Research On Broadband High Power Uni-Traveling-carrier Photodiode For Optical Communication System

With the rapid development of the wireless communication services,there is an urgent need to increase the transmission rate and capacity,radio over fiber communication system emerges as the times require.To improve the data transmission capacity of the system,the photodetector in the optical receiver is required to have the characteristics of broadband and high power output.The recent developments of photodetectors in optical communications systems points out uni-traveling carrier photodetectors(UTC-PDs)is a unique structure that can achieve broadband and high power output performance.The UTC-PD is expected to be used in radio over fiber communication system,radar and satellite communication system,high-speed measurement system,ultra-high-speed optical switch,and become a research hotspot in the field of high-speed optoelectronic devices.This paper focuses on the design,optimization,and fabrication of UTC-PDs with broadband and high output power,while low power consumption.The main research contents and innovations are as follows:1.Based on traditional structure,the high-speed zero bias operational UTC-PDs was modified by following methods.The graded doping distribution and graded bandgap material were introducing into the absorber.The bipolar doping profile was used and the graded-bandgap spacer was inserted between the absorber and the collector.The very low doping concentration was set in the collector,and the doping concentration of the absorber and the spacer were optimized.The thickness of the collector and the spacer were also optimized.2.The main factor restricting the performance of the traditional back-to-back photodetector is the total capacitance,and the capacitance of the bottom photodiode is almost twice that of the top photodiode.Therefore,a novel symmetrical back-to-back structure was proposed,in which the active area of the bottom photodiode is equaled to the top one.Both the bandwidth and output power performance of the proposed structure are improved,compared with traditional back-to-back UTC-PD under the same conditions.3.A charge-compensated UTC-PD with optimized graded doping profile was proposed for high power millimeter wave or sub-terahertz wave generation at zero-and low-bias operation.The charge-compensated UTC-PD can be used to restrain the bandwidth attenuated with the increase of photocurrent,and its bandwidth at high power input is improved,compared to other structural or experimental results.The peak output-power of the device has enhanced at least 7 dB even at 170 GHz and zero-or low-bias operation compared with the structure that has 1014 cm13 doping concentration in the collection layer.4.A novel UTC-PD with mushroom mesa was proposed.The diameter of the absorber in the structure was larger than that of the collector.The optimum performance was obtained by numerical calculation and physical theory when the ratio of the diameter of the collector layer to the diameter of the absorber layer was 0.9.The bandwidth of UTC-PD with mushroom mesa is increased by 4.3%and the RF output power in the linear region is increased by 0.6 dB when the diameter of absorption layer is 5 jam,compared with the traditional structure.The suggested structure can alleviate the contradiction between external quantum efficiency and bandwidth to some extent when miniaturized junction size is required.5.A kind of UTC-PD which can detect the repetition frequency of optical pulse covering microwave to terahertz-wave and convert intense optical pulse into intense electric pulse is designed.The peak output-power of the designed zero-bias operational UTC-PD at 100 GHz,200 GHz and 312.5 GHz pulse train excitation can reach 4.685 dBm,1.128 dBm and-4.653 dBm,improved by 2.05 dB,5.15 dB and 9.36 dB,respectively,compared with the reported device.6.In order to reduce the series resistance,thereby reducing the thermal effect and increasing the RC limited bandwidth of the photodetector,a process for fabricating low ohm contact resistance was presented.Through experimental and theoretical analysis,it is found that the lateral resistance of P-type contact layer is much larger than that of N-type contact layer.Therefore,the conductive type of the contact layer on the top of the photodetector mesa is P-type,and the choice of metal-electrode fully-covered the contact layer is benefit to preparation of high-performance photodetectors.The PIN or UTC Photodetectors with ring-covered P-contact electrode,and PIN or UTC Photodetectors with fully-covered P-contact electrode were fabricated,and the above deduction is proved by experiments.7.The zero-bias operational UTC-PD with traditional and improved structures were fabricated for high-speed communication system.The experimental results show that the modified UTC-PD has improved DC saturation and dark current performance,and its bandwidth is improved when the photocurrent is less than 2 mA,compared with the traditional UTC-PD under the same conditions.The 3dB bandwidth of the improved UTC-PD with 20?m and 14?m mesa diameter can achieve 20.8 GHz and 40 GHz,respectively,under zero-bias operation.The device may find applications in 40 Gb/s optical communications systems,where broadband optical receivers with low power consumption and high-density integration are highly desired.The 3dB bandwidth of the improved UTC-PD with 20?m,14?m,and 10 ?m mesa diameters can reach 36.6 GHz,52.2 GHz,and 75 GHz,respectively,when the reverse bias voltage is 1 V.The maximum RF output power of the improved UTC-PD with 14 um mesa diameter under zero bias operation is-20.2 dBm@20 GHz,-22.8 dBm@30 GHz and-24.4 dBm@40 GHz,respectively.The above research results have reached the existing research level at home and abroad.