We report the direct imaging of Be δ-doped layers in GaAs, in the concentration range (0.5-2)×1014/cm2, comparing samples grown by gas source molecular beam epitaxy (MBE) and conventional MBE. The gas source MBE δ layers are ∼15 Å wide, and at least at high concentrations, consist of clusters ∼12 Å in diameter. At 2×1014 Be atoms/cm2, the MBE δ layer is an order of magnitude wider than that grown at the same temperature by gas source MBE. Our results imply that layers with Be concentrations in excess of 10 21/cm3 can be fabricated by gas source MBE.