No abstract

The quantum efficiency of GaN-based light-emitting diodes (LEDs) is investigated at temperatures 77–300 K. It is found that the efficiency droop is due to a decrease in the internal quantum efficiency (IQE) in the low-energy part of the emission spectrum. The efficiency starts to decrease at a temperature independent forward voltage of Umax≈2.9 V. At this voltage tunneling current through the LED-structure begins to dominate. It is suggested that the external quantum efficiency droop is related to reduction of the IQE due to tunneling leakage of carriers from the quantum well (QW) to defect states in barriers, and to reduction of the injection efficiency by excess tunneling current under QW through deep defect states in barriers.

A direct correlation between efficiency droop and broadening of emission spectrum of InGaN/GaN quantum wells (QWs) with increasing current density is found. A model of incomplete carrier localization in InGaN/GaN QWs is proposed. At low injection, the strong carrier localization and high-energy cutoff of emission spectrum results from fast carrier energy relaxation due to carrier hopping between localized tail states in QWs. At high level injection, the energy relaxation rate decreases due to the partial filling of tail states and high energy slope of the spectrum starts to determine by Boltzmann occupancy of tail states. This results in the incomplete carrier localization and the efficiency droop.

A femtosecond laser focused inside bulk GaN was used to slice a thin GaN film with an epitaxial device structure from a bulk GaN substrate. The demonstrated laser slicing lift-off process did not require any special release layers in the epitaxial structure. GaN film with a thickness of 5 µm and an InGaN LED epitaxial device structure was lifted off a GaN substrate and transferred onto a copper substrate. The electroluminescence of the LED chip after the laser slicing lift-off was demonstrated.