The effect of step-wise surface nitrogen doping in MPECVD grown polycrystalline diamonds

Abstract

Optical centres embedded via nitrogen doping in polycrystalline diamonds (PCDs) are becoming increasingly useful for several wide-area applications including magnetic field sensing. Therefore, investigating the effect of step-wise surface nitrogen doping in PCDs deposited at low pressure is essential. In this study, the influence of a step-wise surface nitrogen doping process on PCDs has been investigated to explain the dominance of neutral charged NV (NV0) centres in PCDs deposited at low pressure in a chemical vapour deposition (CVD) chamber. The surface properties of the films were probed using Raman spectroscopy, Photoluminescence spectroscopy (PL), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS) and Grazing Incidence X-ray Diffractogram (GIXRD). The results obtained from Raman analysis and corroborated by SEM micrographs show the formation of nanocrystalline diamonds (NCDs) with a large quantity of sp2/a-C (amorphous carbon) phases at low nitrogen flow rates on the surface of the doped PCDs. Conversely, increasing nitrogen flow rate ([N/C] > 0.0025) reduced the formation of nano-grains resulting in a decrease in the sp2/a-C contents in the grains and grain boundaries. The increase in the [N/C] ratio and a decrease in the sp2/a-C content at a high surface nitrogen flow rate (10 sccm) enhanced the formation of negatively charged nitrogen-vacancy (NV−) centres. Furthermore, it was shown that the occurrence of stable NV− centres in PCDs deposited at low pressure is also a function of the grain boundary line density: grain boundary line density increases at low surface nitrogen concentration thereby quenching the PL intensity of NV− centres. Samples doped with high surface nitrogen have surface roughness of less than 10 nm. The low surface roughness of highly doped films will optimize the performance of magnetometers that uses these diamond films as NV detectors. Our results give a better understanding of the formation of negatively charged NV centres in PCDs deposited at low pressure.