Huei-Min Huang

Isoelectronic As doping effects on the optical characteristics of GaN films grown by metalorganic chemical-vapor deposition

We have studied the As doping effects on the optical characteristics of GaN films by time-integrated photoluminescence and time-resolved photoluminescence. When As is incorporated into the film, the localized defect levels and donor–acceptor pair transition become less resolved. The recombination lifetime of neutral-donor-bound exciton (I2) transition in undoped GaN increases with temperature as T1.5. However, the I2 recombination lifetime in As-doped GaN first decreases exponentially from 98 to 41 ps between 12 and 75 K, then increases gradually to 72 ps at 250 K. Such a difference is related to the isoelectronic As impurities in GaN, which generate nearby shallow levels that dominate the recombination process.

Photoluminescence and photoluminescence excitation studies of as-grown and P-implanted GaN: On the nature of yellow luminescence

We have studied optical and electronic properties of isoelectronic P-implanted GaN films grown by metalorganic chemical vapor phase epitaxy. After rapid thermal annealing, a strong emission band around 430 nm was observed, which is attributed to the recombination of exciton bound to isoelectronic P-hole traps. From the Arrhenius plot, the hole binding energy of ∼180 meV and the exciton localization energy of 28 meV were obtained. According to first-principle total-energy calculations, the implantation process likely introduced NI and P-related defects. By using photoluminescence excitation technique, we found that the P-implantation-induced localized states not only increase the yellow luminescence but also suppress the transitions from the free carriers to deep levels.

Magnitude-tunable sub-THz shear phonons in a non-polar GaN multiple-quantum-well p-i-n diode

Coherent transverse acoustic phonons are optically generated and detected through the piezoelectric coupling between the build-in electric fields and shear strains of a non-polar GaN multiple quantum wells embedded in a p-n junction. By optical transient transmission change measurement, the phonon frequency is observed to be 0.4 THz which corresponds to a wavelength of 12.5 nm, the periodicity of the multiple quantum wells, and the estimated phonon velocity corresponds to the transverse acoustic phonon velocity in GaN. Moreover, we can magnify the driving amplitude of the generated shear phonons by increasing the reverse bias of the p-i-n diode.

High quality factor nonpolar GaN photonic crystal nanocavities

High quality factor a-plane nonpolar GaN two-dimensional photonic crystal (PC) nanocavities on r-plane sapphire substrates have been demonstrated. Nonpolar GaN PC nanocavities on a thin membrane structure were realized by using e-beam lithography to define the PC patterns and focused-ion beam milling to fabricate the suspended thin membrane. A dominant resonant mode at 388 nm with a high quality factor of approximately 4300 has been demonstrated at 77 K by the micro-photoluminescence system. Moreover, the degree of polarization of the emission from the non-polar GaN PC nanocavity was measured to be 64% along the m crystalline direction.

Microphotoluminescence spectra of hillocks in Al0.11Ga0.89N films

The spatial variation of the optical properties of hillocks in Al0.11Ga0.89N films has been studied by using microphotoluminescence (μ-PL) microscopy. The μ-PL spectrum revealed a strong emission (IH) at 351nm from the hillock, besides the near-band-edge emission (Inbe) at 341nm. Moreover, the IH intensity increases significantly and its full width at half maximum decreases from ∼76to∼53meV by probing across the hillock center. These indicated that the hillock structure is a strong emission center. The temperature-dependent μ-PL measurements showed that the IH also has the S-shape behavior with a transition temperature of ∼120K which is lower than that of Inbe. The redshift of IH is also smaller than Inbe. Both indicated that the Al composition in hillocks is lower than the surrounding area.The spatial variation of the optical properties of hillocks in Al0.11Ga0.89N films has been studied by using microphotoluminescence (μ-PL) microscopy. The μ-PL spectrum revealed a strong emission (IH) at 351nm from the hillock, besides the near-band-edge emission (Inbe) at 341nm. Moreover, the IH intensity increases significantly and its full width at half maximum decreases from ∼76to∼53meV by probing across the hillock center. These indicated that the hillock structure is a strong emission center. The temperature-dependent μ-PL measurements showed that the IH also has the S-shape behavior with a transition temperature of ∼120K which is lower than that of Inbe. The redshift of IH is also smaller than Inbe. Both indicated that the Al composition in hillocks is lower than the surrounding area.

Ultraviolet emission efficiency enhancement of a-plane AlGaN/GaN multiple-quantum-wells with increasing quantum well thickness

The defect-induced carrier localization in nonpolar a-plane (Al,Ga)N/GaN multiple quantum wells (MQWs) structures with different well thickness have been investigated. A strong variation of temperature-dependent photoluminescence peak energy was observed and attributed to the existence of the localized states. The degree of carrier localization in these defect-induced states was more prominent in the case of MQWs with the wider well width. In addition, the ultraviolet light emission efficiency revealed a 3-fold enhancement with increasing the well width from 1.6 nm to 7.3 nm, due to the strong carrier localization generated from the quantum-wire-like features formed by the intersection between basal stacking faults and quantum wells.

Time-resolved photoluminescence study of isoelectronic In-doped GaN films grown by metalorganic vapor-phase epitaxy

Time-resolved photoluminescence spectra were used to characterize isoelectronically doped GaN:In films. Our results indicate that the recombination lifetime of the donor-bound-exciton transition of undoped GaN exhibits a strong dependence on temperature. When In is doped into the film, the recombination lifetime decreases sharply from 68 to 30 ps, regardless of the measured temperature and In source flow rate. These observations might be related to the isoelectronic In impurity itself in GaN, which creates shallow energy levels that predominate the recombination process.

Near-field optical microscopy and scanning Kelvin microscopy studies of V-defects on AlGaN/GaN films

AlxGa1−xN thin film was grown on undoped GaN∕sapphire (0001) substrate by metalorganic chemical vapor deposition. V-defects were directly observed by atomic force microscopy (AFM) with various size of 0.5–2μm in diameter. In a previous study, the microphotoluminescence spectra showed an extra peak (Iv=350nm) inside the V-defect besides the near-band-edge emission (Inbe=335nm). To achieve better spatial resolution, we used near-field scanning optical microscopy (NSOM) and scanning Kelvin-force microscopy (SKM) to probe the V-defect in detail. The NSOM spectra showed that the intensity of the Iv band increased gradually from V-defect edges to its center, while Inbe remained unchanged. Besides, the SKM measurements revealed that the Fermi level decreased from the flat region to V-defect center by about 0.2eV. These results suggest that the Iv band could be related to shallow acceptor levels, likely resulting from VGa defects.

Exciton Localization Behaviors of Basal Stacking Faults in a-Plane AlGaN Alloys

We study the basal plane stacking faults (BSFs) related optical properties in a-plane AlGaN alloys with different Al composition ranging from 0 to 0.28. The low-temperature photoluminescence (PL) spectra for AlGaN show two dominant peaks attributed to the emission of near band edge and BSFs-bound excitons, respectively. The PL integrated intensity ratio of the BSFs to NBE is found to correlate to the density of BSFs observed by the transmission electron microscopy. Finally, the exciton localization behaviors of BSFs in a-plane AlGaN alloys is observed and discussed in this study.

Inverted Octagonal Surface Defects in a-Plane AlGaN/GaN Multiple Quantum Wells

The structural properties of a-plane AlGaN/GaN multiple quantum wells grown on the r-plane sapphire substrate have been characterized. The pentagonal and inverted octagonal surface pits, consisting of several non-polar and semi-polar crystalline facets, are clearly observed and distinguished. The Al incorporation efficiency of the non-polar and semi-polar facets of these special inverted octagonal surface pits has been verified in the order of (1122) <(1012) <(1100) (1120) <(2021) by cathodoluminescence measurements at room temperature. The evolution of these inverted octagonal surface pits could be due to the results of interaction