My current research activity is focused towards: i) spin optoelectronics in multilayered systems ii) large gap diluted magnetic semi-conductors for spin injection iii) magnetic properties of low dimensionality oxides iv) oxides for photovoltaic applications
In all cases the aim is to understand the physical (magnetic, optical, and transport) properties in relation to the morphology and crystalline structure of the materials. Some complex materials are synthesized in the laboratory in powder form using chemical methods, than are sintered to form a target which is further used to deposit thin films by pulsed laser deposition.
i) Spin electronics knows a strong evolution since the discovery of magnetoresistance in systems in which two magnetic layers are separated by a non-magnetic (metallic, insulating or semiconducting) one. This is mainly due the potential application of these systems in devices such as magnetic random access memories, angular sensors, reprogrammable logic circuits…
It is largely known that defects in the tunnel barrier (usually an oxide) of a magnetic tunnel junction lead to a variation of the magnetoresistance ratio. The same defects can also induce a magnetic moment in oxide materials and give raise to a polarized light emission when electronic desexcitations occur by the intermediate of such an impurity level inside the gap of the barrier. It is therefore easy to understand that some defects in the barrier can have in the same time an influence on the transport and optical properties of the junction. The aim is to study the relation between the optical and transport properties in magnetic tunnel junctions. On one hand, we will study the desexcitation on the fundamental level accompanied by emission of light (for which the intensity and polarization can be analyzed) when the electrons tunnel through the barrier. On the other hand, photoluminescence measurements will be carried out in order to determine how polarized light can influence the electron tunneling through the barrier.
ii) Spin injection in semiconductors plays a major role in the performances of spin electronic devices. The efficiency of spin injection is mainly limited by the difference of resistivity between the spin polarizing material (usually a metal) and the semi-conductor. One possible solution is to make wide band gap semiconductors (or oxides) magnetic by doping them with transition metal ions. According to theoretical calculations, such semiconductors should present ferromagnetic properties with a Curie temperature above room temperature. Our studies focus mainly on the origin of magnetism in such systems. If some groups suggest that ferromagnetism has an intrinsic origin (due to the transition metal ions in substituting ions of the host matrix), other groups emphasize the existence of magnetic clusters or defects (such as oxygen vacancies) to explain the observed ferromagnetism.
iii) Low dimensionality magnetic oxides constituted of magnetic chains (1D) or sheets (2D) are particularly interesting due to their strong magnetic anisotropy and their magnetic frustrations leading to a stepped reversal of the magnetization. Moreover, some 2D compounds are constituted by alternating magnetic and non-magnetic sheets, in the same manner as the magnetic and non-magnetic layers of a magnetic tunnel junction. However, due to the natural stacking of the sheets, no diffusion is allowed at the magnetic/non-magnetic interfaces. It should be therefore interesting to obtain model structures and analyze their spin dependent transport properties.
iv) Metallic oxides such as ZnO are particularly interesting for photovoltaic applications. One research direction is oriented towards the increase of the conversion factor of Si based photovoltaic cells. Thin films of ZnO can absorb and convert more efficiently solar light in a Si based cell. These layers can be doped by rare earth ions which constitute additional photon emitting sources, and therefore electric charge generators. When ZnO is doped by rare earth ions, it allows the absorption of photons with energy larger than the gap and an efficient transfer of energy towards the rare earth ions (Yb, Eu, Er…) which emits in the visible range. A second research direction is also oriented towards photovoltaic devices and aims to integrate thin films of ZnO or TiO2 in organic photovoltaic cells such as Glass/ITO/PEDOT:PSS/P3HT:PCBM/Al. Such a layer allows a selection of the electrons that are collected at the anode of the device (here Al) and which leads to an increase of the efficiency of such a cell.