T Pelini

C Elias

R Page

L Xue

S Liu

J Li

J H Edgar

A Dréau

V Jacques

P Valvin

B Gil

G Cassabois

Shallow and deep levels in carbon-doped hexagonal boron nitride crystals

We study shallow and deep levels in carbon-doped hexagonal boron nitride crystals precipitated from a molten metal solution in a high temperature furnace. Reflectance and photoluminescence under deep ultraviolet excitation are complemented by spatially-resolved experiments by means of a scanning confocal micro-photoluminescence setup operating in the ultraviolet. Isotopicallycontrolled carbon doping does not induce any energy shift of the well-known deep level emission at 4.1 eV. Our detailed characterization in a series of carbon-doped crystals reveals that the incorporation of carbon during the growth process results in a distinct class of shallow and deep levels in hexagonal boron nitride, calling into question the exact role of carbon in the growth of hexagonal boron nitride and its direct or indirect influence on the formation of the crystal defects.

I. INTRODUCTION

Hexagonal boron nitride (hBN) is a semiconducting material which has gathered increasing attention due to its unique properties. As a lamellar compound, hBN is a model 2D material along with graphene and transition metal dichalcogenides, and a fundamental building block of van der Waals heterostructures [1]. As a wide-band gap nitride semiconductor, hBN possesses great potential for deep ultraviolet (UV) applications due to the bright luminescent emission of hBN single crystals [2], thus competing with nitride semiconductors of the AlGaN family that are already widely used for blue and UV lighting [3]. The scalable and controlled growth of high-quality hBN, however, is the current challenge precluding future developments in many communities that share a common interest for this outstanding material [4].

A key issue to understanding the growth of a crystal is to understand the nature of its defects. On one hand, the suppression of extended and point defects is a prerequisite for the fabrication of large hBN single crystals. Although recent studies have reported significant progress in the limit of thin samples with the growth of monolayer hBN by chemical vapor deposition [5,6] and molecular beam epitaxy [7], the synthesis of thick epilayers does not lead yet to a crystalline quality competing with bulk hBN crystals grown by precipitation techniques [2,8,9]. On the other hand, the controlled creation of defects is important in the context of quantum technologies where point defects are used as artifical atoms in solid state [10], and also in the traditional prospect of doping for implementing electrical injection in commercial devices.

Carbon was early proposed as an impurity playing an important role in the opto-electronic properties of hBN [11,12]. After the growth of high-purity hBN crystals in 2004 [2], Taniguchi et al. reported a strong increase in the photoluminescence (PL) intensity of the 4.1 eV line in hBN crystals with a high carbon concentration [13]. Still, there is no consensus on the exact role of carbon in the physics of the defects in hBN, with a lively debate on both experimental [14][15][16][17] and theoretical [18][19][20][21] sides.

Here, we have studied the optical properties of carbondoped hBN. We have performed reflectance and PL under deep UV excitation, complemented by spatially-resolved experiments by means of a home-made UV scanning confocal micro-PL setup on a series of hBN crystals grown by precipitation from a molten metal solution in a high temperature furnace. In the spectral range of shallow levels extending from 5 to 5.7 eV, we identify new optical transitions at 5.37 and 5.52 eV that we attribute to defect bound excitons. In the UV range of deep levels emitting around 4 eV, we first focus on the well-known 4.1 eV line and examine if isotopically-controlled doping by 13 C modifies its emission energy. Then, we report the observation of additional optical transitions observed from 4.12 to 4.16 eV, which remain unaddressed in the literature. We combine reflectance, temperature-dependent PL, and spatially-resolved micro-PL to demonstrate they correspond to a class of point defects, distinct from the 4.1 eV line. This paper is organized as follows. Section II is devoted to the presentation of our experiments, with a description of the optical measurements (Section II A) and of our hBN crystals (Section II B). We then present our experimental results on the shallow levels (Section III) and on the deep levels (Section IV) in our carbon-doped hBN crystals.

II. EXPERIMENTS

In this section, we present our experiments dealing with optical spectroscopy by reflectivity, macro-and micro-PL (Section II A), performed in various hBN crystals for which we detail the synthesis protocol and conditions, complemented by basic elementary characterization by PL over a large spectral range from 3.5 to 6 eV (Section II B).

A. Optics

Reflectivity

A home-made reflectance setup was built based on a deuterium lamp (63163 Newport), spectrally filtered through a monochromator (Cornerstone CS130) with 100 µm-slits providing a spectral resolution on the order of 10 meV. The incident light was focused on the sample close to normal incidence, with a rectangular spot of size 100 µm×1.5 mm. All optical elements are reflective mirrors coated for deep UV. For more details on the reflectance setup, please refer to Ref. [7].

Macro-photoluminescence

In macro-PL spectroscopy, we performed above bandgap excitation at 195 nm (6.35 eV) with the fourth harmonic of a cw mode-locked Ti:Sa oscillator with a repetition rate of 82 MHz. The spot diameter is on the order of 50 µm, with a power of 30 µW. An achromatic optical system couples the emitted signal to our detection system, composed of a f=300 mm Czerny-Turner monochromator, equipped with a 1800 grooves/mm grating blazed at 250 nm (providing a spectral resolution of 0.8 nm or 2.5 meV at 200 nm), and with a back-illuminated CCD camera (Andor Newton 920), with a quantum efficiency of 50% at 210 nm, operated over integration times of 5 min. The sample is held on the cold finger of a closedcycle cryostat for temperature-dependent measurements from 10K to room temperature.

Micro-photoluminescence

For micro-PL spectroscopy, the excitation is below the band-gap of hBN with a diode-pumped cw laser providing an optical emission at 266 nm (4.66 eV) coming from the fourth harmonic of a Nd:YAG laser. The setup is a home-made confocal microscope based on a closedcycle cryostat with a high numerical aperture objective (NA=0.4) optimized for UV spectroscopy. Our lateral spatial resolution is about 700 nm and the longitudinal one of about 1µm. The PL signal is detected with an achromatic and geometric aberration-free optical system and coupled, either to hybrid photomultiplier tubes (PMA Series PicoQuant) connected to a single-photon counting acquisition card, or to a spectrometer (Shamrock 500i, f=500 mm Czerny-Turner system) equipped with a grating of 1800 grooves/mm blazed at 250 nm (providing a spectral resolution of 0.02 nm or 0.3 meV at 300 nm) and a back-illuminated UV-enhanced camera (Andor Newton DU940P BU2) with a quantum efficiency of 60% at 300 nm.

B. Samples

Synthesis of hBN crystals

The carbon-doped hBN single crystals were grown from a molten metal solution at atmospheric pressure with a modified version of the growth procedure described in Ref. [8,9]. Three metals were used: nickel, iron, and chromium. Nickel and iron are good solvents for boron, with maximum solubilities of 18.5 at% and 17 at%, respectively. Chromium is good for dissolving nitrogen. Mixing of both metallic solvents forms an eutectic (at 1345 for Ni and Cr). The boron source for all samples were either a commercial hBN powder or Hot Pressed Boron Nitride (HPBN), nominally fabricated from the same commercial powder. The carbon source was graphite powder, with the natural distribution of carbon isotopes, except for sample C40, which used 13 C enriched graphite.

More specifically, the carbon-doped hBN crystals were precipitated from the molten metal solution in a high temperature single-zone furnace. For growing the crystals listed in the first column of were loaded in either an alumina or a HPBN ceramic crucible, and transferred into the furnace. The furnace was evacuated and purged with the atmospheric gases listed in the 6 th column multiple times to remove all residual oxygen. For growing pure hBN crystals, a small amount of hydrogen is typically added with the flowing nitrogen, to reduce and remove any oxides from the source materials. However, to avoid excessive etching of the graphite source for growing these carbon-doped hBN crystrals, carbon monoxide was used instead of hydrogen, as it reacts with, and removes oxygen, but not carbon. The precursor powders were heated up to 1550 to form liquid phase and held at this temperature for 24 hours to thoroughly dissolve the carbon, boron and nitrogen into the molten metal solution. The crystals were then grown by cooling the system to 1300 at rates of 4/hour or 0.5/hour, as listed in the 5 th column. During crystal growth, the atmospheric gases were continuously flowed to the system at the rate of 125 sccm and the system was kept at 850 torr. After crystal growth, the system was then quenched to room temperature at 50/hour.

Iron vs nickel as solvent

As alluded to earlier, the solubility of boron is higher in nickel than iron while the residual amount of carbon impurities in our Fe is smaller than in our Ni. PL is an efficient method to diagnose semiconductors [22] there-fore we use it to get information regarding the structural properties and residual dopings of these BN crystals. For that reason we begin by comparing the 8K PL features in a wide spectral range between 3.5 and 6 eV [Fig. 1], recorded in cases of samples C20 (Fe-Cr solvent) and C22 (Ni-Cr solvent), and of the BN powder used as precursor, here in the form of HPBN. The PL spectrum of the BN precursor displays emission lines near 4 eV (shaded region in Fig. 1). These lines are also found in the PL spectra of C20 and C22. These lines form a more or less documented series, depending on the growth run, with in general, the detection of a strong feature at 4.1 eV. We emphasise that these sharp lines were never recorded in case of samples grown using the protocol of Ref. [23], which led us to revealing the optical performances of mono-isotopically boron-purified BN samples [24]. For these growths, B and N 2 where used instead of a BN powder.

PL intensities have been plotted logarithmically in Fig. 1. The PL spectrum below 5 eV corresponds to the recombination lines arising from deep levels. There is the well-known zero-phonon line (ZPL) at 4.1 eV with its accompanying phonon replicas at lower energies [13,25]. Because of the heavy carbon-doping in our hBN crystals, the PL intensity of the 4.1 eV line is comparable to that of the stacking faults around 5.5 eV [Fig. 1], in contrast to the peak observed in powder samples, if it is seen at all. This is in agreement with similar studies reported in Ref. [13] that led those authors to relate the 4.1 eV line to carbon/oxygen impurities. In Fig. 1, the PL line shape is not influenced by the chemical composition of the solvent (iron or nickel) within the inhomogeneous linewidth of our hBN crystals. It is important to emphasize the occurrence of a doublet feature centered at 4.12 eV, with an inter-level splitting of ∼2 meV in sample C22 (grown using Cr-Ni) which is also recorded but weaker in sample C20 (grown using Cr-Fe), as shown in Fig. 2. The PL feature of the BN powder displayed in Fig. 2 indicates that it is also present in the precursor. Therefore this doublet line is correlated to a contaminant contained in the BN precursor. It has nothing to do with the contamination of Fe, Ni or Cr.

Finally, we observe a broad composite fluorescence band in the visible portion of the spectra (not shown here). There have been many experimental studies of defect-related single photon sources operating in the visible spectrum with recent interpretations suggesting lattice related defects [10,[26][27][28]. The only difference between Ni and Fe as a solvent is the unsystematic recording of a weak and broad PL band at 4.6 eV in case of using the Ni-Cr solvent. We thus conclude that the use of Ni or Fe has only a marginal influence on the optoelectronic properties of our hBN crystals.

In the following, we focus on the impact of carbondoping considering separately shallow levels in the 5-5.7 eV range (Section III), and deep levels around 4.1 eV (Section IV)

III. SHALLOW LEVELS Fig. 3 displays (on a semi-logarithmic scale) the PL spectra recorded at 8 K for different samples: (from top to bottom) a test un-doped hBN crystal purchased from HQ Graphene (HQG), samples C22 and C27 (w/o CO), and sample C33 (w/CO but with different C/BN ratios, see Table 1).

The 5.7-6 eV energy range corresponds to the intrinsic part of the emission spectrum. This series of lines correspond to the well-defined phonon-assisted optical transitions typical of indirect band-gap semiconductors and here, of the hexagonal BN stacking, with their specific spectral distribution [29]. Zooming in on each of these phonon-assisted recombination lines reveals a similar lowenergy fine structure, due to the emission of phonon overtones involving the low-frequency Raman active mode at zone center [30].

Below the energy of the LO-assisted recombination (5.75 eV) starts the spectral range of defect-related emission. From 5.7 eV down to 5 eV, there is a series of discrete lines split from each other by about 147 meV.

Transverse optical phonons at the K point of the Brillouin zone assist inter-K valley scattering, producing a series of discrete recombination bands because the stacking faults in bulk hBN [31,32] provide a density of final electronic states in the Fermi's golden rule, and shape the envelope of the PL spectrum [33]. Two distinct lines are also detected that we labelled D2 and D6 [Fig. 1] in our initial paper [33], where we tentatively attributed them to the BN di-vacancy following Ref. [18]. The PL spectrum is almost identical for the first three samples (HQG, C22, and C27). In contrast, in the heavily-doped C33 sample, the PL spectrum presents less defined features. The sharp and detailed series of phonon-assisted recombination lines above 5.7 eV is strongly broadened and hardly observable and the defectrelated emission lines are also broadened. More importantly we note an apparent blue-shift of the most intense line to 5.52 eV in C33, shifting from 5.47 eV in the other samples.

The higher carbon concentration in C33 may affect the crystal quality and result in a decrease of the intensity of the intrinsic recombination lines together with a larger inhomogeneous broadening, which would contribute to washing out the series of phonon replicas above 5.7 eV. As a matter of fact, this may also impact the defectrelated emission lines, but we note a distortion of the peak around 5.5 eV. In HQG, C22, and C27, the energy of the maximum is 5.47 eV with a high-energy shoulder at 5.5 eV [Fig. 3] whereas we observe a broader asymmetric line in C33 with a maximum at 5.52 eV, an energy that corresponds to a minimum of the PL spectrum in HQG, C22, and C27. Finally, a secondary maximum appears at 5.37 eV in C33, as a high-energy shoulder of the 5.3 eV band, again at an energy where the PL spectrum of HQG, C22, and C27 displays a minimum. These observations suggest the existence of new transitions in the optical response of heavily carbon-doped hBN crystals.

To test this hypothesis, we compare in Fig. 4 the PL FIG. 4. PL (solid lines) and reflectance (dotted lines) spectra on linear scales, recorded in a test un-doped hBN crystal (HQG, black lines) and a heavily carbon-doped one (C33, green lines) at 10 K. Note the coincidences between the reflectance minima and the PL maxima in sample C33.

and reflectance spectra (on linear scales) recorded in HQG and C33. In the former case, the reflectivity (dotted black line in Fig. 4) displays smooth variations with a rather flat spectrum in the spectral domain where the PL spectrum (solid black line in Fig. 4) is on the contrary very rich with many lines due to intervalley scattering into excitonic states bound to stacking faults, as explained above. Such a difference between the spectra is fully consistent with the distinct nature of the microscopic processes involved in PL and reflectivity. While reflectivity probes a material in its thermodynamic equilibrium, PL results from a non-equilibrium state where carrier relaxation and thermalization play a key role. The presence of many defect-related lines around 5.5 eV, in the PL spectrum but not in the reflectance one, is a genuine signature of relaxation effects with resonances of intervalley scattering, further supporting the interpretation initially proposed in Ref. [33].

The heavy carbon-doping in C33 not only modifies the PL spectrum (solid green line in Fig. 4) but also the reflectance one (dotted green line in Fig. 4). Strikingly, there are pronounced reflectance oscillations, in resonance with the new emission lines at 5.37 and 5.52 eV, pointed out above. This observation confirms the existence of new optical transitions associated with heavy carbon-doping in hBN crystals. The incorporation of carbon during the growth process produces complementary states with a high enough density and oscillator strength of their optical transition to significantly change the dielectric constant in the 5.3 eV to 5.6 eV energy region. Following Ref. [18], we tentatively attribute the lines at 5.37 and 5.52 eV to an exciton bound to the defect formed by a carbon substituting a nitrogen (C N ). In Ref. [18], the optical absorption spectrum of hBN calculated by the Bethe-Salpeter equation predicted that native point defects or impurities induce novel optical transitions close to the band-gap, corresponding to defect bound excitons. The heavy carbon-doping in C33 makes the C N -bound exciton a strong candidate for the 5.37 and 5.52 eV lines.

IV. DEEP LEVELS

In this section, we focus on emission lines at lower energy than in the previous part of our paper. In hBN, the emission lines below 5 eV come from deep levels due to intrinsic or extrinsic point defects in the hBN lattice. In Section IV A, we first discuss the well known 4.1 eV line and our original measurements in hBN crystals doped with 13 C, and then address in Section IV B the additional lines in the 4.1-4.2 eV range, which are not discussed in the literature.

A. The 4.1 eV line Carbon was early suggested as the possible origin of the 4.1 eV emission in hBN [11,12]. It was later reported that the intensity of the 4.1 eV line is strongly enhanced in hBN crystals with a high carbon concentration [13]. Advanced spectroscopic studies have been devoted to the vibronic spectrum and the particular structure of the phonon sidebands [25], or to isolated single centers in cathodoluminescence experiments [34]. However, there is so far no definitive proof that carbon enters into the composition of the point defect leading to the 4.1 eV line. Moreover, recent papers reported no positive correlation between the carbon concentration and the intensity of the 4.1 eV emission [14], and excluded C N as its origin on the basis of density functional theory with a hybrid functional [19]. Other possible origins have been proposed in Ref. [15], such as a V B O N complex.

In this controversial context, we have grown hBN crystals with isotopically-controlled carbon doping in order to further test if carbon is a constituent of the defect emitting at 4.1 eV. More specifically, we have performed high-resolution measurements of the energy of the ZPL in hBN crystals doped either with 13 C (sample C40, Table 1) or with nat C (sample C27, Table 1), i.e. the natural mixture of ∼99% in 12 C and ∼1% in 13 C ( nat C being close to pure 12 C).

Isotopic purification is a powerful tool in semiconductor physics enabling the fabrication of chemically and isotopically pure crystals and crystals doped with isotopically pure impurities [35]. For point defects, isotopic control of the atoms forming the host lattice impacts the vibronic part of the emission spectrum with an isotopic shift of the phonon sideband, while isotopic selectivity of the defect itself can shift the zero-phonon lines. Several mechanisms may explain this isotopic shift, such as a modification of the confinement or the electron-phonon interaction [35]. Quantitatively, the energy shifts are very small. For the boron-bound exciton in silicon, the isotopic shift is of the order of 1 µeV when switching from 10 B to 11 B [35], but it can be much larger as for G-centers in silicon with an isotopic shift of ∼15 µeV between 12 C and 13 C doping [36], or even ∼600 µeV between 28 Si and 30 Si for the SiV center in diamond [37].

In the case of hBN doped either with nat C or 13 C, the search for an isotopic shift of the 4.1 eV line requires highspectral resolution measurements. Moreover, one has to pay attention to possible variations of the ZPL energy within a given sample before comparing nat C-and 13 Cdoped hBN in order to exclude any artefact due to a shift not coming from the isotopically-controlled doping.

In the next section (IV A 1), we first study the spatial variations of the energy and linewidth of the ZPL in sample C40, and then compare in section IV A 2 the statistical analysis of our data recorded in nat C-and 13 Cdoped hBN crystals (samples C27 and C40, Table 1).

Spatial inhomogeneity

Fig. 5 displays the analysis of a set of data measured by means of our UV scanning confocal micro-PL setup (Section II A 3) in a 30×30 µm 2 region of sample C40 at 10 K. For each of the 150×150 positions, we acquired the PL spectrum over an integration time of 3 s, resulting in a total acquisition of 27 hours for the full scan. The ZPL was then fitted with a Gaussian function for each point. With ∼ 5.10 4 counts at the maximum of the ZPL, the standard deviation for the estimation of the ZPL energy and width was 6 µeV and 10 µeV, respectively. Compared to the typical shifts discussed above, we conclude that our acquisition parameters are a priori adapted for resolving isotopic effects in hBN.

In Fig. 5(a) we show the energy shift ∆E of the ZPL relative to the average value of 4.0968 eV in this region. The variations of the ZPL Full Width at Half Maximum (FWHM) Γ are plotted in Fig. 5(b). The first finding is that the fluctuations of the ZPL energy over the 30×30 µm 2 scanned region have an amplitude as large as one meV. These fluctuations contribute to the inhomogeneous broadening of the 4.1 eV line. With an average FWHM on the order of 1.3 meV [Fig. 5(b)], the ±0.2 meV variations of the ZPL energy are consistent with the 1.9 meV value of the ZPL width previously reported by macro-PL measurements in Ref. [25], but here we notably demonstrate the sub-µm scale at which the ZPL energy varies.

Interestingly, the ZPL shifts [Fig. 5(a)] appear spatially-correlated with the ZPL broadening [Fig. 5(b)]. Although there is not a one-to-one correlation, the overall pattern is the same in Fig. 5(a)&(b), with lines of different orientations in close analogy to the luminescence maps recorded by cathodoluminescence [31]. We interpret these lines as folds or extended defects in the lamellar structure of our hBN crystal.

The spatial correlation of the ZPL shift and broadening suggests they correspond to the real and imaginary parts of the same perturbation. That they also have the same magnitude (in the 0.1 meV range here) further supports a common origin, presumably mechanical strain related to the stacking faults in hBN.

Maps of the Raman-active mode at ∼1370 cm -1 (170 meV) over the same area were featureless maps (not shown here) with random fluctuations in the range of ± 60 µeV. This may come from the reduction of the stressinduced energy shift for phonons compared to electrons [38,39].

Although detrimental to our objective of resolving an isotopic shift in nat C-and 13 C-doped hBN crystals, the sensitivity of the ZPL energy to the presumable perturbation caused by strain is an important piece of information for elucidating the structure of the point defect leading to the 4.1 eV emission. In this context, PL experiments as a function of pressure could provide interesting complementary inputs [40] for identifying the nature of the defect and reaching a quantitative interpretation of our measurements.

13 C vs nat C doping

Keeping in mind the spatially-resolved ZPL fluctuations in Fig. 5, we now compare the ZPL energy in hBN crystals doped either with nat C and 13 C. Fig. 6 shows the probability density of detecting the ZPL at a given energy. Following the data analysis described in the previous section, we have computed the statistical distributions of the ZPL energy in two 30×30 µm 2 regions, arbitrarily selected in each crystal.

We first note the different shapes and widths of the four statistical distributions, which can be narrow or asymmetric in the two types of samples. More importantly, there is no spectral separation indicative of an isotopic shift of the ZPL, within the inhomogeneous broadening of the 4.1 eV line. Moreover, there does not seem to be any specific feature of the spatial fluctuations of the ZPL energy from 13 C-to nat C-doped hBN crystals. As explained in the previous section, this suggests mechanical strain impacts the ZPL position independently of the carbon isotopes.

We thus conclude that there is either no isotopic shift, or a finite one but smaller than the inhomogeneous broadening. Isotopic selectivity in carbon-doping has not brought any proof to confirm or infirm the presence of carbon in the structure of the defect emitting at 4.1 eV. However, our test demonstrated that the spatial fluctuations of the ZPL were in a sub-µm scale. The correlation of the ZPL shift and broadening indicates a common origin, with original quantitative measurements which hopefully will be useful for testing various models in future calculations.

In the last part of our paper, we finally present our PL experiments dealing with additional spectral lines in the 4.12-4.16 range, which, in contrast to the 4.1 eV line, are not discussed in the literature.

B. The 4.12-4.16 eV lines The 4.1 eV line is not the only feature related to deep levels in the UV spectral range. The existence of additional lines at energies higher than 4.1 eV was never pointed out nor discussed in the literature, to the best our knowledge. In the following, we present extensive characterization of the additional lines in the 4.12-4.16 range, by means of macro-PL and reflectance experiments (Section IV B 1) and spatially-resolved micro-PL measurements (Section IV B 2). We show that their PL intensity increases in heavily carbon-doped hBN crystals and that all the additional lines are independent of each other.

Macro-PL and reflectance experiments

Fig. 7 displays the PL spectra of three samples in the 3.5 to 4.25 eV spectral range. Emission from the 4.1 eV deep level is always recorded with its sharp ZPL, phonon sidebands and prominent optical phonon replicas at 3.9 and 3.7 eV [25,41]. A new line at 4.12 eV (later called UV1) is observed with a doublet fine structure splitting, as seen earlier in Fig. 2 and better resolved below in micro-PL measurements. In sample C27, there are additional weak lines at 4.14 and 4.16 eV (called UV2 and UV3, respectively), all three of these lines being much more intense in the heavily carbon-doped C33 sample. We also note that all of these lines broaden when increasing the amount of carbon involved into the growth protocol, which likely prevents us from observing any fine structure splitting, if any, for UV2 and UV3. Finally, phonon replicas of UV1, UV2, and UV3 are detected, readily observed and red-shifted by 200 meV (LO(Γ) phonon) and 400 meV (2LO(Γ) phonons) from their ZPL [Fig. 7]. The first indication that UV1-3 do not share the same microscopic origin as the 4.1 eV deep level comes from the vibronic part of their PL spectrum. The experimental data recorded in C33 are plotted on a linear scale in Fig. 8 in order to better resolve the different lineshapes of UV1-3 compared to the 4.1 eV line. At low temperature [8K, top spectrum in Fig. 8], we can distinguish the ZPL and the low-energy sideband in the emission spectrum of the 4.1 eV defect as described in Ref. [25], although the carbon-doping leads to broadening of the ZPL. On the contrary, for UV1-3, no phonon sideband is observed and each of the UV1-3 lines can be fairly fitted to a symmetric profile, with a Gaussian function. Moreover, on raising the temperature [Fig. 8], the FWHM of UV1-3 increases from 6 meV at 8K to 9 meV at 120K. Such a temperature dependence contrasts with the phenomenology discussed in Ref. [25] where we observed a 5.5 meV broadening in the 8-120K range, further suggesting a different microscopic structure in UV1-3 compared to the 4.1 eV defect.

Furthermore, looking at the PL signal intensity as a function of temperature [Fig. 8] we do not record any cross-talk between the lines, with a constant intensity ratio among the set of optical transitions in the 3.8 to 4.2 eV range. This observation indicates the absence of interlevel crossing in the relaxation dynamics, either because the spatial separation of the defects is larger than the carrier diffusion length, or because they are formed in distinct mesoscopic domains due to the local fluctuations of the growth conditions during the synthesis of the hBN crystals. Direct access to the oscillator strength of the optical transitions is obtained by reflectivity experiments around 4.1 eV. In Fig. 9, we compare the PL and reflectance spectra recorded at 8 K in sample C33. The reflectivity signal exhibits a marked peak at 4.1 eV, testifying to a substantial oscillator strength. Because of the mirror symmetry between phonon-assisted absorption and phonon-assisted emission, we record a series of blue-shifted minima that are the high-energy analogues of the phonon replicas detected in the PL spectrum [Fig. 9], with an inter-level spacing of 200 meV as discussed above. Note the ZPL is broader in reflectance than in PL, thus smearing out the fine structure of the phonon replicas in reflectivity. Surprisingly, nothing is detected for the UV1-3 deep levels in reflectivity. Although the PL lines of UV1-3 and the 4.1 eV defect are of comparable intensities in this sample, no reflectance minima are seen at the energies of the new carbon-related PL lines. This effect may come from either a smaller oscillator strength, or a lower density of the UV1-3 point defects. In both cases, the interpretation of the high PL signal intensity from UV1-3 implies a more efficient carrier relaxation in these novel carbon-related point defects.

To get further insight in these deep levels, we present in the final part of our paper measurements by means of our UV scanning confocal micro-PL setup.

Micro-PL experiments

In order to get further insight in these deep levels, we finally present measurements by our UV scanning confocal micro-PL setup. We recall these measurements consist in recording the PL spectrum at each point of a given region that is scanned at the surface of the sample. We first studied the spatial variations of the UV1-3 PL signal intensity recorded by means of our UV scanning confocal microscope in sample C27, at 10 K. Carbon-doping in this hBN crystal is moderate to minimise broadening of the PL lines. In Fig. 10(a), the PL map is plotted after integration of the signal intensity from 4.112 to 4.175 eV in order to better visualize the presence of the novel carbon-related UV1-3 lines. With this color code related to a spectral range excluding the 4.1 eV defect, we stress the regions appearing dark in Fig. 10(a) correspond to a large PL intensity of ∼ 5.10 3 counts per second for the 4.1 eV ZPL. In terms of the intensity of the UV1-3 lines, Sample C27 is thus more or less homogeneously dim except for very bright domains. The 4.1 eV defect is also rather uniform and intense with a local increase in some bright regions of Fig. 10(a), as detailed below.

The PL spectra of five representative regions are provided in the lower panels (b-f) of Fig. 10. In the homogeneous part of the sample [circle 1 in Fig. 10(a)], only the 4.1 eV line is detected with a sharp ZPL and the lowenergy acoustic phonon sideband [Fig. 10(b)]. We note the high signal-to-noise ratio with a dynamical range of three decades between the maximum of the ZPL at 4.1 eV and the noise level at higher energy. Such a precision is important in view of claiming the contribution of the novel carbon-related UV1-3 lines is negligible within our signal-to-noise ratio.

Circles 2, 3 and 4 are located in the bright white regions. As expected, we observe that the UV1-3 lines have a PL signal intensity always lower than the 4.1 eV defect, in agreement with the macro-PL measurements presented in Fig. 7. Our spatially-resolved PL measurements reveal the striking absence of spatial correlation between the UV1-3 lines: all three are detected at point 2, only UV1 at point 3, UV1 and UV3 at point 4, and UV1 and UV2 at point 5. We thus conclude that the novel emission lines related to carbon-doping in hBN do not come from the same defect but that they originate from three independent defects. This is a crucial information and a key indication that one has to look for three novel microscopic configurations in order to account for the micro-PL measurements in Fig. 10. It is very tempting, having in mind proposals that were made for porous silicon [42] on ZnO [43], to invoke complexes of the O N H kind, or the C B H one, as candidates for these lattice defects. Unfortunately, theoretical works dedicated to these specific defects have not been performed yet.

Eventually, we point out the doublet structure of UV1, which is much better resolved in Fig. 10 than in Fig. 2. Thanks to the low carbon doping and correspondingly narrow defect lines, as well as the spatial resolution of our microscope, the spectra recorded in positions 3 and 4 show that UV1 is a doublet with a splitting of 2.7 meV and a FWHM of 1.7 meV for the two components. On the contrary, we could not detect any fine structure for UV2 and UV3, supporting the picture that each emission line originates from a distinct defect. We first notice the increased inhomogeneity with many domains giving a large PL signal intensity for the UV1-3 lines. Moreover, there is a significant broadening with a typical FWHM of the order of ∼ 10 meV compared to 1.7 measured in Fig. 10(d&e). Nevertheless, the 4.1 eV defect maintains a narrow ZPL as seen in Fig. 11(c&e), not only where the UV1-3 lines are hardly observable, but also in locations where they are intense [Fig. 11(f)] or even dominate the PL spectrum [Fig. 11(b,d)]. This phenomenology suggests that the UV1-3 defects are not only independent from each other but also uncorrelated to the 4.1 eV deep level. This raises fundamental questions on the exact influence of carbon which is clearly involved here: is carbon itself incorporated into the defect structure, and in which crystallographic configurations with 4 point defects to be explained ? Does carbon play only a side-effect and promote the formation of deep levels without any embedded carbon atom ?

At the very end of Section IV devoted to deep levels, we are thus left with numerous novel issues fed by our measurements in heavily carbon-doped hBN samples. The detailed characterization of the novel deep levels emitting above 4.1 eV shows we are dealing with a new class of point defects in hBN crystals, where carbon plays a key role. Since new shallow levels were observed in heavily carbon-doped hBN (Section III) and interpreted as excitons bound to C N , one may wonder if the UV1-3 lines are themselves the true candidates for point defects made of carbon, and if, on the contrary, the 4.1 eV defect is carbon-free. In future studies the combination of advanced optical spectroscopy with nanoscale electron microscopy appears as a promising strategy for unveiling the nature of the defects.

V. CONCLUSION

We have investigated shallow and deep levels in carbon-doped hBN crystals. The incorporation of carbon during the growth process produces new optical transitions at 5.37 and 5.52 eV, which are observed in both reflectance and PL spectra. These shallow levels are attributed to an exciton bound to the defect formed by a carbon substituting a nitrogen (C N ). For the deep levels around 4 eV, we have first investigated if isotopic selectivity in carbon-doping modifies the energy of the wellknown ZPL at 4.1 eV. We conclude that there is either no isotopic shift due to 13 C, or a finite one but smaller than the inhomogeneous broadening. We have further resolved the spatial fluctuations of the ZPL at 4.1 eV on a sub-µm scale. The correlation of the ZPL shift and broadening indicates a common origin, presumably strain variations in the crystal. Finally, we have shown the existence of additional optical transitions at 4.12, 4.14 and 4.16 eV to which carbon doping induces a strong increase in their PL signal intensity. Reflectance, temperature-dependent PL, and spatially-resolved micro-PL demonstrate they correspond to three deep levels that are independent of each other, and uncorrelated to the 4.1 eV defect. Our detailed characterization of a new class of shallow and deep levels in carbon-doped hBN crystals provides a wealth of novel informations for identifying the influence of carbon in the growth of hBN, and its direct or indirect implication in the formation of the crystal defects.

FIG. 5 .

 5 FIG. 5. Spatial variations of the shift ∆E (a) and broadening Γ (b) of the ZPL in a 30×30 µm 2 region of sample C40 at 10 K. The average ZPL energy is 4.0968 eV.

FIG. 7 .

 7 FIG. 7. Full set of the PL spectra (semi-log scale) of carbondoped hBN samples in the 3.5 to 4.25 eV range. The spectra are vertically shifted for clarity. Note the occurrence of new lines at higher energy than 4.1 eV. The horizontal bars have a 200 meV width, corresponding to the LO(Γ) phonon energy.

FIG. 9 .

 9 FIG.9. PL (green dashed line) and reflectivity (purple solid line) spectrum on linear scales, recorded at 8K for sample C33 in the 4 eV region. The horizontal bars have a 200 meV width, corresponding to the LO(Γ) phonon energy. They are displayed in red for the red-shifted phonon-assisted emission (below 4.1 eV), and in blue for the blue-shifted phonon-assisted absorption (above 4.1 eV).

5 FIG. 10

 510 FIG. 10. (a) Spatial variations of the PL signal intensity recorded by a UV scanning confocal microscope in sample C27, at 10 K, for an excitation power of 0.35 mW. The PL signal is integrated from 4.112 to 4.175 eV. 5 s integration time per acquisition point. (b-f) PL spectrum on a log scale recorded at the five different locations indicated by circles in (a).

5 FIG. 11

 511 FIG. 11. (a) Spatial variations of the PL signal intensity recorded by a UV scanning confocal microscope in the heavily carbon-doped sample C33, at 10 K, for an excitation power of 0.35 mW. The PL signal is integrated from 4.112 to 4.175 eV. 2.5 s integration time per acquisition point. (b-f) PL spectrum on a log scale recorded at the five different locations indicated by circles in (a).

Fig. 11 was

 11 Fig.11was recorded in identical conditions as Fig.10but in a heavily carbon-doped hBN crystal (sample C33). We first notice the increased inhomogeneity with many domains giving a large PL signal intensity for the UV1-3 lines. Moreover, there is a significant broadening with a typical FWHM of the order of ∼ 10 meV compared to 1.7 measured in Fig.10(d&e). Nevertheless, the 4.1 eV defect maintains a narrow ZPL as seen in Fig.11(c&e), not only where the UV1-3 lines are hardly observable, but also in locations where they are intense [Fig.11(f)] or even dominate the PL spectrum [Fig.11(b,d)]. This phenomenology suggests that the UV1-3 defects are not only independent from each other but also uncorrelated to the 4.1 eV deep level. This raises fundamental questions on the exact influence of carbon which is clearly involved here: is carbon itself incorporated into the defect structure, and in which crystallographic configurations with 4

  13 C powder BN pressed boat 10 g Ni + 10 g Cr 0.5 /hour CO + N2

	Sample name Graphite quantity	BN quantity	Solvents	Cooling rate Atmosphere
	C20 C22 C27 C33	1.5 g graphite powder 0.5 g BN powder 10 g Fe + 10 g Cr 4 /hour H2 + N2 1.5 g graphite powder 0.5 g BN powder 10 g Ni + 10 g Cr 4 /hour H2 + N2 graphite piece HPBN 17 g Fe + 17 g Cr 4 /hour H2 + N2 2.0 g graphite powder 1.0 g BN powder 20 g Ni + 20 g Cr 0.5 /hour CO + N2
	C40	0.75 g

TABLE I .

 I Growth parameters of the carbon-doped hBN crystals precipitated from a molten metal solution in a high temperature furnace. HPBN stands for Hot Pressed Boron Nitride.

Table

  1, the precursor powders with compositions listed in the 2 nd -4 th columns

				D 6	D 2
	intensity		C 22
	PL		C 20
			HPBN
	3.5	4	4.5	5	5.5	6
			Energy (eV)
	FIG. 1. Macro-PL spectrum at 8 K (semi-log scale), with an
	excitation energy of 6.35 eV, in bulk hBN synthesized either
	with iron (C22) or nickel (C20), and the hBN powder used
	as precursor, here in the form of Hot Pressed Boron Nitride
	(HPBN).

  . 6. Statistical analysis of the ZPL energy in13 C-and nat C-doped hBN crystals: the normalized probability density of the ZPL energy is plotted for four 30×30 µm 2 regions; two in a 13 C-doped crystal (C40, red data), two in a nat C-doped crystal (C27, blue data). The 100 µeV-large horizontal bar indicates the difference of the mean values of the ZPL energy in13 C-and nat C-doped hBN crystals, the mean values being calculated for a total of ten 30×30 µm 2 regions.

					nat C-doped h-BN 13 C-doped h-BN
	Probability density
	0
	4.0966	4.0968	4.0970 ZPL energy (eV)	4.0972	4.0974

FIG

Acknowledgments We gratefully acknowledge C. L'Henoret for his technical support at the mechanics workshop. This work and the PhD fundings of T. Pelini and C. Elias were financially supported by the network GaNeX (ANR-11-LABX-0014). GaNeX belongs to the publicly funded Investissements d'Avenir program managed by the French ANR agency. R.P. acknowledges support from the Chateaubriand Fellowship Program.


S Riu

T Sugita

T Nakamura

T Hiroi

M Morota

T Popescu

K Michikami

Kitazato

E Tatsumi
email: etatsumi@iac.es

C Sugimoto

L Riu

S Sugita

T Morota

M Popescu

T Michikami

K Kitazato

M Matsuoka

S Kameda

R Honda

M Yamada

N Sakatani

T Kouyama

Y Yokota

C Honda

H Suzuki

Y Cho

K Ogawa

M Hayakawa

H Sawada

K Yoshioka

C Pilorget

M Ishida

D Domingue

N Hirata

S Sasaki

J De León

M A Barucci

P Michel

M Suemitsu

T Saiki

S Tanaka

F Terui

S Nakazawa

S Kikuchi

T Yamaguchi

N Ogawa

G Ono

Y Mimasu

K Yoshikawa

T Takahashi

Y Takei

A Fujii

Y Yamamoto

T Okada

C Hirose

S Hosoda

O Mori

T Shimada

S Soldini

R Tsukizaki

T Mizuno

T Iwata

H Yano

M Ozaki

M Abe

M Ohtake

N Namiki

S Tachibana

M Arakawa

H Ikeda

M Ishiguro

K Wada

H Yabuta

H Takeuchi

Y Shimaki

K Shirai

Y Iijima

Y Tsuda

S Watanabe

M Yoshikawa

Collisional history of Ryugu's parent body from bright surface boulders

Summary Paragraph (201 words excluding references)

(162173) Ryugu and other rubble-pile asteroids are likely re-accumulated fragments of much larger parent bodies that were disrupted by impacts. However, the collisional and orbital pathways from the original parent bodies to subkilometer rubble-pile asteroids are not yet well understood [START_REF] Fujiwara | The rubble-pile asteroid Itokawa as observed by Hayabusa[END_REF][START_REF] Watanabe | Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu-A spinning top-shaped rubble pile[END_REF][START_REF] Lauretta | The unexpected surface of asteroid (101955) Bennu[END_REF]. Here, we use Hayabusa2 observations to show that some of the bright boulders on the dark, carbonaceous ("C-type") asteroid Ryugu [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes[END_REF] are remnants of an impactor with a different composition as well as an anomalous portion of its parent body. The bright boulders on Ryugu can be classified into two spectral groups: most are featureless and similar to Ryugu's average spectrum [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes[END_REF][START_REF] Kitazato | The surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy[END_REF], while others show distinct compositional signatures consistent with ordinary chondrites-a class of meteorites that originate from anhydrous-silicate-rich asteroids [START_REF] Nakamura | Itokawa dust particles: a direct link between S-type asteroids and Ordinary Chondrites[END_REF]. The observed anhydrous silicate-like material is likely the result of collisional mixing between Ryugu's parent body and one or multiple anhydrous-silicate-rich asteroid(s) before and during Ryugu's formation. In addition, the bright boulders with featureless spectra and less ultraviolet upturn are consistent with thermal metamorphism of carbonaceous meteorites [7,[START_REF] Hiroi | Reflectance spectra (UV-3µm) of heated Ivuna (CI) meteorite and newly identified thermally metamorphosed CM chondrites[END_REF]. They might sample a different thermal-metamorphosed regions, which the returned sample will allow us to verify. Hence, the bright boulders on Ryugu provide new insights into the collisional evolution and accumulation of sub-kilometer rubble-pile asteroids.

Main text (1793 words excluding references)

The near-Earth asteroids, Itokawa, Bennu, and Ryugu are rubble-pile asteroids that were formed by the gravitational re-accumulation of rock fragments made by collisional disruption of their parent body [START_REF] Fujiwara | The rubble-pile asteroid Itokawa as observed by Hayabusa[END_REF][START_REF] Watanabe | Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu-A spinning top-shaped rubble pile[END_REF][START_REF] Lauretta | The unexpected surface of asteroid (101955) Bennu[END_REF]. Asteroids are classified in two main spectral groups: 1) S-types, which are suggested to be composed of anhydrous-silicate-rich materials, and 2) C-types, which contain more carbon and hydrated minerals [START_REF] Bowell | Polarizations of asteroids and satellites[END_REF]. The delivery of organics and volatiles from the main asteroid belt has been hypothesized to explain the presence of the oceans and life on the Earth [START_REF] Marty | Origins of volatile elements (H, C, N, noble gases) on Earth and Mars in light of recent results from the ROSETTA cometary mission[END_REF][START_REF] Sarafian | Early accretion of water in the inner solar system from a carbonaceous chondrite-like source[END_REF]. However, the transformation and mixture of materials from the main belt to the Earth crossing orbits is not well understood. Studies of exogenic matter on asteroids can provide some insight on the characteristics of catastrophic disruption of individual asteroids, such as projectile types and impact conditions. Here we report the discovery of bright boulders and discuss the implications on the history of material mixing on Ryugu.

JAXA's Hayabusa2 spacecraft started its investigation of (162173) Ryugu in June 2018 [START_REF] Watanabe | Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu-A spinning top-shaped rubble pile[END_REF]. The initial global observations of Ryugu with the telescopic optical navigation camera ONC-T with seven broad band filters ranging from 0.40 to 0.95 µm have revealed that Ryugu's surface is uniformly dark (the geometric albedo is 4.5% at 0.55 µm) suggesting a thermally metamorphosed carbonaceous chondrite composition [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes[END_REF]. Similarly, Ryugu's near-infrared spectra are uniformly flat and low in reflectance with a weak absorption at 2.7 µm corresponding to a hydroxyl absorption, which is also consistent with heated carbonaceous chondrites [START_REF] Kitazato | The surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy[END_REF]. After the initial global observations of Ryugu, Hayabusa2 conducted several high-resolution observations during proximity operations. As the spacecraft approached the surface, we resolved smaller structures in the ONC-T images, discovering numerous bright boulders at smaller scale (Fig. 1 and Extended Figure 1). Because of the uniformly dark nature of Ryugu, anomalous bright boulders on the surface are potentially exogenic material, which were first reported in the initial observations [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes[END_REF]. Because mostly they are smaller than 10 m, however, spectral characterizations have not been previously conducted and their compositions were not discussed in the initial analyses.

We identified bright boulders as areas with peak normal albedo values >1.5 times brighter than Ryugu's globally averaged albedo, which corresponds to a peak normal albedo of >6.8% [Method 1], while the standard deviation in normal albedo is 4.1 -5.0%. Bright boulders in high-resolution images exhibit inhomogeneous surface brightness (Fig. 1). The presence of fine dust covering the brighter layer, as observed during the touchdown operations [START_REF] Morota | Surface evolution of C-type asteroid 162173 Ryugu revealed from global mapping and touchdown operation of Hayabusa[END_REF], or different degree of space weathering [13] on different boulder parts due to possible motion and cracking could explain the variability in surface brightness. Figure 1(e) shows the cumulative size distribution of the bright boulders found in high-resolution images (down to 6 mm/pix) taken during the rover MINERVA-II deployment operation. The size distribution of boulders is expressed as 𝑁(> 𝐷) ∝ 𝐷 '( , where D is the boulder diameter and 𝛼 is the power-law index. The power-law index 𝛼 > 2 is estimated for the bright boulders in each image [Extended Figure 7], which is similar to the entire boulder distribution in the same images [START_REF] Michikami | Boulder size and shape distributions on asteroid Ryugu[END_REF]. The power-law index of 𝛼 > 2 is compatible with laboratory impact fragments, suggesting that the bright boulders were produced by catastrophic disruption rather than cratering [START_REF] Michikami | Fragment shapes in impact experiments ranging from cratering to catastrophic disruption[END_REF]. The ratio between the distribution of bright boulders and all boulders including bright boulders [START_REF] Michikami | Boulder size and shape distributions on asteroid Ryugu[END_REF] suggests a surface area ratio for bright boulders of ~0.03 -1% [ Method 2].

Using ONC-T multiband images taken on 3 to 4 October 2018 at an altitude of ~3 km (~0.3 m/pixel), the visible spectra of 21 bright boulders (M1 -M21) are examined [Extended Figure 2]. The spectra of bright boulders are classified into two groups depending on the presence or absence of an absorption towards 1 µm [Supplementary Information], which is indicative of the presence of silicates; Six out of 21 bright boulders show the mafic-silicate-like absorption (S-type bright boulders; Fig. 2), while the rest are featureless at 1 µm (C-or X-type bright boulders) [START_REF] Bus | Phase II of the small main-belt asteroid spectroscopic survey: A feature-based taxonomy[END_REF]. The majority of bright boulders have featureless spectral shapes categorized into C/X-types. The spectral trend is continuous with the global spectra of Ryugu, suggesting that C/X-type bright boulders may not be exogenous but an original constituent of Ryugu or its parent body, despite the higher than average albedo. All the C/X-type bright boulders have less upturn towards ul band (0.40 µm), while the average spectrum shows strong upturn in shorter wavelengths. Because the visible to near-infrared spectrum of Ryugu suggests compatibility with thermally metamorphosed carbonaceous chondrites [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes[END_REF][START_REF] Kitazato | The surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy[END_REF], It is important to discuss if the spectral variety in C/X-type bright boulders can be explained by an endogenic process. The spectral shapes of the C/X-type bright boulders are compared with laboratory heating experiments on the hydrated carbonaceous chondrites Murchison and Ivuna [7,[START_REF] Hiroi | Reflectance spectra (UV-3µm) of heated Ivuna (CI) meteorite and newly identified thermally metamorphosed CM chondrites[END_REF] (Fig. 3). Comparisons are made in the spectral slope from 0.55 to 0.86 µm and the degree of upturn in ultraviolet (UV index) space. During the heating of hydrous carbonaceous chondrites, the UV index first increases until 600 -700 ˚C. This is due to the decomposition of hydrated phases and carbonization/graphitization of carbonaceous material. The UV index then decreases by formation of Fe-bearing secondary olivine and small FeNi metal particles and a decrease in carbon abundance by oxidation [START_REF] Nakamura | Post-hydration thermal metamorphism of carbonaceous chondrites[END_REF][START_REF] Applin | Ultraviolet spectral reflectance of carbonaceous materials[END_REF]. These spectral characteristics suggest a possible connection between the different spectral features of bright boulders and different degrees of thermal metamorphism. More specifically, the overall direction and range of spectral variation observed among C/X-type bright boulders in the UV-index/spectral slope space can be reproduced by those heating experiments. It is noted, however, that specific meteorite types or metamorphism temperatures cannot be determined solely based on the spectra. The C/Xtype bright boulders may have experienced different thermal histories in the parent body through heating by decay of short-lived radionuclide 26 Al [START_REF] Grimm | Water and thermal evolution of carbonaceous chondrite parent bodies[END_REF] or/and impact [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes[END_REF]. Those C/X-type bright boulders could be sampled from different parts of the parent body during its catastrophic disruption and subsequent re-accumulations. The returned sample analyses will provide us strong constraints on this senario.

The normal albedos of S-type bright boulders range between 9 -22%, while those of C/X-type bright boulders range between 5 -10% [Extended Figure 6]. The average geometric albedo of S-type asteroids is 22.3 ± 7.3% and that of V-types is 36.2 ± 10.0% [20], supporting the designation that S-type bright boulders are similar to S-type asteroids regarding albedo. Large difference in spectrum and albedo suggest the S-type bright boulders are exogeneous material. The spectra of S-type bright boulders overlap with both ordinary chondrite (OC) and Howardite-Eucrite-Diogenite meteorite (HED) spectra. Furthermore, because both HEDs and OCs display a wide range of spectral variations in this visible color range, near-infrared spectral information is necessary for placing compositional constraints on them. Although near-infrared spectrometer NIRS3 also observed the S-type bright boulders, due to the larger footprint compared with the size of the bright boulders, we examined linear mixture of meteorite spectra with the average spectrum of Ryugu to compare meteorite spectra with the NIRS3 observations. The mixture is comprised of 99% the average Ryugu spectrum and 1% of the meteorite spectrum to mimic the footprint area covered by the bright boulder. To clarify any absorption caused by the bright boulders, the spectrum including the bright boulder is normalized by the spectrum of an adjacent area. After this normalization, the 2-µm band depth is measured [Method 3]. The linearly mixed spectra with HEDs display a 2-μm feature with a 2.5 ± 1.0% absorption, while the mixed spectra with the OCs display a 2μm feature with 0.6 ± 0.3% absorption (Fig. 4). The bright boulders' band characteristics are consistent with either OCs or HEDs, although bright boulders do not present a strong 2 µm band absorption. Considering the albedo values and the small or no 2-µm band absorption, they are more spectrally similar to olivine-rich OCs. Those S-type bright boulders could originate from S-type asteroids, because S-type asteroids are the most abundant in the inner asteroid main belt [START_REF] Demeo | Solar System evolution from compositional mapping of the asteroid belt[END_REF] where Ryugu may have originated [START_REF] Campins | The origin of asteroid 162173 (1999 JU3)[END_REF]. Moreover, more than half of the Nysa-Polana complex, which is Ryugu's likely source family [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes[END_REF], is populated by S-type members with similar orbital elements to C-type members [START_REF] Cellino | The puzzling case of the Nysa-Polana family[END_REF]. Thus, the collisional mixture between C-and S-type asteroids inside the Nysa-Polana complex is highly likely.

Possible origins for the S-type exogenous materials on Ryugu are: 1) they are remnants from the collision at the origin of Ryugu between its parent body and anhydrous-silicate-rich asteroid(s) in the main belt, or 2) they landed at small relative velocity during Ryugu's subsequent orbital evolution. To examine the probability of these hypotheses, comparisons are made with the impact statistical estimates. Four to six Stype bright boulders >0.5 m in diameter have been found in the search area, covering ~40% of the entire surface. For an OC-like boulder to land on the surface without disruption requires the collision speed < 0.2 km/s [Method 4]. For 10 to 15 projectiles >0.5 m in diameter to collide Ryugu at <0.2 km/s , 10000 -15000 of similar-sized asteroids including 2 -8 projectiles 23 -28 m in diameter would also hit Ryugu. This estimate is based on the mean impact velocity distribution between the main-belt asteroids [START_REF] Bottke | Velocity distributions among colliding asteroids[END_REF] and the exponent of -2.55 for the cumulative size distribution of main-belt asteroids [START_REF] Bottke | The fossilized size distribution of the main asteroid belt[END_REF]. A projectile of 23 -28 m is about the critical size of an impactor to disrupt Ryugu catastrophically (assuming the catastrophic disruption threshold, which is the specific energy required to disperse the targets into pieces smaller than the half mass of the original mass, QD * ~800 J/kg, [START_REF] Jutzi | Fragment properties at the catastrophic disruption threshold: The effect of the parent body's internal structure[END_REF]). The soft landing (e.g., ≤0.2 km/s) of several meter-sized objects, during which the impactor does not break up, is much less frequent than the collision of 25-m-sized objects, which would catastrophically disrupt a 1-km body. Thus, all the S-type bright boulders cannot be accreted on Ryugu's surface well after the asteroid was formed; they had to be incorporated in Ryugu's parent body during its catastrophic disruption or before. Furthermore, for a fixed impact speed, a catastrophic disruption event of a larger body involves a larger fraction of projectile, which favors survived exogenic materials since the projectile/target mass ratio of a catastrophic disruption event is larger (i.e., a larger QD * ) for a larger target [START_REF] Jutzi | Fragment properties at the catastrophic disruption threshold: The effect of the parent body's internal structure[END_REF].

Overall, the bright boulders with different spectral types might constitute a witness of the collision between the parent body of Ryugu and one or multiple anhydrous-silicaterich asteroid(s) that led to the formation of Ryugu. Even though several similarities have been discussed between Ryugu and Bennu, such as the region of origin in the main belt [START_REF] Bottke | Velocity distributions among colliding asteroids[END_REF][START_REF] Campins | The origin of asteroid 101955 (1999 RQ36)[END_REF], albedo, and thermal inertia [START_REF] Lauretta | The unexpected surface of asteroid (101955) Bennu[END_REF][START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes[END_REF], the discovery that many of bright boulders on Bennu and Ryugu with anhydrous spectra exhibit different strengths in the 2-µm band absorption [START_REF] Dellagiustina | Material from (4) Vesta on Asteroid (101955) Bennu[END_REF] indicates that Ryugu has gone through a collisional evolution track different from that of Bennu and perhaps also has a different original parent body. The possible minor presence of bright materials in returned sample may provide a key to reveal Ryugu's orbital and collisional evolution history. contribution: E.T., C.S., L.R., T.N., S.Su., T.Mo., M.P., M.M., S.W., T.H., M.T., N.S., D.D., M.A.B., P.M., H.Y. and J.d.L. All authors discussed the results and commented on the manuscript. [7,[START_REF] Hiroi | Reflectance spectra (UV-3µm) of heated Ivuna (CI) meteorite and newly identified thermally metamorphosed CM chondrites[END_REF] in the spectral slope from 0.55 to 0.86 µm (v-to-x slope) and the ultraviolet (UV) index (the upturn of ul-band (0.40 µm) compared with the continuum line defined by v-to-x slope) space. Black cloud describes the deviation of the global spectra of Ryugu from the home position (~20km) observation, which is slightly shifted towards more UV up-turn. The gray line shows the 1-sigma (68% probability) of spectral distribution and the white cross indicates the average spectrum of Ryugu. The Band I depth is absorption at p band (0.86 µm) based on the extrapolated value from vto-w (0.55 to 0.70 µm) slope. The Band II depth of a bright boulder is determined from the NIRS3 spectra after normalization with a spectrum of adjacent area. The band II depth is defined as the absorption at 2 µm from the continuum defined by the interpolation of the normalized reflectance at 1.8 µm and 2.5 µm. Bright boulders occupying ≳1% of the NIRS3 footprint are shown in black diamonds and those occupying ≲ 0.5% are shown in gray diamonds. To compare those spectra with the small occupancy of the bright boulder in the footprint area, the laboratory measured meteorite spectra (OCL: L chondrite, OCLL: LL chondrite, OCH: H chondrite, and HEDs) [Supplementary Information] are linearly mixed with the average Ryugu spectrum, assuming the area of meteorites is 1% of the spectrometer footprint. The dashed line indicates the Ryugu average absorption. Histograms show the distribution of meteorites along both axes. HEDs are scatter large area of band II depth, while OCs shows peak around 0.6%.

Main figure legends

Methods

ONC-T data processing

Calibration and extraction of spectral shape and albedo

The ONC-T images are calibrated from digital number to radiance factor using the same method described in [START_REF] Tatsumi | Updated inflight calibration of Hayabusa2's optical navigation camera (ONC) for scientific observations during the cruise phase[END_REF] using the updated flatfields derived by [START_REF] Tatsumi | Updated flat-fields of ONC-T/Hayabusa2 based on close encounter with Ryugu[END_REF]. Owing to the rotation of the asteroid during an image sequence acquisition over a seven-band observation, the position of bright boulder (BB) in the FOV changes slightly over time. The illumination and observation geometrical information are derived by SPICE toolkit based on the shape model and SPICE kernels. Rotational effect results in sub-degree (< 0.2˚) differences in phase angle (i.e., spacecraft-bright boulder-Sun angle) between different bands. Considering this effect, images were photometrically corrected to incidence (i), emission (e), and phase angles (𝛼) of 30°, 0°, and 30°, respectively, based on the Hapke disk-resolved model [START_REF] Hapke | Bidirectional reflectance spectroscopy: 1. Theory[END_REF] in the supplementary online material of [START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes[END_REF].

Objects smaller than the diameter of 3 pixels cannot be evaluated directly from the image because the point spread function (PSF; FWHM~1.7 pixels, [START_REF] Kameda | Preflight Calibration Test Results for Optical Navigation Camera Telescope (ONC-T) Onboard the Hayabusa2 Spacecraft[END_REF]) blurs the radiance from the BB. The radiances from the BB and adjacent area are contaminated. In that case, we need to remove the radiance of the adjacent area from the radiance measurement of the BB (i.e., similar to the point source stellar radiance measurement). Thus, we measured the spectra in either of two ways depending on the size of BBs. 1) For BBs with a diameter >3 pixels, we acquired spectra of BBs by directly averaging the surface spectra. Specifically more than 15 pixels 2 for each BB in each band are averaged in this study. 2) For BBs with diameter ≦ 3 pixels, we subtracted the contributions from the adjacent area (labeled background) to the BB, from the total radiance of the BB area (ROIA in Supplementary Figure 1). Using the estimated area S, the radiance of the BB 𝐼 .. is calculated by 𝐼 .. = 0 123 4 '0 56789:;<=> ? (S1) where 𝐼 @A0 4 is the total radiance in ROIA and 𝐼 BCDEFGHIJK is the estimated total radiance of the background. 𝐼 BCDEFGHIJK is estimated from the surrounding area near the ROIA (ROIB in Supplementary Figure 1); 𝐼 BCDEFGHIJK ~0123 M @ N 'G N (𝑟 P -𝑆) , where r=7 pixels and R=21 pixels are the size of ROIA and ROIB, respectively. And the normalized spectrum is obtained by

𝐼 ̅ .. = 0 MM 0 MM,U = 0 123 4 '0 56789:;<=> 0 123 4 ,U '0 56789:;<=>,U , ( S2
)
where the subscript v indicates the value for v band. Normal albedo was measured with same way as Eq. (S1) but using the images photometrically corrected to (i, e, 𝛼) = (0°, 0°, 0°).

Extended Figure 6 shows the locations and sizes of the BBs found in the images from the MASCOT hovering operation conducted on 3 to 4 October 2019 at the altitude of ~3 km. Extended Figures 1 and2 show morphologies and spectra of the BBs. The BBs with diameter ≲ 1 m in Extended Figure 6 are measured by the method #2.

Selection Methodology

Bright boulders are identified as areas with peak normal albedo values >1.5 times than Ryugu's globally averaged albedo, with a peak normal albedo >6.8%. For a boulder to be designated as a bright boulder at least 4 adjacent pixels meeting this brightness criteria was required, eliminating false identifications due to pick up noise or cosmic rays. The standard deviation in normal albedo across Ryugu's surface is ± 4.1 -5.0%. The selected normal albedo threshold for bright boulders is well outside this deviation and is distinctly represents an anomalous value.

We tested the robustness of the threshold for detecting bright boulders. If we set the threshold to 1.6 times the global average, we also found both S-and C/X-type bright boulders. However, we could not detected bright boulder M20 due to its small size, while we could identify bright boulders M1, M11, and M5. The reason M20 was not detected is that our detection criteria require more than four adjacent pixels exceed the threshold. Thus, the choice of brightness threshold does not influence the spectral type of bright boulder detected, but limits the size of bright boulders that can be detected.

If we set the threshold to 1.4 times the global average we find ~30 more C/X-type bright boulders. The albedo and spectral variety of C/X-type bright boulders are along a continuous range from Ryugu's average, thus it is difficult to distinguish bright boulders if the threshold is set too low. With too low a threshold the photometric error in the derived normal albedo values does not provide a 1 sigma deviation from the global average. The threshold value needs to provide a clear difference between bright boulders and general boulders. A threshold of 1.5 times the global average albedo tested to be the best value to identify anomalously bright boulders.

It is important to note that classification of a boulder as a 'bright boulder' is based solely on its albedo compared to the global average albedo. The classification into S-type bright boulder and C/X-type bright boulder is based on spectral properties after it has been identified as a bright boulder.

Classification of bright boulders

Based on the taxonomy by [START_REF] Bus | Phase II of the Small Main-Belt Asteroid Spectroscopic Survey[END_REF], we decomposed the spectra of BBs with the same basis vectors, spectral slope and PC2'. The SMASS2 spectral data are available through PDS website (https://sbn.psi.edu/pds/resource/smass2.html, [START_REF] Bus | Small Main-belt Asteroid Spectroscopic Survey, Phase II. EAR-A-I0028-4-SBN0001/SMASSII-V1.0[END_REF]). PC2' was obtained from principal component analysis of SMASS2 after each spectrum was normalized by its fitted slope (Supplementary Figure 4). Component PC2' is sensitive to the strength of the 1-µm band absorption and PC3' is sensitive to the combination of the strength of UV absorption and the 0.7-µm band absorption. Note that because our spectra were obtained using broad band filters, the spectra of BBs are interpolated with a spline fitting before principal component decomposition. Within spectral slope and PC2' space, two distinct grouping of asteroids were suggested [START_REF] Bus | Phase II of the Small Main-Belt Asteroid Spectroscopic Survey[END_REF]; the structures were defined as S-and C/Xclasses. Taxonomic classification defined by [START_REF] Bus | Phase II of the Small Main-Belt Asteroid Spectroscopic Survey[END_REF] is shown in the Fig. 2. Six BBs with PC2' value greater than 0, correspond to deeper 1-µm absorption, are classified as S-type BBs and others as C/X-type BBs.

Color indexes

Spectral slope: Similar to [START_REF] Bus | Phase II of the Small Main-Belt Asteroid Spectroscopic Survey[END_REF], the 7-band or 4-band spectra are fitted to the equation below using the a least-squares method,

𝑅 X Y = 1 + 𝛾(𝜆 Y -0.55),
where 𝑅 X Y is the normalized reflectance at each band (Fig. 2 with i=ul, b, v, Na, w, x, p, and Fig. 4 with i=v, Na, w, x), 𝜆 Y is the wavelength of the band in microns [Supplementary information], and 𝛾 is the slope of the fitted line, constrained to a value of unity at 0.55 µm.

UV index: UV index is defined to measure UV up/down-turn. To consider the effect of the continuum spectral slope, we compute the UV increase/decrease from an extrapolation based on the continuum spectral slope. That is the extrapolated reflectance 𝑅 X I`a at ul band from the v-to-x slope is calculated first and the UV index 𝐶 cd is defined as

𝐶 cd = 𝑅 X IR X I`a -1.
Thus, when the 𝐶 cd is positive, the spectrum has a UV up-turn, and vice versa.

Band I depth: Band I depth is defined to measure the absorption towards 1-µm. To consider the effect of the continuum spectral slope, we compute the p-band increase/decrease from an extrapolation based on the continuum spectral slope. That is, the extrapolated reflectance 𝑅 X e a at p band from the v-to-w slope is calculated first and the Band I depth 𝐶 fghij is defined as

𝐶 fghij = 1 - 𝑅 X e 𝑅 X e a .
Thus, when the larger the value of 𝐶 fghij , the deeper the 1-µm band depth , and vice versa.

Size distribution of bright boulders

We analyzed the same five images used in [START_REF] Michikami | Boulder size and shape distributions on asteroid Ryugu[END_REF] which measured all the boulder size distribution including BBs in the images [START_REF] Michikami | Boulder size and shape distributions on asteroid Ryugu[END_REF]. We picked up the BBs brighter than 1.5 times the median value for each images (Extended Figure 3). The circular shape of BBs is assumed to calculate the diameter, i.e., the diameter DBB is converted from pixel area Spix as

𝐷 .. = k ? lmn `N
Po , where l (m/pixel) is the pixel scale at the altitude of observation. The cumulative number distribution of boulder size can be expressed by 𝑁(> 𝐷) ∝ 𝐷 '( , where D is diameter of the BB, and 𝛼 is the power-law index. The boulder size distribution in each image is fitted by a power-law based on the maximum likelihood method with goodness-of-fit test based on the Kolmogorov-Smirnov statistic [START_REF] Clauset | Power-law distributions in empirical data[END_REF] which is implemented in the python library, powerlaw, by [START_REF] Alstott | Powerlaw: a Python package for analysis of heavy-tailed distributions[END_REF]. We utilized this python library and found the power-law index for datasets in Extended Figure 7. The power-law index value for BBs is similar value to that for all boulders by [START_REF] Michikami | Boulder size and shape distributions on asteroid Ryugu[END_REF] within the error bars except for the image at the altitude of 65 m. Because in this image apparently, several meter-scaled boulders occupy a large part of the FOV (Extended Figure 3(e)), only a little substrate could be observed. This small area may affect the BB statistics. This similarity in power-law index suggests that the BBs and other boulders could be originated from the same mechanism. Area ratio between all boulders and BBs are derived from the comparison between the intercept of cumulative number distributions of boulders from [START_REF] Michikami | Boulder size and shape distributions on asteroid Ryugu[END_REF] and BBs counted in this study. Based on the image at altitude of 636 m comparison of boulder number at D=0.3 m gives an area ratio of 0.03%, and based on the image at altitude of 75 m comparison of boulder number at D=0.07 m gives an area ratio of 1%.

NIRS3 data processing

We use calibrated NIRS3 spectra acquired between 30 June 2018 and 28 February 2019 to characterize the BBs detected by the ONC visible camera. The calibration steps for NIRS3 spectra are summarized in [START_REF] Iwata | NIRS3: The Near Infrared Spectrometer on Hayabusa2[END_REF] and [START_REF] Kitazato | The surface composition of asteroid 162173 Ryugu from Hayabusa2 near-infrared spectroscopy[END_REF]. The raw data is calibrated using a transfer function that was computed from the on-ground calibration of the instrument [START_REF] Iwata | NIRS3: The Near Infrared Spectrometer on Hayabusa2[END_REF]. The obtained radiance is defined as follows:

𝐿(l, 𝑇) = 𝑟(l) × 𝐹 tIJ (l) × 𝑐𝑜𝑠 (𝑖) 𝜋𝐷 tIJ'@{IFI P + 𝜀(l) × 2ℎ𝑐 P l ~× 1 𝑒 ( €D lE M • ) -1 (S3)
where 𝑟(l) is the reflectance, 𝐹 tIJ (l) the sun incident flux, 𝑖 the incidence angle and 𝜀(l) the surface emissivity that is estimated around 1.87 𝜇m. The temperature is retrieved at every location (e.g. for every spectrum), using the Planck function (Eq. ( S3)). The obtained reflectance 𝑟(l) is then corrected to standard viewing geometry, incidence, emission, and phase angles of 30°,0°, and 30°, respectively.

We show in this paper the results for six S-type BBs (Fig. 4). Supplementary Table 1 summarizes the NIRS3 data used in this study. Here we describe the analysis of NIRS3 spectra on one of the largest BBs, M13, as an example. The characterization process is as follows: [START_REF] Fujiwara | The rubble-pile asteroid Itokawa as observed by Hayabusa[END_REF] we search for all NIRS3 spectra that overlap the BB longitude and latitude at ± 1°; [START_REF] Watanabe | Hayabusa2 arrives at the carbonaceous asteroid 162173 Ryugu-A spinning top-shaped rubble pile[END_REF] for each day of observation we extract spectra including the BB and spectra excluding the BB to enable comparison. For M13, we find 8 NIRS3 observations that including it, corresponding to a total of 57 spectra (Supplementary Figure 5).

The NIRS3 instrument presents a rather low resolution compared to the size of the bright boulder (< 3 m). Therefore, for the in-depth characterization, we use the spectra of highest resolution ~6 m x 6 m (7 August 2018). We found 14 spectra that overlap the BB and 32 spectra in the vicinity of the BB (± 5°) for comparison (Supplementary Figure 5c). From those spectra, we computed an average spectrum including M13 (from the 14 blue squares on Supplementary Figure 5c) and an average spectrum excluding M13 (from the 32 yellow squares on Supplementary Figure 5c). There is small variation within those spectra normalized at 2.5 µm. As a result, we also computed error bars for the average spectra by calculating the standard deviation of the normalized reflectance amongst the set of used spectra for both spectra including BB ('in') and adjacent areas ('out') (Extended Figure 4). We observe very slight differences in albedo level between the spectra including and excluding M13 at all resolution (Supplementary Figure 5a). Because the differences in absolute reflectance observed at high resolution may result from shadowing effects which can easily change the reflectance by 10% due to the variation in illumination conditions, we used normalized spectra to evaluate the absorption band. We performed spectral ratio to compare the average normalized spectrum 'in' with the average normalized spectrum 'out' and highlight spectral shape difference (Extended Figure 4). The error bars are calculated from the law of propagation of error as indicated below:

(𝑅 X YJ ± 𝜎 YJ ) (𝑅 X HI ‡ ± 𝜎 HI ‡ ) = 𝑅 X YJ 𝑅 X HI ‡ ± ˆ‰ 𝜎 YJ 𝑅 X HI ‡ Š P + ‹ 𝑅 X YJ 𝑅 X HI ‡ P 𝜎 HI ‡ OE P ,
where 𝑅 X YJ and 𝑅 X HI ‡ are the average normalized reflectance spectra 'in' and 'out', respectively, and 𝜎 YJ and 𝜎 HI ‡ are the corresponding standard deviations.

For the calculation of the Band II depth d2.0, as summarized in Supplementary Table 1, we use only high resolution spectra (spot size on the surface of Ryugu < 12 m). The absorption degree d2.0 at 2 µm is measured by computing the ratio between the reflectance at 2.0 µm to the interpolated continuum based on the reflectance at 1.8 µm and 2.5 µm. 𝑑 P.' = 1 -0.7𝑅 X P.' 0.5𝑅 X "." + 0.2 𝑅 X P.~ where the Rwl is the reflectance ratio at wl=1.8, 2.0, and 2.5 µm. The error in absorption is given by the standard deviation between multiple observations on each site.

Collision on Ryugu with an ordinary chondrite

One possible explanation for the exogeneous materials is that they fall with a slow velocity so that they do not disrupt on collisions to Ryugu. Here we estimate the peak pressure when an ordinary chondrite (OC) impacts Ryugu. Then we discuss the required condition for that type of soft-landing scenario. The peak pressure during a collision between an OC and a carbonaceous one can be calculated based on the Rankine-Hugoniot equations as:

𝑃 ' = 1 2 𝜉𝜌 ' ‡ 𝐶 ‡ P ‰1 + 1 2 𝑠 ‡ 𝜉 𝑣 𝐶 ‡ Š ‰ 𝑣 𝐶 ‡ Š,
where P0 is the initial peak pressure, 𝜌 ' ‡ is the target density, 𝐶 ‡ and 𝑠 ‡ are constants for the Hugoniots state curve, and v is the impact speed. The parameter 𝜉, related to shock impedance matching, can be approximately obtained as:

𝜉~2 ‰1 + 𝜌 ' ‡ 𝐶 ‡ 𝜌 'Y 𝐶 Y Š , ™
where 𝜌 'Y and 𝐶 Y are the impactor density and the Hugoniot parameter for the impactor, respectively. The parameters for the Hugoniot state curve, C and s, were measured by [START_REF] Anderson | Shock Wave Equations of State of Chondritic Meteorites[END_REF] for the CM2 chondrite Murchison (C=1.87±0.07 km/s, s=1.48±0.03) and the L6 chondrite Bruderheim (C=3.11±0.06 km/s, s=1.62±0.02). Therefore, the peak pressure can be estimated as in Extended Figure 5. Comparison with the compressive strength of OC is used to estimate the speed of the impactor allowing its survive at impact. The compression strength of OCs was obtained to be 164 ± 106 Mpa based on a compilation of 22 measurements in the literature [START_REF]Physical Properties of the Stone Meteorites: Implications for the Properties of Their Parent Bodies[END_REF]. The impactor hardly survives at impact speeds ≥ 0.2 km/s. Thus, exogeneous materials must have collided on Ryugu at speeds < 0.2 km/s, unless they are fragments of a much larger impactor. Furthermore, if the impact speed is > 0.2 km/s, the mass of the largest fragment is < 1% of the mass of original projectile [START_REF] Fujiwara | Experimental study on the velocity of fragments in collisional breakup[END_REF]. This suggests that if the BBs larger than 0.5 m were the fragments of a larger impactor with higher velocity, the impactor should be larger than 50 m. However, a 25-m-sized projectile can catastrophically disrupt Ryugu, assuming QD*~250 J/kg for non-porous material [START_REF] Benz | Catastrophic disruptions revisited[END_REF] and QD*~800 J/kg for porous material [START_REF] Jutzi | Fragment properties at the catastrophic disruption threshold: The effect of the parent body's internal structure[END_REF], where QD* is so-called catastrophic specific impact energy threshold which results in the fragmentation and escape of half of the target mass. Thus, if the exogeneous materials are accumulated on Ryugu by collision during its post-formation evolution, slow collisions < 0.2 km/s are needed. 7 Peak pressure during a collision between an ordinary chondrite and a carbonaceous chondrite. The red solid and dashed lines indicate the average and standard deviation of compressive strength among ordinary chondrites, respectively [START_REF]Physical Properties of the Stone Meteorites: Implications for the Properties of Their Parent Bodies[END_REF]. The black solid and dashed lines indicate then peak pressure for the target densities of 2.2 g/cc and 1.2 g/cc (Ryugu bulk density), respectively.

Figure 1 .

 1 Figure 1. Bright boulders observed by ONC-T. (a) hyb2_onc_20190221_215938 (10 mm/pixel), (b) hyb2_onc_20190221_215222 (13 mm/pixel), (c) hyb2_onc_20190221_213610 (25 mm/pixel), (d) The largest bright boulder (M6) during the SCI deployment operation: 07902_20190420130002 (48 mm/pixel). (e) Cumulative size distribution N(>D) of bright boulders (BB) from different image resolutions (dashed lines) compared with that of general boulders (dotted) in [14] during the MINERVA-II deployment operation. Same color plots are derived from the same image. The gray dashed lines indicate the power law fits to the boulder counts. The cumulative size distribution power-index 𝛼 > 2 is consistent with that of the size distribution of fragments produced by catastrophic disruption [SM2].

Figure 2 .

 2 Figure 2. Visible spectral variety of S-type bright boulders. (a) Spectra of the S-type bright boulders normalized at 0.55 µm. They are offset by 1 for clarity. (b) Visible spectral slope (0.40 µm (ul) to 0.95 µm (p)) and PC2' diagram used for classification of the bright boulder spectra [Supplementary Information]. The index PC2' was defined from the principal component analysis on the main belt asteroids indicating an absorption towards 1 µm [16]. S-type bright boulders have positive PC2' values representing the absorption towards 1 µm. Color comparison of bright boulders (S-types: red diamonds, C/X-types: yellow squares) with taxonomy classes [16] (black dots) and laboratory measured meteorite spectra (OCs: blue dots, HEDs: light blue dots, and carbonaceous chondrites: gray dots) [Supplementary Information] are also shown in this space. Meteorite samples are distributed over a large area. S-type bright boulders are well inside of these HED and OC meteorite distributions.

Figure 3 .

 3 Figure3. Visible spectral variety of C/X-type bright boulders. C/X-type bright boulders (black diamonds) compared with the heating experiments on carbonaceous chondrites (Ivuna: blue and Murchison: red)[7,[START_REF] Hiroi | Reflectance spectra (UV-3µm) of heated Ivuna (CI) meteorite and newly identified thermally metamorphosed CM chondrites[END_REF] in the spectral slope from 0.55 to 0.86 µm (v-to-x slope) and the ultraviolet (UV) index (the upturn of ul-band (0.40 µm) compared with the continuum line defined by v-to-x slope) space. Black cloud describes the deviation of the global spectra of Ryugu from the home position (~20km) observation, which is slightly shifted towards more UV up-turn. The gray line shows the 1-sigma (68% probability) of spectral distribution and the white cross indicates the average spectrum of Ryugu.

Figure 4 .

 4 Figure 4. S-type bright boulders absorptions at p-band (0.95 µm), Band I depth, observed by ONC-T and 2-µm band depth , Band II depth, observed by NIRS3.The Band I depth is absorption at p band (0.86 µm) based on the extrapolated value from vto-w (0.55 to 0.70 µm) slope. The Band II depth of a bright boulder is determined from the NIRS3 spectra after normalization with a spectrum of adjacent area. The band II depth is defined as the absorption at 2 µm from the continuum defined by the interpolation of the normalized reflectance at 1.8 µm and 2.5 µm. Bright boulders occupying ≳1% of the NIRS3 footprint are shown in black diamonds and those occupying ≲ 0.5% are shown in gray diamonds. To compare those spectra with the small occupancy of the bright boulder in the footprint area, the laboratory measured meteorite spectra (OCL: L chondrite, OCLL: LL chondrite, OCH: H chondrite, and HEDs) [Supplementary Information] are linearly mixed with the average Ryugu spectrum, assuming the area of meteorites is 1% of the spectrometer footprint. The dashed line indicates the Ryugu average absorption. Histograms show the distribution of meteorites along both axes. HEDs are scatter large area of band II depth, while OCs shows peak around 0.6%.

Extended Data Fig. 1

 1 Bright boulder morphologies. The resolutions of these images are 0.28-0.30 m/pixel, as listed in Extended Data Fig. 4. Scale bars in the images are 5 m. M6 is shown in Fig. 1 of the main text. Extended Data Fig. 3 Normalized reflectance spectra of bright boulders lines) compared with the average spectrum of Ryugu (gray lines). Lines are offset by 0.7 for clarity. The bright boulders which display two spectra were observed twice in different multiband image sets. Extended Data Fig. 5 Bright boulders (enclosed by red lines) in close-up images during the MINERVA-II deployment operation on 21 September 2018. a, hyb2_onc_20180921_043010_tvf, altitude 636 m (68 mm/pixel). b, hyb2_onc_20180921_041826_tvf, altitude of 335 m (36 mm/pixel). c, hyb2_onc_20180921_034938_tvf, altitude of 148 m (16 mm/pixel). d, Extended Data Fig.

Extended Data Fig.[START_REF] Sugita | The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes[END_REF] The locations and sizes of bright boulders.

Data and materials availability:

All images and input data used in this study are available at the JAXA Data Archives and Transmission System (DARTS) at www.darts.isas.jaxa.jp/planet/project/hayabusa2/Tatsumi_2020, and higher-level data products will be available in the Small Bodies Node of the NASA Planetary Data System (https://pds-smallbodies.astro.umd.edu/) 1 year after mission departure from the asteroid.

Acknowledgements:

The Hayabusa2 spacecraft was developed and built under the leadership of JAXA, with contributions from DLR and CNES, and in collaboration with NASA, Nagoya Univ., Univ. of Tokyo, NAOJ, Univ. of Aizu, Kobe Univ., and other universities, institutes, and companies in Japan. We also thank many engineers, including Noriyasu Inaba of JAXA and Tetsuya Masuda, Seiji Yasuda, Kouta Matsushima, and Takeshi Ohshima of NEC Corp. for their dedicated work on the Hayabusa2 mission, Koshi Sato at NEC Corp. for ONC development, and Shingo Kashima at NAOJ for optical calculations. Funding: ET acknowledges financial support from the project ProID2017010112 under the Operational Programmes of the European Regional Development Oz., M.Ab., M.Yo., T.Sa., S.W., and Y.Ts.; Project administration: Y.Ts., S.W., M.Yo., T.Sa., S.Tan., F.T., S.N., Y.Ya., K.K., S.Su., N.N., M.Ab., S.Tac, M.Ar., H.I., M.I., and K.W.; Interpretation and writing Extended Data Fig. 2 Power index for boulder size distribution from close-up images. The power index is calculated based on the maximum-likelihood fitting method by ref. [START_REF] Clauset | Power-law distributions in empirical data[END_REF]. The cumulative boulder size distribution is expressed as 𝑁(> 𝐷) ∝ 𝐷 '( , where D is the diameter of the bright boulder, and α is the power-law index.