Takehiko Nagai

Seeding method with silicon powder for the formation of silicon spheres in the drop method

Silicon spheres with a size distribution around 1.0 mm diameter, which are applicable to spherical solar cells, were formed by dropping molten silicon through a nozzle in a free-fall tube, namely, the drop method. Here we show a seeding technique for the formation of silicon spheres. In this technique, pure silicon powders with a size distribution of 1−75 μm were ejected to the molten silicon droplets at a selected part of the free-fall tube using argon carrier gas. It was considered that the attached silicon powders on the droplets worked as nuclei and stimulated the solidification to occur at low undercooling from one place. Characterizations with scanning electron microscope, carrier lifetime, and photoluminescence measurements demonstrated that the crystallinity of silicon spheres were significant improved by the seeding method. The undercooling of molten silicon droplets at solidification was speculated to decrease from ∼250 °C to below 50 °C by seeding power ejection. This resulted in an increase of...

Characterization of spherical Si by photoluminescence measurement

Spherical silicon (Si) with a size of ∼1mm diameter was fabricated by the dropping method for the applications of spherical Si solar cells. In this research work, we characterized spherical Si by means of photoluminescence (PL) measurement at 4 and 18K. The horn-type spherical Si crystals, formed under large undercooled conditions without a seeding technique, showed D-band luminescence originating from dislocations, whereas intrinsic PL bands of Si were not observed. In contrast, for the tear-type spherical Si crystals, formed under shadow undercooling by a seeding technique with Si powder, the boron (B) bound and Si intrinsic phonon-assisted PL bands were clearly observed both at 4 and 18K. Moreover, the intensity ratio of B bound exciton band to Si intrinsic phonon-assisted PL band showed good correlation to the minority carrier lifetime measured with microwave photoconductance decay method. These experimental results suggested that the crystallinity of the tear-type spherical Si is significantly improv...

Improvement of the Production Yield of Spherical Si by Optimization of the Seeding Technique in the Dropping Method

Spherical Si with a diameter distribution of approximately 1.0 mm, which is applicable to spherical solar cells, was formed by the dropping method. To control the solidification of the molten Si droplet to occur at shallow undercooling, a seeding technique was developed. In this paper, we describe the optimization of the seeding technique with emphases on high production yield and high minority carrier lifetime. An effective seeding supply system was first introduced. It was found that the seeding with Si powder causes impurities, such as Fe, into Si spheres, and the crystallinity becomes poor with increasing yield. SiO, SiO2, Si3N4, and boron nitride powders were shown to have the same seeding effect as Si powder. Among these, SiO2 was the best candidate. The seeding with SiO2 powder did not increase the concentration of Fe and O impurities and did not decrease the crystallinity. The dependences of the yield on the size and the density of SiO2 powder were systemically investigated. The best yield of nearly 70% was realized at a high SiO2 powder density.

Study of spatial distribution of SiH3 radicals in very high frequency plasma using cavity ringdown spectroscopy

Time-resolved cavity ringdown (τ-CRD) spectroscopy has been applied to measure the SiH3 radical density profile between the electrodes in a pulsed SiH4/H2 very high frequency (VHF) plasma under µc-Si:H deposition conditions. On time scales smaller than ~1 s, cavity loss reflects the light absorption by SiH3 radicals, whereas on time scales larger than ~1 s, an additional cavity loss due to light scattering at Si clusters and dust particles, generated in the pulsed SiH4/H2 VHF plasma, is observed. From the measurements of the spatial distribution of SiH3 radicals between electrodes, the incident SiH3 radical flux to the electrode surface is determined, which reveals a significant contribution of SiH3 radicals to µc-Si:H thin film growth.

Formation of SiH3 Radicals and Nanoparticles in SiH4-H2 Plasmas Observed by Time-Resolved Cavity Ringdown Spectroscopy

The growth of nanoparticles between electrodes in a pulsed very high frequency (VHF) silane–hydrogen (SiH4–H2) plasma under hydrogenated microcrystalline silicon (µc-Si:H) growth conditions has been studied by time-resolved cavity ringdown (τ-CRD) spectroscopy. The light absorption of silyl (SiH3) radicals and the light scattering and absorption of nanoparticles have been measured in the UV spectral range (220 and 280 nm). The contribution of the SiH3 radicals and nanoparticles to the measured cavity loss could be distinguished by 1) varying the wavelength of the probe laser pulse, 2) using time-resolved information of the SiH3 radicals and the nanoparticle density, 3) the measured spatial distribution of the species between the electrodes, and 4) the dependence of these distributions on the electrode temperature. From the time evolution of cavity loss related to the nanoparticles growing in plasma, we can determine whether the light losses of the nanoparticles are in the Rayleigh or Mie regime. Subsequently, these measurements also provide information on typical particle size. Additional scanning electron microscopy (SEM) analyses, which reveal the evolution of the nanoparticle size distribution with time, corroborate the results obtained by τ-CRD. Finally, the spatial distribution of the SiH3 radical density and the electrode temperature dependence of the nanoparticles between the electrodes have been studied. These results are discussed in terms of the dominant forces acting on nanoparticles in plasma and the nanoparticle growth mechanism.

Time-resolved cavity ringdown spectroscopy as a monitoring technique of nanoparticles in pulsed VHF plasmas

Time-resolved cavity ringdown (t-CRD) spectroscopy has been applied to monitor the silyl (SiH3) radicals and nanoparticles in pulsed very high frequency (VHF) silane (SiH4)/hydrogen (H2) plasmas under microcryst. silicon (micro c-Si:H) deposition conditions. After the plasma ignition, a small const. cavity loss (.apprx.100 ppm) on time scales smaller than .apprx.1 s has been obsd., whereas on time scales larger than .apprx.1 s after plasma ignition, an addnl. cavity loss is obsd. By variation of the wavelength of the CRD laser pulse, we demonstrate that the cavity loss on time scales smaller than .apprx.1 s reflects the SiH3 absorption. On time scales larger than .apprx.1 s, the addnl. cavity loss corresponds to the loss of light due to mainly scattering at the nanoparticles. Under the conditions studied, the light scattering at nanoparticles can be described by Rayleigh scattering during its initial growth. After .apprx.2.5 s, the cavity loss reflects the transition of the scattering mechanism from dominant Rayleigh to dominant Mie scattering. These results are discussed in terms of nanoparticles growing in time and further confirmed by addnl. SEM analyses on the nanoparticles created in the plasma pulse. [on SciFinder (R)]

Detection of SiH3 radicals and cluster formation in a highly H2 diluted SiH4 VHF plasma by means of time resolved cavity ring down spectroscopy

The spatial distribution of the SiH 3 radicals between the electrodes of a hydrogen diluted silane VHF plasma under thin film hydrogenated microcrystalline silicon (μc-Si:H) growth conditions has been measured using the time resolved cavity ringdown (τ-CRD) absorption spectroscopy technique. The μc-Si:H growth rate is estimated from the measured spatial SiH 3 profiles using a simple model based upon diffusion controlled flux of SiH 3 radicals to the electrode surface, where the SiH 3 can react with the film surface. The calculated value of μc-Si:H growth rate roughly agrees with the value of the experimentally determined growth rate. This agreement implies that the SiH3 radical is the main growth contributor to the μc-Si:H growth. Furthermore, the τ-CRD reveals the growth kinetics of the clusters in the plasma by light scattering at these clusters on time scales of 1 s after the plasma ignition.