Fig 3 - uploaded by Swee Tiam Tan
Content may be subject to copyright.
Schematic energy diagrams for (a) LED I and (b) LED II, along with which four electron transport/transition processes are depicted in the InGaN/GaN MQWs: electrons are captured into the quantum well, electrons recombine with holes and at defects, electrons re-escape from the quantum well and electrons directly fly over to a remote position without being captured by the quantum well.
Source publication
Electron overflow limits the quantum efficiency of InGaN/GaN light-emitting diodes. InGaN electron cooler (EC) can be inserted before growing InGaN/GaN multiple quantum wells (MQWs) to reduce electron overflow. However, detailed mechanisms of how the InGaN EC contributes to the efficiency improvement have remained unclear so far. In this work, we t...
Contexts in source publication
Context 1
... captured by the n-In 0.10 Ga 0.90 N EC layer with LO phonon emission, while the remaining electrons of N 1 directly fly over the EC layer without undergoing thermalization. The electrons of N 2 are then injected into InGaN/GaN MQW region after undergoing thermalization. Here, we correlate the quantum well captured electrons [i.e., process in Figs. 3(a) and 3(b)] with the electron mean free path (MFP) by Eq. (1) and Eq. (2) for LEDs I and II, respectively [20]. Note that the electron loss due to processes and contribute to the electron overflow from the MQW ...
Context 2
... QW t is the thickness of the quantum well, MFP l is the mean free path of electrons within the InGaN/GaN MQWs without electron thermalization and cooler MFP l is the mean free path of electrons in the InGaN/GaN MQWs with electron thermalization in the n-In 0.10 Ga 0.90 N EC layer. Here, the relationship between N 0 and N 2 in Fig. 3(b) can be expressed in Eq. (3), in which we assume the mean free path of electrons in the n-GaN layer before entering the n- In 0.10 Ga 0.90 N EC layer is MFP l . It is shown that, in order to have more electrons thermalized, it is useful to properly increase the thickness of the n-In 0.10 Ga 0.90 N EC layer ( cooler t ...
Context 3
... N EC layer, compared to that in LED I. Meanwhile, the electron current distribution is also depicted in Fig. 6(b). Being consistent with Fig. 6(a), the electron leakage current into the p-type region is reduced from 26.56% to 18.86% at 20 A/cm 2 , if we compare LED II to LED I. It should be noteworthy that the thermionic emission for process in Figs. 3(a) ...
Similar publications
We have developed a dry etch process for the fabrication of lithographically defined features close to light emitting layers in the III-nitride material system. The dry etch was tested for its effect on the internal quantum efficiency of c-plane InGaN quantum wells using the photoluminescence of a test structure with two active regions. No change w...
Optical design in enhancing optical absorption of group-III-nitride- and multiple quantum well-based GaN/In x Ga1- x N/cSi dual-junction tandem solar cells with triangular diffraction grating is simulated and optimized by using combined two-dimensional rigorous coupled wave analysis and transfer matrix methods. This paper thoroughly examines these...
By means of time-resolved photoluminescence (PL) and confocal PL measurements, temporally and spatially resolved optical properties have been investigated on a number of InxGa1-xN/GaN multiple-quantum-well (MQW) structures with a wide range of indium content alloys from 13% to 35% on (11 (2) over bar2) semi-polar GaN with high crystal quality, obta...
The effect of surface states has been investigated in hybrid organic/inorganic white light emitting structures that employ high efficiency, nearfield non-radiative energy transfer (NRET) coupling. The structures utilize blue emitting InGaN/GaN multiple quantum well
(MQW)
nanorod arrays to minimize the separation with a yellow emitting F8BT coating....
In this paper, we report on a detailed spectroscopic study of the optical properties of InGaN/GaN multiple quantum well structures, both with and without a Si-doped InGaN prelayer. In photoluminescence and photoluminescence excitation spectroscopy, a 2nd emission band, occurring at a higher energy, was identified in the spectrum of the multiple qua...
Citations
... The electrostatic field in the active region can be estimated using the following Eqs 3-5. 27 (3) ...
Electron leakage is one of the critical challenges in AlGaN ultraviolet (UV) light-emitting diodes (LEDs). In this regard, a p-type AlGaN electron-blocking layer (EBL) has been utilized to suppress electron leakage. However, it affects the hole injection due to the generation of positive polarization sheet charges at the hetero-interface of the EBL and the last quantum barrier (QB). To address this problem, we propose an EBL-free AlGaN UV LED using polarization-engineered graded QBs instead of conventional QBs. The proposed structure could enhance the carrier confinement in the active region and significantly reduces electron leakage due to the progressively increased effective conduction band barrier heights. Substantially, the proposed structure exhibits higher optical power and wall-plug efficiency at 60 mA current injection, which are boosted by ~85.9% and ~53.6% compared to the conventional structure. Such a unique LED design could pave the way for the next generation of high-power deep UV light sources.
... Most recently, DUV LEDs without p-EBL are also proven to be a potential solution strategy for favoring the hole injection if the electron injection efficiency is not sacrificed [15,16]. Besides engineering the hole injection process, research attention shall also be paid to electrons, such that electrons are difficult to be captured by the multiple quantum wells (MQWs), which is due to the large electron mobility [17]. Therefore, AlGaN quantum barriers (QBs) with the Al composition spike-structured barriers [18] and the n-Al x G 1−x N/n-Al y Ga 1−y N (x > y) electron injection layers [19] are designed to reduce the drift velocity of the free electrons for DUV LEDs. ...
... To reveal the in-depth device physics, we calculate the energy band, the carrier transport and the radiative recombination rate by using APSYS [6,14,17,19]. In our calculation models, we have taken the polarization effect at each lattice-mismatched AlGaN/(Al)GaN heterojunction into consideration by assuming the 40% polarization level [14]. ...
In this work, we report an AlGaN-based ∼275 nm deep ultraviolet light-emitting diode (DUV LED) that has AlGaN based quantum barriers with a properly large Al composition. It is known that the increased conduction band barrier height helps to enhance the electron concentration in the active region. However, we find that the promoted hole injection efficiency is also enabled for the proposed DUV LED when the Al composition increases. This is attributed to the reduced positive polarization charge density at the last quantum barrier (LQB) and p-type electron blocking layer (p-EBL) interface, which can suppress the hole depletion effect in the p-EBL. Thus, the hole concentration in the p-EBL gets promoted, which is very helpful to reduce the hole blocking effect caused by the p-EBL. Therefore, thanks to the improved carrier injection, the proposed DUV LED increases the optical power and reduces the forward voltage when compared with the conventional DUV LED.
... It is clearly understood that processes z and { lead to electron leakage from the active region, which is undesired and needs to be mitigated. The captured electrons (N c 0 ) in the QW of LED0 can be expressed as [20] ...
Electron overflow from the active region confines the AlGaN deep-ultraviolet (UV) light-emitting diode (LED) performance. This paper proposes a novel approach to mitigate the electron leakage problem in AlGaN deep-UV LEDs using concave quantum barrier (QB) structures. The proposed QBs suppress the electron leakage by significantly reducing the electron mean free path that improves the electron capturing capability in the active region. Overall, such an engineered structure also enhances the hole injection into the active region, thereby enhancing the radiative recombination in the quantum wells. As a result, our study shows that the proposed structure exhibits an optical power of 9.16 mW at ${\sim}{{284}}\;{\rm{nm}}$ ∼ 284 n m wavelength, which is boosted by ${\sim}{40.5}\%$ ∼ 40.5 % compared to conventional AlGaN UV LED operating at 60 mA injection current.
... The light extraction efficiency for the investigated devices is set to 75% [20]. The Auger recombination with the recombination coefficient is set to 1 × 10 −31 cm 6 s −1 [21], and the Shockley-Read-Hall (SRH) recombination lifetime for carriers is set to 10 ns [22]. To accurately model the surface recombination, sidewall defects are considered, and relevant trap parameters are summarized in Table 1. ...
In this work, we propose adopting step-type quantum wells to improve the external quantum efficiency for GaN-based yellow micro light-emitting diodes. The step-type quantum well is separated into two parts with slightly different InN compositions. The proposed quantum well structure can partially reduce the polarization mismatch between quantum barriers and quantum wells, which increases the overlap for electron and hole wave functions without affecting the emission wavelength. Another advantage is that the slightly decreased InN composition in the quantum well helps to decrease the valence band barrier height for holes. For this reason, the hole injection capability is improved. More importantly, we also find that step-type quantum wells can make holes spread less to the mesa edges, thus suppressing the surface nonradiative recombination and decreasing the leakage current.
... region is neglected. The captured electrons in the quantum well (Ncapture) are correlated with the electron mean free path (lMFP) as expressed in Equation (2) [31]. To better understand the role of GSQB structure in LED 3, the schematic model for transportation of electrons in LED 1 and LED 3 is depicted in Figure 6. ...
... For the simplicity of the model, electron loss through non-radiative recombination in n-Al 0.6 Ga 0.4 N region is neglected. The captured electrons in the quantum well (N capture ) are correlated with the electron mean free path (l MFP ) as expressed in Equation (2) [31]. ...
... At the same time, l MFP depends on thermal velocity (v th ) and the scattering time (τ sc ) as shown in Equation (3). For LED 1, v th can be further expressed as illustrated in Equation (4) [31]. ...
To prevent electron leakage in deep ultraviolet (UV) AlGaN light-emitting diodes (LEDs), Al-rich p-type AlxGa(1−x)N electron blocking layer (EBL) has been utilized. However, the conventional EBL can mitigate the electron overflow only up to some extent and adversely, holes are depleted in the EBL due to the formation of positive sheet polarization charges at the heterointerface of the last quantum barrier (QB)/EBL. Subsequently, the hole injection efficiency of the LED is severely limited. In this regard, we propose an EBL-free AlGaN deep UV LED structure using graded staircase quantum barriers (GSQBs) instead of conventional QBs without affecting the hole injection efficiency. The reported structure exhibits significantly reduced thermal velocity and mean free path of electrons in the active region, thus greatly confines the electrons over there and tremendously decreases the electron leakage into the p-region. Moreover, such specially designed QBs reduce the quantum-confined Stark effect in the active region, thereby improves the electron and hole wavefunctions overlap. As a result, both the internal quantum efficiency and output power of the GSQB structure are ~2.13 times higher than the conventional structure at 60 mA. Importantly, our proposed structure exhibits only ~20.68% efficiency droop during 0–60 mA injection current, which is significantly lower compared to the regular structure.
... It is assumed that out of No electrons, N2 electrons captured by the first extra QW undergo thermalization with LO phonon emission while remaining electrons denoted as N1, directly travel over the extra QW layer without undergoing thermalization. The captured electrons in the quantum wells are correlated with the electron lMFP [19]. To increase the number of the quantum wellcaptured electrons, the electron lMFP within the InGaN/GaN MQW region must be reduced. ...
In this study, we have proposed and investigated the effect of coupled quantum wells to reduce electron overflow in InGaN/GaN nanowire white color light-emitting diodes. The coupled quantum well before the active region could decrease the thermal velocity, which leads to a reduced electron mean free path. This improves the electron confinement in the active region and mitigates electron overflow in the devices. In addition, coupled quantum well after the active region utilizes the leaked electrons from the active region and contributes to the white light emission. Therefore, the output power and external quantum efficiency of the proposed nanowire LEDs are improved. Moreover, the efficiency droop was negligible up to 900 mA injection current.
... Besides, the left electrons would directly travel from the QB1 to QB2 through process ③. Among the three processes, only process ① is able to realize an effective electron injection, and the concentration of electrons captured by the QW can be expressed by [17]: ...
Carrier velocity modulation by asymmetrical concave quantum barriers to improve the performance of AlGaN-based deep-ultraviolet light emitting diodes has been investigated. The concave structure is realized by inserting a low Al composition AlGaN layer in the barrier, and thus forming a concave region on energy band. The measured device performance shows a significant improvement under 0-250 mA operation current, with the maximum external quantum efficiency and light output power of 1.02% and 8.0 mW, which are more than double (2.32 and 2.35 times) of that for the conventional one, respectively. Simulation confirms that the concave quantum barriers make it possible for electrons to be scattered, which reduce the electron energy before being injected into quantum wells, and thus the capability of quantum wells to capture electrons is enhanced. Meanwhile, the vertical transport of holes within the active region is also enhanced because the asymmetrical setting of the concave layer in the quantum barriers helps holes accelerate under the polarization field, and then get more energy to overcome the barrier.
... Specially, we adopt the very well-developed mean-free-path model when calculating the carrier transport in the active region for DUV LEDs. 23,24 When calculating the polarization induced electric field, we shall take the polarization induced charges at the polarizationmismatched interfaces into consideration. The polarization level is set to 40%. ...
... It is known that the confinement factor for each quantum well can be increased because of the increased quantum well thickness if we use the mean-free-path model to analyze electron transportation. 24 For holes, the mean-free-path model can be revised and modeled to Eqs. (1) and (2). Pi, P i+1 , tQW, and lMFP represent the captured hole density in the i th (i th > 1, note the first quantum wells is the one closest to the n-AlGaN region, according to Fig. 1) quantum well, the incoming hole density which fails to be captured by the i th + 1 quantum well, the quantum well thickness, and the mean free path, respectively. ...
... The scattering time is set to 0.0091 ps. 24,29 The holes have a heavier effective mass and lower mobility, and hence, according to the common belief, the quantum well shall be properly thinned for promoting hole injection, 30 which can also be reflected by Eq. (1), i.e., the next quantum well can capture more holes if the quantum well thickness decreases. In 0.15 Ga 0.85 N/GaN is the active region of blue LEDs. ...
In this work, we simply take advantage of the polarization effect to efficiently improve the hole injection from the p-type electron blocking layer (p-EBL) to the end of the active region for AlGaN based deep ultraviolet light emitting diodes (DUV LEDs). By properly increasing the AlN composition of AlGaN quantum barriers, a smaller positive polarized charge density at the last quantum barrier/p-EBL interface can be obtained, which correspondingly leads to the suppressed hole depletion and the reduced hole blocking effect in the p-EBL. Meanwhile, we properly increase the quantum well thickness so that the polarized electric field can even more accelerate the holes, and this will homogenize the hole distribution more across the MQWs. Therefore, the external quantum efficiency for DUV LEDs can be enhanced.
... Besides, there are still many structure design strategies out of the active region having been adopted to improve the CIE, such as the AlGaN/GaN superlattice p-type layer, 10-13 polarization self-screening effect electron blocking layer (EBL), 14 multiquantum barrier EBL, 15 step graded-composition last quantum barrier, 16 and electron cooler in the GaN n-type layer. 17 Among those approaches, it is noted that almost all designs are aimed at improving the vertical transport of holes and localizing electrons within the active region. Actually, the low hole injection efficiency in AlGaN-based LEDs has been regarded as one of the most serious issues limiting the EQE. ...
AlGaN-based deep-ultraviolet light emitting diodes adopting an embedded delta-AlGaN thin layer with an Al composition higher than that in conventional barriers have been investigated. The experimental result shows that when the current is below 250 mA, the maximum of the external quantum efficiency and light output power for the proposed structure reach severally 1.38% and 10.1 mW, which are enhanced significantly by 160% and 197%, respectively, compared to the conventional ones, showing a tremendous improvement. We attribute that to the inserted delta-thin layer's modulation effect on the energy band, namely, accelerating holes to cross the high barrier with very large kinetic energy, thus increasing the hole injection into the active regions. Meanwhile, the electron concentration within the active regions is enhanced as well because of the accompanying additional effect of the delta-AlGaN thin layer being an electron barrier to block electrons escaping from the active region.
... Parameters regarding the non-radiative recombinations include the Shockley-Read-Hall recombination lifetime of 10 ns and Auger recombination coefficient of 1 × 10 −30 cm 6 · s −1 [22]. The carrier transport is modeled by using the drift-diffusion equations in the non-active layers and the mean-free-path model in the MQW region [23]. The carrier transport in the MQWs is also associated with the energy band offset ratio that is set to 50/50 for AlGaN/AlGaN junctions [24]. ...
It is well known that the p-type AlGaN electron blocking layer (p-EBL) can block hole injection for deep ultraviolet light-emitting diodes (DUV LEDs). The polarization induced electric field in the p-EBL for [0001] oriented DUV LEDs makes the holes less mobile and thus further decreases the hole injection capability. Fortunately, enhanced hole injection is doable by making holes lose less energy, and this is enabled by a specifically designed p-EBL structure that has a graded AlN composition. The proposed p-EBL can screen the polarization induced electric field in the p-EBL. As a result, holes will lose less energy after going through the proposed p-EBL, which correspondingly leads to the enhanced hole injection. Thus, an external quantum efficiency of 7.6% for the 275 nm DUV LED structure is obtained.
