Probing Interface Defects in Top-Gated MoS2Transistors with Impedance Spectroscopy

The electronic properties of the HfO2/MoS2interface were investigated using multifrequency capacitance–voltage (C–V) and current–voltage characterization of top-gated MoS2metal–oxide–semiconductor field effect transistors (MOSFETs). The analysis was performed on few layer (5–10) MoS2MOSFETs fabricated using photolithographic patterning with 13 and 8 nm HfO2gate oxide layers formed by atomic layer deposition after in-situ UV-O3surface functionalization. The impedance response of the HfO2/MoS2gate 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/MoS2surface, consistent with interface traps distributed over a range of energy levels. The interface defects density (Dit) was extracted from the C–Vresponses by the high–low frequency and the multiple-frequency extraction methods, where a Ditpeak value of 1.2 × 1013cm–2eV–1was extracted for a device (7-layer MoS2and 13 nm HfO2) exhibiting a behavior approximating to a single trap response. The MoS2MOSFET with 4-layer MoS2and 8 nm HfO2gave Ditvalues ranging from 2 × 1011to 2 × 1013cm–2eV–1across the energy range corresponding to depletion near the HfO2/MoS2interface. The gate current was below 10–7A/cm2across the full bias sweep for both samples indicating continuous HfO2films resulting from the combined UV ozone and HfO2deposition 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/MoS2interface 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.