Researchers Identify Sp Dangling Bonds on H-C(100) Surfaces for Diamond Technologies

Researchers have long recognised the sp dangling bond on diamond surfaces as a crucial factor influencing the performance and fabrication of advanced diamond technologies. Lachlan Oberg, Yi-Ying Sung, and Cedric Weber, alongside Marcus W Doherty, Christopher I Pakes, and et al, from institutions including the Australian National University and La Trobe University, now present a method for identifying this defect with unprecedented precision. Their work addresses a significant hurdle in the field, as deterministic identification of the sp dangling bond is essential for realising the full potential of atomically-precise diamond device fabrication and optimisation. By establishing a comprehensive experimental and theoretical framework for scanning tunnelling spectroscopy (STS)-based characterisation on hydrogen-terminated diamond, this research paves the way for future studies focused on manipulating these dangling bonds and unlocking next-generation quantum technologies.

Diamond sp3 dangling bonds characterised for quantum devices

These defects, formed by the desorption of hydrogen atoms, introduce paramagnetic and charge trapping behaviour that can impede the functionality of quantum sensors and computers relying on near-surface nitrogen-vacancy (NV) centres. Consequently, they developed a method utilising Scanning tunneling spectroscopy (STS) to directly identify these defects, a technique previously underexplored in the literature. Researchers employed a combined approach of experimental STS measurements and first-principles calculations to align theoretical defect electronic structure with observed spectral features. This alignment was complicated by band bending on the diamond surface, a phenomenon arising from boron doping used to achieve appropriate conductivity under ultra-high vacuum conditions. The work establishes a foundation for future tunneling probe studies aimed at modifying dangling bonds, paving the way for improved control over diamond quantum devices.

STS Characterisation of sp3 Defects on Diamond reveals

Experiments employed ultra-high vacuum conditions and boron-doped diamond samples to achieve appropriate conductivity for tunneling measurements. The team then utilised a hydrogen-functionalised scanning tunneling microscope (STM) to probe the electronic structure of the surface. STS measurements were performed by applying a voltage sweep between the STM tip and the diamond surface, measuring the resulting current as a function of energy. The approach enables differentiation between the sp3 defect and other benign surface features, overcoming limitations of high-resolution STM imaging alone. These calculations predicted the presence of localised states within the diamond band gap, which should manifest as distinct peaks in the STS spectra.

However, the team accounted for significant band bending at the diamond surface, a phenomenon arising from poor electric field screening due to boron doping. This band bending complicates the direct correlation between applied bias and surface state energies, requiring careful calibration and analysis. This alignment provides a foundation for future tunneling probe studies aimed at modifying dangling bonds and creating tailored diamond quantum devices.

Sp3 dangling bonds impair diamond quantum coherence

The research addresses a critical issue in diamond technology, where these defects can negatively impact the performance of sensors and quantum computers due to their magnetic and electric properties. The team measured the deleterious effects of these dangling bonds, demonstrating that they reduce nitrogen-vacancy (NV) centre coherence times and cause electric field screening. Fortunately, recent results have shown that hydrogen capping can neutralise these effects, but current methods lack reliability and spatial resolution, operating at a scale of nanometres. STS offers a direct alternative for identification, which this work substantially explores.

Data shows that scalable diamond quantum devices could be fabricated using hydrogen desorption lithography (HDL), a process involving voltage pulses to remove hydrogen termination and create reactive dangling bond clusters. These clusters then act as active sites for chemical adsorption of nitrogen-based gases, enabling the growth of bulk NV centres via chemical vapour deposition. First-principles calculations previously determined the defect electronic structure, but aligning this structure with experimental STS data has been complicated by band bending on diamond surfaces. Diamond STM/STS typically utilises boron doping, resulting in poorly screened electric fields and significant band bending, altering the relationship between applied bias and surface state energies. Careful analysis was therefore required to account for these modifications in the STS spectra. The work employed STS on an as-grown H, C(100):2 × 1 surface, calculating the defect electronic structure using density functional theory, and modelling band bending using electrostatic simulations.

STS reveals diamond defect energy levels

This work addresses a critical need in diamond technology, where the presence of these defects can hinder both performance and scalable fabrication processes. The findings demonstrate that peak positions in dI/dV curves are not reliable indicators of orbital energies in wide bandgap semiconductors like diamond. Electrostatic modelling reveals that band bending, influenced by doping concentration, tip height, and built-in voltage, significantly affects the energy of occupied surface states. However, first-principles calculations can account for these variations, allowing for accurate defect energy determination from STS spectra.

Specifically, the researchers identified two key features in the dI/dV curve: a consistent tunneling onset at approximately +3.0V corresponding to a mid-gap unoccupied orbital, and a negative-bias peak sensitive to dopant concentration, tip height, and built-in voltage, representing an occupied orbital near the valence band maximum. The authors acknowledge that uncertainty in defining the onset of tunneling into the unoccupied orbital introduces ambiguity in determining the absolute tip height. This is primarily due to the asymmetry in band bending at positive and negative biases. Future research could focus on refining the modelling of band bending effects or exploring alternative STS analysis methods to reduce this uncertainty.