Investigation of the Electroluminescence Mechanism of Gan-Based Blue and Green Light-Emitting Diodes with Junction Temperature Range of 120–373 K

applied sciences

Article Investigation of the Electroluminescence Mechanism of GaN-Based Blue and Green Light-Emitting Diodes with Junction Temperature Range of 120–373 K

Sai Pan, Chenhong Sun, Yugang Zhou * , Wei Chen, Rong Zhang and Youdou Zheng

Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials and the School of Electronic Science and Engineering, Nanjing University, Nanjing 210023, China; [email protected] (S.P.); [email protected] (C.S.); [email protected] (W.C.); [email protected] (R.Z.); [email protected] (Y.Z.) * Correspondence: [email protected]

Received: 10 December 2019; Accepted: 3 January 2020; Published: 8 January 2020

Abstract: Junction temperature (Tj) and current have important effects on light-emitting diode (LED) properties. Therefore, the electroluminescence (EL) spectra of blue and green LEDs were investigated in a Tj range of 120–373 K and in a current range of 80–240 mA based on accurate real-time measurements of Tj using an LED with a built-in sensor unit. Two maxima of the emission peak energy with changing Tj were observed for the green LED, while the blue LED showed one maximum. This was explained by the transition between the donor-bound excitons (DX) and free excitons A (FXA) in the green LED. At low temperatures, the emission peak energy, full width at half maximum (FWHM), and radiation power of the green LED increase rapidly with increasing current, while those of the blue LED increase slightly. This is because when the strong spatial potential fluctuation and low exciton mobility in the green LED is exhibited, with the current increasing, more bonded excitons are found in different potential valleys. With a shallower potential valley and higher exciton mobility, excitons are mostly bound around the potential minima. The higher threshold voltage of the LEDs at low temperatures may be due to the combined effects of the band gap, dynamic resistance, piezoelectric polarization, and electron-blocking layer (EBL).

Keywords: junction temperature; high current; electroluminescence; InGaN; excitons

1. Introduction GaN-based light-emitting diodes (LEDs) have been widely used for industrial lighting and outstanding backlight in liquid crystal displays and have become an increasingly important technology [1–3]. In particular, tunable white-light engines (changing color temperature), high-quality display, and tunable color lighting (changing color) have attracted intense interest [4]. Generally, LEDs are tuned by adjusting the injection currents of the diﬀerent color LEDs. In addition, the driving current, and thus the light output power (LOP), can also be changed to obtain higher current density and light intensity. However, changing the current will lead to a change in the junction temperature (Tj) and emission peak wavelength [5], and the Tj itself is also a key factor inﬂuencing the LED emission peak [6]. For some use in manufacturing applications, LED lighting must be able to function in extreme ambient conditions, such as a high-temperature environment (furnace lighting) and freezing environment (hyperborean lighting). In the future, LED may be used in the much lower temperature, such as aerospace engineering. To fabricate a LED module with better color quality or better color coordinates, or with application in extreme temperature range, it is necessary to understand the electroluminescence mechanism in LEDs under wide range of the Tj and the injection current.

Appl. Sci. 2020, 10, 444; doi:10.3390/app10020444 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, x FOR PEER REVIEW 2 of 13

Appl. Sci. 2020, 10, 444 2 of 13 necessary to understand the electroluminescence mechanism in LEDs under wide range of the Tj and the injection current. InvestigationInvestigation of of photoluminescence photoluminescence (PL) (PL) and and elec electroluminescencetroluminescence (EL) (EL) properties properties of InGaN of InGaN LED LEDin the in low the lowtemperature temperature attracted attracted lots lots of ofresear researchch for for years years because because it it is is necessary necessary for betterbetter understandingunderstanding of the mechanism mechanism of of the the luminescence. luminescence. Kazlauskas Kazlauskas et etal. al. had had used used the the PL PLto study to study the thedynamics dynamics of photoexcited of photoexcited excitons excitons in inInGaN/GaN InGaN/GaN with with different different indium indium compositions compositions in thethe temperaturetemperature rangerange ofof 10–30010–300 KK [[7].7]. WangWang etet al.al. hadhad measuredmeasured thethe PLPL propertiesproperties inin thethe temperaturetemperature rangerange ofof 6–3006–300 K,K, andand foundfound thethe temperaturetemperature dependencesdependences ofof thethe peakpeak energyenergy andand linewidthlinewidth areare inducedinduced byby thethe localized localized carrier carrier hopping hopping and and thermalization thermalization process process [8 ].[8]. PL PL was was used used by by Sabbar Sabbar et al.et toal. calculateto calculate the the spontaneous spontaneous emission emission quantum quantum effi efficiencyciency (QE) (QE) of of blue, blue, green, green, and and redred LEDLED fromfrom temperaturetemperature 7777 KK toto 800800 KK [[9].9]. However, there areare fewfew reports aboutabout thethe EL properties ofof InGaNInGaN/GaN/GaN j LEDsLEDs inin thethe lowlow temperaturetemperature (below (below 220 220 K) K) because becauseT jTis is hard hard to to determine. determine. Generally, Generally, it isit diis ffidifficultcult to j determineto determineTj withT with high high current current injection injection for for temperatures temperatur underes under 213 213 K due K due to the to self-heatingthe self-heating of the of LED,the LED, and itand is alsoit is dialsofficult difficult to perform to perform EL measurements EL measurements at such at low such temperatures. low temperatures. Peter et Peter al. used et al. a highlyused a spectrallyhighly spectrally and spatially and spatially resolved resolved scanning scanning electroluminescence electroluminescence microscopy microscopy to trace detailedto trace spectradetailed of spectra an InGaN of an/GaN InGaN/GaN LED at 300 LED K [10 at]. 300 Hetzel K [10]. et al. Hetzel have measuredet al. have themeasured spectra ofthe the spectra LEDs of at the 4.2 andLEDs 77 at K, 4.2 rather and than77 K, inrather a continuous than in a rangecontinuous of low range temperatures of low temperatures [11]. [11]. InIn thisthis work, work, we we investigated investigated the the electroluminescence electroluminescence (EL) (EL) properties properties of blue of blue and green and green GaN-based GaN- j LEDsbased inLEDs the temperaturein the temperature range ofrang 120–373e of 120–373 K, using K, specially using specially designed designed LEDs with LEDs a built-inwith a built-inTj sensor T unit.sensor To unit. the best To the of our best knowledge, of our knowledge, this work this is the work first is detailed the first report detailed of the report changes of the of changes the EL spectra of the j withEL spectra changes with in changesTj and current. in T and We current. investigated We investigated the mechanisms the mechanisms of the LED ofpeak the LED energy peak changes, energy j andchanges, in particular and in particular the effect ofthe the effect injection of the current injection and currentTj on theand emission T on the peak emission energy. peak It was energy. found It thatwas thefound EL andthat electricalthe EL and properties electrical of properties blue LED are of dibluefferent LED from are different those of green from LED,those and of green the origins LED, ofand these the dioriginsfferences of these are elucidated differences and are described elucidated in and detail. described in detail.

2.2. Materials and Methods

2.1.2.1. LED Epilayer Structure OneOne typicaltypical blueblue LEDLED samplesample andand one typical green LED sample fabricated based on commercial epi-wafersepi-wafers werewere measured.measured. The commercial epi-wafers with InGaNInGaN/GaN/GaN multiplemultiple quantumquantum wellwell (MQW)(MQW) activeactive layerslayers werewere growngrown onon sapphiresapphire substrates.substrates. TheThe detailsdetails ofof thethe LEDLED structurestructure areare shownshown inin FigureFigure1 1.. ThereThere were were 11 11 periods periods of of InGaN InGaN/GaN/GaN MQWMQW layerslayers inin blueblue LED,LED, andand there werewere 99 periodsperiods ofof InGaNInGaN/GaN/GaN MQWMQW layerslayers inin greengreen LED.LED. DueDue toto thethe commercialcommercial secrets,secrets, thethe detaileddetailed widthwidth andand compositioncomposition ofof MQWMQW areare notnot clear.clear.

FigureFigure 1.1. Scheme of the InGaNInGaN/GaN/GaN light-emittinglight-emitting diodediode (LED)(LED) inin thisthis work.work. Appl. Sci. 2020, 10, 444 3 of 13

2.2. Chip and Package Strucure and Tj Measurement Method

Due to nanoscale dimensions of the junction, Tj is hard to determine directly and accurately, especially at low temperatures [11,12]. In the recent years, many researchers have sought to develop methods for accurate Tj measurements at high temperatures [12,13]. Lee and Park reported an advanced direct measurement technique using the nematic liquid crystals thermography in 2004, the measurement accuracy was within 1 K [14]. Xi et al. had developed a theoretical model for ± the relationship between the forward voltage (VF) of LED and the Tj, the theory was verified by experimental data, and they had accurately measured the Tj of ultraviolet LEDs using the VF [15–17]. In this work, Tj was accurately measured in wide range with a built-in Tj sensor unit. The structure and dimensions of the LED with the Tj sensor unit have been described elsewhere [18,19]. The designed chip was composed of a single light-emitting unit with the dimensions of 750 430 µm2, × and a single sensor unit. The chips were packaged without phosphor using a four-lead high-power lead frame, and the packages were mounted on the metal-core printed circuit board (MCPCB). Since the sensor unit is smaller and is located next to the LED unit, the Tj of the LED unit was approximately equal to that of the sensor unit. In the experiment described below, the forward voltage (VF) of the sensor diode was used to characterize the Tj [15,17]. In the ambient temperature range from 77 K to room temperature, the temperature was controlled by placing the samples in a liquid nitrogen tank at different distances above the liquid nitrogen level. For the ambient temperatures in the range from room temperature to ~350 K, the temperature was controlled by placing the samples into an oven whose temperature resolution was 0.1 K and temperature fluctuation within 0.5 K. ± First, the VF–Tj relationship of the sensor unit was calibrated. A Keithley 2636B SourceMeter was used as the current source and voltage/current meter. The package temperature was measured using a thermocouple closely attached to the heat slug of the package. To avoid the generation of a temperature difference between the heat slug and the p–n junction caused by the self-heating during the calibration, a weak testing current of 500 µA was used. The VF of the sensor unit was measured after the reading of the thermocouple was stable for at least 15 min.

2.3. EL Measurement under Diﬀerent Tj

After VF–Tj calibration, the EL spectra were measured using an Everfine ATA-500 auto-temperature LED opto-electronic analyzer (Hangzhou, China). In the ambient temperature range from 77 K to room temperature, the EL spectra were measured using a 4 inch polytetrafluoroethylene (PTFE) integrating sphere (IS). PTFE was used because of its good reflectivity and excellent stability at low temperatures. The samples were mounted on the lower hole of the IS and the fiber was mounted onto the upper hole of the IS, and a PTFE slice was placed in the middle to block the direct light from the LED to the fiber. The IS was connected to the Everfine ATA-500 instrument with a fiber and their positions were fixed during the measurements. The ambient temperature was adjusted by changing the height of the liquid nitrogen tank. For the ambient temperature in the range of ~290–350 K, the auto-temperature stage and IS within the ATA-500 instrument were used for temperature control and EL spectra measurements, respectively. The VF of the sensor unit and the EL spectra were collected simultaneously. EL spectra were measured in the Tj range of about 110 K to about 380 K and with forward current (IF) of 80~240 mA at 40 mA interval.

2.4. VF Measurement of LED Unit under Diﬀerent Tj

Since real-time Tj could be measured by our method, we repeatedly recorded VF of the LED unit and the real-time Tj under certain IF at time intervals of 10 s when Tj slowly changed. Then, we got the VF at certain Tj and forward current from the recorded data. Appl. Sci. 2020, 10, 444 4 of 13

3. Results Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 13

3.1. VF-Tj Calibration3. Results

Figure3.1.2 shows VF-Tj Calibration the V F–Tj relationship of the sensor unit. The results show that VF decreased monotonicallyFigure with 2T jshows. Therefore, the VF–TjT relationshipj was calculated of the sensor by simply unit. The using results the show linear that V interpolationF decreased of the data presentedmonotonically in Figure with2. ItTj. wasTherefore, observed Tj was that calculated at high by temperaturessimply using the (abovelinear interpolation 150 K), V ofF ofthethe green LED was lowerdata presented than that in ofFigure the 2. blue It was LED. observed This that can at be high attributed temperatures to the (above diff erent150 K), energy VF of the band green diagrams LED was lower than that of the blue LED. This can be attributed to the different energy band of the blue LED and the green LED. At the low temperature limit, the V value and the slope of V diagrams of the blue LED and the green LED. At the low temperature limit, theF VF value and the slope F plotted versusof VFT plottedj of the versus green Tj of LED the green were LED both were higher both higher than thosethan those of the of the blue blue LED, LED, whichwhich may may be be due to the differencedue into thethe difference energy band in the diagrams energy band and diagrams p-GaN and doping p-GaN ofdoping the two of the LEDs, two LEDs, and theand ditheff erence in the ohmic contactsdifference betweenin the ohmic the contacts green between and blue the green LEDs and [17 blue,20 ,LEDs21]. [17,20,21].

Figure 2. Relationship of forward voltage (VF) and junction temperature (Tj) for blue and green LEDs Figure 2. Relationship of forward voltage (VF) and junction temperature (Tj) for blue and green LEDs with the sensor unit. with the sensor unit. 3.2. EL Spectra, Peak Energy, Full Width at Half Maximum, and Radiation Power 3.2. EL Spectra, Peak Energy, Full Width at Half Maximum, and Radiation Power

Part of thePart normalized of the normalized EL spectra EL spectra at selected at selectedTj forTj for the the blue blue and green green LED LED at 80 at mA 80 mAand 240 and 240 mA are presentedmA inare Figure presented3 for in demonstration.Figure 3 for demonstration. We extracted We extracted the emission the emission peak peak energies, energies, full full width width at half maximum (FWHM),at half maximum and radiation(FWHM), and power radiation from power the ELfrom spectra. the EL spectra. Figure Figure4 shows 4 shows the the plot plot of of the the emission emission peak energy of the blue and green LEDs versus Tj at different forward currents. As Tj peak energyincreased, of the the blue emission and peak green energy LEDs first versus showedT a jblueshiftat diff erentand then forward a redshift. currents. Green LEDAs showedTj increased, the emissiontwo peak maxima, energy while first blue showed LED showed a blueshift only one and maxi thenmum. a Figure redshift. 5 shows Green the plot LED of showedFWHM of two EL maxima, while bluespectra LED showed for the blue only and one green maximum. LEDs versus Figure Tj at 5different shows forward the plot currents. of FWHM Figure of 6 ELshows spectra the for the radiation power for the blue and green LEDs versus Tj at different forward currents. blue and green LEDs versus Tj at different forward currents. Figure6 shows the radiation power for Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 13 the blue and green LEDs versus Tj at different forward currents.

Figure 3. Normalized electroluminescence (EL) spectra at selected Tj for the blue LED at (a) 80 mA and (b) 240 mA,Figure and 3. for Normalized the green electroluminescence LED at (c) 80 mA (EL) and spectra (d) at 240 selected mA. Tj for the blue LED at (a) 80 mA and (b) 240 mA, and for the green LED at (c) 80 mA and (d) 240 mA.

Figure 4. Relationship between the emission peak energy and Tj for the (a) blue LED and (b) green LED.

Figure 5. Relationship between the full width at half maximum (FWHM) of the EL spectra and Tj for the (a) blue LED and (b) green LED. Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 13 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 13

Figure 3. Normalized electroluminescence (EL) spectra at selected Tj for the blue LED at (a) 80 mA Appl. Sci. 2020andFigure, 10 (b,) 444 2403. Normalized mA, and for electroluminescence the green LED at (c )(EL) 80 mA spectra and (atd )selected 240 mA. T j for the blue LED at (a) 80 mA 5 of 13 and (b) 240 mA, and for the green LED at (c) 80 mA and (d) 240 mA.

Figure 4. Relationship between the emission peak energy and Tj for the (a) blue LED and (b) green Figure 4. Relationship between the emission peak energy and Tj for the (a) blue LED and (b) green FigureLED. 4. Relationship between the emission peak energy and Tj for the (a) blue LED and (b) green LED. LED.

Appl.Figure Sci.Figure 5.2020Relationship ,5. 10 Relationship, x FOR PEER between REVIEWbetween the the full full width width atat half maximum maximum (FWHM) (FWHM) of ofthe the EL ELspectra spectra and andTj for6T ofj for 13 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 13 the (atheFigure) blue (a) blue LED5. Relationship LED and and (b) ( greenb between) green LED. LED. the full width at half maximum (FWHM) of the EL spectra and Tj for the (a) blue LED and (b) green LED.

Figure 6. Relationship between radiation power and Tj of (a) blue and (b) green LEDs at different FigureFigure 6. 6.Relationship Relationship between radiation radiation power power and T andj of (aT) blueof (anda) blue(b) green and LEDs (b) greenat different LEDs at currents. j diﬀerentcurrents. currents. 3.3. Electrical Properties of LEDs 3.3. Electrical3.3. Electrical Properties Properties of LEDsof LEDs VF of the LED unit at different Tj and IF for blue and green LEDs are shown in Figure 7. VF of the LED unit at different Tj and IF for blue and green LEDs are shown in Figure 7. VF of the LED unit at diﬀerent Tj and IF for blue and green LEDs are shown in Figure7.

Figure 7. Forward current vs. forward voltage at different Tj for blue LED (a) and green LED (b). The linearFigure fitting7. Forward (dotted current line) vs. at forwardTj = 193 voltage K for at blue different LED T isj for shown blue LED in Figure (a) and7 a.green The LED intercept (b). The of the Figure 7. Forward current vs. forward voltage at different Tj for blue LED (a) and green LED (b). The fittinglinear line fitting at the (dottedx-axis isline)V at .Tj = 193 K for blue LED is shown in Figure 7a. The intercept of the fitting linear fitting (dotted line)TH at Tj = 193 K for blue LED is shown in Figure 7a. The intercept of the fitting line at the x-axis is VTH. line at the x-axis is VTH. 4. Discussions 4. Discussions 4.1. Emission Peak Energy vs. Tj 4.1. Emission Peak Energy vs. Tj The emission peak shift can be described by a band-tail model [22]: The emission peak shift can be described by a band-tail model [22]: (1) =0 − − (1) =0 −+ − + where α and β are Varshni parameters, and Eg(0) and Eg(T) are the band gap values at 0 K and at where α and β are Varshni parameters, and Eg(0) and Eg(T) are the band gap values at 0 K and at temperature T, respectively. The second term is about the band gap shrinkage with increasing temperature T, respectively. The second term is about the band gap shrinkage with increasing temperature, leading to the redshift of the emission peak in the temperature range of ~175–373 K. The temperature, leading to the redshift of the emission peak in the temperature range of ~175–373 K. The last term describes the redshift of the Stokes type, where σ is the degree of the localized effect of the last term describes the redshift of the Stokes type, where σ is the degree of the localized effect of the carriers, and k is the Boltzmann constant [23]. Indium composition fluctuations lead to the carriers, and k is the Boltzmann constant [23]. Indium composition fluctuations lead to the spontaneous formation of indium-rich regions, and the presence of such regions in addition to the spontaneous formation of indium-rich regions, and the presence of such regions in addition to the presence of doping elements and crystalline quality of the hetero- and homoepitaxial material lead presence of doping elements and crystalline quality of the hetero- and homoepitaxial material lead to potential fluctuations in the InGaN alloy layers [24]. The band-tail states also mean the potential to potential fluctuations in the InGaN alloy layers [24]. The band-tail states also mean the potential fluctuations in energy band structure, and they would capture the carriers and excitons to generate fluctuations in energy band structure, and they would capture the carriers and excitons to generate the localized carriers and bound excitons. The larger the σ value is, the deeper extension of the tails the localized carriers and bound excitons. The larger the σ value is, the deeper extension of the tails into the forbidden band and the greater degree of the localized effect of the carriers [25]. The potential into the forbidden band and the greater degree of the localized effect of the carriers [25]. The potential fluctuations are sufficiently strong to provide efficient luminescence centers because these states can fluctuations are sufficiently strong to provide efficient luminescence centers because these states can Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 13 confine electrons and holes at the same sites [26]. Due to the fact that the density-of-states of the tails is much lower, the occupation of the higher energy states of the tails is more obvious than the occupation in regular energy bands with the increasing Tj [24]. In the temperature range of ~120–175 K, thermalized bound excitons occupy higher energy states, and the effect of the blueshift is stronger than the effect of the temperature-induced band gap shrinkage with temperature increasing, leading to the blueshift of the emission peak energy [22,27]. Using Equation (1) to fit the blue LED data, the fitting curve is presented in Figure 8a and shows good agreement with the experimental data. Two maxima were found in the curve of the emission peak energy as a function of the Tj of green LED, indicating that a transition between two different main luminescence mechanisms occurs as the Appl. Sci. 2020, 10, 444 6 of 13 Tj of the green LED changes. Therefore, the data cannot be fitted with a single set of parameters. As reported previously, excitons play an important role in light emission in GaN and related alloys [28– 4.30]. Discussions Compared to the blue LEDs, larger potential fluctuations, and thus deeper band tails in the green LEDs, cause easier caption of the excitons by defect states. At low temperatures, the recombination 4.1.of the Emission donor-bound Peak Energy excitons vs. Tj (DX) may be the main source of light emission. As the exciton localization energy of DX is low, the DX escape the trap states as the temperature increases, The emission peak shift can be described by a band-tail model [22]: transforming into free excitons (FX), and the amount of free excitons A (FXA) increases rapidly [31]. Therefore, FXA start to play an increasingly importantα Trole2 inσ the2 luminescence at high temperatures. EgT = Eg(0) (1) Due to its large exciton binding energy, FXA is domi− β +nantT − evenkT at room temperature [30]. To fit the data for the green LED in Figure 8b, we used two groups of parameters for the data near the left whereextremeα valueand β andare in Varshni the low parameters, temperature and region,Eg(0) an andd forEg the(T) data are thenear band the right gap valuesextreme at value 0 K andand atin temperaturethe high temperatureT, respectively. region, The respectively. second term Si isnce about the theexciton band localization gap shrinkage energy with of increasing DX was temperature,previously reported leading to to be the approximately redshift of the 6 emission meV [31], peak the inband the width temperature difference range between of ~175–373 the two K. Thefitting last curves term describeswas set to the6 meV redshift in our of curve the Stokes fitting type,. The wherefitting curvesσ is the of degree the green of the LED localized are shown effect in ofFigure the carriers, 8b and agree and k wellis the with Boltzmann the experimental constant [data23]., Indiumexcept that composition due to the fluctuations transition of lead the to main the spontaneousluminescence formation mechanism of indium-richfrom DX-based regions, to FXA-ba and thesed, presence the data of in such the regionsintermediate in addition temperature to the presenceregion are of not doping fitted elements very well. and Thus, crystalline we conclude qualityd that of the the hetero- main luminescence and homoepitaxial mechanism material of green lead toLED potential is dominated fluctuations by inDX the at InGaN low alloytemperatures layers [24, ].and The transitions band-tail statesto FXA-dominated also mean the potential at high fluctuationstemperatures, in giving energy rise band to structure,two extreme and values. they would capture the carriers and excitons to generate the localizedThe value carriers of the andfit σ boundof the blue excitons. LED was The approximately larger the σ value 30 meV, is, the and deeper that of extension the green of LED thetails was intoapproximately the forbidden 34.3 band meV. and A thelarger greater σ means degree a stronger of the localized localization effect effect of the on carriers average, [25 which]. The is potential mainly fluctuationscaused by the are indium sufficiently composition strong to fluctuation provide effi [25].cient In luminescence this work, the centers transition because between these statesthe FXA- can confinedominated electrons mechanism and holes and atDX-dominated the same sites mechanism [26]. Due towas the not fact observed that the in density-of-states the blue LED. This of the is tailsbecause is much of the lower, weaker the occupationlocalization of effect the higher of the energy blue LED, states meaning of the tails the is moreDX is obviousnot a dominant than the occupationluminescence inregular mechanism energy as bandsin green with LED the at increasing low temperaturesTj [24]. In of the this temperature experiment. range Therefore, of ~120–175 a single K, thermalizedpeak energy bound was observed excitons occupyfor the higherblue LED. energy Meanwhile, states, and based the e ffonect the of theprevious blueshift studies, is stronger it is likely than thethat e fftheect transition of the temperature-induced between FXA-dominated band gap and shrinkage DX-dominated with temperature mechanism increasing, should appear leading at tolower the blueshifttemperatures of the [32,33]. emission Due peak to the energy limitations [22,27]. of the experimental setup used in this work, studies of suchUsing a low-temperature Equation (1) toexperimental fit the blue LEDrange data, have the not fitting been completed curve is presented and will in be Figure the subject8a and of shows future goodresearch. agreement with the experimental data.

Figure 8. Fitting curves of Varshni’s formula for (a) blue LED and (b) green LED at 80 mA.

Two maxima were found in the curve of the emission peak energy as a function of the Tj of green LED, indicating that a transition between two different main luminescence mechanisms occurs as the Tj of the green LED changes. Therefore, the data cannot be fitted with a single set of parameters. As reported previously, excitons play an important role in light emission in GaN and related alloys [28–30]. Compared to the blue LEDs, larger potential fluctuations, and thus deeper band tails in the green LEDs, cause easier caption of the excitons by defect states. At low temperatures, the recombination of the donor-bound excitons (DX) may be the main source of light emission. As the exciton localization energy of DX is low, the DX escape the trap states as the temperature increases, Appl. Sci. 2020, 10, 444 7 of 13 transforming into free excitons (FX), and the amount of free excitons A (FXA) increases rapidly [31]. Therefore, FXA start to play an increasingly important role in the luminescence at high temperatures. Due to its large exciton binding energy, FXA is dominant even at room temperature [30]. To fit the data for the green LED in Figure8b, we used two groups of parameters for the data near the left extreme value and in the low temperature region, and for the data near the right extreme value and in the high temperature region, respectively. Since the exciton localization energy of DX was previously reported to be approximately 6 meV [31], the band width difference between the two fitting curves was set to 6 meV in our curve fitting. The fitting curves of the green LED are shown in Figure8b and agree well with the experimental data, except that due to the transition of the main luminescence mechanism from DX-based to FXA-based, the data in the intermediate temperature region are not fitted very well. Thus, we concluded that the main luminescence mechanism of green LED is dominated by DX at low temperatures, and transitions to FXA-dominated at high temperatures, giving rise to two extreme values. The value of the fit σ of the blue LED was approximately 30 meV, and that of the green LED was approximately 34.3 meV. A larger σ means a stronger localization effect on average, which is mainly caused by the indium composition fluctuation [25]. In this work, the transition between the FXA-dominated mechanism and DX-dominated mechanism was not observed in the blue LED. This is because of the weaker localization effect of the blue LED, meaning the DX is not a dominant luminescence mechanism as in green LED at low temperatures of this experiment. Therefore, a single peak energy was observed for the blue LED. Meanwhile, based on the previous studies, it is likely that the transition between FXA-dominated and DX-dominated mechanism should appear at lower temperatures [32,33]. Due to the limitations of the experimental setup used in this work, studies of such a low-temperature experimental range have not been completed and will be the subject of future research.

4.2. Difference of the Infucence of the Current at the High Temperature and the Low Temprature Figure9a shows the relationship between the emission peak energies of the blue and green LEDs and the current for Tj of 250 K, derived from Figure4a. It was observed that the emission peak energy has a linear relationship with the current and increases with increasing current. This relationship is attributed to the electric field formed by the increased carrier concentration that decreases the impact of the quantum-confined Stark effect (QCSE) [26,34] and the energy band filling effect [35,36]. The excitons’ lifetime decreased in the high-temperature range, and therefore, as the current increased the excitons could not occupy the lower energy state prior to the recombination, leading to the blue shift of the emission peak [27]. Since many higher energy extended states may be present, this kind of recombination will be accompanied by the linewidth broadening, as shown in Figure9c, which is derived from Figure5a. The same reason can also cause increase of the radiation power, as can be figured out in Figure6. The slope of the emission peak energy plotted versus the current of the green LED was 1.59 10 4 eV/mA, and that of the blue LED was 6.34 10 5 eV/mA. A higher slope implies × − × − a stronger potential fluctuation, indicating that the potential fluctuation of the green LED is much stronger than that of blue LED. Thus, compared to the blue LED, the potential valley of the green LED is deeper and contains more states, leading to a higher slope. Comparison of Figures6 and9a,c shows that in the high-temperature range (200 K or higher), the emission peak energy, FWHM, and radiation power of both the blue and green LEDs increased with increasing current density. The reason is discussed in the last paragraph. However, in the low-temperature range (below 150 K), the emission peak energy, FWHM, and the radiation power of the blue LED showed only a slight change with increasing current, while the corresponding values of the green LED still increased strongly with increasing current, as shown in Figures6 and9b,d. This difference can be explained by the band filling in real space of the LED at low temperatures, as will be discussed below. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 13

Figure 8. Fitting curves of Varshni’s formula for (a) blue LED and (b) green LED at 80 mA.

4.2. Difference of the Infucence of the Current at the High Temperature and the Low Temprature Figure 9a shows the relationship between the emission peak energies of the blue and green LEDs and the current for Tj of 250 K, derived from Figure 4a. It was observed that the emission peak energy has a linear relationship with the current and increases with increasing current. This relationship is attributed to the electric field formed by the increased carrier concentration that decreases the impact of the quantum-confined Stark effect (QCSE) [26,34] and the energy band filling effect [35,36]. The excitons’ lifetime decreased in the high-temperature range, and therefore, as the current increased the excitons could not occupy the lower energy state prior to the recombination, leading to the blue shift of the emission peak [27]. Since many higher energy extended states may be present, this kind of recombination will be accompanied by the linewidth broadening, as shown in Figure 9c, which is derived from Figure 5a. The same reason can also cause increase of the radiation power, as can be figured out in Figure 6. The slope of the emission peak energy plotted versus the current of the green LED was 1.59 × 10−4 eV/mA, and that of the blue LED was 6.34 × 10−5 eV/mA. A higher slope implies a stronger potential fluctuation, indicating that the potential fluctuation of the green LED is much stronger than that of blue LED. Thus, compared to the blue LED, the potential valley of the green LED is deeper and contains more states, leading to a higher slope. Comparison of Figures 6 and 9a,c shows that in the high-temperature range (200 K or higher), the emission peak energy, FWHM, and radiation power of both the blue and green LEDs increased with increasing current density. The reason is discussed in the last paragraph. However, in the low- temperature range (below 150 K), the emission peak energy, FWHM, and the radiation power of the blue LED showed only a slight change with increasing current, while the corresponding values of the green LED still increased strongly with increasing current, as shown in Figures 6 and 9b,d. This Appl.difference Sci. 2020, 10can, 444 be explained by the band filling in real space of the LED at low temperatures, as will8 of 13 be discussed below.

FigureFigure 9. 9.Relationship Relationship between between the the emissionemission peakpeak energyenergy and and current current for for the the blue blue and and green green LEDs LEDs at at (a)( 250a) 250 K andK and (b )(b 150) 150 K. K. Relationship Relationship between between thethe FWHMFWHM an andd current current of of the the blue blue and and green green LEDs LEDs at at (c)( 250c) 250 K andK and (d )(d 150) 150 K. K.

Compared to high temperatures, for current injection at low temperatures, the exciton lifetime is long enough for the excitons to relax to the potential minima [8,37,38]. Due to the strong spatial potential fluctuations and the low exciton mobility in the green LED, the excitons in the potential minima were restricted and bound. As mentioned above, the excitons play an important role in light emission in InGaN. Thus, exciton recombination mainly occurs at the position of the potential valley [28]. Figure 10 shows a two-dimensional diagram depicting the conduction band, motion of electrons in excitons, and radiative combination due to the potential fluctuation along the real space x-axis. It is a simplified version of the three-dimensional diagram depicting the potential fluctuation along the real space xy-plane [39]. The black solid dots represent excitons (electrons), the curved lines interpret the conduction band, and the dashed line stands for different filling levels of the excitons (electrons) at different currents (I1,I2 and I3). As shown in Figure 10a, when the current increased from I1 to I2 and I3, the increased excitons were distributed among the higher-energy potential minima, leading to the existence of more radiative centers and more recombination excitons. Since potential minima are deep and mobility of excitons is relative low in the green LEDs (due to large potential fluctuation), excitons are inclined to be bound by potential minima with different positions and different energy levels. This leads to the greater FWHM and radiation power of the green LED with the increase of IF. The smaller indium content in the blue LED leads to weaker potential fluctuation and higher exciton mobility. Therefore, the excitons can easily move between the shallower potential valleys, are less likely to be bound in the shallower potential valleys, and thus, those shallower potential valleys do not contribute to the LED luminescence. Only the deeper potential can capture excitons and release radiation emission. A schematic of the blue LED is shown in Figure 10b, and it was observed that when the current was I1, the filling of the potential minima reached saturation. Even when the current increased to I2, the number of excitons participating in radiative recombination did not increase. As the current increased, the increased carriers circulated from the conduction band Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 13

Compared to high temperatures, for current injection at low temperatures, the exciton lifetime is long enough for the excitons to relax to the potential minima [8,37,38]. Due to the strong spatial potential fluctuations and the low exciton mobility in the green LED, the excitons in the potential minima were restricted and bound. As mentioned above, the excitons play an important role in light emission in InGaN. Thus, exciton recombination mainly occurs at the position of the potential valley [28]. Figure 10 shows a two-dimensional diagram depicting the conduction band, motion of electrons in excitons, and radiative combination due to the potential fluctuation along the real space x-axis. It is a simplified version of the three-dimensional diagram depicting the potential fluctuation along the real space xy-plane [39]. The black solid dots represent excitons (electrons), the curved lines interpret the conduction band, and the dashed line stands for different filling levels of the excitons (electrons) at different currents (I1, I2 and I3). As shown in Figure 10a, when the current increased from I1 to I2 and I3, the increased excitons were distributed among the higher-energy potential minima, leading to the existence of more radiative centers and more recombination excitons. Since potential minima are deep and mobility of excitons is relative low in the green LEDs (due to large potential fluctuation), excitons are inclined to be bound by potential minima with different positions and different energy levels. This leads to the greater FWHM and radiation power of the green LED with the increase of IF. The smaller indium content in the blue LED leads to weaker potential fluctuation and higher exciton mobility. Therefore, the excitons can easily move between the shallower potential valleys, are less likely to be bound in the shallower potential valleys, and thus, those shallower potential valleys do not contribute to the LED luminescence. Only the deeper potential can capture excitons and release radiation emission. A schematic of the blue LED is shown in Figure 10b, and it was observed that when the current was I1, the filling of the potential minima reached saturation. Even when the Appl. Sci. 2020, 10, 444 9 of 13 current increased to I2, the number of excitons participating in radiative recombination did not increase. As the current increased, the increased carriers circulated from the conduction band and the andvalence the valence band, band,forming forming a leakage a leakage current current and and not not participating participating in in radiative radiative recombination [40[40].]. Therefore,Therefore, the the current current has has no significantno significant influence influe onnce the on peak the energy,peak energy, FWHM, FWHM, and the and radiation the radiation power ofpower the blue of LEDthe blue at low LED temperatures. at low temperatures. Consequently, Consequently, the difference the indifference the electrooptical in the electrooptical properties betweenproperties the between green LED the and green the LED blue and LED the can blue be explained.LED can be explained.

FigureFigure 10. 10.Transport Transport mechanism mechanism of of the the excitons excitons in in quantum quantum wells wells at at low low temperatures temperatures for for (a ()a green) green LEDLED and and (b) (b)blue blue LED. LED. 4.3. Electrical Analysis 4.3. Electrical Analysis We fitted the linear relationship between the forward current in the 80–240 mA range and VF and We fitted the linear relationship between the forward current in the 80–240 mA range and VF calculated the intercept at the VF axis. The fitting and deriving of intercept with Tj = 193 K for blue and calculated the intercept at the VF axis. The fitting and deriving of intercept with Tj = 193 K for LED is shown in Figure7a. We mention the intercept as threshold voltage ( VTH) below. Figure 11a blue LED is shown in Figure 7a. We mention the intercept as threshold voltage (VTH) below. Figure shows the calculated threshold voltages of the blue and green LEDs at different Tj, and it is observed 11a shows the calculated threshold voltages of the blue and green LEDs at different Tj, and it is that the threshold voltage increased rapidly with decreasing Tj. The relationship between the band gap and Tj was fitted by Equation (1), as discussed above. Based on the aforementioned data, the increase in the band gap was much smaller than the increase in the threshold voltage. Dynamic resistance is the reciprocal of the slope of the above-mentioned I–V curve, and the dynamic resistance results are shown in Figure7b. It was observed that when Tj decreased from 350 to 200 K, the dynamic resistance of the blue LED remained basically unchanged, while that of the green LED increased. When Tj dropped from 200 to 150 K, the dynamic resistance of the blue LED increased, while that of the green LED changed slightly. Overall, the increased dynamic resistance was a minor cause of the increased threshold voltage. The increase in the threshold voltage was much larger than the voltage due to the band gap and dynamic resistance. Due to the fact that the lattice expansion coefficient of GaN was larger than that of InGaN, the piezoelectric polarization caused by lattice mismatch was greater at low temperatures than at higher temperatures [41–43]. An electron-blocking layer (EBL) can decrease the injection efficiency of holes at low temperatures, requiring high voltage to overcome the barrier [44]. The higher threshold voltage of the LED at a low temperature compared to that at a high temperature may be caused by the combined effects of the band gap, dynamic resistance, piezoelectric polarization, and EBL. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 13

observed that the threshold voltage increased rapidly with decreasing Tj. The relationship between the band gap and Tj was fitted by Equation (1), as discussed above. Based on the aforementioned data, the increase in the band gap was much smaller than the increase in the threshold voltage. Dynamic resistance is the reciprocal of the slope of the above-mentioned I–V curve, and the dynamic resistance results are shown in Figure 7b. It was observed that when Tj decreased from 350 to 200 K, the dynamic resistance of the blue LED remained basically unchanged, while that of the green LED increased. When Tj dropped from 200 to 150 K, the dynamic resistance of the blue LED increased, while that of the green LED changed slightly. Overall, the increased dynamic resistance was a minor cause of the increased threshold voltage. The increase in the threshold voltage was much larger than the voltage due to the band gap and dynamic resistance. Due to the fact that the lattice expansion coefficient of GaN was larger than that of InGaN, the piezoelectric polarization caused by lattice mismatch was greater at low temperatures than at higher temperatures [41–43]. An electron-blocking layer (EBL) can decrease the injection efficiency of holes at low temperatures, requiring high voltage to overcome the barrier [44]. The higher threshold voltage of the LED at a low temperature compared to that at a high temperature may be caused by the combined effects of the band gap, dynamic resistance, Appl. Sci. 2020, 10, 444 10 of 13 piezoelectric polarization, and EBL.

Figure 11. (a) Relationship between the threshold voltage and Tj.(b) Relationship between the dynamic Figure 11. (a) Relationship between the threshold voltage and Tj. (b) Relationship between the resistance and Tj. dynamic resistance and Tj. 5. Conclusions 5. Conclusions EL spectra of blue and green LEDs at various junction temperatures and forward currents were investigated.EL spectra The ofT bluej was and measured green LEDs by a built-inat various sensor junction on the temperatures LED chip, enabling and forward accurate currents real-time were Tinvestigated.j measurements The andTj was EL measured spectrum by measurements a built-in sensor for theon LEDsthe LED under chip, a enabling high forward accurate current real-time and inTj ameasurements wide range ofandTj .EL To spectrum the best measurements of our knowledge, for the this LEDs work under is thea high first forward investigation current ofand the in changesa wide range in the of EL Tj. spectrumTo the best under of our high knowledge, current densitythis work in is a the large first temperature investigation range, of the particularly changes in inthe the EL range spectrum of 120–220 under high K. Both current blue density and green in a larg LEDse temperature first showed range, a blueshift particularly and then in the a redshiftrange of of120–220 the emission K. Both peak blue as and the green temperature LEDs first increased showed from a blueshift ~120 K toand ~350 then K. a Green redshift LED of showedthe emission two maximapeak as the in the temperature plot of the increased emission from peak ~120 energy K to versus ~350 K. temperature, Green LED showed which can two be maxima explained in the by plot the transitionof the emission of the peak main energy luminescence versus temperature, mechanism wh ofich the can green be explained LED from by DX the recombination transition of the to FXAmain recombination.luminescence mechanism No change inof the the main green luminescence LED from mechanismDX recombination was observed to FXA in therecombination. blue LED in theNo similarchange temperaturein the main range. luminescence We attribute mechanism this to the was higher observed degree in of localizationthe blue LED and in lower the excitonsimilar mobilitytemperature in the range. green We LED attribute compared this to to the those higher in the degree blue LED.of localization The transition and lower of the exciton luminescence mobility mechanismin the green from LED FXA-dominatedcompared to those to in DX-dominated the blue LED. occurredThe transition at approximately of the luminescence 180 K in mechanism the green LED,from andFXA-dominated it is possible thatto DX-dominated in the blue LED, occurred this transition at approximately occurs at a180 temperature K in the green that LED, is lower and than it is thepossible temperature that in rangethe blue examined LED, inthis this transition work, which occurs needs at a to temperature be investigated that inis thelower future than work. the Accordingtemperature to therange band-tail examined model, in inthis the work, low-temperature which needs region, to be excitons investigated are mainly in the bonded future around work. theAccording potential to minima. the band-tail Therefore, model, in in the the green low-temper LED withature strong region, potential excitons fluctuation are mainly and bonded lower excitonaround mobility,the potential an increase minima. in Therefore, the current in supplied the green a LED greater with number strongof potential excitons fluctuation to be bound and at lower the potential exciton minimamobility, at an di ffincreaseerent positions in the current and di suppliedfferent energy a greater levels, number so that of excitons the emission to be peakbound energy, at the potential FWHM, and radiation power increased rapidly with increasing current at low temperatures (below 150 K). By contrast, in the blue LED, the potential fluctuation was weaker and exciton mobility was higher, leading to only a slight change of the emission peak energy, the FWHM, and radiation power of blue LED at low temperatures (below 150 K). The threshold voltages of the blue and green LEDs were higher at low temperatures, and the increased voltage was most likely due to the combined effect of the band gap, dynamic resistance, piezoelectric polarization, and EBL.

Author Contributions: Conceptualization, Y.Z. (Yugang Zhou); methodology, Y.Z. (Yugang Zhou), S.P., and C.S.; validation, C.S. and W.C.; formal analysis, C.S., W.C., and S.P.; investigation, S.P. and C.S.; resources, Y.Z. (Yugang Zhou); data curation, C.S.; writing—original draft preparation, S.P. and C.S.; writing—review and editing, Y.Z. (Yugang Zhou); supervision, Y.Z. (Yugang Zhou); funding acquisition, Y.Z. (Yugang Zhou), R.Z., and Y.Z. (Youdou Zheng). All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China under Grant (Grant Nos. 61634005), National Key R&D Program of China (Grant No. 2016YFB0400904). Acknowledgments: This work was supported by Collaborative Innovation Center of Solid-State Lighting and Energy-Saving Electronics and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Appl. Sci. 2020, 10, 444 11 of 13

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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