Construction of Atomically Thin Boron Films on Si Heterojunctions Using a First Principles Approach

Deposition of amorphous boron (a-B) onto Si substrates via chemical decomposition of B₂H₆ molecules produces a-B/Si, heterojunctions which are the core parts of photodetectors used in vacuum ultraviolet (VUV) and potentially in extreme ultraviolet (EUV) lithography. However, fundamental questions regarding the limit on the thickness of the deposited a-B thin films and the intrinsic electronic nature of the B atoms adjacent to the Si substrate remain unanswered. Here we investigated the local structural and electronic properties of atomic-thin a-B layers at the Si{001} substrates using ab initio molecular dynamics (AIMD) techniques. The investigation revealed a rich variety of local chemical bonding and consequently interfacial electronic properties. For thin a-B layer(s)/Si systems, most of the a-B atoms at the interface formed (-B-Si-B-Si-) chains on the Si{001} surface. These B atoms were found to occupy the positions of the missing Si atoms and were bonded to the surficial Si atoms. The surficial Si atoms predominantly have two B neighbors. Localized defect states at the Fermi level for the interfacial Si and B atoms were found in the pseudo-gap. These states have a major influence on the electrical properties of the device. The predicted minimum thickness of the a-B films is about 1 to 2 nm, a useful metric for the manufacturing of a-B/Si devices. The information obtained here further helps us to understand the working mechanisms of a-B/Si interfaces for photon detection and constructing new core devices for potential applications in the field of metal/semiconductor heterojunctions for photon detection, photovoltaics, Schottky diodes and semiconductor devices.