Fabrication And Properties Of AlN-based Group â…¢-nitrides Semiconductor Nanomaterials

The family of group III-nitrides semiconductor materials, including Al N, Ga N, In N and their binary, ternary, quaternary compounds, are direct bandgap semiconductors, and the corresponding direct bandgaps range from 0.7 e V for In N, to 3.4 e V for Ga N,to 6.2 e V for Al N in the entire composition region. Therefore, light emission of III-nitrides semiconductors covers continuously from near-infrared to ultraviolet wavelengths. Furthermore, group III-nitrides have outstanding electric properties, excellent mechanical performance, high temperature resistance, corrison resistance, and so on, and provide the wide potential applications in high temperature, high frequency, and high power optoelectronic devices. With the development of nanotechnology, one-dimensional nitrides nanostructures have far-ranging application space and brighter future in light-emission diodes(LED), laser diodes(LD), photodetectors, field-effect transistors, and piezoelectric nanogenerators. In order to meet the requirements of future commercial micro/nano-devices, large scale, high quality, simple growth process and low cost, and highly controllable performance nitrides nanostructures are particularly important.This dissertation will exploy catalyst-free and template-free chemical deposition method(CVD) in a multi-temperature-zone furnace to grow large-scale Al N and AlxGa1-xN nanostructures successfully. We have systematically studied the field emission properties of Al N-based nanomaterials grown on different conductive substrates(graphite sheet and carbon cloth). Meanwhile, the effect of growth conditions, including gas flow rate, growth temperature, growth time, the evaporation temperature of precusors, on the compositions and morphologies of AlxGa1-xN nanostrutures have systematically studied. The growth mechanism of AlxGa1-xN nanostructures with different morphologies has also been discussed in detail. The relationship between the composition and optical, field emission properties of AlxGa1-xN nanomaterials has been investigated deeply. Some valuable and conductive results have been obtained. The main research of this dissertation can be categorized into three parts:(1) Large-scale Al N nanostructures have been successfully fabricated on flexible and conductive graphite sheet and carbon cloth by chemical vapor deposition method. Theeffect of flow rate, growth time, and growth temperature on the density, morphology, and field emission properties of the nanostructures has been systematically investigated. It is demonstrated that field emission of the Al N nanostructures are found to be strongly affected by the tip morphologies, growth density, and the conductive substrates. Based on the analysis of the band diagram, the electron supplement from the conductive substrates contributes to the improvement of the Al N nanostructures grown on conductive substrates. Meanwhile, the reason for the field emission properties of Al N nanostructures grown on carbon cloth better than that on graphite sheet is ascribed to the woven carbon cloth consisted of cylindrical carbon fibers.(2) AlxGa1-xN nanostructures with manifold morphologies, including nanonails, nanoneedles, nanorods, nanoflowers, nanomultipods and nanotowers, have been fabricated on bare Si(100) substrates by a simple halide chemical vapor deposition process. The detailed morphological and structural characteristics of these nanostructures indicate that all the samples are homogenous composition, hexagonal wurtzite single-phase structure, and a preferred orientation along [0001] direction. The evolution of various AlxGa1-xN nanostructures has been systematically investigated in terms of saturation vapor pressure of precursors by tuning the growth parameters, including evaporation of precusors, the flow rate of NH3, the substrate temperature when NH3 is introduced, and growth time. The surface diffusion related Vapor-solid(VS) growth mechanism is responsible for the formation and evolution of the different complex nanostructures. In addition, the structure and growth mechanism of AlxGa1-xN-Ga N core-shell nanorods have been discussed in detail. The enhanced field emission properties of the core-shell nanorods is discussed based on the band diagram, the electrons in the AlxGa1-xN core can be easily emitted into the Ga N shell due to the low electron affinity of AlxGa1-xN, which greatly increases interior electron supply for surface FN tunneling.(3) AlxGa1-xN nanostructures with single phase and different compositions have been fabricated by catalyst-free chemical vapor deposition method, and the effect ofcomposition on the optoelectronic properties of AlxGa1-xN nanostructures has been discussed deeply.① The composition tunable AlxGa1-xN nanowires(0.66≤x≤1) have been fabricated on Si(100) substrates. The characterization of SEM、EDS、XRD、TEM and Raman indicates that AlxGa1-xN are composition tunable,hexagonal wurtzite structure, and a preferred orientation along c axis.② Based on the growth of AlxGa1-xN nanowires(0.66≤x≤1) on Si(100),single phase, homogenous compositon, compositionally tuned ternary AlxGa1-xN(0≤x≤1) alloy nanowires have been successfully grown on flexible carbon cloth by fine-tuning the experimental parameters. The composition manipulation of AlxGa1-xN nanowires in the range from 0 to 1 can be realized by adjusting the evaporation temperature of Al Cl3 and Ga Cl3. The progressive evolution of the lattice constants, characteristic Raman peaks and field emission(FE) not only indicates the continuous composition tunability, but also demonstrates the possibility of the continuous regulation performance. The synthesis of AlxGa1-xN nanowires on carbon cloth provides the new possibility for the further development of flexible nano- and opto-electronic devices.