Quantitative scanning tunneling spectroscopy of non-polar III-V compound semiconductor surfaces

The investigation of non-polar III-V semiconductor surfaces by cross-section scanning tunneling microscopy and spectroscopy as well as transmission electron microscopy revealed physical surface effects that could have a major impact on novel electrical devices, such as light-emitting diodes, lasers, solar cells, but also high-electron-mobility transistors. Furthermore, photo-excited scanning tunneling spectroscopy was performed on non-polar GaAs(110) surfaces. With this promising technique, surface photo-voltages and local charge carrier distributions can be probed with atomic resolution. The general difficulty in a quantitative analysis of scanning tunneling spectroscopy measurements and hence in the determination of physical properties of the semiconductor surface is the $\textit{tip-induced band bending}$: The electrostatic potential of the tip is not completely screened at the surface of the sample, especially for low-doped materials. Hence, the valence- and conduction band edge of these materials are bent to higher or lower values compared to their values deep within the bulk material. Additionally, one has to take into account the generation and the redistribution of light-excited charge carriers for photo-excited scanning tunneling spectroscopy. Thus, in this thesis, a quantitative description of scanning tunneling spectroscopy with and without light-excited carriers is developed. It is based on a finite difference iteration of the electrostatic potential and the carrier distributions in three dimensions, followed by the calculation of the tunnel current that incorporates light-excited carriers. On the basis of this model, the comparison of measured and calculated scanning tunneling spectra enables the determination of the semiconductor's physical properties. At first, the model was applied to scanning tunneling spectra measured on $\textit{p}$-GaAs(110) surfaces with and without laser excitation. It is proven that the model [...

Quantitative description of photoexcited scanning tunneling spectroscopy and its application to the GaAs(110) surface

A quantitative description of photoexcited scanning tunneling spectra is developed and applied to photoexcited spectra measured on p-doped nonpolar GaAs(110) surfaces. Under illumination, the experimental spectra exhibit an increase of the tunnel current at negative sample voltages only. In order to analyze the experimental data quantitatively, the potential and charge-carrier distributions of the photoexcited tip-vacuum-semiconductor system are calculated by solving the Poisson as well as the hole and electron continuity equations by a finite-difference algorithm. On this basis, the different contributions to the tunnel current are calculated using an extension of the model of Feenstra and Stroscio to include the light-excited carrier concentrations. The best fit of the calculated tunnel currents to the experimental data is obtained for a tip-induced band bending, which is limited by the partial occupation of the C3 surface state by light-excited electrons. The tunnel current at negative voltages is then composed of a valence band contribution and a photoinduced tunnel current of excited electrons in the conduction band. The quantitative description of the tunnel current developed here is generally applicable and provides a solid foundation for the quantitative interpretation of photoexcited scanning tunneling spectroscopy

Meandering of overgrown v-shaped defects in epitaxial GaN layers

The meandering of v-shaped defects in GaN(0001) epitaxial layers is investigated by cross-sectional scanning tunneling microscopy. The spatial position of v-shaped defects is mapped on (101¯0) cleavage planes using a dopant modulation, which traces the overgrown growth front. Strong lateral displacements of the apex of the v-shaped defects are observed. The lateral displacements are suggested to be induced by the meandering of threading dislocations present in the v-shaped defects. The meandering of the dislocation is attributed to interactions with inhomogeneous strain fields