In this letter, the reliability of green InGaN-GaN light-emitting diodes (LEDs) has been analyzed by correlating the defect density of wafers with various device parameters, including leakage current, 1/f noise, and degradation rate. It was found that as the wavelength of green LEDs increases from 520 to 550 nm by increasing the indium content in the quantum wells, the defect density also increases, thus leading to larger leakage current, enhanced noise magnitude, and shortened device lifetime. © 2009 IEEE.published_or_final_versio

Choi, HW

Lai, PT

Li, ZL

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Title A reliability study on green InGaN-GaN light-emitting diodesAuthor(s) Li, ZL; Lai, PT; Choi, HWCitation Ieee Photonics Technology Letters, 2009, v. 21 n. 19, p. 1429-1431Issued Date 2009URL http://hdl.handle.net/10722/73854Rights IEEE Photonics Technology Letters. Copyright © IEEE.IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 19, OCTOBER 1, 2009 1429A Reliability Study on Green InGaN–GaNLight-Emitting DiodesZ. L. Li, P. T. Lai, Senior Member, IEEE, and H. W. Choi, Senior Member, IEEEAbstract—In this letter, the reliability of green InGaN–GaNlight-emitting diodes (LEDs) has been analyzed by correlating thedefect density of wafers with various device parameters, includingleakage current,   noise, and degradation rate. It was foundthat as the wavelength of green LEDs increases from 520 to 550 nmby increasing the indium content in the quantum wells, the defectdensity also increases, thus leading to larger leakage current,enhanced noise magnitude, and shortened device lifetime.Index Terms—Light-emitting diode (LED), reliability.I. INTRODUCTIONT HE development of phosphor-free white light-emittingdiodes (LEDs) relies on the availability of high-qualitygreen LEDs at longer wavelengths. LEDs with wavelengthcentered at around 555 nm are particularly important inhigh-brightness RGB-based white lighting systems, coincidingwith the peak photopic response of human eyes. However, it ischallenging to grow InGaN–GaN LED structures for emissionat longer wavelengths due to the difficulty associated with theincorporation of indium (In). This difficulty originates from thestrain condition in the quantum wells (QWs), which limits theextent of In incorporation. Higher In content in a well leads tohigher compressive strain in the layer, resulting in inefficientIn incorporation [1].On the other hand, higher In composition is essential to re-alize longer wavelength emission; it also becomes a threat to thedevice performance and reliability. First, the higher In contentin the multiquantum-wells (MQWs) can introduce large piezo-electric field induced by the large strain. This piezoelectric fieldreduces the overlap of the electron and hole wave functions,and thus results in reduced internal quantum efficiency in greenLEDs [2]. Second, the large lattice mismatch can give rise tohigh defect concentration in the structure in the form of disloca-tions, which result in degraded crystal quality. As a result, thehigher dislocation density of green LED introduced by In incor-poration can often deteriorate its performance and reliability.In this letter, we report on the study of reliability of greenInGaN–GaN LEDs by correlating the defect concentration inwafers of wavelength ranging from 520 to 550 nm with theirlow-frequency noise spectrum and 1000-h lifetime data. Surfacescan by atomic force microscopy (AFM) is used to quantify theManuscript received April 09, 2009; revised June 12, 2009. Current versionpublished September 18, 2009.The authors are with the Department of Electrical and Electronic Engineering,The University of Hong Kong, Hong Kong, Hong Kong (e-mail: hwchoi@hku.hk).Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieeee.org.Digital Object Identifier 10.1109/LPT.2009.2028155exposed defects after reactive ion etching (RIE) to compare thedefect density of the wafers. The electrical properties and op-tical performances of the devices are evaluated. The devices aresubsequently subjected to a 1000-h continuous lifetime test. Weattempt to explain the observed phenomenon in terms of carrierdynamics.II. EXPERIMENTAL DETAILSThe structure of the LED wafers has a GaN buffer layer grownon c-plane sapphire substrate, followed by an n-type GaN layer,active layers with InGaN–GaN as quantum well, and cappedwith a top p-type GaN contact. The emission wavelengths of thewafers are approximately 520, 540, and 550 nm, respectively,obtained by varying the In content from 0.25 to 0.30. Detailsof the growth will be reported elsewhere. The LEDs were fabri-cated by a standard microfabrication process beginning with thedeposition of a semitransparent current-spreading layer com-posed of Ni–Au (10 nm/10 nm) by e-beam evaporation, fol-lowed by annealing in an oxygen environment at a temperatureof 550 C for 5 min. A 500 500 m emission active areawas defined by photolithography and dry etched using a chem-istry comprising CHF and Ar to expose the n-contact regions.Photolithography was again carried out to define the p-pad andn-pad areas. A bilayer of Ti–Al (50 nm/350 nm) was depositedas p-pads and n-pads, and then annealed in N at 350 C for1 min. The chips were diced by laser micromachining and pack-aged into TO-cans. After wire bonding, a silicone encapsulantwas applied to protect the devices.The defect densities of the LED structures were estimated byan AFM using a Seiko Instruments Nanopics system, after de-fect exposure by a plasma treatment similar to the mesa etch.The electrical properties, including the current–voltage (I–V)characteristics and leakage current of three packaged devices,were measured with an HP 4156A precision semiconductor pa-rameter analyzer, while the noise spectra were measuredby a BTA 9603 FET noise analyzer. The optical performancesof the devices were evaluated with an Ocean Optics HR2000fiber-coupled spectrometer. The devices, biased at 20 mA, weresubsequently subjected to a 1000-h continuous stress test atroom temperature.III. RESULTS AND DISCUSSIONThe AFM surface scans in Fig. 1 are used for defect estima-tion of the LED wafers. Defects propagate from the site wherethey are generated, forming 2-D dislocations. These defects in-clude those generated at the GaN–sapphire interface due to lat-tice mismatch and also those generated within the MQWs dueto strain. Since dislocations terminate at an interface as voids,counting the number of such voids within unit area provides1041-1135/$26.00 © 2009 IEEE1430 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 19, OCTOBER 1, 2009Fig. 1. 20  20 mm AFM surface scan of the (a) 520-, (b) 540-, and (c) 550-nmInGaN–GaN LED wafers.Fig. 2. I–V characteristics of the three green LEDs.data on the dislocation density. Nevertheless, such voids are ofsubnanometer dimension. In order to “visualize” these features,they can be enlarged by a plasma treatment. By etching away50 nm of p-type GaN from the surface of the wafer, the de-fects are intentionally enlarged due to preferential etching of re-cessed areas [3], thereby resulting in the formation of V-shapedetch pits. Etch pits are readily revealed by AFM scanning of theplasma-treated surfaces. The dislocation density of the waferscan be thus estimated from the AFM scan, making it an accurateand convenient method for determining defect density. In the20 m 20 m surface scans shown in Fig. 1, the dark dots rep-resent these etch pits. It is estimated that the defect density of the520-, 540-, and 550-nm wafers is about ,and /cm , respectively. These numbers tally wellwith the prediction that In incorporation leads to higher defectdensity.The electrical characteristics of the LED devices fabricatedfrom these wafers are measured. The I–V characteristics of thethree LEDs are presented in Fig. 2. The leakage currents of thedevices have been extracted at a reverse bias voltage of 4 V.Fig. 3.    noise spectrum of the green LEDs.As the wavelength of the LEDs increases, the leakage currentalso increases from 0.99 mA in the 520-nm LED to 2.29 mA inthe 540-nm LED and 4.45 mA in the 550-nm LED. The slopeof the linear region of the I–V curve (in the forward bias re-gion) has been interpolated to determine the static resistance ofthe devices. As evident from the I–V plot, the slope increasesas the wavelength increases. The extracted static resistances are23.5, 33.3, and 52.6 for the 520-, 540-, and 550-nm LEDs, re-spectively. This consistent trend in the leakage current and thestatic resistance can be linked to the increasingly higher defectconcentration as the In incorporation increases. The dislocationsact as vertical current leakage pathway, while the defects lead tocarrier scattering and hence higher resistance. To verify this fur-ther, additional experiments are conducted to study the carrierproperties.Since low-frequency noise is closely related to the presence ofimpurities and defects in semiconductors, it can more accuratelyrepresent the internal crystal quality than other electrical param-eters [4]. The noise spectra of the three LEDs are measured andplotted in Fig. 3. The system floor calibration is conducted fordc bias calculation and noise correction. The range of fre-quency measured is between 3.15 and 105 Hz. The slope can beextracted by the supplied software automatically. In the figure,it is observed that both the magnitude and slope of 520-nm LEDare lower than the 540- and 550-nm LED.In an LED structure, the current spectral density can be ex-pressed by the following equation [5]:(1)where(2)The first term of (1) represents the near-equilibrium steady-state condition, whereby there is no net accumulation of chargesand the generation rate is equal to the recombination rate. Thevariable is the Hooge parameter [6], which depends on theLI et al.: RELIABILITY STUDY ON GREEN InGaN–GaN LIGHT-EMITTING DIODES 1431Fig. 4. 1000-h lifetime degradation test of the green LEDs.quality of the crystal [7] and the scattering mechanisms. A largervalue generally implies poorer crystal quality, and increasesthe magnitude of the noise spectrum. is the current throughthe device; is the lifetime of minority carrier; is the trapdensity, which can increase the magnitude of the noise spectrumwhen the trap density increases; and the other terms have theirusual meanings. The measured noise spectrum thus indi-cates that the crystal quality deteriorates as the emission wave-length increases from 520 to 550 nm due to the increased noisemagnitude, thus supporting the postulation of enhanced defectdensity with increasing In incorporation. Apart from the noisemagnitude, it is also observed that the magnitude of the slope ofthe noise spectrum increases with increasing wavelength. Thismay be explained in terms of occupation of traps within thesemiconductor material. As defect density in the InGaN–GaNlayers increases, transitions between traps become more fre-quent, leading to a reduction in electron mean free time. There-fore, as frequency increases, these trapped electrons cannot re-spond quickly enough and the number of free electron in thecrystal decreases, which subsequently leads to a decrease in thecurrent spectral density at that particular frequency. With moredefects in the crystal, the spectrum decreases at a faster rate, cor-responding to an increase in the slope, which is consistent withour measured data.The results can also be correlated to the lifetime of thedevices. Fig. 4 shows the results of a 1000-h reliability test,whereby a constant bias current of 20 mA was applied to thedevices. While the 520-nm LED maintains 95% of its initialoptical power after 1000 h, the number drops to 91% and 82%for the 540- and 550-nm LEDs, respectively. Assuming lin-earity, the L70 lifetime (L70 lifetime is the time to 70% lumenmaintenance, as proposed by ASSIST [8]) for our 520-, 540-,and 550-nm LEDs is 6000, 3750, and 1667 h, respectively.Green LEDs with increasing wavelength are therefore expectedto have increasingly shorter lifetime.Despite the attractiveness of deep green LEDs in terms ofphotopic response, there are many other issues to be considered.As the In content in the QW regions are increased, a significantincrease in defect density results. These defects act as nonra-diative recombination centers, degrading optical performance.However, there are further side effects. These defects serve asvertical conduction pathways, creating large leakage currents,which subsequently affect the device lifetime. It is therefore per-tinent to reduce the defect densities before longer wavelengthwafers become feasible. One possible approach is to grow theseepilayers on bulk GaN substrates. Based on our experimentaldata, it is deduced that the lifetime of LEDs can be predictedfrom their electrical parameters, particularly from a compara-tive study of their noise spectra.IV. CONCLUSIONIn summary, the role of In incorporation in green InGaN–GaNLED has been investigated. While higher In concentration inthe QW produces LEDs with longer wavelength (particularly,wavelengths around 550 nm with high photopic response forphosphor-free white LEDs), additional defects are introducedinto the devices due to the strain induced. These defects playmultiple roles in affecting device performance, including actingas nonradiative recombination centers and leakage pathways forvertical conduction. Consequently, the reliability and lifetime ofdevices are deteriorated. We propose the use of low-frequencynoise for the prediction of device lifetime, as it is a tool that issensitive to defect density in the material.REFERENCES[1] C. F. Huang, T. Y. Tang, J. J. Huang, W. Y. Shiao, C. C. Yang, C. W.Hsu, and L. C. Chen, “Prestrained effect on the emission properties ofInGaN/GaN quantum-well structures,” Appl. Phys. Lett., vol. 89, pp.051913-1–051913-3, 2006.[2] R. D. Dupuis, J. B. Limb, J. P. Liu, J. H. Ryou, C. Home, and D.Yoo, “InGaN MQW green LEDs using p-InGaN and p-InGaN/p-GaNsuperlattices as p-type layers,” Proc. SPIE, vol. 6894, pp. 68941D-1–68941D-8, 2008.[3] H. W. Choi, C. Liu, M. G. Cheong, J. Zhang, and S. J. Chua, “Exposureof defects in GaN by plasma etching,” Appl. Phys. A, Solids Surf., vol.80, pp. 405–407, 2005.[4] F. N. Hooge, T. G. M. Kleinpenning, and L. K. J. Vandamme, “Experi-mental studies on    noise,” Rep. Progr. Phys., vol. 44, pp. 479–532,1981.[5] J. Hu, L. Du, Y. Q. Zhang, J. L. Bao, and J. Zhou, “Noise as a repre-sentation for reliability of light emitting diode,” Acta Phys. Sin., vol.55, pp. 1384–1389, 2006.[6] F. N. Hooge, “   noise sources,” IEEE Trans. Electron. Devices, vol.41, no. 11, pp. 1926–1935, Nov. 1994.[7] A. Balandin, K. L. Wang, A. Svizhenko, and S. Bandyopadhyay, “Thefundamental    noise and the hooge parameter in semiconductorquantum wires,” IEEE Trans. Electron. Devices, vol. 46, no. 6, pp.1240–1244, Nov. 1999.[8] “LED Life for General Lighting: Definition of Life” Alliance for Solid-State Illumination Systems and Technologies, 2005.

A reliability study on green InGaN-GaN light-emitting diodes

This journal vol. contains invited and contributed papers from the 16th international conference on 'Microscopy of Semiconducting Materials' ... 2009Although InGaN/GaN green light-emitting diodes (LEDs) are widely available, it is still challenging to grow green LED structures for emission at longer wavelengths due to the difficulty associated with the incorporation of In. The higher concentration of In may also affect the performance and reliability of the device. The reliability of green GaN LEDs with three different indium doping concentrations, with centre-wavelength of 520 nm, 540 nm and 550 nm, is studied in this paper. The electrical properties, including I-V characteristics, leakage current and 1/f noise were measured. The optical performance of the devices was also evaluated. The devices were subsequently subjected to a 1000 hours continuous stress test. The defect densities of the LED structures were also determined. Our results show that the 520 nm LED, which contains the lowest indium concentration in its quantum wells, produces highest optical output power at 20 mA. It also degrades slower than 540 nm and 550 nm LEDs. © 2010 IOP Publishing Ltd.link_to_OA_fulltex

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Reliability study on green InGaN/GaN light emitting diodes

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A reliability study on green InGaN-GaN light-emitting diodes

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