Hsin-Yen Lee

Study on the formation of zinc peroxide on zinc oxide with hydrogen peroxide treatment using x-ray photoelectron spectroscopy (XPS)

In a previous study, we found that zinc oxide (ZnO) samples under different hydrogen peroxide (H2O2) treatment durations show the ability to dramatically rectify a diode’s behavior. In this study, the H2O2 mechanism is examined by grazing incidence x-ray diffraction (GIXRD) and x-ray photoelectron spectroscopy (XPS) analyses. In GIXRD, a diffraction peak (111) from the zinc peroxide (ZnO2) was observed for the films grown at low temperatures. The XPS depth profiles of the core O1s clearly indicated oxidation, and an interfacial ZnO2 layer covered the ZnO surface via the H2O2 treatment. The Schottky barrier heights of the treated and untreated samples were illustrated using energy band diagrams.

Bismuth Nanowire Grown Naturally Using a Sputtering System

We report the growth of Bismuth (Bi) nanowires on glass substrates using a radio frequency (RF) sputtering system. The growth temperature and RF power were varied to study the growth mechanism of the nanowires. The scanning electron microscope (SEM)/transmission electron microscope (TEM) images of the samples under various growth conditions were taken to reveal the morphologies of the Bi nanowires and films. We found that the optimal conditions for growing Bi nanowires were 120–160 °C, 0.5 W/cm2 (growth rate 40 A/s), and 240 s.

Fabrication and Properties of Indium Tin Oxide/ZnO Schottky Photodiode with Hydrogen Peroxide Treatment

A transparent, efficient ZnO ultraviolet Schottky detector with indium tin oxide (ITO) as a metallic contact layer is fabricated on ITO-coated glass substrates by cw CO2 laser evaporation. The device behavior changes from near ohmic to Schottky in the current–voltage characteristics after hydrogen peroxide treatment on the ZnO surface with a fitted barrier height of 1.16 eV, an ideality factor of 2.31, and a leakage current of 3.1×10-7 A at -3 V bias. Photoluminescence (PL) data show the effect of hydrogen peroxide, and indicate that the surface defects are removed, and better diode characteristics are shown.

Schottky Photodiode Fabricated from Hydrogen-Peroxide-Treated ZnO Nanowires

An effective, transparent solar-blind Schottky ultraviolet (UV) sensor made of zinc oxide (ZnO) nanowires (NWs) was fabricated by chemical vapor deposition (CVD). Indium–tin oxide (ITO) thin films were deposited by radio frequency (RF) sputtering as Schottky contacts, where the hydrogen peroxide (H2O2) treatment of ZnO NWs played a key role in the rectifying effect. The photodiode showed a fitted barrier height of 0.89 eV, an ideality factor of 1.82, and a rectification behavior of up to three orders of magnitude at a voltage bias between -1 and +1 V. Photoresponse measurement proved a reliable device in the UV region.

Temperature dependence of electron density and electron–electron interactions in monolayer epitaxial graphene grown on SiC

We report carrier density measurements and electron-electron (e-e) interactions in monolayer epitaxial graphene grown on SiC. The temperature (T)-independent carrier density determined from the Shubnikov-de Haas (SdH) oscillations clearly demonstrates that the observed logarithmic temperature dependence of Hall slope in our system must be due to e-e interactions. Since the electron density determined from conventional SdH measurements does not depend on e-e interactions based on Kohns theorem, SdH experiments appear to be more reliable compared with the classical Hall effect when one studies the T dependence of the carrier density in the low T regime. On the other hand, the logarithmic T dependence of the Hall slope δRxy/δB can be used to probe e-e interactions even when the conventional conductivity method is not applicable due to strong electron-phonon scattering.

Charge Trapping in Monolayer and Multilayer Epitaxial Graphene

We have studied the carrier densities n of multilayer and monolayer epitaxial graphene devices over a wide range of temperatures T. It is found that, in the high temperature regime typically T ≥ 200 K, ln⁡n shows a linear dependence of 1/T, showing activated behavior. Such results yield activation energies ΔE for charge trapping in epitaxial graphene ranging from 196 meV to 34 meV. We find that ΔE decreases with increasing mobility. Vacuum annealing experiments suggest that both adsorbates on EG and the SiC/graphene interface play a role in charge trapping in EG devices.

A study on the epitaxial Bi2Se3 thin film grown by vapor phase epitaxy

We report the growth of high quality Bi2Se3 thin films on Al2O3 substrates by using chemical vapor deposition. From the atomic force microscope, x-ray diffraction and transmission electron microscope measurements we found that the films are of good crystalline quality, have two distinct domains and can be grown epitaxially on the Al2O3 substrate. Carrier concentration in the sample is found to be 1.1 × 1019 cm−3 between T = 2 K to T = 300 K, and electron mobility can reach 954 cm2/V s at T = 2 K. Weak anti-localization effect is observed in the low temperature magneto-transport measurement for the sample which indicates that the thin film has topological surface state.

Optical properties of ITO/ZnO Schottky diode with enhanced UV Photoresponse

Photoluminescence (PL) spectra of zinc oxide (ZnO) samples with different hydrogen peroxide (H2O2) treatment durations were measured to examine several point defects on the surfaces of the films. The results suggest successful oxidation through the reaction between oxygen radicals dissociated from H2O2 and the ZnO surface. To further confirm the defect-induced gain mechanism, we fabricated highly transparent indium-tin-oxide (ITO)/ZnO Schottky diodes and measured the key diode characteristics. Photocurrents were measured at different wavelengths, and possible explanations for the high optical gain within the ultraviolet (UV) region are provided.

Towards epitaxial graphene p-n junctions as electrically programmable quantum resistance standards

We report the fabrication and measurement of top gated epitaxial graphene p-n junctions where exfoliated hexagonal boron nitride (h-BN) is used as the gate dielectric. The four-terminal longitudinal resistance across a single junction is well quantized at the von Klitzing constant