Huiqi Gong

The study of radiation effects in emerging micro and nano electro mechanical systems (M and NEMs)

The potential of micro and nano electromechanical systems (M and NEMS) has expanded due to advances in materials and fabrication processes. A wide variety of materials are now being pursued and deployed for M and NEMS including silicon carbide (SiC), III–V materials, thinfilm piezoelectric and ferroelectric, electro-optical and 2D atomic crystals such as graphene, hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS2). The miniaturization, Semiconductor Science and Technology Semicond. Sci. Technol. 32 (2017) 013005 (14pp) doi:10.1088/1361-6641/32/1/013005 15 Author to whom any correspondence should be addressed. 0268-1242/17/013005+14

Total-Ionizing-Dose Effects in Piezoresistive Micromachined Cantilevers

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Influence of LDD Spacers and H + Transport on the Total-Ionizing-Dose Response of 65-nm MOSFETs Irradiated to Ultrahigh Doses

We evaluate the response of T-shaped, asymmetric, piezoresistive, micromachined cantilevers fabricated on p-type Si to 10-keV X-ray irradiation. The resonant frequency decreases by 25 ppm at 2.1 Mrad(SiO2), and partially recovers during post-irradiation annealing. An explanation of the results is proposed that is based on radiation-induced acceptor depassivation. This occurs because radiation-generated holes release hydrogen from previously passivated acceptors, causing the carrier concentration to increase, especially near the surface. Increased carrier concentration decreases Young’s modulus, resulting in a decrease in the cantilever resonant frequency. Finite element simulations show that the effect of a decreasing Young’s modulus in the surface region is consistent with the measured decrease in resonant frequency in the irradiated devices.

Radiation-Induced Charge Trapping and Low-Frequency Noise of Graphene Transistors

The degradation induced by ultrahigh total ionizing dose in 65-nm MOS transistors is strongly gate-length dependent. The current drive decreases during irradiation, and the threshold voltage often shifts significantly during irradiation and/or high-temperature annealing, depending on transistor polarity, applied field, and irradiation/annealing temperature. Ionization in the spacer oxide and overlying silicon nitride layers above the lightly doped drain extensions leads to charge buildup as well as the ionization and/or release of hydrogen. Charge trapped in the spacer oxide or at its interface modifies the parasitic series resistance, reducing the drive current. The released hydrogen transports as H+ with an activation energy of ~0.92 eV. If the direction of the electric field is suitable, the H+ can reach the gate oxide interface and depassivate Si-H bonds, leading to threshold voltage shifts. Newly created interface traps are most prominent near the source or drain. The resulting transistor responses and defect-energy distributions often vary strongly in space and energy as a result, as demonstrated through current–voltage, charge-pumping, and low-frequency noise measurements.

Dose-Rate Effects on the Total-Ionizing-Dose Response of Piezoresistive Micromachined Cantilevers

We have performed a detailed evaluation of radiation-induced charge trapping and low-frequency noise for back-gated graphene transistors fabricated on a thermal SiO2 layer, with Al2O3 or hexagonal boron nitride passivation over-layers. Irradiation with positive or 0 V back-gate bias leads to negative shifts of the charge neutral point (CNP) of the graphene transistors; irradiation under negative back-gate bias leads to positive CNP shifts. The low-frequency noise increases with irradiation and decreases with 400 K postirradiation annealing. The temperature dependence of the noise is described well by the Dutta–Horn model of low-frequency noise. Peaks in effective defect-energy distributions of irradiated devices at ~0.4 and ~0.7 eV are identified via measurements of the temperature dependence of the low-frequency noise. The noise of as-processed devices stored in room ambient also decreases with baking, but does not show the clear peaks observed after irradiation. Density functional theory calculations suggest that OH− and H+ at or near the graphene/dielectric interfaces likely play key roles in both the irradiation and baking response. Low-frequency noise and CNP voltage shifts during switched-bias postirradiation annealing at room temperature also suggest significant roles for O vacancies in the near interfacial SiO2 and/or passivation layers.

Proton-Induced Displacement Damage and Total-Ionizing-Dose Effects on Silicon-Based MEMS Resonators

Total-ionizing-dose-induced resonance frequency shifts in piezoresistive micromachined cantilevers are experimentally shown to be dose-rate dependent. Devices were irradiated to 1 Mrad(SiO2) at rates from 5.4 to 30.3 krad(SiO2)/min, with lower rate exposures producing up to four-times more negative frequency shifts than higher rate exposures. Devices that were hydrogenated in a steam bath for 1 h showed shifts similar to those of control (not hydrogenated) devices at higher dose rates, and larger shifts than control devices at lower rates. All devices recovered to levels close to preirradiation after several hours of post-irradiation annealing. The dose-rate dependence is attributed to differences in carrier concentration caused by varying efficiencies of the depassivation of boron by hydrogen at higher and lower dose rates and/or surface charging effects, and the subsequent differences in Young’s modulus that occur as a result. Many of these processes are similar to effects that lead to ELDRS in linear bipolar transistors, emphasizing the need to include low-dose-rate testing of microelectromechanical systems devices when considering them for use in space systems.

Surface carrier concentration effect on elastic modulus of piezoelectric MEMS silicon cantilevers

The response of silicon-based microelectromechanical systems resonators to proton irradiation is determined by the combined effects of displacement damage and total ionizing dose (TID). Displacement damage (DD) can lead to carrier removal, which tends to decrease the carrier concentration, and TID leads to dopant activation and/or surface charging effects, which tend to increase the carrier concentration. These competing effects lead to changes in carrier concentration that alter Young’s modulus, and consequently the resonance frequency. For higher flux 2-MeV proton irradiation to 1014/cm2, TID effects dominate at low fluence, leading to a decrease in resonance frequency, which is offset by displacement damage effects at higher fluence. Fast recovery is observed as TID effects anneal out and DD effects remain. For lower-flux 0.8-MeV proton irradiation to

Total-Ionizing-Dose Effects on Piezoelectric Micromachined Ultrasonic Transducers

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Scaling Effects on Single-Event Transients in InGaAs FinFETs

/cm2, DD effects are relatively more significant than for 2-MeV proton irradiation, and an increase in resonance frequency is observed at all fluences. Stopping and range of ions in matter calculations reinforce these conclusions.

Total-Ionizing-Dose Responses of GaN-Based HEMTs With Different Channel Thicknesses and MOSHEMTs With Epitaxial MgCaO as Gate Dielectric

This paper provides experimental observation of the variation of the silicon elastic modulus with changing near surface carrier concentration. A piezoresistive MEMS cantilever is used that can simultaneously detect resistance and resonant frequency to probe the correlation between the electrical and mechanical properties of silicon under UV irradiation. Since both resistance and resonant frequency are sensitive to UVs thermal power, an identically processed reference device is used to separate optical effects from thermal effects and other environmental changes. Theory shows that charge carrier generation near the Si surface modifies the elastic modulus in agreement with our observed data.