Semiconductor Physics Group
PhD Projects in the Semiconductor Physics Group
Last updated 1st June 2010
Professor DA Ritchie, Dr CHW Barnes, Dr HE Beere, Dr M Buitelaar, Dr CJB Ford, Dr GAC Jones and Professor CG Smith.
See here for web pages for some individual projects.
Details of graduate admissions for the whole Physics Department are available at http://www.phy.cam.ac.uk/admissions/graduate/.
Prospective PhD students should e-mail admissions@phy.cam.ac.uk for details and application forms.
Introduction
The Semiconductor Physics Group investigates fundamental quantum transport phenomena using advanced and often unique, semiconductor nanostructures. The fabrication of devices also gives rise to challenging projects in applied physics and technology.
The group has extensive clean-room facilities with all necessary lithographic and deposition facilities for advanced semiconductor device fabrication. These include state-of-the-art molecular beam epitaxy and electron beam lithography systems. The group possesses a wide range of measurement facilities including several dilution refrigerators for experiments down to 20 mK in high magnetic fields, and a range of optical and microwave equipment.
Electron-hole interactions in very high quality semiconductor structures
(Prof DA Ritchie)
We have recently developed techniques to fabricate novel devices where very high mobility two-dimensional electron and hole gases are separated by barriers as thin as 10nm with very low current leakage between the two. These devices have recently been used to measure the drag between the two gases at temperatures below 0.5K. As the temperature falls, the drag increases, in some cases it reaches a peak and may even reverse sign. The explanation for this behaviour is as yet unknown but may involve; the formation of excitons (with possible transition to a superfluid state), a Wigner crystal or charge density waves. There is a considerable amount of work to do to investigate these phenomena and build up a phase diagram, varying parameters such as the carrier densities and inter-layer spacing as well as studying the effect of the application of magnetic fields. In addition we will fabricate coupled quantum wires and dots and use them to investigate these interactions in different dimensionalities.
The development and applications of terahertz sources
(Prof DA Ritchie, Dr HE Beere)
We are developing and exploiting high-power sources of coherent terahertz (1 THz = 1012 Hz) radiation. Such sources are potentially of considerable importance for imaging (e.g. in the medical, dental, and pharmaceutical industries) and spectroscopy (e.g. for environmental monitoring, or screening of biological fluids such as blood). Until recently, this significant region of the electromagnetic spectrum has not been exploited owing to the severely limited number of sources previously available, which were bulky, expensive, inefficient, frequently incoherent, and not at all suited to the potential applications. Research topics include: pulsed and continuous (cw) THz generation; mechanisms for visible-to-THz conversion; THz quantum cascade lasers; the fabrication of photonic-band-gap crystals; interactions of THz radiation with organic and inorganic materials. In particular it has been shown that pharmaceutical products can be non-invasively investigated and in the terahertz regime clear differences between healthy and cancerous tissue is observed. CASE awards with TeraView Limited may be available.
The growth of semiconductor nanowires and their incorporation into novel device structures.
(Prof DA Ritchie)
Recent work has established the possibility of growing self-assembled nanowire structures by molecular beam epitaxy, with a core and outer shell of different materials. This project would involve developing growth techniques for structures of this type as well as using electron microscopy, electrical and optical measurements to characterize the wires. Ultimately the aim is to incorporate these wires into novel device structures for a range of applications.
Single and entangled pair photon sources fabricated from InAs quantum dots
(Prof DA Ritchie)
When InAs, is grown on GaAs by molecular beam epitaxy, past a critical thickness, in this case around 1.6 monolayers, the InAs layer self-assembles into quantum dots typically of diameter 25 nm and height 8 nm. InAs dots form near ideal optical emitters with a very narrow linewidth and we are using them to fabricate novel sources emitting over a range of wavelengths. These sources, driven by either optical or electrical excitation have applications in quantum computing and quantum key distribution systems. This work is carried out in a close collaboration with the quantum information group of Toshiba Research Europe Ltd based at the Cambridge Science Park, CASE awards may be available for projects in this area.
Quantum computation using variable g-value systems
(Professor DA Ritchie, Dr CHW Barnes)
As part of a collaborative project with Toshiba Research Europe we have developed devices where by varying the composition the Lande g-value, which determines the spin splitting, will vary with the position of the electron. If coupled dots are formed then the location of the electrons will be determined by the nature of the hybrid state and this can be read off by a “detector” which is a device sensitive to a charge of less than one electron. This opens the way to a number of concepts for quantum computation, as well as “spintronics” in which the transport properties depend on the spin.
Quantum computation with surface acoustic waves
(Dr CJB Ford and Dr CHW Barnes)
We have proposed a solid-state quantum computer based on the transport of individual electrons in the minima of a surface acoustic wave in a set of parallel narrow channels. These projects aim to construct some of the building blocks of such a computer, namely a number of quantum dots travelling along adjacent channels, together with electrostatic gates and nanoscale magnetic structures to initialise, control, entangle and read out the spins of the electrons. This should reveal a considerable amount of interesting physics as well as establishing the feasibility of such a quantum-computational scheme.
Wave-function manipulation and Schrödinger-cat states in moving quantum dots
(Dr CHW Barnes and Dr CJB Ford)
When a quantum box suddenly changes size, an electron in the ground state finds itself in a superposition of excited states of the new system. To cause a fast enough change (in tens of picoseconds) we drag an electron along a narrow channel in a minimum of a surface acoustic wave. Where the channel width changes abruptly, excitation occurs, and the electron’s wave-function starts to evolve in time, oscillating from side to side. This can be detected by measuring oscillations in the small current that tunnels out of a thin barrier forming one side of the channel. This project aims to tailor the channel shape to create particular excited states called coherent states and to detect their evolution. Controlling interactions with a few other electrons in a neighbouring channel will help us try to answer the fundamental question about how the act of measurement causes a wave-function to collapse. Superposition (Schrödinger-cat) states will also be created and measured. Then pairs of electrons will be entangled, separated and measured, giving further insights into the fundamentals of quantum measurement.
Spin and interaction effects in a quantum antidot in the quantum Hall regime (*)
(Dr CJB Ford)
The two-dimensional electron gas formed in a GaAs-AlGaAs heterostructure is a very fruitful system for investigating quantum and interaction effects. At high magnetic fields B, a series of “edge states” forms, with current travelling in opposite directions along the two sides of the sample. If a potential hill is formed in the centre of the sample, edge states may be backscattered to the opposite edge, via circulating edge states that form around this “antidot”. Thus the resistance of the sample can be used to measure the properties of the antidot. The circulating states are quantised, each enclosing one more unit of flux h/e than its neighbour, and so the resistance oscillates with B, with flux period h/e (the Aharonov-Bohm effect). We have shown that the magnetic confinement of electrons around the antidot causes it to behave much like a quantum dot, showing single-electron effects (Coulomb blockade). Very recently, we have injected electrons of up or down spin, and found, surprisingly, that the antidot states are mixed by interactions such that spin and charge excitations have very different energies. These results leave many questions unanswered, and there is now plenty of scope to investigate many-body, spin and charging effects in more detail and in other regimes. With our high-resolution lithography we will also be able to combine antidots, charge detectors and spin filters with ease.
Three-Dimensional Electron-Beam Lithography
(Dr GAC Jones)
High resolution electron-beam lithography is one of the key technologies used to fabricate micro- and nano-scale structures for a range of applications including semiconductor devices, nano-magnetism, optical gratings, MEMS etc. These applications normally require binary lithography i.e. patterns delineated in either “black” or “white”, and currently we use one of the most advanced e-beam lithography systems to carry out such work.
In this project we wish develop our e-beam technology into the field of 3D patterning, where the height of the patterned media is accurately controlled in addition to the usual 2D x-y profile. This capability has many applications in the micro/nano size regime, not only in semiconductor device fabrication, but for physical optics (surface emitting lens structures, phase plates and holographic kinoforms), security tag technology, direct patterning of functional materials etc. The ability to pattern such 3D structures will be particularly timely, as the increasing acceptance of nano-imprint lithography by industry gives an ideal route for mass production of 3D nano-structures from masters written using 3D e-beam lithography.
This research project will be largely experimental but with a significant element of computer programming.
Direct patterning of functional materials on the nano-scale
(Dr GAC Jones)
Inorganic materials such as metallic oxides e.g. ZnO (– which is a wide band gap semiconductor), and particularly mixed metallic oxides e.g. BaTiO3, PbTiO3, PZT (– which are ferroelectric materials), YFeO3 (– a ferromagnetic material) and LaMnO3 (– a multi-feroic material) are interesting, particularly when fabricated on the nano-scale, from the point of view of their basic transport, opto-electronic, nano-photonic, nano-magnetic and nano-ferroelectric properties. Many of these materials can be formed from the decomposition of metal organic precursors, which themselves act as very high resolution resists for electron-beam lithography. Combining these properties gives a route for forming nanostructures in such materials. This experimental project seeks to capitalise on these processes to fabricate and measure the properties of some of these nano-scale functional materials. This project may appeal to those who have a cross disciplinary interest in the areas of physics and chemistry and/or materials science.
Magnetic thin films and spin-injection devices
(Dr GAC Jones, Dr CHW Barnes)
Spin transport through quantum nanostructures is a new field of research that offers the possibility to revolutionize our understanding of the novel states of matter that occur in these systems at low temperature. This understanding will in turn give us the opportunity to create a new class of quantum-information related electronic device. We have recently developed a state-of-the-art facility to create this sort of spintronic device. It involves MBE growth of Fe on a GaAs-AlGaAs-InGaAs heterostructure, e-beam patterning and cleanroom fabrication. Within this project, we will create a broad range of spintronic devices that will enable us to study spin manipulation and spin injection on the nano-scale. CASE awards may be available with Toshiba Research Europe Ltd for applications of spintronic devices.
Engineering of long quantum coherence times in double quantum dots for possible quantum computation applications
(Prof CG Smith)
This project aims at building on our development of a single electron detector capable of measuring the movement of single electrons in sub-micron semiconductor devices. Recently we have been able to start to look at the time dependent evolution of long lived quantum states within a four quantum dot QCA. In this project we want to continue these measurements at high frequencies where the measurement process is on a time scale comparable to the coherence time for the quantum state. This will allow us to explore the fundamental quantum limits of measurement for both charge and spin dephasing.
Microwave-driven transitions in two coupled semiconductor charge qubits”
Petersson K. D., Smith C. G., Anderson D., Atkinson P., Jones G. A. C. and Ritchie D. A., 2009, Phys. Rev. Lett., 103, 016805
Imaging of quantum states in coupled quantum dots defined in agraphene layer using a scanning probe device
(Prof CG Smith)
This project involves using a low temperature scanning force microscope with a conducting tip to modify the transport properties of a sub-micron grapheme device. Graphene (a single layer of graphite) is a gapless two dimensional semiconductor with a high mobility. It is predicted that the edges of graphite are spin active. We will create double quantum dots set up in the spin blockade regime to investigate this. In addition over the last few years we have developed a low temperature scanning probe instrument with 50nm resolution that is able to image the probability distributions of wavefunctions in a 1-D semiconductor device at temperatures below 100mK. We use the conducting tip of the AFM as a movable scatterer, which is able to modify the electron transport in a semiconductor structure. We can use this device to investigate surface states in graphene, and to image individual percolation paths in heavily disordered devices.
“Pauli Spin Blockade in Carbon Nanotube Double Quantum Dots”
Buitelaar M.
R., Fransson J., Cantone A. L., Smith C. G., Anderson D., , Ardavan A.,
Khlobystov A. N., Morley G. W., Porfyrakis K. and Briggs G. A. D.,
2008, Phys. Rev.B., 77, 245439.
“Conductance quantization at a half-integer plateau in a symmetric GaAs quantum wire”
Crook R., Prance J., Thomas K. J., Chorley S. J., Farrer I., Ritchie D. A., Pepper M. and Smith C. G., 2006, Science, 312, 1359–1362.
A scanning capacitance probe to measure polarisation of charge in molecules
(Prof CG Smith)
We have recently been able to measure the change in capacitance that occurs when two states in two quantum dots come into alignment. At this point electrons can tunnel from one dot to the other. This measurement was performed using a resonant circuit connected to one end of the double dot pair resonating at 400 MHz. This measurement can be made when no electron transport is possible through the pair of dots. We will extend this measurement to our low temperature scanning probe. This will allow us to measure capacitance changes with nm resolution and MHz band width, which will allow us to investigate polarization in dimmer molecule and thus identify molecules that could be used for quantum computing applications.
Crook R., Schneble R. J., Kataoka M., Beere H. E., Ritchie D. A., Anderson D., Jones G. A. C., Smith C. G., Ford C. J. B. and Barnes C. H. W., 2008, Phys. Rev. B, 78, 125330.
Reducing Charge dephasing in coupled quantum dots for quantum computing applications.
(Prof CG Smith)
Coupled quantum dots can be used as qubits for quantum computing applications. Both the spin and charge of the electron can be used for this purpose, and the spin life time has been shown to be longer in many systems. However spin quantum computing requires coupling of spin states in different dots via the exchange interaction. During this coupling the spin state is sensitive to charge dephasing. The dominant charge dephasing mechanism is through electron phonon interactions. This project will look at ways of controlling this by using free-standing membranes where the phonon states are quantuized. This allows the dephasing to be controlled.
“Single electron transport in a free-standing quantum dot”
Chorley S. J., Smith C. G., Perez-Martinez F., Prance J.
R., Atkinson P., Ritchie D. A. and Jones G. A. C.,
2008, Microelectronics Journal, 39 No 3-4, 314–317
Cooling electrons below the lattice temperature.
(Prof CG Smith)
We have recently demonstrated that two quantum dots can be used as energy filters at the entrance and exit of a ten microns square region of two dimensional electron gas grown in a semiconductor hetersostructure. By setting up the energy filters appropriately we can drive a current through the central regions that injects cold electrons and remove hot electrons. The resulting cooling will be studied using a third quantum dot, the occupancy of which can be monitored by a near by 1-D channel. We will look at investigating cooling the electrons from 100mK to 10mK using this technique. Cooling quantum systems is a very important way of reducing dephasing and enhancing interaction phenomena.
“Electronic refrigeration of a two-dimensional electron gas”
Prance J. R., Smith C. G., Griffiths J. P., Chorley S. J., Anderson D., Jones G. A. C., Farrer I. and Ritchie D. A.,2008, Phys. Rev. Lett., 102, 146602.
Pumping particles using microfluidic techniques for display applications
(Prof CG Smith)
Recently we have developed a new technique for pumping fluids using patterned electrodes and low voltage ac fields. The technique allows fluids to be pumped along the surface and so enables plug flow to be demonstrated. This technique has applications in the growing area of microanalysis systems where small quantities of chemical reagents are brought together in a small reaction chamber. The project will involve investigating the use of this pumping technique to move reflective particles at each pixel to construct a reflective display.
“Low voltage plug flow pumping using anisotropic electrode arrays”
Mpholo M., Smith C. G. and Brown A. B. D., 2003, Sensors and Actuators B: Chemical, 92, 262–268.
Spin-entangled electron transport in carbon nanotubes and graphene
(Dr MR Buitelaar)
The elementary unit of quantum information is the quantum bit or qubit. Like the classical bit, the qubit is a two-level system but with the intriguing ability to exist in a superposition of states, which means it can be in the on and off state at the same time. This has profound implications if we consider quantum systems of more than one qubit. Instead of each qubit carrying any well-defined information of its own, the information is encoded in their joint properties. In quantum mechanics, the qubits are described as being entangled. The challenge is to find ways to harness quantum phenomena such as superposition and entanglement to construct a quantum computer that is able to perform computational tasks that are unattainable in a classical context. In this project, we will make use of the electron spin as a natural quantum two-level system. More precisely, we will isolate individual spins and couple two spin qubits in a controlled way in carbon nanotube and graphene quantum dots, materials which are expected to have exceptionally long spin coherence times. Using novel techniques developed in our group to convert the spin state of an electron to a much easier measurable electrical signal we will directly determine the spin coherence times and perform landmark experiments aimed at demonstrating quantum entanglement in spin qubits.