Hot Electron Trapping Research Articles

Pushing the Limits of Photoconductivity via Hot Electrons in Deep Trap States in Plasmonic Architectures.

Although extensive research on the mechanisms of photoconductivity enhancement in plasmonic Schottky structures has been conducted, the photoconductive interplay between hot electrons and trapping states remains elusive. In this study, we explored the photoconductive relationship between plasmonic hot-carriers and defect sites present in plasmonic architectures consisting of N-face n-GaN and Au nanoprisms. Our experimental results clearly verified that the plasmonic hot-electrons generated by interband transitions preferentially occupied deep trap levels in n-GaN, thereby considerably enhancing the photoconductivity through the combination of photogating and photovoltaic effects. Our quantitative evaluation demonstrated that a mere 63% increase in hot-electron trapping leads to a 1.7-fold increased photocurrent under localized surface plasmon resonance (LSPR) excitation compared to the figure of photocurrent under non-LSPR stimulus. Our findings provide novel insights into the mechanisms of photoconductive enhancement for advanced plasmonic applications.

SPICE Simulation Assisted-Dynamic Rds(on) Characterization in 200V Commercial Schottky p-GaN HEMTs Under Unstable Phases

Abstract This paper presents a comprehensive analysis of dynamic and static RDS(ON) in Schottky p-GaN High Electron Mobility Transistors (HEMTs), highlighting the impact of off-state and hot electron trapping on device performance. The authors observed significant hysteresis in the transfer characteristics of a 200V commercial Schottky p-GaN, attributing this to charge trapping effects. A novel experimental setup, employing a multi-pulse test synchronous buck converter circuit with additional gate control and a clamping circuit, enabled precise characterization of dynamic RDS(ON) under varying conditions, including unstable phases with overcurrent. This method effectively mimics solar PV input scenarios, exposing the device to high dv/dt and di/dt stresses, which are critical for evaluating GaN device stability under transient conditions. This research also reveals that increased gate resistance reduces energy losses, challenging traditional expectations by demonstrating the nuanced gate charge dynamics of GaN HEMTs. This study overall contributes to the understanding of GaN device behavior, offering a novel approach for accurately characterizing dynamic RDS(ON) under unstable stages, furtherly advances the GaN device in complex renewable energy power converter applications.

Circular polarization sensitive opto-neuromorphic operation at plasmonic hot electron transistor using chiral gold nanoparticles

Opto-neuromorphic operation is critical for biological system to recognize the visual objects and mimicking such operation is important for artificial prosthesis as well as machine vision system for industrial applications. To sophisticatedly mimic biological system, regulation of learning and memorizing efficiency is needed, however engineered synthetic platform has been lack of controllability, which makes huge gap between biological system and synthetic platform. Here we demonstrated controllable learning and memorizing opto-neuromorphic operation at plasmonic hot electron transistor. Especially, circularly polarized light (CPL) sensitive synaptic characteristics and learning experience capability are enabled by incorporating chiral plasmonic nanoparticle. Furthermore, gate voltage gives rise to controllable neuromorphic operation due to hot electron injection and trapping effect, resulting in high remaining synaptic weight of ∼70% at negative gate voltage under CPL excitation. We believe that this discovery makes significant leap toward on-demand in-sensor computing as well as toward bio-realistic device.

Boosting the Plasmon-Mediated Electrochemical Oxidation of p-Aminothiophenol with p-Hydroxythiophenol as Molecular Cocatalyst.

Plasmon-mediated electrochemistry is an emerging area of interest in which the electrochemical reactions are enhanced by employing metal nanostructures possessing localized surface plasmon resonance (LSPR). However, the reaction efficacy is still far below its theoretical limit due to the ultrafast relaxation of LSPR-generated hot carriers. Herein, we introduce p-hydroxythiophenol (PHTP) as a molecular cocatalyst to significantly improve the reaction efficacy in plasmon-mediated electrochemical oxidation of p-aminothiophenol (PATP) on gold nanoparticles. Using electrochemical techniques, in situ Raman spectroscopy, and theoretical calculations, we elucidate that the presence of PHTP improves the hot hole-mediated electrochemical oxidation of PATP by 2-fold through the trapping of plasmon-mediated hot electrons. In addition, the selectivity of PATP oxidation could also be modulated by the introduction of PHTP cocatalyst. This tactic of employing molecular cocatalyst can be drawn out to endorse various plasmonic electrochemical reactions because of its simple protocol, high efficiency, and high selectivity.

Reliability issues of GaN HEMT: Current status and challenges

With the development of power electronic devices, there is an increasing demand for energy-saving, emission reduction, and environmental protection. Thus, higher energy conversion efficiency of power electronic devices is required. While traditional Si-based power electronic devices have the disadvantage of low energy utilization efficiency and high thermal losses, GaN-based HEMT has significant advantages, making it a frontier and hotspot in global semiconductor research. However, the failure mechanisms that affect the reliability of GaN HEMTs are not completely comprehended. Many reliability issues affect devices performance, among which electrical reliability and thermal reliability are widely studied concerns. Electrical reliability issues contain inverse piezoelectric effect, hot electron effect, trapping effect, and mental instability. Thermal reliability issues contain self-heating effects, which are caused by the materials unsatisfying thermal conductivity, and improper structure design. This paper is focused on the current research results of the GaN HEMTs electrical reliability and thermal reliability. The first part of this paper gives a review of GaN HEMT reliability issues at their current status, and the second part outlines the challenges of GaN HEMTs in future development. This review will help researchers to better understand factors that affect GaN HEMTs reliability and help with their device design for further research.

Impact of high-temperature operating lifetime tests on the stability of 0.15 μm AlGaN/GaN HEMTs: A temperature-dependent analysis

We present a detailed analysis of the effect of low- and high-temperature operating lifetime (LTOL and HTOL) carried out on 0.15 μm GaN HEMTs for RF applications. Based on several stress experiments carried out at different temperature levels, a) we demonstrate the existence of two degradation modes consisting, respectively, in the negative and positive shift in the threshold voltage; b) temperature-dependent I-V measurements indicate that no modification in the Schottky barrier height takes place after stress; c) the negative VTH shift is ascribed to the temperature- and field-assisted de-trapping of electrons from the passivation/AlGaN stack under the gate, consistent with previous reports; d) the positive VTH shift, which only occurs at high temperature levels, is ascribed to the trapping of hot electrons, possibly at the AlGaN/SiN interface.

Cumulative Hot-Electron Trapping in GaN-Based Power HEMTs Observed by an Ultrafast (10 V/Ns) On-Wafer Methodology

The goal of this article is to advance the understanding of the impact of hard switching on the dynamic performance of GaN-based high electron mobility transistors (HEMTs). To this aim, we developed a fast (10 V/ns) on-wafer system for testing devices in hard switching. The system has been used to study the reliability of several <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$W_{G}=2$ </tex-math></inline-formula> mm p-type GaN HEMTs with different <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L_{\text {GD}}$ </tex-math></inline-formula> or buffer properties. First, we show that by optimizing the drain node capacitance, we can speed up the hard-switching transition to a few ns, even on-wafer level. Second, repeating the experiment by using multiple frequencies, from 1 to 100 kHz, we demonstrate that, in real-world applications, cumulative turn-on stress has a much stronger effect on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{\mathrm {ON}}$ </tex-math></inline-formula> compared with OFF-state stress. Third, by comparing the results on identical devices having shorter <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L_{\text {GD}}$ </tex-math></inline-formula> , we pinpoint hot electrons as the main mechanism in the device degradation, ruling out the contribution of self-heating. Finally, by comparing three wafers with different processing conditions (different passivation, different buffer), we suggest that trapping phenomena related to hot electrons happen in ns time scale and that the properties of the buffer can significantly impact the dynamic performance of the devices in hard switching.

Analysis of abnormal threshold voltage shift induced by surface donor state in GaN HEMT on SiC substrate

This study investigates the recovery behavior of GaN high-electron mobility transistors on SiC substrates under different hot-carrier stress conditions. The threshold voltage shifts positively due to hot electron trapping at the buffer layer under hot-carrier stress. However, the recovery between semi-on (Vt &amp;lt; VG &amp;lt; 0 V) hot-carrier stress and on-state (VG &amp;gt; 0 V) carrier stress is significantly different. This phenomenon is systematically discussed in terms of the applied gate voltage under stress condition, which affects the occupation of the surface donor states. After applying a semi-on hot-carrier stress, the threshold voltage continues to shift positively after relaxing the applied voltage. In contrast, the threshold voltage exponentially recovers after applying an on-state hot-carrier stress. Silvaco technology computer aided design (TCAD) simulation was performed to verify the effect of the surface donor state on the threshold voltages. Since switching between on-state and off-state in RF-applications includes both conditions, we suggest that the surface donor states are crucial to determine the failure of devices.

Degradation mechanisms for a-InGaZnO thin-film transistors functioning under simultaneous DC gate and drain biases

Degradation of a-InGaZnO thin-film transistors working under simultaneous DC gate and drain bias stress is investigated, and the corresponding degradation mechanism is proposed and verified. The maximum degradation occurs under the bias stress condition that makes the electric field and electron concentration relatively high at the same time. Trapping of hot electrons in the etching-stop layer under the extended drain electrode is proven to be the underlying mechanism. The observed degradation phenomena, including distortion in the transfer curve on a logarithmic scale and two-slope dependence on gate bias on a linear scale, current crowding in the output curve, and smaller degradation in transfer curves measured under large drain bias, can all be well explained with the proposed degradation mechanism.

Boosting the photocatalytic nitrogen reduction to ammonia through adsorption-plasmonic synergistic effects

Ammonia is one of the most essential chemicals in the modern society but its production still heavily relies on energy-consuming Haber-Bosch processes. The photocatalytic reduction of nitrogen with water for ammonia production has attracted much attention recently due to its synthesis under mild conditions at room temperature and atmospheric pressure using sunlight. Herein, we report a high-performance Au/MIL-100(Cr) photocatalyst, comprising MIL-100(Cr) and Au nanoparticles in photocatalytic nitrogen reduction to ammonia at ambient conditions under visible light irradiation. The optimized photocatalyst (i.e., 0.10Au/MIL-100(Cr)) achieved the excellent ammonia production rate with 39.9 µg gcat−1 h−1 compared with pure MIL-100(Cr) (2.73 µg gcat−1 h−1), which was nearly 15 times that on pure MIL-100(Cr). The remarkable activity could be attributed to the adsorption-plasmonic synergistic effects in which the MIL-100(Cr) and Au are responsible to the strong trapping and adsorption of N2 molecules and photo-induced plasmonic hot electrons activating and decomposing the N2 molecules, respectively. This study might provide a new strategy for designing an efficient plasmonic photocatalyst to improve the photocatalytic performance of N2 fixation under visible light irradiation.

Short term reliability and robustness of ultra-thin barrier, 110 nm-gate AlN/GaN HEMTs

Short-term reliability and robustness of 110 nm AlN/GaN HEMTs has been evaluated by means of off-state, semi-on state and on-state step stress tests on devices having different gate-drain distance, LGD. While breakdown voltages and critical voltages scale almost linearly with LGD, failure mode remains almost unchanged in all tested devices, and consists in an increase of gate leakage, accompanied by a positive shift of threshold voltage. In off-state, electroluminescence images detect the presence of localized leakage paths which may act as preferential paths for electron trapping. Degradation does not depend on dissipated power and is preliminary attributed to hot-electron trapping, enhanced by electric fields.

A Physics-Based Approach to Model Hot-Electron Trapping Kinetics in p-GaN HEMTs

Hot electron trapping can significantly modify the performance of GaN-based HEMTs during hard switching operation. In this letter, we present a physics-based model based on rate equations to model the trapping kinetics of hot-electrons in GaN transistors submitted to semi- ON-state stress. The model is validated through comparison with the experimental data, obtained by means of a pulsed-drain current transient setup developed ad-hoc. Hot-electron trapping is found to have logarithmic time dependence; the first 10 μs of operation are critical in determining the current collapse during stress.

Transient Measurement of GaN HEMT Drift Region Potential in the High Power State

This work reports the transient measurement of the AlGaN/GaN high electron-mobility transistor (HEMT) drift region potential in the high power state during switching. The effect of high power stress has been reported by device characterization after stress or during high power DC stress at voltages below the operating voltages to avoid device failure. However, transient measurement during high power stress is challenging, since the device can sustain the high power stress only for a few μs before failure occurs. In this work, a test structure with voltage probe inserted in the drift region between the gate and the drain is used to perform real-time measurement of the drift region potential during the high power state at operating voltages. It is found that the high electric field formed at the gate side during the off-state stress propagates towards the drain side within a few μs. The propagation speed of this high electric field increased with increase in drain voltage. It is proposed that trapping of hot channel electrons is causing this effect, and dynamic on-resistance measurement suggests that the traps are 0.44 eV below the conduction band.

Understanding the effects of off-state and hard-switching stress in gallium nitride-based power transistors

The aim of this paper is to improve the understanding of gallium nitride (GaN) high electron mobility transistors (HEMTs) submitted to hard switching operation, with focus on the hot-electron phenomena. This is becoming a hot-topic both for the scientific community and for the industry. The analysis is carried out through a cross-comparison of three different experimental techniques: conventional Pulsed-IV characterization, a novel pulsed-drain current transient (P-DCT) method, and a custom-developed hard switching test protocol. Hard switching analysis was performed through a novel system able to test the device in hard-switching conditions with an unprecedented turn-on slew-rate of 25 V ns−1 on-wafer level. This allows μs to investigate the impact of hard switching in terms of (i) locus trajectory, (ii) dissipated power, and (iii) dynamic increase. Furthermore, the accumulation of switching stress is assessed by repeating the experiment with increasing frequency, from 1 kHz to 100 kHz. The extensive cross-analysis offers a novel insight on the degradation mechanisms occurring in power GaN HEMTs. The results collected within this paper allow: (1) to evaluate the dynamic behavior under both soft- and hard-switching stress, thus differentiating off-state and semi-on-state stress; (2) to pinpoint hot-electrons as the main cause of the current collapse observed in semi-on; (3) by comparing the results obtained from P-DCT and Hard Switching Analysis we demonstrate that the hot-electron trapping is a very fast process which can happen in few ns. The related trapping and de-trapping kinetics are investigated in detail. The results described within this paper provide novel insight on the important role of hot-electrons in the dynamic increase during hard switching operations.

A novel on-wafer approach to test the stability of GaN-based devices in hard switching conditions: Study of hot-electron effects

In this work, a novel system to investigate the stability of GaN-based HEMT devices is presented and used to investigate hot-electron effects. The developed system is used to study the impact of hard switching on the dynamic on-resistance of such devices. In particular, we were able to obtain (on-wafer level) a very fast turn-ON commutation with dVDS/dt ≈ 10 V/ns (representative of realistic conditions) thanks to the low parasitics at the drain node. As a result, a realistic performance assessment of the dynamic stress of GaN power HEMTs is now available on wafer level, thus shortening the technology development loop. By intentionally tuning the capacitance at the drain node we can accurately control the amount of energy/charge released during each hard switching event, thus being able to evaluate the impact of increasing stress conditions on the devices. The results indicate that even if the hard-switching lasts few nanoseconds, it significantly impacts the dynamic RDSON: we conclude that hot-electron trapping can occur in ns-time scale.

Ultrahigh Negative Infrared Photoconductance in Highly As-Doped Germanium Nanowires Induced by Hot Electron Trapping

Here, we report the observation of negative photoconductance (NPC) effect in highly arsenic-doped germanium nanowires (Ge NWs) for the infrared light. NPC was studied by light-assisted Kelvin probe...

Flexible, ultra-stable and color tunable fluorescent films based on all inorganic perovskite quantum dots embedded in polymer

Colloidal perovskite CsPbX3 (X = Cl, Br, I) nanocrystals (CPNCs)/polymers composites have attracted extensive attention due to their potential to be developed as flexible phosphor films for lighting applications. However, to maintain high quantum efficiency and photo stability of CPNCs in such composites remains a daunting challenge. Here, we have demonstrated a layered composite structure consisting of CPNCs and polydimethylsiloxane (PDMS) with multi-color emission and long-term stability. By tuning the molar ratio between CsPbCl1.58Br1.42 and CsPbBr1.35I1.65, flexible fluorescent films as down-converter layers with a high luminescent efficiency and a controllable color temperature spanning from 3194 K to 5901 K have been demonstrated. Furthermore, due to embedding inside such composites, the quantum efficiency of CPNCs exhibited negligible changes during seven months in ambient conditions. The carrier dynamics based on time-resolved photoluminescence (PL) and transient absorption spectroscopy reveal that the hot electron tunneling and trapping process are significant in the composite film. This work provides a good understanding of CPNC materials in complex composite for the development of flexible, robust, color controllable fluorescent films for lighting applications.

(Invited) Plasmon-Driven Chemical Reactions

Plasmonic metal nanocrystals can interact strongly with light, efficiently converting light into heat and generating hot charge carriers. Both plasmonic photothermal conversion and hot carrier generation can accelerate chemical reactions. The use of plasmons to drive chemical reactions has recently become a new and active research field (Adv. Mater. 2014, 26, 5274). Plasmonic hot charge carriers can not only enhance the reaction yield and selectivity, but also introduce new reaction pathways. The lifetime of plasmonic hot charge carriers is on the femtosecond scale. Without immediate usage, they will rapidly relax, converting their energy into heat. We have employed two approaches to enable the use of plasmonic hot charge carriers. The first is the integration of Au or Ag nanoparticles with Pd or Pt nanoparticles. We have applied this approach to enhance Suzuki coupling reactions by localized plasmons (J. Am. Chem. Soc. 2013, 135, 5588; Adv. Funct. Mater. 2017, 27, 1700016). The second is the integration of plasmonic metals with semiconductors. A barrier is formed at the interface. The generated hot electrons and holes can quickly inject into the conduction or valence band and therefore get separated. The injected charge carriers can then drive chemical reactions. We have synthesized Au/TiO2 (Energy Environ. Sci. 2014, 7, 3431; J. Am. Chem. Soc. 2018, 140, 8497), Au/CeO2 (ACS Nano 2014, 8, 8152) and Au/BiOCl hybrid structures (J. Am. Chem. Soc. 2017, 139, 3513) as plasmonic photocatalysts to drive the photo-generation of reactive oxygen species, the selective oxidation of alcohols, and N2 photofixation. Specifically, oxygen vacancies have been introduced in the metal oxides to “work in-tandem” together with plasmonic hot charge carriers. For example, during the use of Au/BiOCl hybrid structures for the photocatalytic selective oxidation of benzyl alcohol, oxygen vacancies on BiOCl facilitate the trapping and transfer of plasmonic hot electrons to adsorbed O2, producing superoxide anion radicals, while plasmonic hot holes remaining on the Au surface mildly oxidize benzyl alcohol to corresponding carbon-centered radicals. The ring addition between these two radical species leads to the production of benzaldehyde along with an unexpected oxygen atom transfer from O2 to the product. In contrast, the oxygen atom in the product is usually from the benzyl alcohol reactant. In another example, Au nanoparticles are anchored on ultrathin TiO2 nanosheets with oxygen vacancies. The oxygen vacancies on the nanosheets chemisorb and activate N2 molecules, which are subsequently reduced to ammonia by hot electrons generated from plasmon excitation of the Au nanoparticles. The hybrid photocatalyst can accomplish photodriven N2 fixation in the “working-in-tandem” pathway at room temperature and atmospheric pressure. The apparent quantum efficiency of 0.82% at 550 nm for the conversion of incident photons to ammonia is higher than those reported so far. The oxygen vacancies play three roles. They act as the active sites for N2 molecules; they cause defect states within the bandgap for trapping plasmonic hot electrons and therefore lengthening their lifetime; and they function as a bridge between plasmonic hot electrons and the activated N2 molecules. This work offers a new approach for the rational design of efficient catalysts towards sustainable N2 fixation through a less energy-demanding photochemical process compared to the industrial Haber-Bosch process.

Why Does the Photoluminescence Efficiency Depend on Excitation Energy in Case of a Quantum Dot? A Case Study of CdSe-Based Core/Alloy Shell/Shell Quantum Dots Employing Ultrafast Pump–Probe Spectroscopy and Single Particle Spectroscopy

Core/alloy-shell/shell quantum dots (CASS QDs) have been shown to exhibit excitation energy dependent PL efficiency. The magnitude of the normalized transient population has been shown to increase more than 5-fold with decreasing the excitation energy. For high energy excitation cooling of the exciton (predominantly electron) to band edge is much slower (rise time of ∼526 fs) in comparison to low energy excitation (rise time of <∼100 fs). Time constant associated with the excitation energy dependent dynamics of the hot electron trapping is ∼1 ps. Time constant related to excitation energy independent hot hole dynamics is ∼35 ps. Truncation time obtained from single particle investigation has been shown to increase four folds (20 to 80 s) and the magnitude of additional exponential time constant responsible for hole trapping has been shown to increase nearly two folds with decrease in excitation energy. Thus, by employing ensemble level ultrafast dynamics and single particle PL blinking dynamics, it could ...

Hot-Electron Trapping and Hole-Induced Detrapping in GaN-Based GITs and HD-GITs

This paper investigates the trapping mechanisms in gate-injection transistors (GITs) without and with a pdrain electrode, referred to as GITs and Hybrid-Drain-embedded GITs (HD-GITs), respectively, used to inject holes and reduce charge trapping effects. We compare the two sets of devices under both the OFF-state and semi- ON state, to investigate the role of hot electrons in favoring the charge trapping. The analysis is based on combined pulsed characterization, transient measurements, and electroluminescence (EL) characterization. We demonstrate the following relevant results: 1) GITs and HD-GITs have comparable and negligible dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}$ when trapping is induced in the OFF-state; under semi- ON state conditions, GITs suffer from significant dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}$ , while HD-GITs show no additional trapping with respect to OFF-state; 2) EL characterization carried out until ${V}_{\mathsf {DS}}= 500$ V in semi- ON conditions shows comparable features, suggesting that the electric field and hot-carrier density are similar in GITs and HD-GITs. This result indicates that the hot-electron trapping rate is identical for the two sets of samples, so the difference observed in dynamic ${R}_{ \mathrm{\scriptscriptstyle ON}}$ must be ascribed to a different detrapping rate; 3) transient ${R}_{ \mathrm{\scriptscriptstyle ON}}$ measurements indicate that the traps filled in the OFF-state conditions are the same as those filled by hot electrons in semi- ON, with ${E}_{a}= 0.8$ eV (possibly ${C}_{N}$ ); and 4) EL analysis under constant bias indicates that in GITs there is a time-dependent increase in the luminescence signal at the drain terminal, when the devices are biased with ${V}_{\mathsf {DS}}= 300$ V. Such effect, indicative of hot-electron trapping, is not observed in HD-GITs. Based on the experimental evidence collected within this paper, we conclude that—in the semi- ON state—the main difference between GITs and HD-GITs consists in a faster detrapping rate of hot electrons, achieved through hole injection from the pdrain terminal.