Probing Interface Defects\nin Top-Gated MoS₂ Transistors with Impedance Spectroscopy

Abstract

The\nelectronic properties of the HfO₂/MoS₂ interface\nwere investigated using multifrequency capacitance–voltage\n(<i>C</i>–<i>V</i>) and current–voltage\ncharacterization of top-gated MoS₂ metal–oxide–semiconductor\nfield effect transistors (MOSFETs). The analysis was performed on\nfew layer (5–10) MoS₂ MOSFETs fabricated using photolithographic\npatterning with 13 and 8 nm HfO₂ gate oxide layers formed\nby atomic layer deposition after in-situ UV-O₃ surface\nfunctionalization. The impedance response of the HfO₂/MoS₂ gate stack indicates the existence of specific defects at\nthe interface, which exhibited either a frequency-dependent distortion\nsimilar to conventional Si MOSFETs with unpassivated silicon dangling\nbonds or a frequency dispersion over the entire voltage range corresponding\nto depletion of the HfO₂/MoS₂ surface, consistent\nwith interface traps distributed over a range of energy levels. The\ninterface defects density (<i>D</i>_it) was extracted\nfrom the <i>C</i>–<i>V</i> responses by\nthe high–low frequency and the multiple-frequency extraction\nmethods, where a <i>D</i>_it peak value of 1.2\n× 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.\nThe MoS₂ MOSFET with 4-layer MoS₂ and 8 nm HfO₂ gave <i>D</i>_it values ranging from\n2 × 10¹¹ to 2 × 10¹³ cm^–2 eV^–1 across the energy range corresponding to\ndepletion near the HfO₂/MoS₂ interface. The\ngate current was below 10^–7 A/cm² across\nthe 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\napplied to relatively simple top-gated transistor test structures\nprovides an approach to investigate electrically active defects at\nthe HfO₂/MoS₂ interface and should be applicable\nto alternative TMD materials, surface treatments, and gate oxides\nas an interface defect metrology tool in the development of TMD-based\nMOSFETs.