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978-3-8439-1718-6, Reihe Physik
Characterization of femtosecond laser sulfur doped silicon
113 Seiten, Dissertation Technische Universität Clausthal (2014), Softcover, A5
This thesis investigates the structural, electrical and photoelectrical properties of silicon exposed to femtosecond laser pulses under a sulfuric atmosphere. The sulfur atom concentration depth profile of this material is determined with secondary ion mass spectroscopy. A maximum sulfur atom concentration far beyond the solubility limit is found in the top 100 nm and decreases exponentially.
For the determination of the donor concentration, standard Hall effect or capacitance-voltage measurements are rendered useless due to the structured surface and the intrinsic pn-junction. Hence, an advanced capacitance-voltage method which uses impedance spectroscopy is developed. In the process, significant problems with the standard method are pointed out. The new method evaluates the impedance spectrum of a sample and can therefore account for rough interfaces and additional space charges. It is revealed that only about 0.1 % of the sulfur atoms act as donors. Close to the surface, an effective donor concentration is measured, which is again above the solubility limit of sulfur in silicon. Hence, this thesis proves that the femtosecond laser process is suitable to incorporate electrically active sulfur atoms into silicon with higher concentrations than possible with thermal diffusion based methods.
Photoelectrical measurements prove that this material is able to convert infrared light into excess charge carriers. When this material is used as a solar cell, the current-voltage curve shows a photocurrent build-up for monochromatic infrared illumination. An external quantum efficiency measurement verifies an absolute higher efficiency for wavelengths above 1150 nm when compared to a high efficiency reference silicon solar cell. When the lower integral efficiency of the laser-structured solar cell is considered, the effect is even larger, leading to a relative infrared efficiency increase of 35 %. The strong sub-bandgap spectral response likely indicates the presence of the impurity band photovoltaic effect, since the measured photocurrent is solely driven by the internal photovoltaic effect. This means that the concentration of the deep sulfur levels determined by the photocurrent measurements could be high enough that the electrons' wavefunctions overlap and non-radiative recombinations are restrained. If this is actually the case, the femtosecond laser-doped silicon would be one of the first experimental demonstrations of the impurity band photovoltaics theory.