Meoung Whan Cho

Characteristics of dislocations in ZnO layers grown by plasma-assisted molecular beam epitaxy under different Zn∕O flux ratios

We have investigated the characteristic of the dislocations in the ZnO layers grown on c sapphire by the plasma-assisted molecular beam epitaxy under the different Zn∕O flux ratios. The ZnO layers were characterized by the transmission electron microscopy (TEM) and the high-resolution x-ray diffraction (HRXRD). The TEM and HRXRD experiments revealed that the major threading dislocations (TDs) in the ZnO layers are the edge dislocations running along the c axis with Burgers vector of 1∕3⟨11–20⟩. The TD densities are determined to be 6.9×109, 2.8×109, and 2.7×109cm−2, for O-rich, stoichiometric, and Zn-rich grown ZnO, respectively. Different from the O-rich grown ZnO where the dislocations run along the c-axis, several dislocations in the stoichiometric and the Zn-rich grown ZnO are inclined to 20°∼30° from the c-axis. By considering the slip system in the wurtzite-structure ZnO, the glide planes of the dislocations are close to (10-10) for the O-rich grown ZnO and close to (10-11) for the stoichiometric an...

Reduction of dislocations in GaN films on AlN/sapphire templates using CrN nanoislands

We report significant reduction of threading dislocations in GaN films grown by hydride vapor phase epitaxy on AlN/sapphire templates by employing CrN nanoislands on the AlN. High quality GaN films with very small twist mosaic as well as small tilt mosaic have been grown on the AlN/sapphire templates, which had small tilt but very large twist mosaic. The CrN nanoislands were formed by nitridation of a thin Cr film deposited by sputtering on the AlN/sapphire template, where the AlN/sapphire template was prepared by metal organic vapor phase epitaxy. The full width at half maximum values of x-ray rocking curves from the GaN film with the CrN were 114, 209, and 243arcsec for (0002), (10−12), and (11−20) reflections, respectively, while those of the GaN film without the CrN were 129, 1130, and 1364arcsec, respectively. Evaluation of total dislocation density of the GaN films by plan view transmission electron microscopy revealed that the dislocation density was reduced to 2.7×108 from 6.4×109cm−2 by employing...

Experimental verification of effects of barrier dopings on the internal electric fields and the band structure in InGaN/GaN light emitting diodes

We experimentally clarify the effects of barrier dopings on the polarization induced electric fields and the band structure in InGaN/GaN blue light emitting diodes. Both effects were independently verified by using electric field modulated reflectance and capacitance-voltage measurement. It is shown that the Si barrier doping does reduce the polarization induced electric field in the quantum wells. But the benefit of Si-doping is nullified by modification of the band structure and depletion process. With increased number of doped barriers, smaller number of quantum wells remains in the depletion region at the onset of the diffusion process, which can reduce the effective active volume and enhance the electron overflow.

Structural and stimulated emission characteristics of diameter-controlled ZnO nanowires using buffer structure

The diameter of ZnO nanowires grown by chemical vapour deposition was controlled by employing CrN buffer structures on the c-Al2O3 substrate. The nanosized CrN islands with different morphologies were prepared by nitridation of thickness-controlled Cr film in NH3 atmosphere. The ZnO nanowires grew normal to the surface of the CrN/c-Al2O3 templates due to reduction in the lattice mismatch between ZnO and c-Al2O3 by the CrN buffer layer. Investigation of the interface between CrN and ZnO by high resolution transmission electron microscopy revealed the presence of reactive layers such as ZnCr2O4 and Cr2O3. The diameter of nanowires significantly affected their stimulated emission characteristics. At room temperature, the threshold intensity for stimulated emission increased from 35 to above 500?kW?cm?2 as the diameter of ZnO nanowires decreases from 223 to 77?nm. This dependence of threshold intensity for stimulated emission from nanowires is caused by an increase in the surface recombination and/or enhanced leakage of optical field in narrower nanowires.

Heteroepitaxial growth of GaN on various powder compounds (AlN, LaN, TiN, NbN, ZrN, ZrB 2 , VN, BeO) by hydride vapor phase epitaxy

We investigated the nucleation and growth behavior of GaN on various powders by hydride vapor phase epitaxy. In relative comparison, the nucleation tendency of GaN on each powder can be summarized as AlN > LaN, TiN, NbN > ZrN > ZrB2 > VN, BeO, indicating that the number of nucleation sites increased from right to left. LaN and NbN have not yet been reported as buffer materials for GaN growth. Of these, NbN is expected to be a good buffer material because the interatomic distance on the NbN (111) plane has only 2% difference from that on the GaN (0001) plane.

Luminescence properties of ZnSe/ZnS(h 1 1)A low dimensional structures

The power dependence and the temperature dependence of the luminescence intensity of ultra-thin pseudomorphic ZnSe quantum wells, grown on high index GaP substrates were investigated. A blue shift of PL with power excitation is mainly related to the band filling of interfacial ZnSe clusters while it is found that the piezoelectric field has only a small influence. Luminescence spectra of ultra-thin ZnSe/ZnS single quantum wells are characterized by an excitation dependent excitonic localization phenomenon.