Investigations of silicon-based optoelectrical integration have become a development trend for an increased transmission rate in optical networks. Currently, most photonic devices achieve on-chip integration, except for silicon-based lasers, which are essential light sources. Heterogeneous epitaxial growth has been used to construct silicon-based Ⅲ-Ⅴ semiconductor laser structures, and it is one of the most promising solutions offering high yield and low costs. Significant efforts have been made to enhance the performance of silicon-based lasers by improving the quality of the as-grown material. However, only a few studies have been conducted on optimizing the laser-chip structure and the fabrication process that directly influences the lasing modes, differential resistances, and other properties of the lasers. Moreover, high differential resistance can reduce the output power, slope efficiency, and wall-plug efficiency (WPE) of the lasers and can even cause lasing failure owing to excessive waste heat. Therefore, reducing the differential resistance of silicon-based lasers is critical for significantly improving laser performance and realizing high-performance silicon-based lasers.

Combined with the advantages of metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), the quantum-dot (QD) laser structure was grown on a two-inch complementary metal-oxide semiconductor (CMOS)-compatible Si (001) substrate (Fig. 1). Moreover, Fabry-Perot (F-P) laser devices were fabricated using two different chip structures. The ridges were etched using inductively coupled plasma (ICP) via standard photolithography. Ti/Pt/Au and AuGe/Ni/Au were deposited via physical vapor deposition (PVD) as p- and n-type contact electrodes, respectively. A 300 nm thick SiO₂ layer was deposited via plasma-enhanced chemical vapor deposition (PECVD) for electrical isolation. The as-fabricated wafers were fabricated into different chip sizes by adequate cleaving and then mounted on Cu heatsinks with C-mount packages. Finally, the main performance of the lasers with these two chip structures was determined for further comparison and analysis.

The main performance of the silicon-based quantum dot laser was determined under CW conditions at room temperature (25 ℃). The F-P lasers, each with a cavity length of 1.5 mm and a stripe width of 50 μm, achieve a single-facet output power of 70 mW and differential resistance of 1.52 Ω (Fig. 4). The voltage of the lasers with the conventional cathode structure is approximately 3.8 times that with the symmetrical cathode structure under the same injection currents (Fig. 5). The lasing wavelength of the lasers with conventional cathode structure exhibits a red shift by approximately 18.4 nm owing to additional waste heat, whereas the laser with symmetrical cathode structures exhibits a red shift by only approximately 4.1 nm when the injection current increases from 1.2 to 2.8 times the threshold current (Fig. 5). Moreover, compared with the conventional cathode structure, the symmetrical cathode structure can significantly reduce the device differential resistance by approximately 75%, increasing the characteristic temperature from 27.2 to 43.3 K (Fig. 5). In addition, the slope efficiency and maximum wall-plug efficiency increased by 26.4% and 4.7 times, respectively (Fig. 6).

In this study, a new chip structure of lasers on silicon was designed, which could reduce the differential resistance compared with the conventional cathode structure, significantly improving the laser performance. QD lasers on a two-inch CMOS-compatible Si (001) substrate were fabricated using this structure, and the influence of the chip structure on laser performance was investigated experimentally. The results show that the differential resistance of the lasers with symmetrical cathode structures is only 1.52 Ω, which is significantly low differential resistance. Compared with the conventional cathode structure, the chip structure can significantly reduce the differential resistance of the device by approximately 75% and increase the characteristic temperature by approximately 59.6%. In addition, the slope efficiency and maximum wall-plug efficiency increase by 26.4% and 4.7 times, respectively, the output power reaches 70 mW, and the stability improves significantly. In summary, the laser performance can be significantly enhanced by decreasing the differential resistance, which provides another critical approach to enhancing the laser performance and offers an optimized technical solution for producing high-performance and highly reliable lasers on silicon.