Energy-resolved optically stimulated luminescence (OSL) spectra and thermoluminescence (TL) glow curves of a powder sample of YPO
4:Ce
3+,Sm
3+ were measured to investigate the nature of the trapping ...centre and to locate its energy level relative to the valence and conduction bands of the YPO
4 host. The high-temperature glow peak could unequivocally be assigned to Sm
2+ (thus Sm
3+ acts as an electron trap). The trap depth of this centre, as derived from the OSL excitation spectra, is in good agreement with the Dorenbos model prediction. The OSL excitation spectra also reveal excited states of Sm
2+ well below the conduction band. These excited states produce a broadening of the high-temperature TL glow peak and also cause the activation energy determined by the Hoogenstraten method to underestimate the trap depth.
Carbon-based molecular junctions consisting of aromatic oligomers between conducting sp2 hybridized carbon electrodes exhibit structure-dependent current densities (J) when the molecular layer ...thickness (d) exceeds ∼5 nm. All four of the molecular structures examined exhibit an unusual, nonlinear ln J vs bias voltage (V) dependence which is not expected for conventional coherent tunneling or activated hopping mechanisms. All molecules exhibit a weak temperature dependence, with J increasing typically by a factor of 2 over the range of 200–440 K. Fluorene and anthraquinone show linear plots of ln J vs d with nearly identical J values for the range d = 3–10 nm, despite significant differences in their free-molecule orbital energy levels. The observed current densities for anthraquinone, fluorene, nitroazobenzene, and bis-thienyl benzene for d = 7–10 nm show no correlation with occupied (HOMO) or unoccupied (LUMO) molecular orbital energies, contrary to expectations for transport mechanisms based on the offset between orbital energies and the electrode Fermi level. UV–vis absorption spectroscopy of molecular layers bonded to carbon electrodes revealed internal energy levels of the chemisorbed films and also indicated limited delocalization in the film interior. The observed current densities correlate well with the observed UV–vis absorption maxima for the molecular layers, implying a transport mechanism determined by the HOMO–LUMO energy gap. We conclude that transport in carbon-based aromatic molecular junctions is consistent with multistep tunneling through a barrier defined by the HOMO–LUMO gap, and not by charge transport at the electrode interfaces. In effect, interfacial “injection” at the molecule/electrode interfaces is not rate limiting due to relatively strong electronic coupling, and transport is controlled by the “bulk” properties of the molecular layer interior.
We investigated electric field-induced trapped electron lateral migration in a SONOS flash cell. The threshold voltage shift (ΔV t ) and gate-induceddrain leakage (GIDL) current were measured to ...monitor nitride electron movement in retention. We applied different voltages to the gate and the source/drain in retention to vary the vertical and lateral electric fields. Our study shows that: 1) GIDL current can be used to monitor trapped charge lateral migration and 2) nitride charge lateral migration exhibits strong dependence on the lateral electric field. Based on measured temperature and field dependence, a nitride trapped charge emission process via thermally assisted tunneling is proposed for electron lateral migration. The emission rates of thermally assisted tunneling, direct trap-to-band tunneling and Frenkel-Poole emission were compared.
Eliminating the influence of environment temperature is critical for high-accuracy carbon nanotube polymer nanocomposites sensors. In this work, the temperature effects on the nanocomposite are ...studied by both experiment investigation and simulation calculation. Nearly constant resistance values were found at a CNT loading around 3 wt%. By considering the temperature effect and CNT structural distortion in the developed percolation network model, simulation results agree well with experimental data. On this basis, results show that the thermally assisted tunneling on CNT junctions and thermal expansion of polymer matrix are the two core mechanisms, relaxed CNT junctions in CNT/polymer nanocomposite depressed the temperature effect, and the influence of environment temperature could be significantly reduced by adjusting CNT loadings and choosing a matching polymer matrix. All of these findings will benefit for the design of high-accuracy sensors.
In this work, a fundamental problem of the conventional temperature-accelerated life-test methodology is revealed owing to the coexistence of three failure mechanisms in 3-D NAND Flash memories. ...Different from previous studies, for the first time, we are able to separate the roles of trapped electron vertical loss and lateral migration experimentally in multiple bake temperature and different program/erase (P/E) cycle number without any simulation tool and fitting model according to the neighboring data pattern effect on threshold voltage \boldsymbol{(\mathrm{V}_{\mathrm{t}})} traces and the extracted activation energies \boldsymbol{(\mathrm{E}_{\mathrm{a}})} under various conditions. We found that Vt retention loss at lower temperatures tends to be dominated by trapped electron direct tunneling (DT) out from silicon nitride (SiN) to Si channel. At bake temperature rises, Vt loss in non-cycled cells is gradually originated from SiN trapped electron lateral migration via thermally assisted tunneling (ThAT) while Vt loss in P/E-stressed devices is mainly caused by trapped electron vertical escape from SiN storage layer through Frenkel-Poole (F-P) emission and the subsequent positive charge-assisted tunneling (PCAT) process.
Two commonly observed charge transport mechanisms in single-molecule junctions are coherent tunneling and incoherent hopping. It has been generally believed that tunneling processes yield ...temperature-independent conductance behavior and hopping processes exhibit increasing conductance with increasing temperature. However, it has recently been proposed that tunneling can also yield temperature-dependent transport due to the thermal broadening of the Fermi energy of the contacts. In this work, we examine a series of rigid, planar furan oligomers that are free from a rotational internal degree of freedom to examine the temperature dependence of tunneling transport directly over a wide temperature range (78–300 K). Our results demonstrate conductance transition from a temperature-independent regime to a temperature-dependent regime. By examining various hopping and tunneling models and the correlation between the temperature dependence of conductance and molecular orbital energy offset from the Fermi level, we conclude thermally assisted tunneling is the dominant cause for the onset of temperature-dependent conductance in these systems.
A positive bias temperature instability (PBTI) recovery transient technique is presented to investigate trap properties in HfSiON as high-k gate dielectric in nMOSFETs. Both large- and small-area ...nMOSFETs are characterized. In a large-area device, the post-PBTI drain current exhibits a recovery transient and follows logarithmic time dependence. In a small-area device, individual trapped electron emission from HfSiON gate dielectric, which is manifested by a staircase-like drain current evolution with time, is observed during recovery. By measuring the temperature and gate voltage dependence of trapped electron emission times, the physical mechanism for PBTI recovery is developed. An analytical model based on thermally assisted tunneling can successfully reproduce measured transient characteristics. In addition, HfSiON trap properties, such as trap density and activation energy, are characterized by this method.
A novel method to study post-NBTI stress recovery in pMOSFETs is demonstrated by direct measurement of single-hole de-trapping behavior. Individual trapped hole emission in NBTI recovery is observed ...for the first time, which is manifested by the step-like evolution of channel current. By measuring trapped hole emission times and corresponding current jump, the dependence of NBTI recovery on stress hardness, recovery gate voltage and temperature, and gate length is characterized. An analytical model based on thermally-assisted tunneling of trapped oxide charges can successfully reproduce measured recovery characteristics.
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