Yasushi Sobajima

Hybrid-phase growth in microcrystalline silicon thin films deposited by plasma enhanced chemical vapor deposition at low temperatures

Crystallographic studies on microcrystalline silicon (μc-Si) films, which were prepared by very-high-frequency plasma-enhanced chemical vapor deposition at a low temperature of 180°C, have been performed employing thickness evolutions of x-ray and electron diffraction measurements. The experimental results revealed that amorphous phase in μc-Si is transited to crystalline phase in solid phase in whole region from the top to the bottom, and the transition to the (220) orientation is dominantly found. These growth phenomena are interpreted in terms of a proposed model, i.e., the hybrid-phase growth model consisting of conventional vapor-phase growth at the surface plus the solid-phase crystallization occurring in the film. Moreover, the hybrid-phase growth, particularly solid-phase crystallization at low temperatures, is discussed in conjunction with the further results on the thickness evolutions associated with μc-Si films deposited on various underlayers or at substrate temperatures of 160–350°C, or at S...

High-Rate Deposition of Microcrystalline Silicon Photovoltaic Active Layers by Plasma-Enhanced Chemical Vapor Deposition at Kilo-Pascal Pressures

High-rate depositions of microcrystalline silicon (µc-Si) have been investigated for photovoltaic applications employing plasma-enhanced chemical vapor deposition excited at a very high frequency of 100 MHz. Very high-deposition rates of up to 7.3 nm/s have been achieved by introducing depositions at very high pressures of 1.2–3.2 kPa (9–24 Torr). Moreover, by applying the µc-Si deposited at 7.1 nm/s to the photovoltaic active i-layer of the n–i– p solar cells, a conversion efficiency of 5.10% (Jsc = 18.8 mA/cm2, Voc = 0.460 V, FF = 59.0%) has been achieved. We discuss the effects of the microstructures on the photovoltaic performance.

Novel light-trapping structure having smooth surface for silicon thin-film solar cell

A novel light-trapping structure having a smooth surface likewise character ‘T’ (named as Type-T structure) is proposed for Si thin-film solar cells because the conventional structure based on the surface textures of transparent conductive oxide (TCO) film has crucial issues accomplished by surface roughening. The fundamental concept of Type-T structure, consisting of the roughened surface generating diffusive transmittance (or reflectance) of light as well as the TCO film with the plarnarized surface, is described. The key technique, planarization of roughened surface, has been achieved by the sol-gel prepared ZnO:Al films after the improvement of electrical properties. Finally, applying the Type-T structure to the μc-Si thin-film solar cells, the improvement in the photovoltaic performance, particularly in the short circuit current densities due to the light-trapping effects caused by the Type-T structure, is demonstrated.

Fractal study of surface nanostructures of microcrystalline silicon films: From growth kinetics to electronic transport

We studied surface nanostructures of microcrystalline Si (μc-Si) films and their relationship to electronic transport, based on fractal concepts. Two distinctive nanostructures, i.e. columnar and granular structures, are presented together with the crystallographic preferential orientations. The topological evolution of μc-Si surfaces monitored by atomic force microscope measurements is also discussed. The scaling exponents derived from fractal concepts quantitatively revealed a difference in the topological features of surface evolution related to the nanostructures of μc-Si films, thus providing a new insight into the growth kinetics. Growth kinetics were further evaluated using Monte Carlo simulations. Finally, the electronic transport of μc-Si is discussed using the lateral size and fractal boundary determined by fractal concepts.

X-ray pole figure analysis on columnar-structured microcrystalline silicon: Growth-induced crystallographic axial alignment in solid phase

Angular distributions of crystallographic axes of the microcrystalline silicon (μc-Si) thin films have been investigated using the x-ray pole figure measurements with an emphasis on the growth-induced changes in the first growth regions of the μc-Si films. The (220) preferentially oriented μc-Si films containing columnar microstructures were prepared by plasma enhanced chemical vapor deposition with differing thicknesses. The distributions in the tilt angles of ⟨220⟩ axes from the substrate normal decreased with increasing the film thickness, particularly in the first 0.5μm growth regions. Meanwhile, such a change was not found in ⟨111⟩ axes. Moreover, the x-ray pole figure measurements were also carried out for the (111) preferentially oriented μc-Si films containing granular microstructures, revealing that no pronounced change in the tilt angles of ⟨111⟩ axes were found during the film growth. Therefore, the axial alignment should be specific to the ⟨220⟩ axes in the μc-Si films with the (220) preferent...

Fundamental Properties of Titanium-Doped Indium Oxide and Its Application to Thin-Film Silicon Solar Cells

We have investigated the fundamental optoelectronic properties of newly developed transparent conductive oxide (TCO) materials, e.g., titanium-doped indium oxide (InTiO). InTiO films, being deposited at 50 °C by the RF-magnetron-sputtering method followed by thermal annealing at 200 °C, show excellent optoelectronic properties for solar-cell application. We have demonstrated the improved photovoltaic performance of n–i–p microcrystalline-silicon (µc-Si:H) solar cells whose i layer is prepared at a high rate of 2.3 nm/s using a stacked structure of InTiO with aluminum-doped zinc oxide (AZO) as top (front) TCO layers.

Importance of Starting Procedure for Film Growth in Substrate-Type Microcrystalline-Silicon Solar Cells

The starting procedure of hydrogenated microcrystalline-silicon (µc-Si:H) film growth at a high rate has been controlled in a plasma-enhanced chemical vapor deposition process to improve the optoelectronic properties of the resulting n/i interface as well as the intrinsic bulk layer in µc-Si:H-based substrate-type (n–i–p) solar cells. The electron temperature, monitored using optical emission spectroscopy, in the plasma during film growth is successfully controlled by changing the starting procedure, i.e., gradual SiH4 introduction and slow power application, leading to the preparation of high-quality µc-Si:H films with a low dangling-bond defect density. Reduction of the defect density in the intrinsic layer and improvement of the optoelectronic properties at the n/i interface are demonstrated through the fabrication of single-junction n–i–p solar cells showing high photovoltaic performance.

Control of electron temperature in SiH4/H2 plasma for obtaining high photovoltaic performance in microcrystalline silicon solar cells

We have proposed two novel processes for the formation of fine n/i interface to improve the photovoltaic performance in substrate-type (n-i-p type) hydrogenated microcrystalline-silicon (?c-Si:H) solar cells whose i layer is deposited at high growth rate of > 2.0 nm/sec; (1) gradual monosilane-(SiH4)-molecule-introduction method and (2) amorphous silicon (a-Si:H) thin-layer-insertion method. When applying these two methods to the formation process of n/i interface in the solar cells, drastic improvement of the production reproducibility has been achieved in the fabrication process of high efficiency (> 9%) substrate-type ?c-Si:H solar cells.

Influence of impurities on photocarrier diffusion of high-growth-rate microcrystalline silicon

The ambipolar photocarrier diffusion length, L<inf>amb</inf>, observed using the steady-state photocarrier grating technique, has been investigated regarding high-growth-rate microcrystalline Si (μc-Si) together with the lateral size, σ<inf>L</inf>, of grain (column) determined from fractal concepts. Experimental results revealed that the relation between L<inf>amb</inf> and σ<inf>L</inf> is linear, and the slope, L<inf>amb</inf>/σ<inf>L</inf>, being almost unity or less, indicates intra-grain photocarrier diffusion. Furthermore, after exchanging the aged gas-purifiers for SiH<inf>4</inf> and H<inf>2</inf> gases to the fresh ones, L<inf>amb</inf> was increased for each σ<inf>L</inf>, yielding that the slope, L<inf>amb</inf>/σ<inf>L</inf>, was increased from 0.77 to 1.1. In this article, we discuss the effects of impurities, particularly oxygen, on L<inf>amb</inf>, in conjunction with the elemental analyses by secondary-ion-microprobe-mass spectrometry.

Angular Distribution with Respect to Growth Direction of Microcrystalline Silicon Grown in Hybrid Phase

Angular distribution with respect to (220) growth direction of muc-Si method has been studied by employing thickness evolutions of X-ray pole figure measurements. The muc-Si films with the (220) preferential orientation were prepared by plasma enhanced chemical vapor deposition method at low substrate temperatures (~200degC). As for the Si(220) grains, the angular distribution of the tilt angle in the whole growth region was improved during deposition, yielding that the growth directions became almost parallel to the substrate normal with small tilt angles. The decrease in the angular of the Si(220) grains of whole region of film is discussed from the aspect of the hybrid-phase growth