Toshiba Research Europe Ltd., Cambridge Research Laboratory

Quantum Information Group, Quantum Dots

At the heart of Toshiba's single photon devices lie tiny disks of a semiconductor called quantum dots, each measuring just tens of nanometers in diameter and a few nanometers in height. An image of a layer of quantum dots recorded in an Atomic Force Microscope is shown in the figure.

Restricted to such short length scales, the electrons inside the dots display 'quantum' properties not found in larger volumes of semiconductor. Compared to bulk semiconductors, quantum dots can hold only a limited number of electrons. Consequently, they are the natural way of handling single electrons and, in the case of opto-electronic devices, single photons.

In order to display these novel properties, quantum dots must be just tens of nanometres in extent. This can be achieved using a so-called 'self-organising' formation method. It involves growing a highly strained layer during the growth of the device by molecular beam epitaxy (a standard technique for growing high purity layers of semiconductor by depositing one atomic layer at a time).

Initially the strained layer forms a patchy film on the growth surface called the wetting layer, as can be seen in the images in the figure. However, beyond some critical thickness this film breaks up into islands on the growth surface in order to minimise the strain. As more of the strained material is deposited, the islands ripen to an optimum size determined by the strain and growth kinetics. After overgrowth with a suitable capping material, the islands form a layer of quantum dots within the semiconductor crystal.

Quantum dots formed in this way are relatively small; just tens of nanometres wide and a few nanometres tall, have a high density, and are relatively free of defects, which impair device performance. Perhaps the most significant advantage, however, is that such a layer of quantum dots can be easily incorporated into the device structure without complex processing, allowing the technology to be scaled to volume production.

Further Reading

	Type	Title and author(s)	Source
1	Technical	Self assembled quantum dots as a source of single photons and photon pairs by A J Shields et al	Physica Status Solidi (b) 238, 353 (2003)
2	Technical review	Generation of single photons using semiconductor quantum dots by A J Shields et al	Nano-physics and Bio-electronics: A New Odyssey (Elsevier, Amsterdam, 2002)
3	Intermediate	Quantum dots detect single photons by A J Shields and M Pepper	Photonik (February 2001)