## 3 Results and Discussion  
### Transport Properties along the Layers  
Temperature dependences of the conductivity \(\sigma_{\perp C}\) and Hall mobility of electrons \(\mu_{\perp C}\) along the layers, as well as those of the Hall coefficient \(R_{\mathrm{H}}\) for the pristine, vacuum annealed, and RbNO\({}_{3}\)-intercalated InSe crystals are shown in Figures 1-3. Numerical values of the parameters at 80 and 300 K are listed in Table 1. It is established that for the pristine sample, the Hall coefficient decreases approximately twice with increasing temperature from 80 to 300 K. Such dependence is caused by a slight variation of the electron concentration--a shallow donor level (18.5 meV) is preferably ionized already at 80 K, while the deeper donors still do not manifest. It is generally accepted that in undoped InSe crystals, the shallow donor is related to interstitial indium atoms In\({}_{\mathrm{i}}\)[28]. A decrease in the mobility component with increasing temperature within the investigated range is caused by the interaction of electrons with optical phonons, which is the dominant scattering mechanism [29; 30]. Such behavior of \(R_{\mathrm{H}}\) and \(\mu_{\perp C}\) defines a metallic character of the conductivity \(\sigma_{\perp C}\) variation at \(T<370\) K (Figure 1). Only at higher temperatures does the activation of electrons from a deeper donor rapidly increase, resulting in the transition to \(\sigma_{\perp C}\) (\(T\)) dependence which is typical for semiconductors. Figure 1: Temperature dependences of conductivity along the layers for the \(n\)-InSe samples: pristine (1), first-time and repeatedly intercalated with RbNO\({}_{3}\) (2 and 3, respectively), and annealed in vacuum (4 and 5, respectively). Figure 3: Temperature dependences of the Hall mobility along the layers for the \(n\)-InSe samples: pristine (I), first-time and repeatedly intercalated with RbNO\({}_{3}\) (2 and 3, respectively), and annealed in vacuum (4 and 5, respectively). Curve 6 shows drift mobility calculated according to the model in [25; 26]. Figure 2: Temperature dependences of the Hall coefficient for the \(n\)-InSe samples: pristine (I), first-time and repeatedly intercalated with RbNO\({}_{3}\) (2 and 3, respectively), and annealed in vacuum (4 and 5, respectively). According to the theoretical model in [29; 30], the scattering of carriers with homopolar optical phonons, which are polarized along the crystallographic C axis and modulate the thickness of a separate layer, is dominant in layered crystals. In this case, the relaxation time \(\tau_{FS}\) is given by:  
\[1/\tau_{FS}=2g^{2}\omega\Big{[}((\epsilon/\hbar\omega)+1)^{\frac{1}{2}}\;n_{ph} +\Big{(}n_{ph}+1\Big{)}\theta(\epsilon-\hbar\omega)((\epsilon/\hbar\omega)-1) )^{\frac{1}{2}}\Big{]} \tag{1}\]  
Here, \(g^{2}=\mathcal{E}_{0}^{2}m^{*\frac{3}{2}}MN\hbar(\hbar\omega)^{\frac{3}{2}}\) is the electron-phonon coupling constant, \(\hbar\omega\) is the phonon energy, \(\mathcal{E}_{0}\) is the deformation potential, \(M\) is the reduced ionic mass of the phonon mode, \(N\) is the number of cells per unit volume, \(\epsilon\) is the electron energy, \(n_{ph}=[\exp(\hbar\omega/kT)-1]^{-1}\) is the phonon occupation number, \(m^{*}=\left(m_{\perp\mathbb{LC}}^{*}m_{\parallel\mathbb{LC}}^{*}\right)^{1/3}\) is the density-of-states effective mass, and \(\theta(x)=1\) at \(x\geq 0\) and \(0\) at \(x<0\). The drift mobility along the layers caused by this scattering mechanism is:  
\[\mu_{FS}=e<\tau_{FS}>/m^{*} \tag{2}\]  
In the three-dimensional approximation of the scattering with the homopolar optical phonon \(A_{\rm 1g}^{\prime}\) (\(\hbar\omega=14.3\) a\(\mathrm{eB}\)) and \(g^{2}=0.056\), the \(\mu_{FS}(T)\) dependence follows the pattern shown by curve 6 in Figure 3. The difference between the \(\mu_{FS}(T)\) and apparent mobility dependence for the pristine sample can be explained by the effect of additional scattering with ionized impurities.