| Summary: | 碩士 === 國立臺灣大學 === 光電工程學研究所 === 94 === Photodetector (PD) efficiency decreases as bandwidth increases and efficiency- bandwidth (EB) product is an important figure of merit in ultra-high speed PD performances. Generally speaking, side-illuminated PDs have larger EB product than traditional vertically-illuminated PDs. To further increase the EB product over the intrinsic limit of side-illuminated PDs, we here utilize an old effect which recently provokes great interest in nanoscale devices: surface plasmon resonance (SPR). By properly designing optical waveguides to enhance SPR, the inherent tradeoff between efficiency and bandwidth limitation is diminished.
In this thesis, we first demonstrate ways to introduce surface plasmon resonance on a side-illuminated PD. Also, design and analysis of optical and electrical waveguides are made and can be served as design guidelines for devices that introduce surface plasmon resonance. We proposed a novel type of PDs: surface plasmon-enhanced metal-semiconductor-metal traveling wave photodetectors (SPE MSMTWPDs). SPE MSMTWPD is a new kind of MSMTWPD. By ingeniously introducing SP resonances, the field in the active region can be enormously enhanced, which can compensate inefficient optical absorption of the thin absorption layer. That is to say, input power can be completely absorbed in a short device length even though the absorption layer is quite thin. The fact that the absorption layer can be considerably thin means the carrier transverse distance can be small, which reduces carrier transit time. Also, by using nanoscale electrode arrangements of interdigitated fingers to further shorten the carrier transverse distance and increase the drift field, the space charge effect can be greatly diminished, simultaneously increasing the speed and efficiency of the device. As to the microwave properties, it preserves MSMTWPD’s low microwave loss, high microwave propagation velocity, and good impedance matching between the detector itself and the external load circuit. With respect to material, instead of low temperature grown GaAs, one can just use material with high saturation velocity at high field to increase device speed. With these advantages, SPE MSMTWPDs show potential to attain high-efficiency, high-speed performance and are also of great flexibility in choosing material for various applications.
Besides, we built up a practical and comprehensive simulation system to design the novel devices. In this system, the theory of a photo-distributed current model with a microwave transmission line structure was applied to study microwave properties of the novel devices. By utilizing the transfer matrix method to design optical waveguides, one can quickly figure out the optical field distribution and the optical power transfer between guiding modes in optical waveguides. Based on the established simulation system, if other materials are utilized for various applications such as fiber-radio communication system and terahertz (THz) transmitters, the presented design issues can also be served as guidelines for designing new devices.
Two types of SPE MSMTWPDs were investigated in this thesis. One is the device with designed diffraction gratings to enhance filed in the active region (Grating Type), while the other is the device with an optimum-tuned thin dielectric overlayer (Matching Film Type). Generally speaking, Grating Type devices could be useful for broadband applications and investigations about diffraction grating effects on SP waves with optical waveguides. Matching Film Type devices show potential to attain both high efficiency and high speed performances if exact control of the overlayer refractive index is available.
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