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Abstract
The superlattice electron blocking layer (EBL) has been proposed to reduce the electron leakage of the deep ultraviolet light emitting diodes (DUV-LEDs). However, the hole transport is hindered by the barriers of EBL and the improvement of hole injection efficiency still suffers enormous challenges. The superlattice step doped (SLSD) EBL is proposed to improve the hole injection efficiency while enhancing the electron confinement capability. The SLSD EBL enhances the electron confinement capability by multi-reflection effects on the electron wave function. And a built-in electric field towards the active region is generated by superlattice step doping, which facilitates the transport of holes into the multiple quantum wells. The Advaced Physical Model of Semiconductor Devices (APSYS) software is used to simulate the DUV-LEDs with conventional EBL, superlattice EBL, superlattice doped EBL, and SLSD EBL. The results indicate that the SLSD EBL contributes to the increased electron concentration in the multiple quantum wells, the reduced electron leakage in the p-type region, the increased hole injection current, and the increased radiative recombination rate. When the current is 60 mA, the external quantum efficiency of DUV-LED with SLSD EBL is increased to 5.27% and the output power is increased to 13.81 mW. The SLSD EBL provides a valuable reference for solving the problems of serious electron leakage and insufficient hole injection of the DUV-LEDs.
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1. Introduction
AlGaN-based devices experienced rapid development over the past two decades [1]. Due to the wide bandgap of AlGaN-based materials, the wavelength of AlGaN-based light emitting devices can cover almost the entire ultraviolet spectral range (210-400 nm) [2]. AlGaN-based deep ultraviolet light emitting diodes (DUV-LEDs) with wavelength of 220-350 nm have extensive application fields, such as sterilization, information storage, water disinfection, and biomedical research [3–5]. In the field of information storage, DUV-LEDs can be used for ultraviolet curing, enabling the solidification of ink. Therefore, they are widely applied in the printing industry, contributing to information storage [6,7]. By utilizing the ultraviolet radiation emitted by DUV-LEDs, data can be written in photosensitive materials for optical information storage [8,9]. In summary, DUV-LEDs play an indispensable role in the field of information storage. DUV-LEDs have attracted considerable attention and made significant progress in recent years [10,11]. However, the development of DUV-LEDs still faces enormous challenges, such as poor hole concentration due to the low p-doping efficiency of AlGaN with high Al component, severe electron leakage caused by weak confinement capability of electron blocking layer (EBL), and low radiative recombination rate due to quantum-confined Stark effect of multiple quantum wells [12–15]. Achieving high p-doping efficiency in AlxGa1-xN with x > 0.4 has proven to be quite challenging, which results from the interplay of several materials properties, including the increasing ionization energy and the decreasing solubility of Mg dopants with the increase in Al composition [16]. As the Al composition in AlGaN increases from 0% to 100%, the activation energy of Mg increases from 170 meV to 470 meV [17]. The significant increase in activation energy results in few Mg dopants being activated to generate holes in AlGaN materials with high Al composition at room temperature [18]. The solubility of Mg dopants in AlGaN decreases with the increase in Al composition, leading to a low amount of Mg incorporation in AlGaN materials with high Al composition [19]. Therefore, the p-type doping efficiency of AlGaN materials decreases with the increase in Al composition.
The low efficiency of p-type doping leads to inadequate hole generation. The substantial imbalance in electron and hole injection, along with weak electron confinement, results in significant electron leakage [20]. Leaked electrons undergo non-radiative recombination with holes in the p-type region, serving as the primary cause of insufficient hole injection, weak radiative recombination rate and low output power. Cho et al. demonstrated that the electron leakage leads to an electric field in the p-side cladding layer of the junction, further enhancing efficiency droop [21]. Chu et al. fabricated DUV-LEDs and investigated the correlation among efficiency droop, electron leakage and Auger recombination [22]. They found that the electron leakage strongly causes the efficiency droop. Ren et al. demonstrated that the severe leakage current and efficiency droop under high injection current in DUV-LEDs are mainly caused by substantial electron leakage and insufficient hole injection [23]. Therefore, the key to optimizing the performance of DUV-LEDs is to reduce the electron leakage and improve the hole injection.
Many researchers have made efforts and contributions in this field. Schubert et al. designed the superlattice doping structure to enhance acceptor activation in wide-gap semiconductors [24]. Hirayama et al. prepared the multi-quantum-barrier EBL to reduce the electron leakage [25]. Their results demonstrated that the superlattice EBL can effectively reduce electron leakage through multi-reflection effects on electron wave functions. Zheng et al. fabricated multidimensional Mg-doped superlattices in Al-rich AlGaN to enhance the hole concentration, which is an important step for the design of AlGaN-based DUV-LED [26]. Zhang et al. prepared the superlattice p-type EBL to reduce the electron leakage and enhance hole injection, thereby suppressing efficiency droop [27]. And they found that the enhanced hole injection in the multiple quantum wells can more effectively consume electrons through radiative recombination, contributing to the alleviation of electron leakage.
Inspired by the aforementioned studies, the superlattice doped EBL is proposed to improve hole injection efficiency while enhancing electron confinement capability. The p-doping in AlGaN with high Al component is difficult. Therefore, on the premise that the total doping concentration remains unchanged in the superlattice EBL, low doping concentration is used in the regions with high Al component, and high doping concentration is used in the regions with low Al component, which weakens the electron leakage in the p-type doping region and reduces the difficulty of growth. What’s more, superlattice doping generates a built-in electric field towards the active region, which facilitates the transport of holes into the multiple quantum wells, thereby improving the hole injection efficiency of the DUV-LEDs. Based on the superlattice doped EBL, the superlattice step doped (SLSD) EBL is proposed to strengthen the built-in electric field towards the active region and further improve hole injection efficiency. The conventional superlattice EBL may become the recombination centers for electrons [28]. Fortunately, the built-in electric field generated by the superlattice doped and SLSD separates the wavefunctions of electrons and holes, reducing the possibility of recombination between electrons and holes in the EBL [29]. The Advanced Physical Model of Semiconductor Devices (APSYS) software is used to simulate the DUV-LEDs with conventional EBL, superlattice EBL, superlattice doped EBL, and SLSD EBL. The results show that the SLSD EBL effectively increases the electron concentration in the multiple quantum wells, lessens the electron leakage in the p-type region, improves the hole injection current, and increases the external quantum efficiency and the output power of the DUV-LEDs.
2. Proposed structures and simulation parameters
The DUV-LED structure used for reference is based on the LED fabricated by Yan et al., the emission wavelength of which is 284.5 nm [30]. Figure 1 shows the epitaxial layers of the AlGaN-based DUV-LED. The DUV-LED is grown on the AlN/sapphire substrate, and the n-type region is composed of a 3-µm-thick AlGaN basal layer (n-doping = 4${\times} $1018 cm−3). The active region is composed of six 12-nm-thick Al0.5Ga0.5N layers (quantum barriers) and five 3-nm-thick Al0.4Ga0.6N layers (quantum wells) alternately. The p-type region is composed of a 20-nm-thick Al0.65Ga0.35N EBL (p-doping = 2${\times} $1019 cm−3), a 50-nm-thick Al0.5Ga0.5N interlayer layer (p-doping = 2${\times} $1019 cm−3), and a 120-nm-thick GaN contact layer (p-doping = 1${\times} $1019 cm−3). The reference structure is denoted as LED1.
Superlattice EBL, superlattice doped EBL, and SLSD EBL are proposed in this paper. Figures 2(a)-(c) show the epitaxial layers of the superlattice EBL, superlattice doped EBL, and SLSD EBL, respectively. The DUV-LEDs using the above three EBLs are denoted as LED2, LED3, and LED4, respectively. The superlattice EBL forms potential wells and barriers. A significant difference in the energy levels of wells and barriers may lead to the superlattice EBL becoming a recombination center. Therefore, we choose the energy level difference between the wells and barriers in the superlattice EBL to be lower than that in the active region. This ensures a certain blocking capability for electrons while reducing the possibility of recombination between electrons and holes in this region. Superlattice EBL is composed of five 2-nm-thick Al0.62Ga0.38N (p-doping = 2${\times} $1019 cm−3) and five 2-nm-thick Al0.68Ga0.32N (p-doping = 2${\times} $1019 cm−3) alternately. Superlattice doped EBL is composed of five 2-nm-thick Al0.62Ga0.38N (p-doping = 3.5${\times} $1019 cm−3) and five 2-nm-thick Al0.68Ga0.32N (p-doping = 0.5${\times} $1019 cm−3) alternately. SLSD EBL is composed of a 2-nm-thick Al0.62Ga0.38N (p-doping = 5.5${\times} $1019 cm−3), a 2-nm-thick Al0.62Ga0.38N (p-doping = 4.5${\times} $1019 cm−3), a 2-nm-thick Al0.62Ga0.38N (p-doping = 3.5${\times} $1019 cm−3), a 2-nm-thick Al0.62Ga0.38N (p-doping = 2.5${\times} $1019 cm−3), a 2-nm-thick Al0.62Ga0.38N (p-doping = 1.5${\times} $1019 cm−3), and five 2-nm-thick Al0.68Ga0.32N (p-doping = 0.5${\times} $1019 cm−3) alternately.
Fig. 2. The epitaxial layers of (a) the superlattice EBL, (b) the superlattice doped EBL, and (c) the SLSD EBL.
APSYS is a physical model simulation software for semiconductor devices, which solves the Poisson's equation, current continuity equations, carrier transport equations, and photon rate equation for DUV-LEDs [31]. Some material properties for DUV-LEDs in the database of APSYS are outdated and inaccurate. Fortunately, the material properties in the database of APSYS are open to users. Therefore, to ensure consistency between simulation results and experimental results, we adjusted material parameters according to relevant literature. The key material parameters are presented the following context.
In the simulation, the band offset of the materials is set to 0.65/0.35 [32]. Device geometry is fabricated into a rectangular shape of 400${\times} $400 µm2. The non-radiative recombination and material absorption loss are taken into account, the Shockley-Read-Hall (SRH) lifetime, Auger coefficient, and background loss are set to 10 ns, 1.5${\times} $10−30 cm6/s, and 2000m−1, respectively [33]. The ionization energy of acceptors of AlGaN varies within the range of 170 meV (p-GaN) and 470 meV (p-AlN). Light extraction efficiency of AlGaN-based DUV-LED is set to 12.3% [34,35]. The polarization charge caused by spontaneous and piezoelectric polarization effect is calculated by the method proposed by Fiorentini [36].
The spontaneous polarization of the AlxGa1-xN is expressed as
The piezoelectric polarization of the AlxGa1-xN is calculated by the following equations:
The band gap energy at temperature T of AlGaN is calculated by the Eq. (7) [38],
where Eg(0) is the band gap energy at 0 K, and α and β are Varshni coefficients. The value of Eg(0) for GaN and AlN are 3.51 eV and 6.25 eV; the value of α for two alloys are 0.909 meV/K and 1.799 meV/K; the value of β for two alloys are 830 K and 1462 K. T is set to 300 K.The band gap energy of AlxGa1-xN is calculated by the Eq. (8) [39],
The carrier mobility is calculated by Caughey-Thomas approximation theory, which is expressed by the Eq. (9) [40]:
3. Results and discussion
Figures 3 show the experimental data of DUV-LED grown by Yan et al. and the simulation data of LED1. Figure 3(a) shows the P-I curves of the DUV-LED grown by Yan et al. and LED1. Figures 3(b) and (c) show the emission wavelength and the external quantum efficiency of LED1. When the injection current is 60 mA, the output power and emission wavelength of the DUV-LED grown by Yan et al. are 6.75 mW and 284.5 nm. The external quantum efficiency is estimated to be 3.5%. When the injection current is 60 mA, the output power and emission wavelength of the LED1 are 6.75 mW and 284.5 nm. The corresponding external quantum efficiency is calculated to be 3.42%. By employing the material parameters from the references described in Chapter 2, the results show that the output power, emission wavelength, and external quantum efficiency of LED1 are in good agreement with the experimental data of DUV-LED reported by Yan et al.
Fig. 3. (a) The P-I curves of the DUV-LED grown by Yan et al. and LED1, (b) the emission wavelength of LED1, and (c) the external quantum efficiency of LED1.
The energy band and electric field can affect the migration and distribution of carriers, thereby influencing the performance of the devices [43]. Therefore, the energy band and electric field of the DUV-LEDs are analyzed firstly. The effective barrier height is defined as the potential difference between the band edge and its corresponding Quasi-Fermi level [44]. The higher effective barrier height enhances the carrier confinement and the lower effective barrier height improves the carrier injection [45]. The energy band diagrams of the DUV-LEDs are presented in the paper to explain the effects of the superlattice EBL. Figures 4(a) and (b) show the energy band of LED1 and LED2. For EBLs, the effective barrier heights of the electrons in the conduction band of LED1 and LED2 are 259 meV and 293 meV; the effective barrier heights of the holes in the valence band of two structures are 351 meV and 328 meV. Compared with LED1, the effective barrier height of the electrons in LED2 is increased by 34 meV and the effective barrier height of holes is reduced by 23 meV. It indicates that the superlattice EBL enhances the electron confinement and facilitates the hole injection of the DUV-LEDs. However, there is still a high effective barrier height for holes in the valence band of superlattice EBL. Therefore, superlattice doped EBL and SLSD EBL are designed to enhance the hole injection by using the built-in electric field.
Figure 5(a) shows the diagram of superlattice doping, which indicates that the internal electric field is formed between the regions with low doping concentration and the regions with high doping concentration [46]. Figure 5(b) shows the potential of the EBLs of LED2, LED3, and LED4, respectively. Taking the potential on the right side of EBL as the reference potential, the potential on the left side of the EBL of LED3 is lower than that of LED2. It indicates that superlattice doping forms the built-in electric field pointing towards the multiple quantum wells, which enhances the hole injection in the multiple quantum wells. Compared with LED2 and LED3, the potential on the left side of the EBL of LED4 is further reduced. It is concluded that SLSD EBL strengthens the built-in electric field, which is more conducive to the hole injection into the active region.
Fig. 5. (a) The diagram of superlattice doping and (b) the potential of the EBLs of LED2, LED3, and LED4.
The distribution of carriers has a significant impact on the performance of devices. The high carrier injection efficiency in the multiple quantum wells enhances the radiative recombination rate and has a positive impact on the external quantum efficiency and output power of the DUV-LEDs [47]. The carrier injection efficiency is reflected by the carrier concentration in the multiple quantum wells and the carrier leakage in the non-active region. The carrier distribution of four LEDs is studied here. Figures 6(a)-(d) show electron concentration in the multiple quantum wells, electron current density in the p-type region, electron concentration in the EBL, and hole current density in the p-type region, respectively. Figures 6(a) and (b) show that compared with LED1, LED2, and LED3, the electron concentration in the multiple quantum wells of LED4 is improved and the electron current density in the p-type region is reduced significantly. It indicates that the SLSD EBL enhances the electron confinement capability, thereby increasing the electron concentration in the active region and reducing the electron leakage in the p-type region. The improvement of electron confinement capability is attributed to the multi-reflection effects on electron wave functions caused by superlattice structure. In addition, traps generated by the AlGaN with low Al component of SLSD EBL is helpful to confine the electrons. Figure 6(c) shows that LED4 has the lowest electron leakage in the EBL among three LEDs. Figure 6(d) shows that LED4 has the highest hole current density among four LEDs. The previous analysis indicates that SLSD EBL generates the stronger built-in electric field towards the active region, which enhances the hole injection in the multiple quantum wells. The reduction of electron leakage also contributes to the increase of hole concentration in the p-type region. Therefore, the hole injection of LED with SLSD EBL is improved effectively. The above results indicate that the SLSD EBL increases the electron concentration in the multiple quantum wells, reduces the electron leakage in the p-type region, and improve the hole injection current.
Fig. 6. (a) The electron concentration in the multiple quantum wells, (b) electron current density in the p-type region, (c) electron concentration in the EBL, and (d) hole current density in the p-type region.
The external quantum efficiency is used to measure the conversion efficiency of the DUV-LEDs, which is calculated by the following equations [48]:
Fig. 7. (a) The external quantum efficiency of four LEDs and (b) the radiative recombination rate of four LEDs.
The electrons on the conduction band recombine with the holes on the valence band and emit the photons during the operation of DUV-LEDs, which is defined as radiative recombination. The radiative recombination rate is affected by the carrier injection efficiency [50]. The Eq. (11) indicates that the increase of carrier concentration is conducive to the improvement of the radiative recombination rate. Figure 7(b) shows the radiative recombination rate of four LEDs. The results indicate that the radiative recombination rate in the EBL of four LEDs is nearly zero, suggesting that there is no radiative recombination occurring in the EBL regions. And LED4 has the highest radiative recombination rate among four LEDs. It is attributed to the increase of electron and hole injection efficiency in the multiple quantum wells, which improves the probability of radiative recombination. The radiative recombination rate plays an important role in optimizing the output power.
The output power produced at the target emission wavelength is a key parameter for most application fields of the DUV-LEDs. Due to the improvement of the external quantum efficiency and the radiative recombination rate, more photos are produced in the multiple quantum wells and less non-radiative recombination occurs in the non-active region. It means that more electrical energy will be converted into optical energy at the same current, which promotes the improvement of output power. Figure 8 shows the P-I curves of four LEDs. When the injection current is 60 mA, the output power of the four LEDs is 6.75 mW, 11.35 mW, 13.01 mW, and 13.81 mW, respectively. Compared with LED1, LED2, and LED3, the output power of LED4 is increased by 104.6%, 21.7%, and 6.1%, respectively. The above results indicate that the SLSD EBL can effectively optimize the optical and electrical properties of DUV-LEDs. Higher output power means the devices can provide more effective ultraviolet radiation, which is crucial for lots of applications. For instance, the increase in output power can enhance curing efficiency and improve the quality of curing, thereby elevating both the speed and quality of information storage. Additionally, powerful UV-LEDs reduce the reliance on hazardous materials, such as mercury commonly used in traditional UV lamps. Therefore, the enhancement in the output power of DUV-LEDs expands their potential applications and provides ecological advantages.
4. Conclusions
DUV-LEDs still face the challenges of serious electron leakage and the low hole injection efficiency. Superlattice EBL is proposed to enhance the electron confinement by the multi-reflection effects on the electron wave function. But EBL still has high effective barrier height for holes, which hinders the injection of holes into active region. Therefore, SLSD EBL is proposed to reduce electron leakage, improve hole injection, and optimize the output power of DUV-LEDs. APSYS software is used to simulate the DUV-LEDs with conventional EBL, superlattice EBL, superlattice doped EBL, and SLSD EBL. The results indicate that the electron concentration in the multiple quantum wells is increased and the electron leakage in the p-type region is decreased by using the SLSD EBL. In addition, the SLSD EBL generates the built-in electric field towards the multiple quantum wells, which further enhances the hole injection into the multiple quantum wells. The radiative recombination rate is improved effectively due to the increase of electron and hole injection efficiency. When the current is 60 mA, the external quantum efficiency of DUV-LED with SLSD EBL is increased to 5.27% and the output power is increased to 13.81 mW. However, we employed 2D simulation for DUV-LEDs and did not consider composition fluctuations in DUV-LEDs. Therefore, there is still room for improvement in our simulation. In summary, the simulation results using the APSYS software are consistent with the experimental results reported by Yan et al. The SLSD EBL contributes to solving the problems of serious electron leakage and insufficient hole injection, which provides a valuable reference for the fabrication of the high-efficiency DUV-LEDs.
Funding
National Natural Science Foundation of China (62174148); National Key Research and Development Program of China (2016YFE0118400, 2022YFE0112000); Key Program for International Joint Research of Henan Province (231111520300); Science and Technology Innovation 2025 Major Project of Ningbo (2019B10129); Zhengzhou 1125 Innovation Project (ZZ2018-45).
Acknowledgments
We thank Muhammad Nawaz Sharif and Yin Xue for helpful discussions.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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