Electrical transport in metal–carbon hybrid multijunction devices

• The stress reduction of a-C:H thin films by the use of metal/a-C:H hybrid multilayer structures.
• Metal-induced nanostructure formation of amorphous carbon thin films.
• Fabrication and characterisation of metal/a-C:H multilayer multijunction hybrid devices.
• Increased electrical conductivity in a-C:H by the use of metal interlayers.

Understanding the factors that influence the structural, mechanical and electrical properties of hybrid metal–carbon multilayer materials and devices are explored in this study by examining the effects of the choice of metal, Cu or Ti, and the number of metal–carbon bilayers. With up to four bilayers, corresponding to ten discrete metal–carbon electrical junctions, lower interfacial stresses and lower electrical resistance are always found in Cu multilayer structures, when compared with Ti containing multilayers. The lower electrical resistance is a result of a copper–carbon interaction which facilitates a carbon sp3 to sp2 bonding transformation and is accompanied by a metal-induced transformation of the carbon layer from an amorphous to nanostructured morphology which also aids in conduction. Time-of-flight secondary ion mass spectrometry measurements demonstrate that the two selected metals, Cu and Ti, represent extreme examples in their affinity to bond with carbon with Cu (Ti) representing a weak (strong) affinity metal for bonding. This study shows the importance of the metal–carbon interaction in understanding the mechanical stresses and electrical characteristics in particular, and the wider result of the role played by the relative chemical reactivity of the components in multijunction hybrid semiconductor-based devices in general.