Vacancy Exchange Research Articles

Exploration of structural influences on the ferroelectric switching characteristics of ferroelectric thin-film transistors.

In this paper, quantitative analysis was performed focusing on the structural effect on the ferroelectric switching of ferroelectric thin-film transistors (FeTFTs). FeTFTs and ferroelectric capacitor (FeCap) test element groups (TEGs) were designed and fabricated, and positive-up-negative-down (PUND) measurements were performed to analyze the switching characteristics of ferroelectric films in various structures constituting an FeTFT. It was verified that TiN/HZO/a-IGZO/Mo (MFSM, FeTFT source/drain contact) mostly contributed to the memory operation of an FeTFT, while TiN/HZO/a-IGZO (MFS, FeTFT channel) exhibits one-time memory operation with irreversible polarization switching. In addition, the switching characteristics of MFSM and MFS were different from those of MFM, especially after a few cycles, related to the oxygen vacancy migration between a-IGZO channels and HZO films. The extracted 2Pr values for MFS, MFSM and TiN/HZO/Mo (MFM, FeTFT source/drain parasitic capacitor) were 38, 28 and 20 [μC cm-2], respectively. Based on the operation differences according to the device structure, it was found that irreversible switching in the MFS region (channel) causes a rapid decrease in the memory window after the first switching in an FeTFT and degradation of a-IGZO and HZO films in the MFSM region (contact) including oxygen vacancy exchange and related defect generation causes subthreshold slope increases and negative threshold voltage shifts as cycling stress was applied.

Effect of Oxygen Exchange between Two Oxide Layers of a Memristive Bilayer Valence-Change Memory Cell on the Switching Polarity

Valence-change memory (VCM) cells are promising candidates for future nonvolatile memory devices. A special setup of VCM devices consists of bilayer cells where two thin oxide layers are placed in between two metal electrodes. One oxide layer serves as a tunnel barrier, whereas the second oxide layer is a highly doped conductive semiconductor. Experiments show that an exchange of oxygen between the two layers changes the resistance of the cell. However, the exchange process and how it influences the resistance is not well understood yet. With a drift-diffusion model for electrons and oxygen vacancies, we investigate the movement and exchange of oxygen vacancies and their influence on the band structure as well as on the shape of the tunnel barrier. The simulation results show that a high oxygen-vacancy concentration lowers the height of the tunnel barrier; thus it increases the conductivity of the bilayer cell. The effect of the band lowering is stronger in materials with low permittivity. Hence, two different resistance states evolve if there is an exchange of oxygen between the two oxide layers with different permittivities. Thereby, the switching polarity depends on the relation of the permittivities of the two oxide layers. Furthermore, it is revealed that resistance switching can be induced by the movement of vacancies only inside the conductive oxide, without any oxygen exchange between the layers.

Improved Resistive Switching with Low-Power Synaptic Behaviors of ZnO/Al2O3 Bilayer Structure.

In this work, the resistive switching behavior of bilayer ZnO/Al2O3-based resistive-switching random access memory (RRAM) devices is demonstrated. The polycrystalline nature of the ZnO layer confirms the grain boundary, which helps easy oxygen ion diffusion. Multilevel resistance states were modulated under DC bias by varying the current compliance from 0.1 mA to 0.8 mA, the SET operations where the low resistance state of the memristor device was reduced from 25 kΩ to 2.4 kΩ. The presence of Al2O3 acts as a redox layer and facilitates oxygen vacancy exchange that demonstrates stable gradual conductance change. Stepwise disruption of conductive filaments was monitored depending on the slow DC voltage sweep rate. This is attributed to the atomic scale modulation of oxygen vacancies with four distinct reproducible quantized conductance states, which shows multilevel data storage capability. Moreover, several crucial synaptic properties such as potentiation/depression under identical presynaptic pulses and the spike-rate-dependent plasticity were implemented on ITO/ZnO/Al2O3/TaN memristor. The postsynaptic current change was monitored defining the long-term potentiation by increasing the presynaptic stimulus frequency from 5 Hz to 100 Hz. Moreover, the repetitive pulse voltage stimulation transformed the short-term plasticity to long-term plasticity during spike-number-dependent plasticity.

Djurleite Copper Sulfide-Coupled Cobalt Sulfide Interface for a Stable and Efficient Electrocatalyst.

Transition metal sulfides (TMS) exhibit proliferated edge sites, facile electrode kinetics, and improved intrinsic electrical conductivity, which demand low potential requirements for total water splitting application. Here, we have propounded copper sulfide-coupled cobalt sulfide nanosheets grown on 3D nickel as an electrocatalyst for hydrogen (HER) and oxygen evolution (OER) reactions. The formation of djurleite copper sulfide with a Cu vacancy enables faster H+ ion transport and shows improved HER activity with a remarkably lower overpotential of 164 mV at 10 mA/cm2, whereas cobalt-incorporated copper sulfide undergoes cation exchange during synthesis and shows elevated OER activity with a lower overpotential of 240 mV at 10 mA/cm2 for the OER. Moreover, Cu2-xS/Co is said to have a hybrid CoS-CoS2 interface and provide Co2+ active sites on the surface and enable the fast adsorption of intermediate species (OH*, O*, and OOH*), which lowers the potential requirement. The copper vacancy and cation exchange with a hybrid CoS-CoS2 structure are helpful in supplying more surface reactive species and faster ion transport for the HER and OER, respectively. The full-cell electrolyzer requires a very low potential of 1.58 V to attain a current density of 10 mA/cm2, and it shows excellent stability for 50 h at 100 mA/cm2 as confirmed by the chronopotentiometry test.

Plasma-Enhanced High Temperature Solid-Oxide Electrolysis Cells: The Search for Synergy

High temperature solid-oxide electrolysis cells (SOECs) are a promising conversion technology to produce energy-rich molecules such as CO from CO2 [Song2019]. The high energy efficiency and separation of product streams intrinsic to oxygen ion-conducting electrolyte materials are desirable for renewable synthetic fuel and value-added chemical production at industrial scale.While most improvement in SOEC performance in recent years has been achieved by advancements in electrode materials and cell design, the use of plasmas to enhance SOEC performance also shows potential [Mori2017, Patel2019]. It is reported that the oxygen exchange rate for yttria-stabilized zirconia (YSZ) is 100 times higher simply by exposure to plasma [Rohnke2004]. Despite the apparent synergy in such plasma-SOEC approaches, the underlying mechanisms of plasma and surface processes are presently not well understood. A common explanation is that plasma-activated species (e.g., by vibrational excitation, plasma-dissociation and ionization) support the reduction kinetics on the surface. The effects of plasma-induced surface charging and local fields on, for instance, the vacancy concentration and exchange kinetics may also play a significant role. Dedicated experiments aimed to distinguish between these underlying mechanisms are essential to reveal the true nature of the synergy.In this contribution, we will present our novel hybrid plasma-SOEC reactor aimed at investigating fundamental aspects of SOEC-plasma synergy. The plasma is generated in a brushed electrode discharge in the quiescent region [Ratynskaia2015], which provides a highly stable, homogeneous and field-free plasma environment well suited for a controlled exposure of the SOEC. Langmuir probe measurements will help to obtain insights into the plasma properties, complemented with 1D plasma-kinetic modelling of the plasma bulk and wall region. With this combined experimental-modeling approach, we intend not only to quantify the plasma-effect on SOEC performance, but also to develop insights into the rate-limiting kinetics. We will share our first experimental results on plasma-enhanced oxygen pumping in an O2/He plasma environment.Song, Y., Zhang, X., Xie, K., Wang, G., & Bao, X. (2019). High-Temperature CO2 Electrolysis in Solid Oxide Electrolysis Cells: Developments, Challenges, and Prospects. Advanced MaterialsMori, S., & Tun, L. L. (2017). Synergistic CO2 conversion by hybridization of dielectric barrier discharge and solid oxide electrolyser cell. Plasma Processes and Polymers, 14(6), 1600153.Patel, H., Sharma, R. K., Kyriakou, V., Pandiyan, A., Welzel, S., Van De Sanden, M. C. M., & Tsampas, M. N. (2019). Plasma-Activated Electrolysis for Cogeneration of Nitric Oxide and Hydrogen from Water and Nitrogen. ACS Energy Letters, 4(9), 2091–2095.Rohnke, M., Janek, J., Kilner, J. A., & Chater, R. J. (2004). Surface oxygen exchange between yttria-stabilised zirconia and a low-temperature oxygen rf-plasma. Solid State Ionics, 166(1–2), 89–102.Ratynskaia, S., Dilecce, G., & Tolias, P. (2015). BABE – a brush cathode discharge for thermal fluctuation measurements. J. Plasma Physics, 81(2), 345810202.

Modulation of the Conductive Filament Growth During the Electroforming Process in Metal Oxide-Based RRAM

Metal oxide-based Resistive Random-Access Memories (RRAM) exhibit multiple intermediate resistive states, due to the formation/rupture of a conductive filament (CF) inside a switching layer. While there have been extensive studies on the resistive switching behavior caused by a pre-existing CF, the initial stage of the formation and growth of a CF, i.e., the electroforming process has been less understood. In this work, we developed a physical model to fundamentally understand the formation and growth behavior of a CF in a metal oxide layer constrained by the top/bottom electrodes. It is revealed that the CF formation is assisted by the supply of oxygen vacancies at the anode and the oxygen vacancies transport in the oxide layer. The effects of the external conditions (the oxygen exchange rate j 0 and the applied voltage) and the physical properties of the oxides (the electrical σ and thermal conductivity k th ) on the CF morphology evolution, current density evolution, and local temperature and electrical potential distribution have been systematically explored. It is found that larger oxygen vacancy exchange rate and relatively small voltage sweep rate result in a more uniform CF. Furthermore, by choosing oxides with lower Lorenz number (ratio of k th and σ ), CF with more homogeneous morphology can be realized. This work provides a fundamental understanding of the kinetic behaviors of the CF formation and growth during the electroforming process, and could potentially guide the oxide and electrode materials selection, synthesis and working conditions for switching to achieve improved RRAM stability and functionality.

Predicting single phase stability and segregation in the NbMoTaTi–(W,V) high entropy alloy system with the vacancy exchange potential

High entropy alloys (HEAs) are potential next-generation structural materials, yet accurate prediction of phase stability remains a challenge. We study two equimolar refractory high entropy alloys, NbMoTaTi–X (X = W, V). Ab initio calculations are performed to determine the vacancy exchange potential in both alloys. Results and experimental confirmation indicate that a zero/low vacancy exchange potential predicts phase instability in NbMoTaTiW, while elemental trends of the same predict segregation in single phase NbMoTaTiV. If these results hold true across other systems, vacancy exchange potential can serve as a rapid predictor of stability in the vast HEA compositional space.

(Invited) Impact of Stoichiometry and Interface Configuration on the Time Stability and the Speed-Limiting Step in Memristive SrTiO3 Cells

Resistive switching oxides are promising candidates for future non-volatile memory and for functional units of neuromorphic computing. Epitaxial SrTiO3 thin films can be regarded as model system for resistive switching thin films due to their well-known defect chemistry and the absence of grain boundaries. We have recently shown that so-called eightwise switching in SrTiO3 can be attributed to the release and reincorporation oxygen at the interface to a Pt electrode [1,2]. In the light of this knowledge, we studied in detail the impact of the Sr/Ti stoichiometry on the filament formation process, the filament stability [3] and the switching kinetics. We observed that Sr rich thin films exhibit an improved retention of the low resistive state (LRS) at low current compliance values with respect to the stoichiometric thin films. In order to identify the underlying processes, we investigated the filament formation process of SrTiO3 thin films with different stoichiometry by photoelectron emission microscopy. Stoichiometric thin films form a stable LRS state as soon as the current compliance is sufficient to trigger SrO segregation at the surface [4]. We attribute this to the impeded reoxidation of the oxygen deficient filament by the presence of the SrO at the surface. We could mimic this effect by intentionally depositing additional SrO, or other oxygen blocking layers such as Al2O3 on the surface of stoichiometric thin films and were thereby able to improve the memory window as well as the LRS retention. We furthermore investigated the switching kinetics of the SET process for different Sr/Ti stoichiometries and interface layers. We compared these results to analytical models for the switching kinetics based on ion movement, modeled by the Mott-Gurney-Law, and an oxygen exchange reaction at the interface, modeled by the Butler-Volmer-Equation, as the rate-determining steps in combination with simulated temperature and electrical potential data from finite element simulations, as shown in Figures 1 ((a) stoichiometric, (b) Sr rich). For stoichiometric SrTiO3 cells, oxygen transfer at the oxide-metal interface is determined as the rate-determining step. In contrast, for Sr-rich STO cells we propose oxygen diffusion within the SrO layer at the metal-STO interface as rate-limiting. We will discuss possible reasons for the different rate-determining steps, such as different vacancy concentrations and exchange probabilities.

Ab initio study of phosphorus effect on vacancy-mediated process in nickel alloys – An insight into Ni2Cr ordering

The development of long range order in nickel-chromium alloys is of great technological interest but the kinetics and mechanisms of the transformation are poorly understood. The present research utilizes a combined computational and experimental approach to elucidate the mechanism by which phosphorus accelerates the ordering rate of stoichiometric Ni2Cr in Ni-Cr alloys. A series of Ni-33%Cr-x%P samples (in atomic percent) were fabricated with phosphorus concentrations, x = <0.005–0.1 at.% and aged between 373 and 470 °C for times up to 3000 h. The first-principles modeling considers fcc Ni with dilute P as a reasonable approximation for the complex Ni-Cr-P alloy system. Calculation results show a pronounced enhancement of vacancy transport by vacancy-solute pair diffusion via consecutive exchange and rotation jumps of vacancies associated with the phosphorus atom. The energy barriers of these two migration paths are at least 0.35 eV lower than that of vacancy-atom exchange in pure Ni solvent. The analytical diffusion model predicts enhanced solvent diffusion by 2 orders of magnitude for 0.1 at.% P at 400–500 °C. The model prediction is in good agreement with the evolution of micro-hardness. We characterize the micro-hardness result by a kinetic ordering model, showing a significant decrease of the activation energy of ordering transformation. These results help gauge the risk of industrial alloys developing long range order which increases strength but degrades ductility and toughness. Specifically, minor alloying additions that bind with excess vacancies and lower the vacancy migration barrier can greatly accelerate hardening via Ni2Cr precipitation.

Metallic silver doped vanadium pentoxide cathode for aqueous rechargeable zinc ion batteries

Recently, rechargeable aqueous zinc-ion batteries have received considerable attention in relation to large-scale energy storage, mainly because of their advantages of environmental friendliness, non-toxicity, and favorable safety. However, little researches have been conducted on the cathode materials used in zinc-ion batteries. In this work, an aqueous zinc-ion battery is constituted based on metallic silver doped vanadium pentoxide cathode, Zn(CF3SO3)2 aqueous electrolyte, and zinc anode, which exhibits excellent electrochemical performance. At a current density of 0.1 A g−1, the specific capacity of about 215 mAh g−1 was retained after 50 cycles. It also delivers long-cycle stability at 3 A g−1, which remains at 80 mAh g−1 after 700 cycles. Furthermore, the reaction mechanism is confirmed by X-ray diffraction, ex-situ Raman, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy analysis. This outstanding performance is attributed to the high conductivity of the cathode material and the vacancy exchange mechanism of Zn2+/Ag+. This research shows metallic silver doped vanadium pentoxide cathode which could be considered a promising cathode material for aqueous zinc-ion batteries.

High-Temperature Dislocation Climb in the γ′ Rafts of Single-Crystal Superalloys: The Hypothesis of a Control by Dislocation Entry into the Rafts

The good mechanical resistance to high-temperature creep of [001] oriented single-crystal superalloys is due to the properties of the rafts, i.e., platelets of the L12 γ′ phase embedded in a γ matrix. At temperatures higher than 900 °C, the plastic strain of the rafts results from the climb of dislocation pairs with a total $$ a \cdot \left\langle {100} \right\rangle $$ Burgers vector and/or from the climb at the γ/γ′ interfaces of $$ a/2 \cdot \left\langle {110} \right\rangle $$ dislocation segments. This climb motion involves the exchange of vacancies between these dislocations and vacancy sinks such as pores and the specimens’ surfaces. In this paper, we suggest that the entry of $$ a \cdot \left\langle {100} \right\rangle $$ dislocations into the rafts requires the overcoming of a threshold stress and show that this hypothesis gives a natural explanation to some of the most salient aspects of their mechanical behavior.

High-Temperature Ultrasound Attenuation in Langasite and Langatate.

The ultrasound attenuation in langasite crystals increases rapidly at about 800 K with increasing temperature for reasons that are not well understood. In this paper, the attenuation quantified as of the langasite-type materials La3Ga5SiO14 (LGS) and La3Ta0.5Ga5.5O14 (LGT) was studied from room temperature to 1653 and 1608 K, respectively, using resonant ultrasound spectroscopy. Two to three attenuation peaks can be seen. A change of the magnitudes of the largest two attenuation peaks in LGT was correlated with the changing color of an LGT sample, which is related to its oxygen vacancy concentration. Thus, the attenuation likely involves oxygen vacancies. The observed can be explained well by a model based on the anelastic relaxation of two to three noninteracting point defects causing Debye peak-like attenuation maxima. The activation energies of the largest two relaxation peaks match the activation energies for different conductivity mechanisms in LGS and LGT. Thus, the oxygen movement-based conductivity and the relaxation processes seem to involve the exchange of ions and vacancies on the same positions. The largest two attenuation peaks are probably caused by the movement of ions induced by two different phenomena, the deformation of the lattice (point-defect relaxation) on the one hand and the electric field via the piezoelectric effect (piezoelectric/carrier relaxation) on the other hand.

First-principles study of lithium-ion diffusion in β-Li3PS4 for solid-state electrolytes

Understanding the role of partially occupied sites in Li-based superionic conductors is key to improving performance of solid-state electrolyte materials. We study the optimized structure of crystalline β-Li3PS4 and the Li-ion diffusion using first-principles calculations and the nudged elastic band method. Considering diffusion paths through both interstitial and vacancy exchanges, we calculate the migration energies of Li ions. We find that the phonon-mode softening and concurrent inversion symmetry breaking leads to a more stable structure with low symmetry. Atomic distortion from the phonon softening provides diffusion paths for Li ions with less migration energies than the ones in high-symmetry structures. Our results show that diffusion of Li ion is highly anisotropic through the armchair- or zigzag-shaped channels along the b-axis that contain Li-ion sites with fractional occupation.

Solar-Driven Thermochemical Splitting of CO2 and In Situ Separation of CO and O2 across a Ceria Redox Membrane Reactor

SummarySplitting CO2 with a thermochemical redox cycle utilizes the entire solar spectrum and provides a favorable path to the synthesis of solar fuels at high rates and efficiencies. However, the temperature/pressure swing commonly applied between reduction and oxidation steps incurs irreversible energy losses and severe material stresses. Here, we experimentally demonstrate for the first time the single-step continuous splitting of CO2 into separate streams of CO and O2 under steady-state isothermal/isobaric conditions. This is accomplished using a solar-driven ceria membrane reactor conducting oxygen ions, electrons, and vacancies induced by the oxygen chemical potential gradient. Guided by the limitations imposed by thermodynamic equilibrium of CO2 thermolysis, we operated the solar reactor at 1,600°C, 3·10−6 bar and 3,500 suns radiation, yielding total selectivity of CO2 to CO + ½O2 with a conversion rate of 0.024 μmol·s−1 per cm2 membrane. The dynamics of the oxygen vacancy exchange, tracked by GC and XPS, further validated stable fuel production.

Dislocation climb models from atomistic scheme to dislocation dynamics

We develop a mesoscopic dislocation dynamics model for vacancy-assisted dislocation climb by upscalings from a stochastic model on the atomistic scale. Our models incorporate microscopic mechanisms of (i) bulk diffusion of vacancies, (ii) vacancy exchange dynamics between bulk and dislocation core, (iii) vacancy pipe diffusion along the dislocation core, and (iv) vacancy attachment-detachment kinetics at jogs leading to the motion of jogs. Our mesoscopic model consists of the vacancy bulk diffusion equation and a dislocation climb velocity formula. The effects of these microscopic mechanisms are incorporated by a Robin boundary condition near the dislocations for the bulk diffusion equation and a new contribution in the dislocation climb velocity due to vacancy pipe diffusion driven by the stress variation along the dislocation. Our climb formulation is able to quantitatively describe the translation of prismatic loops at low temperatures when the bulk diffusion is negligible. Using this new formulation, we derive analytical formulas for the climb velocity of a straight edge dislocation and a prismatic circular loop. Our dislocation climb formulation can be implemented in dislocation dynamics simulations to incorporate all the above four microscopic mechanisms of dislocation climb.

Advacancy-mediated atomic steps kinetics and two-dimensional negative island nucleation on ultra-flat Si(111) surface

We have investigated Si(111) surface morphology transformations during high-temperature sublimation and oxygen treatments by means of in situ ultrahigh vacuum reflection electron microscopy. By analyzing atomic steps kinetics and two-dimensional negative (vacancy) islands nucleation on ultra-flat Si(111) surface with extremely wide (up to 120μm in size) terraces we have estimated the activation energy associated with the surface-bulk vacancy exchange processes. We show that atomic steps motion and negative islands nucleation kinetics at temperatures above 1180°C can be described by the step-flow model of Burton, Cabrera and Frank taking into account advacancies formation. By comparing experimental results with predictions of model we conclude that the surface mass transport at temperatures above 1180°C is governed by surface vacancies nucleation and interaction with atomic steps rather than via adatoms surface diffusion.

Thermal diffusion boron doping of single-crystal natural diamond

With the best overall electronic and thermal properties, single crystal diamond (SCD) is the extreme wide bandgap material that is expected to revolutionize power electronics and radio-frequency electronics in the future. However, turning SCD into useful semiconductors requires overcoming doping challenges, as conventional substitutional doping techniques, such as thermal diffusion and ion implantation, are not easily applicable to SCD. Here we report a simple and easily accessible doping strategy demonstrating that electrically activated, substitutional doping in SCD without inducing graphitization transition or lattice damage can be readily realized with thermal diffusion at relatively low temperatures by using heavily doped Si nanomembranes as a unique dopant carrying medium. Atomistic simulations elucidate a vacancy exchange boron doping mechanism that occurs at the bonded interface between Si and diamond. We further demonstrate selectively doped high voltage diodes and half-wave rectifier circuits using such doped SCD. Our new doping strategy has established a reachable path toward using SCDs for future high voltage power conversion systems and for other novel diamond based electronic devices. The novel doping mechanism may find its critical use in other wide bandgap semiconductors.

Catalytic Dynamics and Oxygen Diffusion in Doped PrBaCo2O(5.5+δ) Thin Films.

The Sr and Fe codoped double perovskites PrBaCo2O5.5+δ (PrBCO) thin films of Pr(Ba0.5Sr0.5)(Co1.5Fe0.5)O5.5+δ (PBSCFO) were epitaxially grown for chemical catalytic studies. The resistance behavior of PBSCFO epitaxial films was monitored under the switching flow of reducing and oxidizing gases as a function of the gas flow time, t, using an electrical conductivity relaxation (ECR) experimental setup. The R(t) vs t relationships determined at various temperatures show the occurrence of two oxidation processes, Co(2+)/Co(3+) ↔ Co(3+) and Co(3+) ↔ Co(3+)/Co(4+). Mathematical fitting of the observed R(t) vs t relationships was carried out using Fick's second law for one-dimensional diffusion of charge carriers to derive the diffusivity D(T) and τ(T) for the two processes at various temperatures, T. The D(T) vs T relationships were analyzed in terms of the Arrhenius relationship to find the activation energies Ea for each process. Oscillations in the dR(t)/dt plots, observed under oxidation reactions, were discussed in terms of a layer-by-layer oxygen vacancy exchange diffusion mechanism. Our work suggests that thin films of LnBCO (Ln = lanthanide) with their A and B sites doped as in PBSCFO are excellent candidates for the development of low or intermediate temperature energy conversion devices and gas sensor applications.

(Invited) Failure Mechanisms in AlGaN/GaN HEMTs Irradiated with 2MeV Protons

Gallium nitride high electron mobility transistors (HEMTs) have shown the potential to be highly resistant to radiation damage, making them ideal for use in microwave power amplifiers and DC/DC converters in space-based applications. To investigate the mechanisms of radiation-induced degradation in AlGaN/GaN HEMTs, 2MeV protons were used to simulate the space radiation environment, and to probe the second order effect of inherent material defects on the response the initial material quality was intentionally varied by studying HEMTs on sapphire, Si, and SiC substrates. Irradiation was performed incrementally up to a fluence of 6x1014 cm-2. The Hall mobility and 2DEG density was measured on Van der Pauw structures before irradiation and at each dose. The decrease in mobility can be attributed to increased carrier scattering in the 2DEG as a result of radiation-induced defects while the decrease in sheet carrier density is attributed to screening of the 2DEG from charged trap formation [1]. The magnitude of change in the 2DEG density is the same for all substrate materials (1x1012 cm-2), though the percent change varies due to different initial values. Note that mobility begins to degrade at lower fluence than sheet carrier density. FET I-V analysis indicated increased ON-resistance and decreased saturation current, consistent with the reduced mobility and sheet carrier density observed in the Hall measurements. The off-state leakage current decreased and threshold voltage was shifted slightly positive, again due to the reduced 2DEG density. The trapping mechanisms were further probed using pulsed I-V measurements at varying quiescent bias conditions. While the dynamic ON-resistance degradation under quasistatic conditions was consistent with DC measurements, the degradation under off-state stress conditions initiated at a lower fluence, and a culminated with a 10X higher degradation at the final fluence. This implies that dynamic ON-resistance, a parameter related to charge trapping, is much more sensitive to radiation than DC characteristics and must be considered when evaluating the radiation hardness of HEMT devices. TEM imaging was employed to directly probe the mechanisms suggested by the electrical measurements. Process-induced defects were visible in both reference and irradiated samples, notably an overflow of Au in the metal stack at the gate edge, possibly related to PECVD deposition of passivation layers. In addition a new, radiation-induced void at both edges of the gate in the Ni region of the Ni/Au gate metallization was revealed [2]. EDS line scans indicate that, after radiation, the 20 nm Ni layer diffused up into the Au, and to a lesser extent, into the AlGaN, leaving voids protruding ~300 nm under the gate edges. The mechanism of the void formation shows all features of Ni/Au inter-diffusion through vacancy exchange, known as the Kirkendall effect. The exact process is still under investigation, but the voids result in reduced gate area, which is responsible for the degraded off-state performance. It is particularly rare that such pronounced diffusion occurs at room temperature. Since the voids only occur at the gate edges, an additional electrochemical or strain-induced driving force is likely present. Therefore the role of the SiNx passivation layer, as well as strain, at the gate edge will be factor in this ongoing research. 1. T.J. Anderson, et. Al. IEEE Electron Dev. Lett. 35 (8), 826-828 (2014) 2. A.D. Koehler, et. Al. IEEE Electron Dev. Lett. 35 (10) (2014)

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 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . 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.