Ching-Hui Hsu

SiC-capped nanotip arrays for field emission with ultralow turn-on field

Silicon nanotips with tip diameter and height measuring 1 nm and 1 μm, respectively, and density in the range of 109–3×1011 cm−2, were fabricated monolithically from silicon wafers by electron cyclotron resonance plasma etching technique at a temperature of 200 °C. Field emission current densities of 3.0 mA/cm2 at an applied field of ∼1.0 V/μm was obtained from these silicon nanotips. High-resolution transmission electron microscope and Auger electron spectroscopy analyses concluded that the nanotips are composed of monolithic silicon and nanometer-size SiC cap at the top. A 0.35 V/μm turn-on field to draw a 10 μA/cm2 current density was demonstrated, which is much lower than other reported materials. The excellent field emission property demonstrated by these nanotips, which were fabricated by a process integrable to the existing silicon device technology at low temperatures, is a step forward in achieving low-power field emission displays and vacuum electronic devices.

Multilevel resistive switching memory with amorphous InGaZnO-based thin film

Multi-level storage capability of resistive random access memory (RRAM) using amorphous indium-gallium-zinc-oxide (InGaZnO) thin film is demonstrated by the TiN/Ti/InGaZnO/Pt device structure under different operation modes. The distinct four-level resistance states can be obtained by varying either the trigger voltage pulse or the compliance current. In addition, the RRAM devices exhibit superior characteristics of programming/erasing endurance and data retention for the application of multi-level nonvolatile memory technology. Physical transport mechanisms for the multi-level resistive switching characteristics are also deduced in this study.

Bipolar resistive switching characteristics of Al-doped zinc tin oxide for nonvolatile memory applications

Resistive random access memory using Al-doped zinc tin oxide (AZTO) as resistive switching layer was prepared by radio-frequency magnetron sputtering at room temperature. The Ti/AZTO/Pt device exhibits reversible and robust bi-stable resistance switching behavior over hundreds of switching cycles within 2 V sweep voltage. The Ti/AZTO/Pt device showed stable retention characteristics for over 104 s under read disturb stress condition. Besides, the electrical conduction mechanism was dominated by ohmic conduction in low resistance state, while the current transport behavior followed a trap-controlled space-charge-limited conduction process in high resistance state.

Crystal symmetry breaking of wurtzite to orthorhombic in nonpolar a-ZnO epifilms

Crystal symmetry breaking of wurtzite C6V to orthorhombic C2V due to in-plane anisotropic strain was investigated for nonpolar (112¯0) ZnO epifilms grown on the R-sapphire. X-ray diffraction results reveal the epilayer is subjected to a compressive strain along the polar c-axis and tensile strains along both a-[112¯0] surface normal and in-plane p-[11¯00] axis. The polarized Raman spectra of E2 modes reveal violation of the C6V selection rules. Oppositely, the C2V configuration satisfies the selection rules for the Raman modes. The observed E1 and E2 bands in polarized optical reflection and photoluminescence spectra confirm the anisotropic strain causes the structure change to the orthorhombic one.

Hole mobility enhancement of Si0.2Ge0.8 quantum well channel on Si

The ultrathin strained Si0.2Ge0.8 quantum well channel (∼5nm) directly grown on Si substrates is demonstrated with low defect density and high hole mobility. The quantum well Si0.2Ge0.8 channel reveals an ∼3.2 times hole current enhancement and an ∼3 times hole mobility enhancement as compared with the bulk Si channel. The output current-voltage characteristics under the external mechanical strain confirm the compressive strain in the channel. The external compressive strain further enhances the hole mobility in a Si0.2Ge0.8 channel.

SiGe nanorings by ultrahigh vacuum chemical vapor deposition

Formation of SiGe nanorings from Si capped Si0.1Ge0.9 quantum dots (QDs) grown at 500 °C by ultrahigh vacuum chemical vapor deposition was investigated. SiGe nanorings have average diameter, width, and depth of 185, 30, and 9 nm, respectively. Based on both Raman and x-ray diffraction results, the formation of SiGe nanorings can be attributed to Ge outdiffusion from central SiGe QDs during in situ annealing. Moreover, the depth of SiGe nanorings can be controlled by Si cap thickness. The Si cap is essential for nanorings formation.

A transition of three to two dimensional Si growth on Ge (100) substrate

For the initial growth of Si on Ge, three-dimensional Si quantum dots grown on the Ge surface were observed. With increasing Si thickness, the Si growth changes from three-dimensional to two-dimensional growth mode and the dots disappear gradually. Finally, the surface is smooth with the roughness of 0.26 nm, similar to the original Ge substrate, when 15 nm Si is deposited. More Ge segregation on the wetting layer leads to more open sites to increase the subsequent Si growth rate on the wetting layer than on the Si dots. The in-plane x-ray diffraction by synchrotron radiation is used to observe the evolution of tensile strain in the Si layer grown on Ge (100) substrate.