Reduction of Electron Leakage of AlGaN-Based Deep Ultraviolet Laser Diodes Using an Inverse-Trapezoidal Electron Blocking Layer

Zhong-Qiu Xing(邢中秋)$^{1,2,3}$, Yong-Jie Zhou(周勇洁)$^{4}$, Yu-Huai Liu(刘玉怀)$^{1,2,3\hyperlink{s}{**}}$, Fang Wang(王芳)$^{1,2,3\hyperlink{s}{**}}$

doi:10.1088/0256-307X/37/2/027302

⇦Chinese Physics Letters, 2020, Vol. 37, No. 2, Article code 027302⇨ Reduction of Electron Leakage of AlGaN-Based Deep Ultraviolet Laser Diodes Using an Inverse-Trapezoidal Electron Blocking Layer * Zhong-Qiu Xing (邢中秋)1,2,3, Yong-Jie Zhou (周勇洁)4, Yu-Huai Liu (刘玉怀)1,2,3**, Fang Wang (王芳)1,2,3** Affiliations 1National Joint Research Center for Electron Materials and Systems, Zhengzhou University, Zhengzhou 450001 2International Joint Laboratory of Electron Materials and Systems, Zhengzhou University, Zhengzhou 450001 3School of Information Engineering, Zhengzhou University, Zhengzhou 450001 4School of Physics and Electron Engineering, Xinyang Normal University, Xinyang 464000 Received 17 November 2019, online 18 January 2020 ^*Supported by the National Natural Science Foundation of China under Grant No. 61176008, the Special Project for Inter-government Collaboration of State Key Research and Development Program under Grant No. 2016YFE0118400, the Key Project of Science and Technology of Henan Province under Grant No. 172102410062, and the National Natural Science Foundation of China–Henan Provincial Joint Fund for Key Project under Grant No. U1604263.
^**Corresponding authors. Email: ieyhliu@zzu.edu.cn; iefwang@zzu.edu.cn Citation Text: Xing Z Q, Zhou Y J, Liu Y H and Wang F 2020 Chin. Phys. Lett. 37 027302 Abstract To improve the optical and electrical properties of AlGaN-based deep ultraviolet lasers, an inverse-trapezoidal electron blocking layer is designed. Lasers with three different structural electron blocking layers of rectangular, trapezoidal and inverse-trapezoidal structures are established. The energy band, electron concentration, electron current density, $P$–$I$ and $V$–$I$ characteristics, and the photoelectric conversion efficiency of different structural devices are investigated by simulation. The results show that the optical and electrical properties of the inverse-trapezoidal electron blocking layer laser are better than those of rectangular and trapezoidal structures, owing to the effectively suppressed electron leakage. DOI:10.1088/0256-307X/37/2/027302 PACS:73.21.Fg, 73.61.Ey, 78.60.Fi © 2020 Chinese Physics Society Article Text Ultraviolet (UV) photonic devices such as laser diodes (LDs), light-emitting diodes (LEDs), and photodetectors are promising for a wide variety of applications, including biologics testing, water and air purification, dermatology, high-density optical storage, and so on.[1] In addition, UV laser diodes can also be used as a substitute for toxic and inefficient gas lasers and mercury lamps. In terms of its effects on biological and chemical substances, the UV spectrum is usually divided into four bands: UV-AI (340–400 nm), UV-AII (320–340 nm), UV-B (280–320 nm) and UV-C ($ < $280 nm).[2,3] The laser band designed in this study is UV-C, and the target emission wavelength is 267 nm. At present, nitride semiconductor based UV-C LDs suffer from two major challenges. First, due to the high activation energy of Mg in p-type AlGaN and low hole mobility,[4] the efficiency of injecting holes into the active layer is very low.[5,6] Second, electrons are leaking in the active region, and the leaked electrons recombine with the holes in the p-type layer, which has a negative effect on the performance of lasers. To overcome these problems, a single bulk AlGaN electron blocking layer (EBL) was inserted under the p-type layer to block electron leakage,[7] but the valence band offset at the interface formed a barrier to block hole transport.[8] To reduce electron leakage and to improve hole injection efficiency, many studies on LD structure design and optimization have been conducted, such as optimization of quantum wells,[9] quantum barriers[10,11] and electron blocking layers. In these designs, the electron blocking layer plays the most important role in carrier transport, and has been studied with different structures, including tapered EBL,[12] stepped EBL,[13,14] anti-tapered and anti-stepped EBL,[15] double-tapered EBL,[16] and so on. It should be noted, however, that the LD needs to confine the light field in the active layer and oscillate along the cavity, the proposed EBL structure should not deteriorate the optical limitations in the waveguide design of LDs.[17] Therefore, improving optical confinement is also a challenge. To improve the electron concentration and optical confinement factor of the laser in the active region, we propose an inverse-trapezoidal EBL laser structure and compares with the trapezoidal EBL laser and the rectangular EBL laser in the present study. Figure 1(a) is a schematic diagram of a deep ultraviolet laser diode (DUV LD) with sapphire as a substrate. The laser structure consists of a 0.1-µm-thick n-Al$_{0.75}$Ga$_{0.25}$N contact layer, a 1-µm-thick n-Al$_{0.75}$Ga$_{0.25}$N cladding (n-CL), a 0.11-µm-thick n-Al$_{0.68}$Ga$_{0.32}$N lower waveguide (LWG) layer, multi-quantum wells (MQWs) composed of two 3-nm-thick Al$_{0.58}$Ga$_{0.42}$N wells (QWs) and three 8-nm-thick Al$_{0.68}$Ga$_{0.32}$N barriers (QBs), a 70-nm-thick p-Al$_{0.68}$Ga$_{0. 32}$N upper waveguide (UWG) layer, a 10-nm-thick n-Al$_{x}$Ga$_{1-x}$N electron blocking layer, a 0.4-µm-thick p-Al$_{0.75}$Ga$_{0.25}$N cladding (p-CL) and 0.1-µm-thick p-Al$_{0.8}$Ga$_{0.2}$N contact layer. In this simulation, the cavity length of the laser is set to 530 µm, the width is set to 4 µm, and the reflectivity of the front and rear mirrors is set to 30%. In addition, the built-in interface charge caused by spontaneous and piezoelectric polarization is calculated at 40% of the theoretical value,[18–20] the other parameters are set according to Ref. [21]. The spontaneous polarization effect is caused by the difference in lattice constants of different Al composition materials,[22] which will change with the different structures of the EBL.

Fig. 1. (a) Schematics of the deep UV-LD structure and (b) rectangular, (c) trapezoidal and (d) inverse-trapezoidal EBL structures.

Fig. 2. Material gain of the laser ($T=300$ K).

Three types of device structures at room temperature are simulated by Crosslight's Lastip software. Figure 1(b) is a rectangular EBL with a constant aluminum mole fraction. Figure 1(c) is a trapezoidal EBL with a gradually increasing/constant/ gradually decreasing aluminum mole fraction. Figure 1(d) is an inverse-trapezoidal EBL with a gradually decreasing/constant/gradually increasing aluminum mole fraction. The average aluminum compositions of the EBLs in the three structural lasers are equal, and the other layers have the same set parameters to ensure the same emission wavelengths of the three structures. As shown in Fig. 2, the three device structures achieve the same maximum material gain at 267 nm. The value is 8204.9 cm$^{-1}$. The structural design concept of the trapezoidal and inverse-trapezoidal laser EBL comes from the energy band engineering,[23] to obtain an improved lattice matching state around the EBL. Due to the imperfection of the epitaxial growth process, with the increase of the Al composition, there is a serious lattice mismatch in the material during the growth process, which makes it difficult to grow high-quality AlGaN materials, resulting in low efficiency of AlGaN lasers.[24] A lattice matching structure is conducive to smooth epitaxial growth. Therefore, the electron and hole transport efficiency of the laser is further increased. This improved EBL laser is primarily intended to improve the hole injection efficiency and to enhance the electron confinement, as well as to mitigate quantum-confined Stark effects in the active region. The distribution of optical field in the active region is expressed by optical confinement factor ${\it\Gamma}$, $$ {\it\Gamma} =\int_{-\frac{r}{2}}^{\frac{r}{2}} {\varphi^{2}} ({\rm t},y)dt\Big/\int_{-\infty }^\infty {\varphi^{2}} (t,y)dt.~~ \tag {1} $$ In the above formula, $r$ is the width of active region, and $\varphi ({\rm t},y)$ is the wave intensity.[25] The confinement factor for the three different EBLs is firstly considered. Figure 3 shows the real refractive index distribution and optical field distribution of the fundamental waveguide modes of the three different EBLs. In DUV LDs, in order to further confine the light field generated by the carrier radiation recombination in the active region, as shown in Fig. 3, the real refractive index of the material of the multi-quantum well active region is the highest, and the real refractive index of the materials on both sides of the active region show a decreasing trend.[26] Therefore, the effective injection of electrons and holes, as well as the real refractive index difference of each layer of the laser are all aimed at improving the light output performance of the laser. The optical confinement factor of inverse-trapezoidal EBL structure is higher than that of the rectangular structure and trapezoidal EBL structure. The enhancement of optical confinement comes from the larger contrast of real refractive active region and the high electron and hole carrier concentration. This proves that the inverse-trapezoidal EBL structure plays the best role in improving the luminescence performance of the DUV LDs.

Fig. 3. The real refractive index profile (left) and optical field distribution (right) for (a) reference EBL, (b) trapezoidal EBL and (c) inverse-trapezoidal EBL.

The effective barrier height is defined as the potential difference between the conduction band edge and its relative quasi-Fermi level, which is a reliable parameter for evaluating the laser's electron confinement ability and hole injection efficiency.[27] For electrons, the effective barrier characterizes the ability of the active region to bind electrons. The lower the effective potential barrier is, the easier it is for electrons to escape from the active region and to jump over the EBL to the p-type region. For holes, the effective barrier represents the ability of the active region to block the hole injection. The higher the effective barrier is, the more difficult it is to inject holes from the p-type region into the active region, and the lower the injection efficiency is. As shown in Fig. 4, the EBL has the highest energy gap because it prevents electrons from escaping out of the active region to the p-type layer. Figure 4(a) shows that the effective barrier heights of the electrons of the EBL region and the p-CL region of the trapezoidal EBL laser are both low, resulting in the less confined electrons in the active layer, leading to the electron overflow into the p-type layer and increases the nonradiative recombination. The effective barrier of the hole is too high for the holes to transport effectively to the active layer. Figure 4(b) shows that the laser structure benefits from better lattice matching using the inverse-trapezoidal EBL, with increased effective barrier heights of the EBL region and the p-CL region, indicating that the electron confinement ability is enhanced. Meanwhile, the inverse-trapezoidal EBL laser structure is helpful to reduce the barrier height of the holes.

Fig. 4. Energy band diagram and quasi-Fermi level of (a) trapezoidal EBL and (b) inverse-trapezoidal EBL.

As shown in Fig. 5(a), the inverse-trapezoidal EBL laser has a higher electron density than the trapezoidal EBL laser. The electrons are injected from the n-type layer into the active region and recombine with the holes in the quantum well, causing the electron concentration to become smaller and smaller along the growth direction. Part of the electrons overflow to the p-type layer without recombining with the holes in the quantum well, which is defined as electron leakage. Therefore, the electron concentration of the p-type layer can be used to evaluate the extent of electron leakage. The electron concentration of the trapezoidal and inverse-trapezoidal lasers is shown in Fig. 5(b). As is consistent with the energy band diagram, the trapezoidal EBL induces a small effective barrier due to polarization in the conduction band, which increases the electron leakage. However, in the case of the inverse-trapezoidal EBL, the electron leakage is significantly reduced due to the higher effective barrier of electrons, which proves that the inverse-trapezoidal EBL enhances the electron confinement ability. Therefore, the electrons are more concentrated in the active region of the laser, and the radiation recombination is also enhanced, further affecting the electrical characteristics of the laser. As shown in Figs. 5(c) and 5(d), we design and compare the electron concentrations of the tapered EBL, anti-tapered EBL, stepped EBL, anti-stepped EBL, double tapered EBL, anti-double tapered EBL, trapezoid EBL, and inverse-trapezoid EBL in the p-CL region. It is found that the electron concentration of the inverse-trapezoidal EBL laser in the p-type region is the lowest, indicating that the laser of this structure has the least electron leakage among the eight structures.

Fig. 5. (a) Electron current density, (b) electron concentration ($n_{\rm e}$) distribution of trapezoidal EBL and inverse-trapezoidal EBL laser diode, (c) schematic illustration of the six different EBL structures, and (d) electron concentration of eight EBL structures in the p-CL region.

Fig. 6. (a) $P$–$I$ and (b) $V$–$I$ characteristic curves of trapezoidal EBL and inverse-trapezoidal EBL laser diode.

Fig. 7. Photoelectric conversion efficiency of the three lasers.

As shown in Figs. 6(a) and 6(b), the slope efficiency (SE) of the inverse-trapezoidal EBL laser is 1.89 W/A, which is 3.8% higher than that of the trapezoidal EBL laser, and the threshold current $I_{\rm th}$ is 24.06 mA, which is lower than that of the trapezoidal EBL. The inverse-trapezoidal EBL laser is reduced by 4.3% and the threshold voltage $V_{\rm th}$ is 4.92 V, which is 6.2% less than that of the trapezoidal EBL laser. As shown in Fig. 7, with the increase of the injection current, the photoelectric conversion efficiency of the rectangular EBL laser increases sharply, then decreases, and eventually stabilizing at 39%. The photoelectric conversion efficiency of the trapezoidal EBL laser is 41%. Since the threshold current and threshold voltage of the inverse-trapezoidal EBL laser are lowered and the output power is increased, its photoelectric conversion efficiency is the highest, being 47%. This also indicates that the optical characteristics of the inverse-trapezoidal EBL laser are the best among the three structures. In summary, we have compared the rectangular EBL, trapezoidal EBL and inverse-trapezoid EBL structures for deep UV laser diodes. The simulation is carried out based on ensuring the uniform wavelengths of the three structures. The results show that, compared with the references EBL structures, the luminescence of the inverse-trapezoid EBL structure is more concentrated in the active region and the optical confinement factor is increased to 29.65%. The threshold current of the device is found to reduce to 25.06 mA, the threshold voltage is reduced to 4.92 V, and the slope efficiency is 1.89 W/A. The photoelectric conversion efficiency of the inverse-trapezoidal EBL laser reaches 47%. The inverse-trapezoidal EBL has higher lattice matching and higher effective barrier, which effectively suppresses electron leakage. Therefore, the inverse-trapezoidal EBL structure is of great significance for designing deep ultraviolet nitride semiconductor lasers. References Design and Analysis of 250-nm AlInN Laser Diodes on AlN Substrates Using Tapered Electron Blocking Layers231–261nm AlGaN deep-ultraviolet light-emitting diodes fabricated on AlN multilayer buffers grown by ammonia pulse-flow method on sapphireEfficiency droop in 245–247 nm AlGaN light-emitting diodes with continuous wave 2 mW output powerAdvantage of tapered and graded AlGaN electron blocking layer in InGaN-based blue laser diodesObservation of spectral drift in engineered quadratic nonlinear mediaHigher efficiency InGaN laser diodes with an improved quantum well capping configurationImprovement of near-ultraviolet InGaN-GaN light-emitting diodes with an AlGaN electron-blocking layer grown at low temperatureEffect of electron blocking layer on efficiency droop in InGaN/GaN multiple quantum well light-emitting diodesInfluence of Stark Effect and Quantum Wells Thickness on Optical Properties of InGaN Laser DiodesNumerical study of optical properties of InGaN multi-quantum-well laser diodes with polarization-matched AlInGaN barrier layersThe effect of composite GaN/InGaN last barrier layer on electron leakage current and modal gain of InGaN-based multiple quantum well laser diodesImprovement of hole injection and electron overflow by a tapered AlGaN electron blocking layer in InGaN-based blue laser diodesRole of the electron blocking layer in the graded-index separate confinement heterostructure nitride laser diodesEffects of a step-graded Al _x Ga ₁₋ _x N electron blocking layer in InGaN-based laser diodesEditorialReduction of Electron Leakage in a Deep Ultraviolet Nitride Laser Diode with a Double-Tapered Electron Blocking LayerNumerical study of the advantages of ultraviolet light-emitting diodes with a single step quantum well as the electron blocking layerEvidence for nonlinear macroscopic polarization in III–V nitride alloy heterostructuresCharacteristics of polarization-doped N-face III-nitride light-emitting diodesPolarized GaN-based LED with an integrated multi-layer subwavelength structureBand parameters for nitrogen-containing semiconductorsPolarisation fields in III-nitrides: effects and controlAdvantage of InGaN-based light-emitting diodes with trapezoidal electron blocking layerHigh-Temperature Operation of 8.5 μm Distributed Feedback Quantum Cascade LasersConfinement factor and absorption loss of AlInGaN based laser diodes emitting from ultraviolet to greenPerformance improvement of AlGaN-based deep ultraviolet light-emitting diodes by using staggered quantum wells

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