arXiv subjects

Debbie G. Senesky

Publications and source records attributed to Debbie G. Senesky.

18 recordsLinked to original sources

arXiv2026

AlGaN/GaN Hall-Effect Sensor for In-Situ Magnetic Field Monitoring of the HSX Stellarator

Direct magnetic field sensors can address integration drift commonly observed in conventional inductive magnetic diagnostics used in fusion systems. In this work, an AlGaN/GaN Hall-effect sensor was fabricated, packaged, and deployed inside the Helically Symmetric eXperiment (HSX)---the first quasi-helically symmetric stellarator, operating with a 1 T on-axis magnetic field and up to 200 kW of launched electron cyclotron resonance heating (ECRH) power---for in-situ magnetic field monitoring near the plasma edge. The sensor leverages the high-mobility two-dimensional electron gas (2DEG) formed in the AlGaN/GaN heterostructure for sensitive magnetic field measurement, while the wide-bandgap GaN material system provides thermal robustness for harsh-environment operation. During 68 consecutive plasma discharge shots, the sensor remained functional and produced clear transient responses associated with plasma ignition and discharge dynamics. Comparisons between biased and unbiased operation, as well as plasma-discharge and coil-only shots, confirmed that the response originated from the biased Hall-effect sensor element. Furthermore, the sensor output exhibited temporal correlation with the plasma stored energy measured by the HSX diamagnetic loop across high-energy, late-breakdown, and failed-breakdown discharges.

physics.ins-det↗

arXiv2024

Thermal conductivity of macroporous graphene aerogel measured using high resolution comparative infrared thermal microscopy

Graphene aerogel (GA) is a promising material for thermal management applications across many fields due to its lightweight and thermally insulative properties. However, standard values for important thermal properties, such as thermal conductivity, remain elusive due to the lack of reliable characterization techniques for highly porous materials. Comparative infrared thermal microscopy (CITM) is an attractive technique to obtain thermal conductance values of porous materials like GA, due to its non-invasive character, which requires no probing of, or contact with, the often-delicate structures and frameworks. In this study, we improve upon CITM by utilizing a higher resolution imaging setup and reducing the need for pore-filling coating of the sample (previously used to adjust for emissivity). This upgraded setup, verified by characterizing porous silica aerogel, allows for a more accurate confirmation of the fundamental thermal conductivity value of GA while still accounting for the thermal resistance at material boundaries. Using this improved method, we measure a thermal conductivity below 0.036 W/m$\cdot$K for commercial GA using multiple reference materials. These measurements demonstrate the impact of higher resolution thermal imaging to improve accuracy in low density, highly porous materials characterization. This study also reports thermal conductivity for much lower density (less than 15 mg/cm$^3$) GA than previously published studies while maintaining the robustness of the CITM technique.

physics.app-ph↗

arXiv2023

Effect of geometry on the frequency limit of GaAs/AlGaAs 2-Dimensional Electron Gas (2DEG) Hall effect sensors

In this work, we experimentally investigate the frequency limit of Hall effect sensor designs based on a 2 dimensional electron gas (2DEG) gallium arsenide/aluminum gallium arsenide (GaAs/AlGaAs) heterostructure. The frequency limit is measured and compared for four GaAs/AlGaAs Hall effect sensor designs where the Ohmic contact length (contact geometry) is varied across the four devices. By varying the geometry, the trade-off in sensitivity and frequency limit is explored and the underlying causes of the frequency limit from the resistance and capacitance perspective is investigated. Current spinning, the traditional method to remove offset noise, imposes a practical frequency limit on Hall effect sensors. The frequency limit of the Hall effect sensor, without current spinning, is significantly higher. Wide-frequency Hall effect sensors can measure currents in power electronics that operate at higher frequencies is one such application.

physics.ins-det↗

arXiv2021

Effects of Proton Irradiation on Hole Carrier Transport in Hydrogen-Terminated Diamond Surfaces

In this letter, we report the effects of irradiating hydrogen-terminated diamond surfaces with protons. We study the electrical behavior of the two-dimensional hole gas that forms on the diamond surface as a result of hydrogen-termination. Four diamond samples, two of which were passivated with Al2O3 and the other two unpassivated, were irradiated with 2 MeV protons at two fluences, 0.5x10^14 and 1.0x10^14 /cm^2. The hole conductivity degraded at a higher proton fluence, which is attributed to the reduction of the mobility caused by enhanced hole scattering. Both passivated and unpassivated samples exhibited a reduction in the mobility, which can be explained by charging effects of the Al2O3 (for the passivated samples) and degradation of the hydrogen-terminated surface (for the unpassivated samples). To our knowledge, this is the first reported data on 2 MeV proton tolerance of hydrogen-terminated diamond surfaces.

cond-mat.mtrl-sci↗

arXiv2020

Analysis of the Mobility-Limiting Mechanisms of the Two-Dimensional Hole Gas on Hydrogen-Terminated Diamond

Here we present an analysis of the mobility-limiting mechanisms of a two-dimensional hole gas on hydrogen-terminated diamond surfaces. The scattering rates of surface impurities, surface roughness, non-polar optical phonons, and acoustic phonons are included. Using a Schrodinger/Poisson solver, the heavy hole, light hole, and split-off bands are treated separately. To compare the calculations with experimental data, Hall-effect structures were fabricated and measured at temperatures ranging from 25 to 700 K, with hole sheet densities ranging from 2 to 6$\times10^{12}\;\text{cm}^{-2}$ and typical mobilities measured from 60 to 100 cm$^{2}$/(V$\cdot$s) at room temperature. Existing data from literature was also used, which spans sheet densities above 1$\times10^{13}\;\text{cm}^{-2}$. Our analysis indicates that for low sheet densities, surface impurity scattering by charged acceptors and surface roughness are not sufficient to account for the low mobility. Moreover, the experimental data suggests that long-range potential fluctuations exist at the diamond surface, and are particularly enhanced at lower sheet densities. Thus, we propose a second type of surface impurity scattering which is caused by disorder related to the C-H dipoles.

physics.app-ph↗

arXiv2019

Molybdenum Trioxide Gates for Suppression of Leakage Current in InAlN/GaN HEMTs at 300°C

Because high electron mobility transistors (HEMTs) often exhibit significant gate leakage during high-temperature operation, the choice of Schottky metal is critical. Increased gate leakage and reduced ON/OFF ratio are unsuitable for the design of high-temperature electronics and integrated circuits. This paper presents high-temperature characteristics of depletion-mode molybdenum trioxide (MoO${_3}$)-gated InAlN/GaN-on-silicon HEMTs in air. After a room temperature oxidation of the Mo for 10 weeks, the leakage of the HEMT is reduced over 60 times compared to the as-deposited Mo. The use of MoO${_3}$ as the Schottky gate material enables low gate leakage, resulting in a high ON/OFF current ratio of 1.2 x 10${^8}$ at 25°C and 1.2 x 10${^5}$ at 300°C in air. At 400°C, gate control of the InAlN/GaN two-dimensional electron gas (2DEG) channel is lost and unrecoverable. Here, this permanent device failure is attributed to volatilization of the MoO${_3}$ gate due to the presence of water vapor in air. Passivation of the device with SiN enables operation up to 500°C, but also increases the leakage current. The suppression of gate leakage via Mo oxidation and resulting high ON/OFF ratio paves the way for viable high-temperature GaN-based electronics that can function beyond the thermal limit of silicon once proper passivation is achieved.

physics.app-ph↗

arXiv2019

A Laterally Vibrating Lithium Niobate MEMS Resonator Array Operating at 500°C in Air

This paper is the first report of the high-temperature characteristics of a laterally vibrating piezoelectric lithium niobate (LiNbO$_{3}$) MEMS resonator array up to 500°C in air. After a high-temperature burn-in treatment, device quality factor (Q) is enhanced to 508 and the resonance shifts to a lower frequency and remains stable up to 500°C. During subsequent in situ high-temperature testing, the resonant frequencies of two coupled shear horizontal (SH0) modes in the array are 87.36 MHz and 87.21 MHz at 25°C and 84.56 MHz and 84.39 MHz at 500°C, correspondingly, representing a -3% shift in frequency over the temperature range. Upon cooling to room temperature, the resonant frequency returns to 87.36 MHz, demonstrating recoverability of device performance. The first- and second-order temperature coefficient of frequency (TCF) are found to be -95.27 ppm/°C and 57.5 ppb/°C$^{2}$ for resonant mode A, and -95.43 ppm/°C and 55.8 ppb/°C$^{2}$ for resonant mode B, respectively. The temperature-dependent quality factor (Q) and electromechanical coupling coefficient ($k_{t}^{2}$) are extracted and reported. Device Q decreases to 334 after high-temperature exposure, while $k_{t}^{2}$ increases to 12.40%. This work supports the use of piezoelectric LiNbO$_{3}$ as a material platform for harsh environment radio-frequency (RF) resonant sensors (e.g. temperature and infrared).

physics.app-ph↗

arXiv2019

Effect of Proton Irradiation Temperature on Zinc Oxide Metal-Semiconductor-Metal Ultraviolet Photodetectors

The electrical and structural characteristics of 50 nm zinc oxide (ZnO) metal-semiconductor-metal (MSM) ultraviolet (UV) photodetectors subjected to proton irradiation at different temperatures are reported and compared. We irradiated the devices with 200 keV protons to a fluence of 1016 cm-2. Examination of the X-ray diffraction (XRD) rocking curves indicates a strongly preferred (100) orientation for the grains of the as-deposited film, with decreases in crystal quality for all irradiated samples. In addition, peak shifts in XRD and Raman spectra of the control sample relative to well-known theoretical positions are indicative of tensile strain in the as-deposited ZnO films. We observed shifts of these peaks towards theoretical unstrained positions in the irradiated films relative to the as-deposited film indicate partial relaxation of this strain. Raman spectra also indicate increases of oxygen vacancies (V_O ) and zinc interstitials (Zn_i ) relative to the control sample. Additionally, photocurrent versus time measurements showed up to 2x increases in time constants for samples irradiated at lower temperatures months after irradiation, indicating that the defects introduced by suppression of thermally-activated dynamic annealing process has a long-term deleterious effect on device performance.

physics.app-ph↗

arXiv2019

Temperature Dependence of Sensitivity of 2DEG-Based Hall-Effect Sensors

The magnetic sensitivity of Hall-effect sensors made of InAlN/GaN and AlGaN/GaN heterostructures was measured between room temperature and 576°C. Both devices showed decreasing voltage-scaled magnetic sensitivity at high temperature, declining from 53 to 8.3 mV/V/T for the InAlN/GaN sample and from 89 to 8.5 mV/V/T for the AlGaN/GaN sample, corresponding to the decreasing electron mobility due to scattering effects at elevated temperatures. Alternatively, current-scaled sensitivities remained stable over the temperature range, only varying by 13.1% from the mean of 26.3 V/A/T and 10.5% from the mean of 60.2 V/A/T for the InAlN/GaN and AlGaN/GaN samples respectively. This is due to the minimal temperature dependence of the electron sheet density on the 2-dimensional electron gas (2DEG). Both devices showed consistency in their voltage- and current-scaled sensitivity over multiple temperature cycles as well as nearly full recovery when returned to room temperature after thermal cycling. Additionally, an AlGaN/GaN sample held at 576°C for 12 hours also showed nearly full recovery at room temperature, further suggesting that GaN-based Hall-effect sensors are a good candidate for use in high temperature applications.

physics.ins-det↗

arXiv2019

Significant Phonon Drag Enables High Power Factor in the AlGaN/GaN Two-Dimensional Electron Gas

In typical thermoelectric energy harvesters and sensors, the Seebeck effect is caused by diffusion of electrons or holes in a temperature gradient. However, the Seebeck effect can also have a phonon drag component, due to momentum exchange between charge carriers and lattice phonons, which is more difficult to quantify. Here, we present the first study of phonon drag in the AlGaN/GaN two-dimensional electron gas (2DEG). We find that phonon drag does not contribute significantly to the thermoelectric behavior of devices with ~100 nm GaN thickness, which suppress the phonon mean free path. However, when the thickness is increased to ~1.2 $μ$m, up to 32% (88%) of the Seebeck coefficient at 300 K (50 K) can be attributed to the drag component. In turn, the phonon drag enables state-of-the-art thermoelectric power factor in the thicker GaN film, up to ~40 mW m$^{-1}$ K$^{-2}$ at 50 K. By measuring the thermal conductivity of these AlGaN/GaN films, we show that the magnitude of the phonon drag can increase even when the thermal conductivity decreases. Decoupling of thermal conductivity and Seebeck coefficient could enable important advancements in thermoelectric power conversion with devices based on 2DEGs.

cond-mat.mtrl-sci↗

arXiv2019

Stable Operation of AlGaN/GaN HEMTs at 400$^\circ$C in air for 25 hours

In this letter, we report the operation of AlGaN/GaN HEMTs with Pd gates in air over a wide temperature range from 22$^\circ$C to 500$^\circ$C. The variation in the threshold voltage ($V_{th}$) is less than 1$\%$ over the entire temperature range. Moreover, a safe biasing region where the transconductance peak ($g_m$) occurs over the entire temperature range was observed, enabling high-temperature analog circuit design. Furthermore, the operation of the devices over 25 hours was experimentally studied, demonstrating the stability of the DC characteristics and $V_{th}$ at 400$^\circ$C. Finally, the degradation mechanisms of HEMTs at 500$^\circ$C over 25 hours of operation are discussed, and are shown to be associated with the 2DEG sheet density and mobility decrease.

physics.app-ph↗

arXiv2019

Gallium Nitride Photodetector Measurements of UV Emission from a Gaseous CH4/O2 Hybrid Rocket Igniter Plume

Owing to its wide (3.4 eV) and direct-tunable band gap, gallium nitride (GaN) is an excellent material platform for UV photodetectors. GaN is also stable in radiation-rich and high-temperature environments, which makes photodetectors fabricated using this material useful for in-situ flame detection and combustion monitoring. In this paper, we use a GaN photodetector to measure ultraviolet (UV) emissions from a hybrid rocket motor igniter plume. The normalized photocurrent-to-dark current ratio (NPDR) is a performance metric which simultaneously captures the two desired characteristics of high responsivity and low dark current. The NPDR of our device is record-high with a value of 6 x 10$^{14}$ W$^{-1}$ and the UV-to-visible rejection ratio is 4 x 10$^6$. The photodetector shows operation at high temperatures (up to 250°C), with the NPDR still remaining above 10$^9$ W$^{-1}$ and the peak wavelength shifting from 362 nm to 375 nm. The photodetector was placed at three radial distances (3", 5.5", and 7") from the base of the igniter plume and the oxidizer-to-fuel ratio (O2/CH4) was varied. The data demonstrates a clear trend of increasing current (and thus intensity of plume emission) with increasing fuel concentration and decreasing separation between the photodetector and the plume. By treating the plume as a black body, and calculating a radiative configuration factor corresponding to the geometry of the plume and the detector, we calculated average plume temperatures at each of the three oxidizer-to-fuel ratios. The estimated plume temperatures were between 850 and 950 K for all three combustion conditions. The temperature is roughly invariant for a fixed fuel concentration for the three tested distances. These data demonstrate the functionality of GaN as a material platform for use in harsh environment flame monitoring.

physics.ins-det↗

arXiv2018

Micro-Tesla Offset in Thermally Stable AlGaN/GaN 2DEG Hall-effect Plates using Current Spinning

This letter describes the characterization of a low-offset Hall-effect plate using the AlGaN/GaN two-dimensional electron gas(2DEG). Four-phase current spinning was used to reduce sensor offset voltage to values in the range of 20 nV, which corresponds to a low residual offset of 2.6 micro-Tesla when supplied with low voltages (0.04 to 0.5V). These offsets are 50x smaller than the values previously reported for GaN Hall-effect plates, and it is on par with state-of-the-art silicon Hall-effect plates. In addition, the offset does not exceed 10 micro-Tesla even at higher supply voltage of 2.34V. The sensor also shows stable current-scaled sensitivity over a wide temperature range of -100C to 200C, with temperature drift of -125 ppm/C. This value is 3x better than state-of-the-art Silicon Hall-effect plates. Additionally, the sensor's voltage sensitivity (57 mV/V/T) is also similar. Because of their low offset values, AlGaN/GaN Hall-effect plates are viable candidates for low-field and high temperature magnetic sensing in monolithic GaN systems used in extreme temperature environments such as power inverter, down-well, combustion, and space applications.

physics.app-ph↗

arXiv2018

Effect of Geometry on Sensitivity and Offset of AlGaN/GaN and InAlN/GaN Hall-effect Sensors

The current- and voltage-scaled sensitivities and signal-to-noise ratios (SNR) (with respect to thermal noise) of various octagonal AlGaN/GaN and InAlN/GaN Hall-effect sensors were examined in this work. The effect of metal contact lengths on sensitivity and sensor offset was evaluated. Calculations that take into account the shape of the device show that devices with point-like contacts have the highest current-scaled sensitivity (68.9 V/A/T), while devices with contacts of equal length to their non-contact sides have the highest voltage-scaled sensitivity (86.9 mV/V/T). The sensitivities of the two other devices follow the predicted trends closely. All the devices have offsets less than 20 $μ$T at low supply current operation (< 300 $μ$A) and most remain below 35 $μ$T at higher supply current (up to 1.2 mA). The consistent low offsets across the devices imply that the choice of Hall-effect sensor geometry should mainly depend on whether the device is current-biased or voltage-biased and the frequency at which it will operate. This work demonstrates that GaN Hall-effect sensor performance can be improved by adjusting the geometry of the Hall-effect plate specific to its function (e.g., power electronics, navigation, automotive applications).

physics.ins-det↗

arXiv2018

High-temperature Ultraviolet Photodetectors: A Review

Wide bandgap semiconductors have become the most attractive materials in optoelectronics in the last decade. Their wide bandgap and intrinsic properties have advanced the development of reliable photodetectors to selectively detect short wavelengths (i.e., ultraviolet, UV) in high temperature regions (up to 300°C). The main driver for the development of high-temperature UV detection instrumentation is in-situ monitoring of hostile environments and processes found within industrial, automotive, aerospace, and energy production systems that emit UV signatures. In this review, a summary of the optical performance (in terms of photocurrent-to-dark current ratio, responsivity, quantum efficiency, and response time) and uncooled, high-temperature characterization of III-nitride, SiC, and other wide bandgap semiconductor UV photodetectors is presented.

physics.app-ph↗

arXiv2018

High responsivity, low dark current ultraviolet photodetector based on AlGaN/GaN interdigitated transducer

An ultraviolet (UV) photodetector employing the two-dimensional electron gas (2DEG) formed at the AlGaN/GaN interface as an interdigitated transducer (IDT) is characterized under optical stimulus. The 2DEG-IDT photodetector exhibits a record high normalized photocurrent-to-dark current ratio (NPDR, $6\times10^{14}$). In addition, we observe a high responsivity ($7,800$ A/W) and ultraviolet-visible rejection-ratio ($10^{6}$), among the highest reported values for any GaN photodetector architecture. We propose a gain mechanism to explain the high responsivity of this device architecture, which corresponds to an internal gain of $26,000$. We argue that the valence band offset in the AlGaN/GaN heterostructure is essential in achieving this high responsivity, allowing for large gains without necessitating the presence of trap states, in contrast to common metal-semiconductor-metal (MSM) photodetector architectures. Our proposed gain mechanism is consistent with measurements of the scaling of gain with device channel width and incident power. In addition to high performance, this photodetector architecture has a simple two-step fabrication flow that is monolithically compatible with AlGaN/GaN high electron mobility transistor (HEMT) processing. This unique combination of low dark current, high responsivity and compatibility with HEMT processing is attractive for a variety of UV sensing applications.

physics.app-ph↗

arXiv2018

Strain effect in highly-doped n-type 3C-SiC-on-glass substrate for mechanical sensors and mobility enhancement

This work reports the strain effect on the electrical properties of highly doped n-type single crystalline cubic silicon carbide (3C-SiC) transferred onto a 6-inch glass substrate employing an anodic bonding technique. The experimental data shows high gauge factors of -8.6 in longitudinal direction and 10.5 in transverse direction along the [100] orientation. The piezoresistive effect in the highly doped 3C-SiC film also exhibits an excellent linearity and consistent reproducibility after several bending cycles. The experimental result was in good agreement with the theoretical analysis based on the phenomenon of electron transfer between many valleys in the conduction band of n-type 3C-SiC. Our finding for the large gauge factor in n-type 3C- SiC coupled with the elimination of the current leak to the insulated substrate could pave the way for the development of single crystal SiC-on-glass based MEMS applications.

physics.app-ph↗

arXiv2018

Tuning Electrical and Thermal Transport in AlGaN/GaN Heterostructures via Buffer Layer Engineering

Over the last decade, progress in wide bandgap, III-V materials systems based on gallium nitride (GaN) has been a major driver in the realization of high power and high frequency electronic devices. Since the highly conductive, two-dimensional electron gas (2DEG) at the AlGaN/GaN interface is based on built-in polarization fields (not doping) and is confined to very small thicknesses, its charge carriers exhibit much higher mobilities in comparison to their doped counterparts. In this study, we show that this heterostructured material also offers the unique ability to manipulate electrical transport separately from thermal transport through the examination of fully-suspended AlGaN/GaN diaphragms of varied GaN buffer layer thicknesses. Notably, we show that ~$100$ nm thin GaN layers can considerably impede heat flow without electrical transport degradation, and that a significant improvement (~$4$x) in the thermoelectric figure of merit ($\it zT$) over externally doped GaN is observed in 2DEG based heterostructures. We also observe state-of-the art thermoelectric power factors ($4-7\times$ $10^{-3}$$\,Wm^{-1}K^{-2}$) at room temperature) in the 2DEG of this material system. This remarkable tuning behavior and thermoelectric enhancement, elucidated here for the first time in a polarization-based heterostructure, is achieved since the electrons are at the heterostructured interface, while the phonons are within the material system. These results highlight the potential for using the 2DEG in III-V materials for on-chip thermal sensing and energy harvesting.

cond-mat.mtrl-sci↗