Device grade microcrystalline silicon owing to reduced oxygen contamination

As-deposited undoped microcrystalline silicon (<i>µ</i>c-Si:H) has in general a pronounced <i>n</i>-type behavior. Such a material is therefore often not appropriate for use in devices, such as <i>p-i-n</i> diodes, as an active, absorbing <i>i</i> layer or as channel material for thin-film transistors. In recent work, on <i>p-i-n</i> solar cells, this disturbing <i>n</i>-type character had been successfully compensated by the ``microdoping'' technique. In the present letter, it is shown that this n-type behavior is mainly linked to oxygen impurities; therefore, one can replace the technologically delicate microdoping technique by a purification method, that is much easier to handle. This results in a reduction of oxygen impurities by two orders of magnitude; it has, furthermore a pronounced impact on the electrical properties of µc-Si:H films and on device performance, as well. Additionally, these results prove that the unwanted donor-like states within µc-Si:H are mainly due to extrinsic impurities and not to structural native defects.