Ørnulf Nordseth

Modulating the field-effect passivation at the SiO2/c-Si interface: Analysis and verification of the photoluminescence imaging under applied bias method

In this paper, we study the surface passivation properties of thermally oxidized silicon wafers with controlled surface band bending, using a recently developed characterization technique combining calibrated photoluminescence imaging with the application of an external voltage over the rear side passivation layer. Various aspects of the technique and possible errors in the determination of the effective surface recombination velocity are discussed, including lateral carrier diffusion, leakage currents, and optical effects related to the presence of metal electrodes on the investigated samples. In order to quantitatively describe the recombination activity at the SiO2/c-Si interface and the effect of fixed charges in the oxide layer, the measured effective carrier lifetime vs. voltage curves have been analyzed in the framework of an extended Shockley-Read Hall recombination model. Furthermore, the results have been compared with corresponding results from microwave detected photoconductance decay measurem...

Surface structure of mono-crystalline silicon wafers produced by diamond wire sawing and by standard slurry sawing before and after etching in alkaline solutions

Fixed abrasive sawing (FAS) using diamond coated steel wires is an interesting alternative for commercial production of silicon wafers, as it has potential for increasing productivity and reducing consumables costs. The objective of this study has been to understand the differences in surface structure of Cz mono-crystalline silicon wafers produced by diamond wire sawing and by standard slurry sawing, both before and after alkaline etching. Both as-cut wafers and wafers etched in 47% KOH at 75°C for different etching times have been studied. Transmission electron microscope (TEM) investigations of the as-cut slurry wafers show an un-even surface and up to 4 µm deep micro cracks compared to the FAS wafers. Atomic force microscope (AFM) and scanning electron microscope (SEM) analyses of the ascut FAS wafers show a smooth wave-like pattern on the surface with a period of about 15 µm and amplitude of about 1 µm. During the initial part of the etching, square inverted pillars were formed for both types of wafers. The inverted pillars were initially deeper for the slurry cut wafers. The size in lateral direction of the inverted pillars increases with etching time.

Investigation of Carrier Recombination at the SiO

A new technique to analyze the surface recombination for passivated silicon substrates has been employed to study the SiO2/c-Si interface under various band bending conditions. A photoluminescence imaging setup was used to measure the effective minority carrier lifetime of oxidized Si wafers while applying an external bias over the rear side passivation layer. This method was used to investigate both the effect of substrate doping polarity and postoxidation forming gas anneal (FGA) upon the surface passivation properties. The measured carrier lifetimes as a function of voltage were interpreted in the framework of the extended Shockley-Read-Hall theory. The calculated oxide charge density was found to decrease from ~7 × 1011 cm -2 to ~4 × 1011 cm-2 after the FGA treatment for both p-type and n-type substrates, causing a reduction in the field effect passivation. On the contrary, an increased chemical passivation was observed after FGA, shown by a reduction of the effective surface recombination velocity parameters by a factor of 3.8-5.5. In total, a significant increase in the effective carrier lifetime was obtained for both substrate types. Furthermore, the carrier capture efficiency at the surface defects was found to be 2-2.5 times higher for electrons than for holes, regardless of doping polarity and FGA.

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In this work, investigation of a silicon-based tandem heterojunction solar cell was iterated via numerical modeling. The tandem cell was split into a top metal oxide and bottom c-Si subcell, and each subcell was analyzed and compared with experimental data. For the top subcell, Silvaco Atlas was used to ascertain optimum materials for the buffer layer and their impact on the cell performance. For the bottom subcell, a Quokka 2 model has be used to evaluate and compare current-voltage and quantum efficiency curves with experimental data. Transfer matrix algorithm was used to ascertain top subcell optical field characterization. The buffer layer materials for the ZnO/Cu2O subcell that yielded best cell performance are presently TiO2 and Ga2O3 while the Quokka 2 model presents a good fit with the experimental curves.

/c-Si Interface by Photoluminescence Imaging Under Applied Bias

Abstract An investigation of silicon-based tandem solar cells incorporating Al-doped ZnO (AZO) and Cu2O metal oxides, via two of the most efficient methods of optical modeling, specifically ray tracing and transfer matrix algorithms, was performed. The simulations were conducted based on specialized software, namely Silvaco Atlas and MATLAB, as well as on OPAL2 simulation platform. The optical analysis involved the calculation of the spectral curves for reflectance, absorptance and transmittance for different thicknesses of the thin film layers constituting the cell. It was established the optimum thickness of the AZO layer based on the minimum reflectance and maximum transmittance. Moreover, several materials were investigated in order to determine the optimum buffer layer for the tandem solar cell, based on optical modeling. The optical parameters of the ZnO/Cu2O top subcell were optimized, in order to achieve the highest conversion efficiency of such heterojunction solar cell.