Semiconductor nanowires have been intensively explored as building blocks for the next-generation electronic and opto-electronic devices. Further advances towards real-world applications require a reliable and precise control of material properties, which, to a large extent, are determined by impurities. Controlled incorporation of functional impurities enables an impurity-engineering approach, whereby novel material properties can be engineered based on the interactions between impurities and the one-dimensional material host. On the other hand, unintentional impurity incorporation can be significant in determining nanowire electronic properties. Therefore, efforts towards identifying impurity species, especially those incorporated unintentionally, as well as understanding their microscopic structures and effects on material properties, are critical to advancing nanowire-based device technologies.

 

To this end, Raman scattering spectroscopy provides an effective approach to probing impurity incorporation in various materials. When complemented by mass spectrometry studies, this technique can enable unambiguous identifications of impurity species by their vibrational frequencies (i.e. impurity vibrational modes). As impurity vibrational characteristics are sensitive to the surrounding environment, the lattice locations of these impurity atoms can also be determined. Furthermore, the nanoscale spatial resolution of Raman scattering spectroscopy can provide insightful information on the possible routes of impurity incorporation, shedding light on the relationship between nanowire synthesis conditions and material properties.

 

In this work, using Raman scattering spectroscopy complemented by mass-selected time-of-flight particle emission techniques, we show the presence of unintentionally incorporated nitrogen complexes (most likely interstitial nitrogen molecules) in ZnO and GaN nanowires grown via the vapor-liquid-solid (VLS) process. Spatially resolved Raman scattering spectra obtained at various locations on single nanowires suggest a possible route of nitrogen incorporations via metal nanocatalysts during the growth. As nitrogen impurities have profound effects on electronic properties of ZnO and GaN, these results have significant implications for current efforts on realizing high-performance opto-electronic device applications based on these nanomaterials. In addition, with the VLS process as one of the most common growth modes for synthesizing semiconductor nanowires, these experimental findings might be relevant for many nanowire systems, signifying the necessity of more studies on unintentional impurity incorporation in these nanomaterials.