Optoelectronic Performance Variations in InGaN/GaN Multiple-Quantum-Well Light-Emitting Diodes: Effects of Potential Fluctuation

Abstract

We investigate the cause of the optoelectronic performance variations in InGaN/GaN multiple-quantum-well blue light-emitting diodes, using three different samples from an identical wafer grown on a c-plane sapphire substrate. Various macroscopic measurements have been conducted, revealing that with increasing strain in the quantum wells (QWs), the crystal quality improves with an increasing peak internal quantum efficiency while the droop becomes more severe. We propose to explain these variations using a model where the in-plane local potential fluctuation in QWs is considered. Our work is contrasted with prior works in that macroscopic measurements are utilized to find clues on the microscopic changes and their impacts on the device performances, which has been rarely attempted.

Enhanced Radiative Recombination Rate by Local Potential Fluctuation in InGaN/AlGaN Near-Ultraviolet Light-Emitting Diodes

“…Thus, a relatively defect-free potential "valley" was created between the accumulated defective sites. In these potential valleys, carriers were localized as depicted in Figure 7 [30]. Note that the number of defects were much smaller in the potential valley, with better crystal quality that yielded higher strain [30].…”

“…For currents < 200 mA, the increase in IQE was caused by the increase in the radiative recombination rate, which is directly related to carrier localization and the effective radiative-recombination area in QWs. For the case of identical epitaxial structure, the increase in the radiative recombination rate at low currents before the IQE peak was due to the decrease in defects (especially the point defects in the active MQW region) since the defects act as NRCs for carriers, inducing SRH recombination [30] as reflected in the A coefficient. Again, the calculated A coefficient [39] decreased with the sample's In composition (see Table 1), which is consistent with the PHEMOS patterns, n min and S values at low currents.…”

“…However, it needs to be clarified which factor dominantly increased the radiative recombination rate at low currents. In this work, we propose that the dominant factor was spatially localized carriers caused by in-plane potential-energy fluctuation in QWs [18][19][20][21][22][23][24][25][26][27][28][29][30].…”

“…There were inhomogeneously distributed point defects [40] caused by Ga/N vacancies [14,41] in QWs, which were not filled up by neighboring atoms with strong bonds between other atoms. This effect relaxed the neighboring atoms by increasing the lattice constant, which led to the bandgap energy near the point defect increasing [30,33]. Thus, in a site with accumulated defects, the crystal relaxation occurs and the bandgap energy near this site increased like a potential barrier (PB).…”

“…It has been suspected that such high efficiencies are caused by the localization of carriers by the potential-energy fluctuation in the QWs, which prevents the carriers from recombining nonradiatively at defects [13,16,17]. The possible reasons reported for carrier localization include alloy fluctuation with the incorporation of In [18][19][20][21], thickness variation of QWs [20,21], In-N-In chains [22], In clustering [23], crystal quality [24], In/Ga segregation [17], crystallographic defects [14,[25][26][27][28], the piezoelectric field (F PZ ) [21,28,29], and the combined effects of point defects and F PZ [30]. It has been widely accepted that excited carriers are spatially separated from defects owing to higher electrical potential energy at the edge of defects, which prevents carriers from diffusing into them [13,16,25,26,30].…”