Wei-Yen Woon

Characteristics of carbon monoxide sensors made by polar and nonpolar zinc oxide nanowires gated AlGaN/GaN high electron mobility transistor

AlGaN/GaN high electron mobility transistors (HEMTs) with polar and nonpolar ZnO nanowires modified gate exhibit significant changes in channel conductance upon exposure to different concentration of carbon monoxide (CO) at room temperature. The ZnO nanowires, grown by chemical vapor deposition, with perfect crystal quality will attach CO molecules and release electrons, which will lead to a change of surface charge in the gate region of the HEMTs, inducing a higher positive charge on the AlGaN surface, and increasing the piezo-induced charge density in the HEMTs channel. These electrons create an image positive charge on the gate region for the required neutrality, thus increasing the drain current of the HEMTs. The HEMTs source-drain current was highly dependent on the CO concentration. The limit of detection achieved was 400 ppm and 3200 ppm in the open cavity with continuous gas flow using a 50 × 50 μm2 gate sensing area for polar and nonpolar ZnO nanowire gated HEMTs sensor, respectively.

Effect of impurities on thermal stability of pseudomorphically strained Si:C layer

We investigate the thermal stability of pseudomorphically strained Si:C layer using high resolution x-ray diffraction (HRXRD) and Fourier transform infrared (FTIR) spectroscopy. Far below β-SiC precipitation threshold, almost complete strain relaxation is found without significant substitutional carbon (Csub) loss. FTIR shows the strain relaxation is related to volume compensation by Csub-interstitial complex formation through oxidation injection of interstitial. By multilayer HRXRD kinematical simulation, we find correlation of the enhanced strain relaxation to P distribution, implying P’s role as additional interstitial promoter during postannealing treatment. We relate our findings to recent reports on strain relaxation issues in Si:C devices fabrication.

Strain-doping coupling dynamics in phosphorus doped Si:C formed by solid phase epitaxial regrowth

We investigate the solid phase epitaxial regrowth (SPER) dynamics of phosphorus doped Si:C by time resolvedreflectivity and high resolution x-ray diffraction. The effect of SPER kinetics on strain profile and dopant activation is analyzed. The accumulated tensile strain induced by both C and P during SPER synergistically determines the onset of SPER rate retardation and leads to lower strain and electrical conductance near surface. Physical origin for the observed SPER rate evolution is discussed and explained with a strain included solute trapping model. Possibility of tailoring strain and doping profiles is discussed.

Collective sub-diffusive dynamics in bacterial carpet microfluidic channel

We experimentally investigate the collective dynamics in bacterial carpet microfluidic channel. The microfluidic channel is composed of single polar flagellated Vibrio alginolyticus deposited glass substrates. The individual flagellum swimming speed is tuned by varying buffer sodium concentration. Hydrodynamic coupling strength is tuned by varying buffer viscosity. The attached bacteria constantly perform two major modes in flagellum motion, namely, the local rotation and large angle flick. Particle tracking statistics shows high flagellum rotational rate and strong hydrodynamic coupling strength lead to collective sub-diffusive dynamics. The observed effect is strongly correlated to hydrodynamic coupling of flick motions between nearby bacteria.

Synthesis of Ultrathin Composition Graded Doped Lateral WSe2/WS2 Heterostructures

Lateral transition-metal dichalcogenide and their heterostructures have attracted substantial attention, but there lacks a simple approach to produce large-scaled optoelectronic devices with graded composition. In particular, the incorporation of substitution and doping into heterostructure formation is rarely reported. Here, we demonstrate growth of a composition graded doped lateral WSe2/WS2 heterostructure by ambient pressure chemical vapor deposition in a single heat cycle. Through Raman and photoluminescence spectroscopy, we demonstrate that the monolayer heterostructure exhibits a clear interface between two domains and a graded composition distribution in each domain. The coexistence of two distinct doping modes, i.e., interstitial and substitutional doping, was verified experimentally. A distinct three-stage growth mechanism consisting of nucleation, epitaxial growth, and substitution was proposed. Electrical transport measurements reveal that this lateral heterostructure has representative characteristics of a photodiodes. The optoelectronic device based on the lateral WSe2/WS2 heterostructure shows improved photodetection performance in terms of a reasonable responsivity and a large photoactive area.

Graphene reduction dynamics unveiled

The reduction dynamics of micron-sized defects created on chemical vapor deposition- (CVD) grown graphene through scanning probe lithography (SPL) is reported here. CVD-grown graphene was locally oxidized using SPL and subsequently reduced, making use of a focused beam of soft x-rays. During this whole process, the reduction dynamics was monitored using a combination of micro-Raman spectroscopy (μ-RS) and micro-x-ray photoelectron spectroscopy (μ-XPS). After x-ray reduction, the graphene film was found to be chemically identical (μ-XPS) but structurally different (μ-RS) from the original graphene. During reduction the population of C–C bonds was found to first increase dramatically and then decrease exponentially. By modeling the dynamics of the C=O → C–O → C–C → C=C reduction process with four coupled-rate equations and three rate constants, the conversion from C–C to C=C bonds was found to be the limiting rate.

On the doping limit for strain stability retention in phosphorus doped Si:C

Strain stability of phosphorus doped pseudomorphically strained Si:C alloy is investigated via high-resolution X-ray diffractometry, Fourier transform infrared spectroscopy, and Hall measurement. Significant strain relaxations are found under post-annealing treatment far below β-SiC precipitation threshold temperature, especially for the highest phosphorus doped case. Most of the substitutional carbon is retained and no further β-SiC formation can be found for all samples investigated. Volume compensation through gettering of interstitial atoms around substitutional carbon is considered as a probable mechanism for the observed strain relaxation. The strain relaxation effect can be further reduced with HF treatment prior to post-annealing process. We found an upper limit for ion implant dose (<1 × 1014 atom/cm2) for the retention of strain stability in phosphorus doped Si:C.

Collective flow dynamics across a bacterial carpet: Understanding the forces generated

Bacterial carpets consist of randomly anchored uni-polar-flagellated sodium-motive bacterial matrix are prepared by flow deposition. Collective flow dynamics across the bacterial carpets are probed with optical tweezers-microsphere assay. Around the center of a uniform bacterial cluster, collective forces that pull microsphere towards carpet surface are detected at a distance of 10 μm away from carpets. At sodium-motive driving over a critical value, the force magnitudes increase abruptly, suggesting a threshold-like transition of hydrodynamic synchronization across bacterial carpet. The abrupt force increase is explained in term of bifurcation to phase synchronization in a noisy non-linearly coupled rotor array mediated by hydrodynamic interactions.

Vertical Al2Se3/MoSe2 heterojunction on sapphire synthesized using ion beam

The vertical Al2Se3/MoSe2 heterojunction on sapphire was first fabricated via an ion beam-assisted process. The MoSe2 was formed via Mo selenization, while Al2Se3 was formed via Se substitution for O in sapphire. The applications of this heterojunction will be developed in the future.

Impact of ion implantation boundary dimensionality on boron transient diffusion in submicron scale patterns

We investigate two-dimensional boron transient diffusion in sub-micron scale patterns by plane view scanning capacitance microscopy (SCM). Submicron long strips and squares ion implantation windows of systematically varying sizes have been designed and fabricated. Boronion implantation and spike annealing were followed to activate the dopant and cause diffusion. Square opening windows show more enhanced diffusion than the long strip counterparts, especially at larger length scales. We explain the observation and fit the experimental data by a nonlinear logistics model. The implication to modern microelectronic circuit design and conventional dopant profiling methodology are discussed.