Growth of InGaN Light-Emitting Diodes by Metal-Organic Vapor Phase Epitaxy

• Main Authors: Chen-Yu Shieh, 謝鎮宇
• Other Authors: Gou-Chung Chi
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/02005784314033622653

博士 === 國立中央大學 === 光電科學與工程學系 === 103 === The epitaxial layer of InGaN-based light-emitting diodes (LEDs) still contain a high defect density (around 108－1010 cm-2) and large strain-induced piezoelectric field due to the large lattice mismatch and the difference in thermal expansion coefficients of GaN films and sapphire substrates, resulting in the reduction in the external quantum efficiency (EQE) of InGaN-based LEDs devices. In this dissertation including two topics, we demonstrated light-output-power (LOP) enhancement of the InGaN-based light-emitting diodes (LEDs) by using metal-organic vapor phase epitaxy. One is using a new buffer layer for the growth of InGaN-based LEDs, the other is using free-standing GaN (FS-GaN) substrate for the growth of InGaN-based ultraviolet light-emitting diodes (UV-LEDs). First, a low-cost and time-saving buffer layer of nitrided titanium (Ti) achieved through the nitridation of a Ti metal layer on a sapphire substrate was used for the epitaxial growth of LEDs achieved by low pressure metal-organic chemical vapour deposition. It showed that the use of the nitrided Ti buffer layer (NTBL) induced the formation of a nanoscale epitaxial lateral overgrowth layer (NELOG) during the epitaxial growth. The effect of in-situ Ti metal nitridation was then improved on the crystal quality of these InGaN-based LEDs from using X-ray diffraction (XRD) and CL results. When evaluated by Raman spectroscopy, the InGaN-based LEDs with an NTBL exhibited larger in-plane compressive stress releasing 68% than the LEDs with a LT-GaN buffer layer. The electroluminescence (EL) results indicate that the LOP of InGaN-based LEDs with an NTBL can be enhanced by 45% and 42% at 20 mA and 100 mA, respectively. These results suggest that the strain relaxation and quality improvement in the GaN epilayer could be responsible for the enhancement of emission power. Then we investigate the influence of FS-GaN substrates on the performance of 380 nm InGaN-based UV-LEDs with InGaN/InAlGaN MQWs grown atop by atmospheric pressure metal-organic chemical vapor deposition. High-resolution double crystal X-ray diffraction (HRDCXD) analyses demonstrated high-order satellite peaks and clear fringes between them for UV-LEDs epilayers grown on FS-GaN substrate, from which the interface roughness (IRN) was 1.55%. Besides, the full width at half maximum of the HRDCXD rocking curve in the (0002) and (101 ̅2) reflection were reduced to below 90 arcsecond. The Raman results which the calculated in-plane compressive stress was 0.31 GPa indicate that the UV-LEDs epilayers of strain free are grown. Additionally, the effect of FS-GaN substrate on the crystal quality of UV-LEDs epilayers was examined in detail by transmission electron microscopy (TEM). TEM characterizations revealed no defects and V-pits were found in scanned area of InGaN/InAlGaN MQWs. The total defect density including edge, screw and mixed type was considered to be less than 3.6106 cm-2 or less, which agrees well with our HRDCXD rocking curve data, further proving that homo-epitaxial is an effective measure to improve the crystal quality of UV-LEDs epilayers. The LOP of 380 nm UV-LEDs on FS-GaN substrate can be enhanced drastically by 80% and 90% at 20 mA and 100 mA, respectively. Furthermore, an ultra-low efficiency degradation of about 3% can be obtained for 380 nm UV-LEDs on FS-GaN substrate at high injection current. Conclusively, the use of an FS-GaN substrate is suggested to be effective for improving the emission efficiency and droop of UV-LEDs grown thereon. Finally, we investigated the free-standing GaN substrate fabricating from sapphire substrate by hydride vapor phase epitaxy (HVPE) and using these wafers to grow 380 nm InGaN-based UV-LEDs with InGaN/InAlGaN MQWs on them. The LOP of 380 nm UV-LEDs grown on FS-GaN substrate manufacturing from GaAs substrate by HVPE is higher by 70% and 105% than the FS-GaN substrate fabricating from sapphire at 20 mA and 100 mA, respectively. Besides, the efficiency droop was reduced from 21% in the UV-LEDs grown on FS-GaN substrate fabricating from sapphire substrate to 3% in UV-LEDs grown on FS-GaN substrate manufacturing from GaAs substrate. This result was attributed to less difference of thermal expansion coefficient of GaAs substrate than sapphire substrate, resulting in the less residual stress in the 380 nm UV-LEDs on FS-GaN substrate manufacturing from GaAs substrate which the strain relaxation degree was about 47% than UV-LEDs grown on FS-GaN substrate fabricating from sapphire substrate.