Made available in DSpace on 2014-06-11T19:35:45Z (GMT). No. of bitstreams: 0 Previous issue date: 2011-09-22Bitstream added on 2014-06-13T20:07:27Z : No. of bitstreams: 1 gomes_mc_dr_bauru.pdf: 4074042 bytes, checksum: d6522228622688cc96df0482f8a71eca (MD5) / Fundação de Amparo à Pesquisa do Estado de Mato Grosso (FAPEMAT) / Este trabalho é resultado de um estudo teórico sobre o GaN e o Semicondutor Magnético Ga1-xMnxN, tanto para bulk (sólido) como para as superfícies nanoestruturadas mais estáveis, dada sua importância para o desenvolvimento como material spintrônico. Analisamos deste material suas propriedades estruturais, energéticas e eletrônicas, a partir de cálculos periódicos baseados na teoria do Funcional da Densidade (DFT), como o funcional híbrido B3LYP, e também apresentamos resultados preliminares do estudo das propriedades magnéticas deste material. Realizamos um estudo pormenorizado das estruturas de bandas e da densidade de estados, do Ga1-xMnxN bulk (x~0,02 a 0,18) quanto em superfícies (x~0,0 a 0,17) para os modelos de supercélula de 32 e 96 átomos bulk e modelos de superfícies com 12 e 24 camadas. Os resultados obtidos nos mostram que para concentrações acima de 6% ocorre um acréscimo na distância de ligação Mn-N na direção c, pelo fato do Mn apresentar um raio atômico superior ao Ga e propriedades físicas e químicas distintas. Para os cálculos com superfícies, foi realizada a substituição do Ga por Mn em diferentes posições relativas na superfície, sub-superfície e core, ocorrendo o aumento da energia total conforme os átomos de Mn se movem para os sítios mais internos da superfície e ao considerar a forma de equilíbrio baseada na estabilidade termodinâmica, os valores das energias superficiais das superfícies (1010) e (1120) do GaN wurtzita são as mais estáveis para a concentração de ~8%. Com o aumento da concentração, ocorre nas estruturas de bandas uma diminuição do gap, tanto para o bulk quanto para as superfícies, porém ele se mantém direto no bulk, com exceção para concentração de 18% e, na superfície (1010), enquanto que na superfície (1120) o gap é indireto... / This work is the result os a theoretical study concerning GaN and the Magnetic Semiconductor Ga1-xMnxN, in both the bulk and the most stable nanoestructured surfaces, due to its importance in the development as spintronic material. We analyze the structural, energetic and electronic properties of this material, by means of periodic calculations based on the Functional Density Theory (DFT), at the hybrid functional B3LYP level, and also present the preliminary results of the study of the magnetic properties of this material. We carried out a detailed study of the band structures and the density of states, for both the Ga1-xMnxN bulk (x~002 a 0,18) and its surfaces (x~0,0 a 0,17) using supercell models constitued by 32 and 96 atoms for the bulk and 12 and 24 atomic layers for the surface slab model. Our results show that for Mn concentrations above 6% there is an increase in the Mn-N bond distance in the c direction, due to the fact that the Mn has an atomic radium greater than that of the Ga and different physical and chemical properties. For the surface calculations, we substituted the Ga for the Mn in different positions relative to the external surface, sub-surface and corre, it was observed that the total energy increased as the Mn atoms moved from the surface layer to the interior sites and when we consider the equilibrium shape based in the thermodynamic stability, the most stable surface energies for the (1010) and (1020) planos of wurtzite GaN are found for the ~8% Mn concentration. When the Mn concentration increases, the band gap decreases for the bulk as well as for the surfaces, the gap being direct for the bulk, except for the 18% concentration, and for the (1010) surface, whereas the gap is found indirect for the (1120) surface for the concentrations 6 and 17%. The analysis of the density... (Complete abstract click electronic access below)

This thesis aims to develop a chemical vapor deposition (CVD) process for the new directions in both silicon carbon (SiC) and gallium nitride (GaN) epitaxial growth. The properties of the grown epitaxial layers are investigated in detail in order to have a deep understanding. SiC is a promising wide band gap semiconductor material which could be utilized for fabricating high-power and high-frequency devices. 3C-SiC is the only polytype with a cubic structure and has superior physical properties over other common SiC polytypes, such as high hole/electron mobility and low interface trap density with oxide. Due to lack of commercial native substrates, 3C-SiC is mainly grown on the cheap silicon (Si) substrates. However, there’s a large mismatch in both lattice constants and thermal expansion coefficients leading to a high density of defects in the epitaxial layers. In paper 1, the new CVD solution for growing high quality double-position-boundaries free 3C-SiC using on-axis 4H-SiC substrates is presented. Reproducible growth parameters, including temperature, C/Si ratio, ramp-up condition, Si/H2 ratio, N2 addition and pressure, are covered in this study. GaN is another attractive wide band gap semiconductor for power devices and optoelectronic applications. In the GaN-based transistors, carbon is often exploited to dope the buffer layer to be semi-insulating in order to isolate the device active region from the substrate. The conventional way is to use the carbon atoms on the gallium precursor and control the incorporation by tuning the process parameters, e.g. temperature, pressure. However, there’s a risk of obtaining bad morphology and thickness uniformity if the CVD process is not operated in an optimal condition. In addition, carbon source from the graphite insulation and improper coated graphite susceptor may also contribute to the doping in a CVD reactor, which is very difficult to be controlled in a reproducible way. Therefore, in paper 2, intentional carbon doping of (0001) GaN using six hydrocarbon precursors, i.e. methane (CH4), ethylene (C2H4), acetylene (C2H2), propane (C3H8), iso-butane (i-C4H10) and trimethylamine (N(CH3)3), have been explored. In paper 3, propane is chosen for carbon doping when growing the high electron mobility transistor (HEMT) structure on a quarter of 3-inch 4H-SiC wafer. The quality of epitaxial layer and fabricated devices is evaluated. In paper 4, the behaviour of carbon doping using carbon atoms from the gallium precursor, trimethylgallium (Ga(CH3)3), is explained by thermochemical and quantum chemical modelling and compared with the experimental results. GaN is commonly grown on foreign substrates, such as sapphire (Al2O3), Si and SiC, resulting in high stress and high threading dislocation densities. Hence, bulk GaN substrates are preferred for epitaxy. In paper 5, the morphological, structural and luminescence properties of GaN epitaxial layers grown on N-face free-standing GaN substrates are studied since the N-face GaN has advantageous characteristics compared to the Ga-face GaN. In paper 6, time-resolved photoluminescence (TRPL) technique is used to study the properties of AlGaN/GaN epitaxial layers grown on both Ga-face and N-face free-standing GaN substrates. A PL line located at ~3.41 eV is only emerged on the sample grown on the Ga-face substrate, which is suggested to associate with two-dimensional electron gas (2DEG) emission.

The optical properties of GaN nanocolumn structures containing InGaN quantum disks are investigated by optical microphotoluminescence spectroscopy using pulsed lasers, and cathodo- luminescence. The results are analyzed in the context of current theories regarding an inho- mogeneous strain distribution in the disk which has been theorized to generate lateral charge separation in the disks by strain induced band bending, an inhomogeneous polarization field distribution, and Fermi surface pinning. Simulations of the strain distribution for the relevant materials and structures are also performed, and the results analysed. It is concluded from ex- perimental measurements that no extreme lateral separation of carriers occurs in the quantum disks under investigation. Internal field screening by an increased carrier density in the QDisks at higher excitation densities is observed via a blue-shift of the emission and a dynamically changing decay time. Other possible explanations for these effects are discussed and discounted. Microphotoluminescence studies are also carried out on a single GaN nanocolumn struc- ture that has been removed from its growth substrate and dispersed onto a patterned grid. An analysis of the dynamics of the carriers in the nanocolumn is presented. Suppression of the GaN luminescence from the area of the column in the vicinity of the InGaN QDisk in addition to a delayed emission from the QDisk relative to the GaN is observed. Time resolved spatial maps of the luminescence intensity from the column are also presented, illustrating the evolution of the carrier density in the system. Additional, albeit early-stage, work on novel structures based on the production of GaN nanocolumns, namely nanotubes and nanopyramids, is also presented.

We mainly studied the morphology of GaN structures which were grown by plasma-assisted molecular beam epitaxy. The only condition we changed is Ga/N Ratio. Based on observation of reflection high energy electron diffraction (RHEED) patterns, we found all samples belong to two-dimensional (2D) growth mode. Also, based on scanning electron microscope (SEM) analysis, we found when Ga/N Ratio is 0.13 and 0.18, the surface of sample will be smoothest. Furthermore, based on the roughness result derived from atomic force microscope (AFM), we got the same result. Then we observed the surface of samples after etching, we found all samples belong to Ga-face. Also, we can detect the degree of the state of mismatch under X-ray diffraction analysis. We found when Ga/N Ratio is 0.13, we got the lowest screw dislocation density; and when Ga/N Ratio is 0.18, we got the lowest overall dislocation density. In conclusion, we are trying to find sample growing parameters which could generate both better morphology and better structure.

Optical properties of hybrid structures based on zinc oxide nanocrystals (NCs) and Gallium Nitride quantum well (QW) has been studied. The ZnO NCs thin films on the top of GaN QW structures were fabricated using spin coating. The surface morphology was characterized by scanning electron microscopy (SEM). We have performed temperature dependence time-resolved photoluminescence (TRPL) measurements of the bare AlGaN/GaN QW structures and hybrids, containing ZnO NCs. It was found that at some temperatures the QW PL decay has shorter decay time in the presence of ZnO NCs thin film compared to the bare QW. The effect was stronger for the samples with thinner cap layers. The results are discussed in terms of three models such as exciton nonradiative energy transfer (NRET), tunneling effect, and piezoelectric field influence on the QW exciton energy.

Self-organized GaN nanostructures have been accomplished with lattice-(mis)matched using MOCVD. A lattice mismatched system (i.e. GaN nanostructure/ AlN) was utilized with S-K mode mechanism, whereas, metallic droplet method (i.e. Vapor-Liquid-Solid method) was employed in the lattice matched system (i.e. GaN nanostructure / AlGaN). The nanostructure size is adjustable by changing growth parameters (height: 2 ~ 15nm and diameter: 10 ~ 100nm). It has been found that the photon emission energy is tunable relative to the nanostructure size, and smaller nanostructures have larger photon energy. However, a numerical modeling was performed to investigate the relationship between quantum confinement (and/or piezoelectric polarization) and the dot size. For dot height < 4.1nm, the confinement effect is larger than the piezoelectric effect, otherwise the piezoelectric effect is more dominant. In addition, GaN nanostructures grown on Al0.15Ga0.85N have smaller lattice mismatch (less than 0.5%) than the GaN nanostructures grown on AlN. Therefore, the quantum confinement in a GaN/Al0.15Ga0.85N system is more dominant in determining photon emission energy than in a GaN/AlN system. The nanostructure advantages of quantum confinement and high thermal stability have been studied for the achievement of room temperature ferromagnetism using TM (transition metal; Mn or Fe). The transition metal (Mn or Fe) enhances nucleation of islands, resulting in size and density improvements. The magnetization measurements revealed magnetic properties of ferromagnetic nanostructure. Especially, room temperature ferromagnetism was observed in GaFeN nanostructures, which can contribute to ferromagnetic semiconductors operating above room temperature.

We grew Mg-doped of GaN on GaN template by plasma-assisted molecular beam epitaxy (PAMBE) and measured these samples by Hall measurement, I-V curve measurement, PL, Raman scattering, SEM and AFM. The results of Hall measurement of these samples showed that the conducting type is n-type. However, I-V measurement showed these samples to have a behavior of p-n junction between Mg-doped GaN film and GaN template. For optical properties, Raman scattering spectrum did not detect a peak at 656 cm-1 which indicates Mg-N vibration; PL measurement showed the emission peak of growing samples shifted 0.03eV toward to low photon energy. The results of surface analysis showed a smooth surfaces at Ga droplet area while many pin hole was formed at ¡§dry¡¨ area. Those pin hole could be inversion domain. Futher study is going. Based on electrical, optcial, and surface analysis, the Mg-doped GaN thin film has been successful obtained by MBE. However, the hole concentration of these samples is so low that results in difficulty for Hall and Raman scatting measurement.

The study aims at growing nonpolar GaN film on LiGaO2 substrate by chemical vapor deposition (CVD). Metallic gallium and NH3 are the sources of Ga and N. There are two sets of experiment: add NH3 when raising the temperature, and set different reacting pressure at each experiment; add N2 when raising the temperature, and set different reacting temperature at each experiment, while reach the reacting temperature add NH3. Analyze the reacted samples with X-ray diffraction, scanning electron microscope, electron back-scattered diffraction, atomic force microscope and transmission electron microscopy to know the growing direction, morphology, roughness, optical property, and the microstructure of GaN growing situation. Under the experimental conditions, add NH3 when raising the temperature and set the reacting pressure in the range of 50 torr ~ 400 torr at 950¢XC with NH3 flow rate 450sccm for 60 minutes, m-plane GaN can be obtained; setting different reacting temperature(900¢XC ~ 1000¢XC) at 50 torr with N2/NH3 flow rate 450/30sccm for 60 minutes can also get m-plane GaN. Besides, the thin film of pure m-plane GaN can be obtained when setting the reacting temperature at 1000¢XC, but the film peels off seriously. After reacting under the conditions of the first set experiments, the inside LGO substrate become damaged, pores can be observed easily; and the circumstances of LGO is better in second set experiment.

Nonpolar free-standing GaN wafer were fabricated by using the hydride vapor phaseepitaxy(HVPE) technique on £^-LiAlO2 and (010) LiGaO2 substrates. Metallic gallium, NH3 and ultra-purity nitrogen were used as Ga and N sources. Nitrogen and hydrogen was used as the carrier gases. HCl diluted by nitrogen was designed to pass through near surface of liquid Ga to form GaCl fully. Efficaciously GaN deposition was realized by conducted steady NH3 and GaCl flows to the substrate suface, accommodated by additional hydrogen and nitrogen atmosphere flows. The influence of substrate temperature¡Bpressure¡Btime¡Band ratio of NH3/HCl (¢½/¢») on film growth was investigated. Because of the of the lattice-matched theory, Nonpolar m-plane GaN [10-10] growth on the closely lattice-matched (100) £^-LiAlO2 substrat and a-plane GaN [11-20] will growth on the [010] LiGaO2 substrate. In addition, the surface morphologies were characterized by scanning electron microscopy¡BOptical Microscope and Atomic force microscopy. Structural properties of the GaN epilayers are investigated by X-ray diffraction and transmission electron microscopy. High resolution transmission electron microscopy shows the in-plane structure. Photoluminescence (PL) spectroscopy was used to study the optical properties.

In this thesis, we investigated the growth of nonpolar (11-20) GaN films on LiGaO2 substrate by a simple chemical vapor deposition (CVD) process. Metallic gallium, NH3 and ultra-purity nitrogen were used as Ga, N sources and carrier gas. The X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used to study the influence of growth conditions such as reaction pressure, growth temperature and deposition time on the GaN epilayer¡¦s orientation and surface morphology. It¡¦s found that GaN epilayers have different surface morphology grown on LiGaO2 substrates by the change of growth pressures (50 torr ~ 400 torr) under temperature of 950 ¢XC, NH3 gas flow of 450 sccm and the growth time of 60 minutes, and uniform a-plane GaN epilayers are found with growth pressures 50, 200 and 300 torr. In addition, we obtain uniform a-plane GaN epilayers with different surface morphology by the change of growth temperatures (900 ¢XC ~ 975 ¢XC) under pressure of 50 torr, N2/NH3 gas flow of 450/30 sccm and the growth time of 60 minutes. Furthermore, we obtain flatter a-plane GaN epilayer by a longer growth time (120 mins) under temperature of 950 ¢XC, pressure of 50 torr and N2/NH3 gas flow of 450/30 sccm. The orientation relationship between GaN and LiGaO2 was determined as (11-20)GaN // (010)LiGaO2 and (1-100)GaN // (100)LiGaO2 by TEM analysis.