H. Eisele

Surface states and origin of the Fermi level pinning on nonpolar GaN(11¯00) surfaces

GaN(11¯00) cleavage surfaces were investigated by cross-sectional scanning tunneling microscopy and spectroscopy. It is found that both the N and Ga derived intrinsic dangling bond surface states are outside of the fundamental band gap. Their band edges are both located at the Γ¯ point of the surface Brillouin zone. The observed Fermi level pinning at 1.0 eV below the conduction band edge is attributed to the high step and defect density at the surface but not to intrinsic surface states.

Direct measurement of the band gap and Fermi level position at InN(112¯0)

A nonpolar stoichiometric InN(112¯0) surface freshly cleaved inside UHV was investigated by scanning tunneling microscopy and spectroscopy. Due to the absence of intrinsic surface states in the band gap, scanning tunneling spectroscopy yields directly the fundamental bulk band gap of 0.7±0.1u2002eV. The Fermi energy is pinned 0.3 eV below the conduction band minimum due to cleavage induced defect states. Thus, intrinsic electron accumulation can be excluded for this surface. Electron accumulation is rather an extrinsic effect due to surface contamination or material decomposition, but not an intrinsic material property of InN.

Electronic properties of dislocations in GaN investigated by scanning tunneling microscopy

We investigated the type, spatial distribution, line direction, and electronic properties of dislocations in n-type GaN by scanning tunneling microscopy. We found uncharged perfect dislocations with a/3{112¯0} Burgers vectors and negatively charged Shockley partial dislocations with a/3{11¯00} Burgers vectors interconnected by a negatively charged stacking fault. The charges are traced to different charge transfer levels associated with the particular core structure.

Doping modulation in GaN imaged by cross-sectional scanning tunneling microscopy

We investigated the imaging mechanisms of a Si doping modulation in GaN by cross-sectional scanning tunneling microscopy (STM). The Si doping modulation gives rise to a voltage and tip dependent height modulation of at least 0.4 A. The origin of the height modulation in constant-current STM images is traced to two mechanisms. A doping-induced modu-lation of the band edge energies yields a voltage dependent electronic contrast and an additional mechanical relaxation of the doping-induced strain at the cleavage surface is respon-sible for a voltage independent modulation of 0.35 A.

Reverse mass transport during capping of In0.5Ga0.5As/GaAs quantum dots

The rates of indium mass transport between the wetting layer, the quantum dots, and the capping layer are derived from the indium distributions probed by cross-sectional scanning tunneling microscopy of the In0.5Ga0.5As/GaAs quantum dot system. During capping, a lateral back-segregation from the quantum dots toward the wetting layer is found, reversing the Stranski-Krastanov growth mode during quantum dot formation. This lateral back-segregation critically affects the resulting indium distribution in the wetting layer, the apparent segregation coefficients as well as the quantum dot shape. Furthermore, the strain effect on the segregation coefficient is quantified.

Adsorbate-induced restructuring of Pb mesas grown on vicinal Si(111) in the quantum regime

Using scanning tunneling microscopy and spectroscopy, we demonstrate that the adsorption of a minute amount of Cs on a Pb mesa grown in the quantum regime can induce dramatic morphological changes in the mesa, characterized by the appearance of populous monatomic-layer-high Pb nanoislands on top of the mesa. The edges of the Pb nanoislands are decorated with Cs adatoms, and the nanoislands preferentially nucleate and grow on the quantum mechanically unstable regions of the mesa. Furthermore, first-principles calculations within density-functional theory show that the Pb atoms forming these nanoislands were expelled by the adsorbed Cs atoms via a kinetically accessible place-exchange process when the Cs atoms alloyed into the top layer of the Pb mesa.

Intrinsic bandgap of cleaved ZnO(112¯0) surfaces

The existence of intrinsic surface states, the position of the Fermi level, and the size of the surface bandgap of the non-polar ZnO(112¯0) cleavage surfaces were investigated by scanning tunneling microscopy and spectroscopy. The comparison of spectroscopic measurements performed on atomically flat and stepped surfaces reveals the absence of intrinsic surface states within the fundamental bulk bandgap, but shows the occurrence of step-induced gap states. These states lead to a pinning of the Fermi level at the surface within the bandgap and generate a significant defect-related tunnel current, narrowing the measured apparent bandgap.

Cross-sectional scanning tunneling microscopy and spectroscopy of nonpolar GaN(11¯00) surfaces

GaN(11¯00) cleavage surfaces were investigated by cross-sectional scanning tunneling microscopy and spectroscopy combined with calculations of the tunnel current. The different contributions to the tunnel current and their origins are determined by comparison of the experimental results with calculated tunnel currents for both pinned and unpinned GaN surfaces. It is found that both the N and Ga derived intrinsic dangling bond surface states are outside of the fundamental band gap and lead to electron tunneling out of the valence and into the conduction band, respectively. The band edges of both intrinsic surface states are located at the Γ¯ point of the surface Brillouin zone. The observed Fermi level pinning 1.0 eV below the conduction band edge is attributed to the high defect density at the surface, but not to intrinsic surface states. The defects give rise to a current due to electrons tunneling out of the defect states within the band gap.

Strain and compositional fluctuations in Al0.81In0.19N/GaN heterostructures

The strain and compositional fluctuations of nearly lattice-matched Al0.81In0.19N/GaN heterostructures are investigated by cross-sectional scanning tunneling microscopy and selected area electron diffraction measurements in scanning electron transmission microscopy. The presence of strain induces height modulations governed by different roughness components at the cleavage surfaces. The surface height modulations are compatible with a relaxation of alternatingly compressive and tensile strained domains, indicating compositional fluctuations. Changes of the a lattice constant are traced to interface misfit edge dislocations. The dislocations induce steps increasing the roughness within the Al0.81In0.19N layers.