Preparation and characterizations of amorphous nanostructured SiC thin films by low energy pulsed laser deposition

Amorphous silicon carbide (SiC) thin films were deposited on silicon substrates by pulsed laser ablation at room temperature. Thicknesses and surface morphology of the thin films were characterized using optical profilers, atomic force and field emission scanning electron microscopy. Nanohardnes, modulus and scratch resistance properties were determined using XP nanoindenter. The results show that crack free, smooth and nanostructured thin films can be deposited using low laser energy densities.     

采用Si薄膜的光子计数成像系统性能的研究

在陶瓷基底上利用电子束蒸镀方法制备了Si薄膜,用作感应读出方式光子计数成像系统的电荷感应层,并研究了薄膜的结构特征和表面形态.X射线衍射(XRD)测试和场发射扫描电子显微镜(FESEM)图像表明,沉积的Si薄膜为无定形态,由于陶瓷晶界的存在,薄膜较粗糙.搭建了相应的实验系统,对比了采用不同厚度Si薄膜时系统的空间分辨率、计数率、脉冲高度分布曲线等,发现薄膜的厚度对探测器的计数率影响较大.此外,实验还对比了采用相同电阻值的Si薄膜和常用的Ge薄膜时系统的性能.研究表明,采用Si薄膜时系统的畸变较小、计数率高     

Synthesis of composite calcium-phosphate coatings via pulsed photon processing

The phase composition and mechanical properties of coatings generated on a Ti surface via the ion sputtering of a hydroxyapatite (HA) target and a compound (hydroxyapatite and Ti) target with subsequent pulsed photon processing (PPP) with incoherent xenon lamp radiation are investigated. It is found for the first time that pulsed photon processing accelerates the crystallization of amorphous films of Ca–P–O–H and Ca–P–O–H–Ti compositions, during which tricalcium phosphate Ca₃(PO₄)₂, titanium oxide TiO₂ (rutile, anatas), and perovskite CaTiO₃ are formed, depending on the radiation dose and the ratio between Ti and Ca phases (Ti/Ca) with hydroxyapatite structure. It is found that pulsed photon processing of initial amorphous coatings greatly increases their hardness (up to 10.9 GPa) and adhesion (up to 29.0 MPa).     

Growth process of nanostructured silver films pulsed laser ablated in high-pressure inert gas

The growth process of silver thin films deposited by pulsed laser ablation in a controlled inert gas atmosphere was investigated. A pure silver target was ablated in Ar atmosphere, at pressures ranging between 10 and 100 Pa, higher than usually adopted for thin film deposition, at different numbers of laser shots. All of the other experimental conditions such as the laser (KrF, wavelength 248 nm), the fluence of 2.0 J cm⁻², the target to substrate distance of 35 mm, and the temperature (295 K) of the substrates were kept fixed. The morphological properties of the films were investigated by transmission and scanning electron microscopies (TEM, SEM). Film formation results from coalescence on the substrate of near-spherical silver clusters landing as isolated particles with size in the few nanometers range. From a visual inspection of TEM pictures of the films deposited under different conditions, well-separated stages of film growth are identified.     

Femtosecond pulsed laser deposition of nanostructured TiO2 films

Nanostructured deposits of TiO₂ were grown on Si (1 0 0) substrates by laser ablating a TiO₂ sintered target in vacuum or in oxygen using a Ti:sapphire laser delivering 80 fs pulses. The effect of the laser irradiation wavelength on the obtained nanostructures, was investigated using 800, 400 and 266 nm at different substrate temperatures and pressures of oxygen. The composition of the deposits was characterized using X-ray photoelectron spectroscopy (XPS) and the surface morphology was studied by environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM). Deposits are absent of microscopic droplets in all conditions explored. The best deposits, constituted by nanoparticles of an average diameter of 30 nm with a narrow size distribution, were obtained at the shorter laser wavelength of 266 nm under vacuum at substrate room temperature.     

Atomic force microscopy of nanostructured gold films after their irradiation with a powerful pulsed microwave radiation

The possibility of using microwave radiation in combination with metallic nanoobjects for cancer therapy has been studied. The results of optical and atomic force investigations of nanostructured metallic films used for testing the methods of such a therapy have been presented. The size of metal clusters separated from the film by microwave pulses has been estimated. Possible mechanisms for the destruction of cancer cells by nanostructures affected by microwave radiation have been considered.     

Radio-frequency assisted pulsed laser deposition of nanostructured WOx films

The synthesis of tungsten oxide films with large surface area is promising for gas sensing applications. Thin WO_x films were obtained by radio-frequency assisted pulsed laser deposition (RF-PLD). A tungsten target was ablated at 700 and 900 Pa in reactive oxygen, or in a 50% mixed oxygen-helium atmosphere at the same total pressure values. Corning glass was used as substrate, at temperatures including 673, 773 and 873 K. Other deposition parameters such as laser fluence (4.5 J cm⁻²), laser wavelength (355 nm), radiofrequency power (150 W), target to substrate distance (4 cm), laser spot area (0.7 mm²), and number of laser shots (12,000) were kept fixed. The sensitivity on the deposition conditions of morphology, nanostructure, bond coordination, and roughness of the obtained films were analyzed by scanning and transmission electron microscopy, micro-Raman spectroscopy, and atomic force microscopy.     

Preparation and characterization of pulsed laser deposition (PLD) SiC films

Si K-edge XAFS was used to characterize a stoichiometric SiC film prepared by pulsed KrF laser deposition. The film was deposited on a p-type Si(1 0 0) wafer at a substrate temperature of 250 °C in high vacuum with a laser fluence of ∼5 J/cm². The results reveal that the film contains mainly a SiC phase with an amorphous structure in which the Si atoms are bonded to C atoms in its first shell similar to that of crystalline SiC powder but with significant disorder.     

SERS activity of silver and gold nanostructured thin films deposited by pulsed laser ablation

The deposition power dependence of visible transmittance and refractive index of room temperature-deposited ZnO:Al thin films by RF magnetron sputtering has been studied. All films exhibited high visible transmittance and near-complete UV absorption. The refractive index of the films decreased continuously with an increase in the RF power at all photon energies in the visible and near-IR region, which has been partially attributed to the decreased packing density of the films. For each film, the refractive index exhibited strong frequency dispersion in the weak-absorption region. The origin of optical dispersion at different RF power has been discussed in the light of a single-oscillator model.     

Pyroelectric and piezoelectric responses of thin AlN films epitaxy-grown on a SiC/Si substrate

This paper presents the results of pyroelectric and piezoelectric studies of AlN films formed by chloride–hydride epitaxy (CHE) and molecular beam epitaxy (MBE) on epitaxial SiC nanolayers grown on Si by the atom substitution method. The surface topography and piezoelectric and pyroelecrtric responses of AlN films have been analyzed. The results of the study have shown that the vertical component of the piezoresponse in CHE-grown AlN films is more homogeneous over the film area than that in MBE-grown AlN films. However, the signal from the MBE-synthesized AlN films proved to be stronger. The inversion of the polar axis (polarization vector) on passage from MBE-grown AlN films to CHE-grown AlN films has been found experimentally. It has been shown that the polar axis in MBE-grown films is directed from the free surface of the film toward the Si substrate while, in CHE-grown films, the polarization vector is directed toward the free surface.     

Synthesis and microstructure observation of titanium carbonitride nanostructured coatings using reactive plasma spraying in atmosphere

In the present study, nanostructured titanium carbonitride (TiCN) coatings were successfully deposited by reactive plasma spraying (RPS) technology using a self-designed gas tunnel mounted on a normal plasma spray torch. The phase composition and microstructure of the TiCN coatings were characterised by XRD, SEM and TEM. The results indicated that the main phase of the coatings was FCC TiC_0.2N_0.8 with a small amount of Ti₃O. The coating that was deposited using 35 kW displayed better microstructure and properties. The coating exhibited a typical nanostructure including 90 nm diamertrical equiaxed grains and 400 nm long columnar grains by TEM images. The SEM observation further revealed that the equiaxed grains in parallel direction to the substrate surface in TEM images were actually the columnar grains perpendicular to the substrate surface. The formation mechanism of the nanostructured coatings was also discussed. The measured microhardness value of the coating was approximately 1659 Hv_100 g, and the calculated crack extension force was about 34.9 J/m².     

Synthesis of nano-sized SiC precipitates in Si by simultaneous dual-beam implantation of C+ and Si+ ions

Nanometer-sized SiC precipitates were synthesized in situ in Si by simultaneous implantation of two ion beams of C⁺ and Si⁺ ions. The results of simultaneous dual-beam implantation are compared with those of sequential dual-beam ion implantation and of single-beam C⁺ ion implantation. Remarkable differences are observed regarding the content and the crystal quality of SiC precipitates as well as the defect structure of the Si substrate. The SiC precipitation during dual-beam synthesis is found to depend on the ion energy of the second beam and on the implantation mode, simultaneous or sequential. For suitable implantation conditions, simultaneous dual-beam synthesis can improve the in situ SiC formation in comparison to the single-beam synthesis. A higher density of SiC precipitates with better crystal quality was observed, whereas their size was not changed. The second ion beam enables a shift in the dynamic equilibrium of constructive and destructive processes for SiC formation. A model is proposed assuming that SiC precipitation preferentially proceeds in regions with vacancy defects. The implantation process itself creates vacancy-dominated and also interstitial-dominated regions. The balance of the local point-defect composition is shifted under the second ion beam. In this way, the conditions for SiC precipitation can be modified. Received: 18 February 2002 / Accepted: 17 May 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +49-351/260-3411, E-mail: koegler@fz-rossendorf.de     

Polarization transverse distribution of multi-Gaussian Schell photon beams in a turbulent atmosphere

The analytical expression for the polarization transverse distribution of multi-Gaussian Schell photon beams propagating through the modified von Karman turbulence channel are studied, in consideration with the effects of outer and inner scale of turbulence. It is found that the zenith angle has a very slight effect on the degree of polarization of photon beams. In the area near to the beam center, the degree of polarization increases as the source's transverse size and source's transverse coherent width increases, or sub-beam number M decrease. However, in the area away from the beam center, increasing value of sub-beam number M or decreasing the source's transverse coherent width and the source's transverse size, larger the degree of polarization of beams is achieved.     

Self-organization during deposition of semiconductor films in the atmosphere of atomic hydrogen

A scanning tunneling microscope is used to establish that nanostructures with characteristic dimension 30–40 nm are contained in germanium films deposited on copper substrates in an atmosphere of atomic hydrogen. Local conductance, photoconductivity, and nonequilibrium chemiconduction of the films are studied. Etching of germanium and silicon films and also of fullerene soot by atomic hydrogen is observed.     

Growth of stoichiometric and textured LiNbO3 films on Si by pulsed laser deposition

Laser ablation of single-crystal LiNbO₃ in a gas environment is used to grow films on (100) Si substrates heated to 650 °C. The film composition and crystallinity are studied as a function of the nature (reactive, O₂, or inert, Ar) and pressure of the gas environment applied during deposition and cooling-down processes, the laser energy density and the target–substrate distance. Experimental results show that a gas pressure close to 1 mbar is required to produce stoichiometric films in either O₂ or Ar. The modification of the laser energy density and the target–substrate distance allows us to improve the crystallinity of the films that become textured along the (006) direction. The influence of the experimental parameters on the film properties is discussed in the frame of the formation of a blast wave, that leads to the focusing of the expanding Li species and thus, to the increase of the Li content in the films. Received: 8 February 2001 / Accepted: 9 February 2001 / Published online: 3 May 2001     

脉冲激光制备薄膜材料的烧蚀机理

研究了脉冲激光烧蚀靶材的整个过程.从包含热源项的导热方程出发，利用适当的动态边界条件，详细研究了靶材在熔融前后的温度分布规律，并且给出了熔融后的固、液分界面的变化规律.熔融后的温度演化规律和固液相界面均以解析表达式的形式给出.还根据能量平衡原理给出烧蚀面位置随时间的变化规律.以硅靶材为例计算模拟了激光烧蚀的整个过程，与实验结果符合较好. 关键词： 脉冲激光 烧蚀面 熔融 温度演化     

Time resolved calorimetry of te films during pulsed laser annealing

Using a pyroelectric thin-film calorimeter the temperature of 30 nm thick Te films during pulsed laser annealing was studied in real-time. A XeCl excimer laser pumped dye laser with Raman shifter was utilized to study the wavelength and energy dependence. No significant wavelength dependence was noticed. Depending on the pulse energy, however, melting, boiling and crystallization of the Te films was observed. These findings support a strictly thermal model for laser annealing and optical recording with Te based media. In addition boiling was identified as the prevalent mechanism for the loss of material.     

Synthesis of Si nanoparticles with narrow size distribution by pulsed laser ablation

We synthesized Si nanoparticles by pulsed nanosecond-laser ablation. We applied a positive voltage bias during laser irradiation and effectively reduced size distribution. Scanning electron micrographs of samples showed the nanoparticles to be highly non-agglomerated. Si nanoparticles have the average diameter of 4–5 nm, the geometrical standard deviation of 1.35, and the density of 1.6 × 10¹²/cm². A MOS device showed excellent charge trap behavior with a flat-band voltage shift over 7 V, which can be applied for memory device applications.     

Synthesis and characterization of permalloy nanostructured films by deposition of laser ablated nanoparticles

A permalloy (Ni₈₁Fe₁₉; at%) microparticle (MP) aerosol was ablated to produce a nanoparticle (NP) aerosol that was then impacted at high velocity onto a substrate to produce porous thick films. The structure of the NPs was analyzed by transmission electron microscopy and the morphologies of the NPs and the nanostructured films were studied using high-resolution transmission electron microscopy and scanning electron microscopy. These analyses showed that the original composition and structure of the MPs were preserved in the NPs and films. The majority of NPs that were produced ranged in size between 2 and 15 nm with some larger particles present. Magnetization-temperature curves showed that the films consisted of a mixture of small superparamagnetic NPs and larger ferromagnetic NPs. A high saturation magnetization of 62.3 emu/g at 300 K was retained in the films indicating that they remained free of significant oxidation.