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A17.02 - Pulsed-Light Crystallization of Thin Film Silicon, Germanium, and Silicon Germanium Alloy 
Date/Time:
April 25, 2014   8:45am - 9:00am
 
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Thin film silicon and silicon materials used in solar cell and thin film transistors have evolved from amorphous silicon (a-Si:H) to nano-crystalline silicon (nc-Si:H) and large grain polycrystalline silicon (poly-Si). nc-Si:H films have provided significant improvements in the efficiency of thin film silicon solar cells. However, thin film silicon solar modules have lower efficiency than c-Si modules and as a result struggle to compete in the market. One method for improving the efficiency of thin film Si solar cells is to make large grain poly-Si films. Large grain polysilicon thin films (LTPS) with high mobility are also needed for the thin film transistors used in high resolution LCD and OLED displays. Currently LTPS TFTs are made using laser induced crystallization or metal induced solid phase crystallization of a-Si:H. These are expensive methods which are difficult to scale to large size displays. A low cost process for producing thin film poly-Si or possibly poly-SiGe is highly desirable for both photovoltaic and display applications.We present our progresses in the fabrication of poly-Si, poly-SiGe and poly-Ge thin films using a pulsed-light induced crystallization method. We used sputter deposited a-Si, a-SiGe, and a-Ge as the starting materials. Sputtered Si, SiGe and Ge thin films are a lower cost and less complex process than the industry standard PECVD process. Sputtered films also do not require a de-hydrogenation process. We used a pulsed-Xenon-lamp system with multiple-lamps to illuminate large-area samples. This process is much less expensive than the typical excimer laser process used in the display industry and is suitable for large area scale-up. Using this process we have demonstrated that we can uniformly crystallize a-Si, a-SiGe, and a-Ge with a single-pulse or multi-pulse process on 10×5 cm2 glass substrates. We found that the required crystallization power for a-Ge is much lower than a-Si. The power needed to crystallize a-SiGe is between the power required a-Ge and a-Si with power increasing with increasing Si fractions. We did significant material characterizations using SEM, AFM, Raman and Spectroscopic Ellipsometry (SE). The SEM and AFM images showed a significant increase of the surface roughness, which corresponds to the formation of crystallites. Two kinds of features are observed in some poly-SiGe films with large features of a few µm, which could be the large grains; and small features of 0.5 µm, which could be a phase separations of Ge and Si. No Raman signal was measurable in the as-deposited films. Distinct Ge-Ge, Si-Ge, and Si-Si vibration modes were observed at 285 cm-1, 390-1, and 470 cm-1, respectively, in the poly-SiGe films formed after the pulsed-light treatments. Their intensity ratios depend on the Ge/Si ratio and the light intensity used for the crystallization. We are in the process of making TFTs and solar cells. The device characteristics will be presented at the conference.
 


 
 
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