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

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 (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, 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, 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, The Ohio State University, 2019, Electrical and Computer Engineering

Beta-phase gallium oxide (β-Ga2O3) is attracting significant interest for high-power electronics and ultraviolet optoelectronics due to its ~4.8 eV wide bandgap, large predicted breakdown field, ability to support β-(Al1-xGax)2O3/Ga2O3 heterojunctions, and the availability of large area, melt-grown native substrates for homoepitaxial growth. There is also continued interest for space-based applications due to its predicted high radiation hardness compared to contemporary wide bandgap materials (III-nitrides and SiC). However, the integration of β-Ga2O3 into prospective applications will largely depend on device design innovations as well as the availability of high quality and low defect-density materials. This is considerably important as crystalline defects can adversely affect material properties critical to device operation, output power, threshold voltage, and carrier mobility by causing carrier compensation, scattering, and trapping effects. Defect-induced degradation can also dictate the entire behavior of β-Ga2O3 devices in their intended space-based applications where exposure to energetic radiation particles is typical, leading to introducing defect states in the bandgap. Furthermore, there is an intense need of understanding of metal/ β-Ga2O3 contact and interface properties to ensure large Schottky barrier height and low leakage current for high power operations. However, despite remarkable early progress, the underlying knowledge of metal contact properties, dopants, electrically active defects, and their role on material properties is still very limited. Hence, this research aims to pursue a comprehensive investigation of defects in β-Ga2O3 bandgap, building from the native β-Ga2O3 substrate to subsequent homoepitaxial layers. Using deep level transient and optical spectroscopy (DLTS/DLOS) techniques, experiments have been undertaken to understand the formation, physical structure, electronic, and optical properties of defects in β-Ga2O3 bandgap, wit (open full item for complete abstract)

Committee: Steven Ringel (Advisor); Wu Lu (Committee Member); Aaron Arehart (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2018, Electrical and Computer Engineering

AlGaN/GaN high electron mobility transistors (HEMTs) and other devices that take advantage of the particular material properties of the III-nitride material system have become important for a range of applications, including high frequency and high voltage electronics. A crucial approach for further improvement of these devices has been the addition of dielectric layers. Successful implementation of these newer device designs relies on understanding the nature and impact of specific defects present within semiconductor layers, and at interfaces between such materials. Crystallographic defects influence electronic and optical material properties, as well as the entire behavior of devices in their intended application. For instance, defects at the interface of insulators and III-nitrides in HEMTs can seriously degrade electron mobility and increase device switching response times, preventing the devices from operating effectively at the target power and frequency. Interface defect states that respond to electrical signals, through the capture or emission of electrons, are quantified as the interface state defect density (Dit). This Dit is inherently related to the chemistry of an interface and has proven very difficult to quantify in a manner specific to individual defect states and physical sources. With the implementation of GaN-based electronics in so many commercial and government applications, advancing the understanding of interface states related to GaN HEMTs has been one of the major goals of the field over the last decade. With this motivation, this dissertation applies a unique combination of techniques to this problem, notably constant capacitance deep level optical spectroscopy (CC-DLOS) and deep level transient spectroscopy (CC-DLTS), with internal photoemission (IPE) and X-ray photoelectron spectroscopy (XPS). The objective is to link electronic details of interface state distributions with local chemical information as a function of systematic variat (open full item for complete abstract)

Committee: Steven Ringel (Advisor); Siddharth Rajan (Committee Member); Tyler Grassman (Committee Member) Subjects: Electrical Engineering; Engineering; Materials Science

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

Doctor of Philosophy, The Ohio State University, 2009, Electrical and Computer Engineering

Although nitride electronics have matured rapidly, the performance and reliability of nitride high electron mobility transistors (HEMT) and other electronic devices have been hampered by electrically active defects that manifest as deep levels in the bandgap and/or as trap states. To alleviate these problems, not only is a fundamental understanding of the defects in GaN and AlGaN necessary for the continued development of nitride electronics, but also correlations of these defects to the performance and reliability limiting problems is required. Using a multipronged effort, defects were quantitatively studied at the component layer level (i.e. GaN and AlGaN) and also in operational HEMTs using techniques uniquely designed to quantitatively characterize defects as deep levels and traps in these devices.Deep level optical spectroscopy and related methods of trap spectroscopy are applied to several sets of systematically varied GaN and AlGaN materials. Traps in GaN were typically located at EC-0.25, EC-0.60, EC-0.90, EC-(1.28-1.35), EC-2.6, and EC-3.22/3.28 and for AlGaN at EC-0.87, EC-1.5, EC-3.10, and EC-3.93. It was determined that several traps showed specific dependencies on variations in growth parameters, substrate orientation, dislocation density, and growth method. Physical sources were attributed to most of these states for the first time, and this taxonomy is essential for analysis of trap effects in working AlGaN/GaN transistors, which constitutes the second focus of this research. To relate defect incorporation with HEMT performance and reliability, constant drain current deep level optical/transient spectroscopies using gate or drain voltage as the feedback mechanism are developed. This enables simultaneous and quantitative measurement of defect energies and concentrations of individual defects throughout the bandgap in HEMTs, measurement of device relevant parameters (threshold voltage shift and the change in gate-drain access resistance), separation of (open full item for complete abstract)

Committee: Steve Ringel (Advisor); Leonard Brillson (Committee Member); Siddharth Rajan (Committee Member) Subjects: Engineering

Doctor of Philosophy (PhD), Ohio University, 2001, Physics (Arts and Sciences)

Deep levels in GaAs, GaN, ScN and SiC , have been investigated using Deep Level Transient Spectroscopy (DLTS). Properties of deep levels, such as electronic behavior, activation energy, capture cross-section and concentration have been calculated. In order to be able to perform DLTS measurements, Schottky or p-n junctions were fabricated from the material of interest. For this, contact formation and characterization has been studied. For each material, several types of contacts have been investigated. The contacts with the best properties in terms of leakage currents, band bending, and interface states density were used for DLTS measurements. GaN materials have been synthesized using metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and rf-sputtering, in an attempt to compare and correlate the existence of the B defect (activation energy of EC-ET =0.59 eV) with the method of growth. Only material grown using MOCVD could be used for DLTS analysis. ScN material grown using plasma assisted physical vapor deposition (PAPVD) and rf-sputtering, has been used with p-type Si to form p-n junctions. Depending upon the method of growth, different defects are found in the material. A defect with activation energy of 0.51 eV has been identified as an electron trap in the PAPVD material and one electron trap with activation energy of 0.91 eV in rf-sputtered material. The influence of substrate annealing upon the deep levels in two SiC polytypes, 4H- n-type SiC and 6H- p-type SiC has been investigated. For each set of annealed samples, several new defects were found (activation energies of EC-ET = 0.41 eV, 0.50 eV for n-type 4H-SiC, EC-ET = 0.37 eV and 0.33 eV for p-type 6H-SiC), all of them being electron traps, with the exception of one hole trap on the 4H-SiC material (ET-EV=0.14 eV). The activation energies range from (0.14-0.50) eV below the conduction band. The nature of five of the found defects is not clear. For all the other defects, their exist (open full item for complete abstract)

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

Master of Science, The Ohio State University, 2013, Electrical and Computer Engineering

Highly scaled III-V compound semiconductors, particularly n-In0,53Ga0,47As in conjunction with a suitable high-¿ dielectric, has been regarded as a promising channel material for high performance metal-oxide-semiconductor field effect transistors (MOSFETs). The high intrinsic electron mobility and small band gap of n-In0,53Ga0,47As offers the possibility of developing MOSFETs with higher drive currents at low operation voltages. However, the high density of interface states (Dit) at the high-¿/n-In0,53Ga0,47As interface degrades the device performance. Therefore, accurate and quantitative characterization of the interface states is an important issue in the continued development of high quality interfaces to track changes induced by processing and growth optimization. This work demonstrates that high Dit concentrations from high-¿/semiconductor interfaces can be accurately characterized using constant capacitance deep level transient spectroscopy and low temperature C-V (LTCV) method. This is compared with the conductance method, which underestimates Dit magnitude and shows energy dependent distribution.

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