氮化物垂直腔面发射激光器的发展与挑战(特邀)

Development and Challenges of Nitride Vertical-cavity Surface-emitting Lasers(Invited)

垂直腔面发射激光器凭借阈值低、发散角小、调制速率高以及输出光束呈圆斑对称等特点,迅速成为当下半导体激光器的研究热点。氮化镓(GaN)材料是制造紫外到绿光波段光电子器件的理想材料,经过四十余年的研究,蓝光和绿光LED在照明、显示等领域得到广泛应用。技术含量更高的激光器件也已进入了应用的快车道,即将覆盖照明、通信、投影显示、光存储、医疗、微型原子钟及传感器等场景。铝镓氮(AlGaN)是GaN基半导体材料的重要代表之一,其禁带宽度可在3.4 eV(GaN)到6.2 eV(AlN)范围内连续可调,对应波长可覆盖200~365 nm波段,是制造从近紫外波段到深紫外波段紫外垂直腔面发射激光器的理想材料。而铝镓氮(AlGaN)垂直腔面发射激光器经过近20年来的发展,如今已成为半导体激光器的研究热点之一。首先回顾了GaN基垂直腔面发射激光器的发展历史,简要介绍了其在各个波段的主要应用场景;然后介绍蓝光、绿光及紫外垂直腔面发射激光器的研究进展;最后分析了光注入和电注入紫外垂直腔面发射激光器发展过程中的挑战和困难,并介绍了改进和优化的策略。

There are many kinds of semiconductor lasers,such as Distributed Feedback Laser Diode(DFB-LD),Edge Emitting Laser(EEL)and Vertical Cavity Surface Emitting Laser(VCSEL).Among them,VCSEL has many advantages such as large-scale two-dimensional integration,circular far-field spot,single longitudinal mode output,low power consumption,on-wafer testing,low threshold,small divergence angle,high modulation rate and so on.Hence,VCSEL has attracted the attention of many researchers.VCSEL has a history for more than 40 years.It was proposed firstly by Kenichi Iga of the University of Tokyo in 1977.After more than 40 years of research by scientists,applications and development of GaN-based VCSELs have been in the fast lane,including lighting,communication,projection display,optical storage,medical treatment,micro atomic clock and sensor.Nitride is an ideal material for manufacturing optoelectronic devices in ultraviolet to near infrared.Wherein,Aluminum Gallium Nitride(AlGaN)is one of the important materials for nitride semiconductors.Its bandgap is continuously adjustable from 3.4(GaN)to 6.2 eV(AlN),corresponding 365 to 200 nm.It is an ideal material for fabricating ultraviolet VCSEL from near ultraviolet to deep ultraviolet.After nearly 20 years of rapid development,AlGaN-based VCSELs have become the research hotspots of semiconductor lasers.Recently,violet to yellow green VCSELs have been demonstrated with electrical pumping,while there are only a few reports about ultraviolet(UV)VCSELs with optically pumping.Comparing with the visible nitride VCSELs,the development of UV VCSELs faces many challenges,especially in deep ultraviolet(DUV)range(<280 nm),including obtaining high crystalline quality AlGaN epilayer,DBR fabrication,sapphire substrate removal,reducing surface roughness,current spreading structure,high carrier injection efficiency,high doping level of p-AlGaN,and ohmic contact with p-AlGaN layer.Firstly,AlGaN epilayer with higher Al component is desired.However,with the increase of Al composition,the growth of AlGaN epilayer become more difficult,because of high growth temperature,and low surface migration speed of Al element.Secondly,it is difficult to prepare high-quality nitride DBR with high reflectivity in UV range,which is results from the lattice mismatch,thermal expansion coefficient mismatch,uneven in-plane components and small refractive index difference.Thirdly,dielectric DBR is easy to fabricate,however,it requires substrate removal,which is hard to realize.It also absorbs light in DUV.For electrically pumped AlGaN-based VCSELs,the fabrication is more complex and difficult than that of optically pumped,and more challenges need to be overcome.Firstly,high p doping level AlGaN is difficult to obtain,because Mg element is hard to dissolve in AlGaN material,and the activation energy increases with Al component(~200 meV in GaN,~630 meV in AlN).As a consequence,the conductivity of p-AlGaN is low.Secondly,the current spreading structure is required to provide sufficient and stable carriers into the active region,so as to achieve population inversion,and form a continuous output laser.The current spreading layer in VCSEL should be of high conductivity and low optical absorption,which is obstructed by the p doing problem.Thirdly,carrier injection efficiency is often reduced by electron overflowing from active region and low hole injection into active region.For short wavelength VCSELs,especially in DUV,electron leakage is more serious,and hole concentration in p-AlGaN is much lower.Therefore,reasonable design of Electron Barrier Layer(EBL)should be considered,and increasing p doping level is desired.Although there are many technical difficulties,the prospect of AlGaN-based UV VCSEL is still full of opportunities.As the ultraviolet devices,especially in DUV,have great demand in many applications,such as medical diagnosis,biochemical medicine and prevention,atomic capture,spectroscopy,laser lithography,laser high density storage and other important fields,it will be a key component essential to supporting the information society in the future.In this paper,the history of GaN-based VCSELs was reviewed,and its main applications were briefly introduced.Then,the research progress of blue,green and ultraviolet VCSELs was also introduced.Finally,the challenges and difficulties in the development of optically pumped and electrically pumped ultraviolet VCSELs were analyzed,and the improvement and optimization methods were briefly introduced.