**2. Structure and crystal orientation of the AlN material**

In the non-excited state, the aluminum has three electrons in its valence layer, distributed as 3s2 3p1 , so it presents a sublevel s complete with two electrons and one p sublevel semi-complete with an electron and two p sublevels empty, as shown in **Figure 2a**. On the other hand, the nitrogen has five electrons in the valence layer distributed as 2s2 2p3 , where the s sublevel is filled with two electrons and the three sublevels px, py, and pz are semi-complete with one electron each, as shown in **Figure 1b**. Now, in the excited state, the aluminum sublevels 3s2 3p1 rise to four

*Development and Applications of Aluminum Nitride Thin Film Technology DOI: http://dx.doi.org/10.5772/intechopen.106288*

**Figure 2.**

*Hybridization of aluminum and nitrogen. (a) Valence layer in the non-excited state and hybrid sublevels of aluminum, 3sp3 , in the excited state. (b) Valence layer in the non-excited state and hybrid sublevels of nitrogen, 2sp3 , in the excited state.*

hybrids 3sp3 sublevels, in which three sublevels are semi-complete with one electron each, and the remaining sublevel 3sp3 is empty, as indicated in **Figure 2a**.

Also, the nitrogen hybridization rises sublevels 2s2 2p3 , which yields four hybrids 2sp3 sublevels; one is a 2sp3 sublevel full of two electrons, and three sublevels 2sp3 are semi-complete with one electron each. Therefore, in the bonds between the atoms of aluminum and nitrogen, as shown in **Figure 2b**, there are three covalent bonds between three semi-complete sublevels hybrids of one aluminum atom (atom 4) with three atoms of nitrogen (atoms 1, 2, and 3), forming a tetrahedron of a length l1 = 0.1885 nm and 110.5° angle. There is also one covalent bond between the empty hybrid sublevel of aluminum (atom 4) and the complete hybrid sublevel of nitrogen (atom 5), of ionic character, of length l2 = 0.1917 nm, and angle of 107.7° between l1 and l2. The nitrogen atom (atom 5) joins with three additional aluminum atoms (atoms 6, 7, and 8) through covalent bonds between semi-complete hybrid sublevels, forming another tetrahedron of length l1 and angle of 110.5°. The three aluminum atoms (atoms 6, 7, and 8) also bind

#### **Figure 3.**

*(a) Bonds between aluminum and nitrogen atoms forming a hexagonal wurtzite AlN unit cell with the following lattice parameters a = b*≅*0.3100 nm and c = 0.4980 nm. (b) Bonds between atoms of aluminum and nitrogen form a prism of triangular base.*

to other three nitrogen atoms (atoms 9, 10, and 11) through covalent bonds between the aluminum empty hybrid sublevel and the nitrogen complete hybrid sublevel, of ionic character, having a length of l2 and 107.7° angle between l1 and l2. Those 11 atoms of aluminum and nitrogen (atoms 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11) have the form of a prism of triangular base. This prismatic structure originates the wurtzite hexagonal AlN unit cell, as shown in **Figure 3a**, that presents the following lattice parameters a = b≅0.3100 nm and c = 0.4980 nm. The orientation of the crystal axes and of the planes for the hexagonal unit shown in **Figure 4a** are (100, 001), respectively.

The vibrational phonon modes of the AlN film, E1(TO), and A1(TO) active in the IR are associated with the respective covalent bonds l1 and l2 as indicated in **Figure 2b**. In addition, they absorb energy according to the crystal orientation of the AlN film and polarization of the electric field of the IR beam, so this feature can be used to identify the preferred orientation of the film. In the IR analysis, the electric field of the beam is polarized in parallel to surface of the thin film, that is, the incident electric field is perpendicular to the c axis. The ratio between the energy absorbed by the phonon modes E1(TO) and A1(TO) is defined by Eq. (1).

$$A\_f \frac{\text{Energy absorbed by A\_1} \text{(TO)}}{\text{Energy absorbed by E\_1} \text{(TO)}}.\tag{1}$$

If the ratio Af is much smaller than one, it indicates a high degree of AlN (001) crystalline orientation. On the other hand, Af greater than one indicates a degree of AlN (100) crystal orientation. Thus, it can be concluded that when the phonon mode E1(TO) absorbs more energy than mode A1(TO), the crystalline orientation degree tends to AlN (001), whereas if phonon mode A1(TO) absorbs very more energy that mode E1(TO) the preferred orientation tends strongly to AlN (100).

**Figure 5** shows, highlighted in red, the thin film growth units AlN (100) and AlN (001). The atoms and/or clusters of atoms, that reach the surface of the substrate and are adsorbed, move along the surface, colliding and combining, giving rise to nucleation,

#### **Figure 4.**

*(a) Hexagonal unit cell of the AlN (a = b*≠*c,* α *= β = 90°, and γ = 120°) with your crystal axis a1, a2, and a3 plus the identification of the planes (100) and (001). (b) Vibrational phonon modes of the thin film AlN, actives in the IR, E1(TO), and A1(TO).*

*Development and Applications of Aluminum Nitride Thin Film Technology DOI: http://dx.doi.org/10.5772/intechopen.106288*

**Figure 5.** *Aluminum Nitride hexagonal unit cell growth units highlighted in red: (a) AlN (100) and (b) AlN (001).*

which grows with the arrival of more species (atoms and/or clusters) forming larger nuclei (islands), that by coalescence end up forming a larger whole, until a continuous layer emerges, and this whole process repeats itself until a thin film is produced. A better explanation for the formation process of AlN (100)/Si (100) thin films, in which the c-axis is parallel to the substrate surface is: when the mean free path is much smaller than the target substrate distance, the collisions between Al and N species occur more often in the space between the target and the substrate, thus forming dimers many Al-N, which are deposited on the substrate and the AlN (100) preferential orientation is achieved [16]. When the species involved in the growth are atoms and ions, then during the growth of AlN (100), **Figure 4a**, there are four difficult depositions (atoms 5, 6, 7, and 8) and, in the case of AlN (001), **Figure 4b**, there are two difficult depositions (atoms 4 and 5); Consequently, this favors the growth of the AlN with the c-axis perpendicular to the surface of the substrate. If the species involved in the growth are dimers, Al-N, then in the formation of AlN (100), **Figure 4a**, we have two difficult depositions (dimers 5-7 and 8-6) and in the formation of AlN (001), **Figure 4b**, two difficult depositions (dimers 4-3 and 6-5) and also, since all bonds of the AlN (100) growth unit are difficult to break and in the growth AlN (001) unit only one ionic bond easy to break, the AlN film has a good chance of growing with the c-axis parallel to the surface of the substrate.

During the deposition of AlN films (reactive magnetron sputtering and RF source) the degree of crystal orientation of the AlN films, (100) and (001), is strongly influenced by the energy of the species involved in the process. The energy of species that reach the substrate can be controlled by adjusting some parameters during deposition (i.e., mean free path, temperature, pressure, target-substrate distance). In this scenario, the mean free path (L) and the collision probability (Q ) of the species are given by [17]:

B 2 Mean <sup>T</sup> <sup>L</sup> 2(4 r )P κ <sup>π</sup> <sup>=</sup> (2)

and

$$\mathbf{Q} = \mathbf{1} - \exp\left(-\mathbf{D}\_{\mathrm{TS}} / \mathrm{L}\right) \tag{3}$$

#### **Figure 6.**

*Deposition parameters: L versus P. The curves were implemented for and typical plasma temperature values, and rMean = 150 nm. Reproduced from [17] with permission.*

Where T is the absolute temperature, P is the plasma pressure, DTS is the targetsubstrate distance, and rMean is the mean radius of the constituent species of the plasma, and κB = 1.38 × 10−23 J/K is the Boltzmann constant. **Figure 6** shows the estimation of the three deposition parameters: temperature, working pressure, and mean free path. For these deposition parameters, **Figure 7** shows the high probability of obtaining crystalline and highly oriented AlN (100) thin films.

#### **Figure 7.**

*Probability of obtaining crystalline and highly oriented AlN (100) thin films. Curves implemented for glow discharge, different values of DTS, and rMean = 150 nm. Reproduced from [17] with permission.*

*Development and Applications of Aluminum Nitride Thin Film Technology DOI: http://dx.doi.org/10.5772/intechopen.106288*

#### **Figure 8.**

*The substitutional defect: layers of atoms in the formation of the wurtzite lattice of the AlN with the incorporation of some oxygen atoms in place of some nitrogen atoms.*

The species energy can be increased by increasing the RF power or the substrate temperature, or decreasing the working pressure, or the distance between the target and the substrate. The Al and N species arrive with too much energy on the substrate surface, favoring the degree of crystal orientation AlN (001), where the (001) plane is parallel to the substrate surface. Now, the orientation degree of AlN (100) with (100) plane parallel to the substrate surface is strongly favored when the energy of Al-N species (dimer) is smaller, in which case many collisions occur prior to deposition.
