Cathodoluminescence spectroscopy of wide-band-gap nanorods

The quasi one-dimensional systems (micro- or nano-rods) with the diameter comparable to the wavelength of optical transitions characteristic of the semiconductor from which they are made attract interest as microresonators, elements of light-emitting devices, lasers etc. Their optical and electronic properties are investigated in view of revealing the links between their crucial parameters and preparation conditions. Development of reliable but faster, more flexible and cheaper growth methods is also an important research subject. The electron microscopy techniques and related spectroscopic methods, like cathodoluminescence (CL), allow us to study both individual nanorods as well as their ensembles, with the submicron lateral resolution. Two examples of such studies will be shown. GaN nanorods (600-800 nm long) with AlxGa1-xN segments and accompanying Al-rich shell surrounding the part of the nanorod just below the AlxGa1-xN segment were grown on Si(111) substrates by plasma-assisted molecular-beam epitaxy. Morphology and structure of the nanorods were examined with the use of scanning and transmission electron microscopy. The local luminescent properties were studied by low-temperature CL spectroscopy and imaging. Spectral features characteristic of nanorods with the AlxGa1-xN segments of different composition and thickness were observed and their dependence on the strain induced by lattice mismatch was analyzed. The ZnO microrods grown on GaN templates by fast and inexpensive hydrothermal method are vertically well aligned with length, diameter, and distribution density modified in a controlled way by changes in such technological parameters as pH value, temperature, growth time and microwave power. The variations of near-band-edge and defect-related CL of ZnO microrods were observed as the function of growth conditions, as well as the experimental conditions (such as excitation density, irradiated surface area, etc.). The optimized conditions leading to the luminescence intensity patterns and spectral fine structures characteristic of whispering galleries modes in a hexagonal microresonator have been achieved.