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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]

Employed for a long time in optical disks, Ge2Sb2Te5is nowadays considered the most promising material also for phase-change nonvolatile memories. In the current paper, Ge–Sb–Te phase-change material thin films with a nominal composition of Ge2Sb2Te5were grown by molecular beam epitaxy on slightly mismatched GaSb and InAs substrates with (001) and (111) orientations. In situ quadrupole mass spectrometry and reflection high-energy electron diffraction allowed tight control of the growth parameters, revealing that Ge2Sb2Te5grows in epitaxial fashion only within a narrow window of substrate temperatures around 200 °C. Smooth surfaces were achieved solely on (111)-oriented substrates. Rough surfaces and interfaces were observed by transmission electron microscopy for films grown on (001)-oriented substrates. Whereas films deposited on (001) substrates possess two different vertical epitaxial orientations, single-crystalline layers exclusively (111) oriented were achieved on (111) substrates, as shown by synchrotron radiation X-ray diffraction. All the results point to the superior crystalline quality and morphology of Ge2Sb2Te5layers grown on (111) surfaces, independent of the substrate material. [ABSTRACT FROM AUTHOR]

The incorporation of Au during vapor−liquid−solid nanowire growth might inherently limit the performance of nanowire-based devices. Here, we assess the material quality of Au-assisted and Au-free grown GaAs/(Al,Ga)As core−shell nanowires using photoluminescence spectroscopy. We show that at room temperature, the internal quantum efficiency is systematically much lower for the Au-assisted nanowires than for the Au-free ones. In contrast, the optoelectronic material quality of the latter is comparable to that of state-of-the-art planar double heterostructures. [ABSTRACT FROM AUTHOR]

We investigate the nucleation of Ni-induced GaN nanowires by in situ and ex situ experiments. Three nucleation stages are evidenced. In the first two stages, different crystal structures of the Ni collectors are identified. Real-time monitoring of the Ga desorption allows the amount of Ga incorporated in the collectors to be quantified. A transition of their crystal structure prior to nanowire growth is found to be in agreement with the thermodynamically stable phase sequence of the relevant phase diagrams. [ABSTRACT FROM AUTHOR]

The selective area growth of Ga-assisted GaAs nanowires (NWs) with a high vertical yield on Si(111) substrates is still challenging. Here, we explore different surface preparations and their impact on NW growth by molecular beam epitaxy. We show that boiling the substrate in ultrapure water leads to a significant improvement in the vertical yield of NWs (realizing 80%) grown on substrates patterned by electron-beam lithography (EBL). Tentatively, we attribute this improvement to a reduction in atomic roughness of the substrate in the mask opening. On this basis, we transfer our growth results to substrates processed by a technique that enables the efficient patterning of large arrays, nano imprint lithography (NIL). In order to obtain hole sizes below 50 nm, we combine the conventional NIL process with an indirect pattern transfer (NIL-IPT) technique. Thereby, we achieve smaller hole sizes than previously reported for conventional NIL and growth results that are comparable to those achieved on EBL patterned substrates. [ABSTRACT FROM AUTHOR]

Large InN microcrystals grown by molecular beam epitaxy are investigated by micro-Raman spectroscopy and electron backscatter diffraction (EBSD). High-quality (phonon linewidths between 1.5 and 2 cm−1) cubic and hexagonal crystals are identified with Raman mapping by the observation of the respective characteristic phonon modes. The unexpected occurrence of metastable cubic InN crystals is confirmed by EBSD measurements. The cubic microcrystals are revealed by EBSD to be single-crystalline and to exhibit  <1 1 1>  orientation. The transverse (TO) and longitudinal-optical (LO) zone-center phonon frequencies of cubic InN are found to be 463 and 584 cm−1, respectively. The bulk carrier density in the microcrystals lies in the range of 2–3  ×  1017 cm−3 as determined by the analysis of LO phonon-plasmon-coupled modes in the Raman spectra. [ABSTRACT FROM AUTHOR]

We investigated GaN-based heterostructures grown on three-dimensionally patterned Si(111) substrates by metal organic vapour phase epitaxy, with the goal of fabricating well controlled high quality, defect reduced GaN-based nanoLEDs. The high aspect ratios of such pillars minimize the influence of the lattice mismatched substrate and improve the material quality. In contrast to other approaches, we employed deep etched silicon substrates to achieve a controlled pillar growth. For that a special low temperature inductively coupled plasma etching process has been developed. InGaN/GaN multi-quantum-well structures have been incorporated into the pillars. We found a pronounced dependence of the morphology of the GaN structures on the size and pitch of the pillars. Spatially resolved optical properties of the structures are analysed by cathodoluminescence. [ABSTRACT FROM AUTHOR]