Scanning Tunneling Microscopy Studies of Er Adatoms on GaAs (110)*

Rare earth dopants in III-V semiconductors are of interest as high quality optical sources due to the preservation of sharp intra-f-shell transitions. Here we investigate Er interactions with host GaAs (110) surface with atomic resolution using STM. Er atoms were deposited via electron beam evaporation onto the GaAs surface at 5 K. We find three different Er_ad configurations with varying abundance upon deposition, each with a different surface site location. All three configurations exhibit long-range depressions in STM topographic images, attributed to band bending associated with a positive adatom charge state. Individual Er adatoms can be switched between these states by applying a positive voltage pulse with the STM tip. Tunneling spectroscopy on Er adsorbed at the interstitial sites reveals prominent states within the GaAs bandgap, but no evidence of sharp f-shell transitions inferred from bulk optical studies. We also form substitutional Er_Ga by applying a larger positive voltage pulse. Substitutional Er appears neutral, which we attribute to it being isoelectronic with Ga. We will also present preliminary studies with concurrent optical excitation. Shifts in Er defect states are interpreted as a surface photovoltage effect.