Investigation Of High Performance Photodetectors Based On Metallic Interdigitated Electrodes With Resonant-Cavity-Enhanced Structure

Photodetectors are the key devices for optical fiber communication, moreover, they are essential component to generate radio signal based on mixing frequency method. With the development of high speed optical fiber communication and THz wave applications, photodetectors with much higher response speed are required. Metal-semiconductor-metal interdigitated electrodes with low capacity are widely used in high speed photodetectors and THz photomixers (THz photomixers is a kind of ultra-high speed photodetectors).Resposivity is a very important parameter of high speed photodetectors, which determines the lowest detectable power of input signal and the lowest THz output of utilization. Resposivity depends on quantum efficiency and transmittance, therefore, to improve the quantum efficiency and transmittance of high speed devices is a key issue that needs to be solved urgently. Resonant-cavity-enhanced (RCE) structure has the advantage of high quantum efficiency in a thin absorbing layer. Furthermore, when the gap of the metallic interdigitated electrodes decreases to subwavelength range, metallic electrodes can also benefit from transmittance enhancement and act as reflector. In this thesis, different metallic interdigitated electrodes structures coupled with RCE structure are proposed, which are high performance photodetectors with high speed and high respnonsivity. The main research work can be summarized as follows:(1) The basic principle of mixing frequency method to generate THz wave is investigated. Based on the operational principle of THz photomixer, the equation of THz wave output is derived. And different parameters for THz output are discussed. It is found that the best method to increase THz wave output is increasing quantum efficiency. Based on the results, common THz photomixer coupled with RCE structure is proposed and the RCE structure effect on THz output is analyzed. Under optimized the structure, the maximum quantum efficiency is93%, which corresponding THz output ratio of the RCE structure and common structure is about8in the low frequency range.(2) Transmittance equation of subwavelength metallic single slit is derived. Various parameters influence on transmittance is analyzed in detail. Taking the advantages of three functions offered by subwavelength metallic slit, which are forming slit cavity mode resonance, acting as the top reflection mirror and providing low capacitance electrode, a cascaded-cavity structure photodetector with subwavelength metallic slit is proposed. The metallic slit effect on the performance of photodetector is analyzed. The first order resonant peaks don’t depend on the slit width at320nm slit height. Simulation results show that, for60nm slit width,80times responsivity enhancement can be achieved compared to the conventional structure. In addition, its3dB bandwidth can reach to150GHz.(3) Based on subwavelength metallic single slit photodetector with limitations in many applications and subwavelength grating also with previous three functions, in addition, a thin metallic layer with high reflectivity and simple manufacturing process, a cascaded cavity structure photodetector with subwavelength metallic grating is proposed. The optimized SMG structure with820nm period,220nm slit width and272nm heights can bring about27times responsivity enhancement compared to the conventional photodetector. In addition, its3dB bandwidth can reach to220GHz.(4) As defect subwavelength metallic grating can be used to enhance light transmittance and reduce top reflectance, a RCE structure photodetector coupled with defect subwavelength metallic grating is proposed. Refractive index of defect part influence on performance is mainly focused on. With optimized the parameter, responsivity can be increased4.5times, compared to the photodetector with uniform subwavelength metallic grating.