Excimer-laser assisted deposition of carbon and boron nitride-based high-temperature superconducting films

Abstract

In the past two decades, excimer laser ablation has been an extensively explored technique for the deposition of thin films. In particular, the discovery of high-temperature supraconductors and the desire to produce thin films from those ceramic multicomponent materials led in the late eighties to strongly enhanced efforts to understand the mechanisms involved and, moreover, to the application of the method to other materials [1, 2]. While in the beginning the interest in the deposition method was centred on its unique capability to preserve largely the stoichiometry of the target in the films, other unique characteristics of ablation, namely the development of a laser plasma with high degrees of ionization, the high kinetic energies of the ablated species and the two-dimensional film growth became more and more important. By deliberately using them, the method proved to be suited for the preparation of thin films of nearly all materials that are of interest for industrial applications and for which solid or even liquid targets with good absorption at the excimer laser wavelengths are available. The method was used also for the preparation of thin carbon [3, 4, 5, 6, 7, 8] and boron nitride [9, 10, 11] films, and it was shown that metastable superhard phases of those materials can be deposited at parameters superior to other methods. In this chapter, the deposition of such superhard coatings by using excimer laser ablation and the modification of their properties by excimer laser irradiation of growing and as-deposited films is presented and discussed. Due to their outstanding mechanical properties, superhard tetrahedral amorphous carbon (ta-C) and cubic boron nitride (c-BN) films are most interesting for use as wear resistant and, in combination with their optical, electrical and other properties, as multifunctional coatings. Both carbon and boron nitride may have a variety of different bonding configurations. The most important ones for the preparation of thin films are the two-dimensional sp2 bonds that are characteristic for graphite and hexagonal boron nitride (h-BN) and the three-dimensional sp3 bonds characteristic for diamond and cubic boron nitride. Besides the crystalline graphite and diamond there exist amorphous carbon phases with nearly any ratio of sp2 to sp3 bonds, the properties of those phases varying from graphite-like (a-C) to diamond-like(ta-C) with that ratio decreasing. In addition, a lot of hydrogen (up to 40%) may be present in carbon films deposited with methods using hydrocarbons as precursor (termed accordingly a-C:H and ta-C:H). Which bonding configuration prevails depends on the deposition conditions, namely on the kinetic energy of the film-forming species and the substrate temperature during deposition. In hydrogen-free carbon films high sp3 fractions of up to 85% can be obtained at kinetic energies above 30 eV if the substrate temperature during deposition does not exceed 90° C.Boron nitride with perfect stoichiometry exists only in crystalline form, which is due to the ionic part of bonding. In general, the hexagonal phase forms easily during film preparation, whereas the nucleation and growth of the cubic phase requires very special conditions. C-BN nucleation was so far observed to occur exclusively with sufficient energy supply to the growing films and on top of a specially oriented h-BN layer, which forms prior to nucleation at the same conditions as nucleation. Moreover, a minimum substrate temperature of some 160° C is necessary. In the majority of deposition methods, the boron species (atoms) are produced by evaporating or sputtering a target, having thus too low kinetic energies, and the necessary energy supply is usually realized by an ion bombardment using argon/nitrogen ion beams or plasmas with substrate bias. It has been found that the minimum ion energy is some 60 eV and the ion-to-atom (I/A) ratio at the substrate must exceed 1 depending, however, on ion energy itself. Even though the further c-BN growth after completed nucleation requires a somewhat reduced I/A ratio it is still so high that the growth rates are limited to some 10 nm/min. The only way to reduce the I/A ratio is to deliver both the boron and nitrogen components with sufficiently high kinetic energy. The particular significance of excimer laser ablation for the deposition of ta-C and c-BN films lies in the generation of species with high kinetic energies from a solid target. As will be shown in the following sections it provides a source of atoms and ions suited for the preparation of superhard coatings from those materials. © 2005 Springer-Verlag Berlin Heidelberg.