B. Koslowski

Growth of thin, flat, epitaxial (111) oriented gold films on c-cut sapphire

AbstractAnewgrowthprocedurehasbeendevelopedtopreparethin(<50nm),flat(roughness 0.3–1nm rms ),epitaxialAufilmsontopofaninsulatingsubstratewithoutusingamagneticseedlayer.ThiscouldbeobtainedbydepositingfirstaNbseedlayer(61nm)atroomtemperatureontopofc-cutsapphirefollowedbyevaporatingAuatarateof0.05–0.1nm/sat300 C.Thisprocedureresultedin(111)orientedflatAufilmsevenforthicknesswellbelow50nm.Theflatnessofthesefilmswasconsistentlyconfirmedbyatomicforceandscanningtunnelingmicroscopy,theirexcellentepitaxialquality(rocking width 0.1 –0.3 ) byX-raydiffractometryand reflected high energyelectron diffraction. 2001ElsevierScienceB.V.Allrightsreserved. Keywords:Gold;Niobium;Aluminumoxide;Epitaxy;Growth;Metallicfilms;X-rayscattering,diffraction,andreflection;Reflectionhigh-energyelectrondiffraction(RHEED) 1. IntroductionThere is immense interest in flat epitaxial andthin, yet electrically conducting, metallic films,which then can be used for experiments likee.g. attenuated total reflection (ATR) [1], electro-depositionofnanoclusters[2,3]orscanningtunnel-ing microscopy(STM) on organic monolayers [4].In this context, Au films are especiallyattractivedue to their chemical stability, which is essential,if, after film preparation under ultrahigh vacuum(UHV),experimentsdemandfurtherexsitusteps.In our case, aiming at studying surface sensitiveelectrical transport properties of such thin films,the application of magnetic seedlayers had to beexcluded, since otherwise additional spin-depen-dent scattering mechanisms would contributeto the resistance. Furthermore, to make surfacescatteringthedominantprocess,competingdefectscatteringprocesseshavetobeminimizedleadingtothedemandofthin,flat,highqualityepitaxialAu films. As a consequence of these combinedqualityrequirements, the well known substratesusedtogrowepitaxialAufilms,i.e.mica[5–7]andMgO [8] turn out to be unsuitable. On mica, thepercolation thickness of epitaxial Au films neces-saryto observe electrical conductance appears tobe close to 30 nm. Thus, for epitaxial Au filmsbelow30nmelectricalconductancecannolongerbeguaranteed.ForMgO,ontheotherhand,Feor

ON THE ROUGHNESS OF HYDROGEN-PLASMA TREATED DIAMOND(100) SURFACES

Abstract To investigate the possibility of influencing the roughness of diamond(100) surfaces, type Ib or heavily boron (B)-doped HPHT diamond crystals were mechanically and chemo-mechanically polished, and additionally exposed to a microwave-assisted hydrogen plasma. The resulting roughness and surface topology was analyzed on a macroscopic scale by stylus profilometry (PFM), and on microscopic scales by STM and AFM. The following results have been observed: the surface roughness (rms-value) is reduced by mechanical polishing from 4 nm rms (as received) to about 2 nm rms (PFM). This step, however, leads to scratches with depths up to 40 nm pp . Chemo-mechanical polishing with KNO 3 reduces the surface roughness further to typically 100 pm rms (PFM), usually eliminating the above scratches. The roughness determined by STM is typically 5–10-times higher than measured by PFM. After exposing B-doped samples for 3 min to the H-plasma under typical CVD growth conditions, the roughness increases up to 4 nm rms and a “brick-wall” pattern appears formed by weak cusps running along [110]. After exposure for an additional 5 min, the surface roughness of the B-doped samples increases further to 20–40 nm rms , and exhibits a regular pattern frequently with characteristic structures of 60 nm width, 250 nm length, and 160 nm height running along approximately [110]. The “roofs” are faceted with faces of approximately {XX1}. These results will be discussed in terms of strain relaxation, similar to the surface roughening observed on SiGe/Si and anisotropic etching of defects.

Towards quantitative magnetic force microscopy: theory and experiment

We introduce a simple and effective model of a commercial magnetic thin-film sensor for magnetic force microscopy (MFM), and we test the model employing buried magnetic dipoles. The model can be solved analytically in the half-space in front of the sensor tip, leading to a simple 1/R dependence of the magnetic stray field projected to the symmetry axis. The model resolves the earlier issue as to why the magnetic sensors cannot be described reasonably by a restricted multipole expansion as in the point pole approximation: the point pole model must be extended to incorporate a ?lower-order? pole, which we term ?pseudo-pole?. The near-field dependence (?R?1) turns into the well-known and frequently used dipole behavior (?R?3) if the separation, R, exceeds the height of the sensor. Using magnetic nanoparticles (average diameter 18?nm) embedded in a SiO cover as dipolar point probes, we show that the force gradient?distance curves and magnetic images fit almost perfectly to the proposed model. The easy axis of magnetization of single nanoparticles is successfully deduced from these magnetic images. Our model paves the way for quantitative MFM, at least if the sensor and the sample are independent.

Fabrication of regularly arranged nanocolumns on diamond(100) using micellar masks

Cylindrical diamond columns of 15 nm height and 10 nm diameter have been prepared on a diamond(100) substrate by exploiting the self-organization of metal-loaded diblock copolymers. For this purpose, chemomechanically polished substrates of synthetic (high pressure high temperature) diamond exhibiting extremely low roughness (typical root-mean-square roughness 60 pm) were covered by a monolayer of gold-loaded inverse polystyrene-block-poly(2-vinylpyridine) micelles which self-organize in an hexagonal order on the substrate. After burning off the blockcopolymer in an oxygen plasma, the resulting Au nanoparticles act as a mask during further etching the diamond substrate in this plasma. As a result, Au-capped diamond columns are formed with an approximate size of the former gold particles. After removing the Au caps by evaporation at 1100 °C in ultrahigh vacuum, an array of diamond nanocolumns is obtained. As a consequence of this preparation process, the columns form an hexagonal lattice with a separation ...

Characterization of ultrathin insulating Al2O3 films grown on Nb(110)/sapphire(0001) by tunneling spectroscopy and microscopy

Various structural as well as chemical properties of ultrathin Al2O3 films prepared on Nb(110)/sapphire(0001) were analyzed. For this purpose, in a first step, 40-nm-thick Nb(110) films are grown epitaxially by sputtering on top of sapphire(0001). The Nb(110) films are (1×1) reconstructed and exhibit the epitaxial relations Nb(110)‖Al2O3(0001), Nb[001]‖Al2O3[1_010] and equivalents as determined by x-ray diffraction. In a second step, a 1-nm-thick Al film is evaporated on top of the Nb(110) and oxidized ex situ in an oxygen plasma resulting in a polycrystalline Al2O3 film of thickness ∼1.3 nm. This oxide film was characterized by scanning tunneling microscopy and spectroscopy (STS) as well as by x-ray photoelectron spectroscopy. Additionally, the ultrathin Al-oxide films were tested as insulators in large area metal-insulator-metal (MIM) or superconductor-insulator-metal tunneling contacts. According to these experiments, the niobium film underneath is completely protected from oxidation, while the Al film...

STM study on the self-assembly of oligothiophene-based organic semiconductors.

Summary The self-assembly properties of a series of functionalized regioregular oligo(3-alkylthiophenes) were investigated by using scanning tunneling microscopy (STM) at the liquid–solid interface under ambient conditions. The characteristics of the 2-D crystals formed on the (0001) plane of highly ordered pyrolitic graphite (HOPG) strongly depend on the length of the π-conjugated oligomer backbone, on the functional groups attached to it, and on the alkyl substitution pattern on the individual thiophene units. Theoretical calculations were performed to analyze the geometry and electronic density of the molecular orbitals as well as to analyze the intermolecular interactions, in order to obtain models of the 2-D molecular ordering on the substrate.

Terthiophene on Au(111): A scanning tunneling microscopy and spectroscopy study

Summary Terthiophene (3T) molecules adsorbed on herringbone (HB) reconstructed Au(111) surfaces in the low coverage regime were investigated by means of low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) under ultra-high vacuum conditions. The 3T molecules adsorb preferentially in fcc regions of the HB reconstruction with their longer axis oriented perpendicular to the soliton walls of the HB and at maximum mutual separation. The latter observation points to a repulsive interaction between molecules probably due to parallel electrical dipoles formed during adsorption. Constant-separation (I-V) and constant-current (z-V) STS clearly reveal the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals, which are found at −1.2 eV and +2.3 eV, respectively. The HOMO–LUMO gap corresponds to that of a free molecule, indicating a rather weak interaction between 3T and Au(111). According to conductivity maps, the HOMO and LUMO are inhomogeneously distributed over the adsorbed 3T, with the HOMO being located at the ends of the linear molecule, and the LUMO symmetrically with respect to the longer axis of the molecule at the center of its flanks. Analysis of spectroscopic data reveals details of the contrast mechanism of 3T/Au(111) in STM. For that, the Shockley-like surface state of Au(111) plays an essential role and appears shifted outwards from the surface in the presence of the molecule. As a consequence, the molecule can be imaged even at a tunneling bias within its HOMO–LUMO gap. A more quantitative analysis of this detail resolves a previous discrepancy between the fairly small apparent STM height of 3T molecules (1.4–2.0 nm, depending on tunneling bias) and a corresponding larger value of 3.5 nm based on X-ray standing wave analysis. An additionally observed linear decrease of the differential tunneling barrier at positive bias when determined on top of a 3T molecule is compared to the bias independent barrier obtained on bare Au(111) surfaces. This striking difference of the barrier behavior with and without adsorbed molecules is interpreted as indicating an adsorption-induced dimensionality transition of the involved tunneling processes.

Design of an extremely stable low-temperature ultrahigh vacuum scanning tunneling microscope

The design and performance of a scanning tunneling microscope operated under ultrahigh vacuum conditions and at low temperature are presented. It allows operating temperatures between 6K to at least 30K as well as safe and fast tip/sample transfers. Novel design features resulted in an extremely stable instrument with a noise level of only 0.2pmrms in the frequency range of 0.5–500Hz despite a relatively noisy laboratory environment. To demonstrate this behavior, results of test measurements performed on Au(111) and Nb(110) samples are presented.

Ultrathin epitaxial Al2O3 films grown on Nb(110)∕sapphire(0001) investigated by tunneling spectroscopy and microscopy

Structural as well as electronic properties of ultrathin epitaxial Al2O3 films prepared on Nb(110)∕sapphire(0001) were analyzed in situ by applying scanning tunneling microscopy (STM) and spectroscopy as well as ultraviolet photoelectron spectroscopy, cathode luminescence, and low-energy electron diffraction. According to these experiments, the niobium base film is protected from oxidation, while the ultrathin Al film deposited onto the Nb is fully oxidized and (0001)-oriented with a very smooth surface. The STM-imaged topography of the oxide films in most cases reflects monatomic steps of the underlying Nb(110) film. In some cases (10% of all samples with low tunneling barriers) additional ∼0.4-nm-high steps are observed characteristic of monatomic Al2O3 steps. Furthermore, for growing tunneling voltages (>1V), the STM-imaged topographies reveal an increasing density of small hillocks, which are attributed to localized defect states such as oxygen vacancies still present within the oxide layer.

Deconvolution of the density of states of tip and sample through constant-current tunneling spectroscopy

Summary We introduce a scheme to obtain the deconvolved density of states (DOS) of the tip and sample, from scanning tunneling spectra determined in the constant-current mode (z–V spectroscopy). The scheme is based on the validity of the Wentzel–Kramers–Brillouin (WKB) approximation and the trapezoidal approximation of the electron potential within the tunneling barrier. In a numerical treatment of z–V spectroscopy, we first analyze how the position and amplitude of characteristic DOS features change depending on parameters such as the energy position, width, barrier height, and the tip–sample separation. Then it is shown that the deconvolution scheme is capable of recovering the original DOS of tip and sample with an accuracy of better than 97% within the one-dimensional WKB approximation. Application of the deconvolution scheme to experimental data obtained on Nb(110) reveals a convergent behavior, providing separately the DOS of both sample and tip. In detail, however, there are systematic quantitative deviations between the DOS results based on z–V data and those based on I–V data. This points to an inconsistency between the assumed and the actual transmission probability function. Indeed, the experimentally determined differential barrier height still clearly deviates from that derived from the deconvolved DOS. Thus, the present progress in developing a reliable deconvolution scheme shifts the focus towards how to access the actual transmission probability function.