以漸變式氮化鋁鎵超晶格結構提升成長於矽基板上氮化鎵磊晶層之品質;Improving the Quality of GaN Epilayer Grown on Si Substrate Using Graded AlGaN Superlattices

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Title:	以漸變式氮化鋁鎵超晶格結構提升成長於矽基板上氮化鎵磊晶層之品質;Improving the Quality of GaN Epilayer Grown on Si Substrate Using Graded AlGaN Superlattices
Authors:	朱英皓;Ju, Ying-Hao
Contributors:	電機工程學系
Keywords:	氮化鎵;超晶格;高電子遷移率電晶體;氮化鎵磊晶於矽基板;GaN;Superlattice;HEMT;GaN on Si
Date:	2020-08-12
Issue Date:	2020-09-02 18:27:40 (UTC+8)
Publisher:	國立中央大學
Abstract:	本論文研究主題為針對有機金屬氣相磊晶氮化鎵/氮化鋁鎵高電子遷移率電晶體(GaN/AlGaN HEMT)於矽基板上，設計一系列不同漸變超晶格厚度及結構的緩衝層，以提高GaN之材料品質，並藉由電子顯微鏡與X光繞射術分析其原因。磊晶成長GaN於矽基板最大的挑戰是必須克服此二材料因晶格常數與熱膨脹係數不匹配，而產生晶體缺陷，例如差排與龜裂，與晶圓翹曲的問題。一般常見的解決方式是在成長主動層之前，先成長一步進漸變式AlGaN緩衝層，如Al0.8Ga0.2N/Al0.5Ga0.5N/Al0.2Ga0.8N，以避免應力過大所造成的形變。本研究首先提出使用Al0.8Ga0.2N/Al0.2Ga0.8N漸變式應力超晶格取代傳統結構中Al0.5Ga0.5N緩衝層，藉由週期性的調變超晶格內Al0.8Ga0.2N的厚度，可在磊晶過程中抑制晶圓翹曲，不僅維持磊晶片的均勻性也能夠提高氮化鎵的品質。當漸變超晶格的厚度逐漸增加，晶圓的殘餘壓縮應力變小，晶圓翹曲的程度亦有逐漸減少的趨勢。以此超晶格結構，在6吋1 mm低阻矽基板上磊晶總厚度5 μm氮化鎵HEMT磊晶層之晶圓翹曲量僅有44 μm。同時，在良好的應力控制下成長高對數的超晶格可過濾更多差排缺陷，降低電子的差排缺陷散射，氮化鎵/氮化鋁鎵高電子遷移率電晶體磊晶片之低溫霍爾量測結果顯示，在10K時電子遷移率可以高達32,000 cm^2/V-s；X光(002) 與(102)面繞射峰之半高寬分別為472與510 arcsec。上述之Al0.8Ga0.2N/Al0.2Ga0.8N漸變式超晶格亦應用於6吋675 μm低阻矽基板之磊晶，並在Al0.2Ga0.8N步進式緩衝層與GaN緩衝層之間再插入15對Al0.2Ga0.8N/GaN超晶格，在磊晶層總厚度4 μm下，晶圓翹曲量僅2.6 μm。上述結果顯示，本研究所提出之超晶格結構不僅可降低GaN中之差排密度，亦可有效控制磊晶片之翹曲量，達到30 μm以下之量產規格。 ;The objective of this research is to grow high quality GaN/AlGaN high electron mobility transistors (HEMTs) on silicon substrates by metal-organic chemical vapor deposition (MOCVD). A number of graded AlxGa1-xN/AlyGa1-yN superlattice buffer layers with different thickness and aluminum composition are designed to investigate to how these superlattice buffers improve the material quality using transmission electron microscopy and x-ray diffraction. The biggest challenge for epitaxial growth of GaN on silicon substrates is to overcome the problems of crystal defects, such as dislocations, cracks and wafer bow, caused by the mismatch of lattice constant and the thermal expansion coefficient between Si and GaN. A common solution is to grow a step-graded AlGaN buffer layer, such as Al0.8Ga0.2N/Al0.5Ga0.5N/Al0.2Ga0.8N, before growing the active layer to avoid wafer deformation caused by excessive stress. In this study, a graded Al0.8Ga0.2N/Al0.2Ga0.8N superlattice is used to replace the Al0.5Ga0.5N layer in the conventional step-graded AlGaN buffer layer. The quality of GaN and wafer bow can be improved by adjusting the thickness and period number of the superlattice. By increasing the period number of the superlattice, the residual compressive stress becomes smaller and the wafer bow decreases. With this superlattice structure, the wafer bow of a GaN HEMT structure with a total thickness of 5 μm, grown on 6-inch 1 mm-thick low-resistivity silicon substrate, is only 44 μm. Besides, a higher period number of superlattice leads to a lower dislocation density and higher carrier mobility. Low-temperature Hall measurements on the GaN/AlGaN HEMT structure show that electron mobility reaches 32,000 at 10K. The full-width at half maximum of the GaN <002> and <102> x-ray rocking curve is 472 arcsecond and 510 arcsecond, respectively. The aforementioned Al0.8Ga0.2N/Al0.2Ga0.8N graded superlattice is also applied to the epitaxy on 6-inch 675 μm-thick low-resistivity silicon substrates. By inserting another 15 pairs of Al0.2Ga0.8N/GaN superlattice between the Al0.2Ga0.8N step graded buffer layer and the GaN buffer layer, a wafer bow as low as 2.6 μm has been achieved on a 4 μm-thick HEMT wafer. This study shows that the superlattices proposed in this research can reduce not only the dislocation density of GaN, but also the bow of the epitaxial wafer to less than 30 μm, which is the specification of production line.
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