Amorphous Silicon Thin-Films for Uncooled Infrared Microbolometer Sensors

An important application of thin-film hydrogenated amorphous silicon (alpha-Si: H) is infrared detection and imaging with microbolometer focal plane arrays. Key alpha-Si: H electrical transport properties that influence detector design and performance are resistivity and temperature coefficient of resistance (TCR). These properties have been measured over a wide temperature range for p- and n-type doped alpha-Si: H thin-films deposited by plasma enhanced chemical vapor deposition using silane as a precursor gas. Resistivity near and above room temperature follows an Arrhenius thermally activated dependence. At low temperatures, resistivity transitions from Arrhenius behavior to a variable range hopping mechanism described by the Mott relation and TCR changes at a slower rate than predicted by thermally activated transport alone. Resistivity and TCR are affected by doping and film growth parameters such as dilution of the silane precursor with hydrogen. Resistivity decreases with dopant concentration for both p-type and n-type dopants. Resistivity and TCR increase with hydrogen dilution of silane. TCR and resistivity are interrelated and optimization of thin-film preparation and processing is necessary to obtain high TCR with resistivity values compatible with readout integrated circuit designs. Such optimization of transport properties of alpha-Si: H films has been applied to the development of high performance ambient operating temperature (uncooled) microbolometer arrays.