Fabrication and characterization of graphene/silicon heterojunctions

The graphene/silicon (Gr/Si) heterojunction is currently the subject of an intense research activity. It holds promises for a new generation of graphene-based devices such as photodetectors, solar cells and chemical-biological sensors, and offers the opportunity of studying new fundamental physics at the interface between a 2D semimetal and a 3D semiconductor [1]. A Schottky barrier is usually formed at the Gr/Si heterojunction, which consequently shows the typical rectifying behavior of a metal-semiconductor Schottky diode. The Schottky barrier height strongly depend on the fabrication method and on the quality of the interface. Compared to a metal-semiconductor diode, the Gr/Si heterojunction presents important new peculiarities. The vanishing density of states of graphene at the Dirac point enables energy Fermi tuning and hence Schottky barrier height modulation by a single anode-cathode bias [1,2]. Hence, the Gr/Si heterojunction can function as a two terminal barristor. Furthermore, graphene can be used as anti-reflecting and transparent layer, and can facilitate photo-charge separation and transport, thus enabling optoelectronic applications for the Gr/Si heterojunction [1,2]. Here we report extensive I-V and C-V characterization at different temperatures and the response to light of two types of Gr/Si devices, fabricated on nanotip terminated [3] and on flat [4] Si surfaces (device A and B of Fig. 1), respectively. We study several features of the Gr/Si heterojunction and extract relevant parameters such as Schottky barrier height, ideality factor, series resistance, etc. Both Gr/Si devices can be operated as high responsivity photodiodes (up to 3 A/W). We unveil the physical mechanism behind the quite surprisingly high photocurrent, which results in reverse current exceeding the forward one