The aim of this work is to create a process which allows the tailored growth of Ge nanocrystals for use in photovoltic applications. The multilayer systems used here provide a reliable method to control the Ge nanocrystal size after phase separation.
In this thesis, the deposition of GeOx/SiO2 and Ge:SiOx~ 2/SiO2 multilayers via reactive dc magnetron sputtering and the self-ordered Ge nanocrystal formation within the GeOx and Ge:SiOx~ 2 sublayers during subsequent annealing is investigated.
Mostly the focus of this work is on the determination of the proper deposition conditions for tuning the composition of the systems investigated. For the GeOx/SiO2 multilayers this involves changing the GeOx composition between elemental Ge (x = 0) and GeO2 (x = 2), whereas for the Ge:SiOx~ 2/SiO2 multilayers this involves changing the stoichiometry of the Ge:SiOx~ 2 sublayers in the vicinity of stochiometric silica (x = 2). The deposition conditions are controlled by the variation of the deposition rate, the deposition temperature and the oxygen partial pressure.
A convenient process window has been found which allows the sequential deposition of GeOx/SiO2 or Ge:SiOx ~2/SiO2 without changing the oxygen partial pressure during deposition. For stoichiometry determination Rutherford back-scattering spectrometry has been applied extensively.
The phase separation in the spatially confined GeOx and Ge:SiOx ~2 sublayers was investigated by X-ray absorption spectroscopy at the Ge K-edge. The Ge sub-oxides content of the as-deposited multilayers diminishes with increasing annealing temperature, showing complete phase separation at approximately 450° C for both systems (using inert N2 at ambient pressure). With the use of chemical reducing H2 in the annealing atmosphere, the temperature regime where the GeOx phase separation occurs is lowered by approximately 100 °C. At temperatures above 400° C the sublayer composition, and thus the density of the Ge nanocrystals, can be altered by making use of the reduction of GeO2 by H2.
The Ge nanocrystal formation after subsequent annealing was investigated with X-ray scattering, Raman spectroscopy and electron microscopy. By these methods the existence of 2 - 5 nm Ge nanocrystals at annealing temperatures of 550 (GeOx) - 700° C (Ge:SiOx ~2) has been confirmed which is within the multilayer stability range.
The technique used allows the production of extended multilayer stacks (50 periods ~ 300 nm) with very smooth interfaces (roughness ~ 0.5 nm). Thus it was possible to produce Ge nanocrystal layers with ultra-thin SiO2 separation layers (thickness ~ 1 nm) which offers interesting possibilities for charge transport via direct tunneling.:Contents
1 Introduction and motivation 1
2 Basic aspects 6
2.1 Microstructure of sub-stoichiometric oxides (SiOx, GeOx) 6
2.2 Phase transformations 9
2.3 Quantum confinement effect in nanocrystals 12
2.4 Applications of nanostructures in 3rd generation photovoltaics 17
3 Experimental setup 21
3.1 The magnetron deposition chamber 21
3.2 (Reactive) dc sputtering 22
3.3 Annealing processing 26
3.4 X-ray facilities 26
4 Analytical methods 30
4.1 Rutherford backscattering spectrometry (RBS) 30
4.2 Raman scattering 33
4.3 (Grazing incidence) X-ray diffraction (GIXRD) 35
4.4 X-ray reflectivity (XRR) 39
4.5 X-ray absorption near edge structure (XANES) 41
4.6 Transmission electron microscopy (TEM) 42
5 Properties of reactive dc magnetron sputtered Si-Ge-O (multi)layers 44
5.1 Deposition rate and film stoichiometry investigations 44
5.2 Stoichiometry dependent properties of GeOx/SiO2 multilayers 47
5.3 Lateral intercluster distance of the Ge nanocrystals in multilayers 51
6 Confined Ge nanocrystal growth in GeOx/SiO2 multilayers 54
6.1 Phase separation in GeOx single layers and GeOx/SiO2 multilayers 54
6.2 Crystallization in GeOx single layers and GeOx/SiO2 multilayers 58
6.3 Multilayer stability and smallest possible Ge nanocrystal size 60
6.4 Stacked Ge NC films with ultra thin SiO2 separation layers 66
7 Confined Ge nanocrystal growth in Ge:SiOx/SiO2 multilayers 71
7.1 Phase separation in Ge:SiOx/SiO2 multilayers 72
7.2 Crystallisation in Ge:SiOx/SiO2 multilayers 76
8 Summary and conclusions 79
List of Figures 83
List of Tables 85
Bibliography 86
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:25990 |
| Date | 27 April 2012 |
| Creators | Zschintzsch-Dias, Manuel |
| Contributors | von Borany, Johannes, Helm, Manfred, Meyer, Dirk C., Technische Universität Dresden |
| Publisher | Helmholtz-Zentrum Dresden-Rossendorf |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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