Excitons in (Al,Ga)N quantum dots and quantum wells grown on (0001)-oriented AlN templates: Emission diagrams and valence band mixings

Abstract

The luminescence efficiency of AlxGa1−xN quantum dots (QDs) and quantum wells (QWs), buried in AlN cladding layers and emitting in the ultraviolet range between 234 and 310 nm, has been investigated. The growth and optical properties have been done using similar aluminum composition (varying from 0.4 to 0.75) for both QDs and QWs. In order to compare as much as possible the optical properties, the QWs were fabricated with a growth time tuned such that the QW width is similar to the average height of the QDs. The photoluminescence (PL) showed emission ranging from 4 to 5.4 eV, putting into evidence differences in terms of full width at half maximum, PL intensity, and asymmetry of the line shape between QDs and QWs. The results show shorter wavelengths and a slightly narrower PL linewidth for QWs. To determine the light emission dependence with the electric field direction in the crystal, the evolutions of the emission diagrams for all samples were recorded along two orthogonal directions, namely, the “in-plane” (growth) and the “on-side” directions, from which the light emission was collected. For the whole QDs and QWs samples' series, the shapes of the emission diagram indicate emission in both in-plane and on-side directions, as evidenced by intra-valence band mixings caused by strain effects combined with the anisotropic Coulomb interactions that are particularly contributing to the polarization at wavelengths below 260 nm. Furthermore, the degree of polarization, determined for each sample, showed good agreement with results from the literature.

The Influence of Alloy Disorder Effects on the Anisotropy of Emission Diagrams in (Al,Ga)N Quantum Wells Embedded into AlN Barriers

“…Such a behavior is universal for (Al,Ga)N-AlN heterostructures and it was previously reported for quantum dots (QDs), but the Stranski-Krastanow growth modes of QDs prevented us from growing low-dimensional systems with aluminum compositions higher than 70-75%. [23] We demonstrate experimentally that neither the shapes of the polarization-resolved PL recorded for edge emission nor the evolution of the full width at half maximum (FWHM) of the PL lines can be interpreted quantitatively in the framework of the k.p description of the band structure of the QWs. This statement also holds concerning the DOP of the light emitted from the edge of the heterostructures.…”

“…Based on the in situ reflection high energy electron diffraction measurement, a diagram made of streaky lines was continuously observed that is characteristic of a 2D growth mode. [23] The deposited thickness ranged between 7 and 8 MLs, with one ML corresponding to an average thickness of about 0.256 nm. AFM measurements have also been carried out to characterize the surface morphology of the structures, showing surfaces made of surface monoatomic steps and smooth terraces, with a typical RMS roughness of 0.2 nm and an average terrace width of 70 AE 20 nm and a step height of 1 mL, as indicated in Figure 1c.…”

“…The PL setup was described in ref. [23]. The PL signal was excited with the fifth harmonic (corresponding to a wavelength value of 212.8 nm) of Nd:YAG laser with passive Q-switching, resulting in a pulse width of the order of 1 ns and a repetition rate of 10 kHz.…”

“…A specific issue that has been identified for (Al,Ga)N is the switch of the symmetry of the topmost valence band that occurs for a 10% aluminum composition in the bulk [20][21][22] and for a 60% aluminum composition in (Al,Ga)N lattice matched to AlN. [23] For (Al,Ga)N epilayers grown in the (0001) plane, there is a crossover of the photoluminescence (PL) polarization in the 260 nm range, from an emission of (significantly strong) transverse electric (TE) -polarized light to a transverse magnetic-polarized one, [24][25][26] at a critical composition ≥60% in aluminum in these latter strained-layer nitrides. For shorter wavelengths of the PL, a continuous and rapid collapse of the emission intensity is measured that has been interpreted in the context of the elementary k.p description in terms of the effects of the valence band symmetry.…”

“…They can be expanded along appropriate and specific irreducible representations of the wurtzite point symmetry, depending if one considers fluctuations along the growth direction or in the growth plane. These fluctuations of Al composition mix more or less strongly the different valence band states of the simple k.p description, based on the theory of translational invariants in the wurtzite symmetry, [23,59,60] and contribute to the distortion of the shape of the emission diagrams. The effect is found to be particularly strong in the conditions of maximum valence band mixings, that is to say, for a PL emission near 260 nm.…”