对称负极芯片结构改善硅基激光器性能研究

Improved Performances of Lasers on Silicon(001)with Symmetrical Cathode Structures

本课题组设计了一种用于硅基外延激光器的对称负极芯片结构,与传统共面电极芯片结构相比,该结构大幅降低了硅基激光器的微分电阻,使激光器性能显著提升。采用该电极结构以及基于无偏角Si(001)衬底的量子点激光器外延材料进行了激光器芯片制作。芯片尺寸为1500μm×50μm的激光器的微分电阻仅为1.52Ω,单面输出光功率可达70 mW。实验结果表明:相比于传统共面电极芯片结构,该芯片结构可将器件的微分电阻降低约75%;当注入电流从1.2倍阈值电流增大到2.8倍阈值电流时,激射波长红移量减少了约77%,特征温度由27.2 K提高到43.4 K,斜率效率增大了约26.4%,最大光电转换效率增大了约4.7倍。所设计的芯片结构方案为制作高性能硅基外延激光器提供了一种优化的技术途径。

Objective 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-basedⅢ-Ⅴsemiconductor 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.Methods 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.Results and Discussions 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 50μm,achieve a single-facet output power of 70 mW and differential resistance of 1.52Ω(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).Conclusions 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 CMOScompatible 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 1.52Ω,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.