title: Dynamic Charge Carrier Trapping in Quantum Dot Field Effect Transistors.
abstract: Noncrystalline semiconductor materials often exhibit hysteresis in charge transport measurements whose mechanism is largely unknown. Here we study the dynamics of charge injection and transport in PbS quantum dot (QD) monolayers in a field effect transistor (FET). Using Kelvin probe force microscopy, we measured the temporal response of the QDs as the channel material in a FET following step function changes of gate bias. The measurements reveal an exponential decay of mobile carrier density with time constants of 3-5 s for holes and ∼10 s for electrons. An Ohmic behavior, with uniform carrier density, was observed along the channel during the injection and transport processes. These slow, uniform carrier trapping processes are reversible, with time constants that depend critically on the gas environment. We propose that the underlying mechanism is some reversible electrochemical process involving dissociation and diffusion of water and/or oxygen related species. These trapping processes are dynamically activated by the injected charges, in contrast with static electronic traps whose presence is independent of the charge state. Understanding and controlling these processes is important for improving the performance of electronic, optoelectronic, and memory devices based on disordered semiconductors.
authors.affiliation: †Applied Science and Technology Graduate Program, University of California at Berkeley, Berkeley, California 94720, United States.|‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
authors.forename: Yingjie
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: §Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, United States.|∥Miller Institute for Basic Research in Science, University of California at Berkeley, Berkeley, California 94720, United States.
authors.forename: Qian
authors.initials: Q
authors.lastname: Chen
authors.affiliation: ‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.|§Department of Chemistry, University of California at Berkeley, Berkeley, California 94720, United States.|⊥Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, United States.|#Kavli Energy NanoScience Institute, Berkeley, California 94720, United States.
authors.forename: A Paul
authors.initials: AP
authors.lastname: Alivisatos
authors.affiliation: ‡Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.|⊥Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, California 94720, United States.
authors.forename: Miquel
authors.initials: M
authors.lastname: Salmeron
keywords: Colloidal quantum dot; bias stress; charge transport; dynamic charge trapping; field effect transistor; hysteresis