Trans.Nonferrous Met.Soc.China27(2017)694−700
Optimization of parameters,characterization and thermal property analysis of hafnium ethoxide synthesized by electrochemical method
Chang-hong WANG,Sheng-hai YANG,Yong-ming CHEN,Yan-zeng WU,Jing HE,Chao-bo TANG School of Metallurgy and Environment,Central South University,Changsha410083,China
Received2February2016;accepted15June2016
Abstract:Hafnium ethoxide was synthesized using electrochemical method.Optimization experiments were used to optimize various parameters namely Et4NBr concentration(c):0.01−0.06mol/L,solution temperature(t):30−78°C,polar distance(D): 2.0−4.0cm and current density(J):100−400A/m2.The electrolytic products obtained under optimum conditions of c=0.04mol/L, t=78°C,D=2.0cm and J=100A/m2were further isolated by vacuum distillation under5kPa.The product was characterized by Fourier transform infrared(FT-IR)spectra,nuclear magnetic resonance(NMR)spectra.The results indicated that the product was hafnium ethoxide.ICP analysis suggested that the content of hafnium ethoxide in the final product exceeded99.997%.Thermal properties of the product were analyzed by TG/DTG.The vaporization enthalpy of hafnium ethoxide was found to be79.1kJ/mol. The result confirmed that hafnium ethoxide was suitable for the preparation of hafnium oxide by atomic layer deposition.
Key words:hafnium ethoxide;electrochemical synthesis;parameter optimization;characterization;thermal analysis
1Introduction
deposition
Hafnium ethoxide is mainly used for the deposition of hafnium oxide(HfO2)layers by atomic layer dep
osition(ALD),and deposited in this manner,the hafnium-based high-k dielectrics show much more stable electrical characteristics in comparison with the ones formed by sputtering or chemical vapors[1].As the semiconductor industry ushered in nanometer era,a gate dielectric thickness in the field effect transistor(FET) reached its physical limits because of an increase in leakage currents due to tunneling effects[2−4]. Therefore,it is necessary to replace SiO2with a high-k gate dielectric.HfO2layers are amongst the most promising high-k candidates to meet the requirements for replacing the traditional SiO2gate oxide in complementary metal−oxide–semiconductor devices[4,5].
In recent years,the most common method of preparing hafnium alkoxides(Hf(OR)4)has been based on the halide synthesis[6].This method has the disadvantage that HCl gas is liberated,because an apparatus in which the reaction is carried out is subjected to severe corrosion.And the reaction is performed using large amounts of organic solvents which are harmful to the environment.In addition,the necessary reagents are scarce and the reactions involve various side processes which contaminate the products and decrease their yields[7].
For these reasons,the direct electrochemical synthesis of metal alkoxides by anode dissolution of metals in absolute alcohols in the presence of a conductive admixture seems a very promising meth
od. The electrochemical method has great promise for the direct conversion of the less electropositive metals to their alkoxides owing to its simplicity and high productivity as well as its continuous and non-polluting character(with hydrogen as the major by-product)[6].In our earlier works,we prepared several tantalum and niobium alkoxides by electrochemical method[8−10]. Many metal alkoxides,such as Y,Ti,Nb,Ta,Mo,W,Cu, Ge,Sn and other metals were also produced by this technique[6,11−13].Some hafnium alkoxides,such as Hf(OMe)4,Hf(OBu t)4,H(OC5H11t)4and Hf(OEt)4,were prepared using electrochemical method[6].In1995, hafnium ethoxide was obtained for the first time by TUREVSKAYA et al[14]with the electrochemical method involving electrolysis of an ethanol solution containing tetrabutylammonium bromide with a platinum cathode and a hafnium anode.However,parameter
Foundation item:Project(51374254)supported by the National Natural Science Foundation of China Corresponding author:Sheng-hai YANG;Tel:+86-139********;E-mail:Yangshcsu@163 DOI:10.1016/S1003-6326(17)60077-3

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