# Substrate influence on MPCVD boron-doped homoepitaxial diamond

Nada Habka, Marie-amandine Pinault, Christine Mer, François Jomard, Julien Barjon, Milos Nesladek, Philippe Bergonzo

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Nada Habka, Marie-amandine Pinault, Christine Mer, François Jomard, Julien Barjon, et al. Substrate influence on MPCVD boron-doped homoepitaxial diamond. Physica Status Solidi A (applications and materials science), 2008, 205 (9), pp.2169-2172. 10.1002/pssa.200879723. cea-05460076v3

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Substrate influence on MPCVD boron-doped homoepitaxial diamond

Nada Habka*,¹,², Marie-Amandine Pinault*,¹, Christine Mer², François Jomard¹, Julien Barjon¹, Milos Nesladek², and Philippe Bergonzo²

¹ Groupe d'Etude de la Matière Condensée (GEMaC), Université Versailles St Quentin (UVSQ), CNRS, 1 place Aristide Briand, 92195 Meudon, France

² CEA-LIST, Diamond Sensor Laboratory, CEA/Saclay, 91191 Gif sur Yvette, France

* Corresponding author; e-mail nada.habka@cnrs-bellevue.fr, Phone: +33 145 075 431, Fax: +33 145 075 841

** e-mail marie-amandine.pinault@cnrs-bellevue.fr, Phone: +33 145 075 197, Fax: +33 145 075 841

Abstract

We report a study of boron-doped diamond epilayers grown on (100) HPHT diamond substrates by microwave plasma chemical vapor deposition (MPCVD) in simultaneous or successive runs. The structural and crystalline quality of the samples have been investigated by several techniques. The profiles performed by secondary ion mass spectrometry (SIMS) show the homogeneity in depth of the boron concentration in all layers. Cathodoluminescence (CL) spectroscopy indicates state-of-the-art epilayers. The substrate misorientation angles have been investigated by X-ray diffraction (XRD). Confocal micro-Raman mapping of the full width at half-maximum of the 1332 cm⁻¹ diamond line was also preformed on all substrates. It evidences crystalline inhomogeneities that are known to be growth sectors. We show a relation between the epilayer surface morphology, observed by Nomarski optical microscopy, and the substrate crystallinity.

**1 Introduction**

Boron-doped diamond films are attracting much interest because of their specific physical properties. Indeed boron is the p-type dopant that efficiently controls the electrical conductivity in diamond [1]. Recent studies point out the possible conversion from p- to n-type [2]. However, such conversion seems to be strongly dependent on the quality of the boron-doped layers. To improve the doping in diamond, it is necessary to have a better understanding of the parameters that influence the resulting quality of the boron-doped epilayers.

Many growth parameters (rates [3], techniques [4], etc.) were studied in order to evaluate their effect on the boron-doped diamond epilayers. For example, such layers were successfully grown using different boron precursors: diborane (B₂H₆) [5], boron alkoxide [4, 6, 7] or trimethylboron [8]. However, trimethylboron (TMB) is the most suitable precursor to grow boron-doped diamond because, unlike the two other precursors, it has a low toxicity and contains no oxygen.

Recently, the substrate characteristics are being investigated. Takeuchi et al. [9] and Ri et al. [10] have shown a dependence between the substrate misorientation angle and the layer surface morphology. Moreover, a preliminary study tends to show that the substrate homogeneity directly influences the conversion from n- to p-type of diamond [11]. In this work, we evaluate the effect of the substrate misorientation angle, quality and homogeneity (defined by the absence of growth sectors) on the surface characteristics of boron-doped diamond epilayers.

**2 Experimental procedures**

Boron-doped diamond epilayers were grown on Ib (100) HPHT 3 × 3 × 0.5 mm³ substrates by microwave-plasma-enhanced chemical vapor deposition (MPCVD). The substrates were exposed to a H₂ plasma treatment for 20 min before epitaxy. A mixture of methane (CH₄), hydrogen (H₂) and trimethylboron (TMB) gases was used during growth. The growth temperature (Tgr), the pressure (Pgr), the microwave power (PMW) and the ratios of CH₄/H₂ and TMB/CH₄ were fixed at 925 °C (±5 °C),
100 mbar, 600 W, 0.3% and 1.25, respectively. Some of the samples were grown in successive runs (substrates exposed one by one to the plasma), others in simultaneous runs (a maximum of 4 substrates exposed at the same time to the plasma). The substrate misorientation angles, θ<sub>off</sub>, were determined by X-ray diffraction (XRD). A six-axis diffractometer equipped with a (111)-plane Ge monochromator was used. A slit at the exit of the X-ray beam allows selection of only the Cu K<sub>α1</sub> line. Confocal micro-Raman (μ-Raman) mapping of the 1332 cm<sup>-1</sup> diamond line full width at half-maximum (FWHM) was performed on all the substrates before growth. The μ-Raman mappings were recorded in the backscattering geometry using a Jobin-Yvon® (Horiba group) confocal microRaman HR800. Excitation was performed with the 632.81 nm line of a He-Ne laser. The laser beam was focused to 50 μm under the substrate surface with a ×50 objective (spot size ~1 μm and power ~10 mW). By using a 1800 st/mm grating and a confocal hole of 50 μm, the spectral resolution was 0.5 cm<sup>-1</sup> in the range of the diamond peak. The Raman mappings were recorded at a controlled temperature (20 °C). The recording duration was ~4 h for each mapping. Under these conditions, the FWHM of the diamond line depends only on the substrate characteristics and the uncertainty on its values is ±0.05 cm<sup>-1</sup>.

After growth, the surface morphology and the crystalline quality of the boron-doped layers were investigated by Nomarski optical microscopy and cathodoluminescence (CL) spectroscopy. For the CL measurements, the samples were cooled to 103 K and excited by a 10 keV – 7 nA electron beam (~800 nm of penetration length) in a scanning electron microscope. An achromatic and UV-optimized Horiba Jobin Yvon SAS system was used to analyze the CL emission in the 200–1000 nm range. The light was collected with a parabolic mirror and injected with metal optics into a TRIAX550 monochromator equipped with a UV enhanced-silicon CCD camera.

Secondary-ion mass spectrometry (SIMS) using a Cameca IMS 4f equipment was performed on the samples to measure the depth distribution of boron, nitrogen, hydrogen atoms in the diamond films with Cs<sup>+</sup> primary ions accelerated at 10 keV. The concentration of boron was quantified using implanted standards. The hydrogen and nitrogen concentrations are at the SIMS detection limit. The detection limit of boron is obtained using the distribution tail of its ion-implantation standard profiles : ~5 × 10<sup>15</sup> at/cm<sup>3</sup>.

**3 Results and discussion**

Figure 1a shows the SIMS profiles of three boron-doped diamond layers grown in a simultaneous run. All the layers are identical: similar thicknesses (1.5 µm) and the same homogeneous doping levels in depth (2 × 10<sup>18</sup> B cm<sup>-3</sup>). Moreover, their CL spectra in the free-exciton (FE) region are also identical (see Fig. 1b). They exhibit typical transitions of boron bound excitons (BE(B)) that correspond to the state-of-the-art of the optical quality for boron-doped diamond epilayers [1]. All the samples grown in this study exhibit these characteristics of doping homogeneity and optical quality. The SIMS and CL measurements done on each sample show that the boron concentration of each layer is homogeneous over all the layer. We show here that the presence of growth sectors does not modify the layer specificities. The characteristics of the layers are reproducible independently of the substrate used. Boron concentrations vary from one sample to another in the range of 2 × 10<sup>18</sup> to 2 × 10<sup>19</sup> at cm<sup>-3</sup>. All samples are thicker than 200 nm, which is the critical thickness to discuss the morphology [12, 13].
<figure><img src="image_2.png" /><figcaption>Figure 1. (a) Boron concentration profiles obtained by SIMS; (b) CL spectra exhibiting the free exciton (FETO) and the boron bound excitons (BE(B)) of 3 boron-doped diamond layers grown in simultaneous runs. The data superimpose perfectly, showing that the 3 epilayers are identical (thickness, boron concentration and optical quality)</figcaption></figure>

In Fig. 2, typical surface features of two samples grown in a simultaneous run and one sample grown in a successive run are imaged by Nomarski optical microscopy. In Fig. 2a, the surface is free (F) of pyramidal hillocks (PHs) and unepitaxial crystallites (UCs). The surface in Fig. 2b shows PHs, while we can observe UCs with some PHs on the sample of Fig. 2c. Moreover, these features are distributed randomly on the sample surface and not specifically on growth sectors. Even if the epilayers are grown in a simultaneous run, their surface morphologies show differences. As the growth conditions and the growth sectors can not be responsible for this dissimilarity, we suspect the substrate as the key parameter that determines the surface morphology of the boron-doped diamond epilayers.

<figure><img src="image_3.png" /><figcaption>Figure 2. Nomarski optical microscopy images of samples a) 3a, b) 2b and c) 2a as reported in Table 1. Their surfaces exhibit different features: (a) surface free (F) of pyramidal hillocks (PHs) and unepitaxial crystallites (UCs), (b) with PHs and (c) UCs with some PHs</figcaption></figure>
Confocal μ-Raman mappings of the 1332 cm⁻¹ diamond line FWHM are shown in Fig. 3 for two representative substrates. The first one is perfectly homogeneous with no growth sector (see Fig. 3a) with a good crystalline quality as deduced from the low FWHM mean value (⟨FWHM⟩ = 1.72 cm⁻¹). The other one presents growth sectors (see Fig. 3b) that are identified by the variation of the FWHM. Moreover, its crystalline quality is lower than the previous substrate because its FWHM mean value is higher (⟨FWHM⟩ = 2.06 cm⁻¹).

<figure><img src="image_4.png" /></figure>

Figure 3. Typical confocal μ-Raman mappings of the 1332 cm⁻¹ diamond line full width at half-maximum (FWHM) of (a) a homogeneous substrate (no growth sector) with a low FWHM mean value (⟨FWHM⟩) and (b) an in-homogeneous substrate (presence of growth sectors) with lower crystalline quality (high ⟨FWHM⟩).

In a previous work, Takeuchi et al. [9] has established the influence of the CH₄/H₂ ratios and the misorientation angle, θ<sub>off</sub>, on the surface morphology of boron-doped diamond. For the CH₄/H₂ ratio used in our study (0.3%), two types of morphology are expected. The epilayer surface should present UCs and PHs for θ<sub>off</sub> < 1°. For θ<sub>off</sub> > 1°, the density of these defects decreases and it is even possible to obtain a surface free from defects.

Even though these observations were made for growth conditions (T<sub>gr</sub> = 800 °C, P<sub>gr</sub> = 33 mbar, PMW = 750 W) different from this study, we see similar morphology variations (see Table 1). For θ<sub>off</sub> < 1° (samples 1a, 1b and 2a), all the epilayer surfaces contain defects (UCs and PHs). For θ<sub>off</sub> > 1°, the surface contains more or less only PHs. These results are in good agreement with the observations of Takeuchi et al. From the work of Tallaire et al. [14], the absence of UCs at the layer surface, for θ<sub>off</sub> > 1°, is explained by the high lateral growth rate for such misorientations. The UCs are then buried in the layer.
<table><thead><tr><th rowspan="2">sample</th><th colspan="2">FWHM confocal μ-Raman mapping of the substrate</th><th rowspan="2">substrate (± 0.05°)</th><th rowspan="2">θ<sub>off</sub></th><th rowspan="2">layer surface morphology</th></tr><tr><th>⟨FWHM⟩ (cm<sup>-1</sup>)</th><th>Substrate homogeneity</th></tr></thead><tbody><tr><td colspan="6"><strong>θ<sub>off</sub> &lt; 1°</strong></td></tr><tr><td>1a</td><td>1.85</td><td>Yes</td><td>0.67</td><td>UCs + PHs</td><td></td></tr><tr><td>1b</td><td>2.19</td><td>No</td><td>0.36</td><td>UCs + PHs</td><td></td></tr><tr><td>2a</td><td>2.01</td><td>No</td><td>0.27</td><td>UCs + PHs*</td><td></td></tr><tr><td colspan="6"><strong>θ<sub>off</sub> &gt; 1°</strong></td></tr><tr><td>3a</td><td>1.72</td><td>Yes</td><td>1.44</td><td>F</td><td></td></tr><tr><td>3b</td><td>1.84</td><td>No</td><td>3.53</td><td>PHs*</td><td></td></tr><tr><td>2b</td><td>1.87</td><td>No</td><td>1.01</td><td>PHs*</td><td></td></tr><tr><td>3c</td><td>2.06</td><td>No</td><td>1.77</td><td>PHs**</td><td></td></tr><tr><td>2c</td><td>1.93</td><td>No</td><td>4.29</td><td>PHs**</td><td></td></tr></tbody></table>

Table 1. Specific characteristics of the samples: substrate Raman FWHM mean value, substrate homogeneity (absence of growth sectors), substrate misorientation-angle (θ<sub>off</sub>), surface features of the layer (pyramidal hillocks, PHs; unepitaxial crystallites, UCs; or free of defects, F). The index (*) specifies the increase of the defect density (PHs and UCs). Samples grown in the same run are those with the same number in the sample name.

For θ<sub>off</sub> > 1°, the samples present different PH densities. Indeed, the surface degradation is evaluated by this defect density. The index (*) in Table 1 indicates that the layer contains PH defects but could still be used for device fabrication, while the index (**) implicates that the surface is unexploitable because of the high defect density.

Table 1 shows that no relation exists between the θ<sub>off</sub> variation and the surface degradation. However, some correlations can be deduced from the FWHM confocal μ-Raman mappings. We observe that the surface of sample 3a is free of defects, while the layer has been grown on a homogeneous substrate (without growth sector) with the lowest ⟨FWHM⟩, whereas PHs are present in the other samples (3b and c for example) grown on substrates with growth sectors. Moreover, the PHs density increases for high ⟨FWHM⟩. Consequently, for θ<sub>off</sub> > 1°, the boron-doped epilayer surface morphology is controlled by two substrate parameters: its homogeneity and its crystalline quality. We should note that, as mentioned in Ref. [14], Tallaire et al. show that the origin of the PHs is an extended defect from dislocations formed at the layer/substrate interface. A study of the surface state of our substrate is currently ongoing. The aim of this study will be to correlate the formation of PHs to the surface state of the substrate and to relate this surface state to the crystalline quality of the substrate as deduced by Raman mappings.

## 4 Conclusion

We have shown that the substrate misorientation angle, θ<sub>off</sub>, is not the only parameter that influences the surface morphology of the boron-doped diamond epilayers. For θ<sub>off</sub> > 1°, the crystalline quality of the substrates controls the formation of defects at the layer surface. We deduce then that homogeneous diamond substrates with good crystalline quality (low ⟨FWHM⟩) are a crucial parameter in the growth of boron-doped diamond epilayers free from surface defects.

## Acknowledgements

This work is supported by the DOPEN ANR-06-BLAN-0339-01 program.
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