Research On Dark Current Suppression And Wide-spectrum,High-speed And High-power Response Characteristics Of Photodetectors Within Wavelength Region Of Optical Communications

With the continuous development of information technology,people's requirements for network traffic continue to increase.The long-distance rapid transmission of massive data based on optical communication technology has become a long-lasting and important research direction in the information field.In an optical communication system,a photodetector acts as a receiving end for converting an optical signal into an electrical signal,and plays a vital role in information transmission.The continuous development of high-speed and large-capacity optical communication systems requires photodetectors to have excellent characteristics such as low dark current,high speed,high-power output,and wide-spectrum detection.In order to suppress dark current of photodetectors,realize broad spectrum detection in optical communication band,and improve the high-speed and high-power response characteristics of photodetectors,this paper focuses on the wide spectrum and array detection technology of photodetectors and hybrid integration technology of devices,starting from the basic characteristics of dark current mechanism and capacitance of photodetectors.The main innovations and research results of this paper are as follows:1.In order to suppress the dark current of the PIN photodetector,a complete model for calculating dark current and a scheme for suppressing dark current are proposed.The model considers trap-assisted tunneling current caused by the defects in the photodetector and indicates that Shockley-Read-Hall(SRH)generation-recombination and trap-assisted tunneling are the two main mechanisms for generating dark current at low reverse bias.Under the reverse bias voltage of 5V,when the doping concentration of the absorption layer is 1×1015 cm-3,the thickness is less than 1?m,and when the thickness of the absorption layer is 2?m and the doping concentration is more than 7×1015 cm-3,trap-assisted tunneling is the main mechanism of dark current.The total dark current calculated by the model agrees well with the measured.The dark current suppression scheme based on this model is as follows:(1)Reducing the thickness of absorption layer can reduce the dark current.When the doping concentration of absorption layer is 1 × 1015cm-3 and the thickness of absorption layer is less than 1.2?m,the dark current is less than 2nA.However,reducing the thickness of absorption layer affects the responsivity of the photodetector.Therefore,the thickness of absorption layer should be minimized and to meet the requirement of responsivity.(2)Increasing the doping concentration of absorption layer can suppress dark current.When the thickness of absorption layer is 2?m and the doping concentration exceeds 1×1016cm-3,the suppression effect of increasing the doping concentration on the dark current is weakened,and other performance of the photodetector is affected.Therefore,in order to most effectively suppress dark current,the doping concentration of absorption layer should be about 1×1016cm-3.(3)Lowering the temperature can suppress dark current.Since the temperature drop can significantly reduce the SRH generation-recombination current.2.A complete dark current model of uni-traveling-carrier photodetector and a scheme to suppress dark current are proposed.The effect of traps on dark current is considered,and it is found that the dark current in the device is mainly generated in the InGaAs layer near the collection layer.The total dark current calculated by the model is in good agreement with the measured results.The dark current suppression scheme based on this model is as follows:(1)Decreasing the doping concentration of collection layer can suppress dark current.However,when the doping concentration falls below 1×1115cm-3,the suppression effect on dark current is no longer obvious.In addition,if the doping concentration is continued to be lowered,the material growth is difficult.Therefore,in order to obtain a smaller dark current,the optimum doping concentration of collection layer is about 1×1015 cm-3.(2)Increasing the thickness of collection layer is beneficial to reduce the dark current,but the suppression effect is not obvious at low reverse bias voltages.When the reverse bias voltage exceeds 1.5V,increasing the thickness of collection layer can significantly suppress dark current.3.An InP-based photodetector with wide spectrum,low dark current and high responsivity was proposed and demonstrated.By introducing a P+-InAlAs layer,the loss of absorption at 850nm wavelength excitation is reduced.By introducing the InGaAsP intrinsic layer,the absorption of the device at 850nm wavelength excitation is increased,the energy band structure is smoothed,and the capacitance of the device is reduced.The responsivity reaches 0.43A/W,0.47A/W,and 0.4A/W at 850nm,1310nm,and 1550nm operation wavelengths,respectively.When the reverse bias is 0 to 1V,the dark current of the device is less than 1nA.It is worth noting that the 3-dB bandwidth at 1550nm wavelength is 11.2GHz.4.The frequency dependence of the capacitance-voltage curve of the InP-based wide-spectrum photodetector discussed above is studied.A novel negative differential capacitance is observed at the testing frequencies of 50kHz and 100kHz and disappears at a higher frequency of 200kHz.According to the analysis conclusion of predecessors about the cause of negative differential capacitance,we can estimate that the rate of thermionic emission from deep level centers is less than 200kHz in our device.5.A differential capacitance model is proposed,which is used to analyze the dependence of the capacitance on the optical power:It increases slowly in the optical power range of 0-130 mV,increases rapidly in the optical power range of 130-140 mW,and decreases rapidly in the optical power range over 140 mW.In order to achieve better high-speed response characteristics,the upper limit of optical power for photodetectors is given:130mW.The optical power at the maximum capacitance is close to the optical power at the DC saturation point,and after DC saturation,the capacitance is drastically reduced.With the thickness of collection layer increasing,the maximum capacitance decreases and optical power at capacitance maximum point also becomes small.6.A 3-element and 4-element symmetrical connected uni-traveling-carrier photodetector array are proposed and demonstrated,which are hybrid integrated with subwavelength grating based beam-splitter.The beam splitters for the arrays are designed and fabricated.The experiment completes the hybrid integration of the above two.The maximum RF output power of the 3-element hybrid integrated array at 15GHz is 11.5dBm and the corresponding photocurrent reaches 70mA.The maximum RF output power of the 4-element hybrid integrated array at 15GHz is 13.1 dBm and the corresponding photocurrent reaches 91mA.The hybrid integrated structure realizes the absorption of high-power signal light in multiple photodetector elements,improves the high power performance of the device,solves the phase mismatch problem of the optical signal,and reduces the complexity of the device.