In this work a novel 55 kHz plasma-enhanced chemical vapour
deposition (PECVD) technique for deposition of a-SiGe:H films at
high deposition rates is developed. According to infrared
spectroscopy and atomic force microscopy analysis the 55 kHz
a-SiGe:H films possess island-type microstructure. The
Si-Hn and Ge-Hn configurations are clustered on the
island surfaces and do not affect the optical band gap. The Tauc
band gap is determined by the concentration of Ge-Si bonds formed
in the interior of the islands. At low germanium content in the gas
mixture (RGe⩽27.5%) the Si-Hn-related
microstructure controls the Fermi level position while the formation
of Ge-related defects determines which recombination centres are
formed in the a-SiGe:H films. At RGe>27.5% the formation
of Ge-Hn microstructure controls the optoelectronic properties
of 55 kHz PECVD a-SiGe:H films.
The results of the investigation of the properties of a-SiGe:H/c-Si heterostructures fabricated by LF (55 kHz) PECVD are presented. The investigation of spectral characteristics of a-SiGe:H/c-Si heterostructure in the wavelength range from 500 to 1100 nm showed that the short-wave absorption edge is determined by optical gap of a-SiGe:H alloy. The position of maximum of specific photosensitivity shifts from 830 to 940 nm with the increase of germanium concentration in the a-SiGe:H alloy. It was established that in a-SiGe:H/c-Si heterostructures the multitunnelling capture-emission mechanism prevails in low voltage range (<1.0 V). ). In this case the emission of holes to the valence band of a-SiGe:H from the defect states caused by Ge dangling bonds predominate. When applied voltage is more than 1.5 V, space charge limited current is observed. In this case the rise of the current is provided by the thermal emission of electrons from the defect states associated with Si dangling bonds in the upper half of a-SiGe:H.
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