Abstract
Silicon nanocrystals (Si NCs) embedded in a dielectric matrix showing tunable band gap properties have recently emerged as attracting top absorbers in silicon based high efficiency multijunction devices. This paper presents optical and electrical characterization of Si NCs in SiC matrix resulting from annealing at 1100 °C of silicon-rich carbide (SRC)/SiC multilayers produced by Plasma Enhanced Chemical Vapour Deposition (PECVD), varying either the Si content in the SRC or the SiC thickness. Simulation of Reflectance and Transmittance spectra in the UV-Vis revealed that 1) the Si crystallization increases with increasing Si content; 2) a severe shrinkage of the multilayers occurs upon annealing due to the release of hydrogen and to crystallisation; 3) the growth of nanocrystals is affected by atomic environment and diffusivity of involved atoms at the investigated temperature. Temperature dependent conductivity measurements are performed on multlayers and on reference layers. The results show evidence of defect state conduction in the SiC matrix. Copyright © 2012 VBRI Press.
Highlights
The possibility of obtaining band gap modulation in silicon nanodots in dielectric matrix, typically SiO2, has been proven since long [1]
The Si-H stretching mode (2050 cm-1) and the C-Hn stretching mode (2900 cm-1, very weak signal) absorption bands where i=0 refers to silicon and higher numbers refer to the different involved “molecules”, Ei is the atomic concentration of the element
The simulation was obtained with a 159 nm thick effective medium approximation (EMA) composed by 65% SiC, and the remaining Si is composed by 77% a-Si and 23% μc-Si
Summary
The possibility of obtaining band gap modulation in silicon nanodots in dielectric matrix, typically SiO2, has been proven since long [1]. In the case of the Si/SiC system, the analysis of the electrical behaviour of the sample is complicated by the not negligible contribution coming from the SiC matrix, which shorts the conduction through possible barriers between the nanoparticles, the situation being temperature dependent To this purpose, knowledge of the structural composition of the sample (Si to SiC relative volumes; residual amorphous Si and SiC fractions) is an important basis to the understanding of the electrical properties [20]. X=Si/(Si+C) ratio in the SRC in the SL’s, Si and SiC Nominal Volume Fraction after 1100 °C annealing, computed by considering the shrinkage of the Si and SiC components; optically detected SiC volume fraction (the complement to 100% is Si%), amorphous and crystallized Si fraction and total layer thickness; activation energy and exponential prefactor of the conductance as a function of inverse temperature, determined by lateral electrical measurements. A general formula for the Nominal Volume Fraction Vi of a component i for a silicon rich material completely separated into silicon and the silicon alloy(s), such as for instance silicon rich oxides, silicon rich nitrides, silicon rich oxinitrides, is given by: Vi
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