Papers by Author: Michael Seibt

Abstract: This paper investigates the influence of different number of laser pulses on contact behavior and conductivity of the surface layer of femtosecond laser microstructured, sulfur-doped silicon. Single shot laser processed silicon (Pink Silicon) is characterized by low surface roughness, whereas five shot laser processed silicon (Grey Silicon) has an elevated sulfur content with a surface roughness low enough to maintain good contacting. To laterally confine the laser induced pn-junction part of the Grey Silicon sample surface is etched off. The etching depth is confirmed to be sufficient to completely remove the active n-type sulfur layer. While Pink Silicon shows little or no lateral conductivity within the laser processed layer, Grey Silicon offers acceptable conductivity, just as expected by the fact of having incorporated a higher sulfur dopant content. Recombination dominates the irradiated regions of Pink Silicon and suppresses excess charge carrier collection. Grey Silicon, while showing sufficient lateral conductivity, still shows regions of lower conductivity, most likely dominated by the laser irradiation-induced formation of dislocations. According to our results, the optimum laser pulse number for electrical and structural properties is expected to be in the range between one and five laser pulses.

Abstract: We report on a light-beam-induced current (LBIC)-analysis of metal silicide defects arising from co-precipitation of copper and nickel in Cz-silicon-bicrystals produced by wafer direct bonding. Large colonies of silicide precipitates in the one wafer emerging from undisturbed growth from few nucleation sites were observed in different orientations with respect to the surface which correspond to Si {110} planes. From this, the colonies formed during copper-nickel co-precipitation reveal the same attributes as those colonies typical for copper precipitation in the absence of nickel. Oxygen related defects associated with a higher defect distribution in the other wafer were characterized by means of high resolution Transmission Electron Microscopy (TEM) and their temperature dependent LBIC signal.

Abstract: The efficiency of solar cells produced from crystalline silicon materials is considerably affected by the presence of metal impurities. In order to reduce the concentration of metal impurities, gettering processes as phosphorus diffusion gettering (PDG) and aluminum gettering (AlG) are routinely included in solar cell processing. Further development and optimization of gettering schemes has to ground on physics-based simulations of gettering processes. In this contribution we use quantitative simulations to compare the efficiency and kinetics of PDG and AlG in the presence of precipitates for interstitially dissolved metals, like iron, at different gettering conditions. Recently measured segregation coefficients of iron in liquid AlSi with respect to crystalline silicon are used in order to compare with PDG under typical conditions. It is shown that kinetics of both, PDG and AlG, can be separated into two regimes: (i) at low temperatures kinetics are limited by precipitate dissolution, and (ii) at high temperatures kinetics of AlG is mainly limited by metal impurity diffusion while phosphorus in-diffusion is the limiting factor of PDG.

Abstract: We investigated the development of dislocation-related DLTS spectra in n-CZ-Si crystals with small (about 7.104 cm-2) number of long individual dislocations depending on the distance L that dislocations traveled during deformation at 600oC and on the velocity of dislocations. We found that a typical dislocation-related DLTS signal appeared only when dislocations traveled a significant distance that is more than 150-200μm, and it depended strongly on dislocation velocity. The results were interpreted on the assumption that the DLTS signal corresponds to some core defects and atomic impurities accumulated on the dislocations during their slow motion. At high concentration of deep level defects on dislocations a strange “negative DLTS” signal was observed. This can be explained by electron tunneling between deep defects along dislocations.

Abstract: Minority carrier diffusion lengths were measured for the set of n- and p-type silicon samples with NiSi2 precipitates of different electronic structure. We found that the type of precipitate electronic states in the upper part of band gap had no influence on the recombination activity of NiSi2 precipitates. Minority carrier diffusion length L was found to be related to the precipitate density N and L ~ 2 × N -1/3 for n-type Si samples and L ~ 1 × N -1/3 for p-Si samples. Hydrogenation of the p-type Si sample with nanoscale nickel silicide precipitates resulted in an increase of the L value up to a factor of 3, while in n-Si L remained practicaly the same. The only hole emission in the samples of both conductivity types was detected in MCTS measurements and the cross section for the hole capture with the electronic states of the precipitaes was estimated to be as large as 10-11 cm-2.

Abstract: Influence of annealing in molecular hydrogen as well as of treatment in hydrogen plasma (hydrogenation) on the electrical properties of NiSi2 precipitates in n- and p-type silicon has been studied by means of deep level transient spectroscopy (DLTS). Both annealing and hydrogenation gave rise to noticeable changes of the shape of the DLTS-peak and of the character of its dependence on the refilling pulse duration that according to [1] allows one to classify the electronic states of extended defects as “band-like” or “localized”. In both n- and p-type samples DLTS-peak in the initial as quenched samples showed bandlike behaviour. Annealing or hydrogenation of n-type samples converted the band-like states to the localised ones but differently shifted the DLTS-peak to higher temperatures. In p-type samples, the initial “band-like” behaviour of DLTS peak remained qualitatively unchanged after annealing or hydrogenation. A decrease of the DLTS-peak due to precipitates and the appearance of the peaks due to substitutional nickel and its complexes were found in hydrogenated p-type sample after removal of a surface layer of 10-20µm.

Abstract: Deep electronic states associated with iron silicide precipitates have been studied by means of deep-level transient spectroscopy. The observed spectra show the characteristic features of bandlike states at extended defects. From the stability of the states on annealing at moderate temperature they are tentatively attributed to precipitate-matrix interfaces.