The rapid development and widespread application of new broadband services such as cloud computing, big data, and mobile internet have led to an explosive growth in data center traffic. Due to the limitations of traditional electrical interconnects, it is difficult to meet the data transmission requirements of the current data centers, and optical interconnect technology has become the main solution for data transmission issues in data centers. In recent years, the trend in the increase of data center traffic and bandwidth is driving the upgrade of optical modules in data centers, and the demand for optical modules is growing at an accelerated rate. As the core component of the optical module, photodetectors play a key role in converting the transmitted optical signals into electrical signals. In this context, the demand for photodetectors in data centers is growing. Avalanche photodiodes (APD) are one of the photodetectors frequently used in optical modules, which have internal gain and can provide higher receiver sensitivity and dynamic range, and are receiving more and more attention from scholars. As data transmission rates move toward 100 Gb/s and 400 Gb/s, single-channel transmission rates increase to 25 Gb/s and beyond, placing higher demands on the bandwidth of photodetectors. APD can play a vital role in high-speed optical interconnection modules for data centers; therefore, it is of great importance to study APD with high bandwidth and high quantum efficiency.

We use APSYS software to design and optimize the avalanche photodiode with a p-down structure (p-down APD). APSYS is a finite element analysis software that calculates the optical and electrical properties of semiconductor devices by self-consistently solving Poisson’s equation, the carrier continuity equation, and the current density equation with various physical models. The transport mechanism of carriers inside the APD is very complex, and the accuracy and reliability of the results will be affected by the proper choice of physical model. In our simulations, we use the carrier mobility model to describe the variation of carrier mobility with respect to doping concentration, an Shockley-Read-Hall (SRH) model to characterize the process of electron and hole generation and complexation in addition to the diffusion and drift of photogenerated carriers, and the Chynoweth model to characterize the extent of impact ionization accurately.

In this paper, we propose a p-down APD with a hybrid absorber layer, which is designed and optimized to give full play to the advantages of the p-down structure. In the device design process, the hybrid absorber layer structure not only shortens the drift distance of the holes and alleviates the phenomenon of accumulation of holes at the interface but also solves the conflicting relationship between responsivity and bandwidth (Fig. 2). The p-down APD confines the electric field to the central region by using the double-mesa structure (Fig. 3). Due to the limiting electric field at the first mesa, the electric field in the undoped absorber layer and the electric field at the edge of the second mesa are reduced, making the p-down APD with double-mesa structure to have low dark current (Fig. 5), high bandwidth, and gain-bandwidth product (Fig. 6). A triple-mesa p-down APD is obtained by optimizing the mesa and layer structure parameters of the double-mesa p-down APD, which has the advantage of getting stronger electric field limiting effect (Fig. 8), lower dark current (Fig. 10), higher bandwidth (Fig. 11), and being useful in future optical communication systems.

The rapid increase in transmission rate in optical communication systems puts higher demands for the photodetector bandwidth. In this paper, the p-down APD is designed and optimized using the finite element analysis software APSYS. Compared with the conventional structure, the p-down structure can confine the electric field in the central region and reduce the electric field at the edge of the device, which has the advantages of low dark current and high bandwidth. The p-down APD with double-mesa and triple-mesa structures are simulated and compared. The results show that the p-down APD with a double-mesa structure can confine the electric field to the central region and prevent undesirable edge breakdown. The dark current of this APD is about 0.1 nA, the maximum bandwidth is 23 GHz, and the gain-bandwidth product is 276 GHz. A triple-mesa p-down APD with a maximum bandwidth of 31.7 GHz and a gain-bandwidth product of 289.4 GHz are obtained by optimizing the mesa and layer structure parameters of the double-mesa p-down APD, which has the advantage of obtaining stronger electric field limiting effect, lower dark current, and larger bandwidth. Therefore, APD with a p-down structure can play a crucial role in future optical communication systems.