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Phys. Status Solidi C 8, No. 7–8, 2170–2172 (2011) / DOI 10.1002/pssc.201001051

Origin of the “green gap”: Increasingnonradiative recombination inindium-rich GaInN/GaN quantumwell structuresTorsten Langer*, Andreas Kruse, Fedor Alexej Ketzer, Alexander Schwiegel, Lars Hoffmann, Holger Jonen,Heiko Bremers, Uwe Rossow, and Andreas Hangleiter

Institute of Applied Physics, Technische Universitat Braunschweig, Mendelssohnstraße 2, 38106 Braunschweig, Germany

Received 17 September 2010, revised 29 Oktober 2010, accepted 26 November 2010Published online 20 May 2011

Keywords

∗ Corresponding author: e-mail torsten.langer@tu-bs.de, Phone: +49-531-3918528, Fax: +49-531-3918511

Using time-resolved photoluminescence spectroscopyon GaInN/GaN multiple quantum well structures, we an-alyze the radiative and nonradiative processes contribut-ing to the “green gap” in GaN-based light emitting de-vices. We observe that it is only partly caused by a re-duced oscillator strength due to the Quantum ConfinedStark Effect (QCSE) which becomes stronger with in-creasing indium concentration and well width.

As the dominant effect we observe a reduction of non-radiative lifetimes when the indium concentration is in-creased. For higher indium concentrations, we find anadditional nonradiative recombination path that might beattributed to an increased generation of defects like mis-fit dislocations, nitrogen vacancies and/or indium clus-ters within the optically active region.

1 Introduction Optoelectronic devices based onGaInN layers suffer from a steep drop of efficiency whenthe peak emission wavelength is increased towards thegreen region [1]. This phenomenon is well known as“green gap”. It does not only limit the efficiency of greenlight emitting diodes. The performance of green emittinglaser diodes — although functioning laser diodes were re-cently demonstrated up to 532 nm [2] — is subject to itas well. An efficient and reproducible fabrication of greenlaser diodes and LEDs demands a full understanding of theorigin of the green gap.

2 Experimental details Our samples were grownusing low pressure metalorganic vapor phase epitaxy (Aix-tron AIX200RF) on c-plane sapphire. A single sample wasgrown on pseudo-bulk HVPE GaN instead to obtain a re-duction of threading dislocation densities (from 109 cm−2

down to a value in the range of 107 cm−2 and 108 cm−2).All samples consist of a 5-fold GaInN/GaN quantum well(QW) structure on top of a n-type doped GaN buffer layer.

The indium concentration xIn (18% - 38%) and the quan-tum well thickness tQW (1.0 - 2.0 nm) have been varied bychanging growth temperature and duration. Both xIn andtQW were determined by high resolution X-ray diffrac-tometry (HRXRD) using the method described in [3]. Inall samples, the last GaN barrier is the uppermost layer.On bulk GaN, the buffer layer was replaced by a 100 nmhomoepitaxial GaN layer.

The optical properties were analyzed using tempera-ture dependent (5 - 300 K) time-resolved photolumines-cence (PL) spectroscopy. The samples were excited with5 ps laser pulses with an incident energy density of 5 -10 nJ/cm2 and 4 MHz repetition rate. We used resonant ex-citation at a laser wavelength of 380 nm, using the secondharmonic of a mode-locked and cavity-dumped dye laser(dye: Pyridin 2), which is optically pumped by the secondharmonic of a synchronously mode-locked Nd:YAG laser(532 nm). Thus, carrier densities in the order of 109 cm−2

were generated with each laser pulse in each quantumwell. The monochromated (Jobin Yvon - Spex 1680) PL

time-resolved, photoluminescence, recombination, GaInN, quantum wells

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 1 Intensity decay after pulsed excitation at different tem-peratures. Left: low xIn, emission at 450 nm. Lifetimes at 300 Kare comparable to those at low temperatures. Right: high xIn, PLmaximum at 525 nm. Lifetimes at room temperature are stronglyreduced. (PL spectra are shown in Fig. 4).

signal was detected with a Hamamatsu microchannel platephotomultiplier tube and processed using time-correlatedsingle photon counting (PicoHarp 300) with a total time-resolution of 25 ps.

3 Results and discussion The time dependence ofthe PL intensity after pulsed excitation for two sampleswith different indium concentrations in the QWs is shownin Fig. 1. These intensity transients represent the spectralrange of the emission peak where the PL intensity exceeds10% of the peak intensity. They were analyzed at the initialpart of the decay as the generated carrier density is fixedin case of short excitation pulses (� carrier lifetime). Theinitial parts of the transients are fitted with single expo-nential functions to determine the initial lifetime τinit aswell as the initial intensity I0. While the first parameteris a measure of the total recombination rate at the initialcarrier concentration, the latter represents exclusively theradiative part. In this way we have a direct measure of theinternal quantum efficiency η by simply multiplying bothparameters and determining the ratio of this value to itslimit value towards T = 0 K assuming η = 100% at sat-uration. This assumption gives us an upper limit of η(T ),but further analyses show that there is also a lower limit ofη(T = 0) at about 90%.The temperature dependence of these parameters are usedto determine the radiative and nonradiative lifetimes, τr

and τnr respectively:

τr =τinit

η∝

1

I0

τnr =τinit

1− η(1)

These informations were used to analyze the influence ofradiative and nonradiative processes on the green gap byexamining MQW structures with different indium concen-trations and quantum well thicknesses. xIn and/or tQW

have to be increased when longer emission wavelengthsare desired. Unfortunately, both methods also cause a re-duction of oscillator strength [4] due to the QCSE [5]:

Figure 2 Radiative lifetimes vs. the product of indium contentand QW thickness which is a measure for the QCSE (see inset,VB: valence band, CB: conduction band). Dashed line as guidefor the eyes.

Increasing piezoelectric fields in the QWs at higher xIn in-duce a more effective separation of electrons and holes toopposite sides of the QWs. Moreover, the distance betweenthese sides (=tQW) strongly affects the overlap of the en-velope wavefunctions of electrons and holes. Consideringboth, we plotted the radiative lifetimes against the productof xIn and tQW which estimates the strength of QCSE(see Fig. 2). As expected, the QCSE strongly increases theradiative lifetimes, making radiative processes more andmore unlikely when approaching the green region. This isa well known contribution to the green gap that motivatesthe growth of QW structures on non- or semipolar sur-faces, where QCSE plays no (significant) role. However,an efficiency reduction towards the green is also observedon those surfaces [6], implying a significant contributionof nonradiative recombination mechanisms.Figure 3 clearly shows that differences in nonradiativelifetimes are also present in polar QWs. We observe thatnonradiative processes become more and more likely forhigher xIn (see also Fig. 4 (left) showing the evaluatedlifetimes of the transients from the samples in Fig. 1) thussupporting most prominently the green gap. As the life-times represent the recombination at low carrier densities,we exclude that these results were significantly influenced

Figure 3 Strong shortening of nonradiative lifetimes towardshigher indium concentration. Possible explanations: influences ofgrowth temperature, strain, In cluster. Dashed line as guide forthe eyes.

Phys. Status Solidi C (2011) 71

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Figure 4 Left: Increasing in-dium concentration leads tostrong shortening of nonradia-tive lifetimes at room tempera-ture: 0.2 ns for xIn = 30.9% vs.5 ns for xIn = 18.4%. Dashedlines are fits based on thermalactivation of nonradiative pro-cesses. Right: Room tempera-ture PL spectra of both samples.

by Auger recombination.Instead, there are several other possible explanations: Withincreasing xIn, the lattice mismatch between QWs andbarriers becomes larger. This might support the generationof local misfit dislocations slightly reducing the strain inthe QWs [7]. They might act as traps or nonradiative re-combination centers for the excited carriers contributing tothe observed shortening of nonradiative lifetimes.We emphasize that threading dislocations being gener-ated in layers beneath the optically active region are notlikely to explain this observation: Their density primarilydepends on the growth conditions of the subjacent lay-ers and is not (significantly) affected by the growth ofthe active region. Additionally, the indium-rich structureon pseudo-bulk GaN substrate (see Fig. 3, peak emis-sion at 525 nm) exhibits similar nonradiative lifetimes ascomparable MQW structures on sapphire. This indicates,that threading dislocations are a comparably inefficientnonradiative recombination center at elevated indium con-centrations.A further possibility is based on the weak bonds betweennitrogen and indium atoms [8]. It is conceivable that theypartially diffuse out leaving N- and In-vacancies in theepitaxial layers. Besides, cluster-like agglomerations ofindium atoms might occur when In-N bonds are splitted inIn-rich areas.The density of nitrogen vacancies might increase towardshigh indium concentrations also due to another phe-nomenon: Dissociation of ammonia — being the source ofatomic nitrogen — is not as effective as for the growth ofQWs with lower xIn, because the growth temperature hasto be significantly reduced for a better In incorporation.All these effects would end up in an increased density ofdefects at higher indium concentrations acting as nonra-diative recombination centers and thus reducing the non-radiative lifetimes. At this point, we cannot distinguishbetween the nonradiative paths. We only have excess to asingle effective nonradiative lifetime.To gain insight into the nonradiative processes contributingto the short lifetimes, we fitted the temperature dependencewith a model based on thermal activation of nonradiativerecombination mechanisms (see Fig. 4). For the blue emit-ting sample, it can be well described with a single activa-

tion energy whereas the green emitting sample exhibits afurther process with a lower activation energy. Neverthe-less, the dominant contribution at room temperature hasnearly the same activation energy as the low indium sam-ple. This indicates that similar nonradiative mechanismsare active in the analyzed range of indium contents, buttheir effectiveness increase strongly towards higher valuesof xIn.

4 Conclusions We have shown that the efficiencyreduction towards longer peak emission wavelengths ofGaInN/GaN QWs is not solely caused by a reduced os-cillator strength due to QCSE. Most importantly, we haveobserved a strong shortening of nonradiative lifetimes intime-resolved photoluminescence spectroscopy when theindium concentration in the QWs is increased. The tem-perature dependence of nonradiative lifetimes in MQWswith high and low indium concentrations teach us that thedominant nonradiative recombination mechanisms at roomtemperature have nearly the same activation energy.

Acknowledgements We gratefully acknowledge the finan-cial support of the Bundesministerium fur Bildung und Forschung(BMBF) in the frame of the “ERA-SPOT True Green” project.

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