Surface photovoltage spectroscopy and spectral photoconductivity measurements have been carried out in the UV spectral region on GaN nanowires to analyze the near band-edge region. The results reveal the presence of tails in the band−band absorption spectra. Surface Photovoltage spectra performed on the as-grown nanowire ensamble show a long band tail of about 0.1 eV. Spectral photoconductivity on singly contacted nanowires shows that the band tail width strictly depends on the wire diameter. These results are explained by the Franz−Keldysh effect due to the internal electric field induced by Fermi level pinning at the nanowire surface. The experimental values of the absorption tail are well in agreement with the results obtained by simulating the electric field in a cylindrical model.

Electron capture behaviours for major traps in thin epitaxial and thick freestanding GaN samples have been experimentally and theoretically studied by using deep-level transient spectroscopy (DLTS). According to the logarithmic dependence of the DLTS signal on the filling pulse width, most of the traps in thin epitaxial GaN layers with high dislocation density behave as line defects. In sharp contrast, the same traps in thick free-standing GaN layers with low dislocation density behave as point defects. The most likely explanation for these phenomena is that the electron traps in question tend to segregate around dislocations, when present in large numbers.

We show that the usual Arrhenius analysis of the main electron-irradiation-induced defect trap in n-type GaN, observed by deep-level transient spectroscopy (DLTS), is not sufficiently accurate. Instead, an exact fitting of the DLTS spectrum for this trap reveals two components, each of which has a thermal energy near 60 meV, not the apparent 140–200 meV, as given in other DLTS studies. This result resolves the discrepancy between Hall-effect and DLTS determinations of the thermal energy of this defect center.

GaN nanowires with diameters ranging between 50 and 500 nm were investigated by electrical and photoinduced current techniques to determine the influence of their size on the opto-electronic behavior of nanodevices. The conductivity, photoconductivity, and persistent photoconductivity behavior of GaN nanowires are observed to strongly depend on the wire diameter. In particular, by spectral photoconductivity measurements, three main sub-band-gap optoelectronic transitions were detected, ascribed to the localized states giving rise to the characteristic blue, green, and yellow bands of GaN. Photoconductivity with below-band-gap excitation varies orders of magnitude with the wire diameter, similarly to that observed for near-band-edge excitation. Moreover, yellow-band-related signal shows a superlinear behavior with respect to the band-edge signal, offering new information for the modeling of the carrier recombination mechanism along the nanowires. The photoconductivity results agree well with a model which takes into account a uniform distribution of the localized states inside the wire and their direct recombination with the electrons in the conduction band.