Optimizing of the minority carrier lifetime measurements on monocrystalline bulk silicon samples

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Supervisor:
Dr. Mizsei János
Department of Electron Devices

This thesis aims to improve the charge carrier lifetime measurement on monocrystalline silicon ingots. One of the available methods for lifetime assessment at Semilab Co.Ltd. is the e-PCD (Eddy-current Photoconductance Decay) technique. However, the ever-changing needs of the photovoltaic industry has made the optimization of the method a requirement.

First, I present the current economic and technological trends, which trigger the need for better optimized e-PCD techniques in a wide range of applications. To support the understanding of my work, this part is followed by the presentation of the recombination mechanisms and the introduction of different PCD lifetime measurement techniques.

My first task was – as a part of a larger project – the creation of a calculator, which can determine the excess charge carrier density and the injection level in an iterative way, using effective parameters. The calculator considers the specification of the light source and the material properties of the silicon. I compared the results of the finished calculator with a - slower operating – simulation software, and the actual outcome of e-PCD measurement. In the entire range of tested lifetimes, my calculator has shown a difference in injection level below 20%, compared to the simulator. Thus, the calculator proved to be more precise than the formula - used by the measuring device - optimized for Si wafers.

I utilized the completed calculator to propose improvements on the light source, used for the e-PCD measurement. I investigated the impact of replacing the actual laser to another type, emitting longer wavelength light. This modification resulted in a reduced (parasitic) impact of surface recombination (due to the deeper light-penetration), thus possibly enables more precise bulk lifetime characterisation. Furthermore, it improved the signal-noise ratio, in the entire lifetime range.

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