UVB LEDs Grown by Molecular Beam Epitaxy Using AlGaN Quantum Dots

Abstract

AlGaN based light emitting diodes (LEDs) will play a key role for the development of applications in the ultra-violet (UV). In the UVB region (280–320 nm), phototherapy and plant lighting are among the targeted uses. However, UVB LED performances still need to be improved to reach commercial markets. In particular, the design and the fabrication process of the active region are central elements that affect the LED internal quantum efficiency (IQE). We propose the use of nanometer-sized epitaxial islands (i.e., so called quantum dots (QDs)) to enhance the carrier localization and improve the IQE of molecular beam epitaxy (MBE) grown UVB LEDs using sapphire substrates with thin sub-µm AlN templates. Taking advantage of the epitaxial stress, AlGaN QDs with nanometer-sized (≤10 nm) lateral and vertical dimensions have been grown by MBE. The IQE of the QDs has been deduced from temperature dependent and time resolved photoluminescence measurements. Room temperature IQE values around 5 to 10% have been found in the 290–320 nm range. QD-based UVB LEDs were then fabricated and characterized by electrical and electroluminescence measurements. On-wafer measurements showed optical powers up to 0.25 mW with external quantum efficiency (EQE) values around 0.1% in the 305–320 nm range.

1. Introduction

Nitride based light emitting diodes (LEDs) can emit in the ultra-violet (UV) range from the near UV [1], which is a part of the UVA region (400 nm ≤ λ_UVA ≤ 320 nm), down to the UVC region (280 nm ≤ λ_UVC ≤ 100 nm) reaching a minimum wavelength emission of 210 nm achieved by using an AlN p-i-n junction [2]. Compared to the other existing UV sources, LEDs offer strong advantages such as low power consumption, compactness and long lifetimes. Furthermore, they are an environmentally-friendly solution to the commonly used mercury (Hg) lamps, which bring toxicity and recycling issues. Indeed, the replacement of Hg lamps has been planned to start from 2020 by the Minamata Convention on mercury [3]. Aluminum gallium nitride (Al_xGa_1−xN) based LEDs are expected to be the new UV technology and their development has attracted very important research activities over the last decade [4]: In particular, the fabrication of UVB and UVC-LEDs represent major topics. The UVC spectral region is targeting strategic applications for sterilization, water purification and surface disinfection, by inducing bacteria cell inactivation [5]. Another region of interest is the UVB for medical and environmental applications, such as skin treatment and plant growth [4]. In this spectral region, external quantum efficiencies (EQEs) of 4.4% and 2.2% at a current of 20 mA and 50 mA were demonstrated recently for LEDs emitting at 295 nm [6] and 310 nm [7], respectively. Therefore, the EQEs of UVB LEDs remain limited to few percent [8] and novel processes are required to improve these values.

Al_xGa_1−xN LEDs are mainly grown on sapphire, which is well adapted to both UV applications, due to its transparency, and to high temperature growth processes that can play a central role for the reduction of threading dislocation densities (TDD) induced by the low mobility of Al species [9]. EQEs of LEDs are the result of the product of the injection efficiency (IE), the internal quantum efficiency (IQE) and the extraction efficiency (EE) [10]. Therefore, it is crucial to develop processes to improve these parameters to reach high EQEs: Among these, the material crystalline quality plays a central role and the presence of threading dislocations and point defects, which are non-radiative centers, have been shown to strongly decrease the IQE of LEDs [11]. Several methods have been successfully proposed to reach TDD in the low 10⁻⁹ cm⁻² range or below, including an increase of the growth temperatures [12], post-growth high-temperature annealing [13], migration enhanced epitaxy by ammonia pulse-flow method [14], or epitaxial lateral overgrowth [15]. Another way to improve the IQE of LEDs is to favor radiative recombination by an enhancement of the carrier confinement in the active region: This structure design can be obtained by using specific growth conditions to create band structure potential fluctuations [16] or the formation of quantum dots (QDs) [17,18,19,20].

Regarding the use of QDs, molecular beam epitaxy (MBE) has been shown to be an efficient approach to control their fabrication process: The use of in situ reflexion high energy electron diffraction (RHEED) allows for a real-time monitoring of the growth front morphology. 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_yGa_1−yN on Al_xGa_1−xN (with y < x), it was shown that Al_yGa_1−yN 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_yGa_1−yN QDs (with y between 0.1 and 0.4) on Al_xGa_1−xN (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]. In this study, using QD active regions made of Al_0.2Ga_0.8N QDs in Al_0.7Ga_0.3N cladding layers, we have designed and fabricated LEDs and studied their main electro-optical characteristics. First, the structural and photoluminescence (PL) properties of Al_0.2Ga_0.8N QDs were investigated. It was found that the QDs have average heights and diameters varying between 2 nm and 3 nm and between 5 nm and 15 nm, respectively, depending on growth parameters. Regarding photoluminescence (PL), an emission ranging between 300 nm and 340 nm was observed at room temperature. Then, by performing time-resolved and temperature dependent measurements, the IQE has been found between 5% and 10%. Next, QD based LEDs with two different active region designs have been fabricated. The devices electrical and optical properties were characterized and compared: The LEDs show a broad electroluminescence (EL) emission peak centered between 305 nm and 345 nm, depending on the current density and the active region design. The optical power, measured directly on-wafer, is increasing as a function of the current density up to a value of around 0.25 mW at 100 A/cm² for both type of LEDs. At this current density, an EQE of 0.06 % is found for an EL emission peak centered in the UVB region, i.e., in the 305–320 nm range.

2. Materials and Methods

The LED structures were grown on (0001) sapphire substrates by molecular beam epitaxy (MBE) in a RIBER 32 P reactor. Solid sources were used for the III-elements (Al, Ga) and the dopants (Si for n-type layers and Mg for p-type layers). Concerning the nitrogen source, ammonia (NH₃) was chosen as the precursor for the growth except for the fabrication of the QD active region for which a RIBER RF nitrogen (N₂) plasma source was used. This specificity is related to the surface energy contribution during the QD formation process [29]: The use of NH₃ promotes a 2 dimensional (D) growth mode and inhibits the formation of 3D islands and QDs, whereas under N₂ a 2D-3D “Stranski–Krastonov” growth mode can be observed [30]. Two structures (referred as LED-A and LED-B) were fabricated as described in Figure 1. A similar fabrication process was used for both cases except for the active region. At first, a 130 nm-thick AlN buffer layer was grown following the growth conditions described in [31]: A 3 nm thick AlN nucleation layer was initially grown at 500 °C and followed by the growth of an AlN layer at a temperature of 1000 °C. Next, the p-i-n structure was fabricated by first growing a 0.7 µm thick Si-doped n-Al_0.7Ga_0.3N layer at 870 °C, with a Si concentration estimated at around 1 × 10¹⁹ cm^-3, and an active region consisting of three (LED-A) and five (LED-B) (Al,Ga)N QD planes with an Al nominal composition (n.c.) of 0.2, separated by Al_0.7Ga_0.3N barriers lattice matched to the Al_0.7Ga_0.3N template.The Al_0.2Ga_0.8N (n.c.) QDs 2D equivalent thickness is 8 MLs (i.e., about 2 nm with 1 ML being equal to half the c lattice parameter, i.e., 0.257 nm for Al_0.2Ga_0.8N considering a variation of the lattice parameter following Vegard’s law between GaN and AlN). In the case of LED-A, a 10 nm thick cladding layer was grown in between the QD planes whereas the thickness was set to 5 nm for the cladding layer of LED-B. After the deposition of the last QD plane, a 20 nm thick Al_0.7Ga_0.3N layer was grown at 870 °C. This was followed by the p-type region composed of a 10 nm Al_0.8Ga_0.2N:Mg electron blocking layer and a 10 nm Al_0.7Ga_0.3N:Mg layer, by using a Mg flux of 2 × 10⁻⁹ Torr and deposited at 820 °C and 800 °C, respectively. Finally, a 30 nm GaN:Mg contact layer was grown at 770 °C using a Mg flux of 4 × 10⁻⁹ Torr. The Mg concentrations in each layer, based on calibrations by secondary ion mass spectroscopy on reference samples, are estimated to be about 5 × 10¹⁹ atoms·cm⁻³ in both Al_0.7Ga_0.3N and Al_0.8Ga_0.2N layers and 1 × 10²⁰ atoms·cm⁻³ in the top GaN layer. From the growth processes used to fabricate the LED structures and previous studies that are based on X-ray diffraction and transmission electron microscopy measurements performed on reference samples [32], the TDD in the LED structures is estimated to be ranging between: TDD ~ 6-8 × 10¹⁰ cm⁻² for the AlN template and TDD ~3–6 × 10¹⁰ cm⁻² for the Al_0.7Ga_0.3N/Al_0.2Ga_0.8N QD structures [31].

To investigate the optical properties of Al_0.2Ga_0.8N QDs, samples composed of a buried Al_0.2Ga_0.8N QD plane grown in an Al_0.7Ga_0.3N cladding layer were grown and the influence of the Al_0.2Ga_0.8N deposited amount was investigated by changing it from 8 to 12 MLs. In addition, an Al_0.2Ga_0.8N QD plane was grown at the surface of the top cladding layer. The surface QD plane was investigated by atomic force microscopy (AFM) to study the QD morphology. Concerning the optical characteristics, they were studied by photoluminescence (PL) and time-resolved PL (TRPL). The AFM measurements have been done in tapping mode using a diamond coated tip with a typical radius of 5 nm. The PL measurements were carried out in a closed cycle He cryostat using a frequency-doubled Ar laser at 244 nm (5.08 eV) and an excitation power of 20 mW. TRPL measurements were performed at 8K using a mode-locked frequency-tripled titanium–sapphire laser with a wavelength of 260 nm (4.77 eV). The laser excitation power was about 200 μW.

After the growth, LEDs with a squared geometry were processed by using photolithography and reactive ion etching. The mesas are covered by a thin Ni/Au (5/5 nm) semi-transparent current spreading layer plus a Ni/Au (5/200 nm) contact as top electrode to p-GaN. The contact on n-Al_0.7Ga_0.3N consists of a stacking of Ti/Al/Ni/Au (30/180/40/200 nm). LEDs with a squared mesa size of 310 µm (corresponding to mesa areas of ~96000 µm²) have been studied. The LEDs were characterized at room temperature by performing on wafer current–voltage (I–V) and electroluminescence (EL) measurements. The EL was measured from the backside of the LED chips by collecting the light extracted from the sapphire substrate side in an UV-grade optical fiber connected to a linear CCD spectrometer with a spectral resolution of 2.5 nm. The optical power (P_opt) was also measured from the backside of the chips by using a calibrated UV enhanced Si photodiode.

3. Results

In the first part of this section, theAl_0.2Ga_0.8N (n.c.) QDs main structural and optical properties are presented. The second part is dedicated to the investigation of the QD-based LEDs including PL, I–V, and EL measurements.

3.1. Al_0.2Ga_0.8N QDs Structural and Optical Properties

A typical AFM image of Al_0.2Ga_0.8N (n.c.) QDs grown at the surface of an Al_0.7Ga_0.3N layer is shown in Figure 2a. Similarly to previous results [28,33], the QD densities determined from AFM measurements have been found between 2 × 10¹¹ and 6 × 10¹¹/cm² and their diameter varies between 5 and 15 nm. From previous transmission electron microscopy measurements [28], the average QD height is estimated to be of the order of the Al_0.2Ga_0.8N (n.c.) deposited amount ranging from 2 nm (for 8 MLs) to 3 nm (for 12 MLs), with a height distribution of ±20%. In Figure 2b are presented PL spectra at 300 K of a series of samples of Al_0.2Ga_0.8N (n.c.) QDs with deposited amounts ranging from 8 to 12 MLs. Each spectrum is characterized by a broad peak originating from the QD emission: Decreasing the amount of Al_0.2Ga_0.8N from 12 MLs to 8 MLs induces a UV shift of the main PL maximum intensity from 324 to 308 nm (3.83 to 4.02 eV). The PL spectra full width at half maximum (FWHM) are similar and found between 12 and 15 nm (≈150 to 190 meV). This broadening reflects alloy disorder in (Al,Ga)N and QD sizes inhomogeneity, i.e., confinement energies and Stark effect inhomogeneities as discussed in Section 4 below. Next, TRPL measurements are presented in Figure 2c, along with the temperature dependent PL measurement in insert (measured in a temperature range between 8 K and 300 K). At first, it is found that the PL transients exhibit a two-exponential decay behavior. 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:

$$I(t)=A_{fast}\exp (-\frac{t}{{\tau }_{fast}})+A_{slow}\exp (-\frac{t}{{\tau }_{slow}})$$

(1)

with A_fast and A_slow the fast (non radiative) and slow (radiative) decay coefficients, respectively. This bi-exponential behavior is attributed to two different types of regions within the QD plane, i.e., regions with predominant radiative channels and other ones with additional competing non-radiative recombination centers [35]. τ_slow defines the radiative lifetime, while τ_fast is a combination of both radiative and non-radiative lifetimes. Values in the range of 2–2.2 ns and 16.8-18 ns have been determined for τ_fast and τ_slow, respectively, while A_fast and A_slow have values in the 930–970 and 170–185 range, respectively. From these data, it is possible to determine the IQE of Al_0.2Ga_0.8N (n.c.) QDs in Al_0.7Ga_0.3N cladding layers which will be used as the active region of UV LEDs presented in the following section. The IQE at low temperature (LT) can then be calculated as follows:

$$IQE-LT=\frac{A_{fast}{\tau }_{fast}+A_{slow}{\tau }_{slow}}{\left(A_{fast}+A_{slow}\right){\tau }_{slow}}$$

(2)

and IQE-LT values between 24% and 28% are obtained. In order to estimate the IQE at room temperature (300 K), temperature dependent PL measurements have also been realized and the evolution of the integrated PL intensity as a function of temperature determined (insert of Figure 2(c)). As discussed in a previous study [28], the IQE at 300 K is obtained by considering the relation:

IQE = IQE-LT × I_PL(300 K)/I_PL(LT)

(3)

with I_PL(300 K) the integrated PL intensity at 300 K and I_PL(LT) the integrated PL intensity at 8 K. The ratio I_PL(300 K)/I_PL(LT) is found to be between 0.2 and 0.3, giving IQE values at room temperature for Al_0.2Ga_0.8N (n.c.) QDs in Al_0.7Ga_0.3N cladding layers in the range of 4.8%–8.4% in agreement with previous results and confirming the robustness of QDs in structures with high dislocation densities (> 10¹⁰ cm⁻²) [28]. Using similar active regions, QD-LED structures (LED-A and LED-B) were then fabricated as described in Figure 1 and Section 2 and are presented in the next section.

3.2. QD-Based LEDs Characteristics

The LEDs have a squared mesa size of 310 µm and were characterized at room temperature by performing on wafer I–V measurements as well as EL determined from the backside of the device (i.e., extracted from the sapphire substrate).

3.2.1. Photoluminescence

Before processing the heterostructures, their optical properties were studied at room temperature using the frequency-doubled Ar laser at 244 nm. The results for LED-A and LED-B structures are shown and compared in Figure 3. In each spectrum, the photoluminescence covers a very large spectral range spanning wavelengths from 280 nm to above 550 nm. More precisely, the emission is separated between two regions: The most intense one is made of multi-peaks in the UV range with a maximum peak intensity at 314 nm for LED-A and at 320 nm for LED-B. The FWHM of this emission is very broad, with a value of 50 nm and 67 nm (i.e., 630 and 810 meV) for LED-A and LED-B, respectively. This is much broader than the PL bands of single QD planes shown in Figure 2b, and reflects in our opinion the QD size and composition variations between the QD planes along the growth direction. In fact, for both structures, the emission ranges from about 280 nm to 400 nm. Yet, the band gap of Al_0.2Ga_0.8N is estimated at an energy of 3.89 eV that corresponds to a wavelength of 319 nm [32]. Therefore, the broadening of the PL spectrum is attributed to the QD sizes and/or Al compositions fluctuations [24]. Regarding the presence of multi-peaks, it is mainly due to the Fabry–Perot effect resulting from the high refractive index contrasts in the heterostructures. Regarding the emission in the visible (blue–green) range, it is attributed to defects/impurities below the Al_0.7Ga_0.3N cladding layer band gap, p- and n-type, as commonly observed in Al_xGa_1−xN materials [36] and corroborated by the EL measurements for which no emission is observed in this wavelength range due to the electrical injection of carriers in the QD active layers only (not shown).

3.2.2. Electrical Properties

The I–V curves of both devices are presented and superimposed on a semi-log plot in Figure 4. Starting from the characteristics under forward bias conditions, similar turn-on voltages are found around 12 ± 1 V and voltage values of around 15 ± 1 V are measured at 20 mA (i.e., at a current density (J) of 20.8 A/cm²). The high operating voltages are attributed to low conductivities in the doped Al_0.7Ga_0.3N and Al_0.8Ga_0.2N layers, in particular the p-type ones characterized by very high activation energies (≥400 meV) of the Mg dopants [37]. Parasitic resistances have also been extracted and series resistance (R_s) values at around 60 Ω have been determined for both LEDs with a slightly higher one in the case of LED-B. They are the result of excessive contact resistances. Under reverse bias conditions, a leakage current of 1 µA (1 mA/cm²) is found at –6.5 V for LED-A and at –3.3 V for LED-B. The significant reverse saturation current are due to leakage paths in the devices which could be related to the high dislocation densities in our devices (>10¹⁰ cm⁻²). An unusual behavior of the I–V characteristics at low voltages (between ± 1 V) is observed with important variations of voltage values at low currents (<1 µA) attributed to unwanted transport mechanisms caused by structural defects. In addition, sub-threshold turn-on can be seen, in particular in the case of LED-B, which is caused by carrier transport (leakage) caused by surface states or deep levels and resulting in a shunt effect.

3.2.3. Electroluminescence

Electroluminescence (EL) measurements of the two types of LEDs as a function of the input current have been performed and a comparison of the results at four different current values is presented in Figure 5. As observed by PL (Figure 3), a multi-peak emission in the UV spectral range is found, in particular at lower current values for which a wavelength spectral range from nearly 280 nm to 400 nm is covered. In addition, a shift towards shorter wavelengths of the maximum intensity EL peak is occurring when the input current increases.

In order to have more insight into the dependence of the EL emission as a function of the input current density, the variation of the average EL wavelength is reported in Figure 6 and the variation of the full width at half maximum (FWHM) in Figure 7. In fact, since the EL spectra are broad and made of several peaks, the emitted wavelength has been arbitrarily defined as the arithmetic mean between the two wavelength values measured at full width at half maximum of each EL spectrum. For both LEDs, a similar behavior of the wavelength dependence is observed, which involves three distinct regimes (labelled with numbers from 1 to 3 in Figure 6): (1) A fast reduction of the wavelength at lower J (i.e., from 0.1 A/cm² to 40 A/cm²); (2) a saturation of the wavelength at a minimum value (λ_min) for J between 40 A/cm² and 70A/cm²; and (3) an increase of the wavelength at higher J (above 70 A/cm²). Actually, the abrupt wavelength jump at 40 A/cm² in the case of LED B is mainly attributed to a change of Fabry–Perot mode (see Figure 5b).

Regarding the FWHM, two regimes can be distinguished between lower and higher J. At low J (<40 A·cm⁻²), a difference is observed between the two types of LEDs since the FWHM strongly decreases with J for LED-A by 40 nm (from 70 nm to 30 nm), whereas it remains very high (around 50–60 nm) and fairly current independent in the case of LED-B. Actually, below 40 A/cm², the EL spectra of LED B is mainly structured by two Fabry Perrot modes, and only one at higher current which leads to the strong difference in the spectra FWHM. At high J (>40 A·cm⁻²), identical behaviors are found for both LEDs characterized by an almost independence of the FWHM with J and a stabilization at a minimum value around 25 ± 3 nm. This characteristic implies a severe reduction (drop) of the FWHM for LED-B from 51 nm to 24 nm for a variation range of J from 37 to 47 A·cm⁻². In addition, this FWHM narrowing corresponds to an important UV shift of the EL emission from 322 nm to 304 nm (corresponding to the minimum wavelength λ_min). This shift is enhanced by the structuration of the spectra by interferences.

3.2.4. Optical Power and External Quantum Efficiency

The performances of the LEDs as a function of J have been further investigated by analyzing the light power–current density (L–J) characteristics of LED-A and LED-B. The measurements are presented in Figure 8. For both LED, P_opt is in the µW range for current densities of 2 A·cm⁻² or below, around 100 µW in the 40–50 A·cm⁻² range and reaches 230–250 µW at 100 A·cm⁻². However, this increase is not constant as a function of J. Actually, P_opt varies as a function of J at a constant power m, i.e., it is proportional to J^m, and the value of m depends on the current density range. At lower J, below 40–50 A/cm², a linear variation of P_opt is found (m = 1), whereas at higher J an under linear variation (m < 1) is observed. In the case of LED-A, a value of 0.8 is determined for J above 50 A·cm⁻². In the case of LED-B, a much smaller value of 0.2 is found for J above 80 A·cm⁻². However, the behavior is more complex for this device, since between 50 and 80 A·cm⁻², a super linear dependence (m > 1) of P_opt is observed with a value of 1.56, corresponding to an increase of the emitted power from 100 µW to 230 µW.

In Figure 9, the EQE of both LEDs has been reported as a function of J, following the well-known equation:

$$EQE= \frac{P_{opt}e}{\left(\frac{hc}{\lambda }\right)I}$$

(4)

with h the Planck constant, c the speed of light, λ the emitted wavelength, e the electron charge, and I the injected current.

As a general result, EQE values between 0.03% and 0.15% have been found with higher ones at lower J (below 4 A·cm⁻²). The decrease of EQE at high operating current density is commonly observed in LEDs and known as the “efficiency droop”, whose origin is mainly attributed to Auger recombination and to a lesser extend electron leakage contribution [38]. Differences are observed between both LED EQE characteristics, with higher EQEs found for LED-B at J below 4 A·cm⁻² whereas LED-A shows higher ones for J between 4 A·cm⁻² and 60 A·cm⁻². For J above 60 A·cm⁻², fairly similar values are obtained around 0.06–0.07%.

4. Discussion

From the LED characteristics presented above, several points deserve to be commented. At first, the PL emission coming from the QDs, which intensity is maximum in the UVB range at 314 nm and 320 nm for LED-A and LED-B, respectively, is covering a broad emission range from 280 nm up to 400 nm. This feature is in agreement with the EL spectra which show large FWHMs up to 60–70 nm at low J. It is the consequence of fluctuations in the QD active regions in terms of QD size (i.e., mainly the QD height since lateral confinement effects in Al_yGa_1−yN QDs are weak compared to those along the <0001> direction due to a height over diameter ratio of the order of 4 [28]) and Al composition including the intrinsic alloy inhomogeneous broadening as discussed in previous works [24,32]. Regarding the EL and the PL intensity modulation, it is influenced by a Fabry–Perot effect resulting from the high refractive index contrasts at the AlGaN/sapphire and the air/AlGaN interfaces.

It should be noted that the broad spectral range emission is also a direct consequence of the presence of an internal electric field (F_int) in the heterostructures. This field is originating from the interfacial polarization discontinuities, leading to the quantum confined Stark effect (QCSE) in Al_yGa_1−yN/Al_xGa_1−xN 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 Figure 6 and Figure 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. In fact, for J typically above 30–40 A·cm⁻², the LEDs emit, as expected, in the UVB range below 320 nm. This result means that under these conditions, carriers are injected in the main Al_yGa_1−yN QD population with y close to the n.c. of 0.2, since the band gap energy of Al_0.2Ga_0.8N corresponds to a wavelength of 319 nm. The QD emission below the band gap energy of Al_0.2Ga_0.8N is the consequence of the quantum confinement effect together with a minimized influence of F_int as the Al concentration of Al_yGa_1−yN QDs increases (due to the reduction of the piezoelectric polarization originating from the lattice-mismatch between Al_yGa_1−yN QDs and the Al_0.7Ga_0.3N cladding layer). Whereas, at lower J, carriers are preferentially injected in QDs with lower Al concentrations which represent deeper localization potentials, i.e., larger band offsets with the Al_0.7Ga_0.3N cladding layers. Differences in the LED characteristics between the two devices, that are attributed to the design of the active region made of 3 QD planes (LED-A) or 5 QD planes (LED-B), are observed at lower current densities (< 40 A·cm⁻²). 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_yGa_1−yN 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 feature is attributed to a reduced value of the internal electric field in the active region of LED-B as a consequence of thinner barrier layers (5 nm) compared to LED-A (10 nm), limiting the influence of its partial screening when increasing carrier densities in the QDs with the use of larger J. Then, as J is increased above 40 A·cm⁻² and the carriers are mainly injected in Al_yGa_1−yN QDs with y close to the n.c. of 0.2, a strong reduction of the FWHM down to around 25 nm is observed. It should be noted that the minimum FWHM value of 24 nm, compared to 25.5 nm (for similar J) in the case of LED-A, can also be seen as a consequence of a smaller value of F_int in the case of LED-B inducing a smaller inhomogeneous broadening of the QD EL emission [44].

Regarding the variation of the optical power as a function of the current density J, once again similarities and differences are observed between the two types of LEDs. Two or three different regimes are seen in Figure 8 from the variation of P_opt as a function of J at a constant power m. This parameter m is related to the recombination mechanisms involved in LEDs [45]. At lower J, i.e., up to 40 A/cm², a (quasi-)linear dependence (m = 1 ± 0.1) of P_opt is observed for LED-A and LED-B. Such a variation indicates that the IQE of the LEDs is constant in this regime. At larger J, two different behaviors are obtained. A specific regime is seen in the case of LED-B with a super linear dependence (m > 1) of P_opt between 40 and 80 A/cm². It is attributed to an increase of the hole concentration in the active region due to a hole activation mechanism as the LED junction temperature increases with higher J. Above 80 A/cm² for LED-B and 40 A/cm² for LED-A, m is found below 1, i.e., the LED output power increases sub-linearly as a function of J. While in the case of LED-A, the saturation of P_opt is rather limited with a value of 0.8, it is much stronger in the case of LED-B with a value of 0.2. Since this characteristic coincides with the redshift variation of the EL peak as a function of the injection current (Figure 6), it is related to important thermal effects caused by low injection efficiencies and poor current spreading [32]. More insight in the LED performances can be obtained by considering the EQE of the LEDs presented in Figure 9. This value is the product of the IQE, the IE, and the EE, and since the EE is constant and independent of J, the variation of the EQE gives additional information on the current density dependence of the IQE and IE. Therefore, at lower J when the IQE has been found to be constant, the EQE increase in the case of LED-A is related to an increase of the IE whereas in the case of LED-B its decrease involves on the opposite a decrease of the IE which is maximum at the minimum J value of 0.1 A/cm². The EQE of LED-A then reaches a plateau above 0.08% between 1 and 10 A/cm² and starts decreasing for higher J as a consequence of the presence of non-radiative mechanisms attributed to defect states localized in the cladding layers or band gap states in the QD active layers leading to a reduction of the IQE, and thermal effects at larger J leading to a decrease of the IE. For LED-B, an unconventional behavior is observed since after a first decrease of the EQE until 40 A/cm², a sudden increase is observed between 40 and 80 A/cm², which is the consequence of a higher IE as discussed above. Then, a strong reduction is found at larger J which is caused by a drop in the IE. Finally, a similar EQE value of 0.06 % at 100 A/cm² is measured for both devices giving a maximum optical power of 225–250 µW.

The sublinear dependency of the light output at higher current densities indicates the presence of non-radiative mechanisms which are directly related to the high TDDs in the heterostructures and reduce the IQE. In fact, considering the high operating voltages of the LEDs (i.e., in the 10–20V range) heat is certainly the main reason for the EQE decrease observed at 2–3 A/cm², whereas the high TDD is the main origin of the overall low IQE. Therefore, using AlGaN/AlN heterostructures with lower TDDs should enable to further improve the IQE of QD-based LEDs. Concerning the active region design, the insertion of 5 QD planes together with a reduction of the capping layer thickness seem to be an interesting approach to favor carrier injection in the QDs as well as to increase the light output, as observed for LED-B (in the intermediate J regime between 40 and 80 A/cm²). Finally, improvements are required for thermal management, in particular at larger J since a severe drop of the IE has been observed above 80 A/cm². Indeed, considering average values for the different efficiencies in our devices, i.e., with an IQE of 20%, an EE of 8% (estimated from the extraction cone at the sapphire/air interface) and an EQE of 0.05–0.1%, it is then possible to estimate the IE which is in the range of 3 to 6% only. Inefficient hole injection due to low hole concentration and mobility in high Al concentration Al_xGa_1−xN layers leads to electron leakage and overflow out of the active region and will require different layer designs such as tunnel junctions which have been shown as an attractive solution to improve the IE and avoid the use of an absorbing p-type GaN contact [46,47].

5. Conclusions

Al_0.2Ga_0.8N QD-based LEDs emitting in the UVB range have been fabricated by MBE using two different designs for the active region: 3 QD planes separated by 10-nm thick barrier layers (LED-A) and 5 QD planes separated by 5-nm thick barrier layers (LED-B). Despite TDDs above 10⁻¹⁰ cm⁻², IQE values around 5–10% have been estimated on reference samples as a consequence of the 3D carrier confinement in the QDs. Electro-optical characterizations have pointed out at some differences in the LED performances: (1) Higher EQE below 1A/cm² and above 40 A/cm², an emission at shorter wavelengths and an increase in the carrier injection efficiency in the QDs as well as in the optical power (between 40 and 80 A/cm²) were observed for LED-B compared to LED-A; (2) higher EQE between 5 and 40 A/cm² and a reduction of the EL FWHM between 0.1 and 40 A/cm² were found for LED-A. At current densities above 80 A/cm², similar characteristics were found with EQE around 0.06% and optical power in the 0.2–0.25 mW range. Further improvements in the IE together with a reduction of the TDDs are suggested for future research directions to reach higher performances.