There is provided a method of filling one or more gaps by providing the substrate in a reaction chamber and introducing a first reactant to the substrate with a first dose, thereby forming no more than about one monolayer by the first reactant on a first area; introducing a second reactant to the substrate with a second dose, thereby forming no more than about one monolayer by the second reactant on a second area of the surface, wherein the first and the second areas overlap in an overlap area where the first and second reactants react and leave an initially unreacted area where the first and the second areas do not overlap; and, introducing a third reactant to the substrate with a third dose, the third reactant reacting with the first or second reactant remaining on the initially unreacted area.