Epitaxial CoSi2 Research Articles

Sequential growth of self-organized epitaxial FeSi2 and CoSi2 nanostructures on Si(1 1 1)-7 × 7 surfaces

Epitaxial γ-FeSi2 and CoSi2 nanostructures, grown on a vicinal Si(111)-7 × 7 surface by sequentially depositing 1-monolayer (ML) of Fe and 1ML of Co on a hot Si substrate under ultrahigh vacuum condition, have been investigated by in-situ scanning tunneling microscopy, ex-situ field emission scanning electron microscopy and x-ray photoemission spectroscopy (XPS). While γ-FeSi2 nanostructures have grown as nanowires along the three equivalent 〈110〉 directions on the Si(111) surface, CoSi2 nanoislands have grown as equilateral triangular and trapezoidal islands. Such self-organized nanostructures may find applications in nanoscale devices including S-F hybrids for quantum technology, as CoSi2 is a superconductor (S) and γ-FeSi2 is a ferromagnet (F).

Growth, electronic structure and superconductivity of ultrathin epitaxial CoSi2 films

We report growth, electronic structure and superconductivity of ultrathin epitaxial CoSi2 films on Si (111). At low coverages, preferred islands with 2, 5 and 6 monolayers height develop, which agrees well with the surface energy calculation. We observe clear quantum well states as a result of electronic confinement and their dispersion agrees well with density functional theory calculations, indicating weak correlation effect despite strong contributions from Co 3d electrons. Ex situ transport measurements show that superconductivity persists down to at least 10 monolayers, with reduced T c but largely enhanced upper critical field. Our study opens up the opportunity to study the interplay between quantum confinement, interfacial symmetry breaking and superconductivity in an epitaxial silicide film, which is technologically relevant in microelectronics.

Thermal transport across metal silicide-silicon interfaces: First-principles calculations and Green's function transport simulations

In this work, we use a combination of first-principles calculations under the density functional theory framework and heat transport simulations using the atomistic Green's function (AGF) method to quantitatively predict the contribution of the different scattering mechanisms to the thermal interface conductance of epitaxial CoSi$_2$-Si interfaces. An important development in the present work is the direct computation of interfacial bonding from density functional perturbation theory (DFPT) and hence the avoidance of commonly used `mixing rules' to obtain the cross-interface force constants from bulk material force constants. Another important algorithmic development is the integration of the recursive Green's function (RGF) method with B\"{u}ttiker probe scattering that enables computationally efficient simulations of inelastic phonon scattering and its contribution to the thermal interface conductance. First-principles calculations of electron-phonon coupling reveal that cross-interface energy transfer between metal electrons and atomic vibrations in the semiconductor is mediated by delocalized acoustic phonon modes that extend on both sides of the interface, and phonon modes that are localized inside the semiconductor region of the interface exhibit negligible coupling with electrons in the metal. We also provide a direct comparison between simulation predictions and experimental measurements of thermal interface conductance of epitaxial CoSi$_2$-Si interfaces using the time-domain thermoreflectance technique. Importantly, the experimental results, performed across a wide temperature range, only agree well with predictions that include all transport processes: elastic and inelastic phonon scattering, electron-phonon coupling in the metal, and electron-phonon coupling across the interface.

Thermal transport across metal silicide-silicon interfaces: An experimental comparison between epitaxial and nonepitaxial interfaces

Silicides are used extensively in nano- and microdevices due to their low electrical resistivity, low contact resistance to silicon, and their process compatibility. In this work, the thermal interface conductance of TiSi$_2$, CoSi$_2$, NiSi and PtSi are studied using time-domain thermoreflectance. Exploiting the fact that most silicides formed on Si(111) substrates grow epitaxially, while most silicides on Si(100) do not, we study the effect of epitaxy, and show that for a wide variety of interfaces there is no difference in the thermal interface conductance of epitaxial and non-epitaxial silicide/silicon interfaces. The effect of substrate carrier concentration is also investigated over a wide range of p- and n-type doping, and is found to be independent of carrier concentration, regardless of whether the interface is epitaxial and regardless of silicide type. In the case of epitaxial CoSi$_2$, a comparison of temperature dependant experimental data is made with two detailed computational models using (1) full-dispersion diffuse mismatch modeling (DMM) including the effect of near-interfacial strain and (2) an atomistic Green' function (AGF) approach that integrates near-interface changes in the interatomic force constants obtained through density functional perturbation theory. At temperatures above 100K, the AGF approach greatly underpredicts the CoSi$_2$ data, while the DMM prediction matches the data well. The full-dispersion DMM is also found to closely predict the experimentally observed temperature-dependent interface conductance for epitaxial NiSi/Si and non-epitaxial TiSi$_2$/Si interfaces. In the case of epitaxial PtSi/Si interfaces, full dispersion DMM significantly overpredicts the experimental data.

Ultralow 1/f Noise in a Heterostructure of Superconducting Epitaxial Cobalt Disilicide Thin Film on Silicon.

High-precision resistance noise measurements indicate that the epitaxial CoSi2/Si heterostructures at 150 and 2 K (slightly above its superconducting transition temperature Tc of 1.54 K) exhibit an unusually low 1/f noise level in the frequency range of 0.008-0.2 Hz. This corresponds to an upper limit of Hooge constant γ ≤ 3 × 10-6, about 100 times lower than that of single-crystalline aluminum films on SiO2 capped Si substrates. Supported by high-resolution cross-sectional transmission electron microscopy studies, our analysis reveals that the 1/f noise is dominated by excess interfacial Si atoms and their dimer reconstruction induced fluctuators. Unbonded orbitals (i.e., dangling bonds) on excess Si atoms are intrinsically rare at the epitaxial CoSi2/Si(100) interface, giving limited trapping-detrapping centers for localized charges. With its excellent normal-state properties, CoSi2 has been used in silicon-based integrated circuits for decades. The intrinsically low noise properties discovered in this work could be utilized for developing quiet qubits and scalable superconducting circuits for future quantum computing.

Scanning tunneling microscopy observation of ultrathin epitaxial CoSi2(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.

Effects of NH3 flow rate on the epitaxial growth of CoSi2 thin film using a CoN x interlayer deposited by MOCVD

A uniform epitaxial CoSi2 layer could be grown on a Si(100) substrate by deposition of a CoNx interlayer followed by an in situ annealing process. The amorphous Si–N layer which is formed at the CoNx/Si interface plays a role in inhibiting excessive Co incorporation into Si layer and consequently enhancing the epitaxial growth of CoSi2 layer. However, the Si–N layer also limits the growth of thick CoSi2 layer. Therefore, the thickness of the amorphous Si–N layer is an important factor for the epitaxial growth of the CoSi2 layer with uniform structure and low sheet resistance.

Formation of epitaxial cobalt disilicide thin film by solution processing

A precursor obtained by the reaction between cyclopentasilane and dicobaltoctacarbonyl in a toluene solution provides a method for the production of a low-resistivity thin film of cobalt disilicide CoSi2 (ca. 15 µΩ·cm) on a silicon (100) substrate by spin coating and annealing. Several types of analysis have revealed that the films formed on silicon substrates have a double-layer structure consisting of an upper layer (oxidized complex film) and an epitaxial CoSi2 layer with low resistivity along the (100) orientation of the silicon substrate. The former layer can be removed by dry etching or chemical mechanical planarization when used in actual applications.

Field electron emission based on resonant tunneling in diamond/CoSi2/Si quantum well nanostructures

Excellent field electron emission properties of a diamond/CoSi2/Si quantum well nanostructure are observed. The novel quantum well structure consists of high quality diamond emitters grown on bulk Si substrate with a nanosized epitaxial CoSi2 conducting interlayer. The results show that the main emission properties were modified by varying the CoSi2 thickness and that stable, low-field, high emission current and controlled electron emission can be obtained by using a high quality diamond film and a thicker CoSi2 interlayer. An electron resonant tunneling mechanism in this quantum well structure is suggested, and the tunneling is due to the long electron mean free path in the nanosized CoSi2 layer. This structure meets most of the requirements for development of vacuum micro/nanoelectronic devices and large-area cold cathodes for flat-panel displays.

On new two-dimensional structures produced on the Si (111) and Si (100) surface upon molecular-beam epitaxy of cobalt and silicon

Highly perfect epitaxial heterostructure CoSi2 films have been grown on the surface of Si (111) and Si (100) single crystals by the method of molecular-beam epitaxy. The optimal regimes of the film growth with different thicknesses have been determined. It has been shown that short-term annealing of epitaxial films at T = 900–950 K leads to the formation of new CoSi2/Si(111)-2 × 2 and CoSi2/Si(100)−2 × 4 superstructures.

Atomic Layer Deposition of Co Using N2/H2 Plasma as a Reactant

Cobalt thin films were deposited by plasma-enhanced atomic layer deposition (PE-ALD) using CoCp2 as a precursor and N2/H2 plasma as a reactant. We systematically investigated the changes in Co film properties depending on N2/H2 gas flow ratio to study the role of N during PE-ALD Co. With increasing N2 flow ratio, the resistivity decreased reaching minimum value at fN2/H2 = 0.25 ∼ 0.33, which corresponds to the atomic ratio in NH3 molecule, and then increased. With only N2 or H2 plasma, films with very high sheet resistance over 1 M/sq were deposited. The chemical compositions of Co films were analyzed by x-ray photoelectron spectroscopy (XPS) and thickness and conformality were determined by x-ray reflectometry (XRR) and field emission scanning electron microscopy (FE-SEM), respectively. Then, the silicidation of PE-ALD Co films producing epitaxial CoSi2 were investigated by x-ray diffraction (XRD) and high-resolution cross-sectional transmission electron microscopy (HR-XTEM).

Formation of an Epitaxial CoSi2 Layer with Reduced Oxygen Contamination

The formation of epitaxial using an oxide buffer layer is known to be sensitive to the oxygen contamination of the Co film. The oxygen contaminants may originate from various oxygen sources such as ambient annealing. The presence of an oxynitride buffer layer and a Ti-capping layer affect the growth of epitaxial , resulting in good epitaxial quality and excellent interface morphology between the Co film and Si substrate. Almost no interference due to oxygen contamination was observed, in spite of intentional exposure of the Co film to air. The oxynitride buffer layer could effectively control the flux of the Co into the Si, resulting in a coherent interface between the and the Si without the formation of additional . Moreover, the Ti-capping layer could absorb oxygen, resulting in less oxygen contamination. This capping layer positively affected the interface morphology between the Co-silicide and the Si substrate.

Large-area Co-silicide nanodot arrays produced by colloidal nanosphere lithography and thermal annealing

We report here the successful fabrication of large-area size-tunable periodic arrays of cobalt and Co-silicide nanodots on silicon substrates by employing the colloidal nanosphere lithography (NSL) technique and heat treatments. The growth of low-resistivity epitaxial CoSi 2 was found to be more favorable for the samples with smaller Co nanodot sizes. The sizes of the epitaxial CoSi 2 nanodots can be tuned from 50 to 100 nm by varying the diameter of the colloidal spheres and annealing temperatures. The epitaxial CoSi 2 nanodots were found to grow with an epitaxial orientation with respect to the (0 0 1)Si substrates: [0 0 1]CoSi 2//[0 0 1]Si and (2 0 0)CoSi 2//(4 0 0)Si. From the results of planview HRTEM, XTEM, and SAED analysis, the epitaxial CoSi 2 nanodots were identified to be inverse pyramids in shape, and the average sizes of the faceted silicide nanodots were measured to decrease with annealing temperature. The observed results present the exciting prospect that with appropriate controls, the colloidal NSL technique promises to facilitate the growth of a variety of well-ordered silicide nanodots with selected shape, size, and periodicity.

Nanoscale epitaxial cobalt salicide bitlines for charge trapping memory cells

An epitaxial CoSi2 process is presented, which allows the self-aligned formation of bitlines with only a few tens of nanometer width for Twin Flash memory cells in the 63nm generation. The bitlines show a good thermal stability and low resistance for widths down to 35nm, where polycrystalline CoSi2 is known to exhibit a strong narrow linewidth effect. Transmission electron microscopy studies revealed a cube-on-cube epitaxy with only a few twins depending on the annealing conditions. The low bitline resistance results in a linear drain voltage dependence of the programing characteristics and a suppression of secondary electron injection during programing.

Crystallinity of CoSi2 nanolayer grown by refractory metal interlayer and cap layer methods

Epitaxial formation of CoSi2 nanolayer by solid state reaction of Co-Si in refractory metal intermediate-layer and cap-layer systems was investigated. Thin films of Ta and W, as the refractory metal intermediate or cap layers of the Co film, were deposited on Si(100) substrate and then heat-treated. The both interlayers resulted in formation of epitaxial CoSi2 with (100) crystallographic orientation at 900°C. However, in the Ta intermediated system, the grown CoSi2 layer was thermally unstable at 1000°C, unlike the W system with a stable silicide layer. We found that use of W cap-layer cannot yield an epitaxial CoSi2 phase. But, a Ta cap-layer resulted in formation of epitaxial CoSi2(100) layer even in a lower temperature (800°C). Again, in the presence of Ta, the grown CoSi2 layer was thermally unstable at 900°C.

High quality epitaxial CoSi2 using plasma nitridation-mediated epitaxy: The effects of the capping layer

The epitaxial growth of CoSi2 from Co with a Ti capping layer using plasma nitridation-mediated epitaxy (PNME) was demonstrated. By exposing the Si substrate to the NH3 plasma, an a-SiNx layer was formed with atomic scale thickness controllability. After Co and Ti deposition followed by annealing, high quality epitaxial CoSi2 was formed on Si(001) with a perfectly flat interface. However, the epitaxial growth was observed for Co with a Ti capping layer prepared only by ex situ deposition, not by in situ deposition. The epitaxial CoSi2 was analyzed by x-ray diffraction and a scanning transmission electron microscope. Based on these results, the mechanism of PNME and the effects of the Ti capping layer process on silicidation were discussed.

Titanium Interlayer Mediated Epitaxy of CoSi 2 on Si 1− xGe x

Titanium Interlayer Mediated Epitaxy (TIME) has been shown to promote the formation of epitaxial CoSi 2 on Si (100). Similarities between Si and Si 1− x Ge x alloys have motivated a study of whether the TIME process could be successful in forming epitaxial CoSi 2 on Si 1− x Ge x . Titanium layers of varying thickness were deposited as interlayers between a Co layer and c-Si/Si 0.8Ge 0.2 grown epitaxially onto Si (100) to investigate their role in the formation of epitaxial CoSi 2 on Si 1− x Ge x alloys. The effect of Ti interlayer thickness on the orientation of CoSi 2 to the Si 1− x Ge x substrate, and the conditions under which a polycrystalline CoSi 2 film has been formed have been studied. It was found that Ti was beneficial in promoting epitaxy to the substrate in all cases. The experimental results indicate that with a Ti interlayer thickness of ∼ 50 Å, the formation of epitaxial CoSi 2 adjacent to the substrate was achieved, and pinhole formation was minimized. It was also observed that for increased interlayer thickness, Ti reacted with Si to form a titanium silicide.

Growth mechanism of epitaxial CoSi 2 on Si and reactive deposition epitaxy of double heteroepitaxial Si/CoSi 2/Si

The growth mechanism of epitaxial CoSi 2 was studied using Co/Ti/Si multilayer solid phase reaction. Results showed that phase formation was controlled by diffusion of Co through the growing CoSi x , although at the early stage of CoSi 2 growth the diffusion of Co could be controlled by a Ti layer. A reactive deposition technique was also evaluated by using a conventional magnetron sputtering system. Results showed that an epitaxial CoSi 2 layer was formed by controlling the Co sputtering rate not to exceed the Co diffusion rate through CoSi x . However, the surface of CoSi 2 became rough when the deposition rate was much slower than the Co diffusion rate through CoSi x . The roughness was caused by the formation of CoSi 2 (111) facets at the interface between CoSi 2 and the Si substrate. Si/CoSi 2/Si double heteroepitaxial structures were fabricated when Si and Co were sequentially sputter-deposited on a Si (100) substrate.

Roughness improvement of the CoSi 2/Si-interface for an application as buried silicide

The material CoSi 2 is preferred for the fabrication of buried silicide films between silicon device layer and buried oxide of SOI substrates for BICMOS integrations. Such an application needs excellent quality of the interface between the silicide and the silicon device layer. Using the conventional cobalt salicide process the roughness and waviness of the interface is too large for a device application. In this presentation three technologies to improve the CoSi 2/Si-interface quality were characterized. Using the first technology a very thin single crystalline CoSi 2 film was fabricated on a silicon substrate. This film acts as initial layer to produce thicker single crystalline silicide films. By the second technology an interlayer between cobalt and the silicon substrate was used to mediate an epitaxial CoSi 2 growth. Different types and materials were tested. Using the third technique a sacrificial layer of polycrystalline silicon between cobalt and the silicon substrate was consumed during the silicidation reaction. This method gives the best results with interface roughness values of less than 1 nm. The interface roughness was measured after CoSi 2 removal using AFM. A possible epitaxial growth of the silicide films was investigated with XRD analysis. Cross sectional SEM images were prepared to analyze the interface waviness and the CoSi 2 structure.

Localized Charge Trapping Memory Cells in a 63 nm Generation with Nanoscale Epitaxial Cobalt Salicide Buried Bitlines

AbstractA 63nm Twin Flash memory cell with a size of 0.0225μm2 per 2 (or 4) bits is presented. To achieve small cell areas, a buried bit line and an aggressive gate length of 100 nm are the key features of this cell together with a minimum thermal budget processing. A novel epitaxial CoSi2 process allows the salicidation of local buried bitlines with only a few tens of nanometer width.