The Research On Charge Trapping Memory And Ferroelectric Properties Of Gd-doped HfO₂ Thin Films

Since HfO2 thin films were successfully applied as gate dielectrics for the 45nm node CMOS process,novel dielectric materials,such as ferroelectrics and charge trapping layers play a more important role in the logic devices and memory cells for the next-generation.Gd-doped HfO2(GHO),as a dielectric material with excellent performance,has the advantages of high dielectric constant,low leakage current density,low oxygen vacancy concentration and good stability which reveals great potentiality for application.In this paper,we prepared Gd-doped HfO2 films using atomic layer deposition technology,and studied its charge trapping characteristics and ferroelectricity.The details are as follows:(1)MGOS(Metal/GHO/SiO2/Si)structure devices based on single-layer GHO films were fabricated.First,the effect of Gd doping concentration on the charge trapping characteristics of GHO thin films was studied.The test results show that when the Gd doping concentration is 0.5at%,the device has the best charge trapping characteristics.(a memory window of 7.7 V under 10 V sweeping voltage)and stable data retention capability.Besides,performance in terms of P/E speed and fatigue resistance has not been sacrificed.The excellent storage performance is attributed to two aspects:one is the high density of defect states in the GHO film;the second is the reduction of the defect energy level caused by gadolinium doping.(2)A multilayer thin film device with HfO2/GHO/HfO2/SiO2/Si structure was prepared,and C-V test proved that the multilayer film had ferroelectric properties.Subsequently,the device was annealed by Rapid Thermal Processing(RTP)technology.The C-V characteristic test shows that the samples annealed at 400? and 500? under Ar atmosphere show ferroelectricity and the samples annealed at 600? and 700? show charge trapping characteristics.However,the C-V curve of the samples annealed at 800?showed no hysteresis.Combined with high-resolution transmission electron microscopy characterization,we believe that this phenomenon is due to the modulation effect of annealing on the crystal phase composition of the GHO multilayer film.The amorphous GHO film shows good charge trapping characteristics and the orthorhombic GHO film shows good ferroelectricity.(3)The charge trapping memory device and negative capacitance field effect transistor device were prepared using a few layers of MoS2 as the channel material,and the method for obtaining MoS2 was micromechanical stripping.The fabricated charge trap memory cells based on MoS2 showed a 2 V memory window at a scan voltage of 4 V.The considerable memory window at low voltage is due to the high defect state density of GHO films and the reduction of the defect energy level caused by Gd doping.The prepared negative capacitance field effect transistor has low leakage current density and no hysteresis effect,which is attributed to the high dielectric constant and moderate ferroelectricity of GHO film.