Christopher I. Pakes

The nitrogen-vacancy (NV) centre in diamond is a premier solid-state defect for quantum information processing and metrology. An integrated diamond quantum device harnesses the collective properties of multiple NV centres, enabling room-temperature quantum computing and sensing. While large-scale devices are poised to fill an important gap in the burgeoning quantum technology landscape, their practical realisation has not been achieved using current top-down fabrication techniques such as ion implantation. Consequently, this necessitates the development of a bottom-up fabrication technique, which is scalable, deterministic, and possesses atomic-scale precision. Informed by existing methods for fabricating phosphorous defect qubits in silicon, we envision a hydrogen depassivation lithography technique for atomically-precise manufacturing of nitrogen-vacancy centres in diamond. This perspective article outlines a viable multi-step procedure for realising scalable fabrication of diamond quantum devices and identifies the key challenges in its development.

The use of a transition metal catalyzed thermochemical etching method for improving the carrier transport properties of the near-surface two-dimensional (2D) hole gas in surface transfer-doped hydrogen-terminated (111) diamond is demonstrated. Using Ni0.8Cr0.2 films deposited and annealed to a temperature of 900 °C, with up to three etch cycles, preferential (111) surface etching produces large terraces exceeding 10 μm in size with a surface microroughness, σRMS2λ, that is two orders of magnitude lower than for the pre-etched (111) surface. Magnetotransport measurements on hydrogen-terminated Hall bars engineered on the pre- and post-etched surfaces and rendered conductive by the adsorbed water layer formed on exposure to ambient conditions demonstrate that this etching causes an improvement in the hole mobility by an order of magnitude, resulting in a measured sheet resistivity of 1.04 kΩ/sq at a temperature of 4.2 K without gating.

The structure and electron emission properties of scanning tunneling microscope tips electrochemically etched from polycrystalline and recrystallized tungsten wires were investigated using scanning electron microscopy and transmission electron microscopy. Tips etched using the recrystallized wire had single crystal domains larger than those seen in tips etched from the cold drawn wire. The stability of the tips under high electric fields was investigated using field emission. It was found that tips etched from the recrystallized wire tended to have improved stability compared to those etched from the polycrystalline wire and that annealing either type of tip to high temperature in ultra-high vacuum had the greater influence on tip stability.

Sensing and manipulating a particle's spin promise the development of more energy efficient and ultra-fast devices. Diamond is a promising candidate for spintronics and quantum systems, where nano-structuring of diamond surfaces on nanometer length scales is required. Here, direct-write modification of a diamond surface at sub-50 nm resolution is demonstrated. This is achieved with an atomic force microscope tip used as a spatially localized field emission source or so-called field-emission scanning probe lithography on a sample in ambient conditions, which are rendered conductive only through the surface conductivity of hydrogen-terminated diamond. Mask-less direct structuring of the diamond surface is observed, creating features of depth in the range of 4–8 nm and linewidths of 70–150 nm, as well as the desorption of hydrogen from the surface achieving minimum linewidths of 40 nm. Both the linewidth and depth of etching appear to depend on the energy of emitted electrons and the resulting electron exposure dose. These findings demonstrate the possibility of controllably nano-structuring diamond surfaces with features over micrometer length scales and with sub-50 nm resolution, while held under ambient conditions.

Hydrogen-terminated (H-terminated) diamond, when surface transfer doped, can support a sub-surface two-dimensional (2D) hole band that possesses a strong Rashba-type spin–orbit interaction. By incorporating a V₂O₅/Al₂O₃ bilayer gate dielectric in a diamond-based metal–oxide–semiconductor architecture, metallic surface conductivity can be maintained at low temperature, avoiding the carrier freeze out exhibited by devices with an Al₂O₃ gate dielectric alone. Hole densities of up to 2.5 × 10¹³ cm⁻² are achieved by the electrostatic gating of the device, and the spin–orbit interaction strength can be tuned from 3.5 ± 0.5 meV to 8.4 ± 0.5 meV, with a concurrent reduction in the spin coherence length from 40 ± 1 nm to 27 ± 1 nm. The demonstration of a gated device architecture on the H-terminated that avoids the need to cycle the temperature, as is required for ionic liquid gating protocols, opens a pathway to engineering practical devices for the study and application of spin transport in diamond.