高功率邊射型與光子晶體面射型 雷射二極體之研究

Abstract

本篇論文旨在研究以砷化鎵系列材料為基礎所設計製做之雷射元件，其中包含高功率邊射型雷射與面射型的光子晶體雷射。關於邊射型雷射，主要研究為探討結構上的光場侷限對於雷射特性的影響。在條狀窄波導的邊射型雷射中可分別藉由縱向與橫向侷限的最佳化調整，使元件特性提升並達到單模態操作的高功率輸出。在垂直方向的光侷限上，我們採用GRIN-DBSCH的磊晶設計製作出830nm AlGaAs/InGaAs 雷射二極體，其於高功率操作下具有極小的遠場發散角而特徵溫度性質也獲得改善。在DBSCH結構設計下於光波導層與披覆層間插入多階段的漸變結構使得載子侷限提升並有效避免漏電流的發生，而近場端光模態的擴張可以使雷射鏡面出光處的功率密度下降，此對雷射高功率操作大有助益，最大的輸出功率可達21.5 MW/cm2並且提供優異的特徵溫度特性。而水平方向的光侷限實驗方面，我們使用高反射率多層膜的介電披覆層設計可使AlGaInP-GaInP量子井雷射二極體光電特性提升並且維持在一個穩定的遠場發散輸出。三對高低折射率的多層膜批覆層不但可改善金屬吸收效應，高反射介面也能減少散射提升雷射輸出效率，高散熱係數的Al2O3/Ta2O5可改善元件散熱使其穩定的操作於高功率下。其室溫工作下遠場發散角為16.4∘，臨界電流與特徵溫度分別為44.5mA和104.2K。 關於光子晶體結構於光電雷射元件上特性及應用之探討，我們以AlGaAs-InGaAs量子井雷射結構為基礎，利用轉移矩陣法(transfer matrix method)及耦合波理論(coupled-wave theory)先探討於主動層附近不同磊晶層厚度對於閥值增益變化的對應關係。此部分的研究著重於¬¬¬¬¬¬¬¬¬面射型出光的Γ1能帶。此外，光子晶體結構的填充因子也一併列入研究範圍。經過一系列的優化演進，最終結果顯示，對於砷化鎵光子晶體面射型雷射而言，光子晶體層的垂直光學侷限率可大幅提高至12.6%，且結構的閥值增益驟降至50 cm-1左右。此外，為了更加強光子晶體與光場間的耦合作用，我們於p/n磊晶披覆層導入了非對稱組成，該設計可使光子晶體的光學侷限率再上升至13.94%，對應的閥值增益亦降至19.45 cm-1。最後，我們也針對模態頻譜分布與元件電特性做了一些延伸探討。

In this thesis, we investigated the device characteristics of semiconductor lasers fabricated from GaAs-based materials, which including high-power edge emitting laser diodes (EELs) and Photonic Crystal Surface Emitting Lasers (PCSELs). The relationship of optical confinement and laser performance is the main topic in the EEL researches. By optimizing the vertical and horizontal confinement factors in narrow-stripe ridge waveguide structures separately, the laser performance of EEL devices can be enhanced and it contributes to arriving high-power single mode operation. As regards the optimization of vertical optical confinement, 830-nm AlGaAs/InGaAs laser diodes (LDs) adopting multi-step graded index double barrier separate confinement heterostructures (GRIN-DBSCH) with small divergence beams and improved temperature characteristics under a high output power operation are reported. The double barrier separate confinement heterostructure (DBSCH) design provides good carrier confinement and prevents current leakage by adding a multi-step grading layer between cladding and waveguide layers. Meanwhile, the DBSCH design can facilitate to reduce the divergence angle at high power operation and widen the transverse mode distribution to decrease the power density around emission facets. The maxima optical power densities of 21.5 MW/cm2 per laser facet and good characteristic temperature values of threshold current (T0) and slope efficiency (T1) have been achieved. About the investigation of horizontal optical confinement, we demonstrate a high power AlGaInP-GaInP multi quantum wells (MQWs) LD adopted a high-reflectivity passivation to enhance the LI characteristics and keep a suitable far-field divergence angle simultaneously. Under the design of three-pair optical thin films, it cannot only avoid the metal absorption but also enhance emitting efficiency and heat dissipation by using a high reflective and good thermal conductive Al2O3/Ta2O5 multilayer. The measured room-temperature threshold current (Ith) and characteristic temperature (T0) can be arrived 44.5mA and 104.2K at 16.4∘far-field divergence. In the second part research of Photonic crystal, we have presented an AlGaAs-InGaAs multi quantum wells (MQWs) photonic crystal surface emitting lasers (PCSELs) by using the transfer matrix method and coupled wave method to achieve a low threshold operation. We first studied the influence of various thicknesses on different layers around active region in AlGaAs-based laser structure. The investigation has been especially focused on the band edge at Γ1 point because of the characteristic of photonic crystal (PC) surface emitting. The relationship between the threshold gain and filling factor had also been considered. After optimizing the structure of waveguide layers, the best value of vertically optical confinement factor of PC in the optimized AlGaAs-based PCSELs is calculated to be 12.6% and the threshold gain is reduced to 50 cm-1. To keep enhancing the coupling effect between optical mode profile and PC region, asymmetric design of p/n cladding layers is adopted under the above-mentioned AlGaAs-based laser structure. The extremely low threshold gain is achieved by adopting an asymmetric cladding layer design to enhance both of the vertical optical confinement factors for the quantum wells and photonic crystal. The optimized value of vertically optical confinement factor of PC layer is 13.94% and the corresponding threshold gain can be as low as 19.45 cm−1. In addition, we made some extended investigation about mode spectra identification and electrical property discussion.