反射式量子井光調變器之研究

Abstract

本篇論文之目的在於實現一個廣泛應用於將來的光電系統之中的反射 式光調變器．這裡針對兩個主要的研究課題來進行研究：低電壓操作及多 波段操作能力．此外，我們也研製了雙波段操作能力．此外，我們也研製 了雙波段的反射器以因應未來可能的應用． 在操作電壓的研究中，我 們採用堆疊的結構來進行元件設計．首先，我們對相同的砷化鎵╱砷化鋁 鎵量子井結構置於不同成長起始參雜型態之兩種二極體中進行檢驗，實驗 發現起始於ｐ型成長之量子井結構具有較起始於ｎ型成長之量子井結構稍 強之電場效應，這可能是砷化鎵╱砷化鋁鎵及砷化鋁鎵╱砷化鎵接面於成 長時造成的差異性而引發多重量子井的異向性的結果．在探討堆疊結構將 同時使用的兩種量子井的特性之後，我們實際製作了一個堆疊式調變器來 和傳統的調變器做比較，實驗發現操作電壓從原來的14V 有效的降為5V， 之後，我們採用一個高Q 值共振腔結合堆疊結構的設計，把操作電壓降低 至1.75伏特，並得到一個至今最佳的40%/V 電光轉換效率．對於多波段操 作能力的研究，我們提出了一個多共振的結構來進行系統化的分析，並實 際針對雙波段的調變器發展出三種不同的共振腔設計，分別是簡單式共振 腔設計，平衡式共振腔設計，無耦合式共振腔設計，從簡單式共振腔到平 衡式共振腔設計，兩個波段於操作電壓上的差距可以獲得極大的改善，並 且可以藉由此種於長波段低反射的中間反射器設計來獲得較簡化的共振條 件．經由適當的設計中間及前置鏡面，另一型無耦合式共振腔設計得以實 現並且使各波段之等效工作共振腔完全隔離，並因而得以於實際應用中提 供更大的彈性，並且使元件的成長相較於之前的兩種形式更加簡單化，經 過最佳化的設計後，我們獲得可於860nm 及890nm 兩波段操作的雙波段光 調變器，其操作電壓分別為3.4V及6.2V，個別之反射率變化則為62% 及45%．針對反射式光調變器於自發光效應元件上應用之重要性，我們也 將此種雙波段設計應用於常關型之調變器上，實驗上觀察到清晰的雙波段 性能，元件在從正偏0.9V至負偏5V的操作下可分別於859nm 及907nm 波段 獲得約20% 的反射率變化．我們也提出兩種可行的雙波段自發電光效應元 件，可簡化光學對準及提高積體密度．基於各種光電元件對內建反射器之 強烈需求，我們利用砷化鎵╱砷化鋁多層結構設計了雙波段反射器，基本 上，此種反射器結構上利用了相當的相位改變機制來引發雙波段的性能， 實驗結果顯示與理論預期十分一致的結果． The purpose of this dissertation is to implement a reflection modulator which is applicable to future optoelectronic systems. Two primary subjects are studies: the low-voltage and the multi- wavelength capabilities.In addition, dual-wavelength reflectors are also demonstrated for possible applications in dual- wavelength optoelectronic devices. In the study of low operating voltage, the stacked structure is adapted. Two similar GaAs/ AlGaAs quantum well structures in the intrinsic regions of the two p-i-n diodes, one statrting growth from n-layer and the other one from p-layer, are first characterized. A little bit more pronounced field effects are observed in then-i-p-substrate diode. This may result from the anisotropy of the MQW structure due to the difference between the AlGaAs/GaAs and the GaAs/ AlGaAs interfaces caused by crystal growth. After the investigation, a stacked modulator composed of two face-to-face diodes is fabricated. The operating voltage is greatly reduced from 14V, for the conventional modulator with same total QW's, to 5V. With a high-finesse cavity design, a further improvement in operating voltage to 1.75V has been obtained. A state-of-the- art electronic-optical conversion efficiency up to 40%/V has also been achieved. As to the multi-wavelength capability for a single device, a systematic analysis is given based on a multi- cavity configuration. Three different design schemesincluding the simple cavity design, the balanced cavity design and the decoupled cavity design are developed for two-wavelength modulators. From the simple cavity design to the balanced cavity design, the difference in operating voltage of the two wavelengths can be greatly reduced. Simpler matching condition for the long wavelength is also provided in the balanced cavity design due to the transparent property of the middle mirror at the long wavelength. By properly designing the reflective property for both the front mirror and the middle mirror, the decoupled cavity design is built and can make each cavity in the whole two-wavelength structure equivalently isolated from each other for each wavelength. Besides, the decoupled cavity design can also provide more flexibility in real applications and also make the device growth much easier compared to the former two designs. For an optimized design, device operating at 860nm and 890nm with separate operating voltage of 3.4V and 6.2V is achieved with reflectivity changes of 62% and 45% respectively. For the important application of reflection modulators in SEED' s, we apply the methodology of two-wavelength design to a normally-off modulator. A clear two-wavelength operation is observed with reflectivity change of ~20% obtained for 859nm and 907nm under a bias change from +0.9V to -5V. We also propose two possible and accessible configurations for applications in S- SEED's that can simplify the optical alignment and increase the packing density. Based on the demand for built-in reflector in optoelectronic devices, dual-wavelength reflectors with GaAs/ AlAs multilayers are designed. Basically, the reflector structures incorporate proper phase changes to cause two-band reflection. The experimental results show good agreement with the theoretical expectations.