Electronic response of a plasma-facing dielectric solid

By Georgia Longstaff on August 25, 2017

We asked Dr Franz Xaver Bronold to tell us about his research into low-temperature plasmas. His latest article “Kinetic modeling of the electronic response of a dielectric plasma-facing solid”, published in JPhysD, is available now on IOPscience.

Low-temperature plasma physics is a tremendously successful applied science. It is the basis for the management of various kinds of waste and pollutants, as well as for a number of important surface functionalisation techniques. Plasma-enhanced atomic layer deposition and etching, for instance, are essential steps in the chain of processes, turning a waver into an integrated circuit. All these applications take advantage of the richness of plasma chemistry, and can be perhaps traced back to the plasma-assisted production of ozone by dielectric barrier discharges in the middle of the 19th century. In the beginning of the 20th century, low-temperature plasmas were, however, also the workhorse for electronics. Nowadays gaseous electronics of course no longer play a role as far as electronic gadgets are concerned, but this does not have to be the case for ever.

Band edges for intrinsic silicon dioxide (left panel) and intrinsic silicon (right panel) in contact with a hydrogen plasma. Inside the wall solid red (blue) curves are the edges of the conduction (valence) band while in front of it the curves give the potential energy for an electron (ion). Dashed blue curves indicate the edges for the valence band holes. Distances from the interface at z=0 are measured in the wall’s (plasma’s) electron Debye screening length. The profiles inside the light grey regions have no direct physical meaning. They arise from implementing technically the physical boundary conditions for the double layer responsible for the band bending. The electron (ion) temperature of the plasma is 2 eV (0.2 eV). Image taken from Franz X Bronold and Holger Fehske 2017 J. Phys. D: Appl. Phys. 50 294003 © IOP Publishing, All Rights Reserved.

In our work we go back, in some sense, to gaseous electronics, considering the plasma-wall interface as just another kind of electronic interface. An interface on par with, for instance, Schottky contacts or semiconducting heterostructures. In the long run we are interested in engineering plasma-facing solid structures with novel opto-electronic functionalities. As a first step in this direction, in our JPhysD paper we set-up a general kinetic framework capable of analysing the transfer of the electronic non-equilibrium of the plasma (generated by impact ionization of the gas) to the solid, where it shows up as the accumulation of surplus charges. The overall picture emerging from our theory is an electric double layer whose positive space charge on the plasma side is balanced by a thermalised negative space charge inside the solid, while the quasi-stationary electron and ion fluxes released from the plasma source and maintaining the double layer are limited by electron-hole recombination, ambipolar charge transfer across the interface, and quasi-neutrality far away from it. Even in the simplest case, a perfectly absorbing dielectric plasma interface, for which representative results are shown in the figure above, the formalism contains the self-consistent adjustment of the plasma source to the electronic environment inside the solid, and vice versa.

About the Authors:

Dr Franz Xaver Bronold is a senior lecturer at the Institute of Physics of the University of Greifswald, Germany. He is a plasma-physics turned condensed matter theorist. Before focusing his research on charge-transfer processes across plasma-wall interfaces, he investigated Anderson localization of polarons, spin relaxation in nonmagnetic semiconductors, and the possibility of an excitonic insulator phase at a semiconductor-semimetal transition.

Professor Holger Fehske is full professor for theoretical physics at the Institute of Physics of the University of Greifswald, Germany. His research focuses on complex quantum systems. Currently he is mainly concerned with correlated electron and spin systems, charge-transfer processes across plasma-wall interfaces, light matter interaction, optomechanics, and topological insulators. He is also involved in high-performance computing for quantum systems.

This work is licensed under a Creative Commons Attribution 3.0 Unported License. Image taken from Franz X Bronold and Holger Fehske 2017 J. Phys. D: Appl. Phys. 50 294003 © IOP Publishing, All Rights Reserved.