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• Abstract

SECTION I

INTRODUCTION

III-Nitride optoelectronic devices promise more reliability and higher efficiency. High-efficiency light sources based on III-nitride light-emitting diodes (LEDs) are required for general illumination market so as to take the place of traditional lighting source [1]. For high power of application of solid-state lighting, LEDs have to be operated at high current density. However, for InGaN/GaN multiple-quantum-well (MQW) LEDs, the upcoming challenge is the efficiency droop, which means the reduction in the external quantum efficiency (EQE) with increased operation current density. Normally, the EQE of an InGaN/GaN MQW LED reaches its maximum at $\sim\!\! 10\ \hbox{A/cm}^{2}$ and drops to half at $\sim\!\! 100\ \hbox{A/cm}^{2}$. Many possible mechanisms had been suggested to account for the efficiency droop in InGaN-based LEDs as follows: current injection efficiency quenching and carrier leakage [2], [3], [4], [5], nonuniform distribution of holes [6], [7], [8], [9], [10], Auger recombination [11], [12], carrier delocalization [13], polarization field [14], [15], and others. From these works, the carrier-leakage-driven process, as well as inefficient injection of holes, may be attributed as the primary reason. The main reason for electron leakage is the energy barrier reduction by built-in polarization in nitride-based LEDs. The polarization charges at the interface of MQWs and p-GaN are positive, which leads to electron accumulation and strong negative band bending. The polarization effect compensates for the $\hbox{Al}_{\rm x}\hbox{Ga}_{1-{\rm x}}\hbox{N}$ electron blocking layer (EBL) conduction band offset. Therefore, the carrier leakage cannot be retarded effectively. In recent years, several proposals have been made to reduce the efficiency droop by decreasing the built-in polarization. Polarization-matched multiple quantum wells/barriers [10], nonpolar or semipolar GaN substrate are used to reduce polarization field in the active region, which consequently minimize the carrier overflow [16]. Graded-composition multiple quantum barriers (GQB) had been studied to show the improvement in transport of holes in active region and substantial reduction in efficiency droop behavior [17], [18]. However, in order to improve the hole injection efficiency, p-type MQWs has been explored [19]. However, in the p-type MQWs, Mg-dopant will diffuse into the well easily and, consequently, reduce the radiative efficiency.

Extensive works had been reported in the literature on the use of novel InGaN-based QW structures with improved electron hole wave function overlap. The use of these large overlap InGaN QW active regions leads to suppression of charge separation effect due to the lower internal field in the active region [20], [21], [22]. The reduction in charge separation leads to improved internal quantum efficiency and suppression in droop. Several methods for achieving InGaN QW with improved overlap design had been reported by using staggered or graded InGaN QW [23], [24], [25], [26], [27], [28], [29], [30], type-II InGaN-based QW [31], [32], [33], delta-like QW [34], [35], [36], as well as the use of nonpolar/semipolar QWs [37], [38] and ternary substrate [39] approaches. The use of graded growth temperature profiling had also been reported for shaping the In-composition in the QW with improved overlap design, resulting in improved light output power and IQE of the LEDs [22], [23], [24]. In this paper, we report a design based on InGaN QWs by grading the composition of wells from $\hbox{In}_{\rm x}\hbox{Ga}_{1-{\rm x}}\hbox{N}$ to GaN along [0001] direction to form a graded-composition multiple quantum wells (GQW), which results in improved overlap of electron and hole wave function in active region. In addition, the injected carriers do not overflow to p-GaN at high injection level, which reduces the efficiency droop behavior.

The band gap of InGaN-based material can be changed from 0.7 eV (InN) to 3.4 eV (GaN). In our designed GQWs, the indium composition decreased/increased along [0001] direction gradually. Therefore, one can expect that the energy band in QWs will incline by contraries with the direction of the incline arises from polarization field, leading to the weakened polarization effect in the QWs. In this paper, we demonstrate a low droop InGaN/GaN MQW LED with GQW. Compared with that in a conventional LED with constant-composition MQWs, the efficiency droop of LED with GQW was found to be shifted to a higher current density by grading the composition of $\hbox{In}_{\rm x}\hbox{Ga}_{1-{\rm x}}\hbox{N}$ to GaN in QWs.

SECTION II

EXPERIMENTS

The InGaN/GaN MQW LED samples were grown on (0001) sapphire substrate in a low-pressure metal–organic chemical vapor deposition (MOCVD) system. Trimethygallium, trimethylaluminum, trimethylindium, silane $(\hbox{SiH}_{4})$, $\hbox{Cp}_{2}\hbox{Mg}$, and ammonia $(\hbox{NH}_{3})$ were used as sources for Ga, Al, In, Si, Mg, and N, respectively. The typical LED structure was composed of 3-$\mu\hbox{m}$-thick n-GaN with $1 \times 10^{19}\ \hbox{cm}^{-3}$ doping, five pairs of InGaN/GaN MQWs with 16-nm-thick GaN barriers, an $\sim$20-nm-thick p-AlGaN electron blocking layer, and $\sim$200-nm-thick p-GaN layer with $6 \times 10^{17}\ \hbox{cm}^{-3}$ doping. For the conventional LED, the InGaN layers were grown at a constant temperature of 735 °C, and the indium composition in the well is constant, about 15%. While for the GQW LEDs, the InGaN layers were grown under the conditions that the temperature was increased gradually from 680 °C, 730 °C, and 745 °C to 840 °C, respectively; thus, the composition of indium in the quantum well decreases from the n-GaN side toward the p-GaN side. In our case, three samples with composition of indium with different degree of gradation along [0001] direction from 30%, 18%, 13% to $\sim$0%, respectively, were prepared and denoted as GQW LED A, B, C. In addition, in order to investigate the main mechanisms of efficiency droop, the test sample with GQW structure but indium composition in the quantum well increases from the n-GaN side toward the p-GaN side was also grown, the growth temperature of the well layers was decreased gradually from 840 °C to 730 °C, and the indium composition in the well increase from $\sim$0% to 18%, which was denoted as GQW LED D. The thickness of the quantum well for LED A-D is 2.6 nm, 2.4 nm, 2.5 nm, and 2.5 nm, respectively, deriving from fitting XRD results. After growth, the LED chips were fabricated by regular chip process, with indium tin oxide current spreading layer and the size of mesa is $300 \times 300\ \mu\hbox{m}^{2}$. The electrical and luminescence characteristics of the LEDs were measured at room temperature with an integrating sphere. To prevent the self-heating effect partly, a pulsed current in dc mode is injected with a pulse duration of 4 ms and a duty cycle of 20% by a Keithley 2611 source meter.

SECTION III

RESULTS AND DISCUSSION

The concept of band engineering originates from the observation on the band diagram of InGaN/GaN LEDs. For conventional LEDs, the band diagram of the quantum wells shows a triangular shape due to the internal polarization field, as shown in Fig. 1 (solid line). Both conduction and valence bands of the well layer slope downward from the n-GaN side toward the p-GaN side, which leads to the separation of electron and hole wave function on spatial and, therefore, the carrier leakage at high injection level. However, if the composition of indium in the well decreases from the n-GaN side toward the p-GaN side, the band gap broadens gradually. As a result, the slope of conduction band could be leveled down and even overturned, while the slope of the valence band could be enhanced (see the dashed line shown in Fig. 1). The expected reduction in polarization field in active region will be achieved [20], [22]. Hence, the overlap of the electron and hole wave function as well as the efficiency droop will be improved.

Fig. 1. Schematic diagram of band engineering at MQWs with GQWs and conventional QWs.

Fig. 2 shows the fitted and experimental XRD curves. The −9 to +5 and −5 to +3 order secondary satellite peaks are clearly observed for the GQW LED A and conventional LED, respectively. These distinct satellite peaks indicate that the interface of the well and the barrier is smooth and sharp. The secondary peaks are sharper and stronger for GQW LEDs than those for conventional ones. It means that the periodicity of GQW LEDs is better than that of conventional ones. The angular separation between zero order satellite peak of the MQW and GaN (002) peak is larger for the conventional MQW structure than that for GQW structure. It indicates that the GQW structure may be not beneficial to the indium introduction into the strain InGaN QW. Thus, the average indium composition in the GQW structure is lower than that in conventional one. On the other hand, the accurate indium composition obtained from fitting results of X'pert epitaxy 4.0 for conventional LED and GQW LED A are 0.150 and 0.125, respectively.

Fig. 2. Experimental and simulation HR-XRD curves of (a) conventional LED and (b) GQW LED with indium composition grades from 30% to 0%.

The electroluminescence (EL) spectra of the LEDs were measured using pulsed current source. With the injection current of 20 mA, the peak emission wavelength for GQW LED A, B, and C are 473 nm, 433 nm, and 411 nm, respectively, which is attributed to the different degree gradations of indium composition in the quantum well. For conventional LED, the peak emission wavelength is 466 nm. Measurement results of output power for LED A, B, C and conventional LED as a function of injection current density are shown in Fig. 3(a). The output power of the four LEDs increases sublinearly as the current density increases. As shown in Fig. 3(a), LED B and C show higher output power than conventional LED at all current densities. The output power at 20 $\hbox{A/cm}^{2}$, which is the typical operation current density for most LED, is enhanced by 15% and 40% than conventional LED, respectively. There are mainly two factors responsible for the enhancement of output power in LED B and C. The first reason is that the wavelengths of LED B and C are shorter than that from conventional LED. The other reason is the improvement in internal quantum efficiency due to improved electron and hole wave function overlap in our GQW structures. The output power for LED B and C reaches its maximum at current density of 280 $\hbox{A/cm}^{2}$ and 200 $\hbox{A/cm}^{2}$, respectively. As to the LED B and C, the degree gradations of indium composition in the quantum well are different, which leads to different strength polarization field. As a result, the current density corresponding to maximal output power for LED B and C is distinct. In LED B, the indium composition graded from 18% to 0% whereas from 13% to 0% in LED C, and the former compensates much more band incline arising from polarization field than the latter does. In other words, a more flatter band structure will reduce the carrier leakage at high current density. While for LED A, it shows lower output power than conventional LED at low current densities. However, as current density goes on increasing, the output power of conventional LED saturates at 150 $\hbox{A/cm}^{2}$, but the output power of LED A keeps increasing sublinearly and reaches its maximum at about 280 $\hbox{A/cm}^{2}$. It is worth mentioning that this phenomenon has also been observed in other methods related to the low polarization field structure [10], [15]. The undesirable results of LED A could be attributed to nonoptimized epitaxial parameters for graded-composition well. It has been reported that several defects such as V-defects, stacking faults, and dislocations will be formed in the MQWs during sample growth as indium composition is higher at the MQWs, resulting in deterioration in the structure and optical properties of MQWs. In LED A, the high indium composition at the beginning of the quantum well will induce strong strain because of the high lattice mismatch between InN and GaN; therefore, larger numbers of defects may exist in the well and consequently reduce the light output power at the same current density compared to the conventional LED.

Fig. 3. (a) Light output power. (b) Normalized relative efficiency of GQW LEDs A, B, C, and conventional LED as a function of injection current density.

To illustrate the efficiency droop behavior clearly, the normalized relative efficiencies of the four LEDs are plotted in Fig. 3(b) as a function of injection current density. It can be seen that the efficiency of conventional LED shows a maximum at very low current density (around 10 $\hbox{A/cm}^{2}$) and drops rapidly to half at around 100 $\hbox{A/cm}^{2}$. The maximum efficiency $(\eta_{\rm peak})$ of GQW LEDs appeared at an injection current density of 25 $\hbox{A/cm}^{2}$ and showed a slow monotonous decrease as current density increases. The most important result is the reduction in efficiency droop in GQW LEDs. The efficiency droop can be defined as $(\eta_{\rm peak} - \eta_{200\ {\rm mA}})/ \eta_{\rm peak}$, where $\eta_{\rm peak}$ is the maximum efficiency at low current density. It can be seen that the efficiency only drops about 50%, 37%, and 43% for LED A, B, and C from 25 $\hbox{A/cm}^{2}$ to 200 $\hbox{A/cm}^{2}$, respectively. It does not show a significant decrease as seen in conventional LED, which drops about 50% from 10 $\hbox{A/cm}^{2}$ to 100 $\hbox{A/cm}^{2}$. The main reason accounts for this finding: In conventional LED, the conduction and valence bands of the InGaN well layers are largely bent due to the piezoelectric fields in the well and barrier layers, and the energy minimum of the conduction band and the energy maximum of the valence band are largely separated, as shown by solid lines in Fig. 1. Consequently, electrons and holes tend to accumulate at the opposite sides of the well, and the overlap is very small due to the quantum-confined Stark effect (QCSE). By contrast, in GQW LEDs, the lowest energy point of the valence band in the graded QW is still at the bottom of the well and never shifts under the influence of the internal field, but that of the conduction band profile tends from the top toward the bottom of the well due to the graded decreasing indium composition. As a consequence, the energy minimum of the conduction band and the energy maximum of the valence band are closer in GQW LEDs than those in conventional LED, as shown by the dashed line in Fig. 1. Consequently, the separation of electron and hole wave functions in the wells is much reduced in GQW LEDs. The significant improvement in efficiency droop implies that the droop can be alleviated more effectively by reducing the polarization field using graded indium composition in quantum well, especially at high current density. In addition, the GQW LEDs show the potential for high current operation for optical devices. These results indicate that the efficiency droop behavior has been changed by using the GQW in active region.

In Fig. 3(b), it also can be seen that the rising rate as well as the falling rate of the efficiency curve of conventional LED is faster compared to another three efficiency curves derived by GQW LEDs. They can be classified as two distinct types of droop behaviors readily seen in the literature [40], [41]. The slow increasing rate of the efficiency curve at low current density could be attributed to the higher point defects (PDs) density in GQW LEDs. The nonradiative process caused by PDs is more essential in the low current region, because they constraint the mobility of carrier. Nevertheless, PDs usually can be saturated under a moderately high injection level [42]. Consequently, slower rising rate indicates that more carriers are required to saturate the PDs induced by graded-composition ternary InGaN layer in the GQW LEDs. As shown in Fig. 3(b), the different efficiency droop for LEDs A, B, and C from 25 $\hbox{A/cm}^{2}$ to 200 $\hbox{A/cm}^{2}$ could be due to the different degree of indium composition gradation in the quantum well. The efficiency of LED A and C drops all the way, while the efficiency of LED B drops quickly at first and slows down at high current density. The degree of gradation had a decisive influence on the polarization field. Theoretically, larger composition of indium at the beginning of n-side will eliminate more polarization field in active layer. Even the slope of conduction band can be leveled down or overturned as long as the composition of indium is increased large enough at the n-side. However, the graded-composition ternary InGaN was grown with the method of growth temperature ramping, which will change the growth rate, and the lower temperature at the beginning might induce defects due to the big lattice mismatch between InN and GaN and, therefore, damage the quality of QWs. So the efficiency droop of LED A is bigger than that of another two GQW LEDs.

In order to further confirm the band-engineering design, we reverse the variation tendency of indium composition, which increase along [0001] direction gradually from 0% to 18%, labeled as GQW LED D. The peak emission wavelengths for GQW LED B and D are 433 nm and 413 nm, respectively. The normalized relative efficiency and output power for LED B and D are shown in Fig. 4. The normalized relative efficiency for both GQW LED B and D reaches maximum at the current density of about 25 $\hbox{A/cm}^{2}$. The output power of these two LEDs increases with current densities similarly below 150 $\hbox{A/cm}^{2}$. After that, the output power of GQW LED D is lower than that of GQW LED B and saturates at the current density of 220 $\hbox{A/cm}^{2}$. The superiority optical properties of GQW LED D compared to conventional LED are also attributed to the reduced polarization field in the MQWs. In LED D, the lowest energy point of the conduction band in the graded QW is still at the top of the well and never shifts under the influence of the internal field, but that of the valence band tends from the bottom to the top of the well and deviates from the indium maximal point. As a consequence, the separation of electron and hole wave functions is reduced. In addition, as shown in Fig. 4, at the respective onsets of output power saturation, the relative efficiency decreases more dramatically than GQW LED B. This phenomenon indicates that electron overflow is much serious in GQW LED D at high current densities. As the only difference between the two LEDs is the variation direction of indium composition, it is assumed that the LED D suffers from premature electron overflow due to larger band offset in conduction band than that in valence band. As a result, the same indium composition gradient results in different electron blocking effect. Compared to GQW LED B, the onset of electron overflow occurred in GQW LED D at the current density just lower than the onset of rapid droop of efficiency at 220 $\hbox{A/cm}^{2}$. This suggests that efficiency droop at high current densities is either caused or accompanied by electron overflow in these samples.

Fig. 4. Comparison of normalized relative efficiency and light output power of GQW LEDs B and D.
SECTION IV

CONCLUSION

Polarization field was reduced in InGaN/GaN LEDs by indium composition graded MQWs by MOCVD. However, its magnitude was not determined. The GQW LEDs showed better well barrier interface abruptness compared to the conventional MQWs. The relative quantum efficiency results show that efficiency droop for GQW LEDs occurs at a much higher current density than the conventional LEDs. Electron overflow was experimentally observed at high current density in the test structure with the same degree but different indium variation direction. These results suggest that the polarization field in the active layer is the predominant mechanism responsible for the efficiency droop, and electron leakage is responsible for the efficiency droop at high current injection levels.

Footnotes

This work was supported by the National Natural Science Foundation of China under Grant 11104230. Corresponding authors: B.-L. Liu and Y.-J. Liu (e-mail: blliu@xmu.edu.cn; yjlu@xmu.edu.cn).

L.-H. Zhu, Y.-L. Gao, Y.-J. Lu, Z. Chen are with the Department of Electronic Science, Xiamen University, Xiamen 361005, China.

W. Liu, F.-M. Zeng, and B.-L. Liu are with the Department of Physics, Xiamen University, Xiamen 361005, China.

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