Growth mechanism of diamond by laser ablation of graphite in oxygen atmosphere

In this paper, diamond crystals grown on sapphire (0001) substrate by laser ablation in oxygen atmosphere were reported. The experiments were performe d at a substrate temperature of similar to 550 degrees C and in oxygen pres sure of 0.11-0.15 Torr. Field-emission scanning electron microscope (FE-SEM ), x-ray diffraction (XRD), and micro-Raman spectroscopy were applied to ch aracterize the products. FE-SEM observation revealed that hexagonal- and tr iangular-shaped crystals were formed on the sapphire substrate, however, th e crystal nucleation and growth were nonuniform and discontinuous. XRD and micro-Raman analyses indicated the coexistence of hexagonal and cubic diamo nd. Moreover, the secondary nucleation of diamond was observed. The diamond growth mechanism in laser ablation, a physical vapor deposition process, w as different than that in chemical vapor deposition process. In an optimum region of carbon energy, carbon species implanted into substrate to form a local high-density region in which the formation of diamond phase (sp(3) bo nd) rather than graphitic phase (sp(2) bond) was favorable. At suitable sub strate temperature and oxygen pressure, the diamond crystals nucleated and grew, while the graphitic phases were excluded by oxygen preferential etchi ng. Compared to diamond synthesized in a hydrogen environment, the conditio ns for diamond growth in oxygen atmosphere are very critical.