Atomic hydrogen cleaning of GaSb(001) surfaces

We show that the (001) surface of GaSb can be cleaned efficiently by exposure to atomic hydrogen at substrate temperatures in the range 400–470 °C. This treatment removes carbon and oxygen contamination, leaving a clean, ordered surface with a symmetric (1 × 3) reconstruction after a total H2 dose of approximately 150 kL. An ordered but partially oxidized surface is generated during cleaning, and the removal of this residual oxide is the most difficult part of the process. Auger electron spectroscopy and low energy electron diffraction were used to monitor the chemical cleanliness and the ordering of the surface during the cleaning process, whereas high resolution electron energy loss spectroscopy was used to probe the electronic structure in the near-surface region. The results obtained indicates that this cleaning procedure leaves no residual electronic damage in the near-surface region of the Te-doped (n ~ 5 × 1017 cm – 3) samples of GaSb(001) studied

Controlled oxide removal for the preparation of damage-free InAs(110) surfaces

Controlled oxide removal from InAs(110) surfaces using atomic hydrogen (H*) has been achieved by monitoring the contaminant vibrational modes with high resolution electron energy loss spectroscopy (HREELS). The contributing oxide vibrational modes of the partially H* cleaned surface have been identified. Following hydrocarbon desorption during preliminary annealing at 360 °C, exposure to atomic hydrogen at 400 °C initially removes the arsenic oxides and indium suboxides; complete indium oxide removal requires significantly higher hydrogen doses. After a total molecular hydrogen dose of 120 kL, a clean, ordered surface, exhibiting a sharp (1×1) pattern, was confirmed by low energy electron diffraction and x-ray photoelectron spectroscopy. Energy dependent HREELS studies of the near-surface electronic structure indicate that no residual electronic damage or dopant passivation results from the cleaning process

Electron dynamics in InNxSb1–x

Electron transport properties in InNxSb1–x are investigated for a range of alloy compositions. The band structure of InNxSb1–x is modeled using a modified k·p Hamiltonian. This enables the semiconductor statistics for a given x value to be calculated from the dispersion relation of the E– subband. These calculations reveal that for alloy compositions in the range 0.001<=x<=0.02 there is only a small variation of the carrier concentration at a given plasma frequency. A similar trend is observed for the effective mass at the Fermi level. Measurements of the plasma frequency and plasmon lifetime for InNxSb1–x alloys enable the carrier concentration and the effective mass at the Fermi level to be determined and a lower limit for the electron mobility to be estimated

Accumulation layer profiles at InAs polar surfaces

High resolution electron energy loss spectroscopy, dielectric theory simulations, and charge profile calculations have been used to study the accumulation layer and surface plasmon excitations at the In-terminated (001)-(4 × 1) and (111)A-(2 × 2) surfaces of InAs. For the (001) surface, the surface state density is 4.0 ± 2.0 × 1011 cm – 2, while for the (111)A surface it is 7.5 ± 2.0 × 1011 cm – 2, these values being independent of the surface preparation procedure, bulk doping level, and substrate temperature. Changes of the bulk Fermi level with temperature and bulk doping level do, however, alter the position of the surface Fermi level. Ion bombardment and annealing of the surface affect the accumulation layer only through changes in the effective bulk doping level and the bulk momentum scattering rate, with no discernible changes in the surface charge density

Probing the interfacial and sub-surface structure of Si/Si1 – xGex multilayers

The ability to determine structural and compositional information from the sub-surface region of a semiconductor material has been demonstrated using a new time-of-flight medium energy ion scattering spectroscopy (ToF-MEISS) system. A series of silicon–silicon/germanium (Si/Si1 – xGex) heterostructure and multilayer samples, grown using both solid source molecular beam epitaxy (MBE) and gas source chemical vapor deposition (CVD) on Si(100) substrates, have been investigated. These data indicate that each individual layer of Si1 – xGex (x ~ 0.22) in both two- and three-period samples, can be uniquely identified with a resolution of approximately 3 nm. A comparison of MBE and CVD grown samples has also been made using layers with similar structures and composition. The total Ge content of each sample was confirmed using conventional Rutherford backscattering spectrometry

Origin of the n-type conductivity of InN: the role of positively charged dislocations

As-grown InN is known to exhibit high unintentional n-type conductivity. Hall measurements from a range of high-quality single-crystalline epitaxially grown InN films reveal a dramatic reduction in the electron density (from low 1019 to low 1017 cm–3) with increasing film thickness (from 50 to 12 000 nm). The combination of background donors from impurities and the extreme electron accumulation at InN surfaces is shown to be insufficient to reproduce the measured film thickness dependence of the free-electron density. When positively charged nitrogen vacancies (VN+) along dislocations are also included, agreement is obtained between the calculated and experimental thickness dependence of the free-electron concentration

Evidence from scanning tunneling microscopy in support of a structural model for the InSb(001)-c(8×2) surface

In this letter we present evidence from scanning tunneling microscopy studies in support of a recently proposed structural model for the indium-terminated c(8×2) surface of InSb(001). This structural model, by Norris and co-workers, is based on a surface x-ray diffraction study [Surf. Sci. 409, 27 (1998)], and represents a significant departure from previously suggested models for the c(8×2) reconstruction on any (001) surface of the common III–V semiconductor materials. Although filled state images of the InSb(001)-c(8×2) surface have previously been published, empty states image of sufficient quality to extract any meaningful information have not previously been reported. The observations are in excellent agreement with the recently proposed model for this surface reconstruction

Core-level photoemission spectroscopy of nitrogen bonding in GaNxAs1–x alloys

The nitrogen bonding configurations in GaNxAs1–x alloys grown by molecular beam epitaxy with 0.07=0.03, the nitrogen is found to exist in a single bonding configuration – the Ga–N bond; no interstitial nitrogen complexes are present. The amount of nitrogen in the alloys is estimated from the XPS using the N 1s photoelectron and Ga LMM Auger lines and is found to be in agreement with the composition determined by x-ray diffraction

X-ray photoemission spectroscopy determination of the InN/yttria stabilized cubic-zirconia valence band offset

The valence band offset of wurtzite InN(0001)/yttria stabilized cubic-zirconia (YSZ)(111) heterojunctions is determined by x-ray photoemission spectroscopy to be 1.19±0.17 eV giving a conduction band offset of 3.06±0.20 eV. Consequently, a type-I heterojunction forms between InN and YSZ in the straddling arrangement. The low lattice mismatch and high band offsets suggest potential for use of YSZ as a gate dielectric in high-frequency InN-based electronic devices

Gamma-ray Spectral Evolution of NGC1275 Observed with Fermi-LAT

We report on a detailed investigation of the high-energy gamma-ray emission from NGC\,1275, a well-known radio galaxy hosted by a giant elliptical located at the center of the nearby Perseus cluster. With the increased photon statistics, the center of the gamma-ray emitting region is now measured to be separated by only 0.46' from the nucleus of NGC1275, well within the 95% confidence error circle with radius ~1.5'. Early Fermi-LAT observations revealed a significant decade-timescale brightening of NGC1275 at GeV photon energies, with a flux about seven times higher than the one implied by the upper limit from previous EGRET observations. With the accumulation of one-year of Fermi-LAT all-sky-survey exposure, we now detect flux and spectral variations of this source on month timescales, as reported in this paper. The average >100 MeV gamma-ray spectrum of NGC1275 shows a possible deviation from a simple power-law shape, indicating a spectral cut-off around an observed photon energy of E = 42.2+-19.6 GeV, with an average flux of F = (2.31+-0.13) X 10^{-7} ph/cm^2/s and a power-law photon index, Gamma = 2.13+-0.02. The largest gamma-ray flaring event was observed in April--May 2009 and was accompanied by significant spectral variability above E > 1-2 GeV. The gamma-ray activity of NGC1275 during this flare can be described by a hysteresis behavior in the flux versus photon index plane. The highest energy photon associated with the gamma-ray source was detected at the very end of the observation, with the observed energy of E = 67.4GeV and an angular separation of about 2.4' from the nucleus. In this paper we present the details of the Fermi-LAT data analysis, and briefly discuss the implications of the observed gamma-ray spectral evolution of NGC1275 in the context of gamma-ray blazar sources in general.Comment: 20 pages, 6 figures, accepted for publication in the Ap