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Description
Silicon nanowires were grown epitaxially on Si (100) and (111) surfaces using the Vapor-Liquid-Solid (VLS) mechanism under both thermal and plasma enhanced growth conditions. Nanowire morphology was investigated as a function of temperature, time, disilane partial pressure and substrate preparation. Silicon nanowires synthesized in low temperature plasma typically curved compared to the linear nanowires grown under simple thermal conditions. The nanowires tended bend more with increasing disilane partial gas pressure up to 25 x10-3 mTorr. The nanowire curvature measured geometrically is correlated with the shift of the main silicon peak obtained in Raman spectroscopy. A mechanistic hypothesis was proposed to explain the bending during plasma activated growth. Additional driving forces related to electrostatic and Van der Waals forces were also discussed. Deduced from a systematic variation of a three-step experimental protocol, the mechanism for bending was associated with asymmetric deposition rate along the outer and inner wall of nanowire. The conditions leading to nanowire branching were also examined using a two-step growth process. Branching morphologies were examined as a function of plasma powers between 1.5 W and 3.5 W. Post-annealing thermal and plasma-assisted treatments in hydrogen were compared to understand the influences in the absence of an external silicon source (otherwise supplied by disilane). Longer and thicker nanowires were associated with longer annealing times due to an Ostwald-like ripening effect. The roles of surface diffusion, gas diffusion, etching and deposition rates were examined.
ContributorsJoun, Hee-Joung (Author) / Petuskey, William T. (Thesis advisor) / Drucker, Jeff (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2011
Description
Ge1-ySny alloys represent a new class of photonic materials for integrated optoelectronics on Si. In this work, the electrical and optical properties of Ge1-ySny alloy films grown on Si, with concentrations in the range 0 ≤ y ≤ 0.04, are studied via a variety of methods. The first microelectronic devices from GeSn films were fabricated using newly developed CMOS-compatible protocols, and the devices were characterized with respect to their electrical properties and optical response. The detectors were found to have a detection range that extends into the near-IR, and the detection edge is found to shift to longer wavelengths with increasing Sn content, mainly due to the compositional dependence of the direct band gap E0. With only 2 % Sn, all of the telecommunication bands are covered by a single detector. Room temperature photoluminescence was observed from GeSn films with Sn content up to 4 %. The peak wavelength of the emission was found to shift to lower energies with increasing Sn content, corresponding to the decrease in the direct band gap E0 of the material. An additional peak in the spectrum was assigned to the indirect band gap. The separation between the direct and indirect peaks was found to decrease with increasing Sn concentration, as expected. Electroluminescence was also observed from Ge/Si and Ge0.98Sn0.02 photodiodes under forward bias, and the luminescence spectra were found to match well with the observed photoluminescence spectra. A theoretical expression was developed for the luminescence due to the direct band gap and fit to the data.
ContributorsMathews, Jay (Author) / Menéndez, Jose (Thesis advisor) / Kouvetakis, John (Thesis advisor) / Drucker, Jeffery (Committee member) / Chizmeshya, Andrew (Committee member) / Ponce, Fernando (Committee member) / Arizona State University (Publisher)
Created2011
Description
This thesis describes the fabrication of several new classes of Ge1-x-ySixSny materials with the required compositions and crystal quality to engineer the band gaps above and below that of elemental Ge (0.8 eV) in the near IR. The work initially focused on Ge1-x-ySixSny (1-5% Sn, 4-20% Si) materials grown on Ge(100) via gas-source epitaxy of Ge4H10, Si4H10 and SnD4. Both intrinsic and doped layers were produced with defect-free microstructure and viable thickness, allowing the fabrication of high-performance photodetectors. These exhibited low ideality factors, state-of-the-art dark current densities and adjustable absorption edges between 0.87 and 1.03 eV, indicating that the band gaps span a significant range above that of Ge. Next Sn-rich Ge1-x-ySixSny alloys (2-4% Si and 4-10% Sn) were fabricated directly on Si and were found to show significant optical emission using photoluminescence measurements, indicating that the alloys have direct band gaps below that of pure Ge in the range of 0.7-0.55 eV. A series of Sn-rich Ge1-x-ySixSny analogues (y>x) with fixed 3-4% Si content and progressively increasing Sn content in the 4-10% range were then grown on Ge buffered Si platforms for the purpose of improving the material's crystal quality. The films in this case exhibited lower defect densities than those grown on Si, allowing a meaningful study of both the direct and indirect gaps. The results show that the separation of the direct and indirect edges can be made smaller than in Ge even for non-negligible 3-4% Si content, confirming that with a suitable choice of Sn compositions the ternary Ge1-x-ySixSny reproduces all features of the electronic structure of binary Ge1-ySny, including the sought-after indirect-to-direct gap cross over. The above synthesis of optical quality Ge1-x-ySixSny on virtual Ge was made possible by the development of high quality Ge-on-Si buffers via chemical vapor deposition of Ge4H10. The resultant films exhibited structural and electrical properties significantly improved relative to state-of-the-art results obtained using conventional approaches. It was found that pure Ge4H10 facilitates the control of residual doping and enables p-i-n devices whose dark currents are not entirely determined by defects and whose zero-bias collection efficiencies are higher than those obtained from samples fabricated using alternative Ge-on-Si approaches.
ContributorsXu, Chi (Author) / Kouvetakis, John (Thesis advisor) / Menéndez, Jose (Thesis advisor) / Chizmeshya, Andrew (Committee member) / Drucker, Jeffrey (Committee member) / Ponce, Fernando (Committee member) / Arizona State University (Publisher)
Created2013
Description
The thesis studies new methods to fabricate optoelectronic Ge1-ySny/Si(100) alloys and investigate their photoluminescence (PL) properties for possible applications in Si-based photonics including IR lasers. The work initially investigated the origin of the difference between the PL spectrum of bulk Ge, dominated by indirect gap emission, and the PL spectrum of Ge-on-Si films, dominated by direct gap emission. It was found that the difference is due to the supression of self-absorption effects in Ge films, combined with a deviation from quasi-equilibrium conditions in the conduction band of undoped films. The latter is confirmed by a model suggesting that the deviation is caused by the shorter recombination lifetime in the films relative to bulk Ge. The knowledge acquired from this work was then utilized to study the PL properties of n-type Ge1-ySny/Si (y=0.004-0.04) samples grown via chemical vapor deposition of Ge2H6/SnD4/P(GeH3)3. It was found that the emission intensity (I) of these samples is at least 10x stronger than observed in un-doped counterparts and that the Idir/Iind ratio of direct over indirect gap emission increases for high-Sn contents due to the reduced gamma-L valley separation, as expected. Next the PL investigation was expanded to samples with y=0.05-0.09 grown via a new method using the more reactive Ge3H8 in place of Ge2H6. Optical quality, 1-um thick Ge1-ySny/Si(100) layers were produced using Ge3H10/SnD4 and found to exhibit strong, tunable PL near the threshold of the direct-indirect bandgap crossover. A byproduct of this study was the development of an enhanced process to produce Ge3H8, Ge4H10, and Ge5H12 analogs for application in ultra-low temperature deposition of Group-IV semiconductors. The thesis also studies synthesis routes of an entirely new class of semiconductor compounds and alloys described by Si5-2y(III-V)y (III=Al, V= As, P) comprising of specifically designed diamond-like structures based on a Si parent lattice incorporating isolated III-V units. The common theme of the two thesis topics is the development of new mono-crystalline materials on ubiquitous silicon platforms with the objective of enhancing the optoelectronic performance of Si and Ge semiconductors, potentially leading to the design of next generation optical devices including lasers, detectors and solar cells.
ContributorsGrzybowski, Gordon (Author) / Kouvetakis, John (Thesis advisor) / Chizmeshya, Andrew (Committee member) / Menéndez, Jose (Committee member) / Arizona State University (Publisher)
Created2013
Description
The objective of the present investigations is to experimentally determine the fundamental molecular properties of the transient metal containing pieces. The transient molecules have been generated using laser ablation production technique and detected by using laser induced fluorescence technique. Ultra-high resolution spectra of the diatomic molecules, 87SrF, 135&137BaF, YbF, HfF, and IrSi were recorded at a resolution of approximately 30 Mhz. The fine and hyperfine structure of these molecules were determined for the ground and the excited state. The optical Stark splittings of 180HfF and IrSi were recorded and analyzed to determine the permanent electric dipole moments of the ground and the excited state. An effective Hamiltonian operator, including the rotational, centrifugal distortion, spin-orbit, spin-spin, spin-rotation, Λ-doubling, magnetic hyperfine and quadrupole interactions, and Stark effect, was employed to model and analyze the recorded spectra. The electronic spectra of the triatomic molecules, TiO2 and ZrO2, were recorded using pulsed dye laser, LIF, spectrometer at a resolution of 300MHz. These molecules have C2v symmetry. The harmonic frequencies, lifetime measurements were determined. These spectra of ZrO2 and TiO2 were modeled using a normal coordinate analysis and Franck-Condon factor predictions. High resolution field-free and Stark effect spectra of ZrO2 were recorded and for future investigation.
ContributorsLe, Anh Thun (Author) / Steimle, Timothy C (Thesis advisor) / Richert, Ranko (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2013
Description
Group IV alloy films exhibit the ability to tune both band structure and lattice parameters and have recently attracted attention for their potential applications in Si-photonics and photovoltaics. In this work, several new approaches to produce these alloys directly on Si(100) and Ge(100) wafers are developed. For photovoltaics, use of Ge-buffered Si(100) wafers as a low cost platform for epitaxy of In1-xGaxAs layers was explored. The results indicate that this approach has promise for transitioning from bulk Ge platforms to virtual substrates for a significant cost reduction. The electrical and optical properties of Ge and Ge1-ySny layers produced using several different techniques were explored via fabrication of high performance heterostructure photodiodes. First, a new CVD approach to Ge-like materials was developed in which germanium is alloyed with very small amounts of tin. These alloys exhibited no significant difference in their structural properties or band gap compared to pure Ge, however superior photo response and reduced dark currents were observed from fabricated devices relative to pure Ge on Si reference diodes. Additionally, pure Ge/Si(100) photodiodes were fabricated using layers grown via reactions of Ge4H10 on Si(100) and found to exhibit low dark current densities with high collection efficiencies. Ge1-x-ySixSny materials represent the newest member of group IV alloy family. The ability to decouple the lattice constant and the band gap in this system has led to strong interest both for strain/confinement layers in quantum well structures, and as the possible "missing" 1 eV junction in multijunction photovoltaics. Recent progress in this field has allowed for the first time growth, fabrication and measurement of novel photodiodes based on Ge1-x-ySixSny. This work presents the material, electrical and optical properties of Ge1-x-ySixSny layers and photodiodes grown directly on Ge and Si wafers using two different synthetic approaches. A series of photodiodes containing Sn concentrations from 1-5%, all lattice matched to Ge, was fabricated. The devices exhibited low dark current densities with high collection efficiencies as required for photovoltaics. By measuring the photoresponse, tunable band gaps ranging from 0.85 eV to 1.02 eV were observed.
ContributorsBeeler, Richard (Author) / Kouvetakis, John (Thesis advisor) / Menéndez, Jose (Committee member) / Chizmeshya, Andrew (Committee member) / Arizona State University (Publisher)
Created2012
Description
This thesis describes the studies for two groups of molecules in the gas-phase: (a) copper monofluoride (CuF) and copper hydroxide (CuOH); (b) thorium monoxide (ThO) and tungsten carbide (WC). Copper-containing molecules (Group a) are selected to investigate the ionic bonding in transition metal-containing molecules because they have a relatively simple electronic state distribution due to the nearly filled 3d-orbital. ThO and WC (Group b) are in support of particle physics for the determination of electron electric dipole moment (eEDM), de, the existence of which indicates new physics beyond the Standard Model. The determination of the tiny eEDM requires large electric fields applied to the electron. The 3(Delta)1 states for heavy polar molecules were proposed [E. R. Meyer, J. L. Bohn, and M. P. Deskevich, Phys. Rev. A 73, 062108 (2006)] to determine de with the following attractive features: (1) large electric dipole moments; (2) large internal electric fields, Eeff, experienced by valence electrons; (3) nearly degenerate omega-doublets; (4) extremely small magnetic dipole moments. The H3(Delta)1 state for ThO and the X3(Delta)1 state for WC are both good candidates. Spectroscopic parameters (i.e. molecular electric and magnetic dipole moments, omega-doubling parameters, etc) are required for the 3(Delta)1 states of ThO and WC. High resolution optical spectra (linewidth ~50 MHz) of CuF, CuOH, ThO and WC were recorded field-free and in the presence of a static electric field (or magnetic field) using laser ablation source/supersonic expansion and laser induced fluorescence (LIF) detection. The spectra were modeled by a zero-field effective Hamiltonian operator and a Stark (or Zeeman) Hamiltonian operator with various molecular parameters. The determined molecular parameters are compared to theoretical predictions. The small omega-doubling parameter was well determined using the pump/probe microwave optical double resonance (PPMODR) technique with a much higher resolution (linewidth ~60 kHz) than optical spectroscopy. In addition to the above mentioned studies of the two groups of molecules, a resonance enhanced multi-photon ionization (REMPI) combined with a time-of-flight mass spectrometer (TOFMS) has been developed to identify the molecules responsible for observed LIF signals. The operation of this spectrometer has been tested by recording the mass spectrum of Ti/O2 and the REMPI spectrum for TiO using a two-color excitation scheme.
ContributorsWang, Fang (Author) / Steimle, Timothy C (Thesis advisor) / Chizmeshya, Andrew (Committee member) / Wolf, George (Committee member) / Arizona State University (Publisher)
Created2012
Description
In materials science, developing GeSn alloys is major current research interest concerning the production of efficient Group-IV photonics. These alloys are particularly interesting because the development of next-generation semiconductors for ultrafast (terahertz) optoelectronic communication devices could be accomplished through integrating these novel alloys with industry-standard silicon technology. Unfortunately, incorporating a maximal amount of Sn into a Ge lattice has been difficult to achieve experimentally. At ambient conditions, pure Ge and Sn adopt cubic (α) and tetragonal (β) structures, respectively, however, to date the relative stability and structure of α and β phase GeSn alloys versus percent composition Sn has not been thoroughly studied. In this research project, computational tools were used to perform state-of-the-art predictive quantum simulations to study the structural, bonding and energetic trends in GeSn alloys in detail over a range of experimentally accessible compositions. Since recent X-Ray and vibrational studies have raised some controversy about the nanostructure of GeSn alloys, the investigation was conducted with ordered, random and clustered alloy models.
By means of optimized geometry analysis, pure Ge and Sn were found to adopt the alpha and beta structures, respectively, as observed experimentally. For all theoretical alloys, the corresponding αphase structure was found to have the lowest energy, for Sn percent compositions up to 90%. However at 50% Sn, the correspondingβ alloy energies are predicted to be only ~70 meV higher. The formation energy of α-phase alloys was found to be positive for all compositions, whereas only two beta formation energies were negative. Bond length distributions were analyzed and dependence on Sn incorporation was found, perhaps surprisingly, not to be directly correlated with cell volume. It is anticipated that the data collected in this project may help to elucidate observed complex vibrational properties in these systems.
By means of optimized geometry analysis, pure Ge and Sn were found to adopt the alpha and beta structures, respectively, as observed experimentally. For all theoretical alloys, the corresponding αphase structure was found to have the lowest energy, for Sn percent compositions up to 90%. However at 50% Sn, the correspondingβ alloy energies are predicted to be only ~70 meV higher. The formation energy of α-phase alloys was found to be positive for all compositions, whereas only two beta formation energies were negative. Bond length distributions were analyzed and dependence on Sn incorporation was found, perhaps surprisingly, not to be directly correlated with cell volume. It is anticipated that the data collected in this project may help to elucidate observed complex vibrational properties in these systems.
ContributorsLiberman-Martin, Zoe Elise (Author) / Chizmeshya, Andrew (Thesis director) / Sayres, Scott (Committee member) / Wolf, George (Committee member) / School of Mathematical and Statistical Sciences (Contributor) / School of Molecular Sciences (Contributor) / Barrett, The Honors College (Contributor)
Created2019-05
Description
The two-dimensional electron gas (2DEG) at SrTiO3-based oxide interfaces has been extensively studied recently for its high carrier density, high electron mobility, superconducting, ferromagnetic, ferrroelectric and magnetoresistance properties, with possible application for all-oxide devices. Understanding the mechanisms behind the 2DEG formation and factors affecting its properties is the primary objective of this dissertation.
Advanced electron microscopy techniques, including aberration-corrected electron microscopy and electron energy-loss spectroscopy (EELS) with energy-loss near-edge structure (ELNES) analysis, were used to characterize the interfaces. Image and spectrum data-processing algorithms, including subpixel atomic position measurement, and novel outlier detection by oversampling, subspace division based EELS background removal and bias-free endmember extraction algorithms for hyperspectral unmixing and mapping were heavily used. Results were compared with density functional theory (DFT) calculations for theoretical explanation.
For the γ-Al2O3/SrTiO3 system, negative-Cs imaging confirmed the formation of crystalline γ-Al2O3. ELNES hyperspectral unmixing combined with DFT calculations revealed that oxygen vacancies, rather than polar discontinuity, were the key to the 2DEG formation. The critical thickness can be explained by shift of the Fermi level due to Ti out diffusion from the substrate to the film.
At the LaTiO3/SrTiO3 interface, aberration-corrected imaging showed crystallinity deterioration in LaTiO3 films a few unit cells away from the interface. ELNES showed that oxygen annealing did not alter the crystallinity but converted Ti3+ near the interface into Ti4+, which explained disappearance of the conductivity.
At the EuO/SrTiO3 interface, both high-resolution imaging and ELNES confirmed EuO formation. ELNES hyperspectral unmixing showed a Ti3+ layer confined to within several unit cells of the interface on the SrTiO3 side, confirming the presence of oxygen vacancies.
At the BaTiO3/SrTiO3 interface, spontaneous polarization and local lattice parameters were measured directly in each unit cell column and compared with oxidation state mapping using ELNES with unit-cell resolution. The unusually large polarization near the interface and the polarization gradient were explained by oxygen vacancies and the piezoelectric effect due to epitaxial strain and strain gradient from relaxation.
Advanced electron microscopy techniques, including aberration-corrected electron microscopy and electron energy-loss spectroscopy (EELS) with energy-loss near-edge structure (ELNES) analysis, were used to characterize the interfaces. Image and spectrum data-processing algorithms, including subpixel atomic position measurement, and novel outlier detection by oversampling, subspace division based EELS background removal and bias-free endmember extraction algorithms for hyperspectral unmixing and mapping were heavily used. Results were compared with density functional theory (DFT) calculations for theoretical explanation.
For the γ-Al2O3/SrTiO3 system, negative-Cs imaging confirmed the formation of crystalline γ-Al2O3. ELNES hyperspectral unmixing combined with DFT calculations revealed that oxygen vacancies, rather than polar discontinuity, were the key to the 2DEG formation. The critical thickness can be explained by shift of the Fermi level due to Ti out diffusion from the substrate to the film.
At the LaTiO3/SrTiO3 interface, aberration-corrected imaging showed crystallinity deterioration in LaTiO3 films a few unit cells away from the interface. ELNES showed that oxygen annealing did not alter the crystallinity but converted Ti3+ near the interface into Ti4+, which explained disappearance of the conductivity.
At the EuO/SrTiO3 interface, both high-resolution imaging and ELNES confirmed EuO formation. ELNES hyperspectral unmixing showed a Ti3+ layer confined to within several unit cells of the interface on the SrTiO3 side, confirming the presence of oxygen vacancies.
At the BaTiO3/SrTiO3 interface, spontaneous polarization and local lattice parameters were measured directly in each unit cell column and compared with oxidation state mapping using ELNES with unit-cell resolution. The unusually large polarization near the interface and the polarization gradient were explained by oxygen vacancies and the piezoelectric effect due to epitaxial strain and strain gradient from relaxation.
ContributorsLu, Sirong (Author) / Smith, David J. (Thesis advisor) / McCartney, Martha R. (Thesis advisor) / Chizmeshya, Andrew (Committee member) / Crozier, Peter A. (Committee member) / Arizona State University (Publisher)
Created2018
Description
Ge1-xSnx and SiyGe1-x-ySnx materials are being researched intensively for applications in infra-red optoelectronic devices. Due to their direct band gap these materials may in-fact be the enabling factor in the commercial realization of silicon photonics/group IV photonics and the integration of nanophotonics with nanoelectronics. However the synthesis of these meta-stable semiconductor alloys, with a range of Sn-compositions, remains the primary technical challenge. Highly specialized epitaxial growth methods must be employed to produce single crystal layers which have sufficient quality for optoelectronic device applications. Up to this point these methods have been unfavorable from a semiconductor manufacturing perspective. In this work the growth of high-quality Si-Ge-Sn epitaxial alloys on Ge-buffered Si (100) using an industry-standard reduced pressure chemical vapor deposition reactor and a cost-effective chemistry is demonstrated. The growth kinetics are studied in detail in-order to understand the factors influencing layer composition, morphology, and defectivity. In doing so breakthrough GeSn materials and device results are achieved including methods to overcome the limits of Sn-incorporation and the realization of low-defect and strain-relaxed epitaxial layers with up to 20% Sn.
P and n-type doping methods are presented in addition to the production of SiGeSn ternary alloys. Finally optically stimulated lasing in thick GeSn layers and SiGeSn/GeSn multiple quantum wells is demonstrated. Lasing wavelengths ranging from 2-3 µm at temperatures up to 180K are realized in thick layers. Whereas SiGeSn/GeSn multiple quantum wells on a strain-relaxed GeSn buffers have enabled the first reported SiGeSn/GeSn multiple quantum well laser operating up to 80K with threshold power densities as low as 33 kW/cm2.
P and n-type doping methods are presented in addition to the production of SiGeSn ternary alloys. Finally optically stimulated lasing in thick GeSn layers and SiGeSn/GeSn multiple quantum wells is demonstrated. Lasing wavelengths ranging from 2-3 µm at temperatures up to 180K are realized in thick layers. Whereas SiGeSn/GeSn multiple quantum wells on a strain-relaxed GeSn buffers have enabled the first reported SiGeSn/GeSn multiple quantum well laser operating up to 80K with threshold power densities as low as 33 kW/cm2.
ContributorsMargetis, Joseph (Author) / Zhang, Yong-Hang (Thesis advisor) / Chizmeshya, Andrew (Committee member) / Johnson, Shane (Committee member) / Arizona State University (Publisher)
Created2018