Influence of the heterostructure design on the optical properties of GaN and Al0.1Ga0.9N quantum dots for ultraviolet emission

Abstract

International audienceThe optical properties of AlyGa1-yN quantum dots (QDs), with y 1⁄4 0 or y 1⁄4 0.1, in an AlxGa1 xN matrix are studied. The influence of the QD layer design is investigated pointing out the correlations between the QD structural and optical properties. In a first part, the role of the epitaxial strain in the dot self-assembling process is studied by fabricating GaN QD layers on different AlxGa1 xN layers with 0.5 x 0.7. Photoluminescence (PL) measurements show the main influ- ence of the increase of the internal electric field (Fint) on the QD optical response inducing a strong red shift in the emission energy as x increases. Time resolved combined with temperature depen- dent PL measurements enabled the estimation of the QD internal quantum efficiencies at low tem- perature showing values around 50%. In addition, a PL integrated intensity ratio up to 74% is shown, between 300 and 9 K. In the second part, the design of Al0.1Ga0.9N QDs was investigated, by varying the Al0.1Ga0.9N amount deposited. An increase of the transition energy (from 3.65 eV up to 3.83eV) is obtained while decreasing the deposited amount. Calculations of the ground state transition energies as a function of the Al0.1Ga0.9N dot height give a value of Fint around 2.060.5MV/cm. Therefore, the propensity of Al0.1Ga0.9N dots to emit at much higher energies than GaN dots (a PL shift of 1 eV using a low excitation power) is seen as the consequence of the reduced Fint together with their smaller sizes

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

“…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. down to 235 nm.…”

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:…”

“…In this regime, the wavelength and EL spectrum FWHM values are characterized by a strong dependence on J (which varies from 0.1 to 40 A•cm −2 ). For both LEDs, a progressive shift towards shorter wavelengths is observed, which is the consequence of: (i) The partial screening of F int due to an increase of the electron and hole concentrations in the QDs [26,40], (ii) the progressive injection into higher Al concentration Al y Ga 1−y N QDs at larger J and (iii) a band-filling effect in the QDs [41][42][43]. The main difference between LED-A and LED-B is seen in the FWHM dependence of the EL spectra as a function of J: In the case of LED-A, a progressive reduction of the FWHM is observed (from 70 nm down to 30 nm) whereas for LED-B is remains large (between 50 and 60 nm) and fairly independent of J.…”

“…This field is originating from the interfacial polarization discontinuities, leading to the quantum confined Stark effect (QCSE) in Al y Ga 1−y N/Al x Ga 1−x N heterostructures. Indeed, the large values of F int , that can reach several MV/cm in GaN-based QDs [26,39], have been shown to lead to an important red-shift of the QD emission, i.e., at much longer wavelengths than the QD material band gap energy and up to the visible spectral range [25]. However, as presented in Figures 6 and 7, the QD emitted average wavelength as well as the emission range (FWHM) strongly depends on the injected carrier density at lower J (<40 A•cm −2 ): A blueshift from 345 nm to 315 nm for LED-A and from 345 nm to 305 nm for LED-B is observed, together with a reduction of the FWHM from more than 60 nm down to 25 nm.…”

“…Indeed, the QD formation is evidenced by a spotty RHEED pattern coming from the presence of 3 dimensional (D) nanometer-sized islands and eventually chevrons originating from a specific orientation of QD facets [21,22]. Taking advantage of the epitaxial stress in the hetero-epitaxial growth of Al y Ga 1−y N on Al x Ga 1−x N (with y < x), it was shown that Al y Ga 1−y N based nanostructures could be spontaneously formed during growth [23,24] and their shape (i.e., dot or dash) controlled by performing a post-growth step [25,26]. In addition, adjusting the QD growth parameters (Al composition, deposited amount) has allowed covering a broad range from the UVA to the UVC [27,28], and the potential of Al y Ga 1−y N QDs (with y between 0.1 and 0.4) on Al x Ga 1−x N (with x between 0.5 and 0.7) was also shown in the case of very high dislocation densities (above 10 −10 cm −2 ) with IQEs reaching almost 20% [28].…”