Ag growth on Si(111) with an Sb surfactant investigated by scanning tunneling microscopy

We have investigated a room-temperature growth mode of ultrathin Ag films on a Si(111) surface with an Sb surfactant using STM in a UHV system. On the Sb-passivated Si surface, small sized islands were formed up to 1.1 ML. Flat Ag islands were dominant at 2.1 ML, coalescing into larger islands at 3.2 ML. Although the initial growth mode of Ag films on the Sb-terminated Si(111) surface was Volmer-Weber (island growth), the films were much more uniform than Ag growth on clean (Si(111) at the higher coverages. From the analysis of STM images of Ag films grown with and without an Sb surfactant, the uniform growth of Ag films using an Sb surfactant appears to be caused by the kinetic effects of Ag on the preadsorbed Sb layer. Our STM results indicated that Sb suppresses the surface diffusion of Ag atoms and increases the Ag-island density. The increased island density is believed to cause coalescence of Ag islands at higher coverages of Ag, resulting in the growth of atomically flat and uniform Ag islands on the Sb surfactant layer.     

Surfactant-mediated growth of CoSi2 on Si(100)

Thin co-deposited CoSi₂ films grown at 500–600°C on Si(100) are studied by scanning tunneling microscopy (STM). With a Si rich deposition we observe initially the formation of elongated three-dimensional CoSi₂ islands. The use of one preadsorbed atomic layer of As as a surfactant results in a drastic increase of the island density. This effect appears to be a consequence of a decreased rate of surface diffusion of Co and Si. At higher coverages the roughness of the CoSi₂ film is reduced considerably by the surfactant. The results are discussed with regard to the method of allotaxy which allows the fabrication of buried silicide layers. Here, the requirements for small precipitates and high growth temperature can possibly be met more efficiently using As as a surfactant.     

Thickness-dependent coercivity of ultrathin Co films on a rough substrate: Cu-buffered Si(111)

The hysteresis loops, of Co films with thicknesses ranging from 12- to 80-monolayer-equivalent (MLE) coverages grown by thermal evaporation on a Cu-covered Si(111) surface, were measured in situ by the surface magneto-optic Kerr effect (SMOKE) technique. The hysteresis loops were measured as a function of Co coverage under an external sinusoidal magnetic field at fixed driving frequency. The coercivity H_c of the Co film versus thickness t followed a power law t⁻ⁿ with n=0.4±0.1 between 12 and 44 MLE, and stabilized after 44 MLE, up to the 80 MLE studied. The surface morphology of the 80-MLE Co film was imaged ex situ by atomic force microscopy (AFM) and scanning tunneling microscopy (STM), revealing cauliflower-like islands that were rough both in the short and long range. Analysis of the height–height correlation function for the largest image gave measurements of the effective roughness exponent (0.8), the vertical interface width w(2500 Å), and the lateral correlation length ξ(10 000 Å). We suggest that the coercivity changed in part due to changes in roughness of the Co films, deposited on a rough substrate; the spatial roughness would create an additional surface anisotropy, contributing to a fluctuation in the domain wall energy, resulting in a roughness-dependent coercivity.     

Near-surface compositional oscillations of Co diffused into Si(100) at -60 °C: a study by high-resolution Rutherford backscattering

Highly resolved Co depth profiles have been obtained during the initial stages of Co growth on Si(100) at low temperature (-60 °C) by in situ high-resolution Rutherford backscattering spectrometry. We found extensive Co in-diffusion in the submonolayer growth regime even at this low temperature, besides Co on top of the Si surface. The amount of diffused-in Co is larger than the amount of Co at the Si surface. Every second Si layer is depleted of Co, starting at the Si surface, thus giving rise to compositional oscillations of Co in the Si(100) lattice. At this low temperature the growth of metallic Co on the Si surface is observed at 0.1 ML of deposited Co, which continues for higher coverages. At much higher coverage (5.93 ML of Co) almost exclusively low Co content silicides are formed at the Co/Si interface. The data presented here are compared with previous room temperature deposition data and are different in several aspects. PACS 68.35.-p; 68.55.ag; 75.47.-m; 82.80.Yc     

Epitaxy and magnetic properties of surfactant-mediated growth of bcc cobalt

High resolution core level photoemission spectroscopy, photoelectron diffraction, and x-ray magnetic circular dicroism (XMCD) have been used to characterize the structural and magnetic properties of bcc-cobalt films grown on GaAs(110) substrates by using Sb as a surfactant. We have unambiguously disentangled the surfactant role played by the Sb which improves the crystallinity and reduces the lattice distortion of the metallic films as well as changes the interdiffusion process at the interface compared to the Co/GaAs(110) system. As a consequence of these combined effects, an improvement on the magnetic response of the grown Co thin films has been observed by XMCD measurements.     

Homogeneous Si films on CaF2/Si(1 1 1) due to boron enhanced solid phase epitaxy

The structure and morphology of Si/CaF₂/Si(1 1 1) structures have been investigated by X-ray diffraction (XRD, GIXRD) and X-ray photoelectron spectroscopy (XPS). While CaF₂ films were grown via molecular beam epitaxy (MBE), Si films on CaF₂/Si(1 1 1) are fabricated by surfactant enhanced solid phase epitaxy (SE-SPE). Here Boron was used as a surfactant to obtain semiconductor films of homogeneous thickness. The Si films are entirely relaxed while the CaF₂ films have both pseudomorphic and relaxed crystallites. After exposure to ambient conditions, the Si films have a very thin native oxide film. The homogeneous Si film partially prevents the incorporation of impurities at the interface between the Si substrate and CaF₂ via migration along residual defects of the CaF₂ film.     

Scanning tunneling microscopy observation of ultrathin epitaxial CoSi<Subscript>2</Subscript>(111) films grown at a high temperature

Scanning tunneling microscopy (STM) is used to study the basic laws of growth of ultrathin epitaxial CoSi₂(111) films with Co coverages up to 4 ML formed upon sequential deposition of Co and Si atoms taken in a stoichiometric ratio onto the Co–Si(111) surface at room temperature and subsequent annealing at 600–700°C. When the coverage of Co atoms is lower than ~2.7 ML, flat CoSi₂ islands up to ~3 nm high with surface structure 2 × 2 or 1 × 1 grow. It is shown that continuous epitaxial CoSi₂ films containing 3–4 triple Si–Co–Si layers grow provided precise control of deposition. CoSi₂ films can contain inclusions of the local regions with (2 × 1)Si reconstruction. At a temperature above 700°C, a multilevel CoSi₂ film with pinholes grows because of vertical growth caused by the difference between the free energies of the CoSi₂(111) and Si(111) surfaces. According to theoretical calculations, structures of A or B type with a coordination number of 8 of Co atoms are most favorable for the CoSi₂(111)2 × 2 interface.     

Interaction of cobalt atoms with an oxidized Si(100)2 × 1 surface

The initial stages of oxidation of a Si(100)2 × 1 surface and the interaction of cobalt atoms with it were studied by core-level photoelectron spectroscopy. The study was carried out with cobalt coverages of up to 8 ML. Computer modeling of the spectra of photoexcited electrons revealed Co atom penetration under the silicon oxide layer, an effect observed even at room temperature. This process was shown to give rise to the disappearance of silicon interface phases at the oxide-layer-silicon boundary and to the formation of a more complex phase involving atoms of Co, O, and Si. After completion of the process, a Co-Si solid solution forms at the interface.     

Simulation of Surfactant Effects on Growth of Semiconductor Hetero-Epitaxial Sb-Ge/Si(111)

A kinetic Monte Carlo simulation is performed in order to study the effect of Sb as a surfactant on the growth of Ge/Si(111). In our model the exchange mechanism between Ge and Sb atoms and the re-exchange mechanism in which the exchanged Ge adatom re-exchange with the lifted Sb atom to return to the surfactant layer, are considered. Our simulation shows the re-exchange process plays an important role on the growth mode transition in Ge/Sb/Si(111)
system. The influences of the substrate temperature and the deposition rate on the growth of Ge/Sb/Si(111) system is discussed.     

Annealing studies of the Co/Si(1 1 1) interface

The behaviour of the Co/Si(1 1 1) interface upon annealing is investigated by low energy electron diffraction (LEED), angle resolved ultraviolet (ARUPS) and X-ray (XPS) photoemission spectroscopy. According to the Co thickness two regimes can be distinguished. At low coverages (≲ 8 monolayers ML) no well defined bulk silicides other than the silicon rich epitaxial CoSi₂ phase can be identified. In contrast for larger Co thickness (≳ 15–100 ML) it is found that increasing progressively the annealing temperature (up to 600°C) and time (up to ∼ 30 min) leads to the successive arrival of the following silicides phases within the probing depth of our techniques (∼ 5–20 Å): Co, Co₂Si, CoSi, CoSi₂.     

Mechanisms of formation and the electronic properties of the Yb-Si(100) thin-film system

The formation and properties of the Yb-Si(100) thin-film system were studied by high-resolution photoelectron spectroscopy with synchrotron radiation, Auger electron spectroscopy, the contact potential difference technique, and low-energy electron diffraction measurements over a wide range of coverages and at different temperatures. It was established that the formation of the Yb-Si(100) system prepared by solid-phase epitaxy occurs through a mechanism close to the Stranski-Krastanow mechanism. It was shown that, at submonolayer coverages, it is primarily these two-dimensional (2D) 2 × 3 and 2 × 6 structures that are formed, while at higher coverages a three-dimensional Yb silicide film grows. Data on the morphology and phase composition of the silicide film and on the electronic state of the Si atoms and the valence state of the Yb atoms in the silicide and the 2D structures, as well as on the atomic structure of these films, were obtained. The components of the Si 2p spectra were studied for different coverages. The relation between the shape of these spectra obtained for multilayer Yb silicide films and their phase composition was revealed.     

Sb induced (7×7) to (1×1) surface phase transformation of the Si(111) surface

Adsorption of Sb at a very low flux rate results in an epitaxial layer-by-layer growth on Si(111) surface held at room-temperature. Band-bending is not observed for submonolayer Sb coverages while sharp changes in the photoemission features are observed for 1.0 monolayer (ML) Sb adsorption. Changes in the core level binding energy and width in X-ray photoelectron spectroscopy, surface related feature in Electron energy loss spectroscopy and spot intensity ratios in Low energy electron diffraction studies suggest a surface phase transition upon adsorption of 1.0 monolayer of Sb. A plausible model is proposed to explain the abrupt metal-semiconductor transformation at this critical coverage of 1.0 ML.     

Growth of SrTiO3 on Si(001) by hybrid molecular beam epitaxy

We report the heteroepitaxial growth of SrTiO₃ thin films on Si(001) by hybrid molecular beam epitaxy (hMBE). Here, elemental strontium and the metal‐organic precursor titanium tetraisopropoxide (TTIP) were co‐supplied in the absence of additional oxygen. The carbonization of pristine Si surfaces during native oxide removal was avoided by freshly evaporating Sr into the hMBE reactor prior to loading samples. Nucleation, growth and crystallization behavior as well a structural properties and film surfaces were characterized for a series of 46‐nm‐thick SrTiO₃ films grown with varying Sr to TTIP fluxes to study the effect of non‐stoichiometric growth conditions on film lattice parameter and surface morphology. High quality SrTiO₃ thin films with epitaxial relationship (001)SrTiO₃ || (001)Si and [110]SrTiO₃ || [100]Si were demonstrated with an amorphous layer of around 4 nm thickness formed at the SrTiO₃/Si interface. The successful growth of high quality SrTiO₃ thin films with atomically smooth surfaces using a thin film technique with scalable growth rates provides a promising route towards heterogeneous integration of functional oxides on Si. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Strain status of epitaxial Ge film on a Si (001) substrate

An epitaxial Ge film was grown on a Si (001) substrate via a two-step process through the molecular beam epitaxy technique. The strain status of non-annealed and annealed epitaxial Ge films was determined by X-ray diffraction, Raman spectroscopy, and a combination of high-resolution transmission electron microscopy and geometric phase analysis. Results showed that the strain in non-annealed and annealed epitaxial Ge films is nonhomogeneous from the Ge/Si interface to the Ge film surface. The strain parallel to the interface in the non-annealed epitaxial Ge film is compressive; this strain reaches a minimum near the surface and a maximum at the interface. By contrast, the strain parallel to the interface in the annealed epitaxial Ge film is tensile; this strain reaches a minimum at the interface and a maximum near the surface.     

Novel sb induced reconstruction of the (113) surface of ge

Sb induces on Ge(113) a c(2 x 2) reconstruction in which Sb breaks one Ge-Ge bond and occupies an interstitial site, in contrast to Sb adsorption on other Si or Ge surfaces. Sb saturates the three dangling bonds per unit cell of the (113) surface inducing a large strain which is released by occupation of the interstitial site. Two neighboring Sb at interstitial sites form a dimer. The structure has been determined by x-ray diffraction, applying direct methods, and ab initio density-functional-theory calculations. The adsorption geometry and the high binding energy lead one to expect that Sb cannot be used as a surfactant for the growth of Si/Ge layers on the (113) surface.     

Interplay of surface morphology, strain relief, and surface stress during surfactant mediated epitaxy of Ge on Si

Lattice-mismatch-induced surface or film stress has significant influence on the morphology of heteroepitaxial films. This is demonstrated using Sb surfactant-mediated epitaxy of Ge on Si(111). The surfactant forces a two-dimensional growth of a continous Ge film instead of islanding. Two qualitatively different growth regimes are observed. Elastic relaxation: Prior to the generation of strain-relieving defects the Ge film grows pseudomorphically with the Si lattice constant and is under strong compressive stress. The Ge film relieves strain by forming a rough surface on a nm scale which allows partial elastic relaxation towards the Ge bulk lattice constant. The unfavorable increase of surface area is outbalanced by the large decrease of strain energy. The change of film stress and surface morphology is monitored in situ during deposition at elevated temperature with surface stress-induced optical deflection and high-resolution spot profile analysis low-energy electron diffraction. Plastic relaxation: After a critical thickness the generation of dislocations is initiated. The rough phase acts as a nucleation center for dislocations. On Si(111) those misfit dislocations are arranged in a threefold quasi periodic array at the interface that accommodate exactly the different lattice constants of Ge and Si. Received: 1 April 1999 / Accepted: 17 August 1999 / Published online: 6 October 1999     

SiC缓冲层对Si表面生长的ZnO薄膜结构和光电性能的改善

用脉冲激光沉积(PLD)技术制备了ZnO/SiC/Si和 ZnO/Si薄膜并制成了紫外探测器。利用X射线衍射(XRD),光致发光(PL)谱,I-V曲线和光电响应谱对薄膜的结构和光电性能进行了研究。实验结果表明:SiC缓冲层改善了ZnO薄膜的结晶质量和光电性能,其原因可能是SiC作为柔性衬底能够减少ZnO与Si 之间大的晶格失配和热失配导致的界面缺陷和界面态。     

CoSi2/Si (111) Interface study by X-ray standing waves

Summary The CoSi₂/Si (111) interface has been studied with the X-ray standing-wave technique. The interface (a 49? thick CoSi₂ layer) has been epitaxially grown on Si (111) under ultrahigh vacuum andin situ characterized with Auger spectroscopy and low-energy electron diffraction. The perpendicular lattice mismatch between epilayer and substrate has been measured with double-crystal X-ray diffraction. The X-ray standing-wave analysis gives clear indication that the Co atoms are fivefold coordinated at the interface. This work has been partially supported by Progetto Finalizzato ?Materiali e dispositivi per l'Elettronica a Stato Solido?.     

“Silicide-like electron states in the Si-Mo interface grown at liquid nitrogen temperature”

Ultraviolet photoelectron spectra of the Si(111)-Mo interface grown at ～ 85° K are presented and compared with spectra of the Mo 4d states at increasing coverages on a substrate of Al. In the sharp transition region between Si and the metal the interface electron states can be understood in terms of the Si(p)-Mo(d) hybridzation; in spite of the sharpness of the interface the electron states in the transition region show the same qualitative features as refractory metal silicides.     

Gold at the Si(111)-Pd interface; modification of atomic interdiffusion

We present the results of an angle integrated photoemission (hv = 21.2eV) and Auger investigation of the effect of Au interlayers on the growth of the Si(111)-Pd interface. Gold does not remain totally blocked at the interface with increasing amounts of deposited Pd. As a consequence the same amount of Au (around 6 monolayers) acts as a promotor of Si-Pd mixing at low Pd coverages (below about 8 monolayers) and as an inhibitor at higher Pd coverages. The effect is dependent on the amount of Au so that Au at 10 monolayers is an inhibitor independent of the Pd coverage. The implications on the interface growth mechanism are discussed.