Yichun Zhou

Temperature dependence of the elastic and vibronic behavior of Si, Ge, and diamond crystals

The thermally induced softening of the elastic and vibronic identities in crystals and their correlations have long been a puzzle. Analytical solutions have been developed, showing that the detectable elastic and vibronic properties could be related directly to the bonding parameters, such as bond length and strength, and their response to the temperature change. Reproduction of measured T-dependent Young’s modulus and Raman shift of Si, Ge, and diamond reveals that the thermally driven softening of the elasticity and the optical Raman frequency arises from bond expansion and vibration, with derived information about the atomic cohesive energy and clarification of their interdependence.

Hydrogen Bond Asymmetric Local Potentials in Compressed Ice

A combination of the Lagrangian mechanics of oscillators vibration, molecular dynamics decomposition of volume evolution, and Raman spectroscopy of phonon relaxation has enabled us to resolve the asymmetric, local, and short-range potentials pertaining to the hydrogen bond (O:H-O) in compressed ice. Results show that both oxygen atoms in the O:H-O bond shift initially outwardly with respect to the coordination origin (H), lengthening the O-O distance by 0.0136 nm from 0.2597 to 0.2733 nm by Coulomb repulsion between electron pairs on adjacent oxygen atoms. Both oxygen atoms then move toward right along the O:H-O bond by different amounts upon being compressed, approaching identical length of 0.11 nm. The van der Waals potential VL(r) for the O:H noncovalent bond reaches a valley at -0.25 eV, and the lowest exchange VH(r) for the H-O polar-covalent bond is valued at -3.97 eV.Combining the Lagrangian-Laplace mechanics and the known pressure dependence of the length-stiffness relaxation dynamics, we have determined the critical, yet often-overlooked, short-range interactions in the hydrogen bond of compressed ice. This approach has enabled determination of the force constant, cohesive energy, potential energy of the vdW and the covalent segment at each quasi-equilibrium state as well as their pressure dependence. Evidencing the essentiality of the inter-electron-pair Coulomb repulsion and the segmental strength disparity in determining the asymmetric H-bond relaxation dynamics and the anomalous properties of ice, results confirmed that compression shortens and stiffens the OH bond and meanwhile lengthens and softens the covalent bond.

Indentation depth dependence of the mechanical strength of Ni films

The indentation depth effect has been systematically examined on the mechanical properties of electrodeposited nickel films under 0% and 10% tensile strains. It is found that the indentation depth is proportional to the square root of the loads applied and the depth profiles of hardness and elastic modulus follow the similar trend of change with maximal values at the surface skins. The hardness and modulus then attenuate to a value of about half of the maximum, which follows the model proposed by Graca et al., Surf. Coat. Technol. (in press) with the mechanism of geometrically necessary dislocations and surface free energy. We suggest that the effect of surface oxidation and surface bond contraction [C. Q. Sun, Prog. Solid State Chem. 35, 1 (2007)] contributes intrinsically to the anomalous skin strengthening because of the local strain and energy trapping caused by surface bonds breaking.

Local bond average for the thermally induced lattice expansion.

An approach of local bond average (LBA) has been developed by extending the bond-order-length-strength correlation (Sun, C. Q. Prog. Solid State Chem. 2007 , 35, 1- 159 ) to temperature domain for the mechanism of the thermally driven bond expansion in a solid specimen. It is shown that the detectable quantities of a solid can be connected directly to the bonding identities and the response of the bonding identities to the stimulus of temperature change, being free from using the concepts in classical thermodynamics or hypothetical constants. The success of the LBA approach has been evidenced by fitting the observations from a number of specimens with derived information of atomic cohesive energy, which may go beyond the currently available approaches.

Improvement of resistive switching performances via an amorphous ZrO2 layer formation in TiO2-based forming-free resistive random access memory

We present the effects of an amorphous ZrO2 layer on the TiO2-based bipolar resistive switching memory device where the ZrO2 layer plays an important role as a supplementary reservoir of oxygen vacancies. Compared with Pt/TiO2/Pt monolayer device, a remarkably improved uniformity of switching parameters such as switching voltages and resistances in high/low states is demonstrated in the Pt/ZrO2/TiO2/Pt system. The resistive switching mechanism of memory devices incorporating the ZrO2/TiO2 bilayer structure can be attributed to multiple conducting filaments through the occurrence of redox reactions at the ZrO2/TiO2 surface.

Simulation of FeFET-Based Basic Logic Circuits and Current Sensing Amplifier

The usage of ferroelectric-gate field effect transistors (FeFET) in digital circuits typically involved that of memory applications, but the use of FeFET to implement logic circuits was not completely understood and had not been extensively studied. This paper presented the simulation of electrical properties of a single FeFET and the results of behavioral characterization of FeFET-based basic logic circuits in a commercially available TCAD environment. The all-FeFET CMOS inverter, current mirror, 2-input-NAND and 2-input-NOR logic circuits shown almost the same electrical characteristics as standard MOSFET except for the negligible clockwise hysteresis under the condition of low voltage supply. Moreover, the all-FeFET current mode sensitive amplifier whose component units were all-FeFET common-source amplifier, current mirror and differential amplification, also worked reasonably.

Design and Simulation of FeFET-Based Lookup Table

Ferroelectric random access memory (FeRAM) is a promising candidate to substitute static random access memory (SRAM) in lookup table (LUT) design due to its high density, high speed operation, anti-radiation and non-volatility. Ferroelectric gate field effect transistors (FeFETs) have been extensively studied and its usage in memory elements and basic analog circuit configurations has gained much interests. Here, we propose a novel architecture of FeFET-based LUT. An improved timing mode of FeRAM chip is analyzed to satisfy the performance of the FeFET-based LUT. Decoder, driver circuit and sensitive amplifier for FeFET array are also proposed. All the simulation results show that the proposed LUT works properly when the frequency reaches 500 MHz at 0.3 V differential input signal.

Design and implementation of fefet-based lookup table

Ferroelectric random access memory (FeRAM) is a promising candidate to substitute static random access memory (SRAM) in lookup table (LUT) design due to its high density, high speed operation, anti-radiation and non-volatility. Ferroelectric gate field effect transistors (FeFETs) have been extensively studied and its usage in memory elements and basic analog circuit configurations has gained much interests. Here, we propose a novel architecture of FeFET-based LUT. An improved timing mode of FeRAM chip is analyzed to satisfy the performance of the FeFET-based LUT. Decoder, driver circuit and sensitive amplifier for FeFET array are also proposed. Every design process is simulated by Sentarus TCAD. All the simulation results show that the proposed LUT works properly when the frequency reaches 500 MHz at 0.3 V differential input.

Failure mechanisms of guard-drain in mitigating n-hit single-event transient via 45-nm CMOS process

A three-dimensional (3D) technology computer-aided design (TCAD) modeling of guard-drain based on 45-nm CMOS process is presented. Numerical simulations of single-device show that charge collection of the guard-drain technique will higher than that of the conventional technique in different ion incidents. Guard-drain technique fails in helping drain to mitigate single-event transient (SET). Mechanisms of this abnormal phenomenon are analyzed in detail.