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Scanning confocal micro-Raman and PL spectroscopy, Kelvin probe force microscopy (KPFM) and electrostatic force gradient microscopy (EFGM) were applied to study the concentration profiles of free carriers by scanning the cleaved edge along the growth direction of vertical n+/n0/n+-GaN Gunn-diode structure. The vertical frequency profile of the L– branch of the plasmon-LO-phonon modes was used to evaluate the concentration of free charge carriers. Obtained significant contrast through the undoped n0-GaN region validates the expected reduction in carrier concentration. Variation of the local contrast in cross-section KPFM and EFGM electrical images reflects the topology of the device structure and makes it possible to estimate electronic properties of individual GaN layers as well as interface boundaries. The carrier profiles determined from the Raman data and KPFM measurements are in good agreement. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin interactions between particles in quantum dots or quantum dot molecules appear as fine structure in the photoluminescence spectra. Using the understanding of exchange interactions that has been developed from single dot spectra, we analyze the spin signatures of coupled quantum dots separated by a wide barrier such that inter-dot interactions are negligible. We find that electron-hole exchange splitting is directly evident. In dots charged with an excess hole, an effective hole-hole interaction can be turned on through tunnel coupling.

The coherent quantum coupling of carriers in vertically stacked asymmetric pairs of quantum dots in applied electric fields manifests itself in rich photoluminescence spectral patterns of level crossings and anticrossings. These patterns arise from configurations of charges and spins in optically excited coupled quantum dots. We present a theoretical model that provides a useful picture of these optical properties including spin structure in terms of a minimal number of physical parameters.

Scanning confocal Raman spectroscopy was used to non-destructively evaluate the structural and electronic properties of n+/n0/n+-GaN Gunn-diode structures. The depth profiles of the free carrier concentration and mobility were obtained from a line shape analysis of the ω–, ω+ coupled phonon-plasmon modes. It was found that the intensity profiles of the Raman polar A1(LO) phonon mode depend on the thickness of the undoped n0 layer, while the intensity profiles of the nonpolar E2high mode were unaffected by the doping. The change in frequency and linewidth of E2high mode throughout the thickness of the GaN layers, however reveal structural inhomogeneity in the diode structures. The carrier concentration and mobility exhibit only slight variations with thickness with the largest variation being at the free surface of the device. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Here, we report on initial experiments to use the space charge of the changing internal polarization field from graded AlGaN films in a simple p-n junction device. Our devices are fabricated from films which are graded from GaN to AlGaN then reverse graded back to GaN without the intentional addition of impurity dopants. Structural characterization of the films is reported through X-Ray diffraction, reciprocal space maps, and temperature dependent I-V measurements are used to demonstrate the rectifying behaviour of the device (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)