Dielectric properties of Ge_2Sb_2Te_5 phase-change films

摘要

The static (ε_s) and high-frequency (ε_∞) dielectric constants of amorphous and NaCl-type crystalline Ge_2Sb_2Te_5 were measured and the relaxation effects in films were studied using impedance spectroscopy. On the basis of a simple method that allows obtaining the dielectric constant in the low resistivity planar structure, static and high frequency dielectric constants and their temperature dependencies were calculated in both phases. A surprising value of ε_s ≈ 750 in crystalline films was obtained, but the effective dielectric constant, ε_(eff), estimated from the Maxwell-Wagner effective medium model, is significantly lower (e_(eff) ≈ 34.9). Such a high value of ε_s obtained by electrical impedance measurements has been explained by Maxwell-Wagner relaxation, the separation of charges at the interface between grains and grain boundaries. Additionally, three relaxation processes (alpha, beta, and Ohmic relaxation) were observed in the amorphous phase and four relaxations (dipolar relaxation of grains, Ohmic relaxation of grains, dipolar relaxation of grain boundaries, and Maxwell-Wagner relaxation) were observed in the crystalline phase. From these results, a new rule for the selection of materials for optical phase-change data storage is proposed: polycrystalline films must have a high volume fraction of grain boundaries. This requirement increases the effective dielectric constant and the reflectance contrast between amorphous and crystalline phases.