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Novel Semi-Conductor Material Systems: Molecular Beam Epitaxial Growth and Characterization

Description: Semi-conductor industry relies heavily on silicon (Si). However, Si is not a direct-band gap semi-conductor. Consequently, Si does not possess great versatility for multi-functional applications in comparison with the direct band-gap III-V semi-conductors such as GaAs. To bridge this gap, what is ideally required is a semi-conductor material system that is based on silicon, but has significantly greater versatility. While sparsely studied, the semi-conducting silicides material systems offer great potential. Thus, I focused on the growth and structural characterization of ruthenium silicide and osmium silicide material systems. I also characterized iron silicon germanide films using extended x-ray absorption fine structure (EXAFS) to reveal phase, semi-conducting behavior, and to calculate nearest neighbor distances. The choice of these silicides material systems was due to their theoretically predicted and/or experimentally reported direct band gaps. However, the challenge was the existence of more than one stable phase/stoichiometric ratio of these materials. In order to possess the greatest control over the growth process, molecular beam epitaxy (MBE) has been employed. Structural and film quality comparisons of as-grown versus annealed films of ruthenium silicide are presented. Structural characterization and film quality of MBE grown ruthenium silicide and osmium silicide films via in situ and ex situ techniques have been done using reflection high energy electron diffraction, scanning tunneling microscopy, atomic force microscopy, cross-sectional scanning electron microscopy, x-ray photoelectron spectroscopy, and micro Raman spectroscopy. This is the first attempt, to the best of our knowledge, to grow osmium silicide thin films on Si(100) via the template method and compare it with the regular MBE growth method. The pros and cons of using the MBE template method for osmium silicide growth are discussed, as well as the structural differences of the as-grown versus annealed films. Future perspectives include further studies on other semi-conducting silicides material systems in terms ...
Date: December 2013
Creator: Elmarhoumi, Nader M.
Partner: UNT Libraries

MBE Growth and Instrumentation

Description: This thesis mainly aims at application of principles of engineering technology in the field of molecular beam epitaxy (MBE). MBE is a versatile technique for growing epitaxial thin films of semiconductors and metals by impinging molecular beams of atoms onto a heated substrate under ultra-high vacuum (UHV) conditions. Here, a LabVIEW® (laboratory virtual instrument engineering workbench) software (National Instruments Corp., http://www.ni.com/legal/termsofuse/unitedstates/usH) program is developed that would form the basis of a real-time control system that would transform MBE into a true-production technology. Growth conditions can be monitored in real-time with the help of reflection high energy electron diffraction (RHEED) technique. The period of one RHEED oscillation corresponds exactly to the growth of one monolayer of atoms of the semiconductor material. The PCI-1409 frame grabber card supplied by National Instruments is used in conjunction with the LabVIEW software to capture the RHEED images and capture the intensity of RHEED oscillations. The intensity values are written to a text file and plotted in the form of a graph. A fast Fourier transform of these oscillations gives the growth rate of the epi-wafer being grown. All the data being captured by the LabVIEW program can be saved to file forming a growth pedigree for future use. Unattended automation can be achieved by designing a control system that monitors the growth in real-time and compares it with the data recorded from the LabVIEW program from the previous growth and adjusts the growth parameters automatically thereby growing accurate device structures.
Date: May 2006
Creator: Tarigopula, Sriteja
Partner: UNT Libraries

Growth and Characterization of MBE Parametric Variations Upon Polarization Independent Strained Films

Description: This project was designed to comprehensively investigate the effects of molecular beam epitaxy (MBE) growth variations upon tensile-strained films. Using two dimensional (2-D) structures, such as quantum wells of variable configurations, we worked to observe the electro-optical response of polarization independence. The latter had been studied for compressively strained systems. However, the proof of principle for the feasibility of tensile-strained single and double quantum wells, particularly in the GaAs as opposed to the InP material system, had been much less investigated by comparison. The significance of this project, from a Basic Energy Sciences perspective, has been in its contribution to the understanding of how MBE parameters and sample design effect the physics of strained materials which themselves have been tailored to react with polarization insensitivity. Fundamental issues of materials structure control, growth parameter modifications, and characterization methods were investigated. Both single and double quantum well systems were used with both symmetric and asymmetric well widths for the latter. In this work, we essentially investigated both the growth of and potential applications for quantum based structures that use tensile strain to induce polarization independence.
Date: April 30, 2003
Creator: Daniels-Race, Theda
Partner: UNT Libraries Government Documents Department

Defect Doping of InN

Description: InN films grown by molecular beam epitaxy have been subjected to 2 MeV He{sup +} irradiation followed by thermal annealing. Theoretical analysis of the electron mobilities shows that thermal annealing removes triply charged donor defects, creating films with electron mobilities approaching those predicted for uncompensated, singly charged donors. Optimum thermal annealing of irradiated InN can be used to produce samples with electron mobilities higher than those of as grown films.
Date: July 22, 2007
Creator: Jones, R.E.; van Genuchten, H.C.M.; Yu, K.M.; Walukiewicz, W.; Li, S.X.; A ger III, J.W. et al.
Partner: UNT Libraries Government Documents Department

Valence band anticrossing in GaBixAs1-x

Description: The optical properties of GaBixAs1-x (0.04< x< 0.08) grown by molecular beam epitaxy have been studied by photomodulated reflectance spectroscopy. The alloys exhibit a strong reduction in the bandgap as well as an increase in the spin-orbit splitting energy with increasing Bi concentration. These observations are explained by a valence band anticrossing model, which shows that a restructuring of the valence band occurs as the result of an anticrossing interaction between the extended states of the GaAs valence band and the resonant T2 states of the Bi atoms.
Date: July 11, 2007
Creator: Alberi, K.; Dubon, O. D.; Walukiewicz, W.; Yu, K. M.; Bertulis, K. & Krotkus, A.
Partner: UNT Libraries Government Documents Department

Effect of oxygen on the electronic band structure in ZnO{sub x}Se{sub 1-x} alloys

Description: The effect of alloying small amounts of ZnO with ZnSe on the electronic band structure has been studied. Optical transitions in molecular-beam-epitaxy-grown ZnO{sub x}Se{sub 1-x} epitaxial films (0 {<=} x {<=} 1.35%) were investigated using photoreflectance and photoluminescence spectroscopies. The fundamental band-gap energy of the alloys was found to decrease at a rate of about 0.1 eV per atomic percent of oxygen. The pressure dependence of the band gap was also found to be strongly affected by O incorporation. Both the effects can be quantitatively explained by an anticrossing interaction between the extended states of the conduction band of ZnSe and the highly localized oxygen states located at approximately 0.22 eV above the conduction-band edge.
Date: March 14, 2003
Creator: Shan, W.; Walukiewicz, W.; Ager III, J.W.; Yu, K.M.; Wu, J.; Haller, E.E. et al.
Partner: UNT Libraries Government Documents Department

Good electrical contacts for high resistivity (Cd,Mn)Te crystals

Description: We consider that semi-insulating (Cd,Mn)Te crystals may well successfully replace the commonly used (Cd,Zn)Te crystals as a material for manufacturing large-area X- and gamma-ray detectors. The Bridgman growth method yields good quality and high-resistivity (10{sup 9}-10{sup 10} {Omega}-cm) crystals of (Cd,Mn)Te:V. Doping with vanadium ({approx} 10{sup 16} cm{sup -3}), which acts as a compensating agent, and annealing in cadmium vapors, which reduces the number of cadmium vacancies in the as-grown crystal, ensure this high resistivity. Detector applications of the crystals require satisfactory electrical contacts. Hence, we explored techniques of ensuring good electrical contacts to semi-insulating (Cd,Mn)Te crystals. Our findings are reported here. Before depositing the contact layers, we prepared an 'epi-ready' surface of the crystal platelet by a procedure described earlier for various tellurium-based II-VI compound crystals. A molecular beam epitaxy (MBE) apparatus was used to deposit various types of contact layers: Monocrystalline semiconductor layers, amorphous- and nanocrystalline semiconductor layers, and metal layers were studied. We employed ZnTe heavily doped ({approx} 10{sup 18} cm{sup -3}) with Sb, and CdTe heavily doped ({approx} 10{sup 17} cm{sup -3}) with In as the semiconductors to create contact layers that subsequently enable good contact (with a narrow, tunneling barrier) to the Au layer that usually is applied as the top contact layer. We describe and discuss the technology and some properties of the electrical contacts to semi-insulating (Cd,Mn)Te.
Date: October 19, 2008
Creator: Witkowska-Baran, M.; Mycielski, A.; Kochanowska, D.; Szadkowski, A. J.; Jakiela, r.; Witkowska, B. et al.
Partner: UNT Libraries Government Documents Department

Effect of stoichiometry on defect distribution in cubic GaN grown on GaAs by plasma-assisted MBE

Description: High resolution electron microscopy was used to study the structure of {beta}-GaN epilayers grown on (001) GaAs substrates by plasma- assisted molecular-beam-epitaxy. The rf plasma source was used to promote chemically active nitrogen. The layer quality was shown to depend on growth conditions (Ga flux and N{sub 2} flow for fixed rf power). The best quality of GaN layers was achieved by ``stoichiometric`` growth; Ga-rich layers contain a certain amount of the wurtzite phase. GaN layers contain a high density of stacking faults which drastically decreases toward the GaN surface. Stacking faults are anisotropically distributed in the GaN layer; the majority intersect the interface along lines parallel to the ``major flat`` of the GaAs substrate. This correlates well with the observed anisotropy in the intensity distribution of x-ray reflexions. Formation of stacking faults are often associated with atomic steps at the GaN- GaAs interfaces.
Date: December 31, 1996
Creator: Ruvimov, S.; Liliental-Weber, Z.; Washburn, J.; Drummond, T.J.; Hafish, M. & Lee, S.R.
Partner: UNT Libraries Government Documents Department

Automation, Control and Modeling of Compound Semiconductor Thin-Film Growth

Description: This report documents the results of a laboratory-directed research and development (LDRD) project on control and agile manufacturing in the critical metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) materials growth processes essential to high-speed microelectronics and optoelectronic components. This effort is founded on a modular and configurable process automation system that serves as a backbone allowing integration of process-specific models and sensors. We have developed and integrated MOCVD- and MBE-specific models in this system, and demonstrated the effectiveness of sensor-based feedback control in improving the accuracy and reproducibility of semiconductor heterostructures. In addition, within this framework we have constructed ''virtual reactor'' models for growth processes, with the goal of greatly shortening the epitaxial growth process development cycle.
Date: February 1, 1999
Creator: Breiland, W.G.; Coltrin, M.E.; Drummond, T.J.; Horn, K.M.; Hou, H.Q.; Klem, J.F. et al.
Partner: UNT Libraries Government Documents Department

Gallium self-diffusion in gallium arsenide: A study using isotope heterostructures

Description: Ga self-diffusion was studied with secondary-ion mass spectroscopy in {sup 69}GaAs/{sup 71}GaAs isotope heterostructures grown by molecular beam epitaxy on GaAs substrates. Results show that the Ga self- diffusion coefficient in intrinsic GaAs can be described accurately with D = (43{+-}25 cm{sup 2}s{sup -1})exp(-4.24{+-}0.06 eV/k{sub B}T) over 6 orders of magnitude between 800 and 1225 C under As-rich condition. Experimental results combined with theoretical calculations strongly suggest Ga vacancy being the dominant native defect controlling the diffusion. No significant doping effects were observed in samples where the substrates were doped with Te up to 4x10{sup 17}cm{sup -3} or Zn up to 1x10{sup 19}cm{sup -3}.
Date: September 1, 1996
Creator: Wang, Lei; Hsu, L.; Haller, E.E.; Erickson, J.W.; Fischer, A.; Eberl, K. et al.
Partner: UNT Libraries Government Documents Department

III-Sb (001) growth surfaces: Structure and island nucleation

Description: The authors have determined the reconstructions present on AlSb and GaSb(001) under conditions typical for device growth by molecular beam epitaxy. Within the range of Sb flux and temperature where the diffraction pattern is nominally (1 x 3), three distinct (4 x 3) reconstructions actually occur. The three structures are different than those previously proposed for these growth conditions, with two incorporating mixed III-V dimers on the surface. The presence of these hetero-dimers in the top Sb layer leads to an island nucleation and growth mechanism fundamentally different than for other III-V systems.
Date: April 24, 2000
Partner: UNT Libraries Government Documents Department

In situ growth rate measurements by normal-incidence reflectance during MOVPE growth

Description: We present an in situ technique for monitoring metal-organic vapor phase epitaxy growth by normal-incidence reflectance. This technique is used to calibrate the growth rate periodically and to monitor the growth process routinely. It is not only a precise tool to measure the growth rate, but also very useful in identifying unusal problems during a growth run, such as depletion of source material, deterioration of surface morphology, and problems associated with an improper growht procedure. We will also present an excellent reproducibility ({+-}0.3% over a course of more than 100 runs) of the cavity wavelength of vertical-cavity surface emitting laser structures with periodic calibration by this in situ technique.
Date: May 1, 1996
Creator: Hou, H.Q.; Breiland, W.G.; Hammons, B.E. & Chui, H.C.
Partner: UNT Libraries Government Documents Department

Non-equilibrium Approach to Doping of Wide Bandgap materials by Molecular Beam Epitaxy. Final Report

Description: It is well known that it has been difficult to obtain good bipolar doping in a wide bandgap semiconductors. Developed a new doping technique, involving use of a standard dopant, together with a ''co-dopant'' used to facilitate the introduction of the dopant, and have vastly alleviated this problem.
Date: April 19, 2004
Creator: Tamargo, M. C. & Neumark, G. F.
Partner: UNT Libraries Government Documents Department

Effect of hydrostatic pressure on degradation of CdTe/CdMgTeheterostructures grown by molecular beam epitaxy on GaAs substrates

Description: We have shown that external hydrostatic pressure leads to the creation of structural defects, mainly in the vicinity of the II-VI/GaAs interface in the CdTe/Cd{sub 1-x}Mg{sub x}Te heterostructures grown by the molecular beam epitaxy method on GaAs substrates. These defects propagating across the epilayer cause permanent damage to the samples from the point of view of their electrical properties. In contrast, photoluminescence spectra are only weakly influenced by pressure. Our results shed light on the degradation process observed even without pressure in II-VI-based heterostructures.
Date: April 18, 2001
Creator: Wasik, D.; Baj, M.; Siwiec-Matuszyk, J.; Gronkowski, J.; Jasinski, J. & Karczewski, G.
Partner: UNT Libraries Government Documents Department

Surface stress, morphological development, and dislocation nucleation during strained-layer epitaxy

Description: Utilizing marker layer experiments and Z-contrast imaging, we have observed the formation of surface cusps during Si{sub x}Ge{sub l-x} alloy growth. The formation of cusps can be understood in terms of stress-driven surface diffusion, and we consider the large stress build-up at the cusp tip as a potential source for the nucleation of misfit dislocations.
Date: December 1, 1993
Creator: Jesson, D. E.; Pennycook, S. J.; Baribeau, J. M. & Houghton, D. C.
Partner: UNT Libraries Government Documents Department

The nature and origin of lateral composition modulations in short-period strained-layer superlattices

Description: The nature and origin of lateral composition modulations in (AlAs){sub m}(InAs){sub n} short-period strained-layer superlattices (SPS) grown by molecular-beam epitaxy on InP substrates have been investigated by X-ray diffraction, atomic force microscopy, and transmission electron microscopy. Strong modulations were observed for growth temperatures between approximately 540 and 560 C. The maximum strength of modulations was found for SPS samples with InAs mole fraction x (= n/(n+m)) close to approximately 0.50 and when n is approximately equal to m approximately equal to 2. The modulations were suppressed at both high and low values of x. For x &gt; 0.52 (global compression), the modulations were along the &lt;100&gt; directions in the (001) growth plane. For x &lt; 0.52 (global tension), the modulations were along the two &lt;310&gt; directions rotated approximately {+-}27{degree} from [110] in the growth plane. The remarkably constant wavelength of the modulations, between approximately 20--30 nm, and the different modulation directions observed suggest that the origin of the modulations is due to surface roughening associated with the high misfit between the individual SPS layers and the InP substrate. Highly uniform unidirectional modulations have been grown by control of the InAs mole fraction and growth on suitably offcut substrates, which show great promise for application in device structures.
Date: February 28, 2000
Creator: Norman, A. G.; Ahrenkiel, S. P.; Moutinho, H. R.; Ballif, C.; Al-Jassim, M. M.; Mascarenhas, A. et al.
Partner: UNT Libraries Government Documents Department

Diffusion of silicon in crystalline germanium

Description: We report the determination of the diffusion coefficient of Si in crystalline Ge over the temperature range of 550 to 900 C. A molecular beam epitaxy (MBE) grown buried Si layer in an epitaxial Ge layer on a crystalline Ge substrate was used as the source for the diffusion experiments. For samples annealed at temperatures above 700 C, a 50 nm thick SiO{sub 2} cap layer was deposited to prevent decomposition of the Ge surface. We found the temperature dependence of the diffusion coefficient to be described by a single activation energy (3.32 eV) and pre-factor (38 cm{sup 2}/s) over the entire temperature range studied. The diffusion of the isovalent Si in Ge is slower than Ge self-diffusion over the full temperature range and reveals an activation enthalpy which is higher than that of self-diffusion. This points to a reduced interaction potential between the Si atom and the native defect mediating the diffusion process. For Si, which is smaller in size than the Ge self-atom, a reduced interaction is expected for a Si-vacancy (Si-V{sub Ge}) pair. Therefore we conclude that Si diffuses in Ge via the vacancy mechanism.
Date: June 6, 2005
Creator: Silvestri, H.H.; Bracht, H.; Hansen, J. Lundsgaard; Larsen, A.Nylandsted & Haller, E.E.
Partner: UNT Libraries Government Documents Department

Phosphor-Free Solid State Light Sources

Description: The objective of this work was to demonstrate a light emitting diode that emitted white light without the aid of a phosphor. The device was based on the combination of a nitride LED and a fluorescing ZnO substrate. The early portion of the work focused on the growth of ZnO in undoped and doped form. The doped ZnO was successfully engineered to emit light at specific wavelengths by incorporating various dopants into the crystalline lattice. Thereafter, the focus of the work shifted to the epitaxial growth of nitride structures on ZnO. Initially, the epitaxy was accomplished with molecular beam epitaxy (MBE). Later in the program, metallorganic chemical vapor deposition (MOCVD) was successfully used to grow nitrides on ZnO. By combining the characteristics of the doped ZnO substrate with epitaxially grown nitride LED structures, a phosphor-free white light emitting diode was successfully demonstrated and characterized.
Date: February 28, 2007
Creator: Nause, Jeff E; Ferguson, Ian & Doolittle, Alan
Partner: UNT Libraries Government Documents Department

Hole transport and photoluminescence in Mg-doped InN

Description: Hole conductivity and photoluminescence were studied in Mg-doped InN films grown by molecular beam epitaxy. Because surface electron accumulation interferes with carrier type determination by electrical measurements, the nature of the majority carriers in the bulk of the films was determined using thermopower measurements. Mg concentrations in a&quot;window&quot; from ca. 3 x 1017 to 1 x 1019 cm-3 produce hole-conducting, p-type films as evidenced by a positive Seebeck coecient. This conclusion is supported by electrolyte-based capacitance voltage measurements and by changes in the overall mobility observed by Hall effect, both of which are consistent with a change from surface accumulation on an n-type film to surface inversion on a p-type film. The observed Seebeck coefficients are understood in terms of a parallel conduction model with contributions from surface and bulk regions. In partially compensated films with Mg concentrations below the window region, two peaks are observed in photoluminescence at 672 meV and at 603 meV. They are attributed to band-to-band and band-to-acceptor transitions, respectively, and an acceptor binding energy of ~;;70 meV is deduced. In hole-conducting films with Mg concentrations in the window region, no photoluminescence is observed; this is attributed to electron trapping by deep states which are empty for Fermi levels close to the valence band edge.
Date: March 24, 2010
Creator: Miller, N.; Ager III, J. W.; Smith III, H. M.; Mayer, M. A.; Yu, K. M.; Haller, E. E. et al.
Partner: UNT Libraries Government Documents Department

Ferrimagnetic ordering of single crystal Fe1-xGax thin films

Description: Molecular beam epitaxy was used to deposit body centered cubic single crystal Fe{sub 1-x}Ga{sub x} thin films on MgO(001) and ZnSe/GaAs(001) substrates well beyond the bulk stability concentration of about 28%. The crystal quality of the substrate surface and each deposited layer was monitored in situ by reflection high energy electron diffraction. The magnetization of the samples as a function of Ga is found to decrease more rapidly than a simple dilution effect, and element-specific x-ray magnetic circular dichroism ascribes this trend to a decrease in the Fe moment and an induced moment in the Ga that is antialigned to the Fe moment.
Date: October 19, 2009
Creator: McClure, A.; Arenholz, E. & Idzerda, Y. U.
Partner: UNT Libraries Government Documents Department

High-coercivity, c-axis oriented Nd{sub 2}Fe{sub 14}B films grown by molecular beam epitaxy

Description: Thin films of Nd{sub 2}Fe{sub 14}B with a c-axis orientation and bulk- like magnetic properties were grown with thickness as low as 300 {Angstrom}. They were grown on single-crystal MgO(100) wafers overcoated with epitaxial Mo(100) buffer layers. The 2-14-1 phase were crystallized either by sequential deposition or co-deposition of Fe, Nd, and B from pure elemental evaporation sources onto 600-700 C substrates. Structure of each film was characterized in-situ with RHEED and ex-situ with XRD. For the sequentially deposited films, the in-plane saturation field is 60-70 kOe at 300 K, consistent with the bulk anisotropy field of 73 kOe. The spin-reorientation transition at 135 K can also be clearly seen in the in-plane and out-of-plane magnetization vs temperature data. The out-of-plane coercivities range from 15-20 kOe at 20 K and 3-8 kOe at 300 K. Co-deposition results in a multiphase structure, with Nd{sub 2}Fe{sub 14}B now the minority phase. The multiphase structure results in reduced perpendicular anisotropy.
Date: November 1996
Creator: Keavney, D. J.; Fullerton, E. E.; Pearson, J. E. & Bader, S. D.
Partner: UNT Libraries Government Documents Department