Solid Phase Crystallization (SPC) Behavior of Amorphous Si Bilayer Films with Different Concentration of Oxygen: Surface
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SOLID PHASE CRYSTALLIZATION (SPC) BEHAVIOR OF AMORPHOUS Si BILAYER FILMS WITH DIFFERENT CONCENTRATION OF OXYGEN: Surface vs. Interface-nucleation
Myung-Kwan Ryu, Jang-Yeon Kwon, and Ki-Bum Kim, School of Materials Science and Engineering, Seoul National University, Seoul, KOREA ABSTRACT Solid-phase crystallization (SPC) behavior of a-Si film [a-Si(II)] in which oxygen concentration (CO) is higher at the a-Si/SiO2 interface (CO=5x1021 /cm3) than at the film surface (CO=3x1020 /cm3) has been investigated. The results were also compared with that of a-Si single layer [aSi(I), 600 Å] with CO=3x1020 /cm3. It has been found that the interface-nucleation was suppressed in the a-Si(II) and the surface-nucleation occurred to make a poly-Si/a-Si (300 Å/300 Å) bilayer structure. Many equiaxial grains with sizes of 1~2 µm were formed in the surfacenucleated poly-Si layer. Compared with the results of conventional SPC poly-Si (600 Å-thick) in which elliptical grains with sizes of 0.5~1 µm were formed by the interface (a-Si/SiO2)nucleation, we concluded that the poly-Si/a-Si bilayer scheme is a method to improve the microstructure of SPC poly-Si film. INTRODUCTION Solid-phase crystallization (SPC) of amorphous Si (a-Si) has been attracted many interests for the application to polycrystalline Si (poly-Si) thin-film transistors (TFTs) in a liquid-crystal displays (LCDs)[1,2] Many efforts on improving the microstructure of poly-Si film by reducing the grain boundary area and crystalline defects due to their detrimental effects on the performances of TFT. In recent work, we proposed a novel crystallization scheme in which the crystallization start from a film surface, i.e. the surface-nucleation.[3] In that study, the interface-nucleation phenomenon, dominantly occurring in a conventional annealed a-Si, was suppressed by making an oxygen-rich region near the interface between a-Si film and SiO2 substrate with an oxygenblowing process during initial a-Si deposition period. Thus, we could observe a surfacenucleated SPC. However, the oxygen-blowing process still remains to be optimized since it may cause a problem such as a particle formation which is not practical for TFT fabrication process. In this work, we utilized residual oxygen in our deposition chamber for making an oxygenrich region at near the a-Si/SiO2 interface in order to suppress the interface-nucleation. Thus, there is no extra oxygen-blowing process. From the work of crystallization behavior of a-Si films with different CO, we found out the CO at which the crystallization is nearly suppressed. Based on this result, we have deposited an a-Si/oxygen-rich a-Si bilayer and comparatively investigated its SPC behavior with that of a conventional a-Si single layer. EXPERIMENTS AND RESULTS Deposition and annealing of amorphous Si films with different oxygen concentration Amorphous Si films at 425 oC with 1 SCCM of Si2H6 and no carrier gas. Deposition pressure was 0.05, 0.1, 0.2, and 0.4 Torr, which was adjusted by varying the conductance of throttle valve connected to a mechanic
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