| 摘要註: |
因為極性c-平面氮化鎵的自發性與壓電性極化導致巨大內部電場和較低的輻射結合效率，為克服這些障礙和改善元件效率，沿著非極性 和 方向製造磊晶片和元件已經被展示。由於異向性結構，非極性氮化鎵顯現許多獨一無二的特徵，雖然非極性III族氮化物特殊特徵已被研究，異向性應變對條紋的特徵和異向特性的影響在卻沒有被好好的探討。 這篇論文的目的就是探討非極性m-氮化鎵平面條紋與異向性特性之間的關係。我們準備兩個系列m-氮化鎵樣品：氨氣流量與成長壓力變化。藉由控制氨氣流量與成長壓力變化，可以明顯改變異向性特性、條紋結構、和非極性m-氮化鎵的能帶結構。為研究異向性應變對條紋特徵和異向特性的影響，我們進行高解析度電子顯微鏡、陰極發光光譜儀、拉曼光譜儀、x光繞射、和光致螢光光譜量測。當氨氣流量或成長壓力增加，明顯的條紋特徵與大規模的粗糙度是具有高密度表面凹處和堆疊差排的特徵。陰極發光光譜顯示主要峰值(~370 nm)與部份差排終止於堆疊差排有關，而低能量的小肩(~385 nm)是由於稜形堆疊差排。另外，從拉曼光譜和x光繞射量測顯示當氨氣流量或成長壓力增加，伸張應變沿著a軸大幅增加兩倍，同時壓縮應變沿著c軸只增加一點，這應變異向性的劇烈改變預期與條紋圖案和表面形態有關，伸張和壓縮應變各自沿著a與c軸一同存在著，使得條紋圖案將有一較細長條(平行a軸)和較小橫向寬度(平行c軸)。從拉曼光譜和x光繞射量測顯示在各向平面應變 和 具有相同的趨勢，這結果確認了在樣品內在平面應變的分佈。此外，從光致螢光光譜量測到兩個主要發光峰大約在~365 nm (3.397 eV)和~378 nm (3.280 eV)，它們分別是由堆疊差排和稜形堆疊差排所造成。可以發現到較高的氨氣流量與壓力，可得較低的偏極化程度。再者，我們模擬結果顯示除了E3，對於E1、 E2、與所有躍遷的偏極化程度與我們實驗結果相符合。 藉由改變氨氣流量或成長壓力，可以明顯改變異向性特性、條紋結構、堆疊差排密度、和非極性m-氮化鎵的能帶結構。這個研究結果可以提供非極性III族氮化物元件設計與改善輻射結合效率的重要資訊。 The spontaneous and piezoelectric polarizations of c-plane GaN result in a large internal electric field and a lower recombination efficiency. To overcome these obstacles and improve device efficiency, epilayers and devices fabricated along the nonpolar m- and a- planes directions have been demonstrated. Due to the anisotropic structure, nonpolar GaN have shown many unique characteristics. Although the unique characteristics of nonpolar III-nitride have been reported, the effects of anisotropic strain on the striation feature and anisotropic properties were not well discussed. The purpose of this study is to investigate the relation between in-plane strain and anisotropic properties in nonpolar m-GaN samples. Two series m-GaN samples were prepared：NH3 flow rate dependence and growth pressure dependence. By controlling the NH3 flow rate or growth pressure, one can greatly modify the anisotropic property, striation structure, SF density, and band structure of the nonpolar m-GaN. To study the effects of anisotropic in-plane strain on the striation feature and anisotropic properties, SEM, CL, Raman, XRD, and PL measurements were conducted. As the NH3 flow rate or growth pressure increases, the obvious striation feature and larger roughness were characterized by higher densities of surface-pit and SF. The CL spectra show that the main peak (~370 nm) is associated with the partial dislocation terminating BSFs, while the low energy shoulder (~385 nm) is due to the PSFs and/or stair-rod dislocations at the intersection between BSFs. In addition, from Raman and XRD measurement results, as NH3 flow rate or growth pressure increases, a tensile strain along the a-axis increases two orders of magnitude while a compressive strain along the c-axis increases only few folds. The dramatic change of strain anisotropy is expected to correlate the striation pattern and hence the surface morphology. With the existing of a tensile and compressive strains along the a- and c-axes, respectively, the striated pattern would have a longer strip (// a-axis) and smaller lateral width (// c-axis). The in-plane strains and obtained from Raman and XRD measurements show the same trend. This result confirms the in-plane strain distribution inside the samples. Furthermore, two main emission bands around ~365 nm (3.397 eV) and ~378 nm (3.280 eV) in PL were observed, which are attributed to BSFs and PSFs, respectively. It was found that the higher NH3 flow rate or pressure, the lower the degree of polarization. Furthermore, our simulation results show that except E3, the degree of polarization for E1, E2, and all transitions agree with our experimental results. By controlling the NH3 flow rate or growth pressure, one can greatly modify the anisotropic property, striation structure, SF density, and band structure of the nonpolar m-GaN. The research results provide important information to optimize device designs and to improve recombination efficiency for nonpolar III-nitride devices. |