单晶锗的横向外延生长与合并研究

半导体工业是发展新一代信息技术、纳米高新材料等创新产业的关键领域。在当今微电子、光电子和信息产业中，四族（硅、锗及其合金）和三五族（砷化镓、氮化镓等）半导体材料被广泛应用。硅上异质外延生长技术的优点在于可以充分利用其他半导体优异的光电特性，并保持与当下硅基主流半导体产业的兼容性。然而，由于异质外延层与硅基底之间存在晶格失配和热膨胀系数的差异，外延层中会产生高密度的缺陷，从而影响半导体器件的性能。选区外延生长利用外延材料在基底和介电层上完全不同的形核行为，实现外延层在图案化基底上的选择性外延生长。该方法使基底与外延层界面处产生的位错只能通过狭窄的窗口区域传播到外延层中，因此，外延层的位错密度能够被大幅度降低。

选区外延生长已经有六十多年的研究历史，但是由于器件的需求以及材料生长条件复杂，相关研究还存在许多亟待解决的问题。例如，如何调控外延层在掩模板（即介电层）上的横向外延行为；横向生长至完全合并后在外延层内部形成孔洞结构的机理；横向外延层位错密度的降低机制等。本文以锗在图案化硅基底上的横向外延生长为研究对象，围绕掩模板尺寸、几何形状、取向以及生长窗口尺寸等几个自由度进行系统化研究，首次表征了单晶锗内部空腔结构的三维形貌、形成机理以及调制方法。

我们在硅(0 0 1)晶面上设计了与Si <1 1 0>晶向不同夹角的长条状SiO₂掩模板，在0°至45°的范围内逐渐改变掩模板与Si <1 1 0>晶向之间的夹角，系统地研究了锗横向外延生长和合并的角度依赖性。我们发现横向外延生长速度与掩模板的取向呈现强相关：随着掩模板与Si [1 1 0]方向的夹角从0°增加到7.5°，横向外延速度依次增大并在7.5°时达到最大值；继续增加到45°时横向外延速度逐渐减小。长条状掩模板上不同方向横向外延生长的竞争将导致三种合并模式，最终形成不同的孔洞结构。

针对不同形状的掩膜版，横向外延生长总是优先发生在曲率最大的地方。随着生长的进行，横向外延生长从各向异性逐渐转变成各向同性，且该转变过程对于不同形状的掩模板具有普适性。孔洞的形成来源于生长前端的差速生长，这个过程由锗原子在曲率驱动下的表面扩散过程主导。生长前端完全合并后在掩模版上方形成孔洞，且孔洞最终构型受表面能最小化调制。孔洞的尺寸可通过掩模板尺寸调控，在掩模板直径小于~2 μm时孔洞直径随掩模板尺寸单调增加；当掩模板直径超过~2 μm时，因动力学因素主导，孔洞直径趋近恒定。

生长窗口宽度也可以调控孔洞直径。我们设计了宽度和内径可调的硅圆环，并在其上进行锗的横向外延生长。孔洞直径随着生长窗口宽度的减小而增加，表明生长窗口受限时，生长前端原子迁移被曲率驱动，导致加速合并和孔洞尺寸增加。生长窗口足够大时，窗口左右两侧的横向外延生长前端相互独立，因此，继续增加生长窗口尺寸不再改变合并后生成的孔洞直径。

针对外延锗，我们揭示了穿透位错与表面热刻蚀坑的构效关系。外延锗表面热刻蚀坑的形成来源于在850 ℃下真空退火时穿透位错周围锗原子的率先脱附，此外，退火环境中的残余氧也参与了锗原子脱附过程。我们发现热刻蚀坑与穿透位错的一一对应关系表明真空退火可以作为检测外延锗穿透位错密度的新方法。

通过研究锗在图案化硅基底上横向外延生长的角度依赖性、掩模板形状对横向外延生长的影响、孔洞结构形成机理以及调控方法，我们首次发现横向外延生长速度由掩模板的曲率和取向协同调控，导致不同的孔洞结构形成；首次系统性地揭示了掩模板曲率变化时横向外延生长过程由各向异性转变至各向同性的普适性；通过调节掩模板尺寸及生长窗口实现了外延层中孔洞结构的可控构筑。研究结果对于理解和控制锗外延生长过程具有重要意义，对其他四族和三五族横向外延生长和合并也可以提供指导。同时，在外延层中构筑可控孔洞结构可以催生新型器件结构并派生出新的应用，为新型器件的研发奠定重要基础。

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