| With the continuous development of information technology,5G mobile communications,the Internet of Things,and various interactive multimedia communications services have emerged.Optical communication system as the main support for modern communication networks,its transmission capacity and transmission rate have increased exponentially over time.As the key element of high-speed optical communication system,photodetectors need to meet higher development requirements for their response bandwidth and output power performance.Compared with the traditional PIN photodetector,the uni-traveling carrier photodetector(UTC-PD)is faster and has a larger saturation photocurrent and a lower working reverse bias voltage,which is currently a research hotspot in the field of semiconductor optoelectronic devices.This thesis focuses on the study of optoelectronic mixing and phase shift characteristics of the UTC-PD,and the novel photodetectors.The main research results and innovations are as follows:1.The optoelectronic mixing characterizations of the UTC-PD are studied and three different types of optoelectronic mixing modes in UTC-PD,including fundamental mixing,higher-order harmonic mixing,and radio frequency(RF)sub-harmonic mixing,are analyzed.The RF output power bias voltage dependence characteristics of a 60 GHz optical signal with 100%modulation depth are analyzed.It is found that the output power of the 60 GHz signal and its higher-order harmonic signals increases with the reverse bias voltage within a certain bias voltage range.The methods that can significantly improve the power of the optoelectronic mixing output signal are proposed,including setting the peak-to-peak value of RF signal at the bias end in the optimized bias voltage range where the RF output power can obtain a larger increment,and increasing the power of the optical signal to make the increment of the RF output power in the same bias voltage range is further increased.Besides,it is found that when the frequency of the optical signal(or electrical signal)is certain,selecting the frequency of another signal appropriately to make different mixing modes products are in the same target frequency can also enhance the optoelectronic mixing product.2.We have designed and fabricated a UTC-PD.The RF output power bias voltages dependence is tested and analyzed,the optical power and the bias voltage range for optoelectronic mixing is determined.An optoelectronic mixing test system is designed for the UTC-PD on-chip.Based on the UTC-PD with a diameter of 34μm,the fundamental mixing and the 2nd harmonic mixing output of a 3 GHz optical signal and a 60 MHz RF signal are realized.Based on a UTC-PD with a diameter of 24μm,the fundamental mixing and higher-order RF subharmonic mixing products of a 10 GHz optical signal with an RF signal with the frequency range from 50 MHz to 2 GHz are obtained,respectively.The three different optoelectronic mixing modes of the UTC-PD are obtained and the proposed methods that can improve the power of the optoelectronic mixing output signal significantly are proved experimentally.3.Since the AM-PM of UTC-PD exhibits severe optical power dependence characteristics,obtaining the proper working conditions with a low AM-PM coefficient is very important for the application of UTC-PD to generating ultra-stable microwave signal.For this issue,the relationship between the phase and the AM-PM coefficient of the RF output of a 60 GHz optical signal with a 100%modulation depth with the optical power of the UTC-PD is studied.According to the analysis of the equivalent circuit and the carrier dynamic transport process of the UTC-PD,the relationship between the dynamic differential capacitance of the device and the AM-PM coefficient is found.It is proposed that the optimum working conditions of UTC-PD with the AM-PM coefficient at the null point can be obtained by locating the minimum value of the dynamic capacitance of the device when the optical power changes.4.The phase adjustable and controllable characteristics of PIN-PD and UTC-PD are studied intensively.A method to adjust and control the phase of the device by changing the injected optical power and the bias voltage is proposed.Experiment results show that the phase change of UTC-PD is quite sensitive.The adjustable phase change of the output signal of 10 GHz can reach 50 degrees by changing the bias voltage or the optical power.For PIN-PD,changing the bias voltage and optical power of the device has little effect on the phase change of the device.The experimental results show that the dynamic impedance and carrier transit time of the device are the main reasons for the phase change variation.The phase change of UTC-PD has a larger adjustable range than PIN-PD is mainly due to electrons having higher dynamic characteristics than holes when the electric field varies.5.The ohmic contact between nickel oxide(NiO)film with transparent conductive characteristics and P-type In0.53Ga0.47As and unintentionally doped In0.53Ga0.47As material is proposed and realized.It is found that the ohmic contact mechanism is mainly due to the small valence band offset between NiO and InGaAs.In the process of the NiO film fabricating through thermal oxidation,it is found that the oxidation time is the key factor affecting the contact characteristics of NiO/In0.43Ga0.47As.The measured transmittance of the fabricated NiO layer is more than 90%at the wavelength of 1550 nm,and the square resistance of NiO films is less than 5 Ω/square.6.A novel InGaAs/InP photodetector with NiO transparent P-region and electrode is proposed by taking advantage of the ohmic contact characteristics between NiO and InGaAs.By applying a transparent conductive layer of NiO,the device does not require a highly doped P-contact layer and a P-contact metal electrode as the traditional PIN-PD.The epitaxial layer can be simplified,and the optical power loss caused by the additional absorption of the highly doped P-contact layer can be avoided at the same time.With the fully-covered transparent P-contact electrode,the photodetector can be illuminated from P-side at the same time,which can solve the problem that due to the opacity of the metal electrode,the shape of the electrode and the direction of optical injection restricted to each other and degrading the performance of the device.The research results show that the 3dB bandwidth can be improved without sacrificing the output power by applying the novel structure.Compared with the original PIN-PD,the 3dB bandwidth of the NiO-I-N PD with an absorption layer of 600 nm has increased by 4 GHz,and the bandwidth of the NiO-UTC-PD with an absorption layer of 220 nm has increased by 5 GHz compared to the original UTC-PD.7.A novel bias-free symmetrically connected two-element UTC-PD array(SC-PDA)is proposed,and its fabrication and measurement have been realized.It is found that the doping concentration of the collection layer has a great effect on the bias-free UTC-PD.When the doping concentration of the N-type collection layer is at the level of 1×10 cm-3 or below,the capacitance of the device under zero bias voltage reaches its minimum value.Compared with the original same size SC-PDA,under zero bias voltage,the DC saturation photocurrent of the device with a diameter of 60 μm has been improved more than 42 mA,and the 3dB bandwidth of the device with a diameter of 15 μm has been improved 12 GHz approximately.The 3dB bandwidth of bias-free SC-PDA with a diameter of 15 μm reaches 19.4 GHz at 0.5 mA.The AC saturation photocurrent and maximum RF output power of the bias-free SC-PDAs with the diameter of 40 μm,50 μm,and 60 μm at the frequency of 3dB bandwidth exceed 9.31 mA and-5.86 dBm,14.52 mA and 1.17 dBm,13.72 mA and-1.76 dBm,respectively. |
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