Since its discovery in early times, thin films rapidly found industrial applications such as in decorative and optics purposes. With the evolution of thin film technology, supported by the development of vacuum technology and electric power facilities, the range of applications has increased at a level that nowadays almost every industrial sector make use of them to provide specific physical and chemical properties to the surface of bulk materials. The possibility to tailor the film properties through the variation of the microstructure via the deposition parameters adopted in a specific deposition technique has permitted their entrance from the simplest like protective coatings against wear and corrosion to the most technological advanced applications such as microelectronics and biomedicine, recently. In spite of such impressive progress, the connection among all steps of the thin film production, namely deposition parameters-morphology and properties, is not fully accurate. Among other reasons, the lack of characterization techniques suitable for probing films with thickness less than a single atomic layer, along with a lack of understanding of the physics have impeded the elaboration of sophisticated models for a precise prediction of film properties. Furthermore, there remain some difficulties related to the large scale production and a relative high cost for the deposition of advanced structures, i.e. quantum wells and wires. Once these barriers are overcome, thin film technology will become more competitive for advanced technological applications.
- thin films
- deposition techniques
- inorganic and organic
- structure and morphology
This chapter covers the fundamentals of the thin film technology, including deposition techniques, structure and morphology, film properties, characterization techniques and applications. Due to the exceptional diversity of thin films and the large range of applications it is complicated to cover in detail all the areas, however, as many thin films share a number of features in all steps of the production process, characterization, and even applications, it is possible to treat them with a general perspective.
The most widely employed deposition techniques, namely physical vapor deposition (PVD), and chemical vapor deposition (CVD), for thin film production are described with emphasis in the principle, advantages, disadvantages and examples of the common thin film structures produced. Furthermore, the various types of microstructures and the morphological features as a result of the deposition technique and parameters are reported. The growth mechanism is described considering the conditions for the formation of amorphous, polycrystalline or epitaxial structure, and the resulting morphology is explained based on the temperature-dependence zone models. Subsequently, the mechanical, electrical and optical properties are reviewed, stressing the connection with morphological features such as film thickness, material phase, defects, roughness, grain boundaries etc. The principal characterization techniques used for the characterization of chemical composition, morphology, stress, and electrical conductivity are described along with their principle. In addition, the specific information that can be accessed through the data collection is stated. The most technological relevant areas of application are then outlined along with the type of films mostly employed and. Finally, the main challenges for the thin film technology to progress in various areas are discussed.
2. Deposition methods
Thin-films are in general developed to provide special properties, i.e. electrical, optical, mechanical, chemical, that satisfy the needs for specific applications. The desired properties are determined by the resulting film structure, which strongly dependents on the selected deposition method, film material, and substrate. In line with the wide range of applications of thin films, a number of deposition methods have been developed/improved to optimize the film properties, of which, the most commonly employed are described in this section. Broadly speaking, thin-film production can be realized based on two technological groups, namely physical and chemical deposition methods.
2.1 Physical deposition methods (PVD)
Physical deposition methods are usually referred as to physical vapor deposition methods (PVD) because the process entails the generation of vapor. PVD essentially consists in removing growth species from a source or target material via evaporation, then this vapor is transported to the substrate surface, and eventually it solidifies in the surface, forming the film. The evaporation is generally carried out under a reduced pressure chamber to avoid impurities in the film formation which are produced due to collisions between vapor particles and residual gas particles in their displacement from the source to the substrate surface. PVD techniques are known to offer a number of advantages, including the deposition of almost any material, high reproducibility of film properties, the use of a large range of substrate materials, the possibility of tailoring the film properties through modification of deposition parameters in single element deposition, and obtaining films with high purity. On the other side, among the main disadvantages are the use of sophisticated and costly monitoring systems for the control of the deposition rate and film thickness, and the mismatch between the composition of the deposited film and the composition of the evaporant in the case of alloys and compounds.
PVD techniques can be classified according to the method employed in the generation of vapor. The most common PVD techniques are vacuum-based evaporation and its heating versions, sputtering, laser ablation, cluster beam and ion pattering, of which, only the most important will be described in detail in the present section. For further detail on the versions of PVD techniques, the reader is referred to [1, 2, 3].
2.1.1 Vacuum evaporation
It is among the most popular PVD techniques due to its simplicity in operation and high deposition rate. This technique uses heating sources to evaporate the deposition material onto the substrate surface where it condenses forming a thin film, all within a vacuum chamber. This technique is suitable for deposition of elements or compounds at temperatures below 2000 K . According to the method used to evaporate the target material, this technique can be subdivided into resistance-heated evaporation, and electron-beam evaporation.
One of the most important features of this technique is its ability to accurately control the composition and doping profile in the growth direction. This can be realized thanks to the UHV that enables the film growth to occur in the molecular regime. In this regime the atoms and molecules do not collide in their way to the substrate because the mean free path is larger than the distance from the crucible to the substrate. This feature allows to modify as desired the composition of the feeding phase, blocking abruptly one or more streams of atoms being either a constituent atom or a dopant element with the help of mechanical shutters. Regarding that this blocking process can be carried out in very short times, and the low deposition rate from MBE, it is possible to achieve extremely thin thicknesses between the layers of different composition and/or doping. Advanced nanostructures, such as quantum-wells, superlattices and quantum-dots have been successfully fabricated with this deposition technique [5, 6, 7].
Sputtering essentially consists in the bombardment of the target material with energetic particles to dislodge atoms from its surface which travel through the plasma to eventually condensate onto the substrate. Three sputtering techniques are the most employed for thin film growth, including DC diode, RF-diode, and magnetron diode.
2.1.3 Laser ablation
Laser ablation is based on a similar configuration as the previous techniques, i.e. usually an evacuated chamber with a target material to be evaporated, a substrate placed parallel to the target where the film condensates, and in this case, an additional high-power pulsed laser placed outside the deposition chamber which emits the energy for inducing the ablation of the target material . This technique is broadly employed in the deposition of alloys, compounds, polymers, semiconductors, and multilayers due to its excellent stoichiometry transfer from the target to the film. Oxide thin films can also be deposited if oxygen is introduced in the chamber as a background gas. The irradiation from the laser power, i.e. KrF (248 nm), is focused on the target producing a rapid local heating until reaching the melting point, and eventually producing evaporation that will be deposited in the substrate. The laser ablation process and the quality of the sample are affected by a number of parameters, including the characteristics of the target material, deposition conditions, laser parameters, substrate temperature. Although this technique is widely recognized for its diverse and fast applicability, the actual ablation process is yet to be fully understood because the material ejection is not produced solely by a thermal process but also a photochemical is likely present.
2.2 Chemical deposition methods (CVD)
CVD is a deposition method where a volatile compound of a pre-established substance is introduced into a reactor, usually along with an inert gas, to induce a chemical reaction which produces a solid thin film onto a substrate at an elevated temperature. In this technique, unlike PVD, the reaction does not have to be produced under vacuum conditions. Due to its versatility to work with a broad range of reactants and precursors, this technique enables the deposition of a variety of structures, including metal alloys and compound semiconductors with an excellent control of purity and doping (stoichiometric film) . Compared to PVD, this technique offers higher deposition rates, better conformance in rough substrates, easy deposition onto complex surfaces, and high throughput. However, some disadvantages, such as the use of high substrate temperatures, and the toxicity and flammability of the reactive gases have prevented it from being used in low-scaled developments, but is well justified in applications where high-throughput is required, i.e. semiconductor industry. The CVD processes can be classified based on the type of source employed to initiate the chemical reaction, the range of pressure under which the deposition is carried out, and the type of reactant used. The most established CVD method are described below.
3. Thin films structure
Different and complex chemical reaction occur during the deposition process depending on the technique employed and deposition parameters, such as substrate temperature, deposition rate, pressure, and alignment of vapor stream with substrate, which give rise to a variety of microstructures. The resulting microstructure in turn defines the physical and chemical properties of the film, which means that an appropriate management of these variables is essential to obtain tailored film properties. The resulting microstructure can be either amorphous, polycrystalline or epitaxial, which are briefly described below.
4. Thin film morphology
Establishing the correlation between deposition parameters with the resulting morphology of deposited thin films is very complex because of the interaction of a number of factors, which influence the nucleation and growth phases. However, despite the wide spectrum for variability, it is possible to find some typical morphological features that are common in a broad range of thin films.
Probably one of the most common morphological features found in many thin films grown by vapor-based techniques is a columnar structure whose growing direction is not necessarily perpendicular to the substrate. The way how the adsorbed atoms are integrated into the growing process determine the final morphology of thin films. When the atoms strike the substrate surface they may contribute to the creation of islands. These islands increase in size due to the feeding of atoms coming from the diffusion in the substrate surface and also from the vapor flux, forming complex arrangements of islands or compact islands. The flux of atoms diffusing in the substrate is suppressed once the islands start an interfering process between each other due to the progressive growth, and only the contribution from the vapor flux remains active shifting the growth to the thickness direction. The accumulating growth in the thickness direction gives rise to a columnar structure with an appearance of cauliflower-like in the top view [4, 17, 21].
It was reported that, for the formation of a columnar structure with a rough surface, both a limited surface mobility of adatoms and a vapor flux arriving non-normal to the substrate are necessary conditions . The former tends to control de size of the crystallites growing at the interface and defines the area at the base of the columns. The latter gives rise to a geometrical shadowing growth process. The atoms from the source of various deposition methods, i.e. CVD, sputtering, not always strike the substrate surface perpendicularly. This deviation from a normal incidence forces the development of a columnar growth in a direction towards the vapor flux. Overall, it has been observed that the angle between the substrate normal and the column direction falls in between the angle formed by the substrate normal and the vapor flux.
The evolution of the morphology in thin films as a function of substrate temperature (adatom mobility) can be classified according to a model introduced by Movchan and Demchishin in 1969 , named structure zone model (SZM). This model provides three structural zones depending on a homologous temperature (
5. Properties of thin films
In this section a brief description of the mechanical, electrical, and optical properties in connection with the morphological features will be provided.
5.1 Mechanical properties
Thin films, due to their versatility to provide tailored properties, have found application in a number of sectors going from simple coatings for wear and corrosion protection, to more advanced applications such as antireflective coatings, microelectronics, photovoltaics, etc. Although these material structures have been selected due to exclusively their functional properties, they must be able to provide a reliable service operation with a proper mechanical and chemical resistance during the lifetime. These films, during deposition and operation, are prone to develop large stresses that might cause deformation and eventually mechanical failure, and therefore, it is essential to understand the microstructural processes involved in such effects to attempt to mitigate through the control of microstructure during the fabrication. Unlike bulk structures where the mechanical properties do not show a clear dependence on the sample size, in reduced structures like these, these properties are strongly affected by the resulting microstructure giving rise to a different behavior as compared to their bulk counterparts.
Let us assume a thin film deposited on relative tick substrate as illustrated in Figure 1. If strain by any means were produced on the film, it would change its dimensions relative to the substrate where it is deposited to maintain the equilibrium. If, hypothetically, the film were not adhered to the substrate, it would be visible the change in dimensions, for example, when the strain has expanded the original dimensions of the film, as shown in part b of Figure 1. The action of matching again the expanded film into the substrate entails the application of a deformation to force the film to adopt the substrate dimensions as shown in part c, giving rise to the generation of stress within the film. The stored stress naturally tends to be released to reach equilibrium, but depending on the degree of the substrate stiffness, part of this strain can be absorbed if the substrate is compliant which is reflected in a bending produced in the film/substrate system, or can remain entirely in the film if the substrate has a high stiffness.
The means by which strain can be produced in the films during deposition basically derive from the deposition method employed and from thermally-induced effects. Various vapor-based techniques such as sputtering and PECVD are well-known for producing stress due to the incorporation of gas into the microstructure or due to ion bombardment, whose degree can be controlled through a strategic balance of deposition parameters . Impurity incorporation into the microstructure of a host material has also been ascribed as responsible for stress creation. Depending on their size, the introduction of dopant atoms cause a local deformation of the lattice producing stress. A notable example of this case is the fabrication of SiGe thin films , where the introduction of Ge atoms into the Si matrix increases the stress, which collaterally contributes to enhance the thermoelectric properties . Thin film deposition is commonly realized at relative high temperatures and cooled down latter on. This change of temperature along with a difference in the coefficients of thermal expansion between the film and substrate material generate stress in the film microstructure as well . For some thin films like hydrogenated microcrystalline silicon, widely used in the photovoltaic sector, large stress creation has been reported in the phase transition from amorphous to microcrystalline silicon .
The stress contained in the films affect the mechanical properties such as yield strength and hardness. The yield strength of thin films is reported to be higher than the bulk version due to the influence of the microstructure. The value of this property is reported to increase with a smaller grain size, and with a higher density of dislocations present in the microstructure . The deformation mechanism model that may explain the strengthening of thin films is based on the dislocation motion. A number of dislocations present in the microstructure move as a function of the stress applied to the film, and for a dislocation to move, the stress applied must be comparable or higher than the energy necessary to deposit a misfit dislocation. However, it is important to consider that dislocation motion can be constrained by the interaction with other microstructural defects such as extended defect, point defects, and other dislocations which contribute to film strengthening. When the film/substrate system is considered instead of free-standing films, the constraints to the mobility of dislocations provided by the substrate and any oxide present in the substrate surface have to be considered to determine the strength. These additional constraints have demonstrated to strengthen the film in comparison to free-standing ones . These microstructural features also explain the larger hardness exhibited in thin films in comparison to bulk materials.
5.2 Electrical properties
Electrical properties within thin films comprise a broad field if one considers the different resulting microstructure, whether they are metallic films, semiconductors or insulators films, and the type of substrate on which they are deposited. However, much of these films possess some common morphological features that derive in similar transport mechanisms that allow to treat the conductivity in a global perspective.
One of the main factors for the deviation of conductivity in thin films with respect to bulk material is the size effect. The electron mean free path of the bulk material reduces as the material thickness reduces due to the activation of additional scattering mechanisms. In a simplified approximation, dictated by the direct proportionality of conductivity with the electron mean free path in bulk materials, the conductivity undergoes a reduction. This correlation works for either epitaxial, polycrystalline or amorphous structures as the maximum crystallite size is limited by the film thickness, preserving the size effects, considering that all the other constituent components of the conductivity remain unchanged [4, 31]. However, as described in previous sections, the microstructure might contain a large number of structural defects and grain boundaries which can act as scatters for charge carriers, further reducing the conductivity.
In one of the most complex microstructures where the film is a semiconductor formed by small crystallites embedded in amorphous tissue and surrounded by a large number of grain boundaries, and containing either electrons or holes as majority carries, the transport mechanism becomes very complicated owing to the simultaneous interaction of various scattering mechanisms. In this type of microstructure, a larger crystalline volume fraction favors the conductivity by allowing a higher carrier mobility unlike the amorphous phase. The grain boundaries are considered as a disordered region were mobile carriers are scattered in their travel between crystallites. As an example, the electrical conductivity of polycrystalline and nano-crystalline materials is substantially lower than the bulk single crystalline counterpart attributed to a reduced carrier mobility in spite of having a similar carrier concentration . Microstructural defects such as voids, dangling bonds, and localized defects, which are found mostly in the grain boundaries, also contribute to further reduction of conductivity. These defects are known to trap mobile carriers of doped semiconductors, forming a potential energy barrier which limits the motion of charge carriers between crystallites . Accordingly, electrical conductivity is reduced by the decrease in the number of free carriers available for conduction and a reduction of the carrier mobility. Additional scattering mechanisms appear in doped semiconductors such as ionized impurity scattering, carrier-carrier scattering at room temperature, and carrier-phonon scattering at high temperatures which further contribute to the reduction of electrical conductivity.
5.3 Optical properties
The optical behavior of thin films is determined by the resulting microstructure that depends on the deposition parameters. Before explaining the relationship of the optical properties with microstructural features, it is necessary to define the optical coefficients. The optical response of thin films can be characterized based on the reflection and transmission coefficients. In a general arrangement a thin film with a thickness
Due to the consistent results reported in literature, it is possible to generalize the behavior of optical properties with film thickness even though specific details related to film deposition are not provided. Both refractive index and extinction coefficients show a strong dependence on the film thickness. For most metallic films, i.e. Au, Ag, while the former decreases from a value higher than that corresponding to a bulk material as the film thickness increases, the extinction coefficient tends to increase, from a very low value, approaching that of the bulk . These optical coefficients also behave differently depending on the wavelength of the incident beam because they become dominated either for intra-band transitions in the visible, or free electrons in the infrared region. The electronic contribution to the optical behavior explains the effect of film thickness on the optical response. As the thickness is reduced the surface scattering mechanism increases, reducing in turn the electronic mean free path and the scattering time, which limits the contribution to the optical conductivity. The reduction of these both parameters produces an increase of electrical resistivity, and thus, a connection between optical properties and resistivity can be established. This correlation implies that structural defects such as voids, unsaturated bonds, point defects, extended defects, band tail states, and structural features like grain boundaries and oxide, all of which contribute to increase the carrier scattering, influence the optical properties.
The behavior of reflection and transmission is also a function of the degree of roughness in the film created during deposition [34, 35]. Essentially the roughness degrades the film uniformity producing a thickness variation across the film which affect the transmission and optical coefficients.
6. Characterization techniques
In order to understand the connection between deposition parameters, the resulting morphology and the physical properties of thin films, it is necessary to characterize each of these features. The reduced volume of thin films, however, does not allow to employ universally all the characterization techniques developed for bulk materials. Accordingly, in this section the most common characterization techniques suitable for thin film measurements are briefly described.
6.1 Mechanical characterization
The mechanical characterization comprises the determination of residual stress commonly created during deposition, and the physical properties such as Young’s modulus and hardness.
The well-known Stoney Equation  is widely used to characterize the stress generated in a film deposited onto a thicker substrate. The principle behind this equation is the linear correlation between the stress crated within the film, and the amount of bending produced in the substrate as a result of the constraint condition among them. The application of this technique requires the knowledge of the radius of curvature of the substrate before and after the film deposition. The radius of curvature is often measure with an optical profilometer or drawing on any high resolution microscopy method that allows to reproduce the sample profile. The accuracy of this method is subjected to the compliance of certain conditions, i.e. the film thickness is much thinner than the substrate, very small strain and rotations, both film and substrate should be homogeneous and isotropic, equi-biaxial stress in the plane of the film, spherical deformation of the system film/substrate, and spatial invariability of stress and curvature in the whole surface. Even though some of these conditions are not fully in compliance, this technique has still been used for stress determination in a number of investigations including semiconductor-based thin films in the solar sector, thin film transistor industry among others [37, 38].
Physical properties such as hardness and Young’s modulus are usually characterized drawing on the nano-indentation technique [29, 39]. This method consists in moving a sharp indenter towards the surface of the film until making an effective contact, then the applied load and the displacement are repeatedly recorded. In this way a correlation of the applied load through the indenter and the depth of indentation is established. As the indenter penetrates into the film, the slope of the loading curve progressively increases due to stronger contact between them; this correlation allows for the calculation of the hardness at any point of the curve by dividing the load to the contact area in that point. The Young’s modulus of the film instead can be determined from the unloading curve once the contact area is defined considering that the Young’s modulus and Poisson’s ratio of the indenter are known. This is possible thanks to the direct correlation that is established between the backward motion of the indenter and the elastic properties of the film during the unloading.
6.2 Chemical composition
A number of techniques are available to obtain the elemental composition of thin films, which can be classified in two groups, ion scattering and spectroscopic-based techniques. In the first group both Rutherford backscattering spectrometry (RBS) and elastic recoil detection analysis (ERDA) are included. They both are based on the elastic scattering of energetic ions produced when they strike the atoms in the film surface; in this process they transfer an amount of energy to the target species atoms via collisions generating backscattered, forward scattered, and recoils particles. While in RBS the backscattered yield and its energy distribution are measured by a detector, in ERDA the corresponding quantities for the coiled particles are recorded allowing the formation of an energy spectrum from which the compositional depth profile can be extracted. These both techniques can be used as complement of each other as the RBS is suitable for detection of heavy elements while ERDA provides a higher accuracy for light elements. The main disadvantage of RBS is a shallow depth into the film that can be probed while in ERDS complications to distinguish elements with similar masses arise.
In the spectroscopic-based techniques, the X-ray photoelectron spectroscopy (XPS) is one of the most widely employed for elemental composition characterization. The XPS is based on photoelectric effect through which electrons localized either in the core or valence band are emitted when a beam of X-rays, with an energy higher than the binding energy of the electrons, is irradiated onto the film surface. Then, these ejected electrons are driven first to an analyzer to measure their kinetic energy, and subsequently they arrive to a detector where the number of electrons is counted considering their kinetic energies. The information of this kinetic energy along with the known photon energy allow to compute the electron binding energy. A spectra is then formed that correlates the electron count vs. the calculated binding energy. The binding energy constitutes a signature for the identification of each constituent element of the film since each element possesses a unique value. On the other side, the intensity of the spectra instead reflects the concentration of the element. This technique is suitable for probing the sample in a depth of a few nanometers as it is limited by the interactions of emitted electrons with the atoms present in the film.
6.3 Microstructure and morphology
A number of methods are available for the microstructure characterization in terms of crystalline volume fraction, crystallite size, and crystallographic orientation.
The distinctive microstructure and material phase of thin films as a result of deposition conditions can be extracted from Raman microscopy, and X-ray diffraction (XRD). The Raman technique  is a non-destructive method based on the scattering of incident photons, coming generally from a laser beam, caused by the vibrational modes of molecules or atoms present in the film. The scattering of photons can be either elastic, or inelastic; within the latter, photons can be scattered with a frequency lower than incident photons (stokes) or higher than incident photons (anti-stokes). A Raman spectrum is formed considering the intensity and the frequency of the scattered light (inelastic) corresponding to the specific vibrational mode of the molecule in question. The material phase fraction in volumetric terms can be extracted from the Raman line-shape. The different peaks observed in the Raman spectrum designates different material phases, which can be extracted by isolating the integrated intensity of the required peak. This isolation is usually carried out by a deconvolution of the Raman spectra using specialized software that allows to perform a peak fitting analysis. The average crystallite size can also be extracted from the Raman spectrum by means of a correlation length model which is suitable for sizes larger than 5 nm. However, certain considerations are necessary to avoid the superposition of simultaneous effects such as stress and local heating due to a high laser power.
The X-ray diffraction technique  is based on the scattering principle where a monochromatic beam of X-rays is directed onto a specimen which contains a set of lattice planes. The incident X-rays are scattered in different angles by the lattice planes according to the Bragg’s law, giving rise to constructive interference which build up the peaks of the diffraction pattern. The peak shape defines the material phase for the substance under study providing a broad peak for amorphous regions, and sharp peaks for crystalline ones. In this way, the fraction of crystallinity can be determined once the integrated intensities of each characteristic peak is obtained. The peak width also provides information about the crystallite size, which shows a broadening for small crystallites and shrinking for larger ones. The crystallite size is computed drawing on the well-known Scherrer’s formula. In the case of crystalline orientation it can be inferred from the relative change of the peak height.
The morphology of the film can be investigated via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM is one of the most widely used tools to characterize the morphology of thin films. This technique is based on the interplay of a beam of electrons with the sample surface. This interaction produces the emission of electrons from the sample with different energies due to either elastic, inelastic scattering or photons, which are collected by a detector to produce a distribution map based on the intensities of the signal. Each of the three emitted scattered electrons allows the reproduction of images which provide different information about the sample, i.e. images from secondary electrons (inelastic scattering) are suitable for the study of topographical features. From a top view this technique permits the visualization of the crystalline columns emerging at the sample surface, and the different material phases, either crystalline and/or amorphous, due to the contrast created by the secondary electrons, while the film thickness can be evaluated form a cross-sectional SEM analysis.
TEM consists in the emission of a beam of electrons which are directed onto a very thin sample (a few hundred nm) to allow the transmission of such electrons. The intensity with which these transmitted electrons exit the sample depends on the density and thickness of the sample; thus, these structure-dependent intensities give rise to the formation of a contrast that is projected as an image on a fluorescent screen. In essence, a compact structure produces a higher scattering of electrons projecting a darker image while a porous structure projects a brighter image in a bright field image dominated exclusively by transmitted electrons. TEM constitutes a powerful technique with a spatial resolution higher than SEM for the characterization of microstructure, crystallite size, crystalline orientation, and the film thickness can be obtained via a cross-sectional TEM image.
In some characterization cases two or more techniques are complementarily employed to extract thorough information from thin films. For example, due to the in-depth probing the Raman technique can provide the size of the small crystallites that form a columnar structure in the film while the SEM can be used to determine the diameter of the crystalline columns emerging at the sample surface. Furthermore, the selection of the appropriate characterization technique strongly depends on the characteristics of the film such as the thickness, whether it is a conductor, semiconductor or insulator, whether it is possible to achieve a high vacuum, among others.
6.4 Electrical properties
Perhaps the most widely used technique to measure the electrical resistivity is the four-point probe method. It consists of four metal tips linearly arranged keeping the same separation from each one where an electric current is applied in the outer two probes while the potential difference is measured in the inner two probes. From these measurements the sheet resistance (
An extra issue related to the measurement of carrier concentration and carrier mobility appears in semiconductor characterization. The Van der Paw technique  provides a solution to obtain these parameters. This method is based on two independent measurement, i.e. resistivity and Hall coefficient. The characteristic resistances are first obtained by the application of a current in two adjacent contacts while the potential difference is measured in the other two remaining contacts, all of which are located in the periphery of a sample with an arbitrary shape. Then these results are incorporated into an expression developed by Van der pauw where
The Hall effect consists in the creation of a voltage when an electric current (
The carrier Hall mobility (
7. Applications and challenges of thin films
Thin films technology has historically been used in a wide range applications going from decorative purposes in its early stage, evolving for optical purposes latter on, and an almost endless range of applications with the appearance of advanced deposition techniques, supported by the rapid development of vacuum technology and electrical power. Overall, thin films are used to enhance the properties of bulk materials by depositing a layer with the desired physical and chemical characteristics to improve their functionality. In the following section a brief description of the most technological relevant fields of application of thin films is presented.
The use of
The use of thin films has gained a considerable space in
The broad scope of thin film applications require of tailored physical, mechanical and chemical properties which are linked to the resulting structure and morphology, and they in turn depend on the deposition techniques and deposition parameters adopted. Accordingly, a number of challenges remain to be tackled for a complete understanding of the connection among the different phases involved in the fabrication of thin films. Overall, various versions of CVD and PVD deposition techniques present a still expensive final product, lack of reproducibility, inappropriate attachment of the film to the substrate, high deposition temperatures which prevent the use of cheaper substrates, and limited control over the final properties. Thus, deposition technology needs to evolve with a higher precision to control the microstructure, and with a higher deposition rate suitable for large area deposition to reduce the cost. Moreover, although nanostructured thin films are promising for cutting edge applications such as microelectronics, optics, photovoltaics, and biomedicine, some of them need to be transferred to specific substrates for an appropriate operation. Consequently, the now poor transferring technique has to progress to take fully advantage of thin film technology. Even though a number of characterization techniques are available for tracking almost every feature of thin films, many of them struggle when the film thickness approaches a few nanometers. For example Raman or X-ray diffraction characterization might degrade due to the inevitable contribution from the substrate to the acquired spectra. Sophisticated models to analyze the data are therefore needed to isolate the relevant information. Of primary importance for biomedical applications is the knowledge of protein adsorption in substrates for the appropriate selection of materials; however, the characterization tools for this purpose are still at their infant stage, and they are based on complicated models for data analysis. Therefore, more advanced and specific in vitro models can pave the way for a rapid identification of suitable thin films. In spite of the significant progress in deposition and characterization techniques, the prediction of film properties as a function on the microstructure is still very difficult. This occurs due to the complex transport properties derived from the multiple defects, grain boundaries, material phases, quantum confinement effects in very thin films, interface scattering, among others. Thus, advanced models that incorporate all of these structural variants are necessary to establish the appropriate connection between microstructure and film properties to progress in the thin film technology.
This chapter attempted to carry out the study of thin films maintaining a tight correlation among the deposition techniques, the resulting microstructure, and their physical properties. It was found that the development of deposition methods are driven to satisfy the needs for films with specific mechanical, physical and chemical properties. Overall, they have evolved to enable the fabrication of thin films with increasingly higher purity, based on a variety of depositing materials and substrates with high reproducibility. However, the cost of material processing of certain structures such as nanostructures, quantum well, quantum wires, and quantum dots is still a challenge for commercial applications. The selected deposition technique and deposition parameters greatly define the final thin film microstructure being either amorphous, polycrystalline or epitaxial. In particular, the deposition temperature and deposition rate strongly influence the adatom-diffusion at the substrate surface giving rise to the formation of islands whose sizes increase with feeding of atoms. The most common morphological feature in many films is a columnar structure formed due to the feeding of atoms from the vapour flux once the adatom-diffusion in the substrate surface has placated due to the interfering process between islands. The mechanical, electrical, and optical properties of thin films are determined by the final morphology. It is important to remark that these properties normally deviate from the values corresponding to their bulk counterparts. For example, the yield strength and hardness are higher in thin films due to the influence of crystallite size, and movement of dislocations usually found in thin films. Likewise, the electrical conductivity is also affected by additional scattering mechanisms appearing due to the reduced film thickness. There are several techniques available for thin film characterization, but their accuracy strongly depends on the depositing material, substrate material, and film thickness. In most cases, two or more techniques are complementary used to access the required information with high precision. Regardless of the progress in thin film technology, important challenges remain to be tackled, including the accurate prediction of film properties based on the final microstructure, more advanced characterization techniques in the biomedicine field, and sophisticated models for data analysis.
This chapter was produced with the financial support of SENESCYT-ECUADOR, with contract number 564-2012.
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