In this work, we study the basic processes during the initial stages of growth which control polarity in N-polar AlN films grown on c-plane sapphire substrates by metalorganic chemical vapor deposition. More specifically, we study the morphology and atomic structure of the films as dependent on nitridation conditions, i.e., duration and temperature, by atomic force microscopy, high resolution transmission electron microscopy ,and high resolution high-angle annular dark field scanning transmission electron microscopy. Our experimental results show that beyond a critical temperature of 1000 °C in addition to an omnipresent two-dimensional aluminum-oxynitride layer, three-dimensional Al-polar AlN islands form. While the aluminum-oxynitride layer is unstable under high temperature growth conditions and results in N-polar films, Al-polar islands are stable and induce Al-polar columnar inversion domains in the N-polar AlN films. Appropriate nitridation conditions (approximately 10 minutes at T = 850 °C–950 °C) and adjustment of growth parameters (reactor pressure, NH3 flux, etc.) prevent the formation of Al-polar islands, which is essential for achieving N-polar films free of inversion domains.

Undoped GaN and diluted GaNBi alloys were grown on (0001) sapphire substrate by metal-organic vapor phase epitaxy (MOVPE) at 480 degrees C. By using in-situ laser reflectometry, it is found that the increase of TMBi flow rate leads to a reduction of the average value of reflectivity oscillations. Scanning electron microscopy (SEM) images gave a clear observation of the TMBi increasing amount effect on the surface morphology. The appearance of different structure (islands and columns) on GaN surface could be responsible to the reduction of the reflectivity oscillations average value. The energy dispersive X-ray (EDX) analysis showed that the observed structures were only composed of Bi compared to the flat GaN surface. Moreover, the surface morphology between islands and columns is improved when we increase the TMBi flow rate. This improvement is consistent with the decrease of root mean square (RMS) roughness, as measured by atomic force microscopy (AFM). (C) 2014 Elsevier B. V. All rights reserved.

Les transitions des ions Tm 3+ ont un fort potentiel applicatif dans le domaine des fibres optiques amplificatrices. Afin d'ameliorer leurs efficacites d'emission dans des fibres a base de silice, la voie exploree repose sur l'encapsulation des ions Tm 3+ dans des nanoparticules. Dans cette communication, nous etudions un procede de fabrication reposant sur le dopage avec des nanoparticules de LaF3:Tm 3+ . Les traitements thermiques imposes par le procede MCVD conduisent a l'evaporation du fluor et a la formation de nouvelles nanoparticules. Une duree de vie allongee du niveau 3 H4 de Tm 3+ (61 µs) et une faible perte par diffusion de lumiere (0,03 dB.m -1 a 1300 nm) montrent la potentialite de cette voie de synthese pour preparer des lasers ou amplificateurs a fibre a base de silice plus efficaces.

The determination of threading dislocation (TD) densities in c-oriented GaN heteroepitaxial films is of drastic importance as they strongly influence the physical properties and device performance. The goal of this study is to assess different imaging techniques for routine determination of TD densities in a materials laboratory. Transmission electron microscopy in plan-view orientation is very accurate to determine TD densities above the mid-10(8) cm(-2) range. But it is a time-consuming and destructive technique. Our results show that cathodoluminescence does not give the true TD number and should therefore not be used. The most adapted technique is atomic force microscopy (AFM) which images the pits associated with TDs at their termination on the surface. Appropriate annealing processes may be required to extend the use of AFM to low TD densities or to reveal all TDs in the case of molecular beam epitaxy grown GaN. With such annealings, AFM imaging may be used for measuring TD densities from 10(6) to 10(11) cm(-2). Moreover, AFM (and eventual annealing) is easy to implement and can be considered as a non-destructive technique.

Focused ion beam technique is a powerful tool for defining patterns within a semiconductor film. In this paper, we show that it is possible to realize patterns such as disks and columns within thick GaN templates and that it is compatible with the regrowth of GaN based heterostructures. We study the effect of the pattern size and shape on the regrowth by molecular beam epitaxy. We show that the growth using ammonia as the nitrogen source with flux at temperature optimized for 2-dimensional growth leads to the apparition of well defined growth planes. We show that the development of these planes is dependent with the initial pattern size and shape. These results confirm the difficulty for realizing micro or nano-columns with axial heterostructures. At the opposite, these growth conditions seem favourable for core-shell heterostructures column with well defined m-plane and eventually a-plane lateral facets.[GRAPHICS](C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The polarity of GaN micro- and nanowires grown epitaxially by metal organic vapor phase epitaxy on sapphire substrates and by molecular-beam epitaxy, using ammonia as a nitrogen source, on sapphire and silicon substrates has been investigated. On Al2O3(0001), whatever the growth technique employed, the GaN wires show a mixture of Ga and N polarities. On Si(111), the wires grown by ammonia-molecular beam epitaxy are almost entirely Ga-polar (around 90%) and do not show inversion domains. These results can be understood in terms of the growth conditions employed during the nucleation stage.

The ammonia-based molecular beam epitaxy of GaN/(Al, Ga)N quantum dots is investigated using reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy and photoluminescence. The main steps of the formation kinetics are identified and the influence of diffusion and evaporation processes on both the quantum dot and the wetting layer morphology is addressed. The correlation between the optical and structural properties of such structures finally allows for the analysis of matter exchanges between the quantum dots and the wetting layer during capping.

The epitaxial growth of GaN/Al0.5Ga0.5N (11-22) semipolar nanostructures and their structural and optical properties are reported. The nanostructure formation results from a strain induced growth process (Stransky-Krastanov-like growth mode). Atomic force microscopy measurements show that depending on the amount of deposited GaN, the nanostructure shape evolves from an island shape to a string shape aligned along the [1-100] direction. Transmission electron microscopy experiments reveal that (11-20) and (11-23) lateral facets are formed, making with the (11-22) growth plane an angle of 32° and 12°, respectively, and giving a very asymmetric nanostructure shape. Photoluminescence (PL) experiments as a function of the excitation power and temperature show that the internal electric field is very low compared to the case of GaN/Al0.5Ga0.5N (0001) polar quantum dots (QDs). As a consequence, the PL emission is strongly shifted towards the UV range compared to polar QDs and the full width at half maximum of the PL peak is drastically reduced. Furthermore, a room temperature PL nanostructure emission 300 times more intense than that of the semi-polar GaN template is obtained despite a large density of defects in the surrounding matrix, as expected for efficient carrier localization inside the nanostructures.

In this work we study both the structural and electrical qualities of AlGaN/GaN high electron mobility transistor heterostructures grown on silicon(111) by molecular beam epitaxy. Correlations are established between the quality of the structures and the relaxation rate of the mismatch stress in layers grown using ammonia as a nitrogen source. Comparison with layers grown using a nitrogen plasma source confirms the primordial role of the growth temperature for stress relaxation and dislocation filtering.

This work is dedicated to the study of the growth by ammonia source molecular beam epitaxy of AlxGa1−xN/GaN high electron mobility transistors on (111) oriented silicon substrates. The effect of growth conditions on the structural and electrical properties of the heterostructures was investigated. It is shown that even a slight variation in the growth temperature of the thick GaN buffer on AlN/GaN stress mitigating layers has a drastic influence on these properties via a counterintuitive effect on the dislocation density. Both in situ curvature measurements and ex situ transmission electron microscopy and x-ray diffraction experiments indicate that the relaxation rate of the lattice mismatch stress increases with the growth temperature but finally results in a higher dislocations density. Furthermore, a general trend appears between the final wafer curvature at room temperature and the threading dislocation density. Finally, the influence of the dislocation density on the GaN buffer insulating properties and the two-dimensional electron gas transport properties at the AlxGa1−xN/GaN interface is discussed.

In this work, we describe the main features of selective area growth (SAG) of GaN by molecular beam epitaxy (MBE) using ammonia. Different schemes such as growth into micrometer and nanometer size windows in Si3N4 dielectric masks and mesa structures etched on GaN or Silicon substrate are studied.

Thin InAs epilayers were grown on GaAs(100) substrates exactly oriented and misoriented toward [111]A direction by atmospheric pressure metalorganic vapor phase epitaxy. InAs growth was monitored by in situ spectral reflectivity. Structural quality of InAs layers were studied by using high-resolution X-ray diffraction. No crystallographic tilting of the layers with respect to any kind of these substrates was found for all thicknesses. This result is discussed in terms of In-rich growth environment. InAs layers grown on 2° misoriented substrate provide an improved crystalline quality. Surface roughness of InAs layers depend on layer thickness and substrate misorientation.

Cubic SiC/Si (111) template is an interesting alternative for growing GaN on silicon. As compared with silicon, this substrate allows reducing the stress in GaN films due to both lower lattice and thermal expansion coefficient mismatch, and can provide better heat dissipation. In this work, we first developed the epitaxial growth of 3C-SiC films on 50 mm Si(111) substrates using chemical vapor deposition. AlGaN/GaN high electron mobility transistors were grown by molecular beam epitaxy on these films. Both the structural quality and the behavior of transistors realized on these structures show the feasibility of this approach.

Specimens of superior quality are required for quantitative Transmission Electron Microscopy. The classical preparation of heterostructural semiconductors includes stages of mechanical polishing followed by ion-milling up to the transparency of the sample. The last stage is critical, as incorrectly carried out ion-milling brings to the appearance of the different types of preparation defects. Ionic radiation can create crystalline defects which will superpose over the intrinsic defects of the studied materials. The elevation of temperature induced by ion-milling can also cause transformations of phases or metastable phase amorphisation. The most often consequence of the ion-milling is the formation of an amorphous layer on the surface of the sample which can vastly perturb the quantitative analysis. We shall show afterwards that low tension ion-milling allows avoiding some of these defects.

In the frame of studying II–VI oxides of interest in optoelectronic technologies, the structural properties of CdO films grown by metalorganic vapour phase epitaxy on a-plane sapphire substrates have been analysed. The study has been performed by means of X-ray diffraction and cross-sectional transmission electron microscopy measurements. CdO films have been found to grow along [111] with the presence of twinned domains. Asymmetrical reflections have been used to study the crystalline quality of the twinned domains, independent of each other, as well as to determine their relative population. The analysis has been made as a function of growth conditions: VI/II precursors molar ratio and growth temperature.

In this work, we show that, by carefully designing the subsurface Fe doping profile in SI-GaN templates grown by MOVPE and by optimizing the MBE regrowth conditions, a highly resistive GaN buffer can be achieved on these epi-ready GaN-on-sapphire templates without any addition of acceptors during the regrowth. As a result, high-quality high electron mobility transistors can be fabricated. Furthermore, we report on the excellent properties of two-dimensional electron gas and device performances with electron mobility greater than 2000cm2/Vs at room temperature and off-state buffer leakage currents as low as 5μA/mm at 100V.

We report on the growth by molecular beam epitaxy of AlGaN/GaN high electron mobility transistors (HEMTs) on Si(111)/ SiO2/polySiC substrates. The structural, optical, and electrical properties of these films are studied and compared with those of heterostructures grown on thick Si(111) substrates. Field effect transistors have been realized, and they demonstrate the potentialities of III–V nitrides grown on these advanced substrates.

The use of AlN epitaxial films deposited on vicinal Si(111) as templates for the growth of GaN quantum dots is investigated by transmission electron microscopy and atomic force microscopy. It is found that the substrate vicinality induces both a slight tilt of the AlN (0001) direction with respect to the [111] direction and a step bunching mechanism. As a consequence, a dislocation dragging behavior is observed giving rise to dislocation-free areas well suited for the nucleation of GaN quantum dots.