## 3 Results and Discussion
### Conductivity Anisotropy  
Temperature dependences of the conductivity across the layers \(\sigma_{\parallel\text{C}}\) and the conductivity anisotropy \(\sigma_{\perp\text{C}}/\sigma_{\parallel\text{C}}\) for the pristine, intercalated with RbNO\({}_{3}\), and vacuum annealed crystals of InSe are shown in Figures 5 and 6, and the numerical values of the parameters are listed in Table 2. The transversal conductivity component is essentially smaller than the longitudinal one for all the samples. Thermal treatment in vacuum causes a slight  
Figure 4: Temperature dependences of the experimental and calculated (2D and 3D) concentrations. \(n_{\text{dep}}\) is the concentration of depletion. increase of \(\sigma_{\parallel C}\) and a decrease in the anisotropy. Note that for the pristine and annealed samples, there are only slight variations of \(\sigma_{\parallel C}\) and \(\sigma_{\perp C}/\sigma_{\parallel C}\) with temperature. On the contrary, the insertion of RbNO\({}_{3}\) leads to a significant (by a factor of 80 to 800) decrease in the \(\sigma_{\parallel C}\) component, and \(\sigma_{\perp C}/\sigma_{\parallel C}\) considerably increases at 80 K (by a factor of 10 to 40) and exponentially decreases with increasing temperature with a higher slope. The increased anisotropy values are mainly because of the decreased transversal conductivity. Figure 5: Temperature dependences of the transversal conductivity 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). Inset shows the geometry of sample contacts for measurements of \(\sigma_{\parallel C}\). Figure 6: Temperature dependences of the conductivity anisotropy 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). Although many properties of InSe (e.g., mechanical) are highly anisotropic, it is not because of the material's band structure. It is known that the energy bands forming the fundamental absorption edge of InSe are created with the significant participation of the \(p_{z}\)-orbitals of Se, and that they have 3D character [34]. Taking into account that for \(n\)-InSe the longitudinal component of the effective mass is higher than the transversal one (\(m^{*}_{\perp C}=0.13m_{0}\) and \(m^{*}_{\parallel C}=0.08m_{0}\)[41]), in an ideal case of the same averaged relaxation time along and across the layers one can expect the anisotropy ratio to be independent of the temperature, and to be less than 1. Thus, the apparent high conductivity anisotropy should be considered as defect-induced. A low energy of a longitudinal shift of the layers relative to each other leads to disordering in the sequence of layers and the creation of energy barriers between the \(n\)-InSe layers, which explains the anomalously high values of \(\sigma_{\perp C}/\sigma_{\parallel C}\). To participate in conductivity across the layers, charge carriers should be activated over such barriers or tunnel through them. Considering that the conductivity along the layers is  
\[\sigma_{\perp C}=en\mu_{\perp C}, \tag{8}\]  
and assuming that the conductivity across the layers occurs because of thermal activation of the carriers over the energy barrier \(\Delta E_{b}\), in a one-dimensional model:  
\[\sigma_{\parallel C}=en\mu_{\parallel C}exp(-\Delta E_{b}/kT), \tag{9}\]  
and therefore the conductivity anisotropy in the low-temperature range is:  
\[\sigma_{\perp C}/\sigma_{\parallel C}=Aexp(\Delta E_{b}/kT) \tag{10}\]  
At the same averaged relaxation times, the pre-exponential factor \(A=\mu_{\perp C}/\mu_{\parallel C}=m^{*}_{\parallel C}/m^{*}_{\perp C}\). For the pristine \(n\)-InSe sample, the value of the energy barrier is small and is equal to 7.6 meV.