W. Mönch

Surface analysis of 6HSiC

Abstract The composition of {0001} surfaces of 6HSiC samples was studied by using low-energy electron diffraction, Auger electron (AES), and X-ray photoelectron spectroscopy (XPS/SXPS). The samples were cleaned in ultrahigh vacuum by heating them either in the presence of a Si flux at different temperatures or by annealing at 1170 K for 10 min. Depending on the preparation method and temperature used four reconstructions were observed: (1 × 1), (3 × 3), (√3 × √3)R30°, and (6√3 × 6√3)R30°. The compositions of the reconstructions and the chemical bonding of the surface atoms were characterized using AES and XPS/SXPS. Models for the reconstructions are proposed.

Phonons in 3C-, 4H-, and 6H-SiC

Abstract Silicon carbide epilayers of cubic (3C) and hexagonal (4H and 6H) polytypes were investigated by Auger electron spectroscopy, high-resolution electron energy-loss spectroscopy and Raman spectroscopy to determine the excitation energies of the optical Fuchs-Kliewer surface phonons and their relation to bulk phonon frequencies. The surfaces were treated in a buffered hydrofluoric acid solution. Loss structures attributed to excitation of Fuchs-Kliewer phonons were clearly resolved. Their energies were found at 115.9 ± 1 meV irrespective of the SiC polytype. The experimental data agree with values calculated from the experimental bulk phonon frequencies and tabulated dielectric constants.

Oxidation stages of clean and H-terminated Si(001) surfaces at room temperature

Abstract The oxygen uptake on clean Si(001)-2 × 1 and H-terminated Si(001)-l × 1 surfaces at room temperature was investigated by Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED). Surfaces were cleaned by Ar + -ion sputtering and annealing at 1200 K. H-terminated surfaces were prepared by etching of thermally oxidized samples in hydrofluoric acid. The samples were then exposed to research grade oxygen in the range from 10 14 to 10 30 O 2 -molecules/cm 2 . During exposures any excitations of the surface or the gas were avoided. The uptake of oxygen on clean surfaces proceeds in two subsequent steps. The first process saturates at about 1 monolayer and may be attributed to dissociative chemisorption. The second process sets in at a dose of 10 19 O 2 -molecules/cm 2 and follows an inverse-logarithmic growth law. It may be described by field-assisted oxidation (Mott-Cabrera mechanism). The results are compared with similar data for Si(111) surfaces. Irrespective of surface orientation and reconstruction, the oxidation process starts always at the same exposure whereas the initial sticking coefficient and the rate of oxidation depend on the orientation of the investigated surfaces. On HF-treated surfaces, the oxygen adsorption is strongly inhibited: the sticking coefficient amounts only to approximately 10 −12 .

Surface phonons in InP(110)

Abstract The surface phonon dispersion of cleaved InP(110) surfaces was measured by use of high-resolution electron energy-loss spectroscopy (HREELS) along the symmetry directions gGX and gGX′ , i.e., parallel and perpendicular to the surface In-P chains. Besides an acoustic surface phonon branch with energy lower than 9 meV, four optical phonon bands with little dispersion were recorded at 9.5, 19.5, 30.5 to 33.5, and 42.3 meV. The band with excitation energies around 32 meV represents a true surface mode as it is located in the energy gap between the acoustic and optical bulk phonon bands. The results are compared with available calculations of InP(110) surface phonons.

ELECTRONIC PROPERTIES OF CESIUM ON 6H-SIC SURFACES

The adsorption of cesium on clean {0001} surfaces of n‐ and p‐6H‐SiC samples at low temperatures was investigated by using Auger electron, x‐ray photoelectron, and ultraviolet photoelectron spectroscopy as well as a Kelvin probe. At clean surfaces the Fermi level is pinned at 1.2 eV above the valence‐band maximum and the ionization energy measures 5.7 and 5.8 eV on Siand become metallic after the deposition of the first Cs layer. For submonolayer coverages, Cs‐induced surface donors form at 2.96 eV above the valence‐band maximum. They are due to covalent Cs–Si bonds. The barrier height of Cs/6H‐SiC Schottky contacts was found as 0.57±0.05 eV with n‐type and 2.28±0.1 eV with p‐type doped samples. These results confirm the concept that the continuum of metal‐induced gap states determines the barrier heights of ideal metal‐semiconductor contacts.

Dynamical properties of 3C-, 4H-, and 6HSiC surfaces

Abstract Clean 3C-, 4H-, and 6H SiC surfaces were investigated by low-energy electron diffraction, Auger electron spectroscopy, and high-resolution electron energy-loss spectroscopy. The surfaces were treated in buffered HF and then cleaned in ultrahigh vacuum by heating in the presence of a Si flux at different temperatures. Differently reconstructed surfaces ranging from Si-rich to graphitized were examined. On 3C-5 × 2, 3C-2 × 1, 3C-c(2 × 2), and 6H (√3×√3)R30° surfaces the Fuchs-Kliewer (FK) phonons were found around 940 cm−1. At 3C-3 × 2 surfaces the FK phonon energy is shifted to about 935 cm−1 and even lower values are found for 3C-3 × 2 samples with excess Si on top and 6H-3 × 3 surfaces. At well-prepared 3C-3 × 2 samples additional loss structures at 380 and 700 cm−1 are detected. They may be attributed to intrinsic surface-localized vibrations. Heating at high temperatures results in 3C-1 × 1- and hexagonal (6√3× 6√3)R30°- reconstructed surfaces. The FK phonon losses of such graphitized samples are asymmetrically-broadened and shifted to higher loss energies of about 950 cm−1 for 3C- and 6H SiC and 960 cm−1 for 4H SiC. In off-specular scattering geometry an energy loss around 1600 cm−1 is detected. It may be attributed to high-energy optical phonons of the graphite layer.

Vibrational modes of hydrogen and deuterium at InP(110) surfaces

Vibrations at hydrogen- and deuterium-covered InP(110) surfaces were investigated by high-resolution electron energy-loss spectroscopy. With semi-insulating crystals, InH and PD bending modes were found at 438 and 465 cm−1 (54.3 and 57.7 meV), respectively. In the low-coverage regime two components of the PH stretching vibration at 2260 and 2300 cm−1 (280.2 and 285.4 meV) are resolved. They are explained by vibrational motions of mono- and dihydride groups. This indicates that InP(110) surfaces are disrupted even after small H exposures. Bond strengths are estimated from the isotope shifts of the stretching frequencies.

Bending and stretching vibrations of hydrogen and deuterium at GaAs(110) surfaces

Abstract Vibrations at hydrogen and deuterium covered GaAs(110) surfaces were investigated by high resolution electron energy loss spectroscopy. With semi-insulating crystals, bending modes of H and D were found at 515 and 387 cm −1 (63.9 and 48 meV), respectively. After large exposures when substrate bonds are broken scissor modes at 1000 cm −1 (124 meV) for AsH 2 and at 752 cm −1 (93.2 meV) for AsD 2 groups were observed. In the low-coverage regime two components of the GaH stretching vibration at 1830 and 1860 cm −1 (226.9 and 230.6 meV) were resolved. They may be explained by GaH dipole-dipole interactions.

Vibrational properties of GaN(0001) surfaces

Abstract Differently treated hexagonal GaN(0001) epilayers on Al2O3 substrates were investigated by high-resolution electron energy-loss spectroscopy (HREELS) to determine their vibrational properties. X-ray photoemission spectroscopy (XPS), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED) were utilized to monitor the surface composition and structure, respectively. A comparison was made between samples which were cleaned solely by a wet chemical etch prior to insertion into the vacuum system and such which were additionally heated in situ to 800°C first in a Ga-beam and subsequently in UHV. LEED investigations exhibited 1 × 1 patterns for both of the preparation procedures, whereas only for the latter no oxygen contamination was detected by XPS and AES. The AES spectra revealed the presence of C which cannot be traced back to hydrocarbon contamination, since the HREEL spectra showed no such loss features. Electron energy-loss structures at 700 cm−1 (86.8 meV) which are attributed to optical Fuchs-Kliewer phonons were found for both preparation methods. Spectra recorded from heat-treated samples exhibited additional loss features at 565 cm−1 (70 meV). These may be attributed to vibrations between Ga and residual C atoms.

Adsorption of chlorine on GaAs(110) investigated by high-resolution electron energy-loss spectroscopy

Abstract The adsorption of chlorine on cleaved GaAs(110) surfaces at room temperature was investigated by high-resolution electron energy-loss spectroscopy (HREELS) and Auger electron spectroscopy (AES). Chlorine molecules were released from an electrochemical Ag-AgCl-Pt cell. With AES, adsorption of chlorine was observed to saturate at about 0.8 of a monolayer. It is followed by an etching process which removes Ga atoms from the surface. After exposing the surfaces to Cl2, an adsorbate-induced energy-loss peak was found with HREELS which is identified as an adsorbate vibration. The width and the energetic position of this loss peak vary as a function of chlorine coverage. Comparing this loss energy with vibrational energies of AsClBr2 and GaClBr2 molecules suggests to attribute the loss feature to Ga-Cl valence vibrations (430 cm−1, 53.3 meV) for Cl coverages up to one monolayer and to As-Cl valence vibrations (386 cm−1, 47.9 meV) when etching occurs.