Minseo Park

Lasing in whispering gallery mode in ZnO nanonails

We report on the observation of ultraviolet lasing in optically pumped ZnO nanonails synthesized by thermal chemical vapor deposition method. The lasing threshold was found to be 17MWcm−2. Very sharp emission peaks (full width at half maximum of 0.08nm) were observed in the emission spectrum, indicating a high Q factor of the cavity formed by the hexagonally shaped nanonail head. The analysis of the lasing spectra strongly suggests the whispering gallery mode lasing from a hexagonally shaped head of the single ZnO nanonail.

Thermal chemical vapor deposition growth of zinc oxide nanostructures for dye-sensitized solar cell fabrication

Dye-sensitized solar cells (DSSCs) were fabricated using a quasialigned one-dimensional zinc oxide (ZnO) nanostructure. The ZnO nanostructures were grown on indium tin oxide (ITO) coated glass substrate via a thermal chemical vapor deposition (CVD). It has been considered that thermal CVD is not suitable for the growth of ZnO nanostructure on ITO/glass due to the high processing temperature. However, we have demonstrated that a densely populated ZnO nanostructure can be prepared on ITO/glass substrate by a double-source double-tube CVD process. The power conversion efficiency of our device is 0.6%.

Effects of postgrowth annealing treatment on the photoluminescence of zinc oxide nanorods

Postgrowth annealing was carried out to investigate the photoluminescence of zinc oxide (ZnO) nanorods synthesized using a thermal chemical vapor deposition method. The observed change in photoluminescence after the annealing processes strongly suggests that positively charged impurity ions or interstitial Zn ions are the recombination centers for green luminescence observed in the present sample. A model based on the interplay between the band bending at the surface and the migration of positively charged impurity ions or Zn ions was proposed, which satisfactorily explains the observed photoluminescence.

Temperature-dependent electrical characteristics of bulk GaN Schottky rectifier

The temperature-dependent electrical characteristics of Schottky rectifiers fabricated with a SiO2 field plate on a freestanding n− gallium nitride (GaN) substrate were reported in the temperature range of 298–473K. The Schottky barrier heights evaluated from forward current-voltage measurement revealed an increase of Schottky barrier height and series resistance but a decrease of ideality factor (n) with increasing temperature. However, the Schottky barrier heights evaluated from capacitance-voltage measurement remained almost the same throughout the temperature range measured. The Richardson constant extrapolated from ln(J0∕T2) vs 1∕T plot was found to be 0.029Acm−2K−2. A modified Richardson plot with ln(J0∕T2) vs 1∕nT showed better linearity, and the corresponding effective Richardson constant was 35Acm−2K−2. The device showed a high reverse breakdown voltage of 560V at room temperature. The negative temperature coefficients were found for reverse breakdown voltage, which is indicative of a defect-assi...

Raman analysis of longitudinal optical phonon-plasmon coupled modes of aligned ZnO nanorods

The electronic properties of vertically aligned ZnO nanorods have been investigated using micro-Raman spectroscopy. The concentration and mobility of the charge carriers were determined via Raman line shape analysis using longitudinal-optical-phonon-plasmon coupled mode. The local laser heating and the stress effects have been considered when analyzing the Raman spectra. The mobility and carrier concentration of the aligned ZnO nanorods are 84.8cm2∕Vs and 3.8×1017cm−3, respectively. As a comparison, the mobility and carrier concentration of the undoped bulk ZnO were also obtained from the Raman line shape analysis. The mobility of the aligned ZnO nanorods is about 20% lower than that of the undoped bulk ZnO, which can be attributed to enhanced surface scattering due to the reduction in dimension.

High-Mobility Stable 4H-SiC MOSFETs Using a Thin PSG Interfacial Passivation Layer

Phosphorous from P₂O₅ is more effective than nitrogen for passivating the 4H-SiC/SiO₂ interface. The peak value of the field-effect mobility for 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) after phosphorus passivation is approximately 80 cm²/V·s. However, P₂O₅ converts the SiO₂ layer to phosphosilicate glass (PSG)-a polar material that introduces voltage instabilities which negate the benefits of lower interface trap density and higher mobility. We report a significant improvement in voltage stability with mobilities as high as 72 cm²/V·s for MOSFETs fabricated with a thin PSG gate layer ( ~ 10 nm) capped with a deposited oxide.

Epitaxial growth of ZnO films on Si(111)

In this paper, we report the growth of ZnO films on silicon substrates using a pulsed laser deposition technique. These films were deposited on Si(111) directly as well as by using thin buffer layers of AlN and GaN. All the films were found to have c-axis-preferred orientation aligned with normal to the substrate. Films with AlN and GaN buffer layers were epitaxial with preferred in-plane orientation, while those directly grown on Si(111) were found to have random in-plane orientation. A decrease in the frequency of the E 2 (2) Raman mode and a red shift of the band-edge photoluminescence peak due to the presence of tensile strain in the film, was observed. Various possible sources for the observed biaxial strain are discussed.

Synthesis of metal-semiconductor core-shell nanoparticles using electrochemical surface-limited reactions.

We report the synthesis of Au/CuI and Au/CdS core-shell nanoparticle (NP) thin films using codeposition and electrochemical atomic layer deposition (EC-ALD). Au nanoparticle films were prepared on glassy carbon supports by depositing alternating layers of poly(diallyl dimethylammonium)-stabilized Au nanoparticles and CoP(2)W(17)O(61)(8-) polyoxometallate interlayers. From there, CuI was deposited onto the surface of Au nanoparticles using electrochemical atomic layer deposition, while CdS films were grown by an atom-by-atom codeposition method. The semiconductor-Au core-shell nanoparticles were characterized by electrochemistry, photoluminescence spectroscopy, and Raman spectroscopy. Our results indicate that the semiconductors deposit onto the AuNP surface by surface limited electrochemical reactions.

Photoelectron emission microscopy observation of inversion domain boundaries of GaN-based lateral polarity heterostructures

An intentionally grown GaN film with laterally patterned Ga- and N-face polarities is studied using in situ UV-photoelectron emission microscopy (PEEM). Before chemical vapor cleaning of the surface, the emission contrast between the Ga- and N-face polarities regions was not significant. However, after cleaning the emission contrast between the different polarity regions was enhanced such that the N-face regions exhibited increased emission over the Ga-face regions. The results indicate that the emission threshold of the N-face region is lower than that of the Ga face. Moreover, bright emission was detected from regions around the inversion domain boundaries of the lateral polarity heterostructure. The PEEM polarity contrast and intense emission from the inversion domain boundary regions are discussed in terms of the built-in lateral field and the surface band bending induced by the polarization bound surface charges.

Micro-Raman study of electronic properties of inversion domains in GaN-based lateral polarity heterostructures

The electronic properties of inversion domains in a GaN-based lateral polarity heterostructure were investigated using micro-Raman spectroscopy. The piezoelectric polarization of each domain was calculated from strain determined via Raman scattering. The free carrier concentration and electron mobility were deduced from the longitudinal optical phonon–plasmon coupled mode. The electron concentration in the N-face domain was slightly higher than that in the Ga-face domain. It appears that during growth, a larger number of donor impurities may have been incorporated into the N-face domain than into the Ga-face domain.