Chapter 1
Pulsed Laser Deposition for Complex Oxide Thin Film and Nanostructure

Chunrui Ma and Chonglin Chen

1.1 Introduction

Complex oxide thin films and nanostructures are at the heart of new "oxide electronic" applications, such as ultraviolet light-emitting diodes [1-3], resistive switching memories [4, 5], chemical sensor [6, 7], and so on. They are often grown by pulsed laser deposition (PLD) because the technique is believed to be material agnostic. PLD is a thin film deposition technique - a type of physical vapor deposition. A high-power pulsed laser beam is focused on and strikes a target of the material that is to be deposited in a vacuum chamber. This material is vaporized from the target in a plasma plume and deposited as a thin film on a substrate. This process can occur in ultrahigh vacuum or in the presence of a background gas, such as oxygen, which is commonly used when depositing complex oxides.

The synthesis of novel thin films and structures is advancing on two fronts: one is the complexity of materials being deposited; and the other is the reduction in the typical dimensions of the features. As a rule of thumb, any structure that has one or more dimension smaller than about 100 nm is considered to be a nanostructured material. PLD is one of the most promising techniques for the formation of complex oxide heterostructures and nanostructures. The basic setup of PLD is simple relative to many other deposition techniques, and it can stoichiometrically transfer a material from a solid source to a substrate to form its thin film. The first use of PLD to deposit the films of semiconductors and dielectrics by ruby laser is reported in the literature as early as 1965 [8]. PLD for the film growth of SrTiO3 and BaTiO3 was achieved in 1969 [9]. Six years later, stoichiometric intermetallic materials (Ni3Mn and ReBe22) were fabricated by using PLD [10]. In 1987, PLD had a real breakthrough in its successful application to the in situ growth of epitaxial high-temperature superconductor films at Bell Communications Research [11]. Since then, PLD has been used extensively in the growth of high-temperature cuprates and numerous other complex oxides, including materials that cannot be obtained by an equilibrium route [12-16].

This chapter details the PLD setup and focuses primarily on the operating principle, growth mechanism, and parameters of PLD for complex oxide thin film and nanostructure.

1.2 Pulsed Laser Deposition System Setup

The technique of PLD is conceptually simple, as illustrated schematically in Figure 1.1. The system consists of a laser, a vacuum chamber equipped with pumps, a target holder and rotator, and a substrate heater and is typically equipped with various pressure gauges, controllers, and other instruments to control the deposition environment of the system [17]. Film growth can be carried out in reactive environments, such as that for oxides where a partial pressure of oxygen, ozone, or atomic oxygen is carefully controlled. The substrate heater controls the substrate temperature. PLD systems are also often equipped with a set of optics including apertures, attenuators, mirrors, and lenses to focus and direct the laser beam into the target with the right energy density. The bulk material target orients at an angle of 45° toward the incident laser beam. The laser beam is focused onto the target surface by a set of optical components. The target locally absorbs the laser pulse energy and ejects a small amount of target material in the form of a plume containing many energetic species including atoms, molecules, electrons, ions, clusters, particulates, and molten globules. The plasma is then deposited onto a substrate facing the target with a separation distance of 3-5 cm. The substrate temperature can be varied from room temperature to 1000 °C, even higher than 1000 °C, depending on the heater type. The film microstructure depends on various parameters such as substrate temperature; laser energy density and pulse repetition rate; pressure and type of gas inside the chamber; and substrate-to-target distance.

Figure 1.1 Schematic diagram of typical pulsed laser deposition.

1.3 Advantages and Disadvantages of Pulsed Laser Deposition

PLD exhibits many fascinating properties and practical advantages. Firstly, it has the ability to faithfully keep the stoichiometry of the target material, which is the first aspect that draws the attention of the thin film growth community [18]. Secondly, the energy source for material transport (i.e., the laser) is outside the chamber, minimizing any impurities caused by in-vacuum power components; it is very flexible, cost effective, and fast. Many different materials can be ablated by using the same apparatus, and the different laser wavelengths are available in principle. The isolated local heating by the laser spot means that several different materials can be sequentially ablated in a single vacuum chamber by using a carousel system or a segmented target rod to fabricate heterostructures with little of cross-contamination of the source target material. This avoids the interconnected vacuum transfer and is an important advantage in research environment: one laser can serve many vacuum systems in order to save the laser cost, and high-quality samples can be grown in 10 or 20 min. Finally, it is easy to control film thickness and multilayer film by controlling the pulse repetition rate, growth time, and the use of multiple target holders; it demands a much lower substrate temperature than other film deposition techniques because the high kinetic energy (10-100 eV) of species in the ablation plume promotes surface mobility during film growth.

In spite of the above-mentioned advantages of PLD, there are some drawbacks in using the PLD technique. One of the major problems is limited uniformity because the plasma plume ejected from the target can only provide a narrow forward angular distribution. Another problem is high defect or particulate concentration due to surface boiling. The size of particulates may be as large as a few micrometers, which will greatly affect the growth of the subsequent layers as well as the electrical properties of the films. Therefore, these features limit the large-scale film growth. New techniques, such as rotating both target and substrate and using a shadow mask to block the particulates in order to fabricate a large and uniform film, have been developed to improve the film quality.

1.4 The Thermodynamics and Kinetics of Pulsed Laser Deposition

PLD is a nonequilibrium growth technique owing to the high electronic excitation, degree of ionization, and kinetic energies of flux. There are many distinct stages to film growth: [19] the ablation process of the target material by the laser irradiation; the creation of a plasma plume with high energetic ions; and the crystalline growth of the film itself on the heated substrate. In this section, we will thoroughly describe these processes.

1.4.1 Laser-Material Interactions

After the laser pulse is extinguished, a very hot cloud of vaporized material, typically of 104 K or more, has been generated, which is commonly referred to as the ablation plasma or plume. This process is called laser ablation. The mechanisms depend on the laser characteristics as well as on the optical, topological, and thermodynamic properties of the target material. Absorption in a material is defined as

where 1/a is the absorption length, which is approximately 100 nm for many oxide materials at laser wavelengths commonly used in PLD (<400 nm). In this process, electrons in the target are excited and thermalized within several picoseconds or nanoseconds depending on the energy density, duration, wavelength, and shape of the laser pulse as well as on the material properties (reflectivity, absorption coefficient, heat capacity, thermal conductivity, density, etc.). The next step includes surface melting of the target and conduction of heat into the target. The thermal diffusion length is described as

where is the thermal diffusivity, is the thermal conductivity, is the mass density, c is the specific heat, and is the pulse duration. During this process, the temperature rises in the surface of the target. The heating rates as high as 1011 K/s and instantaneous gas pressures of 10-500 atm are observed at the target surface. Then, the target material will vaporize. During this step, there is multiphoton ionization of the gaseous phase creating the characteristic plasma and the temperature at the surface of the target will exceed the boiling point. The final step of the process is the plasma excitation during which further ionization occurs and free electrons are excited, resulting in Bremsstrahlung absorption in which the hot pulse, at nearly 2000 K, expands in a directed manner.

The ablation threshold of materials, or the minimum energy density required in a material to create a plume, will be discussed. In most oxides, the thermal diffusion length is much longer than the absorption length, especially for UV lasers, because of the fact that oxide materials are often opaque and good thermal conductors. An affected volume is related to the spot size times ; thus, a simple estimation of the minimum energy needed to raise this volume to the sublimation point is

where the total energy required (from left to right) is the sum of the energy needed to bring the target material to the melting temperature plus the heat of melting, plus the...