Ultraviolet light emitting diodes using III-N quantum dots

Materials Science in Semiconductor Processing ∎ (∎∎∎∎) ∎∎∎–∎∎∎

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Ultraviolet light emitting diodes using III-N quantum dots Julien Brault a,n, Samuel Matta a,b, Thi-Huong Ngo b, Daniel Rosales b, Mathieu Leroux a, Benjamin Damilano a, Mohamed Al Khalfioui a,c, Florian Tendille a, Sébastien Chenot a, Philippe De Mierry a, Jean Massies a, Bernard Gil b a

CNRS-Centre de Recherche sur l’Hétéro-Epitaxie et ses Applications, Rue B. Gregory, 06560 Valbonne, France CNRS-Université Montpellier 2, Laboratoire Charles Coulomb and Université Montpellier 2, UMR 5221, 34095 Montpellier, France c Université de Nice Sophia-Antipolis, 06103 Nice, France b

art ic l e i nf o

a b s t r a c t

Article history: Received 16 December 2015 Received in revised form 18 February 2016 Accepted 19 February 2016

(Al,Ga)N-based quantum dots (QDs) grown on Al0.5Ga0.5N by molecular beam epitaxy have been studied as the active region for the fabrication of ultra-violet (UV) light emitting diodes (LEDs). In the first part, using both “polar” (0001) and “semipolar” (112̄ 2) surface orientations, the structural and optical properties of different QD structures are investigated and compared. In particular, their propensity to get an emission in the UV range is analyzed in correlation with the influence of the internal electric field on their optical properties. In a second part, (0001) and (112̄ 2)-oriented LEDs using GaN/Al0.5Ga0.5N QD as active regions have been fabricated. Their main current-voltage characteristics and electroluminescence properties are discussed, with a focus on the LED emission wavelength range reached for both surface orientations: it is shown that a large part of the UV-A region can be covered, with longer wavelengthsfrom 415 to 360 nm-for the “polar” LEDs, and shorter ones-from 345 to 325 nm-for the “semipolar” LEDs. In addition, the influence of the internal electric field on the QD-LEDs working operation is shown. & 2016 Elsevier Ltd. All rights reserved.

Keywords: Quantum Dots UV LEDs Semipolar (112̄ 2) orientation AlGaN Molecular beam epitaxy Quantum confined stark effect

1. Introduction UV light emission based on (Al,Ga)N materials have attracted a lot of attention over the last decade owing to the possibility of band gap engineering by simply adjusting the Al concentration: from 3.4 eV (
365 nm) for GaN to 6.0 eV (
205 nm) for AlN. Compared to the mercury vapor lamp, which is presently the most common UV source, light emitting diodes (LEDs) can offer important advantages such as compactness, higher efficiencies, and reduced power consumption [1]. Furthermore, the band gap engineering paves the way to a wide range of applications, from UV curing technology to water and air purification modules [2]. However, a significant drop in the efficiency of (Al,Ga)N LEDs for wavelengths below 350 nm is observed, due in part to the low structural quality of the active region: the typical dislocation densities are in the 109–1010 cm  2 range in (Al,Ga)N layers and have been shown to strongly impact the LED internal quantum efficiency [2]. Indeed, significant improvements of the (Al,Ga)N material quality have led to UV LEDs with higher efficiencies [3,4]. Another approach can strongly limit the influence of dislocations on the internal quantum efficiency of (Al,Ga)N-based n

Corresponding author. E-mail address: [email protected] (J. Brault).

heterostructures: the use of quantum dots (QDs) as the active region, instead of the typically used quantum wells (QWs) [5]. Due to the QD nm-size dimensions along the three directions of space, the electrons and holes are trapped inside the QDs and cannot diffuse towards surrounding defects. Therefore, radiative electronhole recombinations are favored, leading to high radiative efficiencies up to room temperature in the GaN/AlN system [6–8]. However, the use of AlN is a real challenge for the fabrication of LEDs due to the very large ionization energies (i.e. several hundreds of meV), of donors and acceptors [9], giving extremely low injection efficiencies. At this stage, the use of AlxGa1  xN materials with reduced Al-content should be preferred, as initially demonstrated by Tanaka et al. with the fabrication of GaN/Al0.1Ga0.9N QDbased UV LEDs by metal organic chemical vapor deposition (MOCVD) emitting near the GaN bandgap energy [10]. Also, though the ionization energies of doping impurities are strongly increasing with the Al-content of AlxGa1  xN materials, n-type and p-type AlxGa1  xN with x up to 70% have been reported [11–13]. Our approach to fabricate QDs by molecular beam epitaxy (MBE), uses the strain-induced two to three-dimensional (2D-3D) Stranski-Krastanov (SK) type growth mode transition resulting from the lattice-mismatch (ε) between AlyGa1  yN QD layer and the AlxGa1  xN template (with x4 y). In particular, we have shown that GaN QDs can be grown at the surface of AlxGa1  xN layers (with xo 1 [14] down to x¼ 0.35, i.e. for a value of ε reduced to

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0.8% [15]), their formation being monitored in situ by reflection high-energy electron diffraction [15] and their size controlled by the GaN deposited amount [16]. Actually, the interest of MBE for QD-based UV LEDs was shown a few years ago by Verma et al. [17]. Importantly, the versatility of the growth process also enables to grow QDs on different surface orientations, in particular the (0001) [15] and (112̄ 2) [18,19] ones. The use of the (112̄ 2) orientation is particularly interesting since it has been shown to enable the control of the nanostructure dimensionality (i.e. QDs, QWs or quantum wires), and to strongly reduce the internal electric field value [20], which arises from the GaN/AlxGa1  xN interfacial polarization discontinuities in the polar (0001) orientation [21]. This latter property leads to a strong blue-shift of the (112̄ 2)-oriented QD emission in the UV range [18]. In this study, we first present the main structural and optical properties of three different QD active regions, i.e. namely the (0001) and (112̄ 2) GaN/Al0.5Ga0.5N and the (0001) Al0.1Ga0.9N/ Al0.5Ga0.5N QD systems, and discuss their potential for UV emission. In a second part, we present the electro-optical properties of (0001) and (112̄ 2)-oriented UV LEDs with GaN/Al0.5Ga0.5N QD active regions. In particular, the LEDs operating wavelength ranges are shown, and the electroluminescence (EL) characteristics as a function of the injection current densities for both surface orientations are discussed.

typical [Mg]
1–5  1019 atoms cm  3. The LED mesa patterns, i.e. squares with sides ranging between 140 and 460 mm, were fabricated by photolithography and reactive ion etching. The n and p ohmic contacts were then deposited following ref. [22]. AFM measurements were performed in non-contact mode on a Dimension 3100 NanoScope IV. PL experiments at 13 K and 300 K were done by using a frequency-doubled Ar laser emitting at 244 nm. The time-resolved photoluminescence (TRPL) measurements were carried out at 8K by using a temperature-controlled helium flow cryostat. The samples were excited by a pump laser with a wavelength of 266 nm and with repetition rates chosen to correspond to the decay times of each sample. The excitation laser was created from a fundamental wavelength of the mode-locked Ti:sapphire (wavelength of 798 nm, repetition rate at 82 MHz) passed through a pulse picker and a Tripler TP-2000B. Finally, a spectrometer 500IS with diffraction grating 150 groove/mm connected with a Streak camera C10910 was used to detect the emission signal of the samples. The LED characteristics, including current-voltage (I-V) and electroluminescence (EL), were measured on wafer at room temperature. The light output signal was collected by an optical fiber and converted into an electrical signal by a CCD detector.

3. Results and discussion 2. Experimental

3.1. Structural and optical properties of (Al,Ga)N quantum dots

The QD samples were grown on (0001) c-plane and (11̄ 00) m-plane sapphire substrates by MBE in a RIBER 32 P reactor. The heterostructure fabrication process included the growth of an AlN layer grown on top of a GaN buffer layer as described in [14,18] for (0001) and (112̄ 2)-oriented samples, respectively. Then, a 0.8– 1 mm-thick Al0.5Ga0.5N layer (Si-doped for LED structures at a typical [Si] of
1019 atoms cm  3) was grown followed by the AlyGa1  yN/Al0.5Ga0.5N QD active region. Polar (p) GaN and Al0.1Ga0.9N QDs were grown on (0001) surfaces and will be referred to as p-GaN and p-Al0.1Ga0.9N in the following. Semipolar (sp) GaN QDs were also grown on (112̄ 2) surfaces and will be referred as sp-GaN. For samples dedicated to atomic force miscroscopy (AFM), photoluminescence (PL) and time-resolved photoluminescence (TRPL) experiments, the structures consist of three QD planes separated by 30 nm-thick Al0.5Ga0.5N barrier layers and finished with a fourth uncapped QD layer. Finally, for LED structures, the QD active region consists of three GaN QD planes separated by 10 nm-thick Al0.5Ga0.5N barriers and a p-type region made of a Mg-doped Al0.7Ga0.3N electron blocking layer, and a Mgdoped Al0.5Ga0.5N/GaN structure as described in [22], with a

P-GaN, sp-GaN and p-Al0.1Ga0.9N QDs structural properties were investigated by AFM as presented in Fig. 1. In order to get an emission in the UV range, low amounts of GaN and Al0.1Ga0.9N to fabricate the QDs were used, i.e. ranging between 1.5 and 2 nm, to decrease the QD size as much as possible [23,24]. As a general trend, the QD size and QD density are found to increase for larger deposited amounts. However, very different shapes, densities, and spatial distributions are observed, depending on the QD system. In the case of p-GaN (Fig. 1 (a)), isolated QDs with diameters around 20–30 nm, heights ranging between 2 and 4 nm and densities between 5 9  1010 cm  2 are obtained. Compared to polar QDs, sp-GaN QDs present a very different shape and surface distribution (Fig. 1(b)): they are elongated and ordered along a specific in-plane direction, which is the o11̄004 direction [18]. These elongated QDs have lateral sizes between 20 and 40 nm in the o11̄ 004 direction and between 15 and 20 nm in the perpendicular o1̄ 1̄ 234 direction, heights between 2 and 3 nm and densities in the 12  1011 cm  2 range. Finally, p-Al0.1Ga0.9N QDs present a morphology similar to p-GaN QDs (Fig. 1(c)), the main differences being the higher densities, between 1.5 9  1011 cm  2, and smaller sizes, with diameters around 10–20 nm and heights ranging

Fig. 1. (500  500) nm2 atomic force microscopy images of: (a) (0001)-oriented GaN/Al0.5Ga0.5N QDs, (b) (112̄ 2)-oriented GaN/Al0.5Ga0.5N QDs and (c) (0001)-oriented Al0.1Ga0.9N/Al0.5Ga0.5N QDs. The black arrow in (b) indicates the o 11̄ 00 4 direction.

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PL Intensity (a.u.)

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Fig. 3. Low temperature (13.2 K) power dependent photoluminescence spectra of: (0001)-oriented (a) GaN and (b) Al0.1Ga0.9N QDs, and (c) (112̄ 2)-oriented GaN QDs grown on Al0.5Ga0.5N. The power density has been varied on three orders of magnitude for all samples: from 40 mW to 40 mW for Fig. (a) and (b), and from 4 mW to 4 mW for Fig. (c).

between 0.5 and 1.5 nm. The QD spatial distribution is related to specific growth parameters: in our experiments, we attribute it to the surface morphology (in particular the preferential nucleation of QDs on surface steps [25]), and the thermally activated adatom diffusion [26]. Concerning this latter parameter, it is worth noting that it also depends on the atomic species, Al or Ga in our case: for an identical growth temperature (
800 °C for all the structures), the Al adatom diffusion is shorter that the Ga ones, leading to a higher density of nucleation centers and smaller QD diameters as experimentally observed. Finally, the specific ordering and elongation of sp-GaN QDs is related to the in-plane lattice-mismatch (ε) anisotropy between the o11̄004 and o1̄1̄ 234 directions, the larger ε along the o1̄ 1̄234 (i.e. 1.8% compared to 1.2%) favouring the GaN layer elastic relaxation through the formation of elongated 3D islands [18]. The PL spectra of p-GaN, sp-GaN and p-Al0.1Ga0.9N QDs are shown in Fig. 2. The measurements were performed by using the frequency-doubled Ar laser at an excitation power of 23 mW and a spot diameter of
120 mm. Each spectrum is made of a single peak which comes from the emission of the QDs ensemble. Note that the modulated shape of the p-GaN QD spectrum (and to a lesser extends of the p-Al0.1Ga0.9N QD spectrum) is the consequence of interferences coming from the multi-layered structure of the sample. As targeted from the heterostructure design, all the samples are emitting in the UV-A region, between 330 and 390 nm. However, strong differences are observed among them: 1) a strong blue-shift of the emission and 2) a strong reduction of the

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Fig. 2. Room temperature photoluminescence spectra of (0001)-oriented GaN and Al0.1Ga0.9N QDs (p-GaN and p-Al0.1Ga0.9N) and (112̄ 2)-oriented GaN QDs (sp-GaN) grown on Al0.5Ga0.5N.

full-width at half-maximum (FWHM) for sp-GaN and p-Al0.1Ga0.9N QDs compared to p-GaN QDs. In the (0001) orientation, the blueshift is as large as 570 meV, going from 3.18 eV (i.e. 390 nm) for GaN QDs to 3.75 eV (i.e. 330 nm) for Al0.1Ga0.9N ones. The PL energy measured for GaN QDs is found well below the bandgap of GaN strained on Al0.5Ga0.5N (
3.5 eV at 300 K), which is the consequence of the quantum confined Stark effect (QCSE) due to the presence of a large internal electric field (F
3.5 MV/cm in this material system [27]). The blue-shift observed for Al0.1Ga0.9N QDs and the emission at high energy (i.e. above the Al0.1Ga0.9N band gap energy [28]) is the consequence of two characteristics: 1) the increase of the QD bandgap energy (Al0.1Ga0.9N vs. GaN) and 2) the important QD size reduction (in particular the QD height which is mainly responsible for the optical properties of nitride QDs [29]) compared to GaN QDs, which strongly minimizes the influence of F as further discussed in the next paragraph. For the sp-GaN QDs, a PL peak at 3.64 eV (341 nm) is observed. This PL energy, which is close to the energy obtained in the case of Al0.1Ga0.9N QDs, corresponds to an energy shift towards high energy of 460 meV compared to the p-GaN QDs. This difference is mainly attributed to the strong reduction in F [20] and shows the interest of using semipolar orientations for QDs in order to fabricate UV LEDs. The minimized influence of F on the PL properties is also evidenced in the peak FWHM for which a strong reduction is observed, from 300 meV to 220 meV and 120 meV, for p-GaN QDs, p-Al0.1Ga0.9N and sp-GaN QDs, respectively. Indeed, the effect of the QD height variation on the PL FWHM value (mainly caused by inhomogeneous broadening), is strongly amplified by the internal electric field value [18]. In order to get more insights on the influence of F on the QD optical properties, power-dependent PL measurements have been performed, as presented in Fig. 3. It is well-known that a gradual screening of F occurs under increased injection power densities, leading to a progressive blue-shift of the QD PL energy [30,31]. This property is clearly observed for p-GaN QDs in Fig. 3(a), for which an energy shift as large as 420 meV (from 2.80 to 3.22 eV) is found for an excitation power varying by three decades. On the contrary, for a similar excitation power variation, no PL energy shift is observed for p-Al0.1Ga0.9N and sp-GaN QDs, thereby confirming the very weak effect of F in these two cases. To complete the optical characterizations, TRPL decay time measurements at 8 K have been done on the three QD systems (Fig. 4). A first result is that very different decay times (τD) are found: τD in the range of hundreds of ns, tens of ns, and a few ns are observed for p-GaN QDs (Fig. 4(a)), p-Al0.1Ga0.9N (Fig. 4(b)) and sp-GaN QDs (Fig. 4(c)), respectively. The larger τD measured for p-GaN QDs is related to the QCSE which separates the electron and hole wave functions, slowing down the QD radiative recombination rate, i.e. leading to longer radiative recombination times

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[29,32]. Therefore, one to two orders of magnitude differences are found among the samples. As discussed in the previous paragraphs, this feature is a direct consequence of the strong reduction of F and/or its influence on the band structure of p-Al0.1Ga0.9N and sp-GaN QDs. Yet, a precise analysis of the PL transients indicates that they exhibit a two-time-decay behavior: this is typical of the presence of non-radiative recombination channels unsaturated in the photo-injection conditions [33]. From this behavior, the temporal dependence of τD has been fitted using fast (τfast) and long (τslow) decay times and coefficients, enabling us to determine the internal quantum efficiency (IQE) of the QDs, as described in ref. [33]. The lines plotted in Fig. 4 are the result of the fitting, using the following equation:

⎛ ⎛ t ⎞ t ⎞ ⎟⎟ + Aslow exp⎜⎜ − ⎟⎟ I (t ) = Afast exp⎜⎜ − τfast ⎠ τslow ⎠ ⎝ ⎝

(1)

where I refers to the PL intensity, t is the time, and Afast, Aslow correspond to fast (non radiative) and slow (radiative) decay coefficients [33]. From these double exponential fittings, IQE values of 19%, 17% and 27% have been calculated for p-GaN, p-Al0.1Ga0.9N QDs and spGaN, respectively. These similar radiative QD efficiencies, i.e. independently of the QD composition and surface orientation, indicates the high potential of these different QD designs to be used as active regions for the fabrication of UV-LEDs, enabling to fabricate sources covering most of the UV-A region (from 330 to 390 nm). 3.2. (0001) and (112̄ 2)-oriented GaN/Al0.5Ga0.5N QD-based LEDs In this section, LEDs with different active regions (from GaN deposited amounts of 1.5–2 nm) were fabricated as described in the experimental part. 3.2.1. (0001)-oriented LEDs The EL spectra measured at room temperature for three different devices are shown in Fig. 5. The injection current density has been chosen at
30 A/cm2. Each spectrum presents a peak in the near UV range coming from the QD emission. When going from larger to smaller QDs (related to the deposited GaN amount), an EL energy variation from 3.26 to 3.44 eV, i.e. corresponding to a wavelength variation from 380 to 360 nm, is found. As expected, these wavelengths are found in the same range as the PL measurements. However, as discussed above, the QD emitted wavelength strongly depends on the injected carrier density, in particular in the case of larger (higher) QDs. This property is clearly evidenced in Fig. 6(a) which represents the variation of the EL energy as a function of the injection current density for two LEDs with different QD sizes, referred as “large” and “small” QDs in the figure inset. At low injection current densities, the LED with small

Normalized EL Int. (a.u.)

Fig. 4. Time-resolved photoluminescence spectra at low temperature of: (0001)-oriented (a) GaN and (b) Al0.1Ga0.9N QDs, and (c) (112̄ 2)-oriented GaN QDs grown on Al0.5Ga0.5N.

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Fig. 5. Room temperature electroluminescence spectra of (0001)-oriented GaN/ Al0.5Ga0.5N QD-based LEDs (for an injection current density
30 A/cm2) for different QD active regions: going from larger QD sizes (top spectrum) to smaller QD sizes (bottom spectrum).

QDs emits, as expected, at a higher energy than the LED with larger QDs, as a consequence of the QCSE. Then, as the current density increases, the EL energy of both devices shifts towards higher energies, due to the screening of the internal electric field by carrier injection into the QDs [34]. Yet, this effect is limited as the QD size is reduced, going from 3.26 to 3.39 eV (i.e. ΔE¼ 130 meV) for small QDs, whereas it is varying from 3.02 to 3.26 eV (i.e. ΔE¼ 240 meV) over the same current density range for large QDs. This result puts into evidence the weaker influence of F on the QD fundamental energy transition as the QD size is decreased, similarly to the case of (Al,Ga)N/GaN QW heterostructures studied in ref. [35]. Indeed, as the QD-LED emission is pushed towards shorter wavelengths by adjusting the GaN deposited amount to get small QDs emitting below 370 nm (Fig. 6 (b)), we observe a saturation of the QD EL wavelength: although the peak is found at 369 nm (3.36 eV) at lower current densities, it saturates at around 360 nm (3.44 eV) at higher densities, accounting for an energy variation of only ΔE ¼60 meV. Therefore, all these results suggest a limit in the wavelength emission of (0001)-oriented GaN/Al0.5Ga0.5N QD-based LEDs, which saturate at around 360 nm. This result accounts for the existence of a critical thickness (hc) to trigger the QD formation in the two dimensional – three dimensional growth process used here [23]: for GaN QDs grown on Al0.5Ga0.5N, hc has been found at
6 monolayers (MLs) (with 1 ML being equal to half of the lattice parameter c along the o0001 4 axis which is equal to 0.259 nm for GaN) [14], leading to a minimum QD size (height) with this fabrication process.

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Fig. 6. (a) Variation of the electroluminescence energy of (0001)-oriented GaN/Al0.5Ga0.5N QD-based LEDs with two different QD size (large and small QDs) active regions as a function of the current density; (b) Electroluminescence spectra of a (0001)-oriented GaN/Al0.5Ga0.5N QD-based LED with small QDs for different current densities.

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and spectra measured on different (112̄ 2)-oriented devices are reported in Fig. 7(b). EL peak energies with a maximum intensity found between 3.61 and 3.78 eV are shown. These energies correspond to a wavelength emission between 343 and 328 nm, i.e. in good agreement with PL measurements measured from (112̄ 2)oriented GaN/Al0.5Ga0.5N QDs presented in Fig. 2 and in references [18,23]. These results clearly indicate that the EL emission originates from the sp-GaN QDs, showing the potential of this approach for the fabrication of QD-based UV-LEDs. This is further evidenced by the evolution of the EL peak characteristics as a function of the injected current density presented in Fig. 8(a) and compared to the p-GaN QD case (Fig. 8(b)). For a similar current density range, a clear blue-shift from 389 to 376 nm is measured for the p-LED, corresponding to an energy shift ΔE
110 meV, whereas no blueshift of the EL peak is observed for the sp-LED, as a consequence of the strong reduction of the internal electric field in the (112̄ 2)oriented device. Yet, a very weak red-shift of
0.8 nm, i.e. from 332.5 to 333.3 nm and corresponding to an energy shift ΔE
9 meV, can be noticed at larger current densities for the sp-LED as a consequence of the self-heating (by Joule effect) of the device. The appearance of self-heating effects at moderate current densities is directly caused by the LED high series resistance.

4. Conclusions Self-assembled (Al,Ga)N quantum dots (QDs) grown on “polar” (0001) and/or “semipolar” (112̄ 2)-oriented Al0.5Ga0.5N layers have

Normalized EL (a.u.)

Current (mA)

3.2.2. (112̄ 2)-oriented LEDs Based on the PL experiments, the semipolar (112̄ 2) orientation appears as an efficient way to get an emission at shorter wavelengths compared to the (0001) case. Furthermore, we have developed a defect reduction method for the fabrication of semipolar GaN layers on sapphire by MOCVD using asymmetric lateral epitaxy (AS-ELO) [36] or selective growth on patterned facets [37,38]. These methods lead to a strong reduction of heteroepitaxial defects such as basal stacking faults (BSFs) and dislocations. Indeed, we have observed that high BSF densities in LED structures are detrimental to the good working operation of the devices for which the electrical characteristics are dominated by short-circuits of the p-n junction while the dislocations act as non-radiative centers thus reducing the IQE. In this study, sp-GaN QD LEDs obtained using AS-ELO GaN templates were fabricated. At first, after the device processing, I-V measurements were performed and compared to p-GaN QD LEDs (Fig. 7(a)). The characteristics show a clear rectifying behavior. The sp-LED turn-on voltage at 20 mA is
6.6 V, close to the value of 6.0 V found for the p-LED, which compare fairly well with voltage values reported from Al0.5Ga0.5Nbased LED structures [39]. However, the series resistance is found at 100 Ω, with is much higher than the value of 40 Ω measured for the p-LED. This significant difference is attributed to the un-optimized p-type doping of (112̄ 2)-oriented (Al,Ga)N layers, since the same growth conditions than in the (0001) orientation have been used, and could also be the result of a higher residual donor concentration in the sp layers [40]. EL measurements at room temperature were then performed,

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Fig. 7. (a) Current-Voltage characteristics of (0001) and (112̄ 2)-oriented GaN/Al0.5Ga0.5N QD-based LEDs; (b) Electroluminescence spectra of (112̄ 2)-oriented GaN/Al0.5Ga0.5N QD-based LEDs with different QD size active regions.

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0 325 350 375 400 425 450 475 Wavelength (nm)

Fig. 8. Variation of the electroluminescence energy of (a) (112̄ 2)-oriented (sp-LED) and (b) (0001)-oriented (p-LED) GaN/Al0.5Ga0.5N QD-based LEDs as a function of the current density.

been studied and compared. In the polar (p) orientation, the GaN and Al0.1Ga0.9N QDs present a hexagonal base, whereas semipolar (sp) QDs are trapezoidal, with an elongation and ordering along the o11̄ 00 4 axis. QDs with lateral sizes varying between 10 and 40 nm and heights varying between 0.5 and 4 nm have been measured by AFM. In all cases, high QD densities, in the 1010– 1011 cm  2 ranges, are obtained, which is a prerequisite for their use as active layers in ultra-violet (UV) light emitting diodes (LEDs). The photoluminescence properties show a strong blueshift for Al0.1Ga0.9N QDs and sp-GaN QDs compared to p-GaN QDs, going from
3.2 eV to
3.6  3.8 eV. These blue-shifts account for an increase of the bandgap energy (Al0.1Ga0.9N vs. GaN), a size reduction of the QDs and/or the reduction of the internal electric field F. In particular, the influence of F has been evidenced on power-dependent and time-resolved photoluminescence experiments. Next, (0001) and (112̄ 2)-oriented GaN-QDs have been used as the active region of light emitting diodes (LEDs). The LED properties have been studied by electro-optical measurements. The electroluminescence (EL) characteristics of (0001)-oriented LEDs exhibit a strong dependence, i.e. a blue-shift of the EL peak, as a function of the current density, which allows obtaining an emission in the UV-A region between 415 and 360 nm. The difficulty to get an EL emission at shorter wavelengths is related to the influence of F and the QD size which is limited by the minimum GaN deposited amount required to fabricate the QDs. (112̄ 2)-oriented LEDs have been shown to be an efficient approach to get an emission at shorter wavelengths, due to the strong reduction of F as shown by the suppression of the EL blue-shift when increasing the injection current density. In this case, an emission below 345 nm and down to 328 nm has been obtained by varying the QD size.

Acknowledgments The authors would like to thank D. Lefebvre, S. Vézian, and B. Poulet for their invaluable technical and scientific help and discussions. A. Courville and O. Tottereau are acknowledged for atomic force microscopy measurements and expertize. This work is supported by ANR Project oANR-14-CE26-0025-014 “NANOGANUV”. We also acknowledge partial support from GANEX (ANR-11-LABX0014). GANEX belongs to the publicly funded “Investissements d’Avenir” program managed by the French ANR agency.

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