IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 6, JUNE 2001 1065

Effects of Current Spreading on the Performance ofGaN-Based Light-Emitting Diodes

Hyunsoo Kim, Seong-Ju Park, and Hyunsang Hwang

Abstract—Effects of current spreading on the performanceof multiple quantum wells (MQWs) GaN/InGaN light-emittingdiodes (LEDs) were investigated. For the theoretical prediction ofdevice performance, we developed a model using two device-designparameters, which consist of the applied current density and theeffective length for the lateral current path. A comparison ofthe theoretical and experimental results clearly showed that thereliability characteristics and the optical efficiency of device areheavily dependent on the applied current density. In addition, theeffective length for the lateral current path was found to have aprofound effect on the uniform current spreading. Based on thesefindings, an ideal geometrical design of the highly efficient LED isproposed.

Index Terms—Current spreading, GaN, geometrical design,light-emitting diode (LED), model, reliability.

I. INTRODUCTION

GaN-BASED materials have many attractive electronic,optical, and thermal properties, which make them

promising materials for optoelectronic, high-power, andhigh-temperature electronic devices. Recently, significantprogress has been made on our understanding of III–V nitridelight-emitting diodes (LEDs) and laser diodes (LDs) [1], [2].Typically, it is necessary to employ a lateral carrier injectiontype with respect to these diodes due to the insulating sapphiresubstrate. However, an employment of the lateral injection typecan exhibit a problem of nonuniform current spreading duringdevice operation. Recently, Eliashevichet al. [3] reported thatthe conductivity of an n-type GaN layer has a profound effecton uniform current spreading. It was shown that the nonuniformcurrent spreading could significantly degrade the properties ofLED performance due to the current crowding in a localizedregion of the device. However, the role of current spreading inthe GaN-based LEDs has not been clearly understood as yet.

In this paper, we report on the effects of current spreading onthe electrical, optical, and reliability characteristics of the mul-tiple quantum well’s (MQW’s) LEDs using the current-voltage,light output power-injected current density, and the lifetimemeasurements. In order to study this in detail, we propose thedevice operation model using two important parameters suchas the applied current density and the effective length for thelateral current path. It is shown that the LED performances are

Manuscript received September 8, 2000. This work was supported in part bythe Korea Energy Management Cooperation and the Brain Korea 21 Project.The review of this paper was arranged by Editor P. K. Bhattacharya.

H. Kim, S.-J. Park, and H. Hwang are with the Department of Materials Sci-ence and Engineering, Kwangju Institute of Science and Technology, Kwangju500-712, Korea (e-mail: hwanghs@kjist.ac.kr).

Publisher Item Identifier S 0018-9383(01)03237-3.

Fig. 1. Schematic cross-sectional view of the fabricated LED. Current pathsA andB represent two extreme paths from p- to n-pad.

sensitively dependent upon these parameters. Finally, based onour experimental and theoretical results,the highly efficient andreliable LED design is suggested.

II. M ODEL

Fig. 1 illustrates a theoretical prediction of an ideal current,which spreads during LED operation using two main currentpaths for the cross-sectional LED structure. (It should be notedthat “ and ” quoted in this model indicates the extreme cur-rent path from p- to n-pad). The total voltage dropacross anarbitrary current path between two pads is given by [3], [4]

(1)

where represents the current density, andand representthe thickness and the resistivity of the respective layer, respec-tively. The geometrical parameters (and ) represent the arbi-trary length of the lateral current path through the transparentlayer and the n-type layer, respectively. In order to create a sim-pler model, the resistances of the p- and n-ohmic contacts wereignored. In addition, the component of voltage drops which arerelated to the vertical aspects of the transparent layerand the n-layer can also be neglected .Furthermore, that of the voltage drop related to the lateral cur-rent path through the p-layer can be also ignored .Therefore, the relation of the voltage drops for pathcan bewritten as

(2)

where and represent the extreme length of the lateral cur-rent paths through the transparent and the n-type layer, respec-tively. Similarly, the relation of the voltage drops for pathcanbe written as

(3)

0018–9383/01$10.00 © 2001 IEEE

1066 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 6, JUNE 2001

Considering the relationships between (2) and (3), it is evidentthat a perfectly uniform current spreading across the active re-gion of the LED device can be achieved when the total voltagedrop of path is equal to that of path . Thatis, the following condition must be satisfied, and its relation canbe expressed as

(4)

From (4), it can be known that uniform current spreading is afunction of and . Based on this condition,the uniform current spreading can be achieved when the valuesof and are identical. Basically, this is a function of amaterial quality and can be attained via the growth of a high-quality n-type layer. In this paper, using the obtained parameter

(the applied current density) and (the effective length forthe lateral current path) in this model, we will mainly discussthe effects of current spreading on the LED performances.

III. EXPERIMENT

Metalorganic chemical vapor deposition was used to grow1.5- m thick n-GaN:Si layer on a (0001) sapphire sub-strate. This was followed by the growth of 0.05-m thickIn Ga N/GaN MQW layers with five periods and then a0.25- m thick p-GaN:Mg layer. In terms of fabrication of theLED device, the p-GaN layer was selectively etched to exposethe n-type layer using an inductively-coupled plasma system.An Ni/Au (2 nm/6 nm) transparent layer was then depositedonto the surface of p-GaN layer. This was followed by the de-position of an Ni/Au (30 nm/80 nm) layer, in order to achieve ap-ohmic pad. For an n-ohmic pad, a Ti/Al (30 nm/80 nm) layerwas deposited on n-GaN, and the metal-deposited samples werethen annealed at 450C for 40 s in a rapid thermal annealingsystem. All electrical and optical properties of the LED perfor-mance were evaluated via on-wafer probing of the devices. Thecurrent-voltage (- ) characteristics were measured using aparameter analyzer (HP 4155A). The light output power of theLED was measured using a UV/VIS 818 PD. Under a constantstress current, the lifetime of the LED was determined from acomplete turn-off of the light output power.

IV. RESULTS AND DISCUSSIONS

A. The Applied Current Density

The dependence of the applied current density on theLED characteristics was investigated. The output power of light

and the device lifetime as a function of the current den-sity is shown in Fig. 2(a) and (b), respectively. These mea-surements were performed using an LED chip with an overalldevice size of m m. The - characteristic of theLED [Fig. 2(a)] shows that the output power increases with in-creasing current density, reaching a maximum value at criticalcurrent density of 270 A/cm, and then decreases. This indicatesthat the output power is considerably dependent on the currentdensity. Fig. 2(b) shows the relationship between the device life-time and the stress current density. It can be seen that LED life-time significantly decreases with increasing current density. Ac-cording to the theory for a contact electromigration (EM) failure

Fig. 2. (a) Light output power(L) versus current density(J) at roomtemperature continuous-wave operation. (b) The lifetime(t ) as a function offorward current density. The dotted line represents the extrapolation of lifetime,based on the current range withn value of 2.06.

model, the relationship between the lifetimeand the currentdensity can be explained by the empirical expression [5]–[7]

(5)

where is a constant and the exponent. From this theory, it iswell known that is a function of Joule heating and approaches2 for the case of negligible Joule heating at a low stress cur-rent [5], [6]. As shown in Fig. 2(b), it can be seen that the valueof approaches 2 at approximately the critical current densityof 270 A/cm , indicating that Joule heating is negligible. How-ever, the value of significantly increases with increasing cur-rent density. This shows that a high current density could leadto current crowding in a localized area of the device, resultingin the degradation of the device lifetime as the result of signif-icant Joule heating. Using thevalue of 2.06 in the negligibleJoule heating current range, the LED lifetime is predictable. Thecalculation shows that the lifetime of the device under normaloperating condition of 20 mA at room temperature is s.

B. The Effective Length for the Lateral Current Path

Effects of the effective length for the lateral current pathon the LED performance were investigated. For this work, theelectrical, optical, and reliability characteristics of the LEDswere evaluated as a function of device size, as shown in Table I.The results of the threshold voltage and the leakage cur-rent density obtained from a variety of devices are listed

KIM et al.: EFFECTS OF CURRENT SPREADING ON PERFORMANCE OF GaN-BASED LEDs 1067

TABLE IA LIST OF LEDS WITH THE VARIOUS

DEVICESSIZES. THE ACTIVE AREA IS BASED ON THELIGHT-EMITTING AREA

INCLUDING p-TYPE LAYER. THE ACTIVE AREA RATIO IS THE RELATIVE

ACTIVE AREA COMPARED TOTHAT OF SYMBOL “F”. T HE THRESHOLD

VOLTAGE (V ) WAS DETERMINED FROM THEBIAS UNDER CURRENT

LEVEL OF 20 mA.J INDICATES THE LEAKAGE CURRENT DENSITY

EVALUATED UNDER A FORWARD BIAS OF 0.5 V

Fig. 3. (a) The current density(J)-voltage (V ) and (b) the differentialquantum efficiency(�)-current density(J) characteristics for LEDs withvarious device sizes.

in Table I. It is well known that, as the device sizes (or ac-tive area) are scaled down (from the symbol “A” to “F”), thethreshold voltages are increased. This is due to the reducedcross-sectional area for current flow with a scaling down of de-vice size. In fact, however, the current density of a small-size de-vice is even higher than those of larger-size devices, as shown inFig. 3(a). In addition, it can be seen that the differential quantumefficiency can be greatly improved with a scaling down ofdevice size, as shown in Fig. 3(b). Such considerable improve-ments of the electrical and optical characteristics with a scalingdown of device size can be attributed to the reduced effectivelength for the lateral current path , which leads to the uni-form current spreading. These results are consistent with the im-proved reliability characteristics as the device sizes are scaleddown (Fig. 4).

Fig. 4. The LED lifetime(t ) as a function of the active area ratio of device.To evaluate the LED lifetime, a constant stress current density was applied. Theapplied current density was 160 A/cmand 200 A/cm for symbol “A” and “B– F”, respectively.

Considering reliability characteristics, native defects shouldbe also taken into consideration. As given in Table I, ofthe small-size device is much lower than that of the large-sizedevice, which is indicative of the reduced nonradiative recom-bination centers [8]. It is evident that, whether the origin of theleakage current is due to native bulk defects or etch damage-in-duced surface defects, the nonradiative recombination centersare also related to the LED lifetime. Therefore, it is concludedthat the superior reliability characteristics for the small-sizedevices are due to combined effects of the uniform currentspreading and the reduced nonradiative recombination centers.In Fig. 3(b), the presence of the nonradiative recombinationcenters could be confirmed from observation of a lowerefficiency at low current density [8]. The efficiency begins toincrease with increasing current density by means of saturatingnonradiative recombination centers. In addition, the efficiencyof the device is reduced above the critical current density dueto Joule heating of the device.

C. Geometrical Device Design

Based on the condition (4), an ideal geometrical design ofLED is possible for the fabrication of the highly efficient device.Basically, this can be realized by minimizing the effective length

in terms of reducing the device size. However, it should benoted that the threshold voltage increases with a scalingdown of device size as discussed in Table I. This factor preventsthe practical application of the sufficiently small-size device. Inthis regard, it is concluded that device dimensions of m

m or m m are appropriate for both aspectsof LED characteristics and practical applications.

In addition, the LED performance could be consider-ably improved by modifying the p-type pad structures asshown in Fig. 5. Fig. 5 shows the cumulative failure plot for

m m LEDs with different p-type pad structures.The inset (a) is the standard LED design for an evaluation ofthe scaling effect of LED size. Compared to this design, it isclearly shown that the design (b) has the improved reliabilitycharacteristics. This can be attributed to the release of thecurrent crowding problem in a localized region of the devicefor the case of LED design (b). Considering the LED design

1068 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 6, JUNE 2001

Fig. 5. Cumulative failure plots for300 �m� 300 �m LEDs with a differentshape of p-type pad structures. The inset (a) represents the plane-view of thestandard LED design with a rectangular p-type pad structure. The insets of (b)and (c) show the plane-view of the modified LED design. The dotted arrowsin the LED schematic indicate a possible current path for uniform currentspreading during LED performance. The LED lifetimes were measured understress current density of 460 A/cmat 20 C.

Fig. 6. I–V characteristics for LEDs with modified geometrical designs of 1and 2.

(a), the extremely short effective length can be foundbetween the corners of n- and p-type pad (marked in dottedcircle). Thus, it is predictable that the high current density tendsto flow heavily in this localized region. Furthermore, this wasconfirmed by the observation of an optical microscope duringthe lifetime test under high stress current injection. That is, astrikingly bright luminescence was observed in this localizedregion during operation of LED. Therefore, it is concluded thatthe extremely short effect length in a localized region shouldbe avoided for an ideal geometrical design of the LED.

Basically, for the uniform current spreading, the effectivelength should be minimized. As shown in the inset (b),schematically, the parameter can be expressed as the sumof and . It is noteworthy thatthe parameter can be considerably reduced by adding anextra p-type pad as shown in the inset (c). In this case, it wasclearly shown that the lifetimes of devices could be improvedby an order of magnitude compared to those of design (b).Furthermore, we were able to observe the improved light

output characteristics, as shown in Fig. 6. It should be notedthat the output power characteristic was improved, although theadditional p-type pad can reduce the light emitting area. Theseresults show the importance of uniform current spreading bymeans of the geometrical device design for the ideal deviceperformance.

V. SUMMARY

Two important parameters, both current density and the ef-fective length, were proposed to understand the effects of cur-rent spreading on the performance of GaN/InGaN MQW’s LEDfrom the presented simple model. It was clearly shown that twoparameters are sensitively dependent upon the electrical, opticaland reliability characteristics of LEDs, indicating that the pro-posed model and parameters are valid. Based on these results,the geometrical design of the highly efficient and reliable LEDwas successful.

ACKNOWLEDGMENT

The authors would like to thank N.-M. Park and J.-S. Jang atKwangju Institute of Science and Technology for many usefuldiscussions.

REFERENCES

[1] S. Nakamura, T. Mukai, and M. Senoh, “Candela-class high-brightnessInGaN/AlGaN double-heterostructure blue-light-emitting diodes,”Appl. Phys. Lett., vol. 64, pp. 1687–1689, 1994.

[2] S. Nakamura and G. Fasol,The Blue Laser Diode, Berlin, Germany:Springer-Verlag, 1997.

[3] I. Eliashevich, Y. Li, A. Osinsky, C. A. Tran, M. G. Brown, and R. F.Karlicek Jr., “InGaN blue light-emitting diodes with optimizedn-GaNlayer,” in SPIE Conf. Light-Emitting Diodes: Research, Manufacturing,and Applications-Part III, vol. 3621, 1999, pp. 28–36.

[4] H. Kim, J.-M. Lee, C. H, S.-W. Kim, D.-J. Kim, S.-J. Park, and H.Hwang, “Modeling of a GaN-based light-emitting diode for uniform cur-rent spreading,”Appl. Phys. Lett., vol. 77, pp. 1903–1904, 2000.

[5] H. Kim, H. Yang, C. H, S.-W. Kim, S.-J. Park, and H. Hwang, “Elec-tromigration-induced failure of GaN multi-quantum well light emittingdiode,” IEE Electron. Lett., vol. 36, pp. 908–910, 2000.

[6] C. Y. Chang and S. M. Sze,ULSI Technology. New York: McGraw-Hill, 1996.

[7] A. S. Oates, “Electromigration failure of contacts and vias in sub-mi-cron integrated circuit metallizations,”Microelectron. Reliab., vol. 36,pp. 925–953, 1996.

[8] M. Hansen, P. Kozodoy, S. Keller, U. Mishra, J. Speck, and S. Denbaars,“The effect of diode area on the luminescence of InGaN quantum welllight emitting diode grown by MOCVD,” inProc. 2nd Int. Symp. BlueLaser and Light Emitting Diodes, Chiba, Japan, Sept. 29–Oct. 2 1998,pp. 540–543.

Hyunsoo Kim was born in Ulsan, Korea, on October6, 1974. He received the B.S. degree in metallurgicalengineering in 1997 from Pusan National University,Pusan,Korea and the M.S. degree in materials sciencein 1999 from Kwangju Institute of Science and Tech-nology, Kwangju, Korea, where he is currently pur-suing the Ph.D. degree in materials science and engi-neering. His current research interests are device de-sign, modeling, and reliability of GaN-based LEDs.

KIM et al.: EFFECTS OF CURRENT SPREADING ON PERFORMANCE OF GaN-BASED LEDs 1069

Seong-Ju Parkwas born in Korea in 1952. He re-ceived the B.S. degree in chemistry and the M.S. de-gree in physical chemistry from Seoul National Uni-versity, Seoul, Korea, in 1976 and 1979, respectively,and the Ph.D. degree in physical chemistry from Cor-nell University, Ithaca, NY, in 1985.

From 1985 to 1987, he was a Post-doctoralfellow at the IBM T. J. Watson Research Center,Yorktown Heights, NY. From 1987 to 1995, he waswith the Electronics Telecommunications ResearchInstitute,Taejeon, Korea, as a Principal Researcher.

In 1995, he joined the faculty at Kwangju Institute of Science and Technology,Kwangju, Korea, as a Professor in the Department of Materials Science and En-gineering. Presently, he is a Professor of the department and the Director of theCenter for Photonic Materials Research. He has engaged in research on growthand characterization of semiconductor epitaxial structures, characterizationof nanoelectronic and photonic materials, atomic and electronic structures ofsemiconductor surfaces, and plasma etching and reaction mechanism.

Hyunsang Hwangwas born in Korea in 1966. Hereceived the B.S. degree in metallurgical engineeringfrom Seoul National University, Seoul, Korea, in1988, and the Ph.D. degree in materials science fromUniversity of Texas at Austin in 1992.

From 1992 to 1997, he was with the LG SemiconCorporation, Cheongju, Korea, as a Principal Re-searcher. In 1997, he joined the faculty at KwangjuInstitute of Science and Technology, Kwangju,Korea, as a Professor in the Department of MaterialsScience and Engineering. His research interests are

process and device design of deep submicron MOSFET, MOSFET devicereliability, ultrathin dielectric, and optoelectronic devices.