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Advantages of blue InGaN light-emitting diodeswith AlGaN barriers

Jih-Yuan Chang,1 Miao-Chan Tsai,2 and Yen-Kuang Kuo1,*1Department of Physics, National Changhua University of Education, Changhua 500, Taiwan2Institute of Photonics, National Changhua University of Education, Changhua 500, Taiwan

*Corresponding author: ykuo@cc.ncue.edu.tw

Received March 1, 2010; revised March 24, 2010; accepted March 25, 2010;posted April 1, 2010 (Doc. ID 124837); published April 23, 2010

The advantages of blue InGaN light-emitting diodes (LEDs) with AlGaN barriers are studied numerically.The performance curves, energy band diagrams, electrostatic fields, and carrier concentrations are investi-gated. The simulation results show that the InGaN/AlGaN LED has better performance than its conven-tional InGaN/GaN counterpart owing to the increase of hole injection and the enhancement of electron con-finement. The simulation results also suggest that the efficiency droop is markedly improved when thetraditional GaN barriers are replaced by AlGaN barriers. 2010 Optical Society of America

OCIS codes: 230.0250, 230.3670, 230.5590.

The InGaN-based light-emitting diodes (LEDs) havebeen commercialized and widely used in backlight-ing, general illumination, and other applications

[1,2]. Nevertheless, the efficiency droop is still a seri-ous restriction for high-power applications [36]. As aresult, the performance enhancement under highcurrent injection becomes an important issue andarouses massive interests in recent years. Eventhough a plethora of research has been published[3,4,68], the mechanism of efficiency droop is notvery clear, and hence there is no effective method tocompletely solve this problem.

Among the numerous suggestions, the insufficientinjection efficiency of carriers and thereby nonuni-form carrier distributions may play an important rolefor this issue. For GaN-based devices, the holes havea relatively high effective mass and therefore a verylow mobility. In addition, the electron blocking layer(EBL) is found to act as a potential barrier also forholes. To sum up, the holes are more difficult totransport into and between the quantum wells(QWs). It has been reported that a large amount ofholes accumulate in the last QW next to the p-typeregion, and therefore almost only this well contrib-utes to radiative recombination in the InGaN LEDs[810].

One possible factor that can influence the hole in-jection efficiency is the height of the effective poten-tial barrier for holes in the valence band. To investi-gate this topic, the optical and electrical properties of

an InGaN multi-quantum-well (MQW) LED with Al-GaN barriers are studied numerically in this Letter,which is compared with the conventional one withGaN barriers. The use of the AlGaN barrier insteadof GaN barrier can diminish the polarization chargesaccumulated in the last-barrier/EBL interface, whichaccordingly can relax the band bending of EBL. Ow-ing to the above phenomenon, the effective potentialbarrier height for holes in the valence band de-creases, and hence the transportation of holes intothe active region becomes easier. In the meantime,the ability of electron confinement in the last-barrier/EBL interface does not degrade under this variation.

As a result, it is expected that the efficiency of holeinjection can be enhanced for the structure with Al-GaN barriers without the cost in reduction of elec-

tron confinement.The above issues are theoretically studied in detail

by using the APSYS simulation program [11] by solv-ing the Poissons equation, current continuity equa-tions, carrier transport equation, quantum mechani-cal wave equation, and photon rate equation. Thestructure of the original blue LED used as a refer-ence is identical to an already existing actual device.This LED was grown on a c-plane sapphire substrate,followed by a GaN buffer layer, and a 4.5-m-thickn-GaN layer n-doping=51018 cm3. The active re-gion consisted of five 2-nm-thick In0.21Ga0.79N QWs,separated by six 15-nm-thick GaN (or Al0.05Ga0.95Nfor new-designed structure) barriers. On top of theactive region was a 20-nm-thick p-Al0.15Ga0.85N EBLand a 0.5-m-thick p-GaN cap layer p-doping=1.21018 cm3. The device geometry was designed witha rectangular shape of 300 m300 m. To simplifythe simulation, the light extraction efficiency is as-sumed to be 0.78. The simulation results of the struc-ture for reference show very good agreement with theexperimental data, which demonstrates the reliabil-ity of our simulation [10]. The Shockley-Read-Hall(SRH) recombination lifetime and internal loss arethe key parameters used to fit the simulation resultwith the experimental result. In this work, the SRHrecombination lifetime and internal loss are set to be

0.13.9 ns and 3500 m

1

, respectively. Other mate-rial parameters of the semiconductors used in thesimulation can be found in [12].

The electrical and optical performances of theInGaN/GaN and InGaN/AlGaN structures are com-pared in Fig. 1. It is apparent that the InGaN/ AlGaNstructure has lower turn-on voltage, higher internalquantum efficiency (IQE), and output power in thewhole range of current injection under study. In otherwords, the overall performance of the InGaN/AlGaNstructure is superior to that of InGaN/GaN one. Asfor the degree of efficiency droop, defined by the for-mula IQEmaxIQEmin/IQEmax, the efficiency droop

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is 0.56 (0.29) for the structure with GaNAl0.05Ga0.95N barriers. Thus, the efficiency droop ismarkedly improved when the traditional GaN barri-ers are replaced by AlGaN barriers. It is worth notingthat, as the IQE curves indicate, the performance ofthe InGaN/AlGaN structure is good at low-injectioncurrent. For example, when the current ranges from13 to 60 mA, the IQE has a value of higher than 90%.More discussions about this phenomenon are in-

cluded below.Besides the potential difference of the band energyin barriers, the degree of polarization effect near ac-tive region is another major influence for the use ofthe AlGaN barriers. The built-in charge density in-duced by spontaneous and piezoelectric polarizationswithin the interface of the InGaN LEDs can be calcu-lated by the method developed by Fiorentini et al.[13]. In this study, considering the screening causedby the defects, the surface charge density is assumedto be 40% of the calculated value, which is a typical value adopted by several researchers. Besides, thecharge density screened by the injected charge carri-ers is determined self-consistently at different levels

of current injection. For the InGaN/AlGaN structure,as compared with the InGaN/GaN structure, the po-larization induced surface charge density in the well-barrier interface increases (8.931016 m2 versus8.171016 m2), while that in the last-barrier/EBLinterface decreases (1.661016 m2 versus 2.421016 m2), which is consistent with our previousanalysis.

Figure 2 shows the electrostatic fields near the lastthree QWs and EBL at 150 mA. Similar to the theo-retical calculation, the simulation results show thatthere are stronger electrostatic fields in the active re-gion for the InGaN/ AlGaN structure, which may

lead to the situation of band bending, poor overlap ofelectron and hole wave functions, and hence reducedradiative recombination rate. Note that the last QWnext to the p-type region does not show the same ten-dency. Inside this QW, the electrostatic field is lowerthan other QWs, which may be due to the largescreening effect induced by the huge amount of car-riers accumulated in this last QW. Furthermore, theweaker electrostatic fields around the last-barrier/EBL interface are also observed for theInGaN/ AlGaN structure. For this reason, thepolarization-induced downward band bending in thelast-barrier/EBL interface is mitigated, and there

should be more holes that can be injected into the ac-tive region.

These phenomena can be explained by the energyband diagrams shown in Fig. 3.As shown in Figs. 3(c)and 3(d), the effective potential height for holes in

the valence band of the InGaN/AlGaN structure islower than that of the InGaN/GaN one (i.e., 0.255 eVversus 0.282 eV) owing to the slighter polarization ef-fect in the last-barrier/EBL interface. Hence, itshould have better hole injection efficiency. Simulta-neously, more electrons can stay in the QWs and re-combine with holes. Thus, fewer electrons will escapeoutside the active region and overflow to the p-typelayers. Besides, as shown in Fig. 3(e) and 3(f), the ef-fective potential height for the electrons in the con-duction band of the InGaN/AlGaN structure becomeshigher than the other structure (i.e., 0.367 eV versus0.355 eV), which denotes the enhancement of elec-tron confinement. The enhancement of the ability forelectron confinement in InGaN/ AlGaN structure canalso be found in Fig. 3. The potential difference be-tween the conduction band edge and its relativequasi-Fermi level of the p-GaN layer near the EBL islarger than that of the InGaN/GaN structure. Thus,fewer electrons overflow to the p-type region for theInGaN/AlGaN structure.

Figure 4 shows the carrier concentrations ofInGaN/GaN and InGaN/AlGaN structures near theactive region at 150 mA. It is evident that most car-riers accumulate in the last QW for both structures. Ascribing to the enhancement of hole injection andelectron confinement, there are more carriers in the

active region for the InGaN/AlGaN structure, whichresults in better electrical and optical performance.For the InGaN/AlGaN structure, the situation thatmost carriers accumulated in the last QW will causetwo important influences. One is that the large elec-trostatic fields inside the last QW would be compen-sated by the strong screening effect owing to the hugeamount of carriers, as indicated previously in Fig. 2.As a result, the recombination efficiency of the lastQW in the InGaN/AlGaN structure will not be as badas that predicted at first. Another influence is that,because of the severe accumulation of carriers, thecapacity of carrier confinement inside the last QW

Fig. 1. (Color online) (a) Light-current-voltage, (b) IQEperformance curves of the InGaN/ GaN and InGaN/ AlGaNstructures.

Fig. 2. (Color online) Electrostatic fields near last threeQWs and EBL of the InGaN/ GaN and InGaN/ AlGaNstructures at 150 mA (gray regions represent the locationof QWs).

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might not be sufficient at high injection current, es-pecially for the InGaN/ AlGaN structure, owing tothe band-filling effect. This is the main reason for thedegradation of optical performance of theInGaN/ AlGaN structure under high current injec-tion.

In conclusion, AlGaN barriers are proposed to beused in the blue InGaN MQW LEDs. The simulationresults show that when the traditional GaN barriersare replaced by the AlGaN barriers, the carrier injec-tion is enhanced and the electron current leakage isreduced, which in turn result in the improvement ofoverall performance of the proposed InGaN/AlGaN

structure. The physical origin for the performanceimprovement is due mainly to the smaller effectivepotential height for holes, which is induced by the re-duced surface charge density in the last-barrier/EBLinterface.

The authors are grateful to Dr. Jen-De Chen for thetechnical assistance. This work is supported by theNational Science Council (NSC) of Taiwan undergrant NSC 96-2112-M-018-007-MY3.

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Fig. 4. (Color online) Carrier concentrations of the (a)InGaN/GaN and (b) InGaN/AlGaN structures near activeregion at 150 mA

Fig. 3. (Color online) Energy band diagrams of (a)InGaN/GaN and (b) InGaN/AlGaN structures at 150 mA.Enlarged drawings of the valence band near EBL of the (c)InGaN/GaN and (d) InGaN/AlGaN structures. Enlargeddrawings of the conduction band near EBL of the (e)InGaN/GaN and (f) InGaN/AlGaN structures.

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