Temperature-dependent electroluminescence from InGaN_GaN green light-emitting diodes on silicon

Temperature-dependent electroluminescence from InGaN/GaN green light-emitting diodes on silicon with different quantum-well structuresGuangxu Wang,Xixia Tao,Junlin Liu and Fengyi JiangNational Engineering Technology Research Center for LED on Si Substrate,Nanchang University,Nanchang330047,People’s Republic of ChinaE-mail:guangxuwang@Received10July2014,revised8November2014Accepted for publication11November2014Published15December2014AbstractTemperature-dependent electroluminescence from InGaN/GaN light-emitting diodes(LEDs)grown on Si(111)are investigated.With the increase of current density,internal quantumefficiencies(IQEs)firstly rise accompanied by full width at half maximum(FWHM)shrinkageand then IQEs droop combined with FWHM broadening are presented.With the decline oftemperature,both the maximum of IQEs accompanying the minimum of FWHM shift towardsthe direction of low current density.Moreover,the maximum of IQEs shift faster than theminimum of FWHM for single quantum well LED.Furthermore,it was found that the quantumwell close to n-GaN has priority to radiate in small current injection,especially at lowtemperature(100K)for multiple quantum wells LED.Keywords:electroluminescence,IQEs,FWHM,quantum wells,light-emitting diodes(Somefigures may appear in colour only in the online journal)1.IntroductionThe Nobel Prize in Physics2014was awarded for the invention of efficient blue light-emitting diodes,which has enabled bright and energy-saving white light sources.At the current stage of development,light-emitting diodes(LEDs) are discussed as an energy saving replacement for incandes-cent light bulbs,while their price needs to be reduced and light quality needs to be improved for LEDs to be introduced into the general illumination market.InGaN-based LEDs grown on Si substrates are certainly an excellent candidate for their low-cost materials,large substrate diameters and easy-to-operate manufacture process etc.However,there are also lots of challenges to get high efficient InGaN-based LEDs structures on Si for the large lattice constant mismatch and thermal coefficient mismatch between GaN and Si[1].In the past decade,great efforts have been made to grow InGaN-based blue and green LEDs on Si substrates[2].Light outputs in the magnitude of microwatt to milliwatt for LED on Si using different techniques have also been reported[3,4].In 2009,thefirst blue power LED on Si for commercial use was reported with output power of457mW at a driving current of 350mA emitting in453nm for LED die of1mm2after encapsulation[2,5].Zhu et al,fabricated blue and green LED structures on Si(111)substrates,and observed the light out-puts decreased dramatically as the emission wavelength increased from455nm to510nm[6].High efficient green LED on Si has not been reported yet,and the factors that affect the luminescence efficiency of LEDs on Si substrate have not been investigated clearly.The green quantum wells(QWs)have higher In-com-ponents with more serious piezoelectric polarization effect compared with blue QWs,which increases the band tilt of QWs.That affects carrier distribution,and determines the luminous efficiency of QWs[7].In addition,it was found that several peaks appeared in the emission spectrum of multiple quantum wells(MQWs)and varied with the change of current density[8,9],from which it was considered thatmultiple Semicond.Sci.Technol.30(2015)015018(6pp)doi:10.1088/0268-1242/30/1/015018peaks were emitted from different QWs.Therefore,identify-ing which well the light comes from is very important for improving the luminous efficiency of MQW LED.In this paper,we investigate the temperature-dependent electro-luminescence properties of single quantum well(SQW)and MQW green LEDs grown on Si substrate;dependences of internal quantum efficiencies(IQEs)and full width at half maximum(FWHM)on the injection current density are pre-sented.And we have experimentally confirmed that the quantum well close to n-GaN has priority to radiate in small current injection at low temperature(100K).Such experi-mental phenomena will play a guiding role in understanding the luminous mechanisms of LEDs.2.ExperimentsInGaN/GaN SQW and MQW LED,structures are grown on 0.2mm×0.2mm trenched Si(111)substrates in a7×2inch close-coupled showerhead MOCVD reactor.The epitaxial structure consists of a0.06μm thick AlN nucleation layer grown at900°C,and a0.5μm-thick GaN buffer layer grown at1100°C,followed by an0.05μm thick AlN/AlGaN inter-mediate layer grown at930°C,and a2.8μm thick Si-dopedGaN layer grown at1150°C,then an0.08μm-thick InGaN prepare-layer grown at1100°C,subsequently grown InGaN/ GaN QW region,and a0.015μm thick p-type AlGaN(20% Al)electron blocking layer(EBL)and a0.12μm thick p-type GaN layer grown at1000°C for the LED.Other details of the growth process can be found elsewhere[1,10–12].Three samples with different QWs structures have been grown: Sample I includes three cycles of InGaN/GaN(2nm/14nm) MQW,and subsequently grown an InGaN/GaN(2.2nm/ 5nm)structure,where the growth temperature of InGaN is 770°C.Sample II is the same as sample I,except that thefirst period of InGaN is grown at temperature of775°C.Sample III only includes one cycle of InGaN/GaN(2.2nm/5nm) structure,where the InGaN growth temperature is760°C. Schematicfigures of quantum-well structures of three samples are shown infigures1(b)–(d),Samples I,II and III are named MQW-LED,VWT(varied well-temperature)-LED and SQW-LED,respectively.For three samples,SQW-LED and MQW-LED can form a nice contrast in luminous efficiency and luminescence spectra.It is well known[9]that holes are injected and distributed predominantly in the QW close to the p-GaN layer when the LED is in working condition.There-fore,we designed the InGaN/GaN(2.2nm/5nm)structure after three QWs.One side,the thicker InGaN well layer can improve the carrier recombination rate.On the other side,the thickness of GaN-barrier close to p-type layer is thinner than other barriers,which can improve the efficiency of hole injection due to the low mobility of holes in p-GaN.And VWT-LED is designed for identifying which well the light comes from.In order to have the same wavelength,the growth temperature of InGaN in SQW-LED is lower,10°C, than MQW-LED.The grown wafers are processed to vertical N-side up chip structure by using a thin-film approach[13–16]and schematicfigure of chip struture was shown in figure1(a).Wafers were bonded to other p-type(100)silicon substrates by Au–Au diffusion,then N-polar n-type GaN on topside was obtained after removing Si(111)substrate by using HNO3:HF:C2H4O2=5:2:2solutions.After the removal of AlN buffer layer by inductively coupled plasma,SiO2 passivation layer was deposited by PECVD(plasma enhanced chemical vapor deposition)and Al(1500nm)/Ti(100nm)/Au (1500nm)electrode layers were deposited by electron beam evaporation.At last,the wafer was diced into0.2×0.2mm2 LED chips.In this study,electroluminescence(EL)properties of LEDs driven by a dc source are evaluated by using a Keithley2635Source Meter and a compact array spectro-meter(CAS)140CT.Temperature-dependent(100∼350K) EL properties are measured in a heating-cooling system(K-20,MMR Technology).3.Results and discussionsThe dependences of internal quantum efficiencies(IQEs)and full width at half maximum(FWHM)on current density under different temperatures are investigated.We assume the IQE at very small current density and low temperature is 100%.The light extraction efficiency for these type samples is determined to be∼20%(without the use of additional tech-niques to enhance the output coupling,such as surface roughness and Ag-based reflector etc)[17].The values of FWHM are obtained by Gaussfit on each emission spectrum [18].Figure2plots IQEs and FWHM as functions of injection current density for(a)–(c)SQW-LED and(d)–(f)MQW-LED at350K,200K and125K,respectively.With the increase of current density,IQEsfirstly rise and then droop,while FWHMfirstly shrinks and then broadens.We dividethe Figure1.(a)Schematicfigure of thin-film LED chip,(b)–(d) structure schematic of quantum wells for SQW-LED,MQW-LED and VWT-LED,respectively.curves of IQEs and FWHM into three regions marked by (I)–(III)in image (a).In region (I),the injection carrier concentration is low,the rise of IQEs accompanying the shrink of FWHM can be attributed to carrier screening effect.It is reported that the carrier screening effect can offset the polarization charge,level the energy band bending and weaken the carrier space separation [19].With the increase of current density,carrier screening effect gets saturated in region (II),where IQEs and FWHM are both close to their extreme value.In region (III),the carrier filling effect at high injection may contribute to the drop of IQEs,which can increase the rate of auger-like nonradiative recombination.Meanwhile,it is known that carrier filling can also broaden FWHM at high current injection level [20,21].Furthermore,another important rea-son for IQE droop is carrier leakage especially at low tem-perature [22,23].In figure 2,both the current densities corresponding to the minimum of FWHM (marked by the red dashed line)and the maximum of IQE decrease with the decline oftemperature.This is because the defect-related nonradiative centers become more and more inactive as the temperature decreases,which leads to a higher effective carrier con-centration at a lower temperature under the same current injection.Moreover,we also find that the maximum of IQEs and the minimum of FWHM both shift towards the direction of low current density with the decline of temperature.And the maximum of IQEs shifts faster than the minimum of FWHM for SQW-LED,which is because carrier localization in potential fluctuation of QW becomes more severe at a lower temperature [24].According to the literature,the luminous ef ficiency related to carrier localization in potential fluctuation is finer [25,26],and carrier filling in localized state can broaden the FWHM [27,28]at low injection currents.Different from the SQW-LED,there are two in flexions in curves of FWHM especially at low temperature for MQW-LED as shown in figures 2(d)–(f).We conclude that the in flexions are mainly caused by the change of the locations of dominant emitting QWs at different range of currentdensity.Figure 2.Dependences of FWHM and IQEs on the injection current density for (a)–(c)SQW-LED and (d)–(f)MQW-LED at 350K,200K and 125K,respectively.Both SQW and MQW LEDs are emitting in 522nm at 52A cm −2,300K.The curves of IQE and FWHM in image (a)are divided into three regions marked by (I)–(III).The inset of image (e)illustrates that the values of FWHM are obtained by the Gauss fit on the emission spectrum.The red dashed lines mark the minimum of FWHM curves.To demonstrate the above conclusion,the EL spectra of SQW-LED,MQW-LED and VWT-LED were measured.Before EL spectra test,a roughening process on the n-GaN surface was adopted to eliminate the Fabry –Perot interference oscillations [29].EL spectra at 100K of three samples are presented in figure 3.Inspection of the figure reveals that the spectra of SQW LED consist of one Gauss-like wave peak.Meanwhile,three Gauss-like wave peaks marked by W1,W2and W3can be extracted from spectra of VWT-LED.For VWT-LED,the first QW (close to n-GaN)was grown at a higher growth temperature,so the wavelength emitting from the first well is shorter than that from other wells.Thus,the peak marked by W1is emitted from the first well.With the increase of current density,carrier recombination in the last well (close to p-GaN)is dominant [30].For VWT-LED and MQW-LED,the last well is thicker than the other three wells.And by SIMS (secondary ion mass spectroscopy)test,we can know that the In-components of the last well are higher than in the other three wells.So the wavelength emitting from the last well is the longest.Thus,the peak marked by W2is identi fied as emitting from the last well.With the further increase of current density,carriers injected into the last well become gradually saturated,which enhances the rate of auger nonradiative recombination in the last well.In this case,the radiative recombination and emission of the middle wells may be dominant in region W3.Spectra of MQW-LED show the similar but inconspicuous peak evolution with VWT-LED.It seems that this is not consistent with the results reported in literature,which is that the last well would emit fist because of the small mobility of hole at low current injection [30].In our opinion,the inconsistency is probably due to the adjustment of the temperature on the stress,and in fluences the evolution of the band of quantum wells,which is signi ficant for electric injection recombination.However,it also has to be pointed out that the literature has not reported EL spectra at 100K temperature.Temperature-dependent EL spectra of VWT-LED at current density of 0.5A cm −2are shown in figure 4.It is found that peak marked by W1emitted from the first well is inconspicuous when the temperature is higher than 200K,which is due to the fact that the defect density of the first well would be frozen at low temperature.The effect of quantum wells band evolution adjusted by temperature on the recom-bination can also be illustrated in figure 4.4.ConclusionIn summary,InGaN/GaN green LEDs are fabricated on sili-con substrates and their temperature-dependent electro-luminescence properties are investigated.With the increase of current density,IQEs rise accompanied by FWHM shrinkage firstly and then IQEs droop combined with FWHM broad-ening are presented at any given temperatures in the paper.With the decline of temperature,the maximum of IQEs and the minimum of FWHM both shift towards the direction of low current density.Furthermore,the maximum of IQEs shift faster than the minimum of FWHM for SQW LEDs,whichFigure 3.EL spectra at 100K of (a)SQW-LED,(b)VWT-LED and (c)MQW-LED.Different wave peaks dominate in emission spectrumwith increasing current density in region W1,W2andW3.Figure 4.Temperature-dependent EL spectra of VWT-LED atcurrent density of 0.5A cm −2.Two peaks are marked by W1,W2.may be due to carrier localization in potentialfluctuation of QW becoming more severe at a lower temperature.It is found that oscillation-like behavior appearing in the current-depen-dent FWHM curves of MQW-LED can be attributed to that mulitple peaks in spectra emitted from different wells at dif-ferent current density.And we present that the quantum well close to n-GaN has priority to radiate in small current injec-tion at low temperature(100K),which does not conflict with the viewpoint that QWs near p-GaN have priority to emit at working conditions.This is a further understanding of lumi-nescence mechanism of MQWs LED.And the viewpoint of this paper and the point of the previously published paper[8] written by our group complement each other.However,a theoretical model should be set up to support the experimental phenomena of this work,and temperature-dependent photo-luminescence properties were not taken into account,which will be discussed in the future.AcknowledgmentsThis work was supported by the State Key Program of National Natural Science of China(Grant No.61334001),the National Science Foundation 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