Probing Intrinsic Defect-Induced Trap States and Hopping Transport in Two-Dimensional PdSe 2 Semiconductor Devices.
Zhenping WangNasir AliFida AliHyungyu ChoiHoseong ShinWon Jong YooPublished in: ACS applied materials & interfaces (2022)
Palladium diselenide (PdSe 2 ), as an emerging two-dimensional (2D) layered material, is gaining growing attention in nanoelectronics and optoelectronics due to its thickness-dependent band gap, high carrier mobility, and good air stability. However, its asymmetric pentagon structure is inclined to breed defects. Herein, the intrinsic Se vacancy-induced trap states and their influence on the hopping transport in PdSe 2 are systematically investigated. We provide direct evidence that Se vacancies exist in the fresh PdSe 2 samples, which results in the localized trapping states inside the band gap. For the few-layer PdSe 2 , at 77 K, the trap density ( D it ) near the midgap is about 2.2 × 10 13 cm -2 eV -1 , whereas at 295 K, the D it value increases to ∼7.1 × 10 13 cm -2 eV -1 . By comparison, the multilayer PdSe 2 shows nonobvious temperature-dependent trap behaviors with almost unchanged D it values of ∼8.1 × 10 12 cm -2 eV -1 at midgap in the temperature range between 77 and 295 K. Thus, trap states in the few-layer PdSe 2 are more vulnerable to temperature effect. Transport measurements demonstrated that both few-layer and multilayer PdSe 2 field-effect transistor (FET) devices show n-type dominant ambipolar behaviors. The electron mobility in the multilayer PdSe 2 FET is nearly 15-fold higher than that in the few-layer PdSe 2 FET at 315 K, probably owing to the decreased effective mass and suppression of charge impurity scattering in the thicker channel material. However, both FET devices exhibit variable-range hopping over a temperature range from 77 to 240 K and thermally activated hopping at temperatures above 240 K. The hopping transport mechanism is strongly associated with the Se vacancy-induced localized states with poor screening and strong potential fluctuations. This study reveals the important role of structural defects in tailoring and improving the charge transport properties of PdSe 2 .