Optical, Electronic and Visible-Range Photo-Electronic Properties of Boron Carbide-Indole Films

Semiconducting aromatic boron carbide films have been formed by plasma enhanced chemical vapor deposition (PECVD) fromcloso-1,2-dicarbadodecarborane (orthocarborane) and indole precursors. X-ray photoemission and ellipsometry indicate that the films consist of intact indole moieties bonded to the B sites on orthocarborane icosahedra. The ellipsometry measurements, of the PECVD hydrogenated boron carbide alloyed with indole moieties, indicate indirect and direct band gaps of 1.6 eV and 3.5 eV, respectively, in good agreement with density functional theory cluster calculations. These calculations also indicate that states near the valence band maximum and conduction band minimum are associated with indole and carborane moieties, respectively. The current versus voltage (I(V)) curves, of PECVD hydrogenated boron carbide alloyed with indole moieties to n-type silicon diodes, indicate a photocurrent at zero bias, with an appreciable open-circuit voltage of 1 V. The 4th quadrant conductivity and voltage, at the maximum power point, were 1.66 mu A and 0.69 V respectively, under illumination. The frequency- and bias-dependent capacitance versus voltage (C(V)) data yield mean carrier lifetimes from 2.5 ms to 1.5 ms at low and high bias, respectively at 10 kHz, but falling to 0.5 ms, independent of bias voltage, at 100 kHz.