應變導致過渡金屬二硫族化物水平異質結構中之能帶演化

Abstract

近年來，以二維過渡金屬二硫族化物為基礎的半導體異質結構引起了廣大的注意。因其獨特的結構與光學特性，這些二維異質結構被希冀於製作未來的可撓式光電元件。在這些異質結構中，受到晶格不匹配的影響，外圈的材料往往有劇烈的應變變異。例如在二硫化鉬-二硒化鎢系統中便有~1.59%的應變變異。本論文中，我們利用空間解析之光激螢光光譜研究二硫化鉬-二硒化鎢與二硒化鎢-二硒化鉬兩種水平異質結構的應變變異性，發現發光強度與能量有強烈的相關性。我們發現此相關性來自於各能谷中載子佔有率的變化且能夠以波茲曼分布來描述其行為。透過模型的分析，我們可以決定能谷的能量差與形變位能的比值。我們發現在無應力情況下單層二硫化鉬與二硒化鎢分別為直接能隙與間接能隙半導體。我們的發現提供了重要的能帶參數以及水平異質結構中的重要資訊：應變的變異不僅造成少許的能隙變化，更造成了直接能隙與間接能隙的轉變。這將大幅影響以水平異質結構製成元件的特性。

Semiconductor heterostructures based on two-dimensional transition metal dichalcogenides (TMDs) have attracted great interests recently. These two-dimensional heterostructures were given hopes for the future flexible optoelectronic devices due to their unique structural and optical properties. In these heterostructrues, the outer material usually exhibits serious strain variation caused by large lattice mismatch, i.e., ~1.59% variation has been observed in the MoS2-WSe2 system. In this work, the spatial inhomogeneity in lateral MoS2-WSe2 and WSe2-MoSe2 heterostructures were investigated by spatial-resolved photoluminescence (PL), showing strong correlated PL intensity and energy. The PL correlation is found to be caused by carrier occupation between different valleys, which can be described by the Boltzmann distribution. After analyzing by model, the band parameters including energy difference and ratio of deformation potential for involved valleys can be determined. Finally, we demonstrated the strain-free MoS2 and WSe2 monolayers are direct and indirect semiconductors, respectively. Our work presented here provides not only the important band parameters but also information on the lateral heterostructures. Particularly, the local strain variation could result in not only the small change of band gap energy but also the great change of direct/indirect gap, which could largely impact the device properties for future planar TMD heterostructures.