Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / The effects of process conditions on the material’s properties were investigated for the sublimation growth of aluminum nitride and the epitaxial growth of silicon carbide. Since the mid 1990’s, these semiconductors have made new types of high power electronics and short wavelength optoelectronics that were never before feasible. The sublimation growth of AlN crystals on SiC seeds was carried out to produce high quality AlN bulk crystals. Si-face, 3.5 º off-axis 6H-SiC (0001) and 8 º off-axis 4H-SiC (0001) wafers were used as the substrates. An investigation of the initial growth demonstrated 1800 – 1850ºC was the optimum temperature for AlN growth. By optimizing the temperature gradient, large area AlN layer was deposited. Consecutive growths and continuous growth were performed to enlarge the crystal thickness. Single-crystalline AlN layers, each with a thickness of 2 mm and a diameter of 20 mm, were produced. X-ray diffraction confirmed the grown AlN had good crystal quality. Approximately 3 – 6 at% of Si and 5 – 8 at% of C were detected in the crystals by x-ray photoelectron spectroscopy, which came from the decomposition of SiC seeds and the degradation of the graphite components in the furnace. Molten KOH/NaOH etching revealed the dislocation density decreased from 108 cm-2 to 106 cm-2 as the AlN layer thickness increased from 30 μm to 2 mm. Epitaxial growth of SiC was carried out in a chemical vapor deposition system. High-quality 6H-SiC and 4H-SiC homoepitaxial films were produced at growth rates up to 80 μm/hr by using a novel single precursor, methyltrichlorosilane (MTS). Inclusions of 3C-SiC were circumvented by employing 8º mis-orientated substrates. Adjusting the H2/Ar flow ratio in the carrier gas effectively changed the C/Si ratio in the gas phase due to the reaction between H2 and the graphite heater; thereby, influencing surface roughness and dislocation density. Low H2/Ar ratios of 0.1 and 0.125 produced smooth surfaces without step-bunching. Higher H2/Ar ratios of 0.2 and 0.33 enhanced the conversion of basal plane dislocations into threading edge dislocations, and reduced the density of basal plane dislocations to approximately 600 cm-2.

Doctor of Philosophy / Department of Physics / Hongxing Jiang / The correlation between polarity and material quality of un-doped Al[0.81subscript]Ga[0.19subscript]N was studied. The overall material quality is significantly influenced by the growth polarity. The epilayers with aluminum-polarity have a much higher crystalline quality and better surface morphology than those of nitrogen-polarity. Nitrogen-polar growth more readily incorporates unintentional impurities. A-plane AlN epilayers have been grown on r-plane sapphire substrates. The orientation and high crystalline quality were confirmed by x-ray diffraction (XRD) [Theta]-2[Theta] scan exhibiting a reflection peak at 2[Theta] = 59.4[0superscript] and rocking curve of the (110) reflection having a line width of 940 arcsec. Room temperature photoluminescence (PL) spectroscopy showed that the surface emission intensity of a-plane AlN epilayers is comparable to that of c-plane AlN. PL spectra of Mg-doped a- and c-plane AlN revealed that the Mg level in both a- and c-plane AlN is identical and is about ~ 0.5 eV. Identically designed a-plane and c-plane AlN/A1[0.65subscript]Ga[0.35subscript]N QWs have been grown on a-and c-plane AlN/Al[2subscript]O[3subscript] templates respectively, and their PL emission properties were studied. Low temperature PL characteristics of a-plane QWs are primarily governed by the quantum size effect, whereas those of c-plane QWs are significantly affected by the polarization fields. The growth of AlN epilayers on SiC substrates was investigated. A smooth, crack free AlN epilayer with high optical and crystalline quality was achieved. Because of its high quality, AlN was used as active layer in a hybrid Schottky photodetector. Highly conductive Si-doped Al[subscript0.75]Ga[0.25subscript]N alloys were grown on AlN/SiC templates. The effects of using Indium as a surfactant during the growth of Si-doped Al[0.75subscript]Ga[0.25subscript]N epilayers at relatively high temperature 1050 [degrees]C were studied. Indium significantly increases the doping efficiency as shown by RT Hall measurements. RT PL measurements show a clear correlation between emission intensity of the defect related transition and indium flow rate. P-type conductivity has been obtained in beryllium doped GaN by MOCVD. The activation energy of the beryllium acceptor was estimated to be 118 [plus or minus] 4 meV, which is about 40 meV less than the activation energy of the Mg acceptor in GaN.

Master of Science / Department of Chemical Engineering / James H. Edgar / 4H-silicon carbide (4H-SiC) is a wide band gap semiconductor with outstanding capabilities for high temperature, high power, and high frequency electronic device applications. Advances in its processing technology have resulted in large micropipe-free single crystals and high speed epitaxial growth on off-axis silicon face substrates. Extraordinarily high growth rates of high quality epitaxial films (>100 [Mu]m per hour) have been achieved, but only on off-axis substrates (misoriented 4° to 8° from the (0001) crystallographic plane). There is a strong incentive to procure an on-axis growth procedure, due to the excessive waste of high quality single crystal associated with wafering off-axis substrates. The purpose of this research was to develop a reliable process for homoepitaxial growth of 4H-SiC on on-axis 4H-SiC. Typically the use of on-axis SiC for epitaxial growth is undesired due to the increased probability of 3C-SiC inclusions and polycrystalline growth. However, it is believed that the presence of chlorine during reaction may reduce the presence of 3C-SiC and improve the quality of the epitaxial film. Therefore homoepitaxial SiC was deposited using methyltrichlorosilane (MTS) and ethane sources with carrier gases consisting of argon-hydrogen mixtures. Ethane was used to increase the C/Si ratio, to aid in the prevention of 3C-SiC, and to help eliminate silicon droplets deposited during epitaxial growth. Deposition occurred in a homemade, quartz, cold wall chemical vapor deposition reactor. Epitaxial films on on-axis 4H-SiC were deposited without the presence of 3C-SiC inclusions or polycrystalline SiC, as observed by defect selective etching, scanning electron microscopy and optical microscopy. Large defect free areas, [similar to]5 mm[superscript]2, with epitaxial film thicknesses of [similar to]6 [Mu]m were grown on on-axis 4H-SiC. Epitaxial films had approximately an 80%, [similar to]20 cm[superscript]-2, decrease in defect density as compared to the substrates. The growth rate was independent of face polarity and orientation of the substrate. The optimal temperature for hydrogen etching, to promote the smoothest epitaxial films for on-axis substrates (both C- and Si-polarities), is [similar to]1550 °C for 10 minutes in the presence of 2 slm hydrogen. The optimum C/Si ratio for epitaxial growth on on-axis 4H-SiC is 1; excess carbon resulted in the codeposition of graphite and cone-shaped silicon carbide defects.

The goal of this thesis work is to investigate the possibility to grow graphene by Chemical Vapor Deposition (CVD) on copper foil with acetylene as a precursor and varigon (5\% H$_2$ in Ar) as a carrier gas. The possibility of nitrogen doping by ammonia treatment during the growth process is also investigated. The possibility of graphene transfer, with the use of Poly(Methyl Metacrylate) (PMMA), from the copper onto another target substrate, Flourine doped Tin Oxide (FTO), is also explored. The main technique of characterization of the grown and transfered graphene is Raman spectroscopy, a great tool for investigating the number of graphene layers and amount of defects. Other characterization methods used are Scanning Electron Microscopy (SEM) X-ray Photoelectron Spectroscopy (XPS) to investigate morphology and elemental composition, respectively. The result of this thesis study is that graphene growth is entirely possible with acetylene as a precursor, as shown by the Raman spectroscopy, XPS and SEM. The grown graphene has a high quality with few layers and a low number of defects. The ammonia treatment, however, doesn't seem to have an immediate effect on the graphene growth. The XPS data indicates that there are no nitrogen doping in the graphene, though there might be a correlation between the ammonia and the number of layers, but further investigations has to be made. Transfer is also proven possible with the method developed. However, improvements to the transfer method can be done since there are both larger tares, caused by the transfer onto the FTO, as well as microscopic tares, possibly caused by thermal expansion of the PMMA.

Necessity is the mother of all inventions. With Si hitting the speed bottleneck, newer materials to replace Si are being sought out. The ex-foliation based experiments on graphene by Geim and Novoselov at this point was perfect as many of its physical properties were fascinating from an electronics standpoint and hence it was very soon projected as a Si replacement for logic applications. In addition, graphene is also an attractive alternative to applications such as radio frequency devices, ultra-sensitive mass/chemical sensing, high-speed optoelectronics and transparent conductors for photo-voltaic applications. While the widespread success and utility of Si can be attributed to easy availability of source material and the ability to synthesize large areas of ultra high quality material, chemical vapor deposition (CVD) is the only available method to controllably produce large area monolayer graphene. CVD graphene is however polycrystalline and therefore defective. Hence, in order to promote graphene towards large-scale commercialization, it is necessary to be able to grow spatially homogeneous graphene with tailored defect densities. Transfer of atomic layers of graphene from the substrate on which it is grown, a Cu foil typically, on to an insulating substrate for electrical measurements is typically a major defect inducing step. Hence, a direct transfer-free fabrication of suspended device using graphene grown on thin films of electro-deposited Cu was attempted and successfully reported for the first time. Though it was shown that the fabrication process itself did not introduce any additional defects, the maximum obtained mobility on such fabricated structures was 5200 cm2/V·s. This value is lower than reported values in literature and thus improvements for electronic applications warranted further optimization. However, limitations on ability of electro-deposited Cu films (melting point of 1083 ◦C) to withstand high temperatures, 1000 ◦C, impeded further optimizations. Hence, growth on Cu foils was taken up. On Cu foil, we were able to identify the roles of the growth kinetics and system thermodynamics on the final quality of graphene. Specifically, by carefully altering the conditions during appropriate growth phases, we were able to obtain graphene films of tunable defect densities with motilities ranging from 200 - 20000 cm2/V·s. Using a host of characterization Techniques like electrical transport, Raman spectroscopic measurements, TEM imaging and water permeation studies, we find that the defect densities in graphene are largely concentrated at the boundaries, while the bulk of the graphene grain remains pristine. Further investigations revealed a thermodynamic correlation between the growth conditions and quality of the grain boundary in terms of defect density and structure. In addition to the influence of defects in graphene on charge mobility as seen before, their impact on the device contact resistance and charge transport hysteresis in graphene field effect transistors were also investigated. With a careful control on the film defect density, we were able to demonstrate devices with low contact resistance (1000 Ωµm ) and tunable hysteresis behavior. Finally, alternate substrates for graphene and its impact on the carrier densities were explored. Non-polar substrate SiO2 and polar substrates such AlN and AlGaN were chosen. On AlN, we obtained higher carrier mobility due to reduced phonon-electron scattering and a higher ’P’ doping behavior due to piezo-electric effects. Hence, to leverage the previous observation, novel FET device architecture with a HEMT based substrate using AlGaN was demonstrated.

The importance of infrared (IR) technology (for detection in the 3-5 μm and 8-14 μm atmospheric windows) has spread from military applications to civilian applications since World War II. The commercial IR detector market in these wavelength ranges is dominated by mercury cadmium telluride (MCT) alloys. The use of these alloys has, however, been faced with technological difficulties. One of the materials that have been tipped to be suitable to replace MCT is InAs/InxGa1-xSb strained layer superlattices (SLS’s). Atmospheric pressure metal-organic vapour phase epitaxy (MOVPE) has been used to grow InAs/GaSb strained layer superlattices (SLS’s) at 510 °C in this study. This is a starting point towards the development of MOVPE InAs/InxGa1-xSb SLS’s using the same system. Before the SLS’s could be attempted, the growth parameters for GaSb were optimised. Growth parameters for InAs were taken from reports on previous studies conducted using the same reactor. Initially, trimethylgallium, a source that has been used extensively in the same growth system for the growth of GaSb and InxGa1-xSb was intended to be used for gallium species. The high growth rates yielded by this source were too large for the growth of SLS structures, however. Thus, triethylgallium (rarely used for atmospheric pressure MOVPE) was utilized. GaSb layers (between 1 and 2 μm thick) were grown at two different temperatures (550 °C and 510 °C) with a varying V/III ratio. A V/III ratio of 1.5 was found to be optimal at 550 °C. However, the low incorporation efficiency of indium into GaSb at this temperature was inadequate to obtain InxGa1-xSb with an indium mole fraction (x) of around 0.3, which had previously been reported to be optimal for the performance of InAs/InxGa1-xSb SLS’s, due to the maximum splitting of the valence mini bands for this composition. The growth temperature was thus lowered to 510 °C. This resulted in an increase in the optimum V/III ratio to 1.75 for GaSb and yielded much higher incorporation efficiencies of indium in InxGa1-xSb. However, this lower growth temperature also produced poorer surface morphologies for both the binary and ternary layers, due to the reduced surface diffusion of the adsorbed species. An interface control study during the growth of InAs/GaSb SLS’s was subsequently conducted, by investigating the influence of different gas switching sequences on the interface type and quality. It was noted that the growth of SLS’s without any growth interruptions at the interfaces leads to tensile strained SLS’s (GaAs-like interfaces) with a rather large lattice mismatch. A 5 second flow of TMSb over the InAs surface and a flow of H2 over GaSb surface yielded compressively strained SLS’s. Flowing TMIn for 1 second and following by a flow of TMSb for 4 seconds over the GaSb surface, while flowing H2 for 5 seconds over the InAs surface, resulted in SLS’s with GaAs-like interfacial layers and a reduced lattice mismatch. Temperature gradients across the surface of the susceptor led to SLS’s with different structural quality. High resolution x-ray diffraction (HRXRD) was used to determine the thicknesses as well as the type of interfacial layers. The physical parameters of the SLS’s obtained from simulating the HRXRD spectra were comparable to the parameters obtained from cross sectional transmission electron microscopy (XTEM) images. The thicknesses of the layers and the interface type played a major role in determining the cut-off wavelength of the SLS’s.

No description available.

This thesis analyzes the chemical vapour deposition (CVD) growth of vertically aligned carbon nanotube (CNT) forests in order to understand how CNT forests grow, why they stop growing, and how to control the properties of the synthesized CNTs. In situ kinetics data of the growth of CNT forests are gathered by in situ optical microscopy. The overall morphology of the forests and the characteristics of the individual CNTs in the forests are investigated using scanning electron microscopy and Raman spectroscopy. The in situ data show that forest growth and termination are activated processes (with activation energies on the order of 1 eV), suggesting a possible chemical origin. The activation energy changes at a critical temperature for ethanol CVD (approximately 870°C). These activation energies and critical temperature are also seen in the temperature dependence of several important characteristics of the CNTs, including the defect density as determined by Raman spectroscopy. This observation is seen across several CVD processes and suggests a mechanism of defect healing. The CNT diameter also depends on the growth temperature. In this thesis, a thermodynamic model is proposed. This model predicts a temperature and pressure dependence of the CNT diameter from the thermodynamics of the synthesis reaction and the effect of strain on the enthalpy of formation of CNTs. The forest morphology suggests significant interaction between the constituent CNTs. These interactions may play a role in termination. The morphology, in particular a microscale rippling feature that is capable of diffracting light, suggest a non-uniform growth rate across the forest. A gas phase diffusion model predicts a non-uniform distribution of the source gas. This gas phase diffusion is suggested as a possible explanation for the non-uniform growth rate. The gas phase diffusion is important because growth by acetylene CVD is found to be very efficient (approximately 30% of the acetylene is converted to CNTs). It is seen that multiple mechanisms are active during CNT growth. The results of this thesis provide insight into both the basic understanding of the microscopic processes involved in CVD growth and how to control the properties of the synthesized CNTs.

In situ Raman spectroscopy was used to explore etching of carbon nanotubes as well as their growth from C60. The thesis is in three parts: (1) C60 seed particles were partially oxidized in air and were used to grow carbon nanotubes and other nanocarbon structures. Seed oxidization was characterized by monitoring the evolution of the Raman Ag(2) peak and the D band, and oxidation temperature was found to be critical to nanotube growth. (2) To further explore oxidation, carbon nanotubes were thermally oxidized in air at different temperatures, while the evolution of different Raman bands was tracked. Etching dynamics and band intensity evolution were tracked in situ. Notably, metallic species were found to etch much more rapidly than semiconducting species of similar diameter. (3) To confirm and expand on this, a novel, simultaneous two-laser Raman spectroscopy setup was used to track the thermal oxidation of carbon nanotubes in O2 and CO2 gases at different temperatures. Metallic species were resonant with one laser line, while semiconducting species were resonant with the other, so changes to sample metallicity could be tracked unambiguously in two separate spectra. Again, metals were found to etch more rapidly. In situ Raman spectroscopy can track the evolution of nanotubes in real time and provide insight into processing. In general, detailed process monitoring like this can help in the development of selective synthesis and processing.

In Topologischen Isolatoren (TI) werden metallische Zustände an der Oberfläche beobachtet, während die entsprechenden Volumenzustände eine Bandlücke aufweisen. Der Volumenbeitrag zur Leitfähigkeit von TI-Materialien macht eine Synthese von Nanokristallen bzw. eine Dotierung nötig. Der Fokus der Untersuchungen dieser Arbeit liegt dabei auf der Erzeugung von Nanokristallen der TI-Materialien Bi2Te3- und Bi2Te2Se sowie dotierter Bi2Se3-Nanokristallen. Die Synthese der Nanokristalle erfolgte durch den Gasphasentransport im geschlossenen System über den Mechanismus einer Zersetzungssublimation bzw. unter dem Einsatz eines Transportmittels. Für eine erfolgreiche Erzeugung der Nanokristalle sind im Vorfeld thermodynamische Modellierungen des Gasphasentransports sowie Versuche zum chemischen Transport für die quasibinären Systeme Bi2Se3-Bi2Te3, Bi2Se3-Sb2Se3 und Bi2Se3-FeSe sowie für das ternäre System Mn-Bi-Se durchgeführt worden. Durch Versuche zum chemischen Transport konnten die Aussagen der Modellierung bestätigt und im Weiteren der Dotandengehalt in den abgeschiedenen Kristallen sowie der Einlagerungsmechanismus durch Ergebnisse aus XRD- und ICP-OES-Untersuchungen beschrieben werden. Die Synthese bzw. Dotierung der Nanokristalle wurde hauptsächlich durch die Transportrate und den Dampfdruck des Dotanden bestimmt. In den Systemen Bi2Se3-Bi2Te3 und Bi2Se3-Sb2Se3 ist ein Gasphasentransport über eine Zersetzungssublimation durchführbar und resultierte in einer erfolgreichen Darstellung von Bi2Te3- und Bi2Te2Se-Nanokristallen sowie von dotierten (SbxBi1-x)2Se3-Nanokristallen. Entgegen dessen erfolgte der Gasphasentransport in den Systemen Bi2Se3-FeSe und Mn-Bi-Se unter Verwendung eines Transportmittels. Hierbei verringerten die gesteigerten Transportraten das Wachtum von Nanokristallen. Im Weiteren gelang es dotierte (Fe,Mn)xBi2-xSe3-Volumenkristalle sowie MnBi2Se4-Einkristalle darzustellen und mittels XRD, ICP-OES, magnetischer Messungen sowie elektrischem Transport zu charakterisieren.