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

The electronic properties of the HfO 2 /MoS 2 interface were investigated using multifrequency capacitance–voltage ( C – V ) and current–voltage characterization of top-gated MoS 2 metal–oxide–semiconductor field effect transistors (MOSFETs). The analysis was performed on few layer (5–10) MoS 2 MOSFETs fabricated using photolithographic patterning with 13 and 8 nm HfO 2 gate oxide layers formed by atomic layer deposition after in-situ UV-O 3 surface functionalization. The impedance response of the HfO 2 /MoS 2 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 HfO 2 /MoS 2 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 × 10 13 cm –2 eV –1 was extracted for a device (7-layer MoS 2 and 13 nm HfO 2 ) exhibiting a behavior approximating to a single trap response. The MoS 2 MOSFET with 4-layer MoS 2 and 8 nm HfO 2 gave D it values ranging from 2 × 10 11 to 2 × 10 13 cm –2 eV –1 across the energy range corresponding to depletion near the HfO 2 /MoS 2 interface. The gate current was below 10 –7 A/cm 2 across the full bias sweep for both samples indicating continuous HfO 2 films resulting from the combined UV ozone and HfO 2 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 HfO 2 /MoS 2 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.