A-Si and micro-crystalline thin films for solar cells are widely deposited by plasma CVD in industry. To realize combinatorial plasma CVD of such Si thin films, we have developed a multi-hollow discharge plasma CVD method, by which fluxes of H and SiH₃ as well as their flux ratio on the substrate placed perpendicular to the electrodes depend on the distance from the discharges [1-4]. Thus, we can simultaneously deposit Si thin films with various structures and properties. For 60 MHz discharges of H₂+SiH₄ (0.3%), no films were deposited just near the discharge regions due to Si etching by H, micro-crystalline films were deposited in a rather narrow area around the no film regions, and a-Si:H films were obtained in the rest wide area far from the discharges. The spatial distribution of film structures indicate that the density ratio of H to SiH₃ decreases sharply with increasing the distance from the discharges and the surface reaction probability of H is much higher than that of SiH₃, being consistent with the reported surface reaction probabilities [5, 6]. For 2-6 Torr, the micro-crystalline film structure such as crystalline volume fraction and grain size varies sharply not only along the direction perpendicular to the electrodes but also along the direction parallel to the electrodes. These results suggest that the micro-crystalline film structure is highly sensitive to spatial and temporal uniformity of fluxes of H and SiH₃ as well as their flux ratio.

[1] K. Koga, et al., Jpn. J. Appl. Phys. 44, L1430 (2005).

[2] W. M. Nakamura, et al., IEEE Trans. Plasma Sci. 36, 888 (2008).

[3] W. M. Nakamura, et al., J. Phys.: Conf. Series 100, 082018 (2008).

[4] H. Sato, et al., J. Plasma Fusion Res. SERIES, 8, 1435 (2009).

[5] A. Matsuda, et al., Surface Sci. 227, 50 (1990).

[6] J. Perrin, et al., J. Vac. Sci. Technol. A, 16, 278 (1998).

Since the conventional silicon solar-cell conversion is limited to a certain window of solar cell photon energies over 1 eV, a full use of the solar spectrum is one of the crucial issues in order to greatly increase the solar cell efficiency. In this sense, carbon nanotubes (CNTs) are attracting much interest for photovoltaic energy conversion because of their broad absorption bands including the infrared range (0.2 ~ 1.3 eV) as well as other advantages such as large surface areas, high mobility of charge carrier, high mechanical strength, chemical stability, and so on. In this connection, we have developed a plasma-ion irradiation method, which enables pristine single-walled carbon nanotubes (SWNTs) to selectively encapsulate various kinds of atoms and molecules, such as metals and fullerenes, serving as electrons donors or acceptors inside their cavities. Then these enhanced p-type, n-type, and pn-junction housed semiconductor-SWNTs are applied toward the realization of high-efficient photovoltaic devices, which is composed of thin films of p- and n-types semiconductor SWNTs or an individual SWNT with p-n junction inside. Here, as a first step, electrical properties of p-n junctions fabricated using a combination of the thin films of pristine (empty) SWNT or C₆₀-encapsulated SWNT (C₆₀@SWNT), and n-doped Si (n-Si) are investigated.

The electrical properties of these SWNT film/n-Si devices show an obvious rectifying characteristic, and a short-circuit current I_SC and an open-circuit voltage V_OC through a downward shift of I-V curves are observed under illumination of light with wavelength of 1550 nm which corresponds to the photon energy of 0.8 eV. Moreover, it is found that the device fabricated with the C₆₀@SWNT film has a larger V_OC caused possibly by a large diffusion voltage in the interface of p-n junction compared with the device fabricated with the pristine SWNT film, due to the enhanced p-type behavior of SWNTs after C₆₀ encapsulation. To investigate undesirable photovoltaic effects of n-Si, we fabricate a schottky barrier solar cell consisting of silver (Ag) and n-Si in the absence of SWNTs. It is confirmed that the Ag/n-Si schottky barrier solar cell generates photo currents in the visible range (1.5 ~ 3 eV), while there is almost no difference between with and without light in the infrared range (0.8 eV) because the light with photon energy less than 1 eV cannot be absorbed by Si.

Based on these results, high performance solar cells which work in the infrared region are for the first time demonstrated to be formed using SWNTs, especially p-type enhanced C₆₀@SWNT.

Processing a large area substrate in a capacitively coupled plasma (CCP) reactor is becoming increasingly more difficult as the driving frequency required by the process is becoming higher and the size of the substrates is becoming larger. At the VHF (very high frequency) range the wave length of the driving signal is approaching the size of the substrate, and the resulting standing wave causes a severely non-uniform process. In this presentation, we will present a novel approach using magnetic boundary conditions in conjunction with phase modulation between multiple power feed points to improve process uniformity for a CCP reactor operating in the VHF range. The substrate size we consider is Gen 8.5 (2.2 m × 2.6 m substrate) and the VHF power applied to generate the plasma is 40 and 60 MHz. At 60 MHz, with the vacuum wavelength of 5 m, the size of the substrate is approximately ½ of the vacuum wavelength. An electromagnetic simulation with a pseudo plasma showed that, when 60 MHz is applied in a conventional manner, i.e. it is fed from the center of the back of one of the electrodes, it generates a dome shape electromagnetic field profile, which falls off sharply to almost zero at the voltage node before rising again towards the edges. A similar field pattern was also generated even when the VHF was fed from two feed points located at the opposing edges. The plasma distribution pattern measured with a 4 × 8 grid of optical emission spectroscopic (OES) probes revealed that the plasma was localized in the center when VHF the signal applied to the feed points were in phase. To modify the wave propagation pattern to change the shape of the standing wave in the central area, we placed ferrite material along two of the edges (edges that are away from the feed points) of the powered electrode. In this case, the peak in the central area was significantly stretched towards the ferrite-lined edges. We also found that the stretched “bar” of plasma could be moved over the substrate area by dynamically modifying the relative phase between the feed points in a manner similar to the technique employed by Yamakoshi etal. [1], and effectively distribute the processing plasma to much larger area.

[1] H.Yamakoshi etal. Appl. Phys. Lett. 88, 081502 (2006)

Hydrogenated nanocrystalline silicon (nc-Si:H) and amorphous silicon thin film are expected to be promising

materials for solar cell and thin film transistor. Especially, nc-Si:H thin films have been reported to have

an enhanced stability due to its more rigid structure. These thin films are usually grown with

plasma enhanced chemical vapor deposition (PECVD) using silicon-containing gas mixtures such as SiH₄ and H₂.

To increase the photovoltaic efficiency and to improve the mobility of TFT devices, it is necessary to produce

nanocrystalline silicon films with higher crystallization percentages. But it is reported that high H₂. dilution leads to

a significantly lower deposition rate.

In this study, we investigated the influence of He mixture with SiH₄ gas instead of H₂ to improve the crystallization

percentage of the deposited silicon without significantly decreasing the deposition rate.

To find out properties of the thin film deposited with He/SiH₄ such as structural properties, crystalline volume fraction (Xc), active radicals in plasma, Si-H bonding characteristics, and conductivity, Scanning Electron Microscopy

(SEM), Raman spectroscopy, Optical Emission Spectroscopy (OES), Fourier-Transform-Infra-Red (FT-IR), and Keithley

measurement kit, were used respectively. The results showed the increase of crystallization percentage by using

He instead of H₂ as the additive gas and, with the increase of the applied RF power up to 140W, crystalline volume

fraction of about 80% could be observed.