Atmospheric and Long-term Aging Effects on the Electrical Properties of Variable Thickness WSe₂ Transistors

Atmospheric and long-term aging effects on electrical properties of WSe₂ transistors with various thicknesses are examined. Although countless published studies report electrical properties of transition-metal dichalcogenide materials, many are not attentive to testing environment or to age of samples, which we have found significantly impacts results. Our as-fabricated exfoliated WSe₂ pristine devices are predominantly n-type, which is attributed to selenium vacancies. Transfer characteristics of as-fabricated devices measured in air then vacuum reveal physisorbed atmospheric molecules significantly reduced n-type conduction in air. First-principles calculations suggest this short-term reversible atmospheric effect can be attributed primarily to physisorbed H₂O on pristine WSe₂, which is easily removed from the pristine surface in vacuum due to the low adsorption energy. Devices aged in air for over 300 h demonstrate irreversibly increased p-type conduction and decreased n-type conduction. Additionally, they develop an extended time constant for recovery of the atmospheric adsorbents effect. Short-term atmospheric aging (up to approximately 900 h) is attributed to O₂ and H₂O molecules physisorbed to selenium vacancies where electron transfer from the bulk and adsorbed binding energies are higher than the H₂O-pristine WSe₂. The residual/permanent aging component is attributed to electron trapping molecular O₂ and isoelectronic O chemisorption at selenium vacancies, which also passivates the near-conduction band gap state, p-doping the material, with very high binding energy. All effects demonstrated have the expected thickness dependence, namely, thinner devices are more sensitive to atmospheric and long-term aging effects.