基于选区外延技术的单片集成阵列波导光栅与单载流子探测器的端对接设计

Butt-joint design in a uni-traveling carrier photodiode array monolithic with an arrayed waveguide grating by the selective area growth technique

选区外延技术是实现有源与无源光器件单片集成的一种有效的工艺手段,但同时对两种器件在异质生长界面处的对接结构提出了更高的设计要求.本文通过选区外延技术实现了InP基O波段4通道阵列波导光栅与单载流子探测器的单片集成.通过光学仿真重点研究了选区外延后界面处形貌对无源波导结构与有源光探测器间光耦合效率的影响,包括伸长的光学匹配层、二次外延生长边界位置、波导刻蚀边界位置等因素.研究结果表明,在保证二次外延生长边界对准异质对接界面时,将光学匹配层伸出探测器前端10μm并与外延边界无缝对接既可以保证高效的光传输效率(或探测器量子效率),又可以避免外延界面处的异常生长对器件制备工艺的影响,保证生长工艺与器件制备工艺的兼容性.成功制备的单片集成芯片具有高达76%的探测器量子效率,证明了对接方案的有效性.同时,集成芯片的低串扰(<-22 dB)与解复用特性展示出其作为解复用光接收芯片具有巨大潜力.

Monolithic integration of an InP-based O-band 4-channel arrayed waveguide grating （AWG） to a uni-traveling carrier photodiode （UTC-PD） array is realized by the selective area growth （SAG） technique. The passive-active butt- joint design is introduced and experimentally proved to ensure both good compatibility between the PD fabrication process and the SAG technique, and high photodiode quantum efficiency under the complex butt-joint geometry. An extended coupling layer is adopted between the AWG output waveguides and the PD mesa. The extended coupling layer length, the regrowth boundary edge position and the AWG etching edge position relative to the heterogeneous butt-joint boundary, and the refractive indices of the PD collector and coupling layer are optically simulated and optimized by a finite-difference time-domain method. It is found that the extended coupling layer, compared with the un-extended situation, ensures a good matched optical field from AWG to PD and could reduce nearly 30% quantum efficiency loss when connecting seamlessly to the regrown InP AWG top cladding layer. A stable high efficiency around 80% is maintained within an extended layer length from 7.5 μm to 15.0 pro. The regrowth boundary edge into the coupling region will cause a drastic efficiency oscillation up to 20% period with the increase of distance. The efficiency drop is also attributed to the light scattering at the regrowth boundary edge, caused by the optical field mismatch, while the oscillation comes from the alternative light power concentration between the coupling layer and the core layer, for the light scattering is only obvious when the light power is well concentrated in the coupling layer. The AWG etching edge position deviation from the butt-joint boundary, however, exerts little influence on the PD quantum efficiency, which is believed not to bring obvious coupling loss during device fabrication. The higher UTC-PD collector refractive index is proved to be crucial for further better optical coupling from the coupling layer to the PD, with quantum efficiency rapidly increasing from around 0.1 to 0.8 when the index is increased from 3.20 to 3.42. By comparison, the efficiency is little affected by the coupling layer refractive index from 3.34 to 3.42.All things considered, we select a 10 pm extended coupling layer, the refractive indices of both PD collector and the coupling layer to be 3.42, and align both the regrowth boundary edge and the AWG etching edge to the heterogeneous butt-joint boundary, and a PD quantum efficiency of 80% is expected Owing to the extended coupling layer at the butt-joint, the SAG technique facilitates the PD fabrication process The overgrown AWG top cladding layer ridge stretches out 4.67 ~m toward the PD, but not over the mesa yet, hence has little influence on the PD fabrication accuracy. The monolithic chip presents a uniform photodiode quantum efficiency of 76%, which accords well with theoretical value and confirms the butt-joint design. Central wavelengths for the four channels are 1347.0 nm, 1325.0 nm, 1308.0 nm, and 1286.5 nm, respectively. The low crosstalk level （below -22 dB） also indicates a good de-multiplexer performance