Charge Storage Characteristics And Magnetic Anisotropy Of Ag Nanoparticles Embedded HfAlO_x Films

The nano-floating gate non-volatile memory (NFG-NVM) devices based on the MOS capacitor structure for metal nano-particles embedded high-k materials is one of highlighted flash memory device. In this paper, A trilayer capacitor composed by Ag nano-particles as memory nods and HfAlOx films as control/tunneling layers has been fabricated on compressively strained p-type Si83Ge17/Si(100) substrates by radio-frequency magnetron sputtering. Its microstructures, charge storage characteristics and defects-induced magnetic anisotropy have been studied.Microstructures of Ag nano-particles embedded HfAlOx film have been investigated by using XPS, XRD, SEM and HRTEM. The HfAlOx film is oxygen unstoichiometric, and the the2nm thick interface layer between HfAlOx film and Si83Ge17substrate is Ge-enriched mixture of HfSiOx and HfSix. The pure Ag nano-particles embedded the amorphous HfAlOx matrix are well dispersed with a diameter of4-8nm and an area density of5.7×1012/cm2.The dielectric properties and charge storage characteristics of samples have been measured by using Agilent4294A Impedance Analyzer and Keithley-2400Digital Sourcemeter. After embedding Ag nano-particles, the permittivity increases from18.6to30.5at1M Hz, the dielectric loss reduces from0.125to0.048at5M Hz, and the leakage current density decreases from5.0×10-2A/cm2to2.13×10-5A/cm2. Moreover, counterclockwise hysteresis capacitance-voltage curve with a memory window of～2V is observed for this memory capacitor, which corresponds to a charge storage density of～1.39×1013/cm2. However, only54%stored charges are retained in the Ag nano-particles after105electric stressing. Study of the tunneling mechanism reveals the defect-enhanced Poole-Frenkel tunnel should be responsible for the charge loss.We also study the defects-induced magnetism of HfAlOx film and its anisotropy.The room temperature magnetization curves of HfAlOx films deposited on different substrates and annealed at different temperature have been measured by using VSM. HfAlOx films deposited on sapphire substrate exhibit a bigger saturation magnetic moment than that on p-Si83Ge17/Si(100) and p-Si(100) substrates, and show the obvious anisotropy affected by the annealing temperature. When the annealing temperature(Ta) is less than600℃, the saturated magnetic moment with the applied field parallel to the surface of the sapphire substrate (M//) is bigger than that with the perpendicular field (M⊥):M//>M⊥. After annealing at600℃, the separation of amorphous HfOx<2nano-grains from HfAlO*matrixes results in the reverse of anisotropy(M⊥>M//). With the increase of Ta in the range of600-800℃, more and more HfOx<2nano-grains separate, so the anisotropy (M⊥-M//) becomes larger and larger. When Ta increases to900℃, the maximum M⊥and anisotropy are obtained due to the crystallization of amorphous HfOx<2to orthogonal HfO2nanocrystal. When Ta reachs to1000℃, the crystal structure transformation from orthogonal HfO2to monocline HfO2, results in the vanishing of anisotropy. Oxygen vacancies in HfAlOx film are the main origin of its magnetism, and the order distribution of oxygen vacancies is responsible for the anisotropy. After embedding Ag nano-particles, the Ms of HfAlOx films increases while its anisotropy vanishes.Ag nano-particles embedded HfAlOx films have not only acceptable charges storage characteristics but also enhanced saturation magnetic moments. Consequently, it may be potentially applied in the new generation of magneto-electric memory devices.