V. Fellmann

Molecular beam epitaxy growth and optical properties of AlN nanowires

Growth of catalyst-free AlN nanowires has been achieved by plasma-assisted molecular beam epitaxy on SiO2/Si (100), by taking advantage of Volmer–Weber growth mode of AlN on amorphous SiO2. Using a combination of high resolution transmission electron microscopy and Raman spectroscopy, it is found that AlN nanowires are completely relaxed, which has been assigned to the compliant character of SiO2. Elastic strain relaxation of AlN nanowires has been further confirmed by photoluminescence experiments, showing in addition that spectra are dominated by near-band edge emission.

Functionalizing self-assembled GaN quantum dot superlattices by Eu-implantation

Self-assembled GaN quantum dots (QDs) stacked in superlattices (SL) with AlN spacer layers were implanted with Europium ions to fluences of 1013, 1014, and 1015 cm−2. The damage level introduced in the QDs by the implantation stays well below that of thick GaN epilayers. For the lowest fluence, the structural properties remain unchanged after implantation and annealing while for higher fluences the implantation damage causes an expansion of the SL in the [0001] direction which increases with implantation fluence and is only partly reversed after thermal annealing at 1000 °C. Nevertheless, in all cases, the SL quality remains very good after implantation and annealing with Eu ions incorporated preferentially into near-substitutional cation sites. Eu3+ optical activation is achieved after annealing in all samples. In the sample implanted with the lowest fluence, the Eu3+ emission arises mainly from Eu incorporated inside the QDs while for the higher fluences only the emission from Eu inside the AlN-buffer, ...

Selective ion-induced intermixing and damage in low-dimensional GaN/AlN quantum structures

Ion-induced intermixing and damage is evaluated in GaN/AlN superlattices of quantum dots (QDs) and quantum wells (QWs) using 100 keV Ar(+) implantation at low temperature (15 K). Despite the similar damage build up at low fluences, a significant increase of the damage accumulation takes place for QDs at high fluences. Elemental depth profiles were fitted with a diffusion model, revealing the higher intermixing efficiency in QD superlattices, significantly higher than for QWs. The scaling of diffusion length with the local fluence and defect concentration is understood on the basis of cascade mixing and migration of defects in the cation sublattice. The selective intermixing/damage of QDs is explained by the promotion of lateral diffusion mechanisms that result in smooth interfaces, as well as by an enhanced diffusivity due to the characteristic strain distribution in QD superlattices.

Strain assisted inter-diffusion in GaN/AlN quantum dots

The structural and optical properties of high temperature-annealed superlattices of GaN quantum dots embedded in AlN barrier have been studied by a combination of X-ray techniques (reciprocal space mapping, multiwavelength anomalous diffraction, and diffraction anomalous fine structure), high resolution transmission electron microscopy, and photoluminescence spectroscopy. Taking advantage of the disentangling of the chemical and structural information provided by the simultaneous use of X-ray absorption and diffraction data obtained in a synchrotron environment, we provide quantitative determination of strain and composition for each different region of the nanostructures. Eventually, it is shown that strain driven dot/barrier intermixing is present, mostly on top of the dots. These observations have been confirmed by high resolution electron microscopy. A blue shift of photoluminescence peak has been furthermore observed and assigned to GaN/AlN intermixing suggesting a new path for engineering the emissi...

Effect of Eu-implantation and annealing on the GaN quantum dots excitonic recombination

Undoped self-assembled GaN quantum dots (QD) stacked in superlattices (SL) with AlN spacer layers were submitted to thermal annealing treatments. Changes in the balance between the quantum confinement, strain state of the stacked heterostructures and quantum confined Stark effect lead to the observation of GaN QD excitonic recombination above and below the bulk GaN bandgap. In Eu-implanted SL structures, the GaN QD recombination was found to be dependent on the implantation fluence. For samples implanted with high fluence, a broad emission band at 2.7 eV was tentatively assigned to the emission of large blurred GaN QD present in the damage region of the implanted SL. This emission band is absent in the SL structures implanted with lower fluence and hence lower defect level. In both cases, high energy emission bands at approx. 3.9 eV suggest the presence of smaller dots for which the photoluminescence intensity was seen to be constant with increasing temperatures. Despite the fact that different deexcitation processes occur in undoped and Eu-implanted SL structures, the excitation population mechanisms were seen to be sample-independent. Two main absorption bands with maxima at approx. 4.1 and 4.7 to 4.9 eV are responsible for the population of the optically active centres in the SL samples.

Ternary AlGaN Alloys with High Al Content and Enhanced Compositional Homogeneity Grown by Plasma-Assisted Molecular Beam Epitaxy

We have studied the influence of III/N flux ratio and growth temperature on structural and optical properties of high Al-content, around 50–60%, AlGaN alloy layers grown by plasma-assisted molecular beam epitaxy. In a first part, based on structural analysis by Rutherford Backscattering Spectroscopy, we establish that a III/N flux ratio slightly above 1 produces layers with low amount of structural defects. In a second part, we study the effect of growth temperature on structural and optical properties of layers grown with previously determined optimal III/N flux ratio. We find that optimal growth temperatures for Al0.50Ga0.50N layers with compositional homogeneity related with narrow UV photoluminescence properties are in the low temperature range for growing GaN layers, i.e., 650–680 °C. We propose that lowering Ga adatom diffusion on the surface favors random incorporation of both Ga and Al adatoms on wurtzite crystallographic sites leading to the formation of an homogeneous alloy.