Suppression of ion-implantation induced porosity in germanium by a silicon dioxide capping layer

Ion implantation with high ion fluences is indispensable for successful use of germanium (Ge) in the next generation of electronic and photonic devices. However, Ge readily becomes porous after a moderate fluence implant (∼1×10¹⁵ ion cm^-2) at room temperature, and for heavy ion species such as tin (Sn), holding the target at liquid nitrogen (LN₂) temperature suppresses porosity formation only up to a fluence of 2×10¹⁶ ion cm^-2. We show, using stylus profilometry and electron microscopy, that a nanometer scale capping layer of silicon dioxide significantly suppresses the development of the porous structure in Ge during a Sn^- implant at a fluence of 4.5×10¹⁶ ion cm^-2 at LN₂ temperature. The significant loss of the implanted species through sputtering is also suppressed. The effectiveness of the capping layer in preventing porosity, as well as suppressing sputter removal of Ge, permits the attainment of an implanted Sn concentration in Ge of ∼15 at.%, which is about 2.5 times the maximum value previously attained. The crystallinity of the Ge-Sn layer following pulsed-laser-melting induced solidification is also greatly improved compared with that of uncapped material, thus opening up potential applications of the Ge-Sn alloy as a direct bandgap material fabricated by an ion beam synthesis technique.