My research in Molecular Beam Epitaxy is into developing new semiconductor materials, including wide band gap nitrides, ferromagnetic semiconductors and narrow band-gap arsenide-nitrides. The first programme is concerned with the growth of wide band gap (0.7-6.2eV) AlGaInN nitrides using a novel plasma-assisted growth process to incorporate active nitrogen into the films. A key development was the discovery of a method to produce the metastable zinc-blende form of GaN The second programme involves the growth of ferromagnetic semiconductor materials for the spintronics programme. Highlights of this work include the highest ever Curie temperature for thin films of GaMnAs (173K) The third programme involves the growth of dilute nitrides AlGaInAsN for narrow band gap applications, this forms part of an EU programme "Fast Access". In this programme we are studying the use of Ar/N mixtures for rapid switching of nitrogen in the quantum wells forming the active part of the device.
Research at Philips, Redhill 1969-1991
My research is concerned with the growth of compound semiconductor materials using a sophisticated technique known as Molecular Beam Epitaxy (MBE). This allows us to create atomically thin layers of exceptional flatness and purity and, by forming layers of different materials, we can create artificial structures in which the quantum mechanical behaviour of the conduction electrons can be revealed and controlled by varying the composition and thickness of the layers. The confinement of electrons to motion in two dimensions has also led to important technological advances, including quantum well lasers for telecommunication and CD players and to high electron mobility transistor (HEMT) amplifiers used in direct satellite broadcasting and other high frequency and computer applications.
I have been involved in this area since 1970 when MBE was first developed at Bell Telephone Laboratories, New Jersey, at IBM Laboratories, Yorktown Heights and at Philips Research Laboratories, Redhill. I had a leading role in the programme at Philips which for 20 years had a world-wide reputation for its research in the field. My particular work involved the study of basic growth processes for III-V compounds and alloys, the way in which dopant atoms are incorporated in such materials and the electrical and optical properties of the resulting material. I developed a sophisticated Modulated Beam Mass Spectrometer (MBMS) technique which allowed us to study the basic chemistry of the growth process and have published many of the key papers in this area. Because of our leading role in MBE at Philips, we were able to mount the first MBE based device programme in Europe and to take a major part in developing the Low Dimensional Structures (LDS) used in a wide range of research programmes during the last decade. Many University Departments relied on us for the growth of suitable structures and, in addition to supporting the Philips MBE activity, I had a strong involvement in a number of University programmes Highlights of this work have included, the observation of quantised conduction in point contacts, measurement of the charge associated with the quasi-particles in the FQHE, optical detection of the FQHE and observation of the magnetically induced Wigner solid.
My final research programme at Philips concerned fundamental developments taking place in MBE technology. The conventional solid sources for MBE evaporation are being replaced by gaseous sources external to the MBE equipment. This has many advantages and the resulting technologies are called Gas Source MBE (GSMBE): elemental group IIIs + dopants, gaseous group Vs; Metal Organic MBE (MOMBE): gaseous group IIIs with elemental dopants and group Vs and Chemical Beam Epitaxy (CBE): all gaseous sources. I initiated the first fundamental study of MBE with gaseous sources using the MBMS technique which I developed for MBE, combined with Reflection High Energy Electron Diffraction (RHEED) measurements. This produced an improved understanding of some of the key issues involved and I have published invited review papers on this topic.
This includes continuation of all three programmes outlined above with possible increased collaboration with industrial partners. Devices of interest include 1.3 micron lasers for low cost links to the home, UV LEDs for medical and biological application and the use of magnetic quantum dots as the basis for quantum computing.