A new interpretation of the photodoping effect in amorphous As- and Ge-chalcogenides

Abstract

Photodoping is considered as a radiation-enhanced solid state reaction. The reaction kinetics is governed by two physical processes: (i) The generation of electrons and silver (copper) ions a t the Schottky-barrier originated at the metal-chalcogenide interface and (ii) a transport of the electrons and the metal ions through the amorphous reaction product layer silver-( copper-) chalcogenide. The formation of that compound is a three-dimensional intercalation reaction. The intercalation product possesses a high ionic conductivity after reaching a percolation threshold concentration. The percolation mechanism is responsible for the occurrance of a step-like doping profile. Both processes (i) and (ii) are stimulated by absorption of light. Light absorption a t the Schottky-barrier generates electrons and metal ions. The light absorption within the bulk of the doped chalcogenide supplies free photoelectrons that have to maintain charge neutrality during cation migration and for insertion of the cations into the amorphous network. The insertion is based on the capability of the chalcogens to form C, centres and the appearance of CTMf (M: Ag, Cn) ionic bond units within the amorphous chalcogenide (9 + e-+ M+ -CYMf). A quadrupolar deformibility of Ag+ and Cu+ causes the high ionic mobility of these species. Die Photodotierung wird ale strahlungsbeschleunigte Festkorperreaktion betrachtet. Zwei physikalische Prozesse bestimmen die Reaktionskinetik: (i) Die Generation von Elektronen und Silber-(Kupfer-)Ionen an der Schottky-Barriere, die sich am Metall-Halbleiter-tfbergang ausbildet, und (ii) der Transport der Elektronen und Metallionen durch die amorphe Schicht des Reaktionsproduktes Silber-(Kupfer-)Chalkogenid. Die Bildung dieser Verbindung ist eine dreidimensionalc Interkalationsreaktion. Das Interkalationsprodukt besitzt eine hohe ionische Leitfkhigkeit nach Erreichen einer Perkolationsschwellkonzentration. Der Perkolationsmechanismus wiederum ist fur das Auftreten eines stufenformigen Dotierungsprofils verantwortlich. Beide Prozesse (i) und (ii) werden durch Absorption von Licht, stimuliert. Die Absorption des Lichtes a n der Schottky-Barriere generiert Elektronen und Metallionen. Die Lichtabsorption im Inneren dea dotierten Chalkogenids erzeugt freie Photoelektronen, welche zur Aufrechterhaltung der Ladungsneutralitlt wahrend der Kationenmigration sowie fur den Einbau des Kations in das amorphe Netzwerk benotigt werden. Dieser Einbau beruht auf der Fiihigkeit der Chalkogene CY-Zentren zu bilden und dem Auftreten von G M + (M: Ag, Cu) -Bindungseinheiten im amorphen Chalkogenid (C; + e-+ + MC -c C:M+). Die hohe Ionenbeweglichkeit von Agt und Cut wird durch deren quadrupolare Deformierbarkeit verursacht.

Silver photo-diffusion and photo-induced macroscopic surface deformation of Ge33S67/Ag/Si substrate

“…This makes sense if we assume that negatively-charged defects are created on chalcogen (S) sites by the photo-excitation of lone-pair electrons, and that they trap positively-charged silver ions. This type of model is proposed by Kluge 27 and is known as the "intercalation model". In this case, the amount of silver dissolved in the chalcogenide layer depends on the amount of chalcogen atoms in it and, thus, the thickness of chalcogenide layer.…”

Mass transport in chalcogenide electrolyte films – materials and applications

“…So, the penetration of the metal into the chalcogenide during photodoping is actually due to the difference in electrochemical potentials. We can consider the process to be similar to that occurring in a galvanic cell, in which case the more electropositive metal is dissolved into the electrolyte due to an intercalation reaction [31,32]. The main reason that this reaction occurs in chalcogenide glasses is that they possess relatively rigid covalent bonds mixed with soft van der Waals interconnections.…”

Evolution of chemical structure during silver photodiffusion into chalcogenide glass thin films

“…The mechanism of this effect has been studied by different experimental and theoretical methods [4][5][6][7][8], but atomistic and chemical origin of the observed structural transformations is still under discussion. High resolution X-ray photoelectron spectroscopy (XPS) is one of the most effective methods to examine the changes in chemical composition and electronic structure of the materials, which can be utilized also for photodiffusion studies.…”