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We studied the development of V-shaped defects in GaInN±GaN quantum well superlattices. We observed that these defects could not be suppressed by varying growth parameters like strain, In content, GaInN growth temperature etc. However, perfect superlattices without such defects could be grown by cycling the temperature between low (for the GaInN wells) and high temperatures (for the GaN barriers). Although a large hydrogen/nitrogen ratio in the carrier gas seems to hinder the defect formation in GaN±AlGaN superlattices, it was not possible to suppress the defect formation in a GaInN±GaN superlattice by decreasing the total nitrogen flow.

SummaryIn this work the local electronic structure of MOVPE-grown (In, Ga, Al) N heterostructures has been investigated by electron energy loss spectroscopy (EELS). The cold fieldemission scanning transmission electron microscope (VG HB501) used was equipped with a dedicated parallel EELS system which provides high dispersions at an energy resolution of 0·35 eV with the use of subnanometre electron probes the spatial resolution of the measurements depends on the physical localization of the scattering process itself and thus is in the order of nanometres.The low-loss region of the energy spectra gives information on plasmon excitations and transitions across the bandgap. The main problem on looking at the bandgap region of EELS spectra is to separate the bandgap signal from the fading tail of the zero-loss peak. High energy resolution and application of suitable deconvolution routines for removal of the zero-loss peak extract improved information from this energy region.Thus the EEL spectra of different group III nitrides reveal the onset of the bandgap itself and the characteristic shape of the joint density of states. From these results the local optical properties can be deduced via a Kramers-Kronig transformation.The data obtained show detailed structure on the energy scale and are in excellent agreement with optical ellipsometric results. In comparison with these techniques EELS methods yield a superior spatial resolution of better than 10 nm which enables detailed investigation of the effect of local defects and boundaries on the optical properties.

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