At the heart of Toshiba's single photon devices lie tiny semiconductor entities called quantum dots, each measuring just tens of nanometres in diameter and a few nanometres in height. Quantum dots are typically grown by molecular beam epitaxy in a so called self-organising formation mode. Such a method utilises a natural growth progression when a material is grown on the top of another with mismatched lattice constant. These dots nucleate randomly, with the result that their location on the wafer is uncontrolled.
We are working on technology to fabricate quantum dots in controlled locations in device structures. We manipulate the formation of the quantum dots by encouraging their nucleation in pre-defined sites. The nucleation sites are tiny nanoholes manufactured by means of electron beam lithography and dry etching. A wafer substrate patterned with a matrix of nanoholes is then subjected to a complex pre-growth preparation procedure. During growth of the quantum dots the nanoholes attract atoms constituting quantum dots with their low potential energy and increased strain. The figure below is showing randomly nucleated quantum dots (left) and a matrix of positioned dots (right).
In order to study optical properties of such positioned quantum dots a suitable capping material is used to overgrow the wafer.
Our positioned quantum dots are characterised by high emission intensity and narrow linewidths. Moreover, by precise control of the growth parameters we are able to engineer the emission wavelength of the dots. However, the most significant advantage is that such quantum dots can be easily incorporated into the device structure without complex processing, allowing the technology to be scaled to volume production.