Doctor of Philosophy, The Ohio State University, 2022, Electrical and Computer Engineering

Gallium oxide in its beta phase (β-Ga2O3) has compelling material properties that have generated an intense worldwide research interest for applications in high-voltage and high-power electronics, radio frequency (RF) electronics, and ultraviolet optoelectronics. The ultra-wide bandgap (UWBG) of 4.8 eV and large breakdown field of 8 MV/cm lead to a potentially superior performance compared to contemporary wide bandgap materials (SiC, GaN) in high-power and high-frequency applications, as well as in harsh radiation environments, due to the predicted improved radiation hardness. The potentially transformative performance advantages of β-Ga2O3 are further enhanced by the ability to grow low-cost, large-area native substrates through melt-based growth methods. This enables high-quality homoepitaxially grown layers for improved device reliability compared to non-native substrates from the significant reduction in mismatch-related defects such as dislocations. This dissertation is focused on accelerating β-Ga2O3 material and device technology through the characterization and understanding of how atomic-level defects impact the performance, reliability, and radiation hardness of cutting-edge β-Ga2O3 transistors and provide pathways to reduce their effects by a combined understanding of growth, defect engineering, and device engineering. This is done by systematically evaluating the effects of the Fe deep acceptor in molecular beam epitaxy (MBE) grown devices, which is then compared with the use of the Mg deep acceptor in metalorganic chemical vapor deposition (MOCVD) grown devices. Furthermore, the exploration of high energy particle irradiation effects and the implications of the evolving defect spectrum from irradiation are studied for materials and devices. The defects causing dispersion in MBE grown δ-doped MESFETs are determined to be located in the buffer. The close proximity of the Fe from the substrate that also surface rides into the epitaxially grown buffer (open full item for complete abstract)

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

Doctor of Philosophy, The Ohio State University, 2019, Physics

In this work, scanning probe microscopy (SPM) methods are developed and extended to spatially resolve performance-hampering electrically-active defects, known as traps, present in AlGaN/GaN Schottky barrier diodes (SBDs) and high electron mobility transistors (HEMTs). Commercial devices used in these studies were cross-sectioned to expose electrically-active regions which are traditionally inaccessible to SPM techniques. Surface potential transients (SPTs) are collected over the cross-sectioned faces of devices using nanometer-scale scanning probe deep-level transient spectroscopy (SP-DLTS), a millisecond time-resolved derivative technique of scanning Kelvin probe microscopy (SKPM) that was implemented with a custom system designed to study SBDs and HEMTs in cross-section. Detected SPTs are indicative of carrier emission from bulk defect-related trap states. In conjunction with similar measurements of these trap states using macroscopic techniques, finite-element simulations provide strong, corroborating evidence that observable SPTs are produced by traps located in the bulk of these samples and are therefore not a result of surface states or surface-related phenomena. GaN-based materials offer advantages over many alternatives in high-frequency and high-voltage applications. Features including a wide bandgap and a large breakdown voltage often translate to improved efficiency, performance, and cost in many electronic systems. However, GaN-based material research is still maturing, and charge trapping may be a limiting factor in GaN electrical performance and therefore hinder its widespread application and adoption. Determining the signatures and spatial distributions of active traps in GaN devices is critical for understanding trap-related mechanisms of device failure as well as the growth or fabrication steps which may be responsible for introducing these defect states. Powerful techniques like deep-level transient spectroscopy (DLTS) exist for identifying specifi (open full item for complete abstract)

Committee: Jonathan Pelz (Advisor); Ezekiel Johnston-Halperin (Committee Member); Richard Kass (Committee Member); Mohit Randeria (Committee Member) Subjects: Electrical Engineering; Physics

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

Gallium nitride (GaN) based high electron mobility transistors (HEMTs) have shown a lot of promise in high voltage, high power, and high radiation applications. However the full realization of the III-nitride potential and large scale adoption of this technology has been hindered by the existence of electrically active defects that manifest as deep levels in the energy bandgap. These deep levels can potentially act as charge trapping centers limiting device performance and long term reliability. It is therefore imperative to monitor these traps in operational GaN HEMTs as close as possible to their real world operational conditions. With that goal in mind, in this dissertation, a suite of advanced thermal and optical based trap spectroscopy methods and models collectively known as constant drain current deep level (thermal) transient spectroscopy and deep level optical spectroscopy (CID-DLTS/DLOS) were developed and expanded upon to directly probe and track traps in three terminal operational GaN HEMTs. These techniques have allowed an unprecedented ability to quantitatively track trap levels throughout the wide bandgap of operational GaN devices. Depending on their mode of switching (gate-controlled versus drain-controlled) the techniques are able to distinguish between under gate and access region defects irrespective of device design and/or operational history. The devices studied here were subjected to a range of different stressors and very different trap induced degradation mechanisms were identified that further confirms the need for such high resolution defect spectroscopic studies in GaN HEMTs. Specifically the GaN HEMTs studied here were subjected to three very different kinds of stressors, i) high frequency moderate drain voltage (<50 V) accelerated lifetime stressor were applied to GaN HEMTs optimized for radio frequency (RF) applications, ii) very high off-state drain voltage (up to 600 V) stressors were applied to GaN-on-Si MISHEMTs optimized for power (open full item for complete abstract)

Committee: Steve Ringel Prof. (Advisor); Siddharth Rajan Prof. (Committee Member); Aaron Arehart Prof. (Committee Member); Patrick Roblin Prof. (Committee Member) Subjects: Electrical Engineering; Materials Science

Doctor of Philosophy, The Ohio State University, 2013, Physics

In this work, scanned probe microscopy (SPM) based methods are developed and used to spatially resolve particular electrically-active defects that degrade the performance of GaN-based high electron mobility transistors (HEMTs). Surface potential transients (SPTs) resulting from electron emission from defect-related trap states were measured in AlGaN/GaN HEMTs with nm-scale spatial resolution using a modified scanning Kelvin probe microscopy (SKPM) setup. Simultaneous measurements of SPTs and channel resistance transients as a function of temperature allowed particular traps affecting device performance to be resolved both spatially and spectroscopically. GaN-based HEMTs offer high-power radio-frequency (RF) performance, though their ultimate performance and reliability are still limited by electrically-active trap states. Knowledge of the spatial distribution of specific traps that affect device performance is important for understanding and controlling degradation in AlGaN/GaN HEMTs. However, the macro-scale trap spectroscopy techniques typically used to identify traps in HEMTs are sensitive to the transistor output characteristics, and are therefore quite limited in their ability to determine the spatial distribution of traps. To directly probe the spatial distribution of specific traps affecting device performance, nm-scale atomic force microscopy (AFM) based trap spectroscopy techniques, sensitive to the local surface potential, were developed and coupled with macro-scale methods, enabling the spatial distribution of specific trap species to be measured, with both lateral and vertical spatial resolution, in operating AlGaN/GaN HEMTs. To probe traps throughout the AlGaN/GaN bandgaps, both thermal and optical spectroscopy techniques were implemented. The nm-scale trap spectroscopy techniques were used to spatially resolve particular traps in AlGaN/GaN HEMTs grown by metal-organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE). Typically, de (open full item for complete abstract)

Committee: Jonathan Pelz (Advisor); Chris Hammel (Committee Member); David Stroud (Committee Member); Gregory Lafyatis (Committee Member) Subjects: Electrical Engineering; Solid State Physics

Master of Science, Miami University, 2010, Physics

ZnO is a well studied semiconductor that possesses great potential for creating high quality photovoltaic devices. Because of its low exciton binding energy, large band gap, low dielectric constant, and low cost, much effort has been devoted to developing p-type ZnO (necessary for forming robust homojunctions for photovoltaic devices). New samples have been synthesized using a thermal evaporation technique and their photoconductive transient behaviors are characterized versus temperature and incident light energy. A Laplace transform method of Deep-Level Transient Spectroscopy (DLTS) is used to find the distribution of photo-excited states. Results show that these new samples exhibit a two peak energy fingerprint pointing to two defects within the material. Conclusions are that these samples contain high levels of oxygen defects as well as a lower concentration of another defect.

Committee: Jeffrey Clayhold PhD (Advisor); Michael Pechan PhD (Committee Member); Herbert Jaeger PhD (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2020, Physics

Gallium Nitride (GaN) and Gallium Oxide (Ga2O3) are two semiconductors of significant interest for high power and high frequency electronics. However, the performance of these electronics can be inhibited by the presence of defects which can produce "trap" states in the "forbidden" bandgap of semiconductors. These traps can then degrade the output current of transistors and cause undesireable time-dependent phenomenon. This work investigates the physical origin of the most common trap, EC - 0.57 eV, in Gallium Nitride (which happens to be detrimental for certain transistors) by using Scanning Probe - Deep Level Transient Spectroscopy (SP-DLTS) to probe its spatial distribution. For the first time, this trap species is mapped with high spatial resolution and it is found to exhibit strong spatial localization in the form of "trap clusters". Through a correlative study with Electron Channeling Contrast Imaging (ECCI), this trap is found to be located at pure edge dislocations. In another study, the impact of iron on the spatial distribution of this trap is investigated, and it is found that the iron causes a more spatially-uniform trap distribution. One possible explanation is that the EC - 0.57 eV traps are directly related to iron atoms that are gettered by edge dislocations in Gallium Nitride. To better understand how the SP-DLTS maps relate to the trap concentration, simulations are performed. A comparison between the measurement and simulation shows reasonable agreement for the two GaN samples studied here. In collaboration with fellow graduate student Darryl Gleason, a study is conducted on a different device geometry (AlGaN/GaN heterostructures with semi-insulating GaN layers). This study allows for the characterization of two trap species in the GaN layer (one of which is the EC - 0.57 eV trap), and good agreement is found between macroscopic DLTS and SP-DLTS for both trap species. Finally, the first Ballistic Electron Emission Microscopy (BEEM) measurements on (open full item for complete abstract)

Committee: Jonathan Pelz (Advisor); Steven Ringel (Advisor); Nandini Trivedi (Committee Member); Yuri Kovchegov (Committee Member) Subjects: 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, 2016, Electrical and Computer Engineering

The development of the next generation GaN-based electronic and optoelectronic devices primarily depends on the capability of growing high quality novel materials and improving device reliability to support new functionalities. Essential to both is a comprehensive understanding of the electrically active crystalline defects, as these defects may introduce deep states in the bandgap, thus substantially impacting material properties and device performance. To date, in spite of large amounts of research on defects in III-nitrides, there is still an extraordinary gap of knowledge. Thus, the goal of this dissertation has been to explore the presence and properties of defect states in a wide range of state-of-the-art III-nitride materials, and investigate their role in device level degradation, particularly, under high energy proton irradiation as intended for space communication applications. To enable these objectives, a set of capacitance-based measurements, including deep level transient/optical spectroscopy (DLTS/DLOS) that facilitate the quantitative characterization of deep states throughout the 3.4 eV GaN bandgap, have been performed on a variety of materials, such as non-polar m-plane GaN, NH3MBE grown p-type GaN, proton irradiated n-type and p-type GaN. Systematically varied growth, irradiation and annealing conditions allowed for methodical investigation of defect behaviors and properties, thus shedding light on the defect physical sources and atomic configurations. Specifically, by comparing simultaneously grown c-/m-plane GaN, substantial impacts of the growth surface on the defect formation were revealed, as both external and native defects formed with much higher concentrations in m-plane GaN. M-plane growth also created traps at EC - 0.14 eV, EC - 0.20 eV and EC - 0.66 eV that were absent in c-plane GaN, among which the EC - 0.14 eV and EC - 0.66 eV states closely correlated with V/III ratio (and/or oxygen content). In proton irradiation study, monotoni (open full item for complete abstract)

Committee: Steven Ringel (Advisor); Siddharth Rajan (Committee Member); Roberto Myers (Committee Member); Betty Anderson (Committee Member) Subjects: Electrical Engineering

Master of Science, The Ohio State University, 2009, Electrical and Computer Engineering

Despite numerous advances in the growth, fabrication, and characterization of AlGaN/GaN HEMT devices, there remain a number of unknowns related to the impact of deep levels on HEMT performance. Of specific interest to ongoing development of HEMT technology is the development of techniques which can not only detect the specific energy levels of deep levels in operating devices, but can also relate the presence of these defects to changes in specific device parameters. By examining more established techniques and developing new on-device characterization methods, the impact of defects on AlGaN/GaN HEMTs was quantitatively studied. Constant-voltage measurements of current and conductance transients are applied to AlGaN/GaN HEMT devices. Current deep level transient spectroscopy revealed two levels, one with an apparent activation energy that ranged from 0.16 eV to 0.31 eV, and one with an activation energy of 0.52 eV. The manifestation of both of these levels was shown to be affected by the magnitude of the gate-drain electric field. Conductance deep level transient spectroscopy reveled a number of traps with energies between EC- 0.20 eV and EC-0.42 eV, with the presence of a complex field profile making it difficult to determine which peaks were due to unique defects and which were due to the same species of defect emitting under multiple field conditions. Current deep level optical spectroscopy and conductance deep level optical spectroscopy were used to identify various deep levels with onsets at EC-1.55 eV, EC-2.55 eV, EC-2.9 eV, EC-3.25 eV, and EC-3.8 eV. These levels were similar to deep levels previously identified by capacitance measurements on similar material. None of these measurements yielded either trap concentrations or the impact of deep levels on parameters. To facilitate quantitative examination of the effect of deep levels on device parameters, the theory of constant-current measurements is developed. By regulating either the drain or gate voltage (open full item for complete abstract)

Committee: Steven A. Ringel PhD (Advisor); Siddharth Rajan PhD (Committee Member) Subjects: Electrical Engineering; Materials Science