Chen-Yi Su

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 ...

Schottky barrier modulation using ultrathin MgO for metal-silicon (100) contacts

In this work, ultrathin MBE-grown MgO has been employed as a thin tunneling interlayer to modulate the Schottky barrier height (SBH) between metal contacts and Si substrates. The ultrathin MgO films were grown with different starting first monolayers (O2, MgO, and Mg) on Si. With an MgO ultrathin film, all contacts show rectifying behavior for both n- and p-type Si. The SBH generally increases (decreases) with increasing metal work function for n-type (and p-type) and is generally lowest for the oxygen first treatment. To our knowledge it is the first time that the role of the first monolayer of an oxide tunnel barrier on the SBH is revealed. These results indicate a novel, interesting way to modulate the barrier height and hence the contact resistivity in CMOS devices.

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.

Modulation of the Schottky barrier height for advanced contact schemes

Contact schemes for scaled Si, SiGe and Ge channel MOSFETs devices are discussed, consistent with an approach based on SiGe alloys with low Schottky Barrier Height (SBH) for pMOS and Si contacts for nMOS, making reduction of the SBH to nSi critical. Methods for SBH reduction, and their underlying mechanisms, are studied. Accurate cryogenic CV measurements were used to extract SBH. We show that chalcogenide segregation can be effective in lowering the SBH by a dipole effect, while MIS contacts have a partial un-pinning effect. SBH=0.00±0.01 eV was achieved.