Influence of Fe in the buffer layer on the laser lift-off of AlGaN/GaN HEMT film: phenomena and mechanism

Abstract

In this paper, a study is presented of the effect of Fe in the buffer layer on the laser lift-off (LLO) of GaN high electron mobility transistors (HEMTs). AlGaN/GaN HEMTs grown on Fe-doped and unintentionally doped buffer layers were separated from sapphire substrates using a 248 nm KrF excimer laser lift-off system. We analyzed the variations of the crystal characteristics, two-dimensional electron gas (2DEG) characteristics and the strain state of AlGaN/GaN HEMT films before and after LLO by the x-ray diffraction, Hall and Raman methods. The measurement of the distribution of elemental Fe in the GaN buffer layer was performed by secondary ion mass spectroscopy. The results show that a peak Fe concentration of 9.56 × 1018 cm−3 appears at the interface. Moreover, the crystal quality and 2DEG characteristics of the Fe-doped GaN film obviously degenerated, with the formation of micro-cracks under the threshold laser separation energy density (Es). In this case, the threshold laser damage energy density (ED) was used to measure the laser damage tolerance of GaN in LLO. The analysis considers that additional laser absorption centers, induced by the Fe impurity energy level, coupled with a large residual stress of up to 1.0763 GPa in Fe-doped GaN, reduced the threshold laser damage energy density (ED).