Session Tu-B1

Nano- and Microcrystalline Silicon: Growth and Characterization I

Chair: Pere Roca i Cabarrocas, Ecole Polytechnique

Tu-B1.1 8:30–8:50

Investigation of Porosity and Atmospheric Gas Indiffusion in Microcrystalline Silicon Fabricated at High Growth Rates

S. Michard, M. Meier, U. Zastrow, O. Astakhov, A. Gordijn, and F. Finger

IEK5-Photovoltaik, Forschungszentrum Jülich, D-52425 Jülich, Germany

High growth rates (RD) for the deposition of microcrystalline silicon (μc-Si:H) are of interest for the application of intrinsic μc-Si:H as an absorber layer for multi junction thin-film silicon solar cells. It is observed that processes with high RD tend to deteriorate the material properties of μc-Si:H by introducing defects, porosity and high hydrogen content. This material is further subject to metastability effects by in-diffusion of atmospheric gases [1]. In the present study we investigate these relationships on material prepared with very high frequency plasma enhanced chemical vapor deposition.

The material was prepared from different silane-hydrogen mixtures with excitation power between 20 W and 600 W which results in deposition rates between 0.7 and 2.3 nm/s for materials with different crystalline volume fractions (IcRS). A first sample series focused on μc-Si:H films with moderate IcRS considered optimal for the application to thin-film solar cells and referred to as optimal phase mixture (OPM). A second series focused on material with high IcRS (IcRS > 75%). The electronic material quality was investigated by conductivity measurements and ESR. IcRS was evaluated from Raman spectroscopy. Hydrogen and oxygen content were evaluated from FTIR spectra using the absorption bands around 640 cm–1 and 1100 cm–1, respectively [2,3]. Oxygen indiffusion due to storage at ambient was investigated by FTIR after 10, 30, 90, and 180 days. Measurements of profiles of indiffused gas were performed by SIMS on selected samples.

It was found that some of the μc-Si:H material shows good electronic properties in terms of defect density and photosensitivity [4]. There is a clear tendency for increased porosity and resulting gas indiffusion for material prepared beyond 200 W excitation power.

For material prepared with deposition powers below 200 W no increase in oxygen content was observed. It shows that even for high RD, compact material persistent to oxygen indiffusion can be deposited.

[1] S. Veprek, Z. Iqbal, R. Kühne, P. Capezzuto, F. Sarott, and J. Gimzewski, "Properties of microcrystalline silicon: IV. Electrical conductivity, electron spin resonance and the effect of gas adsorption," Journal of Physics C: Solid State Physics, vol. 6241–6262, no. 16, p. 21, 1983

[2] W. Beyer and M. Abo Ghazala, "Absoprtion strengths of Si-H vibrational modes in hydrogenated silicon," in Mat. Res. Soc. Symp. Proc. Vol. 507, 1998, pp. 601–606

[3] A. Langford, M. Fleet, B. Nelson, W. Lanford, and N. Maley, "Infrared absorption strength and hydrogen content of hydrogenated amorphous silicon," Physical Review B, vol. 45, no. 23, pp. 13367–13377, 1992

[4] S. Michard, M. Meier, B. Grootoonk, O. Astakhov, A. Gordijn, and F. Finger, "High deposition rate processes for the fabrication of microcrystalline silicon thin films," Materials Science and Engineering: B, pp. 1–4, Dec. 2012

Keywords: microcrystalline silicon, growth rates, Fourier transform infrared spectroscopy, oxygen in-diffusion

Tu-B1.2 8:50–9:10

Material and Growth Mechanism Studies of Microcrystalline Silicon Deposited using Tailored Voltage Waveforms

Bastien Bruneau (1), Jean-Christophe Dornstetter (1,2), and Erik Johnson (1)

1. LPICM-CNRS – Ecole Polytechnique, 91128 PALAISEAU, France

2. Total RM – Energies Nouvelles, 24 cours Michelet – La Défense 10, 92069 PARIS LA DEFENSE Cedex, France

In capacitively-coupled plasma-processing applications, the parameters of mean ion bombardment energy (IBE), plasma-density, and chemistry are strongly linked. However, this coupling can be partially relaxed through the use of Tailored Voltage Waveforms (TVW's) (also called the Electrical Asymmetry Effect). The excitation of the plasma using non-sinusoidal, TVW's splits the plasma sheath potential unequally between the excitation electrode and the substrate holder. This gives new freedoms in thin-film processing, and we have applied this technique for the deposition of hydrogenated microcrystalline silicon (μc-Si:H) films for photovoltaics, and furthermore, have fabricated un-optimized single junction solar cells with efficiencies above 6% [1]. For this material system, the decoupling of the plasma density and the associated IBE indicates a great advantage for the industrial deposition of μc-Si:H.

Expanding on these results, we further elucidate the impact on material growth dynamics when the IBE is changed but all other process conditions are kept the same. Such effects on the amorphous-to-microcrystalline transition for μc-Si:H are observed in-situ through spectroscopic ellipsometry, and through ex-situ techniques (atomic force microscopy) on the surface morphology. A dramatic change in the surface morphology of the films (even for similar Raman scattering spectra) is seen, and can be linked to the importance of the independently controlled IBE. Furthermore, for the low IBE conditions created using the TVW technique, the nucleation rate can be lowered to a degree to produce surprising structures consisting of randomly initiated crystallites surrounded by regions of no film growth.

These effects have been expanded to explore both SiH4/H2 and SiF4/H2/Ar plasma chemistries, the latter of which has been shown to be extremely effective in producing high efficiency solar cells at high crystallinities [2]. The use of the TVW technique allows the employment of this gas chemistry with a reduced IBE (particularly, allowing the use of lower process pressures). We will further report on progress in the use of the SiF4 chemistry with TVW's, which has improved the efficiency of the solar cells compared to the SiH4/H2 case, even when the P-I and N layers are all deposited in different reactors with air breaks between film depositions.

[1] E. V. Johnson, P-A. Delattre, and J. P. Booth, "Microcrystalline silicon solar cells deposited using a plasma process excited by tailored voltage waveforms", Appl. Phys. Lett. 100 (2012) 133504

[2] J. C. Dornstetter, S. Kasouit, P. Roca i Cabarrocas, "Deposition of High-Efficiency Microcrystalline Silicon Solar Cells Using SiF4/H2/Ar Mixtures", IEEE J. PV. 3 (2013) 581

Keywords: microcrystalline silicon, plasma enhanced chemical vapour deposition, ion bombardment

Tu-B1.3 9:10–9:30

In-situ Detection of Powder Formation via Optical Emission Spectroscopy and Bias-Voltage Measurements for High-Depletion μc-Si:H Deposition Regimes

B. Grootoonk , J. Woerdenweber , M. Meier, and A. Gordijn

IEK5-Photovoltaik, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany

Microcrystalline silicon (μc-Si:H) fabricated by plasma-enhanced chemical vapor deposition (PECVD) is commonly used as absorber material in thin-film tandem solar cells. The source gases used in the μc-Si:H PECVD process are silane and hydrogen. One way to further increase the production effectivity of solar modules is to increase the gas utilization during deposition of the silicon absorber layer. This can be achieved by reducing the hydrogen flow which is equivalent to a high degree of silane-depletion in the plasma. The residence time of the gas precursors in the plasma increases by applying deposition regimes with a reduced hydrogen gas flow. This is known to promote powder formation in silane-hydrogen plasmas, which can be detrimental for the solar cells conversion efficiency.

Since powder formation has an influence on several plasma parameters like electron density and temperature, a transition from a (mostly) powder free to a powder rich deposition regime is likely seen in the optical emissions spectrum of the plasma and in the bias-voltage of the PECVD process. Here, we present an easily applicable approach to determine powder formation in-situ during the PECVD process.

The residence time of the precursor gas was varied by changing the hydrogen gas flow at constant deposition pressures and constant silane gas flow. The experiments were performed with deposition pressures between 4 mbar and 16 mbar. All deposition regimes applied in this study lead to μc-Si:H film growth with a crystalline volume fraction of about 57%–77%. During deposition optical emission spectra (OES) were recorded and the bias-voltage at the powered electrode was measured.

Additionally, long term depositions at different pressures and gas residence times were performed in a cleaned reactor. After each deposition photographs of the chambers interior were taken, to characterize the powder deposition. By means of electrode coverage with powder a correlation between the optical emission spectra, the bias-voltage and the residence time of the precursor gas in the plasma was investigated.

It was found that the OES intensity ratio RSiH*/Hβ between SiH* and Hβ starts to decrease at a characteristic residence time t*res ~ 0.5–0.7 s and increases again for tres >> t*res until it saturates. Likewise, the absolute bias-voltage Vbias starts to increase at t*res and again saturates for tres >> t*res. The characteristic behavior at t*res can be correlated—for both RSiH*/Hβ and Vbias—to the onset of powder deposition on top of the electrode, by means of the photographs taken after long term depositions.

In conclusion, our method provides an easily applicable in-situ approach to determine the onset of electrode coverage with powder, by observing RSiH*/Hβ and Vbias as function of gas precursors residence time. This is helpful to explore new effective deposition regimes for microcrystalline silicon based solar cell devices.

Keywords: high depletion, powder formation, optical emission spectroscopy, bias-voltage

Tu-B1.4 9:30–9:50

Probing Periodic Oscillations in a Silane Dusty Plasma in VHF PECVD Process

A. Mohan (1), C. van der Wel (1), R. E. I. Schropp (2), and J. K. Rath (1)

1. Utrecht University, Faculty of Science, Debye Institute for Nanomaterials Science-Physics of Devices, High Tech Campus 5, 5656 AE Eindhoven, The Netherlands

2. Energy research Center of the Netherlands (ECN), Solar Energy, High Tech Campus Building 5, 5656 AE Eindhoven; and Eindhoven University of Technology (TU/e), Department of Applied Physics, Plasma & Materials Processing, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

Nanoparticle and dust formation in a silane-hydrogen discharge in plasma enhanced chemical vapor deposition can have beneficial or deleterious effect on thin film silicon device property depending on the growth condition of silicon material. Whereas dusts in the plasma can be a source of shunting in thin film silicon solar cells, photoactive amorphous silicon layers incorporating silicon nanoparticles, the so called polymorphous [1] material, on the other hand, delivers high open circuit voltage and has been claimed to have better stability against light soaking. Moreover, use of layers with embedded silicon quantum dots in multijunction solar cells would allow to surpass the Shockley-Queisser efficiency limit. This research attempts to find precise control of the plasma process by which required particles can be embedded in the layers or removed from the gas phase depending on the choice of material for the device.

We performed optical and electrical measurements in both non-dusty and dusty regime. At a plasma frequency of 60 MHz and a substrate temperature of 180°C, the dusty plasma was made at a pressure of 3 mbar, power of 17.5 W, SiH4:H2 flow rate of 5:100 sccm and an electrode distance of 10 mm, whereas the non-dusty regime was obtained at a pressure of 0.16 mbar, power of 5 W, SiH4:H2 flow rate of 35:175 sccm and an electrode distance of 27 mm. The transition from the non-dusty to the dusty regime was observed by the broadening of the plasma sheath and also by the shift in impedance towards a more resistive plasma. The plasma was diagnosed in situ by optical emission spectroscopy in which the emission lines of SiH*, Hα and Hβ were followed and V-I probe in which the potential, current, and impedance of the plasma were recorded. For each measurement the plasma was switched on and left to stabilize for 60 seconds before doing the optical and electrical measurements for 100 seconds. The data was then subjected to discrete Fourier transform (DFT) and were cropped to a range of 5–60 Hz.

The output parameters of both the probes showed a periodic oscillation in the dusty regime. However, for same measurements performed in the non-dusty regime, we found no fluctuation in either optical or electrical measurements, but a weak signal at 50 Hz, which is the line frequency observed in most measurements. Based on our observation, it can be hypothesized that the fluctuations arise from periodic formation and ejection of a dust cloud continuously, probably via the void formation in the plasma bulk when a critical dust size is reached. Further experiments support such a hypothesis; the frequency of fluctuation increased linearly with silane flow rate, which may be attributed to faster growth of dust with increased precursor flux, whereas it decreased linearly with increase in temperature, which is attributed to slower polymerization reaction in gas phase [2] and lower ion drag force.

[1] Pere Roca i Cabarrocas, et al., Plasma Phys. Control. Fusion 46, (2004) B235

[1] M. M. de Jong, A. Mohan, J. K. Rath, and R. E. I. Schropp, AIP Conf. Proc. 1397, (2011) pp. 411–412

Keywords: dusty plasma, VHF PECVD, silicon nanoparticles, plasma fluctuation