Chemical Vapor Deposition of Heteroepitaxial Boron Phosphide Thin Films

Boron monophosphide (BP) is a group III-V compound semiconductor with a wide band gap of 2.3 eV. Its unique electrical properties make it a promising material for use as a room temperature thermal neutron detector and thermoelectric device in high temperature and radiation fields. A thin film of BP enriched in the boron-10 isotope can yield 2.8MeV of energy, which in solid-state BP can yield ∼0.5 million electron-hole pairs that would be detectable with minimal amplification in a device. The high carrier concentration, wide band gap, inertness and refractory nature make it an attractive material for use in the extreme environments of nuclear reactors. The main drawback to BP is the difficulty in synthesizing high quality thin films. The majority of the previous work on BP was performed on Si substrates. The high lattice mismatch between Si and BP incorporates strain in the BP film which causes varying defects and charge traps to be introduced, adversely affecting electrical performance. It is the purpose of this work to identify the parameters necessary to deposit highly ordered zincblende boron phosphide (BP) thin films on 4° off-axis C-face 4H-SiC(0001) substrates by chemical vapor deposition.

SiC only has a 4% lattice mismatch from BP, which could greatly reduce the inherent strain from heteroepitaxial growth. Ultra high purity diborane and phosphineare used as reactive precursors, with hydrogen as the carrier gas. Conditions necessary for high quality BP thin films will be explored. SEM, XRD, TEM and Raman spectroscopy are used to characterize the BP films and identify the temperature, phosphine to diborane flow rate ratios, SiC wafer termination and wafer surface preparation to elucidate optimum BP thin films for eventual device fabrication.