# Transfer of diamond thin films using Smart Cut™ technology

J Chrétien, P Gilles, F Milesi, F Berger, N Bernier, N Gauthier, G Lapertot, C Lecouvey, D Mariolle, L Colonel, et al.

► To cite this version:

J Chrétien, P Gilles, F Milesi, F Berger, N Bernier, et al. Transfer of diamond thin films using Smart Cut™ technology. Diamond and Related Materials, 2025, 155, pp.112295. 10.1016/j.diamond.2025.112295. hal-05357613

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# Transfer of diamond thin films using Smart Cut™ technology

J. Chrétien<sup>a, *</sup>, P. Gilles<sup>a</sup>, F. Milesi<sup>a</sup>, F. Berger<sup>a</sup>, N. Bernier<sup>a</sup>, N. Gauthier<sup>a</sup>, G. Lapertot<sup>b</sup>, C. Lecouvey<sup>a</sup>, D. Mariolle<sup>a</sup>, L. Colonel<sup>a</sup>, P. Acosta Alba<sup>a</sup>, F. Mazen<sup>a</sup>, J. Widiez<sup>a</sup>, I. Huyet<sup>c</sup>, L. Le Van-Jodin<sup>a</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

**Keywords:**

Diamond

Smart Cut™

Ion implantation

Fracture

Bonding

## ABSTRACT

The Smart Cut™ process, originally developed for manufacturing SOI wafers, combines ion implantation and direct bonding techniques to transfer thin monocrystalline films. In this study, the transfer technology is adapted to diamond, leveraging the graphitization induced by high-temperature implantation. A diamond film was successfully transferred onto a silicon substrate using a metallic bonding. The crystallinity and composition of the transferred film were analyzed using Transmission Electronic Microscopy (TEM), Electron Energy Loss Spectroscopy (EELS), Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS). This transfer approach might offer a fast, low-power consumption and efficient method to obtain high quality thin diamond film which can be used as growth template for diamond-based devices.

## 1. Introduction

Diamond is a promising material for wide band gap applications. With a band gap as large as 5.45 eV high breakdown field such as 10 MV/cm can be reached, making it interesting for radiation detector applications [1-4]. Impurity centers in diamond, such as nitrogen-vacancies (NVs) or silicon-vacancies (SiVs), are used for promising applications like scanning magnetometry [5-7], strain measurement [8,9], and quantum computing [10,11]. Its high thermal conductivity (2200 W.m⁻¹.K⁻¹) and high carrier mobility make diamond-base devices attractive candidates for high power applications where diamond can be used as a bipolar [12] junction-gate field-effect transistor (JFET) [13] or Schottky diode [14-16] or just as a sink substrate for thermal dissipation [17]. Finally, with a fracture strength of 10.3 GPa, its mechanical properties are attractive for microelectromechanical systems such as radio frequency switches [18].

However, despite its exciting properties, diamond development is slowed down by the difficulty of manufacturing large-scale substrates at low cost. Strategies have been developed by diamond suppliers to overcome this issue. High Pressure High Temperature (HPHT) method, which was the historical approach, consisted of a growth under thermodynamically stable diamond condition using a metal used as a catalyst [19]. This type of growth still represents a large part of the synthetic

diamonds and remains widely used in industry, particularly for diamond powder production. However, this technique suffers from limitations in handling larger areas, making it unsuitable for surfaces larger than 4 × 4 mm². Chemical Vapor deposition (CVD) growth was then investigated using CH₄ gas diluted with H₂ in the graphite-stable region in the carbon phase diagram [20,21]. CVD Diamond growth by heteroepitaxy on iridium virtual substrate is currently the most promising method to obtain 2 in. wafer or even larger sizes [22,23] despite the high density of dislocations. Another growth method to produce diamond at wafer-scale is the mosaic approach [24-26]. In this case, diamond seeds are firstly implanted to create a buried sacrificial graphite-like layer [27-30]. Then, a CVD homoepitaxy of thick diamond plates is performed on these implanted seeds. After the growth, diamond plates are separated from the seed by electrochemical etching along the implanted layer to get the freestanding diamonds. These plates are collected side by side to form a mosaic array, and final growth is carried out to achieve larger diamond wafers with an area as large as 20 × 40 mm² [31]. These methods required high power consumption due to the growth of thick diamond layers and long etching times.

Smart Cut™ technology is a promising approach to transfer a thin diamond layer onto a cheap handling substrate, such as silicon. This technique of layer transfer was historically developed on silicon to produce Silicon-On-Insulator (SOI) substrates [32] and was later

<footnote><sup>a</sup> Univ. Grenoble Alpes, CEA-Leti, F-38000 Grenoble, France</footnote>

<footnote><sup>b</sup> Univ. Grenoble Alpes, CEA, Grenoble INP, IRIG, PHELIQS, Grenoble, France</footnote>

<footnote><sup>c</sup> Soitec SA, Chemin des Franques, 38190 Bernin, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: jeremie.chretien@cea.fr (J. Chrétien).</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Schematic view of Smart Cut™ process adapted to diamond. Diamond is implanted by hydrogen ions and annealed at the same time (step 2). A thin metal layer is deposited on both diamond donor and Si received substrates (step 3). The substrates are then bonded (step 4) and a final annealing at high temperature separate the diamond donor from the film (step 5).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** (a) 1 × 1 µm² and 10 × 10 µm² AFM scans on the as-grown diamond are represented over a z range between -0.4 nm and 0.4 nm (b) Optical profilometer image of the 4 × 4 mm² diamond (c) SAM image after the bonding of the whole diamond. Bonded area are represented in dark gray.</figcaption></figure>

successfully expanded to other wide bandgap materials like silicon carbide (SiC) [33–35] or gallium nitride (GaN) [36,37]. Smart Cut™ process begins with the implantation of light ions, such as hydrogen into the substrate. During this step, buried nano-cavities called platelets in the case of silicon are formed. This implanted wafer is then bonded onto a receiver substrate and an annealing step causes hydrogen gas to diffuse into the nano-cavities leading them to grow. The nano-cavities coalesce, forming microcracks [38] which keep growing until a spontaneous fracture initiates [39] and propagates [40] throughout the whole wafer.

Regarding the diamond transfer, a similar approach was tested by Pariskh et al. [27] using $O^+$ implantation in order to form $CO_x$ gas but most of the diamond overlayer flaked off the surface during the annealing and thus, film could not be characterized. Another method developed by Suk et al. [41], very similar to Smart Cut™, has achieved the fabrication of a 2 µm thick diamond membrane. This process uses a combination of $C^+$ ion implantation and annealing for graphitization. A subsequent step of $H^+$ implantation followed by annealing allowed the fracture. This promising technique, however, requires high-energy implantation (4
<figure><img src="image_6.png" /><figcaption>Fig. 3. Optical microscope top views on the (a) 4 × 4 mm² diamond donor substrate and (b) the Si receiver substrate. (c) SEM tilted image showing the diamond film bonded onto the silicon substrate.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. (a) Dark-Field STEM image of the unsplit part. (b) Zoom and (c) High-resolution TEM image in the high damage region. d<sub>220</sub> diamond and d<sub>002</sub> graphite planes are highlighted with turquoise and red arrows respectively. A zoom on these planes are provided in the inset (d) Gray intensity along these selected arrows are plotted to determine the distance between planes.</figcaption></figure>

MeV) and the very thin thickness of the membrane without mechanical support makes manipulation challenging. More recently, other approaches for transferring diamond membranes thinner than 1 µm have been developed [42,43] using lighter ion implantation such as helium or hydrogen for the formation of the graphitic layer. The main interests of these methods are that they used lower energy implantation and that the implantation of lighter atoms should create less defects in the diamond layer. In both cases, the graphitic layer was chemically etched, and the transferred diamond surface remains relatively modest and localized (<200 µm²). In this work, we report the successful transfer of diamond films using the Smart Cut™ transfer technique. To achieve this, process sequence is adapted by incorporating a hot implantation and a metallic bonding. We characterize the resulting film with Transmission Electronic Microscopy (TEM), Electron Energy Loss (EEL), Raman and X-ray Photoemission Spectroscopy (XPS) and remove the heavily damaged graphite-like region with plasma etching.

## 2. Diamond film elaboration

### 2.1. *Process overview*

For this study, single crystal diamonds are provided by Orbray with a size and thickness of 4 mm × 4 mm and 500 µm, respectively. These

substrates are grown by CVD heteroepitaxy on the (100) crystal orientation with a misoriented angle of 7°. The Smart Cut™ process for diamond is shown in Fig. 1. Firstly, a hot implantation is performed with H⁺ ions having an energy of 150 keV. Then, the implanted diamond is bonded onto a 200 nm silicon receiver substrate using a metallic interface [44,45]. After a final high temperature annealing, the diamond substrate fractures along the graphite-layer and a thin film of diamond on Si is obtained.

The diamond fracture via a Smart Cut™ process requires the formation of a buried graphitic layer [46–49]. In previous studies, this involved an implantation step to amorphize the diamond to a significant depth, followed by an annealing step to graphitize it. Freestanding diamond films were demonstrated by several groups [41,43,50] using this technique. In our process, we combine both implantation and annealing into a single step of hot implantation as shown schematically in Fig. 1. Similar to the former 2-step process, this hot implantation approach shows blistering on diamonds only in a temperature range [51]. That’s why, the fluence and temperature during implantation are carefully tuned to form a deep buried graphite-like layer as previously reported in [51,52] and detailed in [53]. The implantation fluence is chosen in order to generate a vacancy density above the critical density threshold (D<sub>c</sub> > 10<sup>22</sup> cm<sup>-2</sup>) and the simultaneous annealing allows recrystallization into a graphite-like layer. The temperature needs to be
<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 5.** EELS spectra (a) on the film region at (229.5 nm, 468.2 nm) and (b) on the high damage implanted area at (229.5 nm, 908.8 nm). Blue, turquoise, yellow and orange dash lines correspond to the Gaussian fits for first K-shell ionization loss peak of graphite, first, second and third K-shell ionization loss peak of crystalline diamond, respectively. (c) STEM image of the EELS mapping (d) σ and π contributions along the y-axis. Blue and red circle marks in the STEM image correspond to the locations of the spectra (a) and (b), respectively.</figcaption></figure>

high enough to turn the amorphous layer into a graphite-like layer but not too high to avoid Frenkel pair formation. Post implantation, the formation of defects filled by hydrogen gas called platelets can be observed during this step. These defects are preferentially oriented along the (001) crystal planes.

## 2.2. Requirements for the bonding step

As demonstrated in [54], the bonding success is highly dependent on the surface quality of diamond and thus, a very smooth surface is required for the bonding. Since diamond is one of the hardest materials, polishing its surface remains a real challenge. The surfaces of the supplied diamonds are characterized by Atomic Force Microscopy (AFM) and optical profilometry to check its roughness and flatness. 1 × 1 µm² and 10 × 10 µm² scans (Fig. 2 (a)) show a sufficiently smooth surface for bonding with a Root Mean Square (RMS) roughness of.

0.10 nm and 0.12 nm respectively. No polishing lines are observed

and the surface is free of any particular contamination at this scale. At larger scale, a 4 × 4 mm² optical profilometer mapping of the entire diamond sample is presented in Fig. 2 (b). The peak to valley height $S_x$ for this diamond surface is 401 nm, which is considered flat enough for bonding. Both flatness and roughness values for this sample are close to those required for Surface Activated Bonding (SAB) bonding technique, with values of <300 nm and 0.2 nm on diamond, respectively, as defined in literature [54].

After the bonding, we investigate the bonded interface using Scanning Acoustic Microscopy (SAM), as shown in Fig. 2 (c). The diamond is almost entirely bonded to the silicon over the 4 × 4 mm² surface, with the exception of a few small areas, probably due to particulate contamination during handling of the samples. These unbonded areas are estimated to be <1 % of the total surface.
<figure><img src="image_12.png" /><figcaption>Fig. 6. (a) C 1 s high resolution spectrum and (b) Raman spectrum on the transferred film. Different fitting contributions are also shown: the disorder peak D (in yellow dash line) and G bands (G1 in red and G2 in orange dash lines) and diamond peak (in turquoise dash line). A linear baseline was also extracted (in blue dash line).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 7. (a) STEM image of the transferred part (b) HRTEM images of the top surface (TEOS SiO₂- graphite-like layer) and (c) in the middle of the crystalline diamond film. Graphite and diamond plans are highlighted with orange and blue circles respectively.</figcaption></figure>

2.3. Transferred areas after the splitting

The diamond bonded on Si wafer was placed in an inductive furnace at high temperature, close to 1000 °C. We believe that, as in the silicon case, the annealing treatment causes the diffusion of the implanted hydrogen to the nano-defects in the graphite-like layer, increasing their internal pressure. These defects then develop into micrometer cracks, which lead to a splitting along this graphitic-like layer.

A partial diamond film transfer is finally obtained. The microscope images for the donor.

and receiver substrates are shown in Fig. 3 (a) and (b). The trans-
ferred area appearing in light blue on top view optical image (Fig. 3 (b))
represents roughly 50 % of the initial diamond surface. A near-
symmetrical shape is observed on the donor part from where the dia-
mond film was taken. The tilted Scanning Electronic Microscope (SEM)
image in Fig. 3 (c) shows the film transferred onto silicon with
approximately 750 nm thickness. To estimate the maximum vacancy
concentration peak, we used SRIM simulation software [55] to extract
vacancy density profile. Then, the profile was modified by a python code
to take into account the saturation vacancy and swelling effects as
explained by Bosia et al. [56–58]. The simulated maximum peak is
observed at a depth of 752 nm from the surface (see Fig. SI 1), close to
the experimentally found transferred layer thickness.

The roughness of the transferred film is measured by AFM. The RMS
roughness is 3.7 nm and the maximum peak-to-valley height is 40.5 nm
on the 10 × 10 µm² scan (Fig. SI 2). We have reproduced the experiment

several times with fairly similar results (Fig. SI 3), demonstrating the reproducibility of the process. We also applied Smart Cut™ to a larger diamond surface (18x18mm²), but the flatness did not meet the bonding criteria. Therefore, some areas were not bonded, and no transfer was observed in these unbonded areas. This result demonstrates the importance of flattening for larger diamond wafers when using Smart Cut™.

3. Results and discussion

3.1. Formation of the graphite-like layer in the unsplit region

The Transmission Electron Microscopy (TEM) characterization was
performed on the undetached part in the diamond receiver (Fig. 4) with
TFS Titan Themis tool along the <100> axis. The sample was prepared
by Ga⁺ Focused Ion Beam (FIB) milling using a FEI Helios 450 machine.
A SiO₂ layer was deposited to protect the top surface of the sample from
the tails of the ion beam. The specimen was deposited onto a molyb-
denum TEM grid. Then, we used a 30 kV operation voltage for the initial
thinning and finished with a low beam energy in the range 5–8 kV to
reduce FIB-induced damage. Dark field (DF) scanning transmission
electron microscope (STEM) cross sectional image is provided in Fig. 4
(a). The 90 nm thick damage region represented as the darker layer is
located at a depth of 800 nm indicating the formation of platelets (Fig. 4
(b)), confirming that the fracture occurred in this region. This darkest
layer is surrounded by two other lighter regions with less defects. The
thickness of the lighter region just above, on the film side, is larger than
<figure><img src="image_14.png" /><figcaption>**Fig. 8.** Nano-beam diffraction along the TEM cross-section depth. Annotated pattern in the film region (609 nm) and at 9.7 nm (SiO₂), 30.6 nm (graphite-like layer), 88.7 nm (monocrystalline diamond with high mosaicity), 324.4 nm depth (monocrystalline diamond) from the top surface.</figcaption></figure>

the one on the substrate side. Such asymmetry in the vacancy profile is frequently obtained after implantation process, since it follows a Pearson IV distribution [59].

High Resolution TEM (HRTEM) along the [100]-axis in Fig. 4 (c) on the graphite-like layer in between the two distorted diamond regions, shows the coexistence of graphite and diamond crystallites. Graphite and diamond planes are highlighted in Fig. 4 (d) in red and turquoise, respectively. Using the gray scale of the image, the average distance between the planes is estimated to be 0.335 nm for graphite and 0.126 nm for diamond. Hence, the planes observed on the TEM images correspond to (002) graphite and (022) diamond. These preferential orientations of the graphite with (002) planes and the presence of diamond pockets were previously highlighted with heavy ion implantations [60,61].

Electron Energy Loss Spectroscopy (EELS) measurements have been also carried out at the C-K edge (Fig. 5). An exponential background correction was applied to each spectra. A typical EEL spectrum of the film region is presented in Fig. 5 (a). Three contributions are identified at approximately 294, 299 and 306 eV corresponding to the respective first, second and third K-shell ionization loss peaks of crystalline diamond. In the heavily damaged region (Fig. 5 (b)), a similar spectra is obtained however with an additional peak at lower loss energy (287 eV), a characteristic feature of graphite related to π bonds. Besides, the fine structure above the edge indicates a polycrystalline rather than an amorphous carbon feature, supporting HRTEM observations. Mapping has been done on the cross section STEM image shown in Fig. 5 (c) and all the peaks are fitted by a Gaussian function. Then, amplitudes of the peaks have been summed along the x-axis and divided into 2 contributions: π-contribution (first K-shell ionization loss peak of graphite) and

σ-contribution (first, second K-shell ionization).

loss peak of crystalline diamond). These two contributions are plotted in Fig. 5 (d) as function of the y-axis i.e. depth. The intensity of σ bonds drops significantly from 0.780 µm to 1.063 µm while π bonds intensity increases at the same depth over a thickness of 280 nm. This result is in line with the HRTEM image confirming the formation of a graphite-like layer. As depicted in STEM image in Fig. 4 (a), the profile of this σ bonds is also asymmetrical regarding its minimum value, larger on the film side. Note that the marked difference in intensity of the σ-contribution between the substrate and the implanted film indicates that its entire thickness is slightly damaged by H ion implantation. The entire film, from the top surface to the minimum of π intensity, is approximately 820 nm thick, including a 200 nm thick region of higher damage near the implanted area.

## 3.2. Identification of the graphite-like layer on top of the diamond transferred film

XPS measurements were performed on the transferred parts with a PHI 5000 VersaProbe II XPS Microprobe spectrometer. X-ray light source (Al Kα hν = 1486.6 eV) was focused on the transferred film with a 100 µm diameter spot. The source angle and the photoelectron collecting angle were both fixed at 45° and the high resolution spectra were recorded with a 0.6 eV resolution. Since the inelastic mean free path of the electrons in diamond is 2.6 nm, the maximum penetration depth is estimated here to be at 5.5 nm. Data correction and peak deconvolution were performed with the CasaXPS software: the sp³ component was fitted with a convolution of Gaussian (65 %) and Lorentzian (35 %)
<figure><img src="image_15.png" /><figcaption>**Fig. 9.** (a) Plasmonic peaks for different depths from the top surface graphite-like layer of the diamond film. (b) Density profile derived from the plasmonic peak positions along the TEM cross-section. (c) STEM cross-section image of the mapping area (d) π and σ contributions calculated from the high energy EEL spectrum.</figcaption></figure>

functions and the sp² with a Doniach-Sunjic function adding an asymmetry in the line shape [62,63].

The C 1s photoemission spectrum is presented on Fig. 6 (a) in order to evaluate the sp² and sp³ amount at the very surface of the transferred

layer. The background has been.

subtracted using a Shirley baseline correction. The spectrum for the film substrate exhibits three characteristic peaks at 284 eV, 284.5 eV and 286 eV. C=C (sp²) bonds represent the main contribution, with a percentage of 88 %. In comparison, C-C/C-H (sp³) bonds account for only 11 % in the first 5 nm below the surface. This high sp²/sp³ ratio indicates that a residual graphite-layer is still present on the top surface. This is because when the fracture occurs, it propagates inside the graphite-like layer of the diamond substrate. A thin residual graphite-like layer is therefore expected on the surface after transfer.

Raman spectroscopy was also carried out in the visible range (Fig. 6 (b)). Diamond Raman peak is observed in the transferred film at 1328.5 cm⁻¹ with significantly lower intensity and slight shift compared to the pristine diamond line (1332 cm⁻¹). Three intense additional peaks are also present and are fitted by Gaussian functions. These peaks at 1350 cm⁻¹ and 1550–1600 cm⁻¹ correspond to the disorder D bands and the graphitic G bands, respectively. [64] The spectrum offset is also fitted by a linear baseline.

The relatively high intensity of graphite contributions compared to diamond can be explained by a much larger scattering cross section for sp² sites than for sp³ sites, at least 50 times greater in the visible range [65]. Besides, the graphite-like layer created during the implantation step is on the top surface and has probably absorbed a large among of the incident light. On the contrary, the thin thickness of the diamond film, <750 nm, combined with the 532 nm wavelength excitation below the diamond bandgap will also reduce the light-matter interaction and might explain the low intensity observed for the diamond peak.

### 3.3. Crystallinity of the transferred diamond film

STEM cross-section was also performed on the transferred regions to examine the entire stack as shown in Fig. 7 (a). The film is well bonded on the silicon substrate even after the splitting annealing step thanks to the metallic interface. However, the bonding interface is not well-defined, and this is likely the primary reason for the limited transfer rate of approximately 50 %. The thickness of the transferred film is 825 nm, closed to the estimated value from EELS measurements. On the top of the film, a thin layer appears in darker gray corresponding to the.

graphite-like layer. This observation supports the assumption derived from XPS and Raman measurements in the previous section. These techniques are both very sensitive to the surface and diamond features were partially masked by the presence of graphite phase in the upper part of the film. The HRTEM image of the upper graphite-like layer (Fig. 7 (b)) shows the same crystallites of graphite and diamond just underneath the SiO₂ layer and confirming that the splitting occurred within this graphite-like layer. As the direction beam is along the [110] direction, the observed diamond planes are (11-1). HRTEM performed in the middle of the film (Fig. 7 (c)) shows monocrystalline diamond without any defects.

Nano-beam diffraction mapping along the [110]-axis was also carried out and diffraction patterns at different depths are depicted in Fig. 8. We annotated the diffraction pattern at 605 nm, the L/M ratio and θ are about 1.16 and 54.76° respectively, close to the values expected for a [110] beam axis for a face-centered cubic system. The (002), (1-11), (2-20) diffraction spots are intense indicating a monocrystalline diamond as suggested by the HRTEM (Fig. 7 (c)). As the nano-beam diffraction mapping approaches the splitting interface (88.7 nm), the diffraction pattern of diamond is still intact however it is also associated with higher order diffraction spots such as (004), (-113), (-222) plans. This suggests a higher mosaicity of the monocrystalline diamond in this region. At the top surface (9.7 nm), the SiO₂ layer used for the TEM lamella preparation demonstrates an amorphous pattern. At a depth of 30.6 nm, within the graphite-like layer, some diffraction spots are extinguished from the pristine diamond corresponding to a high damage and a polycrystalline structure, made of (1-11) oriented diamond nano-crystallites appears. A light diffuse halo can be attributed to the
<figure><img src="image_16.png" /><figcaption>**Fig. 10.** Raman spectrum on the film (a) as-split, (b) after 1 min 30 s, (c) 4 min 30 s and (d) 7 min 30 s etching time.</figcaption></figure>

crystallite disorder, and the presence of nano-graphite sheet as shown in Fig. 7 (b) (orange circles). Note that (002) oriented graphite planes are too close to the central spot to be clearly distinguished ($d_{002}^* = 2.987$ nm$^{-1}$).

### 3.4. Density and bond contributions

Similar to the undetached part, high energy loss spectrum were collected along the TEM cross-section to evaluate the amount of $\sigma$ and $\pi$ contributions. Additionally, EELS were also recorded in the low-energy range.

The plasmonic peak in this spectral range is directly related to the density of the material. A Drude model was used for the fitting with an effective mass of $m^* = 0.87m_e$ [66]. Details are provided in Supplementary Information (SI). In Fig. 9 (a), the plasmonic peak is shifting from 22 eV at 44.3 nm (top surface of the film) to 33 eV at 170 nm. This shift in the position of the plasmonic peak along the TEM cross-section indicates the presence of a transition zone from a graphitic-like layer to crystalline diamond film. This result is therefore consistent with TEM imaging, Raman and XPS spectroscopies. The density profile as a function of TEM section is then obtained from this peak position and is shown in Fig. 9 (b) and (c). Close to the surface, the transition zone of around 100 nm is clearly visible, moving from a density of $1.6$ g.cm$^{-3}$ to a value close to that of diamond, $3.5$ g.cm$^{-3}$. The difference with the density of pure diamond can be attributed to the presence of implantation defects still present in the film thickness. Indeed, we had observed in EELS (Fig. 5 (d)) on the TEM section of the donor part with the undetached film that the intensity of the $\sigma$ contribution of the film was slightly lower than that of the diamond substrate. In the same region, the intensity of the $\pi$- contribution measured is important and collapses then deeper in the diamond film, while the $\sigma$- bonds become the only contribution. Finally, the presence of a graphite-like layer on the as-split surface is also confirmed by an increase of $\pi$- contribution in Fig. 9 (d).

### 3.5. Removing the graphite-like layer and recovering the diamond surface properties

As a graphite-like layer was identified on the diamond transferred surface, the next step is to remove this thin layer for a potential epi-ready diamond surface. An $O_2$ plasma was used for this purpose. Raman spectroscopy mapping was carried out at different etching times (see in SI) and representative spectra are presented in Fig. 10. The contribution of G and D bands are gradually decreasing during the etching, until it fully disappears after 7 min 30 s plasma. The intensity of the diamond peak, on the other hand, increases over the same etching time. We represent the G and D bands amplitude over the diamond line amplitude as function of the etching time in Fig. SI 7. After an etching of 4 min 30 s, we observe an average drop from 17.2 to 1.7 for $A_D/A_{diamond}$ and 11.2 to 3.7 for $A_G/A_{diamond}$, testifying the progressive removal of the 100 nm thick graphite-like layer.

Regarding the diamond peak, the average position and the Full-Widh at Half Maximum (FWHM) are respectively at 1324.5 cm$^{-1}$ and 18.0 cm$^{-1}$ after the etching. In other words, the position is red-shifted and linewidth has increased compared to pristine diamond (1332.5 cm$^{-1}$ and 2.4 cm$^{-1}$ respectively), but a symmetrical shape has still remained. A similar trend was also observed experimentally by Orwa et al. [67]. The symmetrical shape of the peak excludes phonon confinement as the main explanation for the change in FWHM and position from a pristine.

diamond. The increase of FWHM corresponds rather to a reduction in phonon lifetime from ion-induced defect scattering. In the anharmonic approximation, FWHM and the peak position are linearly related via Kramers-Koning relations. In [67], two different slopes (reproduced here in Fig. 11 (a)) were highlighted and allocated to two levels of damage. Low redshift positions correspond to isolated point defects while high redshift positions stand for extended defects with overlaps. In our case, the transferred diamond film is in the low damage region with only isolated point defects formed during the ion implantation.

Two additional peaks located at 1447 cm$^{-1}$ and 1490 cm$^{-1}$ appeared on several Raman spectrum from the mapping (Fig. SI 8). They are often observed on implanted diamonds with low damage levels [67,68]. The
<figure><img src="image_17.png" /><figcaption>Fig. 11. (a) Diamond FWHM as function of the peak position. The two lines are reproduced from [57] and represent low levels of damage where the defects correspond to isolated points (in red) and higher levels of damage where defects start to overlap. Experimental measurements are also added for 1 min 30s (orange square), 4 min 30s (blue triangle) and for 7 min 30s (turquoise square). (b) C 1 s high-resolution XPS spectrum (c) 1 × 1 µm² and 10 × 10 µm² AFM scans after the etching are shown in z range between −25 nm and 25 nm.</figcaption></figure>

defect line at 1490 cm⁻¹ is likely associated to a single vacancy or divacancy with the formation of a single and double bonds at the vicinity. This defect seems to affect the compensation of carriers in p-diamond, but has an activation energy of 4.06 eV and can be removed by thermal annealing. Although less well known, the other defect shows similar behavior during annealing [67].

These results suggest that a longer etching and a thermal high temperature annealing are necessary to fully recover diamond features to be as close as possible to those of a pristine diamond. With an annealing of 1 h at 1200 °C [67], it has been demonstrated that the FWHM narrows (6 cm⁻¹) and the diamond peak blue shifts to 1330.2 cm⁻¹.

After etching, the sp³ contribution estimated from C 1 s by XPS measurements (Fig. 11 (b)) reaches 60 % compared with 11 % before etching and becomes the main contribution. At the same time, the sp² contribution decreases from 88 % to 30 %. The rise in C − O, C = O bond contributions is probably due to O₂ plasma etching. AFM scans were finally carried out on the film surface showing a granular texture with an average grain size of 30–40 nm. The RMS roughness and Z range after etching are 10.1 nm and 82.4 nm for 1 × 1 µm² scan and 5.1 nm and 64 nm for 10 × 10 µm². These values are higher than the roughness and Z range measured on the as-split surface (Fig. SI 2) and indicate that the plasma etching here leads to increase in surface roughness. This can be explained by the fact that the O₂ plasma etches sp² bonds faster than sp³ bonds, leading to increased roughness. With such roughness, the surface needs to be planarized by either chemical-mechanical polishing [69,70], plasma-assisted polishing [71,72], or reactive ion beam etching [73] before it can be used for epitaxy.

Similar to the findings from Raman spectroscopy, the remaining sp²

contribution and the RMS roughness of the surface suggest that a longer etching time is needed. Additionally, adjusting etching parameters such as reagents or bias power could help achieve a smoother surface, free of graphitic or oxide contributions, ultimately resulting in an epi-ready diamond film. A tri-acid cleaning solution [74,75] can also be another efficient method to selectively remove the graphite-like layer from the sp³ surface, thereby recovering the diamond surface.

4. Conclusion

In summary, we successfully demonstrate the transfer of a 825 nm-thick diamond film using Smart Cut™ technology. A graphite-like layer at the splitting interface was identified by EELS, TEM, Raman spectroscopy and XPS in both the undetached and transferred regions. This layer is formed during the hot H⁺ ion implantation step, with the splitting occurring and partially propagating along this layer during high temperature annealing. This residual 100 nm-thick residual layer on the top surface of the transferred diamond film was then removed by O₂ plasma etching, as confirmed by the disappearance of the D and G bands in Raman spectroscopy and an increase of the sp³ content observed via XPS. Isolated point defects induced by ion implantation were detected by Raman spectroscopy, which accounts for the slightly lower σ bond intensity and the reduced density observed in the density profile through EELS, compared to the pristine diamond. The high crystallinity of the diamond film was confirmed through HRTEM, Nano-beam analysis and EELS. The current transferred rate of 50 % could be significantly improved by optimizing bonding interface strength and implantation conditions. For instance, with better surface quality or a surface
activation treatment, a direct bonding can be used instead of a metallic bonding, strengthening the interface stability [76]. After removing the graphite-like layer and performing a healing annealing, this transfer technique might provide epi-ready diamond films on Si without requiring electrochemical etching or thick diamond growth, making it a promising method for high power devices, quantum technologies or photonic applications.

## CRediT authorship contribution statement

**J. Chrétien:** Writing – review & editing, Writing – original draft, Visualization, Resources, Formal analysis, Data curation, Conceptualization. **P. Gilles:** Writing – review & editing, Visualization, Resources, Formal analysis, Data curation. **F. Milesi:** Writing – review & editing, Resources, Data curation. **F. Berger:** Writing – review & editing, Resources, Data curation. **N. Bernier:** Writing – review & editing, Visualization, Resources, Formal analysis, Data curation. **N. Gauthier:** Writing – review & editing, Visualization, Resources, Methodology, Formal analysis, Data curation. **G. Lapertot:** Writing – review & editing, Resources. **C. Lecouvey:** Writing – review & editing, Resources, Data curation. **D. Mariolle:** Writing – review & editing, Visualization, Resources, Formal analysis, Data curation. **L. Colonel:** Writing – review & editing, Resources, Formal analysis, Data curation. **P. Acosta Alba:** Writing – review & editing, Validation, Resources, Methodology, Conceptualization. **J. Widiez:** Writing – review & editing, Supervision, Project administration. **I. Huyet:** Writing – review & editing, Validation, Supervision, Project administration, Methodology, Funding acquisition, Conceptualization. **L. Le Van-Jodin:** Writing – review & editing, Writing – original draft, Validation, Supervision, Project administration, Formal analysis, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This work, carried out on the Platform for Nanocharacterisation (PFNC), was supported by the "Recherche Technologique de Base" and "France 2030 - ANR-22-PEEL-0014" programs of the French National Research Agency (ANR). We thank Orbray for providing the substrates used in this work. We greatly acknowledge Audrey JANNAUD for preparing the FIB foils and Dhruba DAS for the helpful suggestions.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.diamond.2025.112295.

## Data availability

Data will be made available on request.

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