Atomic force microscopy of (100), (110), and (111) homoepitaxial diamond films

Abstract

We present atomic force microscopy images of diamond films grown by chemical vapor deposition epitaxially on diamond (100), (110), and (111) substrates. The films were grown from 0.2%–1.6% mixtures of CH4 and C2H2 in H2 in a hot-filament reactor at a total pressure of 25 Torr. The substrate and filament temperatures were held at 810–1000 and 2000–2150 °C, respectively. A (100)-oriented diamond film grown with 0.3% CH4 at a substrate temperature of 810 °C was rough on the μm scale, exhibiting pyramidal features, terraces, and penetration twins, while films grown at higher substrate temperatures and hydrocarbon flow rates were smooth on the nm scale and showed evidence of a (2×1) reconstruction. A (110)-oriented film was very rough on the μm scale but nearly atomically smooth on the 0.5–5 nm scale and exhibited local slopes higher than 40° with no evidence of faceting. A film grown on a diamond (111) substrate underwent spontaneous fracture due to tensile stress and exhibited a roughness of ≊10–50 nm on the ≊100 nm lateral scale in regions far away from any cracks. The implications of the morphological features for diamond growth mechanisms are discussed.

Growth on the reconstructed diamond (100) surface

“…For example, because of the alternating nature of the dimer/trough mechanism, we found that neighboring dimer rows cannot grow until the trough between them has been spanned. However this approach does not account for the relatively smooth surfaces that have sometimes been observed over long ranges on the (100)-(2 x 1):H surface [28,30,31]. On metals and some other types of crystals, smooth surfaces are the result of loosely bound adatoms diffusing to steps and kinks, which lowers their energy (by increasing their coordination numbers) and leads preferentially to growth at those sites.…”

“…Howuver, it is unlikely that such a surface would be stable because of very large H -H steric repulsions [26]. Recent STM and AFM experiments have shown [27][28][29] that diamond surfaces can be rather rough on an atomic scale, but at least some of the surface reconstructs to the (100)-(2x1):H form [28,30,31]. On this surface each carbon atom is bonded twice to carbons in the bulk, once to a surface hydrogen, and once to a neighboring surface carbon making a "dimer" and forming a 5-membered ring, as seen in Figure 2.…”

Pressure and temperature effects on the kinetics and quality of diamond films

“…This surface has been observed experimentally [25,26] postulates that once a CH 3 radical adds to the diamond surface, it has a choice: it can either desorb thermally or it can undergo further reactions with gas phase H 11 atoms that lead to incorpuration of its carbon into the lattice. Since the desorption and incorporation rates can be on the same order of magnitude, the growth rate depends sensitively on the branching ratio into these two pithways.…”

Theoretical study of hydrogen-covered diamond (100) surfaces: A chemical-potential analysis

“…On the right hand side of the vertical line, the formation energy of CH 4 is negative, namely, CH 4 forms spontaneously from the C(100) surfaces in the presence of the hydrogen reservoir. In contrast, the left side of the vertical line has a positive formation energy for CH 4 . The most stable surface phases for this region are: the bare surface when H is smaller than 17.19 eV, the Fig.…”

“…Since the hydrogen is always bonded to a single C atom, so it is reasonable to assume a zero-point energy proportional to the number of H atoms: 27 E 0 = n H e 0 . We estimated e 0 by the sum of vibrational frequencies of a typical hydrocarbon CH 4 and set it to 0.293 eV (0.0215 Ry) per hydrogen. 36 Temperature dependence of the formation energy was mainly incorporated through the chemical potential of hydrogen.…”