The impact of laser lift-off with sub-ps pulses on the electrical and optical properties of InGaN/GaN light-emitting diodes

Abstract

Laser lift-off (LLO) is an important step in the processing chain of nitride-based light-emitting diodes (LEDs), as it enables the transfer of LEDs from the growth substrate to a more suitable carrier. A distinctive feature of LLO with ultrashort pulses is the ability to use either above- or below-bandgap radiation, since nonlinear absorption becomes relevant for ultrashort pulses. This study addresses the differences in the absorption scheme for below- and above-bandgap radiation and investigates the electrical and optical properties of InGaN/GaN LEDs before and after LLO with 347 and 520 nm laser light via current–voltage and power- as well as temperature-dependent photoluminescence measurements. LLO could be successfully realized with both wavelengths. The threshold fluence required for LLO is about a factor of two larger for 520 nm compared to that for 347 nm. Furthermore, an increase in leakage current by several orders of magnitude and a significant decrease in efficiency with laser fluence are observed for below-bandgap radiation. In contrast, leakage current hardly increases and efficiency is less dependent on the laser fluence for samples lifted with 347 nm. This degradation is ascribed to the absorption of laser light in the active region, which facilitates a modification of the local defect landscape. The effect is more severe for below-bandgap radiation, as more laser light penetrates deep into the structure and reaches the active region. Ultimately, we show that LEDs lifted with ultrashort laser pulses can exhibit good quality, making ultrashort pulse LLO a viable alternative to conventional LLO with nanosecond pulses.