Trap-assisted Tunneling Of Electrons Research Articles

Investigation of the Progressive Gate Breakdown Behaviors in p-GaN Gate HEMTs

In this work, multiple time-dependent gate breakdown (BD) experiments are performed on p-GaN gate high-electron-mobility transistors (HEMTs) under constant gate voltage stress. Progressive gate BD behaviors are observed during the time-dependent failure process. It is found that the time-to-BD ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${t}_{\text {BD}}$ </tex-math></inline-formula> ) has a positive dependence on temperature but a weak relevance to p-GaN gate length. Moreover, it is confirmed that the Schottky junction first (1st) degenerates after the 1st stage jump of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${I}_{G}$ </tex-math></inline-formula> and the p-i-n junction fails in the subsequent process. The underlying failure mechanisms are revealed by further analyzing the degradation behaviors and the carrier transport mechanisms before/after the Schottky junction degrades. Hole Fowler–Nordheim (FN) tunneling at the metal gate/p-GaN interface guides the first degradation of the Schottky junction. Afterward, the trap-assisted electron tunneling across the AlGaN layer indicates the failure of the p-i-n junction. These two processes sequentially facilitate defect generation within the p-GaN and the AlGaN layer, resulting in a percolation path between the gate and the source terminal.

AlGaN/GaN Schottky-Gate HEMTs With UV/O₃-Treated Gate Interface

The surface condition under gate of AlGaN/GaN heterostructure plays a critical role in high-electron mobility transistor (HEMT). In this study, the effects of ultraviolet/ozone (UV/O3) treatment applied to Al0.3 Ga0.7 N/GaN heterostructure on the electrical performance of AlGaN/GaN Schottky-gate HEMT were investigated. The reverse-bias leakage current of Schottky diode was reduced by three orders after the treatment. X-ray photoelectron spectroscopy confirms the formation of the Ga2O x layer which serves as an interface passivation layer and thus suppresses trap-assisted electron tunneling. Capacitance–voltage measurements of AlGaN/GaN HEMT show shifts of threshold and on-set voltages, indicating decreased surface states as a result of the treatment. The electrical characteristics of AlGaN/GaN HEMT exhibit improved transconductance and subthreshold swing values after the treatment.

Ultraviolet to near-infrared broadband organic photodetectors with photomultiplication

The broadband organic photodetectors (OPDs) with photomultiplication (PM) phenomenon are fabricated by using Y6 doped with rather less PBDB-T as active layers having electron-only transport characteristic. The PM-OPDs with PBDB-T:Y6 (3:100, wt/wt) as active layers exhibit the external quantum efficiency (EQE) of 18,000% at 350 nm and 15,000% at 870 nm under 1 V bias. The PM-OPDs exhibits a broad spectral response range from 320 to 950 nm, which is mainly determined by the trapped hole distribution near Al electrode. The working mechanism of PM-OPDs should be attributed to interfacial trap-assisted electron tunneling injection, which can be further confirmed from the EQE spectral shape of PM-OPDs with different active layer thickness. The n-type polymer PZ1 was selected as interfacial layer to be inserted between PEDOT:PSS and active layers, which makes PM-OPDs with PZ1 interfacial layer work well under forward and reverse bias. This work may provide a smart strategy to achieve broadband PM-OPDs by adjusting the distribution of trapped charge near electrode.

Study of the electrical stress effect on the SiC-SiO2 interface by inelastic electron tunneling spectroscopy

The influence of electrical stress on the behavior of traps at SiC-SiO2 interface are studied by inelastic electron tunneling spectroscopy (IETS). Measured IETS demonstrate the characteristic vibration modes contributed from defects at both metal-oxide and SiC-SiO2 interface. The observed distinct peaks in IETS provide unambiguous evidence to the existence of disordered carbon cluster at SiC-SiO2 interface. By performing bias-dependent IETS measurement, trap-assisted electron tunneling and charge-trapping process were observed and the defect level were determined for SiC-SiO2 interface. Moreover, the influence of electrical stress on the IETS peak-valley intensity and position was investigated and its origination was discussed.

Increase in the Shockley–Read–Hall recombination rate in InGaN/GaN QWs as the main mechanism of the efficiency droop in LEDs at high injection levels

It is shown that the efficiency droop observed as the current through a GaN-based light-emitting diode increases is due to a decrease in the Shockley–Read–Hall nonradiative lifetime. The lifetime decreases with increasing current because a steadily growing number of traps in the density-of-states tails of InGaN/GaN quantum wells become nonradiative recombination centers upon the approach of quasi-Fermi levels to the band edges. This follows from the correlation between the efficiency droop and the appearance of negative differential capacitance, observed in the study. The correlation appears due to slow trap recharging via the trap-assisted tunneling of electrons through the n-type barrier of the quantum well and to the inductive nature of the diode-current variation with forward bias.

Carrier transport and electroluminescence efficiency of erbium-doped silicon nanocrystal superlattices

A detailed study of transport phenomena and electroluminescence of erbium-doped silicon-rich oxide/silicon oxide superlattices is presented. Extended states conduction is thermally activated from Poole-Frenkel traps located at silicon nanocrystals or its interface. These traps provide bulk limited conduction at low and medium electric fields. In contrast, under high electric fields, conduction is governed by trap-assisted tunneling of electrons from the electrode to the active layer conduction band. Superlattice electroluminescence efficiency at 1.5 μm and injected electron energy distribution in the conduction band are evaluated and compared to a silicon dioxide and a silicon-rich oxide single layer. This work sheds light on the implementation of alternative electroluminescent device architectures with strong emphasis in the hot electron engineering.

Trap-assisted tunneling resistance switching effect in CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3 heterostructure

We reported the resistance switching (RS) behavior in the epitaxially grown CeO2/ La0.7(Sr0.1Ca0.9)0.3MnO3 (CeO2/LSCMO) heterojunctions on SrTiO3 substrate. The CeO2/LSCMO device displayed improved switching characteristics as compared to that of metal/manganite device. The switching threshold voltage showed a strong dependence on the thickness of the CeO2 layer, where a minimum/maximum thickness was required for the appearance of the resistance switching. Both set and reset threshold voltages increase with the increase of the CeO2 layer thickness due to the trap-assisted electron tunneling effect. In the meantime, the defects or vacancies in the CeO2 films, in particular, the concentration of the defects or vacancies in the interface between CeO2 and LSCMO, have a significant impact on the switching effect. These results suggest that the electron tunneling accompanied by a trapping/detrapping process at the interface is likely responsible for the RS effect in the insulator/manganite system.

Model for trap-assisted electron tunneling in thin insulators

The trap-assisted electron tunneling in thin insulators has been modeled to explain the apparition of plateaus in the current-voltage characteristics of some existing experimental data. As a consequence of the electron tight confinement in the trap, the charge therein has a steplike variation with the voltage increase, a feature that further translates into wide plateaus of the current-voltage diagrams, in accordance with some experimental data. The temperature dependence of these diagrams has been included into the model and compares well with the experimental evidence. The possibility of quantitatively comparing the model computations with the measured data opens useful insight into trap’s physical properties such as spatial extent or energy depth.

Core-Shell Germanium–Silicon Nanocrystal Floating Gate for Nonvolatile Memory Applications

We have fabricated germanium-silicon (Si/HfSiOx) core-shell nanocrystal (NC) structures to work as charge storage nodes in NC flash memories. This core shell NC structure was made by doing silane annealing treatment before and after Ge NC deposition. This silicon(Si/HfSiOx) shell layer can separate the Ge NC from HfO2 and ambient oxidants in the following process, and reduces low-quality GeOx, HfGeOx to metallic Ge. Thus, a more robust interface with low trap density between the high-kappa dielectric and the NCs was achieved, which helps suppress the charges loss due to trap-assisted tunneling of electrons and results in better device performance.

IONIZING RADIATION EFFECTS ON ULTRA-THIN OXIDE MOS STRUCTURES

We have briefly reviewed the most important degradation mechanisms affecting ultra-thin gate oxides after exposure to ionizing irradiation. The increase of the gate leakage current seems the most crucial issue for device lifetime, especially for non-volatile memory and dynamic logic. The build-up of positive charge in the oxide and the subsequent threshold voltage shift, which was the major concern for thicker oxide, are no longer appreciable in today's devices due to the reduced oxide thickness permitting a fast recombination of trapped holes with electrons from interfaces. Among the leakage currents affecting thin oxides we have considered here the Radiation Induced Leakage Current (RILC) and the Radiation Soft Breakdown (RSB). RILC is observed after irradiation with a low Linear Energy Transfer (LET) radiation source and comes from a trap-assisted tunneling of electrons mediated by the neutral traps produced by irradiation. RILC depends on the applied bias during irradiation and the maximum is measured when devices are biased in flat band. Contrarily to RILC, RSB is observed after irradiation with high LET ions and derives from the formation of several conductive paths across the oxide corresponding to the ion hits. RSB conduction is explained by the theory of the Quantum Point Contact as also proposed for the electrically induced Soft breakdown. Finally, we present some preliminary results, which indicate that although the direct effects of irradiation (in terms of gate leakage current increase) are small for oxide thinner than 3nm, it is possible that these devices may experience an accelerated wear-out and/or breakdown after subsequent electrical stress relative to a fresh (not irradiated) device.

Group-IV nanocluster formation by ion-beam synthesis

A short review of our investigations devoted to the use of ion-beam-synthesized nanoclusters for silicon-based light emission and nonvolatile memory effects is presented. Blue-violet light emission is demonstrated based on Ge-implanted silicon dioxide layers thermally grown on silicon substrates. This version of silicon-based light emission relies on Ge-related defects in the amorphous ≡Si–O–Si≡ network. The photoluminescence and electroluminescence are excited by a singlet S0–S1 transition of a neutral oxygen vacancy and by electron injection from the silicon substrate into the silicon dioxide layer, respectively. Whereas the photoluminescence excitation is a well-known mechanism, for the case of electroluminescence an interpretation was performed for the first time in the course of our studies. It was found that the most probable way to excite luminescence centers is the impact excitation by hot electrons. Whereas the injection is explained by trap-assisted tunneling of electrons from the substrate into the oxide, the electrons will be transported via traps or in the SiO2 conduction band. The application of the silicon-based light-emitting devices for an integrated optocoupler arrangement is described. Another application of nanoclusters is based on the investigation of thin Si-implanted silicon dioxide layers for nonvolatile memory devices. First promising results demonstrate that the observed programming window can reach several volts and the devices exhibit excellent retention behavior. A 256 K-nv-SRAM is demonstrated showing a programming window of >1 V for write pulses of 12 V/8 ms.

Transient behavior of the strong violet electroluminescence of Ge-implanted SiO 2 layers

Si-based light emitters will be a key element of future optoelectronics. One of the most promising approaches is Ge implantation into thin SiO2 films on crystalline Si. This system exhibits a strong violet electroluminescence with a power efficiency up to 0.5% [18], but the mechanism of electrical excitation is not yet fully understood. In this paper the electrical excitation of the luminescence centers is investigated by means of electrical and electroluminescence transient measurements. It is found that the most probable way to excite luminescence centers is the impact excitation by hot electrons. Whereas the injection is explained by trap-assisted tunneling of electrons from the substrate into the oxide, the electrons will be transported via traps or in the SiO2 conduction band. Furthermore, the electroluminescence rise and decay time is estimated to be of the order of 100 μs.

Trap generation and breakdown processes in very thin gate oxides

Neutral electron traps are generated in gate oxide during electrical stress, leading to degradation in the form of stress-induced leakage current (SILC) and eventually resulting in breakdown. SILC is the result of inelastic, trap-assisted tunneling of electrons that originate in the conduction band of the cathode. Deuterium annealing experiments call into question the interfacial hydrogen release model of the trap generation mechanism. A framework for modeling time-to-breakdown is presented.

Trap-assisted tunneling in high permittivity gate dielectric stacks

The electrical characteristics of SiOx/ZrO2 and SiOx/Ta2O5 gate dielectric stacks are investigated. The current–density JG in these dielectric stacks is shown to be strongly temperature dependent at low voltage (below about 2 V), the more so in the ZrO2 stack. On the other hand, JG is much less temperature dependent at higher voltage. These results are consistent with a model which takes into account the direct tunneling of electrons across the SiOx layer and the trap-assisted tunneling of electrons through traps with energy levels below the conduction band of the high permittivity dielectric layer. The energy levels and densities of these electron trapping centers are estimated by fitting this trap-assisted tunneling model to the experimental results.