Degradation in AlGaN-based UV-C LEDs under constant current stress: A study on defect behaviors

Abstract

Defect behaviors in the degradation of AlGaN-based UV-C light emitting diodes (LEDs) under constant current stress have been intensively investigated in this work. It is found that both the reduction of the optical power and the increase in the leakage current are derived from the newly generated Ga vacancy (VGa) along dislocation, based on the evidence of a strong “yellow” emission peak at 515 nm in the photoluminescence spectra and an energy level of 0.25–0.38 eV. More importantly, the defect evolution behind it was determined through the deep level transient spectroscopy, secondary ion mass spectrometry measurements, and density functional theory. VGa is found to be generated by the departure of the unintentionally doped Mg from MgGa along dislocation in the Si-doped region. The high activity of the unintentionally doped Mg under electrical stress can be an essential factor in the degradation of UV-C LEDs. This study not only provides an in-depth insight into the electrical stress-induced degradation in UV-C LEDs but also sheds light on the way for fabricating AlGaN-based devices with high reliability.

Degradation analysis with characteristics and simulations of 265 nm UV-C LED

“…According to the previous study, the increase of the reverse-bias leakage current during operation could be caused by the increased defect density within or close to the active region, resulting in a higher number of point defect assisted carrier transport and non-radiative recombination paths around the active layers. The forwardbias increased leakage current can be attributed to the defect-assisted tunneling process [19,22]. Degradation may occur from the defect formation process, induced by highly accelerated carriers owing through the active region.…”

“…In previous studies, it has been suggested that the yellow luminescence enhancement at 2.4 eV was attributed to the generation of defects with relatively low formation energy at moderate temperature. A possible explanation for the enhancement is the easy diffusion of and an oxygen atom sitting in a neighboring nitrogen site, which can partly compensate the unintentional n-type doping of the active region [19,[24][25][26].…”

“…LEDs under constant current stress were systematically studied by Wang et al, who found that the degradation was mainly derived from the generation of in the Si-doped region, as both the reduction of the output OP and the increase in the reverse current were linearly related to the increase in [19].…”

Operating Lifetime Study of Ultraviolet (UV) Light-Emitting Diode (LED) Products

“…Although previous studies of the reliability of UV LEDs are more limited in scope than this report, there have been notable studies spanning the UV-A [12][13][14], UV-B [15,16], and UV-C [10, [16][17][18][19][20][21][22] bands. The forward current (If) densities in these studies ranged from 27 amps per square centimeter (A/cm 2 ) to 140 A/cm 2 .…”

“…Several general trends have emerged from these studies, and perhaps the most significant finding involved increased leakage currents at voltages well below the threshold voltage (Vth). The parasitic shunt circuits that give rise to such low voltage leakage currents appeared to reduce active carrier concentration through modifications of Shockley-Read-Hall (SRH) mechanisms and are thought to be directly tied to aging-induced reductions of emissions from UV-A, UV-B, and UV-C devices [10, [12][13][14][15][16][17][18][19][20]22]. Evidence for parallel shunt circuits is provided in the current-voltage (I-V) measurements of an LED by a growing feature at voltages below Vth.…”