Mineral Interface Doping: Hydroxyapatite Deposited on Silicon to Trigger the Electronic Properties

Abstract

Doping silicon wafers without using highly toxic or corrosive chemical substances has become a critical issue for semiconductor device manufacturing. In this work, ultra-thin films of hydroxyapatite (Ca₅(PO₄)₃OH) are prepared by tethering by aggregation and growth (T-BAG), and further processed by spike annealing. Via in situ infrared (IR), the decomposition of hydroxyapatite and intermixing with the native silicon oxide is observed already at temperatures as low as 200 °C. Phosphate transport through the native silicon oxide is driven by a phase transformation into a more stable thermal oxide. At 700 °C, diffusion of phosphorus into the sub-surface region of oxide-free silicon is observed. In situ IR combined with electrical impedance spectroscopy (EIS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and X-ray photoelectron spectroscopy (XPS) measurements allows to conclude that the phosphorus is: i) transported through the silicon oxide barrier, ii)) diffused inside the oxide-free silicon, and iii) finally modified the electrical activity of the silicon wafer. To further explain the experimental findings, density-functional theory (DFT) is used to demonstrate the extent of the effect of phosphorus doping on the electronic nature of silicon surfaces, showing that even small amounts of doping can have a measurable effect on the electrical performance of semiconductor wafers.