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先进纳米薄膜材料：制备方法及应用=Advanced Nano Deposition Methods：英文试读：
书名：先进纳米薄膜材料：制备方法及应用=Advanced Nano Deposition Methods：英文
版权所有 违者必究List of ContributorsHongliang CaoEast China University of Science and TechnologySchool of Materials Science and EngineeringShanghai Key Laboratory ofAdvanced Polymeric Materials Key Laboratory for UltrafineMaterials of Ministry of Education130 Meilong RoadShanghai 200237PR ChinaChonglin ChenUniversity of Texas at San AntonioDepartment of Physics and AstronomyOne UTSA CircleSan Antonio，TX 78249USAXin ChenEast China University of Science and TechnologySchool of Materials Science and EngineeringShanghai Key Laboratory ofAdvanced Polymeric Materials Key Laboratory for UltrafineMaterials of Ministry of Education130 Meilong RoadShanghai 200237PR ChinaandChinese Academy of Sciences Shanghai Institute ofMicrosystem and Information TechnologyState Key Laboratory ofFunctional Materials forInformatics865 Changning RoadShanghai 200050PR ChinaRabi EbrahimUniversity of HoustonCenter for Advanced MaterialsHouston，TX 77204-5504USADaniel FisherUniversity of HoustonCenter for Advanced MaterialsHouston，TX 77204-5504USAMin GaoUniversity of Electronic Science and Technology of ChinaState Key Laboratory ofElectronic Thin Films andIntegrated DevicesNo.4，Section 2，North Jianshe RoadChengduSichuan 610054PR ChinaWen HuangUniversity of Electronic Science and Technology of ChinaSchool of Microelectronics and Solid-State ElectronicsNo.4，Section 2，North Jianshe RoadChengduSichuan 610054PR ChinaAlex IgnatievUniversity of HoustonCenter for Advanced MaterialsHouston，TX 77204-5004USAYanda JiUniversity of Electronic Science and Technology of ChinaState Key Laboratory ofElectronic Thin films andIntegrated DevicesNo.4，Section 2，North Jianshe RoadChengduSichuan 610054PR ChinaYuan LinUniversity of Electronic Science and Technology of ChinaState Key Laboratory ofElectronic Thin films andIntegrated DevicesNo.4，Section 2，North Jianshe RoadChengduSichuan 610054PR ChinaChunrui MaXi'an Jiaotong UniversitySchool of Materials Science and EngineeringState Key Laboratory forMechanical Behavior ofMaterialsNo.28，Xianning West RoadXi'anShanxi 710049PR ChinaTaisong PanUniversity of Electronic Science and Technology of ChinaState Key Laboratory ofElectronic Thin films andIntegrated DevicesNo.4，Section 2，North Jianshe RoadChengduSichuan 610054PR ChinaGoran RasicNorth Carolina Central UniversityDepartment of Physics1801 Fayetteville StreetDurham，NC 27707USAZhitang SongChinese Academy of Sciences Shanghai Institute ofMicrosystem and Information TechnologyState Key Laboratory ofFunctional Materials forInformatics865 Changning RoadShanghaiPR ChinaSerekbol TokmoldinInstitute of Physics and Technology11 Ibragimov StreetAlmaty 050032KazakhstanZhongping WangUniversity of Science andTechnology of ChinaSchool of Physical SciencesCenter for Physics Experiments No.96 Jinzhai RoadHefeiAnhui 230026PR ChinaLiangcai WuChinese Academy of Sciences Shanghai Institute ofMicrosystem and Information TechnologyState Key Laboratory ofFunctional Materials forInformatics865 Changning RoadShanghaiPR ChinaNaijuan WuUniversity of HoustonCenter for Advanced MaterialsHouston，TX 77204-5504USADebiao XieEast China University of Science and TechnologySchool of Materials Science and EngineeringShanghai Key Laboratory ofAdvanced Polymeric MaterialsKey Laboratory for UltrafineMaterials of Ministry ofEducation130 Meilong RoadShanghai 200237PR ChinaWeiqing YangSouthwest Jiaotong UniversitySchool of Materials Science and EngineeringKey Laboratory of AdvancedTechnologies of Materials（Ministry of Education）Ring North of a Section 111ChengduSichuan 610031PR ChinaMukhtar YeleuovUniversity of HoustonCenter for Advanced MaterialsHouston，TX 77204-5504USAHulin ZhangUniversity of Electronic Science and Technology of ChinaSchool of Microelectronics and Solid-State ElectronicsNo.4，Section 2，North Jianshe RoadChengduSichuan 610054PR ChinaZengming ZhangUniversity of Science and Technology of ChinaSchool of Physical SciencesCenter for Physics ExperimentsNo.96 Jinzhai RoadHefeiAnhui 230026PR ChinaWangfan ZhouEast China University of Science and TechnologySchool of Materials Science and EngineeringShanghai Key Laboratory ofAdvanced Polymeric MaterialsKey Laboratory for UltrafineMaterials of Ministry ofEducation130 Meilong RoadShanghai 200237PR China1 Pulsed Laser Deposition for Complex Oxide Thin Film and NanostructureChunrui Ma and Chonglin Chen1.1 IntroductionComplex oxide thin films and nanostructures are at the heart of new“oxide electronic”applications，such as ultraviolet light-emitting diodes［1–3］，resistive switching memories［4，5］，chemical sensor［6，7］，and so on.They are often grown by pulsed laser deposition（PLD）because the technique is believed to be material agnostic.PLD is a thin film deposition technique–a type of physical vapor deposition.A high-power pulsed laser beam is focused on and strikes a target of the material that is to be deposited in a vacuum chamber.This material is vaporized from the target in a plasma plume and deposited as a thin film on a substrate.This process can occur in ultrahigh vacuum or in the presence of a background gas，such as oxygen，which is commonly used when depositing complex oxides.
The synthesis of novel thin films and structures is advancing on two fronts：one is the complexity of materials being deposited；and the other is the reduction in the typical dimensions of the features.As a rule of thumb，any structure that has one or more dimension smaller than about 100 nm is considered to be a nanostructured material.PLD is one of the most promising techniques for the formation of complex oxide heterostructures and nanostructures.The basic setup of PLD is simple relative to many other deposition techniques，and it can stoichiometrically transfer a material from a solid source to a substrate to form its thin film.The first use of PLD to deposit the films of semiconductors and dielectrics by ruby laser is reported in the literature as early as 1965［8］.PLD for the film growth of SrTiO and 3BaTiO was achieved in 1969［9］.Six years later，stoichiometric 3intermetallic materials（NiMn and ReBe）were fabricated by using 322PLD［10］.In 1987，PLD had a real breakthrough in its successful application to the in situ growth of epitaxial high-temperature superconductor films at Bell Communications Research［11］.Since then，PLD has been used extensively in the growth of high-temperature cuprates and numerous other complex oxides，including materials that cannot be obtained by an equilibrium route［12–16］.Advanced Nano Deposition Methods，First Edition.Edited by Yuan Lin and Xin Chen.© 2016 Wiley-VCH Verlag GmbH & Co.KGaA.Published 2016 by Wiley-VCH Verlag GmbH & Co.KGaA.
This chapter details the PLD setup and focuses primarily on the operating principle，growth mechanism，and parameters of PLD for complex oxide thin film and nanostructure.1.2 Pulsed Laser Deposition System SetupThe technique of PLD is conceptually simple，as illustrated schematically in Figure 1.1.The system consists of a laser，a vacuum chamber equipped with pumps，a target holder and rotator，and a substrate heater and is typically equipped with various pressure gauges，controllers，and other instruments to control the deposition environment of the system［17］.Film growth can be carried out in reactive environments，such as that for oxides where a partial pressure of oxygen，ozone，or atomic oxygen is carefully controlled.The substrate heater controls the substrate temperature.PLD systems are also often equipped with a set of optics including apertures，attenuators，mirrors，and lenses to focus and direct the laser beam into the target with the right energy density.The bulk material target orients at an angle of 45°toward the incident laser beam.The laser beam is focused onto the target surface by a set of optical components.The target locally absorbs the laser pulse energy and ejects a small amount of target material in the form of a plume containing many energetic species including atoms，molecules，electrons，ions，clusters，particulates，and molten globules.The plasma is then deposited onto a substrate facing the target with a separation distance of 3–5 cm.The substrate temperature can be varied from room temperature to 1000°C，even higher than 1000°C，depending on the heater type.The film microstructure depends on various parameters such as substrate temperature；laser energy density and pulse repetition rate；pressure and type of gas inside the chamber；and substrate-to-target distance.Figure 1.1 Schematic diagram of typical pulsed laser deposition.1.3 Advantages and Disadvantages of Pulsed Laser DepositionPLD exhibits many fascinating properties and practical advantages.Firstly，it has the ability to faithfully keep the stoichiometry of the target material，which is the first aspect that draws the attention of the thin film growth community［18］.Secondly，the energy source for material transport（i.e.，the laser）is outside the chamber，minimizing any impurities caused by in-vacuum power components；it is very flexible，cost effective，and fast.Many different materials can be ablated by using the same apparatus，and the different laser wavelengths are available in principle.The isolated local heating by the laser spot means that several different materials can be sequentially ablated in a single vacuum chamber by using a carousel system or a segmented target rod to fabricate heterostructures with little of cross-contamination of the source target material.This avoids the interconnected vacuum transfer and is an important advantage in research environment：one laser can serve many vacuum systems in order to save the laser cost，and highquality samples can be grown in 10 or 20 min.Finally，it is easy to control film thickness and multilayer film by controlling the pulse repetition rate，growth time，and the use of multiple target holders；it demands a much lower substrate temperature than other film deposition techniques because the high kinetic energy（10–100 eV）of species in the ablation plume promotes surface mobility during film growth.
In spite of the above-mentioned advantages of PLD，there are some drawbacks in using the PLD technique.One of the major problems is limited uniformity because the plasma plume ejected from the target can only provide a narrow forward angular distribution.Another problem is high defect or particulate concentration due to surface boiling.The size of particulates may be as large as a few micrometers，which will greatly affect the growth of the subsequent layers as well as the electrical properties of the films.Therefore，these features limit the large-scale film growth.New techniques，such as rotating both target and substrate and using a shadow mask to block the particulates in order to fabricate a large and uniform film，have been developed to improve the film quality.1.4 The Thermodynamics and Kinetics of Pulsed Laser DepositionPLD is a nonequilibrium growth technique owing to the high electronic excitation，degree of ionization，and kinetic energies of flux.There are many distinct stages to film growth：［19］the ablation process of the target material by the laser irradiation；the creation of a plasma plume with high energetic ions；and the crystalline growth of the film itself on the heated substrate.In this section，we will thoroughly describe these processes.1.4.1 Laser–Material InteractionsAfter the laser pulse is extinguished，a very hot cloud of vaporized 4material，typically of 10 K or more，has been generated，which is commonly referred to as the ablation plasma or plume.This process is called laser ablation.The mechanisms depend on the laser characteristics as well as on the optical，topological，and thermodynamic properties of the target material.Absorption in a material is defined as−αx
I=Ie0where 1/α is the absorption length，which is approximately 100 nm for many oxide materials at laser wavelengths commonly used in PLD（<400 nm）.In this process，electrons in the target are excited and thermalized within several picoseconds or nanoseconds depending on the energy density，duration，wavelength，and shape of the laser pulse as well as on the material properties（reflectivity，absorption coefficient，heat capacity，thermal conductivity，density，etc.）.The next step includes surface melting of the target and conduction of heat into the target.The thermal diffusion length is described as1∕2
λ=2（αΔt）ththwhereis the thermal diffusivity，K is the thermal conductivity，ρ is the mass density，c is the specific heat，and Δt is the pulse duration.During this process，the temperature rises in the 11surface of the target.The heating rates as high as 10 K/s and instantaneous gas pressures of 10–500 atm are observed at the target surface.Then，the target material will vaporize.During this step，there is multiphoton ionization of the gaseous phase creating the characteristic plasma and the temperature at the surface of the target will exceed the boiling point.The final step of the process is the plasma excitation during which further ionization occurs and free electrons are excited，resulting in Bremsstrahlung absorption in which the hot pulse，at nearly 2000 K，expands in a directed manner.
The ablation threshold of materials，or the minimum energy density required in a material to create a plume，will be discussed.In most oxides，the thermal diffusion length is much longer than the absorption length，especially for UV lasers，because of the fact that oxide materials are often opaque and good thermal conductors.An affected volume is related to the spot size timesλ；thus，a simple thestimation of the minimum energy needed to raise this volume to the sublimation point is
Q=C（T−T）+ΔH+C（T−T）+ΔHheatSmeltmmvapmeltvapwhere the total energy required（from left to right）is the sum of the energy needed to bring the target material to the melting temperature plus the heat of melting，plus the energy needed to bring the melted material to the vaporization point，plus the heat of vaporization.Although this seems like a large amount of energy，a typical instantaneous power density for a single laser pulse with 2energy density of approximately 2 J/cm and 20 ns pulse duration is 82about 10W/cm，which is more than enough to ablate nearly all materials.1.4.2 Dynamics of the PlasmaThe material expands in a plasma form parallel to the normal vector of the target surface toward the substrate owing to Coulomb repulsion and recoil from the target surface.The plasma contains many energetic species such as atoms，molecules，electrons，ions，clusters，particulates，and molten globules with the typical plasma temperature around 10 000 K，above the boiling points of most materials.As the plasma expands，it adiabatically cools to 3000–5000 K.The spatial distribution of the plume is strongly dependent on the background pressure inside the PLD chamber.In vacuum，the plume is very narrow and forward directed；at intermediate pressure，it looks like a shock wave due to the splitting of high energetic ions from the less energetic ions；at high pressure，it undergoes a more diffusionlike expansion.The angular distribution of the plume has been fitted to a ncos（x）function，with the value of n ranging from 2 to more than 20.In general，n is lower when the plasma is at high pressure because multiple collisions broaden the angular distribution［20］.The propagation of the plasma in the background gas follows a distance–time relation described by a blast wave model： where ξ is a constant，ρ is the background gas density，and E is 000the kinetic energy.This model is valid when the mass of the ejected material is smaller than the mass of the background gas.Therefore，the background gas pressure in PLD must be kept at a very low level so that some of the ejected species can avoid being scattered by the background gas and reach the substrate successfully.1.4.3 Nucleation and Growth of the Film on the Substrate SurfaceNucleation and growth of the film are of importance to determine thin film quality，morphology，and stoichiometry［21］.Growth of thin films resulting from a supersaturated gas condensation is a nonequilibrium phenomenon governed by a competition between kinetics and thermodynamics.Nucleation only occurs when the nucleus size r is equal to or greater than the critical nucleus r*，as shown in Figure 1.2.It is worth noting that ΔG*represents a critical energy barrier in the nucleation process.When an aggregate momentarily forms owing to thermodynamic fluctuation but the size of the cluster is smaller than r*，it is unstable and will disappear by shrinking in size，lowering ΔG in the process.Only clusters with size larger than r*can surmount the nucleation energy barrier，and they are stable and tend to grow larger in order to lower the energy of the system.Figure 1.2 Free energy as a function of cluster size r.r*is the critical nucleus size and ΔG*is the critical free energy barrier for nucleation.
Kinetic processes in thin film nucleation on an ideal substrate surface［21，22］are the impinging of the ablated high energetic species from the target onto the substrate surface and the sputtering off of some substrate atoms.These adatoms can subsequently diffuse over the substrate or along the island edge，encounter and bind with each other to form clusters，or attach to an existing island.On the other hand，they can also be re-evaporated from the substrate，from an island，or from a cluster.Meanwhile，it is possible that they are detached from the existing cluster edge but still remain on the substrate surface.In fact，the crystalline surface is not ideally flat but contains lots of defects and step terraces，so other kinetic processes such as interdiffusion and interfacial reactions become significant.
There are several factors that affect the film growth structure.First of all，the nucleation rate is an important parameter：high nucleation rates during deposition will encourage a fine-grained film or even amorphous structure.On the other hand，if nucleation is suppressed，single crystal growth is fostered.Substrate temperature is another factor to influence the film growth.Combining these two parameters，it is concluded that high substrate temperature and low deposition rates favor large crystallites or even monocrystal formation.Alternatively，low substrate temperatures and high deposition rates yield polycrystalline deposits.Basically，there are three thin film growth modes［22，23］：1.Island or Volmer–Weber mode：As shown in Figure 1.3a，island growth occurs when the smallest stable clusters nucleate on the substrate and grow in three dimensions to form islands.The growth of three-dimensional（3D）islands on the substrate is due to the fact that the bonds between the film atoms are stronger than the binding force of atoms to the substrate.Figure 1.3 Basic modes of thin film growth：（a）island or Volmer–Weber mode，（b）layer-bylayer or Frank–van der Merwe mode，and（c）layer plus island or Stranski–Krastanov mode.2.Layer-by-layer or Frank–van der Merwe mode：This kind of thin film growth mode（Figure 1.3b）occurs when the atoms are more strongly bound to the substrate than to each other.Film atoms are completely coalescent to form a monolayer on the surface before they develop into significant clusters on the next layer owing to the no barrier nucleation on this 2D island formation［24，25］.As long as the decrease in bonding energy is continuous toward the bulk crystal value，the layer growth mode is sustained.3.Layer plus island or Stranski–Krastanov mode（Figure 1.3c）：The layer plus island mode follows a two-step process：after forming one or more monolayers，namely，above a critical layer thickness，the layer-by-layer growth mode becomes 3D island forms.The transition mechanism from layer growth mode to island growth mode is unclear until now，but any factor that disturbs the monotonic decrease in binding energy may be the cause，such as the strain from the misfit between the thin film and the substrate，dislocation in the film，chemical potential of the deposited film，and so on.
It is known that the thin film growth mode also depends on the interfacial energies between the three phases present：the substrate，the condensing material，and the vapor［24］.