面向硅基光子集成的片上半导体激光器(特邀)

On-Chip Semiconductor Lasers for Silicon Photonics(Invited)

硅基集成Ⅲ-V族化合物半导体激光器是硅光技术发展的关键,多年来研究人员发展了三大类集成方案。本文对这三类技术进行概括性综述:基于倒装焊等工艺的混合集成方案将预先制备的激光器芯粒通过压焊等手段精确安装至硅光芯片,已实现小批量生产,但在生产效率、良率、成本等方面具有一定的局限性;基于材料转移或键合的异质集成方案通过将晶格匹配衬底上预先外延制备的高质量有源层薄膜贴合至硅光衬底晶圆上,实现了大尺寸硅衬底上的全晶圆级激光器加工,可大幅度提高生产效率,该方案正进一步提高良率;硅基直接外延化合物半导体激光器的异质集成方案作为真正的晶圆级加工技术路线具有极大的吸引力,通过缓冲层、垂直或侧向选区外延等方式,克服材料间的理化性质失配、反相畴以及激光器与硅波导之间的光耦合效率低等技术难题,取得了多项突破,展现了良好的发展前景。

Significance Silicon is an indirect bandgap semiconductor that cannot emit light efficiently.In contrast,Ⅲ-V compound semiconductors such as AlGaAs/GaAs and InGaAsP/InGaAlAs/InP are direct bandgap semiconductors widely used for fabricating efficient electrically driven laser devices.As silicon-based photonic integrated circuits enter commercial application,integratingⅢ-V compound semiconductor lasers on silicon has become a key bottleneck for its future development.Progress Three main kinds of approaches have been developed aimed at integratingⅢ-V semiconductor laser sources on silicon photonic chips.The hybrid integration approach mounts prefabricated semiconductor laser chipsets on silicon photonics substrates by means of precision flip-chip bonding between gold pads.To achieve high optical coupling efficiency between the laser chipset and the silicon waveguide,the main technical challenges include repeatable sub-micron three-dimensional alignment and optical mode matching.Precision alignment mainly relies on high-precision equipment in the horizontal direction and carefully designed mechanical structures in the vertical direction,while optical mode matching relies on predesigned mode converters.This technology has been put into small-volume production but faces challenges in terms of productivity,yield,and cost.The heterogeneous integration approach is based on transferringⅢ-V active devices or films onto silicon by means of transfer printing or die-to-wafer bonding.In transfer printing,preparedⅢ-V lasers are picked up by an elastomer stamp and attached to a silicon substrate by van der Waals force or by an adhesion agent.Compared with flip-chip bonding,transfer printing is a flexible process that can integrate multiple devices of different kinds in one step.Optical coupling with the underlying silicon waveguide is usually achieved via evanescent coupling.In die-to-wafer bonding,Ⅲ-V epitaxial wafers are diced into desired sizes and placed with their active side up on a carrier plate by temporary bonding.The carrier plate with the dies is then activated in plasma and flipped over to bond the dies to the silicon photonics wafer.The carrier plate is then removed,and theⅢ-V dies are thinned to remove their originalⅢ-V substrate,leaving only the active epi-layer attached to the silicon wafer.Semiconductor lasers are then fabricated in the active epi-layer,precisely aligned to the underlining silicon waveguide by means of photolithography alignment markers to ensure high-efficiency evanescent coupling.This technology has successfully realized wafer-scale production.While promoting productivity,the yield of both transfer printing and die-to-wafer bonding approaches is still being improved.The direct epitaxy ofⅢ-V semiconductors on silicon is attractive as a truly wafer-scale production approach but faces significant challenges in terms of crystal lattice mismatch,thermal mismatch,and anti-phase domains at the epi-interface which leads to high stress and high defect densities in theⅢ-V epi-layer.Various schemes have been developed,including blanket growth and selective area growth.In blanket growth,thick buffer layers are needed to reduce the defect density in theⅢ-V active layer,which can prevent optical coupling between theⅢ-V active layer and the underlying silicon waveguide.Growth in etched pits can align the active layer with the silicon waveguide but faces problems in laser facet quality.Using quantum dots instead of bulk or quantum well active layers significantly mitigates adverse effects of defects on laser efficiency and lifetime.Vertical selective area growth can produce high-qualityⅢ-V crystals by means of defect-trapping,but micro-sized material cannot support electrodes for current injection.Horizontal selective area growth enables epi-growth ofⅢ-V materials and quantum-well active structures in the lateral direction,forming larger-sizedⅢ-V films co-planar with the silicon waveguide layer,therefore demonstrating promising potentials for electrically pumped semiconductor lasers efficiently butt-coupled with the silicon passive waveguide.ConclusionsⅢ-V lasers on silicon photonics chips have been realized through hybrid integration,heterogeneous integration(including transfer printing and die-to-wafer bonding),and direct epitaxy.While hybrid and die-to-wafer bonding approaches have been commercialized in small to medium-volume production,further improvement in terms of their productivity and yield is needed.The direct epitaxy approach as a true wafer-scale remains an attractive long-term solution requiring substantial research and development.