En Xia Zhang

Fin-Width Dependence of Ionizing Radiation-Induced Subthreshold-Swing Degradation in 100-nm-Gate-Length FinFETs

The dependence of the subthreshold-swing (SS) degradation on fin width is reported for irradiated 100-nm-gate- length, fully depleted n -channel FinFETs. The wider the fin is, the greater the radiation-induced SS degradation. The higher tolerance to radiation-induced charge for the narrower FinFETs is attributed to the additional lateral gate control over the body potential. The irradiation and room temperature annealing results suggest that the SS increase for wider FinFETs is due primarily to nonuniform trapped charge in the buried oxide (BOX). The subthreshold characteristics of FinFETs with two fins are more likely to exhibit a nonuniform subthreshold slope (NUSS), resulting from fin-to-fin variability, than FinFETs with 20 fins, where the corresponding Id -V gs curve is the composite of the 20 individual Id-V gs curves.

Process Dependence of Proton-Induced Degradation in GaN HEMTs

The 1.8-MeV proton radiation responses are compared for AlGaN/GaN HEMTs grown under Ga-rich, N-rich, and NH3-rich conditions. The NH3-rich devices are more susceptible to proton irradiation than the Ga-rich and N-rich devices. The 1/ f noise of the devices increases with increasing fluence. Density functional theory calculations show that N vacancies and Ga-N divacancies lead to enhanced noise in these devices.

Effect of Transistor Density and Charge Sharing on Single-Event Transients in 90-nm Bulk CMOS

Heavy-ion experiments on spatially isolated inverters and densely populated inverters demonstrate the effects of transistor density on single-event (SE) transients in bulk CMOS. Increased transistor density reduces SE cross section dramatically while having little impact on transient pulse width.

Laser- and Heavy Ion-Induced Charge Collection in Bulk FinFETs

Through-wafer two-photon absorption laser experiments were performed on bulk FinFETs. Transients show distinct signatures for charge collection from drift and diffusion, demonstrating the contribution of charge generated in the substrate to the charge collection process. This result was validated through heavy ion testing on more advanced bulk FinFETs with fin widths as narrow as 5 nm. The drain region dominates the charge collection, with as much as 45 fC of charge collected in the drain region.

Proton-Induced Dehydrogenation of Defects in AlGaN/GaN HEMTs

The responses to 1.8 MeV proton irradiation of AlGaN/GaN HEMTs grown under Ga-rich and ammonia-rich conditions are investigated in this work. Changes in defect energy distributions of AlGaN/GaN HEMTs during proton irradiation are characterized via temperature-dependent low-frequency noise measurements. Density functional theory calculations show these changes are consistent with the reconfiguration and/or dehydrogenation of oxygen-related defects in Ga-rich devices.

Ozone-exposure and annealing effects on graphene-on-SiO2 transistors

We employ resistance measurements and Raman spectroscopy to investigate the effects of UV ozone (UVO) exposure and Ar annealing on graphene-on-SiO2 transistors. Shorter UVO exposures lead to oxygen adsorption and doping; longer exposures lead to significant defect generation and then to etching. Elevated-temperature Ar annealing following UVO exposure leads to local defect healing, as shown by the evolution of the characteristic Raman D- and G-peaks. In striking contrast, the overall graphene transistor resistance increases significantly due to void formation. Density functional calculations show that carbon-oxygen reactions lead to efficient consumption and release of C atoms (as CO or CO2) under conditions of high surface oxygen concentration.

Low-Energy X-ray and Ozone-Exposure Induced Defect Formation in Graphene Materials and Devices

We have evaluated the responses of graphene materials and devices to 10-keV X-ray irradiation and ozone exposure. Large positive shifts are observed in the current-voltage characteristics of graphene-on- SiO2 transistors irradiated under negative gate bias. Moreover, significant radiation-induced increases are found in the resistance of suspended graphene layers; the charge neutral point (CNP) of the graphene layer also shifts positively with increasing total dose. Raman spectroscopy shows that similar defects are generated in graphene-on-SiO2 sheets by 10-keV X-ray irradiation and ozone exposure. First principles calculations of the relevant binding energies, and reaction and diffusion barriers for oxygen on graphene, strongly suggest that oxygen adsorption and reactions, along with the resulting p -type doping, can lead to the observed degradation of irradiated or ozone-exposed graphene materials and devices.

Radiation-Induced Defect Evolution and Electrical Degradation of AlGaN/GaN High-Electron-Mobility Transistors

Threshold-voltage shifts and increases in 1/f noise are observed in proton-irradiated AlGaN/GaN high-electron-mobility transistors, indicating defect-mediated device degradation. Quantum mechanical calculations demonstrate that low-energy recoils caused by particle interactions with defect complexes are more likely to occur than atomic displacements in a defect-free region of the crystal. We identify the responsible defects and their precursors in the defect-mediated displacement mechanism. The electronic properties of these defects are consistent with the increases in threshold voltage and 1/f noise in proton irradiation experiments.

Atomic-scale origins of bias-temperature instabilities in SiC–SiO2 structures

We find that atomic-scale mechanisms for bias-temperature instabilities (BTIs) in SiC/SiO2 structures can differ significantly from those in Si/SiO2 structures. The measured effective-activation energies for BTI in 4H-SiC metal-oxide-semiconductor capacitors, 0.23±0.02 eV for p-type and 0.12±0.02 eV for n-type, are essentially identical to the respective dopant ionization energies, which are much larger than in Si. This suggests a key role for carrier release from deep dopants for BTI in SiC. In addition, asymmetric degradation is observed under switched-bias stress in p-type and n-type SiC, as a result of the reconfiguration of O vacancies in SiO2 layer after hole capture.

Impact of proton irradiation on deep level states in n-GaN

Deep levels in 1.8 MeV proton irradiated n-type GaN were systematically characterized using deep level transient spectroscopies and deep level optical spectroscopies. The impacts of proton irradiation on the introduction and evolution of those deep states were revealed as a function of proton fluences up to 1.1 × 1013 cm−2. The proton irradiation introduced two traps with activation energies of EC - 0.13 eV and 0.16 eV, and a monotonic increase in the concentration for most of the pre-existing traps, though the increase rates were different for each trap, suggesting different physical sources and/or configurations for these states. Through lighted capacitance voltage measurements, the deep levels at EC - 1.25 eV, 2.50 eV, and 3.25 eV were identified as being the source of systematic carrier removal in proton-damaged n-GaN as a function of proton fluence.