Leander Dillemans

Strain relaxation in GaN nanopillars

In this work, we demonstrate the direct measurement of the strain state at the surface of nanostructures by in-plane X-ray diffraction. GaN tapered nanopillars have been fabricated by dry etching of a highly strained epilayer. The strain of the surface as function of pillar height shows an exponential relaxation which can be described by a single relaxation parameter. Additionally, we have simulated the strain relaxation and distribution of nanopillars. The impact of the pillar geometry on the strain relaxation has been discussed. In agreement with the measurements, an exponential relaxation of the strain is observed.

Evidence of the metal-insulator transition in ultrathin unstrained V2O3 thin films

We report the strain state and transport properties of V2O3 layers and V2O3/Cr2O3 bilayers deposited by molecular beam epitaxy on (0001)-Al2O3. By changing the layer on top of which V2O3 is grown, we change the lattice parameters of ultrathin V2O3 films significantly. We find that the metal-insulator transition is strongly attenuated in ultrathin V2O3 layers grown coherently on Al2O3. This is in contrast with ultrathin V2O3 layers grown on Cr2O3 buffer layers, where the metal-insulator transition is preserved. Our results provide evidence that the existence of the transition in ultrathin films is closely linked with the lattice deformation.

Collapse of the low temperature insulating state in Cr-doped V2O3 thin films

We have grown epitaxial Cr-doped V2O3 thin films with Cr concentrations between 0% and 20% on (0001)-Al2O3 by oxygen-assisted molecular beam epitaxy. For the highly doped samples (>3%), a regular and monotonous increase of the resistance with decreasing temperature is measured. Strikingly, in the low doping samples (between 1% and 3%), a collapse of the insulating state is observed with a reduction of the low temperature resistivity by up to 5 orders of magnitude. A vacuum annealing at high temperature of the films recovers the low temperature insulating state for doping levels below 3% and increases the room temperature resistivity towards the values of Cr-doped V2O3 single crystals. It is well-know that oxygen excess stabilizes a metallic state in V2O3 single crystals. Hence, we propose that Cr doping promotes oxygen excess in our films during deposition, leading to the collapse of the low temperature insulating state at low Cr concentrations. These results suggest that slightly Cr-doped V2O3 films can ...

Influence of the Surface Pretreatment on the Electrical Properties of MgO and Al2O3 Gate Stacks grown by MBE

For CMOS devices, the interface between the dielectric and the semiconductor is of crucial importance for the electrical properties. This paper is devoted to examining the role of different substrate preparation procedures on the interface properties. After the surface treatment 5 nm amorphous oxide films were deposited by molecular beam epitaxy (MBE). MgO on Si was chosen as our main model system. Additionally, a higher k dielectric, Al2O3, was also studied. The electrical properties were investigated through measurements of MOS capacitor structures. Mainly capacitance density measurements as a function of voltage C-V were performed and analysed. Moreover, post-metal annealing experiments were performed in order to change the interface configuration and to optimize the electrical properties. The results demonstrate that the interface properties can be well controlled by the pretreatment of the substrates. Surprisingly it was also found that an identical surface pretreatment – despite the different dielectrics – induced a systematic, reproducible and substantial VFB shift of the C-V curves while most other parameters remain unaffected. This opens additional possibilities to fine-tune the flat-band voltage in such high-k transistors.