Browsing by Author "Jagdish Narayan, Committee Chair"

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Electrical and Optical Properties of Ga-Doped Mg1-xZnxO System
(2010-04-05) Wei, Wei; Jagdish Narayan, Committee Chair; Roger Narayan, Committee Co-Chair
The primary aim in this thesis is to investigate Ga-doped Mg1-xZnxO, as well as undoped Mg1-xZnxO for the application of transparent conducting oxide. For this thesis work, the films have been grown on sapphire using pulsed laser deposition technique. The films were grown under various deposition conditions in order to understand the effect of processing on the film properties. The grown films have been characterized using various techniques, including XRD, TEM, XPS, 4-probe resistivity measurements, Hall measurements and absorption/transmission spectroscopy. Undoped Mg1-xZnxO films have been grown at several temperatures between room temperature and 750 oC. Photoluminescence was correlated with Urbach energy values which were determined from absorption spectrum. The film grown at 350 oC exhibited lowest band-tail parameter values and highest photoluminescence values than the other films. The optical and electrical properties of heavily Ga-doped MgxZn1-xO thin films were investigated. The film transparency is greater than 90% in the visible spectrum range. The absorption can be extended to lower wavelength range with higher magnesium concentration, which can improve the transparency in the ultraviolet wavelength range; however, conductivity is decreased. The optimum Ga concentration was found to be 0.5 at. %. At this Ga concentration, the film resistivity increased from 1.9Ã—10-3 to 3.62Ã—10-2 Î©.ï€ cm, as the magnesium concentration increased from 5 at. % to 15 at. %. The optical and electrical properties of Ga-doped MgxZn1-xO thin films were investigated systematically. In these films, the Ga content was varied from 0.05 at.% to 7 at.% and the Mg content was varied from 5 at.% to 15 at.%. X-ray diffraction showed that the solid solubility limit of Ga in MgxZn1-xO is less than 3 at.%. The absorption spectra were fitted to examine Ga doping effects on bandgap and band tail characteristics. Distinctive trends in fitted bandgap and band tail characteristics were determined in films with Ga content below 3 at.% and Ga content above 3 at.%. The effects of bandgap engineering on optical transparency were evaluated using transmission spectra. Carrier concentration and Hall mobility data were obtained as functions of Ga and Mg content. The electrical properties were significantly degraded when the Ga content exceeded 3 at.%. Correlations between conduction mechanisms and Ga doping of MgxZn1-xO thin films were described. In addition, the effect of bandgap engineering on the electrical properties of epitaxial single crystal Ga-doped MgxZn1-xO thin films was discussed. Mott transition in Ga-doped MgxZn1-xO thin films was investigated. 0.1 at.%, 0.5 at.% and 1 at.% Ga-doped Mg0.1Zn0.9O films were selected for resistivity measurements in the temperature range from 250 K to 40 mK. The 0.1 at.% Ga-doped Mg0.1Zn0.9O thin film showed typical insulator-like behavior and the 1 at.% Ga-doped Mg0.1Zn0.9O thin film showed typical metal-like behavior. The 0.5 at% Ga-doped Mg0.1Zn0.9O film showed increasing resistivity with decreasing temperature; resistivity was saturated with a value of 1.15 Ã— 10-2 Î©â€¢cm at 40 mK, which is characteristic of the metal-insulator transition region. Temperature dependent conductivity Ïƒ(T) in the low temperature range revealed that the electron-electron scattering is the dominant dephasing mechanism. MgxZn1-xO/TiN/Si(111) heterostructures were fabricated using pulsed laser deposition. X-ray diffraction and transmission electron microscopy studies showed that both TiN and MgxZn1-xO were grown epitaxially on Si(111). A thin spinel layer (~5 nm) was formed after deposition at the MgxZn1-xO and TiN interface. Current-voltage measurements showed that the electrical contact between MgxZn1-xO and TiN is ohmic contact. These results suggest that the TiN provides a buffer layer to integrate MgxZn1-xO thin films with silicon substrate.
Growth and Characterization of ZnO and ZnO-Based Alloys MgxZn1-xO and MnxZn1-xO
(2004-11-19) Jin, Chunming; Robert M. Kolbas, Committee Member; Carl C. Koch, Committee Member; Jagdish Narayan, Committee Chair; Ronald O. Scattergood, Committee Member
The goals of this work were to synthesize ZnO and ZnO based alloy thin films by using PLD and to study the structural, stoichiometric, optical and electrical properties of these films. Epitaxial hexagonal MgZnO thin films have been grown on sapphire (0001) with domain-matching epitaxy by using PLD. The films show the high single-crystalline quality and bright excitonic luminescence. The maximum Mg concentration was found to be 34 at. %, which is almost ten times of the value allowed by the phase diagram. The bandgap of MgZnO alloy film can be tuned from 3.40 eV to 4.19 eV. Epitaxial ZnMgO thin films with cubic (NaCl) structure were also synthesized on MgO (001) sapphire (0001) and TiN/Si(001) by using PLD. The maximum Zn concentration in these cubic alloy films was 18 at. %. The epitaxial growth of cubic ZnMgO on Si(001) substrate is of significant importance for integrating ZnO-based alloys to the Si-based electronics. The phase stability of MgZnO/ZnO/MgZnO superlattice structures was studied using XRD and HRTEM methods. The diffusion of Mg from the MgZnO barrier to the ZnO well was observed by using the HRTEM and optical measurements. The cubic nanoinclusions were also observed with HRTEM. Epitaxial MnZnO thin films were synthesized on sapphire (0001) substrates. The maximum Mn concentration was 35 at. %. The bandgap of these films shifts to the higher energy side with increasing Mn content. Magnetic investigations indicate that these films are paramagnetic. Epitaxial ZnO films have been grown on Si (111) substrates by using PLD with two different heterostructures, ZnO/AlN/Si(111) and ZnO/MgO/TiN/Si(111). These thin films show the excellent single crystalline quality and extremely bright excitonic emission. C-axis orientated ZnO thin films have been grown on the amorphous silica substrates. The PL characteristics of these films are comparable to that of the films grown on the sapphire substrates. An ultraviolet illumination-enhanced luminescence effect was observed. This new phenomenon is attributed to the oxygen desorption on the surface. A phenomenological model was proposed to explain this new effect.
Growth, Characterization and Magnetic Properties of Epitaxial FePt Nanostructures
(2008-04-08) Trichy, Gopinath R; Jagdish Narayan, Committee Chair
The primary objective of this dissertation was to study the growth, microstructure and magnetic properties of FePt nanostructures so as to demonstrate their suitability for high-density magnetic storage applications. The FePt system is an attractive candidate for high-density storage (greater than 100 Gbits⁄in2) due to its extremely high uniaxial magnetocrystalline anisotropy (Ku = 7.0 x 107 erg⁄cm3). The high magnetocrystalline anisotropy permits the use of smaller particles before the onset of superparamagnetism; this directly leads to larger recording densities. As a part of this study, we have pioneered the epitaxial growth of c-axis FePt on single crystal Si (100) substrates. This holds tremendous future promise in terms of integration of our magnetic structures with the existing Si-based technology. Integration of FePt on Si (100) was achieved by using epitaxial TiN as a template buffer. The TiN template controls the FePt growth along the magnetically hard c-axis and it also acts as an effective diffusion barrier. The FePt⁄TiN⁄Si (100) heterostructure was synthesized using pulsed laser deposition. X-ray diffraction results showed strong c-axis growth and significant L10 order. Transmission electron microscopy revealed the following epitaxial relationship; FePt (001) <001> TiN (100) <001>Si (100) <001>. In spite of the large misfit strains (ε) involved (εFePt⁄TiN = 9.5 % and TiN⁄Si = 22 %), epitaxy was achieved in the FePt⁄TiN⁄Si heterostructure by the unified paradigm of domain matching epitaxy (DME). In this work FePt was synthesized both as continuous thin films and individual discrete nanoparticles. The effect of microstructure on magnetic properties of the epitaxial FePt system was studied in detail. The microstructure was progressively varied from a 9 nm nanoparticle system to a 30 nm thick continuous film. Magnetic hysteresis measurements showed that all the samples were predominantly perpendicularly magnetized with higher coercivity, squareness and remanence when compared to the in-plane loops. The individual nanoparticles, being in a single domain state, showed higher coercivity than the continuous thin film. Within the nanoparticle regime coercivity increased with increasing particle size. The highest coercivity of 13,500 Oe was obtained for a bead-like microstructure, when the individual nanoparticles just begin to merge to form a continuous thin film. The continuous thin film showed least coercivity because of its multi-domain state and lack of defects or pinning sites. The best microstructure in terms of magnetic storage was the 18 nm sized nanoparticle system, with a perpendicular coercivity of 10,000 Oe. For this system, if we assume one bit of information to be stored in each nanoparticle, a storage capacity of 1 Terabit⁄in2 can be realized. All the samples under study showed perpendicular hysteresis loops with remarkable squareness (≥ 0.95). The coercivity of the FePt system can also be controlled via the composition. For the 18 nm system, the perpendicular coercivity was decreased from 10,000 Oe to 3,200 Oe by changing the composition from Fe50Pt50 to Fe41Pt59. A negative magnetoresistance (MR) of 0.57 % was observed at room temperature for the Fe50Pt50 bead-like thin-film system. The MR loops showed hysteretic behavior with maximum resistance at Hc. The values of coercivity in the MR and MH measurements were consistent. The presence of MR in the FePt system was attributed to thin domain walls, whose thickness is comparable to spin diffusion lengths. The negative MR effect was explained based on spin and domain wall dependent electron scattering.
Integration of Functional Oxide Thin Film Heterostructures with Silicon (100) Substrates.
(2010-04-30) Aggarwal, Ravi; Roger Narayan, Committee Chair; Jagdish Narayan, Committee Chair; Nadia El-Masry, Committee Member; John Prater, Committee Member
Integration of VO2 Films on Al2O3 and Si (100) Substrates: Structure-Property Correlations and Applications.
(2011-01-31) Yang, Tsung-Han; Jagdish Narayan, Committee Chair; Veena Misra, Committee Member; Jerome Cuomo, Committee Member; John Prater, Committee Member
Novel Nanostructured Thin Film Heterostructures: Growth, Nanoscale Characterization and Properties
(2006-08-08) Chugh, Amit; Nadia Elmasry, Committee Member; Jagdish Narayan, Committee Chair; Ronald Scattergood, Committee Member; John Muth, Committee Member
During my graduate study, I have been involved in the growth of new nano heterostructures grown by Pulsed Laser Deposition and by Laser MBE with the emphasis on understanding the thin film growth process by a new paradigm of Domain Matching Epitaxy (DME) and to integrate them on substrates like silicon, sapphire and new metallic substrates like Ni RaBiTS with exciting technological applications. The DME involves matching of integral multiples of lattice planes (diffracting as well as nondiffracting) between the film and the substrate, and this matching could be different in different directions. The idea of matching planes is derived from the basic fact that during thin film growth lattice relaxation involves generation of dislocations whose Burgers vectors correspond to missing or extra planes, rather than lattice constants. In the DME framework, the conventional lattice matching epitaxy (LME) becomes a special case where matching of lattice constants results from matching of lattice planes with a relatively small misfit of less than 7-8%. In large lattice mismatch systems, epitaxial growth of thin films is possible by matching of domains where integral multiples of lattice planes match across the interface. The work done in my doctoral study is divided into two main segments, a) Growth of layered nanostructures and b) growth of nanostructured composite thin films. The three systems studied under the first segment are 1) Growth of epitaxial self-aligned insulating films on metals (Cu) and its integration with Si (100). 2) Growth and integration of LSMO with Si (100). 3) Growth of Si on Ni substrates (highly textured) with TiN as a buffer layer. The heterostructures studied under the second part are 1) Role of Self-assembled Gold Nanodots in Improving the Electrical and Optical Characteristics of Zinc Oxide Films and 2) Growth of high quality epitaxial ZnO-Pt Nanocomposite and ZnO/Pt, Nanolayer Structures on Sapphire (0001). The epitaxial growth of these heterostructures was carried out by Pulsed laser deposition and laser MBE. The epitaxial relationships are given in each case are shown to be due to domain matching epitaxy. X-Ray diffraction and Transmission Electron Microscopy studies confirm the relationship between film and substrate. Also, electrical and optical measurements were done, in order to study the change in these properties.
Process and Properties of Nitride-based Thin Film Heterostructures
(2003-11-05) Wang, Haiyan; Jagdish Narayan, Committee Chair; J. Michael Rigsbee, Committee Member; Carl C. Koch, Committee Member; John Muth, Committee Member
The goals of this work were to synthesize nitride-based thin film heterostructures by Pulsed Laser Deposition, study the structural, mechanical, electrical and optical properties of these heterostructures and establish structure-property relations for these materials in order to further improve their properties and design new structures. Domain matching epitaxy was explored in most of these heterostructures and studied in detail for each case. Mechanical and electrical properties of TiN as a function of microstructure varying from nanocrystalline to single crystal TiN films deposited on (100) silicon substrates were investigated. By varying the substrate temperature from 25°C to 700°C during PLD, the microstructure of TiN films changed from nanocrystalline (having uniform grain size of 8 nm) to a single crystal epitaxial film on the silicon (100) substrate. The hardness of TiN films decreased with decreasing grain size. The dependence of resistivity of TiN as a function of the substrate temperature is discussed and correlated with hardness results. High-quality epitaxial B1 NaCl-structured TaN films were deposited on Si(100) and Si(111) substrates with TiN as buffer layer, using pulsed laser deposition. Our method exploits the concept of lattice-matching epitaxy between TiN and TaN and domain-matching epitaxy between TiN and silicon. XRD, TEM, and STEM experiments confirmed the single-crystalline nature of the films with cube-on-cube epitaxy. The stoichiometry of TaN films was determined to be nitrogen deficient (TaN[subscript 0.95]) by RBS. Resistivity of the TaN films was found to be 220μΩ-cm at room temperature with temperature coefficient of resistivity of -0.005K⁻¹. Diffusivity of copper in single-crystal (NaCl-structured) and polycrystalline TaN thin films grown by PLD was investigated. The polycrystalline TaN films were grown directly on Si(100), while single-crystal films were grown with TiN buffer layers. The diffusion distances in epitaxial TaN are found to be about 5nm at 650°C for 30 min annealing. Cu diffusion in polycrystalline TaN thin films is found to be nonuniform with enhanced diffusivities along the grain boundary. By PLD, TiN and TaN targets were arranged in a special configuration that they can be ablated in a sequential manner to obtain TiN-TaN alloy or TiN/TaN superlattice structure. The 60% TaN resulted in superlattice of TaN(3nm) /TiN(2nm), while 30% and 70% TaN generated uniform TaXTi1-XN alloys. TiN buffer layers were deposited first to achieve those epitaxial binary components. XRD and TEM analysis showed the epitaxial nature of these films. Microstructure and uniformity of the superlattice and alloy structures were studied by TEM and STEM. Nanoindentation results suggested high hardness and future hard coating applications for these TiN-TaN composites. Four point probe electrical resistivity measurements and Cu diffusion characteristics study prove that TiN-TaN binary components provide a superior diffusion barrier for copper. Uniform AlxTi1-xN alloys (x up to 70%) and highly aligned TiN/AlN superlattices were deposited by PLD. Microstructure and uniformity for the superlattice structures and alloys were studied by TEM and STEM. Nanoindentation results suggested high hardness for these new structures and four point probe electrical resistivity measurements showed overall insulating behavior for both alloys and superlattices. The eptaxial wurtzite AlN thin films were grown on (0001) &alpha-Al2O3 substrates by PLD. XRD and SAD in TEM revealed the epitaxial growth of AlN on (0001) α -Al2O3 substrate. These AlN films were post-deposition annealed at 1300°C for 30mins. Bright field and dark field TEM and transmittance spectra for the samples before and after annealing prove the annealing can effectively improve the quality of the film. Post-deposition annealing for AlN on α-Al2O3 substrates could be a very promising procedure for high quality optical device fabrications. The eptaxial wurtzite AlN thin films were grown on (111) Si substrates by PLD and Laser-MBE. XRD and SAD in TEM revealed the epitaxial growth of AlN on Si(111) substrate. The interface structure and growth mechanism were studied by high-resolution TEM. Fourier filtered image of cross-sectional AlN/Si(111) samples from both Si (112) zone axes revealed the domain matching epitaxy of 4:5 ratio between a[subscript Si(110)] and a[subscript AlN(2110)].
Self-assembled Magnetic Nanostructures: Epitaxial Ni on TiN (001) Surface
(2006-01-10) Zhou, Honghui; Gerd Duscher, Committee Member; Carl C. Koch, Committee Member; Robert M. Kolbas, Committee Member; Jagdish Narayan, Committee Chair
Systems that contain single domain magnetic particles have been receiving intensive attentions over recent years since they are possible candidates for applications in ultrahigh-density data storage and magnetoelectronic devices. The focus of this research is self-assembly growth of magnetic nickel nanostructures by domain matching epitaxy under Volmer-Weber mode. The growth was conducted by pulsed laser deposition (PLD) technique using epitaxial titanium nitride film as the template, which was in turn grown on silicon (100) substrate. The structural characterization includes X-ray diffraction and both cross-sectional and plan-view transmission electron microscopy. The results showed that the nickel islands formed exhibit a self-assembled nature, i.e., a certain degree of uniformity in orientation, shape, and size. The orientation relationship observed is Ni [100] // TiN [100] // Si [100], the so-called 'cube-on-cube' relationship. The islands are faceted, forming truncated pyramids with walls of (111) planes and a flat top of (100) plane. The base of islands is rectangular with the two principal edges parallel to two orthogonal 011 directions. The size distribution is relatively narrow, comparable to that obtained from self-assembled islands grown under Stranski-Krastanow (S-K) mode. A certain degree of self-organization was also found in the island lateral distribution: island chains were observed along the directions close to 011, which are also the edge directions. The island faceting could be explained by surface energy minimization. The interaction of island edge induced strain field between neighboring islands is believed to be responsible for the size uniformity and the lateral ordering. Magnetic measurements were also conducted on these crystallographically aligned nickel islands using superconducting quantum interference device (SQUID) magnetometer, and the results were compared with that obtained from the ensemble of randomly oriented nickel islands, which were grown on polycrystalline/amorphous Al2O3 matrix layer. It is found that both blocking temperature and coercivity of aligned nickel islands are significantly higher than that of the randomly oriented nickel islands. The enhancement in ferromagnetism is attributed to the increased collective effects resulting from the particle interactions in the ensemble of aligned islands, which are self-assembled and self-organized to some degree.