M. Benaissa

Pyramidal defects in metalorganic vapor phase epitaxial Mg doped GaN

A transmission electron microscopy study of structural defects induced by the introduction of Mg during the growth of metalorganic vapor phase epitaxy GaN is presented. These defects are assumed to be pyramidal inversion domains with an hexagonal base and {1123} inclined facets. The tip of the pyramids is always pointing toward the [0001] direction, i.e., in a Ga-terminated film, toward the substrate and in a N-terminated film, toward the surface. A chemical quantitative analysis shows that these pyramidal defects are Mg rich. They are present in all the studied films, independent of the doping level.

Influence of high Mg doping on the microstructural and optoelectronic properties of GaN

A transmission electron microscopy study of a wide range of p-type GaN samples reveals that high Mg doping has a strong influence on the polarity of GaN. The main characteristic of Mg-doped metal organic vapour phase epitaxy (MOVPE) and bulk GaN is the presence of pyramidal inversion domains (PIDs). It is shown that the appearance of PIDs is correlated with a decrease of the free hole concentration and with the appearance of the blue photoluminescence band which is characteristic of MOVPE-grown Mg-doped GaN. A tentative model based on electrostatic considerations is proposed for this blue luminescence band.

Effects of capping on GaN quantum dots deposited on Al0.5Ga0.5N by molecular beam epitaxy

The impact of the capping process on the structural and morphological properties of GaN quantum dots (QDs) grown on fully relaxed Al0.5Ga0.5N templates was studied by transmission electron microscopy. A morphological transition between the surface QDs, which have a pyramidal shape, and the buried ones, which have a truncated pyramid shape, is evidenced. This shape evolution is accompanied by a volume change: buried QDs are bigger than surface ones. Furthermore a phase separation into Al0.5Ga0.5N barriers was observed in the close vicinity of buried QDs. As a result, the buried QDs were found to be connected with the nearest neighbors by thin Ga-rich zones, whereas Al-rich zones are situated above the QDs.

Structural defects and relation with optoelectronic properties in highly Mg-doped GaN

A transmission electron microscopy of pyramidal inversion domains induced by Mg doping in MOVPE and bulk GaN is presented. Based on high resolution observations and EDX analysis, two atomic models are proposed for the Mg-rich (0001) inversion domain boundaries. These structural defects appearing for Mg concentrations in the 10(19) cm(-3) range are shown to be possible origins for the auto-compensation and the blue luminescence.

Electron energy-loss spectroscopy characterization of pyramidal defects in metalorganic vapor-phase epitaxy Mg-doped GaN thin films

In the present letter, Mg-doped GaN thin films grown by metalorganic vapor-phase epitaxy were studied using parallel electron energy-loss spectroscopy in a transmission electron microscope. A microstructural characterization of such thin films showed the presence of pyramidal defects (PDs) with a density of about 1018 cm−3. Comparison of energy-loss spectra recorded outside a PD and from the PD showed a significant change in the energy-loss near-edge structure strongly reflecting the presence of inclusions (Mg-based), the electronic properties of which differ from those of GaN. Considering, however, their relatively high density (∼1018 cm−3), one can expect that the optical properties of such inclusions may interfere with those of GaN and, therefore, be at the origin of the frequently obtained blue emission at 2.8–2.9 eV in heavily doped samples.

Phase separation in GaN/AlGaN quantum dots

Local investigations using high-angle annular-dark-field imaging combined with electron low-energy-loss spectroscopy were performed to closely characterize the GaN/Al0.5Ga0.5N quantum dots heterostructure. It is found that the Al0.5Ga0.5N barrier tends to exhibit phase separation. Gallium-rich arms arise from the top rims of the truncated quantum dots while the space between these arms is filled with aluminum-rich AlGaN. This phase separation, due to morphological and strain nonuniformities of the GaN front surface, provokes an optical-property modulation in the neighborhood of the quantum dots which, from a practical point of view, could affect the electronic barrier homogeneity.

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.

Plasmon energy from strained GaN quantum wells

Monochromated valence electron energy-loss spectroscopy in a transmission electron microscope has been used to study plasmon energy from strained GaN quantum wells. The width of studied wells was 4 nm, 3 nm, and 2 nm, respectively. A highly resolved line-spectrum recording shows a shift with respect the GaN bulk-value of the plasmon peak as a function of the width of the quantum well. This shift is explained by considering three major effects, namely: (i) relativistic effects due to the travel of fast electrons close to planar interfaces while recording the spectrum, (ii) strain within GaN wells, and (iii) quantum confinement due to the width of the well reduction. The combination of these factors is found to contribute to the observed blue shift in plasmon energies for the strained GaN quantum wells. So the use of high-resolution valence electron imaging has offered the possibility to distinguish the interplay of different confined properties in strained GaN QWs, which is very promising for understanding...

On the impact of indium distribution on the electronic properties in InGaN nanodisks

We analyze an epitaxially grown heterostructure composed of InGaN nanodisks inserted in GaN nanowires in order to relate indium concentration to the electronic properties. This study was achieved with spatially resolved low-loss electron energy-loss spectroscopy using monochromated electrons to probe optical excitations—plasmons—at nanometer scale. Our findings show that each nanowire has its own indium fluctuation and therefore its own average composition. Due to this indium distribution, a scatter is obtained in plasmon energies, and therefore in the optical dielectric function, of the nanowire ensemble. We suppose that these inhomogeneous electronic properties significantly alter band-to-band transitions and consequently induce emission broadening. In addition, the observation of tailing indium composition into the GaN barrier suggests a graded well-barrier interface leading to further inhomogeneous broadening of the electro-optical properties. An improvement in the indium incorporation during growth is therefore needed to narrow the emission linewidth of the presently studied heterostructures.

Mechanism of GaN quantum dot overgrowth by Al0.5Ga0.5N: Strain evolution and phase separation

The capping of GaN quantum dots (QDs) with an Al0.5Ga0.5N layer is studied using transmission electron microscopy and atomic force microscopy in combination with theoretical calculations. The capping process can be divided into several well-distinguishable stages including a QD shape change and a local change of the Al0.5Ga0.5N capping layer composition. The phase separation phenomenon is investigated in relation with the capping layer thickness. Amount of the chemical composition fluctuations is determined from separate analysis of scanning transmission electron microscopy and high-resolution transmission electron microscopy images. The local distortion of atomic lattice in the QD surroundings is measured by high-resolution electron microscopy and is confronted with theoretically calculated strain distributions. Based on these data, a possible mechanism of alloy demixing in the Al0.5Ga0.5N layer is discussed.