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Abstract
We investigated the effect of coupled quantum wells to reduce electron overflow in InGaN/GaN dot-in-a-wire phosphor-free white color light-emitting diodes (white LEDs) and to improve the device performance. The light output power and external quantum efficiency (EQE) of the white LEDs with coupled quantum wells were increased and indicated that the efficiency droop was reduced. The improved output power and EQE of LEDs with the coupled quantum wells were attributed to the significant reduction of electron overflow primarily responsible for efficiency degradation through the near-surface GaN region. Compared to the commonly used AlGaN electron blocking layer between the device active region and p-GaN, the incorporation of a suitable InGaN quantum well between the n-GaN and the active region does not adversely affect the hole injection process. Moreover, the electron transport to the device active region can be further controlled by optimizing the thickness and bandgap energy of this InGaN quantum well. In addition, a blue-emitting InGaN quantum well is incorporated between the quantum dot active region and the p-GaN, wherein electrons escaping from the device active region can recombine with holes and contribute to white-light emission. The resulting device exhibits high internal quantum efficiency of 58.5% with highly stable emission characteristics and virtually no efficiency droop.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
White color light-emitting diodes (white LEDs) based on solid-state technology have attracted great attention because of their tremendous energy saving potentials [1–4], applications for liquid crystal displays [5–7], and general lighting technology. Typically, the white LEDs can be fabricated with the combination of discrete monochromatic LEDs [8,9], and color rendering can be controlled by their light mixing characteristics. An alternative approach for white-lighting is phosphor-converted LED [10–14] typically realized by the luminescence down conversion technique using yellow phosphors such as cerium-doped yttrium aluminum garnet. The conversion efficiency of phosphor-based white LEDs is typically poor due to energy loss of converting short wavelength photons to long wavelength ones known as Stokes effect.
Indium gallium nitride (InGaN) compound semiconductor alloys have been intensively studied and shown tremendous promise for applications in solid-state lighting. Moreover, compared to conventional planar structures, III-nitride nanowires can exhibit significant advantages including greatly reduced dislocation densities and polarization fields, due to the effective lateral stress relaxation. Additionally, the nanowire LED with the incorporation of quantum dots/disks in the active region may lead to superior carrier confinement, promising for high efficiency LEDs with tunable emission [15–23]. However, nanowire LEDs still contain several challenges for further improving the quantum efficiency and light output power, which may include the inefficient carrier confinement in the active region, nonuniform carrier distribution [24,25], electron overflow [26,27], and the presence of large densities of states and defects along the wire lateral surfaces [28–30]. Recently, we have addressed the presence of electron overflow and its effect on the nanowire LED performance [27]. It is believed that electron overflow is a problem that causes efficiency droop [27] and strongly influences the output characteristics of white LEDs, especially at high injection levels. For the emerging nanowire LEDs, compared to conventional planar heterostructures, the device performance is more susceptible to carrier leakage/electron overflow, due to the very large surface-to-volume ratios. Recent research shows the role of carrier leakage and electron overflow in the efficiency droop of GaN-based planar LEDs [31–35], it is even worse in nanowire LEDs due to the highly non-uniform In distribution along the lateral dimension and consequently flowing the current in the near-surface GaN region. Due to the heavy effective mass and small mobility of holes, the hole injection in InGaN/GaN nanowire heterostructures is highly non-uniform. The holes reside close to the p-doped GaN layer, while electrons have a relatively uniform distribution in the active region. Auger recombination and electron overflow are enhanced by highly non-uniform carrier distribution, and consequently adversely affect the device performance. The non-radiative recombination occurs in the p-type GaN region due to the recombination of inefficient injection of holes and electrons leakage.
Recently, some different methods were proposed to prevent electron overflow, including p-type modulation doped active region to enhance the hole transport [16], thin InGaN barrier [36], tunnel junction [37], inserting a p-type AlGaN layer between the p-type GaN layer and active region known as an electron blocking layer (EBL) [38–41]. However, p-type AlGaN with a high Al content may form a high barrier, resulting in the low hole injection into the LED active region. To reduce electron overflow without using an AlGaN electron blocking layer, it is obvious that electron should be slowed down before injecting to the active region. The structure with step-wise increased In composition was utilized between the n-side and the active region of LED to reduce the electron overflow [31,33]. Inserting a layer with the lower In composition reduces the kinetic energy and velocity of the injected electron, and consequently electron overflow could be decreased. However, such phenomenon has not been addressed in the emerging nanowire LEDs. In this paper, we proposed a new method which utilizes InGaN quantum well between the quantum dot active region and n-GaN to reduce electron overflow. Incorporating different compositions of InGaN quantum dots results in an intrinsic white LED. The blue, green and red light from self-organized quantum dots incorporated in a single GaN nanowire with various In compositions emit stable white light emission. We have performed a detailed study, both theoretically and experimentally, of electron overflow in InGaN/GaN dot-in-a-wire white LEDs, wherein an InGaN well is incorporated between the n-GaN and the device active region to effectively control electron overflow. Furthermore, to utilize the electron escaped from the active region, we have employed a second InGaN quantum well between the active region and p-GaN to reduce electron loss to the p-GaN and contribute blue light emission to relatively control the white light emission from the LED device. Consequently, high internal quantum efficiency (IQE) InGaN/GaN nanowire LEDs of 58.5% with stable white-light emission was demonstrated.
2. Simulation and device structure
The effect of coupled quantum wells incorporated in InGaN/GaN white LEDs is studied using commercially available APSYS simulation software. For this study, total three InGaN/GaN nanowire LED structures are considered, schematically illustrated in Fig. 1. The first device is denoted as LED1, consists of a 200 nm thick n-GaN nanowire template, 10 multiple quantum wells (MQWs) of 3 nm GaN quantum barrier (QB)/ 3 nm InGaN quantum well (QW) in the active region and a 100 nm thick p-GaN. The second structure is denoted as LED2, has a similar structure as LED1 except that an EBL of 10 nm thick p-doped Al0.1Ga0.9N is introduced in between the active region and p-GaN layer. Finally, our proposed structure, LED3 has the same structure as LED1, but with an extra 30 nm thick n-doped In0.20Ga0.8N layer incorporated in between the n-GaN template and the active region. Moreover, it has an extra 10 nm thick p-doped In0.2Ga0.8N quantum well in between the active region and p-GaN layer.
The use of the AlGaN EBL has been previously proposed and implemented to overcome the electron overflow outside of the active region in InGaN/GaN LEDs. However, an EBL can adversely affect the hole injection and transportation into the active region. The problems such as electron overflow, inefficient hole injection and transportation in InGaN/GaN white LEDs can be controlled by the incorporation of n-doped and p-doped QWs before and after the active region respectively. The n-doped QW helps to control the electron transportation into the active region. Detailed studies further confirm that the InGaN quantum well can significantly reduce electron overflow through the near-surface GaN region. Compared to the commonly used AlGaN EBL between the device active region and p-GaN, the incorporation of a suitable p-doped InGaN quantum well between p-GaN and the active region does not adversely affect the hole injection process. Moreover, the overflowed electrons from the active region can be utilized by p-doped QW to recombine with holes and contribute to white light emission. As the electron effective masses are much lighter than those of holes and consequently higher thermal velocities, some electrons are not captured and recombine in the active layer and results in efficiency reduction. To increase efficiency of the proposed LED structure, the capture rates for electrons and holes in the active layer should be accelerated. The first quantum well which is closed to the n-GaN segment is employed to control electron flow in the active region while another is used to utilize electron leakage out of the active region for the blue light emission. The n-GaN section as an electron emitter is coupled to the active region via a barrier. Electron current flows from n-GaN section into the electron injector quantum well which acts as a current spreading layer, then electrons tunnel through the barrier into the active region and recombine with the holes. The electron injector quantum well suppresses the electron leakage into the p-GaN segment and eliminates the undesired light generated outside the active region.
Figures 2(a) and 2(b) present the distribution of electron and hole concentrations across the device structures, respectively. Figure 2(c) shows the radiative recombination rates in different LED structures at 750A/cm2. It is consistent with the previous findings that in typical MQW active region based InGaN/GaN LEDs, both electrons and holes concentrations are high in the QW closest to the p-doped region and dominant radiative recombination also occurs therein [42]. In case of LED2, EBL mitigates the electron overflow into the p-region up to some extent and carrier concentration in the active region is slightly higher compared to LED1. However, in case of LED3, the n-doped QW is able to trap the electrons which leads to slow down the electron flow and can be observed from Fig. 2(a). Moreover, in this LED, the carrier transportation is controlled by both QWs and leads to having additional radiative recombination in these QWs along with active region as shown in Fig. 2(c). The total radiative recombination of LED3 is 6.33×1030/cm3s which is higher than other LEDs as the total radiative recombination of LED1, and LED2 are 4.59×1030/cm3s, and 5.74×1030/cm3s, respectively.
Fig. 2. (a) Electron concentration, (b) Hole concentration, (c) Radiative recombination of different simulated LED structures.
It is well known that in nitride-based LEDs, electrons have relatively low effective mass as compared to holes and thus a very high mobility and insufficient carrier confinement capability of QW barriers [43] cause electrons to easily escape from the QW active region and enter the p-type region. To estimate the electron leakage current, the electron current density of LEDs is calculated and is shown in Fig. 3(a). LED3 has less electron leakage from the active region compare to other devices, which supports the above-mentioned results. Hence consumption of holes in p-region is reduced due to less electron overflow into p-region that enhances the hole injection efficiency into the active region, which can be understood from Fig. 3(b). Due to the reduction of electron overflow and improvement of hole injection efficiency in the case of coupled quantum well LED (LED3), the IQE and output power are also improved compared to other cases and as shown in Figs. 3(c) and 3(d).
Fig. 3. Normalized (a) Electron current density, (b) Hole current density, (c) Internal quantum efficiency, (d) L-I characteristics of different simulated LED structures.
3. Experiment and results
The experiment throughout this study was designed according to the results obtained from the simulation. Vertically aligned InGaN/GaN dot-in-a-wire heterostructures, illustrated in Fig. 1, were grown on Si (111) substrates by radio-frequency (RF) plasma-assisted molecular beam epitaxy (Veeco Gen II MBE) under nitrogen-rich condition. GaN nanowires were grown at ∼ 730°C; nitrogen flow rate was kept at 1 sccm with a forward plasma power of ∼ 350W. The grown nanowires vertically aligned to the substrate and exhibit uniform height. The device active region contains ten InGaN/GaN quantum dots which were grown at relatively lower temperatures (550–600°C) to enhance In incorporation in the dots. InGaN quantum dot thickness is ∼ 3 nm and capped by ∼ 3 nm GaN layer. For LED2, the AlGaN EBL layer was grown at 800°C with the same conditions of plasma power and nitrogen flow rate as that for LED1. For LED3, the device active region is sandwiched by two InGaN/GaN quantum wells which were grown at 630°C to control electron flow in the active region and utilize electron leakage out of the active region for the blue light emission. As shown in Fig. 4(a), a 45° tilted scanning electron microscopy (SEM) image of the LED3 grown on Si (1 1 1), the wire diameters and densities are in the range of 60 to 100 nm and ∼ 1×1010 cm−2, respectively. The scanning transmission electron microscope (STEM) (inset in Fig. 4(b)) clearly shows the structure of the active region in one nanowire; Utilizing Z-contrast image which provides the most convenient incoherent image of crystals at atomic resolution shows two InGaN quantum wells and ten quantum dots are brighter compared with the surrounding region. The scanning electron dispersive spectra of the red line in inset were shown in Fig. 4(b), and the distribution of indium in quantum wells and ten dots were confirmed. Strong photoluminescence intensity has been measured for these nanowire LEDs as shown in Fig. 5(a). The peak emission wavelength at ∼430 nm originates due to the emission from the coupled quantum wells while emission peak at ∼550 nm corresponds to the emission from the quantum dot active region.
Fig. 4. (a) 45° tilted SEM image of LED3 on Si substrate. (b) Annular dark-field STEM image and EDXS signals for In and Ga of LED3.
Fig. 5. (a) Normalized photoluminescence spectra of three LEDs measured at 300K. (b) Room temperature electroluminescence spectra of LED1 measured at different injection currents. (c) Room temperature electroluminescence spectra of LED3 measured at different injection currents. (d) The 1931 Commission International l’Eclairage chromaticity diagram presents stable white light emission characteristics of the LEDs.
The nanowire LED fabrication process includes the following steps. First, a polyimide resist was spin-coated to fully cover the nanowires, followed by O2 plasma etching process to expose the top portion of nanowires. Thin Ni(5 nm)/Au(5 nm) and Ti(20 nm)/Au(120 nm) layers, were deposited on the nanowire surface and backside of the Si substrate, respectively. A 200 nm indium tin oxide (ITO) layer was coated on the device top surface to serve as a transparent electrode and current spreading layer. The fabricated devices with Ti/Au and Ni/Au contacts were annealed at 500°C for 1 min in nitrogen ambient, while the complete devices with ITO contacts were annealed at 300°C for 1 hour in vacuum. The device area is ∼ 300×300 µm2. Figures 5(b) and 5(c) shows the normalized electroluminescence spectra of LED1 and LED3 under various injection currents, respectively. The peak at ∼550 nm related to the emission from the quantum dot active region is well agreed with the photoluminescence results as shown in Fig. 5(a). It is obvious that the emission at ∼430 nm becomes progressively stronger with increasing current due to the carrier recombination in the InGaN/GaN quantum wells at higher current. This shows the role of InGaN/GaN quantum wells in emission characteristics of the LED; at higher current, more injected electrons can escape from the quantum dot active region and have more chance to recombine with holes in InGaN/GaN quantum wells. The electroluminescence spectra cover the whole visible range and show the balanced RGB distribution. Under different applied current, spectroscopic shape is slightly changed, exhibit acceptable stability.
Experimentally, we have also demonstrated that electron overflow can be effectively controlled in those nanowire LEDs. In addition, a blue-emitting InGaN quantum well is incorporated between the quantum dot active region and p-GaN, wherein electrons escaping the device active region can recombine with holes to contribute to white-light emission. The resulting device exhibits highly stable emission characteristics, illustrated in Fig. 5(c) which is for LED3. Figure 5(d) illustrates the possibility of white light from the proposed LED by the Commission International de l'Eclairage (CIE) chromaticity diagram which shows a highly stable dot-in-a-wire white-light LED3. It exhibits CIE coordinates of (0.33, 0.36) nearly close to the standard white light point of (0.33, 0.33).
The fabricated LED3 devices exhibit excellent current-voltage characteristics with very small leakage current which is just ∼8 µA at -6 V, presented in Fig. 6(a). The output powers of LED1, LED2 and LED3 are shown in Fig. 6(b). It is seen that, LED3 demonstrates the highest output power which agrees well with the simulation results presented in Fig. 3(d).
Fig. 6. (a) Room temperature current-voltage curve of LED3. The inset shows I-V characteristic of LED3 in semi-log scale. (b) Light-current curves measured for LED1, LED2 and LED3 at room temperature. (c) Relative EQEs measured for LED1, LED2 and LED3 at room temperature.
Moreover, no efficiency droop was observed in LED3 up to injection current density of 1000 A/cm2, shown in Fig. 6(c). In addition, shown in Fig. 7, the IQE of LED3 was measured to be ∼ 58.5% by comparing the EL intensity of this LED at 300 K and 10 K assuming that the emission of the LED at 10 K is unity. We have further studied the loss mechanism in this LED by using ABF model which is described by the following equation:
Here N, A, and B are the carrier density of the active region, the Shockley-Read-Hall nonradiative recombination, and the radiative recombination coefficients. $f(N )$ is used to describe any other higher order carrier loss processes, such as Auger recombination and electron overflow [44,45]. $f(N )$ an be written as $C{N^3} + D{N^4}$. We assumed D is negligible in our fitting. The values of A, B, and C are calculated as 2.9×108 s−1, 5×10−10 cm3s−1, and 1×10−35 cm6s−1 at 300K, respectively. The calculated C value is significantly smaller than the commonly reported values in InGaN thin-film LED structures which are in the range of 10−29–10−31 cm6s−1 [46–48]. Therefore, it is suggested that Auger recombination plays an essentially negligible role on the performance of InGaN/GaN nanowire LEDs which is consistent with other theoretical and experimental studies [49,50]. The reduced Auger recombination coefficient presented in this study may also be resulted from the strong carrier confinement in the dot-in-a-wire nanoscale heterostructure [51]. The derived A, B, C values in this study agree well to those calculated values for other nanowire LEDs [18,30]. The quantum efficiency of the proposed LED reaches its peak value at significantly higher current densities ∼500A/cm2, compared to that of conventional InGaN/GaN quantum well blue-emitting LEDs ∼20A/cm2 [47,52]. Nonradiative surface recombination caused by the very large surface-to-volume ratio of nanowires results in higher Shockley-Read-Hall recombination coefficient as compared to InGaN/GaN quantum well blue-emitting LEDs [48].Compared to the InGaN/AlGaN core-shell nanowire LEDs [43] that we reported earlier, the EBL-free InGaN/GaN nanowire LEDs have better hole transport properties and more uniform electron-hole distribution. However, the large nonradiative recombination on nanowire surfaces significantly affect the current injection efficiency and the output power of the InGaN/GaN EBL-free nanowire LEDs, resulted in lower output power and external quantum efficiency compared to the InGaN/AlGaN core-shell nanowire LEDs. Our future work will be related to the integration of the coupled quantum wells in the InGaN/AlGaN core-shell nanowire LEDs to improve the uniformity of carrier distribution, and enhance the radiative recombination in the active region, enabling high EQE and light output power of the core-shell LED devices.
4. Summary
In summary, we have demonstrated the high efficiency and truly white LEDs using InGaN/GaN dot-in-a-wire structure with the usage of two additional InGaN quantum wells. Using the electrically coupled InGaN quantum wells allows reducing electron leakage into the p-GaN segment, enhancing the electron capture into the active region and eliminating the undesired light generated outside the active region. The resulted EBL-free nanowire white LEDs have relatively high IQE of ∼ 58.5% with remarkably stable white-light emission and virtually no efficiency droop up to injection current density of 1000 A/cm2.
Funding
This work is being supported by the New Jersey Institute of Technology and this research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under the grant number 103.03-2017.312.
Disclosures
The authors declare no conflicts of interest.
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