The presently unknown band offset in nonpolar cubic GaN/AlN superlattices is investigated by inter-sub-band and interband spectroscopies as well as ab initio calculations. On one hand, the conduction-band offset (CBO) has been determined from the comparison of the measured transition energies with model calculations within the effective mass approximation. On the other hand, the valence-band offset (VBO) and the CBO are accurately simulated by calculating many-body corrections within the GW approximation on top of hybrid-functional density functional theory calculations. Thus, a CBO of (1.4 ± 0.1) eV and a VBO of (0.5 ± 0.1) eV is obtained as a result of both approaches.

The intersubband absorption of cubic GaN/Al(Ga)N quantum wells is studied experimentally and theoretically over a wide spectral range. By changing the quantum well thickness it is possible to tune the intersubband absorption peak wavelength from 1.4 μm (214 THz) to 63 μm (4.76 THz). Comparing the experimental results with simulations based on the effective-mass model we demonstrate that the GaN/AlN conduction-band offset is higher than 1.2 eV. The best fit with the experimental data is achieved for a conduction-band offset of 1.4 eV and for a GaN effective mass of 0.11m 0 .

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)

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Molecular beam epitaxy (MBE) of cubic group III-nitrides is a direct way to eliminate polarization effects which inherently limit the performance of optoelectronic devices containing quantum well or quantum dot active regions. In this contribution, the latest achievement in the MBE of phase-pure cubic GaN, AlN, and AlN/GaN quantum wells will be reviewed. The structural, optical, and electrical properties of state of the art cubic nitrides and AlGaN/GaN will be presented. We show that no polarization field exists in cubic nitrides and demonstrate intersubband absorption at 1.55 mm in cubic AlN/ GaN superlattices. Comparing the experimental results with simulations based on an effective-mass model the GaN/AlN conduction-band offset is demonstrated to be higher than 1.2 eV. The best fit with the experimental data is achieved for a conduction-band offset of 1.4 eV. Further the progress toward the fabrication of cubic GaN/AlGaN superlattices for terahertz applications will be discussed and our first experiments on cubic AlN/GaN resonant tunneling devices will be reported.