Tuning of the metal-insulator transition in magnetic phase-change material Ge1−xFexTe

Summary form only given. Chalcogenide phase-change materials (PCMs) are technologically important since they are capable of undergoing fast and reversible transitions between the amorphous and crystalline phase. <;sup>1<;/sup> The two phases have markedly different resistivity, which makes them currently exploited in non-volatile memory technology. In order to enable the multilevel memory in phase-change storage technology, spin is injected to synthesize the magnetic PCMs.<;sup>2<;/sup> The magnetic properties, as well as electrical properties, could be conveniently controlled by the phase change feature. Even though many effects have been made to increase the memory levels in phase-change storage, most of them are based on phase change feature. Thus it is necessary to tune and study electrical properties in a specific phase (amorphous or crystalline phase) of magnetic PCMs to further increase the memory levels. In our work, we report the metal-insulator transition (MIT) in magnetic PCM Ge_1-xFe_xTe by varying the Fe composition, which is considered to be of significance to enable the multilevel memory in phase-change storage technology. The electrical properties reveal that Fe drives the system from insulating to metallic state at the critical doping concentration, x~0.14. An unusual variation of temperature coefficient of the resistivity (TCR) is witnessed, which is related to both Fe impurity and intrinsic Ge vacancy in Ge_1-xFe_xTe. X-ray photoemission spectra (XPS) near the valence-band maximum (VBM) indicate that the Fermi level of Ge_1-xFe_xTe is shifted downward with Fe fraction x exceeding 0.14, which is consistent with the electrical transport studies . A modified Mott-Hubbard-Anderson model is proposed to further interpret the metal-insulator transition. The temperature-dependent resistivity (ρ-T) is used to separate metallic (dρ/dT>0) from ins- lating behavior (dρ/dT<;0). Figure 1 shows the temperature dependence of the resistivity, which is normalized by the value of the resistivity at 300K. The temperature coefficient of the resistivity (TCR) keeps negative when x≤0 .08 and then jumps to positive value, which is in accordance with variation of the hole density. An MIT occurs at the Fe concentration value of 6.5×10²⁰cm^-3 (x=0.14) or hole density value of 3.5×10²⁰cm^-3, which is two orders of magnitude larger than the Mott´s critical concentration. This discrepancy is attributed to the high degree of disorder in PCM GeTe. As Fe composition increases and depletes Ge vacancy, Fe atoms act to introduce holes into system and consequently shift the Fermi Energy downward away from valence-band maximum. An Anderson-type metal-insulator transition occurs when the Fermi energy passes the mobility edge .