Evidence for Primal sp² Defects at the Diamond Surface: Candidates for Electron Trapping and Noise Sources

Abstract

COMMUNICATION (1 of 8)Diamond materials are central to an increasing range of advanced technological demonstrations, from high power electronics to nanoscale quantum bioimaging with unprecedented sensitivity. [1] However, the full exploitation of diamond for these applications is often limited by the uncontrolled nature of the diamond material surface, which suffers from Fermi-level pinning and hosts a significant density of electromagnetic noise sources. [2] These issues occur despite the oxide-free and air-stable nature of the diamond crystal surface, which should be an ideal candidate for functionalization and chemical engineering. In this work, a family of previously unidentified and near-ubiquitous primal surface defects, which are assigned to differently reconstructed surface vacancies, is revealed. The density of these defects is quantified with X-ray absorption spectroscopy, their energy structures are elucidated by ab initio calculations, and their effect on near-surface quantum Many advanced applications of diamond materials are now being limited by unknown surface defects, including in the fields of high power/ frequency electronics and quantum computing and quantum sensing. Of acute interest to diamond researchers worldwide is the loss of quantum coherence in near-surface nitrogen-vacancy (NV) centers and the generation of associated magnetic noise at the diamond surface. Here for the first time is presented the observation of a family of primal diamond surface defects, which is suggested as the leading cause of band-bending and Fermi-pinning phenomena in diamond devices. A combination of density functional theory and synchrotron-based X-ray absorption spectroscopy is used to show that these defects introduce low-lying electronic trap states. The effect of these states is modeled on band-bending into the diamond bulk and it is shown that the properties of the important NV defect centers are affected by these defects. Due to the paramount importance of near-surface NV center properties in a growing number of fields, the density of these defects is further quantified at the surface of a variety of differently-treated device surfaces, consistent with best-practice processing techniques in the literature. The identification and characterization of these defects has wide-ranging implications for diamond devices across many fields.

Spatial mapping of band bending in semiconductor devices using in situ quantum sensors

“…2d), and a reduction in the mean NV − depth from ≈ 23 nm to ≈ 19 nm. These trends are broadly consistent with direct measurements of D sd reported recently [26]. We note that another avenue to reduce D sd is by etching the diamond, as shown in the SI.…”

“…To estimate E z , we first consider the case of commonly used oxygen-terminated diamond. It was recently found that such samples typically host surface defects that introduce an acceptor level into the band-gap, with densities (D sd ) as high as 1 nm −2 [26]. An example of a calculated electric field profile for this scenario (with parameters representative of our implanted samples) is plotted in Fig.…”

“…a, Principle of the experiment, where NV sensors (green dots) probe the electric field associated with surface band bending, here visualised as a distribution of space charge density. b, Calculated electric field profile for a typical (001)-oriented, oxygen-terminated diamond surface, modelled as a layer of surface acceptor defects with a density of states centred at 1.5 eV above the valence band maximum [26] and a surface density D sd = 0.1 nm −2 (see details in SI). The implanted substitutional nitrogen and NV defects are taken to be uniformly distributed over the range d = 0 − 20 nm (i.e.…”

“…Looking forward, our method could be used to validate models of band bending used in semiconductor electronics, or to facilitate the optimisation of surfaces for semiconductor-based quantum technologies, including the improvement of the charge stability and quantum coherence of near-surface NV centres in diamond, which remains an outstanding problem to date [26,32]. Another exciting prospect is to combine electric field mapping with other quantum sensing modalities, e.g.…”

Imaging Graphene Field-Effect Transistors on Diamond Using Nitrogen-Vacancy Microscopy

“…This effect has been investigated previously by varying the surface chemistry of the diamond [38,39] and by electrostatic gating of NV centres [19,40,41]. Here, we suggest that an increasing gate potential populates an acceptor layer at the diamond surface [42], given the low carrier density within the implanted region, and hence increases the band bending across the NV-layer. As the band bending increases, the Fermi level falls below the NV − charge state at greater depths within the diamond.…”