Research efforts undertaken by the group led by Prof. Yanli Zhao at Wuhan National Optoelectronics Laboratory, Huazhong University of Science and Technology have shown that InGaAs/Si avalanche photodiode based on silicon-on-insulator substrate can be improved by using structured photon trapping for the first time. Their results are published in the journal Sensors.
Study: Photon trapping microstructure for InGaAs/Si avalanche photodiodes operating at 1.31 μm. Image Credit: atdigit/Shutterstock.com
High-sensitivity avalanche photodetectors (APDs) are becoming increasingly crucial in optical communication as advances in optical network infrastructure develop rapidly. Optical communication links are used in data centers to handle data traffic and increased capacity bandwidth.
Avalanche photo detectors (APD)
Most current APD technologies are based on Group III-V materials. Group III-V materials, such as InGaAs/InP, are materials that, depending on external factors or chemical changes, can function as an electrical conductor or insulator.
InGaAs/InP APD are limited by their excessive noise and bandwidth performance due to their higher ratio of impact ionization coefficients.
Ge and Si materials have lower electron mobility and lower absorption in the optical communication window than that of III-V compounds. These advantages make APD devices based on InGaAs/Si or Ge more efficient for optical links.
InGaAs materials were traditionally grown on Si by epitaxy. But using this method was inefficient due to the large dislocations and defects in the material. A more efficient technique to create an InGaAs/Si APD with better performance is to bond the InGaAs material to the Si material to create a new InGaAs/Si heterojunction material.
Besides improving fabrication routes, recent research has focused heavily on improving the light absorption of photodetectors using the resonant cavity effect or light trapping structures. Previous research efforts had designed and tested a single resonant cavity photodetector (RCE-PD). By etching micro- and nano-holes, also known as a “photon trapping” (PT) structure, Si photodetectors with intrinsic layer thicknesses less than 2 meters have seen an order of magnitude improvement. magnitude of their absorption efficiency. With further optimization, this efficiency could reach over 70%.
The team investigated an InGaAs/Si APD enhanced by the PT structure. Their new design is built with the optical simulation model used for the cell structure by Wanhua Zheng et al.
(a) 3D schematic of InGaAs/Si PT-APD; (b) cross-sectional diagram of the InGaAs/Si PT-APD.
The experimental details
A unique InGaAs/Si photon-trapping APD detector (PT-APD) was designed with a photon-trapping nano-hole array topology to improve responsiveness. The PT structure is a square lattice with periodic nano-holes in the device core. The distribution of the light field in the device can be modified by modifying the characteristics of the PT structure, such as the depth, the period and the diameter of the holes. The PT structure is built on a vertical InGaAs-Si APD. To provide the high refractive index difference required to limit photons in the active layer of the detector, the substrate uses a silicon-on-insulator (SOI) wafer.
The fabrication methods of PT-APD and the underlying theory of the photon trapping structure are described in detail in the publication.
The design of a highly reactive InGaAs/Si APD at the wavelength of 1310 nanometers is the main objective of this work. The period of the PT structure is first optimized in the design process. The channeling mode is created in the PT structure when the period is shorter than the incident wavelength, and the development of the channeling mode is useful to improve the optical coupling between the incident light and the model.
By simultaneously scanning the hole radius and depth, the absorption efficiency of InGaAs/Si APD with the PT structure was simulated to further increase the absorption efficiency of PT-APD.
The Poynting light energy distribution in particular orientations was also studied to assess the optical process of the device
To evaluate the electrical characteristics that an InGaAs/Si APD should have, electrical simulations for the stacked arrangement were performed. The results showed that the internal electric field distribution is entirely different with and without bright illumination.
Simulated dark current, enhanced photocurrent and responsiveness of InGaAs/Si APD were also tested as a function of bias voltage.
Comments and perspectives
Prof. Zhao’s group has created a PT structure coupled with an InGaAs/Si APD based on an SOI wafer that can achieve higher responsiveness in linear operation mode. Improved absorption, gradation, charge and multiplication-InGaAs/Si APD have been proposed to enable the use of the device in the optical communications sector. By adjusting the parameters of the PT structure, the performance of the PT absorption APD could be significantly improved.
For a standard operating voltage, the responsiveness could be increased by 1.6 times. This study investigated an entirely new technical viability for high-sensitivity, low-noise optical communication systems.
Zhang, H., Tian, Y., Li, Q., Ding, W., Yu, X., Lin, Z., Feng, X. and Zhao, Y., (2022) Photon-Trapping Microstructure for InGaAs/ Si avalanche photodiodes operating at 1.31 μm. Sensors, 22(20), p.7724. https://doi.org/10.3390/s22207724
Peng, H., Qu, H. & Zheng, W., (2020) A promising low-noise, high-gain InGaAs/Si avalanche photodiode. AOPC 2020: Optoelectronics and Nanophotonics; and quantum information technology,. https://doi.org/10.1117/12.2579756