Solid State Phenomena Vols. 99-100

Abstract: The paper presents a review of the results of the synthesis and characterization of Si nanoclusters and Er co-doped glasses for optical amplifiers obtained in the framework of the EC IST-SINERGIA Project.

Abstract: Luminescent and non-luminescent nano materials are used today in a variety of lighting applications. One example is the thin layer of densely packed nano alumina between glass and phosphor in fluorescent lamps that serves to increase the efficiency, to improve the maintenance and to reduce the mercury consumption of the lamps. Further, non-luminescent nano materials may be used to form a thin conformal coating around phosphor particles that protects the phosphor particles and improves the efficiency and maintenance of fluorescent lamps. Luminescent nano materials have been the subject of extensive research in the last 15 years. For particle sizes below 20 nm, the optical properties of nano phosphors change, e.g. the exciton energy increases and perhaps even more important, scattering decreases. We have synthesized nano Y2O3:Eu of particle sizes down to 10 nm, derived its optical properties, and investigated its use in Hg-free fluorescent lamps based on the Xe-excimer discharge. The possibility of using nano phosphors, e.g. semiconducting nano phosphors as CdSe or rare-earth doped phosphors as LaPO4:RE in LEDs, will be discussed.

Abstract: Nanocrystalline powders of Lu2O3:Eu with activator content varying between 0.2%-10% were prepared using four different methods of synthesis. The products differed in their microstructure and crystallites sizes. Combustion of Lu(NO3)3 with urea produced strongly agglomerated material, most probably with significantly non-uniform distribution of the Eu3+ dopant. Replacing urea with glycine for the combustion produced only slightly agglomerated, voluminous, fluffy powder. Applying the Pechini technique resulted in significantly agglomerated powder while the homogeneous precipitation of Lu(OH)3 with urea at 90 °C and its subsequent decomposition to Lu2O3 at 650 °C resulted in a powder of perfectly spherical particles with a uniform size of about 130 nm with very low agglomeration. The efficiency of X-ray excited luminescence of our nanocrystalline Lu2O3:5%Eu was compared to that of the commercial microcrystalline Gd2O2S:Eu. It was found that the commercial phosphor performed four times more efficiently than our nanocrystalline powder. We consider this to be rather encouraging as the fabrication of our powder is not optimized yet. It seems that Lu2O3:Eu, even in nanocrystalline form, can perform much more efficiently which would make it a promising X-ray phosphor.

Abstract: Recent reports on experimental observation of optical gain in silicon nanostructures in the visible region, performed at several laboratories all over the world, have triggered an extraordinary surge of interest in silicon lasing. However, attempts aimed at reproducing the red stimulated emission from „standard“silicon nanocrystals (sized 3-5 nm) at some other laboratories either failed, or. did not come to definite conclusions. Therefore, more detailed measurements of optical gain in a wider variety of samples containing Si nanocrystals are required in order to unravel whether or not the observation of optical gain is an intrinsic property of Si nanocrystals. We have performed a detailed study of optical gain in layers of densely packed Si nanocrystals in SiO2, prepared on the basis of porous Si, using the variable-stripe-length (VSL) method in combination with the shifted-excitation-spot (SES) method. In selected samples we have observed a distinct difference in behaviour between VSL and SES curves, indicating the occurrence of positive optical gain of ~ 24 cm-1. Preliminary reports on transport and electroluminescence measurements in thin films of SiO2 doped with porous silicon grains, prepared by spin-coating technique, are also discussed.

Abstract: The incorporation of Si-nc in Er doped silica is known to strongly enhance the infrared luminescence of Er3+ at 1.54µm. The enhancement is believed to be due to an energy transfer process from Si-nc to Er. In this work we investigate the formation of Si nano-aggregates and their role in the energy transfer process to Er3+ ions for a multi-component glass host. These materials can offer better performances than silica in terms of Er solubility and band broadness for integrated Er-doped optical amplifiers and investigation is therefore very interesting for optoelectronic applications. Si and Er were co-implanted by choosing the implantation energies in order to optimize the overlap between the concentration profiles. The precipitation of Si and the enhancement of the 1.54 µm Er emission were studied for different post-implantation annealing temperatures. In particular the optical properties of the glass were investigated by means of photoluminescence (PL) spectroscopy and the results are discussed in relation to a standard silica substrate. These data are presented and related to the structural properties of the material. Moreover the implications on the future development of an Er doped optical amplifier are discussed.

Abstract: The temperature dependence of the photoluminescence (PL) emission spectra of self-organized InAs/GaAs quantum dots (QDs) grown under different growth conditions in the range 20-300K has been investigated. Three InAs QD samples were grown on (100) 2º-tilted toward (111)A Si-doped GaAs substrates by metal-organic chemical vapour epitaxy (MOVPE), with various size uniformities and dot densities. Observing the measured PL spectra at 20K, the differences caused by size uniformities among the three samples were obvious. The PL spectra were simulated with rate equations, taking into account the carrier relaxation between the first excited state and the ground state. Interestingly, the calculated relaxation lifetimes at 20K were 198ps, 139ps and 54ps for the samples. The temperature dependent PL spectra were also simulated using the same model. Based on the calculated values of temperature dependent relaxation lifetimes, the differences in changes with temperature among the three samples are discussed. The results are consistent with the thermal redistribution effect.

Abstract: Optical communication wavelength emissions from the quantum dots (QDs) structures prepared on (001)-oriented GaAs substrates are discussed. A new growth technique of low-stressed InAs QDs on the AlGaSb layer in a low lattice-mismatched (1.3%) InAs/AlGaSb system is presented. The average height and diameter of the 4-ML InAs QDs on AlGaSb are evaluated to 5.8 nm and 45.2 nm respectively with an average density of 2.18 x 1010 /cm2 using atomic force microscope (AFM) measurements. There is structural selectivity between the QDs layer and the flat hetero-interface under changing growth conditions in the InAs/AlGaSb system. Long-wavelength PL emissions around 1.3 µm and 1.55 µm can be achieved by embedding InAs QDs in AlGaSb layers. Therefore it is expected that low-stressed InAs QDs grown on a AlGaSb layer prepared on a GaAs substrate will be useful in the fabrication of novel QDs devices for optical-communication networks.

Abstract: We review our work on two complementary and compatible techniques, namely direct laser writing and holographic lithography which are suitable for fabricating three-dimensional Photonic Crystal templates for the visible and near-infrared. The structures are characterized by electron micrographs and by optical spectroscopy, revealing their high optical quality.