Periodically Polarity-inverted Research Articles

Optical characterization of periodically polarity-inverted ZnO structures on (0001) Al2O3 substrates

We investigated the optical properties of periodically polarity-inverted (PPI) ZnO structures formed on (0001) Al2O3 substrates by evaluating the cathodoluminescence (CL) as a function of the acceleration voltage (Vacc). At surface regions with a low Vacc, the emission from the interface region was dominant but the dominant emission source changed to the Zn-polar regions as Vacc was increased, which can be explained by the different exciton diffusion lengths. The dependence of the emission properties on Vacc showed a slight red shift as Vacc increased and a larger red shift was observed in the Zn-polar regions. Moreover, the interface regions showed the highest intensity compared to other regions, which can be explained by the results of the geometrical analysis conducted using a finite-differential time domain simulation.

Vertical Array of ZnO Submicrorods on Periodically Polarity-Inverted Templates Using Solution Method

Vertically aligned ZnO nano/submicrorods are grown on periodically polarity-inverted (PPI) ZnO templates by a solution-based growth method without any catalyst. For the selective growth of ZnO submicrorods, PPI ZnO structures are used for templates made by using a polarity control technique of ZnO with CrN and Cr2O3 intermediate layers. After syntheses of ZnO nanostructures on PPI ZnO, the vertically aligned ZnO rods were grown only onto the Zn-polar regions.

Growth and characterization of periodically polarity-inverted ZnO structures on sapphire substrates

We report on the fabrication and characterization of periodically polarity inverted (PPI) ZnO heterostructures on (0001) Al2O3 substrates. For the periodically inverted array of ZnO polarity, CrN and Cr2O3 polarity selection buffer layers are used for the Zn- and O-polar ZnO films, respectively. The change of polarity and period in fabricated ZnO structures is evaluated by diffraction patterns and polarity sensitive piezo-response microscopy. Finally, PPI ZnO structures with subnanometer scale period are demonstrated by using holographic lithography and regrowth techniques.

Position-controlled vertical arrays of single-crystalline ZnO nanowires on periodically polarity inverted templates

Position controlled single crystalline ZnO nanowires are grown on periodically polarity-inverted (PPI) ZnO heterostructures without catalyst. PPI ZnO templates were fabricated by using selective growth method of polarity with Cr-compound intermediate layers by plasma assisted molecular beam epitaxy. In order to control the position, we used the in-plane two-dimensionally discrete PPI ZnO templates. The lateral polarity inversion in PPI template was confirmed by piezo response microscopy. After syntheses of ZnO nanowires, vertically aligned ZnO nanowires were grown only onto the Zn-polar regions without the O-polar regions. The results clearly show that the position control of ZnO nanowires is possible using the PPI ZnO templates.

Anisotropic properties of periodically polarity-inverted zinc oxide structures

We report on the anisotropic structural properties of periodically polarity-inverted (PPI) ZnO structures grown on patterned templates. The etching and growth rates along ⟨112¯0⟩ direction of ZnO structures are higher than those of ⟨101¯0⟩ direction of ZnO films. From the strain evaluation by Raman spectroscopy, compressive strains are observed in all PPI ZnO samples with different stripe pattern size and the smaller pattern size is more effective to residual stress relaxation. The detailed structures at transition region show relationship with the anisotropic crystal quality.

Second harmonic generation in periodically polarity-inverted zinc oxide

We report on the second harmonic generation (SHG) in 2D periodically polarity-inverted (PPI) ZnO heterostructures. The grating structures with nanometer-scale periodicity are fabricated on (0001) Al(2)O(3) substrates by using the in situ polarity inversion method. The achievements of SHG with grating in fabricated PPI ZnO structures are demonstrated under consideration of quasi phase matching conditions. In general, grating formation using the this periodical array of differnet polar surface can be extended to the other heteroepitaxial systems with polarity characteristics.

Fabrication of periodically polarity-inverted ZnO structures on (0001) Al 2O 3

Two-dimensional periodically polarity-inverted (PPI) ZnO structures were fabricated on (0001) Al 2O 3 substrates using plasma assisted molecular beam epitaxy. Holographic lithography was carried out for the realization of nanometer-scale patterns of CrN which was obtained by nitridation of Cr metal. The PPI ZnO structures consisted of Zn-polar regions grown on CrN and O-polar regions grown on Zn-terminated AlN/Al 2O 3. The periodical polarity inversion was confirmed by voltage-piezo response curves and contrast in the piezo response microscopy images. A diffraction pattern of the fabricated PPI ZnO sample was observed when a He–Ne laser beam was incident on the sample surface.

Fabrication of one-dimensional and two-dimensional periodically polarity inverted ZnO structures using the patterned CrN buffer layers

By employing the simple patterning and regrowth procedures, one-dimensional and two-dimensional (2D) periodically polarity inverted (PPI) ZnO structures are fabricated on (0001) Al2O3 substrates. For the selection of Zn- and O-polarity of ZnO films, patterned CrN buffer layers and Al2O3 substrates are used, respectively. The periodical change of the polarity is clearly confirmed by the polarity sensitive piezoresponse microscopy images, which are evidences for the successful fabrication of periodical polarity inversion structures. Cathodoluminescence investigation revealed that both polar layers have high crystal quality with strong free exciton emission. Moreover, 2D PPI ZnO structures with subnanometer scale periodicity are demonstrated by using the holographic lithography.

Ordered Arrays of ZnO Nanorods Grown on Periodically Polarity-Inverted Surfaces

Periodically polarity inverted (PPI) ZnO templates were fabricated using molecular beam epitaxy by employing MgO buffer layers. The polarity of ZnO film was controlled by the transformation of crystal structure from hexagonal to rocksalt due to the thickness of the MgO buffer layers. The polarity of ZnO in the PPI template was confirmed by AFM and PRM measurement. Higher growth rate and lower current value under positive supplied voltage in the region of Zn-polar were measured with comparing to that of O-polar. Holographic lithographic technique was employed for the realization of submicron pattern of periodical inverted polar ZnO over large area. After reaction using a carbothermal reduction, spatially well-separated ZnO nanorods with pitch of submicron were only observed in the Zn-polar regions. The possible reason for the difference of surface characteristics was considered as being due to the configuration of dangling bonds according to polarity.

Fabrication of periodically polarity-inverted ZnO films

One-dimensional periodically polarity-inverted (PPI) structures of ZnO for nonlinear optical devices are fabricated on c-plane Al2O3 substrates. To do so, corrugated MgO buffer layers are fabricated by etching after patterning, which is followed by the growth of ZnO layers by plasma-assisted molecular beam epitaxy. The polarity-inverted structures are confirmed by scanning piezoresponse microscopy and atomic-force microscopy. PPI structures with submicron periodicity are fabricated to satisfy the quasiphase matching condition for second harmonic generation of light.

Fabrication of Well-aligned ZnO Nanorods using Periodically Polarity-inverted Templates for Photonic Devices

ZnO has hexagonal wurtzite structure that is characterized by a mixture of ionic and covalent bondings between Zn and O atoms. A Zn atom is surrounded by four O atoms, or vice versa, which are located at the edges of a tetrahedron. Such coordination gives rise to polar symmetry along the c-axis of the hexagonal structure. The polarity affects electrical, optical, mechanical, and chemical properties of ZnO. In 2000, we have reported the successful selective growth of polarity-controlled ZnO films by molecular-beam epitaxy (MBE) using either GaN templates or MgO buffer. We confirmed different growth behaviors on the surfaces of ZnO layers with different polarities. Recently, we have explored the effect of crystal polarity on the formation of ZnO nanostructures. The periodically polarity-inverted (PPI) ZnO templates with submicron line-pitch over a large area were fabricated using MBE by employing MgO buffer layers and lithography technique. We grew ZnO nanorods using chemical transport and condensation method. The ZnO nanorods grew only on Zn-polar regions with the same periodicity as that of PPI template, which eventually produces well-aligned ZnO nanorod arrays.