Semiconductor nanowires and nanowire heterostructures: Fundamental transport phenomena and application in nanoelectronics

Semiconductor nanowires are emerging among the most promising family of materials to impact future electronics owing to the highly controlled growth, which has enabled predictable variation of structure and composition on multiple length scales. This thesis presents study on two critical scientific areas: understanding the potentially unique nature of 1D electrical transport in nanowires and corresponding enhancements in device performance.; First, we describe using solid state reaction to transform single crystal silicon nanowires into metallic nickel silicide nanowires, which possess low resistivity and high current density. NiSi/Si nanowire heterostructures were also created with atomically sharp metal/semiconductor interface and shown to enable FETs with outstanding performance.; Next we will focus on exploring the unique physics of 1D transport. Inspired by band structure engineering in planar 2D electron gas systems, we have designed and synthesized undoped Ge/Si core/shell nanowire heterostructures using the CVD method. Transport measurements on individual nanowire confirmed the formation of a hole gas and an absence of Schottky barrier at the metal contacts. Significantly, conductance quantization was observed at low temperature suggestive of ballistic transport through discrete 1D subbands. This 1D hole gas has created a new platform for studies in low-dimensional physics. Here we show studies of mesoscopic Josephson junctions using Ge/Si core/shell nanowires with superconducting contacts, which exhibit a dissipationless supercurrent. A systematic investigation of the multiple Andreev reflection phenomena as well as supercurrent quantization from quantum confinement effect in the narrow nanowire channel will be presented.; Furthermore, we have utilized the 1D hole gas and incorporated high-kappa dielectrics using atomic layer deposition and metal top-gate to fabricate high performance Ge/Si nanowire FETs with scaled transconductance and on-current values 3-4 times greater than state-of-the-art MOSFETs. The intrinsic delay metric showed that nanowire performance exceeds substantially the length-dependent scaling of planar MOSFETs. The effect of device geometry on important parameters such as threshold voltage and ambipolar transport will also be discussed. Such clear performance advantage of nanowire materials shows their potential as alternative to conventional CMOS in future electronics.; Lastly, in addition to devices based on the hole gas, we demonstrate a high mobility electron gas system based on undoped GaN/AlN/AlGanN radial nanowire heterostructures. Transport studies reveal an intrinsic mobility as high as 21,000 cm2/Vs at low temperature. Studies on other parameters such as transconductance, on current and subthreshold slope are presented.