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Free keywords:
Surfaces; Semiconductors; GaAs; Molecular Beam Epitaxy; Scanning Tunneling Microscopy; Quantum Dots
Abstract:
The aim of this thesis was to investigate GaAs surfaces whose orientations are located inside the stereographic triangle, i.e., far from the low-index planes (001), (110) and (111). Various surfaces were prepared by using molecular beam epitaxy (MBE), and characterized in situ by scanning tunneling microscopy (STM) and electron diffraction (LEED, RHEED). GaAs(112)A is not stable but decays into facets. Some of these facets are oriented in a direction close to {124}. This plane is located inside the stereographic triangle. On GaAs(113)A-(8x1) a highly anisotropic step structure was observed. While steps along [33-2] are straight for up to 2000Å without any kinks, steps along [1-10] are very rough. This phenomenon was explained on the basis of the electron counting rule (ECR). Under certain preparation conditions, mesoscopic valleys appear along [33-2]. Parts of their sidewalls are of the orientation {3 7 15}. This plane is also located inside the stereographic triangle. In order to check the aforementioned indications that there is a stable surface inside the stereographic triangle, spherical depressions were ground into GaAs(113)A samples. A hitherto unknown stable surface was discovered in these depressions. Further experiments on suitably cut planar samples revealed that the Miller indices of this new surface are (2 5 11). The (1x1) unit mesh of the reconstruction contains an inclined series of three arsenic dimers and fulfills the ECR. The surface consists of large terraces without any islands or holes. Narrow stripes with the orientation (3 7 15) form on the surface. The angle between this plane and (2 5 11) is only 1°. GaAs(3 7 15) violates the ECR, but the density of dangling bonds is only 0.1% greater than on GaAs(2 5 11). It was concluded that, in general, on semiconductor surfaces the gain in stability arising from the minimization of the number of dangling bonds is significantly greater than the gain arising from reaching a semiconducting ground state. On the basis of the results for GaAs(2 5 11), the bounding facets of InAs/GaAs(001) quantum dots were unambiguously identified in STM images as {137}. The discovery of GaAs(2 5 11), the first stable surface inside the stereographic triangle, raises the question of what other surfaces there may be.
