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Ternary semiconductors such as InGaN thin films, quantum wells, and superlattices commonly exhibit alloy fluctuations that become increasingly pronounced with higher In-content. The thickness fluctuations of quantum wells and their thin cap-layers further introduce nanoscale inhomogeneities that alter the potential landscape. In this work, we present a combined theoretical and experimental study of InGaN single quantum wells with thin GaN cap-layers to unravel the influence of cap-layer thickness, compositional inhomogeneity, and thickness fluctuations on their electronic and optical properties. A pronounced spectral shift of quantum well emission for thin cap-layers between 1 and 10 nm is observed by micro-photoluminescence spectroscopy. The origin of this shift is explained by calculations of electronic band profiles and probability density overlap of carriers in the quantum well. The impact of alloy fluctuations and homogeneity for different cap-layer thicknesses is studied on both the microscale and nanoscale using UV micro-Raman scattering and tip-enhanced Raman spectroscopy (TERS). On the microscale, the alloy composition as determined by micro-Raman mapping appears very homogeneous except for the thinnest 1 nm cap-layer where small fluctuations are visible. On the nanoscale, TERS reveals local fluctuations on a 20–30 nm length scale. The influence of the cap-layer thickness on the TERS spectra is discussed regarding both the nanoscale homogeneity and the depth resolution of the near-field Raman scattering technique. Our results demonstrate the capabilities of TERS to resolve nanoscale thickness fluctuations and compositional inhomogeneities in ultra-thin semiconductor layers, even when they are buried by thin cap-layers with thicknesses below 10 nm. [ABSTRACT FROM AUTHOR]

The p-type doping of GaN with Mg, in particular doping of p++ cap layers and its influence on the resistivity of Ni/Au contacts on semipolar (20-21) GaN, has been investigated. For this purpose, we have compared GaN:Mg grown on several semipolar and polar orientations with respect to the acceptor concentration NA measured by electrochemical capacitance voltage techniques. For the same Mg precursor flow and Mg/III ratio, we observe very similar acceptor densities NA of up to 1 × 1019 cm−3 for (0001), (20-21), (20-2-1), and (11-22) GaN:Mg. Furthermore, the impact of the II/III ratio for the p++ cap layer ( NA > 1 × 1019 cm−3) on I- V characteristics of Ni/Au (20 nm/30 nm) contacts on (20-21) oriented GaN:Mg has been investigated. Ohmic I- V characteristics were observed for Mg/III ratios >1 × 10−2. Specific contact resistivities as low as 2.4 × 10−3 Ω cm2 could be achieved. Inclusion of this p++ cap layer resulted in reduced turn on voltages in light emitting diodes with 450 nm emission wavelength. [ABSTRACT FROM AUTHOR]