  We investigate the effect of oxygen vacancies and hydrogen dopants at the
surface and inside slabs of LaAlO3, SrTiO3, and LaAlO3/SrTiO3 heterostructures
on the electronic properties by means of electronic structure calculations as
based on density functional theory. Depending on the concentration, the
presence of these defects in LaAlO3 slab can suppress the surface conductivity.
In contrast, in insulating SrTiO3 slabs already very small concentrations of
oxygen vacancies or hydrogen dopant atoms induce a finite occupation of the
conduction band. Surface defects in insulating LaAlO3/SrTiO3 heterostructure
slabs with three LaAlO3 overlayers lead to the emergence of interface
conductivity. Calculated defect formation energies reveal strong preference of
hydrogen dopant atoms for surface sites for all structures and concentrations
considered. Strong decrease of the defect formation energy of hydrogen adatoms
with increasing thickness of the LaAlO3 overlayer and crossover from positive
to negative values, taken together with the metallic conductivity induced by
hydrogen adatoms, seamlessly explains the semiconductor-metal transition
observed for these heterostructures as a function of the overlayer thickness.
Moreover, we show that the potential drop and concomitant shift of (layer
resolved) band edges is suppressed for the metallic configuration. Finally,
magnetism with stable local moments, which form atomically thin magnetic layers
at the interface, is generated by oxygen vacancies either at the surface or the
interface, or by hydrogen atoms buried at the interface. In particular, oxygen
vacancies in the TiO2 interface layer cause drastic downshift of the 3d e_g
states of the Ti atoms neighboring the vacancies, giving rise to strongly
localized magnetic moments, which add to the two-dimensional background
magnetization.
