Yen-Sheng Lu

Anion detection using ultrathin InN ion selective field effect transistors

Ultrathin (∼10nm) InN ion selective field effect transistors (ISFETs) have been demonstrated to perform ion sensing in aqueous solutions with a sensitivity of 5μA/decade and a response time smaller than 10s. The positively charged surface states on InN surfaces selectively adsorb anions, building Helmholtz voltages in solutions and modulating the drain-source current of the ISFETs. The ISFET performance is greatly enhanced by depleting carriers in the ultrathin InN channel where the film thickness is close to depth of surface electron accumulation.

InN-based anion selective sensors in aqueous solutions

We demonstrate that indium nitride (InN) can be used as ion selective electrode (ISE) for anion concentration measurements. The InN ISE reveals remarkable selectivity, response time, signal stability, and repeatability for chlorine and hydroxyl ions. The selective interaction of Lewis bases in solutions with the N-polarity InN epitaxial layer grown on silicon is confirmed by potentiometric responses. The Helmholtz potential of the InN ISE, generated at the InN/solution interface, satisfies the Nernst equation. The observation of anion attraction to the InN surface further demonstrates the existence of donor-type surface states on InN.

Highly Sensitive pH Sensing Using an Indium Nitride Ion-Sensitive Field-Effect Transistor

We demonstrated an ultrathin (~10 nm) ifndium nitride (InN) ion-sensitive field-effect transistor (ISFET) for pH sensing. The native indium oxide formed on the InN surface functions as a chemical binding layer with a high pH sensitivity, while the strong surface electron accumulation of InN along with the ultrathin conduction channel results in a large ion-induced surface potential to current transconductance. The ultrathin InN ISFETs were characterized to show a gate sensitivity of 58.3 mV/pH in the pH range of 2-12, a current variation ratio of 4.0%/pH, a resolution of less than 0.03 pH, and a response time of less than 10 s.

Tunable dielectric liquid lens on flexible substrate

We demonstrate the fabrication of a tunable-focus dielectric liquid lens (DLL) on a flexible substrate made of polydimethylsiloxane, which was wrapped onto a goggle surface to show its functionality. As a positive meniscus converging lens, the DLL has the focal length variable from 14.2 to 6.3 mm in 1.3 s when the driving voltage increases to 125 Vrms. The resolving power of the DLL is 17.95 line pairs per mm. The DLL on a flexible, curvilinear surface is promising for expanded field of view covered as well as in reconfigurable optical systems.

Self-assembled high NA microlens arrays using global dielectricphoretic energy wells

Microlens arrays were self-assembled from microballs using dielectrophoretic energy wells. Energy wells defined by patterned dielectric were used to produce microlens arrays with array patterns of desire. Microballs of 25microm in diameter were measured to have numerical aperture of 0.8 and focal length of 15.5microm. The optical resolution was found to be 0.4microm. Both the numerical aperture and the focal length were further adjusted to 0.66 and 19.0microm by a post-assembly heat treatment at 190 degrees C for 5 minutes.

Effects of (NH4)2Sx treatment on indium nitride surfaces

Indium nitride (InN) surfaces treated with ammonium sulfide [(NH4)2Sx] are investigated using Hall effect measurement, x-ray photoelectron spectroscopy (XPS), and scanning Kelvin probe microscopy (SKPM). Upon the (NH4)2Sx treatment, the sheet carrier density is reduced by (0.8–0.9)×1013 cm−2, leading to an increase in the sheet resistance. By numerically solving the Poisson’s equation, the associated upward shift of the surface band bending is derived to be 0.3 eV. XPS characterization shows, on the (NH4)2Sx treated InN surface, the formation of native oxide is effectively suppressed and a covalently bonded sulfur layer with surface In atoms is formed. This surface In–S dipole layer results in an increase in the electron affinity, thus giving rise to a lower surface bending shift (0.2 eV) observed in XPS. The electron affinity increase of 0.1 eV can be deduced, which is consistent with the result obtained by SKPM. Thus, the (NH4)2Sx treatment has been demonstrated to be an effective method for reducing th...

Investigation on −c-InN and a-InN:Mg field effect transistors under electrolyte gate bias

The electrical properties of N-polar undoped InN and nonpolar a-InN:Mg ion sensitive field effect transistors (ISFETs) have been investigated by electrolyte-gate-biased current-voltage (IDS-VGS) measurements. IDS-VGS characteristics reveal that the a-InN:Mg ISFETs have a large (∼52%) current variation ratio at a gate bias of 0.3 V with respect to the unbiased one, which is higher than that from the undoped InN ISFETs (∼18% and <0.1% for 10-nm and 1-μm-thick −c-InN epilayers, respectively). The a-InN:Mg ISFETs can also function as a pH sensor with a sensitivity of 56.5 mV/pH and a response time less than 10 s.

Image driven cell manipulation using optical dielectrophoresis (ODEP)

We demonstrated a novel approach to trigger the dielectrophoretic force to manipulate biological cells via projected flash images. The attractive and repulsive force regions are formed due to effectively initiated dielectrophoretic force, where these regions are the bright and dark area among a projected image on a liquid reservoir depending on the polarization states. In present study, the attractive force draw in live cells, saccharomyces cerevisiae, and live cells can be manipulated inside reservoir without any microchannel confinement in a programmable manner. The relation between distance from the boundary and average velocity is also discussed.

An investigation of the Young's modulus of single-crystalline wurtzite indium nitride using an atomic force microscopy based micromechanical bending test

High quality single-crystalline wurtzite indium nitride (InN) thin film was first demonstrated to have a Youngs modulus of 149 ± 5 GPa along a-axis using atomic force microscopy microbending test since the revision of InN energy gap. These released InN cantilever beams were examined to have ignorable in-plane residual stress using micro-Raman spectroscopy, where the E2 (high) mode at 490 cm−1 exists zero shift because of the perfect lattice match (8:9 commensurate) between InN and underneath aluminum nitride buffer. The experimental value of Youngs modulus agrees well with a number of theoretical estimations ranging from 146 to 159 GPa.

Three-dimensional illumination system using dielectric liquid lenses

A 25-pixel illumination system composed of a 5 × 5 dielectric liquid-lens (DLL) zoom module array, 25 light-emission diodes (LEDs), and a secondary optical lens demonstrates 3D light field manipulation. LEDs function as 2D illumination pixels while the DLL module array performs longitudinal illuminance adjustability by zooming each illumination pixel. A test on the similarity of two illuminance patterns between experiments and simulations shows a normalized cross correlation (NCC) higher than 0.8, indicating the feasibility of the system design. Also, the illumination system is further applied to correct a distorted light pattern on a 45° tilt screen as well as to perform light compensation on distance-differential objects.