Manipulation of Si and Ge Crystallization

Citation

Yang, Chih Meng (1997) Manipulation of Si and Ge Crystallization. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3693-9p48. https://resolver.caltech.edu/CaltechETD:etd-01162008-112621

Abstract

This thesis discusses methods for altering the crystallization kinetics of Si and Ge in order to obtain large-grained polycrystalline semiconductor thin films or size-selected semiconductor nanocrystals in silicon dioxide. Reduction of grain boundaries in polycrystalline semiconductor thin films is important for improving the performance of microelectronic devices because grain boundaries act as traps for charge carriers. Control of nanocrystal size and concentration in silicon dioxide is important in controlling the nanocrystal photoluminescence and electroluminescence characteristics. Results of modification of crystal nucleation and growth rate via ion beam irradiation, thermal annealing, metal-induced crystallization, and dopant enhanced solid phase epitaxy are presented.

Ion beam irradiation was used to induce amorphization of 1-50 nm Si crystals in amorphous Si. A size-dependent amorphization rate was calculated from the temporal evolution of the crystal size distribution under ion irradiation. A model for irradiation- induced, size-dependent crystal growth/amorphization is developed and it shows good quantitative agreement with the present experiment as well as other experiments.

Precipitation of 1-10 nm Ge nanocrystals and 1-2 nm Si nanocrystals in silicon dioxide was accomplished by ion implantation followed by thermal annealing. Nucleation of 1-2 nm Ge nanocrystals occurred during implantation, and annealing at temperatures higher than 600°C induced coarsening. Increasing the Ge implantation dose resulted in increased nanocrystal concentration but not size. In contrast, no Si nanocrystals were observed in as-implanted samples or samples annealed at less than 1000°C. Samples annealed at 1000°C for 40 min contained 1-2 nm Si nanocrystals.

Large grained polycrystalline Ge thin films, with controlled grain location and size, were synthesized at low temperatures. Grain sizes of 10-20 µm in 50-nm-thick amorphous Ge were obtained at temperatures less than 475°C, which represents a two-orders-of-magnitude improvement over previous efforts. Selective nucleation was achieved by deposition of an array of 5-micron-diameter metal islands on top of amorphous Ge and annealing at low temperatures. During subsequent anneal at higher temperatures crystals that selectively nucleated underneath the metal islands grew tens of microns before random nucleation impeded their growth. The crystal growth rate was enhanced by doping Ge with B or P, resulting in even larger crystal sizes.

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