Composition-Dependent Charge Transport in Boron Carbides Alloyed with Aromatics: Plasma Enhanced Chemical Vapor Deposition Aniline/Orthocarborane Films

Boron carbide films, alloyed with aniline moieties, were deposited by plasma enhanced chemical vapor deposition (PECVD) from aniline and orthocarborane precursors and were found to exhibit composition-dependent drift carrier lifetimes as derived from I(V) and C(V)) measurements. For a film with an aniline/carborane ratio of 5:1, the effective drift carrier lifetimes are ∼80 μs at low bias voltage but quickly drop to a few microseconds with increasing bias. A film with a 10:1 aniline/carborane ratio, however, exhibited lifetimes of ∼6 μs, or less, at 1 kHz, and much smaller values at 10 kHz. These lifetimes are orders of magnitude longer than those in polyaniline films and comparable to those in PECVD carborane films without aromatic content. X-ray photoelectron spectroscopy (XPS), FTIR, and ellipsometry, combined with density functional theory (DFT)-based cluster calculations, indicate that aniline and orthocarborane moieties are largely intact within the films. Bonding occurs primarily between aniline C sites and carborane B sites, and the aniline coordination number per carborane icosahedron is ∼2 as the aniline/carborane ratio is increased from 3:1 to 10:1. This aniline/carborane coordination ratio independent of aniline/orthocarborane stoichiometry is consistent with the dependence of charge transport properties on aniline film content at high bias voltage.