Semiconductor Physics Research Group

---

Semiconductor physics has been amongst the most exciting and rapidly advancing areas of physics in the past decade. Enormous leaps have been made in our understanding of semiconductor processes, and in the ways we can make semiconductors. Engineering of the electronic properties of materials can now be achieved at the atomic level, and totally new phenomena have resulted. The quantum Hall effect and the quantum-confined Stark effect are just two examples of new and exciting physics which have emerged as a result of activity in this area. The great potential of the new phenomena for optical and electronic devices has led to a high demand for scientists with research experience in semiconductor physics. This means that job prospects in the international market are excellent for people with a PhD in semiconductor physics.

The Semiconductor Physics Group at Exeter is concerned with both the experimental and theoretical properties of semiconductors. Started in 1985 it has brought together a complement of 8 senior staff, 4 experimental and 4 theoretical. Together with postdoctoral workers and research students it comprises over 27 people. The group has a high level of research activity and has attracted substantial support from the University, EPSRC and elsewhere. Close co-operation is maintained with external groups in Universities, Government Establishments and Industry. The present activities and facilities are outlined in more detail below.

Experimental Projects

Experimental studies in the Semiconductor Physics Group concentrate on two main areas, each of considerable interest from both fundamental and applied points of view: the effects of electron-electron interactions on the properties of electron systems of reduced dimensionality, and quantum interference and mesoscopic effects in sub-micron semiconductor structures near the metal-insulator transition. Our study of low dimensional electron systems has led to the observation of dramatic effects on the conduction and optical response of high mobility electron systems at very low temperatures and high magnetic fields, caused by electron-electron interactions. The group has developed considerable expertise in optical spectroscopy thermodynamic measurements and in electrical conductivity measurements which forms the basis for several of the projects described below. The list is not exhaustive, but is intended to give some idea of the nature of the experimental work on offer to students interested in a PhD in experimental semiconductor physics:

1. Optical and transport studies of the integer and fractional quantum Hall effects are being undertaken at temperatures down to 20mK, in magnetic fields of up to 17T. Under these extremes of temperature and field, the two-dimensional electron gases studied condense into a quantum fluid state. Magnetoresistance measurements reveal a rich spectrum of quasiparticle excitations (the fractional quantum Hall effect), and the luminescence from electrons in this state is a powerful probe of these quasiparticles.

2. At the extremes of our experimental capabilities there are clear signs in the two-dimensional electrons' luminescence of the formation of another type of electronic state, tentatively identified as an electron solid. We are currently extending our investigations of this solid state to 2D systems of holes, in which we expect to be able to examine the transition region between classical and quantum solid.

3. We have developed a new capability to perform magnetization measurements at millikelvin temperatures and high magnetic fields. The technique of torsion magnetometery yields direct thermodynamic information about the phases of matter that form in this regime, and also represents a contact-less measurment of the quantum Hall effect.

4. Investigations are underway of the properties of electrons confined to less than two dimensions: quantum wires and quantum dots. These structures have enormous device potential and their physics is particularly interesting because the confinement of electrons in them mimics the confinement of electrons in nature: a quantum dot for instance behaves something like an atom. The experiments performed at Exeter measure the electronic density of states in these structures and the effects of reduced dimensionality on electron-electron interactions.

5. Transistor structures are convenient objects in which different conduction mechanisms can be studied. In different transistors we investigate the fundamental properties of the metal-insulator transition realised by the variation of the electron concentration or magnetic field. One phenomenon of our interest is quantum interference of electrons. It governs the conductance of semiconductors at low temperatures and is responsible for a number of unusual effects such as negative magnetoresistance: the increase of the conductance with magnetic field. We were the first to observe this effect in the hopping regime of conduction and now our aim is to understand the origin of this negative magnetoresistance at ultra low temperatures where it becomes 'giant': at small magnetic fields the conductance increases by several orders of magnitude.

6. If the transistor size is decreased to sub-micron its low temperature conductance is not averaged over random impurity positions. This gives rise to mesoscopic effects: any smooth dependence for a large sample becomes fluctuating in a small device. These fluctuations are very reproducible and reflect the exact impurity configuration of the sample - its 'fingerprint'. We study these fluctuations to understand elementary electron processes involved in the conduction and noise of semiconductors. In small GaAs transistors we have resolved several elementary effects in electron hopping along impurities. Our objective now is to decrease further the temperature and sample size and study electron tunneling through one or two impurities. One phenomenon of particular interest is resonant tunneling, when for a symmetrical position of impurities along the conducting channel the transmission probability increases enormously and gives rise to metallic like conductance.

Experimental Facilities

The group is rapidly expanding its own experimental facilities as well as drawing upon those provided elsewhere. The group's own laboratories include a range of equipment for optical and transport measurements. Our optical laboratories are equipped with argon-ion pumped titanium sapphire laser, providing tunable excitation sources near the GaAs bandgap. In addition, state-of-the-art detection systems on our various spectrometers enable very weak signals to be detected. Supply of specimens grown by Molecular Beam Epitaxy is guaranteed under EPSRC contracts and is further facilitated by our collaborations with various Universities and Electronics Companies in the U K and Worldwide.

Our laboratories are equipped with cryostats covering the range of temperatures from 20mK to room temperature, and superconducting solenoid magnets providing fields of up to 17T. Optical access is also available in these systems using optical fibres or windows. High-quality signal recovery apparatus is used in computer-controlled experiments on electronic systems confined to two or fewer dimensions. Appropriate diagnostic and measuring equipment is available, together with various group facilities for specimen handling. The group has a clean room for sample fabrication, with facilities for optical lithography, etching and annealing contacts.

The laboratories are supported by the Department's very well staffed Mechanical and Electronics Workshops which are used to undertake tasks ranging from the construction of complete helium dilution refrigerators to microprocessor-controlled monitoring apparatus. The Department also runs its own helium and nitrogen liquefiers.

Theoretical Studies

Theoretical work within the Group covers a wide range of topics in condensed matter physics, using both formal and computational techniques, and has an international reputation. We have major efforts in the areas of many body theory, the electronic properties of surfaces, heterojunction systems, low-dimensional semiconducting structures, and the electronic and mechanical properties of non-metallic solids. Close co-operation between the experimental and theoretical group members is maintained by regular informal seminar meetings and joint research programs.

The development of computational power and sophisticated analytical formulations has increased the physicists' ability to treat real systems and relate theory to experimental developments. At Exeter we have developed a whole range of techniques, which are amongst the most powerful currently available anywhere, for the investigation of complex solid state problems.

1. Many body theory is concerned with the interactions of the atoms, ions and electrons in solids and liquids. These interactions result in the appearance of new elementary excitations or "particles" such as plasmons, phonons, excitons, polaritons, rotons etc, and new states of matter such as superconductors and quantum Hall liquids. It is a quantum field theory in which we know the basic fields but not the particles, whereas in elementary particle theory we know the particles but not the fields. Much of the formalism is common to these two subject areas, and a constant interchange takes place between them.

2. The low-dimensional work includes the study of electron states, plasmons and phonons in quantum well, quantum wires, and superlattice systems formed from combinations of different semiconductors. A wide expertise in the area of semiconductor physics, interacting systems and quantum transport has been developed allowing a range of disparate problems to be tackled. This includes the theoretical modelling of the GaAs/AlAs superlattices, quantum dots and fractional quantum Hall systems which are being investigated experimentally within the group.

3. First Principles methods are used to investigate the structural, electronic and dynamic properties of a wide range of materials from fullerenes to diamond. We are particularly interested in defects within them and the ways in which these defects influence their mechanical properties.

4. The use of numerical techniques in the study of surfaces with particular reference to pseudo potential total energy and force methods has been extensively developed in Exeter. These state-of-art computational techniques are used to study atomic structure and electronic states on surfaces, at metal-semiconductor interfaces (Schottky barrier systems), and in thin semiconductor superlattices. These techniques are also used for ab-initio phonon calculations in bulk semiconductors and at semiconductor surfaces.

Computational Facilities

The theoretical group has three CONVEX mini-supercomputers, five Silicon Graphics work stations, five IBM RISC 6000 work stations and a SUN workstation. Purchased with help of the SERC Computational Science Initiative, this forms a very comprehensive local dedicated computer resource. The group also has access to national supercomputers, Cray YMP8 at the Rutherford Laboratory and Fizutsu at University of Manchester Computer Centre, as well as a set of Silicon Graphics machines provided by the University's Computer Unit.

---

Up to Physics Research Groups at Exeter.