Surface Morphological Evolution

The development of nonplanar surfaces during transition metal nitride thin film growth results in surface mound structures that cause atomic shadowing which, under limited adatom mobility conditions, leads to the formation of voids, as described above, and more importantly, the formation of nanopipes, as described below. Surface morphological evolution, including surface diffusion, surface island growth and decay kinetics, and surface mound formation have been investigated for TiN(001) single crystal layers using in-situ scanning tunneling microscopy (STM) studies [20, 23-29].

Karr et al. [23] were the first to report in-situ STM results from TiN(001) surfaces that were grown on Mg0(001) by reactive magnetron sputter deposition at 750 °C. Figure 1 shows the surface morphology of layers with thicknesses t = 25, 50, 100, and 230 nm. In all images, atomic height steps (0.21 nm) are clearly observable. The surface morphology on a large scale is controlled by a regular array of growth mounds, ~1 nm high, that are separated by a characteristic length d ~100 nm. The sequence of images indicates that both d and the vertical amplitude of the surface roughness A slowly increase with film thickness, indicating growth of the surface mounds in both lateral and vertical directions. The formation of the mounds is due to the asymmetry in the attachment of adatoms at ascending versus descending steps, which destabilizes growth on low miscut surfaces [20], and has been reported to lead to mound formation on both metal and semiconductor surfaces [30-32]. Although this process is well established, the reported roughening rates vary widely, with exponents ranging between 0.16 and ~1 [23]. The roughening and coarsening rates for TiN(001) layer growth are within the range of rates reported for other materials and follow a power law with the exponent 0.25, that is, d a h0 25 and A a h0 25. Increasing the growth temperature does not affect the roughening exponent; however, it slightly increases d and decreases A [20]. Thus, the surface mounds become wider and less high at higher temperature, due to larger adatom diffusion lengths, similar to results from low-temperature growth of Ge(001) [33]. Ion irradiation during growth reduces the effective strength of the growth instability that drives the formation of growth

Figure 1. Scanning tunneling microscopy images of TiN(001) films grown on Mg0(001) by reactive magnetron sputter deposition at T = 750 °C as a function of film thickness t = 25, 50, 100, and 230 nm. Reprinted with permission from [23], B. W. Karr et al., Appl. Phys. Lett. 70, 1703 (1997). © 1997, American Institute of Physics.

Figure 1. Scanning tunneling microscopy images of TiN(001) films grown on Mg0(001) by reactive magnetron sputter deposition at T = 750 °C as a function of film thickness t = 25, 50, 100, and 230 nm. Reprinted with permission from [23], B. W. Karr et al., Appl. Phys. Lett. 70, 1703 (1997). © 1997, American Institute of Physics.

mounds [20] and also reduces the diffusion length of Ti adatoms and consequently d, due to the presence of atomic N on the surface [15].

A study by Wall et al. [29] focused on surface diffusion and nucleation kinetics to develop a quantitative understanding of the relevant processes determining the surface morphological evolution of TiN(001). In this study, 0.3 monolayers of TiN were deposited on large, atomically smooth TiN(001) terraces. The resulting surface islands, which are one atomic step high, are dendritically shaped and increase in size with increasing temperature. The nucleation length Ln, which is defined as the characteristic separation between these islands, increases from 6 nm at 510 °C to 9 nm at 600 °C to 17 nm at 800 °C. These data, together with a derivation by Venables [34] showing the exponential growth of Ln versus T and the relationship between Ln and the surface diffusion, are used to determine the effective adatom surface diffusion activation energy for TiN(001), found to be Es = 1.4 ± 0.1 eV [29].

An alternative approach to studying surface diffusion and island kinetics was reported by Kodambaka et al. [25-28], who examined TiN(001) island decay kinetics by STM annealing experiments. The decay of islands in the presence of larger islands is due to coarsening (Ostwald ripening), as described by the Gibbs-Thompson equation showing that the equilibrium adatom concentration around an island increases with decreasing radius r. Small islands have a higher curvature and hence a higher two-dimensional spreading pressure than larger islands, resulting in the decay of the smaller and growth of the larger islands. Kodambaka et al. [26] modeled island decay, adatom diffusion, and adatom density distributions and extracted from the experimental data a value for the activation energy for adatom detachment plus diffusion of 3.4 ± 0.3 eV

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