Control of Zn(S, Se) composition using reflection high energy electron diffraction oscillations

Abstract

Abstract Observation of reflection high energy electron diffraction (RHEED) oscillations during molecular beam epitaxy (MBE) growth of ZnSxSe1−x from elemental selenium and compound ZnS sources is reported. Oscillations are readily observed and may be used to control the sulfur content. In particular, they may be used as feedback during growth to maintain “lattice-matched” composition during growth on GaAs substrates. The RHEED oscillations undergo distinctive changes as various growth parameters (substrate temperature, flux ratio and growth rate) are varied. The growth conditions may be adjusted to obtain a particular form of oscillation, corresponding to a known sulfur content. In general, when growth begins under condition yielding oscillations, the intensity level at which the first oscillation occurs is different from the intensity level of later oscillations. As the ZnS flux changes, these two intensity levels change position relative to one another and, for a particular ZnS flux, merge. For a growth rate of 0.5 μm h−1 and an indicated substrate temperature of 275 °C, if growth is done near the point where the two levels merge, then the resulting ZnSxSe1−x is lattice matched (x ≈ 7%). However, if the substrate temperature is raised to 300 °C, then growth near conditions where the levels merge yields a lower sulfur content (x ≈ 5%). Therefore, because the point at which the levels merge does not occur at the same sulfur composition for various substrate temperatures, the RHEED oscillations are not actually directly sensitive to the amount of strain or to the sulfur content and thus are not directly sensitive to the lattice-matching condition. Confirmation of this is provided by a similar evolution of the RHEED oscillations during simple ZnSe growth, with varying zinc-to-selenium ratio. A possible explanation is that the oscillations are responding to a surface stoichiometry change that occurs during the first monolayer of deposition. Nevertheless, for a given substrate temperature and growth rate, the sulfur content obtained for growth yielding a particular form of oscillation is reproducible. Use of RHEED oscillations to maintain the lattice-matched composition has yielded layers of good structural quality. Layers have been growth as thick as 3.5 μm without misfit dislocations. The minimum full width at half-maximum for a lattice-matched layer, as measured by X-ray rocking curves is 28″.