Eugene Freeman

Intrinsic electronic switching time in ultrathin epitaxial vanadium dioxide thin film

This letter investigates the intrinsic electronic switching time associated with the insulator-to-metal phase transition in epitaxial single crystal vanadium dioxide (VO2) thin films using impedance spectroscopy and ac conductivity measurements. The existence of insulating and metallic phase coexistence, intrinsic to the epitaxial (001) oriented VO2 thin film grown on a (001) rutile TiO2 substrate, results in a finite capacitance being associated with the VO2 films in their insulating phase that limits the electronic switching speed. Insights into the switching characteristics and their correlation to the transport mechanism in the light of phase coexistence are obtained by performing a detailed scaling study on VO2 two-terminal devices.

Transport properties of ultra-thin VO2 films on (001) TiO2 grown by reactive molecular-beam epitaxy

We report the growth of (001)-oriented VO2 films as thin as 1.5 nm with abrupt and reproducible metal-insulator transitions (MIT) without a capping layer. Limitations to the growth of thinner films with sharp MITs are discussed, including the Volmer-Weber type growth mode due to the high energy of the (001) VO2 surface. Another key limitation is interdiffusion with the (001) TiO2 substrate, which we quantify using low angle annular dark field scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy. We find that controlling island coalescence on the (001) surface and minimization of cation interdiffusion by using a low growth temperature followed by a brief anneal at higher temperature are crucial for realizing ultrathin VO2 films with abrupt MIT behavior.

Nanoscale structural evolution of electrically driven insulator to metal transition in vanadium dioxide

The structural evolution of tensile strained vanadium dioxide thin films was examined across the electrically driven insulator-to-metal transition by nanoscale hard X-ray diffraction. A metallic filament with rutile (R) structure was found to be the dominant conduction pathway for an electrically driven transition, while the majority of the channel area remained in the monoclinic M1 phase. The filament dimensions were estimated using simultaneous electrical probing and nanoscale X-ray diffraction. Analysis revealed that the width of the conducting channel can be tuned externally using resistive loads in series, enabling the M1/R phase ratio in the phase coexistence regime to be tuned.

Field Effect and Strongly Localized Carriers in the Metal-Insulator Transition Material VO(2).

The intrinsic field effect, the change in surface conductance with an applied transverse electric field, of prototypal strongly correlated VO(2) has remained elusive. Here we report its measurement enabled by epitaxial VO(2) and atomic layer deposited high-κ dielectrics. Oxygen migration, joule heating, and the linked field-induced phase transition are precluded. The field effect can be understood in terms of field-induced carriers with densities up to ∼5×10(13)  cm(-2) which are trongly localized, as shown by their low, thermally activated mobility (∼1×10(-3)  cm(2)/V s at 300 K). These carriers show behavior consistent with that of Holstein polarons and strongly impact the (opto)electronics of VO(2).

Characterization and modeling of metal-insulator transition (MIT) based tunnel junctions

Continued physical scaling will reduce power dissipation primarily through the reduction in device capacitance; however, a far greater benefit would result if the CMOS FET could be replaced by a fundamentally new device scheme that operates under very low supply voltages. Recently, semiconductor based inter-band tunnel field effect transistors (TFET) have been explored due to their potential to achieve sub kBT/q steep switching swings, enabling low voltage operation. In this work, we explore the abrupt metal to insulator transition (MIT) of vanadium dioxide (VO2) based tunnel junction - a first step towards a correlated electron based steep switching TFET. As illustrated, the metal insulator transition MIT in materials with strong electron correlation can be utilized to modulate the tunnelling current by opening an energy gap around the Fermi level in the OFF-state, and a metal-insulator-metal tunnelling current by collapsing the gap in the ON-state.

On-Chip Glass Microspherical Shell Whispering Gallery Mode Resonators

Arrays of on-chip spherical glass shells of hundreds of micrometers in diameter with ultra-smooth surfaces and sub-micrometer wall thicknesses have been fabricated and have been shown to sustain optical resonance modes with high Q-factors of greater than 50 million. The resonators exhibit temperature sensitivity of −1.8 GHz K−1 and can be configured as ultra-high sensitivity thermal sensors for a broad range of applications. By virtue of the geometry’s strong light-matter interaction, the inner surface provides an excellent on-chip sensing platform that truly opens up the possibility for reproducible, chip scale, ultra-high sensitivity microfluidic sensor arrays. As a proof of concept we demonstrate the sensitivity of the resonance frequency as water is filled inside the microspherical shell and is allowed to evaporate. By COMSOL modeling, the dependence of this interaction on glass shell thickness is elucidated and the experimentally measured sensitivities for two different shell thicknesses are explained.

Improving the magnetoelectric performance of Metglas/PZT laminates by annealing in a magnetic field

A comprehensive investigation of magnetostriction optimization in Metglas 2605SA1 ribbons is performed to enhance magnetoelectric performance. We explore a range of annealing conditions to relieve remnant stress and align the magnetic domains in the Metglas, while minimizing unwanted crystallization. The magnetostriction coefficient, magnetoelectric coefficient, and magnetic domain alignment are correlated to optimize magnetoelectric performance. We report on direct magnetostriction observed by in-plane Doppler vibrometer and domain imagining using scanning electron microscopy with polarization analysis for a range of annealing conditions. We find that annealing in an oxygen-free environment at 400 °C for 30 min yields an optimal magnetoelectric coefficient, magnetostriction and magnetostriction coefficient. The optimized ribbons had a magnetostriction of 50.6 ± 0.2 μm m-1 and magnetoelectric coefficient of 79.3 ± 1.5 μm m-1 mT-1. The optimized Metglas 2605SA1 ribbons and PZT-5A (d31 mode) sensor achieves a magnetic noise floor of approximately 600 pT Hz-1/2 at 100 Hz and a magnetoelectric coefficient of 6.1 ± 0.03 MV m-1 T-1.

Hubbard Gap Modulation in Vanadium Dioxide Nanoscale Tunnel Junctions

We locally investigate the electronic transport through individual tunnel junctions containing a 10 nm thin film of vanadium dioxide (VO2) across its thermally induced phase transition. The insulator-to-metal phase transition in the VO2 film collapses the Hubbard gap (experimentally determined to be 0.4 ± 0.07 V), leading to several orders of magnitude change in tunnel conductance. We quantitatively evaluate underlying transport mechanisms via theoretical quantum mechanical transport calculations which show excellent agreement with the experimental results.

Glass microbubble on-chip packaged ferrofluid based magnetoviscous magnetometer

We present a new packaging method for improving the lifetime of ferrofluid-based magnetoviscous magnetometer. The concept of a ferrofluid based magnetometer has been previously reported where the viscoelastic response of a thin interfacial ferrofluid layer loaded atop a high frequency shear wave quartz resonator to applied magnetic field is monitored. The magnetic field can be sensitively quantified by the changes in the at-resonance admittance characteristics of the resonator. However, under open conditions, continuous evaporation of the ferrofluid compromises the long term performance of the magnetometer. In this work, we integrate glass hemispherical microbubbles, used as vessels of ferrofluid, on the resonator chip to seal and prevent the evaporation of the ferrofluid liquid and drying out. A layer of high relative permeability thin film Metglas (Fe85B5Si10) is deposited on the resonator chip to improve the sensitivity. Using these improvements, a minimum detectable field of 650 nT/yHz at 1 Hz is achieved. Moreover, comparing with the unsealed ferrofluid device, the lifetime of the glass microbubble integrated chip packaged device improved significantly from only few hours to over a month and continuing.

Optimization of Metglas 2605SA1 and PZT-5A magnetoelectric laminates for magnetic sensing applications

Via optimization of the mechanical coupling, alignment of Metglas® magnetic domains, relief of residual stress, and operation of the PZT-5A under a DC electric field of 2 kV/cm an unprecedented magnetoelectric voltage coefficient of 9.52 V/cm-Oe is achieved; resulting to a magnetic field sensitivity of 150 pT at 20 Hz for a d3i Metglas®/PZT-5A laminate. Mechanical coupling is improved by reducing the thickness and porosity of the epoxy. The Metglas® residual stress reduction and easy axis alignment is accomplished by a 30 minute 400 °C anneal under a 1600 Oe magnetic field in vacuum. Finally, a DC electric field bias is applied to increase the <¡3i coefficient of the PZT-5A piezoelectric.