Electronic structure and transitions of REs
Rare-earth metals defined as the 15 lanthanides located below the main body of the periodic table plus Scandium and Yttrium elements as shown in fig. (1.6).
These lanthanide elements are characterized by occupancy of the 4f level by electrons (from 0 in La to 14 in Lu). The properties that depend mainly on the occupancy of 4f level make the interest of lanthanides. Although, the term Lanthanides in general is restricted to the 14 elements from Ce to Lu that contain partially or totally filled 4f electrons, Scandium (Sc), Yttrium (Y) and lanthanum (La) can be considered in the same group. They exhibit similar chemical and physical properties to the 14 lanthanides elements although they haven’t any electrons in the 4f level. The reason behind joining these three elements is the analogy of their electronic configuration of the outer shell three electrons with the lanthanides; (3 d 4s)3 for Sc, (4 d 5 s)3 for Y and (5d 6 s)3 for La. Thus, it shares all the lanthanides properties that are not depend on the 4f electrons. As we are interested in this work by the optical properties of cerium and ytterbium, we will focus our attention on the properties of the 14 lanthanides elements that contain electrons in its f orbital. Although, the lanthanide elements have similar basic electronic structures, the small differences between the individual elements result in diversity in their properties, in particular the optical behaviors. Therefore, in order to understand the basic characteristics of lanthanides, the electronic configuration should be considered. In the following table (1.2), all the lanthanides’ electronic configurations in metallic and ionic forms have been stated.
Rare earth doped in solid hosts (semiconductor/insulators)
Rare earth-doped solid hosts attract great attention from luminescence devices point of view by combining the luminescence properties of REs with the electronic features of the host matrix. It also attracts a particular interest in the field of integrated optoelectronic devices. Rare earth based-devices have extended to cover many sectors from energy harvesting (e.g. solar cell) moving on telecommunications to lighting devices (e.g. lasers and displaying units). Although, the trivalent REs are of a great technological interest due to their sharp emission lines, some divalent REs ions that exhibit broad emission (e.g. Sm+2 and Eu+2) find their way to the devices required broad band emission such as w-LEDs. As consequence of shielding the 4f electrons by 5s and 5p electrons, it is believed that the electronic energy levels of the 4f are influenced mainly by the spin-orbit interaction while the electron phonon coupling with the host becomes weak. Nevertheless, the energy levels of intra-f electrons have been found slightly affected by the applied crystal field. For instance, this can be observed in the difference of the emission lines and photoluminescence excitation of Er doped hexagonal and cubic GaN, which referred to the difference in the local symmetry around Er ions.Hence one can conclude that, although the strong shielding of the intra-f electrons of the REs, the significant influence of change in the local environment around it cannot be excluded.
Issues related to the luminescence of RE-doped solids
Beside the luminescence behavior, incorporation of rare earth in solid hosts raises some key questions about the lattice location of the RE inside the host and the kind of interaction with the host material. In the following section some of these points will be briefly covered.
• RE site location in the host lattice structure:
As mentioned before, the local environment around the RE in the host material can induce modifications in the electronic structure of the RE. These modifications, sometimes, are of great importance such that it can modify the optical response of the RE by altering the transition probabilities of the electronic levels. For example, Steckl, A.J. et al[57-58], demonstrated how the Eu lattice site can play a role in changing the emission wavelength, emission cross section and the PL decay rate of two different Eu sites in GaN, as shown in fig (1.11).  They attributed their results to the different local environment for each site.
More precisely, the Eu ions located very close to the center of Ga substitution site (Euy) are subjected to the local symmetry and participate to the spontaneous emission with low emission cross section. In contrast, the Eu ions located in interstitial-like site (Eux) suffer from a distorted local environment and contribute in stimulation emission with high emission cross section. Furthermore, Gan, L., et al attributed the effect of the crystallographic site of Ce on the structure and the luminescence properties of β-SiAlON:Ce phosphor to the different number of coordination with the surrounding N(O) atoms. Their experimental observations proposed three locations for Ce ions in this phosphor: two of them are in the interstitial sites with 6 and 9 coordination numbers and the third one is in substitution site for Si(Al) atoms. These different coordinations with Ce lead to structure and luminescence wavelength modifications. However, in general the common location of RE in the crystal hosts, in particularly at high concentration of RE, is segregation close to the grain boundaries due to the large ionic sizes of the REs and the less rigid structure at the crystal grain boundaries.[60-62] Therefore, the dependency of the luminescence of such phosphors on the local structure of the RE opens the door for intensive structure investigations in order to better understand the luminescence from atomic scale point of view.
• RE ion- host interaction:
One of the most important process results from the RE ions-host interaction is the multiphonon relaxation. In this process, the emitting photons energy are used to excite the vibration modes of the host material instead of de-activate via radiative pathways which, in turn, lead to luminescence quenching or suppression some of optical transitions in the activator. Generally speaking, the contribution of multiphonon relaxation can be estimated considering the phonon cut off energy of the matrix relative to the energy of the emitting photons of the activator. If the phonon cut off energy is higher than 25% of the photon energy, complete luminescence quenching can be occurred. In contrast, if the phonon cut off energy less than 10% of the photon energy, the multiphonon relaxation can be neglected. Between 10% and 25%, quenching is temperature dependent. Thus, the prior knowledge about the phonon energy of the matrix and the optical transition of the activators is essential for appropriate selection of good matrix for certain activator. For example, in case of erbium doped silica matrix, the phonon cut off energy of silica is 1100 cm-1 then erbium optical transition I13/2 – I15/2 emitting at 1535 nm (6500 cm-1) is weakly quenched at room temperature (1100/6500 = 16%). In contrast, the erbium transition I11/2 – I13/2 at 2700 nm (3700 cm-1) is totally quenched by multiphonon (non-radiative) relaxation (1100/3700= 30%). Therefore, low phonon cut off energy host matrices are desirable to avoid such unwanted multiphonon relaxation. It is worth to note that the maximum phonon energy of AlN material is about 700 cm-1, which is low and compatible with many of rare earth ion transitions. In other words, the multiphonon relaxation process in AlN starts to have a very weak role for optical transitions at 7000 cm-1 (i.e. 10% of the optical transition energy).
• RE ion- ion interactions:
Ion- ion interaction process is a well known and characteristic phenomenon taking place with the RE ions. This process can occur between similar RE ions, for instance at high RE concentrations, and in some case leads to luminescence quenching due to re-absorption process or can lead to RE oxidation conversion by electron exchanges. On the other hand, the same process can occur between two different RE ions in order to sensitize one RE ion by another one. In a well managed sample doped by two different REs, one of them can absorb the excitation pump and transfer the energy to the other ion, which opens several pumping schemes for the sample.[66-69].
Rare earth-doped semiconductors (RE-Sc)
RE-doped semiconductors have attracted great attention from the scientific and industrial societies in order to integrate the RE emission in microelectronic technology. Combining the advantages of the electronic structure of semiconductor as well as its compatibility to the chip technology with the emission properties of the RE will open the door to new and wide spread lightening technology. Exploiting these features has been reflected in many applications especially in color displays and flat panel technologies. RE-doped silicon is the most studied system in this direction due to the compatibility of silicon with silicon microelectronic fields. However, silicon has serious drawbacks like having indirect bandgap which leads to significant unwanted non-radiative processes. Moreover, its low bandgap value (1.12 eV) makes the doping with RE ions is limited to few elements that emit in the IR region. Therefore, intensive research for new semiconductor hosts has been stimulated to overcome the silicon drawback points. The main characteristics that should be considered during searching for new hosts are: the compatibility with the silicon substrates, the direct bandgap (to reduce the non-radiative process) and the wide bandgap value (to cover the visible light range). It was found that III-V semiconductor compounds efficiently offer the integration compatibility with the silicon microelectronic technology. Among them, III-nitride compounds attract particular interest, especially GaN and AlN due to their direct and wide bandgap properties, see fig (1.12).
X-ray diffraction (XRD)
X-ray is considered as an electromagnetic radiation with very high photon energies (i.e. very short wavelength of few angstroms). Thanks to these very short wavelengths which are comparable to the size of atoms, X-rays are suited for probing the structural arrangement of atoms and molecules in a wide range of materials. When rays incident to a material, some photons are elastically scattered and deflected ‘diffracted’ from their original direction. These diffracted x-rays carry information about the atomic arrangement in the material. The diffracted waves from different atoms can interfere, and the resultant spatial intensity distribution is controlled by the atomic arrangement. Thus, X-ray diffraction (XRD) technique is considered as a powerful tool to explore the crystal structure of materials. The atomic spacing of crystalline material can be also extracted from (XRD) data. In addition, the shape, intensity, peak width and position of the XRD signal can be used to extract information about the crystal size and the internal stress of sample. The physical concept behind XRD principle is based on Bragg’s diffraction phenomenon The distribution of the diffracted waves is governed by the Bragg’s law:
Where d is the inter-planar distance,is the wavelength of the x-ray,the scattering angle, and n an integer representing the order of the diffraction peak. Fig (2.6) shows the ideal graphical representation for the Bragg’s diffraction process The constructive interference of the diffracted rays occurs when the previous Bragg’s condition, equation (2.1), is valid. Then, the inter-planar distance for specific crystallographic planes d can be estimated. The average size of crystallites can be also estimated from the XRD data by using Scherrer’s equation:.
where k is the dimensionless shape factor, λ the X-ray wavelength, β the line broadening at half the maximum intensity (FWHM) in radians, θ the Bragg angle.
The basic construction of an XRD apparatus (diffractometer) consists of a source of radiation (x-ray tube) followed by a monochromator to select the X-ray wavelength. Set of slits to adjust the shape and size of the x-ray beam are used to illuminate the sample with an X-ray beam of specific section. A sample holder is placed on a goniometer circle to control the sample’s center position with respect to the position of the incident x-ray and the detector.
In this thesis, the crystal structure and growth orientation of the films were analyzed by X-ray diffraction (XRD) in a Brucker D8 Advance system with Cu Kα1 radiation (λ= 0.15406 nm) in Bragg-Brentano geometry. A LINXEYE XE linear detector was used that exhibits excellent filtering of K radiation without the need of secondary monochromator. All samples were positioned on substrate holders and aligned with a reference plane of the holders. Holders were enabled to spin around their vertical axis during measurement with a total integration time of 1 hr.
Transmission electron microscopy (TEM)
TEM is an electron microscopy technique used for material imaging and diffraction purposes. Electrons are the main probe used in the processes. The TEM principle of work is based on the transmission of electron beam through an ultra-thin specimen. Interactions between these transmitted electrons and the specimen bring specific information about the microstructure, the atomic arrangements, the crystallographic and sometimes the morphology of the investigated materials. The main components of a TEM apparatus are a source of electrons (electron gun), a sample holder, electromagnetic lenses and detectors. Set of electron lenses and apertures are and placed before and after the sample holder to control and manipulate the emitted and the transmitted electron beams. All the setup has to be operated under high vacuum conditions in order to avoid scattering of the electrons by the background gas. The schematic diagram of the ray path in TEM is presented in fig. (2.7a). The electrons are generated by electron gun and accelerated to the required high energy directed to the specimen. Then, the transmitted and scattered electrons are collected by objective lens forming a diffraction pattern (DP) in the back focal plane and intermediate image in the image plane. Therefore both the image and the diffraction pattern (DP) are present simultaneously in the TEM. Switching between the two modes (image and DP) can be achieved by changing the strength of the intermediate lens and inserting aperture called (objective aperture) in the back focal plane in case of image mode or inserting aperture called (SAED aperture) in the image plane in case of DP mode.
Table of contents :
CHAPTER I General introduction
1.1 Aluminium nitride (AlN):Basic properties
1.1.1 AlN crystal structure
1.1.2 Electronic structure of AlN
1.1.3 Type of defects in AlN
1.1.4 AlN: Preparation methods
1.1.5 AlN: Applications
2.1 Rare-earth elements
2.1.1 Electronic structure and transitions of REs
2.1.2 Luminescence principles
2.1.3 Rare earth doped in solid hosts (semiconductor/insulators)
2.1.4 Issues related to the luminescence of RE-doped solids
2.1.5 Rare earth-doped semiconductors (RE-Sc)
2.1.6 Cerium in AlN
2.1.7 Ytterbium in AlN
CHAPTER II Experimental techniques
1.1 Thin film synthesis
1.1.2 Thermal annealing of the thin films
2.1 Thin film characterization
2.1.1 Structural characterization
2.1.2 X-ray diffraction (XRD)
2.1.3 Transmission electron microscopy (TEM)
2.1.4 Rutherford Backscattering Spectrometry (RBS)
2.2 Optical characterization
2.2.1 Fourier transform infrared spectroscopy (FTIR)
2.2.3 Photoluminescence (PL) spectroscopy
CHAPTER III Synthesis and characterizations of AlN thin films
2.1 Sputtering conditions
2.1.1 Crystallographic orientations
2.1.2 Microstructures and morphology
2.1.3 Optical properties
3.1 The optimum deposition conditions
CHAPTER IV Rare earth-doped AlN: Cerium-doped AlN
2.1 Results and discussion
2.1.1 Structures and compositions
2.1.2 Structures and optical analyses of Ce-doped AlN
2.1.3 X-ray diffraction (XRD)
2.1.4 Fourier Transform Infrared Spectroscopy (FTIR)
2.1.5 Microstructure and composition
2.1.6 Photoluminescence (PL)
2.1.7 The role of oxygen
2.2 Intentionally doping oxygen during Ce AlN growth
2.3 Low temperature photoluminescence dependence (LTPL)
3.1 Application perspective
CHAPTER V Yb-doped AlN and (Ce, Yb) co-doped AlN
2.1 Structures and compositions
3.1 Photoluminescence (PL)
4.1 Structure of (Ce,Yb) co-doped Al(O)N sample
6.1 Low temperature photoluminescence