Following D. Allan Bromley's plenary lecture at the 1991 MRS Fall Meeting, the MRS Bulletin conducted a telephone interview with him on December 27. The following is an edited version of that interview. Bromley is Assistant to the President for Science and Technology and also Director of the Office of Science and Technology Policy (OSTP). His plenary address is published in the Bulletin's Material Matters column this month.

Bulletin: You have obviously managed to sustain excellent relations with the administration while also being an effective spokesperson for science. During your term in office, both the FCCSET (Federal Coordinating Council for Science, Engineering, and Technology) and PCAST (President's Council of Advisers on Science and Technology) committees have been revitalized. Could you briefly describe the impact of their increased effectiveness?

Bromley: I think the best way to do that is to indicate what they're doing now. The FCCSET was established in 1976 at the same time that the OSTP was put in place through joint action of the administration and Congress. The charge to the Council was that of integrating and coordinating issues that transcend the boundaries of any single agency, so it focused on interdisciplinary and multi-agency programs. In the early days, however, it did not function as had been intended because the members were at too low a political level. Decisions reached in the Council could, later in the budget process, be disowned by more senior people in the agencies. Having recognized that problem and with strong support from President Bush, I was able to reconstitute the FCCSET so that its members now consist of secretaries or deputy secretaries from all the relevant agencies, as well as heads of independent agencies like EPA, NSF, and NASA.

Research on materials grown by pulsed laser deposition, or PLD, has experienced phenomenal growth since late 1987 when T. Venkatesan (one of the authors for this issue) and co-workers pointed out that extreme nonequilibrium conditions created by pulsed laser melting of YBaCuO allowed in-situ preparation of thin films of this high transition temperature (Tc) superconducting material. Since then, PLD has emerged as the primary means for high throughput deposition of high-quality superconducting thin films for research and devices. This probably came as no surprise to J.T. Cheung (another of this issue's authors), who performed original research in this area and tirelessly labored during the 1980s to convince a skeptical audience of the advantages of PLD.

Along with the success of PLD in the arena of high-temperature superconductivity, however, is the explosion of activity in the deposition of many other materials, made possible by the unique features of pulsed laser deposition, materials previously not amenable to in-situ thin film growth. Creative minds reasoned that since PLD can deposit a demanding, complex material such as the perovskite structure Y1Ba2Cu3O7-δ, why not other perovskites or multicomponent oxide materials? It also turns out that the range of properties of multicomponent oxides is virtually limitless. They can be metallic, insulating, semiconducting, biocompatable, superconducting, ferroelectric, piezoelectric, and so on. One is not limited to the properties of elements or binary compounds on which the electronics and microelectronics industries are based. Indeed, in a recent review of hybrid ferromagnetic- semiconductor structures, G. Prinz states, “… there has been little work devoted to incorporating magnetic materials into planar integrated electronic (or photonic) circuitry there are potential applications that have no analog in vacuum electronics but that remain unrealized, awaiting the development of appropriate materials and processing procedures.” In pulsed laser deposition, we may well have in hand the “appropriate processing procedure” to deposit sequential epitaxial layers of high quality materials that possess profoundly different properties.

The laser, as a source of “pure” energy in the form of monochromatic and coherent photons, is enjoying ever increasing popularity in diverse and broad applications from drilling micron-sized holes on semiconductor devices to guidance systems used in drilling a mammoth tunnel under the English Channel. In many areas such as metallurgy, medical technology, and the electronics industry, it has become an irreplaceable tool.

Like many other discoveries, the various applications of the laser were not initially defined but were consequences of natural evolution led by theoretical studies. Shortly after the demonstration of the first laser, the most intensely studied theoretical topics dealt with laser beam-solid interactions. Experiments were undertaken to verify different theoretical models for this process. Later, these experiments became the pillars of many applications. Figure 1 illustrates the history of laser development from its initial discovery to practical applications. In this tree of evolution, Pulsed Laser Deposition (PLD) is only a small branch. It remained relatively obscure for a long time. Only in the last few years has his branch started to blossom and bear fruits in thin film deposition.

Conceptually and experimentally, PLD is extremely simple, probably the simplest among all thin film growth techniques. Figure 2 shows a schematic diagram of this technique. It uses pulsed laser radiation to vaporize materials and to deposit thin films in a vacuum chamber. However, the beam-solid interaction that leads to evaporation/ablation is a very complex physical phenomenon. The theoretical description of the mechanism is multidisciplinary and combines equilibrium and nonequilibrium processes. The impact of a laser beam on the surface of a solid material, electromagnetic energy is converted first into electronic excitation and then into thermal, chemical, and even mechanical energy to cause evaporation, ablation, excitation, and plasma formation.

The discovery of high transition temperature, Tc, superconductivity in copperoxide-based ceramics by Bednörz and Miiller, and the subsequent increase in Tc above the boiling point of liquid nitrogen (77 K), renewed interest in employing superconducting thin films in high-speed, low-power electronic device applications (e.g., compact high-quality factor filters, delay lines, and Josephson elements for high-speed, low-power switching). However, realization of these benefits requires well-controlled, reliable superconducting thin film technology which addresses not only the growth of superconducting thin films, but also the development of a multilayer device technology encompassing materials with metallic, semiconducting, and insulating properties.

The successful use of pulsed laser deposition (PLD) to fabricate thin film superconductors has generated interest in using the technique to deposit thin films of other materials. The compositional fidelity between laser target and deposited film and the ability to deposit films in reactive gas environments make the PLD process particularly well suited to the deposition of complex multicomponent materials. Cheung and Sankur recently provided an excellent review of the PLD field, including a table of over 100 elements, inorganic and organic compounds, and superlattices that have been laser evaporated. Over 75 of these materials were deposited as thin films.

The goal of this article is to provide an introduction to some of the newer applications of PLD for thin film fabrication. Four classes of materials are highlighted: ferroelectrics, bioceramics, ferrites, and tribological materials. Ferroelectric materials are structurally related to the high-temperature superconducting oxides and therefore are a direct extension of the recent superconducting oxide work. Bioceramics are dissimilar in structure and application to both ferroelectrics and superconducting oxides, but they are complex multicomponent oxides and, therefore, benefit from the use of PLD. Ferrites, also complex, multicomponent oxides, represent another exciting, but only lightly explored opportunity for PLD. In contrast, tribological materials are typically neither complex nor multicomponent. Nevertheless, interesting structures and properties have been produced by PLD. A few of the more important ones will be discussed. These different types of materials demonstrate the diversity of capabilities offered by PLD.

Despite the discovery of the laser a few decades ago, the potential for pulsed laser deposition (PLD) of thin films has remained unexploited. Despite the sustained pioneering work at Rockwell in laser deposition, it took the development of high-temperature superconductors to fully realize the technique's potential. Early work on PLD of high-temperature superconductors demonstrated for the first time that the composition of rather complex multi-elementary materials can be reproduced in the deposited film under appropriate conditions of laser energy density and deposition angle. These features made PLD unique; and once the recipe for making in-situ crystalline films of proper stoichiometry was known, the technique's popularity was significantly enhanced in the research community.

The features of laser deposition that make the process so unique, and that are discussed throughout this issue, are recapped below:

1. Rather complex multi-elementary materials can be deposited well if a single-phase, homogeneous target can be fabricated. The complexity of the deposition process is translated to the relatively easier process of fabricating a high-quality target.

2. The chamber pressure, target-substrate distance, target orientation with respect to the laser beam, etc. are significantly de-coupled, enabling significant freedom in deposition system design. The target is decoupled from the substrate in the sense that a small target can be used to deposit film over a fairly large area substrate with the appropriate scanning schemes.

3. The efficiency of the target use is superior compared to any other technique since a predominant amount of the evaporated material is forward directed and can be collected with a high degree of efficiency. For example, in a production environment, more than 100 YBCO films (ranging 3,000-4,000 Å thick) on 1 × 1 cm2 substrates have been fabricated from a 0.25-inch-thick one-inch target with a majority of the target still left over. The cost of raw materials in a production environment may become significant, and for toxic elements particularly there is a further advantage in minimizing the spread of contaminants.