# Impurity characterization in diamond for quantum and electronic applications: advances with time-resolved cathodoluminescence

Christophe Arnold, Solange Temgoua, Julien Barjon

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Christophe Arnold, Solange Temgoua, Julien Barjon. Impurity characterization in diamond for quantum and electronic applications: advances with time-resolved cathodoluminescence. Nanotechnology, 2024, 35 (35), pp.355705. 10.1088/1361-6528/ad4f94. hal-04837102

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# Impurity characterization in diamond for quantum and electronic applications: advances with time-resolved cathodoluminescence

C. Arnold¹, S. Temgoua¹, J. Barjon¹

¹ Université Paris-Saclay, UVSQ, CNRS, GEMaC, 78000, Versailles, France.

E-mail: christophe.arnold@uvsq.fr

## Abstract.

The ultimate purity of synthetic diamond crystals is currently limited by traces of boron and nitrogen. Here we study diamond crystals grown at high-pressure high-temperature, which are made of 3D growth sectors with variable residual impurity contents. The boron concentration is found in the 0.5-6.4 ppb range thanks to continuous cathodoluminescence (cw-CL) analysis. Time-resolved cathodoluminescence (TRCL) experiments complete the impurity analysis with measurements of free exciton lifetimes. From them, we deduced an estimate of the nitrogen concentration at the ppb level, from 0.6 to 30 ppb depending on the growth sectors. We identified n-type, p-type and highly compensated regions, which illustrates the potential of cathodoluminescence as a local characterization tool for qualifying diamond for electronic and quantum applications.

*Keywords:* diamond, cathodoluminescence, boron, nitrogen, HPHT, free exciton, TRCL, blanker, time resolved

## 1. Introduction

The development of demanding applications often requires intensive progress on materials. As enlightening examples of this are diamond-based applications: In electronics, limiting the presence of residual impurities is a critical step to master its electrical conductivity. Also, in quantum technologies based on the spin of nitrogen-vacancy (NV) color centres, the residual impurities surrounding a single spin generally limit its temporal coherence, and finally the quantum performances in computing [1] or magnetometry [2]. Whether when diamond is used for electronics or for quantum applications, an ultimate control of the single crystal purity is required, in the range of the part per billion (ppb) of carbon atoms, or below. Nowadays, efforts remain to be done on the ultrapure diamond synthesis and the development of reliable analysis to characterize the concentration of impurities in diamond is thus crucial.
Considering the technics available for measuring the residual impurity level, optical absorption (OAS) or magnetic resonance, such as electron paramagnetic resonance (EPR) are the most common. They require bulk crystals with thicknesses in the millimetre range to get a sufficient sensibility for reaching the ppb range. However, these technics are macroscopic, they only provide impurity concentrations averaged over the whole crystal thickness, which turns out to be extremely limiting. Concerning the lateral resolution, it is rather low for OAS, while ESRS is not resolved spatially. More local technics are required for thin films and microsystems. Secondary ion mass spectrometry (SIMS) detects parts per million (ppm) to parts per billion (ppb) of most chemical elements at the scale of typically 100 µm, but is destructive. The analysis derived from electron beams focused at typically 1 nm are by far more local. Among them, the bound exciton spectrocopy using continuous cathodoluminescence (cw-CL) has already proven to be effective for high purity diamond analysis [3].

Substitutional boron and nitrogen are the most common contaminants of synthetic diamonds. The first is an acceptor of electrons, while the second is a donor, so they compensate each other. It raises the issue of identifying the type of free charge carriers (electrons or hole) when nitrogen and boron are present in comparable concentrations. Knowing the semiconductor type is obviously important for electronics but it is also crucial for quantum-defect engineering, since the Fermi level position in the bandgap controls the charge state of color centres [4].

Traces of boron and nitrogen are generally present even in the purest diamonds. This is true either for diamond grown at high pressure high temperature (HPHT) or by plasma-assisted chemical vapour deposition (CVD). Among all technics to date cw-CL provides the lowest detection limit for the boron impurity. At liquid helium temperature, it is below 0.05 ppb (9 × 10¹² cm⁻³) which represents ~10 boron atoms per cubic micrometre of diamond. Unfortunately, cw-CL is of no use for nitrogen, which is a too deep donor in the diamond bandgap to produce a detectable luminescence of its bound exciton. In this work, we show how time-resolved cathodoluminescence (TRCL) and continuous cathodoluminescence (cw-CL) can be combined to push forward the limitations for nitrogen detection in diamond by optical means.

TRCL consists in exciting temporarily a sample with the electron beam and measuring the signal of luminescence over time. This allows to measure the dynamic of recombination during the periods of time with no excitation. In the pioneering work in the 1980s [5], the beam extinction was obtained by applying a bias on electrodes installed on the path of the electron beam. The interest of TRCL has been revitalized by recent technical developments bringing high temporal resolution [6, 7]: samples are excited by very short pulses of electrons (~10 ps) created by focusing pulsed lasers on photo-cathodes. Nowadays, TRCL experiments are used for various types of analyses, such as charge carrier dynamics [8], surface recombination [9], spatially resolved exciton lifetimes [6] or excitonic diffusion [10]. To highlight the interest of the cross analysis between cw-CL and TRCL, we study a high-purity HPHT diamond, whose residual
<figure><img src="image_2.png" /><figcaption>**Figure 1.** IIa HPHT diamond sample (a) as-grown crystal ; (b) CL image in the visible range of a plate extracted from the crystal shown in (a) ; (c) same as (b) but saturated to better identify the growth sectors and (d) X-ray topography. Scale bar is 1 mm.</figcaption></figure>

nitrogen and boron uptake varies among growth sectors. We show how measuring both the cw-CL exciton spectra and the temporal decay of free-excitons by TRCL allows to determine the concentration of both boron and nitrogen impurities in diamond and to deduce the type, (p) or (n), of its majority charge carriers.

**2. Samples and experimental methods**

We have implemented this analysis to a high purity single crystal IIa diamond, synthetized by high-pressure high-temperature (HPHT), shown in Fig.1(a). The as-grown crystal is facetted. The uptake of impurities during growth depends on the crystallographic orientation of the growth facets, leading to different growth sectors with variable impurity contents. Measurements were done on the four different sectors of a diamond plate cut and polished from the whole crystal. The growth sectors of the plate are identified thanks to the blue luminescence associated to boron doping [11, 12] observed in visible cathodoluminescence images (Fig.1b and c). The images are simply recorded with a color CCD camera while the sample is excited by a uniform electron beam of 7 mm diameter. They reveal the sectors 100 and 113 at the center and 111 and 110 sectors on the edges of the plate. The structural quality of the HPHT crystal is excellent as shown by the X-ray topography image (Fig.1d): only a few stacking faults are observed in the lateral sectors while the central part appears free of defects.

As a reference for boron impurities, we also use in this work a p-type homoepitaxial layer grown by chemical-vapor-deposition (CVD) with intentional boron doping. The boron concentration was found homogeneous in the layer and measured at $2.5 \times 10^{16} \text{cm}^{-3}$ by secondary ion mass spectrometry (10% uncertainty in the best conditions). More interestingly, this layer presents an extremely low compensation ratio ($10^{-4}$) as shown in Hall effect experiments [13]. This ensure the substitutional nitrogen concentration is below 0.06 ppb ($10^{13} \text{cm}^{-3}$) in this sample. These properties make the CVD sample a reliable standard for the boron impurities in diamond. It corresponds to the limit case of single impurity doping.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Schematics of the cw- and time-resolved cathodoluminescence setup. For time-resolved experiments, blanker electrodes are inserted in the SEM column to turn off the excitation at will. In continuous experiment, the electrodes are grounded or retracted from the column.</figcaption></figure>

The diamond samples are characterized by continuous (cw-CL) and time-resolved cathodoluminescence (TRCL) with the setup depicted on figure 2. Samples are mounted on a GATAN cryogenic stage inside a 7001F JEOL scanning electronic microscope (SEM), allowing measurements close to liquid helium temperature. In all the experiments, the sample holder was cooled at 5K while the exciton gas temperature is estimated at about 15K-20K from Maxwell-Boltzmann fitting of the measured free-exciton lineshape. The electron beam is focused at the nanometer scale on the region of interest, thanks to the field-emission electron gun of the microscope. The emitted photons are collected by a parabolic mirror then focused on the aperture slit of an HORIBA TRIAX550 spectrometer. For the time-resolved analysis, we position two planar electrodes from a retractable beam-blanker on both sides of the electron beam. This blanker is a prototype system including a Faraday cup designed for the JEOL field-emission microscope, with piezoelectric actuators allowing the alignment of the electrodes with the electron optical axis. It allows to stop the excitation by deflecting the continuous electron beam by the electrostatic force created when the electrodes are biased. The beam blanking is achieved periodically by polarizing the electrodes with a square profile. To that end, an AVTCH (AVNN-1-C) or a SEFRAM 4433 pulse generator are used depending on the working frequency. The photons are filtered through the spectrometer and are finally detected with a prototype fast micro-channel photomultiplier tube from PHOTONIS, having a sufficiently high sensitivity and low noise in the 200-550 nm range to operate at the single photon level. Time-varying luminescence is then reconstructed by making the histogram of the arrival times of photons thanks to a BECKER&HICKL 130M counting card. The time resolution of the TRCL experiment is 100 ps in the best conditions. For cw-CL measurements,
the electrodes are simply grounded (or retracted) to get a continuous excitation of the samples. The cw-CL spectra are recorded thanks to a silicon charge-coupled-display (CCD) camera, mounted on the second exit port of the monochromator.

# 3. Results with continuous and time-resolved cathodoluminescence

## 3.1. Continuous cathodoluminescence

We start by briefly reminding that the impurity level quantification in cw-CL is based on the properties of excitons. The electron beam impinging the crystal releases part of its energy by creating electron-hole pairs that stay bonded together by Coulomb interaction at low temperature. Once created, these free excitons diffuse into the material and are captured by impurities, forming bound excitons. When the impurity is a shallow donor or acceptor, bound excitons usually recombine radiatively, giving rise to a characteristic luminescence for each impurity.

We first measured the cw-CL spectra of the different samples to determine their boron concentration. Figure 3(a) shows the emissions normalized by the intensity measured at 225 nm, which correspond to the transverse optical (TO) phonon-assisted recombination of free excitons ($X_{TO}$) in diamond. The emission at 237 nm comes from the recombination of neutral-boron bound exciton assisted with transverse optical phonons ($B^0X_{TO}$). The ratio between the two lines is proportional to the boron acceptor concentration in diamond, as established in previous works [3, 14]. Instead of using calibration relations as in Ref. [15], the standard is systematically measured together with the specimen mounted together in the same experimental run. This method solves the uncertainties issues related to the temperature dependence of the cw-CL sensitivity [3]. Measuring the standard in the same conditions than the specimen allows to determine rather accurately (~20%) the boron concentration in the HPHT sample. It is found to vary from 0.5 ppb ($8.4 \times 10^{13} \text{cm}^{-3}$) to 6.4 ppb ($1.1 \times 10^{15} \text{cm}^{-3}$) depending on the regions of the crystal (see Table 1).

Note that the boron concentration evaluated with cw-CL corresponds to the total amount of substitutional boron atoms (acceptors $N_A$), since the dopants are efficiently neutralized under the charge carrier injection occurring under e-beam excitation. This is a remarkable difference with optical absorption or magnetic resonance, where some acceptors are ionized by compensation. Given that these techniques are only sensitive to neutral impurities, they just provide the concentration of uncompensated impurities, i.e. $N_A - N_D$. In case of highly compensated crystals, the total amount of substitutional boron atoms is then under-estimated, and in the limit case ($N_A = N_D$), it is not measurable in optical absorption or magnetic resonance.

## 3.2. Time-resolved cathodoluminescence

The temporal decay of the luminescence at 235 nm filtered by the monochromator gives access to the recombination dynamics of free excitons in the crystal. A particular atten-
<figure><img src="image_4.png" /><figcaption>**Figure 3.** (a) cw-CL spectra measured from the different growth sectors and normalized by the intensity at the free exciton wavelength (235nm). (b) Temporal profile of the free exciton luminescence measured on the same area by TRCL at 235 nm, with a vertical offset added for clarity. All CL data are recorded at T=5K.</figcaption></figure>

tion to the effect of the surface on the exciton dynamics has to be paid: surface is indeed an efficient channel of recombination for excitons. An excitation too close to the surface leads to decays faster than the lifetime of excitons in the bulk crystal (i.e. far from the surface), which is the useful information for impurity characterization. In this work the incident electrons were accelerated at 5 keV, which corresponds to a relatively weak penetration of electrons in the crystal (~280 nm stopping depth). A way to mitigate the surface effects is to play on the temporal profile of the excitation: if we use a pulse of electrons generated as in Ref. [9], because of surface recombinations an extremely fast decay of luminescence is obtained on the CVD boron standard, as shown on figure 4(a). Instead, we apply on the electrodes periodical square voltages, alternating between levels at 0V (excitation time) and ~5V (blocking time), with a duration of the excitation level sufficiently long to reach the steady-state regime of the diffusion dynamic of the excitons in the crystal. Such an excitation improves the signal-to-noise ratio (see Fig. 4(a, c, d, e, f)) - TRCL decays could be recorded on 3 orders of magnitude - and favors recombinations in the bulk crystal, thanks to a deeper distribution of excitons in the steady-state regime before the interruption [16].

This excitation method has been decisive for measuring the bulk lifetimes of excitons in diamond. After abruptly stopping the continuous excitation, the temporal profiles of luminescence are well reproduced with a single exponential decay (Fig. 4) indicating negligible surface effects. The time constants then correspond to the free-exciton lifetime in the bulk. The lifetimes extracted from fits are found to scale on more than two orders of magnitude, from 220 ns (HPHT 100) to 630 ps (CVD diamond standard for single boron doping), shorter at high boron doping. We also note that the exciton lifetime is found reproducible and homogeneous within 10% inside each growth sector and this including the 111 sector that contains a few stacking faults.

The dependence of the bulk free-exciton lifetime, $\tau$, on the impurity content is
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Luminescence decays (black lines) measured from the diamond samples after an abrupt interruption of the continuous e-beam excitation (a, c, d, e, f) and after a pulsed excitation (b). Decays are fitted (red lines) with a mono-exponential function $y = y_0 + Ae^{-t/\tau}$ where $y_0$ is related to the noise of the detector.</figcaption></figure>

explained in the absence of extended structural defects, by the capture dynamics of free excitons:

$$ \frac{1}{\tau} = \frac{1}{\tau_R} + \sum_i C_i[i] \qquad (1) $$

where $\tau_R$ is the free-exciton radiative lifetime, $C_i$ are the capture probabilities of every impurities present in the material at the concentration $[i]$. Indirect bandgap semiconductors such as diamond have quite long radiative lifetimes due to the intervention of phonons in the photon emission process. In diamond, $\tau_R$ is about 1.5 µs [17, 18]. This is very long compared to the measured lifetimes which implies that the contribution of the radiative recombinations on the exciton lifetime can be neglected compared to the recombinations by capture on impurities.

The capture probability of free excitons by boron impurities has been determined thanks to the measurement of the exciton decay on the boron-doped-diamond CVD standard. We found $C_B = 6.3 \times 10^{-8} \text{ s}^{-1}\text{cm}^3$ at the temperature of experiments, which is consistent with previous time-resolved photoluminescence results [19]. Figure 5 shows the free-exciton lifetimes measured by TRCL as a function of the boron concentrations measured by cw-CL on the samples. The blue solid line drawn in Fig. 5 is $\tau = (C_B[B])^{-1}$. It corresponds to the lifetime expected when boron is the only impurity of the crystal,
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Free exciton lifetime as a function of boron concentration in diamond for the HPHT growth sectors (full squares) and the CVD diamond standard for boron (open square). The continuous blue line indicates the free exciton lifetime expected when boron is the only impurity present in diamond. The dashed blue line indicates the expected lifetime in case of a full compensation by nitrogen [N]=[B], separating the p-type and n-type regions.</figcaption></figure>

i.e. when other impurities are negligible regarding the boron concentration. The lifetime recorded from the 100 growth sector in HPHT diamond falls on the blue curve, which indicates that boron is by far the majority impurity in this growth sector. Interestingly Shimomura et al. [20] have clearly shown that nitrogen was the majority impurity in a serie of diamond samples with comparable concentrations of residual impurities. This highlights a disparity in the conductivity type, p or n, among the high purity crystals graded as IIa diamonds.

Looking at the summary of our measurements presented in Fig.5, a deviation from the “boron only” situation (blue solid line) is remarkable for the 111 growth sector of the HPHT sample. Although this sector has a boron level comparable to the 110 one, the exciton lifetime is found 4 times shorter in the 111 sector. We interpret this result by the presence of a significant amount of nitrogen in the 111 growth sector. Nitrogen incorporation is known to be at the highest level in 111 sectors among all other growth sectors [21]. This is also in line with ref [22], in which important nitrogen concentration were obtained on the sector 111 of a IIa crystal (500 ppb). We conclude that boron is not the majority impurity in 111 growth sectors of the HPHT sample investigated. It is more likely to be nitrogen instead. In the last session, we push a step further our analysis toward a nitrogen concentration estimation in diamond growth sectors.
<table><caption>**Table 1.** Dependance of the residual impurity uptake on the HPHT growth sectors. The experimental uncertainties are too high to be totally conclusive on the type of sectors 113 and 110.</caption><thead><tr><th>growth sector</th><th>[B] (cm<sup>-3</sup>)</th><th>(ppb)</th><th>[N] (cm<sup>-3</sup>)</th><th>(ppb)</th><th>type</th></tr></thead><tbody><tr><td>100</td><td>8.4 &times; 10<sup>13</sup></td><td>0.5</td><td>&lt; 3 &times; 10<sup>13</sup></td><td>&lt; 0.2</td><td>p</td></tr><tr><td>113</td><td>1.7 &times; 10<sup>14</sup></td><td>1.0</td><td>1 &times; 10<sup>14</sup></td><td>0.6</td><td>likely p</td></tr><tr><td>110</td><td>1.1 &times; 10<sup>15</sup></td><td>6.4</td><td>5 &times; 10<sup>14</sup></td><td>3</td><td>likely p</td></tr><tr><td>111</td><td>9.3 &times; 10<sup>14</sup></td><td>5.4</td><td>5 &times; 10<sup>15</sup></td><td>30</td><td>n</td></tr></tbody></table>

## 4. Nitrogen concentration and type of majority charge carriers

The nitrogen probability for the free exciton capture by nitrogen in diamond is still unknown and hardly measurable with the required accuracy. However, as discussed in Shimomura et al. [20], the excitons having a larger Bohr radius than impurities, it is expected following classical collision theory between hard spheres, comparable capture probabilities on the different impurities. Shimomura et al. indeed verified it between nitrogen and boron and Barjon et al. also showed the same order of magnitude on the cross-sections of boron acceptors ($4 \times 10^{-14}$ cm$^2$) and phosphorus donors ($6 \times 10^{-14}$ cm$^2$) [19]. So, it is reasonable for the following to use for the capture probability on nitrogen the same value as the one we measured for boron ($C = 6.3 \times 10^{-8}$ s$^{-1}$cm$^3$).

With this, we determine the order of magnitude of the nitrogen content [$N$] = $\frac{1}{C\tau} - [B]$, $\tau$ being measured by using TRCL, while [B] by cw-CL (see Table 1). We find that [$N$]=5 ± 0.8 × 10$^{15}$ cm$^{-3}$ in the 111 growth sector. Paying attention to the smaller deviations to the “boron only” situation observed in Fig. 5 for the 110 and 113 growth sectors, we deduce their nitrogen concentration to be 5 ± 4 × 10$^{14}$ cm$^{-3}$ and 1 ± 0.6 × 10$^{14}$ cm$^{-3}$ respectively, meaning these p-type sectors are highly compensated by nitrogen donors ($N_D/N_A$ of 45 and 60 % respectively). The uncertainties on the nitrogen concentrations are estimated considering uncertainties of 10% on the determination of the lifetimes by TRCL and of 20% on the boron concentrations by cw-CL.

As a summary of our investigations on the uptake of residual impurities during HPHT growth of high purity diamonds (see also Table 1): $[B]_{100} < [B]_{113} \ll [B]_{110} \approx [B]_{111}$ and $[N]_{100} \ll [N]_{113} \approx [N]_{110} \ll [N]_{111}$. The sectorial dependence of boron content that we found in HPHT diamond is in good agreement with previous reports [21]. However for nitrogen, our finding deviates from the results of Ref. [21] based on macroscopic optical absorption experiments using ultraviolet and infrared nitrogen-related signals. They found that sector 100 is the second sector more susceptible to incorporate nitrogen during growth, in contradiction with our results. This highlights the difficulty to estimate concentration of impurities with absorption experiments or magnetic resonance techniques, which are blind for the minority impurities that are fully ionized by compensation of the majority ones. The results obtained here with CL clarify the interplay between boron and nitrogen in diamond growth sectors that are highly-compensated.
As a major outcome, we deduce the type of the majority charge carriers present in the HPHT growth sectors, which is a crucial information to control the charge state of single color centres and improving the spin coherence of points defects for quantum applications [23, 24]. We remind that the limit case where only boron is present is drawn as a continuous blue line in Fig.5. The situation of full compensation where [N]=[B] leads to an exciton lifetime decrease by half the one obtained when only boron is present, plotted with a dashed line in Fig.5. In between the two lines, the p-type region where [B]>[N] is shadded in grey. The points situated below indicate n-type diamond where nitrogen is the majority dopant. Even if difficult to confirm with other technics given the local aspect of the problem and the low impurity concentrations, our results indicate that the IIa diamond plate probably presents p-type regions (sectors 100), where boron acceptors are the majority, and a least one kind of n-type regions, the sectors 111 at the edges of the crytal, where nitrogen donors are dominant. The sectors 113 and 110 seem to be p-type but the uncertainties estimated on the measured concentrations are too high to be totally conclusive. Indeed, in the highly compensated sectors, we have a difference between the concentrations of nitrogen and boron smaller than our experimental uncertainties, i.e. $|[N] - [B]| < \Delta[N] + \Delta[B]$.

The impact of this study for diamond-based electronics might be important. Using such multisector HPHT substrates for fabricating electronic devices present the risk of generating strong non-uniformities in their electrical performances. For exemple a p-type layer on such a substrate would form a p-n junction above the 111 sectors. The resulting depletion region should decrease the effective layer thickness for hole transport in the p-type layer. The associated built-in electrical field would also modify the H⁺ diffusion in p-type diamond [25] and might be responsible for the Hall voltage discrepancies measured in hydrogen-passivated boron-doped diamond at the macroscopic scale [26].

Finally, we discuss the limits of detection of impurities in the present approach. Due to the temporal resolution of the TRCL set-up (100 ps), the highest measurable concentrations are limited by $[N] + [B] < 5 \times 10^{16} \text{ cm}^{-3}$, reasonable when dealing with high purity diamonds. A remaining blind situation for nitrogen detection still occurs when $[N] \ll [B]$ in p-type diamond, such as in the weakly compensated 100 growth sector. Given the uncertainties discussed above, we estimate that the nitrogen concentration is accessible when $\sim 0.5[B] < [N] < 5 \times 10^{16} \text{ cm}^{-3}$. In that case, we could determine [N] values in the order of 1 ppb on the HPHT sample, which is comparable to the best values of macroscopic technics. The high impurity sensitivity of TRCL demonstrated here also benefits of the local aspect of focused e-beam methods. Here, the probed area for impurity quantification has the size of the free-exciton cloud established in the crystal before interrupting the excitation. It is typically of micrometer size in the high purity crystals studied here.
## 5. Conclusions

As a summary, TRCL associated with cw-CL has proven to be effective for the characterization of ultrapure diamonds contaminated with both nitrogen and boron. By abruptly interrupting the continuous e-beam excitation, we could measure the free-exciton lifetime in the bulk crystal without interferences from surface recombinations. TRCL then well completes the boron quantification by continuous CL, indicating neither nitrogen or boron is the majority impurity or else pointing out a high compensation situation. The nitrogen concentration could be further evaluated in many situations inaccessible so far, such as in highly compensated diamonds where absorption and magnetic resonance technics are basically limited. With that, we could study and clarify the dependence on growth sectors for the residual impurity uptake during HPHT diamond growth. We found that the central p-type 100 sector is weakly compensated contrary to the 113 and 110 ones, while the lateral 111 growth sectors are n-type.

The proposed method is well adapted for pure crystals: too high concentrations of impurities would lead to exciton lifetimes shorter than the temporal resolution of TRCL. Nitrogen concentration can be deduced in the condition that the other impurities are properly estimated, here thanks to the well-established spectroscopy of bound excitons in diamond. With these conditions fullfilled, high sensitivity can be reached, with nitrogen detection limit pushed forward to the extremely low ppb level, with a local probe of micrometer size typically for the range of impurity concentration investigated.

## Acknowledgments

The authors acknowledge C. Vilar for her help on the SEM characterizations, S. Tarelkin from TISNCM, Moscow, Russia, for providing the IIa crystal and O. Brinza and colleagues from LSPM, Villetaneuse, France for the CVD layer.

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