GaAsP-GaAs nanowire quantum dots for novel quantum emitters

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.

The photonics industry is a critical area of the UK and EU economy which is estimated to reach over 600 billion Euros by 2020 and currently employs over 300,000 people directly, with a total impact of around 30 million jobs (figures for the EU). The EU has a number of world leading companies with global market share in some areas as high as 40%. Photonics technology is rapidly evolving with significant current interest in non-classical light sources for quantum cryptography and computing applications and nanoscale lasers for reduced power on-chip and inter-server communications. In addition photonics is at the heart of solutions for a number of key societal challenges including digital security, energy generation, energy efficiency, climate change and healthy aging of the population. Maintaining the competitiveness of the UK and EU photonics industry requires the continuous development of new materials, structures and devices with much of the fundamental research occurring in universities.
Our research will benefit a range of photonics companies, three of which are project partners. In addition it will benefit other academic groups working in the general area of quantum dot physics whose research in turn will have a direct impact on the photonics industry.

There are a range of benefits that will arise from our research. These include:

Development of a fundamentally different quantum dot system with a number of advantageous properties in comparison to the more widely studied self-assembled quantum dots (e.g. better control of shape, size, composition and position, easier integration with Si electronics).

Development of improved non-classical light sources (e.g. sources of single and entangled photon pairs) and ultra-low power nanoscale lasers with direct integration to Si electronics.

Provision of a pool of highly skilled PhD and PDRAs suitable for subsequent employment by relevant photonic companies. We will ensure that staff are exposed to the full range of skills (growth, optical and structural characterisation, device fabrication, development and testing) across all three academic partners.

Benefits to society span a number of areas and include: enhancement of ICT systems for the digital economy and reduced energy consumption in terms of both fabrication and operation. The provision of single and entangled photon sources will find application in digital security. Similar components may find application in quantum computers. Nano-scale lasers grown on Si will allow continued development of computing power by increasing the number and speed of interconnects whilst reducing power consumption. The direct integration of photonics and Si electronics will open up new areas in remote sensing combined with control and processing electronics. Growth on Si substrates will reduce the reliance on scarce III-V materials which are often sourced from countries whose political system may result in future supply issues.