As transistors continue to shrink toward nanoscale dimensions, their characteristics are increasingly dependent on the statistical variations of impurities in the semiconductor material. The scanning tunneling microscope (STM) can be used to not only study prototype devices with atomically precise placement of impurity atoms, but can also probe how the properties of these impurities depend on the local environment. Tunneling spectroscopy of Mn acceptors in GaAs indicates that surface-layer Mn act as a deep acceptor, with a hole binding energy that can be tuned by positioning charged defects nearby. Band bending induced by the tip or by these defects can also tune the ionization state of the acceptor complex, evident as a ring-like contrast in STM images. The interplay of these effects is explored over a wide range of defect distances, and understood using iterative simulations of tip-induced band bending.

Size, shape, composition, and electronic properties of InAs/GaAs quantum dots by scanning tunneling microscopy and spectroscopyScanning tunneling microscope studies of individual impurities in semiconductors explore challenges associated with future nanoscale electronics and can provide insight into how new materials properties such as ferromagnetic ordering arise from impurity interactions. Atomic manipulation and tunneling spectroscopy were employed to characterize and control the acceptor states of Co atoms substituted for Ga in the GaAs(110) surface. Three states were observed whose appearance in tunneling spectra was sensitive to the tip position within the acceptor complex. The energy of these states did not follow bending of the host bands due to the tipinduced electric field, but did respond to the Coulomb potential of nearby charged defects, such as As vacancies. By applying voltage pulses with the scanning tunneling microscope tip, the vacancies could be positioned on the surface, thus enabling tunable control over the Co acceptor states.

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