Characterization of Diamond Grown on Diamond-Deposition Equipment

Characterization of Diamond Grown on Diamond Deposition Equipment

Diamond samples grown using deposition equipment must first undergo Raman spectroscopy, a highly important and critical characterization technique that allows for a preliminary assessment of whether the epitaxial single-crystal sample is indeed diamond and whether it contains carbon in an sp² hybridized state. If Raman spectroscopy confirms the material to be a diamond single crystal, further characterization of its crystalline quality can be performed: X-ray rocking curves are used to evaluate the crystal quality, while photoluminescence, infrared absorption, and ultraviolet absorption spectra help assess the optical properties and can also provide a quick indication of the types and concentrations of defects within the crystal. Optical microscopy, meanwhile, is employed to examine the morphology of the single-crystal growth surface.

Optical microscope

Optical microscopy (OM) is a commonly used and straightforward method for characterizing diamonds. By selecting an appropriate magnification to observe the diamond’s growth faces, one must balance the trade-off: excessively high magnification can result in insufficient depth of field, leading to blurred images due to sample surface irregularities, while too low a magnification may fail to resolve the epitaxial steps on the diamond surface.

Scanning electron microscope

To further examine the micro‑morphology of diamond sample surfaces, metallographic analysis using a scanning electron microscope is required. The SEM can resolve surface features at the submicron scale and offers substantial depth of field and high resolution.

Raman spectroscopy

Raman spectroscopy can be used to characterize the quality of epitaxial diamond. Raman spectra are highly sensitive to the state of carbon–carbon bonds; by analyzing the Raman shifts arising from phonon modes and other excitations induced by laser irradiation, one can assess the crystalline quality of epitaxial single crystals. In diamond, carbon atoms are sp³‑hybridized, and the corresponding first-order Raman peak appears at 1332 cm⁻¹, whereas graphitic carbon exhibits a peak near 1580 cm⁻¹. Furthermore, amorphous carbon with sp² hybridization manifests as a broad scattering band in the 1350–1600 cm⁻¹ region of the first-order Raman spectrum, with its exact position depending on the fraction of non‑diamond phases.

In addition to phase analysis of diamond, Raman spectroscopy can also be used to assess the crystal quality of diamond. The full width at half maximum (FWHM) of a diamond’s Raman spectrum is closely related to its intrinsic crystal quality: the greater the number of crystalline defects and the more disordered the lattice structure, the broader the FWHM. The Raman spectral FWHM of natural diamond typically ranges around 2 cm⁻¹, while that of CVD diamond can reach up to 2.3 cm⁻¹; for diamond thin films, the FWHM varies between 5 and 25 cm⁻¹. Furthermore, shifts in the characteristic Raman peaks of diamond can provide insights into the internal stress within the crystal.

Photoluminescence spectrum

Photoluminescence (PL) spectroscopy is a nondestructive technique for probing the electronic structure of materials. Its fundamental principle is that when a material absorbs photons, electrons are excited to higher energy states; upon returning to lower states, they emit photons. PL spectroscopy can be used to investigate the types of defects in diamond crystals. Crystal defects arising from the combination of nitrogen atoms and vacancies exhibit distinct characteristic peaks in the PL spectrum—575 nm (NV⁰) and 637 nm (NV⁻)—while nitrogen‑doped diamonds display a pronounced fluorescence background.