Effects of annealing on chemical-vapor deposited PureB layers

Chemical-vapor-deposited pure boron (PureB) layers can be used as a source of p-type boron dopants for thermal diffusion into silicon during a drive-in anneal. In this work, the effect of thermally annealing PureB layers is investigated in terms of surface morphology and electrical properties. The presence of a few nanometer-thick PureB layer on the Si surface was found to increase the silicon oxide growth rate by several factors during annealing in an oxygen-containing atmosphere. The oxide thickness was dependent on the initial PureB layer thickness and oxygen concentration during anneal. In an oxygen-limited atmosphere, the final thickness is insensitive to the anneal temperature as the reaction is diffusion-limited and, after oxide removal, a hydrophilic boron rich layer remains on the Si surface. With a high oxygen concentration, the boron is depleted by an oxidation of the boron-doped silicon resulting in a lower surface concentration and higher sheet resistance. A reaction mechanism involving the oxidation of Si-B compounds to form B ₂ O ₃ is proposed to explain the experimental observations. With solar cell and other photodiode applications in mind, the sheet resistance and carrier lifetime measurements were performed and show that a one-step oxidation process can simultaneously drive-in the dopants and form an oxide passivation layer.