Mechanism of Phosphorus Transport Through Silicon Oxide During Phosphonic Acid Monolayer Doping

Monolayer doping (MLD) is a relatively new method to incorporate shallow dopants from an adsorbed organic monolayer. To prevent evaporation of the dopant-containing organic layer during thermal processing, an oxide capping layer has typically been used, without clearly understanding surface mass transport. In this work, we investigate the thermal evolution of a phosphorus-containing organic layer grafted on oxide silicon surfaces, to determine whether phosphorus can diffuse through the oxide into silicon in the absence of a capping oxide layer. Self-assembled monolayers (SAM) of phosphonic acid are grown by tethering by aggregation and growth (T-BAG) on native oxide-terminated silicon wafers, and in situ characterization is performed by infrared spectroscopy and X-ray photoelectron spectroscopy, with complementary ex situ time-of-flight secondary ion mass spectrometry and impedance spectroscopy measurements, supported by ab initio density-functional theory (DFT) calculations. We find that annealing to 700 K initiates a self-decomposition of the chemisorbed phosphonic acid molecules at the SAM/oxide interface. As the temperature is further increased, the P-C bond, which is the weakest link of the adsorbed molecule, breaks and releases the organic ligand, followed by a molecular rearrangement of the bonding configuration. Then, phosphorus transport through the silicon oxide is mediated by PO3-x species, further driven by the transformation of the native silicon oxide to a thermal silicon oxide phase. At 1000 K, diffusion of phosphorus into the subsurface region of silicon is finally observed, without evidence for P desorption or C contamination. Our DFT results provide a mechanistic understanding of the pathway followed by the phosphorus atoms. Together, these findings provide a fundamental platform for MLD of silicon and other semiconductors in general.