以金屬有機化學氣相沈積法成長長波長面射型雷射

Abstract

本論文在研究以金屬有機氣相化學沉積法(Metalorganic chemical vapor deposition, MOCVD)製作長波長面射型雷射。波長範圍在1.3到1.5微米的面射型雷射，因其具有圓形光束輸出、低製作成本、單一縱膜操作、以及整合二維陣列的潛在特性，因此在光纖通信及中、短距數據通信上，成為極具潛力的發光源。但因在傳統製作長波長雷射的材料中，不容易找到具有折射率差異大的材料組合以製作高反射率的布拉格反射鏡，加上傳統長波長雷射的主動層材料，其高溫特性不佳，使得長波長面射型雷射的發展遲緩。

因此，在本研究中，我們從最基本的設計與模擬出發，先找出適合長波長雷射主動層的材料，然後將主動層材料應用在傳統邊射型雷射上，變化各種參數，我們發現量子井中的應力參數、多量子井的應力補償、以及參雜條件的多寡皆會影響雷射特性，我們優化各項參數，得到最好的臨界電流密度為1.4 kA/cm2。另一方面，我們使用MOCVD成長磷化銦系列的布拉格反射鏡，以優化過的磊晶參數成功製作出高反射率的布拉格反射鏡，並研究其光學及電氣特性。同時，我們也嘗試製作以磷化銦搭配空氣，在只有三對組合下成功得到高反射率的布拉格反射鏡。

為了要製作出擁有良好溫度效應的長波長面射型雷射，我們也發展了晶圓融合的技術將長波長雷射主動層的材料和砷化鎵系列的布拉格反射鏡結合成長波長面射型雷射，我們成功的製作出以光激發的方式操作的融合型長波長面射型雷射，其等效的臨界電流密度為4 kA/cm2。最佳的等效臨界電流密度值是以磷化銦系列的布拉格反射鏡，加上週期性增益之共振腔，再加上以介電材料製成的布拉格反射鏡組合而成的長波長面射型雷射，以連續光激發的方式操作，其等效的臨界電流密度為2 kA/cm2，雷射波長為1562奈米。

我們最終的目的是要製作出電激發式的長波長面射型雷射，雖然在本研究中尚未達成這個目標，但為發展製作電激發式的長波長面射型雷射的過程中，我們發展了磊晶再成長技術、埋藏式穿透接面元件等技術，成功應用在長波長發光二極體，另外，我們同時也遇到了量子井相互擴散的情況、觀察到在傳統氧化侷限面射型雷射中的雙模態情形，這些研究都提供了良好的基礎朝向製作出電激發式的長波長面射型雷射。

In this study, we have developed the process for fabrication of long wavelength vertical cavity surface emitting lasers (LW-VCSELs) by metal organic chemical vapor deposition (MOCVD). LW-VCSELs with emission wavelength ranging from 1.3 mm to 1.5 mm featuring circular-beam output, low production-cost, single longitudinal-mode operation, and possible integration of two-dimensional array are potentially suitable for light sources in fiber communication systems and in medium and short distance data transmission systems. However, the absence of high refractive index contrast in InP-lattice-matched materials impeded the development of 1.3-1.5 mm VCSELs. In addition, active layers with insufficient gain at elevated temperature, absence of natural oxidized current aperture and poor heat conductance in material systems for long wavelength range are problems in making LW-VCSELs.

Therefore, we started this study from design and simulation to obtain appropriate gain materials for LW-VCSELs. We have determined InGaAlAs as the gain material and applied it into the conventional edge emitting lasers to find out the optimized conditions of the active layers. The amount of compressively strain in quantum wells, the net amount of strain in multiple quantum wells (MQWs) with more pairs, and the impurity concentration strongly influenced the performance of edge emitting lasers. The overall optimization of these factors makes us obtaining low threshold current density of 1.4 kA/cm2. On the other hand, we have fabricated InP/InGaAlAs-based distributed Bragg reflectors (DBRs) with excellent electrical and optical properties using MOCVD and the growth interruption technique. Meanwhile, we have successfully fabricated, and demonstrated a rigid InP/airgap structure with high reflectivity at 1.54 mm using InGaAs as the sacrificial layer. The 3-pair InP/airgap DBR structure has a peak reflectivity at 1.54 mm with a stop-band width of about 200 nm.

In addition, we have developed wafer-fusion technique to combine the conventional InP-based active layers with GaAs-based DBRs in order to simultaneously have the superior gain performance of InP-based active layers and the high reflectivity, high thermal conductivity and capability of oxidized layers of the GaAs-based DBRs. We demonstrated the optically pumped VCSEL structure with the fused bottom 30 pairs GaAs/AlAs DBR, InGaAlAs MQW and the fused top 25 pairs GaAs/AlAs DBR. The equivalent threshold current density is calculated to be 4 kA/cm2. The lowest threshold was obtained in InP-based LW-VCSELs. We successfully demonstrated the optically pumped InP-based VCSELs with the 35 pairs InP/InGaAlAs DBRs and 10 pairs SiO2/TiO2 top dielectric mirrors and a 2l thick cavity composed of periodic strain compensated MQWs to fully utilize the gain in every quantum well. The optically pumped VCSELs operated at room temperature with equivalent threshold current density calculated to be 2 kA/cm2. The wavelength of the output beam is 1562 nm.

Although our goal to fabricate electrically driven continuous wave LW-VCSELs with single mode operation has yet to be fulfilled, this process has led to many other developments. For example, we have developed the MOCVD regrowth technique to fabricate buried tunnel junction devices and have applied this technique to fabricate long wavelength light emitting diodes with buried tunnel junction. At the same time, we have studied the quantum well inter-mixing effect, and the coexisting two-cavity configuration in conventional oxide confined VCSELs. All in all, basic physical phenomenon and material issues observed in this study will turn into useful information in making electrically driven LW-VCSELs in the future.