Untersuchungen zum Ladungsträger-Transport in amorphen Festkörpern mittels Ionen- und Elektronenbestrahlung sowie grenzflächensensitiver Sekundärionenmassenspektrometrie
Das Ziel dieser Arbeit besteht in der Entwicklung und Charakterisierung einer niederenergetischen Elektronenkanone. Diese soll als Erweiterung der Hochvakuum-Methode des Bombardement induzierten Ionentransportes[1–11] (englisch: bombardment induced ion transport, kurz: BIIT, neu: charge attachment i...
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The scope of this work is the development and characterization of a low-energy electron gun as implementation for the high-vacuum BIIT-technique (short for: bombardment induced ion transport. New deontation is charge attachment induced ion transport, short: CAIT).[1–11] With the new gun the ion dynamics and redox processes in solid electrolytes such as sodium containing calcium phosphate glasses, lithium containing vanadophosphate glasses and sodium and potassium containing borosilicate glasses are examined. The results of the processes inside the sample are investigated by means of time-of-flight secondary ion mass spectrometry (short: ToF-SIMS). The first part of this work includes the construction and optimization of the low-energy electron gun. The electron optics were based on the concept by ERDMAN and ZIPF. A hairpin filament is installed as emitter. The main challenge is to construct an optics that is capable of accelerating and guiding the electrons towards the surface of the sample without inducing any damage to it. This soft-landing of the electrons on the specimen’s surface is accomplished by using a steel mask which is in touch with the sample and charged to the potential of the incoming electrons. Thus, in the region where the mask is open the electrons homogeneously charge up the surface of the specimen. This in turn leads to a potential and charge gradient inside the sample material and transport processes are induced. In this way, two different modes of operation are possible. Conductivity measurements are realized by contacting the sample at the backside i.e. the side which is not irradiated by the beam and measuring the current flow through the material while varying the acceleration voltage of the electrons. Constant voltage measurements are used to imprint concentration-depth profiles into the sample material in order to gain information about the diffusion and electro-diffusion processes and the charge carriers involved in the transport. These profiles are accessible by means of ToF-SIMS.[1,6,7,10,11] Further setup development is concerned with the design of two new sample holders. The challenge in the case of the first new design is that the sample mount shall combine the tasks of sample heating and measuring the backside current with the ability of rotating the mounting along the z-axis such that subsequent irradiation of the specimen is possible without venting the vacuum chamber. For the second new design simultaneous irradiation of the specimen’s front- and backside is desired. Therefore, heating as well as the measurement of the backside current are relinquished and the fixation of the sample is realized by two identical steel masks between which the specimen is pressed. In order to verify the measurement principle, sodium and potassium containing borosilicate glasses are investigated. These glasses are chosen because reference measurements with cation beams have been carried out earlier. The comparison between cation and electron irradiation shows almost perfect agreement in the values for absolute conductivity and activation energy. Subsequently to the proof of principle, samples of the borosilicate glass with differing backside electrode materials are analyzed. The aim of this analysis is to explore the influence of the electrode material on the transport processes inside the glass. The materials chosen are gold, platinum, chromium and copper. It is revealed that on a short time scale i.e. on a time scale on which a small amount of charge (Q < 2.5 mC) is transported through the sample the chemical identity of the backside electrode does not have a significant influence on the transport behavior.[15,16] On the longer time scale however, copper ions become mobile and begin to move from the electrode into the sample. This behavior is exclusively observed with the copper electrode. For further information on the processes leading to the mobilization of the copper electrode material, conductivity measurements as well as constant voltage measurements are carried out with copper electrodes of varying thicknesses. The electrode thickness is linked the way the electrode is attached to the sample. Thus, the electrodes are sputtered on to the specimen for thicknesses of 100 nm and 500 nm, mechanically pressed to the sample in the case of a 500 nm thin foil and a 0.05 mm thin copper sheet and glued to the glass in the case of a 2 mm thick copper plate. A significant influence of the thickness as well as the way of attaching the electrode to the glass is distinguishable. In another series of measurements, electro-thermal poling [17–19] is performed by means of electron irradiation of a sample covered with metal electrodes at the front- and the backside. The metals are chosen as in the former case, gold, platinum, chromium and copper. The results show that poling can be performed by applying the voltages by beam irradiation which might be of advantage for very thin or brittle samples. A different project in this thesis is concerned with the investigation of the lithium vanadophosphate glass system with the general composition x Li2O (55-x) V2O5 45 P2O5. Glasses of the molar fractions x = 0, 15, 20, 25, 30, 35, 40, 55 are synthesized and subsequently studied by combined electron irradiation and ToF-SIMS. Furthermore, the glass systems transition and melt temperature are determined. Several examinations of similar glass systems pointed at a mixed ionic and electronic conductivity in this sort of sample, but an exact distinction between the two mechanisms could not be made so far.[20–27] By systematic electron irradiation experiments of the specimen, it is possible to identify the transport mechanisms and to distinguish between them. Additionally, detailed information on the redox processes at the interphases of the sample is gained by electron beam irradiation and subsequent ToF-SIMS analysis. This includes the interphases at the front- as well as at the backside. Sodium containing calciumphosphate glasses are used as biomedical tissue for enhanced prosthesis adaption.[28–32] In order to reveal the underlying physico-chemical processes for enhanced bone and tissue growth, several glass samples of the composition 25 Na2O 30CaO 45 P2O5 were synthesized. In another project the second newly constructed sample holder for combined irradiation of three samples with electrons and potassium ions is used. The imprinted concentration-depth profiles are revealed by means of ToF-SIMS. The samples investigated are a borosilicate glass, a 40 Li 2O 15 V2O5 45 P2O5 glass as well as a 15 Na2O 40 CaO 45 P2O5 glass. Interestingly, only the first two specimens react to the irradiation. The 40 Li2O 15 V2O5 45 P2O5 glass shows an intensive green precipitation in the area irradiated by the electron beam. The ToF-SIMS profiles of the vanadophosphate and the borosilicate specimen reveal enrichment and depletion zones at both sides of the sample. These observations point to two different transport mechanisms inside the material. 15 Na2O 40 CaO 45 P2O5 does not show any modification of the sample material. The last project of this work is concerned with the variation of the ToF-SIMS sputter depth as function of the material examined.