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Chencheng Xu

Bio: Chencheng Xu is an academic researcher from Forschungszentrum Jülich. The author has contributed to research in topics: Pulsed laser deposition & Thin film. The author has an hindex of 8, co-authored 13 publications receiving 249 citations.

Papers
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TL;DR: In this paper, the authors studied the pulsed laser deposition of homoepitaxial SrTiO3 thin films in different deposition regimes in order to elucidate the possibility to promote two-dimensional growth by increasing the kinetic energy of the oncoming particles.
Abstract: We studied the pulsed laser deposition of homoepitaxial SrTiO3 thin films in different deposition regimes in order to elucidate the possibility to promote two-dimensional growth by increasing the kinetic energy of the oncoming particles. The kinetic energy of the oncoming species is determined by exploiting plume diagnostics techniques and the resulting nucleation and growth processes are analysed by reflection high-energy electron diffraction and atomic force microscopy. We could show that although the kinetic energy of the oncoming species varies to a great extent, the diffusion process is mostly influenced by the stoichiometry. Under stoichiometric conditions, obtained only in a limited window of process parameters, the adatoms on the surface have the highest diffusivity, thus promoting a step-flow growth mode. Under nonstoichiometric conditions, both Sr- and Ti-rich, the diffusivity is strongly reduced. This results in a transition from a two-dimensional to a three-dimensional growth under Sr-rich conditions. Conversely, in the Ti-rich case, obtained at high laser fluence, the two-dimensional growth sustains until the end of the growth process. We attribute this to the high island density available at high laser fluence which facilitates the diffusion of adatoms to step edges despite of their reduced diffusion length.

59 citations

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TL;DR: The results indicate that equilibrium defect formation is the dominant process for establishing the properties of the two-dimensional electron gas (2DEG), while growth dynamics play a minor role in the typical LAO/STO growth regime.
Abstract: The influence of non-equilibrium and equilibrium processes during growth of LaAlO3/SrTiO3 (LAO/STO) heterostructures is analyzed. We investigate the electronic properties of LAO/STO heterostructures obtained at constant growth conditions after annealing in different oxygen atmospheres within the typical growth window (1 × 10−4 mbar –1 × 10−2 mbar). The variation of annealing conditions is found to cause a similar change of electronic properties as observed for samples grown in different oxygen pressure. The results indicate that equilibrium defect formation is the dominant process for establishing the properties of the two-dimensional electron gas (2DEG), while growth dynamics play a minor role in the typical LAO/STO growth regime. Furthermore, the effects of non-equilibrium processes occurring during growth are investigated in detail by quenching just-grown LAO/STO heterostructures directly after growth. We show that during growth the sample is pushed into a non-equilibrium state. After growth, the sample then relaxes towards equilibrium, while the relaxation rate strongly depends on the ambient pressure. The observed relaxation behavior is mainly associated with a reoxidation of the STO bulk, while the 2DEG is formed immediately after the growth.

37 citations

Journal ArticleDOI
TL;DR: A complete picture of another influence parameter to be considered during pulsed laser depositions is provided and the mechanism behind persistent-photo-conductivity in SrTiO3 is unraveled, finding a UV enhanced oxygen-vacancy incorporation rate as responsible mechanism.
Abstract: Pulsed Laser Deposition is a commonly used non-equilibrium physical deposition technique for the growth of complex oxide thin films. A wide range of parameters is known to influence the properties of the used samples and thin films, especially the oxygen-vacancy concentration. One parameter has up to this point been neglected due to the challenges of separating its influence from the influence of the impinging species during growth: the UV-radiation of the plasma plume. We here present experiments enabled by a specially designed holder to allow a separation of these two influences. The influence of the UV-irradiation during pulsed laser deposition on the formation of oxygen-vacancies is investigated for the perovskite model material SrTiO3. The carrier concentration of UV-irradiated samples is nearly constant with depth and time. By contrast samples not exposed to the radiation of the plume show a depth dependence and a decrease in concentration over time. We reveal an increase in Ti-vacancy–oxygen-vacancy-complexes for UV irradiated samples, consistent with the different carrier concentrations. We find a UV enhanced oxygen-vacancy incorporation rate as responsible mechanism. We provide a complete picture of another influence parameter to be considered during pulsed laser depositions and unravel the mechanism behind persistent-photo-conductivity in SrTiO3.

37 citations

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TL;DR: The results prove the attached measurement devices to be primarily responsible for the reduction of SrTiO3 in the deposition chamber by shifting the thermodynamic equilibrium to a more reducing atmosphere.
Abstract: The reduction of oxides during annealing and growth in low pressure processes is a widely known problem. We hence investigate the influence of mere annealing and of growth in vacuum systems to shed light on the reasons behind the reduction of perovskites. When comparing the existing literature regarding the reduction of the perovskite model material SrTiO3 it is conspicuous that one finds different oxygen pressures required to achieve reduction for vacuum annealing and for chemically controlled reducing atmospheres. The unraveling of this discrepancy is of high interest for low pressure physical vapor depositions of thin films heterostructures to gain further understanding of the reduction of the SrTiO3. For thermal annealing, our results prove the attached measurement devices (mass spectrometer/ cold cathode gauge) to be primarily responsible for the reduction of SrTiO3 in the deposition chamber by shifting the thermodynamic equilibrium to a more reducing atmosphere. We investigated the impact of our findings on the pulsed laser deposition growth at low pressure for LaAlO3/SrTiO3. During deposition the reduction triggered by the presence of the laser plume dominates and the impact of the measurement devices plays a minor role. During post annealing a complete reoxidization of samples is inhibited by an insufficient supply of oxygen.

35 citations

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TL;DR: It is demonstrated that the analysis of angle dependent X-ray photoelectron intensity ratios provides a unique tool to determine both termination and stoichiometry simultaneously in a straightforward experiment, and insights about oxide thin film growth can be utilized for interface engineering of oxide heterostructures.
Abstract: Emerging electrical and magnetic properties of oxide interfaces are often dominated by the termination and stoichiometry of substrates and thin films, which depend critically on the growth conditions. Currently, these quantities have to be measured separately with different sophisticated techniques. This report will demonstrate that the analysis of angle dependent X-ray photoelectron intensity ratios provides a unique tool to determine both termination and stoichiometry simultaneously in a straightforward experiment. Fitting the experimental angle dependence with a simple analytical model directly yields both values. The model is calibrated through the determination of the termination of SrTiO3 single crystals after systematic pulsed laser deposition of sub-monolayer thin films of SrO. We then use the model to demonstrate that during homoepitaxial SrTiO3 growth, excess Sr cations are consumed in a self-organized surface termination conversion before cation defects are incorporated into the film. We show that this termination conversion results in insulating properties of interfaces between polar perovskites and SrTiO3 thin films. These insights about oxide thin film growth can be utilized for interface engineering of oxide heterostructures. In particular, they suggest a recipe for obtaining two-dimensional electron gases at thin film interfaces: SrTiO3 should be deposited slightly Ti-rich to conserve the TiO2-termination.

34 citations


Cited by
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01 Jan 1999

643 citations

Journal ArticleDOI
TL;DR: In this paper, a transverse Ising model was applied to analyze the finitesize effect in 2D ferroelectric films, and the authors showed that when the thickness of a thin film is below several to tens of nanometers, spontaneous polarization for traditional perovskite oxide usually disappears with a dropped transition temperature due to the reduced long-range Coulomb coupling, significantly enhanced depolarizing electrostatic field, surface-reconstruction caused by the surface energy, electron screening and interfacial bonding related strain, the chemical environment, etc.
Abstract: DOI: 10.1002/aelm.201900818 phase transitions by Ginzburg et al.[7] In the 1950s, ferroelectric thin films of perovskite structures were first deposited. After successful integration into a semiconductor, ferroelectric thin films exhibit as an important component in the application of electronics and microtransducers. However, most early experiments were performed using a range of film thickness from 100 nm to several micrometers. It has been known since the late 1950s that ferroelectric thin films exhibit quite different phase transition characteristics compared with those of bulk materials. The finitesize effect in ferroelectric films was analyzed and applied to ferroelectrics based on a transverse Ising model.[8] When the thickness of a thin film is below the critical value of around several to tens of nanometers, the spontaneous polarization for traditional perovskite oxide usually disappears with a dropped transition temperature due to the reduced longrange Coulomb coupling, significantly enhanced depolarizing electrostatic field, surface-reconstruction caused by the surface energy, electron screening and interfacial bonding related strain, the chemical environment, etc.[9,10] However, with the rapid development of microelectronics, higher density electronic devices are urgently needed and the scaling trends continue to shrink the feature size of materials. Controllable thinner, that is, nanoscale ferroelectric materials, are essential not only for understanding the size effect but also for electronic applications. Although the dilemma has posed unprecedented challenges to the entire industry, many efforts have been put forward to overcome the difficulties. As early as the 1980s, ferroelectricity was introduced in polyvinylidene fluoride[11] and its copolymers with trifluoroethylene P(VDFTrFE).[12] In 1986, P(VDF-TrFE) films as thin as 60 nm with a saturated polarization of 100 mC m–2 were reported.[13] Later in 1991, Scott reported the phase transitions in ferroelectric films with a thickness of 70–400 nm both theoretically and experimentally.[14] Significantly, in 1998, a ferroelectric transition in a two monolayers thick random copolymer of vinylidene fluoride and trifluoroethylene was observed.[15] Then, in 2000, Blinov et al. discussed the unique ferroelectric properties in two monolayers (≈1 nm) of P(VDF-TrFE), which were fabricated by the Langmuir–Blodgett technique and demonstrated a breakthrough in original critical thickness, thus encouraging researchers to further explore The investigation of two-dimensional (2D) ferroelectrics has attracted significant interest in recent years for applications in functional electronics. Without the limitation of a finite size effect, 2D materials with stable layered structures and reduced surface energy may go beyond the presence of an enhanced depolarization field in ultrathin ferroelectrics, thereby opening a pathway to explore low-dimensional ferroelectricity, making ultra-high-density devices possible and maintaining Moore’s Law. Although many theoretical works on potential 2D ferroelectric materials have been conducted, much still needs to be accomplished experimentally, as it is rare for 2D ferroelectric materials to be proven and plenty of 2D ferroelectrics are waiting to be discovered. First, experimental and theoretical progress on 2D ferroelectric materials, including in-plane and out-of-plane, is reviewed, followed by a general introduction to various characterization methods. Intrinsic mechanisms associated with promising 2D ferroelectric materials, together with related applications, are also discussed. Finally, an outlook for future trends and development in 2D ferroelectricity are explored. Researchers can use this to obtain a basic understanding of 2D ferroelectric materials and to build a database of progress of 2D ferroelectrics.

209 citations

Journal ArticleDOI
TL;DR: In this article, the relevant regimes of concentrations and associated phenomena arising from oxygen vacancies are discussed, and experimental techniques available for observing oxygen vacancies at widely different levels of concentrations are discussed.
Abstract: Oxygen vacancies play crucial roles in determining the physical properties of metal oxides, representing important building blocks in many scientific and technological fields due to their unique chemical, physical, and electronic properties. However, oxygen vacancies are often invisible because of their dilute concentrations. Therefore, characterizing and quantifying their presence is of utmost importance for understanding and realizing functional metal oxide devices. This, however, is oftentimes a non-trivial task. In this Perspective paper, we discuss the relevant regimes of concentrations and associated phenomena arising from oxygen vacancies. We then focus on experimental techniques available for observing oxygen vacancies at widely different levels of concentrations. Finally, we discuss current challenges and opportunities for utilizing oxygen vacancies in metal oxides.

194 citations

Journal ArticleDOI
Da Chen1, Feng Niu1, Laishun Qin1, Sen Wang1, Ning Zhang1, Yuexiang Huang1 
TL;DR: In this article, surface oxygen vacancies were introduced into hydrothermally-synthesized BiFeO 3 (BFO) nanocrystals through high pressure hydrogenation treatment, and with increasing hydrogenation temperature the oxygen vacancy concentration would increase.

113 citations