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Effect of Laser Sources on the Structural Damage Mechanisms and Reverse-bias Leakages of Laser Lift-off GaN-based LEDs

Sapphire substrate has been used for high temperature GaN growths. However, the performance of GaN device was limited to the high series resistance and the poor heat dissipation caused by its insulating nature. In order to solve such problems, the laser-lift off technique has been developed for removal of GaN-based device layers , and transfer to the better heat and electronic conductivity substrate. Two kinds of LED were investigated in this study, one is KrF Excimer laser system at 248nm and another is 355nm frequency-tripled Nd YAG laser system. Figure 1(a) and 1(b) are the cross-section transmission electron microscopy (TEM) image of KrF-LED and YAG-LED. The screw dislocation density in the bulk region (which penetrate the MQW) of YAG-LLO LED was 2.9 10 cm, which was 10 times higher than that of the Excimer-LLO LED (3.75 10 cm). As a result, the leakage current of YAG-LED (1.65×10 nA) was 10,000 times higher than that of the KrF-LED (0.17 nA). Figure 1(c) and 1(d) is the enlargement of the part in the LLO interface region. We found that there are lots of dislocations located at 200nm away from LLO interface in KrF-LED, which was different from YAG-LED. Inverse fast Fourier transform (IFFT) image using a diffraction spots long (0002) direction of part of 200nm around the interface (Fig. 1(e) and 1(f)) show sequential dislocations with have the same center in KrF-LED (the white dashed line) but not observed in YAG-LED. We believe those phenomenon was due to the laser pulse time and the difference absorption edge between the GaN and laser wave length. This is because Nd YAG has thicker absorption depth, which may generate more screw dislocation density during the rapid thermal decomposition.