Field-Effect Transistors Based on ZnO Nanowires

Semiconductor nanowires are an emerging class of materials with great potential for applications in future electronic devices. The small footprint and the large charge-carrier mobilities of nanowires make them potentially useful for applications with high-integration density, but also to replace thin-film transistors in large-area electronics. This thesis investigates the use of wet-chemically grown ZnO nanowires for the fabrication of high-performance, low-voltage field-effect transistors (FETs). The first part of this thesis addresses the electrical characteristics of the wet-chemically grown ZnO nanowires. The as-grown ZnO nanowires are highly conductive due to a large charge-carrier concentration caused by dopants unintentionally incorporated into the ZnO nanowires during the synthesis. This large charge-carrier concentration makes it difficult to modulate the conductivity of the nanowires by an external electric field, so that the as-grown wires are not suitable for FETs. A post-growth annealing step is employed to dramatically reduce the charge-carrier concentration of the nanowires which makes it possible to fabricate FETs with useful characteristics. ZnO-nanowire FETs in a global back-gate geometry based on annealed ZnO nanowires have a transconductance of 300 nS, an on/off current ratio of 107 and a subthrehold slope of 500 mV/decade. The maximum field-effect mobility of the annealed ZnO nanowires is around 50 cm2/Vs. An important requirement to obtain good FET characterisitcs is to minimize the influence of the source and drain contact resistance on the total device resistance. The material used for the source and drain contacts in this thesis is aluminum. It is demonstrated that the use of a plasma treatment in the contact regions prior to the evaporation of the aluminum contacts can greatly influence the contact properties between ZnO and aluminum. An argon-plasma treatment locally increases the charge-carrier concentration in the ZnO. It is shown that the doping effect of the argon-plasma treatment can be exploited to reduce the contact resistance between alumiunm and ZnO and improve the electrical performance of the FETs. In the second part of this thesis, the influence of the ambient air on the electrical characteristics of the ZnO-nanowire FETs is investigated. The electrical conductivity of the FETs is strongly affected by the distinct atmospheric conditions, which makes it difficult to obtain reliable FET characteristics. The potential of a self-assembled monolayer (SAM) based on fluoroalkylphosphonic acid molecules to passivate the ZnO nanowires against the undesirable effects of the ambient and to stabilize the electrical performance of ZnO-nanowire FETs is demonstrated. The stabilizing effect is attributed to the formation of a densely packed, hydrophobic SAM on the surface of ZnO and aluminum oxide. The quality of the hydrophobic SAMs is confirmed by means of water-contact-angle measurements on SAM-covered ZnO single crystal