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- All Subjects: Materials Science
- Creators: Alford, Terry L.
Description
The object of this body of work is to study the properties and suitability of zinc oxide thin films with a view to engineering them for optoelectronics applications, making them a cheap and effective alternative to indium tin oxide (ITO), the most used transparent conducting oxides in the industry. Initially, a study was undertaken to examine the behavior of silver contacts to ZnO and ITO during thermal processing, a step frequently used in materials processing in optoelectronics. The second study involved an attempt to improve the conductivity of ZnO films by inserting a thin copper layer between two ZnO layers. The Hall resistivity of the films was as low as 6.9×10-5 -cm with a carrier concentration of 1.2×1022 cm-3 at the optimum copper layer thickness. The physics of conduction in the films has been examined. In order to improve the average visible transmittance, we replaced the copper layer with gold. The films were then found to undergo a seven orders of magnitude drop in effective resistivity from 200 -cm to 5.2×10-5 -cm The films have an average transmittance between 75% and 85% depending upon the gold thickness, and a peak transmittance of up to 93%. The best Haacke figure of merit was 15.1×10-3 . Finally, to test the multilayer transparent electrodes on a device, ZnO/Au/ZnO (ZAZ) electrodes were evaluated as transparent electrodes for organic light-emitting devices (OLEDs). The electrodes exhibited substantially enhanced conductivity (about 8×10-5 -cm) over conventional indium tin oxide (ITO) electrodes (about 3.2×10-5 -cm). OLEDs fabricated with the ZAZ electrodes showed reduced leakage compared to control OLEDs on ITO and reduced ohmic losses at high current densities. At a luminance of 25000 cd/m2, the lum/W efficiency of the ZAZ electrode based device improved by 5% compared to the device on ITO. A normalized intensity graph of the colour output from the green OLEDs shows that ZAZ electrodes allow for a broader spectral output in the green wavelength region of peak photopic sensitivity compared to ITO. The results have implications for electrode choice in display technology.
ContributorsSivaramakrishnan, Karthik (Author) / Alford, Terry L. (Thesis advisor) / Schroder, Dieter K. (Committee member) / Newman, Nathan (Committee member) / Theodore, David N (Committee member) / Arizona State University (Publisher)
Created2010
Description
The object of this study is to investigate and improve the performance/stability of the flexible thin film transistors (TFTs) and to study the properties of metal oxide transparent conductive oxides for wide range of flexible electronic applications. Initially, a study has been done to improve the conductivity of ITO (indium tin oxide) films on PEN (polyethylene naphthalate) by inserting a thin layer of silver layer between two ITO layers. The multilayer with an optimum Ag mid-layer thickness, of 8 nm, exhibited excellent photopic average transmittance (~ 88 %), resistivity (~ 2.7 × 10-5 µ-cm.) and has the best Hackee figure of merit (41.0 × 10-3 Ω-1). The electrical conduction is dominated by two different scattering mechanisms depending on the thickness of the Ag mid-layer. Optical transmission is explained by scattering losses and absorption of light due to inter-band electronic transitions. A systematic study was carried out to improve the performance/stability of the TFTs on PEN. The performance and stability of a-Si:H and a-IZO (amorphous indium zinc oxide) TFTs were improved by performing a systematic low temperature (150 °C) anneals for extended times. For 96 hours annealed a-Si:H TFTs, the sub-threshold slope and off-current were reduced by a factor ~ 3 and by 2 orders of magnitude, respectively when compared to unannealed a-Si:H TFTs. For a-IZO TFTs, 48 hours of annealing is found to be the optimum time for the best performance and elevated temperature stability. These devices exhibit saturation mobility varying between 4.5-5.5 cm2/V-s, ION/IOFF ratio was 106 and a sub-threshold swing variation of 1-1.25 V/decade. An in-depth study on the mechanical and electromechanical stress response on the electrical properties of the a-IZO TFTs has also been investigated. Finally, the a-Si:H TFTs were exposed to gamma radiation to examine their radiation resistance. The interface trap density (Nit) values range from 5 to 6 × 1011 cm-2 for only electrical stress bias case. For "irradiation only" case, the Nit value increases from 5×1011 cm-2 to 2×1012 cm-2 after 3 hours of gamma radiation exposure, whereas it increases from 5×1011 cm-2 to 4×1012 cm-2 for "combined gamma and electrical stress".
ContributorsIndluru, Anil (Author) / Alford, Terry L. (Thesis advisor) / Schroder, Dieter (Committee member) / Krause, Stephen (Committee member) / Theodore, David (Committee member) / Arizona State University (Publisher)
Created2011
Description
Miedema's plot is used to select the Cu/metal barrier for Cu metallization.The Cu/metal barrier system selected should have positive heat of formation (Hf) so that there is no intermixing between the two layers. In this case, Ru is chosen as a potential candidate, and then the barrier properties of sputtered Cu/Ru thin films on thermally grown SiO2 substrates are investigated by Rutherford backscattering spectrometry (RBS), X-ray diffractometry (XRD), and electrical resistivity measurement. The Cu/Ru/SiO2 samples are analyzed prior to and after vacuum annealing at various temperatures of 400, 500, and 600 oC and at different interval of times of 0.5, 1 and 2 hrs for each temperature. Backscattering analysis indicate that both the copper and ruthenium thin films are thermally stable at high temperature of 600 oC, without any interdiffusion and chemical reaction between Cu and Ru thin films. No new phase formation is observed in any of the Cu/Ru/SiO2 samples. The XRD data indicate no new phase formation in any of the annealed Cu/Ru/SiO2 samples and confirmed excellent thermal stability of Cu on Ru layer. The electrical resistivity measurement indicated that the electrical resistivity value of the copper thin films annealed at 400, 500, and 600 oC is essentially constant and the copper films are thermally stable on Ru, no reaction occurs between copper films and Ru the layer. Cu/Ru/SiO2 multilayered thin film samples have been shown to possess good mechanical strength and adhesion between the Cu and Ru layers compared to the Cu/SiO2 thin film samples. The strength evaluation is carried out under static loading conditions such as nanoindentation testing. In this study, evaluation and comparison is donebased on the dynamic deformation behavior of Cu/Ru/SiO2 and Cu/SiO2 samples under scratch loading condition as a measure of tribological properties. Finally, the deformation behavior under static and dynamic loading conditions is understood using the scanning electron microscope (SEM) and the focused ionbeam imaging microscope (FIB) for topographical and cross-sectional imaging respectively.
ContributorsVenkatesh, Srilakshmi Hosadurga (Author) / Alford, Terry L. (Thesis advisor) / Krause, Stephen (Committee member) / Theodore, David (Committee member) / Arizona State University (Publisher)
Created2010
Description
This research emphasizes the use of low energy and low temperature post processing to improve the performance and lifetime of thin films and thin film transistors, by applying the fundamentals of interaction of materials with conductive heating and electromagnetic radiation. Single frequency microwave anneal is used to rapidly recrystallize the damage induced during ion implantation in Si substrates. Volumetric heating of the sample in the presence of the microwave field facilitates quick absorption of radiation to promote recrystallization at the amorphous-crystalline interface, apart from electrical activation of the dopants due to relocation to the substitutional sites. Structural and electrical characterization confirm recrystallization of heavily implanted Si within 40 seconds anneal time with minimum dopant diffusion compared to rapid thermal annealed samples. The use of microwave anneal to improve performance of multilayer thin film devices, e.g. thin film transistors (TFTs) requires extensive study of interaction of individual layers with electromagnetic radiation. This issue has been addressed by developing detail understanding of thin films and interfaces in TFTs by studying reliability and failure mechanisms upon extensive stress test. Electrical and ambient stresses such as illumination, thermal, and mechanical stresses are inflicted on the mixed oxide based thin film transistors, which are explored due to high mobilities of the mixed oxide (indium zinc oxide, indium gallium zinc oxide) channel layer material. Semiconductor parameter analyzer is employed to extract transfer characteristics, useful to derive mobility, subthreshold, and threshold voltage parameters of the transistors. Low temperature post processing anneals compatible with polymer substrates are performed in several ambients (oxygen, forming gas and vacuum) at 150 °C as a preliminary step. The analysis of the results pre and post low temperature anneals using device physics fundamentals assists in categorizing defects leading to failure/degradation as: oxygen vacancies, thermally activated defects within the bandgap, channel-dielectric interface defects, and acceptor-like or donor-like trap states. Microwave anneal has been confirmed to enhance the quality of thin films, however future work entails extending the use of electromagnetic radiation in controlled ambient to facilitate quick post fabrication anneal to improve the functionality and lifetime of these low temperature fabricated TFTs.
ContributorsVemuri, Rajitha (Author) / Alford, Terry L. (Thesis advisor) / Theodore, N David (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2013
Description
InAsBi is a narrow direct gap III-V semiconductor that has recently attracted considerable attention because its bandgap is tunable over a wide range of mid- and long-wave infrared wavelengths for optoelectronic applications. Furthermore, InAsBi can be integrated with other III-V materials and is potentially an alternative to commercial II-VI photodetector materials such as HgCdTe.
Several 1 μm thick, nearly lattice-matched InAsBi layers grown on GaSb are examined using Rutherford backscattering spectrometry and X-ray diffraction. Random Rutherford backscattering measurements indicate that the average Bi mole fraction ranges from 0.0503 to 0.0645 for the sample set, and ion channeling measurements indicate that the Bi atoms are substitutional. The X-ray diffraction measurements show a diffraction sideband near the main (004) diffraction peak, indicating that the Bi mole fraction is not laterally uniform in the layer. The average out of plane tetragonal distortion is determined by modeling the main and sideband diffraction peaks, from which the average unstrained lattice constant of each sample is determined. By comparing the Bi mole fraction measured by random Rutherford backscattering with the InAsBi lattice constant for the sample set, the lattice constant of zinc blende InBi is determined to be 6.6107 Å.
Several InAsBi quantum wells tensilely strained to the GaSb lattice constant with dilute quantities of Bi are characterized using photoluminescence spectroscopy. Investigation of the integrated intensity as a function of carrier excitation density spanning 5×1025 to 5×1026 cm-3 s-1 indicates radiative dominated recombination and high quantum efficiency over the 12 to 250 K temperature range. The bandgap of InAsBi is ascertained from the photoluminescence spectra and parameterized as a function of temperature using the Einstein single oscillator model. The dilute Bi mole fraction of the InAsBi quantum wells is determined by comparing the measured bandgap energy to that predicted by the valence band anticrossing model. The Bi mole fraction determined by photoluminescence agrees reasonably well with that estimated using secondary ion mass spectrometry.
Several 1 μm thick, nearly lattice-matched InAsBi layers grown on GaSb are examined using Rutherford backscattering spectrometry and X-ray diffraction. Random Rutherford backscattering measurements indicate that the average Bi mole fraction ranges from 0.0503 to 0.0645 for the sample set, and ion channeling measurements indicate that the Bi atoms are substitutional. The X-ray diffraction measurements show a diffraction sideband near the main (004) diffraction peak, indicating that the Bi mole fraction is not laterally uniform in the layer. The average out of plane tetragonal distortion is determined by modeling the main and sideband diffraction peaks, from which the average unstrained lattice constant of each sample is determined. By comparing the Bi mole fraction measured by random Rutherford backscattering with the InAsBi lattice constant for the sample set, the lattice constant of zinc blende InBi is determined to be 6.6107 Å.
Several InAsBi quantum wells tensilely strained to the GaSb lattice constant with dilute quantities of Bi are characterized using photoluminescence spectroscopy. Investigation of the integrated intensity as a function of carrier excitation density spanning 5×1025 to 5×1026 cm-3 s-1 indicates radiative dominated recombination and high quantum efficiency over the 12 to 250 K temperature range. The bandgap of InAsBi is ascertained from the photoluminescence spectra and parameterized as a function of temperature using the Einstein single oscillator model. The dilute Bi mole fraction of the InAsBi quantum wells is determined by comparing the measured bandgap energy to that predicted by the valence band anticrossing model. The Bi mole fraction determined by photoluminescence agrees reasonably well with that estimated using secondary ion mass spectrometry.
ContributorsShalindar Christraj, Arvind Joshua Jaydev (Author) / Johnson, Shane R (Thesis advisor) / Alford, Terry L. (Committee member) / Goryll, Michael (Committee member) / Arizona State University (Publisher)
Created2016
Description
In this work a newly fabricated organic solar cell based on a composite of fullerene derivative [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) and regioregular poly (3-hexylthiophene) (P3HT) with an added interfacial layer of AgOx in between the PEDOT:PSS layer and the ITO layer is investigated. Previous equivalent circuit models are discussed and an equivalent circuit model is proposed for the fabricated device. Incorporation of the AgOx interfacial layer shows an increase in fill factor (by 33%) and power conversion efficiency (by 28%). Moreover proper correlation has been achieved between the experimental and simulated I-V plots. The simulation shows that device characteristics can be explained with accuracy by the proposed model.
ContributorsHossain, Nazmul (Author) / Alford, Terry L. (Thesis advisor) / Theodore, David (Committee member) / Krause, Stephen (Committee member) / Arizona State University (Publisher)
Created2015
Description
Microwave properties of low-loss commercial dielectric materials are optimized by adding transition-metal dopants or alloying agents (i.e. Ni, Co, Mn) to tune the temperature coefficient of resonant frequency (τf) to zero. This occurs as a result of the temperature dependence of dielectric constant offsetting the thermal expansion. At cryogenic temperatures, the microwave loss in these dielectric materials is dominated by electron paramagnetic resonance (EPR) loss, which results from the spin-excitations of d-shell electron spins in exchange-coupled clusters. We show that the origin of the observed magnetically-induced shifts in the dielectric resonator frequency originates from the same mechanism, as described by the Kramers-Kronig relations. The temperature coefficient of resonator frequency, τf, is related to three material parameters according to the equation, τf = - (½ τε + ½ τµ + αL), where τε, τµ, and αL are the temperature coefficient of dielectric constant, magnetic permeability, and lattice constant, respectively. Each of these parameters for dielectric materials of interest are measured experimentally. These results, in combination with density functional simulations, developed a much improved understanding of the fundamental mechanisms responsible for τf. The same experimental methods have been used to characterize in-situ the physical nature and concentration of performance-degrading point defects in the dielectrics of superconducting planar microwave resonators.
ContributorsZhang, Shengke (Author) / Newman, Nathan (Thesis advisor) / Alford, Terry L. (Committee member) / Chamberlin, Ralph (Committee member) / Flores, Marco (Committee member) / Singh, Rakesh K. (Committee member) / Arizona State University (Publisher)
Created2016
Description
Nanocrystalline (NC) materials experience inherent microstructural instability when exposed to elevated temperature, deformation rates or loads over long periods of time which limits its applications as well as processing. The instability arises due to the predominance of grain boundary (GB) diffusional processes which hastens coarsening. This dissertation aims to provide a solution for the very first time, through the development and characterization of a bulk NC alloy system. The NC-Cu-Ta discussed here offers exceptional thermal stability in addition to superior strength and creep resistance. The systematic study of the behavior of this material will pave the way for future development of NC materials with a multitude of optimized properties for extreme applications.
In-situ and ex-situ TEM characterization, multiple strain-rate compression testing and atomistic modeling were employed to investigate the behavior of NC-Cu-Ta under intense heating, stress/strain-rate and creep conditions. Results reveal, that temperature influences the misfit strain, leading to a significant change in flow stress, despite which (strength) remains greater than all known NC metals. Further, this alloy was found to achieve and retain strengths which were over two orders of magnitude higher than most NC metals under elevated temperature conditions. Dislocation-based slip was found to predominate at elevated temperatures for both high- and low-strain rate testing whereas twinning was favored during low temperature high-strain rate testing. The solute concentration was also found to play a role in dictating the deformation where heterogeneous twinnability was found to decrease with an increase in Ta concentration.
A paradigm-shift in the creep response of NC-materials with unprecedented property combinations is also reported, i.e., high strength with extremely high temperature creep resistance (6-8 orders higher than other NC materials), in this NC-Cu-Ta-alloy. The unique combination of properties in these NC-alloys is achieved through a processing route that creates distinct GB-pinning nanoclusters of the solute that favor kinetic stability of grains.
Overall, this dissertation provides an understanding of the mechanical response of a stable alloy system to extreme conditions, which was previously unattainable, and a perspective on the design of a new class of NC alloys exhibiting a multitude of optimized high temperature properties.
In-situ and ex-situ TEM characterization, multiple strain-rate compression testing and atomistic modeling were employed to investigate the behavior of NC-Cu-Ta under intense heating, stress/strain-rate and creep conditions. Results reveal, that temperature influences the misfit strain, leading to a significant change in flow stress, despite which (strength) remains greater than all known NC metals. Further, this alloy was found to achieve and retain strengths which were over two orders of magnitude higher than most NC metals under elevated temperature conditions. Dislocation-based slip was found to predominate at elevated temperatures for both high- and low-strain rate testing whereas twinning was favored during low temperature high-strain rate testing. The solute concentration was also found to play a role in dictating the deformation where heterogeneous twinnability was found to decrease with an increase in Ta concentration.
A paradigm-shift in the creep response of NC-materials with unprecedented property combinations is also reported, i.e., high strength with extremely high temperature creep resistance (6-8 orders higher than other NC materials), in this NC-Cu-Ta-alloy. The unique combination of properties in these NC-alloys is achieved through a processing route that creates distinct GB-pinning nanoclusters of the solute that favor kinetic stability of grains.
Overall, this dissertation provides an understanding of the mechanical response of a stable alloy system to extreme conditions, which was previously unattainable, and a perspective on the design of a new class of NC alloys exhibiting a multitude of optimized high temperature properties.
ContributorsRajagopalan, Mansa (Author) / Solanki, Kiran N. (Thesis advisor) / Alford, Terry L. (Committee member) / Jiao, Yang (Committee member) / Darling, Kris A. (Committee member) / Arizona State University (Publisher)
Created2016
Description
The state of the solar industry has reached a point where significant advancements in efficiency will require new materials and device concepts. The material class broadly known as the III-N's have a rich history as a commercially successful semiconductor. Since discovery in 2003 these materials have shown promise for the field of photovoltaic solar technologies. However, inherent material issues in crystal growth and the subsequent effects on device performance have hindered their development. This thesis explores new growth techniques for III-N materials in tandem with new device concepts that will either work around the previous hindrances or open pathways to device technologies with higher theoretical limits than much of current photovoltaics. These include a novel crystal growth reactor, efforts in production of better quality material at faster rates, and development of advanced photovoltaic devices: an inversion junction solar cell, material work for hot carrier solar cell, ground work for a selective carrier contact, and finally a refractory solar cell for operation at several hundred degrees Celsius.
ContributorsWilliams, Joshua J (Author) / Honsberg, C. (Christiana B.) (Thesis advisor) / Goodnick, Stephen M. (Thesis advisor) / Williamson, Todd L. (Committee member) / Alford, Terry L. (Committee member) / King, Richard R. (Committee member) / Arizona State University (Publisher)
Created2016
Description
The inexorable upsurge in world’s energy demand has steered the search for newer renewable energy sources and photovoltaics seemed to be one of the best alternatives for energy production. Among the various photovoltaic technologies that emerged, organic/polymer photovoltaics based on solution processed bulk-heterojunctions (BHJ) of semiconducting polymers has gained serious attention owing to the use of inexpensive light-weight materials, exhibiting high mechanical flexibility and compatibility with low temperature roll-to-roll manufacturing techniques on flexible substrates. The most widely studied material to date is the blend of regioregular P3HT and PC61BM used as donor and acceptor materials. The object of this study was to investigate and improve the performance/stability of the organic solar cells by use of inexpensive materials. In an attempt to enhance the efficiency of organic solar cells, we have demonstrated the use of hexamethyldisilazane (HMDS) modified indium tin oxide (ITO) electrode in bulk heterojunction solar cell structure The device studies showed a significant enhancement in the short-circuit current as well as in the shunt resistance on use of the hexamethyldisilazane (HMDS) layer. In another approach a p-type CuI hole-transport layer was utilized that could possibly replace the acidic PEDOT:PSS layer in the fabrication of high-efficiency solar cells. The device optimization was done by varying the concentration of CuI in the precursor solution which played an important role in the efficiency of the solar cell devices. Recently a substantial amount of research has been focused on identifying suitable interfacial layers in organic solar cells which has efficient charge transport properties. It was illustrated that a thin layer of silver oxide interfacial layer showed a 28% increase in power conversion efficiency in comparison to that of the control cell. The optoelectronic properties and morphological features of indium-free ZnO/Ag/MoOx electrodes was also studied. Organic solar cells on these composite electrodes revealed good optical and electrical properties, making them a promising alternative indium free and PEDOT:PSS-free organic solar cells. Lastly, inverted solar cells utilizing zinc oxide and yttrium doped zinc oxide electron transport was also created and their device properties revealed that optimum annealing conditions and yttrium doping was essential to obtain high efficiency solar cells.
ContributorsDas, Sayantan (Author) / Alford, Terry L. (Thesis advisor) / Petuskey, William (Thesis advisor) / Buttry, Daniel (Committee member) / Krause, Stephen (Committee member) / Arizona State University (Publisher)
Created2015