Pulsed laser deposition (PLD) is a conceptually and experimentally simple yet highly versatile tool for thin-film and multilayer research. Its advantages for the film growth of oxides and other chemically complex materials include stoichiometric transfer, growth from an energetic beam, reactive deposition, and inherent simplicity for the growth of multilayered structures. With the use of PLD, artificially layered materials and metastable phases have been created and their properties varied by control of the layer thicknesses. In situ monitoring techniques have provided information about the role of energetic species in the formation of ultrahard phases and in the doping of semiconductors. Cluster-assembled nanocrystalline and composite films offer opportunities to control and produce new combinations of properties with PLD.

Synthesis of Novel Thin-Film Materials by Pulsed Laser Deposition

The broad spectrum of electronic and optical properties exhibited by oxides offers tremendous opportunities for microelectronic devices, especially when a combination of properties in a single device is desired Here we describe the use of reactive molecular-beam epitaxy and pulsed-laser deposition to synthesize functional oxides, including ferroelectrics, ferromagnets, and materials that are both at the same time Owing to the dependence of properties on direction, it is often optimal to grow functional oxides in particular directions to maximize their properties for a specific application But these thin film techniques offer more than orientation control; customization of the film structure down to the atomic-layer level is possible Numerous examples of the controlled epitaxial growth of oxides with perovskite and perovskite-related structures, including superlattices and metastable phases, are shown In addition to integrating functional oxides with conventional semiconductors, standard semiconductor practices involving epitaxial strain, confined thickness, and modulation doping can also be applied to oxide thin films Results of fundamental scientific importance as well as results revealing the tremendous potential of utilizing functional oxide thin films to create devices with enhanced performance are described

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A Thin Film Approach to Engineering Functionality into Oxides

Thin films of YBa2 Cu3 O7−x have been grown epitaxially in c‐axis orientation on 〈100〉 SrTiO3 by pulsed excimer laser evaporation from a stoichiometric 1‐2‐3 target. The substrate temperature was adjusted between 720 and 780 °C, and the oxygen partial pressure during the deposition was chosen in the range 0.1–0.3 mbar. Cooled to ambient temperature in situ for 1 h in flowing oxygen gas, the films showed complete diamagnetism and zero resistance at 90 K with a transition width of 2 K. Critical current densities of 2.2×106 A/cm2 in zero magnetic field and 1.5×105 A/cm2 at 2 T were measured at 77 K. The resistivity at 100 K was about 60 μΩ cm.

Epitaxial growth of YBa2Cu3O7−x thin films by a laser evaporation process

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

Using Rutherford backscattering technique, we have measured the angular distribution of the composition and thickness of the Y‐Ba‐Cu oxide film deposited by firing excimer laser (30 ns, 248 nm) pulses at a stoichiometric Y1Ba2Cu3O7−x pellet. The angular distribution consisted of two distinct components: one a cos θ component, a result of evaporation, and the other a highly forward directed component, a result of a secondary ejection process. The evaporated component is nonstoichiometric, as one would expect, whereas the forward‐directed component has a composition close to that of the pellet. Further, the forward‐directed stoichiometric component increases with the laser energy density in comparison with the evaporated component. These observations are discussed in the context of current models of laser‐induced material ejection at surfaces.The laser energy dependence of the deposition is of critical importance in controlling the film stoichiometry.

Observation of two distinct components during pulsed laser deposition of high Tc superconducting films

2000 A as‐deposited high Tc superconducting Y1Ba2Cu3O7−x films with zero‐resistance temperatures of ∼89 K and critical current densities about 0.7×106 A/cm2 at 77 K have been reproducibly fabricated at a substrate holder temperature of 650 °C, using pulsed laser deposition, with no post‐annealing. One key to these results is the injection of gaseous oxygen into the laser‐produced plume just in front of the target. In this way, the correct amount of oxygen is incorporated into the as‐grown film so that post‐deposition treatment becomes unnecessary. The in situ growth of such high Tc and Jc films is an important step in the use of the laser deposition technique to fabricate multilayer structures such as Josephson junctions.

As‐deposited high Tc and Jc superconducting thin films made at low temperatures

Thin films of Y‐Ba‐Cu‐O superconductors on (100) SrTiO3 were prepared using the pulsed excimer laser evaporation technique. After an annealing treatment in oxygen the films exhibited superconductivity with zero resistance at 85 K with transition widths (90%–10%) as narrow as a 2 K. A considerable amount of orientation of the films with respect to the substrates was observed by x‐ray diffraction and ion channeling techniques. Interface reaction revealed by Rutherford backscattering and Auger electron spectrometry resulted in a thickness dependence of the superconducting properties of the films.

Epitaxial ordering of oxide superconductor thin films on (100) SrTiO3 prepared by pulsed laser evaporation

Thin superconducting films of Y‐Ba‐Cu‐O were prepared at 650 °C using the pulsed laser deposition technique. The as‐deposited films were fully superconducting at low temperature (30 K). After annealing in oxygen at 450 °C for 3 h, the films exhibited superconductivity with zero resistance at temperatures as high as 83 K. Film‐substrate interface reaction was minimal as revealed by Rutherford backscattering and Auger electron spectrometry. These films processed at such low temperatures are also found to have excellent planar surface morphology and high critical current density.

Low‐temperature preparation of high Tc superconducting thin films

By depositing YBaCuO films at low substrate holder temperatures (600 °C) using a pulsed laser, films with the orthorhombic phase exhibiting superconductivity with high zero resistance temperatures (Tc0) were produced. Tc0 values as high as 50 and 80 K were seen on films deposited directly on Si and on Si with only a 50 nm ZrO2 buffer layer, respectively. The key to the successful film deposition is the low processing temperatures involved, which minimized the interface reactions as observed by Auger electron spectrometry.

Superconducting Y1Ba2Cu3O7−x films on Si

High oxygen partial pressure has been found to be an important parameter for the pulsed laser deposition of as‐deposited superconducting thin films with high Tc and Jc. The optical emissions from both elemental and oxide species ejected from the target of YBa2Cu3O7−x during the pulsed laser deposition process increase with the oxygen pressure, with the oxide emission showing a stronger pressure dependence than the elemental emission. The dynamics of the interaction between the oxygen atoms and the species in the laser‐produced plume were studied by a wavelength and time‐resolved measurement. The results are qualitatively explained using a simple model based on optical emission arising from inelastic and recombination collision between the elemental species and electronically excited oxygen atoms. The formation of oxides in the plume is shown to be essential for the production of higher quality superconducting films, indicating the value of optical spectroscopy as a diagnostic tool.

E. W. Chase

Papers

As‐deposited high Tc and Jc superconducting thin films made at low temperatures

Epitaxial ordering of oxide superconductor thin films on (100) SrTiO3 prepared by pulsed laser evaporation

Low‐temperature preparation of high Tc superconducting thin films

Superconducting Y1Ba2Cu3O7−x films on Si

Optical spectroscopy: An in situ diagnostic for pulsed laser deposition of high Tc superconducting thin films