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Internal quantum efficiency of AlGaN/AlN quantum dot superlattices for electron-pumped ultraviolet sources

In this paper, we describe the growth and characterization of 530-nm-thick superlattices (100 periods) of AlxGa1-xN/AlN (x = 0, 0.1) Stranski-Krastanov quantum dots for application as the active region of electron-beam pumped ultraviolet lamps. Highly dense (>10e11 cm-2) quantum dot layers are deposited by molecular beam epitaxy, and we explore the effect of the III/V ratio during the growth process on their optical performance. The study considers structures emitting in the 244-335 nm range at room temperature, with a relative linewidth in the 6-11% range, mainly due to the QD diameter dispersion inherent in self-assembled growth. Under electron pumping, the emission efficiency remains constant for acceleration voltages below 9 kV. The correlation of this threshold with the total thickness of the superlattice and the penetration depth of the electron beam confirms the homogeneity of the nanostructures along the growth axis. Below the threshold, the emission intensity scales linearly with the injected current. The internal quantum efficiency is characterized at low injection, which reveals the material properties in terms of non-radiative processes, and high injection, which emulates carrier injection in operation conditions. In quantum dots synthesized with III/V ratio<0.75, the internal quantum efficiency remains around 50% from low injection to pumping power densities as high as 200 kW/cm2, being the first kind of nanostructures that present such stable behaviour.


Introduction
Semiconductors emitting in the deep ultraviolet (UV) range are in high demand for various applications in disinfection, water purification and related fields. This is due to the fact that most viruses and bacteria experience photochemical changes to the nucleic acids (for example, by the formation of dimers in deoxyribonucleic or ribonucleic acids) when exposed to UV-C radiation, which damages their ability to reproduce and therefore makes them inactive [1]. Mercury lamps are being used at present for this purpose; however, they are highly toxic and present low shelf life. Without question there is a strong need to replace them. Light emitting diodes (LEDs) based on AlGaN semiconductors are being pushed as a replacement [2,3] as their emission is spectrally tunable in the entire UV range, they are non-toxic, eco-friendly and exhibit long lifetime and fast switching capabilities. However, despite there being a plethora of research in III-V LEDs [1,4,5], their performances are still not at the level of their arc lamp counterparts.
Laboratories report record LED external quantum efficiencies (EQE) close to 20% at 275 nm [6], yet commercial devices exhibit efficiencies lower than 1%. The limitations are mostly attributed to doping, asymmetric carrier transport and metal contacting.
To circumvent these issues we propose electron-beam pumped UV lamps based on AlxGa1-xN/AlN quantum dot (QD) superlattices as active region. Devices based on this concept consist of a vacuum tube with a cold cathode (e.g. a carbon nanotube grid) which injects electrons in the semiconductor chip and generates electron hole pairs in the active region by impact ionization [7][8][9]. In principle, they can be miniaturized to the millimetre size. No longer needing p-type doping or contacts, we already tackle a few major problems concerning LEDs, namely the asymmetric carrier mobility, ohmic-contacting, and issues related to doping ionization energy. Electron-pumped emission from GaN-based quantum wells has been reported [8,[10][11][12][13][14], but the performance was mostly limited by the light extraction efficiency. Electron-pumped lasing around 350 nm has also been demonstrated [15,16].
Carrier localization in potential fluctuations leads to an enhancement of the emission yield at room temperature [17][18][19][20][21], which has motivated research on QDs for the development of high efficiency UV emitters [22][23][24][25][26][27]. In particular, plasma-assisted molecular beam epitaxy (MBE) is known to produce layers of close-packed Stranski-Krastanov (SK) GaN QDs with high internal quantum efficiency [17,23,24]. We have previously reported AlxGa1-xN QDs emitting at wavelengths as short as 235 nm at room temperature with internal quantum efficiency IQE  30% [23]. We have also demonstrated IQE higher than 50% in the 276-296 nm spectral range, keeping in mind their high sensitivity to the amount of AlxGa1-xN in the QD layer [22].
In this paper, we explore the effect of the III/V ratio during the deposition of AlxGa1-xN/AlN Stranski-Krastanov (SK) QDs on their optical properties, in view of their application as the active region for electron-beam pumped UV emitters. Their IQE is characterized as a function of the optical pumping power, covering low excitation densities, which reveals the material properties in terms of nonradiative processes, and high excitation densities, which emulates carrier injection in operation conditions. In the case of electron-beam pumping, the variation of the efficiency as a function of the acceleration voltage and injection current is also discussed.

Methods
The periodicity of the structures was analysed by X-ray diffraction (XRD) in a Rigaku SmartLab diffractometer using a 2-bounce Ge (220) monochromator and a long plate collimator of 0.228° for the secondary optics. The morphology of the QDs was analysed by atomic force microscopy (AFM) using a Bruker ICON SPM system operated in the tapping mode using TEXPA-V2 probes. Additional structural studies were conducted using high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) performed on a FEI Tecnai microscope operated at 200 kV.
Photoluminescence (PL) measurements under continuous-wave excitation were obtained by pumping with a frequency-doubled solid-state laser (λ = 244 nm), with an optical power of 100 μW focused on a spot with a diameter of  100 μm. PL measurements under pulsed excitation used an Nd-YAG laser (266 nm, 2 ns pulses, repetition rate of 8 kHz). In both cases, samples were mounted on a cold-finger cryostat, and the PL emission was collected by a Jobin Yvon HR460 monochromator equipped with a UVenhanced charge-coupled device (CCD) camera.
Cathodoluminescence (CL) experiments were carried out using a FEI Inspect F50 field-emission SEM equipped with a low-temperature Gatan stage to cool the sample down to 6 K, and with an iHR550 spectrometer fitted with a Andor Technology Newton DU940 BU2 spectroscopic CCD camera. The beam spot diameter was  10 nm on the focal point, the accelerating voltage was varied from 2 to 20 kV, and the electron beam current was kept below 150 pA. Additional CL experiments were performed using a Kimball Physics EGPS-3212 electron gun operated in direct current mode, under normal incidence, with a beam spot diameter of 4±1 mm. The acceleration voltage was in the range of 3 to 10 kV, injecting up to 800 µA of current. The CL emission arrived to an ANDOR ME-OPT-0007 UV-NIR light collector coupled with an ANDOR Shamrock500i spectrograph connected to an electron-multiplying CCD Newton 970 from ANDOR operated in conventional mode.
The electronic structure of the QDs was modelled in three dimensions (3D) using the Nextnano 3 8-band k·p Schrödinger-Poisson equation solver [28] with the material parameters described in ref. [29]. For the AlxGa1xN alloys, all the bowing parameters were set to zero. The simulated structure consisted of 10 layers of AlxGa1-xN QDs embedded in AlN. The period of the structure (QD layer + AlN barrier) was fixed to 5.3 nm.
In each QD layer, 7 QDs were defined as a hexagonal truncated pyramid with {10-13} facets [28], distributed in-plane in a hexagonal close-packed configuration, and connected by a wetting layer. The 3D strain distribution was obtained by minimization of the elastic energy through the application of periodic boundary conditions along the <1-100> and <11-20> in-plane directions. For the calculation of the band diagram, the spontaneous and piezoelectric polarization and the band gap deformation potentials were taken into account. It was assumed that the lattice was at room temperature, and the temperature dependence of the band gap follows Varshni equation. The quantum confined levels for electrons and holes and their associated square wavefunctions, | ( )| 2 , were calculated in the QD located in the centre of the structure.

Material growth
In order to explore the advantages of using AlxGa1xN/AlN Stranski-Krastanov (SK) QDs as the active region for electron-pumped UV emitters, we synthesized samples consisting of a stack of 100 layers of self-assembled AlxGa1xN QDs with x = 0 or 0.1, separated by 4 nm of AlN as barrier. Such QD superlattices were grown using plasma-assisted MBE on 1-µm-thick (0001)-oriented AlN-on-sapphire templates at a temperature TS = 720°C. The growth process was monitored by reflection high-energy electron diffraction (RHEED).
The active nitrogen flux, N, was tuned to achieve a growth rate vGN0.52 monolayers/s (ML/s) under metal rich conditions. Note that 1 monolayer (ML) of AlN or GaN is  0.25 nm. The AlxGa1xN dots were grown using N-rich conditions, which are known to lead to a high density of small QDs [17,18,22,29]. We explored the effect of varying the Ga flux (i.e. the gallium-to-nitrogen flux ratio) in the range of Ga = 0.149-0.441 ML/s (Ga/N = 0.29-0.85), keeping the Ga deposition time constant (12 s). In the case of ternary AlxGa1xN QDs, we added a flux of aluminium Al so that the targeted Al mole fraction in the dots is x = Al/(Al + Ga). The deposition of the QDs was followed by a growth interruption of 15 s. At the end of this process, a spotty RHEED pattern confirmed the presence of QDs. To favour the charge evacuation during the electron pumping process, the QDs were doped n-type with [Si] = 5×10 18 cm 3 (value estimated from Hall effect measurements using the Van der Pauw method on planar Si-doped GaN layers). The presence of silicon during the growth process does not have any effect on the growth kinetics or in the resulting QD shape/density [29]. The AlN sections were grown under Alrich conditions (Al/N = 1.1), followed by a growth interruption under nitrogen to consume the accumulated Al excess. At the end of the growth of each AlN barrier, the RHEED pattern showed the straight lines characteristic of a planar surface. The growth process was sharply interrupted (the precursor fluxes were shuttered, and the substrate was rapidly cooled down) after the deposition of the last QD layer, to enable AFM characterization of the QD shape and density. A schematic description of the general structure is depicted in figure 1(a) and a summary of the samples under study is presented in table 1.

Results and discussion
The structural properties of the QD superlattices were characterized by XRD. In a reciprocal space map around the (10-15) reflection, depicted in Figure 1 The calculation assumes a period of 5.43 nm and that average out of plane lattice parameter of the QD superlattice is that of Al0.90Ga0.10N fully strained on AlN. In all the samples, the period was 5.3±0.3 nm, in good agreement with the nominal growth parameters. Therefore, the total thickness of the QD stack is  530 nm.
The morphology of the topmost QD layers was analysed by AFM, with the result presented in figure 2 for samples SG3 (GaN/AlN QDs) and SA1 (Al0.1Ga0.9N/AlN QDs). In such images, the QD density is 3.3±0.4×10 11 cm -2 and 4.6±0.5×10 11 cm -2 , respectively, and the height of the dots is about 0.46±0.20 nm and 0.64±0.25 nm, respectively, i.e. approximately 2-3 ML. Note that these values should correspond to the QD height above the wetting layer.
In the case of high-density (close to 10 12 cm -3 ) QDs, as the samples presented here, it is difficult to extract reliable measurements of the QD base diameter from AFM. The dimension of the AFM tip is comparable to the dimension of the dots, and deconvolution of the effect of the AFM tip would require precise knowledge of the tip shape. Therefore, to assess the QD geometry, samples SG2, SA1 and SA3 were analysed using cross-section HAADF-STEM and HRTEM, with the results presented in figure 3. The QDs are clearly resolved, with facets that form an angle of  32° with the (0001) plane. The QD base diameter is 6.9±1.0 nm, 6.2±1.0 nm and 6.5±1.0 nm, and the QD height (including wetting layer) is 5±1 ML, 5.5±1.0 ML and 4±1 ML, for samples SG2, SA1 and SA3, respectively. In addition, the wetting layer thickness is estimated at 2 ML, 2-3 ML and 1-2 ML for SG2, SA1 and SA3, respectively. The average values and error bars were extracted by analysing images of at least 10 QDs per sample. In terms of QD height, the results are consistent with AFM measurements. Comparing the results of SG2 (GaN/AlN QDs) and SA1 (Al0.1Ga0.9N/AlN QDs), grown with the same Ga flux, we conclude that the additional Al flux in SA1 does not introduce a significant distortion of the QD morphology. In the case of SA3, the metal fluxes are reduced by approximately a factor of two with respect to SA1, keeping the same Al/Ga ratio. As a result, the QD height reduces drastically, but the diameter of the dots remains approximately constant. This observation points to a temperature-limited QD diameter, i.e. the diameter is given by the adatom diffusion length, which is determined by the substrate temperature.
To study the optical performance of the QDs, the CL emission of the dots at room  Let us now turn to consider the behaviour of these QD superlattices when pumped with an electron beam. To attain high conversion efficiencies, the active region should be able to collect a maximum number of electron-hole pairs generated by the impinging electrons via impact ionization. Therefore, we need to be sure to create the electron-hole pairs within the active region, to ensure minimum loss of energy. To estimate the penetration depth of the beam, we performed Monte Carlo simulations using the CASINO software. Figure 6 ), and the penetration depth of the electron beam is inversely proportional to the material density [32].
Experimental measurements of CL as a function of the accelerating voltage were also conducted to assess the penetration depth of the electron beam in operating conditions.
We studied all the samples at acceleration voltages ranging from 2 kV to 30 kV. As an example, the spectra of SA3, normalized by their maximum and shifted vertically for clarity, are presented in figure 6 Using the electron gun focused on a 4±1 mm spot, we measured the QD emission as a function of the injection current to assess if our samples could work under high currents without saturation. As an example, the results obtained for SA3 are presented in figure   7(a). We first fixed VA = 5 kV (which we assume is close to operation conditions, to prevent x-ray emission) and observed no saturation up to 800 µA. In contrast, for VA = 10 kV we observed a saturation of the emission intensity for injection current higher than 400 µA, presumably due to charging. This could be explained by the fact that, at 10 kV, part of the electron-hole pairs are generated in the AlN template, which makes it more difficult to evacuate the excess electrons. Interestingly, the saturation is associated to a spectral red shift of the emission, as illustrated in figure 7(b). This shift can be partially due to thermal effects, and to the fact that smaller QDs, emitting at shorter wavelengths, are more prone to charging effects than larger QDs, emitting at longer wavelengths.
To quantify the IQE of the structures, temperature-dependent PL measurements were QDs grown with metal/N = 0.8-0.9 [18,23]. If we compare with the results obtained by Himwas et al. for Al0.1Ga0.9N/AlN QDs with metal/N = 0.63 [22], IQEs around 50% were only achieved for QD layers generated from 4-5 ML of Al0.1Ga0.9N. Our results here point to the fact that further reduction of the metal-to-nitrogen ratio increases the efficiency of small QDs, so that IQE around 50% is possible for QDs generated from only 3 ML of either GaN or Al0.1Ga0.9N.
It is however true that most IQE values reported in literature are measured with a pulsed laser and at much higher power densities [14,21,[36][37][38][39][40][41][42][43][44], in the range of 5-1000 kW/cm 2 to be close to the operation conditions of LEDs. This procedure leads to higher IQE values, which depend on the pulse width, repetition rate and power density of the pumping laser [36][37][38][39]41,45]. Nevertheless, our samples have been characterized with a pulsed laser (266 nm, 2 ns pulses, repetition rate of 8 kHz) and the IQE variation as a function of the excitation power density was recorded purely for the sake of comparison with literature. Measurements were performed at 6 K and at 300 K. Under high excitation, the calculation of the IQE at room temperature must take into account the drop of the PL efficiency at low temperature due to the many-body effects induced by high-power excitation [41,46] so that This proves our QDs are by far superior to other heterostructures in terms of stability under high excitation. However, it should be kept in mind that, in an electron-pumped UV lamp using an acceleration voltage of 5 kV and an injection current of 1 mA to irradiate a spot with a diameter of 1 mm, the excitation density would be below 1 kW/cm 2 . This situation is clearly unfavourable for the use of quantum well structures.