# Detection limit of phosphorus in diamond by high mass resolution secondary ion mass spectrometry (HMR-SIMS)

M.A. Pinault-Thaury, François Jomard

► To cite this version:

M.A. Pinault-Thaury, François Jomard. Detection limit of phosphorus in diamond by high mass resolution secondary ion mass spectrometry (HMR-SIMS). M.-A. Pinault-Thaury and F. Jomard 25th International Hässelt Diamond Workshop on Surface and Bulk Defects in Diamond (SBDD XXVI), Mar 2022, Hässelt, Belgium. pp.2200324, 10.1002/pssa.202200324. hal-04396253

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# Detection Limit of Phosphorus in Diamond by High Mass Resolution Secondary-Ion Mass Spectrometry

Marie-Amandine Pinault-Thaury* and François Jomard

In diamond, secondary-ion mass spectrometry (SIMS) is usually performed in order to detect and measure the depth distribution of impurities. The SIMS measurements are then performed using parameters allowing high sensitivities. In the classical configuration for diamond analysis, the mass resolution is usually set at the lowest value and gives access to the concentration of most dopants. In the case of phosphorus, the detection limit is then in the range of a few $10^{15}$ at cm$^{-3}$ (≈20 ppb). Herein, several diamond samples are studied and the SIMS detection on the secondary ions of masses 12 (carbon) and 31 (phosphorus) is focused. It is shown that SIMS analyses require high mass resolution (HMR) to accurately separate unexpected molecular ions detected at 31 a.m.u. In such HMR-SIMS analyses, the detection limit of phosphorus by one decade is improved and the value of $3 \times 10^{14}$ at cm$^{-3}$ is achieved.

## 1. Introduction

Diamond presents outstanding properties[1-7] that make it a promising material for many applications in several fields.[8-15] Through years, many works have been performed in order to study the nature and the impact of impurities in diamond thin films synthetized by chemical vapor deposition (CVD).[11,16-19] Dopants may be added in a controlled manner to the gas flow during the growth process and incorporated into CVD diamond films. Among them, phosphorus (P) in substitutional site is a donor impurity in diamond. It gives the highest n-type conductivities reported so far in diamond.[20] However, phosphorus doped-diamond is a challenge because its incorporation in diamond is extremely difficult. This is mainly due to a high positive formation energy of phosphorus in diamond (+6.67 eV).[21] Indeed, the incorporation of phosphorus during CVD diamond

synthesis is possible only in narrow growth conditions with high quantity of phosphorus precursors[22] Note that the incorporation efficiency of phosphorus is quite small (e.g., it does not exceed 0.03% in (100) diamond[23]). Another well-known impurity in synthetic diamonds is nitrogen (N). When coupled with vacancies (V), nitrogen forms N-V complexes with a defect state interesting for optically active centers.[24]

In quantum physics, the electron spin plays a significant role. Recently, Herbschleb et al.[25] show that P donors in the diamond crystal extend the coherence times of NV centers in diamond, opening the way of new applications in quantum technology. Such breakthrough

was reached, thanks to progress in the quality of CVD diamond as well as techniques allowing suppression or reduction of unwanted defects as nuclear spins from the 13-carbon isotope (13C). For obtaining these results, the phosphorus doping range was between $3 \times 10^{15}$ and $1 \times 10^{17}$ at cm$^{-3}$.

In this work, we investigate the detection limit of phosphorus in diamond by SIMS analysis. We study several diamond samples and focus the SIMS detection on the secondary ions of masses 12 (carbon) and 31 (phosphorus).

## 2. Experimental Section

### 2.1. Dynamic SIMS Technique

The SIMS technique[26,27] consists of sputtering a solid sample, placed under vacuum, by a focused primary-ion beam, which is rastered over a square area. This destructive analytical technique can be applied to any type of solid material that can be kept under vacuum. The sputtering by primary ions of a few keV energy induces the ejection of particles from the sample. Part of the ejected particles are ionized (positively or negatively): it is the yield of ionization. The sputtered secondary ions are analyzed with a mass spectrometer. The SIMS technique provides a unique combination of extremely high sensitivity for all chemical elements of the periodic table with a very low background. This allows high dynamic range (up to more than 5 decades) and low detection limits (down to ppb level, depending on the SIMS settings, the analyzed element, and the studied material).

With dynamic SIMS, the bulk composition and the in-depth distribution of trace elements are investigated with a good depth resolution (typically lower than 10 nm). This technique gives

M.-A. Pinault-Thaury, F. Jomard
Groupe d'Etude de la Matière Condensée (GEMaC-UMR8635)
CNRS

Université de Versailles Saint Quentin en Yvelines (UVSQ)
Université Paris-Saclay, 45 av des Etats Unis
78035 Versailles Cedex, France

E-mail: marie-amandine.pinault-thaury@uvsq.fr

<figure><img src="image_2.png" /></figure>

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/pssa.202200324.

© 2022 The Authors. physica status solidi (a) applications and materials science published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/pssa.202200324
<figure><img src="image_2.png" /></figure>

# Detection Limit of Phosphorus in Diamond by High Mass Resolution Secondary-Ion Mass Spectrometry

Marie-Amandine Pinault-Thaury* and François Jomard

In diamond, secondary-ion mass spectrometry (SIMS) is usually performed in order to detect and measure the depth distribution of impurities. The SIMS measurements are then performed using parameters allowing high sensitivities. In the classical configuration for diamond analysis, the mass resolution is usually set at the lowest value and gives access to the concentration of most dopants. In the case of phosphorus, the detection limit is then in the range of a few $10^{15}$ at cm$^{-3}$ (≈20 ppb). Herein, several diamond samples are studied and the SIMS detection on the secondary ions of masses 12 (carbon) and 31 (phosphorus) is focused. It is shown that SIMS analyses require high mass resolution (HMR) to accurately separate unexpected molecular ions detected at 31 a.m.u. In such HMR-SIMS analyses, the detection limit of phosphorus by one decade is improved and the value of $3 \times 10^{14}$ at cm$^{-3}$ is achieved.

## 1. Introduction

Diamond presents outstanding properties[1-7] that make it a promising material for many applications in several fields.[8-15] Through years, many works have been performed in order to study the nature and the impact of impurities in diamond thin films synthetized by chemical vapor deposition (CVD).[11,16-19] Dopants may be added in a controlled manner to the gas flow during the growth process and incorporated into CVD diamond films. Among them, phosphorus (P) in substitutional site is a donor impurity in diamond. It gives the highest n-type conductivities reported so far in diamond.[20] However, phosphorus doped-diamond is a challenge because its incorporation in diamond is extremely difficult. This is mainly due to a high positive formation energy of phosphorus in diamond (+6.67 eV).[21] Indeed, the incorporation of phosphorus during CVD diamond

synthesis is possible only in narrow growth conditions with high quantity of phosphorus precursors[22] Note that the incorporation efficiency of phosphorus is quite small (e.g., it does not exceed 0.03% in (100) diamond[23]). Another well-known impurity in synthetic diamonds is nitrogen (N). When coupled with vacancies (V), nitrogen forms N-V complexes with a defect state interesting for optically active centers.[24]

In quantum physics, the electron spin plays a significant role. Recently, Herbschleb et al.[25] show that P donors in the diamond crystal extend the coherence times of NV centers in diamond, opening the way of new applications in quantum technology. Such breakthrough

was reached, thanks to progress in the quality of CVD diamond as well as techniques allowing suppression or reduction of unwanted defects as nuclear spins from the 13-carbon isotope (13C). For obtaining these results, the phosphorus doping range was between $3 \times 10^{15}$ and $1 \times 10^{17}$ at cm$^{-3}$.

In this work, we investigate the detection limit of phosphorus in diamond by SIMS analysis. We study several diamond samples and focus the SIMS detection on the secondary ions of masses 12 (carbon) and 31 (phosphorus).

## 2. Experimental Section

### 2.1. Dynamic SIMS Technique

The SIMS technique[26,27] consists of sputtering a solid sample, placed under vacuum, by a focused primary-ion beam, which is rastered over a square area. This destructive analytical technique can be applied to any type of solid material that can be kept under vacuum. The sputtering by primary ions of a few keV energy induces the ejection of particles from the sample. Part of the ejected particles are ionized (positively or negatively): it is the yield of ionization. The sputtered secondary ions are analyzed with a mass spectrometer. The SIMS technique provides a unique combination of extremely high sensitivity for all chemical elements of the periodic table with a very low background. This allows high dynamic range (up to more than 5 decades) and low detection limits (down to ppb level, depending on the SIMS settings, the analyzed element, and the studied material).

With dynamic SIMS, the bulk composition and the in-depth distribution of trace elements are investigated with a good depth resolution (typically lower than 10 nm). This technique gives

M.-A. Pinault-Thaury, F. Jomard
Groupe d'Etude de la Matière Condensée (GEMaC-UMR8635)
CNRS

Université de Versailles Saint Quentin en Yvelines (UVSQ)
Université Paris-Saclay, 45 av des Etats Unis
78035 Versailles Cedex, France

E-mail: marie-amandine.pinault-thaury@uvsq.fr

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

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/pssa.202200324.

© 2022 The Authors. physica status solidi (a) applications and materials science published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/pssa.202200324
information about the elemental and isotopic composition
through the material thickness (up to a few tens of µm). The
secondary-ion yields will vary greatly according to the chemical
environment and the sputtering conditions (ion, energy, angle).
Even if this adds complexity to the quantitative aspect of the tech-
nique, SIMS is recognized as the most sensitive elemental and
isotopic microanalysis technique.[28]

## 2.2. Standard SIMS Conditions for Diamond Analysis

The SIMS analysis were performed in dynamic mode with mag-
netic sector mass spectrometer (IMS7f-CAMECA, Gennevilliers,
France). For diamond homoepitaxy, SIMS is usually performed
in order to measure the depth distribution of dopants (boron or
phosphorus) and contaminants (as hydrogen) in the films. In
routine analysis, the raster size was 150 × 150 µm² meaning that
the primary beam (≈few µm of diameter) sweeps across the sam-
ple surface over a 150 × 150 µm² area. An electronic gate was set
to 70% and the analyzed zone was restricted to a diameter of
33 µm to avoid the crater edge effects. The depth of the resulting
SIMS crater was measured with a Dektak8 step-meter from
Veeco-Bruker (Wissembourg, France).

The analysis chamber was kept under high vacuum (≈5 × 10¹⁰ mbar). As the SIMS technique is mostly used in diamond to detect impurities, measurements were performed with parameters allowing high sensitivities. Namely, the Cs⁺/M⁻ configuration was classically used: positive primary ions with a Cs⁺ source and detection of negative secondary ions of mass M.[²⁹] The energy of the Cs⁺ primary beam was set to 10 keV. Secondary ions were detected in the negative mode by biasing the sample to -5 kV, leading to an impact energy of the primary ions of 15 keV and an incidence angle of ≈23° with respect to the normal of the sample. Using a primary intensity of 60 nA, leading to a sputtering rate of around 0.4 nm s⁻¹ in diamond matrix, such diamond SIMS conditions do not require time-consuming adjustment.

We applied our standard SIMS conditions for diamond analy-
sis either with low-mass-resolution (LMR) settings (usually used
to ensure a high level of transmission) or high-mass-resolution
(HMR) settings (that induces lower transmission). To limit the
decrease of the transmission between LMR and HMR, the diam-
eter of the contrast aperture was changed to 400 µm at HMR,
instead of 150 µm at LMR, while keeping the same field aperture
(400 µm of diameter).

We measured four different (100)-oriented diamond samples: an implanted HPHT diamond substrate with a phosphorus-implanted dose of 3 × 10¹⁴ at cm⁻² and an energy of 180 keV (named “phosphorus-implanted reference” in the following), a commercial HPHT diamond substrate, an undoped CVD diamond homoepilayer, and a boron-doped CVD diamond homoepilayer. Note that the CVD epilayers were grown in growth reactors that were never exposed to phosphorus. We detected the 12C⁻ ion as matrix element that has an in-depth constant level and 31P⁻ that is the element of interest.[³⁰]

**3. Results and Discussion**

Phosphorus is a chemical element that has a unique stable iso-
tope, ³¹P, at 30.974 a.m.u. Up to your study, no molecular ions

are expected at 31 a.m.u. This means that no combination of ions
coming from diamond matrix elements, diamond impurities, or
impurities from the close environment of the sample in the anal-
ysis chamber should interfere at such mass value. As a result,
LRM settings (M/ΔM = 400) are usually set for SIMS analysis
of phosphorus-doped diamond.[³⁰] **Figure 1a** shows the mass
spectrum at the 31 a.m.u. recorded from the phosphorus-
implanted reference with LMR settings. The peak related to phos-
phorus is dominant and presents an intensity of 1.4 × 10³ cps
while a weak shoulder is observed at the upper mass values
(≈20 cps).

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

**Figure 1.** Phosphorus-implanted reference analyzed in standard SIMS diamond conditions with LMR settings (M/ΔM = 400): a) mass spectrum in the range of the phosphorus mass (31 a.m.u.), b) raw depth profile of 12C (carbone) and 31P (phosphorus) mass, and c) depth profile of the carbone and the phosphorus content (in at cm⁻³).
While making depth-profile SIMS analysis, the intensity of the detected elements (in count per second, cps) is recorded in function of the sputtering time (in second, s). The resulting graph is a raw profile with intensity in Y axis and time in X axis. Figure 1b shows the raw profile of our phosphorus-implanted diamond reference where the intensities of the matrix element, 12C, and of the element of interest, 31P, are recorded. At the end of the profile, the stopping value of the time corresponds to the total sputtering time, $t_{total}$, that has created the resulting crater. The measurement of the crater depth (in nanometer, nm), $p_{crater}$, allows to determine the sputtering rate, $S_r$ as $S_r = p_{crater}/t_{total}$ in $nm s^{-1}$, and to convert the X axis from time (in s) to depth (in nm).

To quantify an element in the depth of a material, it is necessary to analyze a reference sample. Such reference sample is usually made of the material and specifically implanted with a known dose, $D_{implant}$, of the requested element (phosphorus in our case). The SIMS analysis of the reference sample will allow the determination of the relative sensitivity factor of the element (RSF<sub>element/matrix</sub>) to quantify the element of interest. By recording the signals of the matrix element, $I_{matrix}$, and the element of interest, $I_{element}$, in the implanted reference, the RSF<sub>element/matrix</sub> is given by $D_{implant} * I_{matrix} / (Sr * \int_{0}^{t} I_{element} dt)$. The definite integral between 0 and t is indeed the area under the intensity curve of the element of interest. To convert the intensity (in cps) in concentration (in at/cm³), the following equation is then used: $[element] = RSF_{element} * I_{element} / I_{matrix}$.

An example of the quantification of a raw profile (Figure 1b) is given in Figure 1c for our phosphorus-implanted diamond reference. The quantified profile with phosphorus concentration, [P] (at cm⁻³), is then given in function of depth (nm). In our case, our phosphorus-implanted diamond reference gives a RSF<sub>31P/12C</sub> of 1.8 × 10<sup>23</sup> at cm<sup>-3</sup>. Note that the quantification is based on knowledge of the implanted dose which is given by the implanters to within 10%. The phosphorus profile shows the expected implanted peak around 150 nm with a maximum content of ≈3 × 10<sup>19</sup> at cm<sup>-3</sup>.

The profile tail of the implanted reference gives the value of the detection limit of the element.[28] As a result, the phosphorus

detection limit is ≈3 × 10¹⁵ at cm⁻³ (≈17 ppb) while using LMR-SIMS diamond conditions. This means that such SIMS conditions are fully adapted to measure phosphorus-doped diamond with content above 4 × 10¹⁵ at cm⁻³.

However, one can expect from SIMS analysis lower detection limit, typically a few ppb. Moreover, the weak shoulder observed on Figure 1a suggests that they might have interfering molecular ion(s) with masses close to the one of phosphorus ions. To separate ions having comparable masses, HMR is used. It is done by closing the entrance and exit slits of the spectrometer just enough to clearly separate each ion mass. In our case, a moderate HMR of $M/ΔM = 1600$ is appropriate as seen on the mass spectrum in Figure 2a from our phosphorus-implanted reference. The intensity of the phosphorus related peak is of $1 × 10^3$ cps while the ones of the interfering species do not exceed 5 cps. The useful yield (UY), that is the ratio between the number of detected ions and the number of sputtered ions, is then of $6.6 × 10^{-3}$ count/at at LMR and $3.4 × 10^{-3}$ count/at at HMR. As a result, the transmission at HMR is reduced only by a factor ≈2 lower than the transmission at LMR.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Mass spectra in the range of the phosphorus mass (31 a.m.u.) recorded in a crater bottom of our reference, far from the implanted zone, by varying the electronic gate from 100% down to 30% and analyzed in HMR-SIMS diamond conditions.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** a) Mass spectrum in the range of the phosphorus mass (31 a.m.u.) and b) depth profile of the carbone and the phosphorus contents (in at cm⁻³) of our phosphorus-implanted reference while analyzed in standard SIMS diamond conditions with HMR settings ($M/ΔM = 1600$).</figcaption></figure>
While making depth-profile SIMS analysis, the intensity of the detected elements (in count per second, cps) is recorded in function of the sputtering time (in second, s). The resulting graph is a raw profile with intensity in Y axis and time in X axis. Figure 1b shows the raw profile of our phosphorus-implanted diamond reference where the intensities of the matrix element, 12C, and of the element of interest, 31P, are recorded. At the end of the profile, the stopping value of the time corresponds to the total sputtering time, $t_{total}$, that has created the resulting crater. The measurement of the crater depth (in nanometer, nm), $p_{crater}$, allows to determine the sputtering rate, $S_r$ as $S_r = p_{crater}/t_{total}$ in $nm s^{-1}$, and to convert the X axis from time (in s) to depth (in nm).

To quantify an element in the depth of a material, it is necessary to analyze a reference sample. Such reference sample is usually made of the material and specifically implanted with a known dose, $D_{implant}$, of the requested element (phosphorus in our case). The SIMS analysis of the reference sample will allow the determination of the relative sensitivity factor of the element (RSF<sub>element/matrix</sub>) to quantify the element of interest. By recording the signals of the matrix element, $I_{matrix}$, and the element of interest, $I_{element}$, in the implanted reference, the RSF<sub>element/matrix</sub> is given by $D_{implant} * I_{matrix} / (Sr * \int_{0}^{t} I_{element} dt)$. The definite integral between 0 and t is indeed the area under the intensity curve of the element of interest. To convert the intensity (in cps) in concentration (in at/cm³), the following equation is then used: $[element] = RSF_{element} * I_{element} / I_{matrix}$.

An example of the quantification of a raw profile (Figure 1b) is given in Figure 1c for our phosphorus-implanted diamond reference. The quantified profile with phosphorus concentration, [P] (at cm⁻³), is then given in function of depth (nm). In our case, our phosphorus-implanted diamond reference gives a RSF<sub>31P/12C</sub> of 1.8 × 10<sup>23</sup> at cm⁻³. Note that the quantification is based on knowledge of the implanted dose which is given by the implanters to within 10%. The phosphorus profile shows the expected implanted peak around 150 nm with a maximum content of ≈3 × 10<sup>19</sup> at cm⁻³.

The profile tail of the implanted reference gives the value of the detection limit of the element.[28] As a result, the phosphorus

detection limit is ≈3 × 10¹⁵ at cm⁻³ (≈17 ppb) while using LMR-SIMS diamond conditions. This means that such SIMS conditions are fully adapted to measure phosphorus-doped diamond with content above 4 × 10¹⁵ at cm⁻³.

However, one can expect from SIMS analysis lower detection limit, typically a few ppb. Moreover, the weak shoulder observed on Figure 1a suggests that they might have interfering molecular ion(s) with masses close to the one of phosphorus ions. To separate ions having comparable masses, HMR is used. It is done by closing the entrance and exit slits of the spectrometer just enough to clearly separate each ion mass. In our case, a moderate HMR of $M/ΔM = 1600$ is appropriate as seen on the mass spectrum in Figure 2a from our phosphorus-implanted reference. The intensity of the phosphorus related peak is of $1 × 10^3$ cps while the ones of the interfering species do not exceed 5 cps. The useful yield (UY), that is the ratio between the number of detected ions and the number of sputtered ions, is then of $6.6 × 10^{-3}$ count/at at LMR and $3.4 × 10^{-3}$ count/at at HMR. As a result, the transmission at HMR is reduced only by a factor ≈2 lower than the transmission at LMR.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Mass spectra in the range of the phosphorus mass (31 a.m.u.) recorded in a crater bottom of our reference, far from the implanted zone, by varying the electronic gate from 100% down to 30% and analyzed in HMR-SIMS diamond conditions.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** a) Mass spectrum in the range of the phosphorus mass (31 a.m.u.) and b) depth profile of the carbone and the phosphorus contents (in at cm⁻³) of our phosphorus-implanted reference while analyzed in standard SIMS diamond conditions with HMR settings ($M/ΔM = 1600$).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Mass spectra recorded at HMR (M/M = 1 600) in the 31 a.m.u. range for a) a commercial HPHT diamond substrate, b) a CVD undoped diamond homoepilayer, and c) a CVD boron-doped diamond homoepilayer. None of those samples contain phosphorus.</figcaption></figure>

The corresponding depth profile of the phosphorus-implanted reference is given in Figure 2b. As expected, the phosphorus profile shows the implanted peak with a better counting dynamic than in Figure 1b. The profile tail of the implanted reference reveals that HMR settings improve by one decade the phosphorus detection limit. The value is then of ≈3 × 10¹⁴ at cm⁻³ (≈1.7 ppb) while using HMR-SIMS diamond conditions. This means that HMR-SIMS conditions are essential to measure phosphorus-doped diamond with content between 4 × 10¹⁴ and 4 × 10¹⁵ at cm⁻³.

As interfering species were not expected, we try to identify the two observed around 31.000 and 31.020 a.m.u. For that purpose, we activate the dynamic transfer mode (in order to collect all ions coming from the sputtered crater) and play on the electronic gate, from 100% to 30%, thereby reducing progressively the detection area. We record mass spectra in a crater bottom of our phosphorus-implanted reference, well beyond the micrometer depth. This allows to perform measurements far from the phosphorus-implanted zone, meaning in the HPHT diamond substrate itself. The spectra are reported in **Figure 3** with intensities normalized to the upper mass interfering species (≈31.020 a.m.u.). The 31P peak intensity decreases from 100% to 70% electronic gates and is constant for lower electronic gate. The two other peak intensities are unchanged from an electronic gate to another. This indicates that the edge effects coming from the implanted zone are properly managed from 70% of electronic gate. Moreover, the three peaks with unchanged intensities are not related to the implanted zone but come from the diamond matrix and its close environment in the analysis chamber. For the two upper masses, we propose the following identification. The first interference around 31.000 a.m.u. might be a molecular combination of carbon (12 a.m.u.) and fluorine (18.998) ions because fluorine is expected in trace in the analysis chamber due to the pumping system. The second interference around 31.020 a.m.u could be a molecular combination of carbon, oxygen (15.995 a.m.u.), and 3 hydrogen (1.008 a.m.u.) ions because 16O and 1H, as atmospheric elements, could be in trace in the analysis chamber as well as in the diamond matrix.

A difficulty is now to identify the ions located at the exact mass position of the 31P ions. In order to check whether this peak is an artefact due to the presence of the above-implanted zone, we

investigate the detection of ions in the 31 a.m.u. in other samples that cannot contain phosphorus because they have not been exposed to phosphorus during their synthesis but also because of the extremely low incorporation efficiency of phosphorus in diamond. For that purpose (see **Figure 4**), we analyze a commercial (100) HPHT diamond substrate and two (100) CVD diamond homoepilayers grown in separate reactors (one is an undoped film and the other a boron-doped layer). All samples present the 2-interfering species that we previously identify as ¹²C¹⁹F and ¹²C¹⁶O¹H₃.

Surprisingly, the peak located at the exact mass of phosphorus is even observed in the three samples in which phosphorus is not expected. However, in such samples this peak is not supposed to be related to phosphorus. The nature of this peak remains unclear and its identification is still under investigation.

**4. Conclusion**

We show that standard SIMS diamond analysis with LMR induce a detection limit of phosphorus around 3 × 10¹⁵ at cm⁻³, allowing the measurement of phosphorus content in diamond above 4 × 10¹⁵ at cm⁻³. While applying HMR, we evidence unexpected interfering species. Two are identified as coming from diamond matrix and its close environment while the third one, at the exact phosphorus mass, is not yet identified. We point out that HMR-SIMS diamond conditions improve the detection limit of phosphorus by one decade, down to 3 × 10¹⁴ at cm⁻³, that is essential to measure ultralightly phosphorus-doped diamond.

**Acknowledgements**

This work was supported by the French National Research Agency (ANR) through the LAPIN113 program (ANR-20-CE05-0032). The implantation of phosphorus-implanted reference was performed by Ion Beam Services (IBS) company in Peynier (France) through the DIAMONIX 2 program (F1110024M) led by the competitiveness cluster of Aerospace Valley and funded by the French Unique Interministerial Fund (FUI). Authors are grateful to C. Mer-Calfati from CEA-LIST in Saclay (France) for providing the boron-doped chemical vapor deposition homoepilayer.
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Mass spectra recorded at HMR (M/M = 1 600) in the 31 a.m.u. range for a) a commercial HPHT diamond substrate, b) a CVD undoped diamond homoepilayer, and c) a CVD boron-doped diamond homoepilayer. None of those samples contain phosphorus.</figcaption></figure>

The corresponding depth profile of the phosphorus-implanted reference is given in Figure 2b. As expected, the phosphorus profile shows the implanted peak with a better counting dynamic than in Figure 1b. The profile tail of the implanted reference reveals that HMR settings improve by one decade the phosphorus detection limit. The value is then of ≈3 × 10¹⁴ at cm⁻³ (≈1.7 ppb) while using HMR-SIMS diamond conditions. This means that HMR-SIMS conditions are essential to measure phosphorus-doped diamond with content between 4 × 10¹⁴ and 4 × 10¹⁵ at cm⁻³.

As interfering species were not expected, we try to identify the two observed around 31.000 and 31.020 a.m.u. For that purpose, we activate the dynamic transfer mode (in order to collect all ions coming from the sputtered crater) and play on the electronic gate, from 100% to 30%, thereby reducing progressively the detection area. We record mass spectra in a crater bottom of our phosphorus-implanted reference, well beyond the micrometer depth. This allows to perform measurements far from the phosphorus-implanted zone, meaning in the HPHT diamond substrate itself. The spectra are reported in **Figure 3** with intensities normalized to the upper mass interfering species (≈31.020 a.m.u.). The 31P peak intensity decreases from 100% to 70% electronic gates and is constant for lower electronic gate. The two other peak intensities are unchanged from an electronic gate to another. This indicates that the edge effects coming from the implanted zone are properly managed from 70% of electronic gate. Moreover, the three peaks with unchanged intensities are not related to the implanted zone but come from the diamond matrix and its close environment in the analysis chamber. For the two upper masses, we propose the following identification. The first interference around 31.000 a.m.u. might be a molecular combination of carbon (12 a.m.u.) and fluorine (18.998) ions because fluorine is expected in trace in the analysis chamber due to the pumping system. The second interference around 31.020 a.m.u could be a molecular combination of carbon, oxygen (15.995 a.m.u.), and 3 hydrogen (1.008 a.m.u.) ions because 16O and 1H, as atmospheric elements, could be in trace in the analysis chamber as well as in the diamond matrix.

A difficulty is now to identify the ions located at the exact mass
position of the 31P ions. In order to check whether this peak is an
artefact due to the presence of the above-implanted zone, we

investigate the detection of ions in the 31 a.m.u. in other samples
that cannot contain phosphorus because they have not been
exposed to phosphorus during their synthesis but also because
of the extremely low incorporation efficiency of phosphorus in
diamond. For that purpose (see **Figure 4**), we analyze a commer-
cial (100) HPHT diamond substrate and two (100) CVD diamond
homoepilayers grown in separate reactors (one is an undoped
film and the other a boron-doped layer). All samples present
the 2-interfering species that we previously identify as ¹²C¹⁹F
and ¹²C¹⁶O¹H₃.

Surprisingly, the peak located at the exact mass of phosphorus
is even observed in the three samples in which phosphorus is not
expected. However, in such samples this peak is not supposed to
be related to phosphorus. The nature of this peak remains
unclear and its identification is still under investigation.

**4. Conclusion**

We show that standard SIMS diamond analysis with LMR induce a detection limit of phosphorus around 3 × 10¹⁵ at cm⁻³, allowing the measurement of phosphorus content in diamond above 4 × 10¹⁵ at cm⁻³. While applying HMR, we evidence unexpected interfering species. Two are identified as coming from diamond matrix and its close environment while the third one, at the exact phosphorus mass, is not yet identified. We point out that HMR-SIMS diamond conditions improve the detection limit of phosphorus by one decade, down to 3 × 10¹⁴ at cm⁻³, that is essential to measure ultralightly phosphorus-doped diamond.

Acknowledgements

This work was supported by the French National Research Agency (ANR) through the LAPIN113 program (ANR-20-CE05-0032). The implantation of phosphorus-implanted reference was performed by Ion Beam Services (IBS) company in Peynier (France) through the DIAMONIX 2 program (F1110024M) led by the competitiveness cluster of Aerospace Valley and funded by the French Unique Interministerial Fund (FUI). Authors are grateful to C. Mer-Calfati from CEA-LIST in Saclay (France) for providing the boron-doped chemical vapor deposition homoepilayer.
Conflict of Interest

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

Keywords

depth profiles, detection limits, diamonds, high mass resolution, ion
analyses, phosphorus, secondary-ion mass spectroscopy

Received: May 17, 2022

Revised: October 7, 2022

Published online: December 13, 2022

[1] S. Koizumi, K. Watanabe, M. Hasegawa, H. Kanda, *Science* **2001**, *292*, 1899.

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[8] C. D. Clark, P. J. Dean, P. V. Harris, *Proc. R. Soc. London A* **1964**, *277*, 312.

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[10] F. Nava, C. Canali, C. Jacoboni, L. Reggiani, S. F. Kozlov, *Solid State Commun.* **1980**, *33*, 475.

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[25] E. D. Herbschleb, H. Kato, Y. Maruyama, T. Danjo, T. Makino, S. Yamasaki, I. Ohki, K. Hayashi, H. Morishita, M. Fujiwara, N. Mizuochi, *Nat. Commun.* **2019**, *10*, 3766.

[26] L. Sangely, B. Boyer, E. de Chambost, N. Valle, J.-N. Audinot, T. Ireland, M. Wiedenbeck, J. Aléon, H. Jungnickel, J.-P. Barnes, P. Bienvenu, U. Breuer, in *Sector Field Mass Spectrometry for Elemental and Isotopic Analysis* (Eds: T. Prohaska, J. Irrgeher, A. Zitek, N. Jakubowski), Royal Society of Chemistry, United Kingdom **2015**, p. 439, Ch. 15.

[27] P. Williams, *Annu. Rev. Mater. Sci.* **1985**, *15*, 517.

[28] Fred A. Stevie, in *Secondary Ion Mass Spectrometry – Applications for Depth Profileing and Surface Characterization* (Ed: C. Richard Brundle), Momentum Press, LLC, New York, NY **2016**.

[29] M.-A. Pinault-Thaury, F. Jomard, C. Mer-Calfati, N. Tranchant, M. Pomorski, P. Bergonzo, J.-C. Arnault, *Appl. Surf. Sci.* **2017**, *410*, 464.

[30] M.-A. Pinault-Thaury, I. Stenger, R. Gillet, S. Temgoua, E. Chikoidze, Y. Dumont, F. Jomard, T. Kociniewski, J. Barjon, *Carbon* **2021**, *175*, 254.
Conflict of Interest

The authors declare no conflict of interest.

Data Availability Statement

The data that support the findings of this study are available from the
corresponding author upon reasonable request.

Keywords

depth profiles, detection limits, diamonds, high mass resolution, ion
analyses, phosphorus, secondary-ion mass spectroscopy

Received: May 17, 2022

Revised: October 7, 2022

Published online: December 13, 2022

[1] S. Koizumi, K. Watanabe, M. Hasegawa, H. Kanda, *Science* **2001**, *292*, 1899.

[2] J. Isberg, J. Hammersberg, H. Bernhoff, D. J. Twitchen, A. J. Whitehead, *Diamond Relat. Mater.* **2004**, *13*, 872.

[3] J. Isberg, J. Hammersberg, D. J. Twitchen, A. J. Whitehead, *Diamond Relat. Mater.* **2004**, *13*, 320.

[4] J. Achard, F. Silva, H. Schneider, R. S. Sussmann, A. Tallaire, A. Gicquel, M. C. Castex, *Diamond Relat. Mater.* **2004**, *13*, 876.

[5] T. Makino, N. Tokuda, H. Kato, M. Ogura, H. Watanabe, S. G. Ri, S. Yamasaki, H. Okushi, *Jpn. J. Appl. Phys., Part 2* **2006**, *45*, L1042.

[6] D. Takeuchi, T. Makino, H. Kato, M. Ogura, N. Tokuda, K. Oyama, T. Matsumoto, I. Hirabayashi, H. Okushi, S. Yamasaki, *Appl. Phys. Express* **2010**, *3*, 041301.

[7] T. Makino, K. Yoshino, N. Sakai, K. Uchida, S. Koizumi, H. Kato, D. Takeuchi, M. Ogura, K. Oyama, T. Matsumoto, H. Okushi, S. Yamasaki, *Appl. Phys. Lett.* **2011**, *99*, 061110.

[8] C. D. Clark, P. J. Dean, P. V. Harris, *Proc. R. Soc. London A* **1964**, *277*, 312.

[9] A. T. Collins, A. W. S. Williams, *J. Phys. C: Solid State Phys.* **1971**, *4*, 1789.

[10] F. Nava, C. Canali, C. Jacoboni, L. Reggiani, S. F. Kozlov, *Solid State Commun.* **1980**, *33*, 475.

[11] J. Isberg, J. Hammersberg, E. Johansson, T. Wikstrom, D. J. Twitchen, A. J. Whitehead, S. E. Coe, G. A. Scarsbrook, *Science* **2002**, *297*, 1670.

[12] J. E. Butler, M. W. Geis, K. E. Krohn, J. Lawless Jr., S. Deneault, T. M. Lyszczarz, D. Flechtner, R. Wright, *Semicond. Sci. Technol.* **2003**, *18* S67.

[13] J. Isberg, A. Lindblom, A. Tajani, D. Twitchen, *Phys. Status Solidi (A)* **2005**, *202*, 2194.

[14] H. Windischmann, in *Properties, Growth and Applications of Diamond, Vol. 26 in EMIS Datareviews Series* (Eds: M.H. Nazare, A.J. Neves), INSPEC, Institute of Electrical Engineers, London, **2001**, pp. 410, Ch. C2.2.

[15] H. Kagan, *Nucl. Instrum. Methods A* **2005**, *546*, 222.

[16] T. Teraji, T. Ito, J. Cryst, *Growth* **2004**, *271*, 409.

[17] A. Tallaire, J. Achard, F. Silva, R. S. Sussmann, A. Gicquel, *Diamond Relat. Mater.* **2005**, *14*, 249.

[18] J. Achard, F. Silva, O. Brinza, A. Tallaire, A. Gicquel, *Diamond Relat. Mater.* **2007**, *16*, 685.

[19] J. Achard, R. Issaoui, A. Tallaire, F. Silva, J. Barjon, F. Jomard, A. Gicquel, *Phys. Status Solidi A* **2012**, *209*, 1651.

[20] H. Kato, M. Ogura, T. Makino, D. Takeuchi, S. Yamasaki, *Appl. Phys. Lett.* **2016**, *109*, 142102.

[21] J. P. Goss, P. R. Briddon, R. J. Eyre, *Phys. Rev. B* **2006**, *74*, 245217.

[22] H. Kato, T. Makino, S. Yamasaki, H. Okushi, *J. Phys. D: Appl. Phys.* **2007**, *40*, 6189.

[23] M.-A. Pinault-Thaury, B. Berini, I. Stenger, E. Chikoidze, A. Lusson, F. Jomard, J. Chevallier, J. Barjon, *Appl. Phys. Lett.* **2012**, *100*, 192109.

[24] E. Neu, P. Appel, M. Ganzhorn, J. Miguel-Sánchez, M. Lesik, V. Mille, V. Jacques, A. Tallaire, J. Achard, P. Maletinsky, *Appl. Phys. Lett.* **2014**, *104*, 153108.

[25] E. D. Herbschleb, H. Kato, Y. Maruyama, T. Danjo, T. Makino, S. Yamasaki, I. Ohki, K. Hayashi, H. Morishita, M. Fujiwara, N. Mizuochi, *Nat. Commun.* **2019**, *10*, 3766.

[26] L. Sangely, B. Boyer, E. de Chambost, N. Valle, J.-N. Audinot, T. Ireland, M. Wiedenbeck, J. Aléon, H. Jungnickel, J.-P. Barnes, P. Bienvenu, U. Breuer, in *Sector Field Mass Spectrometry for Elemental and Isotopic Analysis* (Eds: T. Prohaska, J. Irrgeher, A. Zitek, N. Jakubowski), Royal Society of Chemistry, United Kingdom **2015**, p. 439, Ch. 15.

[27] P. Williams, *Annu. Rev. Mater. Sci.* **1985**, *15*, 517.

[28] Fred A. Stevie, in *Secondary Ion Mass Spectrometry – Applications for Depth Profileing and Surface Characterization* (Ed: C. Richard Brundle), Momentum Press, LLC, New York, NY **2016**.

[29] M.-A. Pinault-Thaury, F. Jomard, C. Mer-Calfati, N. Tranchant, M. Pomorski, P. Bergonzo, J.-C. Arnault, *Appl. Surf. Sci.* **2017**, *410*, 464.

[30] M.-A. Pinault-Thaury, I. Stenger, R. Gillet, S. Temgoua, E. Chikoidze, Y. Dumont, F. Jomard, T. Kociniewski, J. Barjon, *Carbon* **2021**, *175*, 254.