PHYSICAL-PROPERTIES OF A-SI-H BASED COMPOSITIONAL PERIODIC MULTILAYERS

Compositional periodic multilayers of amorphous hydrogenated silicon (a-Si:H) and either the wideband-gap alloy a-Si1-xCx:H or the low-band-gap alloy a-Si1-xGex: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 Angstrom 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 mu m, 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. This parallel behavior of different physical quantities is inconsistent with an explanation solely in terms of quantum-size effects. Alternative qualitative explanations are presented in terms of an artifact of the Tauc band gap, mutual influence of the structural disorder, Fermi-level adjustment, interfacial transition layers, and deviations from the nominal composition. In addition, for the blueshift of the optical band gap, it is shown that it can, under some assumptions, even be reproduced quantitatively by purely classical calculations of the optical transmission behavior of the amorphous multilayer films. It is concluded that the contribution of quantum-size effects to the characteristic dependences of physical quantities on electronic well widths has been overestimated so far. A qualitatively consistent physical explanation is even possible without assuming any quantization in subbands in this class of materials. A smaller quantum-size effect, however, can, of course, not be excluded stringently.