Observation Of Visible Photoluminescence Research Articles

Photoluminescence and cathodoluminescence of GaN doped with Pr

We report on our observation of visible photoluminescence and cathodoluminescence of Pr-implanted GaN. The implanted samples were subjected to isochronal thermal annealing treatments at a temperature of 1100 °C in N2 at atmospheric pressure to recover from implantation damage and activate the rare earth ions. The sharp characteristic emission lines corresponding to Pr3+ intra-4fn-shell transitions are resolved in the spectral range from 400 to 1000 nm and observed over the temperature range from 12 to 335 K. We have developed an energy level diagram for Pr3+ ions in GaN using recorded spectra. The photoluminescence decay kinetics measurements of P13, P03, and D21 levels and quenching mechanism analysis allow us to conclude that the dominant de-excitation process is of electric dipole–electric quadrupole type. We found also that the D21 level separated from the upper P03 level by 3755 cm−1 energy gap can be populated in several ways, by direct energy transfer processes, cross relaxation |3P0,3H4〉→|1D2,3H6〉 and cascade processes from the P03 or higher levels. The full width at half maximum of the strongest photoluminescence line at 653 nm, the P03 level, is 2.9 meV at 13 K, with a 1.86 meV blue peak shift in going from 13 to 330 K temperature, while the line at 670 nm, P13 level, has a full width at half maximum of 4 meV at 13 K and a 1.6 meV red peak shift. The thermal stability of GaN:Pr3+ epilayers indicates the suitability of this material for visible optoelectronic devices.

The diffusion redistribution of hydrogen and oxygen in porous silicon films

A porous silicon (PS) layer prepared by electrochemical anodization of a single crystal silicon surface has been known for many years. Crystalline structure of the PS is network of silicon is nanometer-sized regions, surrounded by void space, with very large surface to volume ratios [1]. The PS surfaces are covered by silicon hydrides and silicon oxides. Observation of visible photoluminescence from PS has generated widespread interest because of the potential application of PS in Si-based optoelectronic technology. The luminescence, optical and other properties of PS are determined by incorporation of hydrogen and oxygen on surfaces and the formation of Si-H, Si-H2 and Si-O bonds. Therefore stability of luminescence and optical characteristics of PS depend on processes of concentration redistribution of hydrogen and oxygen atoms under treatments. This, in turn, is limited by oxygen and hydrogen diffusion along inner PS surfaces with simultaneous penetration into silicon. The formation mechanism of PS and the luminescence mechanism have been actively discussed, whereas the composition changes of PS caused by hydrogen and oxygen diffusion, and by appropriate changes in the number of Si-H, Si-H2 and Si-O bonds due to thermal treatments are rarely considered. From FTIR measurements, a reduction of photoluminescence intensity of PS with simultaneous decrease of intensity of Si-H2 peaks as results of annealing at 25–400 ◦C was observed by Tsai et al. [2]. Hydrogenand oxygenrelated bonds changes during PS oxidation for the range 20–500 ◦C at atmospheric pressure has also been reported [3, 4]. The aim of the present work is to determine the diffusion parameters of hydrogen and oxygen atoms in free-standing PS films from analysis of changes intensity of Si-H and Si-O absorption peaks as a function of temperature and duration of annealing in air. The PS films were prepared on n-type Si (100) substrates of resistivity ρ= 0,09A cm by anodic etching in a solution HF : H2O= 1 : 3 at a current density of 15 mA/cm2 under the white light illumination. The PS films were then detached from silicon substrate by electropolishing under former solution with a current density of 0.8–1.0 A/cm2. The free-standing PS films and PS film on Si substrates were characterized by porosity, photoluminescence and FTIR measurements at room temperature. Porosity of films was determined by gravimetry. Freestanding PS films of thickness 10–20μm and porosities 70–80% were obtained. FTIR spectra of free-standing PS films were measured at room temperature by using a Mattson 1000 spectrometer. The photoluminescence spectra were excited by the 325 nm line of He-Cd laser under a power output of 20 mW. This investigation focuses on the analysis of changes of absorption coefficients of Si-H, Si-H2 (doublet at 2100 cm−1) and Si-O (1100 cm−1) peaks as a result of isothermal annealing at temperatures in the range 65–185 ◦C at atmospheric pressure. Isothermal annealings of PS films were carried out at 65, 80, 110, 150 and 185 ◦C. Time-dependence FTIR and photoluminescence spectra were measured after 5-min cycles of isothermal annealings. The distribution of photoluminescence intensity along thickness of PS films (on silicon substrates) has been analyzed by sequential removal of thin layers (about 1μm) from PS film and the photoluminescence intensity measurements. The absorption coefficient (α) is deduced from transmission spectra by solving α in the equation

PHOTOLUMINESCENCE FROM NANO-CRYSTALLITES OF SILICON FILMS PREPARED BY PECVD

We report in this paper the observation of visible photoluminescence (PL) at room temperature from nano-crystallites of silicon films, which are prepared in a PE CVD system by using strong hydrogen diluted silane as the reactant gas source, without any post-processings. We attribute the PL to the photons above the band gap of bulk crystal silicon created by the carriers in the nano-crystallites under the function of quantm size effect. The effective deposition parameters are concerned.

Visible photoluminescence from porous Si formed by annealing and chemically etching amorphous Si

We have studied the visible photoluminescence (PL) and microstructure of amorphous Si (a-Si) after annealing and etching. The a-Si layers were grown on (100)Si substrates and partially crystallized by annealing between 550–1150 °C. Porous Si layers (PSLs) were then produced by etching in HF-HNO3. While no visible PL was observed from unannealed and etched a-Si, visible PL was detected after annealing and etching. The observation of visible PL after etching coincided with the observation of Si microcrystallites in the annealed layer. The results suggest that an initial crystalline structure is important for fabricating luminescent PSLs.