Haoshen Zhu

Diameter dependence of electron mobility in InGaAs nanowires

In this work, we present the diameter dependent electron mobility study of InGaAs nanowires (NWs) grown by gold-catalyzed vapor transport method. These single crystalline nanowires have an In-rich stoichiometry (i.e., In0.7Ga0.3As) with dispersed diameters from 15 to 55 nm. The current-voltage behaviors of fabricated nanowire field-effect transistors reveal that the aggressive scaling of nanowire diameter will induce a degradation of electron mobility, while low-temperature measurements further decouple the effects of surface/interface traps and phonon scattering, highlighting the impact of surface roughness scattering on the electron mobility. This work suggests a careful design consideration of nanowire dimension is required for achieving the optimal device performances.

AlN piezoelectric on silicon MEMS resonator with boosted Q using planar patterned phononic crystals on anchors

We report an approach to suppress anchor loss in thin-film piezoelectric-on-silicon (TPoS) micromechanical (MEMS) resonators by patterning 2D phononic crystals (PnCs) externally on the anchors. The PnCs serve as a frequency-selective reflector for outgoing acoustic waves through the tethers of the TPoS resonator. According to our experimental results, combining the PnCs with the conventional TPoS resonator significantly enhances the quality factor (Q) and correspondingly lowers the insertion loss (IL). The measured improvement is reproducible over multiple samples and consistent with the simulations by tuning the PnC bandgaps, suggesting significant reduction of acoustic leakage to the substrate by adopting the PnCs.

Shear dependent nonlinear vibration in a high quality factor single crystal silicon micromechanical resonator

The frequency response of a single crystal silicon resonator under nonlinear vibration is investigated and related to the shear property of the material. The shear stress-strain relation of bulk silicon is studied using a first-principles approach. By incorporating the calculated shear property into a device-level model, our simulation closely predicts the frequency response of the device obtained by experiments and further captures the nonlinear features. These results indicate that the observed nonlinearity stems from the material’s mechanical property. Given the high quality factor (Q) of the device reported here (∼2 × 106), this makes it highly susceptible to such mechanical nonlinear effects.

Piezoresistive Readout Mechanically Coupled Lamé Mode SOI Resonator With

This paper describes the use of the coupling beam in a pair of mechanically coupled Lamé mode resonators to enhance electromechanical transduction by piezoresistive sensing, while at the same time maintaining a high quality factor of a million. This corresponds to an fQ product of 1.3 × 1013, which approaches the Akhiezer limit of silicon. With a 15 mA bias current, electrical characterization of the array using the piezoresistive readout via the coupling-beam provides a 25 dB enhancement over a single Lamé mode resonator using capacitive readout. In this paper, we have modeled the piezoresistive electromechanical frequency response function of the device both analytically and by finite elements. The models mutually agree and are experimentally verified by measured results of fabricated resonators. The model indicates that the transduction factor is independent of the lateral dimensions and thickness of the resonator.

Q

This paper describes the phase velocity dispersion characteristics of Lamb waves propagating in AlN-on-Si plates, which achieves high quality factor (Q > 10000) and low motional resistance (R_m <; 50 Ω) at 260 MHz in air by using the fundamental quasi-symmetrical (QS₀) mode. We have found that the ratio of the Si layer thickness (h_Si) over the acoustic wavelength (λ) sets a limit for the QS₀ mode resonance at higher frequencies as increasing the ratio of h_Sl/λ reduces the electromechanical coupling for the QS₀ mode while enhancing the spurious quasi-antisymmetrical (QA₀) mode. Lastly, we also experimentally demonstrate that using multiple tethers helps suppress spurious modes.

of a Million

This paper presents experimental results showing reduced phase noise in a very high frequency band microelectromechanical systems (MEMS) oscillator. This has been achieved by engineering the embedded MEMS resonator with phononic crystal structures. The aluminum nitride on silicon MEMS resonator with phononic crystal tethers achieves an unloaded quality factor (Qu) 2.3 times of the same resonator with a simple tether design. The increase in Qu leads to a measured 6dB close-to-carrier phase noise reduction in an MEMS-based oscillator that sustained by a transimpedance amplifier implemented in 65nm CMOS process. The 141 MHz MEMS-CMOS oscillator with phononic crystal tethers has a phase noise better than -83dBc/Hz at 1 kHz offset and -134dBc/Hz at the far-from-carrier range while consuming less than 3mW.

High-Q low impedance UHF-band ALN-ON-SI mems resonators using quasi-symmetrical Lamb wave modes

It has been demonstrated that the piezoresistive effect in silicon can be useful for boosting electromechanical transduction in Micro Electro Mechanical Systems (MEMS) resonators. Piezoresistive sensing has been applied to a number of different extensional mode resonator topologies. In comparison, flexural modes are more compliant and of greater interest for mechanical sensing applications. To adopt piezoresistive sensing, flexural-mode resonators require patterning and doping to define piezoresistors at given locations. In this paper, we report a MEMS flexural-mode double-ended tuning fork (DETF) resonator that employs piezoresistive readout using the whole beam as the piezoresistive strain gauge. We show that the boundary conditions of the DETF allow for linear piezoresistive readout at the fundamental resonant frequency. In our device characterization results, we show that a bias current of 10 mA increases the transduction by 22 dB over the capacitive readout. We also model the coupling between beam deformations and the resulting changes in piezoresistivity.

Phase Noise Reduction in a VHF MEMS-CMOS Oscillator Using Phononic Crystals

We present a 13 MHz strongly coupled bulk Lamé mode silicon MEMS resonator-pair with quality factor (Q) of 106 (i.e. f·Q product of 1.3×1013) in addition to a 28 dB increase in transduction by uniquely adapting the coupling spring as an integrated piezoresistor. Our device exploits and preserves the intrinsic high Q of the isochoric Lamé mode while also tapping into the high concentration of stress in the coupling spring as the pair of Lamé mode resonators is synchronized to resonate in phase. This concentration of stress along the coupling beam benefits the output transduction efficiency, which in turn results in enhancing overall transduction while preserving very high Q. Q of the device also remains stable with increasing bias current.

Piezoresistive Transduction in a Double-Ended Tuning Fork SOI MEMS Resonator for Enhanced Linear Electrical Performance

This paper reports the first results that correlate nonlinear Duffing behavior and the temperature coefficient of frequency (TCf) in single-crystal-silicon (SCS) micromechanical resonators vibrating in lateral shear-dominant bulk modes, i.e. face shear (FS) and Lamé modes. Based on the anisotropic material property of SCS, we have derived a model to capture the nonlinear responses and the TCf for these devices in different orientations. The model predicts a clear orientation dependence of nonlinearity and temperature stability in SCS microresonators. These results suggest the possibility of reducing both nonlinearity and TCf by engineering the material properties (e.g. doping).

Piezoresistive sensing in a strongly-coupled high Q Lamé mode silicon MEMS resonator-pair

In this work, we report the experimental observation of quality factor (Q) variation in piezoresistively sensed silicon micromechanical resonators vibrating in a lateral contour mode. By fine tuning the bias current, the gradual descending and ascending trend of Q is distinctively observable. It shows a drastic drop in Q by nearly one order of magnitude (from 105 to 104) at a certain bias current level. The observed Q variation is replicable even when applying a capacitive sensing configuration with bias current running through. This anomaly is consistently detected from multiple die samples including resonators aligned along both <;110> and <;100> crystal orientations in the (100) plane. A detailed quantitative study is currently underway.