This process can be unreliable for micrometer-scale contact pads: no spikes may form or the penetration depth may be too shallow. An Al “spiking” approach, in which metal pads are deposited and annealed at approximately 350 ☌, produces a random distribution of Al spikes between the Si/Al interface and the δ-doped structure. Several electrical contact techniques are currently in use. These issues underlie the prominent challenge in forming low-resistance, Ohmic contacts with high yield, and require a reexamination of contact technology. Additionally, the Si:P device layer is buried beneath 30 nm of undoped Si, further complicating contact strategies. The atomically thin structure of Si:P differentiates this material from traditional doped Si, introducing contact challenges similar to those in other two-dimensional (2D) systems. In Si:P quantum devices, even dopant diffusion at the atomic scale can substantially alter device operation and performance. For instance, atomically precise devices present both an extremely small contact area (approximately equal to 1 nm thick) and an extremely restrictive thermal budget (ideally ≤250☌) to minimize dopant diffusion and retain the precision nature of the device. While Si:P devices have been demonstrated in the laboratory, fabrication challenges intrinsic to this material make it difficult to produce the yield necessary to truly exploit the system. Fabrication of δ-doped Si:P nanostructures with atomistic precision is the subject of intense ongoing study due to the potential to utilize this system in high-performance electronics and quantum computation.
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