Electron microscope phase images, produced by direct reconstruction of the scattered electron wave from a focal series of high-resolution images, were used to determine the nature of defects formed in GaN:Mg crystals. We studied bulk crystals grown from dilute solutions of atomic nitrogen in liquid gallium at high pressure and thin films grown by the MOCVD method. All the crystals were grown with Ga-polarity. In both types of samples the majority of defects were three dimensional Mg-rich hexagonal pyramids with bases on the (0001) plane and six walls on {1123} planes seen in cross-section as triangulars. Some other defects appear in crosssection as trapezoidal (rectangular) defects as a result of presence of truncated pyramids.

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Electroreflectance bias-wavelength mapping is used to analyze electric polarization in AlGaN/GaN heterostructures. The electric field in the structure was calculated by applying the Fourier transform to Franz-Keldysh oscillations. The bias dependence of electric field allowed determining the width of AlGaN and GaN layers, the concentration of positive charge located on the AlGaN/GaN interface and furthermore the difference of polarization between these two layers. X-ray diffraction allowed calculating the piezoelectric contribution to the polarization and furthermore it was possible to calculate the difference of spontaneous polarization between GaN and AlGaN layers. Numerical modeling of potential and electric field in the structure, based on the capacitance-voltage profiling, shows that the fields measured by the electroreflectance mapping are generated by redistribution of electric charges caused by the different polarization of AlGaN and GaN and bias applied to the sample.

Results of photoluminescence (PL) study of the self-organized InGaAs/GaAs quantum dots (QDs) in a field-effect structure grown by metalorganic vapor phase epitaxy are presented. It has been found that the PL from the QDs strongly depends on the bias voltage. No PL from the QDs ground state can be observed from the reverse biased structure, whereas the PL signal recovers in the forward biased structure. It is proposed that the bias dependence of the PL signal results from the QDs electron occupancy changes driven by the electric field within the structure. Due to a long thermalization time, the photogenerated electrons are swept out of the QDs by the electric field before radiative recombination. The electrically modulated PL (e-m PL), making use of the bias dependence of PL signal, is proposed as a tool for QD investigation. The e-m PL spectra at T=300 and T=4.2 K are analyzed and discussed.

V-like columnar inversion domains with a divergence angle of about 4.5° ± 1° grown in AlN films with N-polarity were studied using transmission electron microscopy (TEM) and atomic force microscopy. Such domains emerge at the surface forming a small islands in form of hexagonal, truncated pyramids. A model of such pyramid was proposed. TEM studies indicate a displacement of c/2 along the [0001] direction at the inversion domain boundary. A boundary itself is composed of long segments located on the {1100} planes, which are alternated by short segments on some inclined planes.