Electroluminescence (EL) from AlInGaN-InGaN multiquantum-well violet light-emitting diodes is investigated as a function of forward bias. Two distinct regimes have been identified: 1) quantum-confined Stark effect at low and moderately high forward biases; 2) heating effect at high biases. In the different regimes, the low-temperature EL spectra exhibit different spectral features which are discussed in detail. © 2007 IEEE.published_or_final_versio

Li, J

Lu, F

Shi, SL

Wang, YJ

Xu, SJ

Yang, H

Zhao, DG

Zhu, JJ

IEEE Photonics Technology Letters

HKU Scholars Hub

TitleViolet electroluminescence of AlInGaN-InGaN multiquantum-welllight-emitting diodes: Quantum-confined stark effect and heatingeffectAuthor(s) Li, J; Shi, SL; Wang, YJ; Xu, SJ; Zhao, DG; Zhu, JJ; Yang, H; Lu,FCitation Ieee Photonics Technology Letters, 2007, v. 19 n. 10, p. 789-791Issued Date 2007URL http://hdl.handle.net/10722/80612Rights©2007 IEEE. Personal use of this material is permitted. However,permission to reprint/republish this material for advertising orpromotional purposes or for creating new collective works forresale or redistribution to servers or lists, or to reuse anycopyrighted component of this work in other works must beobtained from the IEEE.IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 10, MAY 15, 2007 789Violet Electroluminescence of AlInGaN–InGaNMultiquantum-Well Light-Emitting Diodes:Quantum-Confined Stark Effect and Heating EffectJun Li, S. L. Shi, Y. J. Wang, S. J. Xu, D. G. Zhao, J. J. Zhu, H. Yang, and F. LuAbstract—Electroluminescence (EL) from AlInGaN–InGaNmultiquantum-well violet light-emitting diodes is investigated as afunction of forward bias. Two distinct regimes have been identi-fied: 1) quantum-confined Stark effect at low and moderately highforward biases; 2) heating effect at high biases. In the differentregimes, the low-temperature EL spectra exhibit different spectralfeatures which are discussed in detail.Index Terms—AlInGaN, electroluminescence (EL), InGaN,light-emitting diodes (LEDs), multiple quantum wells (QWs).SUCCESSFUL demonstration and commercialization of IIInitrides-based light-emitting devices including laser diodes(LDs) might be one of the biggest technology breakthroughsin the past decade due to their tremendous application value[1]. Usually, strained GaN–InGaN quantum wells (QWs)act as the light-emitting active layer in this new class oflight-emitting diodes (LEDs) and LDs. When many studieshave been continuously devoted to increase understandingof fundamental physical mechanisms of GaN–InGaN QWs[2]–[7], AlInGaN–InGaN QWs have attracted an increasinginterest due to the ability for emission wavelength and latticemismatch engineering [8]–[12]. To date, the knowledge ofoptical properties of AlInGaN–InGaN QWs is quite limitedpartially due to the difficult growth of these heterostructureswith device quality.In this letter, we present a study on the electroluminescence(EL) of AlInGaN–InGaN QW LEDs emitting violet light. Thespectral evidence for the quantum-confined Stark effect (QCSE)and heating effect has been observed.The sample used in the present study was grown on sapphireby low-pressure metal–organic vapor phase epitaxy. DuringManuscript received September 22, 2006; revised February 24, 2007. Thiswork was supported in part by the Hong Kong RGC-CERG under Grant HKU7036/03P.J. Li is with the Department of Physics, Fudan University, Shanghai 200433,China, and also with the Department of Physics, The University of Hong Kong,Hong Kong SAR, China.S. L. Shi and S. J. Xu are with the Department of Physics, The University ofHong Kong, Hong Kong SAR, China (e-mail: sjxu@hkucc.hku.hk).Y. J. Wang was with the Department of Physics, The University of HongKong, Hong Kong SAR, China. She is now with the Clarendon Laboratory, Ox-ford OX1 3PU, U.K.D. G. Zhao, J. J. Zhu, and H. Yang are with the State Key Laboratory onIntegrated Optoelectronics, Institute of Semiconductors, Chinese Academy ofSciences, Beijing 100083, China.F. Lu is with the Department of Physics, Fudan University, Shanghai 200433,China.Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LPT.2007.896575the growth of the sample, ammonia (NH3), trimethylaluminum(TMAl), trimethylgallium (TMGa), and trimethylindium(TMIn) were used as N, Al, Ga, and In precursors, respectively.High pure hydrogen gas was used as the carrier gas. Followinga 3- m-thick n -GaN (Si: 5 10 cm ) bottom contactlayer, three periods of undoped AlInGaN–InGaN QWs with28-nm-thick barrier layers and 2-nm-thick well layers weregrown. Then the top contact layer of 200-nm-thick p-GaN (Mg:cm ) was grown. During the growth of AlInGaNbarrier layers, the molar flows of TMAl, TMIn, and TMGawere controlled at 2.5, 22.7, and 9.47 mol/min, respectively.For the growth of InGaN well layers, molar flows of TMInand TMGa were 87.5 and 5.12 mol/min, respectively. Underthese conditions, the elemental compositions of barrier layersand well layers are Al In Ga N and In Ga N,respectively. The sample was finally processed into mesa de-vice structures. For the low-temperature EL measurements, thediode was mounted on the sample holder of a liquid nitrogenDewar. An Acton SP307 monochromator equipped with a1200-grooves/mm grating was employed to disperse the ELsignal from the top of diode and a Hamamatsu R928 PMTwas used to detect the dispersed signal. Variable-temperaturecurrent–voltage ( – ) curves of the diode, which was mountedon the cold finger of a Janis closed-cycle cryostat, were mea-sured by using Keithley source meter.First, EL spectra (thin solid lines) of the device were mea-sured at room temperature as a function of forward bias asshown in Fig. 1. The turn-on voltage of the diode is about 2.70 Vat room temperature. The spectra show oscillatory fine struc-tures which are frequently observed in the emission spectra ofInGaN-based LEDs due to the Fabry–Pérot interference effect[1], [13]. The period of oscillatory structures is about 5 nm. Inorder to analyze the spectral parameters such as peak position,integrated intensity, and full-width at half-maximum (FWHM)height of the EL spectra, the EL spectra were simply averagedand the resulting curves were shown as thick solid lines in thefigure. Unlike the observation in some GaN–InGaN QW LEDs[13], [14], the peak position of EL spectra of AlInGaN–InGaNQW LED does not exhibit a clear blues shift with the forwardbias (hence injection current) at room temperature. Reversely,red shift of the peak position begins accompanying with theevident broadening of the lineshape for the forward bias higherthan 3.75 V. In particular, the peak position red shifts fasterwhen the bias is beyond 4.90 V. At the same time, the ELintensity starts to drop. All these behaviors can be consistentlyexplained by the heating effect of the device, which is evi-denced by the low-temperature EL and variable-temperature– data discussed below.1041-1135/$25.00 © 2007 IEEE790 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 10, MAY 15, 2007Fig. 1. Representative room-temperature EL spectra (thin solid lines) of thedevice at different forward biases. The influence of interference fringes due tothe Fabry–Pérot effect can be efficiently eliminated through conducting averageto the experimental spectra, as indicated by thick solid lines in the figure. Thepeak position is indicated by a vertical arrow.Fig. 2. Representative 77 K EL spectra (thin solid lines) of the device at dif-ferent forward biases. Averaging curves (thick solid lines) of the experimentalspectra are also given for eliminating the influence of interference fringes dueto the Fabry–Pérot effect. The peak position is indicated by a vertical arrow.Fig. 2 shows the EL spectra of the device at 77 K under dif-ferent forward biases. The spectral parameters extracted fromthe spectra were shown in Fig. 3. Clearly, blue shift of the ELpeak takes place with increasing the forward bias lower than5.60 V. However, when the bias is higher than 6.0 V, the ELred shifts rapidly. A more careful look at the spectral parame-ters’ evolution with the forward bias tells us that the three dif-ferent evolution regions can be resolved: 1) the slow blue-shiftregion for the low biases below 5.20 V, i.e., the peak positionblue shifts from 3.006 eV at 3.50 V to 3.019 eV at 5.20 V, ac-companying with linear increase of the emission intensity andalmost unchanged FWHM; 2) the fast blue-shift region for themoderately high biases from 5.20 to 5.60 V, for example, theblue-shift amount is as much as about 18 meV in this bias range,and both intensity and FWHM increase quite rapidly; 3) the redshift region for the high biases beyond 6.0 V, simultaneously,the emission intensity starts to decreases while the FWHM con-tinuously increases.The blue shift of the EL peak position with the forward biascan be well understood in terms of the QCSE effect discoveredFig. 3. Peak positions (solid circles), integrated intensity (solid squares), andFWHM (solid up triangles) extracted from the 77 K EL spectra.by Miller et al. [15]. The QCSE effect manifests in a red shiftof the energy of excitons confined in quantum structures underaction of external or internal perpendicular electric field. It iswell known that the lattice mismatch strain usually exists in theInGaN-based QWs including AlInGaN–InGaN QWs [11], [16].Due to the piezoelectric nature of the InGaN alloy, the mismatchstrain can introduce a huge electrical field in the well regions.Another origin causing the built-in electric field in the active re-gion is due to the P-N junction effect of the p-i-n structured LEDdiode. When the carriers are electrically or optically injectedinto the InGaN well regions, this built-in electric field couldbe effectively screened out by the carriers. As a result, a blueshift of the exciton energy and thus photonic energy of the ex-citonic absorption or emission should take place [16], [17]. Forthe AlInGaN–InGaN QW LEDs under study, the observed slowblue shift of EL peak in the low bias region can be attributedto the screening of the built-in electric field by the electricallyinjected carriers. Such an attribution is supported by the –characteristics of the diode shown in Fig. 4. At 77 K, the in-jection current of the diode steadily grows as the forward biasincreases below 5.20 V.When the forward bias becomes moderately high, the injec-tion current and, hence, carrier concentration rapidly increases.The injection of high density of carriers can lead to the pres-ence of band-filling effect. The spectral features for the band-filling effect include the significant increase of the EL inten-sity at higher energy side and noticeable shift of the EL peaktowards the higher energy side [5], [13], [18], as observed inFig. 3. It is worth mentioning that the bandgap renormaliza-tion could occur when the high density of carriers sustain in theInGaN active layers [5]. The bandgap renormalization tends tonarrow the bandgap. For the case studied here, it is difficult todistinguish the bandgap renormalization from the heating effectthat will be discussed below.LI et al.: VIOLET EL OF AlInGaN–InGaN MULTIQUANTUM-WELL LEDs 791Fig. 4. I–V characteristics of the diode at different temperatures. The insetshows I–V curve of the diode measured at 77 K.Under high forward bias, i.e., higher than 6.0 V, the injec-tion current and thus carrier density becomes very large. Undersuch conditions, nonradiative recombination of a large numberof carriers will make the device hot quickly. Increase of lat-tice temperature will result in shrinking of the material bandgapand broadening of the emission peak due to phonon scattering[19]. That is why the EL peak rapidly red shifts and the inten-sity drops at 77 K under the condition of large injection current.At room temperature, the situation becomes more serious. Onone hand, due to the thermal activation of more nonradiativecenters and increase of phonon-assisted nonradiative recombi-nation, the nonradiative recombination rate of carriers in semi-conductors at room temperature is usually larger than at lowtemperature. In other words, LEDs are much easier to becomehot at room temperature and under the same carrier injection.On the other hand, the injection current of the diode is a strongfunction of temperature as shown in Fig. 4. For example, the in-jection current of the diode at 300 K under 4.0 V is comparablewith that at 77 K under 5.1 V. Therefore, both the large injec-tion current and nonradiative recombination rate of the diode atroom temperature can lead to a significant heating effect. As aconsequence, the blue shift of the EL peak of AlInGaN–InGaNQW LEDs becomes nonobservable at room temperature.In summary, the forward-bias voltage dependence of ELspectra of AlInGaN–InGaN QW LEDs has been studied atroom temperature and low temperature. Three distinct evolu-tion periods, the efficient screening of the built-in electric field,the band-filling effect, and heating effect, have been revealedaccording to the observed spectra features of device EL spectrain various bias ranges. The data provide useful information forthe design and performance improvement of AlInGaN–InGaNQW LEDs.ACKNOWLEDGMENTS. J. Xu would like to thank V. A. L. Roy for his help in the– measurements of the diodes and R. X. Wang for helpfuldiscussions. The authors would like to thank Y. Wen andX. M. Dai for participating in the variable-temperature –measurements.REFERENCES[1] S. Nakamura, S. Pearton, and G. Fasol, The Blue Laser Diode. Berlin,Germany: Springer-Verlag, 2000.[2] K. Okamoto, A. 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Violet electroluminescence of AlInGaN-InGaN multiquantum-well light-emitting diodes: Quantum-confined stark effect and heating effect

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Violet electroluminescence of AlInGaN-InGaN multiquantum-well light-emitting diodes: Quantum-confined stark effect and heating effect

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