Plasma etching | EPFL Graph Search

Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge (plasma) of an appropriate gas mixture being shot (in pulses) at a sample. The plasma source, known as etch species, can be either charged (ions) or neutral (atoms and radicals). During the process, the plasma generates volatile etch products at room temperature from the chemical reactions between the elements of the material etched and the reactive species generated by the plasma. Eventually the atoms of the shot element embed themselves at or just below the surface of the target, thus modifying the physical properties of the target. Mechanisms Plasma generation A plasma is a high energetic condition in which a lot of processes can occur. These processes happen because of electrons and atoms. To form the plasma electrons have to be accelerated to gain energy. Highly energetic electrons transfer the energy to atoms by collisions. Thr

Fabrication and Optoelectronic Properties of Graphene Nanoribbons

Eberhard Ulrich Stützel

Graphene, a single layer of carbon atoms, exhibits excellent charge transport properties. However, due to the absence of a band gap in this two-dimensional carbon nanostructure, graphene-based field effect transistors cannot be turned off. One strategy to increase the on/off ratio relies on patterning graphene into narrow stripes, so-called graphene nanoribbons (GNRs). The present thesis aimed at developing novel fabrication and chemical functionalization methods for GNRs. Along these lines, electrical transport studies and spectroscopic investigations were envisioned as a major tool to monitor the changes brought about by the functional groups attached to the GNRs. GNRs were fabricated with the aid of V2O5 nanofibers or CdSe nanowires as an etching mask during the plasma etching of graphene. The resulting GNRs exhibited good electrical conductivity for ribbon widths as small as 10 nm. Moreover, the transport gap in the GNRs was found to scale inversely with the ribbon width. Scanning tunneling microscopy and surface enhanced Raman spectroscopy testified a good structural quality of the GNRs. Electrical characterization of GNRs revealed a pronounced hysteresis, which was exploited for the fabrication of electrically switchable GNR memory cells. Dynamic pulse response measurements demonstrated reliable switching between two conductivity states for clock frequencies of up to 1 kHz and pulse durations as short as 500 ns for >10^7 cycles. As the most likely switching mechanism, charge trapping within a water layer in the GNR surrounding could be identified. Furthermore, the optoelectronic properties of individual GNRs were studied by scanning photocurrent microscopy. The pronounced photocurrent signal close to the nanoribbon/metal contacts was found to be directly proportional to the conductance of the devices, suggesting that a local voltage source is generated at the nanoribbon/metal interface by the photo-thermoelectric Seebeck effect. The dominance of this mechanism over charge separation via built-in electrical fields was attributed to strong local heating of the GNRs by the laser spot, combined with the reduced thermal conduction capability of the nanoribbons in comparison to extended graphene sheets. Chemical functionalization of graphene and GNRs was attempted via different gas- and liquid-phase approaches. While electrical transport and Raman measurements revealed the presence of significant doping effects, it was not possible to unequivocally prove the covalent attachment of atoms at the GNR edges, neither through changes in the charge transport characteristics, nor via scanning microscopy studies.

EPFL2013

APCVD of dual layer transparent conductive oxides for photovoltaic applications

Benoît Florent Paul Delaup, Monica Morales Masis

We report the atmospheric pressure chemical vapour deposition (APCVD) of a dual layer transparent conductive oxide (TCO). This combines a fluorine doped tin oxide (FTO) base layer with a fluorine doped zinc oxide (FZO) top layer, where we seek to utilise the respective advantages of each material and the differences in their associated industrial deposition process technologies. Deposition of a 250 nm thick FZO layer on FTO was enough to develop features seen with FZO only layers. The crystallographic orientation determined by the FZO dopant concentration. Changes to the deposition parameters of the underlying FTO layer effected stack roughness and carrier concentration, and hence optical scattering and absorption. Photovoltaic cells have been fabricated using this TCO structure showing promising performance, with efficiencies as high as 10.21% compared to reference FTO only values of 9.02%. The bulk of the coating was FTO, providing the majority of conductivity and the large surface features associated with this material, whilst keeping the overall cost low by utilising the very fast growth rates achievable. The FTO was capped with a thinner FZO layer to provide a top surface suitable for wet chemical or plasma etching, allowing the surface morphology to be tuned for specific applications. (C) 2015 Elsevier B.V. All rights reserved.

Elsevier2015

Advanced Diamond Microfabrication for Microoptics and Photonics

Marcell Kristof Kiss

Diamond is an exceptional material - hard, stiff, transparent, which makes it ideal for the fabrication of optical and mechanical systems that take advantage of these properties. Diamond is not only "better", but it offers the possibility of integrating bright colour centres. These optically active defects can be exploited for a variety of applications, including the study of fundamental science, magnetometry, biosensing and new types of lasers. Despite the attractive properties of diamond for optics, no standard platform exists to create photonic circuits and devices. This thesis shows the realisation of a diamond-on-insulator photonics platform, that aims to combine the versatility of the silicon-on-insulator photonics with the capability of performing diamond science in single crystal guided-wave devices. The diamond-on-insulator substrate is achieved via ion implantation-based cut of a single crystal membrane and bonding of the membrane to an insulator layer. This approach provides a monolithic fabrication process that scales to commercial exploitation, bringing convenient access to the study of diamond light-matter interactions without requiring custom substrate fabrication. Furthermore, the possibility of releasing the diamond devices adds access to freestanding structures, opening the way to diamond micro-opto-electro-mechanical systems and optomechanics. Diamond also makes an excellent microoptics substrate, due to its high laser damage threshold, transparency and high refractive index, which is an attractive combination for high-power, compact optical systems operating in the UV, visible and near-infrared. Different diamond etching techniques are developed and investigated in this thesis, that enable the realisation of unique features. These techniques are employed to create diamond diffractive microoptical components, which are then characterised, showing high-quality surfaces that closely match the designed features, indicating reliable fabrication and resulting in excellent optical performance. These devices have applications in high power beamsplitters, beam shapers and compact spectrometers.

EPFL2019