H. Rinnert

Photoluminescence and electroluminescence of size-controlled silicon nanocrystallites embedded in SiO2 thin films

Silicon nanocrystallites (nc-Si) were obtained by thermal annealing treatments of SiO∕SiO2 multilayers, prepared by evaporation. The nc-Si size was controlled by the SiO thickness. In this study the SiO2 thickness was maintained at 5nm and the SiO thickness was varied from 2to6nm. The film’s microstructure was studied by transmission electron microscopy. A strong photoluminescence was obtained in the visible range which corresponds to the radiative recombination of electron-hole pairs in the nc-Si. The electroluminescence signal is weaker and broader than the photoluminescence one. A model taking into account SiO2 defects and nc-Si is proposed to explain the electroluminescence results.

Evolution with the annealing treatments of the photoluminescence mechanisms in a-SiNx:H alloys prepared by reactive evaporation

A wide range of amorphous hydrogenated silicon nitride thin films with an excess of silicon was prepared by evaporation of silicon under a flow of nitrogen and hydrogen ions. A strong visible photoluminescence at room temperature was observed for the as-deposited films as well as for films annealed up to 1100°C. The chemical composition and the structure of the films were investigated using x-ray photoelectron, thermal desorption, and Raman spectroscopies, infrared absorption measurements, grazing incidence x-ray diffraction experiments, and transmission electron microscopy. Two luminescence mechanisms were identified for the films depending on the annealing temperature. For annealing temperatures below 650°C, the films are made of amorphous silicon-rich phases mixed with nitrogen-rich phases. These inhomogeneities in the chemical composition, coupled with the evolution of the photoluminescence energies and intensities with the hydrogen content, suggest that the emission is due to the recombination proces...

Structural, optical, and electrical properties of Yb-doped ZnO thin films prepared by spray pyrolysis method

Yb-doped ZnO thin films were prepared on glass substrates by spray pyrolysis technique in order to investigate the insertion of Yb ions in the ZnO matrix and the related optical properties of the films. The molar ratio of Yb in the spray solution was varied in the range of 0–5 at. %. X-ray diffraction patterns showed that the undoped and Yb-doped ZnO films exhibit the hexagonal wurtzite crystal structure with a preferential orientation along [002] direction. No secondary phase is observed in Yb-doped ZnO films. All films exhibit a transmittance between 75 and 90% in the visible range with a sharp absorption onset about 375 nm corresponding to the fundamental absorption edge at 3.3 eV. The photoluminescence measurements show a clear luminescence band at 980 nm that is characteristic of Yb3+ transition between the electronic levels F25/2 and F27/2. This is an experimental evidence for an efficient energy transfer from ZnO matrix to Yb3+. Hall effect measurements showed low resistivities and high carrier mob...

Visible photoluminescence in amorphous SiOx thin films prepared by silicon evaporation under a molecular oxygen atmosphere

A simple reactive evaporation method is proposed to prepare light-emitting amorphous SiO x thin films. By evaporating pure silicon in a controlled molecular oxygen atmosphere, it is possible to obtain a very large composition range. By changing the pressure in the preparation chamber, x can be varied from 0.7 to 1.85. The composition and the structure of the films were investigated using energy dispersive x-ray, infrared absorption and x-ray photoelectron spectroscopies. The samples contain amorphous silicon clusters dispersed inside an insulating silicon oxide matrix. The room-temperature photoluminescence properties were then measured. By conveniently choosing the oxygen pressure, the as-deposited films exhibit visible photoluminescence without any annealing post-treatments. The luminescence intensity initially increases with excess silicon concentration and then disappears for a too-high silicon excess. The above effect is interpreted in terms of confinement of the amorphous silicon clusters in the insulating matrix.

Visible photoluminescence in amorphous SiNx thin films prepared by reactive evaporation

Photoluminescence in the visible domain can be observed in amorphous silicon nitride (a-SiNx) alloys prepared by evaporation of silicon under a flow of nitrogen ions. A strong improvement of the photoluminescence intensity was obtained with annealing treatments in the range 500–1150 °C. Structural investigations were performed by infrared and Raman spectrometry experiments. The optical gap was obtained from transmission measurements in the ultraviolet, visible, and near infrared range. The evolutions of the structure and the optical properties with annealing treatments are correlated to the evolution of the photoluminescence.

Structure and photoluminescence properties of evaporated GeOx thin films

Amorphous GeOx alloys were prepared by evaporation of GeO2 powder on substrates maintained at 100°C. The evolution of the structure was investigated by infrared-absorption spectrometry, Raman spectrometry and transmission electron microscopy experiments for annealing temperatures less than 600°C. These experiments allowed us to follow the phase separation of the alloy and to observe the appearance of amorphous and crystallized Ge aggregates. The evolution of the photoluminescence in the range of 560–1550nm was correlated to the structure of the films.

Photoluminescence of Nd-doped SnO2 thin films

Structural, optical, and electrical properties of Nd-doped SnOx thin films are reported. The atomic structure was characterized by x-ray diffraction and infrared absorption spectrometry. Investigation of the photoluminescence properties revealed Nd-related bands at 920 and 1100 nm for samples annealed at 700 °C, which present the tetragonal structure of the SnO2 rutile phase. Nd3+ ions can be indirectly excited and no concentration quenching was observed up to 3 at. %. It is concluded that Nd3+ ions are efficient optically active dopants in addition to be responsible of the observed electric conductivity improvement. These materials are then interesting for solar cell applications.

Optical and structural properties of Nd doped SnO2 powder fabricated by the sol–gel method

We report on the structural and optical properties of undoped and neodymium doped SnO2 powders (0, 1, 3, and 5 at% of Nd) synthesized by the sol–gel method. SEM and TEM microscopy techniques reveal a nanometric scale of the powders. We show that the tetragonal rutile phase is achieved after annealing at 700 °C. The crystallite size of the doped SnO2 is found to decrease gradually with the increase of Nd content without changing the SnO2 structure. A strong decrease in the intensity of the Raman peaks is noted for doped powders, which can be attributed to the location of Nd3+ ions at the Sn sites indicating Nd incorporation into the host matrix. For the first time the optical properties were studied by UV-Vis-NIR spectroscopy and revealed Nd related absorption bands in the SnO2 matrix. The investigation of the photoluminescence properties shows broad emission centred around 550–650 nm originating from defects present in the SnO2 host matrix. Under 325 nm laser excitation, a strong photoluminescence of trivalent Nd is observed in the infrared region and shows Nd related emission peaks at 885, 1065, and 1336 nm. Such a strong PL signal under laser excitation indicates that Nd3+ is optically active. The excitation dependent PL (PLE) recorded in the 450–700 nm range confirms the presence of active Nd3+ successfully inserted into the SnO2 host matrix.

Improvement of the photoluminescence properties in a-SiNx films by introduction of hydrogen

Photoluminescence properties of amorphous hydrogenated silicon nitride thin films with various compositions are presented. The as-deposited samples prepared by evaporation of silicon under a flow of nitrogen and hydrogen ions exhibit visible photoluminescence at room temperature without any annealing treatment. The evolution of the photoluminescence properties with increasing nitrogen concentration in the films is correlated to structural investigations performed with Fourier-transform infrared spectroscopy and optical characterization obtained from transmission measurements in the ultraviolet-visible-near-infrared range. It is shown that the introduction of hydrogen is of prime importance to improve the photoluminescence intensity of the films.

Structure and photoluminescence properties of evaporated GeOx∕SiO2 multilayers

Amorphous GeOx∕SiO2 multilayers were prepared by successive evaporations of GeO2 and SiO2 powders onto substrates maintained at 100°C. The evolution of the structure was investigated by x-ray reflectometry, transmission electron microscopy, infrared-absorption spectrometry and Raman spectrometry for annealing temperatures less than 800°C. These experiments allowed us to follow the phase decomposition of the GeOx alloy and to observe the appearance of amorphous and crystallized germanium aggregates. The evolution of the photoluminescence in the range of 0.8–2.2eV was correlated to the structure of the films.