Recent Advances in the STM-Based Luminescence Microscopy of Cu(In,Ga)Se2 Thin Films

We report on recent advances in the development of a luminescence spectroscopy based on scanning tunneling microscopy (STM) and its application to fundamental aspects of Cu(In,Ga)Se₂ (CIGS) thin films. Relevant to our discussion is the specifics of the surface electronics. The CIGS shows pronounced stoichiometric deviations at the surface and, consequently, distinct surface electronics that has been shown to be critical in achieving high efficiency. Cathodoluminescence (CL), a luminescence spectrum imaging mode in scanning electron microscopy (SEM), provides a direct correlation between the microstructure of the CIGS and its electronic properties. As such, cathodoluminescence can resolve the emission spectrum between grain boundaries and grain interiors or be used to investigate the influence of local orientation and stoichiometry on the electronic properties of the CIGS at the microscale. Cathodoluminescence is not a surface microscopy, however, and resolving the electronic structure of the CIGS surface remains elusive to all luminescence microscopies. With this motivation, we have developed a luminescence microscopy based on STM, in which tunneling electrons are responsible for the excitation of luminescence (scanning tunneling luminescence or STL). The hot-tunneling- electron excitation is confined to the surface and, consequently, the tunneling luminescence spectrum reveals the electronic states near the surface. The STM is integrated inside the SEM and, therefore, both CL and STL can be measured over the same location and compared. Using this setup, the transition from the grain interior to the surface can be investigated. We have improved the collection of our optics to a level in which tunneling luminescence spectrum imaging can be performed. Here we present a detailed account on our investigation of the surface electronics in CIGS deposited in the regime of selenium deficiency as defined by 〈Se〉/(〈Cu〉 + 〈In〉 + 〈Ga〉) = 1.