Insulator-to-metal transition with deep-level impurities in silicon achieved by compensated hyperdoping

Hyperdoping (achieved via nanosecond pulsed laser melting and rapid resolidification) allows the substitutional incorporation of impurities at concentrations orders of magnitude beyond the equilibrium solubility limit. This technique opens the door for studying the insulator-to-metal transition (IMT) in silicon doped with impurities for which the critical concentration necessary to drive the transition is inaccessible by conventional doping techniques; specifically, impurities that introduce deep, highly localized states. IMTs have already been observed for silicon hyperdoped with sulfur and with selenium. It may be possible to use these deep impurities to create an intermediate band semiconductor in which there is a delocalized band of impurity states isolated within the conventional band gap. We will discuss the possible nature of these IMTs (impurity band merging with the conduction band vs. closing of the Hubbard gap), and we will present further observations of a metal-to-insulator transition in highly compensated sulfur-doped samples. Sulfur is a double donor in silicon, and by adding varying concentrations of boron, a shallow acceptor, we demonstrate a tunable depletion of the impurity band as evidenced by the materials' optoelectronic properties.