S. K. Dixit

Radiation Induced Charge Trapping in Ultrathin

Radiation induced charge trapping in ultrathin HfO2 -based n-channel MOSFETs is characterized as a function of dielectric thickness and irradiation bias following exposure to 10 keV X-rays and/or constant voltage stress. Positive and negative oxide-trap charges are observed, depending on irradiation and bias stress conditions. No significant interface-trap buildup is found in these devices under these irradiation and stress conditions. Enhanced oxide-charge trapping occurs in some cases for simultaneous application of constant voltage stress and irradiation, relative to either type of stress applied separately. Room temperature annealing at positive bias after irradiation of transistors with thicker gate dielectric films leads to positive oxide-trapped charge annihilation and/or neutralization in these devices, and net electron trapping. The oxide thickness dependence of the radiation response confirms the extreme radiation tolerance of thin HfO2 dielectric layers of relevance to device applications, and suggests that hole traps in the thicker layers are located in the bulk of the dielectric. A revised methodology is developed to estimate the net effective charge trapping efficiency, fot, for high-kappa dielectric films. As a result, estimates of fot for Hf silicate capacitors and Al2O3 transistors in previous work are reduced by up to 18%.

{\rm HfO}_{2}

The effects of gate length and drain bias on the off-state drain leakage current of irradiated fully-depleted SOI n-channel MOSFETs are reported. The experimental results are interpreted using a model based on the combined effects of band-to-band tunneling (BBT) and the trapped charge in the buried oxide. For negative gate-source voltages, the drain leakage current increases with the drain voltage because the electric field in the gate-to-drain overlap region is increasing. The off-state current in these devices increases with total ionizing dose due to oxide trapped charge build up in the buried oxide, enhanced by the BBT mechanism. The experimental data show that these effects are more significant for devices with shorter gate-lengths. Simulation results suggest that the BBT-generated holes are more likely to drift all the way from the drain to the source in shorter devices, enhancing the drain leakage current, while they tend to tunnel across the gate oxide in longer devices.

-Based MOSFETs

Nitrogen incorporation at the SiO2/SiC interface via high temperature nitric oxide annealing leads to the passivation of electrically active interface defects, yielding improved inversion mobility in the semiconductor. However, we find that such nitrided oxides can possess a larger density of hole traps than as-grown oxides, which is detrimental to the reliability of devices (e.g., can lead to large threshold voltage instabilities and to accelerated failure). Three different charge injection techniques are used to characterize this phenomenon in metal–oxide–semiconductor structures: x-ray irradiation, internal photoemission and Fowler–Nordheim tunneling. Some nitrogen-based atomic configurations that could act as hole traps in nitrided SiO2 are discussed based on first-principles density functional calculations.

Gate-Length and Drain-Bias Dependence of Band-to-Band Tunneling-Induced Drain Leakage in Irradiated Fully Depleted SOI Devices

The total dose radiation response of nitrided and non-nitrided n-type 4H-SiC is reported for metal oxide semiconductor capacitors exposed to 10-keV X-rays under positive bias. The radiation response is affected strongly by differences in the SiC band gap and interface/near interface SiO2 trap density from typical Si MOS devices. Significantly higher net trapped positive charge densities were observed in nitrided n-SiC MOS capacitors compared to the non-nitrided samples. The mechanisms contributing to the differences in the charge trapping in these devices are discussed. Differences in the interfacial layer between SiO2/Si and SiO2/SiC are responsible for the observed dissimilarities in charge trapping behavior

Increase in oxide hole trap density associated with nitrogen incorporation at the SiO2/SiC interface

A continuous analytical model for radiation-induced degradation in fully-depleted (FD) silicon on insulator (SOI) n-channel MOSFETs is presented. The combined effects of defect buildup in the buried oxide and band-to-band tunneling (BBT) have been shown to be the primary mechanisms that determine the radiation effects on the electrical characteristics. Closed-form expressions for the front and back-gate surface potential incorporate these effects, thereby enabling accurate modeling of the degraded current voltage characteristics that result from ionizing radiation exposure.

Total Dose Radiation Response of Nitrided and Non-nitrided SiO

Colloidal PbS/PbSe nanostructures with core-shell type morphology have been synthesized for the first time using a simple chemical procedure where template PbS nanocrystals (NCs) were treated with Se solution in tributylphosphine at elevated temperature. The Se-coated PbS NCs were structurally and optically characterized by high resolution transmission electron microscopy (HRTEM), x-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS) analysis, absorption and photoluminescence (PL) spectroscopy. Synthesized PbS/PbSe structures can be of particular importance in photovoltaic applications where fabrication of heterostructures with compositional modulation on the nanometer scale is essential.

_{2}

Radiation-induced charge trapping and mobility degradation are measured on uniaxially stressed HfO2-based nMOSFETs. Controlled external mechanical stress is applied via a four-point bending jig while the samples are irradiated using 10-keV X-rays. Positive charge trapping is observed for unstressed devices, and for devices irradiated under both compressive and tensile stress. Reduced trapped charge is measured as the uniaxial stress level increases. These results suggest that the increased stress leads to a reconfiguration of defect microstructure, as compared to the unstressed devices. The reconfiguration consists of changes in bond lengths and angles and a corresponding change in trap energy levels, which can (1) reduce the probability that a defect can capture holes, (2) increase the probability for electron and trapped-hole recombination, and/or (3) increase the mobility of the transporting holes in the oxides of the devices that are irradiated under stress. These results show that increased mechanical stress in high-k dielectrics does not degrade their radiation hardness. We also observe that the post-irradiation channel mobility degrades less in uniaxially stressed devices than unstressed devices.

/4H-SiC MOS Capacitors

Different amounts of degradation for n-Si and p-Si are observed after X-ray, H+, and He+ irradiations. Recombination lifetime and forward I-V measurements made on abrupt-junction diodes are compared to theory. Ionizing damage and displacement damage associated with surface and bulk trapping mechanisms, respectively, compete with each other and lead to different behaviors according to the doping type of the silicon on the lightly doped side of the junction. Surface effects are dominant in the n+/p diodes compared to the p+/n diodes; bulk trapping prevails in the n-Si compared to p-Si. Independently of ion type or fluence, the lifetime damage factor due to irradiation is worse in the p-Si than in the n-Si by a factor of 2-3 times.

Modeling the Radiation Response of Fully-Depleted SOI n-Channel MOSFETs

We report an observation of low-temperature, athermal, ion-induced decay of infrared-active bond-center hydrogen (BCH) in silicon. Specifically, the infrared intensity of BCH is found to decay monotonically as a function of ion dose with a decay constant determined by the electronic energy deposited by each ion. Our data indicate that ion-induced decay of BCH results in a different final configuration when compared to the thermal decay process. These findings provide insight into the structure and stability of hydrogen related defects in silicon, and thus have implications for the reliability of state-of-the-art semiconductor devices, radiation damage, and ion-beam characterization studies of hydrogen containing solids.