Methane adsorption and hydrogen desorption kinetics during diamond gas source molecular beam epitaxy

Diamond epitaxial films with a thickness of 200-350 Å were grown on C(001) surfaces by gas source molecular beam epitaxy (GSMBE) with pure methane (CH₄) and without the addition of hydrogen or oxygen. From Arrhenius plots of the growth rates, the activation energy (E_a) of the growth was found to be 15 kcal/mol. To clarify the rate-limiting reaction of the diamond epitaxial growth in this GSMBE, the reaction coefficients of CH₄ adsorption and H₂ desorption were obtained by measuring the time evolution curves of the surface hydrogen coverage (θ_H) by electron stimulated desorption of H⁺ ions. From Arrhenius plots of the reaction coefficients, the E_a's of CH₄ adsorption and H₂ desorption on C(001) surface were estimated to be 7.3 ± 1.7 and 21 ± 4.9 kcal/mol, respectively. E_a for H₂ desorption is close to E_a for diamond growth, which suggests that the rate of GSMBE diamond growth is limited by hydrogen desorption. This is further supported by the experimental observation that the growth rate is largely independent of the CH₄ pressure. It was also found that θ_H does not disappear completely but has a large saturation value of ∼30-60% of initial θ_H, depending on the desorption temperature, which will be discussed in terms of surface segregation of hydrogen from the bulk.