H.-W. Lin

Enhancement of ferromagnetic properties in Zn1−xCoxO by additional Cu doping

The high-temperature ferromagnetism in Co-doped ZnO samples fabricated by the standard solid-state reaction method is reported. Additional Cu doping into bulk Zn0.98Co0.02O is essential to achieve room-temperature ferromagnetism. Structure and composition analyses revealed that cobalt is incorporated into the lattice structure, forming a solid solution instead of precipitates. In the case of Zn0.97Cu0.01Co0.02O, the coercive field measured by a vibrating sample magnetometer at room temperature is 60Oe. The implication of the effect of Cu doping in bulk Zn0.98Co0.02O is also discussed.The high-temperature ferromagnetism in Co-doped ZnO samples fabricated by the standard solid-state reaction method is reported. Additional Cu doping into bulk Zn0.98Co0.02O is essential to achieve room-temperature ferromagnetism. Structure and composition analyses revealed that cobalt is incorporated into the lattice structure, forming a solid solution instead of precipitates. In the case of Zn0.97Cu0.01Co0.02O, the coercive field measured by a vibrating sample magnetometer at room temperature is 60Oe. The implication of the effect of Cu doping in bulk Zn0.98Co0.02O is also discussed.

Near-infrared photoluminescence from vertical InN nanorod arrays grown on silicon: Effects of surface electron accumulation layer

We demonstrate that vertically aligned InN nanorods can be grown on Si(111) by plasma-assisted molecular-beam epitaxy. Detailed structural characterization indicates that individual nanorods are wurtzite InN single crystals with the growth direction along the c axis. Near-infrared photoluminescence (PL) from InN nanorods can be clearly observed at room temperature. However, in comparison to the InN epitaxial films, the PL efficiency is significantly lower. Moreover, the variable-temperature PL measurements of InN nanorods exhibit anomalous temperature effects. We propose that these unusual PL properties are results of considerable structural disorder (especially for the low-temperature grown InN nanorods) and strong surface electron accumulation effects.

Intense terahertz emission from a-plane InN surface

We report a significant enhancement in terahertz emission from the indium nitride (InN) films grown along the a axis (a-plane InN), relative to the InN films grown along the c axis. The primary radiation mechanism of the a-plane InN film is found to be due to the acceleration of photoexcited carriers under the polarization-induced in-plane electric field perpendicular to the a axis, which effectively enhances the geometrical coupling of the radiation out of semiconductor. In addition, azimuthal angle dependence measurement shows that the p-polarized terahertz output consists of a large angularly independent component and a weak component with a distinctive fourfold rotation symmetry.

Direct evidence of 8:9 commensurate heterojunction formed between InN and AlN on c plane

We show that, despite a large difference in lattice constants, high-quality InN∕AlN heterostructures can be formed on Si(111) due to the existence of “magic” ratios between the lattice constants of comprising material pairs: 2:1 (Si∕Si3N4), 5:4 (AlN∕Si), and 8:9 (InN∕AlN). For InN growth on AlN with nitrogen polarity, by using reflection high-energy electron diffraction and cross-sectional transmission electron microscopy, we have found that the pseudomorphic to commensurate lattice transition occurs within the first monolayer of growth, resulting in an abrupt heterojunction at the atomic scale. This new route of lattice match allows the formation of commensurate and nearly strain-free interface with a common two-dimensional superlattice.

Experimental and theoretical studies of lattice dynamics of Mg-doped InN

Raman studies of Mg-doped InN films with a Mg content from NMg=3.3×1019to5.5×1021cm−3 are reported. Raman and secondary ion mass spectroscopy data on the Mg content have been found to correlate well. Lattice dynamics of hexagonal InN with substitutional impurities and vacancies has been investigated in the framework of the cluster approach. Energy positions of local vibrational modes in InN have been calculated and compared with experimental findings. It is concluded that Raman spectroscopy is a good tool for quantitative characterization of Mg-doped InN.

Ultrafast E1(LO) phonon and plasma dynamics in a-plane wurtzite InN

Coherent phonon spectroscopy of an a-plane wurtzite InN epitaxial thin film is demonstrated with time-resolved second-harmonic generation technique. Coherent E1(LO) phonons are launched via a transient electric field screening process in the near surface region of the a-plane wurtzite InN. Because of the macroscopic electric dipole interaction between coherent E1(LO) phonons and photoexcited plasmons, two hybridized vibration modes, phononlike (L−) and plasmonlike (L+) E1(LO) phonon-plasmon coupling modes, are identified. The investigation of these coupling modes allows us to understand the ultrafast carrier and phonon dynamics in a-plane wurtzite InN.