The operating voltage, light output power, and efficiency droops of GaN-based light emitting diodes (LEDs) were improved by introducing Mg-doped AlGaN/InGaN superlattice (SL) electron blocking layer (EBL). The thicker InGaN layers of AlGaN/InGaN SL EBL could have a larger effective electron potential height and lower effective hole potential height than that of AlGaN EBL. This thicker InGaN layer could prevent electron leakage into the p-region of LEDs and improve hole injection efficiency to achieve a higher light output power and less efficiency droops with the injection current. The low lateral resistivity of Mg-doped AlGaN/InGaN SL would have superior current spreading at high current injection.
© 2014 Optical Society of America
InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) have attracted much attention because of their application on liquid crystal display back lighting and solid-state lighting. Immense effort has been exerted in the advancement of material quality [1,2], light extraction efficiency (LEE) [3,4], and metal-semiconductor ohmic contacts  to improve drastically the luminous efficiency of GaN-based LEDs. Recent works based on photonic crystals  and microsphere array [7,8] have been performed to enhance LEE in GaN-based LEDs. Novel methods of growing epitaxial layers on the nano-patterned sapphire substrate have also been applied to reduce dislocation density in these devices [9–11]. The output power of current LEDs is already high; nevertheless, the output efficiency of these LEDs requires further improvement to achieve feasible solid-state lighting. Furthermore, the carrier injection efficiency also has key functions in the light emission efficiency of GaN-based LEDs. Meyaard et al. reported asymmetry during carrier transport because of the much lower concentration and mobility of holes in p-GaN and p-AlGaN compared with electrons [12,13]. The asymmetry in carrier transport reduces the light emission efficiency of LEDs and enlarges the efficiency droop. Therefore, several reports have focused on the structure design of the LED p-side, such as p-AlGaN/GaN superlattice (SL) [14,15], graded composition p-AlGaN/GaN SL , InAlN/GaN SL , and p-InGaN/GaN SL last barrier  to improve the injection of the hole. More recently, Chen et al. reported a numerical analysis of green LEDs with p-AlGaN/InGaN SL EBL to improve hole injection efficiency . In the current study, the effects of InGaN thickness of p-AlGaN/InGaN SL EBL on the optoelectrical properties of GaN-based LEDs are discussed. GaN-based LEDs with varying InGaN thicknesses of p-AlGaN/InGaN SL EBL were prepared and fabricated in chips to study the changes in LED optoelectrical properties.
All samples were grown on a 2 in (0001) patterned sapphire substrate (PSS) through a Thomas Swan close-coupled showerhead 19 × 2 metalorganic chemical vapor deposition (MOCVD) system. The PSS periodic convex pattern was formed using an inductively coupled plasma etcher. The pattern diameter, spacing, and height of the PSS were 3.5, 2, and 1.3 µm, respectively. During MOCVD growth, trimethylindium, trimethylgallium, trimethylaluminum, and ammonia were used as the source materials of In, Ga, Al, and N, respectively.
Bicyclopentadienyl magnesium and silane were used as the p-type and n-type doping sources. The reactor temperature was raised to 900 °C to grow a 10 nm-thick AlN nucleation layer on the PSS in situ. The reactor temperature was then raised to 1050 °C to grow a 2 µm-thick undoped GaN epitaxial layer followed by the deposition of a 2 µm-thick n-GaN layer. Subsequently, a 9-pair InGaN (3 nm)/GaN (8 nm) light-emitting MQW structure was produced via a high-low temperature scheme. The growth temperatures of the InGaN “well layers” and the GaN barrier layers were kept at 730°C and 880°C, respectively. After the growth of the light-emitting MQWs structure, a 20 nm-thick Mg-doped Al0.15Ga0.85N EBL was fabricated for conventional LEDs as LED I. Aside from the single AlGaN EBL, an 8 pairs AlGaN/InGaN SL EBL was also introduced with varying InGaN layer thicknesses. The AlGaN thickness and Al% of the AlGaN/InGaN SL was fixed at 2 nm and 15%, respectively. The InGaN In% in AlGaN/InGaN SL was fixed at 2%. The InGaN thickness in AlGaN/InGaN SL, however, were variably set at 1, 2, and 4 nm and were labeled as LED II, III, and IV, respectively. A 0.1 µm-thick p-GaN layer was deposited following the EBL layer.
Subsequently, standard processing steps were performed to fabricate 430 µm × 860 µm LED chips with indium tin oxide at the upper contact. The current–voltage (I–V) characteristics of the fabricated LEDs were then measured using an HP-4156C semiconductor parameter analyzer with a 100 mA current limit. At a high current injection, a Keithley 2400 source meter was used to measure the I–V characteristics of these LEDs. The output powers and emission wavelength of the LEDs were measured using a calibrated integrating sphere and a spectrometer (Ocean Optics USB2000) at room temperature.
3. Results and discussion
Figure 1 shows the forward I–V characteristics and dynamic resistance of all fabricated LEDs (averaged the I–V characteristics of 10 samples of each LEDs). The 100 mA forward voltages (Vf) of LEDs I to IV were 4.02, 3.71, 3.56, and 3.40 V, respectively. The dynamic resistance defined as R = dV/dI at 3 V of LEDs I to IV were 22, 14, 10, and 7 Ω, respectively. The conventional LED with p-AlGaN EBL exhibited the largest Vf. However, the Vf of LED decreased when p-AlGaN EBL was replaced with p-AlGaN/InGaN SL. Moreover, the Vf of LEDs slightly decreased with increasing InGaN thickness of p-AlGaN/InGaN SL EBL. The Vf reduction of LEDs with p-AlGaN/InGaN SL EBL might be due to the reduction of the effective potential barrier on the hole carrier between p-GaN and p-AlGaN/InGaN SL. The reduced effective potential barrier on the p-side of LEDs may also reduce the series resistance of the LEDs. Therefore, LEDs with p-AlGaN/InGaN SL exhibit lower dynamic resistance than that of conventional LEDs.
LEDs with p-AlGaN/InGaN SL may improve hole injection into InGaN/GaN MQWs to enhance light emission of the output power. Figure 2 shows the measured output power and external quantum efficiency (EQE) as a function of the injection current density for all LEDs. The light output powers of the LEDs driven at a current density of 30 A/cm2 were 114.4, 122.3, 129.7, and 131.8 mW, which correspond to EQEs of 41.8%, 44.7%, 47.5%, and 48.2% for LEDs I to IV (without an encapsulated epoxy optical lens), respectively. Hence, the output powers of LEDs II, III, and IV at a current density of 30 A/cm2 were enhanced by magnitudes of approximately 6.9%, 13.4%, and 15.2%, respectively, compared with those of LED I. Furthermore, the light output power increased with increasing thickness of the InGaN layer in AlGaN/InGaN SL EBL. The power enhancements in the light output of LED II to IV might be due to the improvement in hole carrier injection efficiency, which was introduced by AlGaN/InGaN SL EBL. The efficiency droops (efficiency degradation from the peak of EQE to the EQE of 100 A/cm2) of LEDs I to IV were 41.2%, 38.1%, 35.0%, and 31.4%, respectively. Hence, LEDs II to IV have less efficiency droops than LED I. Moreover, increasing the InGaN thickness of InGaN/AlGaN SL EBL would also reduce the efficiency droops of LEDs.
Due to the polarization effect in GaN-based materials, the large electrostatic fields bend the energy band downward at the last-barrier/AlGaN EBL interface. It reduces the effective barrier height for electron blocking and increases barrier height of the hole injection of AlGaN EBL. Lin et al. reported that the AlGaN/GaN SL EBL increases the effective barrier height for electron blocking and reduces the barrier height of the hole injection . Therefore, the EBL of AlGaN/InGaN SL should function similarly as that of AlGaN/GaN SL. To determine the electrostatic fields (Fig. 3) and the band diagrams (Fig. 4) of LEDs I to IV at a current density of 30 A/cm2, we performed the simulation for LEDs I to IV by the APSYS software which was developed by Crosslight Software Inc. It was found that the electrostatic field in AlGaN of AlGaN/InGaN SL EBL is larger than that of in single AlGaN EBL. The enlarged electrostatic field in AlGaN of AlGaN/InGaN SL EBL should be attributed to the increment polarization-induced sheet charges in the AlGaN/InGaN interface with increasing thickness of the InGaN layer as shown in Figs. 3(a)–3(d). Then the energy band at the last barrier/EBL interface could be pulled up as shown in Figs. 4(a)–4(d). LEDs II to IV with AlGaN/InGaN SL EBL showed a larger effective potential height (i.e., 513, 559, and 626 meV) of electrons at the conduction band near the last quantum barrier and the EBL than that of LED I with single AlGaN EBL (i.e., 450 meV). By contrast, the effective potential height for holes at the valence band was reduced from 450 meV to 432 meV when AlGaN EBL of LED I was replaced with AlGaN/InGaN SL EBL of LED II.
Since polarization-induced sheet charges at the AlGaN/InGaN interfaces induced the electrostatic field, the potential drop across one period SL EBL is equal to zero. That isEq. (1), the electrostatic field of AlGaN in AlGaN/InGaN SLs EBL can be increased with incensing the InGaN thickness. Due to the enhancement of the electrostatic fields, the effective potential height of electron for the AlGaN/InGaN SL EBL also increased from 513 meV to 626 meV with increasing thickness of the InGaN layer. The effective potential height could be further decreased from 432 meV to 386 meV by increasing the InGaN thickness of AlGaN/InGaN SL EBL from 1 nm to 4 nm. The high effective potential of AlGaN/InGaN SL could block electron leaking to the p-side of the LEDs. However, a reduction in the effective potential height of holes could enhance the injection of the holes into the active layers.
Figure 5 illustrates the simulation results of the electron and hole concentrations in InGaN wells at a current density of 30 A/cm2. All LEDs exhibited the largest electron and hole concentrations at the last InGaN well. The hole concentration of the last InGaN well increased when AlGaN EBL was replaced with AlGaN/InGaN SL EBL and when the InGaN thickness of AlGaN/InGaN SL EBL was increased. These results could be attributed to the reduction in the effective potential height of the hole. By contrast, the electron concentration at the last InGaN well went towards the reverse direction. The emission efficiency of the LEDs was dominated by the hole concentration because the hole concentration is less than the electron concentration in the active layers of LEDs.
The increased hole concentration at the last InGaN well resulted in an improvement in LED emission efficiency because of recombination efficiency enhancement in the electron hole carriers. This enhancement effectively reduced electron concentration. Therefore, LED IV exhibited the largest light output power at a current density of 30 A/cm2. Aside from hole injection improvement, the potential height of the electron was enlarged by AlGaN/InGaN SL EBL. The increased InGaN thickness of AlGaN/InGaN SL EBL also suppressed electron leakage into the p-region of the devices, as shown in Fig. 6.Hence, the reduction in electron concentration at the last InGaN well and the suppression of electron leakage would cause LED IV, which contains AlGaN/InGaN SL EBL with an InGaN thickness of 4 nm, to achieve the smallest efficient droop of 31.4%.
The emission intensity distribution for the entire LED chip with current densities of 30 and 100 A/cm2 for LED I, II, and IV were determined, as illustrated in Fig. 7.At a current density of 30 A/cm2, all LEDs showed a similar emission intensity distribution. However, at a high current density of 100 A/cm2, LEDs IV and II showed a larger emission area (red and yellow region of the images) than LED I. The line profile of the emission intensity at a driving current density of 100 A/cm2 at the middle of the longitudinal side of the LED chips is represented by broken arrow lines in Fig. 7(g). LEDs IV and II have a larger emission intensity and slower intensity decay rate than LED I because the distance from the p-pad fingers increases. The slower intensity decay rate with increasing distance of LED IV and II indicates superior current spreading at a higher current density than LED I. Kozodoy et al. reported that Mg-doped AlGaN/GaN SL could have high hole concentration and low lateral resistivity because of severe band bending via piezoelectric and spontaneous polarization of AlGaN . The Mg-doped AlGaN/InGaN SL should have similar properties as the Mg-doped AlGaN/GaN SL. On the other hand the electrical field in AlGaN EBL is less than that of AlGaN/InGaN SLs EBL as shown in Fig. 3. The increased electrostatic field in the Mg doped AlGaN/InGaN would help to ionize the Mg dopants. And the high electrostatic field in Mg-doped AlGaN/InGaN SL EBL could also severely bend the band to have high hole concentration and low lateral resistivity compared with the Mg-doped single AlGaN EBL. The low lateral resistivity of Mg-doped AlGaN/InGaN SL could reduce the current crowding effect of the high current density injection and further reduce the efficiency droops at an injection of high current density.
In summary, the Vf, light output power, and efficiency droops of GaN-based LEDs can be improved by introducing Mg-doped AlGaN/InGaN SL EBL. GaN-based LEDs with AlGaN/InGaN SL EBL may have a larger effective electron potential height and lower effective hole potential height than AlGaN EBL. Moreover, the thicker InGaN layer of the Mg-doped AlGaN/InGaN SL EBL changes the effective potential height of electron and the hole may increase and decrease, respectively. The enlarged and reduced effective potential barrier of the electron and hole would prevent electron leakage into p-region of LEDs and improve the hole injection efficiency to have a higher light output power and less efficiency droops with the injection current. Moreover, the Mg-doped AlGaN/InGaN SL EBL could have a low lateral resistivity. The low lateral resistivity of Mg-doped AlGaN/InGaN SL reduces the current crowding effect of the high current density injection and could further reduce the efficiency droops at a high current density injection. Therefore, LED IV with thick InGaN layers of AlGaN/InGaN SL of EBL, which have improved hole injection efficiency, suppressed electron leakage, and superior current spreading at a high injection current density, had the largest light output power and less efficiency droops with injection current density.
The authors are grateful to the National Science Council of Taiwan for their financial support under Contract Nos. NSC101-2221-E-006-066-MY3 and 102-3113-P-009-007-CC2. This research was also made possible through the Advanced Optoelectronic Technology Center, National Cheng Kung University, as a project of the Ministry of Education of Taiwan, and through the financial support of the Bureau of Energy, Ministry of Economic Affairs of Taiwan, under Contract No. 102-E0603.
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