Leandro R. Tessler

"Chapter 3 ‘Sex, Drugs and Communism’" published on 15 Feb 2023 by Brill.

Trivalent rare earth ions efficiently emit luminescence in amorphous silicon based alloy hosts. The luminescence details depend mostly on the immediate chemical neighborhood of the ions. In crystals the luminescence peaks at low temperatures can be adjusted by lorentzian lineshapes with energies determined by the symmetry of the well defined lattice sites. In disordered materials however not only the local symmetry can change but also the local stoichiometry is not well defined. As a result the luminescence peaks become gaussian in shape. Nevertheless, by carefully adjusting the luminescence spectra information can be obtained about the local properties. We studied the luminescence of Eu³⁺ and Tb³⁺ in amorphous silicon alloys at low temperatures. The results indicate that Eu³⁺ is mostly on distorted cubic symmetry sites, while for Tb³⁺ it is not possible to determine the symmetry reliably. In any case, the results are compatible with the rare earth ions coordinated to residual oxygen in the alloys.

Rare earth doped semiconductors have received much attention in the last decades because of potential applications in photonics and illumination. Because silicon is the most widely employed material in the semiconductor industry, much of this effort is devoted to obtain efficient light emitters involving silicon based alloys. The excitation cross section of rare earth ions is orders of magnitude higher in silicon-rich amorphous alloys than in ordinary oxides. Energy transfer is often very efficient in the presence of silicon nanocrystals or nanoclusters. This has led to energy transfer models involving quantum confinement and the electronic structure of silicon nanocrystals. Although these models are elegant and persuasive, they demand a highly homogeneous nanoparticle size distribution, absence of states associated with surface or strain in very small particles and the excitation of many rare earth ions by a single nanoparticle, all very unlikely in real systems. We have studied the excitation of Tb³⁺ doped a-Si₃N₄:H. Our results show a correlation between the density of silicon dangling bonds and the luminescence efficiency under matrix excitation. These findings lead to a reinterpretation of published results in samples containing silicon nanoparticles. When nanoparticles are formed the density of silicon dangling bonds in fact increase as silicon-silicon, silicon-oxygen, silicon-nitrogen or silicon-hydrogen bonds are broken. Thus, the main effect of the formation of nanoparticles, crystalline or not, to the energy transfer to rare earth ions is through the increase of the density of silicon dangling bonds and consequently of midgap states. A very short range Auger process involving carrier recombination at silicon dangling bonds is most probably the main mechanism for excitation from a silicon rich host to rare earth ions. To optimize the luminescence efficiency, the density of dangling bonds must match the density of active rare earth ions.

Rare earth doped semiconductors have received much attention in the last decades because of potential applications in photonics and illumination. Because silicon is the most widely employed material in the semiconductor industry, much of this effort is devoted to obtain efficient light emitters involving silicon based alloys. The excitation cross section of rare earth ions is orders of magnitude higher in silicon-rich amorphous alloys than in ordinary oxides. Energy transfer is often very efficient in the presence of silicon nanocrystals or nanoclusters. This has led to energy transfer models involving quantum confinement and the electronic structure of silicon nanocrystals. Although these models are elegant and persuasive, they demand a highly homogeneous nanoparticle size distribution, absence of states associated with surface or strain in very small particles and the excitation of many rare earth ions by a single nanoparticle, all very unlikely in real systems.

The influence of the substrate temperature and argon gas flow on the compositional, structural, optical, and light emission properties of amorphous hydrogenated silicon carbonitride (a-SiC_xN_y:H) thin films were studied. Thin films were fabricated using electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD) at a range of substrate temperatures from 120 to 170°C (corresponding to deposition temperatures of 300 to 450°C) in a mixture of SiH₄, N₂, and CH₄ precursors. Variable angle spectroscopic ellipsometer (VASE), elastic recoil detection (ERD), and Rutherford backscattering spectrometry (RBS) verified optical bandgap widening, layer densification, and an increase of the refractive index at higher substrate temperatures. The microstructure of a-SiC_xN_y:H_z thin films was determined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy. The substrate temperature strongly affected the binding state of all atoms, and in particular, carbon atoms attached to silicon and nitrogen, as well as hydrogen-terminated bonds. We correlated the films' microstructural changes to a higher species' mobility arriving on the growin layer at higher temperatures. Photoluminescence (PL) measurements showed that the total intensity of visible light emission increased. A systematic blueshift of the centroid of the wide PL peak was observed following the increase of optical gap.

X-ray absorption spectroscopy (both XANES, X-ray Absorption Near-Edge Structure and EXAFS, Extended X-ray Absorption Fine Structure) is a powerful tool to determine the chemical environment of selected elements in solids. In this paper we present results obtained for the rare earth elements Eu and Tb in hydrogenated amorphous silicon alloys. Whenever possible, rare earth atoms coordinate to oxygen present in environments that depend on the host and can be very distorted. Annealing at high enough temperatures makes the coordination evolve towards oxide-like 6-fold environments. Europium is a special case. It may assume either 2+ or 3+ oxidation states. The X-ray absorption peaks can be used to determine the relative concentration of each. When the concentration is above 4 at% it is possible to completely oxidize the Eu²⁺ ions by annealing in an oxygen atmosphere. Efficient luminescence of the 3+ ions is associated with low coordinated oxygen environments. This environment does not change significantly at the relatively low annealing temperatures (up to 500°C) that maximize photoluminescence. The main effects of the annealing are not related to the rare earth chemical neighborhood.