Doctor of Philosophy, The Ohio State University, 2021, Electrical and Computer Engineering

A rapid advancement and adoption of the light emitting diodes (LEDs) based solid state lighting (SSL) technology is projected to reduce the annual lighting energy consumption, which currently accounts for a large share of the total worldwide electricity consumption, by more than half within the next decade. An ultimate SSL white light source would be based on monolithic integration of blue, green, amber and red LEDs. Till now, the efficiency of such color-mixed LEDs is still low due to the challenge of `green gap', which refers to the low quantum efficiency (QE) of the LEDs emitting in the green and amber spectral region. The efficiency of InGaN based beyond-blue visible light emitters has remained an issue despite rigorous efforts by researchers for more than three decades. On the other hand, performance evolution of (AlGaIn)P based red and shorter visible wavelength emitters have plateaued due to the direct-to-indirect bandgap crossover of the alloy at about 2.3 eV. Therefore, a new nitride semiconductor-based beyond-blue light emitter with high efficiency is essential for achieving ultimate SSL technology. Two ternary heterovalent II-IV-N2 compounds, namely, ZnGeN2 and ZnSnN2 are deemed promising for application in nitride based high efficiency LEDs emitting in green and beyond thanks to their band alignment with III-nitrides. The bandgap and lattice parameters of ZnGeN2 and ZnSnN2 are very close to those of GaN, and InxGa1-xN (x~0.3-0.5), respectively. For both materials, the position of the valence band maxima is more than 1 eV above that of GaN. Such a large valence band offset at InGaN/ZnGeN2 or InGaN/ZnSnN2 heterointerfaces can enable achieving (i) long emission wavelength with much lower In-content than conventional InGaN quantum wells (QWs); and (ii) increased emission efficiency by mitigating the problem of low overlap between electron and hole wavefunctions which is severe in conventional InGaN QW. By using numerical simulations, it was shown that an I (open full item for complete abstract)

Committee: Hongping Zhao (Advisor); Leonard J. Brillson (Committee Member); Kathleen Kash (Committee Member); Shamsul Arafin (Committee Member) Subjects: Electrical Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2024, Electrical and Computer Engineering

Solid state lighting technology enabled by III-nitride material system has emerged as the ultimate choice of next-generation technology for applications in wearable devices, AR/VR/MR, phones, tablets, televisions, automotive vehicles along with visible light communication, Li-Fi, biomedical sensing, etc. These direct band gap materials offers efficient light emitters spanning a wide range of the visible spectrum as well as ultra-violet wavelength and has the possibilities of replacing conventional phosphor converted lighting solutions with true R-G-B color mixed technologies. Most display and lighting application space require high performing emitters in blue, green, and red wavelength. While the blue LEDs are nearing theoretical efficiency limits, longer wavelength LEDs like green and red still suffers from higher efficiency droop at their operating conditions. This is partly due to the challenges associated to obtain high quality high composition (Ga, In)N alloys during material growth and partly from the larger polarization induced electric fields from high composition (Ga, In)N alloys. This dissertation focuses on understanding the effect of the polarization induced fields on the active region of long wavelength LEDs, mostly green wavelength, and their integration with switching devices. Alloys of GaN-InN, GaN-AlN or InN-AlN contain nanoscale compositional fluctuation which a crucial role in LED electrical and optical behavior. A simulation model incorporating alloy fluctuations is developed in this dissertation to understand their effect on the electrical properties of c-plane green LEDs. The model provides insights on the vertical and lateral carrier injection in the active region and recombination dynamics and can accurately predict the electrical behavior of experimental III-Nitride LEDs. The model is used in the later part of the work to design and understand green LEDs with reversed-polarization field configuration. Reversed-polarization green LEDs dev (open full item for complete abstract)

Committee: Siddharth Rajan (Advisor); Steven Ringel (Committee Member); Jamie Jackson (Committee Member); Robert Armitage (Committee Member); Hongping Zhao (Committee Member) Subjects: Electrical Engineering; Solid State Physics

Doctor of Philosophy (PhD), Ohio University, 2017, Physics and Astronomy (Arts and Sciences)

This dissertation presents a study on the fabrication of Indium Gallium Nitride (InGaN) based heterojunction solar cells using RF magnetron sputtering method. The goal of the study includes improving the electrical, optical and structural properties of InGaN thin films and examining their potential for photovoltaic applications and to reduce the parasitic resistive loses in solar cells. Reactive radio-frequency (RF) magnetron and Direct Current (DC) sputtering are deposition methods for thin films. The characterization techniques include Hall Effect measurement system for electrical properties, UV-Visible Spectroscopy for optical properties, X-Ray Diffraction (XRD) and Energy Dispersive X-Ray Spectroscopy (EDXS) for structural properties and AM 1.5 G irradiance spectrum to measure current-voltage (IV curves) and photovoltaic measurements. Copper Oxide thin films and Beryllium Zinc Oxide thin films are fabricated and their properties are examined for their potential to pair with n-InGaN to form a p-n junction. We conclude Silicon (111) wafer has better electrical properties than RF deposited Copper oxide and BeZnO and used as p-type layer. Aluminum (Al) and Indium Tin Oxide (ITO) are used as back and front metallic contacts respectively. In this study, we present a simple method for optical bandgap tuning of Indium Gallium Nitride (InGaN) thin films by controlling the growth conditions in magnetron RF sputtering. Thin films with different Indium (In) atomic compositions, x = 0.02 to 0.57 are deposited on high temperature aluminosilicate glass and Silicon (111) substrates. Substrate temperature is varied from 35 oC to 450 oC. Total pressure of sputtering gas mixture is kept constant at 12 mTorr but partial pressures of Ar and N2 are varied. Ar partial pressure to total pressure ratio is varied from 0 to 0.75. Optical bandgap values from 1.4 eV to 3.15 eV, absorption coefficient values of ~ 104 /cm to ~ 7 x 105 /cm and critical film thickness values of 0. (open full item for complete abstract)

Committee: Martin Kordesch (Advisor) Subjects: Condensed Matter Physics; Physics

Doctor of Philosophy (PhD), Ohio University, 2016, Electrical Engineering & Computer Science (Engineering and Technology)

The technological advantages of III-nitride semiconductors (III-Ns) have been demonstrated among others in the area of light emitting applications. Due to fundamental reasons limiting growth of InGaN with high Indium content, rare earth (RE) doped III-Ns provide an alternative way to achieve monolithic red, green, blue (RGB) emitters on the same III-Ns host material. However, the excitation efficiency of RE3+ ions in III-Ns is still insufficient due to the complexity of energy transfer processes involved. In this work, we consider the current understanding of the excitation mechanisms of RE3+ ions doped III-Ns, specifically Yb3+ and Eu3+ ions, and theories toward the excitation mechanism involving RE induced defects. In particular, we demonstrate and emphasize that the RE induced structural isovalent (RESI) trap model can be applied to explain the excitation mechanism of III-Ns:RE3+. Specifically, we have investigated the Yb3+ ion doped into III-Ns hosts having different morphologies. The observed emission peaks of Yb3+ ion were analyzed and fitted with theoretical calculations. The study of Yb3+ ion doped InxGa1-xN nano-rod films with varied indium (In) concentration shown the improvement of luminescence quality from the nanorod due to the presence of Yb dopant. Then we report the optical spectroscopy and DLTS study toward an Eu and Si co-doped GaN and its control counterpart. The Laplace-DLTS and optical-DLTS system developed in this work improved spectrum resolution compared to the conventional DLTS. The high resolution L-DLTS revealed at least four closely spaced defect levels associated with the Trap B, identified with regular DLTS, with activation energy 0.259±0.032 eV (Trap B1), 0.253±0.020 eV (Trap B2), 0.257±0.017 eV (Trap B3), and 0.268±0.025 eV (Trap B4) below the conduction band edge, respectively. Most importantly, a shallow hole trap was observed at energy 30±20 meV above the valence band edge of the GaN:Si,Eu3+ which can be attributed to the RESI hole (open full item for complete abstract)

Committee: Wojciech Jadwisienczak (Advisor); Savas Kaya (Committee Member); Martin Kordesch (Committee Member); Eric Stinaff (Committee Member); Kodi Avinash (Committee Member); Harsha Chenji (Committee Member) Subjects: Electrical Engineering; Materials Science; Nanotechnology; Optics

Doctor of Philosophy, The Ohio State University, 2015, Electrical and Computer Engineering

This dissertation describes the design, fabrication and characterization of high-k dielectric enhanced Gallium Nitride (GaN)-based devices. Interface properties of atomic layer deposited (ALD) Aluminum Oxide (Al2O3) on GaN was initially investigated. The conduction band offset of Al2O3/GaN was experimentally found as 2.1 eV. High density of positive interface fixed charge (2.72x1013 cm-2) was observed in the Al2O3/GaN. These interface fixed charges can not only induce electrical field in the oxide which increases the reverse gate leakage, but shift the threshold voltage toward negative to prevent E-mode operation. A theoretical study using remote impurity scattering along with other scattering models showed that these interface fixed charges are able to degrade the electron mobility in the channel.

Committee: Siddharth Rajan (Advisor); Steven Ringel (Committee Member); Aaron Arehart (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2014, Electrical and Computer Engineering

This thesis describes the design, molecular beam epitaxy growth, fabrication and characterization of Gallium Nitride (GaN)-based interband tunnel junctions (TJs) surpassing the state-of-the-art device performance. GaN and AlGaN-based TJs are attractive for hole injection in visible, and ultraviolet light emitters, respectively. Tunnel junctions enable monolithic integration of multiple active regions for multi-color light emitters and multi-junction solar cells. A major outstanding issue with the visible emitters is the efficiency droop problem, which refers to the reduction in efficiency of a light emitting diode at higher operating current density. Efficient TJs are attractive to overcome this issue as it enables epitaxial cascading of identical active regions, and a low current-high voltage operation of the cascaded structure. In such a structure, the carriers are regenerated at the tunnel junction sites and each of the individual active regions can be operated at its peak efficiency. In this work, two nanoscale heterostructure band engineering approaches, namely, polarization engineering and midgap states assisted tunneling, are used to demonstrate low resistance Gallium Nitride tunnel junctions. In the case of the polarization-based approach, the high spontaneous and piezoelectric polarization sheet charge at the GaN/InGaN heterointerface is utilized to create large band bending over a few nanometers, thereby reducing the tunneling barrier width. Such polarization engineered GaN/InGaN/GaN tunnel junctions are used to demonstrate record reverse current and tunneling under forward bias, leading to the first observation of interband tunneling-related negative differential resistance in III-nitrides. Tunnel hole injection in a GaN PN junction with a tunneling specific resistivity as low as 10-4 Ohm cm2 is achieved using this approach. The second approach involves the use of embedded Gadolinium Nitride (GdN) nano-islands for inter-band tunneling in GaN. By creating (open full item for complete abstract)

Committee: Siddharth Rajan (Advisor); Steven Ringel (Committee Member); Aaron Arehart (Committee Member) Subjects: Electrical Engineering