Probing Interface Defects in Top-Gated MoS₂ Transistors with Impedance Spectroscopy

The electronic properties of the HfO₂/MoS₂ interface were investigated using multifrequency capacitance-voltage (C-V) and current-voltage characterization of top-gated MoS₂ metal-oxide-semiconductor field effect transistors (MOSFETs). The analysis was performed on few layer (5-10) MoS₂ MOSFETs fabricated using photolithographic patterning with 13 and 8 nm HfO₂ gate oxide layers formed by atomic layer deposition after in-situ UV-O₃ surface functionalization. The impedance response of the HfO₂/MoS₂ gate stack indicates the existence of specific defects at the interface, which exhibited either a frequency-dependent distortion similar to conventional Si MOSFETs with unpassivated silicon dangling bonds or a frequency dispersion over the entire voltage range corresponding to depletion of the HfO₂/MoS₂ surface, consistent with interface traps distributed over a range of energy levels. The interface defects density (D_it) was extracted from the C-V responses by the high-low frequency and the multiple-frequency extraction methods, where a D_it peak value of 1.2 × 10¹³ cm^-2 eV^-1 was extracted for a device (7-layer MoS₂ and 13 nm HfO₂) exhibiting a behavior approximating to a single trap response. The MoS₂ MOSFET with 4-layer MoS₂ and 8 nm HfO₂ gave D_it values ranging from 2 × 10¹¹ to 2 × 10¹³ cm^-2 eV^-1 across the energy range corresponding to depletion near the HfO₂/MoS₂ interface. The gate current was below 10^-7 A/cm² across the full bias sweep for both samples indicating continuous HfO₂ films resulting from the combined UV ozone and HfO₂ deposition process. The results demonstrated that impedance spectroscopy applied to relatively simple top-gated transistor test structures provides an approach to investigate electrically active defects at the HfO₂/MoS₂ interface and should be applicable to alternative TMD materials, surface treatments, and gate oxides as an interface defect metrology tool in the development of TMD-based MOSFETs.