A. Eisenhardt

Confirmation of intrinsic electron gap states at nonpolar GaN(1-100) surfaces combining photoelectron and surface optical spectroscopy

The electronic structure of GaN(1–100) surfaces is investigated in-situ by photoelectron spectroscopy (PES) and reflection anisotropy spectroscopy (RAS). Occupied surface states 3.1 eV below the Fermi energy are observed by PES, accompanied by surface optical transitions found in RAS around 3.3 eV, i.e., below the bulk band gap. These results indicate that the GaN(1–100) surface band gap is smaller than the bulk one due to the existence of intra-gap states, in agreement with density functional theory calculations. Furthermore, the experiments demonstrate that RAS can be applied for optical surface studies of anisotropic crystals.

Surface states and electronic structure of polar and nonpolar InN – An in situ photoelectron spectroscopy study

Valence band structure and surface states of InN with (0001), (000-1), (1-100), and (11-20) orientation were investigated in situ after growth using photoelectron spectroscopy. Depending on surface orientation, different occupied surface states are identified and differentiated from bulk contributions. For N-polar, m-plane, and a-plane InN, the surface states are located at the valence band maximum, while In-polar InN features surface states close to the Fermi level. The surface band alignment correlates with the position of surface states. For InN(0001), a much larger surface downward band bending is observed compared to N-polar, m-plane, and a-plane InN, where almost flat band conditions occur.

N-type conductivity and properties of carbon-doped InN(0001) films grown by molecular beam epitaxy

In this work, we have analyzed the effect of intentional carbon doping on molecular beam epitaxy grown In-polar InN epilayers using carbon bromide (CBr4) as dopant source. Hall effect measurements, high resolution X-ray diffraction, atomic force microscopy, transmission electron microscopy, secondary ion mass spectrometry, spectroscopic ellipsometry, as well as X-ray photoelectron spectroscopy were employed to characterize the influence of different dopant concentrations on the electrical, optical, crystallographic, morphological, and electronic properties of InN. It was found that the electron concentration increases linearly with the incorporation of carbon pointing towards the effect of n-type doping and that incorporated C impurities reduce the electron mobility within the InN films. This correlation is further reflected in associated properties such as the onset of optical absorption, the plasmon frequency, the effective electron mass and the position of the bulk and surface Fermi level. Furthermore,...

Reduction of electron accumulation at InN(0001) surfaces via saturation of surface states by potassium and oxygen as donor- or acceptor-type adsorbates

The influence of selected donor- and acceptor-type adsorbates on the electronic properties of InN(0001) surfaces is investigated implementing in-situ photoelectron spectroscopy. The changes in work function, surface band alignment, and chemical bond configurations are characterized during deposition of potassium and exposure to oxygen. Although an expected opponent charge transfer characteristic is observed with potassium donating its free electron to InN, while dissociated oxygen species extract partial charge from the substrate, a reduction of the surface electron accumulation occurs in both cases. This observation can be explained by adsorbate-induced saturation of free dangling bonds at the InN resulting in the disappearance of surface states, which initially pin the Fermi level and induce downward band bending.