Compared with other types of laser diodes, semiconductor laser diodes have excellent properties such as high optical power, high efficiency, low cost, and good laser quality. Thus, laser diodes, especially those with short excitation wavelengths, have important and wide applications in solid-state illumination, high-density optical storage, radio-optical communication, biomedical technology, and chemical analysis. To continuously meet the increasing performance requirements of InGaN-based violet laser diodes in the above fields, we select an experimental sample from the reference as reference structures and theoretically investigate the effect of structural changes in the laser diodes on their optoelectronic performance. Meanwhile, we are motivated by the current problems in the actual growth epitaxy process and operation of violet laser diodes that hinder the performance enhancement, such as difficulties in p-type doped epitaxial growth, lattice mismatch, polarization of the GaN material, and electron leakage. Typically, the mobility of electrons in semiconductors is higher than the mobility of holes, resulting in electron leakage from the active region. This is the region where the radiative composite of electrons and holes is supposed to occur, to the p-cladding layer where holes inject, and non-radiative composite with holes occurs therein, which significantly reduces the carrier radiative composite rate. Additionally, the polarization effect of the GaN material results in a shift in the internal energy band structure of the laser diodes and also reduces the optoelectronic performance. Thus, many researchers have proposed a number of optimization schemes to reduce electron leakage and minimize the influence of the polarization effect, and most of the schemes are the growth optimization in the actual epitaxy process and the structure optimization of the waveguide layer and active region in the laser diodes. We conclude that by directly modifying the structure of the electron blocking layer (EBL) such as thickness and contents, the energy band structure can be adjusted and the electron leakage rate can be reduced without decreasing the hole injection to improve the wall plug efficiency.

The experimental InGaN-based violet laser diode sample in the reference is selected, and the completely identical laser diode sample is constructed by the PICS3D software to simulate the optoelectronic performance. During the simulation, we set the simulation parameters exactly according to the experimental parameters in the reference. After simulation, the luminescence power variation of the simulation results with the injection current is plotted and strictly compared with the luminescence power of the experimental structure to confirm the accuracy and rationality of the obtained simulation results. Subsequently, a series of simulated structures are designed based on the reference structure. The basic concept is to modify the EBL to reduce the electron leakage. Therefore, we first divide the EBL into two layers and optimize the Al content and thickness of the insert layer close to the active region by comparing the wall plug efficiency, and then illustrate the reasons for the performance optimization in carrier distribution and energy band structure distribution. Subsequently, we optimize the Al content of the original EBL far from the active region to increase hole injection and radiative recombination rate and also explain the reasons for the performance optimization by comparing the carrier distribution and energy band structure distribution. Finally, the optoelectronic performance of the optimized structure is compared with that of the reference structure to illustrate the performance enhancement significance.

First, we simulate the reference structure in PICS3D software according to the experimental structure, plot its optical power-injection current curve and compare it with the optical power curve measured from the experimental structure, and the results indicate the validity of the proposed simulation (Fig. 2). Subsequently, a series of insert layer structures are designed by changing the Al content in the insert layer and keeping the total thickness constant, and the wall plug efficiency of different Al content in the insert layer of this series is compared (Fig. 4). By adjusting the thicknesses of the insert layer and the original EBL, the Al content of the insert layer are varied in the same way with the total thickness of the composite EBL constant, and five series of wall plug efficiency is obtained (Fig. 5). As a result, we obtain the optimized structure B. Furthermore, to obtain the reason for the better performance of the optimized structure B, we select three samples with the same thickness and different Al content of the insert layer and compare their energy band structures and carrier distribution (Figs. 6 and 7). The reason is that the change of energy band structure leads to easier carrier injection. Additionally, five samples with different thicknesses and the same insert layer Al content are also selected to compare their optical powers and voltages (Fig. 8), and the reason for optimization is illustrated in terms of the hole barrier height and energy band tilt (Table 3 and Fig. 10). Finally, a series of samples are designed by changing the Al content of the original EBL from a constant Al content to a gradient to obtain their wall plug efficiency (Fig. 11). Finally, we obtain the final optimized structure C. Images of the energy band structure and carrier distribution are plotted according to a similar method (Figs. 12 and 13), which illustrates that the performance optimization is due to the easier hole injection led to by the gradient of the Al content of the original EBL. The optimized samples show an increase in wall plug efficiency relative to the reference structure (Table 4 and Fig. 14).

The effects of the thickness and Al content of the composite EBL on InGaN-based violet laser diodes are investigated. The results show that adjusting the thickness and content of the insert layer and changing the Al content of the original EBL to gradient can reduce the electron leakage of the reference structure and increase its hole injection, thus significantly improving the wall plug efficiency. Setting the insert layer thickness to 15 nm and the Al atomic number fraction to 0.30 can reduce the electron leakage and thus increase the radiative recombination rate for improving the wall plug efficiency from 8.365% to 10.72%. On this basis, the Al atomic number fraction of the original EBL are set to a gradient of 0.24 to 0.06, which further increases the hole injection and radiation recombination rate, and improves the wall plug efficiency to 11.45%. Finally, the wall plug efficiency is improved by 36.9% at 1200 mA injection current compared with the reference structure, and the results provide references for improving the InGaN-based violet laser diodes.