Preparation and Properties of Thin Film Boron Nitride

Clear, vitreous films of boron nitride up to 6000Aå thick have been deposited on a variety of substrates at 600 °–1000 °C by a reaction between diborane and ammonia in hydrogen or inert carrier gas. Deposition rate may be readily adjusted to 50–1000 Aå/min. Most samples were made at either 600° or 800°, with some attendant variation in film properties. The 600° material contains some residual B‐H bonding. The film is essentially amorphous to electron diffraction. The refractive index is 1.7–1.8, the 1 MHz dielectric constant ∼ 3 1/2, the dielectric strength , and the The band gap is 3.8 ev and the phonon temperature in the neighborhood of 2000°K. For semiconductor junction protection boron nitride has no advantage over silicon nitride. 600° deposition directly on Si has produced surface charges as low as , but there are room‐temperature drifts, and high‐field conduction also. BN deposited at 800° on Si is electrically similar to silicon nitride. Etching of BN film also presents the same problems as does silicon nitride. BN is not as good a barrier against sodium ion permeation. Attack by atmospheric moisture over a long period has varied from insignificant to extensive conversion to orthoboric acid. BN film on Si dopes the substrate with boron at temperatures above 900 °C in inert ambient. Uniform junction depths are produced. D‐C conductivity in 500–4000Aå films has been studied from room temperature to 270 °C. With fields ≥ 10 6 v/cm BN film shows stable, nonohmic conductivity which is independent of polarity. The 25 °C d‐c conduction is describable over at least seven decades of current by , , where , . The 600°‐deposited BN is the more conductive and can carry indefinitely. is linear, and the slope of the curve is in good agreement with the theoretical value for a Frenkel‐Poole conduction mechanism. Possible use of BN as a thin film varistor is discussed.