| Abstract [eng] |
Gallium nitride based LEDs emitting blue light, combined with red and green light LEDs or yellow phosphors (i.e. cerium doped yttrium aluminum garnets Ce:YAG) makes a white light. This type of lighting is more effective, it lasts longer so it could change fluorescent lights and incandescent bulbs. Lift-off is a technological process when GaN film is transferred from sapphire/silicon carbide substrate to the substrate of better thermal conductivity, like copper. Currently there is only a few researches, where lift-off is done with ultrashort laser pulses. Smaller heat affected zone of ultrashort laser pulses could mean lesser number of defects and better quality overall in the transferred film. In this thesis, λ=343 nm wavelength laser beam with the pulse width of 300 fs (MGF „Light Conversion“ laser „PHAROS“) was used. The damage threshold of 4 μm GaN film grown on sapphire substrate was found when laser beam propagates through the sapphire. The investigation of the effect of number of pulses hitting the same spot on the film was made, which showed that number of pulses lowers the damage threshold. The damage threshold of one pulse was 24 mJ/cm^2 and the damage threshold of one thousand pulses affecting same spot was 12 mJ/cm^2. These values are approximately ten times lower than ablation threshold values of GaN. There could be two reasons for this phenomenon: thermal stresses from the temperature gradient between laser beam heated GaN film and cooler sapphire substrate; immense pressure of nitrogen gasses in the junction of GaN and sapphire, where they have nowhere to go. There was an attempt to lift-off 500 x 500 〖μm〗^2 GaN segment. It was unsuccessful as lifted-off film was fractured. The peaks of ~800 nm can be seen on this lifted-off segment. |
