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Abstract
AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) still suffer from poor quantum efficiency and low optical power. In this work, we proposed a DUV LED structure that includes five unique AlxGa1-xN quantum barriers (QBs); Each QB has a linear-increment of Al composition by 0.03 along the growth direction, unlike those commonly used flat QBs in conventional LEDs. As a result, the electron and hole concentration in the active region was considerably increased, attributing to the success of the electron blocking effect and enhanced hole injection efficiency. Importantly, the optical power was remarkably improved by 65.83% at the injection current of 60 mA. After in-depth device optimization, we found that a relatively thinner graded QB layer could further boost the LED performance because of the increased carrier concentrations and enhanced electron and hole wave function overlap in the QW, triggering a much higher radiative recombination efficiency. Hence, the proposed graded QBs, which have a continuous increment of Al composition along the growth direction, provide us with an effective solution to boost light output power in the pursuit of high-performance DUV emitters.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
III-nitride deep-ultraviolet light-emitting diodes (DUV LEDs) have been recognized as one of the ideal candidates to replace the toxic mercury-based UV-lamps for the application of air/water purification, disinfection/sterilization and biochemical environmental sensing [1–3]. However, the low optical power of the DUV LEDs is still limited by the poor external quantum efficiency (EQE), internal quantum efficiency (IQE) and carrier injection efficiency [4–6]. How to confine the electrons effectively while injecting sufficient holes into the active region remains a challenge to the community [7–9]. Owing to the high mobility of electrons and poor electron confinement in the active region [10–12], a large number of electrons can easily leak to the p-side of the DUV LED, causing a severe electron leakage current and a poor radiative recombination efficiency. Those leaked electrons could subsequently recombine with the holes in the p-type region, thereby preventing the holes from being injected into the active region [7]. On the other hand, in general, the Mg doping efficiency in high Al-content AlGaN epilayers is as low as ∼10−9 [13], and the relatively lower mobility of the holes in AlGaN alloys further constrains the hole injection. To address this issue, by far, extensive studies have been carried out in the design of a proper electron blocking layer (EBL) to overcome the electron overflow and enhance the hole injection efficiency simultaneously [14,15]. An AlGaN p-EBL structure with graded Al composition was proposed to facilitate the hole injection by reducing the potential barrier height at the valence band [16]. Besides, special EBLs have been designed including the incorporation of multi-quantum barrier EBL [17], AlGaN/AlGaN superlattice EBL [15,18] and inverted-V-shaped graded Al-composition EBL [19], etc, aiming to improve the carrier concentration and their distribution, and radiative recombination in the active region.
Besides the optimization of the EBL structure, an appropriate design of the QBs in the active region has also been extensively investigated [14,20–22]. A gradual increment of the QB thickness from the n- to p-side of the LED could tune the electron and hole distribution thereby benefiting the radiative recombination [20]. Furthermore, compositionally graded QBs have been proposed to suppress the electron overflow effect [21]. Recently, He et al. found that the band-engineered composition graded last QB could dramatically improve the electron blocking ability of the EBL, which further suggests that the importance of the QB structure [22]. All these exciting progresses have shed light on a proper design of the QBs for efficient DUV LEDs. In this work, we proposed an unique DUV LED architecture with five AlGaN-based QBs which have a continuously increased aluminum composition (IAC) in each QB to replace the traditional flat QBs, aiming to increase the electron blocking ability and hole injection efficiency. The optical and electrical characteristics of such DUV LEDs were comprehensively investigated and analyzed.
2. Device structures and parameters
To prove the effectiveness of the proposed DUV LED having an IAC-QBs structure, a conventional DUV LED emitting at 284.5 nm which was experimentally reported by Yan et al [23] has been used as the reference sample (denoted as Sample A). Sample A has a chip size of 400 × 400 μm2 and is composed of a 3-μm-thick n-type Al0.6Ga0.4N layer (Si: 5×1018 cm−3 and the activation energy is set to be 15 meV [12]), followed by the active region, and then capped with the p-type region which consists of a 20-nm-thick p-Al0.65Ga0.35N EBL layer, a hole injection layer of 50-nm-thick p-Al0.5Ga0.5N (Mg: 2×1019 cm−3), and finally a heavily doped 120-nm-thick p+-GaN contact layer (Mg: 1×1020 cm−3). In this numerical calculation, the ionization energy of acceptors (EA) for AlxGa1-xN (0 ≤ x ≤ 1) alloy scales linearly from 170 to 670 meV [13]. In the active region, Sample A comprises five pairs of 3 nm Al0.4Ga0.6N/12 nm Al0.5Ga0.5N multiple-quantum-wells (MQWs). The layer structure of Sample B is identical to that of Sample A except for the QBs, which has a constant Al composition of 0.5, 0.53, 0.56, 0.59, 0.62, 0.65, respectively. Lastly, Sample C also has an identical layer structure except for the five AlxGa1-xN QBs and each QB has a linear-increment of Al composition by 0.03 along the growth direction. The details of the LED architecture for Sample A, B, C are schematically shown in Fig. 1.
Fig. 1. Schematic of the reference DUV LED (Sample A) and Al composition profiles for Sample A, B, and C respectively.
The numerical calculations are conducted by using a commercial software Advanced Physical Models of Semiconductor Devices (APSYS) provided by Crosslight Inc., which can self-consistently solve Schrödinger and Poisson’s equations with proper boundary conditions, drift-diffusion equations and material parameters for III-nitride semiconductors [24,25]. The Shockley-Read-Hall (SRH) recombination lifetime, Auger recombination coefficient, radiative recombination coefficient, and light extraction efficiency are set to be 15 ns, 2.88 × 10−30 cm6/s and 2.13 × 10−11 cm3/s, and 15%, respectively [21,26]. Furthermore, the conduction and valence band offset ratio for the AlGaN alloy is set to be 0.65/0.35 [27]. For the [0001] orientated DUV LEDs, the polarization effect is assumed to be 50% of total charges considering the screening effect of defects [28]. Some other band structure parameters used in the simulation can be found elsewhere [29].
3. Results and discussions
First of all, the measured as well as the calculated light output power (LOP) and forward voltage under different injection current for Sample A have been plotted in Fig. 2(a). Our numerically calculated light-current-voltage (L-I-V) curves (black curves) are perfectly matching with the experimentally measured L-I-V curves of the Sample A (purple dots) [23], suggesting the reliability of our device model and parameters implemented in the calculation. A remarkable enhancement of the LOP by approximately 40.68% and 65.83% can be obtained in Sample B and C, respectively, in comparison to that of Sample A at the injection current of 60 mA. Importantly, the I-V curves and the turn-on voltage extracted from the I-V curves are nearly the same for all three samples, suggesting similar electrical properties of the three devices. Furthermore, the EQE as a function of injection current (as shown in Fig. 2(b)) suggests that Sample C exhibits a higher EQE of 4.36% than that of Sample A (2.61%) and Sample B (3.71%) at the injection current level of 60 mA.
Fig. 2. (a) Measured and calculated L-I-V characteristics of Sample A, B, C. (b) Calculated EQE under different injection current for Sample A, B, and C, respectively.
To reveal the reason for the observed performance improvement in Sample B and C, the density of leaked electrons, electron and hole concentration, and the radiative recombination rate in the MQWs have been calculated. Figure 3(a) shows the electron concentration profile in the last three QWs and p-region (the hole injection layer), which indicates that the electron leakage for Sample B and Sample C has been significantly suppressed. Besides, more holes can be injected into the active region instead of recombining with those leaked electrons in the p-side. Therefore, the electron and hole concentration in the MQWs of Sample B and Sample C increased drastically when compared to that of Sample A, as shown in Figs. 3(b) and 3(c). As a result, a much higher radiative recombination rate has been observed in both Sample B and C due to the enhanced electron blocking ability and hole injection, as illustrated in Fig. 3(d).
Fig. 3. (a) Electron concentration profiles in the last three QWs and p-type region. (b) Electron concentration profiles, (c) Hole concentration profiles and (d) Radiative recombination rate profiles in the MQWs. In order to present the curves more clearly, the relative position of the peaks was deliberately changed in figs. 3(b), 3(c) and 3(d). The data are calculated at the injection current of 60 mA.
To further explain the underlying mechanism of the enhanced electron and hole concentration within the MQWs, the energy band diagram profiles for the three samples are presented in Fig. 4. We defined Фe and Фh as the effective barrier heights for electrons and holes, respectively. Specifically, Фe represents the energy difference between the quasi-Fermi level for electrons and the conduction band while Фh represents the energy difference between the quasi-Fermi level for holes and the valence band.
Fig. 4. Energy band profiles of the MQW region, p-EBL layers and part of the p-type hole injection layers for (a) Sample A, (b) Sample B and (c) Sample C at the injection current of 60 mA.
Firstly, Фe is calculated to be 238.4 meV, 279.7 meV and 311.5 meV for Sample A, Sample B, and Sample C, respectively. The larger values of Фe in Sample B and Sample C demonstrate a higher electron blocking capability which enables the suppression of electron leakage. It is worth noting that a sharp downward bending in the conduction band was formed at the interface of the last QB (LQB) and p-EBL in Sample A due to the positive polarization charge induced at the hetero-interface. In this position, a large number of electrons can be accumulated inside this dip area. These electrons will be eventually consumed through the non-radiative recombination process, rather than radiative recombination [30]. In contrast, the band dips at the LQB/p-EBL interface in Sample B and Sample C were significantly alleviated. As a result, the electrons will be less likely to accumulate in the LQB. This helps to pull the quasi-Fermi level away from the conduction band and thereby increases the effective conduction band barrier height. Hence, both Sample B and Sample C have a reduced electron leakage level compared with Sample A, as exhibited in Fig. 3(a). In addition, Sample B and Sample C have smaller Фh values of 313.2 meV and 299.3 meV in comparison to Sample A, respectively, suggesting that the hole transport across the p-EBL for these structures was significantly improved and therefore a higher hole concentration within the active region can be obtained (see the Fig. 3(c)).
Moreover, the main function of such unique QBs in the active region requires further investigation. We defined the energy barrier height for electrons (ΔΦCB) as the energy difference between the quasi-Fermi level for electrons and the highest point of the conduction band in each QB. The energy barrier height for holes (ΔΦVB) was defined as the energy difference between the quasi-Fermi level for holes and the highest point of the valence band in each QB. The values of ΔΦCB and ΔΦVB inside each QB, as marked in Fig. 4, are summarized in Table 1. Both Sample B and Sample C possess much higher ΔΦCB in comparison to Sample A. The higher ΔΦCB can strongly enhance the electron confinement inside the active region and further prevent the electron leakage. In regards to the ΔΦVB, Sample A was found to have the smallest ΔΦVB and Sample B has the highest ΔΦVB because ΔΦVB increases with the increment of Al-composition in the AlGaN QBs due to the increased band offset in the valence band. Such a large value of ΔΦVB definitely hurts the hole transport in the active region. Interestingly, Sample C has a smaller value of ΔΦVB in comparison to Sample B owing to the successful implementation of the graded QB structure along the [0001] direction. Such IAC grading AlGaN structure introduces a polarization-induced sheet charge which compensates the existing strong interface charges generated at the QW and QB interface. As a result, the valence band of the QBs in Sample C becomes flatter in comparison to Sample B. However, by adopting the conventional flat QBs, such as in Sample B, it is unavoidable to create a higher energy barrier for holes at the QW and QB interface. In Sample C, we can take advantage of IAC grading in the QBs to achieve an enhanced electron confinement effect without sacrificing too much hole injection efficiency. Therefore, the overall performance of Sample C exceeds both Sample A and Sample B.
Table 1. Summarized values of ΔΦCB and ΔΦVB of the QBs for Sample A, Sample B, and Sample C, respectively.
The graded AlGaN QBs not only significantly influence the electron and hole transport and distribution in the active region, but also can affect the notorious quantum confined Stark effect (QCSE) which is directly related with the design of QW and QB structure [31,32]. It is commonly accepted that the QCSE strongly correlates with the overlap of electron and hole wave functions (Гe-hh) which determines the carrier radiative recombination rate. In this work, the values of Гe-hh of the three samples were calculated and summarized in Table 2. Sample B has much lower Гe-hh than that in Sample A. However, Sample B has much higher electron and hole concentration (see Figs. 3(b) and 3(c)). As a result, Sample B still possesses higher LOP than Sample A. Most importantly, we found a larger Гe-hh in Sample C which is possibly attributed to the self-screening effect [33–35]. To further explain this phenomenon, the net polarization charge density (ΔP) and the electric field (Eb) in QB were defined by the following equations [34,36]:
Table 2. The electron and hole wave function overlap for QW 1, QW 2, QW 3 and QW 4, QW 5 in Sample A, Sample B, and Sample C.
Moreover, we can further reduce the QCSE by optimizing the IAC grading QB structure via a variation of the QB thickness (lb). Here, we calculated the evolution of LOP and Гe-hh when the thickness of the QBs increases from 6 nm to 14 nm. As shown in Fig. 5(a), it is observed that the value of LOP increases when the QB thickness decreases from 14 nm to 6 nm, partially attributing to the rapid increase of the Гe-hh. Figs. 5(b) and 5(c) show the electron and hole concentration profiles in the MQWs which have a QB thickness of 6 nm, 8 nm, and 12 nm, respectively. We found that the electron and hole concentration increase drastically as the thickness of QB drops from 12 nm to 8 nm. As a result, owing to the increased Гe-hh along with the improved carrier concentration in the QWs, the radiative recombination rate is remarkably enhanced, as shown in Fig. 5(d). However, the further increment of the LOP is limited as the thickness of QBs goes below 8 nm. Although the Гe-hh continues to increase when the lb reaches 6 nm, the total amount of electron and hole concentration in the active region does not increase but rather reduces as shown in Figs. 5(b) and 5(c). Thus, it is vital to choose a proper thickness of the graded QBs in the process of device optimization by considering how to achieve the minimized QCSE and maximized electron/hole concentration in the QWs at the same time. We may consider the value of 8 nm as a relatively optimum thickness for the QBs. Furthermore, it is also worth mentioning that if we continuously decrease the QB thickness, it becomes more difficult to grow high quality graded AlGaN QB structure with desirable grading profile (A drastic change of the Al composition within a short time period could be difficult as the film thickness decreases). Consequently, we may have to call for a balance in choosing the thickness of the QBs by considering whether the device is relatively easy to make. Although this is a numerical work focusing on enhancing carrier injection efficiency by using the unique graded QB structure to improve the DUV LED optical performance, we expect that it is straightforward to grow such DUV LEDs by metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) by precisely controlling the Al/III molar ratio in monolayer-scale during the epitaxial process [37–41].
Fig. 5. (a) LOP and Гe-hh as a function of the QB thickness for step-like QB with IAC grading structure. (b) Electron concentration profiles. (c) Hole concentration profiles. (d) The radiative recombination rate profiles in the MQWs for IAC grading step-like QB structure with a thickness of 6 nm, 8 nm, and 12 nm, respectively. The data are calculated at the injection current of 60 mA.
4. Conclusions
In summary, we numerically investigated the optical characteristics of DUV LEDs by incorporating various QB structures in the active region. Simulation results suggest that DUV LEDs with increased Al-composition QBs exhibit less electron current leakage, better hole injection, and increased electron and hole wave function overlap (Гe-hh), thus achieving an enhanced LOP and EQE performance. Additionally, we found that the thickness of the proposed graded QBs plays a critical role in high-efficiency UV illumination because it not only affects the electron/hole concentration and distribution, and their transport property in the active region, but also determines QCSE in the QWs which is directly related to radiative recombination rate. Therefore, it is crucial to choose a proper thickness of the graded QB to maximize carrier concentrations and minimize the QCSE in the QWs, aiming to improve the LOP and EQE of DUV LEDs. Throughout the careful investigation involved in this study, we strongly believe that our proposed DUV LEDs with grading QB structures could be an effective solution on the way for efficient UV lighting of the future.
Funding
National Natural Science Foundation of China (61905236); University of Science and Technology of China (KY2100000081); Chinese Academy of Sciences.
Acknowledgment
This work was partially carried out at the University of Science and Technology of China Center for Micro and Nanoscale Research and Fabrication.
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