Electronic properties of assembled islands of hydrogen-saturated silicon clusters on Si(111)-(7 x 7) surfaces studied by scanning tunneling spectroscopy

We present results of scanning tunneling spectroscopy (STS) measurements of hydrogen-saturated silicon clusters islands formed on Si(111)-( 7×7) surfaces. Nanometer-size islands of Si₆H₁₂ with a height of 0.2-4 nm were assembled with a scanning tunneling microscope (STM) using a tip-to-sample voltage larger than 3 V. STS spectra of Si₆H₁₂ cluster islands show characteristic peaks originating in resonance tunneling through discrete states of the clusters. The peak positions change little with island height, while the peak width shows a tendency of narrowing for the tall islands. The peak narrowing is interpreted as increase of lifetime of electron trapped at the cluster states. The lifetime was as short as 10^-13 s resulting from interaction with the dangling bonds of surface atoms, which prevents charge accumulation at the cluster islands. Received 30 November 2000     

Single-crystal Al-catalyzed Si nanowires grown via hedgehog-shaped Al-Si aggregates

Single-crystal, Al-catalyzed Si nanowires (SiNWs) were grown under atmospheric pressure using the dimpled feature of Al metal that remained after the removal of an anodic aluminum oxide (AAO) template directly formed on a Si substrate. During the H₂ preannealing prior to growth, the dimpled surface morphology of the remaining Al changed as the Al formed agglomerations with each other and subsequently formed Al-Si alloy islands on the silicon surface. Silicon nanowires were found to only grow on these islands, resulting in the final hedgehog-shaped morphology. The amount of Al agglomeration which controlled the overall size of the alloy islands was determined by varying the H₂/Ar flow ratio during preannealing. High-density growth of SiNWs was observed at a lower ratio of the H₂/Ar flow rates.     

Si<Subscript>x</Subscript>Ge<Subscript>1-x</Subscript> ultrahigh-vacuum chemical vapor deposition on Si(111)(7×7) from GeH<Subscript>4</Subscript>/Si<Subscript>2</Subscript>H<Subscript>6</Subscript> mixtures

We report the results of a study on ultrahigh-vacuum chemical vapor deposition of Si_xGe_1-x layers on Si(111)(7×7) with GeH₄ and Si₂H₆ mixtures. Using combined scanning tunneling microscopy and X-ray photoelectron spectroscopy, structural properties, the growth kinetics and the composition of the deposited alloys are analyzed as a function of the growth temperature for two different GeH₄:Si₂H₆ mixture ratios. The mutual influence of the precursors is shown by comparing the structures formed during deposition and the sticking coefficients of Si₂H₆ and GeH₄ with results obtained from exposure of Si(111) to the pure gases. Received: 28 July 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-731/502-5452, E-mail: hubert.rauscher@chemie.uni-ulm.de     

INCREASING THE PHOTOLUMINESCENCE INTENSITY OF Ge ISLANDS BY CHEMICAL ETCHING

Self-assembled Ge islands were grown on Si(100) substrate by Si₂H₆-Ge molecular beam epitaxy. After being subjected to chemical etching, it is found that the photoluminescence from the etched Ge islands became more intense and shifted to the higher-energy side compared to that of the as-deposited Ge islands. This behaviour was explained by the effect of chemical etching on the morphology of the Ge islands. Our results demonstrate that chemical etching can be a way to change the luminescence property of the as-deposited islands.     

Growth of oriented crystalline Ag nanoislands on air-exposed Si(0 0 1) surfaces

Growth of Ag islands under ultrahigh vacuum condition on air-exposed Si(0 0 1)-(2 × 1) surfaces has been investigated by in-situ reflection high energy electron diffraction (RHEED). A thin oxide is formed on Si via exposure of the clean Si(0 0 1)-(2 × 1) surface to air. Deposition of Ag on this oxidized surface was carried out at different substrate temperatures. Deposition at room temperature leads to the growth of randomly oriented Ag islands while well-oriented Ag islands, with (0 0 1)_Ag||(0 0 1)_Si, [1 1 0]_Ag||[1 1 0]_Si, have been found to grow at substrate temperatures of ≥350 °C in spite of the presence of the oxide layer between Ag islands and Si. The RHEED patterns show similarities with the case of Ag deposition on H-passivated Si(0 0 1) surfaces.     

Effect of oligomers on the growth of amorphous silicon films in a PECVD reactor

A plasma-chemical model of processes in a PECVD reactor is constructed that is an extension of the earlier model and takes into account the formation of oligomers Si_nH_m (n ≤ 5). The corresponding scheme of chemical reactions is developed, and simulation of film growth is carried out. It is found that Si₂H₅ and Si₃H₇ components strongly influence the film growth. It is of interest to obtain more reliable experimental data dramatizing these effects.     

CO2 laser CVD of a-Si:H: in situ gas analysis and model calculations

4 and disilane Si₂H₆ induced by continuous wave CO₂ laser irradiation has been investigated under the conditions of chemical vapor deposition (CVD) of amorphous hydrogenated silicon a-Si:H. At the very position of depositing the thin film the stationary chemical composition of the processing gas is probed in situ by an effusive molecular beam which passes through a differential pumping stage into a quadrupole mass spectrometer (QMS). With SiH₄ as educt and SF₆ as a sensitizer, SiH₄ and Si₂H₆ are found in the processing gas while Si₃H₈ or higher silanes are lacking. Si₂H₆ and SF₆ lead to SiH₄, Si₂H₆, and Si₃H₈, but higher silanes are missing. The experimentally determined composition of the processing gas is semi-quantitatively reproduced by model calculations based on the assumption of stationary local equilibrium conditions and applying thermodynamic and spectroscopic data (molecular statistics). The mass balance of the processing gas entering and leaving the CVD chamber states an atomic ratio Si:H of 1:2 for the gas phase species forming the solid deposit on the reactor walls. This finding together with theoretical considerations indicates the intermediate Si₂H₄ to be the dominating gas phase species forming the a-Si:H thin films. Received: 17 July 1998/Accepted: 20 July 1998     

A valence-band and core-level photoemission study of a-SixC1-x thin films grown by low-temperature low-pressure chemical vapour deposition

Amorphous Si_xC_1-x thin films have been grown by low-pressure chemical vapour deposition at 800 K from Si₂H₆ and C₂H₂ in the x concentration range 0.5≤x<0.7. Measurements of the valence-band and core-level photoemission spectra using X-ray photoemission spectroscopy and synchrotron radiation have shown a clear change in the electronic structure for 0.55xC_1-x films obtained by excimer laser annealing during growth have shown the formation of a dominant 3C-SiC phase up to x=0.6, while for higher x the growth of a poly-Si phase has been observed . PACS 81.15.Gh; 79.60.Dp     

Epitaxial growth of fully relaxed Si0.75Ge0.25 on SOI substrate

A fully relaxed Si_0.75Ge_0.25 film with low dislocation densities is fabricated by epitaxial growth on SOI substrate without depositing graded buffers. The relaxation mechanism of the SiGe layer directly grown on SOI substrate is also analyzed. For SiGe grown on SOI with low Ge content, the strain is redistributed between SiGe and the top Si of SOI substrate, and the strain residing in SiGe layer can be fully relaxed by the formation and expansion of dislocation half-loops near the SiGe/Si interface. The surface morphology and crystal quality of all samples are analyzed by optical microscopy and transmission electron microscopy (TEM), respectively. Compared to the Si_0.75Ge_0.25 layer epitaxially grown on graded buffer, the Si_0.75Ge_0.25 directly grown on SOI substrate appears good surface morphology and perfect crystal quality.     

Mechanism of two-dimensional chiral growth of 6,13-pentacenequinone thin films on Si(1 1 1)

Thin film growth of 6,13-pentacenequinone (C₂₄H₁₂O₂, PnQ) on Si(1 1 1)-7 × 7 at room temperature (RT) was studied by low-energy electron microscopy (LEEM) and ab initio density functional theory (DFT) calculations. Our experiments yielded direct microscopic observation of enantiomorphic evolution mechanism in the initial stage of the chiral-like growth of PnQ islands, under kinetic growth conditions. We observed that the faster growth direction aligns with the direction of easier molecule incorporation, or lowest kink formation energy, rather than along the lowest energy step. Real time observation of the growth and subsequent relaxation of island shape revealed that kinetically stiff direction differs from the thermodynamic one. This feature together with anisotropic mass incorporation determines the enantiomorphic evolution and rotational arrangement of crystallites during the growth of elongated organic molecules, like PnQ.     

Growth of Co-Silicides from single crystal and evaporated Si

We have investigated the reaction of a thin Co film with a (100) Si (Si ^c ) or an evaporated Si (Si ^e , which is amorphous) substrate during thermal annealing. On either substrate, Co₂Si and CoSi form simultaneously and the growth of each phase has a square root of time dependence. Both silicides grow faster on Si ^c than on Si ^e . A model is proposed to calculate the effective diffusion constant in each silicide from the growth data of the silicides. The activation energies of the effective diffusion constants in Co₂Si and CoSi grown on Si ^c are 1.7±0.1 eV and 1.8±0.1 eV, respectively; while those on Si ^e are 1.85±0.1 eV and 1.9 ±0.1 eV, respectively. The differences observed for the two substrates are tentatively attributed to the presence of impurities in Si^e and to the microstructural differences of the silicides formed on either substrate.     

Endoepitaxial growth of hexagonal-Fe13Ge8 islands on Cubic-Ge(001)

The growth and shape evolution of epitaxial Fe₁₃Ge₈ (hexagonal lattice) islands on single crystal Ge(001) (cubic lattice) substrate was observed in real time using an in situ ultra-high vacuum transmission electron microscope (TEM). Post-deposition high-resolution TEM in conjunction with stereographic projection enabled the identification of the interface structure between the Fe₁₃Ge₈ islands and the Ge substrate. Only one low-energy coherent interface formed via Fe₁₃Ge₈ islands growing into the substrate along the inclined Ge(11?1) plane. This indicates that minimization of net interfacial energy is the driving force for hexagonal Fe₁₃Ge₈ islands formation on Ge(001).     

An effective compliant substrate for low-dislocation relaxed Si1-xGex growth

An effective compliant substrate for Si_1-xGe_x growth is presented. A silicon-on-insulator substrate was implanted with B and O forming 20 wt % borosilicate glass within the SiO₂. The addition of the borosilicate glass to the buried oxide acted to reduce the viscosity at the growth temperature of Si_1-xGe_x, promoting the in situ elastic deformation of the thin Si (∼20 nm) layer on the insulator. The sharing of the misfit between the Si and the Si_1-xGe_x layers was observed and quantified by double-axis X-ray diffraction. In addition, the material quality was assessed using cross-sectional transmission electron microscopy, photoluminescence and etch pit density measurements. No misfit dislocations were observed in the partially relaxed 150-nm Si_0.75Ge_0.25 sample as-grown on a 20% borosilicate glass substrate. The threading dislocation density was estimated at 2×10⁴ cm^-2 for 500-nm Si_0.75Ge_0.25 grown on the 20% borosilicate glass substrate. This method may be used to prepare compliant substrates for the growth of low-dislocation relaxed SiGe layers. Received: 4 January 2001 / Accepted: 30 May 2001 / Published online: 17 October 2001     

Microstructure of epitaxial superconductive YBa2Cu3O7 thin films prepared at different deposition rates

A study is reported on the growth mechanism of YBa₂Cu₃O₇ with different growth speeds by high resolution transmission microscopy (HRTEM) and analysis of the interface and thin film microstructure. Two thin films were synthesized by pulse laser deposition on [100], miscut 5°, SrTiO₃ substrate at 820 °C, one with a pulse laser frequency of 1 Hz and one with 6 Hz. Cross-sections were studied by an H-9000 NAR HRTEM along the [010] direction. The growth process of the sample made at 1 Hz was as follows. First, distorted step flow growth occurred on a step-mediated substrate surface of 3–4 cells thickness. Second, about a 15 nm thickness of island shape growth becomes superimposed on the area of the step flow layer. Finally, thin film growth occurred but with growth fluctuation. The sample made at 6 Hz showed the characteristics of island growth; the growth area of island or ball shape was of small size and dense distribution, and seemed to be a confused mosaic stack. The influence of growth speed on YBCO epitaxial film microstructure was studied explicitly by HRTEM.     

Formation of relaxed SiGe on the buffer consists of modified SiGe stacked layers by Si pre-intermixing

High-quality relaxed SiGe films on Si (0 0 1) have been demonstrated with a buffer layer containing modified SiGe (m-SiGe) islands in ultra-high vacuum chemical vapor deposition (UHV/CVD) system. The m-SiGe islands are smoothened by capping an appropriate amount of Si and the subsequent annealing for 10 min. This process leads to the formation of a smooth buffer layer with non-uniform Ge content. With the m-SiGe-dot multilayer as a buffer layer, the 500-nm-thick uniform Si_0.8Ge_0.2 layers were then grown. These m-SiGe islands can serve as effective nucleation centers for misfit dislocations to relax the SiGe overlayer. Surface roughness, strain relaxation, and crystalline quality of the relaxed SiGe overlayer were found to be a function of period's number of the m-SiGe-dot multilayer. By optimizing period number in the buffer, the relaxed Si_0.8Ge_0.2 film on the 10-period m-SiGe-dot multilayer was demonstrated to have a threading dislocation density of 2.0 × 10⁵ cm⁻² and a strain relaxation of 89%.     

Si3N4/Si表面Si生长过程的扫描隧道显微镜研究

利用原位扫描隧道显微镜和低能电子衍射分析了Si的纳米颗粒在Si₃N₄/Si(111)和Si₃N₄/Si(100)表面生长过程的结构演变.在生长早期T为350—1075K范围内,Si在两种衬底表面上都形成高密度的三维纳米团簇,这些团簇的大小均在几个纳米范围内,并且在高温退火时保持相当稳定的形状而不相互融合.当生长继续时,Si的晶体小面开始显现.在晶态的Si₃N₄(0001)/S 关键词： 氮化硅 扫描隧道显微镜 纳米颗粒     

Experimental and theoretical studies of interactions between Si<Subscript>7</Subscript> clusters

The possibility of using magic Si₇ clusters to form a cluster material was studied experimentally and theoretically. In experiments Si₇ clusters were deposited on carbon surfaces, and the electronic structure and chemical properties of the deposited clusters were measured using X-ray photoelectron spectroscopy (XPS). A non bulk-like electronic structure of Si₇ was found in the Si 2p core level spectra. Si₇ is suggested to form a more stable structure than the non-magic Si₈ cluster and Si atoms upon deposition on carbon surfaces. Theoretically it was possible to study the interaction between the clusters without the effect of a surface. Density functional theory (DFT) calculations of potential curves of two free Si₇ clusters approaching each other in various orientations hint at the formation of cluster materials rather than the fusion of clusters forming bulk-like structures.     

Effect of dislocations on the shape of islands during silicon growth on the oxidized Si(111) surface

The formation of dislocation-rich and dislocation-free silicon islands during growth in the absence of mechanical stresses has been studied by scanning tunneling microscopy. The rounded shape of islands obtained at growth temperatures of 400–500°C on the oxidized Si(111) surface is associated with the presence of dislocations within them. The transfer of atoms from the oxidized surface to the islands occurs due to the barrier of the potential energy at the SiO₂/Si boundary. The {111} and {311} facets dominate in the shape of the islands grown at 500–550°C. Their appearance indicates the absence of the threading dislocations in the islands and that the growth is limited by the stage of the nucleation of a new atomic layer.     

Surface science investigations of atomic layer deposition half-reactions using TaF5 and Si2H6

Fundamental adsorption and surface chemistry studies of TaF₅ and Si₂H₆ on a polycrystalline Ta surface were performed in ultra-high vacuum (UHV) in the range 303-523 K to understand if these precursors may be used in applications to grow Ta-based films by atomic layer deposition. TaF₅ uptake saturated in UHV conditions for less than 100 L exposure for adsorption on both clean Ta and on 144 L Si₂H₆-treated Ta. TaF₅ dissociatively adsorbed on clean Ta, and F ligands were determined to govern the self-limiting adsorption behavior. The extent of each “half-reaction”, the reaction of TaF₅ with a Si₂H₆-treated surface and the reaction of Si₂H₆ with a TaF₅-treated surface, increased with surface temperature and was dependent on SiH_xF_y byproduct desorption. Neither Si₂H₆ adsorption nor Si₂H₆ half-reaction reached saturation in UHV conditions, as measured by the surface Si concentration. F ligands were removed during the Si₂H₆ half-reaction, and residual F asymtotically approached a constant value.     

Bimodal growth of manganese silicide on Si(1 0 0)

Two different growth modes of manganese silicide are observed on Si(1 0 0) with scanning tunneling microscopy. 1.0 and 1.5 monolayer Mn are deposited at room temperature on the Si(1 0 0)-(2 × 1) substrate. The as-grown Mn film is unstructured. Annealing temperatures between room temperature and 450 °C lead to small unstructured clusters of Mn or Mn_xSi_y. Upon annealing at 450 °C and 480 °C, Mn reacts chemically with the Si substrate and forms silicide islands. The dimer rows of the substrate become visible again. Two distinct island shapes are found and identified as MnSi and Mn₅Si₃.