<p></p><p>High mobility 2DEG (two-dimensional electron gas) confined in GaAs is a good platform to understand correlated electron systems and a promising candidate for qubit devices. For example, the non-Abelian feature of Fractional Quantum Hall state enabling topological quantum computation is only found in GaAs with high mobility. Theoretical calculations have shown that the mobility is inversely proportional to impurities in GaAs/AlGaAs heterstructures grown by Molecular Beam Epitaxy (MBE). In recent MBE experiments, the source Ga was found to be more important in the limitation of mobility than Al and As. A high mobility of 35 million cm²/Vs was recently observed when an 8N Ga (total nominal impurity concentration of ~10 ppb) source was used compared to 25 million cm²/Vs for a 7N Ga source. In addition, significant mobility increase was observed after in-situ distillation of the source Ga before growth. In order to clarify the mechanism of how the distillation contributed to the Ga purification, thus resulting in the mobility increase, the MBE in-situ distillation was analyzed by molecular distillation theory. Evaporation behavior of solvent Ga was analyzed including effects of evaporation from a crucible with receding liquid depth. Then impurity removal through molecular distillation was analyzed with molecular evaporation kinetics. The remaining 7N and 8N Ga after in-situ MBE distillation and growth were elementally analyzed by ICP-MS (Inductively Coupled Plasma Mass Spectrometry) and compared with analyses of the starting 7N and 8N Ga from same lots. Due to the increased detection limit of ICP-MS in metal analysis, the concentrations of most impurity elements reached the detection limit of ~1-10 ppb. However, unusual high concentration of 690 ppb Ge was found in the 7N Ga, exceeding the nominal concentration of 7N (100 ppb). Significant decrease in Ge concentration was found in the comparison of initial ultra-pure Ga and remaining Ga for both grades of 7N and 8N. The significant Ge losses cannot be explained by atomic Ge evaporation due to the low vapor pressure of Ge. However, a hypothesis of Ge evaporation as GeO(g) by Ge active oxidation was proposed. In order to test the active oxidation of very dilute Ge in Ga in the MBE conditions with very low P(O₂), the equilibrium P(GeO)-P(O₂) vapor species diagram was calculated from thermodynamics. The analysis shows that even very dilute Ge in Ga of ~ 1 ppm concentration can be <a>actively oxidized in the extremely low P(O₂) of MBE</a>. In order to prove active oxidation of Ge, molecular distillation of 7N Ga was performed in <a>a specially constructed high vacuum chamber. The 7N Ga with unusual high Ge concentration of 440 ppb (by GDMS analysis) was distilled for 16 h at 1360 K under the starting P(O₂) of 3 x 10^-6 torr and the total pressure of 10^-5 torr. The chamber vacuum was monitored by Residual Gas Analyzer (RGA) and the residual Ga after 16 h distillation was analyzed by GDMS. In the GDMS analysis, significant Ge loss was found from 440 ppb to below the detection limit of 10 ppb, confirming Ge active oxidation hypothesis. The oxygen-assisted impurity removal in distillation also may be applicable to other impurities with high vapor pressure gaseous oxide, but low vapor pressure itself, such as Al, Si and Sn. </a></p><br><p></p>

Thesis: Elec. E., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 1975 / Includes bibliographical references. / Elec. E. / Elec. E. Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science

Group III-nitride materials have attracted great attention for applications in high efficiency electronic and optoelectronics devices such as high electron mobility transistors, light emitting diodes, and laser diodes. On the other hand, group VI transition metal dichalcogenides (TMDs) in the form of MX2 has recently emerged as a novel atomic layered material system with excellent thermoelectric, electronic and optoelectronic properties. Also, the recent investigations reveal that the dissimilar heterojunctions formed by TMDs and III-nitrides provide the route for novel devices in the area of optoelectronic, electronics, and water splitting applications. In addition, integration of III-nitrides and TMDs will enable high density integrated optoelectronic circuits and the development of hybrid integration technologies. In this work, we have demonstrated kinetically controlled growth processes in plasma assisted molecular beam epitaxy (PAMBE) for the III-nitrides and their engineered heterostructures. Techniques such as Ga irradiation and nitrogen plasma exposure has been utilized to implement bulk GaN, InGaN and their heterostructures in PAMBE. For the growth of III-nitride based heterostructures, the in-situ surface stoichiometry monitoring (i-SSM) technique was developed and used for implementing stepped and compositionally graded InGaN-based multiple quantum wells (MQWs). Their optical and microstrain analysis in conjunction with theoretical studies confirmed improvement in the radiative recombination rate of the graded-MQWs as compared to that of stepped-MQWs, owing to the reduced strain in graded-MQWs. Our achievement also includes the realization of the p-type MoS2 by engineering pristine MoS2 layers in PAMBE. Mainly, Ga and nitrogen plasma irradiation on the pristine MoS2 in PAMBE has resulted in the realization of the p-type MoS2. Also, GaN epitaxial thin layers were deposited on MoS2/c-sapphire, WSe2/c-sapphire substrates by PAMBE to study the band discontinuity at GaN/TMDs heterointerface. The determination of band offset parameters for both GaN/MoS2 and GaN/WSe2 heterostructures revealed realization of type-II band alignment. Also, heterojunctions such as AlGaN/MoS2 is implemented to achieve type-I heterojunction. This work may open up a new avenue towards photonic quantum devices based on the integration of III-nitrides with 2D TMDs.

The goal of this thesis is to present the design and development of a chemical vapor deposition reactor for the growth of high quality homoepitaxy silicon carbide films for electronic device applications. The work was performed in the Nanomaterials and Nanomanufacturing Research Center at the University of South Florida from 8/2001-5/2003. Chemical vapor deposition (CVD) is the technique of choice for SiC epitaxial growth. Epitaxial layers are the building blocks for use in various semiconductor device applications. This thesis reports on a SiC epitaxy process where a carrier gas (hydrogen) is saturated with reactive precursors (silane and propane) which are then delivered to a semiconductor substrate resting on a RF induction heated SiC coated graphite susceptor. Growth proceeds via a series of heterogeneous chemical reactions with several steps, including precursor adsorption, surface diffusion and desorbtion of volatile by-products. The design and development of a reactor to make this process controlled and repeatable can be accomplished using theoretical and empirical tools. Fluid flow modeling, reactor sizing, low-pressure pumping and control are engineering concepts that were explored. Work on the design and development of an atmospheric pressure cold-wall CVD (APCVD) reactor will be presented. A detailed discussion of modifications to this reactor to permit hot-wall, low-pressure CVD (LPCVD) operation will then be presented. The consequences of this process variable change will be discussed as well as the necessary design parameters. Computational fluid dynamic (CFD) calculations, which predict the flow patterns of gases in the reaction tube, will be presented. Feasible CVD reactor design that results in laminar fluid flow control is a function of the prior mentioned techniques and will be presented.

This study explores in detail design options and simulations of multi-junction solar cells that utilize silicon-germanium-tin (SixGe1-x-ySny or SiGeSn) to achieve high-efficiency solar power conversion devices. SixGe1-x-ySny is an emerging system of alloys that can lattice match with germanium and gallium arsenide and can provide a bandgap higher than that of germanium; useful in the development of multi-junction solar cells. The results herein include designs of four devices: a triple-junction, a quadruple-junction, a seven-junction, and a six-junction, with estimated efficiencies of 41.6%, 42.6%, 41.2%, and 39.2% respectively under 1000x concentrated AM1.5D illumination, where the seven- and six-junction devices relax the thickness requirement of the germanium layer, and have room for improvement via the development of an advanced tunnel-junction component. Visualizations of the potentially available SiGeSn bandgaps are developed. The documentation supports further work in modelling additional compositions of SiGeSn. Loss mechanisms of the devices are calculated and plotted, enabling the design of the device layer components. Tools and techniques are developed to determine and control the resultant output error, and a generalized simulation mesh definition is given that efficiently controls the primary source of error of the calculation, which is related to the optical interaction. Lateral currents and surface recombination effects are included. The software is modularized to enable the development of higher-order segmented devices.

No description available.

Self-catalyzed GaAs nanowires (NWs) are grown epitaxially on Si(111) substrates using molecular beam epitaxy (MBE). The dynamics of the droplet are examined to improve NW yield and to control NW morphology. Control and understanding of the NW diameter via droplet dynamics is applied to NW photovoltaics and to novel corrugated NW distributed Bragg reflectors (DBRs). At the beginning of the MBE growth, a Ga pre-deposition step, between 0 s and 500 s in duration, is introduced to improve the yield of the NW arrays. The effect of the pre-deposition time was examined for five different hole diameters and yield was increased to nearly 100% for the appropriate combination of hole diameter and pre-deposition time. Two models were used to model the NW growth progression under different atomic flux ratios. The first model considers the contributions from direct and diffusion fluxes to the droplet and solves coupled equations for the droplet contact angle and the NW radius. The second model treats the contact angle as constant. Both models explained the accompanying experimental observations. Both models could be used to model future NW growths and the choice between the two would depend on the availability of contact angle data and whether the crystal phase must be considered. Absorption in NWs is determined by the diameter and the HE1n modes. The effectiveness of a linearly tapered inverted conical NW is demonstrated using finite element simulations. The photocurrent of an optimized inverted conical NW array is found and shown to be similar to that achieved by optical nanocones and nanowires. Diameter modulations can also be introduced into NW structures periodically to produce corrugated NW distributed Bragg reflectors (DBRs). The tunability of the reflectance peaks is demonstrated and explained by changes to the effective refractive index of the structure. / Thesis / Doctor of Philosophy (PhD) / This thesis seeks to understand the growth processes behind self-catalyzed nanowire growth. Nanowires (NWs) are very thin, vertical columns of semiconducting material. Self-catalyzed growth is a method of producing these structures that uses a droplet at the top of the structure to add material to the structure over time. These structures have numerous applications. This thesis focuses on solar cells and distributed Bragg reflectors (DBRs). Experiments show how control over the droplet can improve NW yield and give significant control of the NW diameter. These experiments are supported by mathematical models. Control over the diameter is important for the applications discussed. Using numerical simulations, it is shown how control over the diameter of the structure can lead to improvements in light absorption in NW solar cells. Additionally, periodic changes to the diameter can be used to create novel NW structures such as DBRs, which is a promising new application.

Nitride semiconductor materials have been used in a variety of applications, such as LEDs, lasers, photovoltaic cells and medical applications. If incandescent bulbs could be replaced by white GaN LEDs, they would not only provide compactness and longer lifetime, but this would also result in huge energy savings. A renewed interest in InGaN emerged recently after it was discovered that the band gap for InN is 0.7eV, instead of the previously published value of 1.9eV. Thus InGaN solid solutions cover almost the whole visible spectrum, from a band gap of 3.34eV for GaN and 0.7eV for InN. Hence, InGaN can have excellent applications for photovoltaic cells. The objective of this work was to investigate and search for new ways of synthesis of nitrides. We studied the thermodynamics and evaluated chemical compatibilities for the growth of AlN, GaN, InN and their solid solutions from metallic solvents. The compatibility between potential substrate, crucible and solvent materials and various growth atmospheres was evaluated from Gibbs free energy calculations. Most of the nitride synthesis experiments performed by other groups were at higher temperatures (around 2,000C) and pressures up to 1GPa using different growth methods. Therefore, their results could not be extrapolated to our growth system, as their growth conditions were significantly different from ours Moreover, to the best of our knowledge; no-one has ever evaluated such compatibilities by thermodynamic calculations. We used those calculations to design our experiments for further studies on nitrides. Experimentally, we encountered fewer issues such as corrosion problems than others observed with their growth procedures, because near-atmospheric pressures and temperatures not exceeding 1,000C could be used. Preliminary experiments were performed to confirm the thermodynamic computations and test the behavior of the chosen system. A suitable configuration was found that allowed to nucleate films of InGaN on the templates. Nitride templates or 'Buffer layers' were used to saturate the solution and grow the films. A relatively simpler configuration, to create a temperature gradient in the solution was used. Two templates were placed in the crucible, one at the top and the other one at the bottom. The temperature was raised to 950C and they were soaked there for 15-20hrs. After the growth the surface morphology was analyzed using an optical microscope and it was found to be entirely different for both the templates. The atoms from the top template dissolved and attached at the bottom template. This can be explained by the thermal gradient between the two templates: one at the bottom was at lower temperature than the top template, so there was diffusion from the top substrate towards the bottom one. AFM studies were carried out on the film to study the surface morphology of the top and the bottom templates. Growth hillocks having step height typically between 15 and 50 nm were observed. Such hillocks were not present on the templates before the experiment.

<p> Highly orientated BaTi1-xNbx03 thin films, spanning the entire range of x, have been successfully deposited on (001) MgAl20 4 substrates by the pulsed laser deposition (PLD) method. The structure of the films is characterized with a range of techniques. It is found that increasing x gives rise to a Ti4+ to Ti3+ transformation in the oxidation state accompanied by increased conductivity with a semiconductormetal transition near x = 0.2. Temperature dependent magnetic measurements show an anomalous rise in the spin moment. In order to further reduce the lattice mismatch and keep the conductivity at the same time, a partial strontium-for-barium substitution, (Ba1-ySry)Ti0.5Nb0.5O3 withy = 0, 0.4, 0.5 and 0.6, were used. Such a substitution provides a means for independently tuning the lattice parameter and conductivity over a significant range of compositions. The y = 0.6 composition show a sharp interface with flawless epitaxy and good quality films. We attribute the improvements in the film quality to a decrease in the lattice misfit strain made possible through the superior lattice match to the substrate obtained through strontium substitution. Electronic structure calculations were carried out by the 1st principle method using the WIEN2k program in order to understand the electronic structure of these compounds. Based on the assumed ordered structures, the Fermi level of BaTi1-xNbxO3 gradually moved to the lower energies as x increase, while the valence bands were not significantly altered with the Nb ions substitutions. The fraction of each Ti4+ and Ti3+ component in BaTi1-xNbxO3 samples was extracted by the linear profile fitting of the corresponding Ti-L2,3 edge obtained by the electron energy loss spectra. The fitting results indicate a high fraction of Ti3+ is present than excepted as Nb content increase, which could arise from the loss of oxygen stoichiometry. The electron energy loss spectra of the 0-K edge is analyzed by comparison to the partial density of states calculation. The evolutions of 0-K edge features are explained in terms of the decrease of the Ti 3d band contribution and the increase of the Nb 4d band contribution as the Nb content increase.</p> / Thesis / Doctor of Philosophy (PhD)

No description available.