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Enhancing Blue Emission in Ce Doped Silicon Oxynitrides Based Electroluminescent Devices

Abstract : Ce-doped SiO x N y and SiAlON matrices are promising materials for blue LED applications. The uniqueness of this approach stems from the fact that SiO x N y , as a host, combines specific properties of individual SiO x and SiN y matrices like solubility, efficient emission, 5 eV gap, with a broad excitation range from 400 nm to 500 nm of Ce 3+ ions due to the 4f-5d transitions. Furthermore, the co-doping with aluminum enhances the Ce 3+ emission. In this work, we fabricated electroluminescent devices using SiO x N y :Ce 3+ and SiAlON:Ce 3+ as active layers and investigated the resulting emission under optical and electrical excitations as a function of nitrogen, cerium and aluminum concentrations. I-V measurements were conducted to determine the SiO x N y :Ce 3+ layer electrical parameters. Charge transport through the devices obeys the Poole-Frenkel conduction mechanism. It was demonstrated that by optimizing the SiO x N y :Ce 3+ growth parameters, an improvement of electroluminescence yield can be achieved with a maximum intensity obtained for devices with cerium content of 4 at.%. Rare earth (RE) doped silicon based materials have been extensively investigated in the past few years. Various hosts were doped with Er 3+ ions that emit at 1.5 μm corresponding to the maximum transparency of silica used in telecommunications. 1 For the Ce 3+ ion, up to date, only a few studies have been reported on its electrolu-minescence (EL). 2 Among the silicon-based matrices, silica (SiO 2) and silicon nitride (Si 3 N 4) have been explored; however, each of them present certain advantages and drawbacks. In the case of silica matrices , achievement of strong RE 3+ ions emission is limited by a low excitation cross section, 3 a low RE solubility as well as RE clusters formation. 4,5 However, the main drawback limiting SiO 2 : RE 3+ light emitting applications comes from the large bandgap of the matrix (∼9 eV) resulting in low electrical conductivity. On the other hand, Si 3 N 4 with a smaller energy bandgap (4 eV) and reduced tendency of the RE to form clusters, seems to be more suitable for RE doping. 6-8 However, despite these advantages, the emission efficiency from RE 3+ ions in a nitride matrix is much lower than in silica matrix. 9 To capitalize on the RE doping advantages offered by both oxide and nitride silicon matrices, a Ce-doped SiO x N y matrix has been explored by Ramirez et al. 10 It was reported that the maximum EL peak from Ce 3+ ion shifted from 400 nm to 476 nm as function of the nitrogen concentration (i.e. the nephelauxetic effect). 10,11 Koao et al. showed that aluminum co-doping Ce-doped SiO 2 glasses lead to an enhancement of photoluminescence emission. 12 In this work, Ce-doped Si(Al)O x N y layers with a typical thickness of 50 nm were grown by sputter de-position. Photoluminescence (PL) from the SiO x N y :Ce 3+ layers and EL from this active layer were examined for device performance as a function of growth parameters and material composition. Experimental Active layer preparation.-The devices were fabricated in a few step processes. First, the Ce-doped SiO x N y active layer was grown by magnetron reactive sputtering with 8 sccm for Ar and 2 sccm for N 2 on 2-inch diameter (001) p-type silicon wafers. During the growth, the chamber base pressure was fixed at 3 mTorr and the substrate temperature was at room temperature (RT). Additional information on the growth process can be found in the following Reference 11. z Samples were deposited from CeO 2 , Al and Si targets with density of power varied from 0 to 2.1 −2 , 0.3 to 0.75 −2 and fixed at 4.5 W/cm 2 , respectively. As-deposited films were then thermally annealed in the 600°C to 1200°C temperature range for 1 h, in nitrogen atmosphere at ambient pressure. Device fabrication.-Figure 1 illustrates the typical fabricated device structure with the indicated specific layers thicknesses. The individual indium tin oxide (ITO) top electrical contacts were deposited on the SiO x N y :Ce 3+ layer by electron beam evaporation using a shadow mask having a set of circular holes with a diameter of 200 μm. In 2 O 3 /SnO 2 (90%/10%) pellets with a diameter of 1 mm or 2 mm were used as sputtering targets. An oxygen flux was maintained in the sputtering chamber during deposition cycle to prevent the formation of oxygen defects in the transparent conducting layer, which would potentially cause EL from ITO layer. The ITO layer thickness was 200 nm. The SiO x N y :Ce 3+ /ITO structure was heated up at a ramp rate of 15°C/min from RT to 600°C and annealed for 1 h in a nitrogen atmosphere at ambient pressure. The bottom metal contact, a 200-nm-thick Al layer, was deposited at RT on the back of the silicon substrate. Figure 1. Schematic layout of fabricated Al/p-Si/SiO x N y :Ce 3+ /ITO device.
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F. Ehré, C. Dufour, O. Blázquez, B. Garrido, W. Jadwisienczak, et al.. Enhancing Blue Emission in Ce Doped Silicon Oxynitrides Based Electroluminescent Devices. Ecs Journal of Solid State Science and Technology, 2019, 8 (12), pp.R157-R163. ⟨10.1149/2.0031912jss⟩. ⟨hal-02378617⟩



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