Abstract
本論文主要的目的在探討具有應力補償的反對稱耦合量子井激子在外加 電場的作用下，對入射光的吸收特性，並以這些特性為基礎，設計開發一 些操作在波長為 1.3 微米且對入射光吸收與不吸收具有高對比量的光電 調製器。量子井光電調製器裡，最重要的特性要求便是對入射光束吸收與 不吸收的切換能力，而量子井內激子對入射光的吸收配合外加電場所產生 的史塔克效應（Stark effect），對入射光束通過或被吸收，提供了最方 便且有效的控制方法，因此量子井的史塔克效應便成為研究開發量子井光 電調製器裡最重要的課題。耦合量子井結構本身是由兩個單一量子井中間 隔了一層很薄的能障所組成，由於中間這一層能障相當薄，這兩個單一量 子井內電子或電洞的能階彼此間有很強的作用，所以耦合量子井本身具有 很強的史塔克效應。和傳統的單一量子井結構比較，耦合量子井本身提供 了更多的設計參數，除了量子井寬度及深度外，尚多出了中間能障的寬度 、高度及位置，因此耦合量子井具有較大的彈性來控制量子井內電子或電 洞的次能帶能階位置及波函數，所以耦合量子井非常適用於不同特性之光 電元件的研製。到目前為止國內外已有許多相關的研究群已積極地投入耦 合量子井內的光電特性之理論與實驗上的分析探討，並進一步應用於不同 光電元件的研究開發。另外， 電子、電洞在耦合量子井內的能階位置及 電子電洞的波包函數之間的重疊量隨外加電場的變化也較傳統量子井大， 所以外加電場很容易改變耦合量子井內激子對某一入射光波長的吸收特性 ，這個特性揭示了耦合量子井激子對於對某一入射光波長吸收或不吸收可 以外加電場來控制，因此耦合量子井比起其他傳統量子井結構更適合用於 開發研製具有入射光吸收與不吸收高對比量的光電調製器。 然而，一般 的光電調制器的吸收係數與入射光的極化方向有密切關係，不同極化方向 的光有不同吸收量，這種差異主要來自輕電洞與重電洞在量子侷限效應中 有不同的光極化選擇定理；電子第一能階到重電洞第一能階的躍遷決定橫 向電場波 (TE) 的光吸收，而電子第一能階到輕電洞第一能階的躍遷決定 橫向磁場波 (TM) 的光吸收。光電調制器的吸收係數若與入射光的極化方 向有關，這會導致光電調制器只能針對特定極化方向的光才會有反應，往 往限制調制器的應用範圍；經由理論及實驗數據知，若要得到光吸收係數 與入射光的極化方向無關的光電調制器必須改變結構之應力，藉由伸張應 力 (tensile) 及壓縮應力 (compressive) 可以改變輕電洞與重電洞的能 帶變化，進而改變量子井能帶電位，得到一個與入射光的極化方向無關的 光電調制器。在實際的光電調制器均會採用多次重複磊晶成長同樣的量子 井結構，以提高光吸收量，若只用單一應力之量子井結構，則重複成長的 次數不能太多，否則晶格會因為應力太大而破裂；採用耦合量子井兩個量 子井結構，一個使用伸張應力，另一個使用壓縮應力，兩者交互成長，可 以減低晶格應力，重複磊晶成長的次數便可大大提高，因此這個要素也是 本論文研究的課題。基於以上各要素，本論文針對磷化銦/砷化銦鎵/磷化 銦/磷砷化銦/磷化銦具有應力補償的反對稱耦合量子井結構，有系統的研 究並設計出操作在波長為1.3微米、與入射光的極化方向無關、可適合在 低電場操作且吸收係數對比可高達 6 的光電調制器。 A new alternative-strain compensated InP/InGaAs/InP/InAsP/InP polarization independent anti-symmetric coupled-quantum-well (CQW) structure with both blue and red quantum-confined Stark shift for the first heavy-hole-to-electron (or the first light- hole-to-electron) excitonic transition is studied theoretically in this thesis. In the anti-symmetric coupled-quantum-well (CQW), an anti-symmetric-like pair of potential profile between the shallow-deep conduction band profile and the deep-shallow valence band profile is formed. The subband eigenenergies and the associated envelope wave functions in the CQW structures with or without applied electric field are calculated by the transfer matrix method. The effects of the strain on the pseudomorphic layers has been taken into account. The influence of various anti-symmetric CQW structure parameters such as the total well width, the individual well width, the central barrier thickness and the composition of the strained layer on transition energy, QCSE, the envelope wavefunction overlap, and on/off ratio are studied systematically. In addition, the tradeoff between the polarization-independent requirement and the strength of the blue Stark Effect has been discussed. Results indicated that: First, the use of the tensile-strained well is to help to meet the polarization-insensitive condition while that of the remaining compressive-strained well and middle barrier is to help tuning the transition energy to the desired wavelength and simultaneously achieving larger blue shift and wavefunction overlap. Second, in either blue- or red- shift applications, the alternative-strain compensated anti-symmetric polarization-independent CQW structure could gather around superior properties to design the polarization independent electroabsorption modulator because the novel structure could give large enhancement of both blue and red Stark Effect. The design of the novel polarization-independent electroabsorption modulator operated at 1.3mm utilizing either blue or red Stark Effects in the alternative-strain compensated polarization-independent anti-symmetric couple quantum well (CQW) is proposed and studied theoretically in this thesis. The InP/In0.379Ga0.621As(40A)/InP(6A)/InAs0.4P0.6(40A)/InP alternative-strain compensated polarization-independent anti- symmetric CQW is adopted. A value of as high as 5.7 can be achieved by either blue or red Stark Effects shift of the anti- symmetric CQW in the electric field range of 0 to 125kV/cm for blue-shift and 0 to -250kV/cm for red-shift applications. Also, we use In0.409Ga0.591As(52A)/InP operated at 1.3 mm to design the modulator for comparison. Our result shows that in virtue of our novel strain compensated polarization independent anti- symmetric CQW structure, the performance of our novel device, such as switching energy, drive voltage, and the net strain in the device, could be dramatically improved in comparison with the conventional InGaAs/InP SQW modulator.