在現代光通訊系統中，傳統之垂直入射型光電探測器（normal-incidence photodetectors）普遍面臨頻寬與量子效率（quantum efficiency, QE）之間的設計取捨，特別是在長波長高資料傳輸速率之應用下，該限制尤為嚴重。例如，目前市售操作於1310 nm波長且支援25 Gbps傳輸速率之光電探測器，其最大響應度（responsivity）通常受限於0.8 A/W以下。為克服上述挑戰，本研究設計並展示一高速度、背面入射（backside-incidence）、臨界耦合（critically-coupled）之SOI（silicon-on-insulator）基板上鍺（Ge）光電探測器，成功實現高量子效率與高頻寬之雙重優勢。該元件在-1 V偏壓條件下，量測所得之體（表面）暗電流密度分別為13 mA/cm²（0.79 µA/cm）。此外，針對直徑20 µm之元件，於1310 nm波長下量測得到響應度為0.87 A/W，且其3 dB光學頻寬達26 GHz。
進一步地，本研究提出一創新之光酸擴散（photon-acid diffusion）技術，成功實現高速度、多通道（four-channel）光接收器之聚合物微透鏡（polymer microlenses, MLs）整合。該光接收器採用製作於200 mm矽基板上之垂直入射Ge p-i-n光電二極體（photodiode）。對於直徑29 µm之光電二極體，搭配直徑54 µm之微透鏡，於850 nm波長與-3 V偏壓操作條件下，量測所得暗電流為138 nA，響應度為0.6 A/W，3 dB頻寬為21.4 GHz，有效入光孔徑為54 µm。藉由增大有效入光孔徑，有效緩解光電探測器在孔徑尺寸與頻寬之間的設計取捨，並大幅提升光纖與偵測器在模組對準容差（optical fiber misalignment tolerance），由傳統之±5 µm擴展至±15 µm，進一步優化模組封裝容忍度。
此外，本研究所開發之光接收器，於資料傳輸速率為25.78 Gb/s，非歸零（non-return-to-zero, NRZ）編碼下，達成10⁻¹²位元錯誤率（bit error rate, BER）之靈敏度為-9.2 dBm。元件可靠度測試結果證實，本研究製作之鍺光電二極體整合聚合物微透鏡後，符合GR-468可靠度標準。此一新穎之光接收器為業界首創，展現卓越之元件特性，包括高性能、高良率、高產能、低製造成本，且具備與CMOS製程兼容之優勢，極具應用於400 Gb/s脈衝振幅調變四階（pulse-amplitude modulation four-level, PAM4）光通訊系統之潛力。

In contemporary optical communication systems, conventional normal-incidence photodetectors often face a tradeoff between bandwidth and quantum efficiency, particularly detrimental for long-wavelength communication systems operating at elevated data rates. For instance, the maximum responsivity of commercially available 25 Gbps photodetectors at a 1310 nm wavelength is restricted to below 0.8 A/W. To address this challenge, we have designed and demonstrated a high-speed, backside-incidence, critically-coupled Ge on SOI photodetector featuring improved quantum efficiency and bandwidth. The photodetector exhibits a low bulk (surface) dark current density of 13 mA/cm² (0.79 µA/cm) at a -1 V bias. It demonstrates a responsivity of 0.87 A/W at 1310 nm with a measured 3 dB optical bandwidth of 26 GHz for a 20 µm diameter device. Additionally, we introduce a novel photon-acid diffusion method that facilitates the integration of polymer microlenses (MLs) on a four-channel, high-speed photo-receiver comprised of normal-incidence Ge p-i-n photodiodes fabricated on a 200 mm silicon substrate. For a 29 µm diameter photodiode capped with a 54 µm diameter microlens, we measured a dark current of 138 nA, a responsivity of 0.6 A/W, a 3 dB bandwidth of 21.4 GHz, and an effective aperture size of 54 µm at -3 V bias and a wavelength of 850 nm. The enlarged aperture significantly mitigates the tradeoff between aperture size and bandwidth and enhances optical fiber misalignment tolerance from ±5 µm to ±15 µm, thereby improving module packaging tolerance. The sensitivity of the photo-receiver was determined to be -9.2 dBm at a data rate of 25.78 Gb/s with a bit error rate of 10⁻¹² using non-return-to-zero (NRZ) transmission. Reliability tests validate that the fabricated Ge photodiodes integrated with polymer microlenses comply with the GR-468 reliability assurance standard. This unique photo-receiver, as the first of its kind, presents excellent performance, high yield, high throughput, low cost, and compatibility with CMOS fabrication processes, making it a promising candidate for applications in 400 Gb/s pulse-amplitude modulation four-level (PAM4) communication systems.

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