Investigation of Al_yGa_{1− y}N/Al_0.5Ga_0.5N quantum dot properties for the design of ultraviolet emitters

Abstract

Self-assembled Al y Ga1− y N quantum dots (QDs), with y = 0 and 0.1, have been grown by molecular beam epitaxy on Al0.5Ga0.5N(0001) oriented layers using sapphire substrates. The QD formation has been followed in situ by reflection high energy electron diffraction (RHEED). A two- to three-dimensional (2D–3D) transition of the layer morphology is observed, characterized by a change of the RHEED pattern from streaky lines to Bragg spots. High QD densities, from 1010 up to near 1012 cm−2, have been obtained. By decreasing the GaN QD size and incorporating Al inside the QDs, a strong variation in the photoluminescence (PL) emission has been observed, enabling to cover a large spectral range from near UV (3 eV) to UV-B (3.95 eV). By combining temperature-dependent and time-resolved PL measurements, the internal quantum efficiency of the QDs has been determined at both low and high temperatures as a function of the PL energy.

Internal quantum efficiencies of AlGaN quantum dots grown by molecular beam epitaxy and emitting in the UVA to UVC ranges

“…In fact, high QD densities (above 10 11 cm -2 ) are systematically observed in the case of AlyGa1-yN QDs compared to GaN QDs 23 and this characteristic is attributed to the lower surface mobility of Al adatoms compared to Ga ones. Regarding the QD average height, it was estimated from the cross-section HAADF-STEM images (figure 3) and from previously characterized samples of Al0.1Ga0.9N QDs grown on Al0.5Ga0.5N, 22 as in the case of samples A and B. The images show AlyGa1-yN QDs for y values of 0.1, 0.2, 0.3 and 0.4 in figures (a) to (d) (corresponding to samples C, D, E and F, respectively).…”

“…19 Another possibility is the formation of AlyGa1-yN quantum dots (QDs), which is well mastered using MBE. 20,21,22 Such engineering of the active region may inhibit the transfer towards non radiative centers of excitons leading to a weak temperature dependence of the photoluminescence emission and to higher IQE. 23,24 The use of a compressively strained AlyGa1-yN layer on top of AlN has been shown to efficiently lead to the formation of QDs, and to get an emission in the deep UV, i.e.…”

“…27 Furthermore, LED structures based on QDs as active regions were recently reported using different designs, i.e. tunnel injection, 28 polar 20 or semi-polar orientations, 29 varying the QD heights 30 and compositions, 31 and have shown their potential for UV sources covering a broad wavelength range.…”

UVB LEDs Grown by Molecular Beam Epitaxy Using AlGaN Quantum Dots

“…This particular decrease of the PL intensity is attributed to the presence of non-radiative recombination channels unsaturated in the photo-injection conditions. As detailed in [26,34], the time dependence of the PL intensity I(t) can be fitted by considering fast (τ fast ) and long (τ slow ) decay times using the equation:…”

GaN quantum dot polarity determination by X-ray photoelectron diffraction

“…through the use of quantum dots (QDs) instead of the classical unidirectional confinement in QWs [4]. Indeed, Al y Ga 1−y N QDs can be grown on Al x Ga 1−x N layers by triggering a three dimensional (3D) growth mode during the epitaxial process [5,6,7]. Taking advantage of the compressive epitaxial stress between Al y Ga 1−y N and Al x Ga 1−x N (with x>y), Al y Ga 1−y N QDs are formed through a 2D -3D growth mode transition by molecular beam epitaxy (MBE).…”