Layer intermixing in MeV Si-implanted quantum well superlattices (SLs) has been studied by transmission electron microscopy, secondary ion mass spectrometry and Rutherford backscattering spectroscopy. Molecular beam epitaxially grown GaAs(200Å) - Al0. 5Ga0.5As(200Å) SLs were implanted with 1 MeV Si+ at doses between 3 × 1014 and 1 × 1016/cm2. The implanted SLs were either furnace annealed at 850°C for 3 hr or rapid thermally annealed at 1050°C for 10 sec, both under GaAs proximity capping conditions. Totally mixed regions were observed only for the SLs implanted with 1 × 1016 Si/cm2 and then furnace annealed at 850°C for 3 hr. For the same dose, the RTA annealed SLs only showed slight layer intermixing. At lower doses, no appreciable intermixing was detected in either furnace or RTA annealed samples. By contrast, under either annealing condition extensive intermixing has been demonstrated for lower energy (220 keV) implantation, but at doses almost two orders of magnitude lowerl XTEM showed that in all the annealed samples, a defect-free zone existed in the near-surface region, followed by a band of secondary defects, with the maximum density located at about 1 μm below the surface. In the disordered samples, the position of the intermixed layers correlated with the defect band maximum. Under both annealing conditions, Si concentration profiles only showed slight broadening, and they correlated with the distribution of secondary defects as well as with the depth of the intermixed layers. The effects of dynamic annealing and surface on the implantation energy dependence, i.e., MeV vs. keV, of layer intermixing are discussed.