The quantum state of a single atom of dopant in a silicon semiconductor has been measured.

Scientists in the Netherlands have successfully probed the electronic and quantum mechanical properties of a single atom of dopant in a silicon transistor. The research demonstrates the feasibility of addressing single atoms held within a silicon lattice and could provide important information necessary for the development of quantum computers.

Sven Rogge and colleagues at the Delft University of Technology used a prototype of an advanced industrial transistor fabricated by colleagues at the InterUniversity Microelectronics Center in Leuven, Belgium, to investigate single atoms of the dopant, arsenic. The issue of dopants is causing the electronics industry a big headache at the moment. Dopants - trace amounts of impurity - are required in silicon transistors to give the semiconductor appropriate electronic properties. When transistors were relatively large, the random dispersion of dopant atoms throughout the material did not present a problem. Now, with the crucial dimensions of the conducting channels within the transistor measuring only a few tens of nanometres, the number of dopant atoms per transistor has become small - a few dozen typically - causing the position and effect of each individual atom to influence how the entire transistor functions. Consequently the industry is finding it difficult to manufacture transistors with identical characteristics.

The flip side of the problem is that it allows physicists an unprecedented opportunity to focus on single dopant atoms within the material. The transistors used by the Delft team consist of silicon nanowires around 35 nm across, where the electrical current flows through a single atom of arsenic. A fine metal contact called a gate is connected to the nanowires; a voltage applied to a gate permits electrons to flow through the discrete arsenic atom.

By applying a voltage to the gate at low temperatures and precisely monitoring the subsequent current the researchers could manipulate and measure the quantum mechanical behaviour of an arsenic atom as first one and then two electrons were introduced into a particular orbital shell of the atom.

’The key point is that we demonstrated that is it possible to address a single atom in this environment,’ Rogge told Chemistry World. ’There is a huge effort under way to try to understand and utilise quantum effects to harness them for quantum computing. With the dopant system we have essentially a single quantum entity that is isolated, and we can address it to modify the wave functions of the atom and observe how it behaves within the silicon nanostructure.’

Jeremy Baumberg, director of the Nanomaterials Facility at the University of Southampton, UK, said   ’The authors show that you can make a device with a single dopant atom. Although it can release an electron, it all remains embedded within the transistor and acts as a stable valley for electrons to rest at. What you can see is the effect of quantum mechanics coming into the behaviour of this transistor - which we normally don’t have to worry about greatly. Electrons can now tunnel through the insulator on either side of this dopant atom, rest there and then tunnel out. Classically they could not get through the device. Hence a current flows quantum mechanically.’

Several challenges remain, however, if such a system is to find its way into quantum computing, said Baumberg. These include the ability to scale it up to room temperature; to place dopant atoms precisely where they are needed; to work with tiny currents of only a few electrons, and to maintain stability of systems for several years.

Lloyd Hollenberg from the University of Melbourne, Australia said that the research represented an important development that would ’add considerable momentum to the revolution that is happening in semiconductor nano-electronics.’

Hollenberg added, ’Such results are important not only to the extreme atom-scale miniaturisation of conventional CMOS [complementary metal oxide semiconductor] systems, but even more so to the exciting proposals for scalable quantum computer architectures based on information encoded on single donor atoms.’

Simon Hadlington