Microwave Characterization of Transparent Conducting Films

The high frequency conductivity of thin metallic and graphitic nano-films attracts interest due to many potential applications in spin electronics, electromagnetic shielding, flexible antennas, displays, and in solar cells. Surface morphology of thin conducting nano-films typically consists of an isolated clustering structure, which can evolve into a conducting percolated network [1]. The high frequency conductance of such materials is not well understood. We present measurements of microwave conductivity of thin optically transparent films in a coplanar waveguide (CPW) configuration [2]. Fig. 1 shows a signal flow graph of a two port microwave network representing a section of CPW with a conducting thin film specimen. The CPW outside the specimen section has a real characteristic impedance Z0, while the material’s properties in the specimen section are represented by the complex impedance Zs that depends on the reflection (Γ) and transmission (e ) coefficients; propagation constant (γ ) and propagation length (l). We determine the relation between the experimentally measured scattering parameters (S11) and (S21), complex impedance (Zs) and propagation constant (γ ) for the CPW test structure through a signal flow graph method. Once the signal flow is solved for γ and Zs , then the conductance Gs and the capacitance Cs of the specimen can be determined from conventional transmission line relations.