Jordan D. Greenlee

Substrate-Dependent Effects on the Response of AlGaN/GaN HEMTs to 2-MeV Proton Irradiation

AlGaN/GaN high electron mobility transistors grown on Si, SiC, and sapphire substrates were exposed to 2-MeV proton irradiation in incremental fluences up to 6 × 1014 cm-2. The devices were characterized initially and after each irradiation by Hall and dc I-V measurements to probe the mechanisms associated with radiation-induced degradation and failure. It was determined that defects created at the AlGaN/GaN interface introduce scattering centers near the two-dimensional electron gas (2DEG), which result in degraded mobility. Additionally, charged traps in the structure serve to screen the 2DEG resulting in reduced sheet carrier density. These two effects are responsible for degraded I-V behavior, including reduced saturation current and transconductance, increased ON-resistance, and positive threshold voltage shift. Interestingly, the sample with the most pre-existing defects was the most tolerant of radiation-induced damage.

Structural, Optical, and Electrical Characterization of Monoclinic β-Ga2O3 Grown by MOVPE on Sapphire Substrates

Epitaxial growth of monoclinic β-Ga2O3 on a-plane and c-plane sapphire substrates by metalorganic vapor-phase epitaxy (MOVPE) is reported. Crystalline phase, growth rate (∼150xa0nm/h), and energy gap (∼4.7xa0eV) were determined by x-ray diffraction and optical reflectance measurements. Film density of ∼5.6xa0g/cm3 measured by x-ray reflectivity suggests the presence of vacancies, and the O-rich growth regime implies the presence of Ga vacancies in the films. O/Ga ratio of 1.13, as measured by XPS for Ga2O3 grown on c-plane Al2O3, suggests that, near the surface, the film is O-deficient. Atomic force microscopy revealed smoother, smaller grain size when films were grown on c-plane Al2O3. Raman spectroscopy suggested inclusions of α-Ga2O3, likely present at the sapphire interface due to growth on nonnative substrate. Samples of β-Ga2O3 were selectively implanted with Si in the source/drain regions and subsequently annealed at 1000°C for 10xa0min. Normally-off transistors (VTxa0≅xa04.7xa0V) with 20-nm-thick Al2O3 gate oxide were fabricated, and a maximum drain–source current of 4.8xa0nA was measured.

Observation and control of the surface kinetics of InGaN for the elimination of phase separation

The growth of InGaN alloys via Metal-Modulated Epitaxy has been investigated. Transient reflection high-energy electron diffraction intensities for several modulation schemes during the growth of 20% InGaN were analyzed, and signatures associated with the accumulation, consumption, and segregation of excess metal adlayers were identified. A model for shuttered, metal-rich growth of InGaN was then developed, and a mechanism for indium surface segregation was elucidated. It was found that indium surface segregation only occurs after a threshold of excess metal is accumulated, and a method of quantifying this indium surface segregation onset dose is presented. The onset dose of surface segregation was found to be indium-composition dependent and between 1 and 2 monolayers of excess metal. Below this surface threshold off excess metal, metal-rich growth can occur without indium surface segregation. Since at least 2 monolayers of excess metal will accumulate in the case of metal-rich, unshuttered growth of InG...

Effect of Reduced Extended Defect Density in MOCVD Grown AlGaN/GaN HEMTs on Native GaN Substrates

AlGaN/GaN high-electron mobility transistor (HEMT) structures were grown by metal-organic chemical vapor deposition on SiC, hydride vapor phase epitaxy (HVPE) GaN, and ammonothermal GaN substrates to achieve HEMTs with over five orders of magnitude variation in extended defect density. This enables a direct comparison of the effect of extended defects on device performance to achieve the best possible reliability. As-grown material was characterized by atomic force microscopy, electron channeling contrast imaging, and Raman spectroscopy. Devices were characterized by Hall, dc I-V, and pulsed I-V behavior. Reduced threading dislocation density provides an increased 2-D electron gas mobility, but inhibits ohmic contact formation resulting in high contact resistance. Transistor characteristics were nominally identical, with higher OFF-state leakage in the HEMTs on ammonothermal GaN. The pulsed I-V response indicated significantly reduced current collapse in the HEMT on HVPE GaN due to reduced buffer trapping.

Comparison of Interfacial and Bulk Ionic Motion in Analog Memristors

Analog LiNbO2 memristors are characterized using potentiodynamic electrochemical impedance spectroscopy. It is shown that LiNbO2-based devices exhibit analog memristive and meminductive or memcapacitive behavior depending on the applied frequency. The impedance spectra are fit to a circuit model whose elements correspond to ionic and electronic effects in the bulk and at the electrode-semiconductor interface. By separating out the bulk and interfacial effects, it is shown that the majority of the ionic movement in the cation-based analog memristor is at the interfaces. It is also shown that ionic capacitance results from a drift field-driven phase shift between the ac bias and the ionic motion, whereas the ionic inductance phase shift results from diffusion relaxation. Thus, to maximize resistance changes due to this ionic movement, analog memristors should be fabricated with negligible bulk dimensions.

Proton Radiation-Induced Void Formation in Ni/Au-Gated AlGaN/GaN HEMTs

AlGaN/GaN high-electron mobility transistors (HEMTs) were exposed to 2-MeV protons irradiation, at room temperature, up to a fluence of 6 × 1014 H+/cm2. Aside from degradation resulting from radiation-induced charge trapping, transmission electron microscopy and electrical measurements reveal a radiation-induced defect located at the edges of the Ni/Au Schottky gate in the proton-irradiated devices. At the edges of the Ni/Au gate, the Ni of the Ni/Au gate diffused up into the Au layer and migrated into the AlGaN barrier, leaving voids in the Ni layer at the gate edges after irradiation. These radiation-induced voids are caused by diffusion of Ni through vacancy exchange, known as the Kirkendall effect, resulting in reduced gate area and degrading the HEMT performance.

In-situ oxygen x-ray absorption spectroscopy investigation of the resistance modulation mechanism in LiNbO2 memristors

In situ near edge x-ray absorption fine structure spectroscopy (NEXAFS) is performed on LiNbO2 analog memristors to identify the underlying analog resistance modulation mechanism. Empty electronic state gradients in the NEXAFS difference spectra are observed in biased devices indicating a gradual movement of lithium. This movement of lithium supports the assertion that simple ion dopant drift and diffusion dominate the analog memristor’s resistance response. By identifying the physical memristance mechanism in analog LiNbO2 memristors, suggestions are made for additions to the memristor to modify device performance for both neuromorphic computing and memory applications.

Ultraviolet detector based on graphene/SiC heterojunction

There has been significant research on graphene as a sensor owing to the inherent high sensitivity and surface area associated with two-dimensional (2D) materials. Often, the ability of graphene to form heterojunctions with wide-bandgap semiconductors is overlooked. In this study, we present a detector based on an epitaxial graphene/SiC heterojunction, exploiting the 2D nature of graphene to minimize absorption losses for high-efficiency sensing while simultaneously taking advantage of the epitaxial p–n junction to achieve low reverse leakage. We measured a quantum efficiency above 80% at 4 eV using a graphene/SiC p–n heterojunction with a dark current <1 nA/cm2.

Degradation mechanisms of 2 MeV proton irradiated AlGaN/GaN HEMTs

Proton-induced damage in AlGaN/GaN HEMTs was investigated using energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM), and simulated using a Monte Carlo technique. The results were correlated to electrical degradation using Hall measurements. It was determined by EDS that the interface between GaN and AlGaN in the irradiated HEMT was broadened by 2.2u2009nm, as estimated by the width of the Al EDS signal compared to the as-grown interface. The simulation results show a similar Al broadening effect. The extent of interfacial roughening was examined using high resolution TEM. At a 2u2009MeV proton fluence of 6 × 1014 H+/cm2, the electrical effects associated with the Al broadening and surface roughening include a degradation of the ON-resistance and a decrease in the electron mobility and 2DEG sheet carrier density by 28.9% and 12.1%, respectively.

Multicycle rapid thermal annealing optimization of Mg-implanted GaN: Evolution of surface, optical, and structural properties

The first step of a multi-cycle rapid thermal annealing process was systematically studied. The surface, structure, and optical properties of Mg implanted GaN thin films annealed at temperatures ranging from 900 to 1200u2009°C were investigated by Raman spectroscopy, photoluminescence, UV-visible spectroscopy, atomic force microscopy, and Nomarski microscopy. The GaN thin films are capped with two layers of in-situ metal organic chemical vapor deposition -grown AlN and annealed in 24 bar of N2 overpressure to avoid GaN decomposition. The crystal quality of the GaN improves with increasing annealing temperature as confirmed by UV-visible spectroscopy and the full widths at half maximums of the E2 and A1 (LO) Raman modes. The crystal quality of films annealed above 1100u2009°C exceeds the quality of the as-grown films. At 1200u2009°C, Mg is optically activated, which is determined by photoluminescence measurements. However, at 1200u2009°C, the GaN begins to decompose as evidenced by pit formation on the surface of the samp...