Yong-Tae Moon

Temperature dependence of photoluminescence of InGaN films containing In-rich quantum dots

The temperature dependence of the photoluminescence (PL) of InGaN films, grown by metalorganic chemical vapor deposition, has been investigated. A strained InGaN thin film which contains composition-fluctuated regions shows the so-called S-shaped temperature dependence of the dominant PL peak energy. However, an InGaN thick film which contains quantum dot-like In-rich regions shows a sigmoidal temperature dependence of the dominant PL peak energy, as the result of a transfer of carriers from the band-edge related luminescent centers to quantum dot-like In-rich regions. It is also found that the activation energy for the thermal quenching of PL intensity in the InGaN thick film which contains quantum dot-like In-rich regions is larger than that in the strained InGaN thin film which contains composition-fluctuated regions.

Effect of Barrier Thickness on the Interface and Optical Properties of InGaN/GaN Multiple Quantum Wells

We investigated the effect of barrier thickness on the interfacial and optical properties of the InGaN/GaN multiple quantum wells (MQWs) grown in a low-pressure metalorganic chemical vapor deposition system. The GaN barrier thickness in the InGaN/GaN MQWs was found to play a key role to determine the interfacial structural and optical characteristics of the MQWs. As the thickness of the GaN barrier layer was increased, the abruptness of the interface between InGaN and GaN layers deteriorated, probably due to the generation of defects induced by the strain accumulation in the MQWs. Accordingly, the intensity and the line-width of the photoluminescence taken from the MQWs were reduced and broadened, respectively with the increase of the GaN barrier thickness. The InGaN/GaN MQWs grown with an optimized barrier thickness showed an intense room-temperature photoluminescence at the wavelength of 479.5 nm with a very narrow full width at half maximum of 40.82 meV.

Structural and optical properties of InGaN/GaN multiple quantum wells: The effect of the number of InGaN/GaN pairs

The effect of the number of InGaN/GaN quantum well (QW) pairs on the interfacial structural and optical properties of InGaN/GaN multiple quantum wells (MQWs), as grown by low-pressure metalorganic vapor-phase epitaxy was examined. As the number of QW pairs increased, In-rich InGaN precipitates were more readily detected in the InGaN/GaN MQWs by cross-sectional transmission electron microscope. The intensity of the photoluminescence (PL) peak was decreased and the PL peak was red-shifted with an increase in the number of QW pairs. X-ray diffraction measurements revealed that the interfacial structure between InGaN and GaN were also deteriorated with the increasing number of QW pairs. These results can be attributed to the relaxation of an accumulated strain through the dislocations induced by an increase in the total thickness of the MQWs with an increase in the number of QW pairs. These results suggest that the defects such as dislocations facilitate the formation of In-rich phases in the InGaN layers in the MQWs.

Determination of absolute indium content in InGaN/GaN multiple quantum wells using anomalous x-ray scattering

We have determined the absolute indium content incorporated in the crystalline lattice of InGaN films and InGaN/GaN multiple quantum wells using anomalous x-ray scattering (AXS). AXS spectra were obtained near the In K absorption edge at the InGaN (0006) Bragg peak where the InGaN Bragg reflection is well-resolved from the GaN reflections. By comparing the indium composition obtained by AXS to regular x-ray scattering results, which are also sensitive to the lattice strain, we determine the Poisson ratio of InGaN to be ν≈0.23. The AXS method can be effective in determining absolute chemical composition of InGaN independent of the lattice strain, which is especially valuable for InGaN multiple quantum wells.

Activation of Mg Acceptor in GaN : Mg with Pulsed KrF (248 nm) Excimer Laser Irradiation

We report on the activation of Mg acceptors in Mg-doped GaN films, grown by metalorganic chemical vapor deposition, via the use of a pulsed KrF (248 nm) excimer laser irradiation. The as-grown GaN:Mg, which was irradiated by the KrF excimer laser at a laser energy density of 590 mJ/cm 2 in a N 2 ambient showed a hole concentration of 4.42 × 10 17 cm -3 . Furthermore the hole concentration in GaN:Mg, which was activated by a conventional rapid thermal annealing, was increased from 4.3 × 10 17 to 9.42 × 10 17 cm -3 as the result of subsequent laser irradiation. These results suggest that a pulsed KrF excimer laser irradiation can dramatically enhance the p-type conductivity of GaN:Mg by efficiently dissociating the Mg-H complexes.

Effect of growth interruption and the introduction of H2 on the growth of InGaN/GaN multiple quantum wells

The effects of the growth interruption and the introduction of H2 during interruption time on the optical and structural properties of InGaN/GaN multiquantum wells (MQWs), grown by metalorganic chemical vapor deposition, were investigated. When the growth was interrupted during the formation of interfaces in the MQWs, the intensity of photoluminescence (PL) was greatly increased and the formation of InN-rich regions near the surface of the InGaN well layer was suppressed. As the interruption time increased, however, the PL intensity decreased and the average In composition of InGaN/GaN MQWs decreased. When H2 was introduced during the growth interruption, the intensity of the PL was significantly enhanced by eliminating the impurities at the interface and the PL peaks were blueshifted due to the reduction in the thickness of the InGaN well layers, as a result of H2 etching of well and barrier layers.

Recovery of dry-etch-induced surface damage on Mg-doped GaN by NH3 ambient thermal annealing

We report that NH3 ambient thermal annealing is a promising method for recovering the dry- etch-induced damage on Mg-doped GaN surfaces. The surface electrical properties of dry-etched Mg-doped GaN can be fully recovered by thermal annealing using NH3 as an ambient gas at temperatures above 900 °C. The complete recovery of sheet hole concentration in dry-etched Mg-doped GaN can be attributed to a reduction in excess nitrogen vacancies in the damaged surface region by reactive nitrogen atoms supplied during NH3 ambient thermal annealing.

In situ normal incidence reflectance study on the effect of growth rate of nucleation layer on GaN by metalorganic chemical vapor deposition

The effect of the growth rate of the nucleation layer on the growth of a high temperature GaN layer has been studied by observing the in situ normal incidence reflectance during the growth of GaN by metalorganic chemical vapor deposition. This study revealed that the lateral growth and coalescence of a high temperature GaN layer was enhanced as the growth rate of the nucleation layer was increased. However, the measurement of (102) hk-circle scan using x-ray diffraction showed that the in-plane structural qualities of GaN were found to be deteriorated by increasing the growth rate of the nucleation layer. The electron mobility was also decreased by increasing the growth rate of the nucleation layer. These results suggest that the nucleation sites are well oriented on the sapphire substrate at a low growth rate of the nucleation layer but that the lateral growth and coalescence of the GaN layer is hindered due to the limited surface diffusion of adatoms at a fast growth rate of the nucleation layer.

Effects of a two-step rapid thermal annealing process on Mg-doped p-type GaN films grown by metalorganic chemical vapor deposition

A two-step rapid thermal annealing (RTA) process is proposed in order to improve the electrical properties, the crystal quality, and the surface roughness of Mg-doped p-type GaN films. In the two-step RTA process, the first low temperature step (600 °C) with a long annealing time (5 min) was followed by the second high temperature (950 °C) step with a short annealing time. These results show that the two-step RTA process significantly improves electrical properties and reduces the surface roughness of p-GaN compared to the one-step RTA process.

Effects of KrF (248 nm) excimer laser irradiation on electrical and optical properties of GaN:Mg

The electrical and optical characteristics of GaN:Mg irradiated by a pulsed KrF (248 nm) excimer laser have been studied. When an as-grown Mg-doped GaN film was irradiated by an excimer laser at an energy density of 590 mJ/cm2 in a nitrogen atmosphere, the hole concentration was drastically increased up to 4.42×1017 cm−3. Furthermore, a GaN:Mg thin film, which was treated by laser irradiation following a conventional rapid thermal annealing process, showed a very high hole concentration of 9.42×1017 cm−3. The GaN:Mg samples, which were activated in a nitrogen ambient by the KrF excimer laser irradiation, showed two photoluminescence peaks at 2.95 eV and 2.7 eV. The intensities of both photoluminescence peaks were increased with increasing laser energy density and number of pulses. The changes in photoluminescence peaks depending on the laser energy density further suggest that the pulsed KrF excimer laser irradiation dissociates the Mg–H complexes and allows the hydrogens to diffuse out, thus significantl...