International audienceIntrinsic ferromagnetism in high quality wurtzite Ga0:937Mn0:063N semiconductor is unambigu- ously demonstrated by both macroscopic magnetization measurements and x-ray magnetic circular dichroism. The structural quality of the samples grown by plasma-assisted molecular beam epitaxy is confirmed by x-ray di®raction and x-ray linear dichroism. The Curie temperature of a (Ga,Mn)N sample with 6.3 % Mn is 8 K with a spontaneous magnetic moment of 2.4 Bohr magneton per Mn at 2 K

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Controlling the polarity of ZnO nanowires in addition to the uniformity of their structural morphology in terms of position, vertical alignment, length, diameter, and period is still a technological and fundamental challenge for real-world device integration. In order to tackle this issue, we specifically combine the selective area growth on prepatterned polar c-plane ZnO single crystals using electron-beam lithography, with the chemical bath deposition. The formation of ZnO nanowires with a highly controlled structural morphology and a high optical quality is demonstrated over large surface areas on both polar c-plane ZnO single crystals. Importantly, the polarity of ZnO nanowires can be switched from O- to Zn-polar, depending on the polarity of prepatterned ZnO single crystals. This indicates that no fundamental limitations prevent ZnO nanowires from being O- or Zn-polar. In contrast to their catalyst-free growth by vapor-phase deposition techniques, the possibility to control the polarity of ZnO nanowires grown in solution is remarkable, further showing the strong interest in the chemical bath deposition and hydrothermal techniques. The single O- and Zn-polar ZnO nanowires additionally exhibit distinctive cathodoluminescence spectra. To a broader extent, these findings open the way to the ultimate fabrication of well-organized heterostructures made from ZnO nanowires, which can act as building blocks in a large number of electronic, optoelectronic, and photovoltaic devices.

Crystal defects in unintentionally doped ZnO nanowires grown by chemical bath deposition (CBD) play a capital role on their optical and electrical properties, governing the performances of many nanoscale engineering devices. However, the nature of these crystal defects is still highly debated. In particular, the hydrogen-related defects have not been explored in detail yet although the growth medium operates in aqueous solution. By using four-point probe resistivity measurements, we show that ZnO nanowires grown by CBD using zinc nitrate and hexamethylenetetramine exhibit a high electrical conductivity with electron densities ranging from 2.7 x 10 18 to 3.1 x 10 19 cm -3 . Most of them have a metallic electrical conduction. By combining density-functional theory calculations with cathodoluminescence and Raman spectroscopy, we reveal that the high electrical conductivity mostly originates from the formation of interstitial hydrogen in bond-centered sites (HBC) and of zinc vacancy -hydrogen (VZn-nH) complexes. In particular, the HBC and (VZn-3H) complex are found to act as two shallow donors with a very low formation energy, for which the most stable configurations are reported. Additionally, this combined theoretical and experimental approach allows us to revisit the highly debated origin of the visible and ultra-violet emission bands in the luminescence spectra. They are found to be mostly related to VZn and (VZn-nH) complexes located in the bulk and on the surfaces of ZnO nanowires. These findings represent an important step forward in the identification of the predominant native and extrinsic defects driving the electronic structure properties of ZnO nanowires grown by CBD. They further reveal the significance of hydrogen engineering to tune the source of crystal defects for optimizing the physical properties of ZnO nanowires.

International audienceZnO nanowires grown in liquid phase are considered as promising building blocks for a wide variety of optical and electrical devices. However, their structural morphology is still limited by the lack of understanding of their growth mechanisms. We have systematically investigated the effects of orientation and polarity of ZnO monocrystals acting as substrates on the formation mechanisms of ZnO by chemical bath deposition. Under identical growth conditions, two-dimensional layers develop on nonpolar m- and a-plane ZnO monocrystals. In contrast, nanowires form on O-polar c-plane ZnO monocrystals, while more complex nanostructures including nanowires grow on Zn-polar c-plane ZnO monocrystals. All of the structures have homoepitaxially nucleated. Very specifically to chemical bath deposition, both O- and Zn-polar c-planes are found to be active, and no polarity inversion domain boundary is observed on O-polar c-plane ZnO monocrystals, allowing the growth of O-polar ZnO nanowires. These findings reveal the crucial role of crystal orientation and polarity in the growth of ZnO nanowires in liquid phase similarly to their growth in vapor phase. They further cast a new light on the general understanding of the growth of ZnO nanowires and enable the revisiting of their formation mechanisms in liquid phase on seed layers consisting of ZnO nanoparticles

The effects of AlN overgrowth on the structural properties of GaN nanostructures (quantum wells and quantum dots) grown by plasma-assisted molecular beam epitaxy have been investigated using Rutherford backscattering spectroscopy, transmission electron microscopy, and reflection high-energy electron diffraction. The capping process induces a remarkable change in the dimensions of the nanostructures. The overgrowth process implies a thinning of the GaN quantum well and an isotropic reduction of the GaN island size. We demonstrate that this thickness/size reduction affects only the top GaN/AlN interface. The phenomenon is attributed to an exchange mechanism between Al atoms from the cap layer and Ga atoms in the nanostructures. We also demonstrate that this exchange is thermally activated and depends on the strain state of the nanostructures.

It is shown that a two-dimensional GaN layer grown on (0001) AlN under Ga-rich conditions remains two-dimensional while annealing under a Ga flux due to a surfactant effect of Ga. In contrast, further annealing under vacuum without the Ga flux leads to evaporation of excess Ga and to spontaneous transformation of the GaN layer into islands if the initial layer is thicker than about 2.5 monolayers. The resulting morphology is studied by atomic force microscopy and transmission electron microscopy. The latter reveals that these islands sit on top of a continuous 2.5 monolayer thick wetting layer, i.e., they represent a Stranski–Krastanow structure.

Thick layers of GaN on AlN, AlN on GaN, and InN on GaN were grown by plasma-assisted molecular beam epitaxy. Their plastic strain relaxation was studied by reflection high-energy electron diffraction (RHEED) and high resolution x-ray diffraction (HRXRD). The results are consistent with a mechanism of progressive introduction of misfit dislocations based on the coalescence of dynamically formed platelets. Due to the lack of proper gliding planes in the wurtzite structure, such dislocations are not mobile, leading to inhomogeneity of the strain state along the growth axis. The agreement between in situ RHEED and ex situ HRXRD measurements provides evidence that the strain state is frozen in during growth.