• Proceedings of the Korean Vacuum Society Conference
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• 2013.02a
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• pp.461-461
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• 2013

Over the last decade, zinc oxide (ZnO) thin films have attracted considerable attention owing to large band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature [1-3]. Recent interest in ZnO related researches has been switched into the fabrication and characterization of low-dimensional nanostructures, such as nano-wires and nano-dots that can be applicable to manufacture the optoelectronic devices such as ultraviolet lasers, light-emitting-diodes and detectors. Since the optical properties of ZnO nano-structures might be distinct from those of bulk materials or thin films, the low-dimensional phenomena should be examined further. In order to utilize such advanced optoelectronic devices, one of the challenges is how to control the surface state related emissions that are drastically increased with increasing the density of the nano-structures and the surface-to-volume ratio. This paper reports the synthesis and characterization of self-assembled ZnO hexagonal nano-disks grown by radio-frequency magnetron sputtering. X-ray diffraction data and scanning electron microscopy data showed that ZnO hexagonal nano-disks were nucleated on top of the flat surfaces as the film thickness reached to 1.56 ${\mu}m$ and then the number of nano-disks increased with increasing the film thickness. The lateral size of hexagonal nano-disks was ~720 nm and height was ~74 nm. The strong photo luminescence spectra obtained at 10 K was also observed, which was assigned to a surface exciton emission at 3.3628 eV arising from the surface sites of hexagonal nano-disks.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.218-218
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• 2012

Recently, hexagonal manganites have attracted much attention because of the coexistence of ferroelectricity and antiferromagnetic (AFM) order. The crystal structure of hexagonal manganites consists of $MnO_5$ polyhedra in which $Mn^{3+}$ ion is surrounded by three oxygen atoms in plane and two apical oxygen ions. The Mn ions within Mn-O plane form a triangular lattice and couple the spins through the AFM superexchange interaction. Due to incomplete AFM coupling between neighboring Mn ions in the triangular lattice, the system forms a geometrically-frustrated magnetic state. Among hexagonal manganites, $YMnO_3$, in particular, is the best known experimentally since the f states are empty. In addition, for applications, $YMnO_3$ thin films have been known as promising candidates for non-volatile ferroelectric random access memories. However, $YMnO_3$ has low magnetic order temperature (~70 K) and A-type AFM structure, which hinders its applications. We have synthesized $YMn1_{-x}Cr_xO_3$ (x = 0, 0.05 and 0.1) samples by the conventional solid-state reaction. The powders of stoichiometric proportions were mixed, and calcined at $900^{\circ}C$ for $YMn1_{-x}Cr_xO_3$ for 24 h. The obtained powders were ground, and pressed into 5-mm-thick disks of 1/2-inch diameter. The disks were directly put into the oven, and heated up to $1,300^{\circ}C$ and sintered in air for 24 h. The phase of samples was checked at room temperature by powder x-ray diffraction using a Rigaku Miniflex diffractometer with Cu $K{\alpha}$ radiation. All the magnetization measurements were carried out with a superconducting quantum-interference-device magnetometer. Our experiments point out that the Cr-doped samples show the characteristics of a spin-glass state at low temperatures.