Simulation of KrF laser ablation of Al2O3-TiC

Previous work by the authors on micromachining of Al₂O₃-TiC ceramics using excimer laser radiation revealed that a columnar surface topography forms under certain experimental conditions. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations show that the columns develop from small globules of TiC, which appear at the surface of the material during the first laser pulses. To understand the mechanism of formation of these globules, a 2D finite element ablation model was developed and used to simulate the time evolution of the temperature field and of the surface topography when a sample of Al₂O₃-TiC composite is treated with KrF laser radiation. Application of the model showed that the surface temperature of TiC rises much faster than that of Al₂O₃, but since TiC has a very high boiling temperature, its vaporization is significant only for a short time. By contrast, the surface temperature of Al₂O₃ rises above its boiling temperature for a much longer period, leading to a greater ablation depth than TiC. As a result, a small TiC globule stands above the Al₂O₃ surface. The results of the model are compared with experimental measurements performed by AFM. After three pulses, the height of the globules predicted by the model is about 340 nm, in good agreement with the height measured experimentally, about 400 nm.     

Laser-induced etching of polycrystalline Al2O3TiC in KOH aqueous solution

Laser-induced etching of polycrystalline Al₂–O₃TiC material by a tightly-focused cw Ar ion laser has been investigated in a KOH solution with different concentrations. It is found that the KOH concentration can strongly affect the etching quality where low KOH concentration can result in rough and irregular patterns. Laser-induced etching of polycrystalline Al₂O₃TiC in a KOH solution is found to be a photothermal reaction in which a threshold laser power exists. With an appropriate set of etching parameters, well-defined grooves can be obtained with clean side walls and with an etching rate up to several hundred micrometers per second. The etching behavior is also found to depend on laser scanning direction. It is also found that the grains in the polycrystalline Al₂O₃TiC material play an important role in the etching dynamics and etching quality. This etching process is believed to be applicable to the formation of a slider surface of magnetic heads in the future.     

Deep UV laser induced luminescence in oxide thin films

Time-resolved luminescence experiments have been set up in order to study the interaction of 193-nm laser radiation with dielectric thin films. At room temperature, Al₂O₃ coatings show photoluminescence upon ArF excimer laser irradiation, with significant intensity contributions besides the known substrate emission. Time- and energy-resolved measurements indicate the presence of oxygen-defect centers in Al₂O₃ coatings, which suggests a strong single-photon interaction at 193 nm by F⁺ and F center absorption. Measurements on highly reflective thin-film stacks, consisting of quarter-wave Al₂O₃ and SiO₂ layers, indicate similar UV excitations, mainly from color centers of Al₂O₃. Received: 20 February 2002 / Accepted: 11 April 2002 / Published online: 5 July 2002     

Preparation and characterization of ferroelectric SrBi2Ta2O9 thin films on Si using Al2O3 buffer layers

SrBi₂Ta₂O₉ (SBT) thin films were prepared on p-type Si(100) substrates with Al₂O₃ buffer layers. Both the SBT films and the Al₂O₃ buffer layers were deposited by a pulsed laser deposition technique using a KrF excimer laser. An Al prelayer was used to prevent Si surface oxidization in the initial growth stage. It is shown that Al₂O₃ buffer layers effectively prevented interdiffusion between SBT and Si substrates. Furthermore, the capacitance–voltage (C-V) characteristics of the SBT/Al₂O₃/Si heterostructures show a hysteresis loop with a clockwise trace, demonstrating the ferroelectric switching properties of SBT films and showing a memory window of 1.6 V at 1 MHz. Received: 17 July 2000 / Accepted: 16 August 2000 / Published online: 30 November 2000     

Laser–MBE growth of high-quality ZnO thin films on Al2O3(0001) and SiO2/Si(100) using the third harmonics of a Nd:YAG laser

High-quality ZnO thin films were grown on single-crystalline Al₂O₃(0001) and amorphous SiO₂/Si(100) substrates at 400–640 °C using laser molecular beam epitaxy. For film growth, the third harmonics of a pulsed Nd:YAG laser were illuminated on a ZnO target. The ZnO films were epitaxially grown on Al₂O₃(0001) with the narrow X-ray diffraction full width at half maximum (FWHM) of 0.04° and the films on SiO₂/Si(100) exhibited a preferred c-axis orientation. Furthermore, the films exhibited excellent optical properties in photoluminescence (PL) measurements with very sharp excitonic and weak deep-level emission peaks. At 15 K, PL FWHM values of the films grown on Al₂O₃(0001) and SiO₂/Si(100) were 3 and 18 meV, respectively. Received: 8 May 2001 / Accepted: 18 September 2001 / Published online: 20 December 2001     

Silver macro‐texture substrates fabricated by plasma selective etching for surface‐enhanced Raman scattering

We have demonstrated a novel method to generate the nanostructured substrate that shows a large enhancement with a spatially uniform enhancement factor of approximately 10⁶ in surface enhanced Raman scattering (SERS). The substrates are fabricated using plasma selective etching. First, the Al₂O₃–TiC template which contains mixed Al₂O₃ and TiC grains with the diameters of ~400 nm is selected as a base plate. The Al₂O₃ and TiC grains have different physical properties, such as hardness, which corresponds to different etching rate in a plasma gas. Then, the Al₂O₃–TiC substrate is selectively etched to generate a random macro‐texture (MT) with different depths using the plasma of mixed gas of Ar and C₂H₄. Third, the MT substrate is deposited with a silver film (Ag). We further demonstrate that by varying the thickness of Ag layer, the EF is different which is confirmed by the plasmonic localized electric fields calculations using finite difference time domain. Finally, we combine this novel Ag MT substrate with ultrathin dielectric film, and the prepared substrates are coated with a 10 Å ta‐C film. The 10 Å ta‐C film can protect the oxygen‐free Ag in air and prevent Ag ionizing in aqueous solutions. More importantly, the ultrathin ta‐C can release the strongest plasmonic electric field to the outside of ta‐C layer and get a higher electric field than the uncoated Ag substrate. We expect that this method has more potential applications in analytic assays using SERS technology. Copyright © 2012 John Wiley & Sons, Ltd.     

Effective surface passivation of crystalline silicon using ultrathin Al2O3 films and Al2O3/SiNx stacks

We measure surface recombination velocities (SRVs) below 10 cm/s on p‐type crystalline silicon wafers passivated by atomic–layer–deposited (ALD) aluminium oxide (Al₂O₃) films of thickness ≥10 nm. For films thinner than 10 nm the SRV increases with decreasing Al₂O₃ thickness. For ultrathin Al₂O₃ layers of 3.6 nm we still attain a SRV < 22 cm/s on 1.5 Ω cm p‐Si and an exceptionally low SRV of 1.8 cm/s on high‐resistivity (200 Ω cm) p‐Si. Ultrathin Al₂O₃ films are particularly relevant for the implementation into solar cells, as the deposition rate of the ALD process is extremely low compared to the frequently used plasma‐enhanced chemical vapour deposition of silicon nitride (SiN_x). Our experiments on silicon wafers passivated with stacks composed of ultrathin Al₂O₃ and SiN_x show that a substantially improved thermal stability during high‐temperature firing at 830 °C is obtained for the Al₂O₃/SiN_x stacks compared to the single‐layer Al₂O₃ passivation. Al₂O₃/SiN_x stacks are hence ideally suited for the implementation into industrial‐type silicon solar cells where the metal contacts are made by screen‐printing and high‐temperature firing of metal pastes. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ab initio thermodynamic properties for different isomers of the AlO molecule

We present ab initio thermodynamic properties for seven different geometric isomers of molecular Al₂O₃ over a wide temperature range. The rigid rotator-harmonic oscillator approximation is used to calculate the partition function as it is generally applied in thermodynamic studies of polyatomic molecules. The molecular data employed in setting up the partition functions are taken from the theoretical results of ab initio MP2/6-31G(d) calculations. An analysis of the effects of the various contributions is made. The resulting thermodynamic functions of Al₂O₃ are consistent with the JANAF thermochemical data compilation. Some thermochemical implications are discussed. Received: 9 September 1998 / Received in final form: 2 October 1998     

Structural, optical and electrical properties of ZnO and ZnO-Al2O3 films prepared by dc magnetron sputtering

ZnO and ZnO-Al₂O₃ thin films were prepared by dc magnetron sputtering and their structural, optical and electrical properties were studied comparatively. It is discovered that the ZnO-Al₂O₃ thin films remain transparent in a shorter wavelength range than the ZnO films, resulting from the increase of their band gap. Their resistivity decreases by seven orders of magnitude, which is caused by doping of Al to ZnO grains in the film. Preferential orientation of ZnO grains in the ZnO-Al₂O₃ thin films deteriorates because of the existence of Al₂O₃ impurity phase in the film. Received: 5 January 1999 / Accepted: 11 October 1999 / Published online: 8 March 2000     

Physical studies on composites Ag7I4VO4-Al2O3

The electrical properties of the solid electrolytes Ag₇I₄VO₄-Al₂O₃ (0-40 mol% Al₂O₃) are investigated. The electrical conductivity, dielectric constant and dielectric loss are increased by increasing the concentration of Al₂O₃; showing a maximum at 30 mol% Al₂O₃. The conductivity is found to be increased by decreasing the particle size of Al₂O₃. The results are explained using the random resistor network model (RRN). This is due to the formation of a highly conducting interface layer along the matrix-particle interface. This layer is destroyed at concentrations higher than 30 mol% Al₂O₃.     

Artificially nanostructured Cu:Al2O3 films produced by pulsed laser deposition

The processes leading to the formation of Cu:Al₂O₃ composite films on Si (001) with a well defined nanostructure by alternate pulsed laser deposition (a-PLD) in vacuum are investigated. Alternately amorphous Al₂O₃ layers and Cu nanocrystals nucleated on the Al₂O₃ surface are formed, according to the PLD sequence. The Al₂O₃ deposited on the Cu nanocrystals fills in the space between them until they are completely buried, and subsequently, a continuous dense layer with a very flat surface (within 1 nm) is developed. The nucleation process of the nanocrystals and their resulting oblate ellipsoidal shape are discussed in terms of the role of the energetic species involved in the PLD process and the metal–oxide interface energy. Received: 4 July 2000 / Accepted: 5 July 2000 / Published online: 13 September 2000     

Electrical and structural properties of Ce0.9CoFe3Sb12 thermoelectric thin films

In this work, we show experimental results for growth conditions of thermoelectric Ce_0.9CoFe₃Sb₁₂ thin films. An rf-magnetron sputtering system has been used to grow the films on single crystal substrates of sapphire (Al₂O₃), silicon (Si), and magnesium oxide (MgO) at different substrate temperatures between 250 and 450 °C. The films were thermoelectrically characterized with resistivity and thermopower measurements as functions of temperature. The results show linear behavior of resistivity with temperature, and thermopower growth with the temperature increase. Such behavior is typical for metallic materials. The structure and surface morphology of the samples were analyzed by X-ray diffraction pattern and atomic force microscopy (AFM), respectively.     

Excited state absorption and stimulated emission of Nd3+ in crystals. Part I: Y3Al5O12, YAlO3, and Y2O3

4 F_3/2 excited state of the Nd³⁺ ion in Y₃Al₅O₁₂, YAlO₃, and Y₂O₃ were measured in a continuous wave pump- and probe experiment in a wide spectral range from 850 nm (780 nm for Y₃Al₅O₁₂) to 1500 nm. The cross sections were determined from a comparison with the emission spectra and the simultaneously measured ground state absorption bleaching. The strongest excited state absorption transitions were found in the 1220–1400 nm spectral region due to transitions to the ²G_9/2 and ⁴G_7/2 levels. The spectral positions of the measured transitions are in good agreement with the theoretically expected transitions calculated from the known Stark-level splittings. Received: 4 December 1997/Revised version: 8 May 1998     

Structure, electrical and chemical properties of zirconium nitride films deposited by dc reactive magnetron sputtering

2 and Ar ambient. X-ray diffraction indicated that growth of ZrN with a preferred (111) orientation over Si(100) was achieved. The resistivity of the films varies from 200 μΩcm to 15 μΩcm depending on the N₂ content in the working gas. The square resistance of the films deposited on 96% Al₂O₃ ceramic wafers is stable below 300 °C. Received: 17 June 1996/Accepted: 9 December 1996     

Characterization of SnO2 obtained from the thermal oxidation of vacuum evaporated Sn thin films

Tin oxide has been prepared by thermal oxidation of evaporated tin thin films onto pyrex glass substrates. Films oxidation was achieved in air at a temperature of 600 °C with varied duration from 20min to 3 h. Structural, optical and electrical properties of the films were characterized by means of X-ray diffraction, UV–vis spectroscopy and electrical resistivity measurements respectively. The X-ray analysis revealed the transformation of Sn into SnO₂ with preferential orientation along (101) plans. No intermediate phases such as SnO and Sn₃O₄ were evidenced. It was also found that the SnO₂ crystallites orientation changed with the annealing time due to the strain energy effect. Both band gap energy and electrical resistivity decrease with annealing time due to the crystalline quality improvement and films densification. We have noticed that oxidation at 600 °C for 3 h leads to transparent and conductive films with suitable properties for photovoltaic applications.     

Analysis of alumina-based titanium carbide composites by laser-induced breakdown spectroscopy

In this work, alumina (Al₂O₃) containing different volume % of titanium carbide (TiC) ranging from 0 to 30 were consolidated by the novel spark plasma sintering. The spectroscopic analysis of the plasma generated by irradiation of laser Nd:YAG (λ = 1,064 nm) on different concentrations of the composites in air atmospheric pressure was performed. The qualitative examination of the composites confirms the presence of aluminum, titanium, and carbon as major elements, while magnesium and sodium have been found as minor trace elements. Plasma parameters were estimated by assuming the LTE conditions for optically thin plasma. The electron density and temperature were evaluated by using the Stark broadening and intensity of selected aluminum emission lines, respectively. The addition of TiC to Al₂O₃ shows a linear behavior with plasma temperature corroborated by the calibration curve of Ti in the composites. The results suggest that calibration curve between plasma temperature and the composites can be used to estimate different concentrations of TiC in Al₂O₃ without analyzing the whole elements in the composites and thus opens up new applications of LIBS in ceramic industry.     

Molecular beam epitaxy of GaN on a substrate of MoS2 layered compound

The feasibility of MoS₂ layered compound as a substrate for GaN growth was investigated. GaN films were successfully grown on MoS₂ by plasma-enhanced molecular beam epitaxy and the crystal quality of GaN on MoS₂ was compared with that on Al₂O₃. For GaN grown on MoS₂ substrate, it was found that the surface flatness observed by atomic force microscopy, stress in the film measured by Raman spectroscopy, optical properties measured by photoluminescence spectroscopy, and threading dislocation density observed by transmission electron microscopy show superior properties compared with that grown on Al₂O₃. These results suggest the layered compound such as MoS₂, which has no dangling bonds on the surface and has lattice mismatching of 0.9% to GaN, has high potential for a substrate of GaN growth. Received: 1 March 1999 / Accepted: 8 March 1999 / Published online: 26 May 1999     

AlNxOy thin films deposited by DC reactive magnetron sputtering

AlN_xO_y thin films were produced by DC reactive magnetron sputtering, using an atmosphere of argon and a reactive gas mixture of nitrogen and oxygen, for a wide range of partial pressures of reactive gas. During the deposition, the discharge current was kept constant and the discharge parameters were monitored. The deposition rate, chemical composition, morphology, structure and electrical resistivity of the coatings are strongly correlated with discharge parameters. Varying the reactive gas mixture partial pressure, the film properties change gradually from metallic-like films, for low reactive gas partial pressures, to stoichiometric amorphous Al₂O₃ insulator films, at high pressures. For intermediate reactive gas pressures, sub-stoichiometric AlN_xO_y films were obtained, with the electrical resistivity of the films increasing with the non-metallic/metallic ratio.     

The effect of ferrimagnetic ordering in insulating component of composites HTSC+Yttrium Iron Garnet on its transport properties

Composites Y_3/4Lu_1/4Ba₂Cu₃O₇+Y₃Fe₅O₁₂ modeling random network superconductor-ferrimagnetic-superconductor have been prepared and their transport properties (temperature dependences of resistivity ρ and critical current density j_c, current-voltage characteristics) have been studied. Below the superconducting transition temperature T_c, the ρ(T) dependences exhibit a kink at some temperature T_m. The crossover of current-voltage characteristics from ohmic-like behavior in range T_m/T_c to non-linear one below T_m is observed. Transport properties of the ‘benchmark’ composites Y_3/4Lu_1/4Ba₂Cu₃O₇+Y₃Al₅O₁₂ are typical for network superconductor-insulator-superconductor Josephson junction. The behavior observed for Y_3/4Lu_1/4Ba₂Cu₃O₇+Y₃Fe₅O₁₂ composites is attributed to peculiarities of supercurrent carriers tunnelling through ferrimagnetic barriers.     

Preparation and characterization of GaN films by radio frequency magnetron sputtering and carbonized-reaction technique

Radio frequency magnetron sputtering/post-carbonized-reaction technique was adopted to prepare good-quality GaN films on Al₂O₃(0 0 0 1) substrates. The sputtered Ga₂O₃ film doped with carbon was used as the precursor for GaN growth. X-ray diffraction (XRD) pattern reveals that the film consists of hexagonal wurtzite GaN. X-ray photoelectron spectroscopy (XPS) shows that no oxygen can be detected. Electrical and room-temperature photoluminescence measurements show that good-quality polycrystalline GaN films were successfully grown on Al₂O₃(0 0 0 1) substrates.