Cher Xuan Zhang

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.

Electron-Induced Single-Event Upsets in Static Random Access Memory

We present experimental evidence of single-event upsets in 28 and 45 nm CMOS SRAMs produced by single energetic electrons. Upsets are observed within 10% of nominal supply voltage for devices built in the 28 nm technology node. Simulation results provide supporting evidence that upsets are produced by energetic electrons generated by incident X-rays. The observed errors are shown not to be the result of “weak bits” or photocurrents resulting from the collective energy deposition from X-rays. Experimental results are consistent with the bias sensitivity of critical charge for direct ionization effects caused by low-energy protons and muons in these technologies. Monte Carlo simulations show that the contributions of electron-induced SEU to error rates in the GEO environment depend exponentially on critical charge.

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.

Charge Trapping Properties of 3C- and 4H-SiC MOS Capacitors With Nitrided Gate Oxides

Silicon-carbide-based MOS capacitors were formed on either 3C (epitaxial on Si) or 4H substrates and using SiO2 gate dielectrics both with and without interfacial nitrogen. The charge trapping properties of these structures were examined after exposure to ionizing radiation. In all cases interfacial nitrogen results in improved trap density and increased oxide charge trapping. For equivalent nitrogen content, 3C-based devices exhibit more charge trapping than the 4H-based equivalents.

RF Performance of Proton-Irradiated AlGaN/GaN HEMTs

AlGaN/GaN high electron mobility transistors (HEMTs) irradiated with 1.8-MeV protons show more relative degradation in RF power/current gain, cutoff frequency fT, and maximum oscillation frequency fmax than DC transconductance. These result from radiation-induced increases in fast bulk and surface trap densities, as well as increasing impedance mismatch at high frequencies with increasing proton fluence. NH3-rich MBE devices show less degradation in DC transconductance, but more degradation in RF gain than Ga-rich devices.

Electrical Stress and Total Ionizing Dose Effects on Graphene-Based Non-Volatile Memory Devices

Electrical stress and 10-keV x-ray and 1.8-MeV proton irradiation and annealing responses are evaluated for graphene-based non-volatile memory devices. The memory characteristics of these structures derive primarily from hysteretic charge exchange between the graphene and interface and border traps, similar to the operation of metal-nitride-oxide-semiconductor memory devices. Excellent stability and memory retention are observed for ionizing radiation exposure or constant-voltage stress. Cycling of the memory state leads to a significant reduction in memory window.

Effects of Bias on the Irradiation and Annealing Responses of 4H-SiC MOS Devices

The irradiation and annealing responses of 4H-SiC MOS devices with nitrided oxides are investigated at varying biases. Radiation-induced hole trapping dominates the radiation response of these devices. Switching the bias between irradiation and annealing can lead to significant enhancement of the degradation in some cases. Positive irradiation bias followed by negative annealing bias is found to be the worst-case for the combined degradation. After irradiation, significant reversibility of charge trapping is observed in switched-bias annealing of n-substrate capacitors, while enhanced charge trapping is found in p-substrate capacitors. These results show the potential importance of evaluating the combined radiation and reliability responses of SiC MOS devices for long-term use in high total-ionizing-dose space applications.

Bias dependence of total ionizing dose effects in SiGe-SiO2HfO2 p MOS FinFETs

The total ionizing dose (TID) response of double-gate SiGe- SiO₂/HfO₂ pMOS FinFET devices is investigated under different device bias conditions. Negative bias irradiation leads to the worst-case degradation due to increased hole trapping in the HfO₂ layer, in contrast to what is typically observed for devices with SiO₂ or HfO₂ gate dielectrics. This occurs in the devices because radiation-induced holes that are generated in the SiO₂ interfacial layer can transport and become trapped in the HfO₂ under negative bias, leading to a more negative threshold voltage shift than observed at 0 V bias. Similarly, radiation-induced electrons that are generated in the SiO₂ interfacial layer can transport into the HfO₂ and become trapped under positive bias, leading to a more positive threshold voltage shift than observed at 0 V bias.