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

摘要

The investigation of non-polar III-V semiconductor surfaces by cross-section scanning tunneling microscopy (X-STM) and spectroscopy (X-STS) with and without laser excitation may reveal physical effects that could have a major impact on novel electrical devices, such as light-emitting diodes, lasers, and solar cells. For a quantitative analysis and hence the extraction of physical properties, a quantitative description of the tunnel current is needed, taking into account the tipinduced band bending and light-excited carriers. Laser-excited X-STS on non-polar p-GaAs(110) surfaces show an increased tunnel current under illumination for negative sample voltages. For positive voltages, no difference between the current measured under illuminated and dark conditions is detectable. Qualitatively, this result is attributed to the generation of light-excited minority carriers (electrons), screening the electric field at the sample surface and tunneling from the sample into the tip at negative voltages. Here, a quantitative description of the tunnel current is developed, employing a finite-difference algorithm that solves not only the Poisson equation, but also the continuity equations of electrons and holes. Light-excited carriers are introduced in the electrostatic part of the calculation as well as in the derivation of the tunnel current by generation and recombination terms and a modified tunnel model, respectively. Using the irradiation of the laser beam as only fitting parameter for the current under illumination, good agreement between the measurement and the calculation is obtained, validating the accuracy of the developed model. Furthermore, it is found that the GaAs(110) C3 surface state can be charged by light-excited electrons, limiting the band bending. Focusing on nitride-based semiconductors, X-STS on n-GaN(1010) surfaces and the application of the newly developed tunnel current simulation reveal a polarity depended Fermi level pinning: The intrinsic empty surface state in the fundamental band gap pins the Fermi level at small positive voltages only, but not at negative voltages. In equilibrium, the surface state is shifted below the Fermi energy by band bending for negative voltages. Hence, it is expected to be partially filled. However, in non-equilibrium, when electrons can tunnel from the surface state into the tip, one has to take into account the rates of the charging and discharging processes. It is found that the optical transition of electrons from the conduction band to the surface state (charging process) is prohibited by quantum mechanical selection rules due to the same orbital character of the surface state and the conduction band minimum. Hence, the surface state does not reach its equilibrium occupation and therefore does not pin. Furthermore, InN layers grown on GaN(0001) are probed by combining the advantages of X-STM, X-STS, the simulation model, and transmission electron microscopy (TEM). The lattice mismatch of GaN and InN is found to be dissipated directly at the GaN/InN interface by introducing a dislocation network. As a result, the overgrown InN layer is of highest quality with a low defect concentration. On the m-plane cleavage surface, the interface dislocations induce steps in [0001] direction in the InN region. These steps lead to an extrinsic pinning of the Fermi level. An electron accumulation layer near the surface (previously observed on low-quality InN layers), induced by intrinsic surface states, is not observed on the high-quality samples. Thus, the electron accumulation layer is not an intrinsic property and hence InN appears to be as conventional as other III-V semiconductors.