Growth of undoped and doped IIInitride nanowires and their characterization

摘要

In the twenty first century, the rapid development of science, engineering and technology is blessed by the application of nanotechnology. It has become an attractive field of research among the scientists and created a lot of attention of the general public. Fabrication and characterization of various kinds of nanostructures such as carbon nanotubes, quantum wires and dots etc. have enabled to realize the possible applications as the building blocks of new structures and devices. Among those nanostructures, nanowires (NWs) are particularly attractive for future nanotechnology application due to their unique properties. For opto-electronic application, III-Nitride NWs are expected to further improve the performance and efficiency of optoelectronic device structures. III-Nitride NWs (GaN and InN) have been grown on different substrates by Plasma-assisted Molecular Beam Epitaxy (PAMBE). It has been found that the change of growth parameters (e.g. III-V ratio, growth temperature etc.) greatly influences the morphology of NWs. Nitrogen-rich condition is necessary to have columnar growth for both GaN and InN NWs as the surface diffusivity is reduced and the anisotropic growth is initiated. Various growth conditions for NW growth will be explained later on. A systematic analysis is carried out to understand the nucleation process for GaN NWs. For this purpose, a set of samples has been grown at different duration and their Scanning Electron Microscopy (SEM) images have been studied. The density of the wire increases with time until it saturates. A long incubation time indeed results as each wire has different nucleation time. A linear relationship between length and diameter has been established for well-nucleated wires in the nucleation stage. This helps to estimate the critical diameter for the nucleation cluster and it is found to be about 15 nm. Growth modeling of NWs has been performed by taking into account two different growth mechanisms: one is the direct impingement, which is independent of the diameter of the wire and the other is diffusion-induced (D-I) contribution, in which the adatoms are adsorbed on the substrate or wire surface and diffuse along the sidewalls to the top of the wire. A simple diffusion model is implemented, which gives a reciprocal relationship between length (L) and diameter (D) for the final growth i.e. diffusion dominates for thinner wire as described by: L=C1(1+C2/D). In this equation, C1 and C2 are the constants. Further experimental evidence shows no droplets on the top of NW and fabrication of heterostructures with sharp interfaces confirms that Vapor-Liquid-Solid (VLS) mechanism is not responsible for the growth. Furthermore, an interruption during the growth of GaN and InN nano wires does not influence their morphology (no steps are visible) and the growth rate does not change as compared to the wires grown without any interruption. As far as the growth of GaN NWs is concerned, the nitrogen-rich condition has been achieved by increasing the growth temperature, which enhances the Ga desorption or reducing the Ga flux and as a result, III–V ratio is reduced. Manipulation of various growth parameters will determine the wire morphology. The optimum growth takes place at 785°C whereas at 815°C, no growth takes place due to higher Ga desorption. On the other hand, NW growth takes place even at higher temperature of 820°C by increasing the Ga flux. The density and the size distribution of the wires change depending on the Ga flux used. GaN NWs grown on dot templates become longer as compared to the direct growth on Si(111). This is due to the reduction of nucleation time. Also, all the wires have uniform length and are vertically aligned with the substrate. Columnar growth also takes place on Si(100) and SiO2 substrates apart from Si(111). Selective etching of SiO2/Si helps to retain NWs on the patterned areas. The scenario is different for InN NWs as the growth takes place comparatively at low temperature. An increase of the growth temperature will enhance the dissociation of InN due to the evaporation of nitrogen. That’s why, a special attention has been paid for InN growth. An optimum growth temperature of 475°C has been determined and a suitable In flux has been chosen to have desired morphology. Tapering can be reduced by increasing the flux at optimum temperature and further increase of In flux shows broadening effects at the top of the wire. The growth of InN also takes place on Ge(111) substrate. Optical properties of both GaN and InN give evidence of good crystalline quality NWs. Particularly, lower Ga flux or higher growth temperature are necessary for GaN NWs for good quality. Growth parameters for InN have been optimized for obtaining good optical properties. A lower bandgap of InN NWs has also been determined which agrees with the recent literature value of high quality InN films. Raman scattering measurements have been performed to calculate the carrier concentration and mobility of nanowires. TEM results show the formation of an amorphous silicon nitride wetting layer on the substrate surface during the growth of GaN NW and small GaN crystalline clusters on the top of the interface amorphous layer. Lattice constants determined from the TEM results show the wurtzite structure comparable with the literature values and high quality GaN and InN NWs. Doping by Si or Mg greatly changes the morphology of NWs. By tuning the growth parameters, the size and density of the doped wires can be controlled. Optical measurements give the evidence of incorporation of dopants rather than their segregation on the surface. Finally, GaN nanodots have been successfully fabricated by droplet-epitaxy technique in PAMBE. The size of these droplets has been varied by changing the growth temperature. TEM investigation reveals the formation of crystalline dots as well as a “wetting” layer of GaN on Si(111). Surface spectroscopy measurements further confirm that a spreading mechanism takes place during nitridation process to form a GaN wetting layer and estimates the GaN composition.