Physical properties ofa-Si:H based compositional periodic multilayers

Compositional periodic multilayers of amorphous hydrogenated silicon (a-Si:H) and either the wide-band-gap alloy a-${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{C}}_{\mathit{x}}$:H or the low-band-gap alloy a-${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ge}}_{\mathit{x}}$:H have been fabricated in a deposition system specially optimized for this purpose. Electronic well and barrier widths were systematically varied between 10 and 100 Å in different series of multilayers with and without keeping the mean composition of the multilayer films nominally constant. All multilayer films had a total thickness of 1 μm, corresponding to a number of periods up to 400. A comprehensive structural, optical, and electronic characterization of the multilayer films was performed. X-ray diffraction under grazing incidence, secondary-ion-mass spectroscopy depth profiling, and infrared absorption were used for a structural characterization and a verification of the multilayer structure. Optical transmission, photothermal deflection spectroscopy, temperature-dependent dark conductivity, steady-state photoconductivity, and the steady-state photocarrier grating were used to determine optical and electronic parameters. The dependence of optical band gaps, Urbach energies, and activation energies of the coplanar dark conductivity on well and barrier widths shows not only an increase with decreasing electronic well widths, but also a decrease with decreasing barrier widths, when the mean composition of the multilayer films is not kept constant.