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B4: Lateral Transport in Oxidic Field-effect-Structures

We investigate interface properties and the influence of gate heterostructures on the lateral transport in oxidic heterostructures. The correlation of macroscopic electric parameters (output- and transfer characteristics) with microscopic transport properties on the scale of µm and nm (µ-PL, CL) shall lead to a consistent understanding of the electronic and the ambipolar transport mechanisms. Rectifying metal oxides (e.g. Ag_xO) and insulating oxides without (e.g. Al₂O₃), with (e.g. Mg_xZn_1-xO) and with switchable electric polarization (e.g. BaTiO₃) form novel interface affected structures with multiferroic properties.

The lateral transport in oxide heterostructures is investigated. With the correlation of parameters obtained from electric characteristics with microscopic transport properties on the scale of µm (micro photoluminescence) and nm (cathodoluminescence) a consistent understanding of the electronic and ambipolar transport mechanisms will be developed. The characteristic temperature and energy dependencies give information about the underlying scattering mechanisms. Transport properties will be attributed to structural (ideal and non-ideal) properties of the gate heterostructures and their interfaces, the channel and its interfaces as well as the band diagram, strain and point defects (doping and defects).

The prototypes for the planned investigations will be field-effect transistors with insulating (MISFET) and non-insulating (MESFET) gate. A comparison of both types of FET enables to investigate the influence of the interface between insulating and non-insulating oxide and the semiconductor. The channel will consist of ZnO and ZnO-based heterostructures with different crystallographic orientations, i.e. different orientations of the spontaneous polarization. Besides the two-dimensional electron gas (2DEG) formed at the hetero interfaces, the buried ZnO surface below the gate will be interesting, because, dependent on its history (temperature, chemical environment), it can form a conductive layer itself. On the one hand, the gate will consist of different insulating oxides without electrical polarization (e.g. Al₂O₃), with spontaneous polarization (e.g. Mg_xZn_1-xO) and with switchable polarization (e.g. BaTiO₃). Reactively (i.e. under influence of oxygen ions) grown metal oxides (e.g. Ag_xO) will serve as rectifying gate contacts. They exhibit a considerably higher work function compared to pure metals. Perspectively, also ferromagnetic semiconducting oxides will be applied as channel material in order to obtain an interface affected structure with switchable and coupled ferromagnetic and ferroelectric properties.