Research on long wavelength and short wavelength Vertical-Cavity Surface-Emitting Laser Process Techniques
|摘要:||本論文研究與製作以金屬有機氣相化學沉積法 (Metalorganic chemical vapor deposition, MOCVD)成長之長波長與短波長面射型雷射。面射型雷射其具有圓形光束輸出、低製作成本、單一縱模操作、以及整合二維陣列的潛在特性，因此在長波長範圍之光纖通信、短距數據通信與短波長範圍之高密度光學資訊儲存、高速掃瞄輸出、商業化光源及顯示應用上，成為極具潛力的發光源。本論文針對此二主要課題進行研究：研究及製作0.45 mm短波長氮化鎵系列之面射型雷射與1.3-1.5 mm長波長磷化銦系列的面射型雷射。
對於氮化鎵系列之面射型雷射，我們研究了氮化銦鎵/氮化鎵主動層材料之最佳條件。這些最佳條件包含量子井的數目、厚度及矽摻雜於位障層，皆會影響主動層之發光效率，這些優化之參數已由發光二極體 (LED) 證實。低銦含量(<20 %)量子井之LED，在20毫安培之最大光輸出其最佳量子井數目為6個週期、量子井厚度為3.2 nm。矽摻雜於位障層可明顯提升發光效率，矽摻雜流量=0.19 sccm於位障層之LED在20毫安培之最大光輸出比矽摻雜流量=0.12 sccm於位障層之LED高了約20 %。我們也製作了使用一維光子晶體反射鏡得到高取出效率之藍光LED。此一維光子晶體反射鏡包含了14週期堆疊之TiO2/SiO2介電材料，在大角度範圍比傳統布拉格反射鏡 (DBR)具有較高反射率。在相同波長比傳統LED之輸出功率有80 %的提升。我們也已經實現了光激發之氮化鎵面射型雷射，其結構包含了由MOCVD成長之25對AlN/GaN 布拉格反射鏡 (DBR)、3λ長共振腔以及8對Ta2O5/SiO2介電材料。已成功光激發的氮化鎵面射型雷射，其雷射波長為448 nm，線寬只有0.25 nm。
在傳統製作長波長面射型雷射的材料中，缺乏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，也缺乏可氧化之電流侷限材料，使得不易製作長波長面射型雷射。因此，在長波長面射型雷射研究中，我們優化長波長雷射主動層的各種參數，我們發現量子井中的應力參數、多量子井的應力補償、以及摻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，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99 %高反射率的布拉格反射鏡。我們已成功的製作出以光激發操作的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 nm。
In this study, we have developed the fabrication of long wavelength and short wavelength vertical cavity surface emitting lasers (VCSELs) by metal organic chemical vapor deposition (MOCVD). The vertical cavity surface-emitting laser (VCSEL) featuring circular-beam output, low production-cost, single longitudinal-mode operation, and possible integration of two-dimensional array are potentially suitable for fiber communication systems and short distance data transmission systems in long wavelength range, and for high-density optical data storage (for example: CDs and DVDs) and high-scan-speed laser printing, commercial lighting source and display applications in short wavelength range. In this thesis, we concentrate on these two topics. To investigate and develop the 1.3-1.5 mm InP-based VCSELs and ~0.45 mm GaN-based VCSELs. For InGaN/GaN-based VCSELs, we investigated the optimized conditions of the InGaN/GaN active layers. The overall optimization of these factors consists of quantum well number, thickness and Si-doped barrier. The optimization of InGaN/GaN MQW is established by light-emitting diodes. It is shown clearly that the EL output power at 20 mA of low In-content (< 20 %) LED sample with six-wells and 3.2 nm-thick thickness is highest. We have studied the effect of Si doping on the GaN barriers. As increasing Si doping in the barriers, the PL shows an increase of emission intensity and a blueshift of peak energy. With the same MQW emission peak at about 460 nm and driving current of 20 mA, it is found that the light output power of the LED with Si flow rate of 0.19 sccm is 20 % of magnitude higher than that of LED with Si flow rate of ~ 0.12 sccm. This result shows that PL intensity and LED output power of the MQW samples with Si-doped barrier layers are dramatically increased. Additionally, we also fabrication high extraction efficiency of InGaN LEDs with an omnidirectional One-dimensional Photonic Crystals (1D PC) incorporated into the bottom of InGaN blue LED chips. The designed omnidirectional 1D PC composed of two different transparent optical materials TiO2 and SiO2 layers stacked alternately. It is shown that the omnidirectional 1D PC has a higher reflectance and a wider reflection angle than a conventional distributed Bragg reflector (DBR). With the same MQW emission peak at about 450 nm and at 20 mA, it is found that the light output powers of the LED with 1D PC is about 11.7 mW and an up to 80 % enhancement in the extracted light intensity is demonstrated. We have demonstrated the optically pumped GaN-based VCSELs structure grown by MOCVD. The 25 pair AlN/GaN DBR structure and 3λ cavity layer were consisted in this VCSEL structure. The AlN/GaN mirror with 25 pairs of DBR can achieve the high reflectivity of 94 % and the wide FWHM of reflectance spectrum about 33nm. The Ta2O5/SiO2 mirror with 8 pairs of DBR can achieve the high reflectivity of 97.5 % and the wide FWHM of reflectance spectrum about 115nm. The PL emission of and the FWHM of emission spectrum of overall VCSEL structure were 448 nm and 1.4 nm, respectively. The narrow FWHM of 1.4 nm is attributed to the Febry-Perot cavity effect. The stimulated emission of fabricated GaN-based VCSEL was achieved and observed by using the optical pumping system. The GaN VCSEL emits 448nm blue wavelength with a linewidth of 0.25 nm. It evidently expresses the behavior from spontaneous emission to stimulated emission. For InP-based VCSELs, 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. 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 DBR 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. We also successfully have 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. We fabricated electrically driven continuous wave LW-VCSELs by using selectively etched undercut apertures technique, Si-implant technique, regrowth technique. Electrically driven continuous wave operation has not yet to be achieved. Many issues in making electrically driven LW-VCSELs still need to be resolved in the future.
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