Electroluminescence power profiles and 2D micropatterns have been obtained from InAsSb/InAs planar LEDs tuned at several wavelengths within the 3–5 μm band. Light

confined to a small region around the top opaque contact was observed (cw mode, I > 10 mA, T = 300 K). A computer simulation showed that the reason behind the decrease of the emitting area is the current crowding that ensures non-uniform injection into the active region. The

effect becomes more apparent in longer wavelength devices (emitting areas of 3.4 and 4.2 μm emitting devices are related as >10 : 1), providing direct evidence that the current crowding is affected by the bandgap energy of an active layer

Malyutenko, O. Y.

Malyutenko, V. K.

Zinovchuk, A. V.

English

Zhytomyr State University Library

IOP PUBLISHING SEMICONDUCTOR SCIENCE AND TECHNOLOGYSemicond. Sci. Technol. 23 (2008) 085004 (3pp) doi:10.1088/0268-1242/23/8/085004Bandgap dependence of current crowdingeffect in 3–5 µm InAsSb/InAs planarlight emitting devicesV K Malyutenko, A V Zinovchuk and O Yu MalyutenkoLashkaryov Institute of Semiconductor Physics, 03028 Kiev, UkraineE-mail: malyut@isp.kiev.uaReceived 4 April 2008, in final form 16 May 2008Published 13 June 2008Online at stacks.iop.org/SST/23/085004AbstractElectroluminescence power profiles and 2D micropatterns have been obtained fromInAsSb/InAs planar LEDs tuned at several wavelengths within the 3–5 µm band. Lightconfined to a small region around the top opaque contact was observed (cw mode, I > 10 mA,T = 300 K). A computer simulation showed that the reason behind the decrease of the emittingarea is the current crowding that ensures non-uniform injection into the active region. Theeffect becomes more apparent in longer wavelength devices (emitting areas of 3.4 and 4.2 µmemitting devices are related as >10 : 1), providing direct evidence that the current crowding isaffected by the bandgap energy of an active layer.1. IntroductionA key challenge for infrared (IR) light emitting diode (LED)development is in accessing power output >10 mW in the3–5 µm band, where a potential exists for applications indynamic IR scene projection technology [1, 2]. To thisend, IR LEDs have attracted a lot of attention due to avery short time constant, the ability to simulate cold [3, 4]and hot scenes and easy fabrication. These devices forma platform for photonic ‘synthetic’ scene projectors that arecapable of competing with advanced thermal microemittertechnology [5] in testing IR sensors, including forward-looking missile warning systems, search-and-track devices andmissile seekers. However, the current generation of IR LEDsenables power outputs of about 1 mW at room temperature [6],and, therefore, these devices cannot simulate targets with hotengine exhausts or rocket plumes. One reason that limitsthe output power of IR LEDs is the non-radiative Augerrecombination of carriers. This process ensures the internalquantum efficiency of uncooled devices to be as low as 20%(<0.03% the external efficiency due to refractive losses at a flatLED surface [7]), causes sub linear light-current dependences[2], and makes LEDs operate at high current density only ina pulsed mode. The attempts to increase the power emittedby scaling up dimensions of the junction area do not boost theoutput significantly because of a problem posed by ‘currentcrowding’ (CC). In planar structures, the CC manifests itselfby forming a region of high current density in the vicinityof the top contact [8–10], resulting in a reduction of theeffective emitting area [2, 10] and the local overheating ofa structure [2, 10, 11]. As a result, this phenomenon causesthe internal quantum efficiency to decrease [12, 13] as well asa catastrophic degradation of the device [14]. Since the reasonbehind the CC lies in the interaction of the lateral currentcomponent with the sheet resistances of a many-layer diodestructure, numerous efforts in minimizing CC are focusedon the resistance and thickness of the substrate, cladding orspreading layer, as well as contact geometry. Contrary tothis tendency, in what follows, the CC is analyzed in termsof the bandgap energy Eg of the active layer in LEDs thatare tuned at several wavelengths within the 3–5 µm band.We show both theoretically and experimentally that the CC ismost pronounced in the longer-wavelength emitting devices,making the conventional planar LED design inefficient forapplication to fields that require intense IR light.2. ExperimentThe LEDs obtained from an industrial manufacturer wereInAsSbP/InAsSb double heterostructures grown on 200 µmthick p-InAs substrates (ps ∼ 1018 cm−3) by liquid phaseepitaxy. The structures consisted of a 0.5 µm thickp-InAsSbP confining (pc ∼ 1017 cm−3) layer, a 2 µm thickn-InAsSb active (na ∼ 2 × 1016 cm−3) layer and a 5 µm0268-1242/08/085004+03$30.00 1 © 2008 IOP Publishing Ltd Printed in the UKSemicond. Sci. Technol. 23 (2008) 085004 V K Malyutenko et alFigure 1. (a) Micrograph of an LED with the top point p-contact,(b) equivalent circuit model, real layer thicknesses are shown in thetext and (c) current density profiles in devices tuned at different λp(along the diagonal crossing, theory).thick n-InAsSbP confining (nc ∼ 2 × 1017 cm−3) transparentlayer. Their advantage consisted of the fact that the peak-emitting wavelength (λp) could be tailored over a 3–5 µmband only by changing the Sb content (which determinesthe bandgap value) in the active layer. The emitting chip(figure 1(a)) was packaged into the TO-39 case as a substrate-down planar 450 µm square structure with a top centrally-located metal contact (d = 100–150 µm). The point ofinterest lies in the lateral distribution of current in the activelayer (computer simulation) and spatial non-uniformity of light(experimental micro mapping) in the devices tuned at differentλp and operating in the pulsed mode (160 ms pulse duration)at T ∼ 300 K. The light pattern in four LEDs (λp = 3.4, 3.6,3.8, 4.2 µm) was mapped by a test system based on an IRcamera operating in a 3–5 µm band while Joule heating wasmonitored by an 8–12 µm camera. Used in conjunction withan IR microscope, this two-band set-up has been routinelyachieving the 10 µm lateral scanning step [2, 8]. To minimizethe LEDs overheating caused by the CC and non-radiativerecombination (<0.1 ◦C), the study was limited by the 40 mAbias current which is a recommended value for the cw mode.3. ModelThe lateral distribution of current density in the active layerof a device was simulated using the finite-element approach.It was assumed that the electric charges are localized in thespace-charge region of the junction, that the other regionsof a structure are neutral, and that the diffusion componentof the current may be neglected. Therefore, the potentialdistribution follows from the Laplace equation, while the localcurrent density is connected to the potential gradient via theOhm law. The p–n junction was represented as a laterallydistributed nonlinear resistor (Rj) with a diode-like current–voltage (I–V) dependence I = I0[exp(Vj/mVT) − 1], whereI0 is the saturation current, Vj the voltage drop across thejunction, m the ideality factor and VT the thermal voltage.The contact resistance was ignored and the condition m = 1was supposed to be valid. In such approximations, the onlydifference between the structures tuned at different λp wasthe I0 value that is connected to the square of intrinsic carrierconcentration ni with the concentration being dependent on amaterial bandgap energy(I0 ∼ n2i ∼ exp(−Eg)).Figure 1(b) shows the cross-section of an LED structureand two possible current paths from the n to p contact. AsRn1 < Rn2, Rp1 + Rps1 = Rp2 + Rps2, then the voltage dropacross the junction 1 is higher than that of the junction 2. Dueto the nonlinear nature of the junction resistance, the inequalityRj1 < Rj2 becomes valid at Vj1 > mVT. Moreover, the higherthe forward-biased voltage, the more preferable is the verticalcurrent path (I1 > I2) and the current crowds more effectivelyunder the top contact.4. Results and discussionThe lateral distribution of the current density in the activeregion of LEDs tuned at different λp is shown in figure 1(c).There are two specific features in this figure. At a low forwardbias (I < 10 mA), the CC effect is negligible in all LEDs and thecurrent spreads practically over the whole emitting region; thecurrent density at the perimeter is only 7–12% less (dependingon the Eg) than that in the center. This is due to the fact that ata low voltage the relation Rj1 ∼= Rj2 remains valid (there is nojunction barrier and depletion length modulation) and of thetwo current routes the central one is only slightly preferablesince Rn1  Rn2. At a higher bias, a stronger dependenceof the CC on the injection current comes from the nonlinearresistance of a junction that exponentially decreases when thevoltage increases. As Vj1 > Vj2, it is apparent that the higherthe applied voltage, the stronger is the inequality Rj1 < Rj2, andthe central current route through the junction 1 becomes morepreferable; at I = 40 mA, the current density at the perimeter ofthe emitting area decreases by 1.5–4.5 times. Most importantis that at the longer wavelength emitting devices, the CCeffect is stronger and the magnitude of the injection currentat the perimeter of a device is much lower. This trend is thedirect effect of the dependence of the junction resistance on amaterial bandgap: at a longer wavelength emitting LED witha lower junction resistance, the n-confining layer cannot servesimultaneously as a spreading layer.Figure 2 illustrates how the CC modifies the light patternand prevents IR LEDs from having a large emitting area.Shown in figure 2(a) is the diagonal crossing of light emittedby several devices at I = 40 mA. The central dip at the curvesand maps is due to the top contact that prevents some of thelight from passing through the contact area while the non-zero2Semicond. Sci. Technol. 23 (2008) 085004 V K Malyutenko et alFigure 2. (a) Experimental electroluminescence power profiles inLEDs with different λp (note, the diameters of top contacts areslightly different) and (b) spatial view of the power emitted. All thetests are taken at I = 40 mA.signal in the dip is due to reflection of a Lambertian lightpattern by the contact. When conditionally determined as thedistance from the center of the top contact to the position ofhalf maximum of power emitted, the CC lengths were >300,300, 220, and only 100 µm for the devices emitting at 3.4, 3.6,3.8 and 4.2 µm, respectively. Such light that is confined to asmall region around the top non-transparent contact causes theemitting area to catastrophically decrease in longer wavelengthdevices (figure 2(b)). When subtracting the size of the topcontact platform, the effective emitting area is compared witha LED surface in both of the shorter wavelength emittingdevices, but it decreases by half (48%) in 3.8 µm LEDs andfinally reduces to 7% in 4.2 µm LEDs.Of two ways toward increasing the emitting area inlonger wavelength LEDs (increasing the doping density orthe thickness of the n-type confining layer), layer thicknesscontrol appears to be preferable.5. ConclusionsThis study has provided direct evidence of the materialbandgap effect on the CC in 3–5 µm InAsSb/InAs planarLEDs through the observation of spatial non-uniformityof light in the micro scale. It has been experimentallydemonstrated that in structures with a centrally located topcontact, the modification of the light pattern by the CCcannot be neglected even at current as low as 40 mAwhich is a recommended value for cw operation. This pureelectronic effect that is connected to the nonlinear resistance ofp–n junction drastically affects the performance of red-shiftedLEDs. Our calculations and tests have shown that devicesmade of a narrower bandgap material (λp = 4.2 µm) havemuch less uniform injected current density, compared to thosewith a wider bandgap value (λp = 3.4 µm). Therefore, thisleads to a catastrophic decrease in the effectively emittingarea (emitting areas of 3.4 and 4.2 µm emitting devices arerelated as >10:1) as well as a higher local heating in red-shifted devices. Finally, the lateral thermal gradient maycause thermal lensing and uncontrollable deformation of thelight pattern. Such observations can lead to an improveddevice design with a larger emitting area, more uniform currentdensity in the confining layers, a lesser thermal gradient in theactive region and, lastly, a higher efficiency at an extremeoperation mode. In our opinion, the room for improvement islimited due to the trade-off between CC impact and opticaltransparency of the top confining layer. The calculationshows that for 4.2 µm LEDs the five-fold increase in dopinglevel or thickness of the layer causes only slightly uniformcurrent spreading (Id/ID = 2.25 compared to Id/ID = 5in the LED under test). However, it results in the five-foldincrease of absorption coefficient due to activation of the freecarrier absorption.AcknowledgmentThe authors gratefully acknowledge financial support by theMinistry of Education and Science within the Ukrainian–Russian Nanotechnology Project (contract M/190-2007).References[1] Das N C, Olver K, Towner F, Simonis G and Shen H 2005Appl. Phys. Lett. 87 041105[2] Malyutenko V K, Malyutenko O Yu and Zinovchuk A V 2006Appl. Phys. Lett. 89 201114[3] Malyutenko V 2004 Physica E 20 553[4] Lindle J R, Bewley W W, Vurgaftman I, Meyer J R,Thomas M L, Piquette E C, Edwall D D and Tennant W E2007 Appl. Phys. Lett. 90 241119[5] Robinson R, Oleson J, Rubin L and McHugh S 2000 Proc.SPIE 4027 387[6] Sherstnev V V, Monakhov A M, Krier A and Hill G 2000Appl. Phys. Lett 77 3908[7] Kane M J, Braithwaite G, Emeny M T, Lee D, Martin T andWright D R 2000 Appl. Phys. Lett. 76 943[8] Matveev B A, A’daraliev M, Zotova N V, Karandashev S A,Remenniy M A, Stus’ N M, Talalakin G N,Malyutenko V K and Malyutenko O Yu 2001 Proc. SPIE4285 109[9] Malyutenko V K, Malyutenko O Yu, Podoltsel A D,Kucheryavaya I N, Matveev B A, Remennyi M A andStus’ N M 2001 Appl. Phys. Lett. 79 4228[10] Malyutenko V K, Malyutenko O Yu, Dazzi A, Gross N andOrtega J M 2003 J. Appl. Phys. 93 9398[11] Monakhov A, Krier A and Sherstnev V V 2004 Semicond. Sci.Technol. 19 480[12] Kettle J, Perks R M and Dunstan P 2006 Electron. Lett.42 1122[13] Guo X and Schubert E F 2001 Appl. Phys. Lett. 78 3337[14] Malyutenko V K, Malyutenko O Yu, Zinovchuk A V,Zakheim A L, Zakheim D A, Smirnova I P and Gurevich SA 2005 Proc. SPIE 5941 59411K3

Bandgap dependence of current crowding effect in 3–5 μm InAsSb/InAs planar light emitting devices

Abstract

Similar works

Full text

Available Versions

Zhytomyr State University Library