Ming-Yau Chern

Electroluminescence from ZnO/Si-Nanotips Light-Emitting Diodes

A new and general approach to achieving efficient electrically driven light emission from a Si-based nano p-n junction array is introduced. A wafer-scale array of p-type silicon nanotips were formed by a single-step self-masked dry etching process, which is compatible with current semiconductor technologies. On top of the silicon nanotip array, a layer of n-type ZnO film was grown by pulsed laser deposition. Both the narrow line width of 10 nm in cathodoluminescence spectra and the appearance of multiphonon Raman spectra up to the fourth order indicate the excellent quality of the ZnO film. The turn-on voltage of our ZnO/Si nanotip array is found to be approximately 2.4 V, which is 2 times smaller than its thin film counterpart. Moreover, electroluminescence (EL) from our ZnO/Si nanotips array light-emitting diode (LED) has been demonstrated. Our results could open up new possibilities to integrate silicon-based optoelectronic devices, such as highly efficient LEDs, with standard Si ultralarge-scale integrated technology.

Roughness-enhanced electroluminescence from metal oxide silicon tunneling diodes

An approximate two-order increase in magnitude in electroluminescence was observed for the metal-oxide-silicon tunneling diodes with oxide grown at 900/spl deg/C, as compared to 1000/spl deg/C. The X-ray reflectivity revealed that the oxide grown at 900/spl deg/C has rougher interface than that grown at 1000/spl deg/C. The role of interface roughness can be understood in a model composed of phonons and interface roughness. An external quantum efficiency of /spl sim/10/sup -6/ was obtained using Al electrodes.

Study of BixY3-xFe5O12 Thin Films Grown by Pulsed Laser Deposition.

Epitaxial films of Bi\sitxY3-\sitxFe5O12 (Bi:YIG) have been grown on [111]-oriented gadolinium-gallium-garnet substrates, Gd3Ga5O12 (GGG), by pulsed laser deposition (PLD). Complete epitaxial substitution of the bismuth, Bi3Fe5O12 (BIG), was achieved at a growth temperature of 500 °C and an O2 atmosphere of 200 mTorr. The epitaxial nature of the films was indicated by the diffraction patterns from reflection high-energy electron diffraction (RHEED). X-ray diffraction (XRD) also showed that there were no peaks other than multiples of the (444) reflection. The Faraday rotation angle, θ F, of these films increases linearly with the Bi content x yielding θ F/x = -1.9 deg/µm at λ= 633 nm and T = 300 K. From magnetooptical Kerr measurements (MOKE) we conclude that the uniaxial anisotropy constant, K u, of the BIG film is positive, which could be due to the expansive stress caused by the difference in thermal expansion of the film and the substrate.

Characterization of the Ultrathin

The physical properties of HfO₂ and Hf-silicate layers grown by the atomic layer chemical vapor deposition are characterized as a function of the Hf concentration and the annealing temperature. The peaks of Fourier transform infrared spectra at 960, 900, and 820 cm_-1 originate from Hf-O-Si chemical bonds, revealing that a Hf-silicate interfacial layer began to form at the HfO₂/SiO ₂ interface after post deposition annealing process at 600 degC for 1 min. Moreover, the intensity of the peak at 750 cm^-1 can indicate the degree of crystallization of HfO₂. The formed Hf-silicate layer between HfO² and SiO² is also confirmed by X-ray photoelectron spectroscopy

\hbox{HfO}_{2}

Surface morphology of polycrystalline silicon prepared by excimer laser annealing has been investigated. It was found that when a thin amorphous Si film is irradiated by excimer laser, pillars are formed on the surface of the crystallized polysilicon. To find out the mechanism, various preparation parameters such as laser power densities, shot numbers and the thickness of the amorphous Si have been studied. Several intermediate patterns have been found which illustrate the sequential formation of pillars. A surface tension model is proposed to explain these patterns. This mechanism can be applied to prepare the self-assembled silicon quantum dots, whose average size and height are 14.2 and 3.7 nm, respectively.

and Hf-Silicate Films Grown by Atomic Layer Deposition

The magnetooptical Faraday rotations of epitaxial films of BixY3-xFe5O12 (Bi:YIG) grown on [111]-oriented gadolinium gallium garnet (GGG) substrates by pulsed laser deposition (PLD) were studied with bismuth content x = 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0. The Faraday rotation angles, θF, of the films were measured by the method of rotating analyzer ellipsometry (RAE) with the photon energy varied from 1.5 to 3.5 eV. It was shown that in addition to the increase of the Faraday rotation with increasing x, the peaks of θF shifted toward the red region as x changed from 1.0 to 1.5. The peak positions of θF for the completely Bi-substituted iron garnet, Bi3Fe5O12 (BIG), were found at 2.4 and 2.8 eV with peak values as large as -23 deg/µm and 44 deg/µm, respectively

Mechanism for pillar-shaped surface morphology of polysilicon prepared by excimer laser annealing

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.

Red shift of Faraday rotation in thin films of completely bismuth-substituted iron garnet Bi3Fe5O12

The electron effective mass in n-type InNxAs1−x (with x up to 3.0%) grown by gas-source molecular-beam epitaxy was obtained from infrared reflectivity and Hall-effect measurements. The large increase of the effective mass due to the incorporation of nitrogen is attributed mainly to the nitrogen-induced modification on the electronic states near the conduction-band edge. The well-known band anticrossing (BAC) model for the electronic structure of the III-N-V alloys cannot well describe the experimental data, especially in the region of higher electron concentration. This result provides an opportunity to examine the “universality” of the BAC model.

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

This study reports the preparation of self-organized 1-dimensional magnetic structures of Fe on Al2O3 (0001) by oblique deposition. The x-ray diffraction (XRD) results in this study show the preferred (110) texture of the Fe films. XRD and extended x-ray adsorption fine structure measurements indicate larger oblique deposition angle (65°) leads to more disorder in the Fe crystalline structure. After capping with a Pd overlayer, the Pd/Fe/Al2O3 (0001) still exhibits uniaxial magnetic anisotropy induced by the underlying 1-dimensional Fe nanostructure. This uniaxial magnetic anisotropy changes with the variation in Fe thickness and oblique deposition angle. These results clearly indicate the feasibility of manipulating uniaxial magnetic anisotropy and crystalline order through the oblique deposition of magnetic materials.

Nitrogen-induced enhancement of the electron effective mass in InNxAs1-x

High quality Y3Fe5O12/Gd3Ga5O12 (YIG/GGG) superlattices have been grown on (111)GGG substrates by pulsed laser deposition. The superlattices are investigated with the thickness of the YIG layer varied from six to one unit cells while keeping the GGG layer fixed at one unit cell. The thicknesses of the YIG and GGG layers are confirmed with grazing angle x‐ray reflectivity (GAXR). The good quality of the samples is indicated by the small interface roughness, no more than 6 A, also measured with GAXR. While the magnetization of the samples decreases when the thickness of the YIG layer is decreased, the superlattice remains magnetic even when the YIG layer is only one unit cell. Ferrimagnetic resonance indicates that there exists a surface anisotropy favoring in‐plane magnetization.