Non-Equilbrium Chalcogen Concentrations in Silicon: Physical Structure, Electronic Transport, and Photovoltaic Potential

Abstract:

This thesis explores the structure and properties of silicon doped with chalcogens beyond the equilibrium solubility limit, with a focus on the potential presence of an impurity band and its relevance to photovoltaics. The investigations that we report here shed new light on the electronic role of sulfur dopants in particular, and also provide new evidence of a semiconductor-to-metal transition consistent with the formation of an electron-conducting impurity band. The thesis is divided into three primary studies. First, we describe doping silicon with a single fs-laser pulse. We find that irradiation above the melting threshold is sufficient for doping a thin layer of silicon to non-equilibrium sulfur concentrations. Next, we explore the interaction of many fs-laser pulses with a silicon substrate. Temperature-dependent electronic transport measurements indicate metallic conduction, while a form of Fermi level spectroscopy and optical absorption data indicate the presence of an impurity band located 200 − 300 meV below the conduction band edge. Third, we investigate silicon doped to non- equilibrium concentrations using a different technique: ion-implantation followed by pulsed laser melting and crystal regrowth. We determine one of the sulfur states present at low sulfur dose. Additional transport measurements point to the presence of a semiconductor-to- metal transition at sulfur doses corresponding to implanted sulfur concentrations just above 10^20 cm−3 . Finally, in the appendices of this thesis, we describe methods to laser-dope silicon while avoiding the development of significant surface roughness that typically characterizes such samples. Additionally, we present the status of investigations into laser-doping silicon with selenium to non- equilibrium concentrations.