Probing Interface Defects in Top-Gated MoS Transistors with Impedance Spectroscopy.

The electronic properties of the HfO/MoS interface were investigated using multifrequency capacitance-voltage (C-V) and current-voltage characterization of top-gated MoS metal-oxide-semiconductor field effect transistors (MOSFETs). The analysis was performed on few layer (5-10) MoS MOSFETs fabricated using photolithographic patterning with 13 and 8 nm HfO gate oxide layers formed by atomic layer deposition after in-situ UV-O surface functionalization. The impedance response of the HfO/MoS 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/MoS surface, consistent with interface traps distributed over a range of energy levels. The interface defects density (D) was extracted from the C-V responses by the high-low frequency and the multiple-frequency extraction methods, where a D peak value of 1.2 × 10 cm eV was extracted for a device (7-layer MoS and 13 nm HfO) exhibiting a behavior approximating to a single trap response. The MoS MOSFET with 4-layer MoS and 8 nm HfO gave D values ranging from 2 × 10 to 2 × 10 cm eV across the energy range corresponding to depletion near the HfO/MoS interface. The gate current was below 10 A/cm across the full bias sweep for both samples indicating continuous HfO films resulting from the combined UV ozone and HfO 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/MoS 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.