Veeco Applied EPI 930 molecular beam epitaxy system equipped with a radio frequency plasma assisted nitrogen source has been introduced and the growth procedure and some specialized measurements are also described. The GaN thin films grown by RF-MBE have been talked about nitridation, low temperature GaN buffer layer, and GaN epilayer. The nitridation is a necessary for grown GaN on c-sapphire. From the result of the HRXRD symmetric (002) rocking curve, the magnitude of the FWHM of the GaN films without nitridation was larger than the films with nitridation. The growth temperature of the LT GaN buffer layer was the major factor on the quality of GaN epilayer which grown on the almost without nitridated sapphire substrate. The growth condition of high growth temperature, thin, low growth rate, and low N/Ga ratio of the LT GaN buffer layer can improve the sequent GaN epilayer quality. On the other hand, in the N/Ga flux ratio of GaN epitaxy layer experiment, we have found three interesting results. First, the narrowest peak width of PL spectrum appeared in a slight Ga-rich condition. Second, the smallest of HRXRD FWHM appeared in the nearly stoichiometry condition. Third, the highest electron mobility and less overall dislocations appeared in a slight N-rich condition. Finally, we report the results about AlGaN/GaN heterostructure grown by metalorganic chemical vapor deposition. The piezoelectric effect on the Alx-£_In£_Ga1-xN/GaN heterostructures was investigated and we found that a little In atom in the spacer (£_< 0.01 %) will substantially reduce the strain at interface due to the much larger size of the In atom in comparison to Al and Ga atoms. The electric field at the interface is reduced one order of magnitude smaller than that of the heterostructure without In atom. Two SdH oscillations beat each other due to the population of the lowest two subbands was been seen. Another two SdH oscillations beating have been observed in modulation-doped AlxGa1-xN/GaN heterostructures caused by the finite zero-filed spin splitting due to the inversion-asymmetry-induced bulk k3 term.

In order to develop high speed photo-electronic device, first, we grew one layer of GaN by MOPVE to decline lattice mismatch. Then we used PA-MBE to grow AlxGa1-xN/GaN heterostructure III-V semiconductor. Via changing the content of aluminum, we can confer the characteristic of these samples. In these samples, we controlled the content of aluminum by changing the vapor of aluminum. And then we used X-ray diffraction, SEM, AFM, low temperature Hall measurement and SdH to study the characteristic of these samples. Throughout X-ray diffraction, the aluminum content x are 1.76%, 2.3%, 14.33%, 22.03% and 37.26%. Due to (004) AlGaN Rocking Curve F.W.H.M. are only 300 arcsec, the quality of the five samples are extraordinary. In addition, SEM and AFM measurement indicate that this series samples¡¦ interface are very smooth, and the roughest sample is only 2nm. It can make sure that samples were grown in mode of two-dimensional (2D). With low temperature Hall measurement, we can find out the Coulomb scattering which is from the defect are very small in the sample A, B, C, D. And the mobility of this series samples are very high, the highest mobility is sample A at 8K which is 19593 cm2/Vs. We can observe the oscillation of the sample C, D obviously in SdH measurement indicate that the 2DEG is confined in the potential well.

Many scholars suggest that Minority Business Enterprises help disadvantaged populations and achieve greater equity in society. Rooted in the affirmative action policies of the 1960s and 1970s, Minority Business Enterprise designations have become a standard way for the federal government to assist minority entrepreneurs as well as protect against discrimination in contracting. Some scholars even suggest that these policies go beyond protection from discrimination and actually foster economic development in minority communities. This thesis examines those claims and utilizes an example from 12 years of the Georgia Department of Transportation's records on contracting with MBEs to answer the question: who is helped by these federal policies? This examination sheds light on the current literature linking MBEs with economic development as well as adds to the sparse literature on outcomes for MBE policy. The results of data analysis show that , over a 12-year period, White female business enterprises receive the twice as many contracting dollars as all other Minority Business Enterprises combined.

Im Rahmen dieser Diplomarbeit wurden Si(001)-Oberflächen mit Selen modifiziert und anschließend charakterisiert. Die Herstellung der Selenpassivierungsschichten wurde in einer MBE-Anlage durchgeführt. Die Charakterisierung erfolgte durch RHEED-, LEED- und RBS-Untersuchungen.

abstract: HgCdTe is the dominant material currently in use for infrared (IR) focal-plane-array (FPA) technology. In this dissertation, transmission electron microscopy (TEM) was used for the characterization of epitaxial HgCdTe epilayers and HgCdTe-based devices. The microstructure of CdTe surface passivation layers deposited either by hot-wall epitaxy (HWE) or molecular beam epitaxy (MBE) on HgCdTe heterostructures was evaluated. The as-deposited CdTe passivation layers were polycrystalline and columnar. The CdTe grains were larger and more irregular when deposited by HWE, whereas those deposited by MBE were generally well-textured with mostly vertical grain boundaries. Observations and measurements using several TEM techniques showed that the CdTe/HgCdTe interface became considerably more abrupt after annealing, and the crystallinity of the CdTe layer was also improved. The microstructure and compositional profiles of CdTe(211)B/ZnTe/Si(211) heterostructures grown by MBE was investigated. Many inclined {111}-type stacking faults were present throughout the thin ZnTe layer, terminating near the point of initiation of CdTe growth. A rotation angle of about 3.5° was observed between lattice planes of the Si substrate and the final CdTe epilayer. Lattice parameter measurement and elemental profiles indicated that some local intermixing of Zn and Cd had taken place. The average widths of the ZnTe layer and the (Cd, Zn)Te transition region were found to be roughly 6.5 nm and 3.5 nm, respectively. Initial observations of CdTe(211)B/GaAs(211) heterostructures indicated much reduced defect densities near the vicinity of the substrate and within the CdTe epilayers. HgCdTe epilayers grown on CdTe(211)B/GaAs(211) composite substrate were generally of high quality, despite the presence of precipitates at the HgCdTe/CdTe interface. The microstructure of HgCdSe thin films grown by MBE on ZnTe/Si(112) and GaSb(112) substrates were investigated. The quality of the HgCdSe growth was dependent on the growth temperature and materials flux, independent of the substrate. The materials grown at 100°C were generally of high quality, while those grown at 140°C had {111}-type stacking defects and high dislocation densities. For epitaxial growth of HgCdSe on GaSb substrates, better preparation of the GaSb buffer layer will be essential in order to ensure that high-quality HgCdSe can be grown. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2011

Essay 1 Abstract Prior research indicates that equity markets assign a higher valuation to firms that sustain a string of earnings increases (earnings string) and a string of meeting or beating analysts’ earnings expectations (MBE string). However, to date, there is little evidence on the response of debt investors when firms sustain a long string of meeting/beating earnings benchmarks. This study fills the gap in the literature by analyzing the impact of sustaining an earnings string/MBE string on the cost of debt. I find evidence of a positive (negative) association between the length of the earnings string/MBE string and the bond yield spreads (credit ratings). These results suggest that debt holders assess a higher risk to firms that sustain a string of earnings benchmarks and require a higher risk premium, contrary to equity holders, who reward firms that sustain a string of earnings benchmarks. Additional analyses indicate that this discrepancy is attributable to the different investor compositions between debt and equity markets. Essay 2 Abstract This study extends the existing literature by investigating the impact of sustaining a string of earnings increases (earnings string) on stock returns using the time-series asset pricing approach. Using both Fama-French (1993) three-factor and Carhart (1997) four-factor models, I find that the average abnormal return of a zero investment arbitrage portfolio that longs the highest earnings string portfolio and shorts the lowest earnings string portfolio is approximately negative 65 (75) basis points per month. These results provide further insight into the existing literature by demonstrating that earnings string firms initially experience higher stock returns. However, as earnings strings become longer, the market reaction becomes weaker.

abstract: CdTe/MgxCd1-xTe double heterostructures (DHs) have been grown on lattice matched InSb (001) substrates using Molecular Beam Epitaxy. The MgxCd1-xTe layers, which have a wider bandgap and type-I band edge alignment with CdTe, provide sufficient carrier confinement to CdTe, so that the optical properties of CdTe can be studied. The DH samples show very strong Photoluminescence (PL) intensity, long carrier lifetimes (up to 3.6 μs) and low effective interface recombination velocity at the CdTe/MgxCd1 xTe heterointerface (~1 cm/s), indicating the high material quality. Indium has been attempted as an n-type dopant in CdTe and it is found that the carriers are 100% ionized in the doping range of 1×1016 cm-3 to 1×1018 cm-3. With decent doping levels, long minority carrier lifetime, and almost perfect surface passivation by the MgxCd1-xTe layer, the CdTe/MgxCd1-xTe DHs are applied to high efficiency CdTe solar cells. Monocrystalline CdTe solar cells with efficiency of 17.0% and a record breaking open circuit voltage of 1.096 V have been demonstrated in our group. Mg0.13Cd0.87Te (1.7 eV), also with high material quality, has been proposed as a current matching cell to Si (1.1 eV) solar cells, which could potentially enable a tandem solar cell with high efficiency and thus lower the electricity cost. The properties of Mg0.13Cd0.87Te/Mg0.5Cd0.5Te DHs and solar cells have been investigated. Carrier lifetime as long as 0.56 μs is observed and a solar cell with 11.2% efficiency and open circuit voltage of 1.176 V is demonstrated. The CdTe/MgxCd1-xTe DHs could also be potentially applied to luminescence refrigeration, which could be used in vibration-free space applications. Both external luminescence quantum efficiency and excitation-dependent PL measurement show that the best quality samples are almost 100% dominated by radiative recombination, and calculation shows that the internal quantum efficiency can be as high as 99.7% at the optimal injection level (1017 cm-3). External luminescence quantum efficiency of over 98% can be realized for luminescence refrigeration with the proper design of optical structures. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2016

abstract: The research described in this dissertation involved the use of transmission electron microscopy (TEM) to characterize II-VI and III-V compound semiconductor quantum dots (QDs) and dilute-nitride alloys grown by molecular beam epitaxy (MBE) and intended for photovoltaic applications. The morphology of CdTe QDs prepared by the post-annealing MBE method were characterized by various microscopy techniques including high-resolution transmission electron microscopy (HR-TEM), and high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM). Extensive observations revealed that the of QD shapes were not well-defined, and the QD size and spatial distribution were not determined by the amount of CdTe deposition. These results indicated that the formation of II-VI QDs using a post-annealing treatment did not follow the conventional growth mechanism for III-V and IV-IV materials. The structural properties of dilute-nitride GaAsNx films grown using plasma-assisted MBE were characterized by TEM and HAADF-STEM. A significant amount of the nitrogen incorporated into the dilute nitride films was found to be interstitial, and that fluctuations in local nitrogen composition also occurred during growth. Post-growth partial relaxation of strain resulted in the formation of {110}-oriented microcracks in the sample with the largest substitutional nitrogen composition. Single- and multi-layered InAs QDs grown on GaAsSb/GaAs composite substrates were investigated using HR-TEM and HAADF-STEM. Correlation between the structural and optoelectronic properties revealed that the GaAsSb barrier layers had played an important role in tuning the energy-band alignments but without affecting the overall structural morphology. However, according to both XRD measurement and electron microscopy the densities of dislocations increased as the number of QD layers built up. An investigation of near-wetting layer-free InAs QDs incorporated with AlAs/GaAs spacer layers was carried out. The microscopy observations revealed that both embedded and non-embedded near-wetting layer-free InAs QDs did not have well-defined shapes unlike conventional InAs QDs. According to AFM analysis and plan-view TEM characterization, the InAs QDs incorporated with spacer layers had smaller dot density and more symmetrical larger sizes with an apparent bimodal size distribution (two distinct families of large and small dots) in comparison with conventional InAs QDs grown without any spacer layer. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2014

Kyoto University (京都大学) / 0048 / 新制・論文博士 / 博士(工学) / 乙第12695号 / 論工博第4084号 / 新制||工||1555(附属図書館) / 29947 / (主査）教授 平尾 一之, 教授 田中 勝久, 教授 三浦 清貴 / 学位規則第4条第2項該当

abstract: Sn-based group IV materials such as Ge1-xSnx and Ge1-x-ySixSny alloys have great potential for developing Complementary Metal Oxide Semiconductor (CMOS) compatible devices on Si because of their tunable band structure and lattice constants by controlling Si and/or Sn contents. Growth of Ge1-xSnx binaries through Molecular Beam Epitaxy (MBE) started in the early 1980s, producing Ge1-xSnx epilayers with Sn concentrations varying from 0 to 100%. A Chemical Vapor Deposition (CVD) method was developed in the early 2000s for growing Ge1-xSnx alloys of device quality, by utilizing various chemical precursors. This method dominated the growth of Ge1-xSnx alloys rapidly because of the great crystal quality of Ge1-xSnx achieved. As the first practical ternary alloy completely based on group IV elements, Ge1-x-ySixSny decouples bandgap and lattice constant, becoming a prospective CMOS compatible alloy. At the same time, Ge1-x-ySixSny ternary system could serve as a thermally robust alternative to Ge1-ySny binaries given that it becomes a direct semiconductor at a Sn concentration of 6%-10%. Ge1-x-ySixSny growths by CVD is summarized in this thesis. With the Si/Sn ratio kept at ~3.7, the ternary alloy system is lattice matched to Ge, resulting a tunable direct bandgap of 0.8-1.2 eV. With Sn content higher than Si content, the ternary alloy system could have an indirect-to-direct transition, as observed for Ge1-xSnx binaries. This thesis summarizes the development of Ge1-xSnx and Ge1-x-ySixSny alloys through MBE and CVD in recent decades and introduces an innovative direct injection method for synthesizing Ge1-x-ySixSny ternary alloys with Sn contents varying from 5% to 12% and Si contents kept at 1%-2%. Grown directly on Si (100) substrates in a Gas-phase Molecular Epitaxy (GSME) reactor, both intrinsic and n-type doped Ge1-x-ySixSny with P with thicknesses of 250-760 nm have been achieved by deploying gas precursors Ge4H10, Si4H10, SnD4 and P(SiH3)3 at the unprecedented low growth temperatures of 190-220 °C. Compressive strain is reduced and crystallinity of the Ge1-x-ySixSny epilayer is improved after rapid thermal annealing (RTA) treatments. High Resolution X-ray Diffraction (HR-XRD), Rutherford Backscattering Spectrometry (RBS), cross-sectional Transmission Electron Microscope (XTEM) and Atomic Force Microscope (AFM) have been combined to characterize the structural properties of the Ge1-x-ySixSny samples, indicating good crystallinity and flat surfaces. / Dissertation/Thesis / Masters Thesis Chemistry 2019