Analysis of the microstructure of silicon quantum dot superlattice embedded microcrystalline silicon carbide for solar cell application

Abstract

We have recently demonstrated [1] p-i-n structure diodes with the intrinsic (i) layers comprising of periodic depositions of silicon quantum dot embedded microcrystalline silicon carbide and silicon rich microcrystalline silicon carbide layers by rf (13.56 MHz) plasma enhanced chemical vapour deposition method. The rf power was toggled between 80 mW/cm3 and 40 mW/cm3 in successive layers. The high power layer (HPL) contains silicon nanocrystallites which forms a layer of silicon quantum dots in HPL having thickness within the limit of silicon excitonic Bohr radius (~5nm). The low power layer (LPL) forms a silicon rich microcrystalline silicon carbide layer when thickness is large but becomes nearly amorphous at low thicknesses. By optimizing the layer thicknesses of HPL and LPL the interaction between the quantum dots in alternate HPL layers gave rise to the formation of an intermediate band gap resulting in good photovoltaic properties of the diode. In this paper we report an analysis of the ellipsometric data of a series of the multilayered samples of HPL having thickness at 5 nm and LPL thickness controlled between 13 nm and 2 nm to observe the evolution of the microstructure near the formation of the intermediate band. The imaginary part of the pseudodielectric function data of the multilayered samples obtained from ellipsometry have been fitted with the help of effective medium approximation method using the respective microcrystalline silicon carbide films i.e. HPL or LPL together with the phase separated poly silicon and voids as the reference materials for modeling, we observe that the volume fraction of the nanocrystalline silicon increases while the void fraction decreases within the superlattice with the lowering of the LPL thickness from 13 nm to 2 nm. These data are consistent with the evolution of the short range order within the multilayer samples and may serve as a guideline for designing the silicon quantum dot based solar cells.