Study On High-speed Avalanche Photodetector For Data Center

In response to the huge bandwidth demands from emerging businesses such as unmanned driving,VR/AR,smart cities,and cloud computing,data centers continue to be built and updated,becoming an important pillar for handling the massive influx of data.The traditional electric interconnection methods,limited by its own characteristics,is difficult to meet the current traffic transmission demand of data centers.Therefore,high-speed optical interconnection ushers in broad development prospects and great development opportunities.Thanks to the large-scale deployment of data centers and the rapid development of optical interconnection technology,the market demand for optical modules is expanding dramatically.As the core of the receiving component in an optical module,the performance of the photodetector directly determines the transmission rate of the optical receiving component.With the upgrade of optical interconnection architecture and capacity of data centers,optical modules evolve into ones with high-speed rate,low cost and low power consumption,which promotes the development of the photodetector with a focus on its high performance.The research heat of avalanche photodetectors increases day by day as they can provide higher sensitivity and dynamic range for the receiver due to their internal gain.However,the conventional structure of InP/InGaAs APD often witnesses problems such as excessive edge electric field,high dark current and mutual constraints of responsiveness and bandwidth,which limit the improvement of the device performance.Aiming at the 1310 nm avalanche photodetector required for data center applications,this paper proposes an inverted P-down InGaAs/InAlAs APD structure with a hybrid absorption layer with the core problem of achieving high speed,high stability and low dark current.Apart from that,this paper gives full play to the advantages of the P-down structure through design optimization.The main findings and innovations of this paper can be seen as follows:1.A P-down APD based on a double mesa structure is designed.To address the inherent contradiction between bandwidth and responsiveness of conventional APDs,a hybrid absorption layer structure is formed by incorporating a P-type absorption layer into the intrinsic one.The optimal value of the thickness ratio of the P-type absorption layer to the total absorption layer can be obtained by numerical calculation to minimize the carrier transit time and improve the high-speed performance without sacrificing device responsiveness.According to the results,when the total thickness of the absorption layer is 0.5μm,the best effect of bandwidth enhancement can be achieved if the ratio is designed to be 0.5.The device has a bandwidth of 22GHz and a gain-bandwidth product(GBP)of 214 GHz.To achieve precise control of the internal electric field,the field control layers are systematically analyzed and optimized to reduce the generation of tunneling dark current.The resulting APD dark current is less than 0.1 nA.2.The performance difference between the conventional APD and the P-down APD based on a double mesa structure is compared and analyzed,and the advantages and research significance of the P-down structure are elaborated and indicated.In terms of the internal electric field,the first mesa of the P-down APD embraces the effect of limiting the electric field,and the edge electric field decreases by about 87.97kV/cm as compared to the central electric field,effectively avoiding the occurrence of edge breakdown.With regard to Ⅰ-Ⅴ characteristics,the photocurrent of the Pdown APD is 10.9μA.Compared with the conventional APD,the photocurrent of the P-down APD increases by about 76%,with the increasing sensitivity of the device.Concerning the frequency characteristics,the bandwidth of the P-down APD is 22GHz,and the gain-bandwidth product is about 1.42 times that of the conventional APD.3.A P-down APD based on a triple mesa structure is proposed through the optimization of the study of the P-down APD based on a double mesa structure.By introducing a new InAlGaAs gap-grading layer,the effect of valence band discontinuity at the heterojunction interface on the motion velocity of the photocarrier is reduced.Through optimization of the field control layers,mesa diameter and mesa spacing,the device performance is further improved.The results indicate that the electric field intensity of the avalanche layer of the triple mesa P-down APD is significantly reduced from 825.3kV/cm to 4.6kV/cm when the diameter and the spacing of the first mesa witness 14μm and 3μm respectively.In addition,the dark current of the device decreases with its bandwidth up to 32GHz and its gainbandwidth product about 285GHz,which is about 33.2%higher than that of the double mesa P-down APD.