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We present a systematic study of top-down processed GaN/AlN heterostructures for intersubband optoelectronic applications. Samples containing quantum well superlattices that display either near- or mid-infrared intersubband absorption were etched into nano- and micro-pillar arrays in an inductively coupled plasma. We investigate the influence of this process on the structure and strain-state, on the interband emission and on the intersubband absorption. Notably, for pillar spacings significantly smaller (≤1/3) than the intersubband wavelength, the magnitude of the intersubband absorption is not reduced even when 90% of the material is etched away and a similar linewidth is obtained. The same holds for the interband emission. In contrast, for pillar spacings on the order of the intersubband absorption wavelength, the intersubband absorption is masked by refraction effects and photonic crystal modes. The presented results are a first step towards micro- and nano-structured group-III nitride devices relying on intersubband transitions. [ABSTRACT FROM AUTHOR]

Electron irradiation of GaN nanowires in a scanning electron microscope strongly reduces their luminous efficiency as shown by cathodoluminescence imaging and spectroscopy. We demonstrate that this luminescence quenching originates from a combination of charge trapping at already existing surface states and the formation of new surface states induced by the adsorption of C on the nanowire sidewalls. The interplay of these effects leads to a complex temporal evolution of the quenching, which strongly depends on the incident electron dose per area. Time-resolved photoluminescence measurements on electron-irradiated samples reveal that the carbonaceous adlayer affects both the nonradiative and the radiative recombination dynamics. [ABSTRACT FROM AUTHOR]

Basal-plane stacking faults are an important class of optically active structural defects in wurtzite semiconductors. The local deviation from the 2H stacking of the wurtzite matrix to a 3C zinc-blende stacking induces a bound state in the gap of the host crystal, resulting in the localization of excitons. Due to the two-dimensional nature of these planar defects, stacking faults act as quantum wells, giving rise to radiative transitions of excitons with characteristic energies. Luminescence spectroscopy is thus capable of detecting even a single stacking fault in an otherwise perfect wurtzite crystal. This review draws a comprehensive picture of the luminescence properties related to stacking faults in GaN. The emission energies associated with different types of stacking faults as well as factors that can shift these energies are discussed. In this context, the importance of the quantum-confined Stark effect in these zinc-blende/wurtzite heterostructures, which results from the spontaneous polarization of wurtzite GaN, is underlined. This discussion is extended to zinc-blende segments in a wurtzite matrix. Furthermore, other factors affecting the emission energy and linewidth of stacking fault-related peaks as well as results obtained at room temperature are addressed. The considerations presented in this article should also be transferable to other wurtzite semiconductors. [ABSTRACT FROM AUTHOR]

(In,Ga)N insertions embedded in self-assembled GaN nanowires (NWs) are of current interest for applications in solid-state light emitters. Such structures exhibit a notoriously broad emission band. We use cathodoluminescence spectral imaging in a scanning electron microscope and micro-photoluminescence spectroscopy on single NWs to learn more about the mechanisms underlying this emission. We observe a shift of the emission energy along the stack of six insertions within single NWs that may be explained by compositional pulling. Our results also corroborate reports that the localization of carriers at potential fluctuations within the insertions plays a crucial role for the luminescence of these NW based emitters. Furthermore, we resolve contributions from both structural and point defects in our measurements. [ABSTRACT FROM AUTHOR]

This paper assesses the effects of Ge-doping on the structural and optical (band-to-band and intersubband (ISB)) properties of GaN/AlGaN multi-quantum wells (QWs) designed to display ISB absorption in the short-wave, mid- and far-infrared ranges (SWIR, MIR, and FIR, respectively). The standard c-plane crystallographic orientation is considered for wells absorbing in the SWIR and MIR spectral regions, whereas the FIR structures are grown along the nonpolar m-axis. In all cases, we compare the characteristics of Ge-doped and Si-doped samples with the same design and various doping levels. The use of Ge appears to improve the mosaicity of the highly lattice-mismatched GaN/AlN heterostructures. However, when reducing the lattice mismatch, the mosaicity is rather determined by the substrate and does not show any dependence on the dopant nature or concentration. From the optical point of view, by increasing the dopant density, we observe a blueshift of the photoluminescence in polar samples due to the screening of the internal electric field by free carriers. In the ISB absorption, on the other hand, there is a systematic improvement of the linewidth when using Ge as a dopant for high doping levels, whatever the spectral region under consideration (i.e. different QW size, barrier composition and crystallographic orientation). [ABSTRACT FROM AUTHOR]

We present an innovative approachto integrate arrays of isolated,strain-free GaN crystals on patterned Si substrates. First, micrometer-sizedpillars are patterned onto Si(0 0 1) substrates. Subsequently, 2.5μm Si substrates are deposited by low-energy plasma-enhancedchemical vapor deposition, forming crystals mostly bounded by {1 11}, {1 1 3}, and {15 3 23} facets. Plasma-assisted molecular beamepitaxy is then used for GaN deposition. GaN crystals with slanted{0 0 0 1} facets having a root-mean-square surface roughness of 0.7nm are obtained for a deposited material thickness of >3 μm.Microphotoluminescence measurements performed at room and cryogenictemperature show no yellow luminescence and a neutral donor-boundA exciton transition at 3.471 eV (10 K) with a full width at half-maximumof 10 meV. Microphotoluminescence and micro-Raman spectra reveal thatGaN grown on Si pillars is strain-free. Our results indicate thatthe shape of GaN crystals can be tuned by the pattern periodicityand that a reduction of threading dislocations is achieved in theirtop part. [ABSTRACT FROM AUTHOR]

Hexagonal GaN microcrystals of a size between 1 to 3 μm are obtained by the pendeoepitaxial overgrowth of a GaN nanowire template on Si(111). The GaN microcrystals are free of threading dislocations and exhibit an atomically smooth surface (roughness of 0.2 nm). Photoluminescence spectra of these microcrystals are dominated by an intense donor-bound exciton transition at 3.471 eV with a width of 1 meV reflecting strain-free GaN of exceptional structural quality. [ABSTRACT FROM AUTHOR]

Nanowire photodetectors are investigated because of their compatibility with flexible electronics, or for the implementation of on-chip optical interconnects. Such devices are characterized by ultrahigh photocurrent gain, but their photoresponse scales sublinearly with the optical power. Here, we present a study of single-nanowire photodetectors displaying a linear response to ultraviolet illumination. Their structure consists of a GaN nanowire incorporating an AlN/GaN/AlN heterostructure, which generates an internal electric field. The activity of the heterostructure is confirmed by the rectifying behavior of the current–voltage characteristics in the dark, as well as by the asymmetry of the photoresponse in magnitude and linearity. Under reverse bias (negative bias on the GaN cap segment), the detectors behave linearly with the impinging optical power when the nanowire diameter is below a certain threshold (≈80 nm), which corresponds to the total depletion of the nanowire stem due to the Fermi level pinning at the sidewalls. In the case of nanowires that are only partially depleted, their nonlinearity is explained by a nonlinear variation of the diameter of their central conducting channel under illumination. [ABSTRACT FROM AUTHOR]