Multiphonon-assisted Tunneling Research Articles

Modeling the threshold voltage instability in SiC MOSFETs by multiphonon-assisted tunneling

Threshold voltage instability is a main reliability issue of silicon carbide MOS transistors submitted to gate bias stress. A new time and temperature-dependent TCAD model based on multiphonon-assisted tunneling is proposed. The dynamics during both stress and recovery phase are studied and are compared with experimental data.

Combined high permittivity and high electrical conduc­tivity of carbon nano-onion/polyaniline composites

We report high dielectric permittivity values for electrically conducting carbon nano-onion/polyaniline (CNO/PANI) composites. Broadband Dielectric Spectroscopy (BDS) measurements conducted under isothermal conditions from 16K to room temperature in the frequency range between 1mHz and 1MHz, revealed a Cole–Cole relaxation mechanism. The intensity of the relaxation is comparable with the observed huge static dielectric constant values, and indicates a correlation between the intense capacitance effects and the relaxation. The relaxation dynamics involve single or two phonon assisted tunneling of the relaxing electric charge carriers (protons and electrons) at low temperatures, which transition to multi-phonon assisted tunneling relaxation above 100K. Within the Mott–Davis model describing phonon-assisted tunneling relaxation in disordered media, the typical spatial scale relaxation occurs and is found to correlate with the size of the CNOs. The capacitance correlates with the large effective CNO surface areas.

Mechanism of GeO2 resistive switching based on the multi-phonon assisted tunneling between traps

Model of evenly distributed traps in bulk dielectric is proposed for the resistive memory switching mechanism. Switching from high resistance to the low resistance state is explained by several-fold increase in trap concentration after the application of switching voltage. Both high and low resistance conductivities are governed by multi-phonon ionization and tunneling between neighboring traps. Thermal trap energy for oxygen vacancy and electron effective mass for crystal α-GeO2 were calculated using density functional theory and used for the fitting of our charge transport model of resistive memory. The model was verified on the TaN-GeO2-Ni structure with good semi-quantitative agreement with experiment.

Optical properties of CdTe∕ZnTe quantum dots sandwiched between two quantum wells with ZnTe separation barriers

Temperature-dependent photoluminescence (PL) measurements were performed to investigate the optical properties of CdTe∕ZnTe nanostructures. The activation energy of the CdTe∕ZnTe quantum dots (QDs) sandwiched between two quantum wells (QWs) with ZnTe separation barriers, as obtained from the temperature-dependent PL spectra, was much larger than the activation energies of the QDs alone and of the QDs combined with a single QW. This behavior can be attributed to a transfer of carriers from the QWs to the QDs through a separation layer due to a nonresonant multiphonon-assisted tunneling process resulting from carrier thermal emission.

Photoluminescence study of carrier transfer among vertically aligned double-stacked InAs/GaAs quantum dot layers

Photoluminescence (PL) properties of self-organized quantum dots (QDs) in a vertically aligned double-layer InAs/GaAs QD structure are studied as a function of temperature from 10 to 290 K. The QDs in a sample with a 1.8 ML InAs seed layer and a second 2.4 ML InAs layer are found to self-organize in pairs of unequal sized QDs with clearly discernible ground-states transition energy. The unusual temperature behavior of the PL for such asymmetrical QD pairs provides clear evidence for carrier transfer from smaller to larger QDs by means of a nonresonant multiphonon-assisted tunneling process in the case of interlayer transfer and through carrier thermal emission and recapture within one layer.

Self-assembled InAs/GaAs quantum dots under resonant excitation

The energy structure and the carrier relaxation in self-assembled InAs/GaAs quantum dots (SADs) is investigated by photoluminescence excitation spectroscopy (PLE) and photoluminescence (PL) at resonant excitation (below the GaAs and the wetting layer bandgap). In PLE measurements we find a clear resonance from the first excited hole state as well as resonances from a relaxation via different phonons. From a comparison of the PL-rise times in time resolved spectroscopy, we conclude on a fast electron relaxation (⩽50 ps) and a slow hole relaxation with a time constant of about 400 ps. Different relaxation paths are observed in the InAs/GaAs quantum dot system and allow us to identify the hole relaxation in the SADs as multiphonon assisted tunneling. The PL-decay time in the SADs after resonant excitation (about 600 ps) is attributed to the lifetime of the quantum dot exciton. In agreement with theoretical predictions, we find a constant lifetime of about 600 ps for temperatures below 50 K and a linear increase of the lifetime between 50 and 100 K with a slope of 26 ps/K.

Intrinsic Carrier Relaxation and the Exciton Lifetime in InAs/GaAs Quantum Dots

The carrier relaxation in self-assembled InAs/GaAs quantum dots (SADs) is investigated by photoluminescence spectroscopy (PL) at resonant excitation (below the GaAs and the wetting layer bandgap). Different relaxation paths are observed in the InAs/GaAs quantum dot system and allow to identify the hole relaxation in the SADs as multiphonon assisted tunneling. The PL decay time in the SADs after resonant excitation (about 600 ps) is attributed to the lifetime of the quantum dot exciton. In agreement with theoretical predictions, we find a constant lifetime of about 600 ps for temperatures below 50 K and a linear increase of the lifetime between 50 and 100 K with a slope of 26 ps/K.

Deep impurity-center ionization by far-infrared radiation

An analysis is made of the ionization of deep impurity centers by high-intensity far-infrared and submillimeter-wavelength radiation, with photon energies tens of times lower than the impurity ionization energy. Within a broad range of intensities and wavelengths, terahertz electric fields of the exciting radiation act as a dc field. Under these conditions, deep-center ionization can be described as multiphonon-assisted tunneling, in which carrier emission is accompanied by defect tunneling in configuration space and electron tunneling in the electric field. The field dependence of the ionization probability permits one to determine the defect tunneling times and the character of the defect adiabatic potentials. The ionization probability deviates from the field dependence e(E) ∝ exp(E 2/E c 2 ) (where E is the wave field, and E c is a characteristic field) corresponding to multiphonon-assisted tunneling ionization in relatively low fields, where the defects are ionized through the Poole-Frenkel effect, and in very strong fields, where the ionization is produced by direct tunneling without thermal activation. The effects resulting from the high radiation frequency are considered and it is shown that, at low temperatures, they become dominant.

Multiphonon-assisted tunneling through deep levels: A rapid energy-relaxation mechanism in nonideal quantum-dot heterostructures.

The presence of a deep-level trap coupled to a quantum-dot heterostructure is shown to provide a rapid energy-relaxation pathway through which electrons may thermalize. A capture process is considered whereby a free conduction-band electron is captured into the ground conduction-band state of a quantum dot by multiphonon-assisted tunneling through the trap. As an example calculation, transition rates for a 5 nm radius ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As/GaAs quantum dot coupled to the defect M1 are calculated as a function of separation between the quantum dot and the deep level. For separations less than \ensuremath{\approxeq}10 nm these rates are found to be in excess of ${10}^{10}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at 4.2 K. The result suggests that the presence of point defects may serve to enhance the luminescence efficiency of quantum-dot material. The physical situation described in this paper could only arise if the spatial distribution of defects were strongly correlated with that of the quantum-dot structures, e.g., through formation of interface states or point defects as a consequence of the growth process. With this caveat, the proposed mechanism may possibly explain the failure to observe a significant phonon bottleneck effect in recent work on ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As quantum-dot structures [e.g., Appl. Phys. Lett. 64, 2815 (1994)].

Field and high-temperature dependence of the long term charge loss in erasable programmable read only memories: Measurements and modeling

High-temperature charge transport across an oxide-nitride-oxide sandwich of erasable programmable read only memories is mainly governed by the oxide conductivity as experimentally determined. It was verified in the examined devices that charge loss is not due to mobile ions. Since hole injection from the control gate into the nitride can be blocked by a 70-Å-thick top oxide we conjecture that charge loss is due to leakage of electrons; however, the observed leakage current is too large to be explained by pure electrode-limited charge transport (Richardson emission and direct tunneling). It was also verified that field gain on asperities and along edges cannot increase the charge loss current to the required range. Numerical evaluation of trap tunneling and resonant tunneling indicated that both mechanisms are weakly temperature dependent while charge loss has a typical activation energy of 1.2 eV in the range of 250–350 °C. Consequently, a multiphonon-assisted tunneling mechanism is proposed where electrons stored on the floating gate tunnel to oxide traps, then are emitted into the nitride. The coupling of the trap level to oxide phonons results in virtual energy levels in the oxide which allow for more effective transition paths. As a consequence of the electron-phonon coupling, the emission occurs close to the oxide conduction-band edge at temperatures between 250 and 350 °C, producing a strong temperature dependence for the mechanism.

Electron transport in Ta2O5 films

Present work deals with the studies of the voltage and temperature dependences of current in p-Si/Ta 2 O 5 /Al structures aiming to determine the dominating mechanism of charge transport. The measured dependences are explained on the basis of the multiphonon-assisted tunneling of electrons from deep centres in high electric fields