The Inverted Meyer-Neldel Rule in the Conductance of Nanostructured Silicon Field-Effect Devices

Abstract

AbstractThin film transistors (TFTs) offer the possibility to study the electronic transport properties of an intrinsic semiconductor as a function of the Fermi level position without the introduction of dopants and/or doping related defects. Recently, we reported on the first TFTs incorporating nanostructured silicon deposited with the Hot-Wire Chemical Vapor Deposition technique. These structures offer significant advantages over conventional plasma-deposited amorphous silicon TFTs. First of all, the HW deposited nanocrystalline silicon (nc-Si:H) TFTs do not show any threshold voltage shift upon prolonged gate voltage stress. Therefore, it is now possible to study the transport characteristics at a relatively large gate voltage in a controlled fashion, unhampered by any drift of the characteristics due to the creation of metastable electronic defect states and/or charge trapping. Second, the result of the field effect is that the Fermi energy moves into the conduction band of the virtually defect-free nanocrystalline domains in the channel region of the TFT. As the effective mobility gap of the surrounding amorphous phase is higher than that of the silicon crystallites, the Fermi energy is driven deep into the band-tail distribution of the amorphous phase, a situation that could never be achieved in purely amorphous silicon TFTs nor by heavily doping an amorphous semiconductor. Thus, the nanostructured nature of the silicon thin film near the gate insulator allows to shift the Fermi level far into the tail states region of the amorphous phase. This situation reveals for the first time the inverted Meyer-Neldel relationship in an intrinsic semiconductor.