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ABSTRACT: BiFeO3(BFO) films were deposited by pulsed laser deposition on GaN(0002)/c-sapphire. X-ray diffraction data show that BFO(111) epitaxial films was deposited on SrTiO3/TiO2 double-layer buffered GaN, compared to the polycrystalline film grown directly on GaN. The epitaxial relationships of BFO film with SrTiO3/TiO2 buffer layers were BFO [11-2](111)//GaN [1-100](0002) as revealed by XRD phi scans. Furthermore, BFO films on SrTiO3/TiO2 buffer exhibited enhanced electric properties compared to these deposited directly on GaN. The remnant polarization was improved about 30% while the leakage current was reduced by nearly three orders of magnitude. The epitaxial growth promoted by the SrTiO3/TiO2 buffer layer is a pivotal parameter in the improved electric properties of BFO films on GaN (0002).
Journal of Applied Physics 05/2011; 109(10):104108-104108-5. · 2.17 Impact Factor
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ABSTRACT: LiNbO3 film (LNO)/AlGaN/GaN heterostructure was fabricated and its memory characteristics were studied. The heterostructure exhibited a wide range clockwise hysteresis (0.3–12.1 V) likely due to the electrons trapping and distrapping from the Li vacancies in the LNO film. After 10 years retention, 10% of the window could remain. In addition, a slight decrease for the memory window happened after 105 cycles. These results indicated that LNO film combined with AlGaN/GaN would hold promise for next-generation nonvolatile memory devices. Possible operating mechanism for the memory effect in the heterostructure was explained qualitatively by the energy band diagram.
Applied Physics Letters 01/2010; 96(3):032103-032103-3. · 3.84 Impact Factor
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ABSTRACT: Epitaxial growth of (111)-oriented BaTiO3 thin films on GaN/Al2O3 (0001) substrates has been realized by intervening epitaxial (111) SrRuO3/(100)TiO2 buffer layers. Rutile TiO2 was deposited by Laser molecular beam epitaxial firstly and the deposition process of TiO2 layer was in-situ monitored by reflective high energy electron diffractions. After that SrRuO3 conductive films and BaTiO3 ferroelectric films were fabricated via pulsed laser deposition. The perovskite BaTiO3 films were epitaxially grown on wurtzite GaN as revealed by X-ray diffraction. Electrical measurements demonstrate that the BaTiO3 films possess a saturation ferroelectric hysteresis loop, butterfly-shape C-V curve and low leakage current density. These results show that BaTiO3 films with favorable electrical performance could be epitaxially grown on GaN using SrRuO3/TiO2 bufferlayer.
Ferroelectrics. 01/2010; 406(1):56-61.
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ABSTRACT: Epitaxial SrTiO <sub>3</sub> films were fabricated by laser molecular beam epitaxy on bare and TiO <sub>2</sub> buffered GaN(0002), respectively. The whole deposition processes were in situ monitored by reflection high energy electron diffraction (RHEED). X-ray diffraction (XRD) was carried out to study the growth orientation and crystalline quality of STO films. The interfacial characters and epitaxial relationships were also investigated by high revolution transition electron microscope and selected area electron diffraction (SAED). According to the RHEED observation, the lowest epitaxy temperature of STO on TiO <sub>2</sub> buffered GaN was decreased compared with the direct deposited one. The epitaxial relationship was (111)[1 1 0] STO //(0002)[11 2 0] GaN in both cases as confirmed by RHEED, XRD, and SAED. The full width at half maximum of ω -scan and Φ -scan of STO on TiO <sub>2</sub> buffered GaN was reduced compared with that deposited on bare GaN, indicating that epitaxial quality of STO film is improved by inserting TiO <sub>2</sub> layer. In summary, the lattice mismatch was reduced by inserting rutile TiO <sub>2</sub> . As a result, the crystalline temperature was reduced and enhanced epitaxial quality of STO thin film was obtained.
Journal of Applied Physics 12/2009; · 2.17 Impact Factor
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ABSTRACT: Hf-doped Bi <sub>4</sub> Ti <sub>3</sub> O <sub>12</sub> (BTH) ferroelectric films with excellent electrical properties were epitaxially integrated with GaN semiconductor using (111) SrTiO <sub>3</sub> (STO)/rutile (200) TiO <sub>2</sub> as buffer layer. The STO / TiO <sub>2</sub> buffer layer was deposited by laser molecular beam epitaxy. The structural characteristics of the buffer layer were in situ and ex situ characterized by reflective high energy electron diffraction, x-ray diffraction (XRD), and high resolution transmission microscopy. The overlaying SrRuO <sub>3</sub> (SRO) and BTH films were then deposited by pulsed laser deposition. XRD spectra, including θ-2θ and Φ scans, show that the (208) BTH films were epitaxially grown on GaN, and the BTH films inherit the in-plane twin-domain of STO buffer layer. Electrical measurements demonstrate that the non- c axis BTH films possess a large remnant polarization (2P<sub>r</sub>=45 μ C / cm <sup>2</sup>) , excellent fatigue endurance (10.2% degradation after 1.1×10<sup>10</sup> switching cycles), and a low leakage current density ( 1.94×10<sup>-7</sup> A / cm <sup>2</sup> at an electric field of 200 kV/cm). These results reveal that the (208) BTH films with favorable electrical performance could be epitaxially grown on GaN template using <font face=' ||
'roman'>STO / TiO <sub>2</sub> buffer layer.
Journal of Applied Physics 06/2009; · 2.17 Impact Factor
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ABSTRACT: Cubic AlN thin films have been fabricated on SrTiO3(100) substrates by pulsed laser deposition (PLD) at different substrate temperatures and ambient nitrogen pressures. The microstructure and surface morphology of the deposited films were characterized by reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), atomic force microscopy (AFM) and X-ray photoelectron spectrometry (XPS). AlN films fabricated at 450°C had polycrystalline structures. Epitaxial cubic AlN films were obtained when the deposition temperature was increased to 650°C. The epitaxial relationship of cubic AlN film on SrTiO3 substrate was AlN[100]∥SrTiO3[100] and AlN(200)∥SrTiO3(100). The degradation of the film crystalline quality was found if the growth temperature was further increased to 800°C. AFM investigation revealed that the surface morphology of AlN films strongly depended on N2 partial pressure. The root mean square (RMS) roughness values for AlN films deposited at 650°C and at 10Pa N2 were about 0.674nm. In addition, XPS results demonstrated that no oxide phase existed in epitaxial AlN films. Thus, epitaxial cubic AlN films could be fabricated on STO substrates under the optional deposition conditions by PLD.
Journal of Crystal Growth - J CRYST GROWTH. 01/2008; 310(4):731-737.
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ABSTRACT: Oxygen ions are implanted into BiFeO3 films on LaNiO3/SrTiO3(1 0 0) substrates. The evolution of films is characterized by x-ray diffraction, scanning electron microscopy and cross-sectional transmission electron microscopy. The leakage current density of the implanted films is lowered by two orders of magnitude in comparison with unimplanted BFO films. The mechanisms for reduced leakage current in BiFeO3 thin films are discussed.
Journal of Physics D Applied Physics 08/2007; 40(18):5775. · 2.54 Impact Factor
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ABSTRACT: BiFeO3 films have been grown on SrTiO3 (001) substrates by pulsed laser deposition. It was found that oxygen partial pressure is crucial to phase purity, surface morphology, and surface chemistry. Single-phase BFO films were obtained at 1 Pa O2, while Bi2O3 appeared in the films deposited at 0.01 Pa as confirmed by x-ray diffractions. It was revealed that Fe2+ and metallic Bi exist in the films fabricated at 0.01 Pa by x-ray photoelectron spectroscopy investigation. Owing to Fe2+ in the samples deposited at 0.01 Pa, the saturation magnetization is much larger than the ones fabricated at 1 Pa. A well-saturated ferroelectric hysteresis loop with a polarization of 23.6 μC/cm2 was observed in the single-phase samples. In contrast, the films deposited at 0.01 Pa exhibited poor ferroelectric properties.
Applied Physics Letters 08/2007; 91(8):082501-082501-3. · 3.84 Impact Factor
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ABSTRACT: Epitaxial ZnO thin films with different orientations have been grown by laser molecular beam epitaxy on (001)-, (011)-, and (111)-orientated SrTiO3 single-crystal substrates. The growth behavior was in situ monitored by reflection high-energy electron diffraction, and the epitaxial orientation relations were reconfirmed by ex situ x-ray diffraction measurements. In the case of ZnO on SrTiO3(001), four orthogonal domains coexisted in the ZnO epilayer, i.e., ZnO(110)‖SrTiO3(001) and ZnO[−111]‖SrTiO3〈100〉. For (011)- and (111)-orientated substrates, single-domain epitaxy with c axial orientation was observed, in which the in-plane relationship was ZnO[110]‖SrTiO3[110] irrespective of the substrate orientations. Additionally, the crystalline quality of ZnO on SrTiO3(111) was better than that of ZnO on SrTiO3(011) because of the same symmetry between the (111) substrates and (001) films. The obtained results can be attributed to the difference of the in-plane crystallographic symmetry. Furthermore, those alignments can be explained by the interface stress between the substrates and the films.
Applied Physics Letters 04/2007; 90(15):151918-151918-3. · 3.84 Impact Factor
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ABSTRACT: BaTiO3 (BTO) films have been epitaxially grown on SrTiO3(001) and SrTiO3(111) substrates with a LaNiO3 conductive layer as a template using pulsed laser deposition. X-ray diffraction θ–2θ and ψ scanning reveal that the epitaxial relationship for these samples fabricated at 600 °C and 700 °C are BTO[001]//LNO[001]//STO[001] and BTO[111]//LNO[111]//STO[111], respectively. Ex situ reflective high energy electron diffraction patterns also clearly confirm this epitaxial relationship. The epitaxial growth can be more easily realized on LNO-inserted STO(001) substrates at a relatively wide range of temperatures. From C–V measurements, the dielectric constant of the epitaxial (001)BTO film is determined as 159.5, which is larger than the value 89.6 of (111)BTO film. The remanent polarization (2Pr) of the epitaxial BTO(001) thin film on the LNO-coated (001)STO substrate is 39.8 µC cm−2, which is much larger than that of the BTO(111) film on the LNO-coated STO(111) substrate. The result is also consistent with the direct observation made by piezoresponse force microscopy investigation. Thus, with the epitaxial LNO electrode template layers, the epitaxial BaTiO3(001) ferroelectric films exhibit high-quality structure and good electrical properties.
Journal of Physics D Applied Physics 05/2006; 39(11):2438. · 2.54 Impact Factor
