gan-based ultraviolet light-emitting diodes with multifinger contacts

Phys. Status Solidi A 207, No. 11, 2585–2588 (2010) / DOI 10.1002/pssa.201026193 p s sa

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pplications and materials science

GaN-based ultraviolet light-emittingdiodes with multifinger contacts

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Hernan Rodrıguez*,1, Neysha Lobo2, Sven Einfeldt1, Arne Knauer1, Markus Weyers1, and Michael Kneissl1,2

1 Ferdinand-Braun-Institut, Leibniz-Institut fur Hochstfrequenztechnik, Gustav-Kirchhoff-Str. 4, 12489 Berlin, Germany2 Institute of Solid State Physics, Technische Universitat Berlin, Hardenbergstr. 36, 10623 Berlin, Germany

Received 12 April 2010, revised 31 May 2010, accepted 10 June 2010

Published online 15 July 2010

Keywords current crowding, device design, GaN, light-emitting diodes, semiconductors

* Corresponding author: e-mail [email protected], Phone: þ49 30 6392 2759, Fax: þ49 30 6392 2642

GaN-based ultraviolet light-emitting diodes with identical

contact areas but different contact shapes are studied.

Interdigitated multifinger contacts with reduced finger width

result in a lower series resistance and a thermal roll-over of

the output power at higher cw currents in comparison to

square-shaped contacts. Under pulsed operation, the external

quantum efficiency in these devices is increased due to a

reduction of the efficiency droop at elevated currents.

Simulations of the current distribution explain the exper-

imental data with an improved uniformity of the current

density in multifinger contacts as current crowding at the

contact edges is largely suppressed.

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction The development of ultraviolet (UV)light-emitting diodes (LEDs) has recently attracted a lot ofattention. Water purification, sterilization, and decontami-nation in biochemical processes are only a few examples forthe potential applications of theses devices. Although theperformance of UV LEDs has been continuously improvedover the years, their external quantum efficiency (EQE) isstill low compared to that of devices emitting in the visiblespectral range [1]. Several factors are responsible for the lowEQE: (a) The efficiency of carrier injection in the activeregion is low due to a poor carrier confinement. (b) The rateof nonradiative recombination is higher than the rate ofradiative recombination due to a high density of threadingdislocations [2, 3]. (c) The emitted light is partially absorbedin the epitaxial layers or the contact metals. (d) The lightis inefficiently extracted from the chip due to total reflectionat the layer interfaces or at the chip surface. (e) The lowconductivity of the epitaxial layers results in currentcrowding at the edges of the contacts [4]. Finally, a lowEQE results in self-heating which further impairs theperformance of UV LEDs and makes it difficult to operatethe devices at high currents [5].

Many strategies have been pursued to increase the EQEof UV LEDs. On the side of the epitaxy, for instance,quaternary InAlGaN quantum barriers were used tocompensate for the strain in the active region, and electronor hole blocking layers were employed to reduce the carrier

leakage [6]. On the side of the chip processing, surfaces havebeen modified by roughening [7], texturing [8], or theimplementation of photonic crystal structures [9] to enhancethe light extraction efficiency. Moreover, the currentcrowding at the facing edges of the coplanar n- and p-contacts was studied, and a current spreading length wasderived to describe the nonuniform distribution of the current[4, 5]. Current crowding becomes a more and more severeproblem the further the emission wavelength of the LEDs ispushed deeper into the UV spectral region since the layerresistivity usually increases along with the bandgap [10].Multifinger contact geometries were reported to reducecurrent crowding. For example Guo et al. [11] showed thatthe optical power of 460 nm LEDs increases when thecurrent distribution gets more uniform by the use of aninterdigitated contact pattern. However, these authorsalso observed that the leakage current increases with theperimeter of the p-contact. Chitnis et al. [12] compared theefficiency of 315 nm LEDs with a multifinger and a squarecontact geometry and reported on the superiority of multi-finger contacts.

In this paper, GaN-based UV LEDs emitting at awavelength of 380 nm and having different contact geome-tries are studied. While the p-contact had always the samearea, its shape was either a square or a multifinger structurewith fingers of different widths. It is shown that devices withmultifinger contacts are superior in terms of the efficiency at


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Figure 2 (online color at: www.pss-a.com) Optical power versuscurrent for cw and pulsed operation of the LEDs with square andmultifinger contacts. Inset shows the I–V characteristics of the LEDwith square and multifinger contacts.

elevated cw currents. Based on measurements under pulsedcurrent injection and simulations of the current distributionin the device it is concluded that narrower contact fingersimprove the uniformity of the current injection and reduceself-heating.

2 Experimental The LED layer structures weregrown by metalorganic vapor phase epitaxy on 2 in. (0001)oriented sapphire substrates [13]. They consist of a 3.4mm-thick GaN:Si buffer layer, a 10 nm Al0.23Ga0.77N:Sihole-blocking layer, an InGaN/InAlGaN multiquantumwell (MQW) active region, followed by a 10 nmAl0.23Ga0.77N:Mg electron blocking layer and a 200 nmthick GaN:Mg contact layer [14]. Devices were fabricatedusing standard chip-processing technologies. Ohmic p-contacts to the GaN:Mg were fabricated by depositing Ni/Au, annealing it in an oxygen ambient and reinforcing it byTi/Au. The GaN:Si buffer layer was uncovered by plasmaetching and contacted by the deposition of Ti/Al/Mo/Au. Thearea of the p-contact of all devices was 0.0225 mm2 but theshape of the contact was varied. A 150mm� 150mm squareis compared with interdigitated multifingers whose widthis 40, 20, and 10mm, respectively, corresponding to 6, 10,and 14 fingers, respectively, as shown in Fig. 1. The devicemeasurements were performed on-wafer at room tempera-ture by analyzing the light emitted through the backside ofthe sapphire substrate. The emission spectra measured at acurrent of 20 mA showed a single peak at a wavelength of383 nm. Besides cw measurements the LEDs were alsooperated in pulsed mode with a pulse width of 3ms and apulse repetition rate of 1 kHz. The distribution of the current

Figure 1 (online color at: www.pss-a.com) Optical microscopyimages of the studied LED structures with different shapes of the p-contact: Square contact (a), multifinger contact with a finger width of40mm (b), 20mm (c), and 10mm (d), respectively.


injection across the active region was simulated using theSpeCLED software package [15].

3 Results and discussion The operation voltage ofthe devices was found to depend on the shape of the p-contact. The inset in Fig. 2 shows the current–voltagecharacteristics, from the linear part differential seriesresistances of 2.5, 2.0, 1.4, and 1.5V were obtained for thesquare contact and the multifinger contacts with a fingerwidth of 40, 20, and 10mm, respectively. Since the contactresistance between the metal and p-GaN can be assumedto be the same for all devices, the variation in the seriesresistance can be attributed mainly to a variation inthe current distribution in the devices. As the width of thep-contact decreases, the current distribution across thep-contact becomes more uniform. Therefore, the effectivearea of the current flow through the p-contact increaseswhich lowers its resistance. This idea is consistent with othermeasurements showing that the resistance of the p-contactaccounts for >70% of the total differential series resistanceof our LEDs.

Figure 2 shows the light output power versus currentcharacteristics (L–I) under cw and pulsed current operationof the different LEDs. For cw operation the L–I curves belowapprox. 70 mA are linear and almost similar for all devices,i.e., the EQE is independent of the contact shape in the lowcurrent range. Beyond a cw current of 70 mA the L–I curvesare nonlinear, and they roll-over at different currents. Thepoint of roll-over shifts to larger currents when the fingerwidth decreases. Therefore, the maximum output powerunder cw operation is achieved with the LED having amultifinger contact with the narrowest fingers of 10mmwidth. The increase of the output power by shifting from asquare contact to a multifinger contact is up to 50% at thesame cw current. Assuming that the roll-over of the L–Icurves solely results from self-heating of the devices, the

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Phys. Status Solidi A 207, No. 11 (2010) 2587

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Figure 3 (online color at: www.pss-a.com) External quantumefficiency versuspulsedcurrent ofLEDs with square and multifingercontacts (solid lines are guides to the eye).

Figure 4 (online color at: www.pss-a.com) Shift of the peak emis-sion wavelength with the current for cw (closed symbols) and pulsedoperation (open symbols) of LEDs with square and multifingercontacts.

heating in a multifinger contact is lower than the one in asquare contact due to the lower series resistance and/or amore efficient extraction of the heat from the contact.Considering the proposed non-uniform current distribution,the roll-over may also reflect a non-linear dependence of theoptical power on the carrier density in the active region of adevice operated at a constant temperature.

To distinguish between the effect of self-heating and anintrinsic dependence of the EQE on the carrier density,pulsed measurements have been performed. The correspond-ing L–I curves in Fig. 2 show that the output power under cwand under pulsed current operation is almost the same forcurrents below approx. 70 mA, whereas it is much larger forpulsed operation at elevated currents. Therefore, the roll-over under cw operation is largely due to self-heating of thedevice. However, also for pulsed operation theL–I curves arenot perfectly linear, and their slope decreases slightly withincreasing finger width. Figure 3 shows the EQE for pulsedoperation of the LEDs. Even at low pulsed currents of<100 mA the EQE is largest for the LEDs with the smallestwidth of the contact fingers. Since self-heating of the devicescan be excluded at these low pulsed currents, the data shownin Fig. 3 has to be discussed in terms of different carrierdistributions in the MQW rather than different devicetemperatures. The drop in efficiency with increasing current,as shown in Fig. 3, can be attributed to the widely discussedefficiency droop at high current densities in nitride basedLEDs [16]. The efficiency droop shifts to higher currentswhen the width of the contact fingers decreases. This fact isconsistent with the assumption that the current distributionbecomes more uniform when the width of the contact fingersis reduced. A reasonably uniform current injection avoidsareas with current densities largely above average where theefficiency would be reduced.

The self-heating of the LEDs can be studied by the shiftof the emission wavelength with the operation current, asshown in Fig. 4. For cw operation, the emission peak shifts tolonger wavelengths with increasing current. Assuming a

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linear dependence of the emission wavelength on the devicetemperature with a slope of 0.05 nm/K, the data suggests aheating of the LED with the square contact by approx. 300 Kat 300 mA. In comparison, the temperature of the LEDs withthe multifinger contacts increases only by 80–120 K at thesame current. These results agree with the differences in thethermal roll-over of the L–I curves discussed above.Contrary to cw operation, the shift of the emissionwavelength for pulsed operation is opposite. The shift toshorter wavelengths with increasing current can be attributedto a carrier induced screening of the quantum confined Starkeffect (QCSE) which is partially reduced by band gaprenormalization [17]. However, the shift of the emissionwavelength under pulsed operation is almost the same for allcontact shapes. Further studies must be done in order toinvestigate the influence on the blue shift of the local carrierdistribution in the active region for the different contactgeometries. An estimation of the thermal impedance ofdevice for cw and pulsed operation was done from the shift ofthe wavelength and the net input power. For cw mode thethermal resistance of the LED with square contact is268 K/W whereas that for the finger contacts is approx.110 K/W. Under pulsed mode the self-heating can beignored, however, for current beyond 800 mA a slightlyshift to longer wavelength was observed, which results in athermal resistance of a few mK/W for all contacts.

The spatial distribution of the current in the LEDs withdifferent contact shapes has been simulated. The devicedimension and material properties used in simulation are asfollow: thickness of p-GaN and n-GaN are tp¼ 0.2mmand tn¼ 1.7mm; electron and hole concentration aren¼ 4� 1018 cm�3 and p¼ 1� 1017 cm�3; mobilities ofholes and electrons are mp¼ 10 cm2/Vs and mn¼ 95 cm2/Vs;specific contact resistance to p-GaN and n-GaN are 7� 10�3

and 10�5V cm2. Figure 5 shows the results for a cwoperation at 100 mA. For the square-shaped contact it can


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Figure 5 (online color at: www.pss-a.com) Simulation of thespatial distribution of the current density across the active regionof LEDs with square and multifinger contact shapes at a total cw of100 mA. The operation voltage is 4.25, 4.19, 4.17, and 4.15 V for thesquare contact and the multifinger contacts with a finger width of40, 20, and 10mm, respectively.

be seen that significant current crowding occurs next to thecontact edges, as the current density varies by 20% from edgeto center. As the width of the fingers decreases, the currentdensity becomes more uniform. For the LED with thenarrowest contact fingers the variation of the current densityis less than 3%. This result agrees well with an estimation ofthe current spreading length based on the equation given inRef. [5] which gives a value of about 80mm when assumingtypical values for our contact and sheet resistances. The datain Fig. 5 also indicate that the forward voltages decreasewhen the contact fingers become narrower. This loweringindicates a reduction in the series resistance, which agreeswith the evaluation of the experimental I–V curves. Thecurrent crowding effect indicated by the simulationsconsistently explains the experimental data for the cwoperation of the LEDs, i.e., the roll-over in theL–I curves andthe current induced wavelength shift.

4 Summary UV LEDs with an emission wavelengtharound 380 nm having the same area of the p-contact butdifferent contact shapes have been studied experimentallyand by simulations. Differences in the series resistance, themaximum output power, the efficiency, and the currentinduced wavelength shift can be attributed to differentuniformities of the current density and different efficienciesof the heat extraction from the p-contact. Due to currentcrowding at the edges of the p-contact, the current uniformityis superior for LEDs with interdigitated contact fingersof small width. Therefore, those devices also exhibit the


smallest series resistance, the highest efficiency and thelowest heating of the active region under cw operation.Moreover, an increasing perimeter of the p-contact formultifinger contacts enhances the heat extraction from thecontact. Self-heating of the LEDs can be largely suppressedby pulsed current operation thus revealing the EQE droop.The enhanced uniformity of the current density in LEDs withmultifinger contacts of small width shifts the efficiencydroop to larger total currents. In conclusion, the overallperformance of UV LEDs is suggested to gain considerablyfrom the use of multifinger p-contacts of small width.

Acknowledgements The authors thank M. Hoppe for hisassistance in the electrical measurements. This work was supportedby the German Federal Ministry of Education and Research withinthe joint research project Deep-UV LEDs under the contractnumbers 13N9933 and 13N9934.

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gan-based ultraviolet light-emitting diodes with multifinger contacts

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