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Analysis of the stability of InGaN/GaN multiquantum wells against ion beam intermixing

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2015 Nanotechnology 26 425703

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Analysis of the stability of InGaN/GaNmultiquantum wells against ion beamintermixing

A Redondo-Cubero1,2, K Lorenz1, E Wendler3, S Magalhães1, E Alves1,D Carvalho4,5, T Ben4,5, F M Morales4,5, R García4,5, K P O’Donnell6 andC Wetzel7

1 Instituto Superior Técnico, Universidade de Lisboa, Campus Tecnológico e Nuclear, Estrada Nacional 10,2695-066 Bobadela LRS, Portugal2Departamento de Física Aplicada y Centro de Microanálisis of Materiales, Universidad Autónoma deMadrid, E-28049 Madrid, Spain3 Friedrich-Schiller-Universität Jena, Institut für Festkörperphysik, Max-Wien-Platz 1, D-07743 Jena,Germany4Department of Materials Science and Metallurgic Engineering, and Inorganic Chemistry, Faculty ofSciences, University of Cadiz, Spain5 IMEYMAT, Institute of Research on Electron Microscopy and Materials, University of Cádiz, Spain6 SUPA Department of Physics, University of Strathclyde, Glasgow, G4 0NG, Scotland, UK7Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 EighthStreet, Troy, NY 12180-3590, USA

E-mail: andres.redondo@uam.es

Received 7 June 2015, revised 27 July 2015Accepted for publication 5 August 2015Published 1 October 2015

AbstractIon-induced damage and intermixing was evaluated in InGaN/GaN multi-quantum wells(MQWs) using 35 keV N+ implantation at room temperature. In situ ion channelingmeasurements show that damage builds up with a similar trend for In and Ga atoms, with a highthreshold for amorphization. The extended defects induced during the implantation, basal andprismatic stacking faults, are uniformly distributed across the quantum well structure. Despite theextremely high fluences used (up to 4×1016 cm−2), the InGaN MQWs exhibit a high stabilityagainst ion beam mixing.

Keywords: ion beam mixing, InGaN, quantum wells, implantation

(Some figures may appear in colour only in the online journal)

1. Introduction

InGaN/GaN multiple quantum wells (MQWs) are the basis ofmany modern optoelectronic devices, including bright lightemitting diodes, laser diodes, light displays, solar cells, etc.However, the strain induced by the lattice mismatch, inaddition to the spontaneous polarization in III-N hetero-structures, affects the internal quantum efficiency through theso-called quantum-confined Stark effect (QCSE), whichinduces a spatial separation of electron and hole wavefunc-tions [1], in particular for high InN-content devices, with wideMQW emitting in the green spectral region. One of the

strategies suggested to mitigate QCSE in GaN-based materi-als is the fabrication of intermixed or graded quantum struc-tures by ion beam mixing [2, 3]. This approach has beensuccessfully applied in AlGaAs/GaAs [4, 5], AlGaInP/GaInP [6] and MgZnO/ZnO [7] MQWs, but it remainsunexplored in InGaN/GaN systems.

Despite the outstanding radiation resistance of GaN (withamorphization thresholds above 10 displacements per atom),recent experimental results in GaN/AlN MQWs [8] haveproved that partial intermixing can be induced at low tem-peratures (15 K) for sufficiently high Ar fluences. Thisintermixing induced by collision cascades was shown to be

Nanotechnology

Nanotechnology 26 (2015) 425703 (6pp) doi:10.1088/0957-4484/26/42/425703

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more efficient than thermal interdiffusion [8], and motivatesthe present studies. We consider here conventional InGaN/GaN nanostructures under more adaptable conditions forindustrial scalability, i.e., room temperature implantation withnative ions. Thus, in this work we investigate the damagebuild-up during ion mixing processes in InGaN/GaN MQWswith 35 keV N+ ions, comparing the behavior with thickInGaN layers.

2. Experiment

The InxGa1−xN/GaN MQW sample was grown by metal-organic chemical vapor deposition on a (0001) GaN/sapphiretemplate [9]. The active layers consist of five InxGa1−xN/GaN periods with thicknesses of 3 nm/20 nm (measured bytransmission electron microscopy (TEM)) and an average InNmolar fraction in the QWs of x=10% (determined byRutherford backscattering spectrometry (RBS)). A ∼20 nmGaN capping layer was deposited to protect the active layers.Photoluminescence and ionoluminesce measurements reveal agreen QW emission centered at ∼530 nm (2.34 eV).

Ion implantation and in situ RBS in channeling mode(RBS/C) studies were carried out in the double-beamchamber at the Institut für Festkörperphysik (Jena) [10]. Theimplantations were performed at room temperature with35 keV N+ ions, being the energy selected to place themaximum of the damage profile in the central region of theMQWs. According to Monte Carlo simulations with theSRIM code the mean N range is 55 nm, with a longitudinalstraggling of 25 nm [11]. The ion fluence was increasedsequentially from 5×1012 to 4×1016 cm−2 and the samplewas tilted by 7° off-axis during the implantation to avoidchanneling effects. After each implantation step, the damageaccumulation was evaluated in situ by RBS/C spectra of thesample using a 2.2 MeV He+ collimated beam (∼1 mm2).RBS/C spectra were taken in the 〈0001〉 aligned direction,using a 3-axis goniometer for sample orientation.

The final as-implanted sample, corresponding to a totalfluence of 4×1016 cm−2, was further studied ex situ withx-ray diffraction (XRD), TEM, and grazing incidence RBS.XRD data were acquired in a D8Discover high resolutiondiffractometer (Bruker-AXS) using Cu(Kα1) radiation, anasymmetric two-bounce Ge(220) monochromator, and a NaIscintillation detector. To reduce the divergence, the incomingx-ray beam is collimated with a 0.2 mm slit while the dif-fracted beam is collimated with a 0.5 mm motorized slit and a0.1 mm fixed slit, being the approximate final 2θ/ω angularresolution ±0.01°. XRD 2θ/ω curves were simulated usingthe dynamical theory of XRD [12] The influence of theimplantation is taken into account by including the staticDebye–Waller parameter (describing the attenuation causedby thermal motion) on the structure factor of individual lay-ers, thus attenuating the derived intensity and considering a c-lattice expansion following the work of Boulle andDebelle [13].

TEM experiments were carried out using a JEOL 2010Fmicroscope operated at 200 kV. Structural and compositional

features were studied from micrographs collected by HR-TEM and high angle annular dark field (HAADF) imaging inscanning-TEM (STEM) mode, while nanoprobe analyses oflocal proportions of atomic elements were based upon energydispersive x-ray spectroscopy done in the same mode (EDX-STEM). Grazing incidence RBS experiments were carried outwith a 2MeV He+ beam using the Van de Graaff acceleratorat LATR/IST (Portugal).

3. Results and discussion

Figure 1(a) shows RBS/C spectra of the sample after theimplantation to different total N fluences. Remarkably, the Insignal does not show any shift to higher energies even for thehighest fluence, what confirms that there is neither appreci-able diffusion of In towards the surface, nor significantsputtering of the GaN capping layer. Two energy windowswere defined for In(w1) and Ga(w2) at equivalent depthscorresponding to the central region of the damage profile inthe MQWs and avoiding the surface peak zone. The minimumyield (χmin), determined as the ratio of the aligned and therandom backscattering yield, was obtained for both windows.The increase of χmin with fluence determines the damageaccumulation. The damage level is then described as thedifference in the minimum yield of the implanted and virginsample, Δχmin=χmin(implanted)−χmin(virgin). As a refer-ence, the initial χmin values for the as-grown sample were 4.4(1)% and 2.2(1)% for In and Ga, respectively. For the max-imum fluence of 4×1016 cm−2 the aligned level almostoverlaps with the random level (χmin=100%), indicating alevel of damage that is maximal for measurement by RBS/C.

Figure 1(b) shows the damage level obtained for In(w1)and Ga(w2) for the entire range of fluences. Both curves havea similar behavior in two main regions of interest. For lowfluences (

Solid lines in figure 1(b) represent the fits using Heck-ing’s model, which takes into account the production (Ppd),recombination (Rpd), and clustering (Cpd) of point defects insemiconductors, as well as the production of amorphousregions (Pa) and their growth (Ga) [17, 18]. The results

obtained from these fits are summarized in table 1. The var-iations observed with the values obtained from Wendler et al[16], in particular the low values of Pa obtained here, areascribed to the different temperature and not to the thicknessor the structure of the InGaN layers. This is supported by thefact that the damage build-up curve does agree well with thedata from Kucheyev et al [14] at room temperature, pointingout the relevant role of dynamic annealing processes.

Figure 2(a) shows the XRD 2θ/ω scans for the 0002reflection of the sample before and after the implantation(corresponding to a total fluence of 4×1016 cm−2 andnamed as-implanted) as well as the fits to these curves. Thespectrum of the as-grown sample is well described con-sidering the nominal structure without any roughness, defor-mation or disorder (Debye–Waller factor of ∼1) but allowinga slight variation of individual layer thicknesses below0.5 nm. The as-grown sample shows up to 13 superlatticepeaks, in good agreement with the simulated structure,reflecting excellent crystal and interface quality. After theimplantation, a clear broadening of the main peaks isobserved and several superlattice peaks are missing as a resultof the damage build-up. The fit assumed the same SL struc-ture as the as-grown sample but allowing slightly higher

Figure 1. (a) RBS/C spectra for different N+ fluences (note thesemi-log scale). The surface energy positions for In and Ga aremarked for better identification of the elements. Energy windowsused for the individual analysis of damage are marked as w1 (In) andw2 (Ga). Fluence values are given in cm

−2. (b) Damage level forwindow w1 (In) and w2 (Ga) as a function of the fluence. Thecorresponding fits using Hecking’s model are also shown (lines).Results for thin InGaN films implanted with Au at room and lowtemperature are also shown as a reference (adapted from [14, 16]).

Table 1. Main parameters obtained from the Hecking model.

Parameter In (w1) Ga (w2)

Ppd (10−16 cm2) 4.5(5) 2.9(5)

Rpd (10−14 cm2) 1.5(2) 1.2(2)

Cpd (10−16 cm2) 1.2(2) 0.4(2)

Pa (10−18 cm2) 1(1) 1(1)

Ga (10−17 cm2) 4(1) 8(1)

Figure 2. (a) HR-XRD (0002) 2θ/ω scans and simulations for as-grown and as-implanted (4×1016 cm−2) MQWs. (b) and (c)Reciprocal space mappings for both samples. The color barrepresents intensity in a log scale.

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Nanotechnology 26 (2015) 425703 A Redondo-Cubero et al

variation of layer thicknesses (up to 0.7 nm deviation from thenominal structure) and a decreased Debye–Waller factor(∼0.5 for the first three periods increasing until reachingagain unity for the deeper layers). Furthermore, a deformationof the implanted layers was taken into account yielding aperpendicular strain of maximum ∼1% for the three firstperiods and decreasing to zero for the deeper unimplantedlayers.

The in-plane (Qx) and out-of-plane (Qz) components ofthe scattering vector were obtained from the corresponding1015( ¯ ) reciprocal space maps of both samples, shown infigures 2(b) and (c). In addition to the broadening, the mapsconfirm the strong suppression of the SL peaks (the signalabove GaN being due to the remaining Kα2 contribution fromthe monochromator). The elongated peak below GaN isassociated to the expansion of the c-lattice parameter in theheterostructure, an effect well-documented for implantation inthin nitride films [19, 20]. It should be noted that the dyna-mical theory may not be well suited to describe XRD of thehighly damaged sample after high fluence implantation.Nevertheless, the results are in good agreement with the RBSand TEM results presented in this paper suggesting a

deterioration of the crystal quality, in particular of the firstthree periods, while intermixing remains below the detectionlimits of the employed techniques.

Figure 3 shows TEM images of the defects formed in theas-implanted maximum-fluence sample. Dark field images,recorded using 0002, 1–120 and 1–100 reflection g vectors,respectively (figures 3(a)–(c)), reveal a heavily damagedsurface region. Figure 3(a), taken under the g=0002 con-dition, indicates the presence of a large number of pointdefect clusters, interstitials or vacancies, also confirmed infigure 3(b). Figure 3(c), exciting the g=1–100 reflection,shows that, in addition to the defect clusters, a complexnetwork of basal stacking faults (BSFs), both intrinsic (I1 andI2 type) and extrinsic (E), and prismatic stacking faults[21, 22] has been generated during the implantation. A highresolution example of one I2 BSF inside the implanted regionis exhibited in figure 3(d). The formation of these extendeddefects agrees well with previous reports on both GaN[15, 19] and InGaN [23], where clusters and planar defectshave been pointed out for relatively high implantation flu-ences. A similar defect structure of the ternary and binarycompounds can explain the homogenous distribution of

Figure 3. Dark field TEM images of the as-implanted sample (4×1016 cm−2) acquired with g=0002 (a), g=1–120 (b), and g=1–100(c) orientations. (d) HR-TEM image corresponding to the implanted region, where an I2 intrinsic BSF is visible.

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Nanotechnology 26 (2015) 425703 A Redondo-Cubero et al

defects observed in the present case, where the presence ofthe QWs does not seem to affect the defect distribution andthe migration/annihilation properties.

Figure 4 shows HAADF micrographs of (a) the as-grownand (b) the as-implanted samples. After the implantation, thetop first QWs are not clearly distinguishable due to the highdamage levels, which are known to distort the unit cell eitherby accumulation of point defects or strain fields associatedwith extended defects, both cases disimprove on the contrastof HAADF-STEM images [24, 25], and can render the directinterpretation of contrasts in terms of chemical compositionsineffective. However, local EDX spectra reveal that most ofthe In remains close to the original QW structure (not shown).This fact was confirmed by grazing incidence (60° tiltbetween the surface normal and the incoming beam) RBSmeasurements, which clearly resolve the first two InGaNQWs which are completely separated in the spectrum.Figure 4(c) directly compares the RBS spectra of the as-grown and as-implanted sample; no major difference can bediscerned. Both spectra are compatible with the targeted ori-ginal depth profile. Nevertheless, some degree of intermixingcannot be completely excluded due to the limited depthresolution of the technique. These results differs from thoseobtained for AlN/GaN MQWs where a partial intermixingcan be induced by 100 keV Ar+ implantation [8]. This fact

might be related to the different efficiency of the ballisticintermixing in both cases (which is expected to be three timeslower in the current study due to the lower energy and ionmass) but other factors such as the electronic energy-losscontribution, the stress state of the material, and the solubilitylimits of the elements cannot be ruled out and could also playa role in this regard.

4. Conclusions

We have demonstrated that InGaN/GaN MQWs exhibit ahigh resistance to ion beam induced mixing at room tem-perature. Damage builds up with a similar trend for In and Gaatoms, leading to the formation of defect clusters and exten-ded planar defects. Such defects are homogeneously dis-tributed throughout the MQWs, but despite the high damagelevel attained both EDX-STEM and grazing incidence RBSconfirmed that compositional intermixing in the QWsremains low.

Acknowledgments

We thank Dr P Ruterana for fruitful discussions and sug-gestions. We acknowledge support by FCT Portugal (bilateralproject DAAD/FCT 2011-2012, PTDC/FIS-NAN/0973/2012, SFRH/BPD/74095/2010, Investigador FCT) andJuan de la Cierva program (under contract number JCI-2012-14509). TEM experiments were carried out in the ElectronMicroscopy -DME- and Sample Preparation -LPM- Divisionsof the Central Services of Science and Technology of theUniversity of Cádiz (SCCYT-UCA), being financed by theprojects MAT2010-15206 (CICYT, Spain), EU-COST ActionMP0805, and P09-TEP-5403 (Junta de Andalucía with EU-FEDER participation).

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1. Introduction2. Experiment3. Results and discussion4. ConclusionsAcknowledgmentsReferences