Seeding Solid Phase Crystallization of Amorphous Silicon Films with Embedded Nanocrystals
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1066-A06-14
Seeding Solid Phase Crystallization of Amorphous Silicon Films with Embedded Nanocrystals Curtis Anderson, and Uwe Kortshagen Department of Mechanical Engineering, University of Minnesota, 111 Church St. S.E., Minneapolis, MN, 55455 ABSTRACT Silicon nanocrystals with diameters up to 30 nm are used as nucleation seeds for fast solid phase crystallization of amorphous silicon films. Purely amorphous films required an incubation time of up to 12 hours at 600°C prior to the onset of nucleation, while films with nanocrystals embedded between layers of amorphous silicon grew immediately upon annealing in a quartz tube furnace. Structural characterization was performed by heated-stage transmission electron microscopy and Raman spectroscopy. INTRODUCTION In recent years, thin film Si photovoltaic (PV) cells have gained in popularity compared to wafer-based modules. The last several years have seen a significant increase in the number of thin film PV modules sold [1]. While partially attributed to the recent shortage of crystalline Si wafers, the fast production of high quality Si thin films is critical for the advancement of costeffective PV devices. Further, the conversion efficiency of amorphous Si (a-Si) solar cells will need to improve dramatically if they will ever compete with mono-crystalline wafer devices [2]. Large grain polycrystalline Si films (poly-Si) are viewed as a possibility to fill this role. The techniques used to produce poly-Si range from the classic direct deposition via thermal chemical vapor deposition [3], to the more recent “recrystallization” methods [4,5]. Furnace recrystallization of amorphous Si films (a-Si) has been studied for many years [6], and is understood as a classical nucleation and grain growth mechanism. The limitation of the process comes from the steady-state nucleation rate; one needs to limit or control the nucleation process in order to maximize the final annealed grain size. Other approaches have been successful in producing poly-Si with grains larger than several µm, such as excimer laser annealing [7], metalinduced crystallization [8], or even epitaxial growth over poly-Si seed layers [9]. These approaches find their respective limitations due to either fabrication scaling issues or additional processing steps to remove metal impurities. We introduce here a radically new yet simple approach for fast recrystallization of a-Si, wherein the nucleation of grains is controlled. This was accomplished by embedding freestanding Si nanocrystals in between separately deposited layers of a-Si film. Upon annealing in a typical tube furnace at 600°C, the embedded nanocrystals act as nucleation “seeds” for the surrounding amorphous film. The structure of the films is reported, along with initial measurements of the crystallization kinetics compared to films without nanocrystals embedded.
EXPERIMENT A-Si:H films with embedded Si nanocrystals were produced using a multi-layer deposition process. Initially, a 100 nm thick a-Si:H film was deposited at 250°C in a parallelplate pl
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