⇦Chinese Physics Letters, 2019, Vol. 36, No. 5, Article code 057301 Reduction of Electron Leakage in a Deep Ultraviolet Nitride Laser Diode with a Double-Tapered Electron Blocking Layer * Yi-Fu Wang (王一夫)1,2, Mussaab I. Niass1,2, Fang Wang (王芳)1,2,3**, Yu-Huai Liu (刘玉怀)1,2,3** Affiliations 1National Joint Research Center for Electronic Materials and Systems, Zhengzhou University, Zhengzhou 450001 2International Joint Laboratory of Electronic Materials and Systems, Zhengzhou University, Zhengzhou 450001 3School of Information Engineering, Zhengzhou University, Zhengzhou 450001 Received 23 January 2019, online 17 April 2019 *Supported by the National Key Research and Development Program under Grant No 2016YFE0118400, the Key Project of Science and Technology of Henan Province under Grant No 172102410062, the National Natural Science Foundation of China under Grant No 61176008, and the National Natural Science Foundation of China Henan Provincial Joint Fund Key Project under Grant No U1604263.
**Corresponding author. Email: ieyhliu@zzu.edu.cn; iefwang@zzu.edu.cn Citation Text: Wang Y F, Niass M I, Wang F and Liu Y H 2019 Chin. Phys. Lett. 36 057301    Abstract A double-tapered AlGaN electron blocking layer (EBL) is proposed to apply in a deep ultraviolet semiconductor laser diode. Compared with the inverse double-tapered EBL, the laser with the double-tapered EBL shows a higher slope efficiency, which indicates that effective enhancement in the transportation of electrons and holes is achieved. Particularly, comparisons among the double-tapered EBL, the inverse double-tapered EBL, the single-tapered EBL and the inverse single-tapered EBL show that the double-tapered EBL has the best performance in terms of current leakage. DOI:10.1088/0256-307X/36/5/057301 PACS:73.21.Fg, 73.61.Ey, 78.60.Fi © 2019 Chinese Physics Society Article Text Optoelectronic devices such as laser diodes (LDs) have enormous application markets, including chemical analysis, medical diagnostic equipment, biological reagent detection system, high-density data storage, water purification and material processing.[1–4] As promising LDs materials, group-III nitride semiconductors have been attracting much attention for many manufacturers and scholars.[5–9] The UV band can be divided into four parts: UV-AI (340–400 nm), UV-AII (320–340 nm), UV-B (280–320 nm) and UV-C ($ < $280 nm).[10–12] UV-C LDs can emit light in 210 nm band theoretically because these materials have large band gap energy.[13,14] The progress about short wavelength UV-LDs has been limited in recent years although the stimulated emission has been realized in deep ultraviolet region under optically pumping. UV-LDs require lower dislocation density, more complex structure and thicker layer than light emitting diodes, to achieve high emission efficiency, suitable electrical and optical confinement.[15] Another major challenge is that multiple stacked thick AlGaN layers with different AlN mole fractions will crack because of the accumulated tensile stain. The AlGaN material with high AlN mole fraction and high performance is indispensable for fabrication of devices.[16] In addition, the p-type conductivity problem, which is due to a high activation energy of Mg-dopant, is also one of the major challenges.[17] There have been very few reports on deep UV-LDs, especially for electron blocking layers (EBLs), which is a significant part of the laser diode. To improve the performance of UV-LDs, in this Letter we investigate the influence of EBLs.
cpl-36-5-057301-fig1.png
Fig. 1. (a) Schematic illustration of the layer structure of the ultraviolet laser diode, the double-tapered EBL and the inverse double-tapered EBL. (b) Material gain of the laser diode.
Figure 1(a) shows a schematic diagram of the deep ultraviolet laser diode with an Al$_{0.78}$Ga$_{0.22}$N layer as the substrate. The laser structure was composed of a 0.5-µm-thick n-cladding layer with Al$_{0.78}$Ga$_{0.22}$N, a 0.1-µm-thick n-Al$_{0.75}$Ga$_{0.25}$N guiding layer, the multiple quantum well (MQW) with two 3-nm-thick Al$_{0.56}$Ga$_{0.44}$N wells and three 4-nm-thick Al$_{0.66}$Ga$_{0.34}$N barriers, a 15-nm-thick AlGaN electron blocking layer without doping, a 0.15-µm-thick p-Al$_{0.75}$Ga$_{0.25}$N guiding layer and a 0.5-µm-thick Al$_{0.78}$Ga$_{0.22}$N p-cladding layer. In this simulation, the cavity length was set to 500 µm. The width of this laser was 4 µm. The refractive index of the left and right mirror faces was set to 5%. The back loss was set to 2400. All the characteristics of the deep-UV laser diode were simulated under room temperature. The laser gain reaches the maximum at approximately 261 nm as shown in Fig. 1(b). The $L$–$I$ curves of the two EBLs are shown in Fig. 2(a). An observation from Fig. 2(a) shows that the threshold current ($I_{\rm th}$) of the double-tapered EBL and the inverse double-tapered EBL are 81.6 mA and 79.0 mA, respectively.[18] Although the difference is very slight, it is obvious that the double-tapered EBL structure makes the slope efficiency (SE) increase from 2.34 to 3.15, which is increased by about 35%. The double-tapered EBL shows increases in the SE, compared to the case of the inverse double-tapered EBL. However, the increase of $I_{\rm th}$ is probably due to the change of the energy level from 469 meV to 631 meV, as shown in Fig. 3(a). Hence, the increased energy directly induces more current to flow and to accumulate. This enhancement will definitely increase the radiation and recombination in the active layer. Then the laser output power will also increase. The $V$–$I$ curves of the two structures are shown in Fig. 2(b). The threshold voltages ($V_{\rm th}$) of the two different EBLs are both 4.62 V and the resistance for current under the double-tapered EBL is slightly higher than that of the inverse double-tapered EBL.
cpl-36-5-057301-fig2.png
Fig. 2. All mode laser power at both facets (a) and voltage (b) versus injection current of two EBLs.
cpl-36-5-057301-fig3.png
Fig. 3. (a) Conduction band diagram and quasi-Fermi level. (b) Electron concentration ($n_{\rm e}$) distribution. (c) Valence band diagram and quasi-Fermi level. (d) Hole concentration ($n_{\rm h}$) distribution of two EBLs.
To demonstrate the improved performance clearly, the double-tapered EBL and the inverse double-tapered EBL are used to simulate and are compared using the Crosslight software. Figures 3(a) and 3(b) show the conduction band and electron concentration distribution of two EBLs. It can be observed that the last quantum barrier is mitigated, when the AlGaN with a high Al mole fraction is placed on the outermost layer of the EBL. The double-tapered EBL shows the higher conduction band, which results in more effective electron confinement in the MQW region. Furthermore, the effective potential height of the EBL for electron increases from 469 meV to 631 meV, indicating a more efficient electron blocking effect by the double-tapered EBL. In Fig. 3(b), the electron concentration of the double-tapered EBL shows a more drastic decline than that of the inverse double-tapered EBL. In other words, the double-tapered EBL confines the higher concentration of electrons in the MQW region. The electrons escaping from the MQW active region can recombine with the hole in the p-type region. It will lower the LD gain. The double-tapered EBL exhibits lower leakage current and higher laser efficiency. Figures 3(c) and 3(d) show the valence band and the hole concentration distribution of two EBLs. The effective potential height of the EBL for holes has declined from 370 meV to 338 meV, indicating a few of more efficient holes via the double-tapered EBL. In Fig. 3(d), in the middle part of the EBL, the hole concentration in the double-tapered EBL is significantly higher than that of the inverse double-tapered EBL. Two different EBLs have the same hole concentration at the last quantum barrier. This phenomenon shows uniform ability to promote holes passing through the EBL to the MQW region for the two EBL structures. When the inverse double-tapered EBL is replaced by the double-tapered EBL, the carrier concentration in the MQW region is improved and the efficiency of recombination is enhanced. This is a very valuable improvement for the laser performance.
cpl-36-5-057301-fig4.png
Fig. 4. (a) Electron concentration, (b) hole concentration, and (c) radiative recombination rate distribution in MQW region and (d) electron current density of two EBLs.
cpl-36-5-057301-fig5.png
Fig. 5. (a) Schematic illustration of the structures of the single-tapered EBL and inverse single-tapered EBL. (b) Electron concentration of four EBLs in the guiding layer.
To observe the changes of carrier concentration clearly, an adjustment of the observation position is settled. The profiles of electron and hole concentration distribution within the MQW are shown in Figs. 4(a) and 4(b), respectively. Compared with the inverse double-tapered EBL, the double-tapered EBL shows a higher concentration of electrons in the MQW. The electron concentration shows no significant changes in the first QW, while it increases by 10.7% and 53.6% for the second QW and third QW, respectively. The hole concentration has increased slightly in comparison to the increase in electron concentration. We can observe the carrier concentration in Fig. 4(c). The radiative recombination of the double-tapered EBL has been enhanced by about 4.9% compared with the inverse double-tapered EBL. The radiative recombination of electrons and holes mainly occurs in the last QW, which is adjacent to the p-type region. This is consistent with the conclusions given by David et al.[19] Hence, the increasing concentration of electrons and holes in the last well can explain the increasing light output power by enhancing the radiative recombination. Figure 4(d) illustrates the electron current density distribution of the two EBLs. After the inverse double-tapered EBL is replaced by the double-tapered EBL, the capacity for lager electron current density has been obviously promoted. In addition, the electron concentrations for four different EBLs in guiding layer are shown in Fig. 5(b). For a clear observation, the electron concentrations of the four EBLs at position 60 nm are compared. As shown in Fig. 5(b), the double-tapered structure has the lowest electron concentration about approximately 9.65. Compared with the inverse double-tapered EBL, the single-tapered EBL and the inverse single-tapered EBL, the double-tapered EBL has a significant effect on blocking electrons.[20,21] Therefore, the double-tapered EBL is more conducive to improving the laser performance such as output power. In conclusion, numerical research between the double-tapered EBL and the inverse double-tapered EBL has been investigated and proposed. It is found that the double-tapered EBL has a higher slope efficiency and better effect for electron blocking. Compared with the inverse double-tapered EBL, the single-tapered EBL and the inverse single-tapered EBL, the double-tapered EBL has the best effect for inhibiting the current leakage. References Pseudomorphically Grown Ultraviolet C Photopumped Lasers on Bulk AlN SubstratesMaster Recording for High-Density Disk Using 248 nm Laser Beam RecorderDemonstration of an ultraviolet 336 nm AlGaN multiple-quantum-well laser diodeImprovement of hole injection and electron overflow by a tapered AlGaN electron blocking layer in InGaN-based blue laser diodesEntirely Crack-Free Ultraviolet GaN/AlGaN Laser Diodes Grown on 2-in. Sapphire SubstrateUltraviolet AlGaN multiple-quantum-well laser diodes365 nm Ultraviolet Laser Diodes Composed of Quaternary AlInGaN Alloy350.9 nm UV Laser Diode Grown on Low-Dislocation-Density AlGaNHigh efficiency GaN-based LEDs and lasers on SiC231–261nm 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 power250nmAlGaN light-emitting diodesAn aluminium nitride light-emitting diode with a wavelength of 210 nanometresStructure optimization of 266 nm Al0.53GaN/Al0.75GaN SQW DUV-LDCharacteristics of InGaN-Based UV/Blue/Green/Amber/Red Light-Emitting DiodesEffect of electron blocking layer on efficiency droop in InGaN/GaN multiple quantum well light-emitting diodesImprovement of peak quantum efficiency and efficiency droop in III-nitride visible light-emitting diodes with an InAlN electron-blocking layerReduced Droop Effect in Nitride Light Emitting Diodes With Taper-Shaped Electron Blocking LayerCarrier distribution in (0001)InGaN∕GaN multiple quantum well light-emitting diodesAdvantage of tapered and graded AlGaN electron blocking layer in InGaN-based blue laser diodesRe-Examining the Doping Effect on the Performance of Quantum Well Infrared Photodetectors
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