| The energy storage materials and systems have become focused areas because the energy shortage since the 21st century.Dielectric film capacitors store electrical energy in the form of an electrostatic field via polarization,which exhibit high power density,high-speed charge and discharge,excellent fatigue-resistance and small size compared with batteries and supercapacitors.Dielectric capacitors are widely used in many pulsed-discharge and power conditioning electronic applications,these include medical equipment(X-ray units,surgical lasers),energy systems(high-frequency inverters,grid-connected photovoltaics),scientific research(nuclear effects simulation,high-intensity magnetic field experiments)and avionics(space-shuttle power systems,rocket propulsion systems),and have broad application prospects in microelectronic integrated circuits.However,dielectric film capacitors show low energy storage density during the charge-discharge process due to the dielectric loss and leakage current,severely restrict its wide application in microelectronic integrated circuits.Therefore,how to improve the energy storage performance of dielectric film capacitors is an urgent problem to be solved.Moreover,the temperature limit of the existing CMOS-Si process is 500?,and it is necessary to deposit dielectric/ferroelectric film capacitors with excellent energy storage performance below the temperature.Barium titanate(BTO)ferroelectric film is one of the ideal ferroelectric film capacitors because of its simple and lead-free chemical composition with good ferroelectric and dielectric properties.The BTO ferroelectric film has significant anisotropy,introducing the buffer layer to deposit highly-oriented BTO films,and its electrical properties are optimization via strain engineering and size effect,investigating the influence of microstructure,residual strain and size effect on its electrical properties which have important theoretical significance and subsequent application value for improving the energy storage performance of ferroelectric film capacitors.In the part of experimental research,BaTiO3(BTO)ferroelectric films deposited at low temperature(300?-500?)on Si by using multiple target radio frequency magnetron sputtering,the rules of orientation growth and structure-property relationship were investigated.The main research contents are summarized as follows:(1)Highly-oriented BTO ferroelectric films sputtered at low temperature on Si by using LaNiO3(LNO)buffer layer,which has a closely-matched lattice with BTO,using the "degree of under cooling" to predict the effect of deposited temperature on microscopic morphology,grain orientation and grain size.The results display the grain size increases with increase deposition temperature,while the in-plane strain decrease.XRD patterns reveal(001)-textured growth in these BTO films deposited at 350?-450?,350? and 450?-deposited films possess(111)-oriented grains.TEM indicate all the BTO films display columnar nanograins microstructure throughout the film thickness.Small grain size and large in-plane strain lead to large breakdown electric field and large polarization,revealed 350?-deposited BTO films have optimal energy storage performance,450?-deposited BTO films exhibit superior dielectric tunability,respectively.(2)P-E hysteresis loops of BTO films with epitaxial,polycrystalline,columnar large grain and columnar nanograin microstructure were simulated by phase field modeling(PFM),columnar-nanograined BTO film display a delayed saturation under an electric field,can be attributed to a strong polarization-constraining effect from the densely packed,non-ferroelectric,periodic grain boundaries,which is supported by a PFM.On the basis of(1),An optimal deposition temperature of 350? is chosen,controlled the grain size and in-plane strain of BTO films by lowering the thickness of LNO buffer layer and BTO ferroelectric layer.XRD patterns exhibit a broadened(001)BTO peak reveal that the nanocrystalline grain sizes are 11nm and 14nm,as well as in-plane strain of-2.9±0.04%and-2.1±0.02%,respectively.Therefore,these BTO films show"slim" P-E loops with large maximum polarization(81?C/cm2 for 11nm BTO film and 59?C/cm2 for 14nm BTO film)and breakdown electric field(8.1MV/cm for 11nm BTO film and 6.5MV/cm for 14nm BTO film),leading to a record-high energy density(Wrec)of all the ferroelectric film deposited on Si,Wrec?221 J/cm3 with high energy efficiency ??79.5%,as well as a wide operating temperature range,high fatigue resistance and excellent frequency stability.(3)"Device-level" charge-discharge for energy storage performance tests were carried out by using the high-speed RC circuitry assembled in-house,verified a similar energy density value integrated from P-E loop under same applied voltage.Finally,700±30nm-thick BTO and 1200±70nm-thick BTO film exhibit high Wrec?128J/cm3 and?144J/cm3,reveal a thickness-scalable BTO films capacitors with similar grain aspect ratio.It is also noted that areal energy density of our 1200±70nm-thick BTO film is comparable to that of a?10 times(?10?m)thicker ferroelectric polymer nanocomposite. |
PDF Full Text Request