Matching Items (45)
Filtering by
- All Subjects: Materials Science
- Genre: Doctoral Dissertation
- Creators: Alford, Terry
Description
Thin films of ever reducing thickness are used in a plethora of applications and their performance is highly dependent on their microstructure. Computer simulations could then play a vital role in predicting the microstructure of thin films as a function of processing conditions. FACET is one such software tool designed by our research group to model polycrystalline thin film growth, including texture evolution and grain growth of polycrystalline films in 2D. Several modifications to the original FACET code were done to enhance its usability and accuracy. Simulations of sputtered silver thin films are presented here with FACET 2.0 with qualitative and semi-quantitative comparisons with previously published experimental results. Comparisons of grain size, texture and film thickness between simulations and experiments are presented which describe growth modes due to various deposition factors like flux angle and substrate temperature. These simulations provide reasonable agreement with the experimental data over a diverse range of process parameters. Preliminary experiments in depositions of Silver films are also attempted with varying substrates and thickness in order to generate complementary experimental and simulation studies of microstructure evolution. Overall, based on the comparisons, FACET provides interesting insights into thin film growth processes, and the effects of various deposition conditions on thin film structure and microstructure. Lastly, simple molecular dynamics simulations of deposition on bi-crystals are attempted for gaining insight into texture based grain competition during film growth. These simulations predict texture based grain coarsening mechanisms like twinning and grain boundary migration that have been commonly reported in FCC films.
ContributorsRairkar, Asit (Author) / Adams, James B (Thesis advisor) / Krause, Stephen (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2011
Description
ABSTRACT Along with the fast development of science and technology, the studied materials are becoming more complicated and smaller. All these achievements have advanced with the fast development of powerful tools currently, such as Scanning electron microscopy (SEM), Focused Ion Beam (FIB), Transmission electron microscopy (TEM), Energy dispersive X-ray spectroscopy (EDX), Electron energy loss spectroscopy (EELS) and so on. SiTiO3 thin film, which is grown on Si (100) single crystals, attracts a lot of interest in its structural and electronic properties close to its interface. Valence EELS is used to investigate the Plasmon excitations of the ultrathin SrTiO3 thin film which is sandwiched between amorphous Si and crystalline Si layers. On the other hand, theoretical simulations based on dielectric functions have been done to interpret the experimental results. Our findings demonstrate the value of valence electron energy-loss spectroscopy in detecting a local change in the effective electron mass. Recently it is reported that ZnO-LiYbO2 hybrid phosphor is an efficient UV-infrared convertor for silicon solar cell but the mechanism is still not very clear. The microstructure of Li and Yb co-doped ZnO has been studied by SEM and EDX, and our results suggest that a reaction (or diffusion) zone is very likely to exist between LiYbO2 and ZnO. Such diffusion regions may be responsible for the enhanced infrared emission in the Yb and Li co-doped ZnO. Furthermore, to help us study the diffusion zone under TEM in future, the radiation damage on synthesized LiYbO2 has been studied at first, and then the electronic structure of the synthesized LiYbO2 is compared with Yb2O3 experimentally and theoretically, by EELS and FEFF8 respectively.
ContributorsYang, Bo (Author) / Alford, Terry (Thesis advisor) / Jiang, Nan (Committee member) / Theodore, N. David (Committee member) / Arizona State University (Publisher)
Created2013
Description
Ball Grid Array (BGA) using lead-free or lead-rich solder materials are widely used as Second Level Interconnects (SLI) in mounting packaged components to the printed circuit board (PCB). The reliability of these solder joints is of significant importance to the performance of microelectronics components and systems. Product design/form-factor, solder material, manufacturing process, use condition, as well as, the inherent variabilities present in the system, greatly influence product reliability. Accurate reliability analysis requires an integrated approach to concurrently account for all these factors and their synergistic effects. Such an integrated and robust methodology can be used in design and development of new and advanced microelectronics systems and can provide significant improvement in cycle-time, cost, and reliability. IMPRPK approach is based on a probabilistic methodology, focusing on three major tasks of (1) Characterization of BGA solder joints to identify failure mechanisms and obtain statistical data, (2) Finite Element analysis (FEM) to predict system response needed for life prediction, and (3) development of a probabilistic methodology to predict the reliability, as well as, the sensitivity of the system to various parameters and the variabilities. These tasks and the predictive capabilities of IMPRPK in microelectronic reliability analysis are discussed.
ContributorsFallah-Adl, Ali (Author) / Tasooji, Amaneh (Thesis advisor) / Krause, Stephen (Committee member) / Alford, Terry (Committee member) / Jiang, Hanqing (Committee member) / Mahajan, Ravi (Committee member) / Arizona State University (Publisher)
Created2013
Description
I studied the properties of novel Co2FeAl0.5Si0.5 (CFAS), ZnGeAs2, and FeS2 (pyrite) thin films for microelectronic applications ranging from spintronic to photovoltaic. CFAS is a half metal with theoretical spin polarization of 100%. I investigated its potential as a spin injector, for spintronic applications, by studying the critical steps involved in the injection of spin polarized electron populations from tunnel junctions containing CFAS electrodes. Epitaxial CFAS thin films with L21 structure and saturation magnetizations of over 1200 emu/cm3 were produced by optimization of the sputtering growth conditions. Point contact Andreev reflection measurements show that the spin polarization at the CFAS electrode surface exceeds 70%. Analyses of the electrical properties of tunnel junctions with a superconducting Pb counter-electrode indicate that transport through native Al oxide barriers is mostly from direct tunneling, while that through the native CFAS oxide barriers is not. ZnGeAs2 is a semiconductor comprised of only inexpensive and earth-abundant elements. The electronic structure and defect properties are similar in many ways to GaAs. Thus, in theory, efficient solar cells could be made with ZnGeAs2 if similar quality material to that of GaAs could be produced. To understand the thermochemistry and determine the rate limiting steps of ZnGeAs2 thin-film synthesis, the (a) thermal decomposition rate and (b) elemental composition and deposition rate of films were measured. It is concluded that the ZnGeAs2 thin film synthesis is a metastable process with an activation energy of 1.08±0.05 eV for the kinetically-limited decomposition rate and an evaporation coefficient of ~10-3. The thermochemical analysis presented here can be used to predict optimal conditions of ZnGeAs2 physical vapor deposition and thermal processing. Pyrite (FeS2) is another semiconductor that has tremendous potential for use in photovoltaic applications if high quality materials could be made. Here, I present the layer-by-layer growth of single-phase pyrite thin-films on heated substrates using sequential evaporation of Fe under high-vacuum followed by sulfidation at S pressures between 1 mTorr and 1 Torr. High-resolution transmission electron microscopy reveals high-quality, defect-free pyrite grains were produces by this method. It is demonstrated that epitaxial pyrite layer was produced on natural pyrite substrates with this method.
ContributorsVahidi, Mahmoud (Author) / Newman, Nathan (Thesis advisor) / Alford, Terry (Committee member) / Singh, Rakesh (Committee member) / Chen, Tingyong (Committee member) / Arizona State University (Publisher)
Created2013
Description
Of the potential technologies for pre-combustion capture, membranes offer the advantages of being temperature resistant, able to handle large flow rates, and having a relatively small footprint. A significant amount of research has centered on the use of polymeric and microporous inorganic membranes to separate CO2. These membranes, however, have limitations at high temperature resulting in poor permeation performance. To address these limitations, the use of a dense dual-phase membrane has been studied. These membranes are composed of conductive solid and conductive liquid phases that have the ability to selectively permeate CO2 by forming carbonate ions that diffuse through the membrane at high temperature. The driving force for transport through the membrane is a CO2 partial pressure gradient. The membrane provides a theoretically infinite selectivity. To address stability of the ceramic-carbonate dual-phase membrane for CO2 capture at high temperature, the ceramic phase of the membrane was studied and replaced with materials previously shown to be stable in harsh conditions. The permeation properties and stability of La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF)-carbonate, La0.85Ce0.1Ga0.3Fe0.65Al0.05O3-δ (LCGFA)-carbonate, and Ce0.8Sm0.2O1.9 (SDC)-carbonate membranes were examined under a wide range of experimental conditions at high temperature. LSCF-carbonate membranes were shown to be unstable without the presence of O2 due to reaction of CO2 with the ceramic phase. In the presence of O2, however, the membranes showed stable permeation behavior for more than one month at 900oC. LCGFA-carbonate membranes showed great chemical and permeation stability in the presence of various conditions including exposure to CH4 and H2, however, the permeation performance was quite low when compared to membranes in the literature. Finally, SDC-carbonate membranes showed great chemical and permeation stability both in a CO2:N2 environment for more than two weeks at 900oC as well as more than one month of exposure to simulated syngas conditions at 700oC. Ceramic phase chemical stability increased in the order of LSCF < LCGFA < SDC while permeation performance increased in the order of LCGFA < LSCF < SDC.
ContributorsNorton, Tyler (Author) / Lin, Jerry Y.S. (Thesis advisor) / Alford, Terry (Committee member) / Lind, Mary Laura (Committee member) / Smith, David (Committee member) / Torres, Cesar (Committee member) / Arizona State University (Publisher)
Created2013
Description
Soft magnetic alloys play a significant role for magnetic recording applications and highly sensitivity magnetic field sensors. In order to sustain the magnetic areal density growth, development of new synthesis techniques and materials is necessary. In this work, the effect of oxygen incorporation during electrodeposition of CoFe alloys on magnetic properties, magnetoresistance and structural properties has been studied. Understanding the magnetic properties often required knowledge of oxygen distribution and structural properties of the grown films. Transmission electron microscopy (TEM) was a powerful tool in this study to correlate the oxygen-distribution nanostructure to the magnetic properties of deposited films. Off-axis electron holography in TEM was used to measure magnetic domain wall width in the deposited films. Elemental depth profiles of Fe, Co, O were investigated by secondary ion mass spectroscopy (SIMS). Magnetic properties have been determined by superconducting quantum interference device (SQUID) measurements. Oxygen content in the CoFe deposited films was controlled by electrolyte composition. Films were deposited on Si 100 substrates and on other substrates such as Cu and Al. However, a good film quality was achieved on Si substrate. Electron energy loss and x-ray spectroscopies showed that the low oxygen films contained intragranular Fe2+ oxide (FeO) particles and that the high oxygen films contained intergranular Fe3+ (Fe2O3) along grain boundaries. The films with oxide present at the grain boundary had significantly increased coercivity, magnetoresistance and reduced saturation magnetization relative to the lower oxygen content films with intragranular oxide. The differences in magnetic properties between low oxygen and high oxygen concentration films were attributed to stronger mobile domain wall interactions with the grain boundary oxide layers. The very high magnetoresistance values were achieved for magnetic devices with nanocontact dimension < 100 nm and oxide incorporation in this nanoconfined geometry. The content of oxide phase in nanocontact was controlled by concentration of the Fe3+ ions in the electrodeposition solution. Magnetic device integrity was improved by varying amount of additive into plating solution. These results indicated that electrodeposited CoFe nanocontact is a novel class of materials with large application for magnetic field sensors.
ContributorsElhalawaty, Shereen (Author) / Carpenter, Ray (Thesis advisor) / Chamberlin, Ralph (Committee member) / McCartney, Martha (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2012
Description
In 2022, integrated circuit interconnects will approach 10 nm and the diffusion barrier layers needed to ensure long lasting devices will be at 1 nm. This dimension means the interconnect will be dominated by the interface and it has been shown the interface is currently eroding device performance. The standard interconnect system has three layers - a Copper metal core, a Tantalum Adhesion layer and a Tantalum Nitride Diffusion Barrier Layer. An alternate interconnect schema is a Tantalum Nitride barrier layer and Silver as a metal. The adhesion layer is removed from the system along with changing to an alternate, low resistivity metal. First principles are used to assess the interface of the Silver and Tantalum Nitride. Several stoichiometric 1:1 Tantalum Nitride polymorphs are assessed and it is found that the Fe2P crystal structure is actually the most stable crystal structure which is at odds with the published phase diagram for ambient crystal structure. The surface stability of Fe2P-TaN is assessed and the absorption enthalpy of Silver adatoms is calculated. Finally, the thermodynamic stability of the TaN-Ag interconnect system is assessed.
ContributorsGrumski, Michael (Author) / Adams, James (Thesis advisor) / Krause, Stephen (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2012
Description
The goal of this work is to develop low cost and highly efficient hybrid solar cells based on semiconductor nanoparticles (NPs). Hybrid solar cells have been demonstrated to take advantages of both inorganic and organic semiconductors by employing simple soluble process. In order to improve the power conversion efficiency (PCE), the bulk heterojunction (BHJ) of cadmium selenide (CdSe) tetrapods (TPs) and poly (3-hexylthiophene) (P3HT) are introduced as an electron acceptor and donor, respectively. The dimension of CdSe TPs and the 3D spatial distribution of CdSe TPs:P3HT photoactive blends are investigated to improve optical and electrical properties of photovoltaic devices. Hybrid solar cells having long-armed CdSe TPs and P3HT establish higher PCE of 1.12% when compared to device employing short-armed TPs of 0.80%. The device performance are improved by using longer armed CdSe TPs, which aids in better percolation connectivity and reduced charge hopping events, thus leading to better charge transport. The device architecture of hybrid solar cells is examined to assist vertical phase separation (VPS). Improvement of VPS in hybrid solar cells using CdSe TPs:P3HT photoactive blends is systematically manipulated by solution processed interfacial layers, resulting in enhanced device performance. Multi-layered hybrid solar cells assist better light absorption, efficient charge carrier transport, and increase of the surface contact area. In this work, hole transport assisting layer (HTAL)/BHJ photoactive layer (BPL)/electron transport assisting layer (ETAL) or HTAL/BPL/ETAL (HBE) multi-layered structure is introduced, similarly to p-type layer/intermixed photoactive layer
-type layer (p-i-n) structure of organic photovoltaic devices. To further control the improvement of the device performance, the effects of nano-scale morphology from solvents having different boiling points, the various shapes of semiconductor NPs, and the emergence of blending NPs are demonstrated. The formation of favorable 3D networks in photoactive layer is attributed to enhance the efficient charge transport by the optimized combination of semiconductor NPs in polymer matrix.
-type layer (p-i-n) structure of organic photovoltaic devices. To further control the improvement of the device performance, the effects of nano-scale morphology from solvents having different boiling points, the various shapes of semiconductor NPs, and the emergence of blending NPs are demonstrated. The formation of favorable 3D networks in photoactive layer is attributed to enhance the efficient charge transport by the optimized combination of semiconductor NPs in polymer matrix.
ContributorsLee, Kyu Sung (Author) / Jabbour, Ghassan E. (Thesis advisor) / Alford, Terry (Thesis advisor) / Krause, Stephen (Committee member) / Arizona State University (Publisher)
Created2012
Description
Programmable Metallization Cell (PMC) is a resistance-switching device based on migration of nanoscale quantities of cations in a solid electrolyte and formation of a conducting electrodeposit by the reductions of these cations. This dissertation presents electrical characterization results on Cu-SiO2 based PMC devices, which due to the na- ture of materials can be easily integrated into the current Complimentary metal oxide semiconductor (CMOS) process line. Device structures representing individual mem- ory cells based on W bottom electrode and n-type Si bottom electrode were fabricated for characterization. For the W bottom electrode based devices, switching was ob- served for voltages in the range of 500mV and current value as low as 100 nA showing the electrochemical nature and low power potential. The ON state showed a direct de- pendence on the programming current, showing the possibility of multi-bit storage in a single cell. Room temperature retention was demonstrated in excess of 105 seconds and endurance to approximately 107 cycles. Switching was observed for microsecond duration 3 V amplitude pulses. Material characterization results from Raman, X-ray diffraction, Rutherford backscattering and Secondary-ion mass spectroscopy analysis shows the influence of processing conditions on the Cu concentration within the film and also the presence of Cu as free atoms. The results seemed to indicate stress-induced void formation in the SiO2 matrix as the driving mechanism for Cu diffusion into the SiO2 film. Cu/SiO2
Si based PMC devices were characterized and were shown to have inherent isolation characteristics, proving the feasibility of such a structure for a passive array. The inherent isolation property simplifies fabrication by avoiding the need for a separate diode element in an array. The isolation characteristics were studied mainly in terms of the leakage current. The nature of the diode interface was further studied by extracting a barrier potential which shows it can be approximated to a Cu-nSi metal semiconductor Schottky diode.
Si based PMC devices were characterized and were shown to have inherent isolation characteristics, proving the feasibility of such a structure for a passive array. The inherent isolation property simplifies fabrication by avoiding the need for a separate diode element in an array. The isolation characteristics were studied mainly in terms of the leakage current. The nature of the diode interface was further studied by extracting a barrier potential which shows it can be approximated to a Cu-nSi metal semiconductor Schottky diode.
ContributorsPuthenthermadam, Sarath (Author) / Kozicki, Michael N (Thesis advisor) / Diaz, Rodolfo (Committee member) / Schroder, Dieter K. (Committee member) / Alford, Terry (Committee member) / Arizona State University (Publisher)
Created2011
Description
Transparent conductive oxides (TCOs) are used as electrodes for a number of optoelectronic devices including solar cells. Because of its superior transparent and conductive properties, indium (In) tin (Sn) oxide (ITO) has long been at the forefront for TCO research activities and high-volume product applications. However, given the limited supply of In and potential toxicity of Sn-based compounds, attention has shifted to alternative TCOs like ZnO doped with group-III elements such as Ga and Al. Employing a variety of deposition techniques, many research groups are striving to achieve resistivities below 1E-4 ohm-cm with transmittance approaching the theoretical limit over a wide spectral range. In this work, Ga-doped ZnO is deposited using pulsed laser deposition (PLD). Material properties of the films are characterized using a number of techniques. For deposition in oxygen at pressures >1 mTorr, post-deposition annealing in forming gas (FG) is required to improve conductivity. At these higher oxygen pressures, thermodynamic analysis coupled with a study using the Hall effect measurements and photoluminescence spectroscopy suggest that conductivity is limited by oxygen-related acceptor-like defects in the grains that compensate donors, effectively reducing the net carrier concentration and creating scattering centers that reduce electron mobility. Oxygen is also responsible for further suppression of conductivity by forming insulative metal oxide regions at the grain edges and oxygen-related electron traps at the grain boundaries. The hydrogen component in the FG is thought to passivate the intra-grain acceptor-like defects and improve carrier transport across these grain boundaries. Given this deleterious effect of oxygen on conductivity, depositions are performed in pure argon (Ar), i.e., the only oxygen species in the growth ambient are those ejected directly from the PLD solid source target. Ga-doped ZnO deposited in Ar at 200 °C and 10 mTorr have resistivities of 1.8E-4 ohm-cm without the need for post deposition annealing. Average transmittance of the Ga-doped films is 93% over the visible and near infrared (IR) spectral regions, but free carrier absorption is a limiting factor further into the IR. After annealing in FG at 500 °C, a 300 nm Ar film has a Haacke figure of merit of 6.61E-2 sq. ohm.
ContributorsScott, Robin Charis (Author) / Zhang, Yong Hang (Thesis advisor) / Alford, Terry (Committee member) / Krause, Stephen (Committee member) / Leedy, Kevin (Committee member) / Arizona State University (Publisher)
Created2011