Invited: Point Defects in Chalcogenide Compound Semiconductors

The II-VI chalcogenide semiconductors are used widely in applications ranging from phase change memory materials to photovoltaics.  In some cases, such as phase change materials, defects are relatively unimportant while in optoelectronic devices such as photovoltaics, these defects are critical.  Indeed, this ultimately led to the choice of III-V nitrides over the II-VI compounds for visible light emitters.  This talk focuses on materials primarily used for photovoltaics and photosensors, as this has been the most successful application of the chalcogenides in optoelectronic devices.  When one considers the series of increasingly ionic semiconductors Ge, GaAs, ZnSe, and CuBr, the impact of grain boundaries on device performance continuously decreases, allowing practical devices made from chalcogenide polycrystals.  Remarkably, many devices work better as polycrystals than as single crystals.  This is because valence compensation is increasingly important and point defects are easily created that passivate extended defects.  At the same time, when one considers the sequence ZnSe, CuInSe₂, Cu₂ZnSnSe₄, the increasing complexity of the compounds leads to an ever increasing potential for formation of detrimental point defects and second phases.  Ordering of point defects is common in these materials and can mitigate their effects and lower their formation energy, allowing significant deviation from stoichiometry without loss of performance.  These topics and the experimental evidence related to their activity are reviewed in this talk, primarily focusing on the chalcopyrite materials as examples and photovoltaics as the application.  Selected advanced characterization methods such as scanning microwave microscopy for study of the materials are also noted.