Controlling crystal quality and orientation of pulsed-laser-deposited BaTiO3 thin films by the kinetic energy of the film-forming particles

The deposition of BaTiO₃ thin films by pulsed excimer laser radiation (248 nm) on Pt/Ti/Si(111) and Pt/Ti/Si(100) substrates is investigated as a function of the processing variables laser fluence, processing gas pressure, and target-to-substrate distance under conditions of temporal and spatial properties of the involved vapour and plasma states. The kinetic energy of the species in the laser-generated plasma, as measured by time-of-flight optical emission spectroscopy and time-of-flight quadrupole mass spectrometry, is described as a function of the pressure of the processing gas, the distance from the target, and the laser fluence. The influence of the kinetic energy of the film-forming particles on the crystalline structure, defects, and orientation, and on the resulting electrical properties of the films is investigated. X-ray diffraction measurements and polarisation-dependent micro-Raman measurements reveal a c-axis orientation normal to the substrate surface, in the case of high particle energy (>50 eV), whereas at low kinetic energies (<30 eV) a [111]_pc or [110]_pc orientation is preferred. The ferroelectricity and the dielectric constant of the films, determined by impedance measurements, decrease with increasing kinetic energy of the film-forming particles from )_r=1000-2200 to )_r=200-700. This decrease correlates with the change of the orientation and with an increasing lattice constant of the films, indicating that particles with high kinetic energies produce crystal defects and stress in the growing film.     

A photoelectron spectroscopy study of ultra-thin epitaxial alumina layers grown on Cu(111) surface

Thin epitaxial alumina layers were grown on the Cu(111) surface using simultaneous aluminum deposition and oxygen exposure. Different substrate temperatures during the deposition resulted in layers with different thicknesses, growth rates, crystallinity and epitaxy. Low energy electron diffraction measurements confirmed the epitaxial growth for substrate temperatures above 870 K. The Al 2p doublet was studied by means of photoelectron spectroscopy in order to determine the alumina termination at the metal-oxide interface. A strong dependence on the preparation temperature was found and both aluminum and oxygen terminated interfaces were created.     

High-quality epitaxial LaNiO3 thin films on SrTiO3(100) and LaAlO3(100)

LaNiO₃ thin films have been deposited by pulsed laser deposition on SrTiO₃(100) and LaAlO₃(100) substrates. The processing conditions have been investigated in order to optimize electrical resistivity, crystal quality, and surface morphology. Excellent properties are achieved at moderate substrate temperature and relatively low oxygen pressure, without the need for annealing. Thickness exerts an important influence on electrical transport, as the electrical resistivity increases quickly in films thicker than a few tens of nanometer. The surface of the films on LaAlO₃ is very flat in all the studied thickness range, but the films on SrTiO₃ develop a pattern of boundaries and even cracks as the thickness is higher. Below the critical thickness, high-quality epitaxial films with very smooth surface and low electrical resistivity are obtained under the optimum conditions of substrate temperature and oxygen pressure. The optimum processing conditions are different depending on the substrate, and control is especially critical in films deposited on SrTiO₃.     

Nanocrystalline growth and diagnostics of TiC and TiB2 hard coatings by pulsed laser deposition

Nanocrystalline coatings of TiC and TiB₂ were grown by pulsed laser deposition on Si(100) and on X155 steel at low substrate temperatures ranging from 40 °C to 650 °C. A pulsed KrF excimer laser was used with the deposition chamber at a base pressure of 10^-6 mbar. The morphology and structure of the films, studied with SEM, XRD, and TEM, showed that nanocrystalline films with a fine morphology of TiC and TiB₂ were deposited with a grain size of 10 nm-70 nm at all substrate temperatures. The growth of the polycrystalline coatings possessed a columnar morphology with a 𘜄¢ preferred orientation. The hardness of the coatings was determined to be 40 GPa and the elastic modulus, 240 GPa. The composition and the kinetics of the plume produced during the pulsed laser deposition of TiC and TiB₂ was studied under film growth conditions. The mass analysis of ions of the ejected material was performed by time-of-flight mass spectroscopy (TOF-MS) and showed the presence of Ti⁺ and C⁺ during TiC ablation and B⁺, B₂⁺, and Ti⁺ during TiB₂ ablation. The kinetic energies (KE) of the ions depended on the laser fluence which was between 0.5 eV and 340 eV. The kinetic energy and the evolution of the plasma was studied with a streak camera. The velocity of the plasma was of the order of 10⁶ cm/sec and was linearly dependent on the energy fluence of the laser. The emission spectroscopy of the plasma plume confirmed the atomic neutral and single excited species of Ti. These results show that coating growth basically occurs by the recombination of the ionic species at the surface of the substrate.     

Low-temperature growth of epitaxial ZnO films on (001) sapphire by ultraviolet- assisted pulsed laser deposition

ZnO thin films have been grown on thin Si₃N₄ membranes and (001) sapphire substrates by an ultraviolet-assisted pulsed laser deposition (UVPLD) technique. The microstructure of the films grown on Si₃N₄ membranes, investigated by transmission electron microscopy, showed that crystalline and textured films can be grown by UVPLD at a substrate temperature of only 100 °C. For deposition temperatures higher than 400 °C, ZnO films grown on sapphire substrates were found to be epitaxial by Rutherford backscattering (RBS) and X-ray diffraction measurements. The minimum yield of channeling RBS spectra recorded from films deposited at 550 °C was around 2% and the FWHM of the rocking curve for the (002) diffraction peak was 0.17°; these values are similar to those recorded from ZnO layers grown by conventional PLD at 750 °C.     

An atomically controlled Si film formation process at low temperatures using atmospheric-pressure VHF plasma

To grow epitaxial Si films with atomic- and electronic-level perfection, a high-temperature chemical vapor deposition (CVD) process (>1000 °C) has been generally employed. To reduce the growth temperature below 600?°C but keeping a high deposition rate, other energy sources than thermal heating are required. Atmospheric pressure plasma CVD (AP-PCVD) is considered to be suitable for fabricating high-quality films at high deposition rates due both to the high radical density and to the low ion bombardment against the film surface, because the collision frequency among ions and neutral atoms is high. The present study focuses on the low-temperature growth of epitaxial Si, and experimentally demonstrates that AP-PCVD is capable of growing epitaxial Si films with high perfection applicable for semiconductor devices. It is found that the pre-growth cleaning of the Si surface by H(2) AP plasma is effective to grow high-purity Si films, and that the exposure of a film-growing surface to AP plasma during growth is important to form particle-free and defect-free Si films. From the experimental results and the first-principles molecular dynamics simulations of surface atomic reactions, it can be mentioned that both H atoms in the AP plasma and high-density He atoms having thermal kinetic energy contribute to the reduction of growth temperature by supplying considerable energy to the surface.     

Epitaxial Properties of Co-Doped ZnO Thin Films Grown by Plasma Assisted Molecular Beam Epitaxy

High quality Co-doped ZnO thin films are grown on single crystalline Al2O3（0001） and ZnO（0001） substrates by oxygen plasma assisted molecular beam epitaxy at a relatively lower substrate temperature of 450℃. The epitaxial conditions are examined with in-situ reflection high energy electron diffraction （RHEED） and ex-situ high resolution x-ray diffraction （HRXRD）. The epitaxial thin films are single crystal at film thickness smaller than 500nm and nominal concentration of Co dopant up to 20%. It is indicated that the Co cation is incorporated into the ZnO matrix as Co^2＋ substituting Zn^2＋ ions. Atomic force microscopy shows smooth surfaces with rms roughness of 1.9 nm. Room-temperature magnetization measurements reveal that the Co-doped ZnO thin films are ferromagnetic with Curie temperatures Tc above room temperature.     

XPS and LEED study of Pd and Au growth on alumina/Cu-Al surface

Metal-insulator-metal system was prepared using the single-crystalline Cu-9at.% Al(1 1 1) support. Oxidation of the substrate under well-controlled conditions at elevated temperature leads to the formation of well-ordered aluminium oxide layer. The Pd-Au topmost layer was prepared by a step-by-step deposition of both metals afterwards on the oxide layer at room temperature. Low energy electron diffraction (LEED) measurement did not confirm epitaxial growth of the metal overlayer and gave only a rise of diffuse background after each deposition step. The growth of Pd-Au overlayer exhibited Stranski-Krastanov mode influenced by intermetallic interaction between those metals. No binding energy shifts were visible for the core-level photoelectron peaks of the substrate and the oxide using X-ray photoelectron spectroscopy (XPS). In contrast, the binding energy shifts of Pd 3d and Au 4f photoelectron levels in both directions were observed during all depositions. Bimetallic interactions between the metals as well as size effects are further discussed.     

Epitaxial growth of Au and Pt on Fe3O4(1 1 1) surface

We have studied the epitaxial growth of Au and Pt layers on Fe₃O₄(1 1 1) as a function of the deposition temperature and thickness. The layers were deposited by UHV sputtering and the structural properties were investigated by reflection high energy electron diffraction (RHEED), X-ray experiments and transmission electron microscopy (TEM). The epitaxial growth of both metals was obtained whatever the deposition conditions but the wetting is however different for the two metals. Comparison between the coverage ratios of Au and Pt is correlated with their surface and interfaces energies. The optimum conditions to achieve a 2D flat epitaxial metallic layer are determined.     

高脉冲功率能量PLD法制备MgZnO薄膜中的沉积机理

用PLD法成功制备了一系列高质量的MgZnO薄膜。实验中发现高脉冲能量沉积薄膜的结构和发光特性随基片温度的变化规律与低脉冲能量下的结果不一样:基片在室温时高脉冲能量制备薄膜的XRD峰的半峰全宽比高基片温度时的结果相对更小;AFM显示其颗粒变大,柱状生长突出;PL谱紫峰与绿峰强度比最大,结晶质量反而提高。另一方面,与低脉冲能量时相反,增大氧气压强后高脉冲能量沉积的薄膜XRD半峰全宽变窄。结合实验现象和表征,合理解释了高脉冲能量沉积的机理。室温制备高质量MgZnO薄膜的PLD沉积机理对于以后在柔性衬底上沉积薄膜的研究有重要的参考价值。     

Influence of the laser wavelength on the epitaxial growth and electrical properties of La0.8Sr0.2MnO3 films grown by excimer laser-assisted MOD

Epitaxial La_1−xSr_xMnO₃ (LSMO) films were prepared by excimer laser-assisted metal organic deposition (ELAMOD) at a low temperature using ArF, KrF, and XeCl excimer lasers. Cross-section transmission electron microscopy (XTEM) observations confirmed the epitaxial growth and homogeneity of the LSMO film on a SrTiO₃ (STO) substrate, which was prepared using ArF, KrF, and XeCl excimer lasers. It was found that uniform epitaxial films could be grown at 500 °C by laser irradiation. When an XeCl laser was used, an epitaxial film was formed on the STO substrate at a fluence range from 80 to 140 mJ/cm² of the laser fluence for the epitaxial growth of LSMO film on STO substrate was changed. When the LaAlO₃ (LAO) substrate was used, an epitaxial film was only obtained by ArF laser irradiation, and no epitaxial film was obtained using the KrF and XeCl lasers. When the back of the amorphous LSMO film on an LAO substrate was irradiated using a KrF laser, no epitaxial film formed. Based on the effect of the wavelength and substrate material on the epitaxial growth, formation of the epitaxial film would be found to be photo thermal reaction and photochemical reaction. The maximum temperature coefficient of resistance (TCR) of the epitaxial La_0.8Sr_0.2MnO₃ film on an STO substrate grown using an XeCl laser is 4.0%/K at 275 K. XeCl lasers that deliver stabilized pulse energies can be used to prepare LSMO films with good a TCR.     

Silicon nanoclusters formed through self-assembly on CaF2 substrates: morphology and optical properties

We studied the influence of the size and shape of silicon nanoclusters on their optical response. For this purpose, clusters were prepared by deposition, and subsequent diffusion and nucleation, of Si atoms on CaF₂ substrate surfaces. By varying the growth parameters, oblate aggregates with sizes of between 5 and 30 nm and axial ratios between almost unity and 0.1 were generated. We found that the substrate temperature during growth predominantly influences the diameter and number density of the particles, whereas the coverage and deposition rate determine the axial ratio. Optical extinction was measured in the photon energy range between 1.0-6.0 eV and compared to model calculations. The mean size of the Si clusters mainly determines the absolute magnitude of the optical extinction. In contrast, the axial ratio drastically affects the overall structure of the spectra and the relative importance of three identified maxima. The most essential and interesting reason for the pronounced influence of the particle shape on the optical properties is a shift of the valence band plasmon in the silicon nanoparticles from an energy of above 9.0 eV for spherical clusters into the ultraviolet spectral range to about 5 eV for axial ratios below 0.3.     

X-ray pole-figure study of the epitaxial growth of C60 thin films on mica (001)

The growth of epitaxial C₆₀ thin films on mica(001) by thermal evaporation has been studied in detail by X-ray pole-figure measurements. The influence of the deposition rate, the substrate temperature and the film thickness on the in-plane epitaxial arrangements and the formation of twins has been investigated. It has been demonstrated that the C₆₀ growth is determined by two independent and equivalent C₆₀-crystal grain alignments (type-A and type-B). The nearly six-fold symmetry of the mica(001)-substrate surface offers the three-fold fcc-(111)-oriented C₆₀-crystal grains two equivalent crystal alignments. A high deposition rate of 0.5 Å/s is responsible for the formation of twins at a substrate temperature of 150°C, which diminishes by a higher substrate temperature of 200°C. By a decrease of the deposition rate down to 0.08 Å/s the twins vanish at a film thickness of 200 nm and at the substrate temperature of 150°C. Under the same sublimation conditions, in addition to the type-A and type-B crystal orientations, the growth of the thin C₆₀ films starts with a slight fibre texture which does not appear at a larger film thickness.     

Effect of the substrate temperature and deposition rate on the initial growth of thin lithium niobate-tantalate films deposited from a thermal plasma

Thin lithium niobate-tantalate (LiNb_0.5Ta_0.5O₃) films are studied at the initial stage of deposition from a thermal plasma. The effect of two deposition parameters (the substrate temperature and the deposition rate) on the film morphology, the film crystallinity, and the density of nuclei growing on a (0001) sapphire substrate are investigated. It is shown that the crystalline structure and roughness of a film are determined, for the most part, in the initial growth stage and therefore depend directly on both parameters. At the optimum temperatures and growth rates for obtaining good characteristics of (0006) texture, crystallinity, and surface roughness of the films, the film nuclei on the substrate have a high density and good epitaxial orientation to it. If the growth conditions are not optimum, the islands are either amorphous or have a low density on the substrate surface. The nucleation activation energy is observed to decrease as the deposition rate increases, which supports the assumption that the species that are active in film deposition are “hot” clusters forming in an oxygen-argon plasma in the immediate vicinity of the substrate.     

An investigation of the internal temperature dependence of Pd-Pt cluster beam deposition: A molecular dynamics study

We investigated the internal temperature dependence of the Pd_1−aPt_a cluster beam deposition in the present study via the molecular dynamics simulations of soft-landing. By analysis of the velocity distribution and diffusion coefficient of the bimetallic cluster, Pd atoms with better mobility improved the diffusibility of Pt atoms. The radial composition distribution showed that a Pt-core/Pd-shell structure of the cluster formed at high internal temperatures through migrations of the Pd atoms from inner to surface shells. In the soft-landing process, the diffusing and epitaxial behaviors of the deposited clusters mainly depended on the internal temperature because the incident energy of the cluster was very small. By depositing clusters at high internal temperatures, we obtained a thin film of good epitaxial growth as the energetic cluster impact. Furthermore, nonepitaxial configurations such as scattered nonepitaxial atoms, misoriented particles, and grain boundaries of (1 1 1) planes were produced in the growth of the cluster-assembled film. As the size of the incident cluster increased, the internal temperature of the cluster needed for better interfacial diffusion and contact epitaxy on the substrate also rose.     

Determining the properties of pulsed laser deposited thin films by controlling the kinetic energy of the film-forming particles

The deposition of Al₂O₃ thin films by pulsed KrF excimer laser radiation (248 nm) on fused silica substrates is investigated as a function of the processing variables: laser fluence, processing gas pressure and target-to-substrate distance. The kinetic energy of the Al species in the laser-generated plasma, as measured by time-of-flight optical emission spectroscopy and time-of-flight quadrupole mass spectrometry, is described as a function of the type and pressure of the processing gas, the distance from target, and the laser fluence. The influence of the kinetic energy of the film-forming particles on the density and the refractive index of the resulting films, determined by ellipsometry, is investigated. The densification of the Al₂O₃ thin films to 94% of the bulk value is achieved by film-forming Al particles impinging on the growing surface with mean kinetic energies of about 25 eV.     

Simulation of island aggregation influenced by substrate temperature, incidence kinetic energy and intensity in pulsed laser deposition

Using kinetic Monte Carlo method, we have simulated a pulsed energetic growth process in pulsed laser deposition. During the growth of film, substrate temperature mainly influences upon film morphology by directly enhancing the adatom mobility through the temperature-dependent thermal vibration. By contrast, the effect of incidence kinetic energy on film growth is complex resulting from the collisions between the incident particles and the adatoms. The results show that improving incident kinetic energy cannot significantly accelerate the migration rate of adatom but change surface microstructure and promote single adatom formation resulting in more island aggregation density. Moreover, since pulse-influx characterizes pulsed laser deposition, the intensity per pulse contributes to the evolvement of nucleation density and the results illustrate that a general scaling law different from ordinary power law still exists in energetic growth of pulsed laser deposition.     

4H-SiC同质外延生长Grove模型研究

本文通过对4H-SiC同质外延化学反应和生长条件的分析,建立了4H-SiC同质外延生长的Grove模型,并结合实验结果进行了分析和验证.通过理论分析和实验验证,得到了外延中氢气载气流量和生长温度对4H-SiC同质外延生长速率的影响.研究表明:外延生长速率在衬底直径上为碗型分布,中心的生长速率略低于边缘的生长速率;随着载气流量的增大,生长速率由输运控制转变为反应速率控制,生长速率先增大而后逐渐降低;载气流量的增加,会使高温区会发生漂移,生长速率的理论值和实验出现一定的偏移;随着外延生长温度的升高,化学反应速率和气相转移系数都会增大,提高了外延速率;温度对外延反应速率的影响远大于对生长质量输运的影响,当温度过分升高后,外延生长会进入质量控制区;但过高的生长温度导致源气体在生长区边缘发生反应,生成固体粒子,使实际参与外延生长的粒子数减少,降低了生长速率,且固体粒子会有一定的概率落在外延层上,严重影响外延层的质量.通过调节氢气流量,衬底旋转速度和生长温度,可以有效的控制外延的生长速度和厚度的均匀性.     

用甲醇-氢混合气源在不锈钢上生长金刚石薄膜的研究

 利用甲醇-氢（CH₃OH-H₂）混合气体为气源，30 nm厚的无定形硅为过渡层，借助于微波等离子体化学气相沉积（MWCVD）成功地将金刚石薄膜生长在不锈钢上，其最低生长温度可至420 ℃，并且甲醇-氢混合气体比传统的甲烷-氢（Ch₄-H₂）更具优势，测试表明这种金刚石薄膜有希望作为耐磨层在工业上应用。     

Adsorption-desorption studies of Zn on GaAs

By employing a wide range of techniques to study adsorption-desorption behaviour in the ostensibly simple system of the metal Zn on the semiconductor GaAs it has been found that many complicating factors can occur, and reliance on any one of the techniques would have given a totally misleading picture. The methods used were temperature programmed thermal desorption, modulated atomic beam adsorption measurements, AES, high resolution UHV SEM combined with AES and high resolution (300 Å) AES, and RHEED. GaAs substrate surfaces were cleaned in-situ by thermal treatment or inert gas ion bombardment followed by annealing. It was established by SEM and RHEED observations that surface topographic and compositional changes could occur at this stage. Zn sticking coefficient measurements by modulated beam and AES techniques showed that it could vary widely for minor changes in surface composition, and that it was also a strong function of deposition time and substrate temperature. However, initial growth of the Zn deposit was always two dimensional (within the resolution limits of the SEM) and epitaxial, for the range of substrate conditions used. Thermal desorption spectra were also found to depend rather critically on the substrate surface, with very pronounced difference between the Ga and As stabilized forms. An attempt has been made at a systematic interpretation of the kinetic data based on reasonably simple models, and also to relate it to previously published work on this system, but the profound influence of substrate surface effects makes a fully quantitative evaluation extremely difficult. Nevertheless, the value, and perhaps the necessity, of employing a wide range of techniques to investigate metal-semiconductor systems is clearly demonstrated.