Katsuhiko Shirasawa

Probing the surface potential of oxidized silicon by assessing terahertz emission

Using laser terahertz emission microscopy, we measured laser-excited terahertz (THz) emission from silicon wafers with silicon-oxide passivation layers, revealing a strong correlation between the THz waveform and the surface potential. The surface potential was electrically tuned by a semitransparent top electrode disc and evaluated by measuring capacitance–voltage characteristics. The waveform changed with external bias and inverted near the flatband voltage, and changes appeared in the peak amplitude were similar to the capacitance–voltage characteristics. These results indicate that by analyzing the waveform of laser-excited THz emission generated by laser terahertz emission microscopy, we could quantitatively measure and map the internal field of surface band bending in semiconductors.

Method of Removing Single-Side Doped Layer While Maintaining Pyramid Textured Surface of n-Type Bifacial Solar Cells

BBr3 and POCl3 thermal diffusion is a widely used technique for p- and n-type emitter formation in bifacial solar cell fabrication. However, single-side doping of BBr3 or POCl3 is difficult to achieve through gas diffusion carried out at high temperatures. Thus, removal of the unexpected doped layer on one side is undertaken in the fabrication of bifacial solar cells. We have implemented a spin-etching technique to remove the doped layer; however, the removal leads to a planar surface, and the related optical losses affect the bifaciality of the cell through a lowering of the short-circuit current. In this paper, we demonstrate a new procedure to maintain the pyramid texture on the side from which the doped layer is removed. The effect of the texture-maintaining removal process was examined using scanning electron microscopy (SEM) and reflectance measurements. SEM images show that the edges of the texture pyramids become wider. The optical losses due to the wider edges were investigated through reflection measurements. To investigate the potential of this new procedure, an n-type bifacial cell was fabricated on a 156 mm × 156 mm 180-μm-thick n-type pseudo Czochralski-Si wafer, and the current–voltage parameters were compared with the single-side planar n-type bifacial solar cell.

A novel approach for suppression of oxygen precipitation in CZ silicon wafers of solar cells by pre-thermal treatment

We report on the bulk lifetime improvement in Czochralski (CZ) silicon wafers of solar cells by pre-rapid thermal processing (RTP). The commercially available n-type CZ silicon wafers with the thickness of 180 µm were used. Silicon wafers were subjected to pre-RTP followed by the three-step thermal annealing: the first was oxidation at 900 °C for 60 min, the second was isothermal annealing at 1050 °C for 30 min in a nitrogen atmosphere, and the third was oxidation at 900 °C for 10 min, which corresponded to the typical processes of a fully ion-implanted passivated emitters, rear totally diffused silicon solar cell. We investigated the impact of pre-RTP condition on the oxygen precipitation in CZ silicon wafer. A pre-rapid thermal oxidation (RTO) at 1050 °C for 30 sec suppressed the oxygen precipitation during the above three-step annealing, resulting in the lifetime improvement.We report on the bulk lifetime improvement in Czochralski (CZ) silicon wafers of solar cells by pre-rapid thermal processing (RTP). The commercially available n-type CZ silicon wafers with the thickness of 180 µm were used. Silicon wafers were subjected to pre-RTP followed by the three-step thermal annealing: the first was oxidation at 900 °C for 60 min, the second was isothermal annealing at 1050 °C for 30 min in a nitrogen atmosphere, and the third was oxidation at 900 °C for 10 min, which corresponded to the typical processes of a fully ion-implanted passivated emitters, rear totally diffused silicon solar cell. We investigated the impact of pre-RTP condition on the oxygen precipitation in CZ silicon wafer. A pre-rapid thermal oxidation (RTO) at 1050 °C for 30 sec suppressed the oxygen precipitation during the above three-step annealing, resulting in the lifetime improvement.

Evaluation of Si-SiO x interface using laser terahertz emission microscope (LTEM)

Terahertz emission from a Si metal-oxide-semiconductor (MOS) structure with transparent indium tin oxide electrodes was measured using a Laser Terahertz Emission Microscope (LTEM) with time-domain spectroscopy technique. The THz waveform and amplitude at various external voltage were compared with the capacitance-voltage characteristics. The observed peak amplitude strongly correlated with the surface potential and the THz waveform inverted near the flat-band voltage. This result strongly suggest that the LTEM can quantitatively measure the surface band bending of Si and can be applied as an evaluation technique of the Si MOS devices and the passivated surfaces of Si solar cells.

Phosphorus-Implanted Emitter Crystalline Silicon Solar Cell with AL-BSF

As-implanted local phosphorus profiles in a single texture are discussed, taking into account of the incident angle of ion beam into the pyramidal textured (111) surface. The impact of tilt and twist angle on the electrical characteristics of phosphorus-implanted homogeneous emitter silicon solar cells with aluminum back surface field, 156 × 156 mm2 in size, are investigated. Conversion efficiency increases with increase of tilt angle within a range of 7° to 35°, or decrease of rotation angle within a range of 15° to 90°. Thus, tilt and twist are the key factors of as-implantation depth profile, resulting in the improvement of conversion efficiency. Based on scanning capacitance microscopy observation, two-dimensional carrier distribution in phosphorus-implanted emitter is discussed.