為了降低氮化物的磊晶成本，本研究以濺鍍的方式，在矽基板及多晶氮化鋁基板上形成緩衝層，以提供氮化物磊晶之同調性環境。由於矽基板與氮化鎵之間有極大的晶格與熱膨脹係數差異，使得氮化鎵的晶格應力在隨著厚度增加之後，易導致磊晶層龜裂。在矽基板上，我們以氧化鋅薄膜做為氮化鎵的磊晶緩中層，這是因為氧化鋅與氮化鎵的晶格差異只有1.8%。在研究的過程中，我們發現氧化鋅薄膜的厚度與氮化鎵的磊晶品質間有密切的關聯。 本研究所使用的多晶氮化鋁基板以粉末壓製而成，由於基板表面過於粗糙，不適於磊晶成長。為了解決此問題，我們利用射頻磁控濺鍍系統，在氮化鋁基板上鍍上一層類單晶氮化鋁，並透過鍍膜過程中的氬氣與氮氣流量來控制薄膜的品質。此氮化鋁濺鍍層，不但能提升基板表面的平整度，也能改善基板表面的晶格品質。 在磊晶的過程中，我們發現成長於氮化鋁基板上的磊晶面呈現金字塔狀的形貌，即半極性晶面。此半極性晶面在發光二極體的應用上有兩個優點:1.增加發光面積2.增強載子發光結合的效率。此外，我們也利用脈衝式的氨氣氣流來增加晶格側向成長的速度，以控制磊晶表面的形貌。在攝氏1180度的環境中，以脈衝式氣流成長之氮化鋁磊晶層有更平整、均勻的表面，晶格品質也更佳。 ;In this study, we deposit buffer layers by sputtering on Si (100) and poly-AlN substrates, providing a cost-effective and compliant surface for epitaxial growth of III-nitride semiconductors. Owing to the huge mismatches in lattice constant and thermal expansion coefficient mismatch between GaN and Si substrate, the strain induced during GaN growth often leads to cracks on epi-layer’s surface, resulting in deteriorated crystal qualities. ZnO, with merely 1.8% lattice mismatch to GaN, was utilized as the buffer material to improve the crystal qualities of GaN-on-Si. By optimizing the buffer layer thickness, the undesired stain can be mitigated, deferring the formation of threading dislocations. The poly-AlN substrate was produced with compressed AlN powder. To smoothen the surface for epitaxial growth, the poly-AlN substrate was coated with an AlN buffer layer by RF sputtering prior to the growth. According to the results of scanning electron microscopy (SEM) and x-ray diffraction (XRD), the gas mixture of Ar/N2 and sputtered AlN films thickness play important roles in the optimization of AlN epitaxy. During the epitaxial growth, it is found that the AlN epilayer grown on the poly-AlN substrate exhibited pyramid-like semipolar facets. The semipolar facets have two advantageous properties for solid state lighting: i). Increasing the emission area. ii). Enhancing the radiative recombination efficiencies in the quantum wells. Moreover, pulsed-flow of NH3 precursor was adopted to enhance the lateral growth rate of AlN. The enhanced lateral growth was found effective in improving the smoothness and lattice qualities of the AlN epilayer. This study demonstrates an alternative route for the growth of ultraviolet light emitting diodes.
