行波式表面電漿強化光偵測器之模擬與分析

光偵測器的效率隨著其電頻寬的增加而遞減。因此效率－電頻寬的乘積在超高速光偵測器表現中是非常重要的指標。其中，側向入射式的光偵測器較傳統垂直入射式普遍具有較高的效率－電頻寬乘積。為了進一步增加側向入射式光偵測器在效率－電頻寬乘積方面的上限，我們應用一個古老但在近年來引起奈米元件廣大興趣的光學效應：表面電漿共振。藉由適當設計的光波導結構增強表面電漿共振，能有效減輕效率和電頻寬互相牴觸的限制。在這篇論文中，我們首次展示表面電漿共振如何應用在側向入射式的光偵測器上，並且分別針對光波導以及微波結構進行模擬與分析，對於未來應用表面電漿共振的元件能提供設計上的準則。我們提出一種新穎的光偵測器：行波式表面電漿強化光偵測器(SPE MSMTWPD)。SPE MSMTWPD是一種新型的行波式金屬半導體金屬光偵測器(MSMTWPD)。我們巧妙地利用表面電漿共振來增強主動區的電場，補償主動區過薄造成吸光效率不佳的影響。換言之，即使光吸收層相當薄，輸入能量仍可在很短的元件長度下被完全吸收。而吸收層較薄能縮短載子移動距離，因而有效減少載子傳動時間。除此之外，更利用奈米尺寸的電極分布，進一步使載子移動距離變小且增大漂移電場(drift field)，空間屏蔽效應因此被大幅減低，同時提高元件的速度與效率。在微波特性方面，保有MSMTWPD其低微波損耗，高微波速度以及與外加負載電路之間良好阻抗匹配的優點。並且在材料方面我們不需使用低溫成長的砷化鎵(LTG-GaAs)，只需使用在高電場之下有高載子飽和速率的材料，元件即能有高速表現。具有上述特性，SPE MSMTWPD顯露了表現高效率，高電頻寬的潛力，同時在選擇材料方面並擁有很大的彈性。更者，我們建立了一套實用且一般化的模擬系統來設計此元件。我們使用分布式光電流模型來模擬此元件的微波特性。而利用轉移矩陣法(transfer matrix method)來設計光波導能迅速了解光波導中的場形分布以及模態間能量傳遞現象。利用我們所建立的模擬系統，若於各種應用上採用其他材料，例如光纖通訊系統或是兆赫波發射器，此元件的設計與分析亦能當作設計新元件的的準則。本論文中我們探討了兩種SPE MSMTWPD，一種是以散射光柵來增強主動區電場(Grating Type)，另一種則是利用覆蓋一層薄膜於元件上(Matching Film Type)。大致上，Grating Type 元件能應用在需要高頻寬的應用上或是探討散射光柵對於表面電漿波以及光波導的效應。而若能精確控制覆蓋層的光學性質，Matching Film Type元件則展現了同時達到高效率以及高電頻寬的表現。

關鍵字

高速光偵測器；表面電漿

並列摘要

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.

並列關鍵字

high-speed photodetectors ； surface plasmon

參考文獻

List of References

[1] P. S. Henry, “High-capacity light wave local area networks,” IEEE Commun.

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GHz InGaAs multimode waveguide photodetector,” IEEE J. Quantum Electron, vol. 28, pp. 2728-2735, 1992.

[5] J.-W. Shi, Y.-H. Chen, K. G. Gan, Y. J. Chiu, C.-K. Sun, and J. E. Bowers, “High Speed and High Power Performances of LTG-GaAs Based Metal-Semicondutor-Metal Traveling-Wave-Photodetectors in 1.3 μm Wavelength Regime” IEEE Photon. Techno. Lett., vol. 14, pp. 363-365, 2002.

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行波式表面電漿強化光偵測器之模擬與分析

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