Hsin-Hung Yao

Crack-free GaN∕AlN distributed Bragg reflectors incorporated with GaN∕AlN superlattices grown by metalorganic chemical vapor deposition

A crack-free GaN∕AlN distributed Bragg reflector (DBR) incorporated with GaN∕AlN superlattice (SL) layers was grown on a c-plane sapphire substrate by metalorganic chemical vapor deposition. Three sets of half-wave layers consisting of 5.5 periods of GaN∕AlN SL layers and GaN layer were inserted in every five pairs of the 20 pair GaN∕AlN DBR structure to suppress the crack generation. The grown GaN∕AlN DBRs with SL insertion layers showed no observable cracks in the structure and achieved high peak reflectivity of 97% at 399nm with a stop band width of 14nm. Based on the x-ray analysis, the reduction in the in-plane tensile stress in the DBR structure with insertion of SL layers could be responsible for the suppression of crack formation and achievement of high reflectivity.

Room-Temperature Operation of Optically Pumped Blue-Violet GaN-Based Vertical-Cavity Surface-Emitting Lasers Fabricated by Laser Lift-Off

Room-temperature optically pumped GaN-based vertical-cavity surface-emitting lasers (VCSELs) were demonstrated by laser lift-off. A VCSEL was fabricated by combining a GaN-based cavity with two dielectric distributed Bragg reflectors: SiO2/TiO2 and SiO2/Ta2O5. The Q factor of the VCSEL is 518 indicating a good interfacial layer quality of the structure. The laser emits blue-violet wavelength light at 414 nm under optical pumping at room temperature with a threshold pumping energy of 270 nJ. The laser emission has a narrow linewidth of 0.25 nm and a degree of polarization of 70%. The laser emission patterns clearly indicate a vertical lasing action of the VCSEL.

The lasing characteristics of GaN-based vertical-cavity surface-emitting laser with AlN-GaN and Ta/sub 2/O/sub 5/--SiO/sub 2/ distributed Bragg reflectors

The characteristics of a GaN-based vertical-cavity surface-emitting laser (VCSEL) with 25 pairs AlN-GaN distributed Bragg reflector (DBR) and eight pairs Ta/sub 2/O/sub 5/--SiO/sub 2/ DBR was investigated and analyzed under the optical pumping at room temperature. The GaN-based VCSEL emits a blue wavelength at 448 nm with a linewidth of 0.17 nm with a near-field emission spot diameter of about 3 /spl mu/m. The laser beam has a near linear polarization with a degree of polarization of about 84%. The laser shows a high spontaneous emission coupling efficiency of about 5/spl times/10/sup -2/ and a high characteristic temperature of about 244 K.

Aluminum incorporation into AlGaN grown by low-pressure metal organic vapor phase epitaxy

Aluminum (Al) incorporation in AlxGa1−xN films grown by low-pressure metal organic vapor phase epitaxy using trimethylaluminum (TMAl) and trimethylgallium as group III precursors has been systematically studied. The solid phase Al composition of the AlxGa1−xN films varied nonlinearly with the Al gas phase composition. The incorporation kinetics of AlxGa1−xN alloy has been analyzed by using an adsorption-trapping model. Two parameters were used to characterize the properties of Al incorporation, i.e., the capture radius and the adsorption time of Al atoms. An exponential function of the Al composition of the AlxGa1−xN films versus the TMAl gas flow rate was obtained. It was demonstrated that the adsorption time of the Al atom was larger than the growth time of one atomic layer. The effects of ammonia flow rate, crystal growth rate, and growth temperature on the adsorption parameters were also discussed.

High-speed Modulation of InGaAs: Sb-GaAs-GaAsP quantum-well vertical-cavity surface-emitting lasers with 1.27-/spl mu/m emission wavelength

1.27-/spl mu/m InGaAs: Sb-GaAs-GaAsP vertical-cavity surface-emitting lasers (VCSELs) were grown by metal-organic chemical vapor deposition and exhibited excellent performance and temperature stability. The threshold current changes from 1.8 to 1.1 mA and the slope efficiency falls less than /spl sim/35% as the temperature raised from room temperature to 70/spl deg/C. With a bias current of only 5 mA, the 3-dB modulation frequency response was measured to be 8.36 GHz, which is appropriate for 10-Gb/s operation. The maximal bandwidth is measured to be 10.7 GHz with modulation current efficiency factor (MCEF) of /spl sim/5.25 GHz/(mA)/sup 1/2/. These VCSELs also demonstrate high-speed modulation up to 10 Gb/s from 25/spl deg/C to 70/spl deg/C.

Formation and optical properties of GaN/AlN multi-stacks with GaN nano-structures by metallorganic chemical vapour deposition

A 16-pair GaN/AlN structure with GaN nano-structures at the hetero-interface was grown by a metallorganic chemical vapour deposition (MOCVD) system. From the atomic force microscopy (AFM) analysis of the AlN surface grown on the GaN/sapphire template, there are many nano-trenches and nano-holes with a density of about 5 × 108 cm−2. These GaN nano-structures fill the AlN nano-holes with a dimension of 40 × 40 nm2 in width and depth, and with a line density of 6 × 104 cm−1. Furthermore, these GaN nano-structures will be propagated and self-aligned in the growth direction. The photoluminescence (PL) measurement showed that there are bright and dark emission areas on the surface of the 16-pair GaN/AlN structure; the PL intensity at the bright region was ten times stronger than the dark region, and this stronger GaN peak has the blue-shift property caused by a quantum confined effect. In the end, we concluded that the V-shape GaN nano-structures filling the AlN nano-hole structure have a strongly quantum confined effect depending on the temperature and the power dependent PL results.

Strong Ultraviolet Emission from InGaN/AlGaN Multiple Quantum Well Grown by Multi-step Process

We propose a method of realizing strong ultraviolet emission from InGaN/AlGaN multiple quantum wells (MQWs) grown by a multi-step process. During growth of the quantum well layer, only trimethylindium (TMIn) and ammonia were introduced into the reactor, followed by a growth interruption treatment before growth of AlGaN barriers. The growth temperature of QWs dominates the photoluminescence (PL) emission peak position and surface morphologies of the films. It was found that the PL spectra of the samples with MQWs grown at 685 °C showed a strong UV emission at 380 nm. The correlation between surface structures and optical characteristics was studied by cathodoluminescence microscopy. The electroluminescence spectra under various injection currents showed a weak carrier localization effect induced by a quantum-confined Stark effect in the MQW.

Lasing characteristics of GaN vertical-cavity surface-emitting lasers with dielectric DBRs fabricated by laser-lift-off technique

The lasing characteristics of a GaN VCSEL with two dielectric DBRs of SiO₂/TiO₂ and SiO₂/Ta₂O₅ were investigated. The laser emits wavelength at 414 nm under optical pumping at room temperature with a threshold energy of 270 nJ.

Effects of growth interruption time on InGaN/GaN quantum dots grown by metal organic chemical vapor deposition

Wide-bandgap gallium nitride (GaN) quantum dot (QD) structure is attractive because it is a zero-dimensional (0-D) confinement structure and has many unique physical characteristics. We have successfully grown self-assembled InGaN QDs structure by metal organic chemical vapor deposition. A high quality GaN/sapphire template with a flat surface and the suitable growth condition including low growth temperature and low V/III ratio were used to grow InGaN QDs structure. The density of InGaN QDs is about 4.5 × 1010 cm-2 with an average lateral size of 11.5 nm and an average height of 1.6 nm. The effect of the interruption growth for InGaN QDs structure was systematic studied with the growth temperature of 660°C. The surface morphology and optical property was measured by atomic forced microscopy and various temperature PL, respectively. The results indicated that as increasing interruption time from 30s to 120s, QDs area occupied on the surface above the wetting layer increases from 5.2% to 7.2%, and the In composition decreases from 25% to 21%. The results were discussed by considering the influences of ad-atom desorption and diffusion effect between wetting layer and InGaN QDs structure. Our results suggest that the interruption growth during an optimum time can modify the size of InGaN QDs and extend the emission wavelength to short wavelength, and at the same time improve the QD optical quality. Using this technique was feasible for formation of multi layer InGaN QDs structures and applicable for the fabrication of GaN-based light emitting devices.

Fabrication and characteristics of GaN-based Microcavity LEDs with high reflectivity AlN/GaN DBRs

`Abstract GaN-based micro cavity light emitting diodes (MCLEDs) which composed of 25 pairs of in-situ epitaxially grown GaN/AlN Distributed Bragg Reflector (DBR) with high reflectivity (94%), and 6 pairs ex-situ deposited SiO2/TiO2 dielectric mirrors (97.5%) was reported. The electroluminescence peak of this structure well matched with the dip of the reflectance within the stop band and the fabricated device showed excellent optical performances including the narrowed emission width (6.7 nm), and more stable emission peak wavelength (redshift free) as varying injection current density and operating temperature than the regular LED.