Kirstin Alberi
National Renewable Energy Laboratory, USA
Title: Strongly localized isoelectronic impurities for synthesizing novel semiconductor alloys
Biography
Biography: Kirstin Alberi
Abstract
Isoelectronic impurities whose electronegativity and size are very different than the host atom the substitute can introduce resonant states in either the conduction or valence band that induce substantial changes in the electronic bandstructure. Nitrogen is well known to create a resonant state above the conduction band minimum that causes band mixing and a 180 meV/% N drop in the bandgap energy. Bismuth also produces a similar effect in the valence band of GaAs that leads to a bandgap reduction of 88 meV/% Bi. This strong dependence of the bandgap energy on the alloy composition makes both GaAs1-xNx and GaAs1-xBix alloys potentially important materials for high efficiency multijunction solar cells, infrared lasers, LEDs and detectors. Moreover, they provide rich systems for exploring the interaction of localized states dispersed throughout a host matrix. Statistically occurring N pairs and clusters strongly localize electrons in the bandgap, while analogous Bi-related states localize holes. These states can then be made to interact by increasing their concentration. For example, wavefunction overlap between localized N pair states creates delocalized superclusters that further perturb the host GaAs electronic structure. Application of a magnetic field can then be used to reduce the Bohr radius of excitons bound at these states, fragmenting the superclusters. The investigation of such phenomena provide insight into the evolution of localized impurity states into delocalized alloy states and shed light on the birth of novel semiconductor alloys.