Camille Barbier
email: camille.barbier@c2n.upsaclay.fr

Tao Zhou

Gilles Renaud

Olivier Geaymond

Patrick Le Fèvre

Frank Glas

Ali Madouri

Antonella Cavanna

Laurent Travers

Martina Morassi

Noelle Gogneau

Maria Tchernycheva

Jean-Christophe Harmand

Ludovic Largeau

In situ X-ray diffraction study of GaN nucleation on transferred graphene

Keywords: A1. Nucleation, A1. High resolution X-ray diffraction, A1. Growth modelling A3. Molecular beam epitaxy, B1. GaN, B1. Graphene

We investigate by in situ X-ray diffraction the early stage of GaN nanowire growth on a single-layer of polycrystalline graphene transferred onto a Si (100) / SiO 2 carrier substrate.

The experiment was carried out in a molecular beam epitaxy chamber installed on a synchrotron line equipped with a X-ray diffractometer and a nitrogen plasma source. The evolution of the in-plane lattice parameters of the crystals is monitored. The in-plane lattice parameter of graphene shows only little variation between room temperature (RT) and GaN growth temperature (720 °C). As for GaN, an incubation time of about 1 h precedes nucleation. Just after nucleation, the GaN nanocrystals are under tensile strain and they relax as growth proceeds. The critical nucleus size and the parameters governing nanowire growth dynamics are obtained from modeling. We discuss the possible incorporation of N atoms in the graphene lattice during the incubation time. These atoms could be involved in chemical bonds between GaN and graphene, which could explain the initial strain observed experimentally.

INTRODUCTION

GaN-based compounds are among the most coveted semiconductors for applications in optoelectronics and high-power electronics. Though efficient GaN-based devices are nowadays commercially available, the epitaxy of high-quality single-crystalline III-N layers on a low-cost substrate is still challenging. In particular, high electron mobility transistors could benefit from a reduction of the threading dislocation density which is generally quite high in these epilayers [START_REF] Besendörfer | Methodology for the investigation of threading dislocations as a source of vertical leakage in AlGaN/GaN-HEMT heterostructures for power devices[END_REF] . Van der Waals epitaxy is an alternative to conventional epitaxy [START_REF] Koma | Fabrication and characterization of heterostructures with subnanometer thickness[END_REF] . This method relies on weak van der Waals interactions between two atomic planes without the formation of covalent bonds between them. Thereby, the deposited layer is expected to be free from epitaxial strain and the amount of defects in the grown structure is expected to decrease drastically.

Several groups have attempted to grow GaN thin films on layered materials such as h-BN [START_REF] Kobayashi | Layered boron nitride as a release layer for mechanical transfer of GaN-based devices[END_REF] or graphene [START_REF] Kim | Principle of direct van der Waals epitaxy of singlecrystalline films on epitaxial graphene[END_REF] . In these studies, a crystalline substrate (sapphire [START_REF] Kobayashi | Layered boron nitride as a release layer for mechanical transfer of GaN-based devices[END_REF] , SiC [START_REF] Kim | Principle of direct van der Waals epitaxy of singlecrystalline films on epitaxial graphene[END_REF] ) was present beneath the layered 2D material. The latter was shown to be efficient for the mechanical peel-off of the epitaxial stack and its transfer on a host support, but the expected reduction of defect density was not achieved: a dislocation density around 10 9 -10 10 cm -2 was observed [START_REF] Kobayashi | Layered boron nitride as a release layer for mechanical transfer of GaN-based devices[END_REF][START_REF] Kim | Principle of direct van der Waals epitaxy of singlecrystalline films on epitaxial graphene[END_REF] . This is possibly due to the underlying crystalline substrate preventing the graphene compliance. Its presence may lead to a situation similar to what happens in conventional epitaxy, via a remote epitaxy mechanism [START_REF] Kim | Remote epitaxy through graphene enables twodimensional material-based layer transfer[END_REF] . Other groups have succeeded in growing GaN nanowires on graphite [START_REF] Tabata | GaN Nanowires Grown on a Graphite Substrate by Radio Frequency[END_REF][START_REF] Fernández-Garrido | Molecular Beam Epitaxy of GaN Nanowires on Epitaxial Graphene[END_REF] and on graphene transferred on single-crystalline substrate 8,9,10, or amorphous support [START_REF] Chung | Growth and characterizations of GaN micro-rods on graphene films for flexible light emitting diodes[END_REF][START_REF] Kumaresan | Epitaxy of GaN Nanowires on Graphene[END_REF] . A few of these studies have demonstrated unambiguously that vertical III-V nanowires can grow with a well-defined basal orientation. In-plane alignment was reported on multilayer graphene obtained from SiC [START_REF] Fernández-Garrido | Molecular Beam Epitaxy of GaN Nanowires on Epitaxial Graphene[END_REF] and on graphene transferred on an SiO 2 [START_REF] Kumaresan | Epitaxy of GaN Nanowires on Graphene[END_REF] , the GaN < 21 1 0 > directions being parallel to the graphene zigzag edges. While in the first case, the monocrystalline bulk substrate beneath the layered 2D material may influence the epitaxial alignment, the second case definitely shows that the sole interaction between the 2D material and the threedimensional (3D) semiconductor can induce an epitaxial stacking. This observation raises the fundamental question of the exact nature of the interaction between GaN and graphene. How do the sp 2 orbitals of graphene interact with the sp 3 orbitals of GaN at the interface?

This particular material combination presents a large lattice misfit (+29.2%) which seems difficult to fully accommodate by elastic strain. Ref. 12 highlights a possible coincidence of the two lattices. Adopting the observed in-plane alignment, a (33) GaN supercell has a moderate misfit with a (44) graphene supercell: namely -3.1 % at room temperature and -2.8 % at a growth temperature of 720 °C. If chemical bonds form at the interface, this misfit could reasonably be accommodated via elastic strain at the beginning of growth. Thus, it appears interesting to investigate whether such a strain is present or not during the initial stage of growth. In the present work, we track this hypothetical strain by measuring in situ the lattice parameters of GaN NWs as they start to form on a graphene monolayer transferred on a SiO x carrier substrate.

EXPERIMENTAL METHODS

The experiments were conducted on the BM 32 beamline at the European Synchrotron Radiation Facilities (ESRF). The beamline is equipped with an ultra-high vacuum growth chamber coupled with a goniometer through a large Be window for X-ray transparency.

During this experiment, we have characterized the early stage of the GaN NW growth on a graphene substrate by grazing incidence X-ray diffraction (GIXRD).

The GaN NWs were synthesized by plasma-assisted molecular beam epitaxy (PA-MBE) on a polycrystalline monolayer of graphene transferred on an amorphous carrier substrate. The commercial film used for this experiment was a polycrystalline single-layer graphene, with grain sizes in the tens of micrometers range, grown by chemical vapor deposition (CVD) on a copper foil. The graphene layer was transferred by a wet process from this copper substrate to a 1 cm 2 Si (100) substrate covered with a 300 nm layer of thermal SiO x (the transfer procedure is provided as Supporting Information). This relatively thick layer of amorphous material guarantees that the interaction between the graphene and the monocrystalline silicon carrier substrate underneath is negligible. Before its introduction in the ultra-high vacuum growth chamber, the sample was outgassed at 600 °C for 8 hours in a buffer chamber, in order to eliminate any potential contamination originating from the chemical transfer process. The base pressure in the growth chamber was 5x10 -10 mbar before the growth. N-rich conditions were used for the growth of the GaN NWs. The N-plasma source was powered at 350 W with radio frequency and supplied with a 2 sccm N 2 flow. From a calibration performed in a standard MBE chamber in the same conditions, we estimated the N flux to be equivalent to 0.7 monolayer (ML) per second. The Ga effusion cell was heated up to 930 °C, resulting in an atomic flux equivalent to a 0.06 ML/s growth rate on a flat surface. Then, the graphene patch was exposed to Ga and N fluxes simultaneously for 150 minutes. The sample temperature was set at 720 °C, as measured by a 2-waves pyrometer, and the pressure in the chamber was 1.310 -5 mbar during growth.

The X-ray energy used during the experiment was 18 keV (0.688 Å wavelength), with a relative energy resolution of 10 -4 keV. This energy corresponds to the highest power delivered by the beam line and is much higher than the Ga absorption edge, which avoids most of the fluorescent signal in the measurement. A schematic of the experimental set-up is shown in Figure 1. The incident angle α was set to 0.05° (well below the critical angle for total external reflection) in order to maximize the signal diffracted by the GaN nanocrystals and to minimize the background coming from the carrier substrate. The incident beam spot was 20 μm and 300 μm wide in the directions perpendicular and parallel to the surface, respectively.

The beam divergence was 1 mrad and 0.1 mrad, perpendicular and parallel to the surface, respectively. A two-dimensional pixel detector (MAXIPIX, with 1280  256 pixels of 55  55 μm [START_REF] Koma | Fabrication and characterization of heterostructures with subnanometer thickness[END_REF] ), positioned at a distance of 700 mm from the sample, was used to map reciprocal space with two possible motions, along β or δ angles (see Figure 1). The in-plane lattice parameters, associated to the crystallographic planes perpendicular to the sample surface, were measured by scanning the detector along the δ angle. The signal was integrated over 5° out-of-plane to increase the counting statistics. These radial scans allowed us to investigate in situ the in-plane deformation of the two materials at the early stage of the growth. The GaN out-of-plane lattice parameter was measured by scanning the detector along the β angle. This second configuration was used to characterize the system after growth by performing a wide out-of-plane reciprocal space mapping (RSM). Finally, the presence of GaN nanocrystals grown on graphene was confirmed by ex situ scanning electron microscopy (SEM) using a ZEISS-Ultra 55 instrument.

RESULTS

Since the transferred graphene is polycrystalline, all the in-plane directions are expected to be equivalent for diffraction. Figure 2 shows radial scans around the Bragg angle of {101 0} graphene planes, collected at an arbitrary in-plane azimuthal direction. The two sets of symbols correspond to the experimental in-plane diffraction of the polycrystalline graphene at room temperature (RT) and at the GaN growth temperature (720 °C). The data were fitted by pseudo-Voigt functions. For clarity, Figure 2 shows only the fit of the data at RT. Remarkably, the positions and widths of the peaks obtained at room and growth temperatures are very similar. The peak positions correspond to a lattice parameter of 2.447±0.002 Å and 2.445±0.002 Å at RT and at 720 °C, respectively. During GaN growth, radial scans around the Bragg peaks of {101 0} planes of GaN and graphene were performed. We scanned alternatively a δ angular range of 0.8° around the graphene Bragg peak for 3 min of acquisition time, and a δ angular range of 2.5° around the GaN Bragg peak for 2 min. Figure 3a shows some typical graphene radial scans collected during the growth process after 30 to 110 min exposure to Ga and N fluxes. For better clarity, we only present scans at 15 min intervals. Figure 3b shows the GaN radial scans collected in situ from 65 min to the end of growth, with 5 min intervals. The experimental data are fitted with a pseudo-Voigt function for graphene and with a Pearson VII function for GaN. We detect significant diffraction from graphene until the end of the growth. Between the different scans, the graphene peak intensity varies slightly but without any systematic trend.

The peak position does not show any clear evolution either. This indicates that the graphene layer remains crystalline and does not suffer severe structural degradation such as the partial etching sometimes reported after long exposure to nitrogen plasma [START_REF] Hayashi | Self-organization of dislocation-free, high-density, vertically aligned GaN nanocolumns involving InGaN quantum wells on graphene/SiO2 covered with a thin AlN buffer layer[END_REF] . The GaN signal appeared after 65 min of sample exposure to the matter fluxes. Then, the diffracted intensity of GaN increases steadily, with a clear shift of the peak position toward smaller lattice parameters during the first 15 min of GaN growth. After this period, the GaN peak position remains fixed until the end of the growth.

After the growth, the GaN nanostructures on graphene were analyzed at RT by performing an out-of-plane reciprocal space map (RSM), shown in Figure 4a. This RSM was built by scanning the in-plane angle δ by 42° and the out-of-plane angle β by 24°, with 0.05° and 3° angular steps respectively. The diffraction from the monolayer graphene is not visible on the map due to the short counting time. All the Bragg peaks of the hexagonal wurtzite GaN structure are found, with no additional contribution, except a diffuse spot which is attributed to the silicon substrate. The GaN Bragg reflections from the different planes, namely {101 l}, {112 l} and {123 l}, are clearly aligned along the Ɩ miller index, which indicates that the GaN crystallites are vertically c-aligned. These peaks extend as arcs centered on the origin of the reciprocal space. This indicates that the growth direction of some GaN nanocrystallites slightly deviate from the normal axis. We have estimated the tilt distribution by measuring the full width at half maximum (FWHM) of the 101 1, 112 l, 123 1, 203 1 and 303 2 arcs. We obtain a tilt of 5.6°0.4° around the substrate normal. This value is larger than the typical tilt distribution of GaN NWs reported in the literature [START_REF] Fernández-Garrido | Molecular Beam Epitaxy of GaN Nanowires on Epitaxial Graphene[END_REF][START_REF] Chung | Growth and characterizations of GaN micro-rods on graphene films for flexible light emitting diodes[END_REF][START_REF] Kumaresan | Epitaxy of GaN Nanowires on Graphene[END_REF] . It could be due to the roughness of the SiO x layer and a non-ideal transfer process of the graphene monolayer on this layer. This tilt is confirmed by post-growth observations of the samples by SEM (Figure 4b) which show short GaN NWs with diameters between 10 and 30 nm. In figure 4a, we do not observe diffracted rings. This indicates that the GaN volume irradiated by the X-rays is equivalent to a polycrystalline material with a c-axis fiber texture. Note that, with a grazing incidence of 0.05° and a beam height of 20 µm, the probed area is several mm long and thus contains many graphene grains with various in-plane orientations. Hence, although the GaN NWs grow with the same in-plane orientation on a specific graphene grain [START_REF] Kumaresan | Epitaxy of GaN Nanowires on Graphene[END_REF] , our GIXRD experiments probe GaN NWs of different in-plane orientations, but which all have their c-axis perpendicular to the substrate surface.

DISCUSSION

First, we will comment in detail our measured value of the graphene lattice parameter and its negligible variation between room and growth temperatures. We will then discuss the evolution of the diffracted signal related to GaN growth, as regards peak position, width and intensity.

We have measured by GIXRD the average in-plane projection of the lattice parameter of the single-layer polycrystalline graphene transferred on a SiO x substrate. Our experimental value of 2.447±0.002 Å at RT, is close to those theoretically calculated for free-standing graphene [START_REF] Pozzo | Thermal expansion of supported and freestanding graphene: Lattice constant versus interatomic distance[END_REF][START_REF] Zakharchenko | Finite temperature lattice properties of graphene beyond the quasiharmonic approximation[END_REF] , or measured by scanning tunneling microscopy (STM) on graphene grown on Cu (111) [START_REF] Gao | Epitaxial graphene on Cu(111)[END_REF] and by GIXRD for decoupled graphene on silicon carbide oxide [START_REF] Schumann | Effect of buffer layer coupling on the lattice parameter of epitaxial graphene on SiC[END_REF] (see Table 1). The in-plane lattice parameter that we measure is 0.013 Å smaller than the theoretical graphene lattice parameter of 2.46 Å. It is known that graphene may form ripples or wrinkles and hence may not be totally flat [START_REF] Mounet | First-principles determination of the structural, vibrational and thermodynamic properties of diamond, graphite, and derivatives[END_REF][START_REF] Shaina | Estimating the thermal expansion coefficient of graphene: The role of graphene-substrate interactions[END_REF][START_REF] Magnin | Thermal expansion of free-standing graphene: Benchmarking semi-empirical potentials[END_REF] . By assuming that our measurements yield the in-plane projection of the actual lattice parameter, a deviation angle of 5.9°±0.5° from the exact surface plane could explain our experimental value smaller than the theoretical one. Furthermore, such a deviation angle is in agreement with the angle of corrugation measured by Meyer et al. [START_REF] Meyer | The structure of suspended graphene sheets[END_REF] . We can then deduce that our graphene layer is rippled. According to ref. 22, when transferred by a wet process on an amorphous layer, graphene is mechanically freestanding, with no pinning to the carrier substrate [START_REF] Ferralis | Determination of substrate pinning in epitaxial and supported graphene layers via Raman scattering[END_REF] . This is consistent with our observations. Indeed, if the graphene interacted strongly with the SiO x layer underneath, it should at least partially follow its thermal expansion, which, in turn, is imposed by the thermal expansion of the Si (100) bulk substrate. Between RT and 720 °C, the Si lattice parameter is expected to undergo a thermal expansion of 0.014 Å, corresponding to a relative expansion of ~2.610 -3 23 . In the same range of temperature, the graphene peak position does not vary more than 0.002 Å, which corresponds to a relative expansion of 8.110 -4 . This expansion is thus well below what it would be if graphene followed the thermal expansion of the Si bulk substrate. This indicates that the graphene layer has a thermal free-standing behavior and undergoes no or negligible expansion. Ab initio molecular dynamics [START_REF] Pozzo | Thermal expansion of supported and freestanding graphene: Lattice constant versus interatomic distance[END_REF] and Monte Carlo simulation [START_REF] Zakharchenko | Finite temperature lattice properties of graphene beyond the quasiharmonic approximation[END_REF] have predicted the lattice expansion of free-standing graphene to be between 0.001 Å and 0.003 Å, over this temperature range. Our results are consistent with these values. Actually, the temperature-dependence of graphene structural properties was studied by several groups, mainly by theoretical calculation and Raman spectroscopy experiments. Depending on the simulation method [START_REF] Magnin | Thermal expansion of free-standing graphene: Benchmarking semi-empirical potentials[END_REF] , the temperature range [START_REF] Mounet | First-principles determination of the structural, vibrational and thermodynamic properties of diamond, graphite, and derivatives[END_REF][START_REF] Yoon | Negative thermal expansion coefficient of graphene measured by raman spectroscopy[END_REF][START_REF] Lee | Study of the Substrate-Induced Strain of As-Grown Graphene on Cu(100) Using Temperature-Dependent Raman Spectroscopy: Estimating the Mode Grüneisen Parameter with Temperature[END_REF] , and the type of substrate [START_REF] Shaina | Estimating the thermal expansion coefficient of graphene: The role of graphene-substrate interactions[END_REF] , the reported thermal expansion coefficient of graphene differs widely in sign and value. Our measurement indicates that, between RT and 720 °C, either the thermal expansion coefficient (TEC) always remains very small, or it varies from negative to positive value or the opposite.

Ref

From the fits of the in situ radial scans around the Bragg peaks of graphene {101 0} and GaN {101 0} planes, we extract different quantities, namely the positions, widths and areas of the peaks. Figure 5a shows the area of the GaN diffraction peak as a function of growth time. This peak is not detected before 65 min. This delay corresponds roughly to the incubation time during which no GaN nucleus could form. After 65 minutes, the peak area increases with time until the end of the growth. Figure 5b shows the evolution with time of the experimental GaN inplane domain size (D ) extracted (figure 3b) from the width of the diffraction peak. D amounts to about 10 nm shortly after nucleation (65 min) and then increases slowly to reach about 30 nm at the end of growth, in good agreement with our post-growth SEM observations (Fig. 4b).

Obviously, the irradiated GaN volume (V) and D increase with time because of the continuous growth of each NW, but the evolution of these quantities may also depend on a distribution over time of the nucleation of the NWs. Modeling the evolution of V and D gives insights into the various parameters involved. Specifically, each NW was assumed to obey the scaling growth model developed by Dubrovskii et al. for single cylindrical NWs [START_REF] Dubrovskii | Scaling growth kinetics of self-induced GaN nanowires[END_REF] . Namely, if a NW nucleates at time t , its diameter D and height L vary over time t as:

D = D 1 + A(t t ) , L = L D D (1)
with D and L the dimensions at nucleation and A and α model parameters, which may depend on growth conditions. In addition, we assumed that the total number of NWs varies with time according to an error function law, n(t) = 1 + erf , such that a number dt of new NWs nucleate during dt. At time t , half of the final number of NWs have nucleated and t accounts for the width of the distribution of nucleations over time. In the context of GaN NW growth, other sigmoid functions were employed previously, albeit to describe the "gallium incorporation rate per unit area", not the NW number or density [START_REF] Fernández-Garrido | Monitoring the formation of nanowires by line-of-sight quadrupole mass spectrometry: A comprehensive description of the temporal evolution of GaN nanowire ensembles[END_REF][START_REF] Sobanska | Self-assembled growth of GaN nanowires on amorphous AlxOy: from nucleation to the formation of dense nanowire ensembles[END_REF] . The total GaN volume and the average NW diameter are obtained by convoluting functions πD L and D (obtained from Eq. ( 1)), respectively, with dn dt ⁄ (we actually started the integrations at a time t close to the experimentally observed incubation time, at which a number n(t ) of NWs was assumed to form, but this has very little effect on our results). The joint fit of volume and size involves 6 parameters, namely D , A, α, t , t and a parameter aggregating L and N. Figure 5 shows that our model allows us to reproduce the experiments very well.

From this, we deduce a NW diameter at nucleation D ≈ 1.5 nm, a characteristic width of the nucleation distribution t ≈ 8 min and a scaling parameter α = 2.49. The values of the fit parameters are expected to depend on growth conditions. This said, the present t falls within the range (up to 15 minutes) obtained by direct measurement of the NW density in other experiments that we carried out at different temperatures. The diameter at nucleation is not accessible experimentally; as expected, our D is below the size of the smallest islands (4 nm) observed by Consonni et al. [START_REF] Consonni | Nucleation mechanisms of epitaxial GaN nanowires: Origin of their self-induced formation and initial radius[END_REF] . Our α is very close to the value obtained by Dubrovskii et al.

under different growth conditions [START_REF] Dubrovskii | Scaling growth kinetics of self-induced GaN nanowires[END_REF] , which might suggest that it applies to GaN NWs in general.

We now focus on the evolution of the GaN and graphene in-plane lattice parameters given by the diffraction peak positions during growth (Figure 3). To confront our results to the coincidence supercell proposed in our previous work [START_REF] Kumaresan | Epitaxy of GaN Nanowires on Graphene[END_REF] , we plot the evolution of the supercell dimensions 4a graphene and 3a GaN with growth time (Figure 6). Within the experimental errors, the graphene lattice parameter is constant over the whole duration of the experiment. Note that after growth, only a small part of the graphene area is covered by GaN NWs, so that the diffracted signal is dominated by the uncovered area, i. e.

the part which does not interact with GaN. On the other hand, just after nucleation, we observe a rapid evolution of the GaN lattice. The in-plane parameter decreases between 65 and 80 min and then stabilizes at a constant value close to 3.1986 Å ± 0.0006 Å. Within the uncertainties associated to the available data, this value corresponds to the unstrained GaN inplane lattice parameter at 720 °C30,31,32 . This result is compatible with the idea that the upper part of the nanowires is fully relaxed. The initial value after nucleation corresponds to a tensely strained GaN, with a lattice parameter 0.8 % larger than the relaxed one. The rapid evolution suggests strain relaxation, or more exactly, an increasing contribution of a GaN volume adopting its stress-free parameter. It is well established that strain does not build up in the upper part of NWs, even if they are grown on a mismatched substrate [START_REF] Glas | Critical dimensions for the plastic relaxation of strained axial heterostructures in free-standing nanowires[END_REF][START_REF] Ye | Plastic relaxation of mixed dislocation in axial nanowire heterostructures using Peach-Koehler approach[END_REF] . This is why, upon increasing the height of the NWs during growth, the average lattice parameter that we measure tends rapidly to the nominal stress-free value, while at the nucleation stage, only the very first interfacial GaN monolayers contribute to the diffracted signal. Thus, we conclude that GaN nucleates on graphene in a tensile strained state. This suggests that covalent bonding is involved in the cohesion of the interface between graphene and GaN. For a one-monolayer (ML)-high GaN nucleus, the strain should be shared almost equally between the two materials since they have comparable thicknesses. At the growth temperature (720 °C), the experimental lattice mismatch between the (44) supercell of graphene and the (33) supercell of GaN is -2 %. Assuming a 1:1 partition of strain in the two materials of 1 ML height, the GaN and the graphene should accommodate a misfit of about -1 % and + 1%, respectively. The largest tensile strain detected after nucleation amounts to -0.8 %. This agrees well with our rough estimate, which does not account for the different elastic properties of the materials. Moreover, the first diffraction measurement after nucleation already corresponds to GaN crystals having on the average more than one monolayer. The strain in GaN observed at nucleation indicates possible covalent bonding between the two materials.

However, the exact origin of this bonding remains to be clarified. Indeed, due to its sp 2 hybridization, pristine graphene cannot form strong chemical bonds with the first layer of GaN. Nucleation of GaN at pre-existing graphene defects, such as grain boundaries, wrinkles or scratches, is often noticed [START_REF] Heilmann | Vertically Oriented Growth of GaN Nanorods on Si Using Graphene as an Atomically Thin Buffer Layer[END_REF][START_REF] Fernández-Garrido | Molecular Beam Epitaxy of GaN Nanowires on Epitaxial Graphene[END_REF] . Dangling bonds may be associated to some of these defects, which thereby become favorable nucleation sites. However, the fact that, in our conditions, a particularly long incubation time precedes the detection of the first GaN nanocrystals, tends to show that nucleation on pre-existing defects is not predominant. It is more plausible that our transferred graphene was modified upon the N-plasma exposure. This was checked by X-ray photoelectron spectroscopy (see supporting information). This analysis clearly reveals that N incorporates in the graphene lattice before the nucleation of GaN. Nitrogen is a well-known dopant of graphene [START_REF] Wang | Review on recent progress in nitrogen-doped graphene: Synthesis, characterization, and its potential applications[END_REF][START_REF] Jeong | Nitrogen-doped graphene for high-performance ultracapacitors and the importance of nitrogen-doped sites at basal planes[END_REF] . Specifically, three main incorporation sites of elemental nitrogen are reported with various proportions, depending on the method employed for doping [START_REF] Wang | Review on recent progress in nitrogen-doped graphene: Synthesis, characterization, and its potential applications[END_REF][START_REF] Joucken | Localized state and charge transfer in nitrogen-doped graphene[END_REF][START_REF] Scardamaglia | Nitrogen implantation of suspended graphene flakes: Annealing effects and selectivity of sp2 nitrogen species[END_REF][START_REF] Rybin | Efficient nitrogen doping of graphene by plasma treatment[END_REF][START_REF] Soin | Enhanced and stable field emission from in situ nitrogen-doped fewlayered graphene nanoflakes[END_REF] :

graphitic, pyrrolic and pyridinic, each being bound differently to C atoms. In the pyrrolic configuration, lattice compression was reported [START_REF] Zafar | Evolution of Raman spectra in nitrogen doped graphene[END_REF] . In our case, we do not observe a graphene lattice distortion associated with this N incorporation, which suggests that the N sites are not dominantly pyrrolic. The incorporation of N atoms in the graphene lattice has a strong impact on its electronic structure. Some configurations may lead locally to sp 3 coordination, which could promote the nucleation of GaN by forming a few covalent bonds between the two materials. This scenario also implies local modifications of the graphene lattice parameter under the GaN nuclei. Such modifications cannot be observed in our GIXRD experiments because the diffraction by unstrained graphene surrounding the NWs dominates the signal.

CONCLUSION

Our in situ study of GaN NW nucleation and growth on graphene has focused on the possible interactions between these two materials. We took care to eliminate extraneous interactions by The impact of the N-plasma on our transferred graphene was characterized by X ray photoelectron spectroscopy realized at Cassiopée beam line of SOLEIL synchrotron, in a previous study. The photon energy was set to 650 eV. In order to analyze the incorporation of N atom, we measured a transferred graphene without exposure to N-plasma (as a reference sample) and a transferred graphene exposed for 1 hour to N-plasma. This exposure time corresponds to the incubation delay of the GaN growth in our system. From the reference sample, only the C-1s peak was present, which was expected. For the graphene exposed to Nplasma, the C-1s peak was still present but also the N-1s peak and weak N-KLL peaks were present. The appearance of these N peaks confirms the efficient incorporation of N atoms into the transferred graphene during the incubation time.

Figure 1 .

 1 Figure 1. Schematic of the in situ Nanostructures and Surface (INS) set up at the ESRF/BM32 beamline, which couples UHV-chamber with a diffractometer.

Figure 2 .

 2 Figure 2. Experimental radial scans around the Bragg peak of {101 0} graphene planes,

Figure 3 .

 3 Figure 3. In situ experimental radial scans (dots) around the Bragg Peak of (a) the {101 0}

Figure 4 .

 4 Figure 4. (a) The reciprocal space mapping as a function of parallel (Q para ) and perpendicular

Figure 5 .

 5 Figure 5. (a) Experimental diffraction peak area (blue dots) and fit of the GaN volume (red

Figure 6 .

 6 Figure 6. Evolution of the supercell lattice parameters of graphene (grey squares) and GaN

Figure A :

 A Figure A: XPS experimental data on transferred graphene (black) and on transferred

Table 1 .

 1 Comparison of the graphene lattice parameters obtained by different methods at room temperature and high temperature.

	.	Method	Graphene type	a RT (Å)	a 720 °C (Å)
	(14)	Ab initio simulation	Free standing	2.464
	(15)	Monte Carlo simulations	Not specified	2.457	2.455
	(17)	X-Ray Diffraction	Decoupled monocrystalline graphene on SiC oxide	2.460 ± 0.001	-
	This work	X-Ray Diffraction	Transferred on Si/SiO2	2.447 ± 0.002	2.445 ± 0.002

using an amorphous carrier substrate (thermal SiO x ). Moreover, in our temperature range, the transferred graphene behaved as free-standing, with no evidence of thermal stress. As expected for a sp 2 material, the graphene surface is weakly reactive under the Ga flux and the N plasma exposure: GaN growth was not detected before a long incubation time. While the graphene remains crystalline over the whole growth process, we argue that it becomes Ndoped before GaN can nucleate. At nucleation, GaN is found to be tensely strained with an inplane extension of 0.8%. This initial strain suggests that covalent bonds are formed at the interface between the two materials. The N atoms incorporated in the graphene lattice before GaN nucleation, might be involved in this chemical bonding. Such a scenario requests further investigation to determine the sites of N incorporation before GaN nucleation, and to identify the signature of a specific interface bonding after GaN nucleation. The measured strain tends to zero as the GaN NWs grow, because of the increasing contribution of their upper parts, which free sidewalls permit efficient lattice relaxation

Supporting Information

Graphene transfer process: The single-layer polycrystalline (grain size up to 20 µm) graphene grown by CVD on copper foil (60 mm x 40 mm) were purchased from Graphenea supermarket. To transfer the graphene patches, from the Cu substrate to 1 cm² Si (100) wafers covered with a 300 nm layer of thermal SiO x , we realized a wet-transfer process. First, the Cu/graphene foil was spin-coated with a 50-nm-thick layer of poly(methyl methacrylate)

(PMMA-A2) to protect the graphene. Then, a 1 cm² piece of Cu/graphene/PMMA was deposited on the surface of a (NH 4 ) 2 S 2 O 8 bath solution, in order to etch the copper substrate at the backside. The acid solution was prepared by diluting 2.4 g of ammonium persulfate powder with 100 ml of deionized (DI) water. After 5 hours etching time, the floating graphene/PMMA piece was rinsed into three DI water baths during 10 minutes each, fished with the Si/SiO x substrate and dried in ambient atmosphere. The PMMA layer on top of the graphene was removed by solvent cleaning (acetone and isopropyl alcohol) and rinsed in DI water. Then, the sample was introduced in the outgassing chamber.

X-ray photoelectron spectroscopy (XPS):


Latifa Benhamou

Pierre Fenies

Vincent Giard

Delayed differentiation in chemical supply chains by Reverse Blending

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15/11/2020 à 19:38


Myriam Boudadi-Maligne
email: myriam.boudadi-maligne@u-bordeaux.fr

Jean-Baptiste Mallye
email: baptiste.mallye@u-bordeaux.fr

Jean-Georges Ferrié
email: jean-georges.ferrie@inrap.fr

Sandrine Costamagno
email: costamag@univ-tlse2.fr

Carolyn Barshay-Szmidt

Marie-France Deguilloux
email: marie-france.deguilloux@u-bordeaux.fr

Marie-Hélène Pémonge
email: marie-helene.pemonge@u-bordeaux.fr

Michel Barbaza
email: michel-barbaza@wanadoo.fr

Boudadi-Maligne Myriam

Costamagno Sandrine

Barshay-Szmidt Carolyn

Barbaza Michel

The earliest double dog deposit in the Palaeolithic record: the case of the Azilian level of grotte-abri du Moulin (Troubat, France)

Keywords: Domestication, Palaeolithic dog, Azilian culture

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INTRODUCTION

Documenting the processes that led to the eventual domestication of the wolf by prehistoric hunter-gatherers remains a heavily debated topic in Archaeology. This is partly due to difficulties in reliably establishing the role of these animals in past societies. This domestication during the Upper Palaeolithic [START_REF] Davis | Evidence for domestication of the dog 12,000 years ago in the Natufian of Israel[END_REF][START_REF] Müller | The domestication of the wolf -the inevitable first? In The first Steps of Animal Domestication[END_REF][START_REF] Vigne | The origins of animal domestication and husbandry: A major change in the history of humanity and the biosphere[END_REF] certainly brought several advantages to hunter-gatherer groups (hunting assistance, group protection, etc) but these gains remain difficult to see in the archaeological record. The timing of this innovation is still debated but we now consider that dogs lived with humans for at least the last 15,000 years [START_REF] Boudadi-Maligne | A biometric re-evaluation of recent claims for Early Upper Palaeolithic wolf domestication in Eurasia[END_REF][START_REF] Janssens | An evaluation of classical morphologic and morphometric parameters reported to distinguish wolves and dogs[END_REF][START_REF]A wolf in dog's clothing: initial dog domestication and Pleistocene wolf variation[END_REF][START_REF] Pionnier-Capitan | New evidence for Upper Palaeolithic small domestic dogs in South-Western Europe[END_REF]. Several recently re-evaluated Western European Upper Palaeolithic sites [START_REF] Boudadi-Maligne | Des restes de chiens magdaléniens à l'abri du Morin (Gironde, France). Implications socioéconomiques d'une innovation zootechnique[END_REF], 2018;[START_REF] Janssens | A new look at an old dog: Bonn-Oberkassel reconsidered[END_REF][START_REF] Napierala | A "new" Palaeolithic Dog from Central Europe[END_REF][START_REF] Pionnier-Capitan | New evidence for Upper Palaeolithic small domestic dogs in South-Western Europe[END_REF] have produced small canid remains, including the Grotte-abri du Moulin in Southwestern France.

Discovered in 1860 by Piette, who carried out the first archaeological work at the site, the Grotte-abri du Moulin is located in the central Pyrenees, at the confluence of a vast network of rivers (Fig 1a.). Following multiple visits by illicit diggers, Barbaza carried out the first scientific excavations of the cave over fifteen field seasons, beginning in 1986 [START_REF] Barbaza | Le Magdalénien supérieur final et l'Azilien dans les Pyrénées centrales. La grotte-abri du Moulin à Troubat (Hautes-Pyrénées) et son context[END_REF][START_REF] Barbaza | L'Azilien classique pyrénéen. L'Azilien de la grotte de Troubat dans ses divers contexts[END_REF][START_REF] Barbaza | Environmental changes and cultural dynamics along the northern slope of the Pyrenees during the Younger Dryas[END_REF][START_REF] Barbaza | La grotte-abri du Moulin (Troubat, Hautes-Pyrénées). Environnement et Archéologie[END_REF][START_REF] Costamagno | Mobilité, territoires de chasse et ressources animales au Magdalénien final en contexte pyrénéen : le niveau 7A de la grotte abri du Moulin (Troubat, Hautes-Pyrénes)[END_REF][START_REF] Heinz | Environmental changes during the Late Glacial and Post-Glacial in the central Pyrenees (France): new charcoal analysis and archaeological data[END_REF]. The stratigraphic sequence demonstrates a continuous human occupation of the site from the Middle Magdalenian to the Sauveterrian period (Fig 1b). Layer 6 produced over 10,000 identifiable mammal remains, several painted pebbles and numerous bone harpoons, including two with buttonhole perforations typical of the Pyrenean Azilian [START_REF] Barbaza | Le Magdalénien supérieur final et l'Azilien dans les Pyrénées centrales. La grotte-abri du Moulin à Troubat (Hautes-Pyrénées) et son context[END_REF][START_REF] Barbaza | L'Azilien classique pyrénéen. L'Azilien de la grotte de Troubat dans ses divers contexts[END_REF][START_REF] Chevallier | Exploitation du milieu montagnard sur le versant nord des Pyrénées entre 20 000 et 12 000 cal BP: que nous apprend la faune?[END_REF][START_REF] Heinz | Environmental changes during the Late Glacial and Post-Glacial in the central Pyrenees (France): new charcoal analysis and archaeological data[END_REF]. The faunal assemblage identified from layer 6 is dominated by red deer (Cervus elaphus), roe deer (Capreolus capreolus), wild boar (Sus scrofa), chamois (Rupicapra pyrenaica), ibex (Capra pyrenaica) and salmonids that reflect the unique ecological context of the site (540 m asl) allowing Azilian groups to exploit resources from the Garonne plain, steep mountainous areas and nearby rivers. Two radiocarbon dates on this layer, one on a deer bone (12,092-11,817 cal BP;[START_REF] Cheung | L'Azilien pyrénéen parmi les sociétés du tardiglaciaire ouesteuropéen: apport de l'étude des industries lithiques[END_REF] the other on a charcoal fragment (12,877-12,438 cal BP;[START_REF] Barbaza | Le Magdalénien supérieur final et l'Azilien dans les Pyrénées centrales. La grotte-abri du Moulin à Troubat (Hautes-Pyrénées) et son context[END_REF], are fully consistent with the known chronology of the Pyrenean Azilian (Fat [START_REF] Cheung | Comparaison des séquences aziliennes entre Dordogne et Pyrénées. État des travaux en cours. In Les groupes culturels de la transition Pléistocène-Holocène entre Atlantique et Adriatique: Séance de la société préhistorique de Bordeaux[END_REF].

MATERIALS AND METHODS

We focused our study on faunal material from Layer 6 because this layer stands apart from the other archaeological levels by the presence of an important number of previously identified canid remains (NISP = 84). For this work, we re-examined all undetermined faunal remains from layer 6 to ensure that other Canis remains had not been missed. All Canis isolated remains were then measured with callipers according to the measurements established by von den Driesch (1976) and completed with the ones defined by [START_REF] Boudadi-Maligne | Les Canis pléistocènes du Sud de la France : approche biosystématique, évolutive et biochronologique[END_REF]. Canis remains were also systematically observed using a monocular magnifying glass (magnification x20). We performed multivariate analysis to compare Troubat's canid bones to several comparative groups. Three comparative groups (Tab. 1) were used for the analysis of calcanei dimensions: modern medium-sized dogs (n=8), modern wild grey wolves from Portugal (n=20) and late Pleistocene wolves (i.e. from between 27 ky and 18 ky cal BP; Boudadi-Maligne, 2010) from Southwestern France (n=21). The same comparative groups (Tab. 2) including 16 modern dogs, 20 modern wild wolves, 7

Pleistocene wolves and the dog from Pont d'Ambon [START_REF] Boudadi-Maligne | L'Aquitaine à la fin des temps glaciaires -Les sociétés de la transition du Paléolithique final au début du Mésolithique dans l'espace Nord aquitain, Actes de la table ronde organisée en hommage à Guy Célérier[END_REF][START_REF] Célérier | Un chien dans l'Azilien de "Pont d'Ambon" (Dordogne)?[END_REF][START_REF] Pionnier-Capitan | New evidence for Upper Palaeolithic small domestic dogs in South-Western Europe[END_REF] were used to analyze the dimensions of the ulna.

Both tibias were also sampled for palaeogenomic analyses in order to document the genetic affinities of Troubat individuals with ancient wolves and dogs. All sampling steps and analyses were conducted in the ancient DNA facilities of the UMR PACEA (Bordeaux University). Bone samples were first irradiated with UV-light for 30 min on all sides. A layer of the bone surface was then mechanically removed and powder was drilled from the inner bone region, in the diaphysis. DNA was extracted following the protocol described in [START_REF] Dabney | Complete mitochondrial genome sequence of a Middle Pleistocene cave bear reconstructed from ultrashort DNA fragments[END_REF], using the MinElute columns (Qiagen). Double-stranded libraries were then built using the QIAseq Ultra Low Input kit (Qiagen) (following the kit's recommendations) and tested for sufficient DNA concentration.

RESULTS

Our sorting of all undetermined faunal material from layer 6 and overlying layer 5b, which also contained Azilian material, produced 63 additional remains, bringing the total number of Canis remains to 147. Based on the anatomical representation (Fig. 2 & Tab. 3), two individuals can be identified. A systematic re-fitting programme demonstrated that all canid bones from layer 5b belong to the two individuals from layer 6 (Fig. 2).

Biometric analysis of the mostly complete canid remains demonstrates their dimensions to fall clearly outside the known variability of modern and fossil wolves (Fig. 3). We used a Linear Discriminant Analysis (LDA) of four commonly employed measurements of the calcaneus (Tab. 1) to compare the two individuals and discuss their species attribution. Important size differences between the left and right calcaneus exclude them from belonging to the same individual (Fig. 2, Fig. 4a). The LDA indicates that the two Canis from the Grotte-abri du Moulin clearly fall outside the known variability (95% confidence ellipses) of modern and fossil wolves (Fig. 4a). Equally we carried out a Principal Component Analysis (PCA) of three measurements from the proximal part of the ulna (Tab. 2). The distinction between wolves and dogs is clear with the Grotte-abri du Moulin individuals falling once again, outside the known variability of modern and fossil wolves (Fig. 4b).

The stature of the two dogs could be estimated using the total length of the calcaneus and long bones [START_REF] Harcourt | The Dog in Prehistoric and Early Historic Britain[END_REF]. The first dog corresponds to a young adult (none of the vertebral discs are fused, aged < 2 years: Barone, 1986) whose height at the withers can be estimated at around 57 cm and whose body mass can be estimated from the dimensions of the tibias and ulna (Boudadi-Maligne, 2010) to about 20kg. The second dog corresponds to an osteologically mature adult of slightly taller build (3cm taller) and whose body mass can be estimated, using the talus dimensions, to about 25 kg. The stature of these two individuals is most comparable with current Siberian Huskies, whose height at the withers and body mass average, respectively, between 53 to 60 cm and 20 to 27 kg for males and between 51 to 56 cm and 16 to 23kg for females.

The unusual aspect of the Grotte-abri du Moulin is that 141 of the 147 Canis remains were recovered from five successive spits covering a depth of 11 cm in a single square (H17). The majority of the canid bones (119/147) are complete and several were discovered in anatomical connection or loose connection (Fig. 2). The most labile elements were discovered only slightly dispersed and there is no evidence of human or carnivore modification.

In order to test the association of the dog remains with the Azilian occupation, we sampled one bone of each individual for radiocarbon dating. Due to their good general preservation (i.e. no bone cracking or use of glue), the right tibias of the two dogs were sampled. Initially, the Oxford Laboratory could not obtain dates due to the low collagen content of both samples. Subsequently, a second, larger sample from the complete tibia attributed to the young adult returned an ultrafiltrated radiocarbon date of 10,600 ± 45 BP (OxA 36550) [12,521 (87,6%) and 12,475-12,429 (7,8%) cal BP; OxCal 4.3; Bronk [START_REF] Ramsey | Bayesian analysis of radiocarbon dates[END_REF]. This age is consistent with two previous dates from layer 6 (Fig. 1b) and demonstrates the Grotte-abri du Moulin dogs to be contemporary with the Azilian occupation of the cave.

Unfortunately, the palaeogenomic analyses conducted on the Troubat remains did not permit us to obtain exploitable DNA. Libraries constructed for both samples presented concentrations too low to be sequenced, indicating that Troubat remains did not preserve sufficient endogenous DNA to be genetically analysed.

DISCUSSION AND CONCLUSION

The cave of Troubat produced the remains of two partial Azilian dogs. The biological study and estimation of their statures makes it possible to describe them as medium-sized dogs without any visible pathology. We are unable to determinate the sex of the two individuals.

Biological comparisons with early Palaeolithic dogs remain difficult to establish due to their rarity and fragmentation. Often different bones are preserved from site to site making direct same-bone comparisons impossible. Comparing at the regional scale, we can nevertheless observe that the dogs from Troubat cave are slightly larger than the individuals from Pont d'Ambon that are associated with Laborian artefacts. This suggests a continuous decrease in the size of domesticated individuals from the Magdalenian to the Neolithic period, although conclusions remain tentative until more samples can be compared.

While layer 6 of the grotte-abri du Moulin yields an abundant collection of ungulate remains with numerous evidence of human modification, no anthropogenic marks were identified on the 147 dog remains. Moreover, the bones of the two individuals were found in an extremely small surface area and some bones still preserved anatomical connections. The lack of evidence of human intervention and the preservation of anatomical connections are commonly used criteria in paleoanthropology to establish the intentional deposition of bodies [START_REF] Henry-Gambier | Comportement des populations d'Europe au Gravettien : pratiques funéraires et interprétations[END_REF]. According to the typology used for dog deposition in archaeological contexts (Perri, 2017), we are faced with an "expedient deposition" of dogs at the Grotte-abri du Moulin. Some articulations are preserved and dogs' remains, although not bearing any traces, were discovered within a layer containing many faunal and lithic remains.

The intentional burials or deposits of dogs are extremely rare in the Palaeolithic and Epipalaeolithic record. The best-known example is by far the Natufian associated deposition at Ain Mallaha, where a young dog was found interred with a woman [START_REF] Davis | Evidence for domestication of the dog 12,000 years ago in the Natufian of Israel[END_REF].

Two other instances of intentional dog burials are known, one from the Natufian of Hayonim Cave in Israel [START_REF] Tchernov | Two new Dogs, and other Natufian Dogs, from the Southern Levant[END_REF], the other from the Late Glacial site of Bonn-Oberkassel in Germany [START_REF] Janssens | A new look at an old dog: Bonn-Oberkassel reconsidered[END_REF][START_REF] Street | The age of the Oberkassel burial in the context of climate, environment and the late glacial settlement history of the Rhineland. In The Late Glacial Burial from Oberkassel Revisited[END_REF]. The Grotte-abri du Moulin dogs constitute a unique case whereby two dogs are well preserved, making them the earliest known example of a double intentional dog deposit in the European archaeological record.

The place of dogs in prehistoric human societies appears highly variable, a fact that the sparse fossil data are currently at pains to document. At the end of the Palaeolithic, dogs could have been a potential source of food as at Le Morin (ca. 14.5 ky cal BP, [START_REF] Boudadi-Maligne | Des restes de chiens magdaléniens à l'abri du Morin (Gironde, France). Implications socioéconomiques d'une innovation zootechnique[END_REF] or Pont d'Ambon (ca. 11.7 ky cal BP, [START_REF] Boudadi-Maligne | L'Aquitaine à la fin des temps glaciaires -Les sociétés de la transition du Paléolithique final au début du Mésolithique dans l'espace Nord aquitain, Actes de la table ronde organisée en hommage à Guy Célérier[END_REF][START_REF] Célérier | Un chien dans l'Azilien de "Pont d'Ambon" (Dordogne)?[END_REF][START_REF] Pionnier-Capitan | New evidence for Upper Palaeolithic small domestic dogs in South-Western Europe[END_REF], where multiple cutmarks have been described on the dogs' remains. In other contexts, dogs could be considered as members of the group as they had been associated with human remains (Bonn-Oberkassel, ca 14 ky cal BP, [START_REF] Baales | Environnement et archéologie durant le Paléolithique final dans la région du Rhin moyen (Rhénanie, Allemagne): conclusions des 15 dernières années de recherches[END_REF][START_REF] Janssens | A new look at an old dog: Bonn-Oberkassel reconsidered[END_REF][START_REF] Street | The age of the Oberkassel burial in the context of climate, environment and the late glacial settlement history of the Rhineland. In The Late Glacial Burial from Oberkassel Revisited[END_REF]. In other instances, dogs remain of unknown status, like at Kesserlerloch (ca. 14 ky cal BP, Napierala & Uerpmann, 2012), Hauterive-Champreveyre (Upper Magdalenian 14-16 ka cal BP, [START_REF] Morel | Hauterive-Champréveyres, 11. Un campement magdalénien au bord du lac de Neuchâtel : étude archéozoologique (secteur 1)[END_REF] or the Upper Magdalenian site of Monruz where only deciduous teeth have been discovered (ca 15 ky cal BP, [START_REF] Müller | Le site magdalénien de Monruz, 3. Acquisition[END_REF]. All these examples illustrate the fact that it remains difficult, when studying dog remains, to establish a direct link between presence and function. In several publications the idea of using dogs as hunting aids has been put forward [START_REF] Lupo | A dog is for hunting[END_REF][START_REF] Lupo | When and where do dogs improve hunting productivity? The empirical record and some implications for early Upper Palaeolithic prey acquisition[END_REF].

The question remains, though, what direct evidence of such a dog function can we find in the archaeological record? At Troubat, the fact that the two dogs were not sources of meat is demonstrated by the lack of human modification on the bones. The dog is the only taxon for which complete and modification-free remains have been identified in the Palaeolithic levels of the cave. In the same way, more than 4% of the identified ungulate remains from layer 6 bear digestion marks.

These semi-digested bones belong to red deer, roe deer, wild boar, chamois and ibex: the same species as those hunted by Azilian groups from the Grotte-abri du Moulin. This indicates that dogs had access to the same carcasses (or at least to part of the carcasses) as those consumed by hunter-gatherer groups. Without necessarily constituting definitive evidence that dogs played an active role in hunting practices, this suggests at the very least, that a close relationship existed between dogs and humans regarding food. Whichever were the function(s) of dogs, the intentional double dog deposit at the Grotte-abri du Moulin is clearly the earliest robust evidence for the privileged place of dogs in human societies during the Pleistocene. Stratigraphy is modified from Barbaza 1996. These previously done radiocarbon dates are available in [START_REF] Barbaza | Le Magdalénien supérieur final et l'Azilien dans les Pyrénées centrales. La grotte-abri du Moulin à Troubat (Hautes-Pyrénées) et son context[END_REF][START_REF] Barbaza | L'Azilien classique pyrénéen. L'Azilien de la grotte de Troubat dans ses divers contexts[END_REF]Barbaza & Heintz, 1992;[START_REF] Costamagno | Mobilité, territoires de chasse et ressources animales au Magdalénien final en contexte pyrénéen : le niveau 7A de la grotte abri du Moulin (Troubat, Hautes-Pyrénes)[END_REF][START_REF] Cheung | L'Azilien pyrénéen parmi les sociétés du tardiglaciaire ouesteuropéen: apport de l'étude des industries lithiques[END_REF][START_REF] Heinz | Environmental changes during the Late Glacial and Post-Glacial in the central Pyrenees (France): new charcoal analysis and archaeological data[END_REF]. Dates are uncalibrated. Principal Component Analysis (PCA) on ulnae. Specimens used for these analyses are detailed in Table 1 & Table 2, respectively). Troubat remains are well within the modern dog ellipses and outside the ranges of wolves.

Figure 1 :

 1 Figure 1: Location (a.), stratigraphy and chronology (b.) of the "Grotte-abri du Moulin".

Figure 2 :

 2 Figure 2: Examples of complete anatomical portions (a.) Restitution of a part of the vertebral column (a.1: T10 to L5), complete ulna (a.2), end of a left front leg in anatomical connection (a.3), right tibia of the tallest dog (a.4) right tibia of the smallest dog (a.5), articular restitution of right metatarsals (a.6), left calcaneus belonging to the tallest dog (a.7), right calcaneus from the smallest one (a.8). b: Skeletal representation of the dog remains

Figure 4 :

 4 Figure 4: Multivariate analysis. a: Linear Discriminant Analysis (LDA) on calcanei and b:

Table 1 :

 1 Identification Number, Location, Group of canids and measurements used for the Linear Discriminant Analysis on the calcanei (Fig4.a.) MD: Modern dogs; LPW: Late Pleistocene wolves; MWWP: Modern Wild Wolves Portugal

Table 2 :

 2 Identification Number, Location, Group of canids and measurements used for

	Principal Component Analysis on the ulnae (Fig4.b.) MD: Modern dogs; LPW: Late
	Pleistocene wolves; MWWP: Modern Wild Wolves Portugal

Table 3 :

 3 Canid remains from the Grotte-abri du Moulin: Identification number, anatomy, portion and lateralisation of each element.

ACKNOWLEDGEMENTS

This work was supported by the French National Research Agency ANR-10-LABX-52 Labex LaScArBx, through the TeHoTeCa project and by Fondation Fyssen which funded the DOGMATIQ project. The authors would like to thank the Midi-Pyrénées Regional Archaeology Service for granting access to the Grotte-abri du Moulin faunal remains. We also thank Maxime Pelletier for his help with the photogrammetry, Brad Gravina for his careful proofreading and the reviewers whose suggestions helped improve this paper.