Research Of Novel Uni-traveling-carrier Photodetectors For Fiber-optic Communication Systems

Photodetectors(PDs)with high-speed,high-responsivity and high output saturation current are essential components in many fiber-optic communication systems.However,the conventional PIN-PDs,whose output saturation current is limited by space charge effect and the trade-off exists between bandwidth and responsivity,can not meet the the requirements of fiber-optic communication systems.In order to solve this problem,uni-traveling-carrier photodetector(UTC-PD)with unique epitaxial structure in which only electrons are used as active carriers for enhancing frequency response,saturation output current,is presented.The UTC-PD,which can be widely used in ultrafast optical gate,transmitter,high-speed measurement systems,millimeter and submillimeter-wave generator in addition to using as a photoreceiver for ultra-high bit-rate communication systems,has become a hot topic in semiconductor field.Based on the UTC-PD theory,this thesis mainly focuses on the structure design,performance simulation,experimental design,fabrication,quasi-monolithic integration and measurement of UTC-PD used in fiber-optic communication systems.The main contributions are as follows:1.A novel modified uni-traveling-carrier photodetector(MUTC-PD)with Gaussian gradient doping distribution in absorber and collector is proposed to achieve high-speed and high saturation output current.And its parameters are being carefully studied,such as char factor,peak doping concentration,the location of peak doping concentration and doping concentration of cliff layer related to Gaussian gradient doping.For a device area of 14 ?m2,the simulated results are as follows at the wavelength of 1550 nm under the reverse bias voltage of 1.5 V.(1)The proposed MUTC-PD has achieved a 3dB bandwidth of 195 GHz at the char factor of 0.052 and the peak doping concentration of 6.08×1018cm-3 in absorber.Compared with conventional UTC-PD,the bandwidth performance of MUTC-PD is improved,while its responsivity remains the same as 0.11 A/W.Additionally,the location of peak doping concentration shows that the more it keeps close to the P ohmic contact,the better the bandwidth will be.The MUTC-PD achieves the bandwidth of over 200 GHz and the output saturation current of?50 mA when the doping concentration of cliff layer is increased to 4×1018 cm-3.(2)The bandwidth and the output saturation current of MUTC-PD will increase by 4.5%(195 GHz vs.199.5 GHz)and 11.5%(34.9 mA vs.38.9 mA)whel the average doping concentration arrives at 5×1017 cm-3(the corresponding peak doping concentration is 1.92×1018 cm-3),if we keep the Gaussian gradient doping distribution in absorber unchanged and the doping concentration of cliff layer of 3×1018 cm-3.(3)The bandwidth of the MUTC-PD can be maintained over 160GHz when the injected light is about 2×106 W/cm2,which shows the good ability to deal with the high injected light.2.A hybrid absorber uni-traveling-carrier photodetector(HA-UTC-PD)is presented and explored to achieve high speed,high responsivity and high output saturation current under the circumstance of zero bias voltage and low bias voltage.For a device area of 14 ?m2 and the absorber thickness of 220 nm,the simulated results are as follows when the illuminated light wavelength is 1550 nm.(1)The HA-UTC-PD has been obtained the maximum bandwidth of60.6' GHz under the zero-bias condition when the optimized intrinsic absorber thickness is 64 nm.Besides,the maximum bandwidth of 110.9 GHz,104.1 GHz and 120 GHz with the bias voltage of-0.5V,-1.0 V and-2.0 V has been achieved when the optimized intrinsic absorber thickness is 150 nm.(2)The parameter of Bandwidth Saturated Current Product(BSCP)is employing to evaluate the operation performance of HA-UTC-PD.The BSCP and responsivity of HA-UTC-PD are 1916 GHz·mA and 0.21 A/W under the optimized intrinsic absorber thickness of 64 nm at the zero-bias condition.Moreover,the BSCP of HA-UTC-PD are 3660 GHz·mA,4807 GHz·mA and 6508 GHz·mA under the bias voltage of-0.5 V,-1.0 V and-2.0 V when the optimized intrinsic absorber thickness is 150 nm,while the responsivity remains the same,is still 0.26 A/W.3.The high-speed and high responsivity UTC-PD structure has been designed and optimized based on MUTC-PD and HA-UTC-PD design theory while the Gaussian gradient doping distribution is introduced in absorber and the intrinsic absorber is optimized.Then,the epitaxial growth,quality measurement,fabrication and relevant performance measurment of the UTC-PD have been completed and the results are as follows.(1)The fabricated UTC-PD with a 40?m diameter shows the dark current of 8.0 nA,the responsivity of 0.68 A/W and a 11 GHz 3dB bandwidth at the wavelength of 1550 nm under the reverse bias voltage of 3.0 V.(2)The responsivity of the fabricated UTC-PD can be increased by 3%,4.4%and 3.1%at the wavelength of 1550 nm,1560 nm and 1590 nm,respectively,when the reverse bias voltage is ranged from 1.0 V to 6.0 V.The increased responsivity derives from the collision ionization effect combined with the Franz-Keldysh effect in the depleted absorber,which can be overcome by adopting a thinner depleted absorber.4.In order to solve responsivity-bandwidth trade-off in UTC-PD,a novel UTC-PD(UTC-PD with high reflectivity DBR mirrors,HR-UTC-PD)is proposed by employing benzocyclobutene(BCB)wafer bonding technology integrated with GaAs/AlGaAs-DBR stacks.A 40-?m-diameter HR-UTC-PD is measured and the results are as follows.(1)The measured responsivity of HR-UTC-PD is up to 0.88 A/W,a 24.4%increase than that of UTC-PD wihout high reflectivity DBR mirrors at a wavelength of 1560 nm under the reverse bias voltage of 3.0 V.(2)The 3dB bandwidth of 13.87 GHz is obtained with the 1550 nm illuminated light at a reverse bias of 6.0 V.Besides,the RF output power of-3.52 dBm at 10 GHz has been achieved while the output saturation current is 50 mA.5.A novel quasi-monolithically integrated UTC-PD structure bonded to the Non-periodic Concentric Circular Subwavelength Grating(NP-CC-SWG)with 85.4%reflectivity is proposed to overcome the responsivity-bandwidth trade-off in UTC-PD.This quasi-monolithic structure is realized by employing BCB bonding technology and the measured results of a 40-?m-diameter device are as follows.(1)The responsivity of 0.86 A/W and the bandwidth of 18 GHz can be obtained from the fabricated device at a wavelength of 1550 nm under the reverse bias voltage of 3.0 V.Compared with the conventional UTC-PD,the responsivity of the quasi-monolithic device has been increased by 36.5%.(2)The RF output power of-1.77 dBm at 10 GHz and the output saturation current of 17.56 mA have been obtained at a 6.0 V reverse bias voltage,while the 3 dB bandwidth is near to 18 GHz.