Dielectric Properties of Phase Change Thin Films at Millimeter Waves

We present the characterization of the dielectric permittivity and loss tangent of germanium telluride (GeTe) and germanium antimonium telluride [Ge2Sb2Te5 (GST)] phase change thin films (less than <tex-math notation="LaTeX">1 μ m </tex-math> thicknesses) in the millimeter-wave (mmW) domain. The dielectric permittivities in the amorphous (insulator) state of GeTe and GST were measured using two independent differential methods: a wide bandwidth characterization based on the measurement of the propagation constant of a coplanar waveguide (CPW) and a single frequency characterization based on the measurement of the resonant frequency of a planar resonator. This differential approach allows addressing the challenges linked to thin-film dielectric characteristics estimation at mmWs without the need of an absolute knowledge of other parameters of the measurement circuit like the conductivity and the thickness of the metals as well as the permittivity and losses of the substrate. The extracted mean values range between 20 and 22 for the GeTe and 30 and 34 for the GST. These values are rather constant over the frequency range from 10 to 60 GHz. Additionally, the loss tangent at 30 GHz of both compositions was extracted giving values of <tex-math notation="LaTeX">3.4× 10⁻² </tex-math> and <tex-math notation="LaTeX">3.2× 10⁻¹ </tex-math> for the GeTe and GST, respectively. These values are among the first reported ones regarding the electromagnetic (EM) properties of GeTe and GST in this frequency band.