Victor M. Bermudez

Study of oxygen chemisorption on the GaN(0001)‐(1×1) surface

Clean, ordered GaN(0001)‐(1×1) surfaces are prepared by sputtering with nitrogen ions followed by annealing in ultrahigh vacuum. The surfaces are subsequently exposed at room temperature to O2 and the chemisorption process studied using Auger, valence and core‐level photoemission and electron energy loss spectroscopies, low‐energy electron diffraction, and work function measurements. Saturation occurs at a coverage of Θox=0.4 ML and is accompanied by the removal of surface states near the band edges. The continued presence of a clear (1×1) diffraction pattern, together with other data, indicates a well‐defined adsorption site, but the relative importance of Ga–O and N–O bonding remains undetermined. The realization that surface states exist near the valence‐band maximum has led to a more accurate determination of the surface Fermi‐level pinning position, and of dependent quantities, than given previously. Clean‐surface data are also compared with those for surfaces prepared by in situ deposition of Ga metal followed by thermal desorption. No significant differences are seen, which suggests that nitrogen‐ion sputtering and annealing is suitable for preparing clean, ordered GaN(0001)‐(1×1) surfaces. The results for O chemisorption on atomically clean surfaces have been applied to evaluating the passivation of surfaces prepared by ex situ wet‐chemical cleaning. The band bending is found to be ∼0.5 eV less than on atomically clean surfaces.

The growth and properties of Al and AlN films on GaN(0001)–(1×1)

The growth, structure, and annealing behavior of Al films, formed by in situ vapor deposition on GaN(0001)–(1×1) near 25 °C, have been studied using Auger, electron energy loss, x ray and ultraviolet photoemission spectroscopies and low‐energy electron diffraction. Film growth occurs by a Stranski–Krastanov process with reaction at the immediate interface leading to metallic Ga. Annealing at ≳800 °C leads to release of N, which reacts with Al to form a (1×1)‐ordered layer of AlN, possibly alloyed with a small amount of Ga. The AlN layer has been characterized using the various spectroscopies, and the work function, band bending, and electron affinity of GaN and of the AlN overlayer have been obtained. The Al/GaN Schottky barrier height has been measured and compared with previous results for Ni/GaN.

Luminescence cycling and defect density measurements in porous silicon: Evidence for hydride based model

Changes in dangling bond densities in porous silicon were measured and results indicate a relatively low dangling bond density (roughly 3×1016 bonds/cm3) in as‐prepared samples, which increases by a factor of 6–7 upon quenching of the photoluminescence (PL). The electron spin resonance (ESR) data suggest the presence of significant disorder in the as‐prepared 1 Ω cm porous silicon samples, which may correlate with an enhanced PL intensity. The results of heat cycling and HF dipping experiments suggest that a continuous decrease in particle size does not result in a continuous PL blue shift, as would be expected in the quantum confinement model. These results will be discussed in terms of a hydride/polysilane luminescence mechanism.

Auger and electron energy‐loss study of the Al/SiC interface

Auger and electron energy‐loss spectroscopies, as functions of Al coverage and annealing temperature, have been used to determine the mechanism of formation of the Al/SiC interface. Al deposited at room temperature forms quasi‐metallic islands randomly distributed over the surface. Annealing at moderate temperature (≤600 °C) causes aggregation of Al at C‐rich sites. At higher temperature, Al reacts with C (but not with Si) to form Al4C3.

Wavelength-scanning polarization-modulation ellipsometry: some practical considerations

A discussion is presented of the practical considerations involved in wavelength-scanning polarization-modulation ellipsometry. Emphasis is placed on factors affecting accuracy and precision and on the alignment of the optical elements. The system described is used to measure the optical properties of air-cleaved KCI and of clean and tarnished Ag surfaces in ultrahigh vacuum in the 250-650-nm range.

Adsorption and co-adsorption of boron and oxygen on ordered α-SiC surfaces

Abstract Boron layers (grown by thermal decomposition of B10H14) on the (0001) Si- and (0001) C-terminated surfaces of α-SiC have been studied using Auger and electron energy-loss spectroscopies and low-energy electron diffraction. Adsorption of O2 on the clean and B-adsorbed surfaces was also studied. Cleaning the (0001) surface by annealing in a flux of Si vapor gives a (3 × 3) structure which converts to 3 × 3 upon further annealing in vacuum. The (3 × 3) consists of an ordered layer of Si chemisorbed on the Si termination layer, while the 3 × 3 involves an ordered arrangement of Si vacancies. A (1 × 1) structure is observed for the clean C-face. Adsorption of B on the (3 × 3) Si-face eliminates the reconstruction, and further annealing produces a complicated superstructure. Stronger interaction occurs on the 3 × 3 Si-face leading to an incommensurately ordered (1 × 1) layer loosely termed “Si boride” due to the substantial changes in SiL2,3VV and B KLL Auger lineshapes. On the C-face, a disordered B layer forms. On B-free surfaces, room-temperature chemisorption of O2 is slower on the (1 × 1) C-face than on the (3 × 3) Si-face, but the rates of O uptake become comparable with increasing coverage as chemisorption gives way to oxidation. For the 3 × 3 Si-face, chemisorption is rapid, but oxidation very slow. For all three surfaces, adsorbed B suppresses chemisorption of O2 but has little or no effect on oxidation.

Characterization of reconstructed SiC(100) surfaces using soft‐x‐ray photoemission spectroscopy

The surface quality of βSiC films grown on Si(100) by chemical vapor deposition has been assessed through synchrotron photoemission measurements of the valence band and of the linewidths and surface‐induced structure in Si 2p core‐level spectra. For these n‐type samples, band bending is small on the c(2×2) and (3×2) surfaces but larger on the (2×1), which also exhibits an increased Si 2p linewidth and evidence of elemental Si patches. All three reconstructions show emission from gap states extending from the valence band maximum to the Fermi level.

Growth and structure of aluminum films on (001) silicon carbide

The formation and the physical and electronic structure of the interface between Al and SiC films, grown epitaxially on Si(001), are studied using x‐ray photoelectron spectroscopy (XPS), low‐energy electron diffraction (LEED), and energy‐loss spectra (ELS). Zr M‐zeta excitation (hν=151.4 eV) is employed to obtain high surface sensitivity in the Si and Al 2p and valence‐band photoemission. The first few monolayers of Al grow as layers, with Al island formation at higher coverage. Al‐Si interaction is apparent as a shift of the Al 2p (Si 2p) to higher (lower) binding energy (BE) for θ≤1. A Schottky barrier height of ≊1.4 eV is estimated. At higher θ the Al 2p assumes the BE and shape characteristic of bulk Al, and the Si 2p shows satellite structure to lower BE suggesting both Si bonded to Al and C as well as Si interacting mainly with Al. Annealing (350≤T≤1050 °C) leads to a reduction in Al coverage and reversal of the trends observed during sequential deposition. Before annealing LEED shows only a weak (1×1) pattern. Annealing at successively higher temperatures leads to a sharper (1×1), followed by two‐domain (4×1) and two‐domain c(8×2) patterns. No clear indication of Al carbide formation is found in Auger electron spectra or in the Al 2p XPS unless the SiC, prior to Al deposition, is first treated at high temperature to generate a C‐rich surface.

Photoemission study of oxygen adsorption on (001) silicon carbide surfaces

X‐ray photoemission (ZrMζ, hν=151.4 eV, and MgKα, hν=1253.6 eV) and electron energy loss spectroscopies, low‐energy electron diffraction and work‐function measurement have been used to study the initial adsorption of oxygen on cubic β‐SiC(001) at room temperature. Three different SiC surfaces have been considered—Si‐rich [two‐domain (3×1) low‐energy electron diffraction pattern], stoichiometric [two‐domain (2×1)], and Si‐deficient [c(2×2)]. Similar data have also been obtained for a Si(001)‐(2×1) surface. For SiC the initial rates of O uptake are in the order (2×1)>(3×1)>c(2×2), and the rates for all three are much less than that for Si (2×1). A model for the initial adsorption of O on SiC is proposed in which the rates for the different SiC surfaces reflect the relative ease of formation of Si‐O‐Si bridges between surface Si atoms while the greater rate for Si versus SiC results from the difficulty in inserting O into SiC backbonds.

Simple interpretation of metal/wurtzite–GaN barrier heights

Photoemission data for the dependence of the Schottky barrier height on the metal work function, for n-type wurtzite GaN, are discussed in terms of the Cowley–Sze model [J. Appl. Phys. 36, 3212 (1965)] for a uniform density of surface states in the band gap. It is suggested that, in the context of this model, such barrier heights can be expressed largely as a sum of the “bare-surface barrier height” (i.e., the band bending before contact formation) and a Mott–Schottky term.