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We demonstrate a novel monocrystalline high-performance thermistor material based on SiGe quantum well heterostructures. The SiGe/Si quantum wells are grown epitaxially on standard Si [001] substrates. Holes are used as charge carriers utilizing the discontinuities in the valence band structure. By optimizing design parameters such as the barrier height (by variation of the germanium content) and the fermi level Ef (by variation of the quantum well width and doping level) of the material, the layer structure can be tailored. Then a very high temperature coefficient of resistivity (TCR) can be obtained which is superior to the previous reported conventional thin film materials such as vanadium oxide and amorphous silicon. In addition, the high quality crystalline material promises very low 1/f-noise characteristics promoting an outstanding signal to noise ratio as well as well defined and uniform material properties.
High-resolution X-ray diffraction was applied to characterize the thickness and Ge content of QWs. The results show sharp oscillations indicate an almost ideal super lattice with negligible relaxation and low defect density.
The impact of growth temperature on the thermistor material properties was characterized by analyzing how the defect density, misfit dislocations and resulting tensile strain primarily affect the performance of the TCR and 1/f noise. Results illustrate a value of 3.3 %/K for TCR with a low 1/f noise.
The thermistor material can be implemented [1] on a bolometer and acts as an uncooled LWIR detector. Due to its silicon nature, it is compatible to low cost devices and has the potential in a wide variety of infrared applications (security, medical, automotive etc.).
REFERENCE:
[1] Jan-Erik Källhammer et al, ” Fulfilling the pedestrian protection directive using a long-wavelength infrared camera designed to meet both performance and cost targets”, Proc. SPIE Vol. 6198, pp. (619809-1) – (619809-11), Strasbourg, France, 2006.
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