C. Adelmann

Self-assembled InGaN quantum dots grown by molecular-beam epitaxy

Self-assembled InGaN islands were grown by molecular-beam epitaxy on GaN, following a Stranski–Krastanow growth mode. Atomic force microscopy revealed that their dimensions were small enough to expect zero-dimensional quantum effects: the islands were typically 27 nm wide and 2.9 nm high. Strong blue-violet photoluminescence of the dots is observed, persisting up to room temperature. The temperature dependence of the photoluminescence is analyzed and compared to that of InGaN quantum well and bulk samples.

Dynamically stable gallium surface coverages during plasma-assisted molecular-beam epitaxy of (0001) GaN

The Ga surface coverage during the growth of GaN by plasma-assisted molecular-beam epitaxy (PAMBE) has been systematically studied by reflection high-energy electron diffraction as a function of the Ga flux and the substrate temperature. As a consequence, a diagram is depicted, which describes the Ga surface coverage during PAMBE as function of growth conditions. In particular, we show that a region exists in this diagram, in which the Ga surface coverage is independent of fluctuations in the Ga flux or the substrate temperature and which forms a “growth window” for GaN growth. The influence of the Ga surface coverage on the GaN surface morphology and the growth kinetics is discussed.

Plastic strain relaxation of nitride heterostructures

Thick layers of GaN on AlN, AlN on GaN, and InN on GaN were grown by plasma-assisted molecular beam epitaxy. Their plastic strain relaxation was studied by reflection high-energy electron diffraction (RHEED) and high resolution x-ray diffraction (HRXRD). The results are consistent with a mechanism of progressive introduction of misfit dislocations based on the coalescence of dynamically formed platelets. Due to the lack of proper gliding planes in the wurtzite structure, such dislocations are not mobile, leading to inhomogeneity of the strain state along the growth axis. The agreement between in situ RHEED and ex situ HRXRD measurements provides evidence that the strain state is frozen in during growth.

GaN islanding by spontaneous rearrangement of a strained two-dimensional layer on (0001) AlN

It is shown that a two-dimensional GaN layer grown on (0001) AlN under Ga-rich conditions remains two-dimensional while annealing under a Ga flux due to a surfactant effect of Ga. In contrast, further annealing under vacuum without the Ga flux leads to evaporation of excess Ga and to spontaneous transformation of the GaN layer into islands if the initial layer is thicker than about 2.5 monolayers. The resulting morphology is studied by atomic force microscopy and transmission electron microscopy. The latter reveals that these islands sit on top of a continuous 2.5 monolayer thick wetting layer, i.e., they represent a Stranski–Krastanow structure.

Indium incorporation during the growth of InGaN by molecular-beam epitaxy studied by reflection high-energy electron diffraction intensity oscillations

The indium incorporation in hexagonal InGaN grown by plasma assisted molecular-beam epitaxy is studied by means of reflection high-energy electron diffraction (RHEED) intensity oscillations. It is demonstrated that, in addition to being incorporated in the alloy, indium acts as a surfactant, significantly changing the gallium incorporation. This surfactant effect has to be taken into account to allow for a precise in situ determination of the alloy composition. The indium concentrations determined in situ by RHEED intensity oscillations were found to be in good agreement with ex situ results of Rutherford backscattering spectroscopy. This method also allows us to directly assess the maximum In incorporation as a function of the substrate temperature.

Growth and characterisation of self-assembled cubic GaN quantum dots

Abstract Self-assembled cubic GaN quantum dots have been grown by plasma-assisted molecular-beam epitaxy on cubic AlN. Atomic force microscopy and transmission electron microscopy reveal islands of a mean height of 1.6 nm and a mean diameter of 13 nm. The influence of stacking faults on island nucleation is discussed. The quantum dots show ultraviolet photo- and cathodo-luminescence with no thermal quenching up to room temperature.

Epitaxial Growth of GaN, AlN and InN: 2D/3D Transition and Surfactant Effects

The influence of foreign species on the growth mode has been studied, with special emphasis on the role of In. It is demonstrated that the growth mode, i.e. 2D or 3D, first depends on the competing kinetics of Ga and N. In this view, we show that the surfactants modify both the Ga droplet formation process and the N desorption rate. Next, we discuss the role of strain relaxation in nitride heterostructures. In particular, we show that GaN and InGaN can experience a Stranski-Krastanow growth mode leading to the formation of quantum dots. A mechanism of quantum dot nucleation is proposed in the case of GaN on AlN.

Structure and ordering of GaN quantum dot multilayers

Grazing incidence x-ray techniques are used to characterize the structure of multilayered GaN quantum dots in an AlN matrix. For a dot lateral size of 170 A, the values of the interdot vertical and lateral correlation lengths are 1500 and 250 A, respectively. The presence of smaller quantum dots is observed only in the layers deposited first. The strain distribution in the multilayer is also investigated as a function of depth. Along the dot columns, the crystal lattice remains coherent, with elastic relaxation from the bottom to the top of the multilayer.

Atomic-layer epitaxy of GaN quantum wells and quantum dots on (0001) AlN

We show that a dynamically stable Ga film is formed on (0001) AlN in a large range of Ga fluxes at a substrate temperature of 740u200a°C. This feature allows for atomic layer epitaxy (ALE) of GaN on AlN by alternate exposure to Ga and N flux. We show that, at a growth temperature of 740u200a°C, one ALE cycle leads to the formation of a two-dimensional GaN layer, whereas further cycles lead to the formation of GaN quantum dots following a Stranski–Krastanov growth mode. This behavior is confirmed by atomic force microscopy, transmission electron microscopy, and cathodoluminescence.

Growth and Optical Characterization of InGaN Quantum Dots Resulting from a 2D–3D Transition

The growth of InGaN on GaN by plasma-assisted molecular beam epitaxy is studied using reflection high energy electron diffraction. We find that InGaN follows a layer-by-layer growth mode for low In contents and a Stranski-Krastanov growth mode for In concentrations above a critical value. Using this 2D–3D transition, nanometric islands are grown. These islands are characterized by atomic force microscopy. Their size is found to be small enough to expect strong confinement effects. Photoluminescence experiments show strong blue light emission at room temperature. The temperature dependence of dots luminescence is compared to that of a quantum well and a bulk sample.