| 摘要: | 本論文旨在利用超高真空分子束磊晶系統，成長高品質之長波長砷化銦量子點異質結構與雷射，並探究其發光特性。 由於成長量子點活性層為研製量子點雷射的關鍵技術，因此如何成長出高品質的量子點活性層，便成了研發量子點雷射最重要的工作。首先我們將量子點的成長著重在量子點成長參數的調校上，包括調校成長速度、溫度、厚度以及成長間歇等參數。藉由將上述量子點成長參數與發光特性最佳化的過程中，我們以原子力顯微鏡以及光激光譜量測發現了量子點高度均勻性對發光特性的重要性，並以長波長量子點雷射證明量子點高度均勻性對雷射元件的重要性。研究結果顯示擁有較均勻量子點高度的量子點脊狀波導雷射，其特性均優於對照組，並可於室溫連續操作於1328奈米。而雷射鏡面雖僅有經過簡單的崩裂製程，卻擁有低臨界電流密度：250 A/cm2，與文獻上經過高反射層鍍膜之相同尺寸雷射相當，相當適合應用於光纖通訊的雷射光源上。 而為能進一步改善長波長量子點雷射的特性，我們對量子點波長延伸的機制，做了詳細的討論，並利用不同材料特性的量子點結構組成來解釋可能的紅移機制。我們發現砷化銦鎵覆蓋層的厚度將會影響紅移的機制，而當覆蓋層的厚度夠薄時，量子點發光波長紅移最主要機制為應力緩衝效應。而若以砷化銦鋁取代砷化銦鎵成為應力緩衝覆蓋於量子點上，由於砷化銦鋁不僅可以抑制銦聚集現象，還可於砷化銦/砷化鎵量子點能帶結構中增加位障高度，所以不僅可以增加量子點的均勻性，更可以增加量子點中載子能態的分離程度。根據此一研究成果，我們研製出具有較大能態分離程度的量子點活性層，此結果將有利於研製高特徵溫度的量子點雷射。 在成功最佳化量子點成長參數，及探究量子點發光波長紅移的物理機制後，我們嘗試成長多層量子點堆疊以提昇雷射元件之增益值。然而研究中卻發現因為砷化鎵間格層與砷化銦量子點之間存在著極大的不匹配性，使量子點活性層中容易形成孔洞狀缺陷。此類缺陷不僅阻礙了均勻量子點的成長，同時將嚴重影響多層量子點堆疊之結晶品質及其發光特性。而欲解決此一問題之方法為在成長砷化鎵間格層時，增加磊晶的成長溫度，並配合一些適當的間歇及高溫熱退火以揮發大顆量子點（缺陷）上方的砷化銦，並提升砷化鎵分子的擴散長度，使其鍵結或沈積於此類缺陷中進而形成平坦的砷化鎵表面，以利多層量子點活性層成長。同時我們亦研製了高品質五層堆疊的1.3微米脊狀波導量子點雷射證明上述方法將可有效消除孔洞狀缺陷。 另外，由於目前在研製長波長量子點雷射上，多在砷化銦量子點上覆蓋一層砷化銦鎵應力緩衝層，以延伸量子點發光波長至1.3微米。然而欲進一步延伸量子點的發光波長至1.5微米，卻往往因為晶格不匹配而於活性層中產生了晶體缺陷，因而降低了量子點的發光效率。除此之外，由於砷化銦鎵應力緩衝層於量子點系統的能態結構中，扮演了載子從量子點內部熱脫逃的媒介。所以此類長波長雷射的熱穩定性，也遠低於理論預期。為改善此問題，我們發現若在應力緩衝層中添加銻成分，使應力緩衝層成為InGaAsSb或是InAlAsSb則可改善量子點活性層的能帶結構。在此研究中，我們探討了應力緩衝層中的銻元素摻雜，對量子點發光強度與熱穩定性的影響。同時我們利用此類含銻化合物成功研製出具有較以往典型量子點活性層，更高的發光效率、熱穩定性及更長波長之量子點活性層。對未來研製高品質量子點光電元件具有極大的助益。 This dissertation is devoted to growing and characterizing high quality long-wavelength InAs quantum dot heterostructures and lasers by an ultra-high vacuum molecular beam epitaxy system. Since the growth of QD active layers is the key factor to the development of QD lasers, it is most important to study the epitaxy growth of a high quality QD active layer. First, we focus on the modification of the QD growth recipe, including growth rate, temperature, nominal thickness and growth interruption. In the optimization of the QD growth recipe for fine optical properties, the importance of dot-height uniformity to the optical properties is disclosed through the employment of atomic force microscopy and photoluminescence measurement. Long-wavelength QD lasers with different dot-height uniformity are also fabricated to demonstrate the significance of dot-height uniformity in achieving high performance QD lasers. The results reveal that a ridge-type QD laser with high dot-height uniformity shows better characteristics than its counterpart, and can be operated at 1328 nm under room temperature. The threshold current density of this as-cleaved laser is 250 A/cm2 , which is comparable to the reported results obtained with high-reflectivity facet coating and similar ridge sizes, indicating the potential of the QD laser in this work for optical-fiber communication. In order to further improve the characteristics of the long-wavelength QD laser, we systemically study and clarify the mechanisms of elongated QD emission wavelength within different matrices. It is found that the thickness of the InGaAs overgrown layer would alter the wavelength-extension mechanisms and that strain-reducing effect is more dominant in extending the emission wavelength of QDs when the overgrown layer is thin. We also overgrow an InAlAs layer on the InAs QDs to act as a strain-reducing layer (SRL). Since the InAlAs layer suppresses indium segregation and increases potential barrier height in InAs/GaAs band diagram, uniform dot size and large state separation results are obtained by QDs with an InAlAs overgrown layer. The mechanisms of red-shifted wavelength discussed here could be constructive in realizing long-wavelength QD LDs with high characteristic temperature when operated above room temperature. After optimizing the QD growth parameters and studying the mechanisms of elongated QD emission wavelength, multi-stack QDs are grown for increasing the modal gain of QD lasers. However, the surface stress caused by lattice mismatch between the InAs and (In)GaAs overgrown layers often results in defects that are detrimental for optical devices. The surface stress would also lead to the formation of pinhole-like defects in the growth of multi-stack QDs. These not only impede the formation of uniform QDs, but also deteriorate the crystal as well as the optical quality of the multi-stack QDs. Growth interruptions during GaAs spacer layer formation and thermal annealing after the GaAs growth are employed to lead to indium-flush behavior and increase the diffusion length of GaAs adatoms for a smooth GaAs surface, and multi-stack quantum dot structures without pinhole-like defects are thus obtained. Based on the investigation, the demonstration of high performance 1.3 um quantum dot lasers with a 5-stack QD active region proves the effectiveness of the novel method in eliminating the pinhole-like defects. Additionally, the employment of an InGaAs strain-reducing layer is now the most favorable approach for long-wavelength (1.3 um) QD lasers. However, further extending the emission wavelength to 1.5 um always induces misfit dislocations and degrades the radiative efficiency of the QD structure because of lattice mismatch. Besides, since the InGaAs strain-reducing layer acts as a transit channel for facilitating the carriers’ thermal escape from InAs/GaAs QD structure, the thermal stability of this kind of laser at elevated temperatures is therefore far from expected. For improving the characteristics of QDs, we employ InGaAsSb or InAlAsSb instead of typical InGaAs or InAlAs strain-reducing layers, and find it can actually improve the band structure of QD active layer. In this investigation, the influence of Sb incorporation in SRL on QD optical properties and thermal stability is comprehensively studied. Enhanced emission intensity, thermal stability and extended emission wavelength of InAs/GaAs QDs is shown by the use of an Sb-contained strain-reducing layer. The superior optical characteristics of this novel structure make it a promising candidate for high-performance QD optoelectronic devices. |
