We have used magnetophotoluminescence to study the impact of different capping layer material combinations (InP, GaInAsP quaternary alloy, or both InP and quaternary alloy) on lateral confinement in InAs/InP quantum dots (QDs) grown on (311)B orientated substrates. Exciton effective masses, Bohr radii, and binding energies are measured for these samples. Conclusions regarding the strength of the lateral confinement in the different samples are supported by photoluminescence at high excitation power. Contrary to theoretical predictions, InAs QDs in quaternary alloy are found to have better confinement properties than InAs/InP QDs. This is attributed to a lack of lateral intermixing with the quaternary alloy, which is present when InP is used to (partially) cap the dots. The implications of the results for reducing the temperature sensitivity of QD lasers are discussed. ©2005 American Institute of Physic

Caroff, P.

Cornet, Charles

Even, Jacky

Folliot, Hervé

Hayne, Manus

Labbé, Christophe

Le Corre, Alain

Levallois, Christophe

Loualiche, Slimane

Moshchalkov, Victor V.

English

Caroff, Philippe

Moshchalkov, V.V.

Portail HAL UNIV-RENNES

Impact of the capping layers on lateral confinement in InAs/InP quantum dots for 1.55 µm laser applications studied by magnetophotoluminescence

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APPLIED PHYSICS LETTERS 87, 233111 2005Impact of the capping layers on lateral confinement in InAs/ InP quantumdots for 1.55 m laser applications studied by magnetophotoluminescenceC. Cornet,a C. Levallois, P. Caroff, H. Folliot, C. Labbé, J. Even,A. Le Corre, and S. LoualicheLaboratoire d’Etude des Nanostructures à Semiconducteurs (LENS), CNRS UMR FOTON 6082 INSA, 20Avenue des Buttes de Coësmes, 35043 Rennes Cedex, FranceM. Hayne and V. V. MoshchalkovPulsed Field Group, Laboratory of Solid-State Physics and Magnetism, K.U. Leuven, Celestijnenlaan 200D,B-3001 Leuven, BelgiumReceived 30 June 2005; accepted 26 September 2005; published online 30 November 2005We have used magnetophotoluminescence to study the impact of different capping layer materialcombinations InP, GaInAsP quaternary alloy, or both InP and quaternary alloy on lateralconfinement in InAs/ InP quantum dots QDs grown on 311B orientated substrates. Excitoneffective masses, Bohr radii, and binding energies are measured for these samples. Conclusionsregarding the strength of the lateral confinement in the different samples are supported byphotoluminescence at high excitation power. Contrary to theoretical predictions, InAs QDs inquaternary alloy are found to have better confinement properties than InAs/ InP QDs. This isattributed to a lack of lateral intermixing with the quaternary alloy, which is present when InP isused to partially cap the dots. The implications of the results for reducing the temperaturesensitivity of QD lasers are discussed. © 2005 American Institute of Physics.DOI: 10.1063/1.2132527In the last few years, there has been a great interest insemiconductor nanostructures, and especially quantum dotsQDs.1 Indeed, the zero-dimensional confinement effect inQDs improves the performance of lasers and wavelengthswitching devices, and can be used for optical memories.1Considering the laser application, such a zero-dimensionalconfinement is of prime importance in order to guarantee theinsensitivity of the device to temperature. Following one ofthe main quality criteria usually assumed for device tempera-ture insensitivity, the difference between ground-state transi-tion and first-excited-state transition should be larger thankbT=25 meV at RT. In this regard, physical parameters likeexciton Bohr radii, exciton binding energy, and energy dif-ferences between ground transition and first excited transi-tions are well adapted to describe the lateral confinementbehavior in QDs.In order to reach the 1.55 m 0.8 eV wavelength usedin optical telecommunications, QDs are usually grown onInP substrates.2–5 Growth on 311B oriented substrates leadsto good quality QD structures with high densities and lowsize dispersion. An original double cap method has been de-veloped for controlling the QDs emission energy.4 The un-derstanding of the impact of both the first cap layer and thesecond cap layer on electronic and optical properties is thuscrucial for laser applications. Several material combinationshave been considered to improve QD laser performance. Us-ing the QDs/1st cap/2nd cap notation, InAs/ InP/ InP QDs areusually considered as a reference system.4,5 But this systemhas very poor optical confinement and is not suitable forlaser applications. InAs/ InP/Q1.18 QDs, where Q1.18 is aquaternary alloy In0.8Ga0.2As0.435P0.565 emitting at a wave-length of 1.18 m, have been used to generate laseraAuthor to whom correspondence should be addressed; electronic mail:charles.cornet@ens.insa-rennes.fr0003-6951/2005/8723/233111/3/$22.50 87, 23311Downloaded 30 Nov 2005 to 134.58.253.114. Redistribution subject toemission.6 Recently, InAs/Q1.18/Q1.18 structures have suc-cessfully produced laser emission.6,7 In this work, we studylateral confinement properties by magnetophoto-luminescence8,9 and high excitation power luminescence inthese three kinds of samples and show that using the quater-nary alloy is necessary for the insensitivity to the tempera-ture of QDs lasers emitting at 1.55 m.Six samples of single layer InAs/ InP/ InP samples Aand A, InAs/ InP/Q1.18 samples B and B andInAs/Q1.18/Q1.18 samples C and C QDs have beengrown on an InP 311B substrate by gas-source molecularbeam epitaxy using the Stranski-Krastanov method Fig. 1.Samples A, B, and C are designed to emit around 0.8 eVat low temperature for magnetophotoluminescencemagneto-PL experiments.4 Samples A, B, and C are de-signed to emit around 0.8 eV at RT for high excitation powerphotoluminescence PL experiments.4 The sample design isdifferent for the two experiments in order to match the de-FIG. 1. 22 m2 atomic force microscopy pictures of uncapped InAs/ InPQDs similar to sample A and A a and uncapped InAs/Q1.18 QDs similarto samples B, B, C, and C b. Density and radius are similar in bothpictures. The z scale goes from 0 nm dark areas to 8 nm bright areas,which corresponds to the highest dots.© 2005 American Institute of Physics1-1 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp233111-2 Cornet et al. Appl. Phys. Lett. 87, 233111 2005tector spectral bandwidth. In Fig. 1, atomic force microscopypictures are presented both for uncapped InAs QDs on InPsimilar to sample A and A and InAs QDs on Q1.18 simi-lar to samples B, B, C, and C. From these pictures, acomparison can be made between these two kinds of QDs.Densities are found to be 5.81010 cm−2 on InP sample, and6.11010 cm−2 on the Q1.18 sample. Dot radii are un-changed from one sample to another, equal to about18±3 nm for Q1.18 and 19±4nm for InP. These valuesare reported in Table I. The similarity between theses twosamples allows us to interpret any further differencesbetween samples A, B, and C as a consequence of thecapping procedure and/or confinement induced by the barriermaterials.Magneto-PL experiments are carried out at 4.2 K in a Hebath cryostat placed in the bore of a pulsed magnet with amaximum field of 50 T.10,11 The field is applied parallel tothe growth direction z. A single 550 m core optical fiberis used to collect the PL signal, which is excited by the lightfrom a cw frequency-doubled Nd:yttrium-aluminium-garnetYAG laser at 532 nm via a second fiber. A cooled InGaAslinear diode array coupled to an optical spectrum analyzer isused to detect the PL. The impact of a magnetic field on QDssamples has been already studied.8,10 Following this exci-tonic model, at zero and low magnetic fields, the electronand hole within the dot are strongly spatially confined by thephysical boundaries of the dot. In these conditions, magneticfield can be treated as a perturbation in the Hamiltonian,leading to a square dependence of the energy shift on themagnetic field: E=e22B2 /8r,8,10 for BBc=2 /e2where 2 is the in-plane effective exciton radius Bohrradius, r is the in-plane exciton effective mass, B is themagnetic field, and Bc is the crossover magnetic field, whichdepends only on 2. At sufficiently high field BBc,when the attempted Larmor radius is smaller than the spatialsize of the dot, the charges become confined by the field inthe plane perpendicular to the applied field, and the energylevels shift linearly with B :E=eB /2r.8,10 Using this ex-pression, measurements can be fitted with only two free pa-rameters: the effective mass and the Bohr exciton radius.Consequently, the exciton binding energy is given by:EBINDINGX=2 /2r2.This analysis has been performed on data from the threesamples. Values for exciton Bohr radii and exciton bindingenergies are presented in Table I. Figure 2 represents theevolution of photoluminescence for samples B and C as afunction of B2. The dotted line represents the extrapolation ofthe linear fit for sample C in the 0;713 T2 range. It is clear2TABLE I. Lateral confinement parameters for samples A, A, B, B, C, andC. QD diameters, exciton binding energies, exciton Bohr radii, and energydifferences between excited-state and ground-state transitions are presented.SamplesAFM diameternmEBINDINGXmeV2nmE1 /E2meVA and AInAs/ InP/ InP38 7 7.7 9/24B and BInAs/ InP/Q1.1836 7 7.8 10/23C and CInAs/Q1.18/Q1.1836 12 6.9 26/39that both curves deviate from a linear B dependence. TheDownloaded 30 Nov 2005 to 134.58.253.114. Redistribution subject tofield at which this deviation occurs depends only on 2,thus it can immediately be seen that the lateral spatial extentof the wavefunction is smaller in sample C than in sample Band in sample A see also Table I. From these data we seethat the first capping layer is of prime importance for thelateral confinement, which is increased by using Q1.18 in-stead of InP in the first capping layer. This finding is furthercorroborated by the exciton binding energy, which is 70%larger for sample C than for samples A and B. The observa-tion of enhanced confinement with Q1.118 as a first cap layeris, however, not consistent with published theory. Parameterslike EBINDINGX,2, or r, representing the lowest limit forelectronic effective mass, have been studied in previousworks.12–14 InP has a larger band gap than Q1.18, whichshould lead to better lateral confinement, i.e., a largerEBINDINGX and a smaller 2. Thus the difference in confine-ment between the samples cannot be explained by the intrin-sic nature of the barrier material, but is a consequence of thegrowth procedure. Moreover, we have demonstrated that thedifference appears when the first capping layer compositionchanges. The strength of the lateral confinement potential islinked to two parameters: the height of the confinement bar-rier, and the spatial extent of the confinement potential.While the total height of the confinement barrier cannot bechanged, the spatial extent of the confinement potential is notfixed during the growth. Lateral diffusion of atoms duringthe growth leads to a spatial gradient of composition andconfinement potential. Several growth phenomena are basedon this mechanism, such as indium segregation, demixing orintermixing.15 When we compare our results with thecalculations,12–14 a larger difference is noticed for InPsamples between theory and experiment than for Q1.18sample. Indeed, these calculations did not take into accountany growth effect during the capping procedure. Consideringthat P2 growth interruption enhances the As/P exchange dur-ing the growth after first cap deposition,4 we propose tointerpret the difference between experiment and calculationsas a result of a lateral intermixing effect between InAs andthe InP first capping layer, during the double cap procedure.This intermixing effect leads to a composition gradient in thelateral plane, reducing the lateral confinement in InP basedsamples A and B. This effect is lower with a Q1.18 firstcapping layer. Therefore, sample C has a better lateral con-FIG. 2. Magnetophotoluminescence measurements for samples B and C.Evolution of ground-state transition peaks is plotted as a function of thesquare of the magnetic field. The dashed line is the extrapolation of a linearfit for sample C between 0 and 713 T2. The influence of the first cappinglayer composition on lateral confinement is shown by the field at which thedata deviate from a straight line.finement than samples A and B. In order to study the conse- AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp233111-3 Cornet et al. Appl. Phys. Lett. 87, 233111 2005quences of such a difference on spectral and electronic prop-erties, high power photoluminescence experiments areperformed on similar samples.High power PL experiments are carried out at RT. Theluminescence is excited with a YAG laser at 1.06 m fo-cused on the sample with a microscope objective, leading toa maximum excitation power of 170 kW cm−2. An InGaAsdetector is used, coupled to an optical spectrum analyzer.Figure 3 represents luminescence spectra for samples A, B,and C. Luminescence is performed for various excitationpowers, so that excited-state transitions can be seen easily.Measured differences between ground and excited state tran-sitions are reported in Table I. Consequently, sample Cseems to be very different from samples A and B. The linkbetween the distance of the excited transition to the groundtransition and the lateral confinement has already beenshown previously.16 The stronger the lateral confinement is,the farther the excited-state transitions are from ground-statetransitions. In this regard, the InAs/Q1.18/Q1.18 samplesare confirmed as the strongest confining structures.These spectral considerations are of direct relevance tolaser applications. One of the main quality criteria usuallyassumed for device temperature insensitivity is given by E1and E2kbT=25 meV at RT. In this respect, samples A andB with a first capping layer of InP are not suitable for laserapplications while sample C, with a first capping layer ofQ1.18, is convenient in order to develop temperature insen-sitive lasers. Moreover, samples C and C have better opticalconfinement, leading to better carrier injection.6 Insensitivityto the temperature of QDs lasers could perhaps be furtherimproved, by using other materials, or by changing the shapeof the QDs and especially their aspect ratio. We should keepin mind that, in order to have a good quality QD laser, wealso need a low threshold current density. In such a Q1.18system, high QDs density can be reached,17 leading to lowthreshold current density lasers.6,7 Thus, theInAs/Q1.18/Q1.18 QD system, appears to achieve both lowthreshold current densities and insensitivity to temperature,which are two of the major performance criteria for QDFIG. 3. High excitation power PL measurements for samples A, B, and Cat room temperature for a InAs/ InP/ InP sample with excited-state transi-tions at E1=E0+9 meV and E2=E0+24 meV. b InAs/ InP/Q1.18 samplewith excited-state transitions at E1=E0+10 meV and E2=E0+23 meV. cInAs/Q1.18/Q1.18 sample with excited-state transitions at E1=E0+26 meV and E2=E0+39 meV.lasers.Downloaded 30 Nov 2005 to 134.58.253.114. Redistribution subject toIn conclusion, InAs/ InP/ InP QDs, InAs/ InP/Q1.18QDs, and InAs/Q1.18/Q1.18 QDs were studied bymagneto-PL and high excitation power PL. On the first twostructures, the lateral confinement is weakened, which is in-terpreted as a consequence of enhanced lateral intermixingduring growth. These electronic properties make such struc-tures unsuitable for laser applications. On the other hand,measurements show that InAs/Q1.18/Q1.18 QDs have avery different electronic and optical signature. The influenceof the first capping layer is thereby demonstrated, and a weaklateral intermixing for InAs/Q1.18/Q1.18 QDs is consideredto be responsible for the improved confinement. Moreover, ahigh QD density has already been reached in this system, asrequired for low threshold current density lasers. In theseregards, the choice of InAs/Q1.18/Q1.18 QDs seems to beat present the best in this system for achieving QDs lasersemitting at 1.55 m with low threshold density currents, andlarge insensitivity to the temperature.This work was supported by the EuroMagNET projectContract No. R113-CT-2004-506239 and the SANDiE Net-work of Excellence Contract No. NMP4-CT-2004-500101of the 6th Framework Programme of the EuropeanCommission.1D. Bimberg, M. Grundmann, and N. N. Ledentsov, Quantum Dot Hetero-structures Wiley, Chichester, 1999.2S. Frechengues, N. Bertru, V. Drouot, B. Lambert, S. Robinet, S.Loualiche, D. Lacombe, and A. Ponchet, Appl. Phys. Lett. 74, 33561999.3C. Paranthoën, N. Bertru, B. 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Labbé, H. Folliot, A. Le Corre,and S. Loualiche, Phys. Rev. B 72, 035342 2005. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp