**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.

*Amorphous thin films* are essentially short-range order structures derived from deviations in the bond length and bond angle from a perfect crystalline lattice [17]. Overall, in most materials the growth of amorphous films take place at low substrate temperatures where the mobility of adatoms (adsorbed atoms) at the substrate surface is very limited. With a low temperature the adatoms approach to a thermal equilibrium with the substrate limiting the energy available for diffusion through its surface. Instead, these nearly immobile adatoms are incorporated almost at the point of strike with the substrate surface. High deposition rate is another parameter to induce amorphous growth because it prevents adatoms from migrating to more energetic sites to reach equilibrium due to the limited time for diffusion, and they are incorporated into the film structure almost at the point of strike with the substrate surface. Some developments have also reported the formation of this disordered microstructure by the incorporation of certain gases, i.e. oxygen, nitrogen, that inhibit the growth of crystallites during the deposition process [13]. Both PVD and CVD techniques are suitable for deposition of amorphous structures. Deposition of amorphous thin films are required in a number of applications, such as solar cells, transistors, optoelectronics, dielectric films, etc.

*Polycrystalline thin films* consist in a large number of nano/micro crystallites with different orientations separated by grain boundaries. The crystallite size is mainly determined by the deposition parameters, i.e. deposition temperature, and deposition rate. A higher deposition temperature than that applied to obtain amorphous structures may lead to the formation of polycrystalline thin films. When adatoms are not in thermal equilibrium with the substrate due to a high substrate temperature, they have enough energy to continue diffusing in the substrate surface until adhering to an existing island or giving rise to new islands. These islands do not become thermodynamically stable until their size reach a nucleation threshold. The stable islands continue growing until saturation, and then, a coalescence process

initiates between islands giving rise to the formation of a polycrystalline layer [4]. As adatom-diffusion is a temperature-dependent process, the crystallite size is expected to increase in line with the increase of substrate temperature. Another driver for the crystallite size is the film thickness. While the crystallite growth in the lateral direction is limited by the coalescence process, in the cross plane direction, the growth is limited by the film thickness. The typical microstructure of this material is of great importance for some applications where the scattering of carriers and phonons need to be controlled, i.e. thermoelectrics [18, 19].

*Epitaxial thin films* consist in a solid crystalline film deposited onto a substrate surface with a nearly perfect lattice structure, whose crystal orientation is aligned with the crystallographic orientation of the substrate surface. Depending on the nature of substrate employed, the epitaxial growth can be divided into homo and hetero-epitaxial, with the former referring to the growth of a film onto a substrate of the same material, and the latter, onto a substrate of different material. Various deposition techniques have been developed to enable the deposition of epitaxial films, including MBE and vapor-phase epitaxy; they are mostly used in the semiconductor industry where high quality and complex films, i.e. quantum wells, quantum wires, are required [20]. Achieving epitaxial film growth depends on a number of factors, such as equilibrium thermodynamics of nucleation, creation of vapor from reactants, substrate surface reactions, and mobility of species through the substrate surface. The complex interaction of these atomic processes may lead to the formation of epitaxial films only if they occur over a certain temperature called "epitaxial temperature", which depends on the specific system and deposition parameters. Epitaxial grow usually takes place at high substrate temperature to promote the mobility of adatoms in the substrate surface forming islands that become stable after they reach a certain size, and their continuous growth increases the nucleation density giving rise to a solid film with a preferential orientation. The quality of epitaxial growth is strongly influenced by the strain created in the film during deposition due to lattice misfit, and thermal strain produced as a result of the different coefficients of thermal expansion between the films and substrate. Substrate surface contamination also influences the growth by interrupting the formation of epitaxial layers. To prevent this, deposition is normally carried out under reduced pressures or vacuum to promote the effusion of impurities from the substrate.

### **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

#### *Thin Films/Properties and Applications DOI: http://dx.doi.org/10.5772/intechopen.95527*

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 nonnormal to the substrate are necessary conditions [4]. 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 [22], named structure zone model (SZM). This model provides three structural zones depending on a homologous temperature (*T*h) which results from the ratio between the substrate temperature (*T*s) and the melting point temperature of the deposited material. In the zone I for a *T*<sup>h</sup> ≤ 0.3, the relative low substrate temperature allows for a low diffusion of adatoms which are not enough to compensate the density of defects. As a result, amorphous or nano-crystalline columns with high density of defects between them are formed with a cauliflower-like appearance in the top surface, which is enhanced by the shadowing process. In the zone II for a 0.3 < *T*h < 0.5, with a higher substrate temperature, the diffusion of adatoms is higher and enough to compensate the defects, producing a columnar structure with a lower concentration of defects and larger crystallites. In the zone III where *T*h > 0.5, the higher substrate temperature produces a dominant bulk diffusion in the layer giving rise to a structure formed by coarse crystallites [23].

## **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 wellknown 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 [24]. 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 [25], where the introduction of Ge atoms into the Si matrix increases the stress, which collaterally contributes to enhance the thermoelectric properties [26]. 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 [27]. 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 [28].

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 [29]. The deformation mechanism model that may explain the strengthening of thin films is based on the dislocation motion.

**Figure 1.** *Schematic of stress/strain creation within thin films.*

#### *Thin Films/Properties and Applications DOI: http://dx.doi.org/10.5772/intechopen.95527*

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 freestanding ones [30]. 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 [32]. 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 [33]. 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 *d1* and refractive index *n1* is deposited onto a substrate of similar/different material with a refractive index *n2*. When a beam of light strikes the interfaces formed by the incident medium and the film, and the film-substrate interface, multiple reflections and transmissions occur at both interfaces at specific angles whose total amplitudes are computed from the sum of individual reflections directed back into the incident medium, and the individual refractions traversing into the substrate, respectively. The main difference between nonabsorbing and absorbing layers is that the refractive index from the nonabsorbing layer becomes more complex by integrating a quantity *k* named *extinction coefficient* in the absorbing layer, which defines the absorption of energy within the film. Thus, it can be inferred that the optical coefficients strongly depend on the refractive indexes of each medium, the extinction coefficient, and the film 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 [4]. 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.
