M. Korytov

Interfacial structure and defect analysis of nonpolar ZnO films grown on R-plane sapphire by molecular beam epitaxy

The interfacial relationship and the microstructure of nonpolar (11−20) ZnO films epitaxially grown on (1−102) R-plane sapphire by molecular beam epitaxy are investigated by transmission electron microscopy. The already-reported epitaxial relationships [1−100]ZnO∥[11−20]sapphire and ⟨0001⟩ZnO∥[−1101]sapphire are confirmed, and we have determined the orientation of the Zn–O (cation-anion) bond along [0001]ZnO in the films as being uniquely defined with respect to a reference surface Al–O bond on the sapphire substrate. The microstructure of the films is dominated by the presence of I1 basal stacking faults [density=(1–2)×105cm−1] and related partial dislocations [density=(4–7)×1010cm−2]. It is shown that I1 basal stacking faults correspond to dissociated perfect dislocations, either c or a+c type.

Indium incorporation dynamics into AlInN ternary alloys for laser structures lattice matched to GaN

Al1−xInxN ternary alloys with solid phase indium compositions between x=0.15 and 0.28 have been grown by metal organic chemical vapor deposition under indium rich conditions within the growth temperature range of 750–810u2009°C. A thermally activated process with activation energy of 1.05±0.05eV is found to compete with indium incorporation. Smooth epitaxial layers with root mean-squares surface roughness of 0.3–0.8nm are obtained. (Al,In)N films lattice matched to GaN have been introduced into laser diode structures for optical confinement. Optical gain is observed.

Tailoring the shape of GaN/AlxGa1−xN nanostructures to extend their luminescence in the visible range

We show that the shape of GaN nanostructures grown by molecular beam epitaxy on AlxGa1−xN (0001) surfaces, for x≥0.4, can be controlled via the ammonia pressure. The nanostructures are obtained from a two dimensional to three dimensional transition of a GaN layer occurring upon a growth interruption. Atomic force microscopy measurements show that depending on the ammonia pressure during the growth interruption, dot or dash-shaped nanostructures can be obtained. Low temperature photoluminescence measurements reveal a large redshift in the emission energy of the quantum dashes, as compared to the quantum dots. By simply adjusting the GaN deposited thickness, it is shown that quantum dashes enable to strongly extend the emission range of GaN/Al0.5Ga0.5N nanostructures from the violet-blue (∼400–470u2002nm) to the green-orange range (∼500–600u2002nm).

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.

Analysis of statistical compositional alloy fluctuations in InGaN from aberration corrected transmission electron microscopy image series

We propose an approach that improves the measuring precision of lattice parameters in semiconductor alloys from high resolution transmission electron microscopy images on the unit cell scale. The method is based on the evaluation of a series of images taken under optimized imaging conditions in an aberration corrected microscope. By comparing image simulations with experimental images, we show that hardly avoidable amorphous surface layers are the main cause for the limited measuring precision. We use our approach to analyze the In distribution within a nominal In0.09Ga0.91N and In0.16Ga0.84N layer and show that both layers do not show a deviation from a random alloy. In addition, we are able to resolve local variations of the in-plane (a-lattice) and out of plane (c-lattice) parameter at the unit cell scale. These fluctuations are due to the local strain within the InGaN alloy caused by the different atomic radii of In and Ga.

Structural properties of InN films grown on O-face ZnO(0001¯) by plasma-assisted molecular beam epitaxy

We study the impact of substrate temperature and layer thickness on the morphological and structural properties of InN films directly grown on O-face ZnO(0001¯) substrates by plasma-assisted molecular beam epitaxy. With increasing substrate temperature, an interfacial reaction between InN and ZnO takes place that eventually results in the formation of cubic In2O3 and voids. The properties of the InN films, however, are found to be unaffected by this reaction for substrate temperatures less than 550u2009°C. In fact, both the morphological and the structural quality of InN improve with increasing substrate temperature in the range from 350 to 500u2009°C. High quality films with low threading dislocation densities are demonstrated.

Polar and semipolar GaN/Al0.5Ga0.5N nanostructures for UV light emitters

AlxGa1−xN-based ultra-violet (UV) light emitting diodes (LEDs) are seen as the best solution for the replacement of traditional mercury lamp technology. By adjusting the Al concentration, a large emission spectrum range from 360 nm (GaN) down to 200 nm (AlN) can be covered. Owing to the large density of defects typically present in AlxGa1−xN materials usually grown on sapphire substrates, LED efficiencies still need to be improved. Taking advantage of the 3D carrier confinement, quantum dots (QDs) are among the solutions currently under investigation to improve the performances of UV LEDs. The objectives of this work are to present and discuss the morphological and optical properties of GaN nanostructures grown by molecular beam epitaxy on the (0 0 0 1) and the (11–22) orientations of Al0.5Ga0.5N. In particular, the dependence of the morphological properties of the nanostructures on the growth conditions and the surface orientation will be presented. The optical characteristics as a function of the nanostructure design (size, shape and dimensionality) will also be shown and discussed. The electroluminescence characteristics of a first series of QD-based GaN/Al0.5Ga0.5N LEDs grown on the polar (0 0 0 1) plane will be investigated.

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.

Stark effect in ensembles of polar (0001) Al0.5Ga0.5N/GaN quantum dots and comparison with semipolar (11−22) ones

This work presents a continuous-wave photoluminescence study of Al0.5Ga0.5N/GaN quantum dots grown by ammonia-assisted molecular beam epitaxy on sapphire, either on the wurtzite polar (0001) or the semipolar (11−22) plane. Due to interface polarization discontinuities, the polar dots are strongly red-shifted by the Stark effect and emit in the visible range. Carrier injection screening of the polarization charges has been studied. A model relying on average dot heights and dot height variances, as measured by transmission electron microscopy, is proposed. It can account for the injection dependent luminescence energies and efficiencies. The electric field discontinuity deduced from the fittings is in good agreement with theoretical expectations for our barrier composition. On the contrary, semipolar quantum dot ensembles always emit above the gap of GaN strained to Al0.5Ga0.5N. Their luminescence linewidth is significantly lower than that of polar ones, and their energy does not shift with injection. Our ...

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.