Main research interest of this project is physics of materials, which can be used in a new branch of nanoscience called spintronics, which is based on the concept that the electron spin may be used instead of, or in combination with its charge, for purpose of information storage, processing and transport. The majority of spintronic devices involve the transport of spin-polarized carriers across the layers of various compositionally modulated magnetic quantum structures. Recent studies have shown that semimagnetic semiconductors represent good candidates for substantial spin injection into nonmagnetic semiconductors. In these alloys some of atoms in host crystal lattice is replaced with atoms (ions) of magnetic materials. The best-known examples of these are II-VI compounds (e.g. ZnSe, ZnTe, CdTe, CdSe) where a percentage of the group-II atoms is replaced by Mn2+ ions. In these materials, the electron spin can be actively manipulated by external magnetic or electric field, or by shining the polarized light.
A remarkable property of these materials is an extremely large Zeeman splitting of the band edges, due to exchange interaction between d electrons of Mn2+ ions and band electrons (of s or p character). As a result of this splitting between electronic states characterized by spin-up and spin-down, compositionally modulated nanostructures incorporating semimagnetic semiconductor layers have the unique property that their conduction and valence band offsets can be tuned over a significant range by externally applied magnetic field.
We want to make improvement towards the application of semiconductors, by combining the influences of doping, changing composition and the grain size with crystal structure. Our basic materials are:
1. Single crystal samples of Zn1-xMnxTe1-ySey, Cd1-xMnxTe1-ySey, Cd1-xMnxTe1-ySy, Hg1-xMnxTe1-ySey и Pb1-xMnxTe, where a few or all of Mn2+ ions are replaced with Fe, Ni, Co or Еu;
2. Pb1-xMnxTe и Pb1-xЕuxTe thin films, with dimension appropriate for detection of quantum effects.
3. Nanoparticles Cd1-xMnxS, Zn1-xMnxS and Pb1-xMnxS. The diameter od nanoparticles provides registering the effects related with decreasing of dimension. The influence of the dimension and density of nanoparticles CdTe on properties of ZnTe/Cd1-xMnxTe nanostructures will be studied, also.
The research will be performed through following tasks:
1. Technological tasks. The researchers will be engaged in process of synthesis and characterization of samples. Single crystal samples will be grown by using Bridgeman method. Thin films and quantum nanostructures will be obtained by molecular beam epitaxy, while the nanoparticles will be formed by processes of precipitation or milling. The composition and structure will be measured by X-ray diffraction, AFM and SEM. Transport and magnetic measurements will be performed by Hall experiment (only in case of single crystals and thin films) and SQUID in temperature range from 1.8 to 300K and magnetic fields up to 7T. Samples with phase transition detected by Hall measurement will be also characterized by photoacustic measurement.
2. The experimental tasks. Raman, FIR, and UV-VIS spectroscopy, as well as photoluminescence spectroscopy are the central part of this project. The measurements will be performed in temperature range from 1.8 to 300K, while photoluminescence in magnetic fields up to 7T. Complex experimental work should reveal phonon and electronic structure of materials mentioned above. The electron-phonon interaction will be studied in mixed crystals, and the interaction between electrons and local vibration phonons of impurities (local mode) in doped systems. Photoluminescnce spectroscopy in magnetic fields and low temperatures should characterize the electronic structure, excitons and magnons, which is very important properties of nanoparticles and quantum nanostructures.
3. Modeling. To describe experimentally obtained effects it is necessary to make new or improve known models. First of all, there is complex model for phonon properties of multicomponent alloys, which takes into account the influence of decreasing of at least one dimension of the crystal. Based on these results, the present models for electron phonon and electron-local mode interaction will be modified.
4. Characterization of DX centers. The effects related to mixed valence, for example the effect persistent photoconductivity in Pb1-xMnxTe и Cd1-xMnxTe doped with In and Ga will be studied.
| Nebojsa Romcevic