Donors in semiconductors - Are they understood in electronic era?

Abstract

The physics of semiconductors and contemporary electronics cannot be understood without impurities. The hydrogen-like shallow donor (and acceptor) state of electron (hole) bound by Coulomb electrostatic force of excess charge of impurity is used to control conductivity of semiconductors and construct semiconductor diodes, transistors and numerous types of semiconductor electronic and optoelectronic devices, including lasers. Recently, surprisingly, the physics of impurity donors appeared to be much reacher. Experimental evidence has been provided for universal existence of other types of electronic states of the same donor impurity: i) mysterious, deep, DX-type state resulting in metastability - slow hysteresis phenomena - understood as two-electron, acceptor-like state of donor impurity, formed upon large lattice distortion or rearrangement arround impurity and accompanying capture of second electron, resulting in negative electron correlation energy U; ii) deep, localized, fully symmetric, A1, one-electron donor state of substitutional impurity. The latter state can be formed from the "ordinary" shallow hydrogen-like state in the process of strong localization of electron by short range, local potential of impurity core, preserving full (A1) symmetry of the substitutional impurity in the host lattice. The "anticrossing" of the two A 1(shallow hydrogenic and deep localized) energy levels upon transformation is observed. All types of electronic states of impurity can be universally observed for the same donor impurity and mutual transformation between different states occur upon changing experimental conditions. The knowledge about existence and properties of these "new", molecular type, donor states in semiconductors seems still await general recognition and positive application in contemporary material and device science and engineering. © 2007 IOP Publishing Ltd.