We report the influence of {111} stacking faults on the cathodoluminescence (CL) emission characteristics of cubic GaN (c‐GaN) films and cubic GaN/AlN multi‐quantum wells. Transmission electron microscopy (TEM) measurements indicate that stacking faults (SFs) on the {111} planes are the predominant crystallographic defects in epitaxial films, which were grown on 3C‐SiC/Si (001) substrates by plasma‐assisted molecular beam epitaxy. The correlation of the SFs and the luminescence output is evidenced with a CL setup integrated in a scanning TEM (STEM). By comparing the STEM images and the simultaneously measured CL signals it is demonstrated that SFs in these films lead to a reduced CL emission intensity. Furthermore, the CL emission intensity is shown to increase with increasing film thickness and decreasing SF density. This correlation can be connected to the reduction of the full width at half maximum of X‐ray diffraction rocking curves with increasing film thickness of c‐GaN films. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The non-resonant carrier transfer in asymmetric double quantum wells is studied. Asymmetric cubic GaN/Al x Ga 1Àx N double quantum wells with Al content of x ¼ 0.26 AE 0.03 were grown on 3C-SiC (001) substrate by radio-frequency plasmaassisted molecular beam epitaxy. The barrier thickness d between a wide quantum well having 2.5 nm thickness and a narrow quantum well with width of 0.7 nm was varied from 1 to 15 nm. Furthermore, high resolution X-ray diffraction reciprocal space maps around the (113) direction provided the Al content and revealed a partially strain in the Al x Ga 1Àx N barriers and QWs. The coupling between the QWs was studied by interband photoluminescence spectroscopy at low temperatures. Four clearly distinguishable emission bands at 3.27 eV, 3.37 eV, 3.60 eV, and 3.74 eV are observed and could be assigned to the different layers. With decreasing barrier thickness d the photoluminescence intensity from the narrow QW is strongly reduced, indicating wave function redistribution from the narrow QW to the wide QW. The emission energies for the QWs are in good agreement with theoretical calculations using a Schrödinger-Poisson solver based on an effective mass model (nextnano 3 ). The PL intensity ratio of the narrow QW to the wide QW for varied barrier thicknesses was calculated by exploiting rate equations, revealing a good agreement between theory and experiment.

Optical transitions involving higher energy levels of cubic AlGaN quantum wells are investigated by means of photoluminescence excitation spectroscopy. An asymmetric cubic GaN/Al x Ga 1%x N double quantum well (QW) structure with an Al content of x = 0.25 + 0.03 was grown on a 3C-SiC(001) substrate exploiting radio-frequency plasma-assisted molecular beam epitaxy. The photoluminescence excitation data reveals two emission bands, which are assigned to the first electron and the third heavy hole (e1-hh3) and the second electron and the second heavy hole (e2-hh2) energy level of the wide QW. Besides in the narrow QW no higher energy levels can be observed. The experimental data is in good agreement with theoretical calculations using a Schrödinger-Poisson solver based on an effective mass model (nextnano 3). The exciton binding energy was calculated considering the confinement of the QWs and also the energy dependency of the effective mass for excited energy levels.

In this contribution we report on the optical properties of cubic AlN/GaN asymmetric multi quantum wells (MQW) structures on 3C-SiC/Si (001) substrates grown by radio-frequency plasma-assisted molecular beam epitaxy (MBE). Scanning transmission electron microscopy (STEM) and spatially resolved cathodoluminescence (CL) at room temperature and at low temperature are used to characterize the optical properties of the cubic AlN/GaN MQW structures. An increasing CL emission intensity with increasing film thickness due to the improved crystal quality was observed. This correlation can be directly connected to the reduction of the linewidth of x-ray rocking curves with increasing film thickness of the c-GaN films. Defects like stacking faults (SFs) on the {111} planes, which also can be considered as hexagonal inclusions in the cubic crystal matrix, lead to a decrease of the CL emission intensity. With low temperature CL line scans also monolayer fluctuations of the QWs have been detected and the observed transition energies agree well with solutions calculated using a one-dimensional (1D) Schrödinger-Poisson simulator.

Abstractauthoren Cubic gallium nitride (GaN) films are analyzed with high‐resolution X‐ray diffraction (HRXRD) and Raman spectroscopy. Several cubic GaN layers were grown on 3C‐SiC (001) substrate by radio‐frequency plasma‐assisted molecular beam epitaxy. The layer thickness of the cubic GaN was varied between 75 and 505 nm. The HRXRD analysis reveals a reduction of the full‐width at half‐maximum (FWHM) of omega scans for growing layer thicknesses, which is caused by a partial compensation of defects. The Raman characterization confirms well‐formed c‐GaN layers. A more detailed examination of the longitudinal optical mode hints at a correlation of the FWHM of the Raman mode with the dislocation density, which shows the possibility to determine dislocation densities by Raman spectroscopy on a micrometer scale, which is not possible by HRXRD. Furthermore, this Raman analysis shows that normalized Raman spectra present an alternative way to determine layer thicknesses of thin GaN films.