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Terbium and ytterbium co-doped aluminum oxynitride thin films were grown onto silicon substrates using radiofrequency magnetron sputtering. Aluminum oxynitride samples doped with 4.6 at. % of Yb3+ and co-doped with 0.4 at. % of Tb3+ were obtained. The prepared samples were annealed from 150°C to 850°C in steps of 100°C. By using energy dispersive X-ray analysis we measured the sample composition and the doping concentration. The emission intensities at different annealing temperatures were characterized using photoluminescence measurements upon excitation at 325 nm. The 5D4 → 7F5 main transition of Tb3+ and the characteristic near infrared emission at 980 nm of Yb3+ were recorded. In order to study the luminescence behavior of the samples in terms of a down conversion process, we have plotted the integrated areas of the main transition peaks versus the annealing temperature.
The luminescence of Tb-doped a-SiC:H thin films with different Tb concentrations under sub-bandgap photon excitation was investigated. Two independent processes were identified. The annealing induced activation of the Tb3+ and the inhibition of host-mediated non-radiative recombination paths. The integrated emission intensity is described by a rate equation model, considering these two. In this study, the luminescence enhancement with increasing annealing temperature is shown. The optimal Tb concentration and annealing temperature for the highest Tb-related light emission intensity is determined. Finally, a parameter proportional to the number of optically active ions is found through the aforementioned model.
A simple model to describe the fundamental absorption of amorphous hydrogenated silicon carbide thin films based on band fluctuations is presented. It provides a general equation describing both the Urbach and Tauc regions in the absorption spectrum. In principle, our model is applicable to any amorphous material and it allows the determination of the bandgap. Here we focus on the bandgap engineering of amorphous hydrogenated silicon carbide layers. Emphasis is given on the role of hydrogen dilution during the deposition process and post deposition annealing treatments. Using the conventional Urbach and Tauc equations, it was found that an increase/decrease of the Urbach energy produces a shrink/enhancement of the Tauc-gap. On the contrary, the here proposed model provides a bandgap energy which behaves independently of the Urbach energy.
We present a systematic study of photo- and cathodoluminescence measurements in the visible of Terbium doped SiC:H and AlN thin films. The Terbium atomic concentrations vary from 0.9 to 10% for the SiC:H and from 0.8 to 6% for the AlN samples. For both materials the increase of the emission intensity with concentration and the subsequent quenching effect can be seen. The optimal concentration for the highest light emission is found. Photoluminescence excitation spectroscopy addresses the enhancement light emission mechanisms of the principal emission electronic transition of Terbium at ∼542 nm.
A simple derivation of sub-bandgap exponential tails and fundamental absorption equations ruling the optical absorption of amorphous semiconductors are presented following the frozen phonon model. We use the Kubo-Greenwood formula to describe the average transition rate for the optical absorption process. Asymptotic analysis leads to the commonly observed exponential tail as well as the Tauc expression for the fundamental absorption. We test our theoretical results with experimental absorption coefficients of amorphous Si:H, SiC:H, AlN and SiN. The validity of the Urbach focus concept is evaluated.