiH (414 nm) and intensity ratio of Hα (656 nm)/SiH plotted as a function of SiH4 concentration R for both G1 and G2 source gas supply configurations. (b) Film deposition rate and Raman crystallinity, Ic/Ia, are shown as functions of SiH emission intensity and Hα/ SiH intensity ratio, respectively. The Fr(H2) and working pressure were constant at 15 sccm and 80 mTorr, respectively, under a substrate temperature Ts of 250 °C.
1011 cm− 3 ranges from the center up to 9 cm in radial direction, indicating that the microwave power can be effectively and uniformly coupled into the discharge by using the spoke antenna. As a consequence, a uniform, high-density and low-
Fig. 5. ESR spin density Ns for corresponding μc-Si films plotted against SiH emission intensity. Insert shows the Ts dependence of Ns for μc-Si films prepared at Fr(SiH4)/Fr(H2) = 5/15 sccm and working pressure of 80 mTorr using this microwave plasma and the reference data from a conventional rf-PECVD.
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Fig. 6. (a) Typical evolution spectra for substrate holder surface temperature and set temperature during plasma process with H2 of 15 sccm, working pressure of 80 mTorr and microwave power of 700 W. (b) Substrate holder surface temperature and set temperature as a function of MW power with H2 of 15 sccm and pressure of 80 mTorr.
method. Size of crystallites in corresponding film calculated using Scherrer formula from the (220) diffraction peak in XRD spectrum was about 100 Å. No saturation trend could be observed for Rd, suggesting further higher deposition rate can be expected. 3.2.2. Reduction of defect density in μc-Si films Fig. 5 shows the ESR spin density, Ns, for corresponding μcSi films fabricated using both G1 and G2 gas supply methods at Ts of 250 °C, plotted as a function of SiH emission intensity (namely the density of deposition precursor). For all the samples, film thickness was ∼2 μm and the ESR measurements were performed directly on those films. It is to be noted that Ns decreases by about one order of magnitude with a increase in SiH emission intensity when the showerhead (in G2 configuration) was used compared with G1 method, despite the other deposition conditions being the same, indicating that highdensity film precursor helps to eliminate defects in deposited films. However, Ns is almost independent of SiH emission intensity on the order of (3–4) × 1016 cm− 3 in G2 method, which is still one order of magnitude larger than that of high quality μc-Si films reported elsewhere [22].
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The inset in Fig. 5 shows Ts dependence of ESR spin density in μc-Si films prepared at Fr(SiH4)/Fr(H2) = 5/15 sccm without the use of the showerhead. The reference data for μc-Si films fabricated using rf-PECVD under optimized conditions are also included [23]. For the MWP case, Ns increases monotonically with Ts, which is contrary to the result of conventional rf plasma where a minimum Ns appears around 250 °C. This suggests that the real temperature of film growing surface is higher than the monitored value because of surface heating effect by plasma [24,25]. To confirm this, temperature of substrate holder surface during plasma process was measured by attaching another thermal couple directly on surface of substrate holder. Temperature measured by the second thermal couple can not give exact value of the surface temperature, but it does provide useful information about the change in temperature. Fig. 6 (a) presents typical temperature evolution spectra for both the bottom thermal couple (set temperature) and the top thermal couple (surface temperature). The measurement was performed using H2 gas of 15 sccm and working pressure of ∼ 10 Pa (80 mTorr) with MW power of 700 W. It is clearly observed that the surface temperature raises rapidly just after the plasma ignition, and after ∼ 100 s it arrives saturation value of ∼ 280 °C while the set temperature keeping at 220 °C. High microwave power made the surface temperature more higher, as shown in Fig. 6 (b). This clarifies that the surface temperature of substrate holder is higher than the set substrate temperature value. This effect results in the promotion of the abstraction of film surface covering H and the creation of dangling bonds during film deposition process even when Ts is below 300 °C. A sufficient supply of SiH3 and atomic H is effective in terminating the generated dangling bonds as in the G2 configuration case. A lower Ts is also one of the probable conditions for further reducing Ns. Consequently, the low Ts of 150 °C in G2 method reduced Ns to as low as (1–2) × 1016 cm− 3, although Raman crystallinity of the film slightly deteriorated with Ic/Ia ∼ 2.5 while maintaining a high deposition rate of 65 Å/s. Dark- and photo-conductivities of the corresponding μc-Si films under 100 mW/cm2 white light exposure were on the order of 10− 7 S/ cm and N 1 × 10− 5 S/cm, respectively. 3.2.3. Microstructure of μc-Si films As has been reported, μc-Si films share common features of growth dynamics, that is a nonequilibrium growth process [26– 28]. It is thought that the formation of μc-Si proceeds through four stages: incubation, nucleation, growth, and steady stage. As a result, a multilayer structure in the direction of growth may result. Spectroscopic ellipsometry has been verified as a powerful tool for the analysis of complex structure of deposited μc-Si films [26,29]. In this study, we also employed the SE technique to discriminate microstructure of μc-Si films fabricated by using the high-density microwave plasma. The imaginary parts of the pseudo-dielectric function bϵ2N spectra of ∼ 2 μm μc-Si films fabricated for different R conditions by using G2 gas supply method are shown in Fig. 7 (a). One can clearly identify the appearance of fine structures at 3.4 eV and 4.2 eV (attributed to the E1 and E2 optical band transitions, respectively), as being a signature of the presence of
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to enhanced reaction between H and a-Si incubation layer. The best fit of volume fractions of c-Si, fc-Si, and voids, fvoid, in the bulk are summarized in Fig. 7 (b) for the corresponding μc-Si samples. No marked decrease in fc-Si in the bulk layer is observed with R up to 67%, which is in the range of 65%–75%. The fvoid in the bulk decreases markedly when R is over 40%. 4. Discussion
Fig. 7. (a) Imaginary parts of pseudo-dielectric function bϵ2N spectra for the resulting μc-Si films fabricated using G2 method as shown in Fig. 4. (b) Changes in fc-Si and fvoid in bulk layer with SiH4 concentration R for some corresponding μc-Si films shown in (a).
crystalline phase [30,31]. This confirmed the formation of μc-Si even for high SiH4 concentrations. Amplitude of the bϵ2N spectrum depends on the crystalline volume fraction, crystallite size, surface roughness and film porosity [27]. In Fig. 7 (a), the magnitude of bϵ2N increases markedly with increasing R, which can be interpreted in terms of improved bulk homogeneity and a lower degree of surface roughness. To better understand film microstructure, some of the bϵ2N spectra were analyzed using a fitting procedure with the Bruggeman effective medium approximation (BEMA) and a four layer model (for detail, see Ref. [32]). This approach, based on the assumption of the linearity of the optical response of a mixed phase material, gives a unique opportunity to define the film structure as expressed in terms of the volume fraction of its different constituents. The fitting results demonstrated that between the initial a-Si incubation layer and the stable bulk layer, there is an interface layer with a high voids volume fraction [32]. Similar report was also given by A. Fontcuberta et al. [33]. In addition, H. Fujiwara et al. reported that an porous amorphous Si:H interface layer with a large amount of SiH2 bonds was formed in μc-Si growth on ZnO substrate [34]. The nucleation process of μc-Si growth is considered to take place in this layer, which is resulted from high concentration and efficient diffusion of atomic H in the initial growth stage leading
A high-electron-density, low-electron-temperature and uniform microwave plasma source utilizing a spoke antenna was demonstrated. It is possible to simply scale up the chamber by modifying the design of the spoke antenna, which shows a great potential for the large-area semiconductor material processing. The fast deposition of highly crystallized μc-Si films with low defect density was realized by using this microwave plasma source in this research. In this study, source gases were introduced by using two different gas supply configurations. When SiH4 was fed independently from the showerhead 2 cm above the substrate holder, Rd was markedly promoted without any degradation in Raman crystallinity. This result is quite different from those of conventional rf-PECVD, in which the μc-Si films are commonly formed only when R is less than 10%. The formation of μc-Si film even for high SiH4 concentration in this experiment is attributed to efficient dissociation of H2 and SiH4 due to the high electron density, providing sufficient atomic hydrogen. Short residence time of SiH4 also suppresses second reaction of H with SiH4 (H + SiH4 → H2 + SiH3), which annihilates atomic hydrogen [32]. On the other hand, the generation rate of film deposition precursors (such as SiH3), d [X]/dt, is determined by the product of electron density and number density of SiH4 as follows [35]: d ½X =dt ¼ rth ½SiH4 Ne
ð1Þ
where σ is the dissociation cross section of silane with electrons having higher energy than the threshold value in the plasma, υth is the thermal velocity of electrons, [SiH4] is the number density of SiH4 molecules, and Ne is the number density of energetic electrons responsible for the reaction. Large SiH3 density is achieved owing to high Ne in this microwave plasma source. However, even if a large number density of SiH3 is generated, the film deposition rate still depends on the position of substrate holder, because it is also determined by the diffusion loss of SiH3 during its transportation to the growth surface. Therefore, supply of SiH4 near substrate holder, wherein higher ne was also observed, is effective in suppressing the diffusion loss of SiH3 and in promoting dissociation efficiency of SiH4. Consequently, as has been shown, the SiH emission intensity (a possible indicator of precursor density) was almost 2 times stronger (G2 configuration) than that for the gas supply from a ring set just beneath the quartz window plate (G1 configuration). The Ns in μc-Si films is one of the most important structural properties, particularly for solar cell applications, because dangling bonds form deeply localized electronic states in the band gap and act as recombination centers for photoexcited
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carriers. Therefore, a lower Ns is one of the crucial demands while maintaining a high crystallinity. As is well known, Ns shows a minimum of (2–3) × 1015 cm− 3 at Ts of ∼ 250 °C and tends to increase at Ts both below and above ∼250 °C in the conventional rf plasma of a SiH4–H2 mixture [23]. However, as shown in the inset of Fig. 5 in the case of high-density microwave plasma, Ns increased with Ts even at Ts below 300 °C due to plasma heating, which indicated that the thermal abstraction of H at the growth surface was promoted for all Ts values. This causes the generation of dangling bonds on the growing surface. According to the surface diffusion model of SiH3 [7], these steady-state surface dangling bonds can be incorporated into the bulk layer, resulting in an increase in number density of defects in resulting films. The steady-state dangling bond density on growing surface is determined by a balance between the generation and saturation rates of dangling bonds. Therefore, it is expected that Nd can be reduced and Rd can be promoted by sufficient introduction of SiH3 (precursorassisted defect suppression) and atomic hydrogen on the film growing surface. On the one hand, generation rate of dangling bonds accelerated by the plasma heating will provide sufficient growth sites (dangling bonds) for the attachment of successive SiH3 to form Si–Si bonds contributing to film growth. Thus, by supplying SiH4 near the growth surface using a showerhead in G2 method, a large number density of SiH3 on the surface saturates the dangling bonds rather effectively, resulting in a significantly enhanced deposition rate and a remarkably reduced defect density in the films than G1 method. In addition, degree of film homogeneity improves with increasing R, which also could be attributed to sufficient termination of dangling bonds on film growing surface by the large amount of film precursor SiH3 arriving on the surface. On the other hand, however, Ns was almost independent of R in G2 method, which was also due to enhanced H desorption caused by the plasma heating. The use of lower Ts could help overcome this difficulty. Lowering Ts and wall temperature also tends to decrease the electron temperature Te, which promotes the preferential growth of (220) orientation beneficial for solar cell performance [36,37]. As a result, Ns was reduced to (1–2) × 1016 cm− 3 while maintaining high deposition rate at a Ts of 150 °C. 5. Conclusions A high electron density of (1–2) × 1011 cm− 3 and a low electron temperature of 1–2 eV (in Ar plasma) were achieved using a microwave plasma source utilizing a spoke antenna. We have applied this plasma to the high-rate synthesis of μc-Si films and proposed guiding principles beneficial for the improvement in film properties. OES measurements revealed that SiH emission intensity and intensity ratio of Hα/SiH were good monitors for the film deposition rate and film crystallinity, respectively. Contrary to the case of a conventional rf plasma, Ns increased with increasing Ts even at Ts below 300 °C in this high-density plasma, suggesting that the real temperature of film growing surface was higher than the monitored value and the abstraction of atomic hydrogen of growing surface was accelerated. A combination of a sufficient supply of SiH3 and
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atomic hydrogen on film growing surface under a lower substrate temperature is effective for fast deposition of highly crystallized μc-Si films without creating additional defects, and for enhancement in film homogeneity and surface smoothness. As a consequence, fast deposition was realized at a very high deposition rate of ∼ 65 Å/s for μc-Si film growth at a Ts of 150 °C with a high Raman crystallinity Ic/Ia N 2.5 and a low defect density of (1–2) × 1016 cm− 3. Acknowledgement The authors thank Dr. T. Kobayashi of the Institute of Physics and Chemical Research (RIKEN) for allowing the use of microRaman spectrometer. References [1] S. Veprek, V. Marecek, Solid-State Electron. 11 (1968) 683. [2] J. Meier, R. Fluckiger, H. Keppner, A. Shah, Appl. Phys. Lett. 65 (1994) 860. [3] R.B. Bergmann, Appl. Phys., A 69 (1999) 155. [4] H. Keppner, J. Meier, P. Torres, D. Fischer, A. Shah, Appl. Phys., A 69 (1999) 169. [5] A. Shah, P. Torres, R. Tscharner, N. Wyrsch, H. Keppner, Science 285 (1999) 692. [6] T. Matsui, M. Kondo, A. Matsuda, Jpn. J. Appl. Phys. 42 (2003) L901. [7] A. Matsuda, J. Non-Cryst. Solids 16 (1998) 365. [8] R. Dewarrat, J. Robertson, Thin Solid Films 427 (1993) 11. [9] A.S. Ferlcuto, R.J. Koval, C.R. Wronski, R.W. Collins, J. Non-Cryst. Solids 299–302 (2002) 68. [10] A.L. Baia Neto, A. Lambert, R. Carius, F. Finger, J. Non-Cryst. Solids 299–302 (2002) 274. [11] A. Matsuda, J. Vac. Sci. Technol., A 16 (1998) 365. [12] M. Kondo, M. Fukawa, L. Guo, A. Matsuda, J. Non-Cryst. Solids 266–269 (2000) 84. [13] E.A.G. Hamers, A. Fontcuberta i Morral, C. Niikura, R. Brenot, P. Roca i Cabarrocas, J. Appl. Phys. 88 (2000) 3674. [14] J.-L. Guizot, K. Nomoto, A. Matsuda, Surf. Sci. 244 (1991) 22. [15] M. Takai, T. Nishimoto, T. Takagi, M. Kondo, A. Matsuda, J. Non-Cryst. Solids 266–269 (2000) 90. [16] M. Shiratani, T. Fukuzama, Y. Watanabe, Jpn. J. Appl. Phys. 38 (1999) 306 (and references therein). [17] A. Shah, E. Vallat-Sauvain, P. Torres, J. Meier, U. Kroll, C. Hof, C. Droz, M. Goerlitzer, N. Wyrsch, M. Vanecek, Mater. Sci. Eng., B 69–70 (2000) 219. [18] J.H. Wu, J.M. Shieh, B.T. Dai, Y.S. Wu, Electrochem. Solid-State Lett. 7 (2004) G128. [19] K. Goshima, H. Toyoda, T. Kojima, M. Nishitani, M. Kitagawa, H. Sugai, Jpn. J. Appl. Phys. 38 (1999) 3655. [20] H. Shirai, Y. Sakuma, Y. Moriya, C. Fukai, H. Ueyama, Jpn. J. Appl. Phys. 38 (1999) 6629. [21] H. Sugai, I. Ghanashev, K. Mizuno, Appl. Phys. Lett. 77 (2000) 3523. [22] G. Ganguly, A. Matsuda, Phys. Rev., B 47 (1993) 3661. [23] A. Matsuda, Jpn. J. Appl. Phys. 43 (2004) 7909. [24] D. Daineka, P. Bulkin, G. Girard, J.-E. Bouree, B. Drevillon, Eur. Phys. J. Appl. Phys. 26 (2004) 3. [25] C. Niikura, M. Kondo, A. Matsuda, J. Non-Cryst. Solids 338–340 (2004) 42. [26] M. Losurdo, R. Rizzoli, C. Summonte, G. Cicala, P. Capezzuto, G. Bruno, J. Appl. Phys. 88 (2000) 2408. [27] M. Fang, B. Drevillon, J. Appl. Phys. 70 (1991) 4894. [28] S. Hanma, P. Roca i Cabarrocas, J. Appl. Phys. 81 (1997) 7282. [29] J. Koh, Y. Lee, H. Fujiwara, C.R. Wronski, R.W. Collins, Appl. Phys. Lett. 73 (1998) 1526.
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