Internal photoemission, photoconductivity, and photodepopulation in ultrathin layers of oxide insulators for memory applications

Over the last decade the spectroscopies of internal photoemission (IPE)and photoconductivity (PC) have established a record of success when applied to the investigation of thin insulating layers and their interfaces. However, when considering challenges related with the application of these techniques to the characterization of solid hetero-structures for the use in electronic memory circuits, two major complications become evident: First, the photoinjected (or photogenerated) charge carriers may not betransported across the entire insulating layer of a charge-trapping memory cell but become trapped inside of it. In this case no reliable interpretation of the steady-state photocurrent is possible, which precludes one from carryingout conventional IPE/PC experiments. Second, with the increasing demand for high-permittivity nano-scaled insulators, the problem of leakage currentbecomes more and more acute since it makes thephotocurrent measurements inaccurate, or even impossible. The goal of the current work is to explore the experimental limits of IPE/PCspectroscopy in relation to the analysis of electronic memory structures. It is shown that photocharging measurements may successfully replace the photocurrent detection and deliver the required information regarding spectral onsets of the IPE and PC. Furthermore, as a bonus, this approach enables the determination of the energy distribution of the gap states ( traps ) in absolute terms a major step forward, indeed. It is also shown that IPE/PC measurements at room temperature can be used to determine the interface barriers and band gaps of insulators down to a gap width of about 3 eV. Furthermore, instead of simply hampering the measurements, assessing the behavior of the dark (leakage) current can provide valuable information concerning the type of the injection process. This range of successful developments firmly establish IPE/PC spectroscopy as a reliable tool of experimental analysis of the memory-related materials and heterostructures.status: publishe