Browsing by Author "Qu, Fei"

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Open Access
Aluminum-induced crystallization of semiconductor thin films
(2015) Qu, Fei; Schmitz, Guido (Prof. Dr.)
Thin film materials of the semiconductors, such as silicon (Si), germanium (Ge) or their alloys, are turning into the most promising functional materials in the energy technology. However, the morphologies of these semiconductor thin films must be varied to be suitable for the different applications, e.g. a large-grained layer as the seed layer of thin film solar cells, a porous structure for anode materials of high energy rechargeable lithium (Li) ion batteries. Due to the collective interdiffusion process during the aluminum (Al)-induced crystallization, in this thesis, the suitable morphologies are achieved for the corresponding applications under the different fabrication conditions. A large-grained Si layer can be formed by the crystallization of Si in a porous Al layer, which is obtained by applying a bias voltage. Since the Al grain boundaries are contaminated by e.g. oxygen (O), the diffusion of Si in the Al grain boundaries is retarded. It can lead to a reduction of the nucleation density of Si. At a certain high temperature, a collective diffusion process of Si in Al is activated. Consequently, a large-grained Si layer with (100) texture can be formed. By purposely interrupting the annealing of nanocrystalline Al/amorphous Si (a-Si) bilayers, a porous structure of the crystallized Si can be developed due to the incomplete intermixing of Si and Al. Due to the different dominant diffusion processes of Si in Al at the different annealing temperatures, the most Si diffuses along the different paths in the Al layer, such as triple junction, grain boundary and Al bulk. Therefore, it can develop the different morphologies of the porous Si layers after the selectively etching of Al. By introducing an amorphous Ge interlayer between the crystalline Al and amorphous Si layer, the Al grain boundaries are not essential for the crystallization of the amorphous Si in contrast to the case in Al/Si bilayer system. Si crystallizes continuously on the pre-crystallized Ge seeds which form initially at the original interface of crystalline Al and amorphous Ge. The thermodynamic models to interpret the fundamentals of these different crystallization behaviors of Si are established based on the change of the interface energy between the different phases of the whole system during the crystallization. Using the effective diffusivity, the dominant diffusion process of Si in Al can be investigated to explore the morphological dependence of the crystallized Si layer on the annealing conditions.