Investigation of Efficiency Droop for InGaN-based LEDs with Carrier Localization State and Polarization Effect

We prepared wavelength-dependent InGaN-based light emitting diodes (LEDs) with peak emissions ranging from 400 to 445 nm, and investigated their efficiency droop characteristics at injection currents of up to 1 A. We found that the emissions of the wavelength-dependent InGaN LEDs underwent blue shifts at elevated currents. In addition, although the external quantum efficiencies (EQEs) changed dramatically when the critical current was less than 350 mA, the efficiency droop of each device exhibited a similar negative slope upon increasing the current from 350 mA to 1 A. Whereas the effects of piezoelectric polarization and different localized states in the active layer of the near-UV–to–blue LEDs influenced the peak EQEs and the dramatic decays of the EQE droops at lower injection currents, they were not responsible for the EQE droops at higher current levels. In addition, the piezoelectric effect and Auger non-radiative recombination were not dominating influences determining the efficiency droops of the wavelength-dependent LEDs at higher carrier densities. INTRODUCTION Gallium nitride (GaN)–based light-emitting diodes (LEDs) operate with several attractive features, including long lifetimes, low energy consumption, high durability, design flexibility, and ecological friendliness. As a result, they are promising candidates for use in solid-state lighting to replace traditional incandescent (containing toxic mercury) and fluorescent (relatively low energy efficiency) light sources. Outdoor and indoor lighting applications require high operation currents—typically greater than 350 mA and in some cases greater than 1 A. Notably, however, high-power LEDs exhibit unsatisfactory efficiency at high injection currents, with the efficiency further declining monotonically upon increasing the current density—a well-established phenomenon known as “efficiency droop.” Many physical mechanisms have been proposed to explain this behavior, including Auger recombination [1], current roll-off [2], carrier delocalization [3-5], and the effects of the carrier injection rate, [6-8] and polarization field [9-10]. Nevertheless, the main principle behind the efficiency droop remains unknown because of the complicated nature of the efficiency droop process. In addition, substantially different wavelength-dependent droop effects have been noted in InGaN-based LEDs [5, 11]. Accordingly, higher peak levels and lower droop decays have been observed in short-wavelength LEDs (near-UV region), with lower peak levels and severe droops in long-wavelength LEDs (green region) Green LEDs have suffered, however, from poor crystal quality in the active layer, due to their relatively low growth temperatures. In addition, at higher indium contents, the effects of strain and indium phase separation tend to deteriorate the crystal quality. UV LEDs, on the other hand, are strongly affected by threading dislocations (TDs), such that the entire external quantum efficiency (EQE) might suddenly deteriorate because of remarkable degrees of nonradiative recombination. Therefore, it remains difficult to objectively determine the dominant phenomena affecting the efficiency droops in such systems. In this paper, we demonstrate the wavelength-dependent droop effects of InGaN-based LEDs emitting in the range from 400 to 445 nm (but not for green-region LEDs—the behavior of which is related to crystal quality issues). From EQE measurements performed while elevating the current density, herein we attempt to realize the leading effects on the efficiency droops in InGaN-based LEDs. EXPERIMENT All the InGaN/GaN multiple quantum well (MQW) LEDs were grown on (0001) sapphire substrates using an atmospheric-pressure metalorganic chemical vapor deposition (AP-MOCVD) system. Prior to growth, the substrate was heated at 1180 °C in H2 ambient to remove any surface contaminants. A 25-nm-thick low-temperature GaN nucleation layer, a 1-μm-thick undoped GaN buffer layer, and a 3-μm-thick n-GaN layer, using SiH4 as the n-type dopant, were then deposited. Next, wavelength-dependent InGaN/GaN active layers with emitting peaks at 401.6, 408.7, 419.5, 426.8, 432.4, and 443.4 nm under 350 mA (chip size: 1 × 1 mm) were fabricated by adjusted the TMI (Trimethylindium) flow rate. Note that, to ensure consistent crystal quality, different growth temperatures were not employed to tune the emitting wavelength of the LEDs. Subsequently, a 20-nm-thick p-AlGaN electron blocking layer (EBL) and a CS MANTECH Conference, April 23rd 26th, 2012, Boston, Massachusetts, USA 100-nm-thick p-GaN layer, using Cp2Mg as the p-type dopant, were deposited. LEDs having dimensions of 1 × 1 mm were formed using conventional photolithography. The electrical characteristics of the wavelength-dependant LEDs were examined through electroluminescence (EL) measurements using an Agilent B1500A semiconductor parameter analyzer. The output power and center wavelength shift were measured using an integrated sphere detector. To determine the effect of current on the EQE, pulsed mode measurement was employed to avoid thermally induced degradation of the LED output power. For pulsed measurement, a pulse width of 1 ms and a duty cycle of 0.1% were used with a maximum injection current of up to 1 A. RESULTS and DISSCUSSIONS To investigate the wavelength-dependent droop effects, we measured the EQEs upon increasing the current with pulse operation (Fig. 1). To systematically analyze the wavelength-dependent droop effects, we separated the EQE droop effects into two regions: one where the peak EQE begins to roll off at a certain current and the other where the EQE characteristics have a negative slope with respect to the increase in current. At emission wavelengths of 401.6, 408.7, 419.5, 426.8, 432.4, and 443.4 nm, the peak efficiencies reached 22, 27, 27.5, 29, 30, and 30%, respectively. In addition, the peak efficiency shifts toward lower current and EQE were also enhanced upon increasing the emission wavelength over the range from 0 to 200 mA. This phenomenon is presumably related to two dominant mechanisms: the localized state and the variation in the internal piezoelectric polarization upon changing the indium content. The localization effect increased dramatically upon increasing the indium content in the active layer [12], exhibiting a trend similar to those previously found experimentally [5, 11] and theoretically [13]. Although a localization effect induced by a higher indium content would prevent nonradiative recombination from TD defects at lower currents, the delocalization effect was revealed by band-filling upon increasing the current, with the EQE beginning to decay spectacularly as a result of the participation of severe nonradiative recombination. In contrast, because the short-wavelength LED featured a less-localized state from the onset, we did not expect the delocalization effect to occur upon increasing the current; therefore, the EQE of the short-wavelength LED rose monotonically under a lower current. On the other hand, we expected the higher indium content of the longer-wavelength LED to result in a strong internal field across the MQWs, naturally inducing a quantum-confined Stark effect (QCSE); this behavior is well established as causing red-shifts of the QW energy and decreases in electron/hole wave function overlap, meaning that a radiative recombination decline could be expected for the longer emission wavelength. From simulations, Chen et al. [13] proposed a relatively uniform electron/hole concentration distribution in short-wavelength LEDs in Fig.1. External quantum efficiency with wavelength dependent LEDs as a function of forward current. addition to a higher-magnitude radiative recombination rate in low-indium-content LEDs, due to the lower piezoelectric field. This phenomenon might be another reason for the lack of a droop effect for the short-wavelength LED under lower current levels. Although the EQE–I curve of the shortest-wavelength LED was quite flat, the total EQE deteriorated dramatically over the whole current level because of sensitive nonradiative recombination with the TDs [3] and the lower barrier potential height in the MQWs. Figure 2 presents the relative center wavelength shifts of the LEDs plotted with respect to the forward current. The 443.4 nm LED featured the largest blue-shift, by almost 10 nm, upon increasing the forward current to 1 A; the total blue-shift decreased upon decreasing the LED wavelength. This behavior resulted from (i) screening of the piezoelectric field in the MQWs by the injected carriers, leading to blue-shifts of emission peaks, and (ii) carriers filling up to a higher energy level upon increasing the current density (the so-called “band-filling effect”). In contrast, the peak shift for the 401.6 nm LED was quite stable over the entire current range, consistent with its lowest internal piezoelectric field and lowest potential barrier height. We used the equation