Dielectric properties of amorphous phase-change materials

The AC conductivities and dielectric properties of five amorphous phase-change materials (PCMs) and three ordinary chalcogenides have been determined by employing a combination of the AC electrical measurement (0.5 Hz – 186.2 Hz), the impedance spectroscopy (9 kHz – 3 GHz) and the optical spectroscopy (20 cm-1 – 12000 cm-1, i.e., 0.6 THz – 360 THz). Those measurements almost range from the DC limit to the first interband transition. In addition, the temperature dependence of the low-frequency dielectric permittivity and the AC conductivities of amorphous PCMs were also investigated by the AC electrical measurement in the range of 4 K – 170 K and by the impedance spectroscopy in the range of 220 K – 350 K. Moreover, the aging effect on these properties of amorphous GeTe thin films annealed for one hour at successively higher temperatures, i.e. 333 K, 353 K, 373 K, 393 K, 403 K was studied by the AC electrical measurement. This work mainly focuses on amorphous PCMs. Firstly, measurements of AC conductivities of amorphous PCMs have been extensively used to understand the conduction process in these materials. No frequency dependence of AC conductivities is discernible in the impedance spectroscopy measurements, which is in line with charge transport via extended states. Secondly, the permittivities of amorphous PCMs are frequency independent among the impedance measurement frequency range. Consequently, there are no dielectric relaxations in this range. Thirdly, the static dielectric constants of amorphous PCMs significantly exceed their optical dielectric constants. This observation is corroborated by transmittance measurements in the far-infrared, which show optical phonons. Particular attention is also paid to the correlation between the dielectric constant and Born effective charge of the amorphous phase-change materials. From the intensity of these phonon modes, a large Born effective charge is derived. Nevertheless, it is known that crystalline PCMs such as GeTe possess even significantly larger Born effective charges. Crystallization is hence accompanied by a huge increase in the Born effective charge, which reveals a significant change of bonding upon crystallization. Interestingly, a clear stoichiometry trend in the static dielectric constant along the pseudo-binary line between GeTe and Sb2Te3 has been identified. On the other hand, there is a comparison of dielectric properties between the PCMs and non-PCMs. The optical dielectric constants of amorphous PCMs increase a lot after crystallization, while there is no difference between the optical dielectric constants of the amorphous and crystalline chalcogenide AgInTe2. This illustrates that the PCMs undergo a change from covalent bonding to resonant bonding on crystallization, but the amorphous and crystalline phases of ordinary chalcogenides are both governed by virtually the same covalent bonds. In addition, the static dielectric constants obtained for PCMs on the pseudo-binary line between GeTe and Sb2Te3 are compared with those obtained for ordinary covalently-bonded chalcogenide semiconductors. The static dielectric constants of both PCMs and non-PCMs significantly enhance from amorphous to crystalline, which hints that the contribution of infrared active phonons is remarkably strengthened in the crystalline states of both PCMs and non-phase-change materials. Moreover, the temperature dependence of dielectric constants of amorphous chalcogenides shows the contribution enhancement of infrared active phonons with temperature. Lastly, the aging effect on the dielectric property of amorphous GeTe thin films derived from the experimental results is in good agreement with the results of density functional theory (DFT) calculations, which at the same time reveal the bonding mechanisms and atomic structures in the representative amorphous phase