Next-generation electronics architectures such as spintronics and quantum computing will rely on precise and efficient structures just nanometers in size. At such scales, however, device performance is severely limited by defects at the interfaces between the many different materials that typically comprise these complex structures. Researchers from the University of New South Wales in Australia, in collaboration with colleagues at the University of Wisconsin-Madison in the USA, have now succeeded in fabricating a complete quantum electronics device from a single crystal of silicon.1

Controlling the flow of electrons — the basic principle of electronics — is remarkably difficult to achieve using a single homogeneous material because there are no restrictions on the directions that charge carriers can move. Michelle Simmons and her colleagues were able to produce planar electron-confining structures in a single crystal of silicon by defining quantum dot patterns using a combination of phosphorus 'doping' and scanning tunneling microscopy.

The researchers took a single-crystal silicon substrate and traced out the structures of their quantum dots using the tip of a scanning tunneling microscope in an ultrahigh-vacuum chamber. By applying a voltage to the tip, hydrogen atoms uniformly adsorbed to the silicon surface could be desorbed with atomic precision. Exposing the patterned substrate to phosphine gas at elevated temperature then caused phosphorus atoms to be incorporated into the silicon crystal lattice wherever the hydrogen had been removed in a process known as doping. “In this way we were able to replace, for example, seven silicon atoms in the crystal with phosphorus atoms,” explains Simmons.

Phosphorus doping increases the electrical conductivity of a semiconductor such as silicon because each phosphorus atom donates one unbound electron to the silicon crystal. This allows a current to flow in the phosphorus-doped regions but, at low enough temperatures, not elsewhere.

Fig. 1: An atomic-scale image of a hydrogen-terminated silicon surface where the hydrogen has been removed using a scanning tunneling microscope tip to form the bright regions. The atomically precise pattern formed is then doped with phosphorus to form a quantum dot less than 10 nm in size with phosphorus-doped source–drain leads.© 2010 M. Fuechsle

Simmons and her team created a quantum dot less than 10 nm across (Fig. 1) coupled with four connections. The two gates connections allow the electrostatic potential of the quantum dot to be controlled, which in turn controls the flow of current via the other two leads. This nanoscale structure was connected to macroscopic wires using conventional semiconductor processing techniques. “For us, this is very exciting as it proves that we can fabricate devices in silicon with atomic precision,” says Simmons. “This is a critical step toward developing a silicon-based quantum computer where we use just one phosphorus atom as the quantum bit, or qubit.”