Temperature-Dependent Electrical Transport and Microwave Network Characteristics of Rare-Earth (Y, Yb)-Coated Ge Thin Films

Amorphous Ge thin films coated with rare-earth yttrium (Y) and ytterbium (Yb) nanosheets were investigated as multifunctional materials for electronic and microwave applications. Electrical characterization of as-grown Ge and coated Ge (Y, Yb) structures over 30–300 K and 0.01–6.0 GHz showed that Y and Yb coatings enhanced the low-temperature electrical conductivity by factors of 23.7 and 11.3, respectively. Charge transport was governed by thermally activated conduction and variable range hopping above about 120 K, while tunneling dominated in the 20–120 K range, indicating that rare-earth coating strongly modifies carrier localization and interfacial electronic states in Ge. In the 0.001–1.8 GHz range, Ge/Y and Ge/Yb also exhibited negative conductance and negative capacitance, supporting their potential for signal processing and sensing technologies. In parallel, sub-6 GHz two-port microwave measurements showed that the Y-coated sample exhibited the broadest common matched frequency range of 2.01–3.90 GHz and a balanced two-port response near 2.40 GHz. For Ge/Y, the minimum reflection coefficients were S11 = −37.84 dB at 2.61 GHz and S22 = −38.06 dB at 2.34 GHz, while S21 and S12 reached −24.12 dB and −23.96 dB, respectively, at 0.21 GHz. These results establish rare-earth coating as an effective approach for tuning the electrical and microwave network response of amorphous Ge thin-film structures and motivate their further investigation for sub-6 GHz microwave and wireless-device applications.