It is shown that diamond nucleation on iridium buffer layers followed by an appropriate textured-growth step offers a viable way to realize single-crystal diamond films. Bias-enhanced nucleation on iridium layers results in heteroepitaxial diamond films with highly improved alignment. By a subsequent textured-growth step, the mosaicity can be further reduced for tilt as well as for twist in sharp contrast to former experiments using silicon substrates. Minimum values of 0.17° and 0.38° have been measured for tilt and twist, respectively. Plan view transmission electron microscopy of these films shows that, for low thicknesses (0.6 μm and 8 μm), the films are polycrystalline, consisting of a closed network of grain boundaries. In contrast, at the highest thickness (34 μm) most of the remaining structural defects are concentrated in bands of limited extension. The absence of an interconnected network of grain boundaries shows that the latter films are no longer polycrystalline.

Heteroepitaxial diamond films with highly improved alignment have been realized by using the layer sequence diamond/Ir/SrTiO3(001). In a first step, epitaxial iridium films with a misorientation <0.2° have been deposited on polished SrTiO3(001) surfaces by electron-beam evaporation. Using the bias-enhanced nucleation procedure in microwave plasma chemical vapor deposition, epitaxial diamond grains with a density of 109 cm−2 could be nucleated on these substrates. The orientation relationship for this layer system is diamond(001)[100]∥Ir(001)[100]∥SrTiO3(001)[100]. The polar and azimuthal spread for the crystal orientation of a 600 nm thick diamond film is about 1° in each case. For an 8 μm thick diamond film a significantly improved alignment of 0.34° (polar) and 0.65° (azimuthal) has been measured. The latter values, which to the best of our knowledge are superior to those of all former reports about epitaxial diamond films on alternative substrates, indicate the high potential of the substrate Ir/SrTiO3 for the realization of large-area single-crystalline diamond films.

Limitations of the process window for the bias enhanced nucleation of heteroepitaxial diamond films on silicon in the time domain

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The residual stress in thin diamond films with a strongly improved heteroepitaxial alignment has been studied by x-ray diffraction and micro-Raman spectroscopy. The measurements have been compared with the predictions from finite element simulations. The diamond films have been deposited by microwave plasma chemical vapor deposition at a temperature of 700°C on thin (200 nm) iridium buffer layers on SrTiO3(001). Three different regions have been found for a 600 nm thick diamond film: (I) a high quality epitaxial central area with >109 cm−2 oriented diamond grains showing a mosaic spread of only ≈1°; (II) a ringlike area of isolated epitaxial islands; and (III) a nontextured closed film at the edge of the sample. In area I the stress tensor was determined from the mean shift of the x-ray Bragg reflections. It can be interpreted in terms of a plane, biaxial stress state with σ=−4.9 GPa which is confirmed by micro-Raman measurements. Analyzing the diamond (004) and (311) peak profiles measured by x-ray diffraction (XRD) using monochromatic CuKα1 radiation allows us to distinguish a strongly shifted main component and a weaker, broader component with a minor shift. Finite element simulations predict a pronounced elastic relaxation of the thermal stress at rugged surfaces thus explaining this minor component. They also substantiate a stress reduction by more than 80% as observed by Raman measurements in area II. Combining all measurements taken in the different areas with the predictions of the simulation allows to separate four contributions, i.e., the thermal stress, elastic stress relaxation at a rugged surface, inhomogeneous stress contributions from the coalescence of the grains, and finally coherence stress due to lattice misfit.