Koji Arafune

Antireflective subwavelength structures on crystalline Si fabricated using directly formed anodic porous alumina masks

A simple fabrication technique for subwavelength structured (SWS) surfaces by means of anodic porous alumina masks directly formed on Si substrates was proposed and demonstrated. By this technique, SWS surfaces were fabricated on polished single-crystalline Si and chemically etched as-cut multicrystalline Si wafers. Smoothly tapered SWS surfaces with a periodicity of 100nm and a height of 300–400nm were obtained. A low reflectivity below 1% was observed from 300to1000nm for both of the wafers, in agreement with numerical simulation. After thermal annealing at 800°C, the reflectivity of the SWS surface increased to 3%.

Wide-Angle Antireflection Effect of Subwavelength Structures for Solar Cells

The angle-dependent reflectivity of several surface structures was analyzed and evaluated with the viewpoint of solar cell applications. Numerical analysis showed that a Si subwavelength structure (SWS) maintains a lower reflectivity at large incident angles than conventional light-trapping techniques such as a random pyramid texture, and that it can contribute to increasing the output power of solar cells under oblique irradiation. This wide-angle antireflection effect was demonstrated by fabricating test crystalline Si cells with several surface structures including a SWS and measuring their angle-dependent short-circuit current densities.

In situ Real-Time X-ray Reciprocal Space Mapping during InGaAs/GaAs Growth for Understanding Strain Relaxation Mechanisms

In situ real-time X-ray diffraction measurements during In0.12Ga0.88As/GaAs(001) epitaxial growth are performed for the first time to understand the strain relaxation mechanisms in a lattice-mismatched system. The high resolution reciprocal space maps of 004 diffraction obtained at interval of 6.2 nm thickness enable transient behavior of residual strain and crystal quality to be observed simultaneously as a function of InGaAs film thickness. From the evolution of these data, five thickness ranges with different relaxation processes and these transition points are determined quantitatively, and the dominant dislocation behavior in each phase is deduced.

Interface engineering for the passivation of c-Si with O3-based atomic layer deposited AlOx for solar cell application

We have investigated the effects of deposition temperature and post-annealing on the passivation performance of AlOx films deposited by O3-based atomic layer deposition for crystalline Si. We found that the dramatic enhancement in the passivation performance of room-temperature deposited AlOx films by post-annealing is due to the phase transformation of aluminum silicate to mullite in an AlOx interlayer and the resulting self-aligned AlOx/SiOx interface. This result is interesting for the fabrication of high-performance silicon solar cells with AlOx passivation layers.

Study on Iron Distribution and Electrical Activities at Grain Boundaries in Polycrystalline Silicon Substrate for Solar Cells

To clarify the role of grain boundaries in iron sinks and carrier recombination centers, iron distributions and their chemical states were studied before and after gettering. They were measured by the X-ray microprobe fluorescence and the X-ray absorption in the near-edge structure using the beamline 37XU at the SPring-8 third-generation synchrotron facility. To determine the crystallographic orientation of the grain boundaries, electron backscatter diffraction measurements were performed. The distribution of electric active defects was characterized by electron-beam-induced current measurements. Before gettering, the iron was distributed in the small grain and its chemical state was similar to that of iron oxide. After gettering, the iron was redistributed along the small angle grain boundary, and its chemical state was similar to the iron silicide complexed with the iron oxide. Regarding the electrical activity, high carrier recombination was observed along the small-angle grain boundary. On the contrary, Σ3 grain boundaries were relatively weak impurity sinks and showed low recombination activity.

Evaluation of defects generation in crystalline silicon ingot grown by cast technique with seed crystal for solar cells

Although crystalline silicon is widely used as substrate material for solar cell, many defects occur during crystal growth. In this study, the generation of crystalline defects in silicon substrates was evaluated. The distributions of small-angle grain boundaries were observed in substrates sliced parallel to the growth direction. Many precipitates consisting of light elemental impurities and small-angle grain boundaries were confirmed to propagate. The precipitates mainly consisted of Si, C, and N atoms. The small-angle grain boundaries were distributed after the precipitation density increased. Then, precipitates appeared at the small-angle grain boundaries. We consider that the origin of the small-angle grain boundaries was lattice mismatch and/or strain caused by the high-density precipitation.

Interaction between Metal Impurities and Small-Angle Grain Boundaries on Recombination Properties in Multicrystalline Silicon for Solar Cells

Recombination properties at small-angle grain boundaries (SA-GBs) in multicrystalline silicon were evaluated. After Fe contamination, the electron-beam-induced current (EBIC) contrast at most SA-GBs became stronger, especially at >1.5°. After Al gettering, EBIC contrast of most 1.5° SA-GBs showed comparable contrast before gettering. In addition, there were SA-GBs which have different recombination properties even with the same misorientation angle. Between these SA-GBs, there were differences in the rotation axis, boundary direction, and existence of defects. The associativity of metals might be affected by the existence of defects caused by these differences.

Surface Recombination of Crystalline Silicon Substrates Passivated by Atomic-Layer-Deposited AlOx

AlOx films as passivation layers for p-type crystalline silicon were prepared by atomic layer deposition with ozone as an oxidant, and the effects of the AlOx film thickness and deposition temperature on the maximum recombination velocity (Smax) were evaluated. Smax is improved by increasing the layer thickness but saturates at a layer thickness of about 30 nm. In the case of samples deposited at room temperature, Smax is improved fivefold when the thickness is increased from 20 to 33 nm. Smax also improved as the deposition temperature was increased to 300 °C then deteriorated when it was further increased to 350 °C. After postdeposition annealing, we obtained an Smax of 8.5 cm/s.

Study of the Degradation of p–n Diode Characteristics Caused by Small-Angle Grain Boundaries in Multi-Crystalline Silicon Substrate for Solar Cells

The causes of degradation of electrical characteristics, which affect the energy conversion efficiency of solar cells, were evaluated using a small p–n diode array fabricated on a multi-crystalline silicon (mc-Si) substrate. Many of the current–voltage (I–V) characteristics of diodes with grain boundaries deteriorated. However, some deteriorated diodes without grain boundaries were also found. We especially evaluated the diodes to determine the causes of degradation by electron-beam-induced current (EBIC) imaging, photoluminescence (PL) mapping, transmission electron microscopy (TEM), and electron backscatter diffractometry (EBSD). As a result, it was clarified that the electrical characteristics severely deteriorated with the existence of small-angle grain boundaries. Mc-Si solar cell efficiency was significantly affected by not only obvious grain boundaries but also small-angle grain boundaries consisting of periodically aligned dislocations and possibly metallic and oxygen impurities.

Impact of Light-Element Impurities on Crystalline Defect Generation in Silicon Wafer

In multi-crystalline silicon grown by unidirectional solidification, there are many origins of crystalline defects. In this study, we investigated the effect of light-element impurities on the generation of crystalline imperfections during crystal growth. In order to control the interfusion of impurities, we regulate the Ar gas flow in the atmosphere on the basis of a computer simulation. The etch pit densities in the sample fabricated without and with Ar gas flow control in the atmosphere were 1.5×105–7.0×107 and 5.0×103–4.0×105 cm-2, respectively. In the sample fabricated without Ar gas flow control, the precipitates consisting of light-elements were observed in the region where the etch pit density markedly increased. In the region with the highest etch pit density, there were small-angle grain boundaries consisting of dislocations. We believed that the precipitates consisting of light-element impurities were the potential origins of small-angle grain boundaries. The light-element impurities should affect the crystalline defect generation induced during crystal growth, and thereby should be controlled.