Pia Homm

Auger electron spectroscopy study of semiconductor surfaces: Effect of cleaning in inert atmosphere

In this paper, the authors demonstrate that Auger electron spectroscopy (AES) is an effective characterization tool in the analysis of the cleaning of semiconductor surfaces under different atmospheres. AES has several advantages for this purpose: it is nondestructive, surface specific {the analysis depth is only 4–50 A [Childs et al., Handbook of Auger Electron Spectroscopy (Physical Electronics, Eden Prairie, MN, 1995)]}, and very sensitive to common contaminants such as carbon and oxygen. Furthermore, the authors have proven that AES allows us to describe the effectiveness of surface cleaning in a quantitative manner by comparing the peak-to-peak height of the oxygen signal for different samples. In this work, the surface cleaning of five semiconductors, namely, Si, Ge, GaAs, In0.5Ga0.5As, and In0.5Al0.5As, was investigated. The same standard HF cleaning procedure was applied in two different atmospheres, air or nitrogen. The latter was used to prevent reoxidation after cleaning. The authors found that...

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 ...

Recent advances in hybrid VO2/Si devices for enabling electro-optical functionalities

The change in the optical properties of VO2 across its insulator to metal to transition could give rise to novel photonic devices with beyond state-of-the art performance in terms of footprint, power consumption and speed operation. However, the final performance strongly depends on the control and efficiency of the phase transition in the VO2. For electro-optical applications, such control and efficiency is highly dependent on the electrode configuration. In this work, the influence of the electrodes is experimentally analyzed and an alternative approach is proposed to switch the VO2 to the metallic state by minimizing the electrical power consumption. The electro-optical performance of the proposed short-circuited electrode is also experimentally demonstrated. An extinction ratio of 12 dB is achieved with a 20μm long hybrid VO2/Si waveguide with an electrical power of only 11mW. The power consumption could be further reduced by decreasing the distance between the electrode and the silicon waveguide, which in this work has been fixed to 1.5μm, without affecting the optical losses.

Ultra-low power hybrid VO 2 /Si photonic microring switch

An ultra-low power 2×2 photonic switch based on VO2/Si technology is proposed. The switch structure consists of an add-drop ring resonator with a footprint below 50 μm2. The waveguide structure is based on a fully etched silicon waveguide with a VO2 film on top of it. Electro-optical switching is achieved by means of the metal-insulator-transition (MIT) that can be induced in VO2. This MIT provides an ultra-large change of the refractive index at 1550 nm optical wavelengths that yields to ultra-low power operation. Waveguide structures have been fabricated by a combination of molecular beam epitaxy (MBE) growth, ex-situ annealing, e-beam lithography and lift-off. Experimental results have also been carried out to evaluate propagation losses as well as the electro-optical performance.

Ultrafast orbital manipulation and Mott physics in multi-band correlated materials

Multiorbital correlated materials are often on the verge of multiple electronic phases (metallic, insulating, superconducting, charge and orbitally ordered), which can be explored and controlled by small changes of the external parameters. The use of ultrashort light pulses as a mean to transiently modify the band population is leading to fundamentally new results. In this paper we will review recent advances in the field and we will discuss the possibility of manipulating the orbital polarization in correlated multi-band solid state systems. This technique can provide new understanding of the ground state properties of many interesting classes of quantum materials and offers a new tool to induce transient emergent properties with no counterpart at equilibrium. We will address: the discovery of high-energy Mottness in superconducting copper oxides and its impact on our understanding of the cuprate phase diagram; the instability of the Mott insulating phase in photoexcited vanadium oxides; the manipulation of orbital-selective correlations in iron-based superconductors; the pumping of local electronic excitons and the consequent transient effective quasiparticle cooling in alkali-doped fullerides. Finally, we will discuss a novel route to manipulate the orbital polarization in a a k-resolved fashion.

Thermo-optical switching in hybrid VO 2 /Sİ waveguides by lateral displaced microheaters

A lateral displaced microheater is demonstrated for switching across the VO2 phase transition in ultra-short hybrid VO2/Si waveguides for both TE and TM light polarizations. Simulation and experimental results are obtained showing a very good agreement with a switching electrical power of around 10mW.

Impact of the external resistance on the switching power consumption in VO2 nano gap junctions

The influence of an external resistance on the performance of VO2 nanogap junctions is analyzed and experimentally characterized. The current-voltage response shows the reversible metal-insulator transition typical of VO2 based devices. When reaching the metallic state, the current through the VO2 junction is abruptly increased, which may result in electrical contact damage. Therefore, an external resistance in series with the VO2 junction is usually employed to limit the maximum current through the device. Our results indicate that the external resistance plays a key role in the switching power consumption showing an optimum value, which depends on the dimensions of the VO2 junction. In such a way, power consumption reductions up to 90% have been demonstrated by selecting the optimum external resistance value.

Electrical switching in hybrid VO 2 /Si photonic structures

The integration of active materials on silicon is emerging as a promising field in silicon photonics to improve the performance metrics of key photonic components, in particular active components. Active materials allow tuning their optical properties as function of external stimuli. Amongst them, vanadium dioxide (VO2) has been largely investigated for different applications due to its controllable change between an insulating and a metallic phase. For photonic applications, VO2 shows a promising performance due to the abrupt change in the refractive index between the two phases across the semiconductor to metal transition (SMT). In this work, we will present our recent results for enabling disruptive electrical switching performance in hybrid VO2/Si photonic structures. Results have been obtained in the framework of the FP7-ICT-2013-11-61456 SITOGA project.

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.