Academic Appointments


  • Sr Research Engineer, Stanford Nanofabrication Facility

All Publications


  • Tuning Electrical and Thermal Transport in AlGaN/GaN Heterostructures via Buffer Layer Engineering ADVANCED FUNCTIONAL MATERIALS Yalamarthy, A., So, H., Rojo, M., Suria, A. J., Xu, X., Pop, E., Senesky, D. G. 2018; 28 (22)
  • Photoelectrochemical Water Oxidation by GaAs Nanowire Arrays Protected with Atomic Layer Deposited NiO (x) Electrocatalysts Zeng, J., Xu, X., Parameshwaran, V., Baker, J., Bent, S., Wong, H., Clemens, B. SPRINGER. 2018: 932–37
  • Temperature-Dependent Thermal Boundary Conductance of Monolayer MoS2 by Raman Thermometry ACS APPLIED MATERIALS & INTERFACES Yalon, E., Aslan, O., Smithe, K. H., McClellan, C. J., Suryavanshi, S. V., Xiong, F., Sood, A., Neumann, C. M., Xu, X., Goodson, K. E., Heinz, T. F., Pop, E. 2017; 9 (49): 43013–20
  • ISFET pH Sensitivity: Counter-Ions Play a Key Role SCIENTIFIC REPORTS Parizi, K. B., Xu, X., Pal, A., Hu, X., Wong, H. S. 2017; 7

    Abstract

    The Field Effect sensors are broadly used for detecting various target analytes in chemical and biological solutions. We report the conditions under which the pH sensitivity of an Ion Sensitive Field Effect transistor (ISFET) sensor can be significantly enhanced. Our theory and simulations show that by using pH buffer solutions containing counter-ions that are beyond a specific size, the sensor shows significantly higher sensitivity which can exceed the Nernst limit. We validate the theory by measuring the pH response of an extended gate ISFET pH sensor. The consistency and reproducibility of the measurement results have been recorded in hysteresis free and stable operations. Different conditions have been tested to confirm the accuracy and validity of our experiment results such as using different solutions, various oxide dielectrics as the sensing layer and off-the-shelf versus IC fabricated transistors as the basis of the ISFET sensor.

    View details for DOI 10.1038/srep41305

    View details for Web of Science ID 000393098100001

    View details for PubMedID 28150700

    View details for PubMedCentralID PMC5288728

  • Engineering a Large Scale Indium Nanodot Array for Refractive Index Sensing ACS APPLIED MATERIALS & INTERFACES Xu, X., Hu, X., Chen, X., Kang, Y., Zhang, Z., Parizi, K. B., Wong, H. P. 2016; 8 (46): 31871-31877

    Abstract

    In this work, we developed a simple method to fabricate 12 × 4 mm(2) large scale nanostructure arrays and investigated the feasibility of indium nanodot (ND) array with different diameters and periods for refractive index sensing. Absorption resonances at multiple wavelengths from the visible to the near-infrared range were observed for various incident angles in a variety of media. Engineering the ND array with a centered square lattice, we successfully enhanced the sensitivity by 60% and improved the figure of merit (FOM) by 190%. The evolution of the resonance dips in the reflection spectra, of square lattice and centered square lattice, from air to water, matches well with the results of Lumerical FDTD simulation. The improvement of sensitivity is due to the enhancement of local electromagnetic field (E-field) near the NDs with centered square lattice, as revealed by E-field simulation at resonance wavelengths. The E-field is enhanced due to coupling between the two square ND arrays with [Formula: see text]x period at phase matching. This work illustrates an effective way to engineer and fabricate a refractive index sensor at a large scale. This is the first experimental demonstration of poor-metal (indium) nanostructure array for refractive index sensing. It also demonstrates a centered square lattice for higher sensitivity and as a better basic platform for more complex sensor designs.

    View details for DOI 10.1021/acsami.6b11413

    View details for Web of Science ID 000388913900047

  • Wafer-level MOCVD growth of AlGaN/GaN-on-Si HEMT structures with ultra-high room temperature 2DEG mobility AIP ADVANCES Xu, X., Zhong, J., So, H., Norvilas, A., Sommerhalter, C., Senesky, D. G., Tang, M. 2016; 6 (11)

    View details for DOI 10.1063/1.4967816

    View details for Web of Science ID 000392082600024

  • Crystallinity, Surface Morphology, and Photoelectrochemical Effects in Conical InP and InN Nanowires Grown on Silicon. ACS applied materials & interfaces Parameshwaran, V., Xu, X., Clemens, B. 2016; 8 (33): 21454-21464

    Abstract

    The growth conditions of two types of indium-based III-V nanowires, InP and InN, are tailored such that instead of yielding conventional wire-type morphologies, single-crystal conical structures are formed with an enlarged diameter either near the base or near the tip. By using indium droplets as a growth catalyst, combined with an excess indium supply during growth, "ice cream cone" type structures are formed with a nanowire "cone" and an indium-based "ice cream" droplet on top for both InP and InN. Surface polycrystallinity and annihilation of the catalyst tip of the conical InP nanowires are observed when the indium supply is turned off during the growth process. This growth design technique is extended to create single-crystal InN nanowires with the same morphology. Conical InN nanowires with an enlarged base are obtained through the use of an excess combined Au-In growth catalyst. Electrochemical studies of the InP nanowires on silicon demonstrate a reduction photocurrent as a proof of photovolatic behavior and provide insight as to how the observed surface polycrystallinity and the resulting interface affect these device-level properties. Additionally, a photovoltage is induced in both types of conical InN nanowires on silicon, which is not replicated in epitaxial InN thin films.

    View details for DOI 10.1021/acsami.6b05749

    View details for PubMedID 27455379

  • Electrochemical Reduction Properties of Extended Space Charge InGaP and GaP Epitaxial Layers JOURNAL OF THE ELECTROCHEMICAL SOCIETY Parameshwaran, V., Xu, X., Clemens, B. 2016; 163 (8): H714-H721
  • VLSI-Compatible Carbon Nanotube Doping Technique with Low Work-Function Metal Oxides. Nano letters Suriyasena Liyanage, L., Xu, X., Pitner, G., Bao, Z., Wong, H. P. 2014; 14 (4): 1884-1890

    Abstract

    Single-wall carbon nanotubes (SWCNTs) have great potential to become the channel material for future high-speed transistor technology. However, as-made carbon nanotube field effect transistors (CNFETs) are p-type in ambient, and a consistent and reproducible n-type carbon nanotube (CNT) doping technique has yet to be realized. In addition, for very large scale integration (VLSI) of CNT transistors, it is imperative to use a solid-state method that can be applied on the wafer scale. Herein we present a novel, VLSI-compatible doping technique to fabricate n-type CNT transistors using low work-function metal oxides as gate dielectrics. Using this technique we demonstrate wafer-scale, aligned CNT transistors with yttrium oxide (Y2Ox) gate dielectrics that exhibit n-type behavior with Ion/Ioff of 10(6) and inverse subthreshold slope of 95 mV/dec. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) analyses confirm that slow (∼1 Å/s) evaporation of yttrium on the CNTs can form a smooth surface that provides excellent wetting to CNTs. Further analysis of the yttrium oxide gate dielectric using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques revealed that partially oxidized elemental yttrium content increases underneath the surface where it acts as a reducing agent on nanotubes by donating electrons that gives rise to n-type doping in CNTs. We further confirm the mechanism for this technique with other low work-function metals such as lanthanum (La), erbium (Er), and scandium (Sc) which also provide similar CNT NFET behavior after transistor fabrication. This study paves the way to exploiting a wide range of materials for an effective n-type carbon nanotube transistor for a complementary (p- and n-type) transistor technology.

    View details for DOI 10.1021/nl404654j

    View details for PubMedID 24628497

  • GaAs buffer layer technique for vertical nanowire growth on Si substrate APPLIED PHYSICS LETTERS Xu, X., Li, Y., Parizi, K. B., Huo, Y., Kang, Y., Wong, H. P. 2014; 104 (8)

    View details for DOI 10.1063/1.4866915

    View details for Web of Science ID 000332619100114

  • N-Type Doping of Carbon Nanotube Transistors using Yttrium Oxide (Y2Ox) PROCEEDINGS OF TECHNICAL PROGRAM - 2014 INTERNATIONAL SYMPOSIUM ON VLSI TECHNOLOGY, SYSTEMS AND APPLICATION (VLSI-TSA) Liyanage, L. S., Pitner, G., Xu, X., Wong, H. P. 2014
  • Dilute phosphide nitride materials as photocathodes for electrochemical solar energy conversion Conference on Physics, Simulation, and Photonic Engineering of Photovoltaic Devices II Parameshwaran, V., Xu, X., Kang, Y., Harris, J., Wong, H. P., Clemens, B. SPIE-INT SOC OPTICAL ENGINEERING. 2013

    View details for DOI 10.1117/12.2003486

    View details for Web of Science ID 000322825200040