砷化銦/砷化銦鎵/砷化鎵量子點雷射之研究

Abstract

本論文分別研究量子點層數、耦合式量子點結構以及雷射InGaP夾層的晶格匹配對量子點雷射特性的影響。在量子點層數的部分，我們發現具有10層量子點的元件雖有較大的飽和增益，但因為層數過多，應變太大造成起振電流等特性的劣化。5層量子點的元件具有最佳的特性，當共振腔長度為3 mm時，室溫放光波長1293 nm、起振電流密度為83.4 A/cm2，其平均每層透明電流甚至低達6 A/cm2和理論計算結果約4~8 A/cm2相當，證明具有不錯的材料品質。3層量子點的元件則因為層數太少，影響增益，造成其雷射特性較5層為差。 我們發現耦合式量子點發現具有較大的量子點和較低的量子點密度。但較低的量子點密度並不影響其雷射特性表現，當共振腔長度為2 mm時起振電流密度為133 A/cm2和一般正常量子點結構相當，然而耦合式量子點結構具有更長的放光波長，其放光波長為1250 nm長於一般正常量子點結構的1211 nm。 最後我們探討應力夾層磷銦化鎵的影響，樣品分別具有晶格匹配 、壓縮應力和弛張應力的夾層。我們發現將量子點成長在具有應變的夾層之上時，會造成波浪狀或細紋結構。量子點更會形成兩個具有不同尺寸的量子點群。所製成的雷射雖然也有很小的起振電流，但由於增益較小無法在基態放光。

In this thesis, we report our studies on the effects of quantum-dot (QD) stack number, coupled QD structure, and the lattice mismatch of InGaP cladding layers on the performances of QD lasers. In the portion of QD stack number, it is found that though the lasers with 10 QD stacks have the highest saturation gain, the accumulated strain is too large to be accommodated by the lattice, which leads very large threshold current density in laser performances. A 3-mm-long as-cleaved device with 5 QD stacks lases at 1293 nm and shows the best performances. In terms of the transparency current density per QD layer, the device demonstrates a value of only 6 A/cm2 which is within the range of the calculated results and justifies the good material quality in our QD lasers. Lasers with 3 QD stacks, however, have lower gain and therefore worse performances than the lasers with 5 QD stacks. In the study of InAs coupled-QD structure, we found that the coupled QDs have lower density than the normal QDs. However, the lower density does not significantly deteriorate the laser performances. A 2-mm-long laser with coupled-QD demonstrates a very low threshold current density of 133A/cm2, which is just slightly lower than the laser with normal QD. In addition, the coupled-QD laser also demonstrates longer emission wavelength than its controlled device. In the final research subject, we investigate the effect of the lattice mismatch in the InGaP cladding layers on the laser performances. Samples with lattice-matched, compressive strained and tensile strained InGaP layers were grown and compared. We found that the lattice-mismatch InGaP layers will result in wavy strip structure or even hatches. QDs deposited on top of these layers will be separated into two groups. Though the lasers with lattice-mismatch cladding layers still have low threshold current density, the gain is too small to support ground state lasing.