A study on the effect of stress on the growth, and electrical, mechanical behaviors of one-dimensional nanomaterials

Abstract

Stress or strain is a universal phenomenon pertinent to the synthesis, fabrication, and application of all types of materials. In the plastic regime, materials generally undergo irreversible changes, such as failure or property degradation. Elastic deformation, on the other hand, can induce reversible changes to materials properties from electronic and chemical to optical. The strength of a material depends very strongly on its dimensions. Typically, smaller structure can tolerate larger deformations before yielding. Therefore, it is anticipated that stress or strain engineering can be profitably explored in materials with reduced dimensions, such as one-dimensional nanowires. Rapid progress in device miniaturization has led to the rise of flexible, nanoscale devices for which one-dimensional nanowires and atomic sheets are particularly promising candidate materials. These nanomaterials possess unusual properties arising from giant surface effect and quantum confinement, and super mechanical properties. Therefore, nanowires are one of ideal platforms to explore novel stress effect and device concepts at the nanoscale, such as energy harvesting piezoelectric nano-generator, stress induced phase transition, and wrinkle based stretchable electronics. In this dissertation, we focused the effect of stress on the growth and piezoresistive properties of one-dimensional nanowires. In a view of growth, we utilized the stress driven single crystalline indium nanowire growth on InGaN substrate by ion beam irradiation. With comprehensive microstructural and chemical analysis, we confirmed that source of indium nanowire growth was originated from Ga+ ion beam induced phase decomposition of InGaN substrate, and compressive stress build up by ion irradiation and atomic migration are responsible for the growth of indium nanowires as a process of stress relaxation. Since focused ion beam can be precisely controlled by changing the accelerating voltage, current density, and location of irradiation, the diameter and length of the nanowires as well as their growth rate could be effectively controlled. Indium nanowires with diameter of 40-200 nm and length up to 120 μm were fabricated at growth rate as high as 500 nm/s, which is remarkable fast compared with other nanowire growth methods. Second, the effect of stress on the phase change nanowire (Ge2Sb2Te5) was explored to tune the electromechanical properties for the application to advanced electronic and memory devices. A single crystalline GST nanowires were grown by the vapor-liquid-solid mechanism. And GST nanowires possess a large piezoresistive effect and strain dependent electrical switching behavior of PCM devices. For example, the longitudinal piezoresistance coefficient along <10-10> direction for GST nanowires reach as high as 440 x 10-11 Pa-1, which is comparable to the values of Si. Resistance change by strain was reversible, thus it restored its original resistance when uniaxial stress was released. Since GST is known as p-type semiconductor with negligible band gap of ~0.3 eV, this great piezoresistivity of GST was originated from the hole mobility change, which induced the tunable switching behaviors of phase change memory with threshold voltage changes. The large piezoresistance and strain dependent behavior of PCM enable the potential application not only for flexible memory device, but also for another candidate of piezoresistive materials for strain sensors.