Metal organic chemical vapor deposition of ultrathin ZrO2 films on Si(1 0 0) and Si(1 1 1) studied by electron spectroscopy

The growth of ultrathin ZrO₂ films on Si(1 0 0)-(2 × 1) and Si(1 1 1)-(7 × 7) has been studied with core level photoelectron spectroscopy and X-ray absorption spectroscopy. The films were deposited sequentially by chemical vapor deposition in ultra-high vacuum using zirconium tetra-tert-butoxide as precursor. Deposition of a > 50 Å thick film leads in both cases to tetragonal ZrO₂ (t-ZrO₂), whereas significant differences are found for thinner films. On Si(1 1 1)-(7 × 7) the local structure of t-ZrO₂ is not observed until a film thickness of 51 Å is reached. On Si(1 0 0)-(2 × 1) the local geometric structure of t-ZrO₂ is formed already at a film thickness of 11 Å. The higher tendency for the formation of t-ZrO₂ on Si(1 0 0) is discussed in terms of Zr-O valence electron matching to the number of dangling bonds per surface Si atom. The Zr-O hybridization within the ZrO₂ unit depends furthermore on the chemical composition of the surrounding. The precursor t-butoxy ligands undergo efficient C-O scission on Si(1 0 0), leaving carbonaceous fragments embedded in the interfacial layer. In contrast, after small deposits on Si(1 1 1) stable t-butoxy groups are found. These are consumed upon further deposition. Stable methyl and, possibly, also hydroxyl groups are found on both surfaces within a wide film thickness range.     

Adsorption and thermal decomposition of C60 on Co/Si(111)-7 × 7

We present a study on the adsorption and thermal decomposition of C₆₀ on Co covered Si(111)-7 × 7 using scanning tunneling microscopy and X-ray photoelectron spectroscopy. Co-induced magic clusters grown on Si(111)-7 × 7 are identified as a possible adsorption site where 51 ± 3% of C₆₀ molecules adsorb at room temperature. On Co/Si(111)-7 × 7, C₆₀ molecules start to decompose at 450 °C, and are completely dissociated to form SiC by 720 °C. This temperature is significantly lower than 910 °C at which C₆₀ completely dissociates on clean Si(111)-7 × 7. This is a possible low temperature method for growing crystalline SiC films using C₆₀ as a precursor molecule.     

Ultra-thin Si overlayers on the TiO2 (1 1 0)-(1 × 2) surface: Growth mode and electronic properties

The growth of thin subnanometric silicon films on TiO₂ (1 1 0)-(1 × 2) reconstructed surfaces at room temperature (RT) has been studied in situ by X-ray and ultra-violet photoelectron spectroscopies (XPS and UPS), Auger electron and electron-energy-loss spectroscopies (AES and ELS), quantitative low energy electron diffraction (LEED-IV), and scanning tunneling microscopy (STM). For Si coverage up to one monolayer, a heterogeneous layer is formed. Its composition consists of a mixture of different suboxides SiO_x (1 < x ? 2) on top of a further reduced TiO₂ surface. Upon Si coverage, the characteristic (1 × 2) LEED pattern from the substrate is completely attenuated, indicating absence of long-range order. Annealing the SiO_x overlayer results in the formation of suboxides with different stoichiometry. The LEED pattern recovers the characteristic TiO₂ (1 1 0)-(1 × 2) diagram. LEED I-V curves from both, substrate and overlayer, indicate the formation of nanometric sized SiO_x clusters.     

Surface chemistry of HfI4 on Si(1 0 0)-(2 × 1) studied by core level photoelectron spectroscopy

The chemistry of HfI₄ adsorbed on the Si(1 0 0)-(2 × 1) surface has been studied by core level photoelectron spectroscopy in ultra-high vacuum. Two stable surface intermediates are identified: HfI₃ and HfI₂, both of which remain upon heating to 690 K. The dissociation of HfI₄ is accompanied by the formation of SiI. In addition, HfI₄ is observed up to 300 K. Complete desorption of iodine occurs in the temperature regime 690-780 K. Deposition of HfI₄ at 870 K results in a layer consisting of metallic Hf, whereas deposition at 1120 K results in the formation of Hf silicide. The results indicate that the metallic Hf formed at 870 K is in the form of particles. Oxidation of this film by O₂ at low pressure does not result in complete Hf oxidation. This suggests that complete oxidation of Hf is a critical step when using HfI₄ as precursor in atomic layer deposition.     

Conformational isomers of stilbene on Si(1 0 0)

Stilbene (1,2-diphenylethylene) has shown an intriguing isomerisation behavior and may serve as a model system for “molecular switches” incorporating a CC double bond. To evaluate the possible use of such molecules as molecular switches on semiconductor surfaces, the adsorption of cis- and trans-stilbene on Si(1 0 0) has been investigated. Identification of both isomers is achieved by differences in adsorption geometry as revealed by NEXAFS, and differences in electronic structure in the occupied and unoccupied molecular orbitals. For both isomers, bonding takes place via the CC double bond to the Si dimer atoms allowing for free movement of the aromatic rings, a necessary prerequisite for photoinduced isomerisation on the surface. Our experimental results agree well with theoretical calculations.     

Mechanism of GeH4 dissociation on Si(1 1 1)-(7 × 7)

Results of an STM study of dissociative GeH₄ adsorption on Si(1 1 1)-(7 × 7) at 300 K show that GeH₄ adsorbs under scission of two Ge-H bonds according to GeH₄(g) + 4db → GeH₂(ad) + 2H(ad). GeH₂ binds to two adatom dangling bonds in a bridged configuration, while the two released hydrogen atoms saturate two additional dangling bonds. The GeH₄ sticking coefficient under these conditions is 1.2 × 10⁻⁶, one order of magnitude smaller than for SiH₄.     

Growth mode and novel structure of ultra-thin KCl layers on the Si(1 0 0)-2 × 1 surface

This study investigates ultra-thin potassium chloride (KCl) films on the Si(1 0 0)-2 × 1 surfaces at near room temperature. The atomic structure and growth mode of this ionic solid film on the covalent bonded semiconductor surface is examined by synchrotron radiation core level photoemission, scanning tunneling microscopy and ab initio calculations. The Si 2p, K 3p and Cl 2p core level spectra together indicate that adsorbed KCl molecules at submonolayer coverage partially dissociate and that KCl overlayers above one monolayer (ML) have similar features in the valance band density of states as those of the bulk KCl crystal. STM results reveal a novel c(4 × 4) structure at 1 ML coverage. Ab initio calculations show that a model that comprises a periodic pyramidal geometry is consistent with experimental results.     

Photoemission study of cobalt interaction with the oxidized Si(1 0 0)2 × 1 surface

The interaction of cobalt atoms with an oxidized Si(1 0 0)2 × 1 surface was studied by photoelectron spectroscopy with synchrotron radiation at room and elevated temperatures. The SiO_x layer grown in situ on the crystal surface was ∼0.3 nm thick, and the amount of deposited cobalt was varied within eight atomic layers. It was found that Co atoms could penetrate under the SiO_x layer even at room temperature in the initial growth. As the Co amount increased, a ternary Co-O-Si phase was formed at the interface, followed by a Co-Si solid solution. Silicide synthesis associated with the decomposition of these phases started under the SiO_x layer at ∼250 °C, producing cobalt disilicide with a stable CaF₂-type of structure.     

Si epitaxial growth on LaAlO3(0 0 1)

We report on the growth of Si on c(2 × 2) reconstructed LaAlO₃(0 0 1) surfaces at high substrate temperature (700 °C) by molecular beam epitaxy. An initial Volmer-Weber mode is evidenced using reflection high energy electron diffraction (RHEED), X-ray photoelectron diffraction (XPD) and atomic force microscopy. After the deposition of a few monolayers, the islands coalesce. Using X-ray photoelectron spectroscopy, we demonstrate that Si islands exhibit an abrupt interface with the LaAlO₃ substrate without formation of silicate or silica. Finally, combined RHEED and XPD measurements show the epitaxial growth of Si with a unique Si(0 0 1)//LaAlO₃(0 0 1) and Si<1 0 0>//LAO<1 1 0> relationship.     

Initial stages of iron silicide formation on the Si(1 0 0)2 × 1 surface

The initial stages of iron silicide growth on the Si(1 0 0)2 × 1 surface during solid-phase synthesis were investigated by photoelectron spectroscopy using synchrotron radiation. The experiments were made on iron films of 1-50 monolayer (ML) thickness in the temperature range from room temperature to 750 °С. Our results support the existence of three stages in the Fe deposition on Si(1 0 0) at room temperature, which include formation of the Fe-Si solid solution, Fe₃Si silicide and an iron film. The critical Fe dose necessary for the solid solution to be transformed to the silicide is found to be 5 ML. The solid-phase reaction was found to depend on the deposited metal dose. At 5 ML, the reaction begins at 60 °С, and the solid-phase synthesis leads to the formation of only metastable silicides (FeSi with the CsCl-type structure, γ-FeSi₂ and α-FeSi₂). A specific feature of this process is Si segregation on the silicide films. At a thickness of 15 ML and more, we observed only stable phases, namely, Fe₃Si, ε-FeSi and β-FeSi₂.     

Hydroxylated α-Al2O3 (0 0 0 1) surfaces and metal/α-Al2O3 (0 0 0 1) interfaces

X-ray photoelectron spectroscopy was applied to study the hydroxylation of α-Al₂O₃ (0 0 0 1) surfaces and the stability of surface OH groups. The evolution of interfacial chemistry of the α-Al₂O₃ (0 0 0 1) surfaces and metal/α-Al₂O₃ (0 0 0 1) interfaces are well illustrated via modifications of the surface O1s spectra. Clean hydroxylated surfaces are obtained through water- and oxygen plasma treatment at room temperature. The surface OH groups of the hydroxylated surface are very sensitive to electron beam illumination, Ar⁺ sputtering, UHV heating, and adsorption of reactive metals. The transformation of a hydroxylated surface to an Al-terminated surface occurs by high temperature annealing or Al deposition.     

Thermal decomposition mechanisms of tungsten nitride CVD precursors on Cu(1 1 1)

Chemisorption and thermal decomposition of metallorganic chemical vapor deposition precursors, (t-BuN)₂W(NHBu-t)₂, bis(tert-butylimido)bis(tert-butylamido)tungsten (BTBTT) and (t-BuN)₂W(NEt₂)₂, bis(tert-butylimido)bis(diethylamido)tungsten (BTBDT), on Cu(1 1 1) have been investigated by means of thermal desorption spectroscopy (TDS) and synchrotron-based X-ray photoelectron spectroscopy (SR-XPS) under ultrahigh vacuum conditions. The precursors remain intact upon chemisorption on Cu(1 1 1) at 100 K, and at 300 K both precursors decompose readily via the characteristic hydride abstraction/elimination pathways to produce two stable surface intermediates for each precursor. For BTBTT, one species is W(=NBu-t)₃ and the other is proposed to be a bridged amido complex, [(t-BuN)₂W(μ-NBu-t)]₂. In comparison, a W-imine complex and a W-N-C metallacycle are two intermediates produced from BTBDT. Annealing toward 800 K further decomposes the intermediates and the detectable desorption species are completely derived from the ligands. The desorption products from BTBTT include t-butylamine generated from α-H abstraction, isobutylene from γ-H elimination, acetonitrile from β-methyl elimination, and molecular hydrogen. In addition to these desorption species, BTBDT produces hydrogen cyanide and imine (EtN = CHMe) via β-H elimination, not possible with BTBTT due to the absence of β-H in the ligands. Eventually, tungsten nitrides incorporating oxygen atoms and a small amount of graphitic carbons are formed and the stoichiometry is approximated as WN_1.5O_0.1. Oxygen incorporation, driven by a large oxide formation enthalpy, is sensitively dependent on the moisture exposure in UHV environment.     

Growth and characterization of thin PTCDA films on 3C-SiC(0 0 1)c(2 × 2)

The growth of thin 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) films on a 3C-SiC(0 0 1)c(2 × 2) substrate has been studied by means of photoelectron spectroscopy (PES) and atomic force microscopy (AFM). In the first monolayer the molecules interact with the substrate mainly through the O atoms in the end groups of the molecule. The O atoms have a higher binding energy in the first molecular layer compared to the following layers. No chemical shifts are observed in the Si 2p spectra or in the C 1s spectra from the perylene core of the molecules. From the VB spectra and LEED pattern we conclude that the substrate remains in the c(2 × 2) reconstruction after PTCDA deposition. For thicker films a Stranski-Krastanov film growth was observed with flat lying molecules relative to the substrate.     

Photon-induced chemical vapor deposition of molybdenum on Pt(1 1 1)

Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) have been employed to study the adsorption and photon-induced decomposition of Mo(CO)₆. Mo(CO)₆ adsorbs molecularly on a Pt(1 1 1) surface with weak interaction at 100 K and desorbs intact at 210 K without undergoing thermal decomposition. Adsorbed Mo(CO)₆ undergoes decarbonylation to form surface Mo(CO)_x (x ? 5) under irradiation of ultraviolet light. The Mo(CO)_x species can release further CO ligands to form Mo adatoms with CO desorption at 285 K. In addition, a fraction of the released CO ligands transfers onto the Pt surface and subsequently desorbs at 350-550 K. The resulting Mo layer deposited on the Pt surface is nearly free of contamination by C and O. The deposited Mo adatoms can diffuse into the bulk Pt at temperatures above 1070 K.     

Band alignment of SiO2/Si structure formed with nitric acid vapor below 500 °C

A relatively thick (i.e., ∼9 nm) SiO₂ layer can be formed by oxidation of Si with nitric acid (HNO₃) vapor below 500 °C. In spite of the low temperature formation, the leakage current density flowing through the SiO₂ layer is considerably low, and it follows the Fowler-Nordheim mechanism. From the Fowler-Nordheim plots, the conduction band offset energy at the SiO₂/Si interface is determined to be 2.57 and 2.21 eV for HNO₃ vapor oxidation at 500 and 350 °C, respectively. From X-ray photoelectron spectroscopy measurements, the valence band offset energy is estimated to be 4.80 and 4.48 eV, respectively, for 500 and 350 °C oxidation. The band-gap energy of the SiO₂ layer formed at 500 °C (8.39 eV) is 0.68 eV larger than that formed at 350 °C. The higher band-gap energy for 500 °C oxidation is mainly attributable to the higher atomic density of the SiO₂ layer of 2.46 × 10²²/cm³. Another reason may be the absence of SiO₂ trap-states.     

Tuning the chemical functionality of a gas sensitive material: Water adsorption on SnO2(1 0 1)

Photoemission and density functional theory studies show that water adsorbs dissociatively on the SnO₂(1 0 1) surface in the presence of terminating oxygen atoms and molecularly if these surface oxygen atoms are removed. The different chemical surface responses of these two bulk terminations of SnO₂ also change the water induced band bending and consequently the conductivity of the gas sensing material.     

The interaction of ultrathin Cr layers with SrTiO3(1 0 0)

The growth of ultrathin Cr overlayers on SrTiO₃(1 0 0) was studied by X-ray photoelectron spectroscopy, scanning tunneling microscopy, and transmission electron microscopy. It is found that the metal-oxide interaction strongly depends on the deposition temperature. At T < 600 °C, the interfaces are atomically sharp. Local charge transfer happens between the interfacial Cr adatoms and the topmost substrate atoms. The binding energy shift of Cr 2p is dominated by the final state effects. In case of T > 600 °C, bulk diffusion of oxygen in the oxide substrate may occur, which results in a redox reaction and the formation of new reaction phases at the interfaces. In this temperature regime, the binding energy shift of Cr 2p is mainly controlled by the initial state effects.     

Photoemission study of the Li/Ge(1 1 1)-3 × 1 reconstruction

In this article we report our findings on the electronic structure of the Li induced Ge(1 1 1)-3 × 1 reconstruction as determined by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and core-level spectroscopy using synchrotron radiation. The results are compared to the theoretical honeycomb-chain-channel (HCC) model for the 3 × 1 reconstruction as calculated using density functional theory (DFT). ARUPS measurements were performed in both the and directions of the 1 × 1 surface Brillouin zone at photon energies of 17 and 21.2 eV. Three surface related states were observed in the direction. In the direction, at least two surface states were seen. The calculated band structure using the single-domain HCC model for Li/Ge(1 1 1)-3 × 1 was in good agreement with experiment, allowing for the determination of the origin of the experimentally observed surface states. In the Ge 3d core-level spectra, two surface related components were identified, both at lower binding energy with respect to the Ge 3d bulk peak. Our DFT calculations of the surface core-level shifts were found to be in fair agreement with the experimental results. Finally, in contrast to the Li/Si(1 1 1)-3 × 1 case, no double bond between Ge atoms in the top layer was found.     

Formation of 10-30 nm SiO2/Si structure with a uniform thickness at ∼120 °C by nitric acid oxidation method

Silicon dioxide (SiO₂) layers with a thickness more than 10 nm can be formed at ∼120 °C by direct Si oxidation with nitric acid (HNO₃). Si is initially immersed in 40 wt.% HNO₃ at the boiling temperature of 108 °C, which forms a ∼1 nm SiO₂ layer, and the immersion is continued after reaching the azeotropic point (i.e., 68 wt.% HNO₃ at 121 °C), resulting in an increase in the SiO₂ thickness. The nitric acid oxidation rates are the same for (1 1 1) and (1 0 0) orientations, and n-type and p-type Si wafers. The oxidation rate is constant at least up to 15 nm SiO₂ thickness (i.e., 1.5 nm/h for single crystalline Si and 3.4 nm/h for polycrystalline Si (poly-Si)), indicating that the interfacial reaction is the rate-determining step. SiO₂ layers with a uniform thickness are formed even on a rough surface of poly-Si thin film.