Nitrogen-Terminated Polycrystalline Diamond Surfaces by Microwave Chemical Vapor Deposition: Thermal Stability, Chemical States, and Electronic Structure

Abstract

In this work, we investigate the interaction of microwave (MW) N2 plasma with polycrystalline diamond surfaces. Optical emission spectroscopy (OES) was used to identify different species present in the plasma. The chemical states were investigated by X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS), and the electronic structure was investigated by ultraviolet photoelectron spectroscopy (UPS). OES showed that the MW­(N2) plasma contains mainly radical species. The nitrogen coverage is estimated to be lower than the diamond average areal density. XPS analysis showed that the nitrogen may have occupied a number of bonding configurations, while upon annealing to 800 °C, some of the incorporated nitrogen onto the diamond was stable. From the HREELS study, the N–H surface bond was stable only up to ∼600 °C. The hydrogen-terminated diamond exhibits negative electron affinity, whereas, there was an increase in the electron affinity upon the nitridation process. Even though the nitrogen concentration decreased upon annealing to 800 °C, the low energy cutoff in the electron distribution in the UP spectrum remained constant.

Atomistic Insight into Nitrogen-Terminated Diamond(001) Surfaces by the Adsorption of N, NH, and NH2: A Density Functional Theory Study

“…By contrast, radio frequency N 2 plasma (∼ 40 kHz) were found by high-resolution electron energy loss spectroscopy (HREELS) to generate multiple surface species of C–N, CN, and CN on diamond(001), demonstrating the tunability of surface doping by varying plasma frequency. Similar results were obtained on polycrystalline diamond samples, which further suggested the involvement of surface hydrogen. , …”

“…The formation of N dimers was proposed by Stacey et al, in which the saturation coverage is 1 ML. However, this value disagrees with most experimental results of ∼0.5 ML. − Our model successfully reconciles this discrepancy.…”

“…For each case, we identify the favored adsorption geometry by total energy calculations and evaluate the nature of surface bonding by analyzing the density of states (DOS) and the corresponding charge distribution. Further calculations of the vibration of the adsorbed nitrogen species and the 1s core-level shift of surface carbons are complementary to the recent works that used HREELS and XPS, − calling for re-interpretation of some spectrum features in the literature. − Lastly, we present simulated images of adsorbed nitrogen species on diamond(001), in preparation for future work using scanning tunneling microscopy (STM). Our results highlight the flexibility of the nitridation of diamond surfaces, offering opportunities of controlling both the number and charge state of the shallow NV centers in the layer-by-layer construction of diamond-based devices.…”

“…It is speculated that this mode might be hidden in the shoulder of 54 meV (CC bounce mode) in the HREELS spectrum. 25 Last, we discuss the vibration modes involving both adsorbate and the underlying substrate. For N(ad), Ndimer(ad), and NH(ad), this mode is best described as a "ring breath mode" because a close ring is formed upon the adsorption at the bridge site of the surface carbon dimers.…”

“…Similar results were obtained on polycrystalline diamond samples, which further suggested the involvement of surface hydrogen. 24,25 Despite the valuable insight gained from the prior works, 23−26 there remains some ambiguity around the nitridated diamond surfaces, such as the composite, structure, and chemical state of the adsorbed nitrogen species. Here, we present DFT studies on the adsorption of representative nitrogen species (N, NH, and NH 2 ) on bare and hydrogenated diamond(001) surfaces, aiming to shed light on the atomistic description of the nitridated diamond surfaces.…”