GaAsP-GaAs nanowire quantum dots for novel quantum emitters

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Semiconductors are able to efficiently convert electrical energy into light; this is the basis of light emitting diodes (LEDs) and semiconductor lasers. Such devices produce classical light, consisting of many trillions of photons every second. However there are applications in quantum computing and cryptography which require non-classical light, for example a regular stream of single photons or entangled photon pairs; two spatially separated photons which form a single quantum system. Such non-classical light can be created by semiconductor quantum dots; semiconductor nanostructures in which the size of the semiconductor in any dimension is no greater than a few 10's nanometres. Electrons trapped within a quantum dot are unable to move; resulting in dramatically different properties compared to conventional bulk semiconductors in which free electron motion is possible. In addition to the production of non-classical light quantum dots can be used to improve the performance of both lasers and solar cells.There are a number of approaches for the formation of quantum dots. The most studied is self-assembly where the dots form spontaneously on a semiconductor surface; this process is driven by the strain that results when the deposited semiconductor has a different atomic spacing to that of the underlying semiconductor. However the spontaneous nature of this process results in the quantum dots having a distribution in their shape and size; no two dots are identical. In addition controlling the position at which the dots form is very difficult. Recently the formation of quantum wires which grow vertically upwards from a semiconductor surface has been demonstrated. Growth of these wires is initiated either by initially depositing tiny metal droplets on the surface or by forming nanoscale holes in an oxide mask. The quantum wires can have lengths in excess of 1um and diameters below 100nm. During the growth of the quantum wire it is possible to change the semiconductor type and hence insert a small disk of a different semiconductor within the quantum wire. This disk forms a quantum dot and it is this new type of quantum dot that forms the subject of our research.These so-called nanowire quantum dots have a number of significant advantages in comparison to self-assembled ones. For example their position can be accurately controlled by placing the hole in the oxide mask at the desired position. There is also much greater control of the quantum dot shape and size; one consequence of this is the possibility to form many closely spaced identical dots within the wire. Such vertical stacking of quantum dots is not possible in the self-assembled system but is advantageous in lasers where a large number of quantum dots are required to achieve sufficient amplification of the light. In addition the nanowire acts as a cavity to confine photons, allowing the fabrication of nanoscale lasers. Nanowire quantum dots is a very immature field and significant growth development complemented by extensive optical and structural characterisation is required to optimise their properties for a range of applications. We will develop the system based on GaAs quantum dots in GaAsP nanowires grown by molecular beam epitaxy on silicon substrates. Growth on silicon is important as it provides the potential for integration with conventional electronics. Structures will be characterised by transmission electron microscopy and optical spectroscopy of single nanostructures. Following optimisation we will develop structures for a number of applications, including sources of single photons and entangled photon pairs, and nanoscale lasers. We will initially develop devices which are excited by light from a laser but a major later aim is to achieve all electrical devices.