Universal peroxide formation and Fermi level alignment in semiconductors due to ambient air-induced surface transfer doping

Abstract

The naturally occurring surface transfer doping phenomenon is a process in which humid atmospheric air exchanges electrons with a semiconductor surface that is in contact with it, thereby modulating the Fermi energy (EF) of the material. In ambient air, the process occurs through an adsorbed water layer on the surface, where an electrochemical reaction serves as an acceptor or donor of electrons in the semiconductor. Despite the broad relevance of this phenomenon for the doping of a wide class of materials, the exact nature of the resulting redox reaction(s) that are operative and responsible for the alignment of EF is not clearly known because of the mixed constituents of atmospheric air. Through systematic titration experiments measuring changes in dissolved oxygen, pH, and reactive intermediates that occur after reaction with diamond and 20 other semiconductor particles of oxides, nitrides, sulfides, and selenides, it is shown that the electrochemical reaction O2+2H++2e−→H2O2 is the primary reaction responsible for surface transfer doping as well as the universal alignment of EF positions of all air-exposed semiconductors to an average value approximately −5.0 eV with respect to the vacuum level. The process appears to occur regardless of the nature of doping, defects, and bandgaps of the materials.