Transistors consisting of single atoms or several-atom clusters may lay the framework for future quantum computing and offer unparalleled memory and processing power.
Researchers at the National Institute of Standards and Technology (NIST) and their colleagues at the University of Maryland have developed a step by step procedure to produce such atomic-scale transistors. They are only the second in the world to construct a single-atom transistor and to fabricate a series of single-electron transistors.
NIST’s fabrication steps
- Silicon chip is covered with a layer of hydrogen atoms, which readily binds to silicon.
- Hydrogen atoms are removed at select locations using the fine tip of a scanning tunneling microscope.
- Phosphine gas was then directed at the silicon surface; individual Phosphine molecules got attached in places where there are no hydrogen atoms.
- The Silicon chip is heated to eject the hydrogen atoms from phosphine, causing phosphorus to embed on the surface.
- The phosphorus is sealed with protective layers of silicon at room temperature, and electrical contacts were made.
With additional processing, the embedded phosphorus atoms created the foundation for a series of highly stable single-atom or a cluster of atoms transistors.
In the past, researchers have applied heat as all layers of silicon was grown. Heat burns off all impurities and ensures that the silicon has the pure crystalline silicon structure required to integrate the single-atom transistors with conventional silicon chip components. NIST scientists found that heating could dislodge the phosphorous atoms and disrupt the single-atom transistors. Hence the team deposited the first few layers of silicon at room temperature and ensured the phosphorous atoms stay put.
The team also have developed a novel technique to make electrical contacts with the buried atoms. They gently heated a layer of palladium metal applied at select regions on the silicon surface. Palladium reacted with silicon, forming a conductive palladium silicide and made contact with phosphorous atoms.
According to NIST researcher Richard Silver, this process of sealing the phosphorus atoms with protective layers of silicon and then making electrical contact with the embedded atoms appears to be essential to fabricate many copies of such transistors.
The scientists demonstrated that they could precisely control the rate at which individual electrons flow through a physical gap or electrical barrier in their transistor. Classical physics would forbid the electrons from doing so because they lack enough energy, but this is a strictly quantum phenomenon known as quantum tunneling. Quantum tunneling is fundamental to any quantum device, including the construction of qubits.
The ability to control the flow of one electron at a time is a significant achievement. It opens up a new foundation for Quantum computing.
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