具高電流開關比之鍺錫n+/p二極體之製作與分析

Abstract

鍺錫合金其高載子遷移率的特性，使其非常有潛力成為下一代邏輯元件的通道材料。當錫比例超過8 ~ 11%時，鍺錫將變成直接能隙材料，此時聚集在Γ能帶中的電子數量會增加，導致較低的電子等效質量及較高的電子遷移率。近來鍺錫n-型電晶體的相關研究顯示其最佳開關電流比值僅10 ~ 10^3 A/A，主因是源極(汲極)p/n界面之漏電流所致，因此在本論文中，我們將針對n-型施體磷離子佈植於應變或鬆弛鍺錫薄膜中，研究其材料品質及載子活化特性，並深入探討不同應變對鍺錫n+/p二極體漏電流的影響。 由於鍺錫在經過離子佈值與熱退火或微波退火製程後，晶格品質會受損，且可能會產生應變鬆弛與錫偏析的現象，我們以X射線繞射(X-ray diffraction, XRD)訊號分析鍺錫的晶格狀態，並進行霍爾實驗以量測片電阻及載子活化率，整體而言，微波退火的表現比熱退火來的好。另外，在製作鍺錫二極體之前，藉由製作純鍺二極體作為參考，並確認各項製程步驟對於元件效能的影響，以作為後續鍺錫二極體製程基礎。本論文中鍺二極體的最高開關電流比值為4.3 x 10^6 A/A。另一方面，藉由不同離子佈值劑量，與熱退火或微波退火條件製作鍺錫二極體元件，電性結果顯示位於鍺錫薄膜之缺陷可能為主要漏電流來源，當鍺錫膜厚越厚，其缺陷造成越大的二極體漏電流，而微波退火相較於熱退火較能抑制漏電流。藉由較高能量離子佈植，可以有效降低漏電流，應變鍺錫二極體之最高的電流開關比可達約10^6 A/A，而鬆弛鍺錫二極體則可達10^4 A/A。

Germanium-Tin (GeSn) alloys with a high carrier mobility are a promising channel material for next-generation CMOS technology. As the Sn fraction of GeSn alloysis higher than 8 ~ 11 %, GeSn becomes a direct-bandgap material, where more electrons would populate in the Γ band, leading to a smaller effective mass and higher electron mobility. Recent works on GeSn n-MOSFET shows the best ION/IOFF ratio is merely 10 ~ 10^3 A/A possibly due to the source/substrate or drain/substrate junction leakage. In this thesis, we focus on the material quality and carrier activation by phosphorus ion implant in strained or relaxed GeSn films to investigate the effects of strains in the epitaxial GeSn films on the n+/p junction leakage. Crystal quality will be degraded with severe Sn segregation after the ion implantation and the subsequent thermal or microwave annealing step. By X-ray diffraction, crystal quality after annealing can be evaluated. For sheet resistance and carrier activation, Hall measurements were performed. The results showed that MWA is better than RTA for carrier activation and crystal quality. To test the pn junction leakage, we fabricated Ge diodes as a baseline to evaluate the effects of process steps on the diode performance. The best JON/JOFF ratio achieved in Ge diode is 4.3 x 10^6 A/A. For GeSn diodes, by different process parameters such as implant doses, annealing temperature and microwave power, and GeSn thicknesses, it is suggested that the defects in the GeSn film would dominate the junction leakage. As the GeSn film is thicker, the leakage current is larger due to more associated defects by the strain relaxation. By MWA, the leakage current can be effectively suppressed. Furthermore, with a deep pn junction by a higher implant energy, the leakage current can be further suppressed. The record-high JON/JOFF ratios are 10^6 and 10^4 A/A for the strained and relaxed GeSn diodes, respectively.