TEM study of GaN/AlN quantum dots deposited on vicinal silicon

Transmission electron microscopy was performed to investigate the use of AlN epitaxial films deposited on vicinal Si(111) as templates for the growth of GaN quantum dots. 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. The microstructure of different QD encountered in the GaN/AlN system is also described.Transmission electron microscopy was performed to investigate the use of AlN epitaxial films deposited on vicinal Si(111) as templates for the growth of GaN quantum dots. 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. The microstructure of different QD encountered in the GaN/AlN system is also described

Filtering of Defects in Semipolar (11−22) GaN Using 2-Steps Lateral Epitaxial Overgrowth

Good-quality (11−22) semipolar GaN sample was obtained using epitaxial lateral overgrowth. The growth conditions were chosen to enhance the growth rate along the [0001] inclined direction. Thus, the coalescence boundaries stop the propagation of basal stacking faults. The faults filtering and the improvement of the crystalline quality were attested by transmission electron microscopy and low temperature photoluminescence. The temperature dependence of the luminescence polarization under normal incidence was also studied

Intentional polarity conversion of AlN epitaxial layers by oxygen

Nitride materials (AlN, GaN, InN and their alloys) are commonly used in optoelectronics, high-power and high-frequency electronics. Polarity is the essential characteristic of these materials: when grown along c-direction, the films may exhibit either N- or metal-polar surface, which strongly influences their physical properties. The possibility to manipulate the polarity during growth allows to establish unique polarity in nitride thin films and nanowires for existing applications but also opens up new opportunities for device applications, e.g., in non-linear optics. In this work, we show that the polarity of an AlN film can intentionally be inverted by applying an oxygen plasma. We anneal an initially mixed-polar AlN film, grown on sapphire substrate by metal-organic vapor phase epitaxy (MOVPE), with an oxygen plasma in a molecular beam epitaxy (MBE) chamber; then, back in MOVPE, we deposit a 200 nm thick AlN film on top of the oxygen-treated surface. Analysis by high-resolution probe-corrected scanning transmission electron microscopy (STEM) imaging and electron energy-loss spectroscopy (EELS) evidences a switch of the N-polar domains to metal polarity. The polarity inversion is mediated through the formation of a thin AlxOyNz layer on the surface of the initial mixed polar film, induced by the oxygen annealing

Polarity Control in Group-III Nitrides beyond Pragmatism

Controlling the polarity of polar semiconductors on nonpolar substrates offers a wealth of device concepts in the form of heteropolar junctions. A key to realize such structures is an appropriate buffer-layer design that, in the past, has been developed by empiricism. GaN or ZnO on sapphire are prominent examples for that. Understanding the basic processes that mediate polarity, however, is still an unsolved problem. In this work, we study the structure of buffer layers for group-III nitrides on sapphire by transmission electron microscopy as an example. We show that it is the conversion of the sapphire surface into a rhombohedral aluminum-oxynitride layer that converts the initial N-polar surface to Al polarity. With the various AlxOyNz phases of the pseudobinary Al2O3-AlN system and their tolerance against intrinsic defects, typical for oxides, a smooth transition between the octahedrally coordinated Al in the sapphire and the tetrahedrally coordinated Al in AlN becomes feasible. Based on these results, we discuss the consequences for achieving either polarity and shed light on widely applied concepts in the field of group-III nitrides like nitridation and low-temperature buffer layers

Luminescence behavior of semipolar (101¯1) InGaN/GaN “bow-tie” structures on patterned Si substrates

In this work, we report on the innovative growth of semipolar “bow-tie”-shaped GaN structures containing InGaN/GaN multiple quantum wells (MQWs) and their structural and luminescence characterization. We investigate the impact of growth on patterned (113) Si substrates, which results in the bow-tie cross section with upper surfaces having the (101¯1) orientation. Room temperature cathodoluminescence (CL) hyperspectral imaging reveals two types of extended defects: black spots appearing in intensity images of the GaN near band edge emission and dark lines running parallel in the direction of the Si stripes in MQW intensity images. Electron channeling contrast imaging (ECCI) identifies the black spots as threading dislocations propagating to the inclined (101¯1) surfaces. Line defects in ECCI, propagating in the [12¯10] direction parallel to the Si stripes, are attributed to misfit dislocations (MDs) introduced by glide in the basal (0001) planes at the interfaces of the MQW structure. Identification of these line defects as MDs within the MQWs is only possible because they are revealed as dark lines in the MQW CL intensity images, but not in the GaN intensity images. Low temperature CL spectra exhibit additional emission lines at energies below the GaN bound exciton emission line. These emission lines only appear at the edge or the center of the structures where two (0001) growth fronts meet and coalesce (join of the bow-tie). They are most likely related to basal-plane or prismatic stacking faults or partial dislocations at the GaN/Si interface and the coalescence region

Recent TEM developments applied to quantum structures

To shed light on these confined properties, a technique with a high energy-and-spatial resolution is of absolute need. Modern transmission electron microscopy (TEM) is the most suitable technique for a direct measurement of optical and structural properties at a nanometer scale. Thanks to the successful construction of aberration corrected transmission electron microscopes, the mapping of atomic positions with high accuracy becomes a routine experiment enabling therefore a more intuitive interpretation of structural deformation (strain). In addition, the recent development in energy-filters, especially when coupled to monochromated electron-beams, measurements of physical properties are achieved with unprecedented performances. The case of individual buried GaN/(AlGaN) quantum dots is discussed

Plastic relaxation through buried cracks in AlGaN/GaN heterostructures

Due to high lattice mismatch, heterostructures of III-nitrides are subject to plastic relaxation. In this paper, we focus on the relaxation of AlGaN films grown on GaN. This relaxation is realised by cracking followed by the introduction of misfit dislocations. We describe those mechanisms and present a method to grow thick high quality crack-free AlGaN layers. This method uses jointly plastic relaxation and lateral growth