Dopant and defect density imaging in crystalline Silicon
Creators: name: Lim, Siew
Publisher: name: The Australian National University
Year: 2014
Subjects: subject: Silicon crystals
subject: Silicon Solar cells Design
subject: Photoluminescence

description: Accurate knowledge of dopant concentration of silicon wafers is of considerable interest for solar cell fabrication. Conventionally, dark conductance measurements, four-point probe analysis or eddy current mapping are used for the determination of dopant concentrations in silicon material. The main drawback of these methods lies in their limited spatial resolution, as well as the slow nature of their point-by-point mapping. The thesis is concerned with the development of a photoluminescence based imaging technique for spatially resolved measurement of dopant density and the applications of the technique in crystalline silicon. The physical mechanism underlying luminescence from band-to-band carrier transition is directly determined by the pn product. Thus, the measurement of photoluminescence intensity can provide information about either the majority carrier or the minority carrier concentration. In this thesis, two techniques for spatially resolved determination of the dopant density in silicon wafers, utilizing either information, are demonstrated. The first technique is based on measuring the formation rate of iron-acceptor pairs, through monitoring excess carrier lifetimes by using band-to-band photoluminescence in low injection. This method provides absolute boron concentration images on p-type wafers, even of compensating dopants such as phosphorus are present, without reference to other techniques. The second technique is based on photoluminescence images of unpassivated wafers, where the excess carrier concentration is pinned by a high surface recombination rate, thus allowing imaging of the majority carrier concentration in the wafer. This rapid technique is applicable to either p- or n-type wafers, when the bulk carrier lifetime is much longer than the transit time to the surface. The technique was also confirmed by results determined using the Sheet Resistance Imaging method. In the thesis, various surface preparation methods to achieve homogeneous, high recombination velocity is also described in detail. Several applications of the dopant density imaging technique are introduced, which include thermal donor imaging, radial dopant density variation analysis in Czochralski and float zone wafers, donor related defects in high resistivity phosphorus-doped float zone silicon wafers. In addition to the above examples, the thesis also presents an application of the technique in the reconstruction of solid-liquid interface shape along the height of a directionally-solidified multicrystalline silicon ingot, by employing Scheil's Law of impurity segregation. The reconstructed three dimensional interface images allows for optimization for the ingot growth process. The last section of the thesis demonstrates a method for determining effective boron-oxygen related defect density on Czochralski-grown silicon. By combining the dopant density imaging technique based on surface limited conditions, and incorporating a microscopic Fourier transform infrared spectroscopy measurements of the local interstitial oxygen concentration, the established dependencies of the BO-related defect density on the interstitial oxygen density and the boron dopant density are confirmed through wafer-based analysis. The result also demonstrates an additional technique for studying this important defect in silicon wafers.