Study Of Drying Control For Preparation Of RE³⁺-doped Alumina Thin Film By A Sol-gel Method

Preparation of the thin films by sol-gel method has many advantages,such as simple process,low cost,easy to form films,etc.,but disadvantages of the cracking in the gel films during forming of drying process and the low deposition efficiency,still have barriers to its application in practical production.The problem of the cracking in the thin films has been resolved by introducing drying control chemical addtive(DCCA)and drying control physical additive(DCPA)in the sols to modify the Al₂O₃ sols,respectively.The deposition efficiency of the thin films without cracking has been improved by introducing the viscosity increaser (â…¥)to increase the viscosity of the sols modified by DCPA.The control mechanisms of the cracking and the icrease of the deposition efficiency have been discussed.1 mol%Er³⁺-(0-10)mol%Yb³⁺codoped Al₂O₃ thin films on thermally oxidized SiO₂/Si(100)substrates have been prepared by dip-coating process from Er³⁺-Yb³⁺codoped Al₂O₃ sols in non-aqueous sol-gel method,using aluminium isopropoxide[Al(OPrⁱ)₃]as precursor,acetylacetone(HAcAc)as chelator,Er(NO₃)₃·5H₂O and Yb(NO₃)₃·5H₂O as doped medium.The sols are modified by introducing N,N-dimethylormamide(DMF)as a DCCA. Homogeneous,uniform and crack-free 1 mol%Er³⁺-10 mol%Yb³⁺codoped Al₂O₃ thin films are formed by 40 dipping and drying cycles sintered at 1000℃under conditions of a molar ratio of 1:1 for DMF:Al(OPrⁱ)₃.The formation of the cracking in the thin films is suppressed by the DMF assisted the deprotonation process of -OH at the surfaces of gel particles, resulting in enhancement of the polymerization degree of sols and improvement of the mechanical properties of gel thin films.The strong photoluminescence(PL)spectra in wavelength range 1.400-1.700μm centered at 1.53μm for the Er³⁺-Yb³⁺codoped Al₂O₃ thin films,obtained by using semiconductor laster diodes pumping at 980 nm,are independent on the DMF additive.The EtOH and PrⁿOH as DCPAs,mixed with PrⁱOH as solvent.Homogeneous,uniform and crack-free 1 mol%Er³⁺- 10 mol%Yb³⁺codoped Al₂O₃ thin films are formed by 40 dipping and drying cycles sintered at 1000℃under conditions of a volume ratio of 1:4 for PrⁱOH:EtOH and 1:1:8 for PrⁿOH:PrⁱOH:EtOH.In the drying process,a lower weight loss of the sols modified by DCPA due to the different evaporative rate of compound solvent, provides a structural relaxation for the gelation of the sols,resulting in the decrease of the stress of non-uniform shrinkage to suppress the cracking formed.The strong PL spectra at 1.53μm for the Er³⁺-Yb³⁺codoped Al₂O₃ are unrelated to the changeable solvent.Polyringlpyrrolidone(PVP)as a viscosity increaser is used to adjust the viscosity of the sols.When the molar ratio of PVP:Al(OC₃H₇)is up to 1:1,the viscosity of the PVP assisted sol increases from 2.3 to 31.5 mPa·s,and homogeneous,uniform and crack-free 1 mol%Er³⁺-10 mol%Yb³⁺codoped Al₂O₃ thin films with the thickness of 1.6μm are formed by 10 dipping and drying cycles sintered at 1000℃.The deposition efficiency of preparation of the thin films by the sol-gel method is increased by about one order of magnitude without cracking.The strong PL spectra at 1.53μm for the Er³⁺-Yb³⁺codoped Al₂O₃ have no significant change in the PL intensity with PVP assisted.