Young Yi Kim

A comparative analysis of deep level emission in ZnO layers deposited by various methods

This study examined the origin of visible luminescence from ZnO layers deposited on p-Si substrates by various growth methods using temperature dependent photoluminescence measurements. The deep level emissions of ZnO layers are found to be strongly dependent on the growth conditions and growth methods used. For the samples grown by sputtering, the visible emission consisted of violet, green, and orange-red regions, which corresponded to zinc interstitial (Zni), oxygen vacancy (VO), and oxygen interstitial (Oi) defect levels, respectively. In contrast, the deep level emissions of metal organic chemical vapor deposition grown samples consisted of blue and green emissions and blue and orange-red emissions at low and high oxygen flow rates, respectively. The ZnO nanorods synthesized by thermal evaporation showed a dominant deep level emission at the green region, which is associated with oxygen vacancies (VO).

Synthesis and characterization of ZnO/MgZnO heterostructure nanorods by simple two-step evaporation.

ZnO-core/MgZnO-shell heterostructure nanorods with high aspect ratio were synthesized using a two-step thermal evaporation procedure, in which the core and the shell layers were formed separately at different temperatures. Microstructural characterization revealed a position dependence of the crystal structure and composition in the shell layer. The shell layer in the upper region consisted of MgO with quantum dot-like structure having cubic phases embedded in an amorphous oxide layer, while a Mg(0.35)Zn(0.65)O shell layer with a self-assembled superlattice structure of triple periodicity was formed in the middle region.

Characterization of Thermal Annealed n-ZnO/p-GaN/Al2O3

This paper reports the effects of post thermal annealing in air and N2 on the structural and optical properties of zinc oxide (ZnO) films deposited on a p-GaN substrate. The properties of annealed ZnO/GaN/Al2O3 films were investigated by atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL), and Auger electron spectroscopy (AES). Our experiments demonstrated that the ZnO film annealed in N2 had better structural properties than as-deposited ZnO film. When annealed in air, the optical property of ZnO film was improved by the oxygen from ambient air, although the structural properties were somewhat poor and the interface of ZnO and GaN was degraded by the formation of a Ga–O mixed phase.

Improvement of the Light Extraction Efficiency in n-ZnO:Ga/p-Si Heterojunction Light Emitting Diodes by a SiO2 Current-Blocking Layer

We studied the role of a patterned SiO2 current-blocking layer (CBL) on the output power in polycrystal n-type ZnO:Ga/p-Si heterojunction light emitting diodes (LEDs). The introduction of a CBL considerably improved the light extraction efficiency by about 200%, despite the increased series resistance. It suppressed the vertical current with a low extraction efficiency by opaque metals and allowed the spreading current component to be enhanced, in contrast to conventional n-ZnO/p-Si LEDs operated only a vertical current component due to the grain boundaries in the ZnO films. This result suggests that the use of CBL is profitable method in the emitter application using the film with the polycrystal structure.

Influence of Growth Temperature on the Characteristics of Ga-Doped ZnO Thin Films Deposited by Magnetron Sputtering

Ga-dope ZnO (ZnO:Ga) thin films with n-type semiconducting behavior were grown on c-sapphire substrates by radio frequency magnetron sputtering at various growth temperatures. The room temperature grown ZnO:Ga film showed a slight preferred orientation along the c-axis with small domain size and high density of stacking faults. The increase in growth temperature from 300 to 550 °C resulted in the growth of granular shaped epitaxial ZnO:Ga films due to insufficient thermal energy and large lattice mismatch. The films grown at above 550 °C showed a flat surface with the simultaneous improvement of the electrical carrier concentration and carrier mobility (6.3×1018/cm-3 and 27 cm2 V-1 s-1, respectively). In addition, there was an increase in the grain size and a decrease in the dislocation density in the high temperature grown films. The low-temperature photoluminescence of the ZnO:Ga films grown at <450 °C showed the redshift in deep-level emission, which was attributed to a transition from Zni to Oi level.

Effect of Buffer Thickness on the Formation of ZnO Nanorods Grown by MOCVD

ZnO semiconductor has a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV, and displays excellent sensing and optical properties. In particular, ZnO based 1D nanowires and nanorods have received intensive attention because of their potential applications in various fields. We grew ZnO buffer layers prior to the growth of ZnO nanorods for the fabrication of the vertically well-aligned ZnO nanorods without any catalysts. The ZnO nanorods were grown on Si (111) substrates by vertical MOCVD. The ZnO buffer layers were grown with various thicknesses at 400 °C and their effect on the formation of ZnO nanorods at 300 °C was evaluated by FESEM, XRD, and PL. The synthesized ZnO nanorods on the ZnO film show a high quality, a large-scale uniformity, and a vertical alignment along the [0001]ZnO compared to those on the Si substrates showing the randomly inclined ZnO nanorods. For sample using ZnO buffer layer, 1D ZnO nanorods with diameters of 150-200 nm were successively fabricated at very low growth temperature, while for sample without ZnO buffer the ZnO films with rough surface were grown.