Quantum Dot Architecture Nanodynamics

Lead Research Organisation: Cardiff University
Department Name: School of Physics and Astronomy


Site controlled semiconductor quantum dots (QDs) are the subject of world-wide interest because of their potential use in quantum information technologies and nanoscale optoelectronics. The goal is to create advanced QD architectures by controlling the precise position of single dots or molecules during epitaxial growth. However, a significant limitation in the realisation of site controlled III-V quantum structures is our inability to observe how they form in real-time and hence understand how to precisely tailor their characteristics. This project will utilise a unique electron microscope in Cardiff to obtain the first real-time movies of how QDs actually form in advanced architectures. This will provide unprecedented feedback to the National Centre for III-V Technologies and allow us to fabricate QDs with the best site control and optical properties ever achieved. These optimal structures will be tested optically with a view to commercial exploitation with Hitachi Europe Ltd for the large scale integration of single-photon (SP) sources into quantum photonic circuits and networks.

Planned Impact

The UK is investing £270M in a National Network of Quantum Technology Hubs. It is widely recognised that the miniaturisation of photonic quantum circuits is essential for technological applications. In particular, the integration of on-demand single photon (SP) sources on a chip is considered indispensable for scalable architectures in quantum information processing. However, the key obstacle preventing such large scale integration into quantum photonic circuits and networks is precise site control and narrow optical line-width of individual QDs.

To overcome this major difficulty we will utilise the Cardiff III-V LEEM to see how QDs form in real-time and rapidly establish optimal growth conditions. This unprecedented feedback to the National Centre for III-V Technologies will allow us to fabricate QDs with the best site control and optical properties ever achieved. These optimal structures will be tested optically with a view to commercial exploitation with Hitachi who have the resources and expertise to rapidly exploit the technology generated by this project.

The creation of on-demand SP sources on a chip will be a major breakthrough in the implementation and commercialisation of quantum technology and transform virtually all areas of society. This ranges from the encryption that underpins most internet/electronic data security to pattern recognition algorithms yielding almost real-time national surveillance. The ability to manipulate extremely large data sets has important implications for fundamental studies in physics, astronomy and the life-sciences while using quantum computation to simulate complex systems has relevance to pharmaceutical design, metrology and energy production. Our proposed programme therefore underpins and addresses EPSRC's four priority challenge themes of 'manufacturing the future', 'energy', 'digital economy' and 'healthcare technologies'.

Cardiff University has recently signed an agreement with leading UK semiconductor wafer company IQE to establish an Institute for Compound Semiconductors (ICS) at Cardiff with a £17.3m UK Government award to underpin the foundation. This is a key step in creating the World's first Compound Semiconductor Cluster, spanning III-V technology from basic research to full scale production. This proposal will establish the III-V LEEM in the UK and facilitate interaction with IQE via ICS to address key technological problems including IQE's specific interest in SP sources and the integration of III-V technology with silicon. This significant benefit to UK industry will maximise impact beyond the immediate scope of the proposal.

This project will provide excellent training opportunities for PDRA and PhD students involving exposure to state-of-the art equipment, unavailable elsewhere, and modelling of experimental movies. Travel to Sheffield and Hitachi Cambridge Laboratory will provide experience in MBE growth, lithography and micro photoluminescence and there will be opportunities for exposure to commercial entities such as Hitachi Europe Ltd and IQE who will also provide advanced courses in semiconductor manufacture.

Movies encapsulating dramatic events, such as QD formation, will have wide ranging appeal and form an ideal basis to communicate nanoscience to the general public. This will be achieved in collaboration with spin-out company 'science made simple' who will provide communication training to the research team.

The III-V LEEM system at Cardiff is directly applicable to many areas of III-V research and several visits to UK III-V groups are planned to communicate these capabilities. The project will bring in leading international fabrication expertise (Bracker, USNRL) to the UK and develop new capabilities in SCQD growth. This knowledge base will be used by the National Centre for III-V Technologies to fabricate SCQD structures for the UK community which will have a significant national impact on the implementation of quantum technology.


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Description We have implemented a technique that enables us to measure complex samples from external laboratories. This provides UK epitaxy community to collaborate and apply our unique experimental technique to understand the dynamic behaviour of their thin film samples.

We have developed a new way of simulating the electron microscope image contrast observed in movies of nanostructure formation. This has been extended to simulate 3D quantum structures. In the process of studying nanoscale dynamics we have developed a new imaging method which allows the discrimination of surface phases. This may have important implications for improving crystal growth.

We have provided unique insight on the stability of compound semiconductors. Our technique enables us to observe dynamic processes in GaAs, and we have observed, for the first time, the metastability of the atomic structure of GaAs (001) at temperatures that are relevant to GaAs growth. Metastability has been observed during evaporation and during growth. We have demonstrated growth can be observed by using the diffusion from a Gallium droplet as metal source.

We have developed a method that enable us to observe all the surface phases of a III-As compound semiconductors by combining the use of metallic droplets on the surface of the thin film and the unique imaging capabilities of our Low Energy Electron Microscope.

We have proved that we can transfer samples grown externally and established collaborations with the National Epitaxy Facility.

We have grown quantum dots within the Low Energy Electron Microscope. We are investigating the different sources of contrast in the Low Energy Electron Microscopy movies.

We are currently developing a method that allow us to measure entropy change between two different phases observed in the microscope. Entropy is a critically important thermodynamic parameter, but is elusive to experimental techniques. Low Energy Electron Microscopy of the phase transition and monitoring of the boundary fluctuations enable us to provide an estimate of the entropy change associated to the phase transition.

The work has continued to provide insight on stability of semiconductor surfaces. Surfaces are key for the fabrication of thin films, as any change in the surface of the material will affect the thin film to be grown on top. We are currently comparing the mechanisms that stabilise Silicon surfaces (the most commonly used material in electronic devices) and compound semiconductor surfaces. We have observed growth of GaAs on GaAs and will report on the detailed growth mechanisms.
Exploitation Route Our results demonstrate a way to measure externally grown samples, therefore it enables the UK research community to use the unique capabilities of our Low Energy Electron Microscope to understand nano-dynamics on thin films.

Methods can be used to understand how nanostructures form. The methods we are developing to measure entropy change can be very helpful to understand phase transitions.

Observations of Quantum dot formation using droplet epitaxy has enabled us to provide insight on GaAs surface phases and the chemical potential at which the different surface structures of GaAs 001 are stable. The technique developed can be used to understand new materials and also to provide feedback to theoreticians developing DFT simulations.

The experiments started during the project continue to produce results on the mechanisms stabilising the surface phases in GaAs and its comparison with Silicon and on the growth of GaAs. Understanding the mechanisms stabilising surface phases can used to improve understanding of thin film nucleation and improve predictions of stability in new materials.
Sectors Digital/Communication/Information Technologies (including Software),Electronics,Energy,Manufacturing, including Industrial Biotechology

URL https://research.cardiff.ac.uk/converis/portal/detail/Dataset/40924851?auxfun=&lang=en_GB
Title Selected energy dark-field imaging using low energy electrons for optimal surface phase discrimination 
Description We propose a general strategy for surface phase discrimination by dark-field imaging using low energy electrons, which maximizes contrast using diffraction spots, at selected optimal energies. The method can be automated to produce composite phase maps in real space and study the dynamics of complex phase transformations in real-time. To illustrate the capabilities of the technique, surface phases are mapped in the vicinity of liquid Ga droplets on the technologically important GaAs (001) surface. The data is in .dat format, each file corresponds with a photogram of a movie taken with the Low Energy Electron Microscope. 3 types of data can be found: Photograms from Low Energy Electron Diffraction profiles. In this cases the x/y plane corresponds to coordinates in the reciprocal space taken at given electron energies. Photograms from real space movies taken with the Low Energy Electron Microscope at given electron energies. In this case the x/y plane corresponds to real space coordinates, where the total size of the image varies between 20 microns and 6 microns. Each file is labelled a,b,c,d depending on the diffracted beam we are selecting, being a.diffracted beam from c(8x2), b. diffracted beam from 6x6 pattern, c. diffracted beam from 3x6 pattern, d. diffracted beam from 2x4 pattern. IV curves: Data extracted from the total intensity of a particular diffracted beam of a certain pattern. The Y axis represents the total intensity, and the X-axis represents the energy. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Title Simulation of Mirror Electron Microscopy Caustic Images in Three-Dimensions 
Description Three-dimensional (3D) image simulation methods are applied to interpret mirror electron microscopy (MEM)images obtained from a movie of GaAs droplet epitaxy. The data for the 3D surface height map of the t = 20 min shape is in Fig2a in the format (x, y, height) with units in microns. The equations used to generate this height function are contained in Fig2a_Eqns. The surface profile (a slice through the 3D height map) and equipotential traces of Fig. 2b are contained in Fig2b_Height and Fig2b_Eqp1-5 respectively, and similarly for Fig. 2c. The format is (x, height) with units in microns. Figure 3 gives an example of the Voronoi method. Figure 3a contains a sample grid of positions contained in file Fig3a in the format (x, y) in arbitrary units (e.g. microns). Figure 3b is made up of a list of electron positions in Fig3b_Points in (x, y) format, and the file generated by the software package qHull (obtained from http://www.qhull.org) is in Fig3b_Voronoi. In this 'Voronoi' file, the first line is the number of dimensions (2); the second line is in the format (total number of Voronoi vertices (V) which form the Voronoi regions, the number of positions (P), the number one); the next 'V' lines give the positions of the Voronoi vertices in (x, y) format starting with the position representing infinity; and the remaining 'P' lines describe the Voronoi region for each position, in the format of (number of vertices used to define the region, the label of each vertex used). Similarly, Fig3c_Points and Fig3c_Voronoi are the relevant data files for Figure 3c. Fig4ab is the experimental MEM image data in (red, green, blue) format. Fig4c,d are the atomic force microscope height data, giving surface height values in nm at equally spaced points or pixels on a 128 by 128 grid. The grid separation is 6229.9/127 nm in x and y. Fig4e,f are line traces through the 3D data in the format of (y, height) in nm. (e) corresponds to a line trace in y along the 71st x pixel in (c), and (f) corresponds to a line trace in y along the 69th x pixel in (d). Figure 5 uses the same format and data file structure as Fig. 2 but for the t = 15 min shape. Data files Fig6a and Fig6b contain simulated MEM images in the format (x, y, intensity) for the surfaces in Fig. 5a and Fig.2a, respectively, with distances in nm and the intensity normalised so that '1' corresponds to uniform or background intensity. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
Description University of Bremen - Ruthenium, Graphene, Arsenene and Diamond 
Organisation University of Bremen
Country Germany 
Sector Academic/University 
PI Contribution Cardiff group will make the unique capabilities of the LEEM in Cardiff available to understand As intercalation in graphene and growth of GaAs and As monolayers on Ruthenium.
Collaborator Contribution Cardiff's LEEM group has established a collaboration with Prof. Jens Falta's group. The collaboration encompasses mainly 4 separate projects: 1. CBLEED - The project aims at developing convergent beam low energy electron microscopy experimentally. Initial testing has been done by our previous post-doctoral research associate (currently in MAXIV in Lund, Sweden). Lens configuration is not yet optimal and significant beam damage was observed. Several strategies have been proposed to optimise the lenses, the collaboration consists of testing complementary configurations in Bremen and Cardiff in different material systems with strong strain gradients across the surface (Dislocation network on Ge/Si system, InAs/GaAs system close to critical thickness, GaN surface and GaN wires). David Jesson will provide his experience on the sensitivity to strain of the technique based on his own simulations. This project should lead to preliminary results for funding proposals and at least one publication. 2. Intercalation of As on Ruthenium - Prof. Jens Falta's group has strong experience on preparing graphene on Ruthenium via high temperature annealing of the samples. Cardiff's LEEM has the unique capability of enabling imaging under high As flux. The goal of this project is to observe the behaviour of As atoms on Graphene on Ruthenium and study possible intercalation of Arsenic between Graphene monolayers. 3. Growth of As or GaAs monolayers on Ruthenium: This experiment is complementary to the previous one. It is based on the study of the growth dynamics of Arsenene or monolayer GaAs on Ruthenium. Both experiments take advantage of the strength of Bremen's group preparing Ruthenium substrates and the unique capabilities of Cardiff's system. We expect these experiments to provide preliminary results for future funding proposals and at least one publication in common. 4. Study of burning Diamond thin films and graphitization of Diamond: The experiment takes advantage of the Cardiff's experience in diamond thin film growth (Prof. Oliver Williams) and the compatibility of two LEEM systems with unique characteristics in Cardiff and Bremen. The project aims at observing the dynamics of the phase transformation of diamond into graphite and the reaction of diamond with oxygen at high temperatures. Bremen's LEEM has been chosen to perform the experiments due to the capability to expose the sample to oxygen flux, which reduces the complications associated to Carbon contamination. Funding from the Cardiff-Bremen alliance will be sought for in order for Dr. J. Pereiro to travel to Bremen to carry out the experiment. Two publications in common are expected from this experiment. Experiments 1 and 4 will be scheduled within the next month.
Impact Not outcomes yet. Several common publications and funding proposals are expected.
Start Year 2020
Description Innovation in Isolation - Dissemination talk 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Dissemination talk through youtube live.
Year(s) Of Engagement Activity 2020
URL https://www.youtube.com/channel/UCXuEKKD55k05QVrn1rbrFlA
Description Press release: A press release for one of our publications was written by Cardiff University press office and distributed by several internet outlets 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact The initial press release and websites that distributed the news are listed below. A couple of outlets for scientific dissemination has contacted the group to write a piece on our instrument.

Year(s) Of Engagement Activity 2019