No description available.

<p>InAlN thin films and InAlN/GaN heterostructures have been intensively studied over recent years due to their applications in a variety of devices, including high electron mobility transistors (HEMTs). However, the quality of InAlN remains relatively poor with basic material and structural characteristics remain unclear.</p><p>Molecular beam epitaxy (MBE) is used to synthesize the materials for this research, as MBE is a widely used tool for semiconductor growth but has rarely been explored for InAlN growth. X-ray photoelectron spectroscopy (XPS) is used to determine the electronic and chemical characteristics of InAlN surfaces. This tool is used for the first time in application to MBE-grown InAlN and heterostructures for the characterization of surface oxides, the bare surface barrier height (BSBH), and valence band offsets (VBOs).</p><p>The surface properties of InAlN are studied in relation to surface oxide characteristics and formation. First, the native oxide compositions are studied. Then, methods enabling the effective removal of the native oxides are found. Finally, annealing is explored for the reliable growth of surface thermal oxides.</p><p>The bulk properties of InAlN films are studied. The unintentional compositional grading in InAlN during MBE growth is discovered and found to be affected by strain and relaxation. The optical characterization of InAlN using spectroscopy ellipsometry (SE) is also developed and reveals that a two-phase InAlN model applies to MBE-grown InAlN due to its natural formation of a nanocolumnar microstructure. The insertion of an AlN interlayer is found to mitigate the formation of this microstructure and increases mobility of whole structure by fivefold.</p><p>Finally, the synthesis and characterization of InAlN/GaN HEMT device structures are explored. The density and energy distribution of surface states are studied with relationships to surface chemical composition and surface oxide. The determination of the VBOs of InAlN/GaN structures with different In compositions are discussed at last.</p> / Dissertation

Abstract In this literary, we discuss with structure and morphology improvement of GaN epilayer on c-sapphire by multi-step method in molecular-beam epitaxy. Our research is caused for the critical results of defect in GaN epilayer and rough surface morphology. In order to solve these problems we used a novel technique which we called multi-step method. In this thesis, the results of X-ray, SEM, AFM all demonstrated the achievement in our composition. However, we obtained the results of full width of half maxima (FWHM) of (0002) and (10 2) XRD rocking curves with 60~120 arcseconds and 700~ 1200 arcseconds from a series of multi-step samples respectively. Comparing with previous measurement, multi-step method is relatively superior, and the measurement of AFM roughness is under 2 nm from the series of multi-step samples. If we discuss the flat area further, we can get smoother surface which roughness is about 0.4 nm. It is obviously to recognize the flat and rough regions, but in SEM image we made sure that the flat region occupied the greater part of surface. So, in this literary we verified that the method of multi-step can improve the structure and morphology of GaN by molecular-beam epitaxy.

"none"

The present study aims at studying the surface morphology, crystallinity and optical emission property of Mg added ZnO grown by molecular beam epitaxy. Zn1-xMgxO epitaxial films were first grown at a fixed Zn flux of 1¡Ñ10-7 mbar and Mg flux of 4¡Ñ10-10 to 6.2¡Ñ10-9 mbar on sapphire substrates at 400 oC. The corresponding Mg content (x) is in a range of <0.01 to 0.17. Scanning electron microscopy observations indicated that the surface of the films are flat. The orientation relationship between the film and the substrate is: (0001)Zn1-xMgxO¡ü(0001)Al2O3©M[101 ¡Â0] Zn1-xMgxO¡ü[112 ¡Â0] Al2O3. The full width at half maximum (FWHM) of the (0002) reflection in rocking curve measurement is in a range of 3.83 o to 4.81 o. Photoluminescence results showed that the intensities of both the near band-gap emission and the deep level emission increases with increasing Mg content. The former has FWHM values of 0.16-0.21 eV. While the epitaxial films were grown at a high Zn flux of 5¡Ñ10-7 mbar and Mg flux of 2¡Ñ10-9 to 9.6¡Ñ10-9 mbar on sapphire and LiAlO2 substrates at 400 oC, the film surface are at high roughness. The FWHM of (0002) rocking curve is 4.43 o to 5.71 o for films grown on sapphire and is relatively larger of 6.88 o to 8.18 o for films grown on LiAlO2, respectively. These films possess a stronger near band-gap emission and a lower deep level emission as compared to the films grown at a low Zn flux. After annealed at 600 oC in oxygen or nitrogen, the FWHMs of the (0002) rocking curve for most of the epilayers decreased slightly. The photoluminescence results were rather distinct. For samples having low Mg content (x<0.05), the intensity of the near band-gap emission increases 50-200 ¢H after annealed in oxygen. The intensity of the near band-gap emission did not change but that of the deep level emission decreases ~50 ¢H for the film having x=0.16 after annealing in oxygen. The emission characteristics basically do not change after annealed in nitrogen.

Zinc selenide is a wide bandgap II-VI semiconductor. The minimum bandgap at £F point (zone center) is direct and has a room temperature value of 2.67eV, corresponding to the blue region of the visible spectrum (464nm). Molecular beam epitaxy (MBE) is an ultra high vacuum technique used for the growth of semiconductors. The molecular beam epitaxy system used for the growth of semiconductors . The molecular beam epitaxy system used for growth of the II-VI semiconductor layers is described in detail in Chapter 2. Chapter 3 describes the substrate preparation procedure and growth of ZnSe epitaxial layers. Last, information from characterization technique has been used to analysis the quality of the layers and hence determine referred growth conditions.

Described in this thesis are the growth and characterization of high quality ZnO epitaxy layers.Zinc oxide (ZnO) epitaxy layers were grown on sapphire and epi-GaN substrates respectively, using plasma assisted molecular beam epitaxy (MBE) . Various growth conditions, such as growth temperature, II/VI ratio, and buffer layers, were employed to optimize the quality of the ZnO film. The subsequent characterization of the films was carried out to evaluate the surface, optical and crystalline properties of the film, using AFM, SEM, PL and XRD techniques. It was found out that the high quality of the ZnO film was grown on epi-GaN substrates under the Low temperature of ~ 300 degrees C, flash annealing up to ~680 degrees C, followed by high temperature growth at ~600 degrees C.

III–V nitride quantum-confined structures embedded in nanowires (NWs), also known as quantum-disks-in-nanowires (Qdisks-in-NWs), have recently emerged as a new class of nanoscale materials exhibiting outstanding properties for optoelectronic devices and systems. It is promising for circumventing the technology limitation of existing planar epitaxy devices, which are bounded by the lattice-, crystal-structure-, and thermal- matching conditions. This work presents significant advances in the growth of good quality GaN, InGaN and AlGaN Qdisks-in-NWs based on careful optimization of the growth parameters, coupled with a meticulous layer structure and active region design. The NWs were grown, catalyst-free, using plasma assisted molecular beam epitaxy (PAMBE) on silicon (Si) substrates. A 2-step growth scheme was developed to achieve high areal density, dislocation free and vertically aligned NWs on Ti/Si substrates. Numerical modeling of the NWs structures, using the nextnano3 software, showed reduced polarization fields, and, in the presence of Qdisks, exhibited improved quantum-confinement; thus contributing to high carrier radiative-recombination rates. As a result, based on the growth and device structure optimization, the technologically challenging orange and yellow NWs light emitting devices (LEDs) targeting the ‘green-yellow’ gap were demonstrated on scalable, foundry compatible, and low-cost Ti coated Si substrates. The NWs work was also extended to LEDs emitting in the ultraviolet (UV) range with niche applications in environmental cleaning, UV-curing, medicine, and lighting. In this work, we used a Ti (100 nm) interlayer and Qdisks to achieve good quality AlGaN based UV-A (320 - 400 nm) device. To address the issue of UV-absorbing polymer, used in the planarization process, we developed a pendeo-epitaxy technique, for achieving an ultra-thin coalescence of the top p-GaN contact layer, for a self-planarized Qdisks-in-NWs UV-B (280 – 320 nm) LED grown on silicon. This process constitutes a significant advancement in simplifying the UV-B and UV-C fabrication process favoring light extraction. Addressing the issue of poor white light quality in the conventional blue laser diode (LD) and YAG:Ce3+ technology, a number of applications related investigations was conducted. Notably, the orange and yellow emitting InGaN/GaN Qdisks-in-NWs LEDs were implemented as an “active phosphor” to achieve intensity- and bandwidth-tunability for high color-quality solid-state lighting.

Submitted by Evandro Martin Lanzoni (evandrolanzoni@yahoo.com.br) on 2018-05-28T18:03:48Z No. of bitstreams: 1 dissertação mestrado evandro lanzoni_versão final.pdf: 4510657 bytes, checksum: dadfb13eef638b712a1e377fddcf85d2 (MD5) / Approved for entry into archive by Lucilene Cordeiro da Silva Messias null (lubiblio@bauru.unesp.br) on 2018-05-28T19:02:57Z (GMT) No. of bitstreams: 1 lanzoni_em_me_bauru.pdf: 4510657 bytes, checksum: dadfb13eef638b712a1e377fddcf85d2 (MD5) / Made available in DSpace on 2018-05-28T19:02:57Z (GMT). No. of bitstreams: 1 lanzoni_em_me_bauru.pdf: 4510657 bytes, checksum: dadfb13eef638b712a1e377fddcf85d2 (MD5) Previous issue date: 2018-03-29 / As técnicas de microscopia de sonda Kelvin (KPFM) e de microscopia de força eletrostática (EFM) são amplamente utilizadas para analisar a distribuição do potencial de superfície, porém com pouca aplicação em nanoestruturas semicondutoras auto-organizadas embutidas em um substrato. Neste trabalho, investigamos diretamente o acúmulo de carga dentro de estruturas mesoscópicas de GaAs (MGS) [1]. As estruturas são fabricadas através do crescimento sobreposto de um modelo de nano orifícios usando epitaxia de feixe molecular. Para tal, uma combinação de desoxidação assistida por Ga e ataque químico por gotículas localizadas foram utilizadas para criar orifícios iniciais com uma profundidade de ca. 10 a 15nm, que são posteriormente cobertos com 15nm de barreira AlxGax-1As e GaAs com 1nm, 2nm, 5nm, 10nm de espessura. Microscopia de força atômica e microscopia eletrônica de transmissão mostraram que a forma do orifício é preservada durante o crescimento de AlGaAs. Em seguida, esses orifícios são preenchidos com GaAs formando uma estrutura alongada sobre o buraco [1]. Investigamos o potencial de superfície local e a distribuição das cargas nestas estruturas com a técnica KPFM de passagem única. Portanto, uma voltagem AC de 5 V é aplicada a uma ponta metalizada e varremos a amostra no modo de contato intermitente. Observamos uma clara diferença de potencial na região central da estrutura, onde esperamos o furo preenchido. Então, um estudo sistemático com a técnica de KPFM mostrou a influência no acúmulo de carga quando a espessura de GaAs é alterada, bem como, quando modificamos a concentração de Al na barreira de AlGaAs. O cálculo simulando um poço de potencial com barreiras semi-finitas e finitas mostrou que não ocorre acúmulo de carga quando a espessura do GaAs é menor que 1,5 nm, corroborando com nossos resultados. Simulações do diagrama de banda e da densidade de elétron da estrutura permitem atribuir o acumulo de carga observado, aos diferentes níveis de energia da estrutura mesoscópica de GaAs em comparação com as camadas de GaAs circundantes. / Kelvin probe force microscopy and electric force microscopy techniques are widely used to analyze the distribution of the surface potential with little application to self-assembled semiconductor nanostructures embedded into a substrate. In this work, we directly investigate the charge accumulation inside mesoscopic GaAs structures [1]. The structures are fabricated by overgrowth of a nanohole template using molecular beam epitaxy. Therefore, a combination of Ga assisted deoxidation and local droplet etching is used to create initial holes with a depth of ca. 10 to 15nm, which are covered subsequently with 15nm of AlxGax-1As barrier and GaAs caps with 1nm, 2nm, 5nm, 10nm thicknesses. Atomic force microscopy and transmission electron microscopy results showed that the hole shape is preserved during the AlGaAs overgrowth. Then filled with GaAs forming an elongated mount over the hole [1]. We investigate the local potential and the charge distribution in these structures with a single pass Kelvin probe force microscopy technique. Therefore, an AC voltage of 5 V is applied to a metalized tip and scanned in tapping mode over the sample. We observed a clear potential difference in Kelvin probe force microscopy measurements in the middle of the structure, where we expect a filled hole. We systematically study by Kelvin probe force microscopy the influence on the charge accumulation when the GaAs thickness is changed, as well as the Al concentration in the AlGaAs barrier. Calculation of the particle in the box for semi-finite and finite barriers were done and show that no charge accumulation is observed for GaAs thickness lower than 1.5nm in the semi-finite barrier, corroborating with our results. Simulations of band gap and electron wavefunction of the structure allow us to ascribe the charge accumulation observed, to the different confinement of carriers inside of the unstrained mesoscopic GaAs structure compared to the surrounding GaAs layers.

The quality of GaN template layer plays a very important role in high electron mobility transistors. We proposed a special method in the growth of molecular beam epitaxy to deal with the dilemma between structure and the morphology of GaN. In our study, we used a nitrogen-rich GaN growth condition to deposit the initial varied layer. After that, we changed the N/Ga ratio stepwise to the growth condition of gallium-rich GaN and grew the epitaxy layer right away. In X-ray diffraction analysis, the full width at half-maximum (FWHM) value of rocking curves of GaN(002) was improved relatively to gallium-rich sample from 531.69 arcsecond to 59.43 arcsecond. In atomic force microscopy (AFM) analysis, the root mean square (rms) roughness of sample surface was improved relatively to nitrogen-rich sample from 18.28 nm to 1.62 nm over 5 £gm ¡Ñ 5 £gm area. The Raman scattering shows there is a slightly tilted plane in gradient layer and the gradient layer can also slash the strain force which is caused from Ga-rich GaN epitaxy layer and AlN buffer layer. A series high mobility AlxGa1-xN/GaN heterostructures samples were grown on MOVPE-grown GaN templates substrate by molecular beam epitaxy with different Al concentrations (x = 0.017~0.355). The quality checked by XRD and AFM indicated that the excellent properties agreed with the GaN-template. The highest mobility in this series samples at 8 K is 19593 cm2/Vs with carrier concentration 3.13 ¡Ñ 1012 cm-2 and Al concentration x = 0.017. In our experiments, the carrier density decreases as Al concentration reduces. In the illuminated Hall measurement, there are only few electrons increased following blue LED illumination. It shows that there are only few deep level defects existing near the heterointerface. From temperature-depended Shubnikov-de Haas (SdH) oscillations, the electron effective mass m* in 2DEG are evaluated as 0.213 mo and for x = 0.207 0.227 moand 0.136 respectively. The high mobility AlxGa1-xN/GaN was fabricated to a series of wires by focused ion beam (FIB) equipment, and the width of the active channel is ranged from 900 nm to 50 nm (900 nm, 500 nm, 300 nm, 200 nm, 100 nm, 80 nm and 50 nm) with the channel orientation in [11 0] direction. The largest spin-splitting energy in the series of wires is 2.14 meV. Due to larger spin-splitting energy and quasi-ballistic transportation, the 200 nm wire is the best candidate to be the channel of the quantum-ring interferometer in our case.