研究目的
Investigating the feasibility of using femtosecond laser pulses at a wavelength of 520 nm for laser lift-off (LLO) of GaN-based LED films from sapphire substrates to produce free-standing GaN LED chips.
研究成果
The presented femtosecond LLO is a viable alternative approach to lift GaN-based LEDs from sapphire. Key advantages are that the approach relies on solid-state lasers instead of excimer lasers and that the technique is less dependent on the band gap of the semiconductor material.
研究不足
The study notes that the impact of laser irradiation on small localized areas in the p-GaN has been found, indicating a higher absorption at these 'hot spots'. The underlying mechanism and effects on LED performance have to be further explored.
1：Experimental Design and Method Selection
The study employs a two-step process for area-selective LLO, involving high fluence ablation for creating the outer frame surrounding the GaN chip and scanning with lower fluence for releasing the GaN chip from the sapphire substrate.
2：Sample Selection and Data Sources
Two types of wafers were utilized: plain n-GaN layers for laser beam characterization and InGaN/GaN LED structures for demonstration of LLO.
3：List of Experimental Equipment and Materials
A solid-state laser with 520 nm emission wavelength, 350 fs pulse width at 200 kHz repetition rate, and up to 40 μJ pulse energy was used. The setup included a laser machining system with a galvanometer scanner and a telecentric f-theta lens.
4：Experimental Procedures and Operational Workflow
Single laser pulses were directed towards plain n-GaN layers for beam characterization. For LLO, the beam was scanned across the sapphire side of the sample in a quadratic pattern.
5：Data Analysis Methods
The sizes of the yielded craters were assessed in SEM. The beam shape at a specific working distance was calculated by directing single pulses at increasing energies to a test sample.
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