AlN Quantum Dots As Revealed by Three

Letter pubs.acs.org/NanoLett

Carrier Localization in GaN/AlN Quantum Dots As Revealed by ThreeDimensional Multimicroscopy Lorenzo Mancini,†,⊥ Florian Moyon,† David Hernàndez-Maldonado,†,‡ Ivan Blum,† Jonathan Houard,† Williams Lefebvre,† François Vurpillot,† Aparna Das,§,∥ Eva Monroy,§,∥ and Lorenzo Rigutti*,† †

Normandie University, GPM, UNIROUEN, INSA Rouen, CNRS, 76000 Rouen, France SuperSTEM STFC Daresbury Laboratories, Warrington WA4 4AD, United Kingdom § Université Grenoble Alpes, 38000 Grenoble, France ∥ CEA-Grenoble, INAC-PHELIQS, 17 avenue des Martyrs, 38000 Grenoble, France ‡

S Supporting Information *

ABSTRACT: The localization of carrier states in GaN/AlN self-assembled quantum dots (QDs) is studied by correlative multimicroscopy relying on microphotoluminescence, electron tomography, and atom probe tomography (APT). Optically active field emission tip specimens were prepared by focused ion beam from an epitaxial film containing a stack of quantum dot layers and analyzed with different techniques applied subsequently on the same tip. The transition energies of single QDs were calculated in the framework of a 6-bands k.p model on the basis of APT and scanning transmission electron microscopy characterization showing that a good agreement between experimental and calculated energies can be obtained, overcoming the limitations of both techniques. The results indicate that holes effectively localize at interface fluctuations at the bottom of the QD, decreasing the extent of the wave function and the band-to-band transition energy. They also represent an important step toward the correlation of the three-dimensional atomic scale structural information with the optical properties of single light emitters based on quantum confinement. KEYWORDS: Quantum dots, III−N semiconductor heterostructures, atom probe tomography, electron tomography, correlative microscopy, carrier localization

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allows predicting the structural properties of the individual dot.11 Conversely, structural studies on QDs such as those carried out by scanning tunnelling microscopy, atomic force microscopy, and cross-sectional electron microscopy on III−V systems since the mid-90s are extremely important for the determination of the growth mode, size and shape distribution,12 of strain and alloy distribution within the dot,13,14 of dot/matrix intermixing effects,15 and of their influence on the optical properties.16 However, if the properties of an individual dot are targeted, the structural information is still not detailed enough to predict the detailed spectral signature of a single QD,

he localization of carriers in quantum confined systems such as quantum wells (QWs) or quantum dots (QDs) may play a crucial role in their optical properties, influencing emission energies, oscillator strength, and eventually the recombination efficiency of the system. The optical effects of localization have been studied for different III−N quantum confined systems.1−6 A number of structural factors can induce a weak three-dimensional (3D) confinement of carriers which adds up to the effect of the potential barriers generated by design. These structural factors include alloy fluctuations,1,3,6 presence of extended defects7,8 and strain inhomogeneities.9,10 Correlative microscopy techniques can be used to identify the structural origin of carrier localization in nanoscale quantum confined systems. Concerning QDs, it has been shown that the detailed knowledge of the spectral emission properties of a single QD © XXXX American Chemical Society

Received: March 20, 2017 Revised: June 14, 2017

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DOI: 10.1021/acs.nanolett.7b01189 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters which can only be attained relying on microscopy techniques in three dimensions (3D) and with a resolution close to the atomic scale. This problem is particularly intriguing in the case of GaN/AlN Stransky-Krastanov QDs,5 whose applications are various and encompass photodetection,17−19 fluorescence markers,20 nano-optomechanical systems,21 and single/entangled photon emitters for quantum cryptography and computation.22,23 These nanoscale emitters have a complex 3D structure, and a full 3D structural study would allow achieving a deep insight into the problem. Laser-assisted atom probe tomography (LA-APT) is the only microscopy technique that allows 3D chemical mapping of semiconductor systems with subnanometric resolution.24,25 APT reconstructed volume data have recently demonstrated the ability to constitute a reliable basis for the calculation of electronic states, even in the basic approach of the effective mass approximation.26−28 Nevertheless, the effectiveness of the technique is limited by the difficulties in performing accurate measurements of elemental fractions due to the complex nature of field evaporation.29,30 Imprecise or not properly spatially resolved composition measurements can hinder a thorough understanding of the optical properties of QDs, where small variations of the elemental fraction at the nanometer scale yield strong variations of the electron or hole energies. However, such limitations may be overcome through the application of complementary microscopy techniques.26,31−34 In this contribution, we studied optically active field emission tips8 containing up to some tens of QDs, each tip prepared by focused ion beam (FIB) and characterized by microphotoluminescence (μPL). A set of these tips were then studied by APT and/or by scanning transmission electron microscopy (STEM) tomography (electron tomography, ET). The correlation of optical and structural properties is obtained in the framework of an effective mass 6 bands k.p model. The QD morphologies obtained by APT and inferred from ET are used for calculating the potential landscape seen by the charge carriers along with the corresponding optical transition energies, which can then be compared to those experimentally obtained. STEM information is also used for ensuring the best APT reconstruction. A good correlation between the structure of the QDs and their optical signature was ensured by the complementary structural information given by ET and APT; whereas the distributions of size and shape of QDs measured by the two techniques show an excellent consistency, the QD composition measured by APT differs from the nominal value, mainly due to the limited lateral resolution. Despite this issue, APT reveals the presence of interface thickness fluctuations at the base of the QDs. Holes are confined at these interface fluctuations with average localization energies of around 50 meV. As a result, the spatial extension of their wave function decreases from 1 nm in the absence of such fluctuations to around 0.75 nm. The system investigated in this work consists of 30 periods of GaN QD layers separated by 3 nm thick AlN barriers, grown along the polar wurtzite c-axis [0001] by plasma-assisted molecular-beam epitaxy (PAMBE) in conditions that are known to rule out the possibility of interdiffusion of Al/Ga between the dot and the matrix.35 Optically active field emission tips such as the one reported in Figure 1a were prepared through standard FIB annular milling. Further details on sample growth and specimen preparation are described in Methods. The needle-like shape of the so-prepared samples allows performing optical spectroscopy (Figure 1b), ET (Figure

Figure 1. (a) SEM image of specimen tip E as prepared by FIB. (b) Micro-PL spectra recorded from the thin film sample (gray dashed line) and from the specimen tip E (black solid line). (c) STEM image of tip E and (d) APT reconstructed volume issued from the analysis of tip E. The red arrows point to an individual dot imaged by both STEM and APT.

1c), and APT (Figure 1d) on the same nano-object. A total of 17 tips were analyzed: 2 by APT only (samples B and C), 11 by optical spectroscopy only (from sample G to sample Q), one by sequential μPL-APT (sample A), 2 by sequential μPL-ET (samples D and F), and only one with all three techniques (Sample E). Note that APT characterization was attempted for tips D and F but due to the small volume probed by the technique no QDs were fully reconstructed. In sample E, only one QD was correctly reconstructed by APT; all the other QDs of the tip were completely or partially outside the analysis field of view. The list of specimen tips and applied techniques of analysis is available in Table SI of the Supporting Information. The optical emission properties of the GaN/AlN QDs were studied by μPL at a temperature of 4 K. Figure 1b displays the μPL spectra acquired from the thin film sample and from the tip specimen E (other spectra are reported in Figure S8 of the Supporting Information). The thin film spectrum contains two main contributions: the main low-energy peak originates from the GaN layer, whereas the QD contribution is a large band peaked at E = 3.76 eV. On the contrary, the spectrum of sample E exhibits narrow lines (the minimum line width being limited by the spectral resolution), distributed in the energy interval 3.65−4 eV and assigned to the emission from single QDs. These spectra show that at least a subset of QDs in the tip is optically active even after the FIB preparation, which is B


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estimated considering the hexagonal truncated pyramid which best fits them, as shown in Figure 2b,c for a QD of sample E. The choice of such geometry, which reflects the hexagonal symmetry of the wurtzite structure, is justified by direct observation and is consistent with previous calculations5 and experimental observations.37,38 On the other hand, short-range reconstructed fluctuations were not considered because they were most likely induced by the missing angles in the tomographic series. Three parameters are thus sufficient for defining the structure of a QD: the length of the longer axis L, the ratio between the longer and the shorter axis η (Figure 2b), and the height of the truncated pyramid h (Figure 2c). A further set of structural analyses were performed by LAAPT. Figure 3a and the animated graphics available as

explained by the carriers being confined in the QDs and spatially separated from the damaged regions at the surface.36 The energy dispersion of the peaks most likely stems from the size dispersion of the dots within the tip. Notice that in the tip specimen there is no signal related to the GaN layer. Here, indeed, the loosely localized transitions suffer from the proximity of the damaged surface. Electron tomography allows the assessment of the spatial dimensions of heterostructures, and the characterization of crystallographic features such as crystal type, orientation, and defects. Such complementary information can be fundamental for the proper analysis of semiconductor heterostructures by APT.8 Furthermore, the interlayer distance measured by STEM constitutes a reference for obtaining the best possible 3D reconstruction in APT. Figure 2a shows the ET reconstruction

Figure 3. Atom probe tomographic study of tip A containing a set of QDs. (a) Reconstructed positions of the Ga atoms within the whole probed volume. (b) Reconstructed positions of the Ga atoms within a 0.5 nm thick slice corresponding to a cross section of a selected stack of QDs. (c) Ga site fraction map within the volume displayed in (b). The red arrows point to interface monolayer fluctuations at the bottom of several QDs.

Supporting Information show the reconstructed positions of Ga ions within the probed volume of tip A, from which GaN wetting layers can be easily identified. GaN QDs appear as high Ga density regions of average 2.5 nm height and 15 nm longer axis. The zoom-in of the reconstruction showed in Figure 3b is a close-up of the 3D reconstruction in a 0.5 nm thick slice approximately cut through the center of a QD stack, highlighting the vertical correlation of QDs.39 Figure 3c displays the Ga site fraction, defined as the local fraction of Ga atoms over the total number of Ga and Al atoms nGa/(nAl + nGa), over the volume defined in (b). The map indicates a compositional interface gradient. However, Al/Ga interdiffusion should not occur in this specific growth regime.40 Thus, the apparent compositional gradient is a consequence of the limited lateral resolution of the APT technique, estimated here at ∼2 nm. In APT, the lateral resolution is significantly worse than the in-depth resolution,41 as the latter is generally not influenced by surface phenomena such as short-range diffusion and local aberration effects, particularly important at interphase boundaries.42 It must also be pointed out that the lateral resolution is here particularly affected because of the strong mismatch in the binding energies of GaN and AlN, which translates into a significant difference of the evaporation fields.31,43 The in-depth resolution is of the order of 0.25 nm, corresponding to an atomic monolayer in the polar direction of the crystal.42 Figure 3c shows interface fluctuations at the bottom GaN/AlN interface of several QDs, as pointed out by the red arrows. These features could be associated with the

Figure 2. Electron tomography study. (a) Tomographic reconstruction of tip E. (b) Top view and (c) side view of a QD in which the fitting hexagonal pyramid and its defining geometrical parameters are shown. The quantities L, η and h correspond to the longer axis of the hexagonal QD basis, the ratio between longer and shorter axis of the hexagon, and the QD height, respectively.

of tip E. Wetting layers and QDs can be identified respectively as planes and trapezoidal areas of brighter contrast in the highangle annular dark-field (HAADF) images used for the reconstruction. Figure 2a and the corresponding animated graphics available as Supporting Information report the 3D reconstruction of the volume of tip E. Further HAADF images and the diffraction pattern analysis allowing the determination of the QD crystallographic orientation are also displayed in the Supporting Information. Thirty QDs organized in five stacks are identified with their wetting layers aligned on the c-plane. The slope of the QD lateral surfaces is nearly constant, α = (20 ± 2)°, while the intersections of the lateral facets with the wetting layer are aligned with the a[112̅0] crystal directions (see Supporting Information). For all three analyzed tips, the size of the reconstructed QDs used in calculations was C


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Figure 4. Correlation between the optical and structural properties of the QDs resulting from calculations of the wave functions within the 6 bands k.p approximation. (a) Cross-section of the APT calculated compositional map for one QD in tip C (top), and of the corresponding electron and heavy hole ground state wave functions (bottom). Interface fluctuations at the base of the QD in the site fraction maps are highlighted by red arrows. Parts (b) and (c) display the same distributions as in (a), but in (b) the sample is simulated as a hexagonal pyramid defined by the L, η, and h parameters as derived by ET. In (c), a cylindrical monolayer fluctuation in the growth direction with 2 nm diameter has been added to (b). Histograms of (d) μPL peak energies, (e) transition energies calculated over the QD compositional distribution measured by APT, (f) transition energies calculated within QDs with hexagonal pyramid shape and L, η, and h parameters extracted by ET, with the addition of a bottom interface fluctuation of 0 ML (top), 1 ML (middle), and 2 ML (bottom). The average transition energies calculated from STEM data are plotted in (g) as a function of the interface fluctuation amplitude, where they are compared with the average μPL peak energy and with the average energy calculated based on the APT composition maps.

strain conditions leading to the QD nucleation during the growth and can possibly originate from steps on the AlN barrier surface, or from extended defects (dislocations) propagating through the structure.44 Their amplitude is approximately that of one or two atomic monolayers (1 ML = 0.25 nm). As we show in detail in the Supporting Information, we can exclude that these fluctuations are merely related to the sampling protocol adopted for the analysis of APT data. From APT data, it is also possible to extract the L, η, and h parameters, as previously shown for the STEM analysis. As illustrated in the Supporting Information, the sets of parameters obtained by the two techniques are consistent. As a summarizing remark on the structural characterization techniques, ET and APT have a complementary role and the application of both allows overcoming the limitations of each. ET provides a reliable assessment of the QD shapes and sizes and a useful reference for obtaining reliable APT volume reconstructions. However, the limitations in spatial resolution due to the missing angles make ET unsuitable for the assessment of small scale features such as interface fluctuations. On the other hand, APT suffers from a limited lateral resolution, around 2 nm in this study, also due to the important mismatch in the evaporation field of AlN and GaN, which severely limits the characterization of the lateral QD interfaces. On the other hand, the axial resolution is sufficient

(0.25 nm) to assess very small scale features at the bottom interface, which play an important role in carrier localization, as it will be shown in the following. The correlation between the optical end structural properties of the analyzed specimens is performed in the framework of a 6 bands k.p model with the calculations implemented by the commercial software Nextnano.45 From the electron and hole single particle ground states, the expected photoluminescence transition energy EPL can be simply defined as EPL = Ee1 − Ehh1 − EX where EX = 35 meV is the exciton binding energy of GaN, as determined for QDs and QWs with height h around 2−3 nm.46 Calculations were performed on three kinds of simulation volumes: (a) The 13 compositional maps directly issued from the 3D APT reconstructed volumes, as shown in Figure 3c, and, more in detail, in Figure 4a. These compositional maps have limited lateral resolution resulting in nonabrupt QD lateral interfaces but display well-resolved interface monolayer fluctuations at the bottom of the QD. (b) The 78 hexagonal pyramids described by the L, η, and h parameters extracted by ET, as displayed in Figure 4b. Such structures have abrupt interfaces and no monolayer fluctuations at the bottom of the QD. (c) The 78 hexagonal pyramids described by the L, η, and h parameters extracted by ET with an extra interface fluctuation D


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eliminated from the tip specimen by FIB or sufficiently far from the QD region to allow for strain relaxation at the free surfaces. The correlation of μPL and of the structural parameters issued by ET has also been performed at the level of tips D, E, and F, as reported in the histograms of Figure S8b, and confirming the results obtained on the ensemble of the dots. Although this correlation performed on single tip specimens remains statistical, the result is particularly significant because it opens the possibility to correlate the optical signature and the 3D structural information on a single quantum dot. Finally, we discuss the effect of interface fluctuations in terms of the in-plane extension of the electron and hole wave functions. The “degree” of localization of charge carriers within the dots can be quantified by the in-plane wave function extension (also defined as “localization length” in ref 1) → ⎯ 2 ⟨Δρe(hh) ⟩ = dr(ρ ⃗ − ⟨ρ ⃗ ⟩)2 |ψe(hh)|2

added in order to reproduce the QD irregular bottom interface as indicated by APT. Such a structure is visualized in Figure 4c. The interface fluctuation is either one or two ML thick, cylindrical, and presents a diameter of 1.5 nm. For the simulated geometries, electrons tend to localize toward the top of the QD and holes localize toward the QD base due to strong built-in piezo- and pyroelectric fields.47,48 In the absence of interface fluctuations (Figure 4b), holes are weakly localized at the center of the bottom interface due to a combination of quantum confinement, strain distribution, and internal electric field. However, interface monolayer fluctuations at the base of the QD can effectively localize the hole state, as reported in the examples of Figure 4a,c. The three simulated geometries yield different statistics of the calculated transition energies, as shown in the histograms of Figure 4e,f. These can be compared to the experimental distribution of the PL peaks, reported in Figure 4d. The energies obtained by calculation performed on APT reconstruction of samples A, B, and C are overestimated (histogram of Figure 4e) due to the limitation of the APT lateral resolution: the “interdiffusion” of Al into the QD artificially increases the bandgap within the QD and decreases the internal electric field. On the contrary, the distribution of the transition energies simulated from ET data (histograms of Figure 4f) is quite close to the energies observed by optical spectroscopy. The effect of interface fluctuations on the transition energies is displayed in Figure 4f,g: the presence of interface fluctuations leads to a narrower distribution with lower average energy. Specifically, the mean and standard deviation of the distribution goes from ⟨E⟩ = 3.96 eV and ΔE = 0.17 eV for QDs with flat bottom interface, to ⟨E⟩ = 3.90 eV and ΔE = 0.15 eV in the QDs with 1 ML fluctuations, and to ⟨E⟩ = 3.855 eV and ΔE = 0.14 eV in the QDs with 2 ML fluctuations. For comparison, the experimental μPL peak distribution displayed in Figure 4d has an average value and a standard deviation of ⟨E⟩ = 3.72 eV and ΔE = 0.15 eV, respectively. Therefore, the best statistical agreement between the optical transition energy distribution obtained by μPL and that issued from the structural analysis is obtained with hexagonal pyramidal shapes parametrized by the L, η, and h parameters extracted by the ET analysis, corrected by assuming an irregular QD bottom interface as indicated by APT. The remaining difference between the calculated and experimental distributions of transition energies can be in part ascribed to strain relaxation effects induced by the geometry of the tips: the GaN wetting layers could partially relax close to the surface, as in nanowires,49,50 inducing a lowering of the QD emission energies. The deviation could also be partially ascribed to the 6-band k.p approximation itself. The accuracy of the 6 bands k.p can be in principle improved by applying the 8-band k.p approximation,51 which, however, requires a much larger computational effort and turned out to be not adapted to the calculation on a large set of QDs. It has been shown on similar structures that an 8-band k.p approach would yield lower transition energies, by around 100 meV,51 in much better agreement with the experimental PL data and with our interpretation. Finally, we remark that the average μPL energy extracted from the statistical distribution, ⟨E⟩ = 3.72 eV, is very close to the peak of the μPL spectrum of the thin film, E = 3.76 eV. For this reason, we can consider that the effect of the thermal stress induced by the Si substrate52,53 should not be taken into account, most likely due to the presence of the GaN/AlN superlattice. In any case, the substrate is either

∫

(1)

with ⟨ρ ⃗ ⟩ =

∫ dr ρ⃗ ⃗ |ψe(hh)|2

(2)

where r ⃗ = (x,y,z) and ρ⃗ = (x,y). Figure 5 depicts histograms of the (a) electron and (b) heavy-hole wave function extensions

Figure 5. Calculation of wave function extension in GaN/AlN QDs. Histograms of (a) electron and (b) heavy-hole wave function extension computed for ET reconstructed QDs are reported. Sparsely patterned columns are used for QDs with flat bottom interface, while densely patterned and full columns are used for QDs with interface fluctuations of 1 and 2 ML amplitude, respectively. The average values of the distributions are reported versus the fluctuation amplitude in part (c) for electrons and in part (d) for heavy holes.

computed for QDs having flat bottom interface or for QDs with 1 or 2 ML interface fluctuations with the structural parameters determined by ET. Considerations about the calculations performed over the APT compositional maps are exposed in the Supporting Information. The average values of the in-plane wave function extension of the electron and heavy hole states are reported in Figure 5c,d, respectively. Typical in-plane dimensions for the thickness fluctuations were defined consistently with those observed by APT reconstruction. Insight on the effect of interface roughness on localization is given by the comparison of localization lengths computed for ET reconstructed QDs with and without interface fluctuations: although these expectedly do not affect the wave function extension of the electron states, localized at E


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Micro-PL. Spectra were acquired from all the prepared samples except for tips A and C (for which a strong enough signal could not be detected probably due to surface carbon contamination of the tip) cooling them down to 4 K in a continuous He flux cryostat. Excitation was provided by a pulsed 260 nm laser at a repetition frequency of 4 MHz focused on a spot of the size of around 1 μm and at an incident power of 100 μm, measured at the entrance of an achromatic reflective objective with numerical aperture NA = 0.35. The emitted signal was collected by a 320 mm focal length grating spectrometer with a spectral resolution of 0.06 nm. Electron Tomography. STEM observations were performed in a JEOL ARM 200F microscope equipped with a Schottky field emitter operating at 200 kV. A probe diameter of 0.13 nm was used for HAADF images, with collection angles in the range 70−250 mrad. Tomographic reconstructions were acquired from samples D, E, and F using the equally sloped tomography algorithm58 for which 59 images were acquired by rotating the specimen tip approximately around its axis for angles between −79.3° and 79.3° with equal slope increment. The method applied for acquisition and reconstruction in ET has been proved to lead to atomic resolution for dedicated experiments.59,60 For the present acquisition parameters, the resolution was in the 0.5−0.7 nm range perpendicular to the axis of rotation, around 0.25 nm along the axis. Atom Probe Tomography. In this contribution samples A,B and C were analyzed in a Cameca laser-assisted wide-angle tomographic atom probe (LAWATAP) equipped with a 8 cm diameter multichannel plate/advanced delay line detector (MCP/ADLD)61 and providing a flight length of around 10 cm, corresponding to a field of view of 22° on the detector and of 35° on the tip. Analyses were performed at 40 K and under the illumination of 400 fs laser pulses at 343 nm and at the repetition frequency of 100 kHz, delivering an average pulse intensity of around 0.22 W μm−2. As displayed in the Supporting Information all peaks appearing in the mass spectra were unambiguously identified. The 3D reconstruction of the analyzed samples was obtained using either the so-called Eβ or the cone-angle protocol42 for which good parameters (those ensuring the good spatial separation between wetting layers) were chosen on the base of scanning electron microscopy images acquired during sample preparation (information on the initial radius and of the shank angle of the tip) and of STEM tomographic reconstruction obtained from tips extracted from the same macroscopic sample. Calculations. Calculations of the electron and hole confined levels and wave functions have been implemented by the commercial software Nextnano,45 using the 6 bands k.p approximation.53 Photoluminescence transition energies are calculated as the difference between electronic and heavy hole ground-state energies (considering the contribution due to the exciton binding energy). Subvolumes of 30 × 30 × 3.5 nm containing one QD were selected for calculating transition energies from the APT reconstructed volume. The size of the subvolumes was chosen in order to contain the QDs and to respect the periodicity of the wetting layers along the growth direction. The reconstruction space within each subvolume was subdivided in cubic bins of 0.5 nm side for ensuring a sufficient number of detected events per bin. For each bin, the Ga site fraction was computed. Subsequently, a finer grid of 0.25 nm side cubic bins was considered for interpolating the site fraction values of the main grid. Electron and heavy-hole ground states were computed from the site fraction maps. First, we calculated

the QD top by the internal electric field, the extension of hole states is significantly affected. The extension of electron states is on the average around 2.2 nm independently of the presence of the fluctuation, while the extension of hole states passes from around 1 nm for flat bottom interfaces to 0.74 nm in the case of 1 ML fluctuations and 0.72 nm in the case of 2 ML fluctuations, that is, a reduction of 25%. In summary, we studied carrier localization within GaN/AlN Stransky-Krastanov QDs by correlative APT-ET-μPL multimicroscopy. The approach was able to correctly correlate optical and structural properties of optically active field emission tips containing a limited number of quantum emitters, overcoming the difficulties arising from a biased APT composition measurement due to the limited lateral resolution of the technique. The complementary APT and STEM structural information was exploited to get insight into localization mechanisms and the influence of surface roughness in transition energies. In particular, we found by APT that the bottom interface of the QD is often affected by interface fluctuations. Taking into account such fluctuations, the calculations of the electron and hole levels within the 6 bands k.p approximation showed that holes effectively localize at such fluctuations. Furthermore, localization was found to lead to a very good agreement with the μPL peak energy statistical distribution. The possibility of analyzing quantum emitters inside FIB-elaborated nanoscale tips not only opens the way toward the sequential analysis of individual quantum light emitters by complementary microscopy techniques, but also to the use of such individual emitters as functional elements for scanning probe microscopies. Methods. Sample Growth. Substrates consisted of commercial 1 μm thick GaN on a 40 period AlN/GaN superlattice with 10 nm AlN and GaN layer thickness, grown on Si(111) templates. The nitrogen-limited growth rate was fixed at 0.3 monolayers per second, and the substrate temperature was TS = 720 °C. Prior to the growth of the QD stack, 130 nm of AlN were deposited under Al-rich conditions, to ensure that the GaN layers would find the compressive stress required for the synthesis of StranskiKrastanov QDs. The growth of GaN QDs was performed by deposition of 2.3 monolayers of GaN under N-rich conditions35,54 The QDs were capped with 3 nm of AlN grown under slightly Al-rich conditions. This thickness is sufficient to achieve a good planarization before the next QD layer deposition. After the growth of each barrier, the Al excess was consumed by exposing the substrate to active nitrogen during 6 s, before opening the Ga shutter. Under these growth conditions, it is unlikely to expect Al/Ga interdiffusion. Specimen Preparation. Tip specimens were prepared by standard FIB-based lift-out and annular milling procedure using Ga+ ions55 and extracting the regions of interest from the thin film sample, previously coated with a Si layer of 200 nm thickness. The annular milling procedure was performed at 30 keV ion energy in a first stage, then at 2 keV for the final milling and cleaning stages. In the 2 keV stage, around 100 nm (20 nm) of material were ablated in the tip axial (radial) direction. In this way, the thickness of the amorphized region and the Ga projected range in the radial direction can be estimated as 3 nm.56,57 This quantity is sufficiently low to conclude that Ga ions do not have an influence on the structural properties of the QDs. F


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ACKNOWLEDGMENTS This work has been funded by the French National Research Agency (ANR) through projects EMC3 Labex ASAP, ANR-13JS10-0001-01 Tapoter, and ANR JCJC TIPSTEM.

calculating numerically the strain within the subvolumes by minimizing the elastic energy assuming coherent interfaces on unstrained AlN. Subsequently, we solved self-consistently the Poisson and Schrödinger equations. Note that in the conditions considered for simulations (equilibrium at 4 K, consistent with experiments) the dots are not populated or very weakly populated, so that the Poisson contribution to computed energies is negligible. The material parameters used for the calculations and the references from which they are issued are reported in the Supporting Information. Periodic boundary conditions are imposed for both strain and self-consistent Poisson-Schrödinger calculations.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Electron Tomography and Atom Probe Tomography reconstructions are available as animated graphics. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b01189. Data and information about sample preparation and about the implementation of electron tomography, atom probe tomography, and 6-bands k.p calculations (PDF) Electron tomography reconstructions (MPG) Electron tomography reconstructions (MPG) Electron tomography reconstructions (MPG) Atom probe tomography reconstructions (AVI) Atom probe tomography reconstructions (AVI) Atom probe tomography reconstructions (AVI) Atom probe tomography reconstructions (AVI)

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +33 (0)2 35 14 71 82. ORCID

Ivan Blum: 0000-0002-4729-6510 Eva Monroy: 0000-0001-5481-3267 Lorenzo Rigutti: 0000-0001-9141-7706 Present Address ⊥

(L.M.) Centre de Nanosciences et de Nanotechnologies, CNRS UMR 9001, University Paris-Sud, Université ParisSaclay, C2N − Orsay, 91405 Orsay cedex, France Author Contributions

L.M. carried out μPL and APT analysis, analyzed the APT data, performed the effective mass 6 bands k.p calculations, coordinated the sample flow through the sequential analysis, interpreted the results, and wrote part of the manuscript. F.M. and D.H.M. carried out the electron tomography analysis and relative volume data reconstruction. I.B. prepared the specimen tips. J.H. carried out part of the μPL analysis. W.L. coordinated the electron tomography analysis and interpreted part of its results. F.V. contributed to the APT volume data reconstruction. A.D. grew the samples with the contribution and under the supervision of E.M., who also wrote part of the manuscript. L.R. designed and coordinated the experiment, interpreted the correlated analysis, and wrote part of the manuscript. All authors discussed the results and contributed to their interpretation. Notes

The authors declare no competing financial interest. G


Letter

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AlN Quantum Dots As Revealed by Three

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