Ultrahigh-vacuum Deposition Research Articles

Enhancement of Memory Performance Using Doubly Stacked Si-Nanocrystal Floating Gates Prepared by Ion Beam Sputtering in UHV

Structures of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> /SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> have been prepared on Si wafers by ion beam sputtering deposition in ultrahigh vacuum (UHV) and subsequently annealed to form single-layer and doubly stacked Si nanocrystals (NCs). Using these two structures, nonvolatile Si-NC floating-gate nMOSFETs were fabricated at x=1.6 following 1.5-mum CMOS standard procedures. The Fowler-Nordheim tunneling of the electrons through the tunnel oxide, their storage into NCs, retention, and endurance are all investigated by varying the device structure and the thicknesses of the NC and oxide layers. It is shown that charge-retention time is longer, and program/erase (P/E) speeds are faster in doubly stacked devices than in single-layer devices, which seem to result from the optimization of device structure, the exclusion of unwanted defects due to the nature of UHV, and the suppression of charge leakage by the multiple barriers/NC layers in the doubly stacked devices. It is also found that the threshold voltages in the endurance characteristics anomalously increase with the P/E cycles, more strongly in the doubly stacked NC memories

Metal organic chemical vapor deposition of ultrathin ZrO 2 films on Si(1 0 0) and Si(1 1 1) studied by electron spectroscopy

The growth of ultrathin ZrO 2 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 ZrO 2 ( t-ZrO 2), whereas significant differences are found for thinner films. On Si(1 1 1)-(7 × 7) the local structure of t-ZrO 2 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-ZrO 2 is formed already at a film thickness of 11 Å. The higher tendency for the formation of t-ZrO 2 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 ZrO 2 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.

Electronic properties of the Ce∕4H-SiC interface studied by x-ray photoemission spectroscopy

The deposition and annealing in ultrahigh vacuum of 5–6 ML (monolayers) of cerium on clean reconstructed Si-face 4H-SiC (0001) are studied by x-ray photoemission spectroscopy and low-energy electron diffraction. Band bending as a function of annealing was studied by shifts of the bulk peak contribution in the C 1s and Si 2p spectra relative to the clean reconstructed surface. Silicide formation was studied by low binding energy components in the C 1s and Si 2p spectra. A large relative upward band bending of 0.3–0.4eV takes place upon deposition of Ce on 4H-SiC at room temperature. Upon annealing to 350°C, a disordered CeSixCy interface layer forms, as observed from chemically shifted components in the Si 2p and C 1s spectra. Annealing to 600°C causes the interface to become CeSi2−x, and carbon desorbs from the interface. A maximum relative band bending of 0.6eV is observed from 400to600°C. Further heating of the sample to 850–1000°C results in a relative total upward band bending of approximately 0.4eV and a relatively sharp CeSi2−x peak in the Si 2p spectrum. SiC bulk bonds appear not to be broken and it is found that a Ce overlayer terminates the layer, with a cerium silicide layer at the interface.

Gas phase preparation and analysis of semiconductor surfaces in a clustered reactor apparatus

An integrated reactor system was built for studying gas phase surface preparation chemistries. The system integrates HF/vapor and UV photochemistry modules with an ultrahigh vacuum deposition reactor and a surface analysis chamber (x-ray photoelectron spectroscopy and Auger) for in situ surface preparation, deposition, and analysis. Each vacuum chamber is mounted on a separate, isolated branch from a main sample transfer tube. The system was designed for samples with variable shapes and thickness, but less than 64mm (212in.) in diameter. This design allows for rapid transfer times between chambers (&amp;lt;5min) and for the simultaneous processing and storage of up to four samples. Use of standard sample transfer and vacuum hardware components minimized initial equipment costs and system maintenance. The capabilities of the clustered reactor apparatus and the importance of surface termination were demonstrated by (1) the removal of a mixed oxide and fluorocarbon residue on silicon, leaving the surface completely terminated with Cl atoms, (2) the removal of copper oxide and copper metal from silicon, (3) the deposition of Ti preferentially on a nonannealed, aqueous-cleaned SiO2 surface relative to an annealed surface, and (4) the use of complementary surface analysis techniques to chemically identify hydrogen-bonded silanol groups on a silicon surface after HF/vapor etching. Gas phase cleaning and surface termination utilized a combination of HF/vapor (100Torr, 27°C for 200s) and UV∕Cl2 (10SCCM Cl2, 90°C for 15min) steps. The results demonstrate that integrated processing provides a means to clean thin layers of organic, oxide, and metal contaminants from semiconductor surfaces and to control the terminating atom or chemical group.

Thermal stability of nanometer range Ti/Ni multilayers

Thermal stability of equidistant nanometer (nm) range Ti/Ni multilayer (ML) structures have been studied to investigate the interfacial microstructural changes upon thermal annealing. Two different ML having bilayer periodicity 10 nm, i.e., [Ti(5 nm)/Ni(5 nm)]10 and 6 nm, i.e., Ti(3 nm)/Ni(3 nm)]10 were prepared under ultra high vacuum deposition condition. It was observed that when the bilayer periodicity is changed from 10 nm to 6 nm, the individual layers grow in an amorphous form as compared to poly-crystalline layers of the former. The X-ray diffraction measurements show significantly different changes in the structural properties of the two ML. Moreover, the interdiffusion measurements carried out on both the ML, using X-ray reflectivity technique indicates slower inter-diffusion in case of samples having smaller periodicity. The hysteresis measurements on a ML with 10 nm periodicity show a drastic change in the magnetization behavior of the sample when annealed at or above 300 °C. The observed magnetization behavior is explained on the basis of structural and chemical changes that have occurred at the interface due to the thermal treatment.

Growth of ultrathin ZrO2 films on Si(100): Film-thickness-dependent band alignment

The band alignment of ultrathin ZrO2 films of different thickness formed on Si(100) have been monitored with synchrotron radiation photoelectron spectroscopy and x-ray absorption spectroscopy. The films were deposited sequentially by way of metal-organic chemical-vapor deposition in ultrahigh vacuum. A significant decrease in the conduction band offset is found for increasing film thickness. It is accompanied by a corresponding increase of the valence band offset. The variations originate in the formation of an interfacial layer characterized by a lower degree of Zr-O interaction than in bulk ZrO2 but with no clear evidence for partially occupied Zr4d dangling bonds.

Observation of competing modes in the growth of diindenoperylene on SiO2

Diindenoperylene (DIP) thin films deposited on SiO2-surfaces by organic molecular beam deposition in ultra-high vacuum at room temperature have been investigated by atomic-force-microscopy and X-ray scattering as a function of film thickness. We observe the evolution of two distinct molecular orientations; the long axis of the DIP molecule is pointing either along the surface normal (σ-orientation) or along the surface (λ-orientation). Initially, the films exhibit predominantly σ-orientated islands. Increasing the film thickness, elongated, fibrous λ-orientated islands nucleate on top of the σ-oriented islands and dominate the growth front due to their fast 3D growth mode. Upon annealing at 150 °C, the fibers disappear. We argue that these observations can be understood as a competition between surface energy (favoring the σ-orientation) and growth kinetics (favoring the λ-orientation).

Thermal stability, atomic vibrational dynamics, and superheating of confined interfacial Sn layers inSn∕Simultilayers

Multilayers composed of materials with low (Sn) and high (Si) bulk melting points were grown at room temperature by ultrahigh vacuum deposition. $^{119}\mathrm{Sn}$ M\"ossbauer spectroscopy has been used to investigate the temperature dependence of the Debye-Waller factor $f$, the mean-square displacement, and the mean-square velocity of $^{119}\mathrm{Sn}$ nuclei in ultrathin ($10\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ thick) $\ensuremath{\alpha}$-like Sn layers embedded between $50\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ thick Si layers. The $f$ factor was found to be nonzero with a value of $0.036\ifmmode\pm\else\textpm\fi{}0.009$ even at $450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. This provides unequivocal proof of the solid state of the confined $\ensuremath{\alpha}$-like Sn layers at least up to $450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. Melting can only be achieved by superheating to $T&gt;450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. This temperature is significantly higher than the melting temperature of bulk $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Sn}$ $(231.9\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ and of a nonconfined epitaxial $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ single layer grown on InSb(111) $(170\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ previously reported in the literature [T. Osaka et al., Phys. Rev. B 50, 7567 (1994)]. Our molecular dynamics calculations show that melting of bulk-like $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Sn}$ starts at $\ensuremath{\sim}380\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ and is complete at $\ensuremath{\sim}530\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ according to the Lindemann criterion. Since we still observe the solid state at $450\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ for the confined $\ensuremath{\alpha}$-like Sn films, considerable superheating is observed for this system. The stability of the ultrathin confined $\ensuremath{\alpha}$-like Sn layers arises from electronic interactions with the surrounding Si layers, as evidenced by the M\"ossbauer chemical shift.

Ultrahigh Vacuum Deposition of l-Cysteine on Au(110) Studied by High-Resolution X-ray Photoemission: From Early Stages of Adsorption to Molecular Organization

We report on a high-resolution X-ray photoemission spectroscopy study on molecular-thick layers of L-cysteine deposited under ultrahigh vacuum conditions on Au(110). The analysis of core level shifts allowed us to distinguish unambiguously the states of the first-layer molecules from those of molecules belonging to the second layer. The first-layer molecules strongly interact with the metal through their sulfur headgroup. The multipeaked structure of the N 1s, O 1s, and C 1s core levels is interpreted in terms of different molecular moieties. The neutral acidic fraction (HSCH2CH(NH2)COOH) is abundant at low coverage likely associated with isolated molecules or dimers. The zwitterionic phase (HSCH2CH(NH3+)COO-) is largely dominant as the coverage approaches the monolayer limit and is related to the formation of ordered self-assembled molecular structures indicated by electron diffraction patterns. The occurrence of a small amount of cationic molecules (HSCH2CH(NH3+)COOH) is also discussed. The second-layer molecules mainly display zwitterionic character and are weakly adsorbed. Mild annealing up to 100 degrees C leads to the desorption of the second-layer molecules leaving electronic states of the first layer unaltered.

Deposition of silver, indium, and magnesium onto organic semiconductor layers: Reactivity, indiffusion and metal morphology

Novel devices based on organic semiconductors such as organic light emitting diodes or organic field effect transistors require metallic contacts. Therefore, the interface between metals and organic semiconductors deserves special attention. In situ Raman scattering was applied here to assess metal/organic interface formation. As model systems molecular layers of two perylene derivatives, viz. 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) and N,N′-dimethyl 3,4,9,10-perylene-tetracarboxylic diimide (DiMe-PTCDI) were investigated. These layers were grown on S-GaAs(1 0 0) substrates by organic molecular beam deposition in ultra-high vacuum. Ag, In, and Mg were thermally evaporated onto the organic layers and the Raman spectra were recorded in situ. The results reveal that Ag and Mg form an abrupt interface while In strongly diffuses into the organic layers. Mg, on the other hand, undergoes strong chemical reaction with PTCDA. The experiments also benefit from a strong enhancement of the scattering intensity which is induced by the rough morphology of deposited metals. The enhancement factors as a function of nominal metal coverage and the subsequent attenuation at large metal coverages deliver valuable information on the metal morphology which is discussed in comparison to additional atomic force microscopy results.

Ultrahigh vacuum low temperature scanning tunneling microscope with ultrahigh vacuum deposition capability

An Omicron ultrahigh vacuum low temperature scanning tunneling microscope has been combined with an ultrahigh vacuum deposition chamber equipped with molecular beam sources and a rotatable liquid helium cooled sample holder. This configuration permits the growth of films and structures on either heated or cooled substrates with the possibility of subsequent study by scanning probe techniques at low temperatures without exposing them to air. The versatility of the fabrication process and the configuration of the equipment lends itself to easily exploring many physical systems. The apparatus has been used to investigate the electronic properties of metal clusters prepared using buffer layer assisted growth.

Thin bismuth film as a template for pentacene growth

Thin Bi(001) films grown by ultrahigh vacuum deposition on Si(111)-7×7 surfaces at room temperature, were annealed at ∼400K in order to improve their morphology by reducing the step density on the surface. Annealed, well-ordered Bi(001) films have been subsequently used as substrates for growth of pentacene (Pn). It has been determined using low-energy electron microscope that Pn nucleates on Bi(001) into a highly ordered, crystalline layer, with Pn molecules “standing up” on the Bi surface, and the (001) plane on the growth front. Moreover, the Pn layer is aligned with the Bi(001) surface having a “point-on-line” commensurate relationship with the substrate. The Pn∕Bi(001) film crystallizes in a bulk-like structure directly from the first Pn layer. Formation of the thin film phase reported for the Pn growth on SiO2 and other inert substrates was not observed in our experiments.

Sharp metal-insulator transition in Sr(Ti 1− xV x)O 3− δ thin films on SrTiO 3 substrates

We fabricated Sr(Ti 1− x V x )O 3 composition-spread films on SrTiO 3 (100) substrates by using pulsed laser deposition in ultra high vacuum. Sharp metal-insulator (MI) transitions were observed in the case of Sr(Ti 1− x V x )O 3− δ composition-spread films grown on an oxygen deficient SrTiO 3 (STO) buffer/SrTiO 3 (STO) substrate in the restricted composition x=0.5–1.0 while no MI transition was observed in the case of Sr(Ti 1− x V x )O 3− δ below x=0.5. In the insulating phase below the MI transition, non-linear I– V characteristics were observed. To explain the appearance and disappearance of MI transitions in Sr(Ti 1− x V x )O 3− δ two possibilities were considered: (i) strain effect from the substrate and (ii) charge-transfer at the interface between Sr(Ti 1− x V x )O 3− δ and STO.

Magnetic-Field-Sensing Materials Composed of Metal–Semiconductor Hybrid Nanostructures

A huge positive magnetoresistance effect, about 4 orders of magnitude at room temperature, was discovered in metal-semiconductor hybrid nanostructures. The hybrid material consisting of metallic nanostructures that are fabricated on a GaAs substrate by ultra-high vacuum deposition method exhibits magnetic field-sensitive current-voltage characteristics. When a constant voltage above the threshold value, is applied to the film, a very steep change in the current, which we term magnetoresistive switch, is driven by the huge magnetoresistance effect under a relatively low magnetic field at room temperature. The magnetoresistance effect is very sensitive to the nanoscale morphology of the hybrid film; in other words, one can control the magnetoresistance function at will by modifying the nanostructure of this material. The origin of the magnetoresistive switch effect and its possible application as a magnetic field sensor is discussed.

Structural and Magnetic Characteristics of Fe∕Si Bilayer and Multilayer Films Obtained by Thermal Deposition in Ultrahigh Vacuum

The structural and magnetic characteristics of Fe/Si bilayer and multilayer films with nanometer-thick layers obtained by thermal deposition in ultrahigh vacuum have been studied by methods of small-angle X-ray scattering, electron spectroscopy, and magnetometry. It is established that the mechanisms involved in the formation of Fe/Si and Si/Fe interfaces are different.

Combined UHV and Liquid Phase (CULP) Processing of Self-assembled Nanostructures

AbstractThe fabrication and structures of nanometer sized metal and semiconductor dots by a new “combined UHV and liquid-phase processing” (CULP) method are investigated. Ultrahigh vacuum deposition of monolayers of material, buffered and co-adsorbed along with chemically-inert working gas molecules, and subsequent pressurization in situ with high purity helium, led to self-assembly of metal and semiconductor nanoparticles in the liquid phase. The density and size distribution of nanoparticles were found to depend on processing parameters including the substrate surface condition and the dilution of the co-adsorbed film. These results are discussed in terms of surface and interface energies and kinetics.

Correlation Between Elastic Constants and Magnetic Anisotropy in Co/Pt Superlattice Thin Films

AbstractIn this paper, we study contribution of the magnetoelastic anisotropy effect on perpendicular magnetic anisotropy of Co/Pt superlattice thin films. The effective-magnetic anisotropy energy was measured for Co/Pt thin films with a constant Co-layer thickness (4 Å) deposited on silicon substrate by the ultrahigh-vacuum deposition method. As the Pt-layer thickness increases, the effective-magnetic anisotropy energy increases and decreases after showing a maximum at the Pt-layer thickness of 12 Å. This behavior can be explained only by the magnetoelastic anisotropy effect, that is, elastic-strain energy released through the magnetostriction effect with the strain-dependent elastic constants.

Octylgermane on Gold: Synthesis, Oxidation, and Pattern Formation

Chemisorbed monolayers of octylgermane (C8H17GeH3) on gold have been formed by vapor deposition in ultrahigh vacuum. The monolayer has been characterized by X-ray photoelectron and reflection absorption infrared spectroscopies (XPS and RAIRS) and scanning tunneling microscopy (STM). XPS data indicate the monolayer can be oxidized by exposure to ozone. STM images exhibit a complex pattern which can be modeled as strain-mediated spinodal decomposition.

The anisotropic dielectric function for copper phthalocyanine thin films

Abstract Copper phthalocyanine (CuPc) thin films were prepared by organic molecular beam deposition (OMBD) in high vacuum and ultra-high vacuum on passivated Si(1 1 1) using β-phase CuPc as source material. The substrates were kept at room temperature during the deposition. The IR peak positions indicate that the films consist mainly of α-phase CuPc, while the relative intensities suggest that the films are anisotropic. The anisotropic dielectric function for these CuPc layers was determined from ellipsometric spectra using uniaxial models in the range from 0.73 to 5 eV.

Ultrahigh vacuum deposition of CdSe nanocrystals on surfaces by pulse injection

The fabrication of thin films of colloidal semiconductor nanocrystals is attracting muchattention due to their exceptional optoelectronic properties. This requires the developmentof new methods for depositing nanocrystals under well-controlled conditions. Here, wereport the use of the pulse injection method to deposit CdSe nanocrystals under ultrahighvacuum (UHV) on clean and well-ordered surfaces. The deposition of nanocrystals has beentested by x-ray photoelectron spectroscopy (XPS) and near edge x-ray absorption finestructure spectroscopy. Special attention has been paid to the preparation of verypure solutions of CdSe nanocrystals using cadmium stearate, trioctylphosphineoxide (TOPO) and the TOP/Se adduct for the nanocrystals synthesis followed bydissolution in pentane. It has been found that CdSe nanocrystals adsorb with similarsticking coefficients on graphite, hydrogenated silicon (100) and hydrogenateddiamond (100) surfaces. Furthermore, the XPS analysis has revealed that thesurface of the CdSe nanocrystal is Cd rich, which has important consequencesfor the optical and chemical properties. This ability to deposit semiconductornanocrystals under UHV conditions on clean and well-ordered surfaces opens up newperspectives for studying in a reliable manner all their chemical, electronic and opticalproperties.