We studied the second order optical nonlinearity of aluminum nitride films grown by sputtering onto silicon substrates. The crystalline properties of the films were investigated by x-ray diffraction measurements. Preliminary linear optical characterization of the films was carried out by spectrophotometric optical reflectance measurements at different incidence angles; thus the dispersion laws for both ordinary and extraordinary refractive indices were retrieved. Finally, second harmonic generation measurements in reflection mode were performed at a fixed angle from a fundamental beam provided by a picosecond Ti:sapphire laser system at λ = 800 nm. In the experiments a high blue light conversion efficiency was found for samples 1.5 and 2 μm thick, and the second order nonlinear coefficient d33 = 11±1 pm/V was found

Some of the trapping mechanisms responsible for the persistent photocurrent effects in GaN metal–semiconductor–metal photodetectors have been studied on a time scale of several hours by analyzing the photocurrent decay kinetic as a function of temperature. The analysis of the Arrhenius plot of the decay kinetic on a long time scale shows two activation energies of about 140meV and 1eV. Such values are in good agreement with two slope changes observed in the room temperature photocurrent spectrum around 2.40 and 3.25eV, in the GaN energy band gap. According to the Lucovsky theory the 140meV activation energy was interpreted as due to the transition from a deep localized state to the conduction band edge, whereas the 1eV activation energy was interpreted as due to the transition from the valence band edge to a deep localized state. Therefore, the persistent photocurrent on long time scale is primarily due to the presence of donor deep and acceptor deep states generated by gallium vacancies, gallium antisites, and carbon impurities.

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Second harmonic generation was observed experimentally from GaN∕Al50Ga50N multilayers grown on sapphire substrate by means of the rotational Maker fringe technique at a fundamental beam wavelength of 1064nm. From a single thick GaN layer (302nm), the d33 of GaN was evaluated and compared to the nonlinear coefficient obtained from measurements on several thin multilayer samples. Results show that the process of growing several thin, alternating layers does not cause the deterioration of the effective nonlinear susceptibility, which is 4.82pm∕V for GaN and 1.20pm∕V for Al50Ga50N, consistent with known values obtained for thick substrates.

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In this work we have studied the role of excitons in two different low barrier metal-semiconductor-metal (MSM) GaN-based UV photodetectors at high temperature. The active material of the two MSM devices consists of bulk GaN grown by metal organic chemical vapour deposition (MOCVD), and of an AlGaN/GaN heterostructure forming a two-dimensional electron gas (2DEG) grown by molecular beam epitaxy/magnetron sputtering epitaxy (MBE/MSE) system. The response of the devices has been characterized at the wavelength of 325 nm (He-Cd laser) as a function of the temperature in the range between room temperature and 700 K. The Arrhenius plot obtained by the decay times of the photocurrent allowed us to calculate the activation energies of some trapping mechanisms responsible of the persistent photocurrent (PPC). This shows that in both device the GaN excitonic resonances provide the most important contribution to the PPC on the millisecond time scale at low temperature.

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