Hydrogenated amorphous silicon thin films with nanocrystalline silicon inclusions ͑a / nc-Si: H͒ have received considerable attention due to reports of electronic properties comparable to hydrogenated amorphous silicon ͑a-Si: H͒ coupled with an improved resistance to the light-induced formation of defects. In this study, a / nc-Si: H thin films are synthesized via radio-frequency plasma-enhanced chemical-vapor deposition with helium and hydrogen diluted silane. The plasma conditions were chosen to simultaneously deposit both Si nanocrystallites and an amorphous silicon matrix. This structure has been confirmed by transmission electron microscopy ͑TEM͒ studies. Both plasma electronic diagnostics and TEM image analysis of a / nc-Si: H films deposited with and without a temperature gradient between the capacitively coupled reactor electrodes suggest nanoparticle formation in the plasma, as opposed to solid-state nucleation of the nanoparticles in the film. Optical-absorption studies of the a / nc-Si: H films indicate electrical properties comparable to a-Si: H. In particular, the evolution of the films' photoconductivity over light exposure time shows a Staebler-Wronski effect similar to a-Si: H.
Recent work has shown that the electrical properties of hydrogenated amorphous Si films with nanocrystalline inclusions (a/nc-Si:H) make this material a promising candidate for applications in solar cells. The present study applies the technique of spherical aberration-corrected high-resolution transmission electron microscopy for the identification and analysis of the crystalline content of an a/nc-Si:H film. By varying both the spherical aberration of the objective lens and the defocus, regions of crystallinity in the a/nc-Si:H film can be identified. This study reports the analysis of Si nanoparticles of approximately 1.5 nm in size. Some of these nanoparticles contain planar defects, such as twin defects and stacking faults. All particles observed were the same crystal structure as bulk Si, which agrees with theoretical cluster calculations. Beam damage was observed in the amorphous matrix for long electron–beam exposures.
Thin films of hydrogenated amorphous silicon containing nanocrystalline silicon inclusions (a/nc-Si:H) have been synthesized in an RF capacitively coupled PECVD system using a mixture of hydrogen diluted silane and helium, under deposition conditions at the edge of powder formation within the plasma. High resolution TEM confirms the presence of nanocrystallites as small as 2 nm in these films. Measurements of the optical absorption spectrum using CPM and PDS indicates a broadening of the Urbach slope in the a/nc-Si:H, compared to a-Si:H films, but no appreciable increase in midgap absorption. Despite the deposition conditions for the a/nc-Si:H being very different from those associated with producing optimal quality a-Si:H, the dark conductivity and photoconductivity values, and the sensitivity to light-induced defect creation in the a/nc-Si:H films are comparable to those in a-Si:H.
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