K. L. Teo

Ultraviolet and visible electroluminescence from n-ZnO∕SiOx∕(n,p)-Si heterostructured light-emitting diodes

n-ZnO∕SiOx∕n-Si and n-ZnO∕SiOx∕p-Si heterostructured light-emitting diodes have been fabricated using metal-organic chemical-vapor deposition for a comparison study. n-ZnO∕SiOx∕p-Si heterostructures show diodelike rectifying current-voltage characteristic with low breakdown voltage, while n-ZnO∕SiOx∕n-Si heterostructures show symmetric nonlinear current-voltage behavior due to the double Schottky barriers at the interface. Both types of diodes emit light when a positive bias applied at Si side. Ultraviolet emission at ∼390nm with an orange-emission centered at ∼600nm were observed in electroluminescence spectra of n-ZnO∕SiOx∕n-Si diodes, while whitish emission centered at ∼520nm was observed for n-ZnO∕SiOx∕p-Si diodes. The emission mechanisms were discussed.

Systematic investigation of structural and magnetic properties in molecular beam epitaxial growth of metastable zinc-blende CrTe toward half-metallicity

We report a systematic investigation on the structural and magnetic properties of molecular-beam epitaxial growth of CrTe thin films with different layer thicknesses and Cr∕Te flux ratios. A phase diagram of the growth parameters is established based on the detailed analyses of the reflection high-energy electron diffraction patterns, atomic force microscopy, and magnetization. Our high-resolution transmission electron microscopy results show that under appropriate growth conditions, a metastable zinc-blende (ZB) phase of CrTe film can be achieved with a nominal thickness of 5nm. The magnetic properties of ZB CrTe exhibit a strong in-plane anisotropy with an easy axis along the [001] direction and hard axes along the [011] and [01¯1] directions. Correspondingly, the uniaxial (KU) and cubic (KC) anisotropy constants are obtained through the fitting of the [011] hard-axis direction. The temperature dependence of the remanent magnetization indicates the TC∼100K of ZB CrTe is attained.

Raman-active Fröhlich optical phonon mode in arsenic implanted ZnO

This work was supported by the Singapore Agency for nScience, Technology and Research A*STAR, under SERC nGrant No. 0521260095.

Two-dimensional electron gas in Zn-polar ZnMgO/ZnO heterostructure grown by metal-organic vapor phase epitaxy

This work was supported by the Singapore Agency for nScience, Technology, and Research A*STAR, under SERC nGrant No. 0521260095 and Infuse Project No. I01-0911-06.

High spin filtering using multiple magnetoelectric barriers

A periodic array of magnetoelectric barriers is modeled to achieve maximum spin polarization (P) at high transmission probability (T). Each double-pair unit of the array consists of four magnetic barriers designed in several ways, such that an electron passing through, in the Landau gauge A=(0,Ay(x),0), acquires zero gain in kinetic energy. This enables multiple double-pairs to be used to enhance P without sacrificing T. By tuning the magnetoelectric barrier heights, a high P of 75%–100% is obtained at 0.8–1.0EF, for a 27 unit array. For antisymmetrical arrays, electrical barriers act as a switch to the polarization capability.

Epitaxially grown n-ZnO∕MgO∕TiN∕n+-Si(111) heterostructured light-emitting diode

Epitaxial n-ZnO∕MgO∕TiN∕n+-Si heterostructured light-emitting diodes have been fabricated. The epitaxial growth of MgO∕TiN on Si(111) was established by pulsed laser deposition, which was further employed as a buffer layer for epitaxial growth of ZnO layer by metal-organic chemical-vapor deposition. Good epitaxial quality was found using high-resolution x-ray diffraction and transmission electron microscopy. A strong wide electroluminescence band, ranging from 350to850nm and centered at ∼530nm, was observed from the diode when a positive voltage was applied on Si substrate. The diode exhibited a linear light-output-current characteristic with an injection current up to 192mA.

Origins of ferromagnetism in transition-metal doped Si

We present results of the magnetic, structural, and chemical characterizations of Mn+-implanted Si displaying n-type semiconducting behavior and ferromagnetic ordering with Curie temperature, TC, well above room temperature. The temperature-dependent magnetization measured by superconducting quantum interference device from 5 to 800 K was characterized by three different critical temperatures (TC*∼45u2002K, TC1∼630–650u2002K, and TC2∼805–825u2002K). Their origins were investigated using dynamic secondary ion mass spectroscopy and transmission electron microscopy (TEM) techniques, including electron energy loss spectroscopy, Z-contrast scanning TEM imaging, and electron diffraction. We provided direct evidences of the presence of a small amount of Fe and Cr impurities which were unintentionally doped into the samples together with the Mn+ ions as well as the formation of Mn-rich precipitates embedded in a Mn-poor matrix. The observed TC* is attributed to the Mn4Si7 precipitates identified by electron diffraction. Poss...

Selective excitation and thermal quenching of the yellow luminescence of GaN

We report the observation of narrower structures in the yellow luminescence of bulk and thin-film n-type GaN, using the technique of selective excitation. These fine structures exhibit thermal quenching associated with an activated behavior. We attribute these fine structures to phonons and electronic excitations of a shallow donor-deep acceptor complex, and determine its activation energy for delocalization. Our results suggest that in addition to distant donor-acceptor pairs, the yellow luminescence can also involve emission complexes of shallow donors and deep acceptors.

Surfactant effect of arsenic doping on modification of ZnO (0001) growth kinetics

The effect of arsenic doping on the growth kinetics of ZnO during metalorganic vapor phase epitaxy has been investigated. Arsenic was found to segregate to the growth surface and facilitate layer-by-layer growth. Such surfactant enhances the lateral expansion of the terraces preferential along [1¯1¯20] direction and also reduces the screw lattice distortion. Arsenic is expected to reduce the total surface energy and diffusion barrier of oxygen adatoms, hence producing Zn-rich surface condition on the growth front, in which two-dimensional growth is thermodynamically and kinetically favored. The origin of tiny hexagonal pits formed on the wide terrace is discussed in terms of the modified step-bunching mechanism.

Magnetic and transport behaviors in Ge1−xMnxTe with high Mn composition

The authors investigate the magnetic and transport behaviors of Ge1−xMnxTe thin films with high Mn composition (x=0.98) grown by solid-source molecular-beam epitaxy. The temperature-dependent magnetization (M-T) gives a Curie paramagnetic temperature θp∼120K, in contrast to the Curie temperature of TC∼95K obtained from the Arrott plot and temperature-dependent resistivity measurement. The resistivity and M-T behaviors can be attributed to weak localization effect of disordering. The authors discussed the ferromagnetism in Ge0.02Mn0.98Te on the basis of the Ruderman-Kittel-Kasuya-Yoshida interaction and clustering effect.