Manipulation and analysis of a single dopant atom in GaAs

This thesis focuses on the manipulation and analysis of single dopant atoms in GaAs by scanning tunneling microscopy (STM) and spectroscopy (STS) at low temperatures. The observation of ionization rings is one of the key results, showing that we can control the charge state of a single dopant atom by the STM tip. Using the ionization rings, we can study properties of a single dopant atom, such as its binding energy. The binding energy is an important property with respect to device applications. A low binding energy is needed for the impurities to act as donors or acceptors, and to induce free carriers in the conduction or valence band, respectively. Defects with a large binding energy act as traps and reduce the carrier density. We found that Si donors in GaAs close to the surface have a higher ionization threshold than donors buried deeper below the surface. This leads to the conclusion that the binding energy is enhanced towards the surface, which is corroborated by the reduced extension of the wave function towards the surface. Also for the Mn acceptor in GaAs, an enhanced binding energy towards the surface was found. This can have important consequences for future devices, where the surface to bulk ratio is larger than in current devices, due to the shrinking device dimensions. The importance of the surface became furthermore apparent for Si atoms in the surface layer of GaAs. They were found to be bistable. A negatively charged acceptor-like configuration is favorable on the bare GaAs surface, which we can force into a hydrogenic donor configuration with the STM tip. From our analyses of the temperature dependence and the dependence on the current set-point of the switching rate belonging to the transition, we found that it probably involves a combination of inelastic excitations and quantum tunneling. The effect of impurities in a functional device is explored as well. We studied double barrier resonant tunneling p-i-n diodes, where the p-layer consists of Ga1-xMnxAs, with a Mn concentration of 5 %. During the growth of Ga0:95Mn0:05As, interstitial Mn ions (Mni) are formed, which act as double donors. The STM and STS measurements show the individual Mni that have diffused towards the GaMnAs-GaAs interface during annealing, and reveal the potential landscape that arises due to clusters of Mni ions. The last chapter of the thesis concerns a very different system, viz.nitrogen-vacancy (NV) centers in diamond, studied with a confocal microscope. We use optically detected electron-spin resonance to measure the Zeeman-splitting of the spin-triplet ground state. The Zeeman-splitting depends on the inner product of the external magnetic field and the NV-axis. We can thus use the NV centers as a magnetic vector field probe. The spatial resolution and magnetic field sensitivity in this thesis are ~ 0:3 mum and ~ 0:2 G, which can be significantly improved, by implementing a few techniques, to the level of magnetic force microscopy and magnetic resonance force microscopy respectively. The important advantages of the technique are operation in ambient pressure, room temperature and low external magnetic fields and the simultaneous measurement of all three magnetic field components in a single pass.