Shinya Yoshidomi

Activation of silicon implanted with phosphorus and boron atoms by microwave annealing with carbon powder as a heat source

We report the activation of silicon implanted with phosphorus and boron atoms by microwave annealing using carbon powder as a heat source. Silicon substrates were covered with carbon powder and then irradiated with 2.45 GHz microwaves using a commercial microwave oven. Carbon powder effectively absorbs microwaves and heats itself at 1000 °C. Silicon substrates are heated by thermal conduction. We carried out implantations of phosphorus atoms at a concentration of 1.0 × 1015 cm−2 at 75 keV and boron atoms at a concentration of 1.0 × 1015 cm−2 at 25 keV for p- and n-type silicon substrates, respectively. Microwave annealing at 1000 W for 120 s achieved sheet resistivities of 140 and 85 Ω/sq for the phosphorus- and boron-implanted samples, respectively. It also realized the recrystallization of surface amorphized regions caused by implantation. Moreover, low surface recombination velocities of 3.8 × 102 and 2.7 × 102 cm/s were obtained at the top implanted surfaces for the phosphorus- and boron-implanted samples, respectively. Typical diode rectified characteristics and solar cell characteristics with a conversion efficiency of 10.1% were successfully obtained.

Photo induced minority carrier annihilation at crystalline silicon surface in metal oxide semiconductor structure

We report the properties of features of photo induced minority carrier annihilation at the silicon surface in a metal–oxide–semiconductor (MOS) structure using 9.35 GHz microwave transmittance measurement. 7 Ω cm n-type 500-µm-thick crystalline silicon substrates coated with 100-nm-thick thermally grown SiO2 layers were prepared. Part of the SiO2 at the rear surface was removed. Al electrode bars were formed at the top and rear surfaces to form the structures Al/SiO2/Si/SiO2/Al and Al/SiO2/Si/Al. 635 nm light illumination onto the top surface caused photo induced carriers to be in one side of the silicon region of the Al electrode bar of the structure Al/SiO2/Si/SiO2/Al. Microwave transmittance was measured on the other side of the silicon region of the Al electrode bars. The measurement and analysis of microwave absorption by photo induced carriers laterally diffusing across the silicon region coated with Al electrodes revealed a change in the carrier recombination velocity at the silicon surface with the bias voltage applied onto the top Al electrode. The applied bias voltages of +2.0 and −2.2 V gave peaks at surface recombination velocities of 83 and 86 cm/s, respectively, for the sample structure Al/SiO2/Si/SiO2/Al, while it was 44 cm/s under the bias-free condition. A peak surface recombination velocity of 81 cm/s was only observed at a bias voltage of −2.0 V for the sample structure Al/SiO2/Si/Al.

Minority Carrier Lifetime Measurements by Photoinduced Carrier Microwave Absorption Method

We propose a measurement system for photoinduced minority carrier absorption of 9.35 GHz microwaves using periodically pulsed light illumination at 620 nm. The ratio of average carrier density when light illumination is ON to that when light illumination is OFF, P, was theoretically analyzed for different light pulse widths. The analysis of P resulted in a formula giving the minority carrier lifetime τeff of silicon under continuous light illumination. τeff for holes was experimentally determined using the formula, and its spatial distribution was obtained to be from 1.0 ×10-3 to 1.28 ×10-3 s for n-type silicon substrates with a thickness of 520 µm coated with 100-nm-thick thermally grown SiO2 layers. We also demonstrated that τeff depended on the means of light illumination for a defective sample. Two different τeff values were obtained, 7 ×10-5 and 1.73 ×10-4 s, in the cases of light illumination to the top surface and rear surface, respectively, when the SiO2 layer was etched up to 2 nm at the top surface.

Indium–gallium–zinc–oxide layer used to increase light transmittance efficiency of adhesive layer for stacked-type multijunction solar cells

We report the increase in transmittance efficiency of the intermediate layer for multijunction solar cells caused by the indium–gallium–zinc–oxide (IGZO) layer used as the antireflection layer. Si substrates coated with a 200-nm-thick IGZO layer with a refractive index of 1.85 were prepared. The resistivity of the IGZO layer was increased from 0.0069 (as-deposited) to 0.032 Ω cm by heat treatment at 350 °C for 1 h to prevent free-carrier optical absorption. Samples with the Si/IGZO/adhesive/IGZO/Si structure were fabricated. The average transmissivity for wavelengths between 1200 and 1600 nm was 49%, which was close to 55% of single-crystal silicon substrates. A high effective transmittance efficiency of 89% was experimentally achieved. The numerical calculation showed in an effective transmittance efficiency of 99% for 170-nm-thick antireflection layers with a resistivity of 0.6 Ω cm and a refractive index of 2.1.

Multi junction solar cells using band-gap induced cascaded light absorption

We propose multi junction solar cells using an optical reflection system formed by arranging plural solar cells in decreasing order of their band gaps for achieving cascaded light absorption by their own band gaps: the first solar cell absorbs some light with a photon energy higher than the highest band gap and reflects the residual light with a lower photon energy to the second solar cell. We further propose to use plural batteries for charging electrical power generated by the individual solar cells to overcome the current matching problem in the multi-junction solar cells. We experimentally demonstrated reflection-type multi junction solar cells using commercially available hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) solar cells using air mass 1.5 light illumination. A high open circuit voltage of 24.3V was achieved, which was a sum of 19.3 and 5.0V for the individual a-Si:H and c-Si solar cells. However, since there was no current matching between the a-Si:H and c-Si solar cells, the a-Si:H–c-Si serially connected solar cell gave a maximum power of 0.057W, which was lower than 0.063W, the sum of those for the individual a-Si:H and c-Si solar cells. The method of charging electrical power from individual solar cells is useful to efficiently achieve electrical power from individual a-Si:H and c-Si solar cells in the absence of current matching in multi junction solar cells.

Minority Carrier Annihilation in Lateral Direction Caused by Recombination Defects at Cut Edges and Bear Surfaces of Crystalline Silicon

We report on the photo-induced minority carrier annihilation effect in the lateral direction caused by cut edges and partially formed bare surfaces for 500-µm-thick n-type silicon substrates coated with thermally grown SiO2 layers. A 9.35 GHz microwave transmittance measurement system with illumination with a 0.2-cm-wide 635-nm continuous wave light beam was used to measure spatial distribution of the minority carrier effective lifetime τeff. A mechanical cut decreased τeff in a 0.9-cm-wide region from the cut edges. τeff decreased from 3.4×10-3 (initial) to 6.5×10-4 s at cut edges. A simple model of carrier diffusion in the lateral direction with a carrier lifetime of 3.4×10-3 s and a recombination velocity of 500 cm/s at the cut edges well explained the experimental spatial change in τeff near the cut edges. A similar widely spatial decrease in τeff was also observed in the region coated with thermally grown SiO2 layers near the bare silicon surface formed by partial etching of SiO2.

Indium gallium zinc oxide layer used to decrease optical reflection loss at intermediate adhesive region for fabricating mechanical stacked multijunction solar cells

Reduction of optical reflection loss is discussed in three mechanical stacked samples: top crystalline silicon and bottom crystalline germanium substrates, top crystalline GaAs and bottom crystalline silicon substrates, and top crystalline GaP and bottom crystalline silicon substrates using an epoxy-type adhesive with a reflective index of 1.47. Transparent conductive Indium gallium zinc oxide (IGZO) layers with a refractive index of 1.85 were used as antireflection layers. IGZO layers were formed on the bottom surface of the top substrate and the top surface of the bottom substrate of the three stacked samples with thicknesses of 188, 130, and 102 nm. The insertion of IGZO layers decreased the optical reflectivity of the stacked samples. The IGZO layers provided high effective optical absorbency of bottom substrates of 0.925, 0.943, and 0.931, respectively, for light wavelength regions for light in which the top substrates were transparent and the bottom substrates were opaque.

Reduction in optical reflection loss at intermediate adhesive layer for mechanical stacked multi-junction solar cells

We propose a method of reduction in optical reflection loss by transparent conductive Indium Gallium Zinc Oxide (IGZO) for the purpose of fabricating mechanical stacked solar cells. Si and Ge substrates coated with 200 nm thick 0.058 Ωcm IGZO layers were stacked with epoxy-type transparent adhesive dispersed with 20 μm sized Indium Thin Oxide (ITO) particles to form a structure of Si/IGZO/adhesive/IGZO/Ge. Marked low reflectively ranged from 0.33 to 0.38 in wavelength between 1150 and 1600 nm were achieved while conventional stacked sample with the structure of Si/adhesive/Ge had reflectivity ranged from 0.51 to 0.52. Numerical analysis revealed that reduction in optical reflectivity was realized by optical interference matching of IGZO layers and proposed the best thickness of IGZO of 185 nm.

Passivation of silicon surfaces by oxygen radical followed by high pressure H 2 O vapor heat treatments and its application to solar cell fabrication

We report passivation on the silicon surfaces by combination of oxygen radical and high pressure H2O vapor heat treatment. The top bear surface of 20 Ωcm n-type silicon substrates with the rear surface coated by 100 nm thermally grown SiO2 layers were treated by oxygen radical generated from oxygen plasma via a metal mesh closing the plasma induced by 13.56 MHz radio frequency induction-coupled remote plasma with mixed gases of O2 and Ar at 10 sccm, 1.0 Pa and at a power of 100 W. The samples were subsequently annealed with 1.3×106 Pa H2O vapor heat treatment at 260°C for 3 h. A high effective minority carrier lifetime of 1.1×10-3 s was achieved in the case of 635 nm light illumination on the top surface in the case of the oxygen radical treatment for 3 min. Metal-insulator-semiconductor-type solar cells were formed by formation Al and Au metal strips with a gap length of 17 μm on the passivated surfaces. When airmass 1.5 at 100 mW/cm2 were illuminated on the rear surface with 100 nm thermally grown SiO2 layers, solar cell characteristics were observed by applying voltage between Al and Au electrodes. The open circuit voltage and efficiency were obtained as 0.51 V and 6.4 %.

Passivation of silicon surfaces by heat treatment in boiled water and its application of solar cells

We report passivation of silicon surfaces by heat treatment in boiled water. 500-μm-thick n- and p-type silicon substrates with bare surfaces were treated in boiled water for 1 h at 97-99°C. High values of the effective minority carrier lifetime were obtained to be 6.3×10-4 and 4.0×10-5 s for n- and p-type samples. The recombination velocity was estimated to be 34 and 680 cm/s for n- and p-type samples. Those results indicate a possibility of passivation of silicon surfaces by simple heat treatment in boiled water. Metal-insulator-semiconductor type solar cells were demonstrated with Al and Au metal formation with gaps of 100 and 90 μm for n- and p-type samples to cause an asymmetric internal built-in potential distribution in silicon because of the difference between their work functions. Rectified current voltage and solar cell characteristics were observed. The open-circuit voltage and short-circuit current density were 0.29 V and 11 mA/cm2 for n-type sample, and 0.42 V, 5.6 mA/cm2 for p-type sample.