O. Tottereau

Growth of high quality crack-free AlGaN films on GaN templates using plastic relaxation through buried cracks

A method is presented to achieve thick high quality crack-free AlGaN layers on GaN. This method uses jointly plastic relaxation and lateral growth. In a first step, plastic relaxation by cracking and misfit dislocation introduction is realized. Then the cracks are overgrown to obtain a smooth surface. By this reproducible technique, we grew smooth metal-organic chemical vapor deposition Al0.2Ga0.8N films with a threading dislocation density as low as 5×108 cm−2. This result is the best ever reported for crack-free AlGaN growth over a large area. The control of the complete plastic relaxation opens up perspectives for the realization of high performance devices. In order to explain the mechanisms involved in the full relaxation of the AlGaN/GaN heterostructure, we propose a relaxation scheme and discuss its different steps.

Long wavelength GaInNAs/GaAs quantum-well heterostructures grown by solid-source molecular-beam epitaxy

We have investigated as-grown Ga1−xInxNyAs1−y/GaAs quantum-well heterostructures (QWHs) prepared by solid-source molecular-beam epitaxy (SS-MBE). We show that the QWH properties appear to depend strongly on the growth technique and that SS-MBE emerges as a technique of choice for growing these QWHs. We demonstrate photoluminescence emission at wavelength as long as 1.43 μm at 295 K, and up to 1.68 μm at 10 K. This shows that development of 1.55 μm optoelectronics based on the Ga1−xInxNyAs1−y/GaAs materials system may now be reasonably thought of.

The critical role of growth temperature on the structural and electrical properties of AlGaN/GaN high electron mobility transistor heterostructures grown on Si(111)

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 ...

On the polarity of GaN micro- and nanowires epitaxially grown on sapphire (0001) and Si(111) substrates by metal organic vapor phase epitaxy and ammonia-molecular beam epitaxy

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.

Relaxation mechanisms in metal-organic vapor phase epitaxy grown Al-rich (Al,Ga)N∕GaN heterostructures

The relaxation mechanisms in metal-organic vapor phase epitaxy grown (Al,Ga)N∕GaN heterostructures are studied. The first stage of the relaxation process is a two-dimensional–three-dimensional growth transition with the formation of mesalike islands separated by V-shaped trenches. The tensile stress relief is obtained by an elastic relaxation of the islands edges. In the case of AlN∕GaN, the apexes of the V trenches reach the heterointerface and misfit dislocations are nucleated at the islands coalescence region. These dislocations are a type and glide in the basal plane to promote further relaxation. For (Al,Ga)N∕GaN with an Al concentration below 70%, the apexes of the V trenches do not reach the heterointerface, prohibiting the nucleation of misfit dislocations. For thicker layers, the next stage of the relaxation is the cracking of the films.

In situ growth monitoring of distributed GaN–AlGaN Bragg reflectors by metalorganic vapor phase epitaxy

A series of distributed GaN-AlGaN Bragg reflectors (DBR) has been grown on Al2O3(0001) substrates by metalorganic vapor phase epitaxy. The growth of the GaN template as well as of the GaN–AlxGa1−xN quarter-wave stack has been monitored by laser reflectometry. The evolution of the in situ reflectivity as well as DBR reflection spectra are discussed as function of the AlxGa1−xN composition x.

Correlation between threading dislocation density and the refractive index of AlN grown by molecular-beam epitaxy on Si(111)

We report on the influence of the structural properties on the refractive index of AlN films grown on Si(111) substrates by molecular-beam epitaxy using ammonia. The structural properties are assessed by reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and x-ray diffraction. Refractive index values are deduced from room-temperature spectroscopic ellipsometry. Optical data analysis is performed using the Kramers-Kronig relation in the transparent spectral region, from 1.6 to 3.2 eV. Evidence is presented showing the influence of strain and dislocation density on the AlN layer refractive index.

GaN/Al0.5Ga0.5N (11-22) semipolar nanostructures: A way to get high luminescence efficiency in the near ultraviolet range

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...

Study of the growth mechanisms of GaN/(Al, Ga)N quantum dots: Correlation between structural and optical properties

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.

Investigation of AlN films grown by molecular beam epitaxy on vicinal Si(111) as templates for GaN quantum dots

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.