2D Atomic Layer Crystals for Nanoelectronics and Applications

Abstract

Since the discovery of graphene in 2004, much research enthusiasm has been evoked for this first discovered two-dimensional (2D) material due to its unique physical and electrical properties, such as an extraordinarily high surface-to-volume ratio, a gapless linear band structure with Fermi level tunability, bipolarity and ultra-high carrier mobility. The following discovery of transitional metal dichalcogenides (TMDCs), with direct band gaps and spin-coupled valleys, adds another crucial piece to the 2D material library. The thesis aims at 2D materials growth and utilizing their unique physical and electrical properties for real-world solid-state device applications. Chemical vapor deposition (CVD) of monolayer MoS2 is first demonstrated, with both single crystalline flake (~15 µm) and large area (centimeter level) polycrystalline continuous film growth achieved. Various characterization methods are subsequently utilized to examine growth morphology and quality such as optical microscopy, scanning electron microscopy (SEM), Raman spectroscopy, photoluminescence (PL), atomic force microscopy (AFM) and electrical measurements. Monolayer growth is confirmed by AFM and PL for monocrystalline flakes and polycrystalline continuous films, respectively. For the application portion, an electrically tunable lateral structured graphene-silicon-graphene bipolar junction transistor (BJT) has been fabricated with direct current gain over 20. During device operation, electrons in the graphene emitter overcome the graphene/Si barrier to diffuse into the silicon base and are subsequently collected by the collector under a large base-collector reverse bias. In contrast with traditional silicon BJTs, the current gain of graphene BJT device can be readily and smoothly tuned. With the doping concentration and Fermi level of the graphene emitter electrically controlled by a top gate electrode, the electron injection barrier height is consequently tuned, leading to tunability in the graphene BJT direct current gain. With decent performance, simplicity and extensibility, this novel graphene BJT device demonstrates a promising way forward for nanoscale BJT applications. With an ultra-high surface to volume ratio, graphene is an ideal platform for sensor applications. A graphene chemical sensor, based on a pristine graphene field-effect-transistor (FET) bonded with a µ-column gas flow channel cap, has been miniaturized for volatile compounds sensing and discrimination. With a small size of 1 cm by 0.7 cm, the miniature graphene sensor remains as sensitive (nanogram detection limit) and even faster (down to sub-second) than its pioneering large footprint counterpart. More importantly, a true label-free nanoelectronic sensing platform is pioneered by combining the electrical gate tunability of graphene sensor responsivity (or gate spectra) with the principal component analysis (PCA) technique. In contrast to conventional electronic sensors or electronic nose technology, surface functionalization is no longer needed in order to achieve chemical discrimination. 11 measured chemicals (acetone, ethyl acetate, chloroform, dichloromethane, pentane, heptane, nonane, benzene, toluene, o-xylene and p-xylene), as represented by 11 clusters of points in PCA plot, are clearly grouped into separate regions with each representing a corresponding chemical category. The identification accuracy, as verified by multiple algorithms such as k-nearest neighbors (KNN), linear discrimination analysis (LDA), support vector machine (SVM) and multi-layer perceptron (MLP), is rather satisfactory with at least 98.8% accuracy for the 11 specific chemicals and at least 93.9% accuracy for the four corresponding categories, indicating the robustness of data acquired by the graphene sensor. This work should lay the groundwork toward true label-free electronic sensor with high sensitivity and selectivity, and a novel electronic nose technology with better simplicity and higher accuracy.