Cross-beam Pulsed Laser Deposition Research Articles

Growth, structure, and properties of BiFeO/sub 3/-BiCrO/sub 3/ films obtained by dual cross beam PLD

The properties of epitaxial Bi(2)FeCrO(6) thin films, recently synthesized by pulsed laser deposition, have partially confirmed the theoretical predictions (i.e., a magnetic moment of 2 micro(B) per formula unit and a polarization of approximately 80 microC/cm(2) at 0 K). The existence of magnetic ordering at room temperature for this material is an unexpected, but very promising, result that needs to be further investigated. Because magnetism is assumed to arise from the exchange interaction between the Fe and Cr cations, the magnetic behavior is strongly dependent on both their ordering and the distance between them. We present here the successful synthesis of epitaxial Bi(2)Fe(x)CryO(6) (BFCO x/y) films grown on SrTiO3 substrates using dual crossed-beam, pulsed-laser deposition. The crystal structure of the films has different types of (111)-oriented superstructures, depending on the deposition conditions. The multiferroic character of BFCO (x/y) films is proven by the presence of both ferroelectric and magnetic hysteresis at room temperature. The oxidation state of Fe and Cr ions in the films is shown to be 3+ only, and the difference in macroscopic magnetization with Fe/Cr ratio composition could only be due to ordering of the Cr(3+) and Fe(3+) cations to the modification of the exchange interaction between them.

Structural properties of Ge nanostructured films synthesized by laser ablation

We compare the structural properties of nanostructured Ge films deposited by pulsed laser deposition (PLD) and crossed-beam pulsed laser deposition (CBPLD). We show that CBPLD is an excellent technique for reduction of micron-sized particles which affect the quality of nanostructured films. We demonstrate that CBPLD in the equivalent dynamic conditions of PLD yields similar nanostructured Ge films with particle sizes of ∼20 nm. Combining different characterization techniques we conclude that under these deposition conditions the Ge nanoparticles are amorphous.

Influence of an inert background gas on bimetallic cross-beam pulsed laser deposition

A cross-beam pulsed laser deposition (CBPLD) system operated at variable pressure in an inert (He) background atmosphere was used to deposit films from two dissimilar targets (Pt–Ru and Pt–Au). Using this setup, we showed that films with mixed Pt–Au and Pt–Ru composition can be prepared over the whole compositional range, from [Pt] = 0 to 100at.%. Films deposited at He pressure higher than 1.6Torr are fairly homogeneous and the standard deviation of the Pt concentration over the whole area of the deposit is less than 1at.%. Using a diaphragm located at the interaction zone between the two plasmas, a drastic reduction of the normalized droplet density was observed, from about 700×102cm−2nm−1 in conventional PLD to 6×102cm−2nm−1 in CBPLD. The deposition rate increases as the pressure is increased from vacuum to an optimal He pressure. The deposition rate decreases again for higher He pressure. The optimal operating conditions are P(He)=2Torr for Pt–Ru and P(He)=4Torr for Pt–Au. In these conditions, the deposition rates are, respectively, ∼32% and ∼22% of what they would be in conventional PLD. The behavior of the deposition rate with the He pressure is consistent with what can be concluded from a visual observation of the interaction of the plasma plumes at various pressures. A simple model considering the quadratic dependence of the velocity on the flow resistance of heavy particles in the rarefied light ambient particles is developed to understand the role of the background gas in the deposition rate. This model succeeds in predicting a maximum in the deposition rate versus He pressure curve, allowing us to get a better physical understanding of what is going on during the interaction between the two plasma plumes.

Crystal structure and microwave resistance of YBa2Cu3O7−δ films prepared by cross-beam pulsed-laser deposition

High-Tc superconducting YBa2Cu3O7−δ (YBCO) films were prepared by a unique pulsed-laser deposition using crossed-laser-beam technique onto LaAlO3 substrates in order to develop a basic technology for microwave-device application. The pulsed-laser deposition set-up uses two Nd–YAG lasers with 1064 nm wavelength and 0.3 J/pulse power. The unique feature of this method is a possibility to get a plasma plume with a low level of ‘droplet’, which essentially improves the film-surface quality. Transmission-electron microscopy and x-ray data show that YBCO films deposited at substrate temperature (Ts) of 780 °C exhibit a minimal density of dislocations in the crystal. YBCO films deposited at Ts = 780 °C with a cooling rate of V = 3 K min−1 have the smallest surface resistance below the transition temperature.

Phase transformations after long-time annealing in metastable Fe–Cr alloy films prepared by pulsed laser deposition

Metastable Fe–Cr alloy films of various composition prepared by cross-beam pulsed laser deposition using two different procedures are investigated by wide-angle X-ray scattering. Depending on the Fe–Cr composition of the samples in an extended range, a body-centered cubic (bcc) phase or metastable phases with body-centered tetragonal (bct), face-centered orthorhombic (fco) or primitive orthorhombic (po) and primitive cubic (pc) lattices are formed in the films prepared by simultaneous co-deposition of Fe and Cr. In the films produced by layer-by-layer deposition of thin separate Fe and Cr layers (thickness of about 1 nm), only bcc and bct Fe–Cr phases were observed. A long-time annealing (∼50 h) at a temperature of 425 °C near the low-temperature existence limit of the σ-phase under equilibrium conditions followed by slow cooling (rate ∼0.5 °C/min) has been performed and various phase transformations were observed. In addition to known equilibrium and metastable Fe–Cr crystalline phases (mainly bcc and bct phases in the films prepared by layer-by-layer technique and bcc, bct and σ-FeCr phases in co-deposited films), a new metastable Fe–Cr superstructure characterised by a primitive tetragonal lattice with parameters a and c of about 0.57 and 0.63 nm, respectively, has been identified. It is shown that the formation of α″-crystallites with preferred orientation in the metastable Fe–Cr alloy films during dedicated long-time annealing gives rise to a spatially periodic modulation of chemical composition resulting in the formation of multilayers with periods of one or a few atomic monolayers of individual Fe and Cr components.

Effect of annealing on the interface structure of cross-beam pulsed laser deposited Co/Cu multilayers

Co/Cu multilayers were prepared by cross-beam pulsed laser deposition and characterised by high-angle X-ray diffraction as well as specular and off-specular reflection before and after annealing (500 °C, 2 h). Using synchrotron radiation at the K-edge energy of Co and Cu to enhance the scattering contrast, the study shows that annealing does not enlarge the interface roughness, σ r.m.s., but strongly influences the interface morphology. In the fractal model of self-affine structures the latter is expressed by the drastic reduction of the extremely large lateral roughness correlation length parameter, ξ, from approximately 4 μm in the as-deposited state to approximately 20 nm after annealing. High-angle X-ray diffraction indicates that the reduction of ξ is caused by grain coarsening due to growth of crystallites and grain boundaries. Thermal treatment enhances the separation of Co and Cu at the interface, i.e. the initially very jagged (roughness exponent, h, between 0.15 and 0.3) structure is smoothed ( h between 0.6 and 0.7).

Thermally stimulated crystalline phase transformations in metastable Fe–Cr alloy films prepared by pulsed laser deposition

Thermally induced crystalline phase formation of metastable Fe–Cr alloy films with different Cr-content (34.5, 43.5 and 48.5 at.% Cr) and thickness of 100 nm prepared by cross-beam pulsed laser deposition was examined by wide angle X-ray scattering. Therefore the annealing temperature was increased stepwise up to 1100 °C, the duration of the treatment per step was 90 min. Through a number of intermediate metastable crystalline phases, above 550 °C the phase composition did not change until the onset of reaction with Si/SiO 2 substrate at ∼900 °C. Equilibrium σ-FeCr is formed in all samples. In addition, body-centered cubic (b.c.c.) and tetragonally distorted b.c.c. Fe–Cr phases are formed in a film with nearly equiatomic composition. Reactions with substrate result in formation of Fe- and Cr-silicides.

X-ray investigation of metastable crystalline phases in co-deposited Fe–Cr alloy nanometer films

Fe 100− x Cr x ( x=24.5–61.7, 100 at.%) alloy nanometer films prepared by crossed-beam pulsed laser deposition were examined by wide-angle X-ray scattering. On varying the Cr content, x, a change of phase content is observed starting from the known body-centered cubic phase at low Cr content. Indications for three new metastable Fe–Cr phases were found to form in body-centered tetragonal, face-centered orthorhombic and primitive orthorhombic lattice types, respectively.

Reflectivity and diffraction study of cross-beam pulsed laser deposited Co/Cu multilayers

Co/Cu multilayers were prepared by cross-beam pulsed laser deposition and characterised by high-angle X-ray diffraction as well as specular and off-specular reflection. Using synchrotron radiation at the K-edge energy of Co and Cu to enhance the scattering contrast, the study shows that the roughness of these multilayers is well described by the fractal model of self-affine structures. For the pulsed laser deposited layers an extremely large lateral correlation length, ξ, of the vertically correlated interfacial roughness is found (ξ>1 μm) which exceeds the ξ-value of the uncorrelated roughness by approximately two orders. The interfaces are very jagged (roughness exponent, h, between 0.15 and 0.3). The root mean square roughness, σ r.m.s., of the Cu/Co and Co/Cu interfaces are of the same order compared with the values reported for sputtered layers.

Comparative study of interfaces of Fe–Al multilayers prepared by direct and crossed-beam pulsed laser deposition

Fe/Al multilayers with different composition were prepared by conventional, direct-pulsed laser deposition (DPLD) and crossed-beam pulsed laser deposition (CBPLD). Films were studied by means of X-ray reflectometry, wide-angle X-ray scattering and X-ray absorption fine structure experiments. Ballistic simulations of the deposition processes were performed. It is found that adjacent Fe and Al layers are partially intermixed during the deposition, forming Fe–Al transition layers of a few nm in thickness. Both DPLD and CBPLD multilayers have comparable interfaces. In Fe-rich layers, a b.c.c. solid solution is formed. Al-rich layers of approximately 5 nm in nominal thickness are amorphous. DPLD samples have a higher Al content and a significant number of incorporated Al droplets.

Cross-beam pulsed laser deposition: General characteristic

Physical background of the operation of the cross-beam pulsed laser deposition (CBPLD) is considered. In this modification of PLD, gas-dynamic interaction of two plasma plumes from separate targets is used for particulate elimination from the plasma flux to the substrate. The interaction of the plasma plumes by CBPLD in a vacuum was investigated with the help of ion collectors and optical spectroscopy. A minimal plasma number density in the intersection region necessary for an efficient plume interaction amounts to ≈1.4×1015 cm−3 in the geometry of the experiments. For the materials with relatively high evaporation rates, the plasma density in the plume intersection region is larger than this value during almost the entire plume existing time, and the ablated material can be effectively utilized for the deposition of films. CBPLD demonstrates a very good effectiveness of the particulate removal from the plasma flux, and it is reasonable to suggest that CBPLD is one of the most effective working methods for particulate suppression in PLD. The interaction of two plumes results in filtering out the most energetic ions and the slowest neutrals of the original plumes from the plasma flux to the substrate, in decrease of the average plasma particle energy, and in increase of the degree of ionization of plasma up to more than 100%. Mechanisms responsible for these effects are analyzed. An asymmetric CBPLD geometry is proposed that makes it possible to mix materials of different targets directly in the laser plasma with good control over film composition.

Thermally stimulated solid state reactions in Fe–Al multilayers prepared by pulsed laser deposition

Two 15-period Fe–Al multilayers (single layer thicknesses ∼5 nm Al and ∼2 nm Fe) prepared by direct and crossed-beam pulsed laser deposition (DPLD and CBPLD, respectively) were subjected to annealing between 50 and 950°C. Phase analysis done by X-ray diffraction and determination of layer morphology using X-ray reflectometry showed that solid state reaction kinetics is different in DPLD and CBPLD samples. This applies in particular to the sequence of intermetallic compounds formed as well as to the temperatures of full intermixing. Pronounced interdiffusion of Fe and Al starts at annealing temperatures of about 110 –140°C and a full intermixing of multilayer material occurs at temperatures between 250°C (DPLD) and 300°C (CBPLD). The difference in kinetics between CBPLD and DPLD samples is due to a difference in the structure of interfaces and in their average chemical composition.

Preparation of granular Co/Cu by ion-beam mixing of laser-deposited multilayers

Co/Cu multilayers were prepared by crossed-beam pulsed laser deposition and characterized by high-angle X-ray diffraction, as well as specular and non-specular reflection. Ion-beam mixing of the multilayers with 150 keV Cu+ ions and a fluence of up to 5×1015 cm-2 leads to the formation of a granular system of superparamagnetic Co clusters in Cu, as shown by magnetic measurements.

Kinetic, crystallographic, and optical studies of GaN and AlxGa1−xN thin films grown on Si(111) by pulsed reactive crossed-beam laser ablation using liquid alloys and N2 or NH3

High quality GaN and AlxGa1−xN films have been grown by reactive crossed-beam pulsed laser deposition at 248 nm (KrF) using liquid Al–Ga alloy targets and a synchronous gas pulse on 7×7 reconstructed Si(111) substrates. The growth kinetics and their effect on the optical properties were studied in detail. The growth rates were lower and the optical properties were superior for GaN films grown with N2 than when NH3 was used as the nitriding source. The ratio of Ga to Al in the alloy was reproduced in AlxGa1−xN films when using NH3, but was in general lower when using N2. These findings can be explained by the lower reaction rate of Ga with N2 and the high vapor pressure of Ga at the adopted substrate temperatures.

Metastable Phase Formation in Fe-Al Thin Films Cocondensed by Cross-Beam Pulsed Laser Deposition

Metastable Phase Formation in Fe-Al Thin Films Cocondensed by Cross-Beam Pulsed Laser Deposition

Metastable Phase Formation in Fe-Al Thin Films Cocondensed by Cross-Beam Pulsed Laser Deposition

Metastable Phase Formation in Fe-Al Thin Films Cocondensed by Cross-Beam Pulsed Laser Deposition

Production of Thin Films of Metastable Materials by Cross-Beam Pulsed Laser Deposition

Production of Thin Films of Metastable Materials by Cross-Beam Pulsed Laser Deposition

Production of Thin Films of Metastable Materials by Cross-Beam Pulsed Laser Deposition

Production of Thin Films of Metastable Materials by Cross-Beam Pulsed Laser Deposition

Thin film mixtures synthesized by cross-beam PLD

Fabrication of metastable binary solid state film mixtures with continuously variable composition by asymmetric cross-beam PLD is demonstrated. In this technique, the materials are mixed directly in the intersection region of colliding plasmas of individual target materials. This technique can be advantageous for synthesis of solid state quasi-solutions of partially miscible materials, where one can expect interesting mechanic properties and magnetic transformations.

Features of the film-growth conditions by cross-beam pulsed-laser deposition

Spatial and energetic characteristics of the plasma plume by cross-beam pulsed-laser deposition (CBPLD) were investigated. Effective droplets filtering together with high efficiency of material usage are observed by this approach. Time-of-flight (TOF) technique with electrostatic ion collectors (Langmuir probes operating in the ion-collecting mode) were applied to obtain kinetic energy distribution functions of ionized particles and to compare the ionization degrees of the plasma by the CBPLD and by the conventional PLD. The average and maximum kinetic energies of the ions by the CBPLD are found to be 2–3 times lower as compared to the conventional PLD. At the same time, the fraction of ionized species and highly exited neutrals (Rydberg atoms) in the CBPLD plasma is 1.5–2 times larger in comparison to the conventional approach. Re-sputtering of the material of the growing film by fast ions is a considerable effect in both the PLD methods by the chosen experimental conditions. The angular width of the directional pattern of the plasma plume by CBPLD is comparable to that typical for the conventional PLD.