The characterization of a wide range of luminescent thin films can be a long and tedious endeavor. With reactive combinatorial sputtering of multiple metal targets, it possible to fabricate thin films with a gradient in composition simply by not rotating the substrate. In this work, combinatorically sputtered thin films of Cr³⁺ and Nd³⁺ doped in the Al₂O₃–Y₂O₃ system (YAlO) are studied for thin film based luminescent solar concentrators (TFLSCs) application. Contrary to mm's thick plate type LSC's, TFLSCs of just several 100 nm thick require much higher Cr³⁺ concentration to achieve 40% absorption which can enable several 10's of W/m² LSC power efficiencies. Our transmission measurements on a Cr₂O₃ film with a thickness gradient result in an absorption cross section at 460 nm of 1.3 ± 0.7 × 10⁻¹⁹ cm² showing that the TFLSC absorption requirement can be fulfilled provided that the Cr³⁺ concentration is in the order of 10²² ions/cm³. The Y:Al ratio of the YAlO host in our films ranged between 0.5 and 3.5, thereby including the monoclinic (Y₄Al₂O₉), perovskite (YAlO₃) and garnet (Y₃Al₅O₁₂) stoichiometry's on a single film. Position dependent XRD, EDX, excitation, emission and lifetime measurements of Cr³⁺ and Nd³⁺ show that the unique gradient film sputtering method is able to characterize thin films as a function of host composition and doping concentration. Energy transfer between Cr³⁺ and Nd³⁺ in co-doped YAlO films is concluded from Cr³⁺ excitation bands observed while monitoring Nd³⁺ emission and from the matching of the rise-time of Nd³⁺ 1340 nm emission (⁴F_3/2 -> ⁴I_11/2) and the decay time of Cr³⁺ 840 nm emission (⁴T₂ -> ⁴A₂). Nd³⁺ lifetime systematically decreases from 0.24 to 0.05 ms with increasing Cr³⁺ concentration in Y₃Al_5-xCr_xO₁₂:Nd (0.05 < x < 2). The constraints of heavily doped Cr³⁺ thin films for enabling adequate absorption and energy transfer to Nd³⁺ in TFLSC applications are the subjects of the discussion.

Silicon-aluminum-oxigen (SiAlO) coatings doped with Sm²⁺ and prepared by reactive magnetron co-sputtering of Si, Al, and Sm targets, are attractive for luminescence solar concentrator applications but suffer from the low absorption between 300 and 600 nm. This article reports that the main cause of low absorption is a high concentration of undesired Sm³⁺. This finding is supported by optical transmittance, photoluminescence emission and excitation characterization, and X-ray photoelectron spectroscopy data of the Sm's 3d^5/2 edge. We present an alternative deposition process for obtaining Sm doped SiAlO layers with enhanced Sm²⁺ absorption by incorporating Sm through the use of multilayer thin-film precursors composed of metallic Sm and SiAlO layers. After thermal post-deposition treatments, diffusion and reaction of the metallic Sm layers with the SiAlO host results in coatings showing the characteristic 5d → 4f transitions of Sm²⁺ in the region between 250 and 600 nm which were not detectable in Sm-doped single layers. This same deposition strategy produces Tm doped SiAlO coatings with Tm²⁺‘s characteristic luminescence at 1132 nm when the SiAlO host is in the mullite composition region. The photoluminescence excitation spectrum of Tm²⁺ is compared to phosphor with similar composition and covers the range between 300 and 700 nm.

This paper reports the fabrication and characterization of several thulium oxide and nitride thin films grown by reactive magnetron direct-current sputtering. Hysteresis curves of the Tm emission spectra of the sputtering plasma versus the flow of N₂ or O₂ around the Tm-metal target were monitored. Emission spectra of atomic transition lines in the region between 370 and 420 nm were identified to be of neutral thulium. The plasma emission was compared to the hysteresis curves generated by monitoring the sputter rate and target voltage. The nitride films' composition and optical properties were determined by X-Ray Diffraction, and optical transmission spectroscopy. The composition of the oxide films was determined by energy dispersive X-ray spectroscopy. The films are initially amorphous but crystallize after thermal treatment at 800°C. The optical bandgap values obtained using the Tauc method are consistent with what has been previously reported for both Tm₂O₃ and TmN prepared by other methods.

The valence stability parameter (E_Ff), defined as the difference between the charge transfer energy to the host intrinsic Fermi energy, was used as criterion to analyze the capability of different host materials within the SiAlON class to stabilize divalent thulium. Available data on charge transfer energies and optical bandgap values are reviewed for Si₃N₄, SiO₂, AlN, and Al₂O₃. In addition, new data on thin films, collected by our gradient sputter deposition and characterization method on silicon and aluminum nitrides (Si_0.75xAl_1-xN), are reported. These data are sufficient to show that, at least in the nitride subsection of the SiAlON class, divalent thulium is not expected to be stable due to the presence of high E_Ff values. The use of sub-stoichiometric silicon nitride and oxide is also briefly considered.