DARWIN Digitale Dissertationen German Version Strich

FU Berlin
Digitale Dissertation

Marten Huisinga :
Ultraviolet photoelectron spectroscopy and electron stimulated desorption from CaF2
Ultraviolett-Photoelektronenspektroskopie und Elektronenstimulierte Desorption von CaF2

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Abstract

Calcium difluoride crystals were investigated with ultraviolet-photoelectron spectroscopy, optical absorption spectroscopy, and electron stimulated desorption. Photoemission from ionic crystals like CaF2 is characterized by some peculiarities that were investigated in detail. Among the investigated effects were the influence of phonons on photoemission (phonon broadening and "quasidirect transitions"), sample charging phenomena, and photoconductivity. It was shown that photoconductivity is caused by thermal detrapping of valence band holes (VK-centers) created in photoemission. Furthermore, the electronic structure of differently prepared CaF2 surfaces was investigated, and first UPS measurements from CaF2 crystals cleaved in UHV are shown. Impurity states on crystals that were cleaved in air and on polished crystals were investigated. By gas dosage experiments, these impurity related states could be assigned to oxygen. Further gas dosage experiments with films of different thickness show that oxygen penetrates the crystal. Changes in the electronic structure of CaF2 were also induced by electron irradiation (1.5 -2.5 keV) and photon irradiation (21.2eV). During electron irradiation of crystals cleaved in air, the oxygen dissolved in the lattice reacts with calcium metal to form CaO.

Desorption of F+ ions from CaF2 single crystals and epitaxial films was investigated using electrons in the energy range from 0.6 to 3.0 keV. Irradiation resulted in a positively charged sample surface, thus giving an undetermined electrostatic contribution to the F+ desorption energies. In this thesis, a method was developed allowing for the first time to measure a positive surface potential of an insulator under electron irradiation. Thus, the kinetic energy of F+ ions desorbing from CaF2 crystals could be corrected for surface charge to determine the energy characteristic for the desorption process. The corrected peak of the F+ kinetic energy distribution was at about 0.9 eV for crystals cleaved in UHV. It was shown that coverage of the surface with oxygen or fluorine results in a high positive potential, while metallization causes a low surface potential. In particular, the surface potential of crystals cleaved in UHV shows a pronounced minimum after about 10 to 20 min of electron irradiation. This effect could be explained by a balance of F+ desorption and diffusion of fluorine from the bulk.

Table of Contents

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1. Introduction

2. The investigated material: Calcium fluoride

2.1. Structure of the CaF2 crystal

2.2. Defects and surface defects in CaF2

2.3. Defect aggregation: Metal colloids

2.4. Stopping power and penetration depth of low energy electrons in CaF2

2.5. Mean free path in insulators

3. Photoemission

3.1. Angular resolved photoemission

3.2. Dipole selection rules

4. Experimental setup

4.1. UHV system

4.2. Ultraviolet photoelectron spectroscopy

4.3. Optical spectroscopy

4.4. Electron stimulated desorption

4.5. Preparation of thin epitaxial CaF2 films on Si (111)

5. Ultraviolet photoelectron spectroscopy (UPS) on CaF2

 5.1. Introduction

5.2. Basic features

5.3. Photoemission from differently prepared CaF2 surfaces

5.4. Irradiation induced effects in CaF2

6. Optical transmission spectroscopy on CaF2

7. Electron stimulated desorption (ESD)

7.1. Introduction

7.2. Surface potential determination

7.3. Charge corrected kinetic energy of F+ ions

7.4. Field driven diffusion

7.5. Desorption from thin epitaxial CaF2 films

8. Summary

Literaturverzeichnis

More Information:

Online available: http://www.diss.fu-berlin.de/1999/24/indexe.html
Language of PhDThesis: english
Keywords: photoemission; CaF2; insulator; desorption; charging
DNB-Sachgruppe: 29 Physik, Astronomie
Classification PACS: 73.61.Ng 79.60.-i 73.20.Hb 79.20.La 79.20.Hx
Date of disputation: 17-Dec-1998
PhDThesis from: Fachbereich Physik, Freie Universität Berlin
First Referee: Prof. Dr. Dr.h.c. Eckart Matthias
Second Referee: Prof. Dr. Karl-Heinz Rieder
Contact (Author): huisinga@matth1.physik.fu-berlin.de
Contact (Advisor): matthias@physik.fu-berlin.de
Date created:22-Apr-1999
Date available:28-Apr-1999

 

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