Structural and electrical properties of solid-source MBE-grown graphene/Ge(001) heterostructures

We report the structural and electrical properties of a monolayer graphene (Gr) directly grown on Ge(001) by solid-source molecular beam epitaxy (SSMBE), without a transfer step, evaluating the impact of this approach on surface homogeneity and the quality of the Gr/Ge interface, two key parameters for the electrical performance of optoelectronic devices. In this work, the electrical and STEM results are discussed for the optimized growth conditions T_G = 920 °C and t_G = 3 h 12 min. The correlation between scanning transmission electron microscopy (STEM), Raman spectroscopy, and electrical measurements performed on the AuPd/a-Si/Gr/Ge(001) structure reveals a pronounced quantum-capacitance-related capacitive signature, supporting the formation of a structurally continuous graphene/Ge interface. The capacitance-voltage (C-V) characteristics exhibit a U-shaped dependence with a marked minimum near the Dirac point (charge neutrality point), followed by an increase in C with |V|; This behavior is consistent with a significant contribution from the finite quantum capacitance of graphene, while the measured capacitance also includes the geometrical capacitance of the a-Si layer, the Gr/Ge interfacial contribution, and the depletion capacitance of the Ge substrate. This quantum-capacitance-related signature persists up to 1 MHz. These properties are attributed to the continuous integration of the graphene monolayer on Ge, with lateral domains reaching ∼200 nm, suggesting complete coverage and an overall abrupt interface. This structural quality is further supported by Raman analysis, which shows an enhanced 2D/G ratio under the optimized growth conditions (T_G = 920 °C, t_G = 3 h 12 min). Current-voltage (I-V) characteristics, measured in darkness and under white light, reveal a significant photocurrent, confirming the role of graphene as an efficient 2D collector electrode for photogenerated carriers generated predominantly in the Ge substrate. Beyond measured performance, SSMBE offers a low-contamination, controlled-atomic-flux integration pathway compatible with CMOS platforms, opening up prospects for devices sensitive to surface and interface properties, such as radio-frequency varactors and capacitive readout sensors exploiting graphene-related quantum-capacitance effects.