Institute Silicon Photovoltaics
The carbon atoms in the graphene form strong sp2 bonds in a hexagonal network resembling the structure of honey combs. Being just one atom thin, graphene is highly transparent for visible and infrared light. Only 2.3% of the light passing through graphene is absorbed in this carbon monolayer. Moreover, graphene is a semi-metal or zero-gap semiconductor with remarkable electronic properties. Valence and conduction band coincide at special points (Dirac points K and K’) forming bands with linear dispersion. Owing to this unique band structure, charge transport in graphene is different from conventional metals or semiconductors. The mobility of charge carriers in graphene is orders of magnitude higher than in conventional bulk material. The combination of optical transparency and electrical conductivity renders this material a potential candidate for transparent front contacts in photovoltaic devices.
Our research focus on the fabrication of large area graphene layers by means of CVD processing using copper foil substrates with subsequent transfer to any substrates. In this context, the compatibility of single-layer graphene with conventional silicon based technology and the impact of various functional layers on the electrical performance of graphene in actual devices are only two of the important questions we address. Bare films as well as graphene buried below amorphous and crystalline silicon capping layers were studied recently by Raman backscattering spectroscopy and Hall-effect measurements. Uncapped films possess charge-carrier mobilities of 2030 cm2/Vs at hole concentrations of 3.6 × 1012 cm−2.
We showed, that graphene withstands the deposition and subsequent crystallization of silicon capping layers . However, the crystallinity of the silicon cap has large influence on the field-induced doping of graphene. Temperature dependent Hall-effect measurements reveal that the mobility of embedded graphene is limited by charged-impurity and phonon-assisted scattering.