Fabrication of bulk and epitaxial germanium field emitter arrays by dry etching techniques

We present the fabrication and characterization of novel high density field emitter arrays (FEAs) on CVD-grown epitaxial germanium on (0 0 1) silicon. In particular we propose a heterostructure made up of silicon as substrate and of germanium as active layer, exploiting the infrared transparency of Si and the infrared sensitivity of Ge to realize a semi-transparent photo-assisted electron beam source. We used a completely dry etching process in fluorinated gases (SF6) due to its significant under-etching for both silicon and germanium. High aspect ratio silicon and germanium FEAs, with minimum tip radii of 25 nm and 40 nm, respectively, and lower aspect ratio Ge/Si FEAs with minimum tip radii of 50 nm were fabricated. The realized FEAs show good emission behavior with field emission characteristics straight related to tip geometry: low electric field threshold for silicon and germanium tips (<18 V/?m) and enhancement factor of more than 250 and 130, respectively; conversely for the epitaxial germanium we obtained 32 V/?m for electric field threshold and 70 for enhancement factor. Current emission time stability for silicon, for germanium and for Ge/Si field emitter arrays were demonstrated.