Xue-Fei Li

Magnetic properties of FePt nanoparticle assemblies embedded in atomic-layer-deposited Al2O3

Self-assembled face-centered cubic FePt nanoparticles on Si substrates were embedded into amorphous Al2O3 capping layers with various thicknesses in the range 5–20 nm using atomic layer deposition (ALD) technology. The effect of the Al2O3 layer thickness on the structure, mono-dispersibility, and magnetic properties of the FePt/Al2O3-matrix composite films was investigated. After annealing at 700 °C in a reducing atmosphere for 1 h, well-dispersed face-centered tetragonal (fct) FePt particles could be obtained for the samples with 10 nm-thick and greater Al2O3 layers. Experimental results suggest that the protection of the amorphous 10 nm-thick Al2O3 matrix can effectively inhibit grain growth and particle aggregation, and preserve the ordered domains of FePt nanoparticles during the L10 ordering transition through annealing. The 5 nm fct FePt-nanoparticles/10 nm-thick Al2O3-matrix sample shows higher coercivity of 5.9 kOe. The combination of ALD-capping layer and self-assembled FePt nanoparticles provides a potential new approach to fabricate patterned magnetic recording media with ultrahigh areal density.

Impact of the Al/Hf ratio on the electrical properties and band alignments of atomic-layer-deposited HfO2/Al2O3 on S-passivated GaAs substrates

HfO2, Al2O3 and HfO2/Al2O3 (AHO) nanolaminates with various Al/Hf ratios (including 1:3.0, 1:2.1 and 1:1.3) were fabricated on S-passivated GaAs substrates by atomic layer deposition (ALD). The interface structure and band alignments of various dielectric/GaAs structures have been investigated systematically. The AHO films with the Al/Hf ratio of 1:1.3 suppress the formation of As oxides and elemental As overlayers around AHO/GaAs interfaces more effectively than other samples, showing higher accumulation capacitance, less hysteresis width (ΔVFB = 415 mV) and lower leakage current density. The band alignments of interfaces of HfO2/GaAs, Al2O3/GaAs and AHO/GaAs were established. The results indicate that ALD HfO2/Al2O3 nanolaminate structures could effectively tune the interface quality and band offset of gate dielectric films on n-GaAs.

Redox-controlled memristive switching in the junctions employing Ti reactive electrodes

We have proposed a kind of memristive device based on the junctions employing Ti as the reactive electrodes. The role of electrically-derived redox of Ti in such memristive switching is shown. The structural and chemical evidence of the electrically-derived oxidation is presented by TEM and XPS experiment, respectively. Due to the redox of the top electrode Ti and the consequent drift of oxygen vacancies, the device shows two distinct resistance states under a sweeping voltage loading. ON state is controlled by tunneling process, while OFF state is controlled by Schottky emission conductive mechanism. The failure behaviors of such memristive junctions are also discussed. In the light of the redox principle, we demonstrate that the devices could be recovered by loading a long electrical reduction treatment.

Growth of high-density Ir nanocrystals by atomic layer deposition for nonvolatile nanocrystal memory applications

A careful investigation is made of the growth of Ir nanocrystals (NCs) on Al2O3 by atomic layer deposition (ALD), and a charge trapping memory device using ALD-grown Ir NCs as the charge trapping layer and ALD-grown Al2O3/HfO2 as the tunneling/blocking layers is fabricated. It is found that the ex situ nucleation of Ir NCs on ALD-grown Al2O3 is difficult, though in situ growth can produce pure metallic Ir NCs with a face-centered cubic crystalline phase directly on ALD-grown Al2O3 at the initial growth stage, which follows the nucleation incubation model. The growth of these metallic Ir NCs is attributed to the presence of a uniform coverage of reactive groups (hydroxyl or dimethylaluminum) on the as-deposited fresh ALD-grown Al2O3 surface, which greatly promotes the uniform nucleation of Ir. Electrical measurements of p-Si/Al2O3/Ir NCs/HfO2 memory cells exhibit a large memory window of 4.2 V at the sweeping gate voltage of ±10 V, and a ∼76% retention property after 104 s at 75 °C. Also, a stable memory w...

Effect of annealing on interfacial and band alignment characteristics of HfO2/SiO2 gate stacks on Ge substrates

The authors have investigated the effect of 500 °C annealing for 60 s in N2 on interfacial and band alignment characteristics of HfO2/SiO2 gate stacks on n-type Ge substrates. X-ray photoelectron spectroscopy analyses reveal that the SiO2 interlayer can effectively suppress Ge outdiffusion during HfO2 growth and subsequent postdeposition annealing process. The electrical measurement shows that capacitance equivalent thickness of 1.75 nm and a leakage current density of 3.9 × 10−3 A/cm2 at gate bias of flatband voltage (Vfb) + 1 V was obtained for the annealed sample. The conduction band offsets at the HfO2/SiO2/Ge with and without annealing are found to be 2.22 and 2.07 eV, respectively.

Characterization of HfO2/Al2O3 gate dielectric nanometer-stacks grown by atomic layer deposition on Ge substrates

We report the characteristics of HfO2/Al2O3 gate dielectric nanometer-stacks deposited on Ge substrates at 250 °C by atomic layer deposition using Hf[N(CH3)(C2H5)]4 and Al(CH3)3 as the precursors. The annealing effect on the interface and electrical properties of stack films was investigated by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and electrical measurements. It is demonstrated that the regrowth of GeOx during annealing is suppressed by Al2O3 layer. And the annealed samples exhibit better electrical properties with reduction hysteresis, high capacitance value and improved leakage current density.

Effects of Pr Substitution on Electrical Properties of Bi(Fe0.95Mn0.05)O3 Thin Films

The effects of Pr3+ substitution on electrical properties of Bi(Fe0.95Mn0.05)O3 (BFMO) thin films are investigated. The leakage current densities of BFMO films can be significantly suppressed by Pr3+ substitution. Dielectric analysis reveals that dielectric constants of the films increase with increasing Pr3+ content. Reduced loss tangents are obtained in Pr3+-substituted BFMO films. Ferroelectric measurements demonstrate that Pr3+ substitution is helpful for lowering the coercive fields of films. In addition, a double hysteresis loop is observed in the BFMO film with 25% Bi3+ substituted by Pr3+. This may be ascribed to the existence of the defect-complexes.

Effects of Postannealing Temperature on the Band Alignments and Interfacial Properties of Atomic Layer Deposited Al2O3 on Ge Substrates

The effect of postannealing temperature (500, 600 and 700°C) on the band alignments and interfacial structures of Al2O3 films grown on Ge by atomic layer deposition was investigated by X-ray photoelectron spectroscopy and electrical measurements. With increasing postannealing temperature, the reaction between Al2O3 and Ge becomes serious and the valence-band offsets of Al2O3/Ge interface is observed to upshift. The conduction-band offsets at the interface of Al2O3/Ge without and with postannealing temperatures at 500, 600 and 700°C are estimated to be 2.90, 2.75, 2.59 and 2.41 eV, respectively.

Patterned distribution of silicon nanocrystals prepared by pulsed laser interference crystallization of an ultrathin a-Si:H single layer

We employ the method of phase-modulated KrF excimer pulsed laser interference crystallization (LIC) to fabricate nanocrystalline silicon with a two-dimensional (2D) patterned distribution within an ultrathin a-Si:H single layer. A local phase transition occurs in the ultrathin a-Si:H film upon LIC with the appropriate energy density. The results from atomic force microscopy, Raman scattering spectroscopy, planar and cross-sectional transmission electron microscopy and scanning electron microscopy demonstrate that Si nanocrystallites are formed within the initial a-Si:H single layer, selectively located in disc-shaped regions with diameters of 250 nm and patterned with the same 2D periodicity of 2.0 μm as the phase-shifting grating.