# Electron Transport in \(n\)-Type InSe van der Waals Crystals with Co Impurities
## 2 Materials and Methods  
Crystal growth.Nominally undoped InSe single crystals were grown by the Bridgman method from an indium-rich melt of components with a nonstoichiometric composition of In\({}_{1.03}\)Se\({}_{0.97}\). This facilitates the formation of indium-interstitial atoms which act as native donors and lead to \(n\)-type conductivity. To obtain a homogeneous ferromagnetic semiconductor system based on In-Se, we have explored different methods for incorporating transition metals in InSe material. These methods involve the incorporation of the transition metals (namely Co in this study) during the Bridgman growth or the intercalation of the transition metals after the growth of InSe. These methods are described below. Doping.The Co dopant was introduced by adding 10 at. % of high-purity (99.999%) Co to the initial load. Since the Co melting point is high (1495 \({}^{\circ}\)C), for the doped ingot the temperature and duration of synthesis, 1000 \({}^{\circ}\)C and two weeks, respectively, were higher than for undoped InSe. It is known that the solubility of dopants in indium monoselenide is very low and most of that is pushed towards the end of ingots when growing layered van der crystals. It means that the nominal amount of a dopant and its real content in different parts of an ingot can be essentially different. For this reason, the Co distribution along the ingot was determined for several pieces of the sample by means of X-ray fluorescence analysis, which revealed that it is inhomogeneous. For electrical measurements, we selected pieces of the ingot with the following composition Co\({}_{0.12}\)InSe. Intercalation.Significantly smaller ionic radius of double-charged Co\({}^{2+}\) ions (0.78 A) compared to the interlayer separation of InSe sheets (3.8 A) enables to perform their intercalation into the van der Waals gaps [29]. The electrochemical insertion of Co ions was carried out by using a method of \"drawing\" electrical field under galvanostatic conditions. As electrolyte we used a saturated aqueous solution of cobalt sulphate CoSO\({}_{4}\) providing a low current density (0.2 mA/cm\({}^{2}\)) through intercalated samples. The required amount of the ions (\(x\) = 0.12 in Co\({}_{x}\)InSe) was calculated according to the [PERSON]'s law and regulated with the amount of transferred electric charge. Structural studies.X-ray diffraction studies were performed by means of a DRON-3 X-ray diffractometer with a monochromatic Cu-K\(\alpha\) radiation of wavelength \(\lambda\) = 1.5418 A. LATTIK-KARTA software was used to analyze the obtained X-ray diffraction patterns. Surface morphology studies.Image acquisitions were performed using a field-emission gun scanning electron microscope (JEOL 7100 FEG-SEM). Prior to studies of surface morphology, the crystals were freshly cleaved to avoid oxidation and degradation over time at ambient conditions. Photoluminescence measurements.The excitation of photoluminescence spectra was done using a current-wave semiconductor laser with \(\lambda\) = 532 nm (\(p\) = 100 mW) equipped by a laser (SL-532-10 Thorlabs) and edge (LP-03-532-RS Semrock's) filters. The experimental set-up for photoluminescence measurements comprised a 0.6 m optical spectrometer MDR-23 (LOMO) with a grating of 600 groves/mm. Electrical measurements.The samples for measurements of electrical conductivity in different crystallographic directions and the Hall effect were prepared using plane-parallel plates mechanically cleaved from the as-grown InSe crystal ingots. It is worth noting that the as-cleaved InSe (0001) surfaces are atomically smooth and do not require any additional chemical or mechanical treatment for electrical and optical applications. After characterization of electrical properties of pristine samples, they were subjected to intercalation with Co\({}^{2+}\) ions.