blue InGaN multiple-quantum well light-emitting diodes with InGaN

Advantages of blue InGaN multiple-quantum well light-emitting diodes with InGaN barriersYen-Kuang Kuo, Jih-Yuan Chang, Miao-Chan Tsai, and Sheng-Horng YenCitation: Appl. Phys. Lett. 95, 011116 (2009); doi: 10.1063/1.3176406View online: /10.1063/1.3176406View Table of Contents: /resource/1/APPLAB/v95/i1Published by the American Institute of Physics.Related ArticlesElectroluminescence and electric current response spectroscopy applied to the characterization of polymer light-emitting electrochemical cellsAPL: Org. Electron. Photonics 5, 211 (2012)Electroluminescence and electric current response spectroscopy applied to the characterization of polymer light-emitting electrochemical cellsAppl. Phys. Lett. 101, 113305 (2012)Uniform and refreshable liquid electroluminescent device with a back side reservoirAPL: Org. Electron. Photonics 5, 208 (2012)Magnetic field dependence of the spin relaxation length in spin light-emitting diodesAppl. Phys. 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Lett.Journal Homepage: /Journal Information: /about/about_the_journalTop downloads: /features/most_downloadedInformation for Authors: /authorsAdvantages of blue InGaN multiple-quantum well light-emitting diodes with InGaN barriersYen-Kuang Kuo,1,a ͒Jih-Yuan Chang,1Miao-Chan Tsai,2and Sheng-Horng Yen 31Department of Physics,National Changhua University of Education,Changhua 500,Taiwan 2Institute of Photonics,National Changhua University of Education,Changhua 500,Taiwan 3R&D Division,Epistar Co.,Ltd.,Science-based Industrial Park,Hsinchu 300,Taiwan͑Received 18April 2009;accepted 17June 2009;published online 10July 2009͒The advantages of blue InGaN light-emitting diodes ͑LEDs ͒with InGaN barriers are studied.The L -I curves,carrier concentrations in the quantum wells,energy band diagrams,and internal quantum efficiency are investigated.The simulation results show that the InGaN/InGaN LED has better performance over its conventional InGaN/GaN counterpart due to the enhancement of electron confinement,the reduced polarization effect between the barrier and well,and the lower potential barrier height for the holes to transport in the active region.The simulation results also suggest that the efficiency droop is markedly improved when the traditional GaN barriers are replaced by InGaN barriers.©2009American Institute of Physics .͓DOI:10.1063/1.3176406͔The quantum efficiency ͑QE ͒of conventional InGaN/GaN multiquantum well ͑MQW ͒light-emitting diodes ͑LEDs ͒reaches its peak at low current density and mono-tonically decreases with further increasing drive current.1–4This phenomenon,commonly referred to as efficiency droop,is a critical restriction for the usage of LEDs in high power applications.Many suggestions to its mechanism have been reported,such as the Auger recombination,5electron leakage,2,3,6,7carrier injection efficiency,6–8polarization field,2,3and filling of localized states.9Nevertheless,the physical origin of efficiency droop remains debatable and,hence,an overall solution is still a lack.For the exploration of efficiency droop in blue InGaN LEDs,the carrier injection and distribution in the active re-gion may be a key issue.It has been reported that only the last QW next to the p -type region contributes to radiative recombination in the InGaN MQW LEDs.8,10This is because that holes in GaN based materials have a relatively high ef-fective mass and therefore a very low mobility.Moreover,the electron blocking layer ͑EBL ͒is considered to act as a potential barrier also for holes.Under this circumstance,it is difficult for holes to inject into the active region and trans-port in the active region.Thus,a large amount of holes ac-cumulate in the last QW next to the p -type region.As for the electrons,due to the opposite characters to holes and the insufficient blocking efficiency of EBL under polarization fields,they can transport easily in the active region and even overflow across the EBL to the p -type region.As a result,nonuniform distribution of carriers is expected,which ex-plains why not every QW contributes to radiative recombi-nation.In this paper,the optical and electrical properties of conventional InGaN/GaN MQW LEDs and new designed InGaN/InGaN MQW LEDs are investigated numerically with the APSYS simulation program,which was developed by the Crosslight Software Inc.11The original blue InGaN LED used as a reference was grown on a c -plane sapphire sub-strate,followed by a 50-nm-thick undoped GaN layer,and a4.5-␮m-thick n -GaN layer ͑n -doping=5ϫ1018cm −3͒.The active region consisted of five 2-nm-thick In 0.21Ga 0.79N QWs,separated by six 15-nm-thick GaN barriers.On top of the active region was a 20-nm-thick p -Al 0.15Ga 0.85N EBL and a 0.5-␮m-thick p -GaN cap layer ͑p -doping=1.2ϫ1018cm −3͒.The device geometry was designed with a rectangular shape of 300ϫ300␮m 2.The internal absorption within the LED device is assumed to be 500m −1and the operating temperature is assumed to be 300K.To simplify the simulation,the light extraction efficiency is assumed to be 0.78.Other material parameters of the semiconductors used in the simulation can be found in Ref.12.The surface charges at the interfaces are calculated by the methods de-veloped by Fiorentini et al.13The experimental and simulated light-current-voltage ͑L -I -V ͒performance curves of the original InGaN/GaN structure are plotted in ͓Fig.1͑a ͔͒,which shows good agree-ment between the experimental data and our simulations.The distribution of carrier concentrations near the active re-gion of the original InGaN/GaN structure at an injection cur-rent of 150mA is shown in ͓Fig.1͑b ͔͒.It indicates that both electrons and holes distributions are quite nonuniform among QWs;namely,most carriers concentrate on the last QW next to the p -type region.There also exists a severe electron leak-age in this LED structure.a ͒Electronic mail:ykuo@.tw.Tel.:ϩ886-4/724-8812.FAX:ϩ886-4/721-1153.FIG.1.͑Color online ͒͑a ͒Experimental and simulated L -I -V performance curves of original InGaN/GaN structure.͑b ͒Distribution of carrier concen-trations of original InGaN/GaN structure at 150mA ͑gray regions represent the location of QWs ͒.APPLIED PHYSICS LETTERS 95,011116͑2009͒0003-6951/2009/95͑1͒/011116/3/$25.00©2009American Institute of Physics95,011116-1For the above phenomenon,Park and co-workers 2,3re-ported that the polarization field induced sloped triangular barriers will hinder the transportation of electrons and cause the conduction band on the n -side to be higher than that on the p -side.This in turn results in a large electron leakage current.In a recent paper,Xu et al.14proposed to use the polarization-matched InGaN/InGaN MQW structure to re-duce the potential difference of conduction band between the n -side and p -side,and to improve the device performance.In their article,however,the experimental data do not show much decrease of forward voltage under the same injection current that can diminish the above potential difference for the InGaN/InGaN MQW LEDs.Therefore,there might exist other mechanisms which are responsible for the improve-ment of LED performance.Figure 2shows the energy band diagram of the original InGaN/GaN structure and its local enlargements at 150mA.In these plots,a severe situation of bend bending,i.e.,sloped triangular barriers and wells,is observed.The sloped trian-gular barriers cause the conduction band edge of barriers to be higher than the conduction band of EBL,as shown in ͓Fig.2͑b ͔͒.Note that the conduction band of EBL is also deformed by polarization fields.Thus,insufficient electron blocking efficiency and thereby serious electron current leak-age can be expected in this structure.7Figure 2͑c ͒shows the enlarged band diagram of the last QW and the electron and hole wave functions.Due to the quantum confined Stark ef-fect ͑QCSE ͒,the electron and hole wave functions separate partially,which results in poor overlap between the two wave functions,as well as the reduction of radiative recombination rate and internal QE ͑IQE ͒.In order to improve the above shortcomings,an InGaN/InGaN MQW structure,with an indium composition of 10%in the InGaN barriers,is proposed.As shown in ͓Figs.3͑a ͒and 3͑b ͔͒,the effective barrier height between the last barrier and EBL is increased dramatically due to the lower conduc-tion band energy of InGaN barrier.The electron blocking efficiency of EBL is influenced by the thickness and alumi-num composition;the thicker EBL and the higher aluminum composition in AlGaN layer,the better electron confinement.However,on the other hand,this also increases the difficulty for holes to transport in valence band.Therefore,replacing the traditional GaN barrier with a lower band gap InGaN barrier is beneficial for enhancing the electron confinement without the price of blocking the holes.Moreover,the InGaN barriers may help the holes to transport in the QW region more easily because of the lower barrier height in valence band,which is also beneficial for reducing the electron cur-rent leakage.Besides,because of the better match of lattice constants between the InGaN barrier and InGaN well,the band bending situation is less severe,as shown in ͓Fig.3͑c ͔͒,which in turn results in less QCSE and better light emission efficiency.Figure 4shows the carrier concentrations of InGaN/InGaN structure near the active region at 150mA.It indi-cates that both electron and hole distributions in the QWs are more uniform than those of the InGaN/GaN structure.Note also that the electron leakage current is minimal,indicating the effectiveness of EBL in the InGaN/InGaN structure.The two peaks in the curve of electron concentration in the left-hand side and right-hand side of QW region represent the massive electrons accumulating in the first and lastbarriersFIG.2.͑Color online ͒͑a ͒Energy band diagram of original InGaN/GaN structure at 150mA.͑b ͒Enlarged drawing of the conduction band near EBL.͑c ͒Enlarged drawing near lastQW.FIG.3.͑Color online ͒͑a ͒Energy band diagram of proposed InGaN LED structure with In 0.1Ga 0.9N barriers at 150mA.͑b ͒Enlarged drawing of the conduction band near EBL.͑c ͒Enlarged drawing near lastQW.FIG.4.͑Color online ͒Carrier concentrations of InGaN/InGaN structure near active region at 150mA.which have sloped triangular barriers,as indicated in Fig.3, due to the large amount of polarization charges at GaN–InGaN and InGaN–AlGaN interfaces.Because of the advantages in carrier injection and elec-tron blocking,the performance of the InGaN/InGaN LED is expected to be superior to the conventional InGaN/GaN LED.Figure5shows the IQE and light output power as a function of current for the two LED structures under study, which indicates that the InGaN/InGaN LED has better light-ing efficiency,especially at high current.It is noteworthy that there is almost no efficiency droop for the InGaN/InGaN LED.Note that the performance improvement of InGaN/ InGaN LED obtained in this study is much better than that observed in the experimental results reported by Xu et al.,10 which might result from the material quality degradation caused by increased indium incorporation into the barriers.In summary,when the GaN barriers are replaced by the InGaN barriers in blue InGaN LEDs,the carriers injection is enhanced,the electron current leakage is markedly reduced, and the efficiency droop becomes less severe due to the rela-tively small polarization effect between the barriers and wells,and the lower potential barrier height in the active region for holes.1Y.-L.Li,Y.-R.Huang,and i,Appl.Phys.Lett.91,181113͑2007͒. 2M.H.Kim,M.F.Schubert,Q.Dai,J.K.Kim,E.F.Schubert,J.Piprek, and Y.Park,Appl.Phys.Lett.91,183507͑2007͒.3M.F.Schubert,J.Xu,J.K.Kim,E.F.Schubert,M.H.Kim,S.Yoon,S. M.Lee,C.Sone,T.Sakong,and Y.Park,Appl.Phys.Lett.93,041102͑2008͒.4J.Xie,X.Ni,Q.Fan,R.Shimada,Ü.Özgür,and H.Morkoç,Appl.Phys. Lett.93,121107͑2008͒.5Y.C.Shen,G.O.Mueller,S.Watanabe,N.F.Gardner,A.Munkholm,and M.R.Krames,Appl.Phys.Lett.91,141101͑2007͒.6I.V.Rozhansky and D.A.Zakheim,Phys.Status Solidi C3,2160͑2006͒. 7I.V.Rozhansky and D.A.Zakheim,Phys.Status Solidi A204,227͑2007͒.8A.David,M.J.Grundmann,J.F.Kaeding,N.F.Gardner,T.G.Mihopou-los,M.R.Krames,T.G.Mihopoulos,and M.R.Krames,Appl.Phys. Lett.92,053502͑2008͒.9A.Y.Kim,W.Go,D.A.Steigerwald,J.J.Wierer,N.F.Gardner,J.Sun, S.A.Stockman,P.S.Martin,M.R.Krames,R.S.Kern,and F.M. Steranka,Phys.Status Solidi A188,15͑2001͒.10J.P.Liu,J.-H.Ryou,R.D.Dupuis,J.Han,G.D.Shen,and H.B.Wang, Appl.Phys.Lett.93,021102͑2008͒.11APSYS by Crosslight Software Inc.,Burnaby,Canada͑http:// ͒.12I.Vurgaftman and J.R.Meyer,J.Appl.Phys.94,3675͑2003͒.13V.Fiorentini,F.Bernardini,and O.Ambacher,Appl.Phys.Lett.80,1204͑2002͒.14J.Xu,M.F.Schubert,A.N.Noemaun,D.Zhu,J.K.Kim,E.F.Schubert, M.H.Kim,H.J.Chung,S.Yoon,C.Sone,and Y.Park,Appl.Phys.Lett. 94,011113͑2009͒.FIG.5.͑Color online͒͑a͒IQE and͑b͒light output power as a function of current for the two LED structures under study.。