Internal stresses in {111} homoepitaxial CVD diamond
Introduction
From its early beginnings, Raman spectroscopy has been recognised to be ideally suited to the study of diamond [1]. During the last two decades, innumerable Raman results have been published on CVD diamond individual grains or continuous films. Recent progress in confocal micro-spectroscopy allows Raman studies at the micrometer scale. For example, in the case of high-quality free-standing polycrystalline boron-doped diamond films [2], 2D stress or defect images were deduced from the Raman spectra. High local stresses (both compressive and tensile) were thus observed within diamond grains, in particular close to defects or boundaries. In this paper, we present some preliminary results obtained by confocal micro-Raman techniques applied to homoepitaxial diamond thin films, in particular phosphorus-doped layers similar to those in which n-type doping of diamond by phosphorus was demonstrated 5 years ago [3]. One of the purposes of this study was to find out whether the presence of phosphorus in the layer could be identified by specific features observable in the Raman spectra. Similar studies have already been performed on boron-doped diamonds and Raman spectroscopy showed to be a well-established method—at least qualitatively—to track the presence of boron in the doped films. Moreover, convincing evidence of phosphorus incorporation on substitutional sites of the diamond lattice has been so far achieved only in the case of the more critical {111}-oriented MPCVD growth, which is known to be difficult to control. Thus, a second motivation of this work was to look for the presence of specific defects in the epilayers. Finally, the presence of residual stress, induced in the epilayers as a consequence of the growth process, was investigated. The study of the structural properties of the epitaxial layers is extremely important as the long term stability of phosphorus-doped diamond-based devices, such as the recently announced UV-emitting p–n junctions [4], are expected to strongly depend on the structural perfection of the deposited films.
Section snippets
Experimental
Different undoped (CN52, CN53 and CN54) or phosphorous-doped samples (CN16, CN23, CN60 and CN63) were grown either on irregular mm-size unpolished {111}-oriented Ib-type crystals normally used as cutting tools or on 2×2 mm2 polished substrates. Both kinds of substrates were purchased from Sumitomo electric Ltd. For comparison purposes, another undoped film (CN67) was grown on a {100}-oriented diamond substrate. Before deposition, substrates were cleaned in a solution of one part of percloric
Results and discussion
At first, to get representative average spectra, line-scans of Raman spectra were obtained from all the samples. A typical set of spectra obtained over a 30 μm linear distance (CN16 sample) is presented in the Fig. 1. The spectra were acquired at 1 μm intervals along the profile. A first observable feature is the presence of a more or less intense photoluminescence (PL) background. When excited with the 514.5 nm line, this PL signal does not show any fine structure, and presents a maximum in
Conclusion
In summary, we have shown that, under tight confocal optical observation, {111}-oriented diamond epilayers yield a significant broadened and down-shifted Raman phonon line that we attribute to an inhomogeneous tensile stress distribution. In this study, such a peak appears on all analysed {111} homoepitaxial layers independently of doping and CVD growth conditions used. In the case of thick films, the internal stress was relieved by cracks leading, in some cases, to the local delamination of
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