Properties Of GaN Quantum Dots Research Articles

Density control of GaN quantum dots on AlN single crystal

Full control over the density and emission properties of GaN quantum dots (QDs) should be feasible, provided that the growth proceeds in the Stranski-Krastanov (SK) growth mode. In this work, we derive the phase diagram for GaN QD formation on AlN by NH3-molecular beam epitaxy and analyze the corresponding optical signature by micro-photoluminescence (μ-PL). Interestingly, the growth window for SK-GaN QDs is very narrow due to the relatively small lattice mismatch of the GaN/AlN system (2.5%), constituting a fundamental challenge for QD growth control. By relying on bulk AlN single crystal substrates, we demonstrate QD density control over three orders of magnitude, from 108 to 1011 cm−2 by changing the growth rate. In contrast, the QD density is pinned to 2 × 1010 cm−2 when growing on AlN/sapphire templates, which exhibit dislocation densities on the order of 1010 cm−2. Thanks to QD densities as low as 108 cm−2 on bulk AlN, we can probe the emission of spatially isolated single GaN QDs by μ-PL on unprocessed samples.

Strain- and surface-induced modification of photoluminescence from self-assembled GaN/Al0.5Ga0.5N quantum dots: strong effect of capping layer and atmospheric condition

We report on the influence of a capping layer on the photoluminescence properties of self-assembled GaN quantum dots grown on an Al0.5Ga0.5N template. Self-assembled GaN quantum dots show a large quantum confined Stark shift and long carrier recombination time due to strong built-in spontaneous and piezoelectric polarization fields. Nevertheless, owing to strong carrier localization and suppressed nonradiative processes, these quantum dots have a high-quantum efficiency even at room temperature. Here, we show that the capping thickness has an important role on the optical properties of the GaN quantum dots. The radiative and nonradiative recombination processes of quantum dots are strongly affected by adjusting the capping thickness, and the GaN quantum dots with 12 monolayers-thick Al0.5Ga0.5N capping layer show a remarkably high internal quantum efficiency of more than 80% at room temperature. We also studied photoluminescence quenching and enhancement for surface (uncapped) quantum dots caused by photoadsorption and photodesorption of oxygen.

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.

Cathodoluminescence characterization of GaN quantum dots grown on 6H–SiC substrate by metal-organic chemical vapor deposition

The luminescence properties of GaN quantum dots (QDs) were characterized by cathodoluminescence (CL) at room temperature via spectroscopy and monochromatic imaging. The correlations between CL results and the QD geometry measured by atomic force microscopy (AFM) were discussed considering the quantum confinement and built-in electric field in the GaN QDs. We demonstrated that CL characterizations supply abundant information about a QD system, e.g. the exciton diffusion length and the ratio of radiative recombination efficiency in the wetting layer and QDs.

Optical properties of GaN quantum dots grown on nonpolar (11-20) SiC by molecular-beam epitaxy

We report on nonpolar GaN quantum dots embedded in AlN, grown on (11-20) 6H–SiC by plasma-assisted molecular-beam epitaxy. These dots are aligned in the growth plane and present a constant aspect ratio of 10. Their optical properties were studied as a function of GaN coverage. Especially, the variation of their emission energy as compared to that of (0001) GaN quantum dots is a clear fingerprint of the reduced internal electric field present in these nonpolar nanostructures. Time-resolved spectroscopy confirmed this result by revealing lifetimes in the few 100 ps range in contrast to the much longer ones obtained for the (0001) GaN quantum dots.

GaN quantum dots: from basic understanding to unique applications

The GaN self-assembled quantum dots constitute a very special and intriguing type of semiconductor nanostructure, mainly because they carry in their structure a giant internal electric field that can reach a value up to 7 MV/cm. In this report, we review the most important structural and optical properties of GaN quantum dots, and we discuss their advantages and limitations for blue-UV optoelectronic applications.

Optical and structural properties of rare earth doped GaN quantum dots

We have studied the structural and optical properties of GaN quantum dots (QDs) doped with Tm and Eu. It has been found that the morphology of the dots was affected by the presence of the rare earth atoms. Differences in incorporation have also been pointed out, Eu being located inside the QDs while Tm is more likely to be found at the interface between GaN QDs and AlN matrix. Intense and sharp Eu- and Tm-related transition lines are observed in the photoluminescence and cathodoluminescence spectra of doped QDs, with no thermal quenching between 20 and 300 K. They exhibit a saturation effect with increasing excitation power, independently of the temperature. It is tentatively proposed that some transition lines in GaN:Tm QDs could be induced by the presence of the internal electric field.

Morphological properties of GaN quantum dots doped with Eu

Morphological properties of Eu-doped GaN quantum dots grown by molecular beam epitaxy have been studied. Eu tends to segregate on the surface of AlN and GaN, leading to drastic changes in adatom kinetics. As a consequence, both size and density of Eu-doped GaN quantum dots strongly depend on the Eu flux used during the growth.

Self-consistent calculations of the optical properties of GaN quantum dots

We present calculations of the transition energies and radiative lifetimes in GaN quantum dots embedded in AlN. The effects of elastic strains, and piezoelectric and pyroelectric fields are included. The electronic structure is described using a tight-binding method which takes into account the screening of the internal electric field by excited carriers in a fully self-consistent procedure. We show that the presence of one electron-hole pair in a quantum dot increases the optical gap by a few tens of meV and decreases significantly the radiative lifetime, which could give rise to very interesting nonlinear optical effects.

Structural and Optical Properties of GaN Quantum Dots

ABSTRACTGrowth conditions, structural and optical properties of GaN quantum dots (QDs) grown by plasma-assisted molecular beam epitaxy will be examined. It will be shown that, depending on the Ga/N ratio value and on growth temperature, the growth mode of GaN deposited on AlN can be either of the Stranski-Krastanow or of the Frank-Van der Merwe type. It will be shown that vertical correlation results in a red shift and in a narrowing of the photoluminescence spectra.Growth of Eu-doped GaN quantum dots embedded in AlN will be described. Intense photoluminescence associated with Eu has been measured, with no GaN band-edge emission, as an evidence that carrier recombination mostly occurs through rare earth ion excitation. Persistent photoluminescence of Eu-doped GaN quantum dots as a function of temperature has been put in evidence, as a further confirmation of the recombination of confined carriers through Eu ion excitation.

Photoluminescence properties of multiple stacked planes of GaN/AlN quantum dots studied by near-field optical microscopy.

We have studied the photoluminescence properties of GaN quantum dots with submicrometre lateral resolution by means of near-field scanning optical microscopy. The instrument operated at room temperature and was implemented for near-ultra-violet spectroscopy in the illumination-mode configuration. The analysed sample consisted of several stacked planes of GaN/AlN quantum dots grown by molecular beam epitaxy on Si(111) substrate. The photoluminescence maps showed islands in the micrometre range emitting at different wavelengths, confirming the atomic force microscopy studies on the morphology of similar uncapped samples.

Near-Field Optical Spectroscopy of Multiple Stacked Planes of GaN/AlN Quantum Dots

We have investigated the photoluminescence properties of GaN quantum dots with sub-micron lateral resolution by means of Near-Field Scanning Optical Microscopy (SNOM) operating in illumination mode. The analyzed sample consists of several stacked planes of GaN/AlN quantum dots grown by molecular beam epitaxy on Si(111) substrate. A clear correlation between the surface topography measured by Atomic Force Microscopy on uncapped samples and the SNOM luminescence maps has been found.

Optical properties of GaN quantum dots

We report on an investigation of the optical properties of GaN quantum dots (QDs) grown by means of metalorganic vapor phase epitaxy. The growth regime for GaN on AlxGa1−xN was observed to change from two- to three-dimensional, forming GaN QDs, when Si was deposited on the AlxGa1−xN surface prior to the GaN growth. These QDs showed a redshift of the photo luminescence (PL) energy from the increased Coulomb energy induced by a compression of the exciton Bohr radius. Furthermore, a diminishing temperature-dependent shift of the PL energy with decreasing QD size caused by a reduction of the longitudinal-optical phonon coupling was found. We also show that the size of the QDs is a critical parameter for the optical nonlinearities. For large dots, the dominant nonlinearity in the PL is the bandgap renormalization but when the size of the dots was reduced below the critical size of 10 nm thick and 30 nm diameter, the state-filling effect became dominant.

Blue-light emission from GaN self-assembled quantum dots due to giant piezoelectric effect

It is shown that the optical properties of GaN quantum dots with the wurtzite structure result from a balance between confinement and piezoelectric effects. In ``large'' quantum dots with an average height and diameter of 4.1 and 17 nm, respectively, the photoluminescence peak is centered at 2.95 eV, nearly 0.5 eV below the bulk GaN band gap. We attribute this enormous redshift to a giant 5.5 MV/cm piezoelectric field present in the dots, in agreement with theoretical calculations.