A study of the nucleation and formation of multifunctional nanostructures using GaN-based materials for device applications.

摘要

The objective of this research is to design multi-functional GaN nanostructures for optoelectronic and spintronic applications and to study the growth conditions for epitaxial material using metal-organic chemical vapor deposition (MOCVD).; III-nitrides have been known as prospective materials for semiconductor device applications for a wide range of wavelengths. However, it is more difficult to produce high quality GaN-based semiconductor compounds, compared to GaAs compounds due to the absence of coherently matched substrates. The most common substrate used for GaN growth is sapphire which has a lattice mismatch of 16%. The high dislocation densities resulting from this lattice mismatch may be detrimental to light output efficiency with optoelectronic semiconductors.; Nanostructures are too small to have dislocations as well as they have spatially quantized electronic energy states. This results in more stable thermal perturbation and high quantum efficiency of light emitting devices. But it is also difficult to grow and control nanostructures. In this thesis, multi-functional self-organized GaN nanostructure was investigated.; We have achieved self-organized GaN nanostructures on AlN system using MOCVD. The MOCVD provided a remarkable control ability to form nanostructures. The growth techniques which are obtained from typical bulk GaN growths encouraged the self-organized nanostructure growth. We have changed growth conditions: temperature, V/III ratio, Ga flux, and introduction of anti-surfactants. Especially, the growth temperature and V/III ratio affecting Ga migration and evaporation were optimized at around 800°C and 300 respectively. Growth pressure was also investigated with a wide range. A good result was obtained with lower pressures from 100 to 200 Torr. We kept the pressure 100 Torr in this work. Under these optimized growth conditions, the nanostructure height and diameter were less than 7 nm and 50nm, respectively. The nanostructure density was also enhanced up to the order of 10 cm-2.; We have also achieved self-organized GaN nanostructures on AlGaN system for nearly lattice-matched system (≤ 0.5%) using a new growth technique: metal droplet method. In the method, a liquid droplet condensing from vapor phase transforms crystal solid under chemical vapor deposition conditions at a high temperature. The nanostructure size and density were improved by changing growth parameters. We have optimized the TMGa flux and flow time and island size was apparently minimized and density was maximized at this optimized TMGa flow rate.; The emission wavelength for GaN nanostructure is dependent both on quantum confinement and piezoelectric effect. It was observed that larger dots obtained at a higher V/III ratio resulted in a red shift. The shift to higher energies with a decrease in nanostructure size is attributed to the decrease in piezoelectric effect rather than quantum confinement. The intensity of the PL is dependent on the density of the nanostructures. A higher PL intensity is obtained with a higher density of nanostructures. In addition, a numerical approach was performed to calculate excitonic and optical properties of GaN nanostructure system and we had a good agreement with experimental data.; The magnetic and structural properties of ferromagnetic self-assembled GaN nanostructures have been studied for spintronic applications. Transition metal (TM; Mn or Fe) was incorporated into GaN nanostructures at a relatively low temperature, and the ferromagnetic behavior of GaN:TM nanostructures at room temperature was observed. In addition, atomic force microscopy measurements revealed that TMs affected the surface morphology of these nanostructures. A small amount of TM incorporation affected both island size reduction and nucleation, enhancing quantum confinement as well as ferromagnetism in the GaN:TM nanostructures. The increased island density improved the magnetic characteristic after minimization of island