Step Flow Growth Analysis of Doped and Undoped Diamond Epilayers

Diamond, with its superior properties, is widely used or anticipated to be used in various applications including thermal management substrates, protective coatings, nuclear radiation detectors, biosensors, and electronic devices. Recently, diamond has gained interest for high power devices due to its high breakdown voltage and exceptional thermal and carrier transport properties. The growth of high-quality, thick diamond films is critical for such applications but often encounters issues such as hillock formation on on-axis (100) surfaces with no miscut. Suppression of hillock formation is possible with miscut surfaces, although this can result in varied surface morphology and non-uniform impurity incorporation.<br/><br/>In this study, we report on the variations in the optical properties of ~30-micron thick and thicker CVD grown diamond films grown by step-flow at different miscut angles from (100) substrates. The substrates used were commercially available high-temperature high-pressure (HTHP) Ib type single crystal diamond. Prior to epitaxial growth, the miscut substrates were prepared through laser cutting, mechanical polishing, and Hydrogen-plasma etching. Among the samples, one was polished by Chemical Mechanical Polishing (CMP) before the ~30-micron diamond growth with a lightly boron-doped concentration. The other samples were mechanically polished followed by a 3-micron etching via Reactive Ion Etching (RIE) before the ~30-micron diamond deposition. Each sample underwent a short (10-minute) hydrogen-plasma etching to clean the surface at the start of the diamond deposition. The homoepitaxial diamond films were grown using a 2.45 GHz microwave plasma-assisted CVD technique, resulting in ~30-micron thick or thicker single crystal diamond films. The diamond deposition was performed with a mixture of hydrogen, methane, diborane, and carbon dioxide as feed gases, achieving a growth rate of 2-3 µm/hr.<br/> <br/>Cross-sections of the grown samples were prepared by laser cutting and polishing. Cross-section cathodoluminescence (CL) imaging reveals variation in band-A emission across the epitaxial growth regions, particularly near the top. For example, the epi-layer grown on a CMP surface was quite smooth. The CL analysis showed a strong overall luminescence near the top region of the diamond film, indicative of high-quality crystalline structure with minimal defects. Additionally, typical defect-related emissions are observed from the high-pressure high-temperature (HPHT) diamond substrate, reflecting the presence of NVN defects characteristic of such substrates. Crucially, no band-A emission is detected near the interface or in the bulk of the diamond film. This absence of band-A emission indicates that there are very few point defects, which signifies that both the interface and the bulk of the diamond film are of high quality. Other samples not subject to the CMP polish and different preparation conditions will also be reported on with respect to their defect properties as analyzed with CL. These observations collectively suggest that the CMP surface preparation and the subsequent CVD process have been effective in producing a high-quality diamond film growth with minimal defects, both at the interface and in the bulk of the thick epi-layer.<br/> <br/><b>Acknowledgement:</b> The Energy Frontier Research Center on Ultrawide Bandgap Materials (EFRC-ULTRA) funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES) division under award number DE-SC0021230.