| 摘要: | 在此篇論文中，我們成功的實現幾種能操作於兆赫茲頻率(THz)之單載子光偵測器(uni-traveling-carrier photodiodes, UTC-PD).首先，其中之一為近彈道單載子光偵測器(Near-ballistic uni-traveling-carrier photodiode, NBUTC-PD)，藉由P型電場控制層(P-type charge layer)調整電場分佈維持電子在最高飄移速度(overshot drift-velocity)以達到高速頻寬表現. 此元件在兆赫茲頻率操作下之最佳偏壓通常位在-2V。然而，在此高偏壓操作下，高功率輸出時亦承受更高機率的熱失效(Thermal failure)。
為了解決此問題，我們提出另一種創新的設計N-UTC-PD，藉由在收集層插入N型電場控制層(N-type charge layer)重新最佳化電場分佈進一步加強抑制空間電荷屏蔽效應(Space charge screening effect, SCS)。此結構即使在較低的電場即-1V偏壓操作下，電子在跨越收集層時依然維持最高飄移速度(overshot drift-velocity)。總結來說，對新結構N-UTC-PD來說，它仍然具有優異的高頻寬表現(~315GHz)，但它在功率輸出表現上會有些許衰減(-3dBm@280GHz)。而對NBUTC-PD來說，雖然它有較佳的輸出功率(-1.8dBm)及頻寬表現(~325GHz)，但設計上需要更有效的散熱方式來避免熱失效。
然而，為了獲得超高速頻寬操作，設計上會使用較薄之吸收層，此類光偵測器會極大的犧牲響應度(Responsivity)進而造成光的吸收效率嚴重降低。為了解決此問題，我們藉由改良磊晶結構來獲取較高的響應度表現。最新設計的另一種GaAs0.5Sb0.5/In0.53Ga0.47As II型混合吸收層之單載子光偵測器。此3μm直徑主動區元件經由覆晶鍵結封裝後從量測結果可得知，與現有相關長波長(1.3–1.55μm)之光偵測器研究相比，亦具有適度合理之響應度值0.11A/W (NBUTC-PD:0.08A/W, N-UTC-PD:0.09A/W), O-E頻寬操作可高達0.33THz。 最後，我們成功實現此結構之光偵測器在正弦波封包(Sinusoidal envelope)產生之射頻頻率(Radio frequency, RF) 0.32 THz及63% 調製深度之光訊號操作下，在 −1 V偏壓擁有13mA之飽和電流及-3dBm毫米波功率輸出。
透過使用此種光偵測器，我們也成功實現具有新穎前端波導耦合(Waveguide, WR6)設計之超寬頻、整合型光子傳輸器( Photonic transmitter)；並藉由創新設計之高增益雙邊山脊型號角天線(Dual-ridge horn antenna)及設計平面電路波導激發，此傳輸器頻率操作可達紀錄之0.1至0.3THz帶寬，在無線傳輸過程中維持0.24THz 頻率操作下，接收端亦偵測高達31.6 W之毫米波輸出功率
;In this thesis, we demonstrated several kinds of uni-traveling-carrier photodiodes (UTC-PDs) for THz operation. One among such is near-ballistic uni-traveling-carrier photodiode (NBUTC-PD) with P-type charge layer in order to sustain electrons overshot drift-velocity for high-speed bandwidth. The optimum bias for THz operation of this device is usually located at −2 V. However, it has high probability of suffering from higher thermal failure under high output power due to high bias voltage.
For the sake of solving this issue, we report another kind of novel collector design by inserting N-type charge layer, N-UTCPD, which optimizes the profile of electric-field distribution in order to further suppress serious Space charge Screening (SCS) effect. The electrons will drift across the collector layer with overshoot velocity even under lower electric-field operation at −1V. Overall, for new structure N-UTC-PD, it still has superiorly high bandwidth (~315GHz) performance, but the new structure also leads to degradation of output power(-3dBm@280GHz); for NBUTC-PD, it has better output power (-1.8dBm) and bandwidth (~325GHz), but this kind of photodiode must need better heat dissipation to avoid thermal failure.
However, by using thin absorption layer for ultra-speed bandwidth operation, low responsivity performance in these ultrafast PDs is a big issue. For this reason, we demonstrate a GaAs0.5Sb0.5/In0.53Ga0.47As type-II hybrid absorbers UTC-PD by improving the epi-layer structure to get higher responsivity. According to the measurement results, the flip-chip bonding packaged device with active diameter of 3μm shows moderate responsivity of 0.11A/W (NBUTC-PD:0.08A/W, N-UTC-PD:0.09A/W) along with the recorded, really wide 3-dB optical-to-electrical bandwidth reaching 0.33 THz, among all those reported for long wavelength (1.3–1.55μm) PDs. A 13-mA saturation current and a continuous wave output with power as high as -3 dBm at −1 V are successfully demonstrated at an operating radio frequency (RF) of 0.32 THz under an optical signal with a sinusoidal envelope and ∼63% modulation depth for PD excitation.
By use of such kind of PDs, the broadband, integrated photonic transmitter front-end with a novel waveguide-coupled (WR6) is demonstrated. According to the measurement results with the novel design in high gain dual-ridge horn antenna and planar circuit for waveguide excitation, such transmitter achieves recorded-wide fractional bandwidth (100%; 0.1 to 0.3 THz) and high detected power in the receiving-end (31.6 W) at 0.24 THz through wireless transmission. |
