Gerichtete Erstarrung von einkristallinen Siliciumkristallen nach dem VGF-Verfahren für die Anwendung in der Photovoltaik

In dieser Dissertation werden das Wachstumsverhalten sowie die Defektbildung während der Züchtung von Quasi-Mono-Siliciumkristallen in einem Tiegel untersucht. Schwerpunkte liegen dabei auf der Ausbildung des multikristallinen Randwachstums und der Versetzungsentstehung an Keimstößen. Das in einem Laborofen hergestellte Kristallmaterial wird hierfür u.a. hinsichtlich der Kristallorientierung (Laue-Verfahren) und des Defekthaushalts (defektselektives Ätzen) charakterisiert

Investigation of dislocation cluster evolution during directional solidification of multicrystalline silicon

Dislocation clusters are the main crystal defects in multicrystalline silicon and are detrimental for solar cell efficiency. They were formed during the silicon ingot casting due to the relaxation of strain energy. The evolution of the dislocation clusters was studied by means of automated analysing tools of the standard wafer and cell production giving information about the cluster development as a function of the ingot height. Due to the observation of the whole wafer surface the point of view is of macroscopic nature. It was found that the dislocations tend to build clusters of high density which usually expand in diameter as a function of ingot height. According to their structure the dislocation clusters can be divided into light and dense clusters. The appearance of both types shows a clear dependence on the orientation of the grain growth direction. Additionally, a process of annihilation of dislocation clusters during the crystallization has been observed. To complement the macroscopic description, the dislocation clusters were also investigates by TEM. It is shown that the dislocations within the subgrain boundaries are closely arranged. Distances of 40-30 nm were found. These results lead to the conclusion that the dislocation density within the cluster structure is impossible to quantify by means of etch pit counting

Modeling of incorporation of O, N, C and formation of related precipitates during directional solidification of silicon under consideration of variable processing parameters

An axisymmetric time-dependent model of the melt region is presented for the diffusive and convective heat and O, N and C transfer as well as the formation of SiO2, Si3N4 and SiC precipitates during crystallization of multi-crystalline silicon ingots. The species model considers different feedstock qualities, the evaporation of SiO from the free melt surface, the incorporation of carbon via CO from the gas atmosphere into the melt, the dissolution of the Si3N4 crucible coating by the silicon melt, a carbon flux into the melt resulting from carbon contamination of the Si3N4 coating and the segregation effect by the moving phase boundary. Beside the development of the species transfer model a detailed parameter variation is shown. The numerical results were compared with experimental findings obtained with a laboratory scale crystal growth facility, wherein Si-ingots with a diameter of 6 cm and a height of 4-5 cm were directionally solidified. It will be demonstrated that the species model can describe the experimental results

Delay of Regeneration by Adding Aluminum in Boron Doped Crystalline Si

In this work, two B-doped Cz-grown Si materials with different Al concentrations are investigated concerning the long-term behavior of excess charge carrier lifetime under injection at elevated temperature. By determining the defect density and the surface saturation current density, a delay in regeneration and a delay in the onset of surface-related degradation is found in the material containing an order of magnitude more Al. Investigations under constant excess carrier concentration reveal that the effect of the delay is still significant, but less pronounced compared to constant generation conditions, so the effect causing the delay seems to be injection-dependent. The findings could be explained by the higher activation energy for the splitting of Al-H pairs compared to splitting of B-H pairs, which might cause a delayed release of H from the dopant-H configuration. Assuming that regeneration depends on this released H, the delay in regeneration could be explained by this model.publishe

Evaluation of improvement strategies of grain structure properties in high performance multi-crystalline silicon ingots

High performance multi-crystalline silicon (HPM-Si) for the use in photovoltaics is characterized by a very fine grain structure and a high content of random grain boundaries, finally resulting in a low dislocation density and consequently in a high material quality. Typically, the grain size increases and the fraction of random grain boundaries decreases over ingot height due to annihilation mechanisms, especially in the first 150 mm. One approach for further material improvement is to further increase the initial random grain boundary fraction and to maintain it as high as possible over the complete ingot height. In this work, several theoretical approaches to achieve these points were evaluated by experiments in G1 scale. Firstly, the influence of the silicon seeding material on the initial grain structure was investigated regarding the effect of extremely fine Si particles in the µm to nm range and the bulk density of the particle layer. Secondly, the effect of the initial geometrical grain boundary arrangement in the seed layer was evaluated. For that purpose, special seed alignments similar to the Quasimono approach were tested. Finally, the process parameter growth rate was varied in a wide range to investigate its influence on the evolution of the grain boundaries during growth. The results show that the optimum for the initial random grain boundary fraction is already reached by the existing methods/commonly used seed materials. Concerning the decrease of the random grain boundary fraction over ingot height, some technical aspects were identified which are able to keep the amount of random grain boundaries at a high level. However, the practical realization within an industrial setup might be difficult

In Situ Imaging of Dislocation Expansion in FZ-Si Seeds During Temperature Ramp Heating Process

International audienceThe impact of the thermal field in a directional solidification furnace on the generation and propagation of dislocations is investigated in intrinsic floating zone single crystal silicon. Seeds with different crystallographic orientations are wire-cut from mono-crystalline wafers and dislocation sources are solely left at the edges. Thermal annealing experiments are carried out in situ at the European synchrotron radiation facility and the evolution of the silicon crystalline quality is studied by X-ray diffraction imaging technique. At 1073 K, dislocations nucleate only at the edges and their strain field remains local. At higher temperature (1373 K), dislocations propagate throughout the entire width of the seed via the preferential activation of slip planes, related to the crystallographic orientation of the seed. These results confirm the high importance of seed preparation in mono – like silicon growth process. and chemical polishing of all seed surfaces, including their edges, are mandatory to prevent dislocation expansion and multiplication