, , , Brian Gorman, David R. Diercks, R Kirchofer
GaN has seen use in many applications such as photovoltaics, blue and ultraviolet lasers, and light-emitting diodes with additional research into expanded and new applications. GaN in the form of nanowires present additional possibilities due to their high aspect ratios and defect-free growth. Three-dimensional characterization of these materials with high spatial and chemical resolution provides important feedback in correlating device performance, processing, and structure. Laser pulsed atom probe tomography (APT) is one particularly well-suited technique for this. With atomic or near atomic spatial resolution and chemical sensitivities of tens of ppm, most of the major aspects of the nanowires can be explored via APT. However, recent work suggests that the nature of field evaporation of these materials is very sensitive to analysis conditions such as laser energy and crystal structure. Therefore, through a combination of APT and transmission electron microscopy (TEM), we have explored these parameters in more detail. The pre- and post-APT TEM images provided confirmation of the evaporated volume and evaporation tip shape. From these, the APT reconstructions were adjusted based on the known values. The formation and ionization of N2 appears to play an important role in the field evaporation of GaN. Under nearly all analysis conditions N2+ is the predominant species. These effects are apparent in the local stoichiometry which appears to be primarily governed by the underlying crystallography and in the global stoichiometry which appears to be governed by laser energy. Additionally, these field evaporation behaviors lead to detection efficiencies of GaN in APT that are significantly lower than have been measured on standard materials such as Al and Si. Many of these aspects are not specific to GaN, but are also observed in other materials which have anions that readily form dimers, such as oxides.
Proceedings of the 2013 Electromagnetics Conference
June 26-28, 2013
South Bend, IN
GaN, nanowires, wide band gap semiconductors, laser pulsed atom prob tomography