Strain-induced self-rolled-up semiconductor micro/nanotubes: Fabrication and characterization.

摘要

Strain-induced self-rolled-up semiconductor nanotubes are a new type of building block for three-dimensional architectures. Semiconductor nanotubes (SNTs) take advantage of the lattice mismatch between different layers and are formed when the strained semiconductor layers are released from the substrate by selective etching of the sacrificial layer. They are produced by a combination of "top-down" and "bottom-up" approaches, using the epitaxial growth of strained films and conventional fabrication processes such as lithography and wet/dry etching. Taking advantage of "bottom-up" approaches, nanoscale objects can be achieved; by using a "top-down" approach, their positions can be precisely controlled, and it is possible to form a large-area assembly of ordered tubes.;Tube diameter is determined by the thickness and amount of misfit strain which is accommodated in epitaxial films and can vary from a few nanometers to tens of microns. The shapes of three-dimensional (3D) structures are determined by the crystal orientation of the defined mesa patterns, and they can be formed as 3D tubular structures, helices, or just curved structure. The tube wall consists of compound semiconductor materials such as GaAs, AlxGa 1-xAs, and InxGa1-xAs and therefore it forms a heterostructure. As a result, optical gain media such as quantum wells and quantum dots can be embedded in the tube wall and these semiconductor tubes have potential for application as optoelectronic devices.;In this dissertation, metal-organic chemical vapor deposition (MOCVD) has been used to grow epitaxial layers. Variation of tube diameter, which depends on the thickness of layers and the amount of In content in the In xGa1-xAs layer, and the orientation dependence of tube formation were systematically investigated. The precise controllability of structural and spatial positioning of tubes has been achieved by understanding effects of geometry on tube formation, and perfectly ordered large arrays of tubes were realized. Also, GaAs quantum wells and InAs quantum dots have been embedded in the tube wall, and their optical properties were studied, using the micro-PL system. In rolled-up tube structures, strain plays a significant role in engineering the band structure, and therefore peak positions in the photoluminescence spectrum can be tuned continuously as a function of tube curvature and were experimentally investigated.;By taking advantage of strain relaxation of the strained films, different types of tubes are formed. SiN tubes, consisting of a compressively and tensile strained bilayer and tubes that were functionalized with different metals have been demonstrated. These tubes are used for the fabrication of high density carbon nanotubes and biosensor application using the micro-Raman system.;Chapter 1 presents a brief overview of a new tubular architecture that is formed by self-rolling of the strained semiconductor films and describes the formation mechanism of SNTs.;Chapter 2 outlines fabrication techniques to produce strain-induced self-rolled-up tubes. Epitaxial growth using a MOCVD reactor and two fabrication methods using wet or dry etching techniques will be discussed.;Chapter 3 demonstrates the formation method, process, and dependence on the crystal orientation and rolling direction of the strained thin membranes. Also, the geometry effect on the rolling behavior of the strained membrane will be discussed.;Chapter 4 presents the optical properties of GaAs quantum well tubes. Optical properties of microtubes where the GaAs quantum well is embedded in the tube wall as the optical gain medium will be studied using the photoluminescence measurement system.;Chapter 5 demonstrates several types of hybrid tubes. Metal/semiconductor, silicon nitride, and metal/silicon nitride tubes will be demonstrated. These different types of tubes can be implemented for a variety of applications and utilized for different purposes.;Chapter 6 shows the preliminary results of SNT applications. SNTs can be utilized for realizing high density ordered arrays of single-wall nanotubes and micro/nano-fluidic channels, by taking advantage of the unique property of the scrolled-up tubular structure which can serve as a rolling vehicle.