XANES studies of low-z dopants and defects in semiconductors

The determination of the local structure of dopants in semiconductors is of paramount importance to understand their influence on the optical and transport properties; this knowledge is of great value since it can lead to insightful materials design. Many commonly used dopants are low atomic number elements, the K-edge of which lie in the soft X-ray region. Application of X-ray absorption spectroscopy, the technique of choice for local structural investigations, to the study of dopants in this energy range has been limited in the past, mainly due to the very small fluorescence yields (3x10-3 for N); fluorescence detection is mandatory in order to guarantee sufficient bulk sensitivity. By taking advantage of the high brilliance of the X-ray beam emitted by undulators on ELETTRA and using a dedicated window-less Ge fluorescence detector, we have been able to overcome this limitation; we have performed a series of experiments in this field on the branch line of the ALOISA beamline, some of which are here briefly described. The method we have used involves recording X-ray absorption near edge spectra (XANES) and their structural interpretation in the framework of multiple scattering theory, using the FEFF code. The clusters used for the analysis were based on density-functional theory simulations. ZnO is a wide band gap, naturally n-type semiconductor with great promise for optoelectronic applications; the main obstacle yet to be overcome is p-type doping. Nitrogen, the most promising candidate currently being pursued as a dopant, has been predicted to preferentially incorporate into the ZnO lattice in the form of a N2 molecule at an O site when a plasma source is used, leading to compensation rather than p-type doping. We demonstrate this to be incorrect and that in fact, N incorporates substitutionally at O sites where it is expected to act as an acceptor. Using high resolution spectra in the region of the pi* transition and comparing to those of molecular nitrogen, we also detect the formation of molecular nitrogen upon annealing at the moderately low temperature of 800 °C. These results suggest that effective p-type doping of ZnO with N may be possible only for low-temperature growth processes. N-dilute GaAsN alloys belong to a novel class of semiconductors with fascinating physical properties. Indeed, a small amount of nitrogen incorporation in GaAs leads to a counterintuitive and large band gap reduction, along with a decrease of the lattice parameter. Even more surprisingly, both electronic and structural changes can be reversed fully and in a tunable manner by hydrogen incorporation. We have investigated the atomic geometry of N-H complexes in hydrogenated GaAsN and we have demonstrated that dihydrogen-nitrogen complexes with C2v symmetry, in which H breaks two N-Ga bonds on the same N atom replacing them by two N-H ones, are the most abundant species in hydrogenated GaAsN. This finding contradicts all previous predictions of “in-line” N-H2 complexes (in which H breaks only one N-Ga bond and the two H atoms bind N and Ga, respectively) as the predominant species, and accounts for many other physical properties. These investigations illustrate that high quality fluorescence-yield XANES spectra for dilute low-Z atoms can now be recorded and that combining structural and spectral simulations, precious information on the three-dimensional structure of dopants and defects can be obtained.