The propagation of laser ablation plumes from 248 nm laser‐irradiated YBCO into vacuum and 100 mTorr ambient oxygen has been photographed with a gated, intensified CCD array camera system. The thermalization of the laser plasma and onset of shock structures due to collisions with the background gas are investigated from two‐dimensional digitized images of the visible plume emission.

Fast intensified‐CCD photography of YBa2Cu3O7−x laser ablation in vacuum and ambient oxygen

We have theoretically and experimentally analyzed the laser‐induced evaporation process for deposition of superconducting thin films from bulk targets. The spatial thickness variations have been found to be significantly different from a conventional thermal deposition process. Unlike a cos θ thickness variation expected from a thermal evaporation process, the laser evaporation process is characterized by a forward‐directed deposit with a sharp variation in its thickness as a function of distance from the center of the deposit. We have studied in detail the interactions of nanosecond excimer laser pulses with bulk YBa2Cu3O7 targets leading to evaporation, plasma formation, and subsequent deposition of thin films. A theoretical model for simulating the pulsed laser evaporation (PLE) process has been developed. This model considers an anisotropic three‐dimensional expansion of the laser‐generated plasma, initially at high temperature and pressure. The forward‐directed nature of laser deposition has been found to result from anisotropic expansion velocities of the plasma edges arising due to the density gradients in the gaseous plasma.The physical process of the laser ablation technique for deposition of thin films can be classified into three separate interaction regimes: (i) interaction of the laser beam with the bulk target, (ii) plasma formation and initial isothermal expansion, and (iii) adiabatic expansion leading to deposition of thin films. The first two regimes occur during the time interval of the laser pulse, while the last regime initiates after the laser pulse terminates. Under PLE conditions, the evaporation of the target is assumed to be thermal in nature, while the plasma expansion dynamics is nonthermal as a result of interaction of the laser beam with the evaporated material. The expansion velocities of the plasma edges are related to the initial dimensions and temperature of the plasma, and the atomic weight of the respective species present in it. Preliminary calculations have been carried out on spatial thickness variations as a function of various parameters in PLE deposited thin films. The effects of the various beam and substrate parameters including energy density and substrate‐target distance affecting the nature of deposition of superconducting thin films have been theoretically examined. Experimental results have been obtained from thin films deposited on silicon substrates by XeCl pulsed excimer laser (λ=308 nm, τ=45×10−9 s) irradiation. The spatial thickness and compositional variations in thin films have been determined using Rutherford backscattering technique and the results compared with the theoretical calculations.

Theoretical model for deposition of superconducting thin films using pulsed laser evaporation technique

The gas flow dynamics of laser ablation plumes is investigated experimentally and theoretically. Experimentally, angular‐resolved time‐of‐flight (ARTOF) measurements are performed on a model system (laser etching of copper in a chlorine environment). The TOF spectra obtained can be fitted by elliptical Maxwell–Boltzmann distributions on a stream velocity. Theoretically, an analytical model is constructed, based on the hydrodynamical problem of an expanding elliptical gas cloud. The model allows semiquantitative prediction of ARTOF distributions and angular intensity distributions. Observed trends in laser ablation deposition such as independence of the angular intensity distribution on mass of the atom and laser fluence, and dependence of the angular distribution on spot dimensions are explained.

Gas flow dynamics in laser ablation deposition

Physics and diagnostics of laser ablation plume propagation for high-Tc superconductor film growth

We have prepared superconducting thin films of Y‐Ba‐Cu‐O with Tc onsets above 95 K by both diode and magnetron sputtering. Films with full superconductivity (R=0) at 85 K have been produced by dc magnetron sputtering. The compositions of the films are fairly uniform across an area 50 mm in diameter and through the film thickness. Structural properties of the films were studied by x‐ray diffraction. Critical current densities in the range of 3000 to 104 A/cm2 have been measured at 4.2 K.We have prepared superconducting thin films of Y‐Ba‐Cu‐O with Tc onsets above 95 K by both diode and magnetron sputtering. Films with full superconductivity (R=0) at 85 K have been produced by dc magnetron sputtering. The compositions of the films are fairly uniform across an area 50 mm in diameter and through the film thickness. Structural properties of the films were studied by x‐ray diffraction. Critical current densities in the range of 3000 to 104 A/cm2 have been measured at 4.2 K.

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Superconducting Y-Ba-Cu-O oxide fi lms by sputt ering

The pulsed laser thin‐film deposition process can enable preparation of thin films of complex composition with good control over the film stoichiometry The film compositions are similar to that of the target pellet and as a consequence this technique appears to be an ideal method for preparing high Tc thin films on a variety of substratesThe factors which contribute to this beneficial phenomenon have been explored by a laser ionization mass spectrometry (LIMS) and a post ablation ionization (PAI) neutral velocity analysis technique in order to determine the mass and velocities of the laser ejected material In addition, x‐ray absorption measurements on films deposited onto substrates at room temperature were performed in order to identify the presence of short‐range crystalline order in the films Both of these studies rule out the ejection of stoichiometric clusters of material from the pellet during the laser ablation/deposition process Instead, binary and ternary suboxides are emitted from the targe

Nature of the pulsed laser process for the deposition of high Tc superconducting thin films

Pulsed laser deposition (PLD) from a single target is simple, easy, and inexpensive compared to most of the other techniques employed for preparation of high Tc superconducting thin films. We present the results of Rutherford Backscattering study of the PLD process and identify the parameters which are critical in controlling the deposited film stoichiometry. Also, by using mass spectrometry the nature of the laser deposition process has been investigated. The PLD technique enables us to get films with stoichiometry close to that of the target. High Tc superconducting thin films of R-Ba-Cu oxide (R,Y, Eu, Gd, Sm, Tm, Er) have been prepared using PLD. Films deposited in vacuum at 450 C were not superconducting and post-annealing at over 850 C in oxygen was necessary to achieve high zero resistance temperatures. Detailed analyses of the films were carried out using RBS, TEM, XRD, Raman, Auger, SIMS, X-ray absorption and resistivity measurements. Recently, we were able to obtain superconducting thin films of Y-Ba-Cu Oxide with zero resistance temperature over 80 K at and below 650 C (processing temperature). The critical current densities in the films prepared at low temperature on SrTiO3 were on the order of 5x105 A/cm2 at 77 K. These films exhibited minimal interface reaction and were very smooth. We will also show that PLD can be utilized to prepare high Te superconducting thin films of Bi-Sr-Ca-Cu oxide.

Y. Jeon

Papers

Nature of the pulsed laser process for the deposition of high Tc superconducting thin films

Versatility Of Pulsed Laser Deposition Technique For Preparation Of High Tc Superconducting Thin Films