Despite of only having 25 years so far, the efficiency of GaN-based light-emitting diodes (LEDs) has multiplied many times, which has been able to produce six times more efficiency than traditional incandescent light bulbs. As the great prospect of next-generation solid-state lighting, their external-quantum efficiency (EQE) still requires further improvement. Currently, the single growth method by patterned-sapphire substrates (PSSs) can not only improve internal-quantum efficiency (IQE) but also enhance light-extraction efficiency (LEE). However, few paper reports the epitaxial critical point for GaN on commercial PSSs, at which residual strain starts relaxing along with defect generation in epitaxial film. In this work, a complete study on material properties of GaN on the commercial PSS is reported through the use of Raman, etch pit density (EPD), XRD and TEM. We demonstrate that the accuracy of Raman measurement is pretty good. Edge- and screw-mixed threading dislocations measured with EPD are strongly correlated to E2-high and A1-LO’s FWHM of the Raman method, respectively. The XRD-measured lattice constants can be calculated through the Raman shift of E2-high and A1-LO, individually. Through the investigation of TEM, the increase in the 3D growth of GaN on the commercial PSS accumulates residual strain inside it, followed by strain saturation in GaN when it approaches to the 2D growth. After reaching the summit of residual strain inside GaN, called the epitaxial critical point, it starts releasing strain in it and forcefully bends threading dislocation to the surface of the commercial PSS.
Vin-Cent Su, Po-Hsun Chen, Shang-Hsuan Wu, and Chieh-Hsiung Kuan, "Raman, EPD, XRD, and TEM analysis of GaN on commercial patterned-sapphire substrates (Conference Presentation)," Proc. SPIE 10554, Light-Emitting Diodes: Materials, Devices, and Applications for Solid State Lighting XXII, 105540C (Presented at SPIE OPTO: January 30, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2292079.5751478522001.
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