Pulsed laser deposition of thin superconducting films of Ho1Ba 2Cu3O7 _ x and Y1Ba 2Cu3O7 _ x D.B. Geohegan, D. N. Mashburn, R. J. Culbertson, S. J. Pennycook, J. D. Budai, R.E. Valiga,B. C. Sales, D. H. Lowndes, L. A. Boatner,E. Sonder.D. Eres, D.K. Christen, and W. H. Christie Solid State Division, Oak Ridge National Laboratory, OakRidge, Tennessee 37831-6056 (Received 1 June 1988; accepted 9 August 1988) Thin films of Ho,Ba2Cu3O7 _ x and Y!Ba2Cu3O7 __ x were deposited on SrTiO3 and A12O3 substrates by pulsed laser deposition of high- Tc bulk superconductor pellets in vacuum. Following annealing in O2 at 800-900 °C the films were superconducting with typical Tc (50%) = 89 K and transition widths of 10 K.Rutherford backscattering spectrometry (RBS) and secondary ion mass spectrometry (SIMS) were utilized to study the stoichiometry of the as- deposited films for laser energy densities between 0.11 and 4.5 J cm" 2 . The films were deficient in holmium and yttrium for energy densities below 0.6 and 0.4 J cm" 2 , respectively. The films were stoichiometric for fluences above 0.6 J cm" 2 . In addition, preliminary time dependence and spectroscopic observations of the laser-produced plasma are presented. The results indicate an ablation mechanism that at high energy densities preserves stoichiometry. TEM and x-ray characterization of annealed, superconducting HO|Ba2Cu3O7 _ x films on (100) SrTiO3 showed mixed regions of epitaxially oriented 1:2:3 material with either the c axis or a axis oriented along the surface normal. The a-axis-oriented material grew preferentially in the films with b, c, twinning. I. INTRODUCTION Since the first characterizations of the interaction of high-powered laser radiation with solid surfaces over 20 years ago,1' 2 the technique of pulsed laser vaporization has been used to deposit a wide range of materials in- cluding metals,3 semiconductors, 4"10 and dielec- trics. 3' 5' 7811' 12 The technique has been limited, however, by problems associated with the complex vaporization process such as incongruent stoichiometry transfer between target and film, particulate formation2'9'10 ("spitting"), ion damage of films by the laser-induced plasma13 and, underlying these, lack of a physical un- derstanding of the laser ablation process. These prob- lems have been overcome experimentally in many cases and, by careful control of the laser power and wave- length, epitaxial semiconductor films91014 and superlat- tices have been grown.15'16 Recently, the pulsed laser deposition technique has been applied successfully to deposit thin films of the high-7]. oxide superconductors YiBa 2 Cu 3 O7 _^ 17 " 22 and La 185 Sr 015 CuO4 _ Jc . 21 The attractiveness of this technique is that the 1:2:3 stoichiometry of the target material (usually a bulk superconducting pellet) can be reproduced relatively easily in the films, to within about 10%. At low laser energy densities, however, deficien- cies of yttrium have been reported in annealed films.18>23 In this paper we describe pulsed laser deposition of thin superconducting films of HojBa2Cu3O7 _x and Y1 Ba 2 Cu 3 O7 _ x on a variety of substrates (SrTiO3, A12O3, and SiO2 on Si). In a previous paper, we reported a lack of stoichiometry in oxygen-annealed thin films formed by pulsed laser ablation of Y1Ba2Cu3O7_ * at low fluences.18 Here the stoichiometry of as-deposited films of Ho^Ba^C^O^ and YxBa^,CuzO5 was charac- terized as a function of the actual laser energy density delivered to the pellet, in order to investigate the mecha- nism of the ablation process and to determine a range of useful deposition conditions. Scanning-electron micro- graphs of the as-deposited films, as well as preliminary measurements of the emission spectra and time-depen- dent intensity of the laser-induced plume also are pre- sented as an aid to understanding the pulsed laser depo- sition process. The superconducting properties and microstructure of the annealed Ho,Ba2Cu3O7 _ x super- conducting films, as revealed by transmission electron microscopy (TEM) and x-ray diffraction (XRD), are also presented. II. EXPERIMENTAL The deposition process utilizes a high-power, pulsed excimer laser to ablate material from the face of a superconducting pellet onto a nearby substrate. A sche- matic of the experimental apparatus has been published elsewhere.18 The pellet (target) was mounted in a cop- per ring and positioned by a vertical support rod 2-3 cm away from a heated substrate under vacuum (1X 10" 6 Torr). The pellet was irradiated by pulsed 248 nm (25 ns FWHM) light from a KrF laser (Questek 2640). An external cylindrical lens focused the rectangular ex- cimer beam to a line on the pellet (typically 1.2x0.015 cm), producing peak intensities of up to 280 MW cm" 2 . J. Mater. Res. 3(6), Nov/Dec 1988 0884-2914/88/061169-11 $01.75 © 1988 Materials Research Society 1169