Studies on B-doping during low-temperature growth of nc-Ge thin films via PECVD to mitigate unwanted conduction characteristics from the post-deposition oxygen absorption

Abstract

Post-deposition O absorption is a common issue in low-temperature grown nc-Ge films, that makes the material unstable and induces high n-type electrical conductivity ∼10⁻² S cm⁻¹. An attempt has been made to mitigate the oxygen absorption issue through B doping of the nc-Ge thin film network at a low deposition temperature of ∼220 °C, employing a conventional capacitively coupled PECVD. At a B₂H₆ flow rate of 3.0 sccm, the incorporation of a restricted quantity of B maintains a narrow band gap of ∼1.04 eV, significantly low conductivity of ∼6.76 ×10⁻⁷ S cm⁻¹, activation energy ∼398 meV, and photosensitivity of 6.0. Spectroscopic studies indicated that B doping facilitated reduced O-intake in the nc-Ge matrix, possibly via passivating the grain boundary defects and dangling bonds in the amorphous matrix and inducing the preferential absorption of O atoms in the Ge–O₂ configuration compared to its Ge–O_x (x < 2) counterpart. The diminished O-induced carriers and the supplied acceptor levels by the B dopants compensate for the inherent n-type carrier concentration. However, at higher B₂H₆ flow rates, significant B incorporation in the film matrix introduces numerous defects, degrading the crystallinity, despite increasing its p-type conductivity to 2.16 ×10⁻⁵ S cm⁻¹ and maintaining a narrow band gap. Organized switching in the type of conductivity from the n-type to p-type ensues via gradually rising the B-dopants within the nc-Ge thin film network during its growth, which deserves attention for specific device applications.