Hole transport improvement in InGaN/GaN light-emitting diodes by graded-composition multiple quantum barriers

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Hole transport improvement in InGaN/GaN light-emitting diodes by graded-composition

multiple quantum barriers

C. H. Wang, S. P. Chang, P. H. Ku, J. C. Li, Y. P. Lan, C. C. Lin, H. C. Yang, H. C. Kuo, T. C. Lu, S. C. Wang, and C. Y. Chang

Citation: Applied Physics Letters 99, 171106 (2011); doi: 10.1063/1.3655903

View online: http://dx.doi.org/10.1063/1.3655903

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/99/17?ver=pdfcov

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Hole transport improvement in InGaN/GaN light-emitting diodes

by graded-composition multiple quantum barriers

C. H. Wang,1S. P. Chang,1,2P. H. Ku,1J. C. Li,1,a)Y. P. Lan,1C. C. Lin,3H. C. Yang,2 H. C. Kuo,1,b)T. C. Lu,1S. C. Wang,1and C. Y. Chang4

1

Department of Photonics and Institute of Electro-Optical Engineering, National Chiao-Tung University, Hsinchu, Taiwan

2

R&D Division, Epistar Co., Ltd., Science-based Industrial Park, Hsinchu, Taiwan

3

Institute of Lighting and Energy Photonics, National Chiao Tung University, Tainan County, Taiwan

4

Institute of Electronics, National Chiao-Tung University, Hsinchu, Taiwan

(Received 4 July 2011; accepted 30 September 2011; published online 25 October 2011)

Graded-composition multiple quantum barriers (GQB) were designed and incorporated in c-plane InGaN/GaN light-emitting diodes (LEDs) grown on c-plane sapphire substrate to improve hole transport and efficiency droop. The simulation of GQB LED design predicts enhancement of the hole transport in the active region at both low and high current densities. The fabricated LED with GQB structure exhibits lower series resistance and substantially reduced droop behavior of only 6% in comparison with 34% for conventional LED, supporting the improvement of hole transport in our design.VC _{2011 American Institute of Physics. [doi:}_{10.1063/1.3655903}_]

InGaN/GaN light-emitting diodes (LEDs) grown on c-plane sapphire substrate have recently become a favorable choice for applications in energy saving solid-state lighting.1 One key feature that still needs to be solved is the so-called “efficiency droop.” Despite the various arguments have been proposed and discussed, electron overflow out of the active region, Auger recombination, and insufficient transport of holes have been identified as the most possible reasons for droop.2–4Among these factors, poor transport of holes might be the most important one. Because it could lead to the accu-mulation of these carriers at the quantum wells nearp-GaN, which increase the possibilities for the overflow of electrons and Auger recombination at high carrier concentration. As a result, improving hole transport in the active region has been proposed as a mean to reduce droop behavior. Various designs such as coupled multiple quantum wells (MQWs),5 non-polarized active region,6 and graded-well-thickness structure7 were reported and proved to be effective. In our former research work, we have demonstrated that leveling the triangular barriers at valence band by using a graded-composition electron blocking layer (GEBL) can effectively enhance the transport of holes across the EBL.8In this paper, we report a new design of the barrier layers in MQWs by grading the composition of barriers from InxGa1xN to GaN

along [0001] direction, to form a graded-composition multi-ple quantum barriers (GQB) and show the improvement in transport of holes in active region and substantial reduction in efficiency droop behavior. The injected holes do not accu-mulate at the well closest top-GaN, denoted as the last well, and uniformly spread in the active region. The droop behav-ior predicted by our simulation agrees well with the experiments.

For conventional LEDs operated under forward bias, the band diagram of multiple quantum barriers (MQBs) shows

triangular shape due to the internal polarization field and for-ward bias,6as shown in Fig. 1. The valance band of MQBs shows an upward slope from the n-GaN side toward p-GaN side, which retards the holes to travel across the triangular barrier. But if the composition of indium in barriers decreases from then-GaN side toward p-GaN side, the band-gap broadens gradually. As a result, the barrier in valance band could be leveled down and even overturned, while the slope of conduction band could be enhanced. This could enhance the hole transporting across the barriers.

We then simulated the band diagrams and carrier distri-butions in GQB LED using APSYSsimulation software. The

simulation LED structures were composed of 4-lm-thick n-type GaN layer (n-doping¼ 2 1018cm3), six pairs of In0.15Ga0.85N/GaN MQWs with 2.5-nm-thick wells and

10-nm-thick barriers, 20-10-nm-thick p-Al0.15Ga0.85N EBL

(p-doping¼ 5 1017cm3), and 200-nm-thick p-type GaN layer (p-doping¼ 1 1018cm3). For the GQB LED, the composition of indium was graded from 5% to 0% along [0001] direction and compared with the conventional LED with GaN barriers. Commonly accepted physical parameters were adopted to perform the simulations, the percentage of screening effect of 50%, the Shockley-Read-Hall recombina-tion lifetime of 1 ns, and the Auger recombinarecombina-tion coefficient in quantum wells with order of 1031 cm6/s, respectively.9 Other material parameters used in the simulation can be referred to Ref.10. The calculated band diagrams of conven-tional and GQB LEDs at current density of 100 A/cm2 are shown in Fig. 1. The triangular barriers in valance band are partially leveled in GQB LED. Unexpectedly, the quasi-Fermi-level for holes in GQB LED is lower than that of con-ventional LED. This phenomenon could further favor the transport of holes in active region.

Figures2and 3show the calculated carrier concentra-tion in the active region for convenconcentra-tional and GQB LEDs at current density of 20 and 200 A/cm2, respectively. At low current density of 20 A/cm2, holes appear to hop out of the

a)_{Electronic mail: jchli@mail.nctu.edu.tw.} b)

Electronic mail: hckuo@faculty.nctu.edu.tw.

0003-6951/2011/99(17)/171106/3/$30.00 99, 171106-1 VC2011 American Institute of Physics APPLIED PHYSICS LETTERS 99, 171106 (2011)

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last well and spread to the others for GQB LED, as shown in Fig.2(b). The hole concentrations in the last well are about 8.5 1028 _{and 3.5}₁₀28_cm3 _{for conventional and GQB}

LEDs, respectively. While they are about 1.3 1028 _and

5.8 1028_cm3 _{in the fifth well for conventional and GQB}

LEDs, respectively. These results indicate that GQB LED has better hole transport even at low current density, lower-ing the hole concentration at the last well. At high current density of 200 A/cm2, hole distribution in GQB LED is more uniform than that at low current density. Such improved hole distribution is useful for droop reduction.5 Meanwhile, the calculated electron concentration in the active region for conventional and GQB LEDs was shown in Figs.2and3. At either low or high current density, electron concentration in GQB LED is higher than that in conventional LED, which could be attributed to lower electron overflow for GQB LED. The distribution of electrons in GQB LED seems to be more uniform than that in conventional LED, which might corresponds to better transport of holes. From the simulation results, hole and electron distributions in GQB LEDs, either

at low or high current density, are favorable for reduction of droop behavior.

In light of simulation results, we have grown the LED structures with GaN barriers and GQB on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). After depositing a low temperature GaN nuclea-tion layer, a 4 lm n-type GaN layer, and ten-pair InGaN/ GaN superlattice prestrain layer, the rest of the LED struc-tures were grown based on our simulation design. The graded composition barriers were grown using In/Ga ratio ramping to prevent the change in the growth rate. Finally, the LED chips were fabricated by regular chip process with ITO current spreading layer and Ni/Au contact metal. The LED has a typical chip size of 300 300 lm2_{. The emission}

wavelengths of both types of LEDs were around 450 nm at 22 A/cm2.

TheI-V characteristics of conventional and GQB LEDs are shown in the inset of Fig. 4. The series resistance is

FIG. 2. (Color online) Calculated carrier concentrations at current density of 20 A/cm2for (a) conventional and (b) GQB LEDs.

FIG. 3. (Color online) Calculated carrier concentrations at current density of 200 A/cm2for (a) conventional and (b) GQB LEDs.

FIG. 4. (Color online) Experiment and simulation normalized efficiency for conventional and GQB LEDs. The inset figure showsI-V characteristics of conventional and GQB LEDs.

FIG. 1. (Color online) Calculated band diagrams of the conventional and GQB LEDs.

171106-2 Wang et al. Appl. Phys. Lett. 99, 171106 (2011)

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reduced from 8.2 X in conventional LED to 6.5 X in GQB LED, which indicates a certain degree of the improvement in hole transport. As a result, the forward voltage at 22 A/cm2 is reduced from 3.4 V in conventional LED to 3.27 V in GQB LED. The normalized efficiency of conventional and GQB LEDs as a function of current density was investigated by experiment and simulation, as illustrated in Fig.4. It can be seen that the experimental data have similar droop behav-ior to simulation results. However, the efficiency of GQB LED shows slightly lower value in experiment. This can be attributed to non-optimized epitaxial parameters for graded-composition barriers. The most important result is the reduc-tion of efficiency droop, defined as (gpeak g200 Acm2)/

gpeak, which is reduced from 34% for conventional LED to

6% for GQB LED. This result confirms that the graded bar-rier design did contribute to the reduction of efficiency droop.

Although the efficiency droop has been reduced, the effi-ciency of GQB LED at 20 A/cm2, which is the typical opera-tion current density for most LEDs, is only 70% of that of conventional LED. This phenomenon has also been observed in other droop-reduction methods related to the layer design of MQBs (Ref.5) or low polarization field structures.6,11To understand this, the radiative recombination distribution in the active region for both LEDs was calculated and illus-trated in Fig.5. The results show that the radiative recombi-nation distribution in GQB LED is more uniform than that in conventional LED, at current density of 20 A/cm2; however, the total amount of radiative recombination in GQW LED is only 70% of that in conventional LED. At 200 A/cm2, the total amount of radiative recombination in GQW LED is 119% of that in conventional LED. The reason could be referred to the poor spatial distribution overlap between holes and electrons. For good hole transport as GQB LED, the spatial distributions of holes and electrons are quite differ-ent. As shown in Figs.2and3, at current density of 20 A/cm2,

most of the electrons still concentrate in the last well, while the hole concentration in the last well is less than that in fifth well. That is, the spatial overlap between electrons and holes in GQB LED is not the optimal situation at low current den-sity. While at 200 A/cm2, such mismatch is alleviated due to much more holes are injected. However, in conventional LED, both holes and electrons concentrate at the wells near p-GaN, so the radiative recombination is quite effective at that location. These results indicate that to reduce droop behavior without deteriorating the total recombination, one should pay more attention to the spatial distribution between holes and electrons.

In summary, we have designed a graded-composition multiple quantum barriers for InGaN/GaN LED to improve the hole transport in active region. The simulation results showed that the triangular barrier of multiple quantum bar-riers at valance band could be balanced by increasing the band gap of InxGa1xN along [0001] direction. As a result,

the hole transport in MQWs was significantly enhanced at ei-ther low or high current density, which is beneficial for droop reduction. The GQB LED was realized by MOCVD, and theI-V curve showed that GQB LED has lower series re-sistance than the conventional one and the efficiency droop was reduced from 34% in conventional LED to only 6% in GQB LED, which is in agreement with our simulation results. Beyond these, the efficiency of GQB LED at 20 A/cm2, was found to be only 70% of that in conventional LED. The reason for such low recombination could be attributed to the poor spatial distribution overlap between holes and electrons. These results indicate that although the improvement in hole transport facilitates the reduction of efficiency droop, spatial distribution between electrons and holes should be taken into consideration.

The authors would like to thank Dr. T.C. Hsu, Dr. M. H. Hsieh of Epistar Corporation for their technical sup-port. This work was funded by the National Science Council in Taiwan under Grant No. NSC 100-3113-E-009-001-CC2.

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FIG. 5. (Color online) Calculated radiative recombinations of conventional and GQB LEDs at current density of (a) 20 A/cm2and (b) 200 A/cm2.

171106-3 Wang et al. Appl. Phys. Lett. 99, 171106 (2011)