University of Bath

Dept. of Physics

Coherent Raman detected electron spin resonance (CRESR)

Optically Detected Magnetic Resonance (ODMR)

Raman spectroscopy of wide bandgap II-VI epitaxial layers and superlattices

This work is concerned with II-VI semiconductors such as ZnSe, (Zn, Mg)(S,Se) and superlattices and heterostructures based on these materials grown by molecular beam epitaxy or by metal-organic vapour deposition. The work was stimulated by the interest in II-VI materials stimulated by the development of the II-VI quantum well blue-green laser. The initial objectives were (i) to obtain a clearer understanding of the problems associated with p-type doping; (ii) to assist in the optimisation of the semiconductor growth processes for the production of high quality, doped materials and (iii) to investigate details of the electronic band structure of quantum-confined systems, taking advantage of the particular characteristics of the II-VI materials compared to the III-V systems. Electronic, vibrational and spin-flip Raman scattering spectroscopies are being used for this purpose. This work  was funded by EPSRC (Materials) and involves collaborations with growth teams in Europe and Japan as well as the two UK II-VI growth teams at NEWI and Heriot-Watt.

Modulated reflectance spectroscopy of semiconductor heterostructures

The optical reflectivity of semiconductor materials changes rapidly around the photon energies corresponding to the semiconductor band gaps. By studying the reflectivity of a sample as a function of wavelength, one can therefore investigate its electronic and excitonic structure. More sensitive variations of the basic reflectivity method use modulation of one of the experimental parameters to yield derivatives of the reflectivity spectrum. This technique has been set up (covering the visible spectral regions) together with the associated tools for the interpretation of the data, and has been applied very successfully, initially in studies of dilute magnetic semiconductor heterostructures. Quaternary phosphide materials used in red and infrared-emitting laser structures have also been studied. A particular interest in this work was the ability of modulation techniques to give insight into the details of vertical cavity laser structures (VCSELs), where independent determination of the quantum well transition energies and of the cavity mode is required. We are also interested in the application of photomodulated reflectivity in studies of wide bandgap II-VI systems (for instance, fractional monolayers of CdSe in ZnSe epitaxial layers) and in the group-III nitride semiconductors.

Raman spectroscopy of dilute magnetic semiconductor superlattices and multiple quantum well structures

Semiconductors of the II-VI series can be alloyed with large concentrations of magnetic ions, such as manganese. The presence of the magnetic ions results in novel properties, including the creation of magnetic polarons and the ability to 'tune' the forbidden bandgap by the application of an external magnetic field. When such materials are incorporated into superlattices and multiple quantum well structures, the reductions in layer thicknesses produce further novel effects that arise because of quantum confinement and because of the reduction in dimensionality from three to two. Raman spectroscopy has been applied to study the vibrational (phonon) and magnetic (spin-flip) excitations in quantum well structures grown by molecular beam epitaxy. Quantum dot structures of the CdTe / Cd(Mn)Te, ZnTe / Zn(Mn)Te and ZnSe / Zn(Mn)Se systems have been prepared by ion beam etching, leading to the first observation of spin-flip Raman scattering in such structures. 

This work is continuing in collaboration with the University of Würzburg, King's College London, CNRS Marcoussis and the Polish Academy of Sciences Institute of Physics, Warsaw. Interest is focusing on modulation-doped dilute magnetic semiconductor structures.