Investigating the Graphene-Metal Interface in micro-electronic devices

Abstract

Today's silicon integrated circuits exhibit significant leakage currents and short channel effects as the transistor per chip ratio has risen substantially in order to sustain the technological growth postulated by Moore's law. Two dimensional graphene consists of π-orbital electrons that produce unique semi-metallic properties of permanent conductivity in an electric field. The metal-graphene contact is critical for graphene transistors to enable integrated circuits applications. However, large discrepancies exist in reported values of contact resistance stemming from nano-scale surface interaction with metal. Current research is focused on developing unit processes that maintain graphene's transport characteristics which degrade in the fabrication process. This study focuses on optimizing ohmic metal–graphene contacts. CVD graphene on copper foils was transferred onto thermally oxidized silicon wafers and the contact resistance evaluated using e-beam evaporated Silver and Titanium side-contacts on two distinct graphene films of different grain sizes. Electrical back-gating of the silicon substrates indicate p-doping at the Dirac point. Two probe electrical testing on Transfer Length Method devices and four probe Kelvin structures measured the contact resistance and sheet resistance respectively. In addition, rapid thermal treatment in N_2 is studied as a technique to improve metal adhesion. Moreover, we performed experiments on pre-transferred graphene and observe that PMMA resist used in our transfer process prior to exfoliation, lowers conductivity and reduces area of functional graphene films. Wrinkle conductivity study of the graphene was performed under various forces upto 300nN. Wrinkles were found to be 14% less conductive than the smooth regions at low forces but the conductivity of wrinkles above 175nN was only 6% less. Titanium is a promising contact metal for graphene transistors, yielding the lowest contact resistance of 283Ω and lowest sheet resistance of 63Ω/sq for graphene films with large grain sizes of 1200nm. This research proposes Titanium as a suitable metal contact for CVD graphene circuit applications.

Date of Award	2014
Original language	American English
Supervisor	Irfan Saadat (Supervisor)