Study On Ge/Si Waveguided Avalanche Photodiodes

With the development of epitaxial techniques, Ge/Si waveguide avalanche photodiodes(WG APDs) become a research focus, which applied to the detection of the near-infrared light and integrated with the CMOS process. On the one hand, they can expand the range of light detection of the device due to use Ge material as absorption layer; On the other hand, the photodetectors have characteristics of high responsibility and high bandwidth by combining the advantages of the waveguide and APDs. With the development of optoelectronic integrated circuit, it requires photodetectors to couple with the optical fiber. So the thesis design a waveguide avalanche photodetectors based on SOI(Silicon-On-Insulator) waveguide. SOI waveguide can easily couple with optical fiber by conbining the tapered waveguide structure. In addition, SOI waveguide not only can reduce the phenomenon of substrate absorption, but also has a high degree of integration with optoelectronic integrated circuit. While, the thesis applied the APDs' structure of separating absorption-charge-multiplication. In the structure, the photons absorption and carriers multiplication respectively occur in different regions, they interfere with each other. Therefore, it improved the APDs' performance. In the thesis, the specific contents of are as follows:Firstly, the design and the performance simulation of the device. After analyzing the Ge/Si heterojunction and the theory of waveguide and APDs, the thesis designed the Ge/Si WG APDs' structure at the angle of coupling with optical fiber. And the thesis made the parameter optimization of the device at angle of the size and doping requirements of core layer of the SOI WG and APDs' own characteristics. And systematically explored APDs' performance at the different doping and at dimension, simulated and analyzed the photocurrent, dark current, response, frequency characteristics and other performance parameters of the device. The research results show that: the avalanche breakdown voltage of the device is-26 V, the higher degree of the responsibility is obtained in the range of 1.0~1.4um, the wavelength of peak responsibility is 1.2um, the highest responsibility is 0.75A/W, the dark current is 51 10-? A, 3dB bandwidth is 10GHz; when the reverse bias voltage and illumination is high, which will reduced avalanche`s gain due to the space charge effects of excess carrier; when the doping of absorption layer and multiplication layer are too high, it will easily leads to the phenomenon of zero electric field of absorption layer; when the doping of charge layer slightly higher than absorption layer and multiplication layer can maintained a low electric field of absorption layer and a high electric field of multiplication layer; the size variation of charge layer and multiplication layer can affect the breakdown voltage of detector; under a high reverse bias voltage, the 3dB bandwidth of the detector will gradually decreases with the increase of the reverse bias voltage.Secondly, the establishment and simulation of equivalent circuit. In order to verify the performance of the Ge/Si WG APD, the thesis established an equivalent circuit of the device. Taking into account the parasitic effects, the thesis builded the equivalent circuit of APDs based on carriers rate equation and the analysis of noise. Used PSpice simulation software to simulate photocurrent, dark current and 3dB bandwidth of the equivalent circuit. The result shows simulations of the equivalent circuit are in agreement with the result of simulations of the device.