Bibliothek

Notizen: 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 aitch-thetaox=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. ©1996 American Institute of Physics.

Notizen: 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 (approximately-greater-than)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.

Notizen: The transient behavior of the electric field during simultaneous growth and in situ poling of LiNbO3 has been studied. The transient features have been correlated with the poling of the multidomain seed and crystal, and domain reversal in poled seed. A steady-state condition is obtained after the shoulder region of the crystal is terminated and the crystal reaches its final diameter. Finally, the critical values of field parameters necessary to obtain complete monodomain crystals without causing melt electrolysis at the solid–liquid interface have been analyzed. © 1997 American Institute of Physics.

Notizen: Electrolytes formed with poly(oxyethylene), POE, and europium picrate, [Eu(pic)2(OH2)6]pic.6H2O (where pic denotes the picrate anion) or simply Eu(pic)3xH2O, have been investigated by infrared and photoluminescence spectroscopies. The complexes have been represented by POEnEu(pic)3xH2O (where n represents the molar ratio of (OCH2CH2) units per Eu3+ ion). Materials with n ranging from 133 to 11 have been examined. A tentative attribution for the absorption bands of the mid-infrared spectra is presented. The spectral changes detected in the mid-infrared region and the modifications observed in the XRD patterns at increasing salt content show that Eu(pic)3xH2O exerts an effective plasticizing role which leads to the complete supression of crystallinity at n=11. The emission spectra of the complexes and their signature in the νC–O spectral region provide conclusive evidence that the Eu3+ ions are coordinated to the oxygens atoms of the polyether chains over the whole range of compositions studied. The photoluminescence spectra of the POEnEu(pic)3xH2O electrolytes indicate that the Eu3+/ether oxygen complexation occurs with the concomitant partial substitution of water molecules from the cation coordination shell. The luminescence data obtained confirm that the latter aspect is quite advantageous from the standpoint of optical properties. The incorporation of the picrate salt into POE results in a substantial increase in the calculated total radiative contribution for the depopulation of the 5D0 level (from 0.58 to 1.94 ms−1) and in the quantum efficiency (from 5.6 to 41.9%). The analysis of the νaNO2, νsNO2 and νphenCO picrate anion characteristic bands of the spectra of the POE-based complexes suggests that the coordinated picrate ligands of the picrate salt are unaffected by the presence of POE and never leave the lanthanide coordination shell. The absence of diffraction peaks of Eu(pic)3xH2O in the diffractograms of the electrolytes and the recognition that the crystalline regions are essentially composed of pure POE allow us to conclude that the complexation effect occurs exclusively in the amorphous phase. It is probable that the charge carrier in these electrolytes is the non-hydrogen-bonded and uncoordinated picrate ligand of the salt, while the complex cation [Eu(pic)2(OH2)y(OR2)z]+ (where R2O stands for POE, y〈6 and z≥1) is expected to be immobile. © 2000 American Institute of Physics.

Notizen: 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.

Notizen: This communication corrects an error in the value previously reported by one of the authors for the electron affinity (EA) of AlN. A brief discussion is given of the potential errors in photoemission measurements of EA which affect this and other studies. Finally, a recommendation is given for 1.9 eV as the "true" EA of wurtzite AlN. © 2001 American Institute of Physics.

Notizen: 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.

Notizen: 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. © 1999 American Institute of Physics.

Notizen: The growth of SiC films on Si by reaction with tetraethyl silane (SiEt4) has been studied using Auger and electron energy-loss spectroscopies, low-energy electron diffraction, and external-reflection infrared reflection absorption spectroscopy (IRRAS). IRRAS is used to monitor the chemisorption of the reagent molecule on polycrystalline Si at substrate temperatures below the point where complete dissociation occurs. The electron techniques are used to characterize the structure and composition of SixC1−x (0≤x≤1) layers formed on Si(100) at higher temperatures during dosing with SiEt4. Near room temperature, IRRAS data indicate adsorption of undissociated -C2H5 groups with the C—C bond oriented nearly normal to the surface. Under growth conditions, the relative rates of deposition and indiffusion of C control the stoichiometry of the initial SixC1−x phase. At lower temperatures (∼750–1000 K), a partially noncarbidic phase occurs. Annealing this surface or dosing the clean surface at higher temperature leads to formation of a SiC layer which then acts to slow further diffusion. Subsequent layer growth occurs mainly by pyrolysis of the SiEt4 molecule. However, the underlying Si substrate continues to function as either a C sink or a Si source.

Notizen: Oxide layers have been formed on (110) and (100) GaAs wafers by exposure to a high kinetic energy beam of atomic O and characterized using x-ray photoemission spectroscopy (with Ar+ ion sputter profiling) and Raman spectroscopy. Photoemission shows the reacted layer, ∼500 A(ring) thick, to be uniform in composition and fully oxidized. Raman spectroscopy shows that the substrate is not appreciably disordered during oxidation and in some cases no free-elemental As is present at the oxide-substrate interface at a detectable level.