Photoluminescence Of Rare Earth Ions Doped Oxides And Their Thin Films Processing In Sol-gel Method

The Er³⁺-doped and Er³⁺-Yb³⁺ codoped Al₂O₃ with the photoluminescence (PL) properties in the near infrared region centered at 1.53μm were prepared by using a non-aqueous sol-gel method. The effects of mixing of SiO₂/P₂O₅ on phase structure and PL properties of the Er³⁺-doped Al₂O₃ were systematically investigated to explore the enhancement mechanism of PL properties of Er³⁺-doped Al₂O₃ due to the mixing of oxides. The cracking in the thin films has been suppressed by introducing drying control chemical additive (DCCA) and drying control physical additive (DCPA) in the sols, respectively. The deposition efficiency of the sol-gel dip-coating has been improved by introducing the viscosity increaser (VI). The mechanisms of suppressed cracking and increase of the deposition efficiency for the rare earth ions doped Al₂O₃ thin films in sol-gel method have been discussed.The Er³⁺-doped Al₂O₃ powders sintered at different temperatures were prepared by using the aluminum isopropoxide [Al(OC₃H₇)₃] as precursor, acetylacetone (AcAcH) as chelating agent, nitric acid (HNO₃) as catalyzer, and hydrated nitrate of Er(NO₃)₃·5H₂O as dopant in the non-aqueous isopropanol environment. The phase contents diagram for the Er-Al-O system with the doping concentration up to 5.0 mol% was described at the sintering temperature from 550℃to 1250℃. There were three crystalline types of Er³⁺-doped Al₂O₃ phases,γ,θandα-(Al,Er)₂O₃, and two stoichiometric compounds composed of Al, Er, and O, the perovskite ErAlO₃ (ErAP) and garnet Er₃Al₅O₁₂ (ErAG), in the Er-Al-O phase contents diagram. The Er³⁺ doping suppressed the crystallization of theγandθphases and delayed the phase transitions ofγ→θandθ→α.The PL spectra in the wavelength range from 1.400μm to 1.700μm with a main peak at 1.535μm and a side peak at 1.556μm were obtained for the Er³⁺-doped Al₂O₃ with theγphase and the mixture ofγandθphases. The PL spectra were splitting with the appearance ofα-Al₂O₃, ErAP and ErAG phases in the matrix, due to a definite local environment in theα-(Al,Er)₂O₃, ErAP and ErAG lattices, and a weaker effect of inhomogeneous widening than in theγandθ-Al₂O₃ phase. The low temperature PL spectra of Er³⁺-dopedα-Al₂O₃ powders in the doping concentration range from 0.002 to 2.5 mol% indicated that a very low concentration of Er³⁺ could resolute inα-Al₂O₃ lattice. The combining photoluminescence of Er³⁺-dopedα-Al₂O₃ unambiguously was mainly derived from the Er³⁺ which replaced with Al³⁺ inα-Al₂O₃ lattice (α-(Al,Er)₂O₃). For the Er³⁺ concentration of 0.5 -5.0 mol%, the luminescence was from two different kinds of optical centers, the ErAG andα-(Al,Er)₂O₃, respectively, and the former mainly benefits to the visible emission, while the latter prefers to contribute to the near-infrared luminescence.The Er³⁺-doped SiO₂-Al₂O₃ powders were prepared by mixing of SiO₂ sol and Al₂O₃ sol with same doping concentration. The phase transitions ofγ→θandθ→αof the 0.1 mol %Er³⁺-doped Al₂O₃ were suppressed by mixing of SiO₂, and the 0.1 mol %Er³⁺-doped Al₂O₃ was mainly of theγandθ-Al₂O₃ phases with a mount ofα-Al₂O₃ and Al₆Si₂O₁₃ phase for the sintering temperature below 1200℃. The PL spectra maintained a single broadband centered at 1.53μm even though the sintering temperature was up to 1200℃. The PL lifetime and intensity at the wavelength of 1.53μm increased obviously with the increase of the sintering temperature and mixing content of SiO₂, due to the decrease in -OH content and the formation of lower symmetrical octahedral ErO₆, respectively.The 1.0 mol% Er³⁺-doped P₂O₅-Al₂O₃ powders sintered at the temperatures of 1000℃were prepared by introduced triethyl phosphate (TEP, (CH₃CH₂O)₃P=O) in the sols. The -OH has been removed completely due to the addition of TEP with an increased molar ratio to 30.0 mol%. The vibrate energy of P-O bond is about 1300-1600 cm^-1, which contribute to the transition of the second excited state ⁴I_11/2 to the first excited state ⁴I_13/2. Both the removing of the -OH and the introducing of the P-0 bond led to the increasing of PL properties. The J-0 parametersΩ_λ(λ=2, 4, 6) and radiative transition probabilities of the Er³⁺-doped P2O5-Al₂O₃ powders were calculated by the J-O theory. The J-O parametersΩ_2,4,6 were all reduced with the mixing content of P₂O₅ increased, indicating that the ionicity of the materials was enhanced. The radiation lifetime of ⁴I_13/2 was increased gently with increasing the mixing content of P₂O₅, which coincided with the variety of measured lifetime.The cracking in the thin films was suppressed by introducing drying control chemical additive (DCCA) and drying control physical additive (DCPA) in the sols to modify the Al₂O₃ sols, respectively. The homogeneous, uniform and crack-free 1.0 mol%Er³⁺-10.0 mol%Yb³⁺ codoped Al₂O₃ thin films on the thermally oxidized SiO2/Si(100) substrate were formed by 40 dip-coating cycles. Polyringlpyrrolidone (PVP) as a viscosity increaser was used to adjust the viscosity of the sols. When the molar ratio of PVP(K90):AlOC₃H₇ was up to 1:1, the viscosity of the PVP assisted sol increased from 2.3 to 31.5 mPa·s, and homogeneous, uniform and crack-free 1.0 mol%Er³⁺-10.0 mol%Yb³⁺ codoped Al₂O₃ thin films with the thickness of 1.6μm were formed by 10 dipping and drying cycles sintered at 1000℃. The deposition efficiency of the crack-free thin films by the sol-gel method was increased by about one order in magnitude. The PL spectra of Er³⁺-Yb³⁺ codoped Al₂O₃ thin films in the wavelength range from 1.400μm to 1.700μm centered at 1.535μm were measured by the face coupling method. The drying control additive and viscosity increaser have no effect on the strong PL of the Er³⁺-Yb³⁺ codoped Al₂O₃ thin films.