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8.9.3 Influence of surface energy anisotropyIn the description of formation of a strained epitaxial island in Section 8.9.1, it was assumed that the surface energy density of the ¯lm material was in- dependent of strain and of surface orientation. It was recognized in Sec- tion 8.8.3 that, if the lateral faces of the island are vicinal surfaces of the crystalline material, then the surface energy density may depend on both orientation and strain. In this section, the question of free energy change in the system during island formation is revisited for the case when the surface energy density varies linearly with surface slope for slopes of small ampli- tude, as indicated by the general expression (8.138) with a = tan j à j . How does this surface energy anisotropy in°uence island formation? The corre- sponding issue for large values of slope is pursued in subsequent sections.The principal change from the development of Section 8.9.1 is that the factor ° in the ¯rst term on the right side of (8.149) must be replaced by (° 0 + ¯ 1 a)= p1 + a 2 , neglecting the step interaction contribution for small values of a. The change in surface energy of the system can again be written | Khác | |
8.9.6 Observations of island formationImages of islands that formed spontaneously during deposition of the al- loy Si 0.8 Ge 0.2 on Si(001) at 755 ◦ C, obtained by means of atomic force mi- croscopy after the deposition was interrupted, are shown in Figure 8.37. The upper image shows very small islands forming on the growth surface dur- ing the early stage of the position. In the lower image, islands have grown nearly to the point of mutual impingement. The edges of the square island bases are aligned with h 100 i directions on the growth surface. Note that the lateral faces of the islands all have approximately the same slope. An enlarged view of a cross-section of one of the islands in the lower portion of Figure 8.37, obtained by transmission electron microscopy, is shown in Figure 8.38. This image reveals that the lateral faces of the island appear to be f 105 g surfaces.1 0 0 n m | Khác | |
3. Continuous measurements of substrate curvature were made during the de- position of a thin ¯lm onto a relatively thick (100) Si substrate; the substrate thickness is 600 ạm. It was determined that ¯lm growth follows the Volmer{Weber mechanism, and that the process of stress evolution could be approx- imated by the island impingement model presented in Section 8.6.2. The surface energy per unit area of the thin ¯lm is approximately 1 J/m 2 . The magnitude of substrate curvature at the instant when the islands impinge on the surface of the ¯lm was measured to be 5.8 £ 10 ¡5 m ¡1 .(a) Estimate the average radius of the islands when they impinge.(b) What is the direction of bending of the substrate?(c) Calculate the volume-averaged stress in the ¯lm | Khác | |
4. The values of the parameters in (8.139) that characterize the dependence of energy U S of a strained surface on step density have been estimated to be¯ 1 = 0:162 J/m 2 , ^ ¯ 1 = 12:5 J/m 2 and ¯ 3 = 2:29 J/m 2 for Ge on the basis of the Terso® interatomic potentials (Shenoy et al. 2002). Determine the angle à ¤ de¯ned in (8.142) for epitaxial growth of Ge on Si(001) | Khác | |
5. Consider the chemical potential expression (8.137) for the special case when the surface energy density U S depends on orientation à but not on surface strain ². Verify that the chemical potential expression reduces to  = U ¡∙(U S + U S 00 ), where the prime denotes diđerentiation with respect to à. The quantity in parentheses is sometimes called surface sti®ness; its existence presumes the function U S (à) is indeed twice diđerentiable | Khác | |
6. The change in elastic energy due to island formation is the elastic energy of the state in part (a) of Figure 8.34 minus that of the state in part (b), or¢W = ZR 12 c ijkl ² ij ² kl dR ¡ ZR 12 c ijkl ² m ij ² m kl dR (8.161) where R is the volume of space occupied by the material system and c ijklis the array of elastic constants which may have di®erent values within the distinct material components. Use the idea of superposition as depicted in Figure 8.34 to show that the expression for ¢W can be expressed in terms of only the relaxation ¯eld in part (c), thereby con¯rming the conclusions of Section 8.9.2 | Khác |
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