  Hydrogenation of nitrogen (N) doped GaAs allows for reversible tuning of the
bandgap and the creation of site controlled quantum dots through the
manipulation of N-nH complexes, N-nH complexes, wherein a nitrogen atom is
surrounded by n hydrogen (H) atoms. Here we employ cross-sectional scanning
tunneling microscopy (X-STM) to study these complexes in the GaAs (110) surface
at the atomic scale. In addition to that we performed density functional theory
(DFT) calculations to determine the atomic properties of the N-nH complexes. We
argue that at or near the (110) GaAs surface two H atoms from N-nH complexes
dissociate as an H$_2$ molecule. We observe multiple features related to the
hydrogenation process, of which a subset is classified as N-1H complexes. These
N-1H related features show an apparent reduction of the local density of states
(LDOS), characteristic to N atoms in the GaAs (110) surface with an additional
apparent localized enhancement of the LDOS located in one of three crystal
directions. N-nH features can be manipulated with the STM tip. Showing in one
case a switching behavior between two mirror-symmetric states and in another
case a removal of the localized enhancement of the LDOS. The disappearance of
the bright contrast is most likely a signature of the removal of an H atom from
the N-nH complex.
