Field effect in co-deposited quantum confined a/nc-Si:H, The

Abstract

When considering new materials for optoelectronic applications, the transport of charge carriers, in the dark and under illumination, in the material is of utmost importance. In this dissertation, the transport of charge carriers in co-deposited mixed phase amorphous/nanocrystalline silicon (a/nc-Si:H), a nano-composite consisting of quantum confined silicon quantum dots (SiQDs) embedded in an amorphous silicon (a-Si:H) matrix, is examined using the field effect. A/nc-Si:H is grown by concurrently embedding crystalline silicon nanoparticles, grown in a flow-through plasma reactor, into an amorphous silicon film that is being grown in a Plasma Enhanced Chemical Vapor Deposition (PECVD) chamber. With this process, we are able to grow a/nc-Si with crystalline phase ranging from about 2nm to 10nm. Unlike conventional nc-Si, where the crystalline phase size and volume fraction are limited, this gives us the opportunity to investigate the effects of quantum confined crystalline phase on the overall properties of the hybrid material. Using a/nc-Si:H thin-film transistors (TFTs), we address the effects of the crystalline phase on the electron and hole mobility, $\mu_{e}$ and $\mu_{h}$ respectively. An increase in $\mu_{e}$ as a function of increasing volume fraction, $\varphi$, of the crystalline phase, was succeeded by a gradual $\varphi$ dependent decline. For the holes, however, $\mu_{h}$ was found to decline over the measured $\varphi$ range. These observed trends in $\mu_{e} (\varphi)$ were interpreted as resulting from the competition between an enhancement in the transport of electrons due to the silicon quantum dots and a simultaneous suppression because of the presence of additional defects. We modeled this effect using a variant of the Maxwell-Garnett Effective Medium Approximation that accounts for carrier scattering and localization attributed to defect density in the host, a-Si:H. For holes, the persistent decrease in the measured mobility suggested \textit{trapping} of thermalized holes in the SiQDs because of the significant offset between the valence bands of the a-Si:H and matrix. Using $1.96eV$ and $1.55eV$ photons, we were able to selectively photo-excite carriers in either the a-Si:H or SiQDs. We took advantage of this to investigate what effects $\varphi$ had on \textit{dynamic} features, like the photosensitive conduction exponent, $\gamma$, and the mobility lifetime product, $(\mu\tau)_{e}$. These dynamic features revolve around charge carrier generation and recombination. From the $\gamma$ measurements, we were able to infer the presence of SiQD inclusions that are not \textit{coupled} to the a-Si:H matrix. We were also able to corroborate the defect density increase with $\varphi$. Ultimately, using a simple yet powerful tool, the thin film transistor, we were able to investigate the macroscopic transport mechanisms involved in an hybrid form of silicon for photovoltaic applications. We were also able to identify and characterize the possible origins of the factors limiting its transport properties.