1762 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 18, SEPTEMBER 15, 2013 Enhancing Light Output of GaN-Based LEDs With Graded-Thickness Quantum Wells and Barriers Bin Cao, Run Hu, Zhiyin Gan, and Sheng Liu, Senior Member, IEEE Abstract— GaN-based light-emitting diodes (LEDs) with graded-thickness quantum wells and barriers (GMQW-LEDs) are fabricated and researched in this letter. The light power and carrier distribution of GMQW-LEDs are compared with those of LEDs with original uniform MQW (OR-LEDs), graded- thickness quantum wells (GQW-LEDs), and graded-thickness quantum barriers (GQB-LEDs) through numerical simulation, respectively. The experimental results show that light power of GMQW-LEDs is enhanced significantly compared with that of OR-LEDs. The simulation results reveal that GMQW-LEDs show light output power enhancements of 25.7%, 14.3%, and 9.2% compared with OR-LEDs, GQW-LEDs, and GQB-LEDs at current density of 100 A/cm2, respectively. This is due to the superior hole distribution in quantum wells, which inhibits the electron leakage and enhances the radiative recombination. Index Terms— Light-emitting diodes (LEDs), quantum wells, quantum barriers, carrier distribution. I. INTRODUCTION GaN-BASED multiple quantum wells (MQWs) light-emitting diodes (LEDs) have been widely used in backlighting sources, indicator lamps and other illumination areas due to the environmental-protection, high efficiency, long lifetime, and high reliability [1]. Although significant progress has been made on GaN-based LEDs, they are still suffering from the efficiency droop [2]–[11]. As a result, the high-power applications of GaN-based LEDs are restricted. The origin of efficiency droop is still under discussion and various mechanisms are proposed to explain the phenomenon, such as Auger recombination [3], polarization field [4], [5], junction heating [6], and non-uniform distribution of holes and consequent electron leakage [6], [7]. Holes always accumulate at the last quantum well (QW) neighboring p-GaN due to their large effective mass and low mobility, resulting in the Manuscript received May 10, 2013; revised July 2, 2013; accepted July 24, 2013. Date of publication July 30, 2013; date of current version August 16, 2013. This work was supported in part by the State Key Development Program for Basic Research of China under Grant 2011CB013103 and in part by the National High Technology Research and Development Program of China under Grant 2011AA03A106. B. Cao is with the School of Optical and electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China (e-mail:
[email protected]). R. Hu is with the School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China (e-mail:
[email protected]). Z. Gan and S. Liu are with the State Key Laboratory for Digital Manufactur- ing Equipment and Technology, School of Mechanical Science and Engineer- ing, Huazhong University of Science and Technology, Wuhan 430074, China (e-mail:
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2013.2275166 low radiative recombination rate in other QWs and serious carrier leakage [7]. Carrier leakage, which could be enhanced by the sheet charges resulting from polarization mismatch [8]–[9], decreases the current injection efficiency and then leads to efficiency droop [10], [11]. In order to increase the uniformity of hole distribution and suppress the carrier leakage, LEDs with graded-thickness QWs or quantum barriers (QBs) were fabricated and stud- ied. Thicker QWs help carrier accumulate and lead to less carrier leakage, however, the separation of electron and hole wave functions becomes larger [12], [13]. Thinner QBs are beneficial to hole transport towards the neighboring QW to increase hole concentration, however, the electrons tend to overflow [14], [15]. Compared to uniform QWs and QBs, graded-thickness QWs or QBs were reported to result in reduced efficiency droop [14]–[18]. LEDs with QW thickness decreasing [14] or increasing [15] from n-side to p-side showed an improved emission intensity, which was attributed to the enhancement of superior hole distribution. It was also found that when the thicknesses of QBs were varied, the relatively uniform carrier distribution among the QWs was obtained [16]–[18]. In order to further improve the hole transport in QWs and decrease the carrier leakage, we try to combine the graded- thickness QWs and QBs to fabricate the GaN-based LEDs. In this letter, the LEDs combining graded-thickness QWs and QBs (GMQW-LEDs) are fabricated and their light output is compared to the original LEDs with uniform thickness QWs and QBs (OR-LEDs). Furthermore, through numerical simulation, the carrier distribution and radiative recombination of GMQW-LEDs are analyzed and compared to those of OR-LEDs, LEDs with graded-thickness QWs (GQW-LEDs) and LEDs with graded-thickness QBs (GQB-LEDs) to illus- trate the effects of the proposed structure on suppression of the carrier leakage and improvement of the radiative recombination. II. METHODOLOGY The blue GaN-based LEDs are grown on a c-plane pat- terned sapphire substrate using metal organic chemical vapor deposition (MOCVD) method. A 20-nm-thick low temper- ature GaN nucleation layer and a 4-μm-thick n-GaN layer (n = 5×1018 cm−3) are grown on the sapphire substrate. The active region consists of six-pair In0.16Ga0.84N/GaN MQWs. The growth rates of QWs and QBs are 0.16 and 0.5 Å/s, respectively. For the OR-LEDs, the thicknesses of QWs and QBs are nominal 3 and 15 nm, respectively. When QWs and/or QBs are varied, the total thicknesses of MQWs for different 1041-1135 © 2013 IEEE CAO et al.: ENHANCING LIGHT OUTPUT OF GaN-BASED LEDs 1763 LEDs are the same. Thus, for the GMQW-LEDs, the thick- nesses of QWs and QBs are nominal 2.8, 2.8, 3.0, 3.0, 3.2, and 3.2 nm and 20, 20, 20, 10, 10, and 10 nm from the n-side to p-side, respectively. The thicknesses of QWs increase along [0001] direction because the holes in thicker QWs tend to escape to the next QWs [16], resulting in a more uniform hole distribution. The thinner QBs close to p-GaN are beneficial to hole injection [15]. A 10-nm-thick p-Al0.15Ga0.85N electron blocking layer (EBL) (p = 1.2×1018 cm−3) and a 100-nm- thick p-GaN cap layer (p = 1.2×1018 cm−3) are deposited subsequently on top of the active layer. During the regular chip process, indium-tin oxide with thickness of 230 nm is deposited on the epitaxial wafer as a current spreading layer. The device dimension is designed with a rectangular shape of 300×300 μm2. The LED chips are measured under pulse current with duty cycle of 1% to eliminate the thermal effect at room temperature and the light power is collected by an integrating sphere. In order to analyze the carrier distributions in QWs, LEDs are theoretically studied in detail by the software APSYS by solving the Poisson’s equation, current continuity equation, and carrier transport equation. The band offset ratio defined as the ratio of the conduction band offset (�Ec) to the valence band offset (�Ev) is set to be 0.7:0.3. The Shockley-Read- Hall (SRH) recombination lifetime and Auger recombination rate are 50 ns and 1×10−30 cm−6/s, respectively. The internal spontaneous and piezoelectric polarization causes the electric field. The polarization charge density is set to be 40% of the calculated total value considering the carrier screening effect. Other material parameters such as the bandgap and carrier mobility are adopted as in Ref. [19], [20]. The GQW-LEDs and GQB-LEDs are simulated for comparison to demonstrate the benefit of the GMQW-LEDs. The GQW-LEDs have uni- form QBs of 15 nm and varied QWs while the GQB-LEDs have uniform QWs of 3 nm and varied QBs. III. RESULTS AND DISCUSSIONS Figure 1 shows the cross-sectional high-resolution transmis- sion electron microscopy (TEM) image of the active layer with varied QWs separated by varied QBs. It can be seen that the QWs are clearly resolved as six dark layers. The interfaces between QWs and QBs are very abrupt and the In0.16Ga0.84N QWs show uniform indium distribution around each QW layer, which illustrates the high quality of the epitaxial layer. The experimental results of OR-LEDs and GMQW-LEDs as well as the simulated results of the four LEDs are plotted as a function of current density in Fig. 2. It can be seen that the simulated results are in good agreement with those of the experimental measurements, which suggests the reliability of the numerical simulation. Obviously, the GQW-LEDs as well as GQB-LEDs show higher light output than that of OR-LEDs as reported in previous studies [15]–[18], owing to the enhancement of hole injection caused by the varied QWs and QBs. Moreover, the simulation illustrates that the influence of varied QBs on the light output is more significant than that of varied QWs. Compared to OR-LEDs, the light powers of GQW-LEDs and GQB-LEDs are improved by 9.9% and 15.0% at current density of 100 A/cm2, respectively. If both Fig. 1. Cross-sectional TEM image of the InGaN/GaN active layer with graded-thickness QWs and QBs. Fig. 2. Simulated and experimental light output-current density-forward voltage (L − I − V ) of four LEDs. the thicknesses of QWs and QBs are varied, the output power can be further enhanced, as shown in Fig. 2. At 100 A/cm2, GMQW-LEDs show light output enhancements of 25.7%, 14.3%, and 9.2% compared to the OR-LEDs, GQW-LEDs, and GQB-LEDs, respectively. The simulated results reveal that the combined action of graded-thickness QWs and QBs has more significant effect on the performance compared to varied QWs or varied QBs. Figure 2 also shows the measured and simulated forward voltages as a function of current density for the different LEDs. GMQW-LEDs have a lower forward voltage than OR-LEDs based on the experimental results. The simulated results show obviously that the OR-LEDs have the highest forward voltage. At the current density of 100 A/cm2, the simulated forward voltages of GQW-LEDs, GQB-LEDs, and GMQW-LEDs decrease to 3.709 V, 3.616 V, and 3.591 V, respectively. The decrease of forward voltage is due to the decrease of series resistance of the device caused by the enhancement of hole transport [21]. For the high-power GaN- based LEDs, the low forward voltage is beneficial to improving the wall-plug efficiency. Figure 3 illustrates the calculated carrier concentrations of the four LEDs at current density of 100 A/cm2. The horizontal 1764 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 18, SEPTEMBER 15, 2013 Fig. 3. Hole and electron concentrations of (a) OR-LEDs, (b) GQW-LEDs, (c) GQB-LEDs, and (d) GMQW-LEDs at 100 A/cm2. The horizontal axis is along the growth direction. axis is along the [0001] direction. It can clearly be seen that the electron concentration decreases in the first three QWs and then increases in the last three QWs for the four LEDs. The last peaks of electron and hole close to p-GaN are caused by the carrier accumulation at the potential well formed at the interfaces of GaN QB/p-AlGaN EBL/p-GaN. The holes accumulate mainly in the last QW near the p-GaN because of the large effective mass and low mobility. Therefore, only the last QW contributes to the radiative recombination in the con- ventional GaN-based LEDs [12]. Compared to OR-LEDs, the carrier concentrations in last QW of GQW-LEDs are increased, especially for the hole concentration, as shown in Fig. 3(b). Fig. 4. Normalized hole and electron current through the four LEDs at current density of 100 A/cm2. This is due to the enhanced confinement of the carriers in the thick QWs. However, the hole concentration in the neighboring QW shows little enhancement, which is probably due to the inhibition of hole tunneling caused by the thick QBs. With the thicknesses of QBs decreasing, hole transport through QBs is enhanced and the hole distribution in QWs is more uniform [17], as shown in Fig. 3(c). The enhancement of hole injection in QWs subsequently reduces the electron leakage. It can be seen that the electron concentration near the EBL is decreased significantly. For the LEDs combining the graded-thickness QBs and QWs, the uniformity of hole distribution and carrier concentration are further improved, as shown in Fig. 3(d). The GMQW-LEDs take the advantage of the graded-thickness QBs and QWs. As a result, it exhibits a superior performance. The uniform hole distribution is beneficial to suppression of the carrier leakage. In order to compare the leakage current, the normalized electron and hole current densities through the MQWs are plotted at current density of 100 A/cm2, as shown in Fig. 4. It can be seen that the hole leakage current is nearly 0 as we expected due to the transport impediment. However, the electron leakage current of the four LEDs are significant. It is as high as 35.6% for OR-LEDs and reaches 33.3% for GQW-LEDs. However, due to the decrease of QB thickness, the hole injection is enhanced and therefore hole distribution is more uniform, the electron leakage current is 23.4% and 19.4% for GQB-LEDs and GMQW-LEDs, respectively. The electron escaped from MQWs participates in non-radiative recombination, resulting in LEDs efficiency droop. Figure 5 shows the radiative recombination rates of the four LEDs at current density of 100 A/cm2. OR-LEDs and GQW- LEDs demonstrate only two peaks, however, the intensity in last QW of GQW-LEDs is higher than that of OR-LEDs due to the increase of hole concentration. The varied QBs lead to relatively uniform hole distribution. As a result, GMQW- LEDs and GQB-LEDs demonstrate three peaks. Similarly, the varied QWs in GMQW-LEDs increase the intensity of radiative recombination rate in last QW. Meanwhile, the carrier screening effect is enhanced with the increase of carrier con- centration and thus, this effect decreases the local piezoelectric CAO et al.: ENHANCING LIGHT OUTPUT OF GaN-BASED LEDs 1765 Fig. 5. Radiative recombination rates of the four LEDs at 100 A/cm2. field across the QWs, resulting in enhancement of the radiative recombination [22]. IV. 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