Hyunjin Cho

Formation mechanism of V defects in the InGaN/GaN multiple quantum wells grown on GaN layers with low threading dislocation density

V-defect formation of the InxGa1−xN/GaN multiple quantum wells (MQWs) grown on GaN layers with different threading dislocation (TD) densities was investigated. From cross-sectional transmission electron microscopy, we found that all V defects are not always connected with TDs at their bottom. By increasing the indium composition in the InxGa1−xN well layer or decreasing the TD density of the thick GaN layer, many V defects are generated from the stacking mismatch boundaries induced by stacking faults which are formed within the MQW due to the strain relaxation. Also, TD density in the thick GaN layer affects not only the origin of V-defect formation but also the critical indium composition of the InxGa1−xN well on the formation of V defects.

Effect of growth interruptions on the light emission and indium clustering of InGaN/GaN multiple quantum wells

InGaN/GaN multiple quantum wells (MQWs) grown with various growth interruptions between the InxGa1−xN well and GaN barrier by metalorganic chemical vapor deposition were investigated using photoluminescence (PL), high-resolution transmission electron microscopy, and energy filtered transmission electron microscopy (EFTEM). The integrated PL intensity of the MQWs with growth interruptions is abruptly reduced compared to that of the MQW without growth interruption. Also, as the interruption time increases the peak emission shows a continuous blueshift. Evidence of indium clustering is directly observed both by using an indium ratio map of the MQWs and from indium composition measurements along an InGaN well using EFTEM. The higher-intensity and lower-energy emission of light from the MQW grown without interruption showing indium clustering is believed to be caused by the recombination of excitons localized in indium clustering regions and the increased indium composition in these recombination centers.

Codoping characteristics of Zn with Mg in GaN

The doping characteristics of Mg–Zn codoped GaN films grown by metalorganic chemical vapor deposition are investigated. By means of the concept of Mg–Zn codoping technique, we have grown p-GaN showing a low electrical resistivity (0.72 Ωu200acm) and a high hole concentration (8.5×1017u200acm−3) without structural degradation of the film. It is thought that the codoping of Zn atoms with Mg raises the Mg activation ratio by reducing the hydrogen solubility in p-GaN. In addition, the measured specific contact resistance of Mg–Zn codoped GaN film is 5.0×10−4u200aΩu200acm2, which is one order of magnitude lower than that of Mg doped only GaN film (1.9×10−3u200aΩu200acm2).

Superlattice-like stacking fault and phase separation of InxGa1−xN grown on sapphire substrate by metalorganic chemical vapor deposition

InxGa1−xN alloys were directly grown on sapphire substrate with a GaN nucleation layer. The degree of phase separation in the InGaN layer on sapphire substrate is maximized at higher growth temperature than for an InGaN layer grown on a thick GaN layer. For high indium composition, a superlattice-like stacking fault in the InxGa1−xN grown on sapphire substrate was detected by the selected area diffraction pattern and high-resolution transmission electron microscopy. The superlattice-like arrangement of stacking faults leads to the formation of split spots and the distance of split spots corresponds to the distance between stacking faults.

Phase separation and stacking fault of InxGa1−xN layers grown on thick GaN and sapphire substrate by metalorganic chemical vapor deposition

Abstract We have studied the phase separation and the stacking faults of In x Ga 1− x N grown on sapphire substrate with a GaN nucleation layer and In x Ga 1− x N grown on a thick GaN layer. For high In composition, the distribution of the periodic stacking faults in the In x Ga 1− x N grown on sapphire substrate was detected by selected area diffraction (SAD) pattern and high-resolution transmission electron microscopy (HRTEM). The periodic arrangement of stacking faults leads to the formation of split spots and the distance of split spots corresponds to the distance between stacking faults. For the InGaN layer grown on the thick GaN layer, misfit strain is relaxed by the formation of V-grooves and the random stacking faults. For the InGaN layer grown on sapphire substrate with a thin buffer layer, a higher residual strain than GaN/sapphire is found, which is relaxed by the formation of the periodic stacking faults in the InGaN layer.

Structural and optical characteristics of InGaN/GaN multiple quantum wells with different growth interruption

InGaN/GaN multiple quantum wells (MQWs) grown with various growth interruptions between InGaN well and GaN barrier by metalorganic chemical vapor deposition were investigated using photoluminescence, high-resolution transmission electron microscopy (HRTEM), and energy filtered transmission electron microscopy (EFTEM). The luminescence intensity of the MQWs with growth interruptions is abruptly reduced compared to that of the MQW without growth interruption. Also, as the interruption time increases the peak emission shows a continuous blue shift. We found that the higher intensity and lower energy emission of the MQW grown without interruption is caused by the recombination of excitons localized from indium clustering regions. Evidence of indium clustering is directly observed by indium ratio map of MQWs and indium composition measurements along an InGaN well using EFTEM.

Structural properties of Si and Mg doped and undoped Al0.13Ga0.87N layers grown by metalorganic chemical vapor deposition

Abstract Structural properties of Si and Mg doped and undoped Al 0.13 Ga 0.87 N layers grown on sapphire substrates by metalorganic chemical vapor deposition were studied using high-resolution X-ray diffraction (HRXRD) and transmission electron microscopy. For both low SiH 4 and low Cp 2 Mg flow rates, the full width at half maximum values of rocking curve and total threading dislocation density in Al 0.13 Ga 0.87 N layers rapidly decrease due to the increased island size by surfactant effect. The origin of the broadening of HRXRD rocking curve in Al 0.13 Ga 0.87 N layers with high SiH 4 and Cp 2 Mg flow rates results from the increase of total threading dislocation density and stacking fault density, respectively. Beside, it was observed that while for Si doping the lattice constant c is decreased continuously with the SiH 4 flow rate, the lattice constant c of Mg doped AlGaN layers is rapidly increased with a Cp 2 Mg flow rate of 3.172xa0μmol/min.

Highly luminescent blue-emitting CdZnS/ZnS nanorods having electric-field-induced fluorescence switching properties

We demonstrate for the first time highly luminescent blue-emitting CdZnS/ZnS wurtzite core/shell nanorods (NRs) that show electric-field-induced fluorescence switching properties. Uniform CdZnS NRs were rapidly synthesized by injecting sulfur powder dissolved in 1-octadecene into a flask containing phosphonic acid ligands, and subsequently ZnS shells were coated using reagents consisting of sulfur powder, zinc sulfate heptahydrate, and oleylamine. The growth of high-quality ZnS shells resulted in a dramatically increased photoluminescence (PL) quantum yield (QY) of ∼40% with a minimal red-shift of the blue PL peak, which indicates that the combination of reagents successfully controlled a large number of defects appearing on the surface of the NR cores. By pre-annealing CdZnS cores before growing ZnS shells, we could achieve an additional increase in the maximum PL QY to 50%, decreases in both the full width at half maximum (FWHM) and the red-shift of the PL peak, and improved electric-field-induced fluorescence switching performance. Density functional theory calculations reveal that the effective relaxation of strain accumulating on the NR core during shell growth is the key to our successful synthesis of blue-emitting NRs, and that the additional improvement in performance obtained through the pre-annealing process results from the elimination of sulfur vacancies appearing at the surface of the NR core.

Chemical mapping of indium rich quantum dots in InGaN/GaN quantum wells

Indium clustering in InGaN/GaN multiple quantum wells (MQWs) is believed to be responsible for the high luminescent efficiency of GaN based light emitting diodes. In this paper we show that substantial clustering can be induced by reducing to zero the interruption time between growth of the GaN barrier layer on the InGaN quantum well. Photoluminescence (PL) shows that this has the effect of increasing the luminescence intensity and decreasing the band gap energy (higher indium concentration). The clusters or quantum dots were examined and quantified by energy filtered transmission electron microscopy (EFTEM), which was used to form chemical distribution maps of indium, gallium and nitrogen. In this paper we will show that this technique can accurately calculate the indium concentration and distribution in the quantum wells. The calculations show that In x Ga 1−x N quantum dots (width = 1.3nm) exhibit an In concentration of up to x = 0.5, which are embedded in a quantum well matrix with x = 0.05.