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- Creators: Alford, Terry
Description
Microwave dielectrics are widely used to make resonators and filters in telecommunication systems. The production of thin films with high dielectric constant and low loss could potentially enable a marked reduction in the size of devices and systems. However, studies of these materials in thin film form are very sparse. In this research, experiments were carried out on practical high-performance dielectrics including ZrTiO4-ZnNb2O6 (ZTZN) and Ba(Co,Zn)1/3Nb2/3O3 (BCZN) with high dielectric constant and low loss tangent. Thin films were deposited by laser ablation on various substrates, with a systematical study of growth conditions like substrate temperature, oxygen pressure and annealing to optimize the film quality, and the compositional, microstructural, optical and electric properties were characterized. The deposited ZTZN films were randomly oriented polycrystalline on Si substrate and textured on MgO substrate with a tetragonal lattice change at elevated temperature. The BCZN films deposited on MgO substrate showed superior film quality relative to that on other substrates, which grow epitaxially with an orientation of (001) // MgO (001) and (100) // MgO (100) when substrate temperature was above 500 oC. In-situ annealing at growth temperature in 200 mTorr oxygen pressure was found to enhance the quality of the films, reducing the peak width of the X-ray Diffraction (XRD) rocking curve to 0.53o and the χmin of channeling Rutherford Backscattering Spectrometry (RBS) to 8.8% when grown at 800oC. Atomic Force Microscopy (AFM) was used to study the topography and found a monotonic decrease in the surface roughness when the growth temperature increased. Optical absorption and transmission measurements were used to determine the energy bandgap and the refractive index respectively. A low-frequency dielectric constant of 34 was measured using a planar interdigital measurement structure. The resistivity of the film is ~3×1010 ohm·cm at room temperature and has an activation energy of thermal activated current of 0.66 eV.
ContributorsLi, You (Author) / Newman, Nathan (Thesis advisor) / Alford, Terry (Committee member) / Singh, Rakesh (Committee member) / Arizona State University (Publisher)
Created2013
Description
Complex perovskite materials, including Ba(Zn1/3Ta2/3)O3 (BZT), are commonly used to make resonators and filters in communication systems because of their low dielectric loss and high-quality factors (Q). Transition metal additives are introduced (i.e., Ni2+, Co2+, Mn2+) to act as sintering agents and tune their temperature coefficient to zero or near-zero. However, losses in these commercial dielectric materials at cryogenic temperatures increase markedly due to spin-excitation resulting from the presence of paramagnetic defects. Applying a large magnetic field (e.g., 5 Tesla) quenches these losses and has allowed the study of other loss mechanisms present at low temperatures. Work was performed on Fe3+ doped LaAlO3. At high magnetic fields, the residual losses versus temperature plots exhibit Debye peaks at ~40 K, ~75 K, and ~215 K temperature and can be tentatively associated with defect reactions O_i^x+V_O^x→O_i^'+V_O^•, Fe_Al^x+V_Al^"→Fe_Al^'+V_Al^' and Al_i^x+Al_i^(••)→〖2Al〗_i^•, respectively. Peaks in the loss tangent versus temperature graph of Zn-deficient BZT indicate a higher concentration of defects and appear to result from conduction losses.Guided by the knowledge gained from this study, a systematic study to develop high-performance microwave materials for ultra-high performance at cryogenic temperatures was performed. To this end, the production and characterization of perovskite materials that were either undoped or contained non-paramagnetic additives were carried out. Synthesis of BZT ceramic with over 98% theoretical density was obtained using B2O3 or BaZrO3 additives. At 4 K, the highest Q x f product of 283,000 GHz was recorded for 5% BaZrO3 doped BZT.
A portable, inexpensive open-air spectrometer was designed, built, and tested to make the electron paramagnetic resonance (EPR) technique more accessible for high-school and university lab instruction. In this design, the sample is placed near a dielectric resonator and does not need to be enclosed in a cavity, as is used in commercial EPR spectrometers. Permanent magnets used produce fields up to 1500 G, enabling EPR measurements up to 3 GHz.
ContributorsGajare, Siddhesh Girish (Author) / Newman, Nathan (Thesis advisor) / Alford, Terry (Committee member) / Tongay, Sefaattin (Committee member) / Chamberlin, Ralph (Committee member) / Arizona State University (Publisher)
Created2022
Description
Diamond transistors are promising as high-power and high-frequency devices having higher efficiencies than conventional transistors. Diamond possesses superior electronic properties, such as a high bandgap (5.47 eV), high breakdown voltage (>10 MV cm−1 ), high electron and hole mobilities [4500 and 3800 cm2 V−1 · s−1, respectively], high electron and hole saturation velocities (1.5 × 107 and 1.05 × 107 cm s−1, respectively), and high thermal conductivity [22 W cm−1 · K−1], compared to conventional semiconductors. Reportedly, the diamond field-effect transistors (FETs) have shown transition frequencies (fT) of 45 and 70 GHz, maximum oscillation frequency (fmax) of 120 GHz, and radiofrequency (RF) power densities of 2.1 and 3.8 W mm−1 at 1 GHz. A two-dimensional-hole-gas (2DHG) surface channel forms on H-diamond by transfer doping from adsorbates/dielectrics in contact with H-diamond surface. However, prior studies indicate that charge transfer at the dielectric/ H-diamond interface could result in relatively low mobility attributed to interface scattering from the transferred negative charge to acceptor region. H-terminated diamond exhibits a negative electron affinity (NEA) of -1.1 to -1.3 eV, which is crucial to enable charge transfer doping. To overcome these limitations modulation doping, that is, selective doping, that leads to spatial separation of the MoO3 acceptor layer from the hole channel on H-diamond has been proposed. Molybdenum oxide (MoO3) was used as dielectric as it has electron affinity of 5.9eV and could align its conduction band minimum (CBM) below the valence band maximum (VBM) of H-terminated diamond. The band alignment provides the driving potential for charge transfer. Hafnium oxide (HfO2) was used as interfacial layer since it is a high-k oxide insulator (∼25), having large Eg (5.6 eV), high critical breakdown field, and high thermal stability. This study presents photoemission measurements of the electronic band alignments of the MoO3/HfO2/H-diamond layer structure to gain insight into the driving potential for the negative charge transfer and the location of the negative charges near the interface, in the HfO2 layer or in the MoO3 layer. The diamond hole concentration, mobility, and sheet resistance were characterized for MoO3/HfO2/H-Diamond with HfO2 layers of 0, 2 and 4 nm thickness.
ContributorsDeshmukh, Aditya Vilasrao (Author) / Nemanich, Robert J. (Thesis advisor) / Alford, Terry (Committee member) / Yang, Sui (Committee member) / Arizona State University (Publisher)
Created2024