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Elaboration techniques for luminescent silicon nanostructures Porous silicon
Porous silicon has been the first silicon-based system on which efficient room temperature photoluminescence has been observed. It is obtained by an electrochemical wet etching process on monocrystalline silicon employing fluoridic acid. To improve the etching process, an electric voltage is applied on the silicon substrate. The etching process leads to the formation of micropores of crystalline silicon. After the appropriate etching time, silicon filaments with a diameter of several nanometers are formed. Dangling bonds at the surface of the micropores are passivated by the hydrogen of the fluoridic acid [12]. During exposure to the air, the silicon filaments are progressively oxidized [13]. After this oxidation, the silicon filaments are sometimes considered as splitted into grains surrounded by silicon oxide [14].
Porous silicon has several very interesting properties for applications such as the high radiative recombination efficiency at room temperature and the fact that the emission wavelength can be shifted throughout the whole visible range. On the other hand, there are some important inconveniences: as a consequence of the large size distribution of the crystallites, the emission bandwidth is very large. In addition, the micropores are not very stable from mechanical and thermodynamical points of view. Finally, the wet etching technique is incompatible to common techniques in microelectronics. More convenient systems are essentially thin films of silicon oxide [15], silicon nitride [16] or multilayers of Si/SiO2 [17], Si/CaF2 [18] and SiO/SiO2 [19]. In the following, we will discuss more in detail silicon nanostructures embedded in silicon oxide.
Silicon nanocrystals embedded in silicon oxide matrix
The fabrication of silicon nanostructures that are more stable than porous silicon can be achieved by embedding the silicon nanostructures into a thermodynamically stable matrix as silicon oxide, e.g. In the following we will detail different methods to fabricate stable silicon nanotructures.
Ion beam implantation: high energetic silicon ions with energies up to 200 keV are implanted into silicon oxide thin films [15, 20, 21]. After the implantation, the silicon ions are distributed in the matrix according to the beam energy and profile. To produce precipitation of the ions in nanoclusters, it is necessary to anneal them at high temperatures up to 1100°C. As a function of the irradiation dose, the concentration of silicon on the film can be precisely controlled and thus the size of the silicon nanoclusters. Even the vertical position of the silicon nanoparticles can be controlled by adjusting the acceleration voltage. Typically, this technique is used for very large scale integration (VLSI) microelectronics.
Segregation in a SiOx thin film (1<x<2): the segregation of a sub-stoichiometric SiOx layer can also be employed to generate silicon nanoparticles [11,22]. High temperature annealing at temperatures over 800°C leads to the decomposition of the SiOx thin film into two thermodynamically stable components following the segregation equation 2SiO SiO2 + Si (1-3).
By controlling the stoichiometry factor, one has the possibility to influence the size of the precipitations: the less excess silicon is in the film, the smaller are the crystallites. Such a kind of film can be produced by different techniques: evaporation [22], sputtering [23] or chemical vapour deposition [11].
Multilayers: in multilayered structures as Si/SiO2 [17, 24, 25, 26] or SiO/SiO2 [10], the size of the silicon nanoparticles can be controlled by the thickness of the Si or SiO layer, respectively. Both systems have to be annealed to obtain silicon nanocrystals. In the first system, the nanocrystals are created by the crystallization of the initially amorphous silicon layers on the system [17]. In the SiO/SiO2 multilayers, segregation takes place as in the case of the SiOx thin films. The aim of this deposition technique is that the intermediary SiO2 layers avoid diffusion of silicon atoms between the silicon-rich layers. Thus, the growth is limited to the thickness of the silicon or SiO layers in the direction of the surface normal of the sample. As a consequence, the size distribution of the silicon nanocrystals is limited, too. Correspondingly, the bandwidth of the photoluminescence band is reduced with respect to the two above presented techniques [27]. At the same time the segregation temperature is increased with respect to SiOx thin films.
Modification of the electromagnetic field by a cavity
Such a planar microcavity – called Fabry-Perot interferometer type microcavities – is outlined in Figure 1.13. Two mirrors M1 and M2 with reflectivities R1 and R2, respectively, are confining a space in between of length Lcav. eventually filled with a medium of refractive index n. The mirrors are aligned parallel to each other so that the light inside the cavity can only propagate along the surface normal of the mirrors.
Ideal loss-less planar cavity: In order to get a good idea of the physical process, let us consider the case of an ideal planar cavity. It is constituted of two planar non-absorbing mirrors of reflectance 1. These mirrors are oriented perpendicularly to the propagation of the light wave in a distance Lcav. from each other. Compared to the emission wavelength, their extension in x and y direction is considered as infinite. The light is reflected on the surface of each mirror by interaction of the electromagnetic wave and surface plasmons of the mirror. Summing up incident and reflected waves, we obtain a stationary wave inside the cavity with oscillation nodes at the position of the mirrors. As the dimensions of the cavity are infinite in the x – y – plane, this resonance condition imposes a quantification of the wave-vector only in the z – direction: kzm = m. /L, m being an integer indicating the order of the resonance. Thus, we obtain an electric field E, which is proportional to sin (kzmz).
If we place an emitter inside a cavity, where the electric field is modified with respect to free space, its coupling to the field and thus its emission depends on its position in the cavity (mainly z – direction). Consequently, the coupling between emitter and field depends on the intensity of the stationary wave E2~sin2(kzmz) at the position of the emitter in the cavity. In a node, E2=0, in an anti-node E2 is maximal.
In the low loss regime, the transmission of the considered cavity can be described in the same way as the transmission of a Fabry-Perot interferometer: T 1 , (1-9) / 2 ) sin 2 1 (4F2 ( /2).
UV-visible-NIR absorption spectrophotometry
The optical measurements were performed on a dispersive dual beam Cary 5 spectrometer by Varian in the transmission configuration. The analysis of the transmission of films was performed in the ultraviolet, visible and near infrared spectral range (UV-Vis-NIR – 200-3000 nm). The spectrometer is equipped by two different radiation sources, a deuterium UV lamp and a tungsten filament lamp for the Vis-NIR spectral range. Two detectors are used, depending on the spectral range to analyze the intensity of the transmitted signal, a PbS photodetector for the NIR and a photomultiplier (PM) for the visible and UV ranges. The spectral resolution of 1nm is chosen which is quite sufficient for the type of measurement that we made (the device can work with a resolution of about 0.1 nm).
The substrates are fused silica plates with a thickness equal to 1 mm. This substrate has the advantage of having very low absorption across the spectral range studied, particularly in the near UV. Moreover, the dispersion relation of the refractive index is well known.
A reference spectrum from the air is recorded for each series of films with the substrate from the same slide to avoid possible effects due to the substrate. The transmission of the film deposited on the substrate is also seen from the air. The signal analyzed is the ratio of the two transmissions (Figure 2.5).
Electron energy loss spectroscopy
Electron Energy Loss Spectroscopy (EELS) is a spectroscopy that serves to analyze the energy spreading of the initially almost mono-energetic electrons of the electron beam of the transmission electron microscope, after their interaction with the sample. The interaction takes place inside the specimen and information about its structure can be obtained by passing the transmitted beam into a spectrometer.
Fast electrons passing through the matter undergo a great number of scattering events, which can be divided into two groups: elastic scattering, where the transmitted electrons do not suffer any energy loss and inelastic scattering of the electrons through interaction with the material.
Elastic scattering is due to Coulomb interaction with the atomic nucleus, which represents a highly repulsive centre. Electrons, approaching to its vicinity, can be deflected to large angles. Actually, most of the incoming electrons pass far from the centre of an atom, where the nuclear field is partially screened by the electronic cloud and electrons, therefore, are scattered only to smaller angles. Elastic scattering provides the most important contribution to the contrast in conventional TEM images.
Table of contents :
Chapter 1. Bibliography and state-of-the-art
1.1 Introduction
1.2 Silicon nanocrystals
1.2.1 Quantum confinement of the exciton in silicon nanostructures
1.2.2 Elaboration techniques for luminescent silicon nanostructures
1.3 Microcavities
1.3.1 Principle
1.3.2 Free space spontaneous emission
1.3.3 Modification of the electromagnetic field by a cavity
1.3.4 Modification of the spontaneous emission rate
1.3.5 Cavity geometries
1.4 Summary & structure of this work
Chapter 2. Experimental & Theoretical Techniques
2.1 Introduction
2.2 Evaporator
2.3 Thermal annealing
2.4 UV-visible-NIR absorption spectrophotometry
2.4.1 Experimental
2.4.2 Principle of the direct simulation method
2.4.3 Swanepoel’s method of envelopes
2.4.4 Principle of the transfer matrix simulation method
2.5 Glancing incidence x-ray reflectivity
2.6 Transmission electron microscopy
2.6.1 Principe of the transmission electron microscopy
2.6.2 Preparation techniques
2.6.3 Electron energy loss spectroscopy
2.6.4 Energy filtered transmission electron microscopy
2.7 Infrared absorption spectroscopy
2.7.1 Principle
2.7.2 Vibrational spectrum in oxygen-rich amorphous silicon
2.7.3 Vibrational characteristics in a- SiO2
2.7.4 Analysis of the stoichiometry
2.8 Raman spectroscopy
2.8.1 Working principle & fundamental notes
2.8.2 Density of vibrational states in silicon
2.8.3 Raman effect in silicon nanocrystals
2.8.4 Discussion on the interpretation of the vibration at 480 cm-1
2.8.5 Raman scattering experiments
2.9 Photoluminescence
2.9.1 Continuous photoluminescence setup – general photoluminescence measurements
2.9.2 Time-resolved photoluminescence setup – lifetime measurements
Chapter 3. Analysis of the silicon nanocrystals
3.1 Introduction
3.2 Preliminary investigations on SiOx thin films
3.2.1 Infrared absorption spectroscopy
3.2.2 Photoluminescence spectroscopy of SiO and SiO1.5 thin films
3.2.3 Photoluminescence and Raman modelization of the experimental spectra
3.3 Formation of the silicon nanocrystals in the SiOx/SiO2 multilayers by RTA annealing
3.3.1 Photoluminescence and Raman models for multilayered samples
3.3.2 EFTEM analysis
3.3.3 Photoluminescence
3.4 Application of lithographic techniques to further reduce the surface density
3.5 Conclusion
Chapter 4. Study of silicon nanocrystals inserted in microcavities
4.1 Introduction
4.2 Choice of the materials for the distributed Bragg reflector
4.3 Optical properties of the Si and SiO2 thin films
4.3.1 Experimental procedure
4.3.2 Properties of the silicon layer
4.3.3 SiO2 layer
4.3.4 Active SiO/SiO2 layer
4.4 Distributed Bragg reflectors
4.4.1 Influence of the number of dielectric pairs
4.4.2 Influence of the annealing temperature
4.4.3 Importance of the layer thickness
4.5 Microcavities
4.5.1 Influence of the number of dielectric pairs in the Bragg mirrors
4.5.2 Fabrication of the cavities and study of their thermal stability
4.5.3 Importance of a constant layer thickness
4.5.4 Influence of the thermal annealing on the transmittance spectra of the cavity
4.5.5 Influence of the thickness of the active layer
4.6 Photoluminescence
4.6.1 Choice of the excitation wavelength
4.6.2 Microcavity with distributed Bragg reflector containing two Si/SiO2 bilayers
4.6.3 Microcavities with distributed Bragg reflectors containing three Si/SiO2 bilayers
4.6.4 Time resolved spectroscopy
4.6.5 Cavity based single dot spectroscopy
4.7 Conclusion
General Conclusion
Bibliography
