Phase reaction of Au/Sn solder bonding for GaN-based vertical structure light emitting diodes

Au/Sn solder bonding on Si substrates was used to fabricate the GaN-based vertical structure light emitting diodes (VSLEDs). The phase reaction of Au/Sn solder under different bonding conditions was investigated by the measurement of electron back scattering diffraction (EBSD), and the characteristics of VSLED were analyzed by scanning acoustic microscope (SAM), Raman scattering, current-voltage (I-V) and light output-current (L-I) curves. After the bonding process, horizontal stripes of Au/Sn phase (δ phase) and Au5Sn phase (ζ phase) were redirected to vertical stripes, and δ phase tended to move to the solder joint. Sn interstitial diffusion led to the distribution of δ phase and voids in Au/Sn solder, which could be seen in SAM and SEM images. Vertical distribution of the δ phase and ζ phase with proper voids in the Au/Sn bonding layer showed the best bonding quality. Good bonding quality led to little shift of the E2-high mode of Raman spectra peak in GaN after laser lift off (LLO). It also caused more light extraction and forward bias reduction to 2.9 V at 20 mA.