We describe the design and development of a scanning tunneling micoscope (STM) working at very low temperatures in ultra-high vacuum (UHV) and at high magnetic fields. The STM is mounted to the He3 pot of an entirely UHV compatible He3 refrigerator inside a tube which can be baked out to achieve UHV conditions even at room temperature. A base temperature of 315 mK with a hold time of 30 h without any recondensing or refilling of cryogenics is achieved. The STM can be moved from the cryostat into a lower UHV-chamber system where STM-tips and -samples can be exchanged without breaking UHV. The chambers contain standard surface science tools for preparation and characterization of tips and samples in particular for spin-resolved scanning tunneling spectroscopy (STS). Test measurements using either superconducting tips or samples show that the system is adequate for performing STS with both high spatial and high energy resolution. The vertical stability of the tunnel junction is shown to be 5 pmpp and the energy resolution is about 100 μeV.

The coverage dependence of the Fe-induced Fermi-level shift on p-and n-InAs(110) was investigated by angle-resolved photoelectron spectroscopy at 300 K. The Fermi-level position was found to be coverage dependent, exhibiting a maximum at 300 meV above the conduction-band minimum. The coverage dependence is explained by the surface doping model, if inhomogeneities in the Fe-adatom distribution and the resulting ionization probabilities are taken into account. The Fe-adatom distribution is determined by scanning tunneling microscopy. Photoemission spectra provided direct evidence of a two-dimensional electron gas at the Fe-covered surface.

We present the design of a new ultrahigh vacuum scanning tunneling microscope (STM) which operates at T<20 K inside the bore of a 2.5 T superconducting split-coil magnet. The tip/sample region can easily be controlled visually, thus allowing safe and fast exchange of samples and tips while the microscope stays at low temperatures. A newly developed rotary motion stepper motor is presented which allows rotation of the sample by >270° about an axis perpendicular to the tip axis. This feature allows metal or molecular beam evaporation normal to the sample surface. Even more important, by means of this device tip and sample can be brought into a parallel or antiparallel magnetic configuration thus opening a novel approach to the study of magnetic phenomena on an atomic length scale. In addition, measurements of the magneto-optical Kerr effect can be carried out without removing the sample from the STM. Also a new tip exchange mechanism is described. The microscopic and spectroscopic performance of the new instrument is illustrated on Au(111)/mica, on Tb(0001)/W(110), and on Gd(0001)/W(110).

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The electronic structure of the narrow gap semiconductor InAs is investigated by scanning tunneling spectroscopy and magnetotransport measurements in the extreme quantum limit. The well-known oscillations of the Hall coefficient are reproduced and the last, most pronounced oscillation is shown to be correlated with the appearance of corrugations in the local density of states. While the increasing part of the Hall constant corresponds to the existence of isolated patterns indicating magnetic field induced localization, the decreasing part correlates with the development of a network which most likely consists of one-dimensional channels. We conclude that the decrease of the Hall constant in the extreme quantum limit is caused by a transition from a purely three-dimensional to a partly one-dimensional transport regime.

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