Characterization of Nitrogen Aggregation in CVD-Grown Diamond

Understanding the mechanisms behind incorporation of dopants in chemical vapor deposition (CVD)-grown diamond is paramount to optimizing nitrogen vacancy (NV)-center based quantum devices and those for power electronics. The mechanism of nitrogen incorporation during CVD growth, and the resulting density and spatial distribution of the nitrogen are not well understood. In the context of limited incorporation of dopants such as nitrogen, there is evidence that features on the growth surface of diamond, like step bunching and hillocks, strongly segregate these impurities. A spatially resolved study of dopant distribution around these growth features would provide more insight into the impact on devices and point to strategies to obtain higher densities of dopants. To this end, we have studied nitrogen aggregation in hillocks formed in (100) homoepitaxial diamond grown via plasma-enhanced chemical vapor deposition (PECVD).[1] ¹⁵N enriched nitrogen gas was utilized during growth to form a near-surface, 7 nm-thick delta-doped ¹⁵N layer. Hillock growth has been studied extensively in diamond and has been attributed to a variety of mechanisms, including spiral growth on a screw dislocation core[2], heterogeneous nucleation at foreign crystallite sites[3] and penetration twins[4], as well as repeated 2D nucleation on miscut diamond substrates.[5] Meynell et al confirmed an inverse relationship between hillock density and substrate miscut and also observed nitrogen accumulation in hillock defects which correlated to a higher NV density.[6] Using panchromatic and spectrally resolved cathodoluminescence (CL) mapping, we observed greater NV emission from hillock edges in particular. We utilized nanoscale secondary ion mass spectrometry (nanoSIMS) to spatially resolve ¹⁵N concentration (measured as the ¹²C¹⁵N^- ion) with ~50-70nm resolution, using a 20 pA Cs⁺ ion beam and a mass resolution greater than 9000 to resolve nearby isobaric interferences. NanoSIMS confirmed ¹⁵N aggregation in hillock edges, which we conclude directly correlates to an increase in NV center density. Previous broad-area SIMS measurements identified 2×10¹⁶cm^-3 ¹⁵N density in the uniform regions of the delta-doped layer. By comparing the local ¹²C¹⁵N^- counts per volume in the hillock defect to that of the uniform regions, we identified ¹⁵N density of 1.2×10¹⁸cm^-3 in the hillock defects, a nearly two order of magnitude increase compared to the delta-doped layer. We will show how volumetric SIMS scans using nitrogen as a tracer provide additional insight into the growth mechanism of these surface features. Understanding these can lead to future NV-based device applications that intentionally harness dopant aggregation.<br/><br/>1. Hughes, L. B. et al. Two-dimensional spin systems in PECVD-grown diamond with tunable density and long coherence for enhanced quantum sensing and simulation. APL Mater. 11, (2023).<br/>2. Tokuda, N. Homoepitaxial diamond growth by plasma-enhanced chemical vapor deposition. in <i>Novel aspects of diamond: from growth to applications</i> 1–29 (2014).<br/>3. Van Enckevort, W. J. P. <i>et al.</i> Thermal chemical vapour deposition of homoepitaxial diamond: dependence of surface morphology and defect structure on substrate orientation. <i>Surf. Coatings Technol.</i> <b>47</b>, 39–50 (1991).<br/>4. Tsuno, T <i>et al</i>. Twinning Structure and Growth Hillock on Diamond (001) Epitaxial Film. <i>Jpn. J. Appl. Phys.</i> <b>33</b>, 4039 (1994).<br/>5. Lee, N. & Badzian, A. A study on surface morphologies of (001) homoepitaxial diamond films. <i>Diam. Relat. Mater.</i> <b>6</b>, 130–145 (1997).<br/>6. Meynell, S. A. <i>et al.</i> Engineering quantum-coherent defects: The role of substrate miscut in chemical vapor deposition diamond growth. <i>Appl. Phys. Lett.</i> <b>117</b>, (2020).