Strain Relaxation at Semiconductor Surface Revealed by X-Ray Diffraction

极不对称X射线衍射法是测量晶体表面或界面附近应变场的一种重要方法。由于X射线的掠射角设置在全反射临界角附近，因此该方法对近表面区域的晶体结构敏感。表面微小的应变场(大于或等于0.1%)会引起摇摆曲线的强度和宽度的变化。因此，我们可以通过分析曲线的形状或曲线的综合强度来评估表面附近的应变。我们展示了两个实验应变评估的例子。一种是硅重构表面。我们定量地计算了Si重构表面，即Si(111)-(7×7)、Si(111)-(√&lt；3&gt；×√&lt；3&gt；)-Al和Si(111)-(√&lt；3&gt；×√&lt；3&gt；)-Ag表面附近的本征应变场。通过对实验摇摆曲线与计算曲线的拟合，我们发现，由于表面晶格松弛，所有重构表面都产生了(111)间距的收缩，并且这种应变在表面下扩展到大约10 nm。另一个例子是硅化物表面。我们评估了在氢终止的Si(111)表面附近由于镍沉积而产生的应变演化。我们发现，在氢端表面附近，随着压缩应变逐渐引入衬底，伴随着“镍扩散层”的生长。

Extremely asymmetric X-ray diffraction is a noble method to evaluate strain fields near crystal surfaces or interfaces. This method is sensitive to crystal structure near surface region because a glancing angle of X-ray is set near a critical angle of total reflection. A minute strain fields (【greater than or equal】 0.1%) at surface brings a variation of intensity and width of rocking curves. Therefore we can evaluate strain near surfaces by analyzing curve-shape or integrated intensity of the curves. We show two examples of experimental strain evaluation. One is Si reconstructed surfaces. We evaluated intrinsic strain fields near Si reconstructed surfaces, i.e., Si(111)-(7×7), Si(111)-(√<3>×√<3>)-Al, and Si(111)-(√<3>×√<3>)-Ag surfaces, in quantitatively. From the fitting of the experimental rocking curves with calculated curves, we found that all reconstructed surfaces bring a contraction of the (111) spacing due to surface lattice relaxation, and such strain extends to some ten nm under the surfaces. Another example is silicide surfaces. We evaluated a strain evolution near hydrogen-terminated Si(111) surface due to nickel deposition. We found that a compressive strain gradually introduces into the substrate accompany with a growth of "Ni diffusion layer" near the hydrogen-terminated surface.