Correlation of grain-to-grain electrical properties with impurities and defects in solar grade polycrystalline silicon

by 1974- Sen, Indradeep

SEN, Indradeep. Correlation of grain-to-grain electrical properties with impurities and defects in solar grade polycrystalline silicon. (Under the guidance of Dr. George Rozgonyi.) In this study a three pronged diagnostic approach has been adopted for the characterization of active recombination centers in thin film polycrystalline silicon. It comprises structural, chemical and electrical analysis of the various lifetime limiting defects and impurities which are responsible for the variations observed in the electrical activity across different grains. Preferential defect etching using Wright and Secco etchant solutions provided for the structural analysis. This approach was useful in determining the nature and distribution of the structural defects and how they affect the growth characteristics. The process induced structural changes were also delineated using this approach and successfully correlated with the electrical activity observed in particular regions. The work then focuses on chemical characterization of the impurities and the analytical tools used for this purpose were Fourier Transform Infra-red Spectroscopy (FTIR) and Deep Level Transient Spectroscopy (DLTS). The chemical data were enhanced by Secondary Ion Mass Spectroscopy (SIMS) data from Astropower (AP). The effect of light elements like oxygen, carbon and nitrogen were studied in relation to their concentration, agglomeration and generation of heterogeneous nucleation centers for metallic precipitation in various grains through FTIR studies. DLTS, on the other hand, identified the metallic impurities (Fe, Cr & Al) which are acting as deep traps in the material and quantified their gettering efficiency at the grain-boundaries and extended defects in the material. The above characterization tools were well complemented by electrical characterization through micro-wave photo-conductance (µ-PCD) decay lifetime mapping and Electron Beam Induced Current (EBIC) techniques. These two techniques facilitated the identification of the electrically active areas in the material while providing images to correlate with the etch pit data. Through the above complimentary set of characterization tools, it was shown that the electrical activity of a grain is determined by its size and local defect density. Generally, grains less than 100 µm in diameter performed worse than the large ones. A large variation in the gettering and precipitation behavior of individual grains has been demonstrated, typically controlled by the defect clusters or “Black Spots” in the grains. Metal precipitation takes place in the as-grown as well as thermally treated samples. In the as-grown samples, SiO x complexes and other micro-defects provide the nucleation sites, whereas the same has been shown to be provided by oxygen and carbon precipitates after thermal treatment. The formation of an impurity free denuded zone occurs along the grain-boundaries (GB) while twin-boundaries act as diffusion barriers for impurities but exhibit no electrical activity, unlike the GB’s. It is also shown that oxygen precipitation after thermal treatment is aided by carbon rather than nitrogen. Furthermore, the presence of SiN inclusions in small grains is discussed in light of increased N-related IR absorption in these regions. Finally, the gettering efficiency of the thermal processes adopted by AP has been characterized by DLTS and µ-PCD measurements and it is shown that the aluminum gettering step is the most effective in this regard and determines the final diffusion length of the material. Correlation of grain-to-grain electrical properties with impurities and defects in solar grade polycrystalline silicon By: