Xue-Fei Li

Nanjing University, Nan-ching, Jiangsu Sheng, China

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Publications (13)27.7 Total impact

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    ABSTRACT: Hydrophilic FePt nanoparticles (NPs) have been embedded into the MgO-matrix systems via a sol–gel process to prevent FePt NPs from aggregating and sintering during the heat-treatment process required for the L10 ordering. The chemically ordered L10-phase FePt can be obtained after annealing at 700 °C for 60 min in atmosphere containing H2. The effect of the pH value of MgO collosol and FePt nanocrystal loading amount on the structure, morphology, and magnetic properties of FePt/MgO nanocomposites has been investigated. The neutral pH value of 7 in MgO sol is beneficial to stabilize FePt NPs and obtain higher chemical ordering parameter S for the face-centered tetragonal -FePt/MgO nanocomposites with larger coercivity. The FePt NPs loading amount also plays a key role in tuning the microstructure and magnetic properties of the nanocomposites. The relatively higher FePt NPs loading with FePt/MgO molar ratio (RFM) of 1:2 leads to relatively perfect hexagonal assembly and pure L10 phase. When the RFM is 1:5 and 1:10, the MgO-matrix in nanocomposites causes the Fe element loss in FePt NPs along with formation of secondary phases such as magnesioferrite or Pt3Fe during the annealing process. Under optimal processing of neutral pH value of 7 and RFM of 1:2, the presence of MgO matrix produces more homogeneous microstructures and better magnetic properties with higher room-temperature coercivity (H C = 4.65 kOe).
    Journal of Sol-Gel Science and Technology 08/2014; 71(2). DOI:10.1007/s10971-014-3365-x · 1.55 Impact Factor
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    ABSTRACT: 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 window of ∼2 V is achieved during the first 105 program/erase cycles under a ±10 V/10 ms program/erase operation. In situ ALD-grown Ir NCs with the highest density of 0.6 × 1012/cm2 provide a potential approach to fabricate large-area high-density NCs for future ultrahigh-density nonvolatile NC memory applications.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 07/2014; 32(4):042201-042201-5. DOI:10.1116/1.4878942 · 1.36 Impact Factor
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    ABSTRACT: A simple approach is developed to fabricate oxide/FePt nanocrystals/oxide composite films by a combination of chemically-synthesized FePt nanocrystals’ self-assembly and atomic layer deposition for ultrahigh-density nonvolatile memory applications. A hexagonally arranged monolayer of well-monodispersed FePt nanocrystals with a grain size of 4.5 nm has been assembled onto atomic-layer-deposited Al2O3 oxide by solution-based dip-coating. The lattice constant of this two-dimensional pattern is about 8 nm with the high density of 1.8 × 1012/cm2. A fraction of the Fe is oxidized during annealing at 500 °C for 5 min in O2 atmosphere, and the core–shell structure is formed with fcc-Fe0.75Pt nanocrystal core and amorphous Fe2O3 shell. The metal–oxide–Si capacitors with unannealed and annealed FePt nanocrystals embedded into Al2O3 films are electrically measured, and exhibit obvious memory effects with a hysteresis memory window of 4.1 and 8.1 V at the sweeping gate voltage of ±8 V, respectively. The enhanced memory window of samples with annealed FePt nanocrystals can be attributed to the existence of the Fe2O3 shell, which introduces additional interface and provides more trap sites for charge trapping storage.
    Journal of Alloys and Compounds 03/2014; 588:103–107. DOI:10.1016/j.jallcom.2013.11.008 · 2.73 Impact Factor
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    ABSTRACT: The growth of Pt nanocrystals (NCs) on Al2O3 surface via atomic layer deposition (ALD) was investigated deeply. A charge trapping memory using Pt NC as charge trapping layer and amorphous ALD Al2O3/HfO2 as the tunneling/blocking layers was fabricated. Pure metallic Pt NC can be formed at the initial growth stage, following the nucleation incubation model. Electrical measurements of p-Si\Al2O3\Pt NCs\HfO2 show a larger memory window of 6.6 V at the sweeping gate voltage of ±12 V and ∼73% retention property after 105 s. Fowler–Nordheim tunneling serves as the dominant tunneling mechanism at the applied gate voltage of 10 V. Compared to that of 4.0 V with HfO2 blocking layer, control sample with Al2O3 blocking layer shows negligible memory window of 0.3 V at ±10 V, which is attributed to the smaller electric field intensity in the Al2O3 tunneling layer of stacking structures of p-Si\Al2O3\Pt NCs\Al2O3. ALD Pt NCs with a high density of 1.0 × 1012/cm2 provides a potential approach to fabricate large area nanocrystals for future ultrahigh density nonvolatile memory applications.
    Applied Surface Science 01/2014; 289:332–337. DOI:10.1016/j.apsusc.2013.10.160 · 2.54 Impact Factor
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    ABSTRACT: BiNbO4 (I) and BiTaO4 (II) powders are obtained via the sol-gel citrate route, starting with 5% excess of Bi(NO3)3 in aqueous solutions of niobium or tantalum citrate peroxide (pH 7—8, solvent evaporation at 180 °C, 4 h, calcination at 550—1200 °C, 3 h).
    ChemInform 07/2013; 44(31):no-no. DOI:10.1002/chin.201331016
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    ABSTRACT: Visible-light photcatalysts of BiNbO4 and BiTaO4 powders have been successfully synthesized by a citrate method. The formation of pure triclinic phase of BiNbO4 and BiTaO4 at low temperature of 700 °C can be attributed to the advantage of the citrate method. The photocatalytic activity and possible mechanism were investigated deeply. For BiNbO4 particles, the mechanism of methyl violet (MV) degradation under visible light irradiation involves photocatalytic and photosensitization pathways and the catalyst specific surface area has dominant influence. While for BiTaO4 particles, the dominant mechanism arises from photosensitization pathways and a trade off between high specific surface area and good crystallinity is achieved. BiNbO4 powder calcined at 700 °C shows the best photocatalytic efficiency among these catalysts, which is ascribed to its large surface area and more positive conduction band level. The optimal catalyst loading, additive H2O2 concentration and pH value is around 1 g/L, 2 mmol/L and 8 mmol/L, respectively.
    Journal of Solid State Chemistry 06/2013; 202:6–14. DOI:10.1016/j.jssc.2013.03.012 · 2.20 Impact Factor
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    ABSTRACT: We report the characteristics of HfO2 films deposited on Ge substrates with and without La2O3 passivation at 250 °C by atomic layer deposition (ALD) using La[N(SiMe3)2]3 and Hf[N(CH3)(C2H5)]4 as the precursors. The HfO2 is observed to form defective HfGeOx at its interface during 500 °C postdeposition annealing. The insertion of an ultrathin La2O3 interfacial passivation layer effectively prevents the Ge outdiffusion and improves interfacial and electrical properties. Capacitance equivalent thickness (CET) of 1.35 nm with leakage current density JA of 8.3 × 10-4 A/cm2 at Vg = 1 V is achieved for the HfO2/La2O3 gate stacks on Ge substrates.
    Applied Surface Science 01/2013; 264:783-786. DOI:10.1016/j.apsusc.2012.10.127 · 2.54 Impact Factor
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    ABSTRACT: The self-cleaning effect of trimethylaluminium (TMA) and tetrakis (dimethyl-amino) hafnium (TDMAH) pretreatments on GaAs substrates was investigated deeply. The chemical states were carefully characterized by the X-ray photoelectron spectroscopy, which demonstrates that the pretreatment before ALD of dielectric films can suppress the formation of native oxides on GaAs substrates effectively. It is found that the combination of TMA and TDMAH pretreatments has better self-cleaning effect than single TMA pretreatment based on a ligand-exchange reaction mechanism between TMA/TDMAH and the native oxide. The transmission electron microscopy images also show a thinnest interlayer thickness of ∼0.2 nm for the TMA + TDMAH pretreated sample. TMA + TDMAH pretreated samples exhibit significantly improved interfacial and electrical properties such as the highest accumulation capacitance, the least stretch-out of capacitance–voltage curves, and the lowest interface trap density. These results indicate that the surface pretreatment by using the combination of TMA and TDMAH pulses may be a promising approach for the realization of high quality GaAs-based transistor devices.
    Applied Surface Science 12/2012; 263:497–501. DOI:10.1016/j.apsusc.2012.09.092 · 2.54 Impact Factor
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    ABSTRACT: 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.
    Integrated Ferroelectrics 01/2012; 134(1):16-21. DOI:10.1080/10584587.2012.663653 · 0.37 Impact Factor
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    ABSTRACT: 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.
    Journal of Vacuum Science & Technology B Microelectronics and Nanometer Structures 01/2012; 30(1):0602-. DOI:10.1116/1.3665416 · 1.36 Impact Factor
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    ABSTRACT: (TiO2) x (Al2O3)1−x (x=0.7,0.8,0.9) gate dielectrics were deposited on Ge by atomic layer deposition using trimethylaluminium and Ti isopropoxide. The interfacial properties and band alignment were investigated by means of transmission electron microscopy (TEM) and X-ray photoemission spectroscopy. High-resolution TEM results show that the (TiO2)0.8(Al2O3)0.2 film annealed at 500°C is amorphous with sharp interface between (TiO2)0.8(Al2O3)0.2 and Ge. The conduction-band offsets are enhanced from 1.04 to 1.40 eV with increasing Al content. Capacitance equivalent thickness of 15.8 Å for (TiO2)0.9(Al2O3)0.1 gate dielectrics is achieved with a gate leakage current of 2.70×10−5 A/cm2 at V g=+1V.
    Applied Physics A 11/2011; 105(3):763-767. DOI:10.1007/s00339-011-6511-0 · 1.69 Impact Factor
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    ABSTRACT: Self-assembled face-centered cubic FePtnanoparticles 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 FePtnanoparticles 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 FePtnanoparticles provides a potential new approach to fabricate patterned magnetic recording media with ultrahigh areal density.
    Journal of Materials Chemistry 03/2011; 21(13):5046-5050. DOI:10.1039/C0JM03268A · 6.63 Impact Factor
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    ABSTRACT: 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.
    Semiconductor Science and Technology 04/2010; 25(5):055012. DOI:10.1088/0268-1242/25/5/055012 · 2.21 Impact Factor