Characterization of charge trapping mechanisms in GaN vertical Fin FETs under positive gate bias

In this paper, we present a comprehensive analysis of the charge trapping mechanisms that affect the GaN based vertical Fin FETs when the devices are submitted to positive gate bias. Devices with higher channel width show lower threshold voltage: with 2D simulations of the electron density we are able to explain the phenomenon and propose a trade-off to improve the technology. By using double pulse measurements and threshold voltage transients, two trapping/detrapping mechanisms under positive gate bias can be identified according to two voltage ranges. At low positive gate bias, electrons (previously trapped inside the oxide during the fabrication process) are detrapped towards the gate metal (mechanism 1). At higher gate bias, electrons are trapped at the GaN/oxide interface, moving the threshold towards positive values (mechanism 2). The second mechanism is observable at higher time of stress and it is predominant for higher voltages. Moreover, mechanism 2 is found to be recoverable only when the device is exposed to UV-light and electrons trapped in a specific level in the oxide acquire the energy necessary to escape and reach the n-type GaN and/or the UV-generated holes accumulate at the interface may reduce the trapped electron density. We demonstrate our hypothesis by calculating the interface state density in trapping/detrapping conditions by using photo-assisted Capacitance-Voltage measurements

Hydrodynamics of local excitations after an interaction quench in 1D cold atomic gases

We discuss the hydrodynamic approach to the study of the time evolution -induced by a quench- of local excitations in one dimension. We focus on interaction quenches: the considered protocol consists in creating a stable localized excitation propagating through the system, and then operating a sudden change of the interaction between the particles. To highlight the effect of the quench, we take the initial excitation to be a soliton. The quench splits the excitation into two packets moving in opposite directions, whose characteristics can be expressed in a universal way. Our treatment allows to describe the internal dynamics of these two packets in terms of the different velocities of their components. We confirm our analytical predictions through numerical simulations performed with the Gross-Pitaevskii equation and with the Calogero model (as an example of long range interactions and solvable with a parabolic confinement). Through the Calogero model we also discuss the effect of an external trapping on the protocol. The hydrodynamic approach shows that there is a difference between the bulk velocities of the propagating packets and the velocities of their peaks: it is possible to discriminate the two quantities, as we show through the comparison between numerical simulations and analytical estimates. In the realizations of the discussed quench protocol in a cold atom experiment, these different velocities are accessible through different measurement procedures. ArXI

Topological order and thermal equilibrium in polariton condensates

We report the observation of the Berezinskii-Kosterlitz-Thouless transition for a 2D gas of exciton-polaritons, and through the joint measurement of the first-order coherence both in space and time we bring compelling evidence of a thermodynamic equilibrium phase transition in an otherwise open driven/dissipative system. This is made possible thanks to long polariton lifetimes in high-quality samples with small disorder and in a reservoir-free region far away from the excitation spot, that allow topological ordering to prevail. The observed quasi-ordered phase, characteristic for an equilibrium 2D bosonic gas, with a decay of coherence in both spatial and temporal domains with the same algebraic exponent, is reproduced with numerical solutions of stochastic dynamics, proving that the mechanism of pairing of the topological defects (vortices) is responsible for the transition to the algebraic order. Finally, measurements in the weak-coupling regime confirm that polariton condensates are fundamentally different from photon lasers and constitute genuine quantum degenerate macroscopic states

Lack-of-correlation anomaly in CMB large scale polarisation maps

We present an assessment of the large-scale CMB anomalies in polarisation using the two-point correlation function as a test case. We employ the state of the art of large scale polarisation datasets: the first based on a 2018 HFI 100 and 143 GHz cross-spectrum analysis, based on processing, and the second from a map-based approach derived through a joint treatment of 2018 LFI and -9yr. We consider the well-known S1/2 estimator, which measures the distance of the two-point correlation function from zero at angular scales larger than 60∘, and rely on realistic simulations for both datasets to assess confidence intervals. By focusing on the pure polarisation field described by either the Q and U Stokes parameters or by the local E-modes, we show that the first description is heavily influenced by the quadrupole (which is poorly constrained in both datasets) while the second one is more suited for an analysis containing higher multipoles up to ℓ∼ 10, limit above which both datasets become markedly noise dominated. We find that both datasets exhibit a lack-of-correlation anomaly in local E-modes, similar to the one observed in temperature, which is better constrained by the less noisy data, where its significance lies at about 99.5%. We perform our analysis using realizations that are either constrained or non-constrained by the observed temperature field, and find similar results in the two cases

Charge trapping in 0.1 μm AlGaN/GaN RF HEMTs: Dependence on barrier properties, voltage and temperature

The goal of this paper is to describe and understand the de-trapping dynamics in AlGaN/GaN transistors for radiofrequency (RF) applications, as a function of three parameters: the aluminium content in the barrier (22.6%, 24.6%, and 26.6%), the stress voltage applied to the drain and temperature. The analysis is based on threshold voltage transient measurements, carried out under at different stress conditions. The original results of this analysis show that: (i) the analysed devices show the presence of a dominant charge-trapping process E1 that, at room temperature, has de-trapping time constants in the range of 10 ms, and activation energy around 0.6 eV. (ii) trap E1 (that is responsible for ~0.5 V threshold voltage shift) is located in the semiconductor material, and its effect increases with increasing aluminium content in the barrier. (iii) leakage measurements indicate that devices with higher Al content in the barrier have a higher gate leakage. Our hypothesis is that under off-state stress, traps in the semiconductor material are charged by electrons injected from the gate metal through defects located in the AlGaN barrier. (iv) a second trap E2, whose signal is independent on the properties of the barrier, was also detected. This trap is supposed to originate from surface defects, is responsible for a minor (~0.1 V) threshold voltage shift and is weakly thermally activated (0.25–0.4 eV). (v) finally, a detailed analysis of the de-trapping kinetics indicated that the signal associated to trap E1 increases linearly with the drain stress voltage, and the related kinetics are ideal exponentials (consistent with semiconductor traps), while the amplitude of trap E2 does not significantly depend on drain voltage, and its kinetics are described by a stretched exponential behaviour (consistent with surface traps)

Degradation mechanism of 0.15 \u3bcm AlGaN/GaN HEMTs: effects of hot electrons

The degradation mechanisms of AlGaN/GaN HEMTs adopting Fe and C co-doping, with high and low carbon doping concentration were investigated by means of hot-electron step stress and 24 h' stress tests. Firstly, DC and EL characterization at room temperature are summarized, then the parametric evolution during hot-electron step stress tests at the semi-on state was compared, the assumption for the degradation mechanism is that hot-electrons activated the pre-existing traps in the buffer, attenuate the electric field in the gate drain access region and damaging the gate contact, the parametric evolution during constant stresses is discussed

Failure mechanisms of GaN HEMTs for microwave and millimeter-wave applications: From interdiffusion effects to hot-electrons degradation

Gallium Nitride High Electron Mobility Transistor are the most prominent devices for application to high power density, high efficiency transmission system in the microwave and millimeter-wave frequency range. High bandwidth requirements of 5G and future telecommunication infrastructure require the adoption of higher frequency bands beyond 60 GHz, thus accelerating the scaling of devices gate lengths below 150 nm, possibly promoting failure mechanisms accelerated by electric field and hot-electron effects. This paper reviews main failure modes and mechanisms of GaN HEMTs for microwave and millimeter-wave applications11Work partially supported by EUGANIC project (EDA Contract B 1447 IAP1 GP), EC Horizon 2020 ECSEL project SG_GaN_2, ESA ESTEC project RELGAN, Italian MIUR PRIN project GANAPP, by the Office of Naval Research award N00014-20-1-2177, under the supervision of Paul Mak

Thermally-activated failure mechanisms of 0.25 μm RF AlGaN/GaN HEMTs submitted to long-term life tests

Reliability and failure mechanism of 0.25 mu m AlGaN/GaN HEMTs under thermal storage tests and high temperature operating life (HTOL) tests have been evaluated. Results show that, during thermal storage tests, Schottky metal interdiffusion and gate sinking took place, possibly accompanied by thermo-mechanical degradation, with an activation energy of 1.8 eV. Failure modes consisted in carrier density decrease and sheet resistance increase, positive V-TH shift and I-DSS decrease. During HTOL tests, the degradation is mainly due to electrochemical oxidation of AlGaN, leading to on resistance increase, and I-DSS and g(m) decrease, with an activation energy of 1.0 eV