Ya-Wen Lin

Investigation of efficiency droop for UV LED with N-type AlGaN layer

The efficiency droop in InGaN-based 380nm UV light emitting device (LED) with n-GaN and n-AlGaN underlayer grown on sapphire substrate by metal-organic chemical vapor deposition (MOCVD) was investigated. From simulation result of high resolution x-ray diffraction (HRXRD) ω-2θ curve by using dynamical diffraction theory, the Al composition in the n-AlGaN layer was determined to be about 3%. The experimental results of temperature dependent photoluminescence (PL) demonstrated that the internal quantum efficiency (IQE) of n-GaN and n-AlGaN UV-LEDs are 43% and 39%, respectively, which are corresponding to an injected carrier density of 8.5 × 1017 #/cm3. It could be explained that the crystal quality of n-GaN is better than of n-AlGaN. In addition, the observation of pit density from atomic force microscopy (AFM) surface morphology is consistent with the interpretation. It was well-known that the pits appearing on the surface in the virtue of the threading dislocations. Thus, it means that defects induce the non-radiative centers and deteriorate the IQE of the UV-LED with n-AlGaN underlayer. Therefore, the light output power of n-GaN UV-LED is slightly higher below the forward current of 250 mA. Nevertheless, the output power was enhanced about 22% as the injection current was increased to 600 mA. Furthermore, the external quantum efficiency (EQE) of n-AlGaN UV-LED was nearly retained at the 600 mA (only 20% droop), whereas the UV-LED with n-GaN exhibits as high as 33%. We attributed this improvement to the less self-absoption by replacing n-GaN underlayer with n-AlGaN.

High performance 375 nm ultraviolet InGaN/AlGaN light-emitting diodes by using a heavily Si-doped GaN growth mode transition layer

High performance 375 nm ultraviolet (UV) InGaN/AlGaN light-emitting diodes (LEDs) with a heavy Si-doped GaN growth mode transition layer (GMTL) were fabricated by metal-organic chemical vapor deposition (MOCVD). From transmission electron microcopy (TEM) image, the dislocation densities are reduced significantly by using the GMTL technique. The threading dislocation (TD) value of AlGaN grown on GMTL was significantly decreased from the control sample value of 8×108 to 8×107 cm-2. Furthermore, the internal quantum efficiency (IQE) of the LEDs with GMTL was measured by power-dependent photoluminescence (PL) to be 40.6% higher than ones without GMTL. After vertical-type (size:1mm×1mm) LED chips were fabricated, the output power were measured by integrating sphere detector under 350 mA injection current driving. The output powers of the LEDs with and without GMTL were measured to be 286.7 and 204.2 mW, respectively. As much as 40.4% increased light output power was achieved. The GMTL leads to the superior IQE performance of the LEDs not only in decreasing the carrier consumption at nonradiative recombination centers but also in partially mitigating the efficiency droop tendency. Therefore, forming the GMTL between un-doped GaN and n-AlGaN to reduce dislocations would be a promising prospective for InGaN/AlGaN UV-LEDs to achieve high IQE.n the abstract two lines below author names and addresses.