Interdiffusion Of Si Research Articles

Selective Epitaxy of Germanium on silicon for the fabrication of CMOS compatible short-wavelength infrared photodetectors

Here we present the selective epitaxial growth of Ge on Si using reduced pressure chemical vapor deposition on SiO2/Si solid masks realized on 200 mm Si wafers, aiming at manufacturing integrated visible/short-wavelength infrared photodetectors. By a suitable choice of the reactants and of the process conditions, we demonstrated highly selective and pattern-independent growth of Ge microstructure featuring high crystalline quality. The Ge “patches” show a distinct faceting, with a flat top (001) facet and low energy facets such as e.g. {113} and {103} at their sidewalls, independently on their size. Interdiffusion of Si in to the Ge microcrystals is limited to an extension of ∼20 nm from the heterointerface. The Ge patches resulted to be plastically relaxed with threading dislocation density values better or on par than those observed in continuous two-dimensional Ge/Si epilayer in the low 107 cm−2 range. A residual tensile strain was observed for patches with size >10 μm, due to elastic thermal strain accumulation, as confirmed by μ-Raman spectroscopy and μ-photoluminescence characterization. Polarization-dependent Raman mapping highlights the strain distribution associated to the tridimensional shape. On this material, Ge photodiodes were fabricated and characterized, showing promising optoelectronic performances.

Refractory high entropy metal sublattice nitride thin films as diffusion barriers in Cu metallizations

The suitability of refractory high entropy metal sublattice nitride (HEAN) thin films to act as diffusion barrier between Cu and Si was evaluated. For this purpose, bi-layers comprising a 150 nm thick Cu layer and a 20 nm thick layer based on (MoNbTaW)N and alloyed with either Ti, V, Cr, Mn, Zr or Hf have been deposited onto Si substrates using high power impulse magnetron sputtering. Subsequently, the bi-layers were annealed up to 950 °C in vacuum and analyzed by X-ray diffraction, confocal laser scanning microscopy and resistance measurements to check for barrier failure. Depending on the elements present in the HEAN layer, two failure modes were identified: (1) interdiffusion of Cu and Si leading to the formation of Cu3Si starting at 850 °C and (2) dewetting of the Cu layer starting at 900 °C. A better barrier performance could be correlated to a lower formation enthalpy of the binary nitride of metals present in the HEAN layer leading to the conclusion that more stable metal‑nitrogen bonds in the HEAN phase are beneficial for its diffusion barrier performance.

Atomic Layer Deposited RuO2 Diffusion Barrier for Next Generation Ru‐Interconnects

AbstractAtomic layer deposition (ALD) is a suitable technology for conformally depositing thin films on nanometer‐scale 3D structures. RuO2 is a promising diffusion barrier for Ru interconnects owing to its compatibility with Ru ALD and its remarkable diffusion barrier properties. Herein, a RuO2 diffusion barrier using an ALD process is developed. The highly reactive Ru precursor [tricarbonyl(trimethylenemethane)ruthenium] and improved O2 supply enable RuO2 deposition. The optimal process conditions [pulsing time ratio (tO2/tRu): 10, process pressure: 1 Torr, temperature: 180 °C] are established for the RuO2 growth. Growth parameters, such as the growth rate (0.56 Å cycle–1), nucleation delay (incubation period: 6 cycles), and conformality (step coverage: 100%), are also confirmed on the SiO2 substrate. The structural and electrical properties of the Ru/RuO2/Si multilayer are investigated to explore the diffusion barrier performance of the ALD‐RuO2 film. The formation of Ru silicide does not occur without the conductivity degradation of the Ru/RuO2/Si multilayer with an increase in the annealing temperature up to 850 °C, thus demonstrating that interdiffusion of Ru and Si is completely suppressed by a thin (5 nm) ALD‐RuO2 film. Consequently, the practical growth behavior and diffusion barrier performance of RuO2 can serve as a potential diffusion barrier for Ru interconnects.

Probing of the influence of bilayer geometry, substrate temperature and post-deposition annealing on Si and Cr thin film interdiffusion through Raman spectroscopy

Probing of the influence of bilayer geometry, substrate temperature and post-deposition annealing on Si and Cr thin film interdiffusion through Raman spectroscopy

Deformation behavior of an A356 alloy containing small sub-grains with wide low-angle boundary

We introduce a new method for overcoming the strength–ductility trade-off by using a non-metallic alloying method, whereby numerous oxygen atoms are introduced in a conventional A356 alloy (I-A356 alloy). Self-organized nanofibers containing oxygen atoms formed in the melt had a chemically coherent interface with the matrix after solidification. Furthermore, the nanofibers (~10 nm in width) developed small sub-grains (<1 μm in size) with wide low-angle boundaries within the grains. Pre-existing dislocations developed by the small lattice mismatch between the nanofibers and matrix promoted strong activities. The sub-grain boundaries do the role of dislocation cells during plastic deformation, imparting high elongation to the I-A356 alloy. In addition, finely dispersed Si-rich nanoprecipitates formed during the aging treatment. The oxygen-rich intermixed layer at the Al–Si interfaces with a small radius of curvature drove Al–Si interdiffusion owing to the high attractive binding energy with Al. Thus, in addition to Mg2Si precipitates, the Si-rich nanoprecipitates further enhanced the alloy strength. Therefore, both, the ultimate tensile strength and elongation to failure of the I-A356 alloy, significantly increased by approximately 69.6 MPa and 14.8%, respectively, in relation to those of the A356 alloy.

Effect of etching solution concentration on preparation of Si holes by metal-assisted chemical etching

The effect of the etching solution concentration on the etching profile of vertical microscale holes formed on a Si(100) substrate by metal-assisted chemical etching (MacEtch) was investigated. MacEtch was performed at different solution concentration ratios (characterized by their molar ratio ), and the Au catalyst split above a certain concentration ratio. The Au–Si interface in the samples after MacEtch was observed using transmission electron microscopy, which revealed that there was a porous SiO x interlayer between the Au and Si, and that the SiO x layer became thinner as the concentration ratio increased. The interlayer almost disappeared under solution conditions when the catalyst was split. We propose two hypotheses for the mechanism of catalytic fracture during MacEtch: chemical fracture due to Si atoms diffusing into the grain boundaries of polycrystalline Au by Au–Si interdiffusion, and mechanical fracture due to stress on the catalyst caused by heterogeneous etching.

Excellent diffusion barrier property of amorphous NbMoTaW medium entropy alloy thin films used in Cu/Si Connect System

In this work, NbMoTaW medium entropy alloy (MEA) films with thickness of 40 nm were prepared by DC magnetron sputtering as a diffusion barrier material for Cu/Si Connect System. The microstructure and thermal property of Cu/NbMoTaW/Si and Cu/Si stacks in varies of annealing temperature (T) were studies by XRD, SEM, TEM and four-point probe measurement system. The results show that the amorphous NbMoTaW films can suppress Si inter-diffusion in Cu film when T is lower than 750 °C while electrical property of Cu/Si system without the barrier fails when T reaches 450 °C. Free of grain boundary, low diffusion coefficient value and stable structure of amorphous NbMoTaW MEA film are suggested to be the main reasons for the excellent barrier property.

Er3+-catalyzed interdiffusion of elements across the interface between Bi2O3 film and SiO2 substrate resulting in host-material-specific photoluminescence spectra of Er3+

Er3+-doped Bi2O3 thin films were sputter-deposited on SiO2 substrates under various process conditions. After post annealing in an O2 atmosphere, near-infrared photoluminescence (PL) spectra from Er3+ ions in specific product oxides were observed. For sufficiently oxidized Bi2O3:Er films with Er contents less than 1.5 at%, α-Bi2O3:Er having a monoclinic structure nucleated with negligible intermixing of the elements across the interface. Its PL spectra exhibited Er3+ luminescence signals consisting of eight Stark splitting peaks. Deposition at temperatures higher than 400 °C and/or with high Er content produced a somewhat reduced Bi2O3 network. Post annealing triggered diffusion of Si into the film as well as diffusion of Bi and Er into the SiO2 substrate, creating Bi2O3 −SiO2 −Er2O3 compound oxides. The original film body crystallized into a δ-Bi2O3 structure, which was stabilized by the presence of a large amount of Er. The emission-active product at Er contents of 2 at% was Bi2O3:Er−SiO2 (Bi2SiO5:Er), which exhibited four-line emission spectra and the intensity was lower than that of α-Bi2O3:Er. For Er3+ contents higher than 4 at%, interdiffusion of Si and O atoms was enhanced and SiOx:Er domains were created in the Bi2O3:Er film. Intense and broad emission peaks at 1530 and 1560 nm were observed in the PL spectra. The emission-active species in the Bi2O3 −SiO2 −Er2O3 compound oxide will be either Er3+ doped in the strongly disordered δ-Bi2O3 lattice involving oxygen deficiencies or Er3+ attached to SiOx domains formed inside the δ-Bi2O3 network.

Interface analysis of SrWO4:Er3+-Yb3+/Si thin films prepared by radio frequency magnetron sputtering for upconversion emission

Upconversion (UC) emission of Er–Yb doped strontium tungstate (SrWO4:Er3+-Yb3+) thin films on silicon substrates prepared by the radio frequency (RF) magnetron sputtering method using different RF powers has been investigated and reported. The X-ray diffraction patterns showed that the tetragonal phase of SrWO4 formed with an improvement in the crystallinity of the films at higher RF power. Atomic force microscopy images demonstrated the homogeneity of the grains on the surface and an increment of grain size with an increase in RF power during deposition. Auger electron spectroscopy was used for the analysis of the interface of the thin films, the Si interdiffusion as well as the role of the thickness with an increase in the sputtering power of the RF deposition. The UC emission of the films prepared at the different RF powers excited at 980 nm was recorded. An enhancement in the intensity with an increase in the RF powers was obtained. Three different bands of UC emission were recorded, which were assigned to the 2H11/2/4S3/2 → 4I15/2 (two green bands) and the one red band (4F9/2 → 4I15/2) transitions for the Er3+ ions. The maximum intensity of UC emission was recorded at 240 W RF power due to a better crystallinity and the bigger size of the grains in the film as well as a better defined interface between the thin film and the substrate.

Establishing multilayered coating on Mo–12Si–8.5 B alloy by Si pack cementation and its oxidation behavior at 900°C–1300°C

AbstractA multilayered oxidation protection coating consisting of MoSi2 outer layer, Mo5Si3 internal layer, and Mo5SiB2/MoB inner layer was developed on the surface of Mo–12Si–8.5B 1.0 wt% ZrB2 alloy via Si pack cementation. The multilayered coating significantly enhanced the oxidation resistance of the alloy at 900°C, 1100°C, and 1300°C in the air by exhibiting negligible oxidation recession. MoSi2 outer layer provided admirable oxidation protection for the alloy at high temperatures by forming a thin and protective SiO2‐rich glass scale on its surface. This was supplemented by the Mo5Si3 internal layer and Mo5SiB2/MoB inner layer that reduced the thermal expansion mismatch between the MoSi2 outer layer and substrate, and therefore no obvious cracks were found in the MoSi2 outer layer. More importantly, the Mo5SiB2/MoB layer as an in situ barriers of Si interdiffusion ensured the stable existence of MoSi2 and Mo5Si3 layers without obvious thickness change during oxidation at 900°C and 1100°C. Mechanical property test indicated that the formation of the coating layers could not affect the fracture toughness of the alloy.

Investigation of an atomic‐layer‐deposited Al2O3 diffusion barrier between Pt and Si for the use in atomic scale atom probe tomography studies on a combinatorial processing platform

In order to enable the application of atomic probe tomography combinatorial processing platforms for atomic‐scale investigations of phase evolution at elevated temperatures, the pre‐sharpened Si tip of 10–20 nm in diameter must be protected against interdiffusion and reaction of the reactive Si with a film of interest by a conformal coating on the Si tip. It is shown that unwanted reactions can be suppressed by introducing a 20‐nm‐thick intermediate Al2O3 layer grown by atomic layer deposition (ALD). As a representative case, Pt is chosen as a film of interest, as it easily forms silicides. Whereas without the ALD coating diffusion/reactions occur, with the protective film, this is prevented for temperatures up to at least 600°C. The effectiveness of the Al2O3 layer serving as a diffusion barrier is not limited to a sharpened Si tip but works generally for all cases where a Si substrate is used.

Nb silicide coatings processed by double pack cementation: Formation mechanisms and stability

Niobium silicide coatings were processed on Nb metal using double pack cementation procedures at temperatures in the range 1000 °C–1200 °C for up to 6 h. Coating thickness and hardness were affected by processing parameters. Lower processing temperatures resulted in single-layer NbSi2 coatings while the highest processing temperature (1200 °C) produced coatings with two layers, NbSi2 and Nb5Si3, the latter at the NbSi2/Nb interface. Analysis of the results shows that Nb5Si3 forms by decomposition of NbSi2 and interdiffusion of Si and Nb. The influence of processing parameters on vacancy density and of vacancy density on the coating formation mechanisms and properties is discussed. Silicide-coated Nb had superior oxidation resistance to pure Nb. However, under the conditions tested a two-layer scale of Nb2O5 and SiO2 was formed. Analysis of the data indicates that Nb2O5 forms rapidly on the surface of the coatings at lower temperatures whereas SiO2 forms at the top of the oxide scale. It is concluded that pre-oxidation treatment is essential to ensure formation of an SiO2 scale, which is associated with enhanced oxidation performance.

Microstructure and morphology of 2D arrays of Ge quantum dots in a Si/Al2O3 matrix

The paper presents the results of the microstructure and morphology study of two-dimensional Ge QD arrays in a Si/Al2O3 matrix formed by annealing multilayer periodic structures with Ge nanolayers in a Si/Al2O3 matrix. The distinctive features of samples in the series are the location and thickness of the Si barrier layers between Ge and aluminium oxide matrix. X-ray reflectometry and diffractometry and transmission electron microscopy studies have shown that large Al6Ge5 crystallites are formed and the multilayer structure is destroyed after annealing of the multilayer sample Al2O3/Ge without Si. It was found that the presence of Si barrier layers in multilayer Al2O3/Si/Ge structures reduces the interdiffusion of Al and Ge, but Ge1-xSix nanocrystallites are formed as a result of Si and Ge interdiffusion. Thus, the introduction of Si barrier layer into the Al2O3/Ge structure allowed obtaining of two-dimensional arrays of Ge1-xSix nanocrystallites with the penetration of Si up to 0.64.

Fabrication of SiGe/Ge nanostructures by three-dimensional Ge condensation of sputtered SiGe on SiO2/Si substrate

In this work, three-dimensional (3D) Ge condensation of sputtered “Si/SiGe” nanostructures on SiO2/Si substrate was investigated. Through varying the width of sputtered Si/SiGe structures from 50 μm to 400 nm, the Ge content of fabricated SiGe structures can be modulated from 0.49 to 1.0 after 3D Ge condensation. By further constructing a width modified nanowire (NW) with adjacent widths of 800 nm and 400 nm before 3D Ge condensation, a Si0.33Ge0.67/Ge heterostructure NW was fabricated after 3D Ge condensation. Due to inter-diffusion of Si and Ge atoms, a ∼2-μm-long region with gradually varied Ge content from ∼0.67 to 1.0 is formed between Si0.33Ge0.67 and Ge. The low Schottky barrier height (0.29 eV) of Ge NW/metal contact and good conductivity of the Ge NW suggest that the 3D Ge condensation technique is very promising for fabrication of scalable and low-cost SiGe or Ge nano-electronic/photonic devices.

Oxide-Free Three-Dimensional Germanium/Silicon Core-Shell Metalattice Made by High-Pressure Confined Chemical Vapor Deposition.

Metalattices are crystalline arrays of uniform particles in which the period of the crystal is close to some characteristic physical length scale of the material. Here, we explore the synthesis and properties of a germanium metalattice in which the ∼70 nm periodicity of a silica colloidal crystal template is close to the ∼24 nm Bohr exciton radius of the nanocrystalline Ge replica. The problem of Ge surface oxidation can be significant when exploring quantum confinement effects or designing electronically coupled nanostructures because of the high surface area to volume ratio at the nanoscale. To eliminate surface oxidation, we developed a core-shell synthesis in which the Ge metalattice is protected by an oxide-free Si interfacial layer, and we explore its properties by transmission electron microscopy (TEM), Raman spectroscopy, and electron energy loss spectroscopy (EELS). The interstices of a colloidal crystal film grown from 69 nm diameter spherical silica particles were filled with polycrystalline Ge by high-pressure confined chemical vapor deposition (HPcCVD) from GeH4. After the SiO2 template was etched away with aqueous HF, the Ge replica was uniformly coated with an amorphous Si shell by HPcCVD as confirmed by TEM-EDS (energy-dispersive X-ray spectroscopy) and Raman spectroscopy. Formation of the shell prevents oxidation of the Ge core within the detection limit of XPS. The electronic properties of the core-shell structure were studied by accessing the Ge 3d edge onset using STEM-EELS. A blue shift in the edge onset with decreasing size of Ge sites in the metalattices suggests quantum confinement of the Ge core. The degree of quantum confinement of the Ge core depends on the void sizes in the template, which is tunable by using silica particles of varying size. The edge onset also shows a shift to higher energy near the shell in comparison with the Ge core. This shift along with the observation of Ge-Si vibrational modes in the Raman spectrum indicate interdiffusion of Ge and Si. Both the size of the voids in the template and core-shell interdiffusion of Si and Ge can in principle be tuned to modify the electronic properties of the Ge metalattice.

Thermodynamic route for self-forming 1.5 nm V-Nb-Mo-Ta-W high-entropy alloy barrier layer: Roles of enthalpy and mixing entropy

This study reports a thermodynamic route for self-forming an ultrathin V-Nb-Mo-Ta-W high-entropy alloy layer for potential use as a promising diffusion barrier. In Cu alloy films minor-doped with 1.2 at.% of one-to-five metallic elements (V, Nb, Mo, Ta and W), the alloying elements spontaneously segregated. Under the competition of enthalpy and mixing entropy that determines the delta Gibbs free energy, one and, in particular, five alloying element(s) formed an alloy solution layer at the Cu/Si interface, whereas three alloying elements differently formed intermetallic compound clusters at the grain boundaries of Cu. Dominant factors for the final states of the alloying elements include the large positive enthalpy between Cu and the alloying elements, the negative enthalpy among the alloying elements, and the low-to-high mixing entropy among the alloying elements. The self-forming quinary alloy layer of only 1.5 nm thick provided excellent resistance to the interdiffusion of Cu and Si up to 700°C, better than practical and other newly developed barrier materials.

Applications of High Diffusion Resistance Multi‐component AlCrTaTiZrRu/(AlCrTaTiZrRu)N0.7 Film in Cu Interconnects

Herein, a multi‐component AlCrTaTiZrRu/(AlCrTaTiZrRu)N0.7 double‐layer structure is prepared by direct current (DC) magnetron sputtering as a diffusion barrier for Cu interconnection. To verify the diffusion‐barrier properties at high temperatures, the Cu/AlCrTaTiZrRu/(AlCrTaTiZrRu)N0.7/Si structures are annealed at 700–900 °C for 30 min in high vacuum. The barrier layer remains amorphous with few nanocrystalline structures after annealing at 800 °C. Cu particle agglomeration begins to appear on the surface, but no Cu‐silicide compound is found on the Si substrate, indicating that the layer can effectively defer the interdiffusion of Cu and Si. Under annealing at high temperature of 900 °C, Cu silicides and other intermetallic compounds are formed on the surface of the Cu film, with the film resistivity increasing significantly, indicating that the AlCrTaTiZrRu/(AlCrTaTiZrRu)N0.7 double‐layer structures have completely failed.

Molecular dynamics studies on spark plasma sintering of Ge–Si based thermoelectric material

The development of new fabrication technologies for Ge–Si thermoelectric materials requires a corresponding theoretical description of physical processes lying behind the synthesis. In the present paper, we investigated the interdiffusion of Si and Ge atoms at the Ge/Si interface, which takes place during spark plasma sintering of Ge and Si powders for fabrication of thermoelectric bulk. The calculation was performed using numerical simulation based on the classical molecular dynamics method. The diffusion coefficients of Si in Ge and vice versa were found at sintering temperatures of 900 K–1300 K and an external pressure of 7 MPa. The calculation results were used to analyze the experimental data derived from the measurements of Ge and Si profiles at the interface of thin Ge/Si plates subjected to spark plasma sintering at the temperature of 1160 K (887 °C). The comparison of measured and calculated diffusion profiles has shown good agreement with one another.

Micrographic Properties of Composite Coatings Prepared on TA2 Substrate by Hot-Dipping in Al–Si Alloy and Using Micro-Arc Oxidation Technologies (MAO)

A composite coating composed of intermetallic compounds, Al–Si alloys, and an oxide ceramic layer was prepared on TA2 substrate by hot-dipping Al–Si alloy and micro-arc oxidation (MAO) methods. The microstructure and composition distribution of the resulting hot-dipped Al–Si alloy layer and MAO-caused ceramic layer were studied by scanning electron microscope (SEM) and energy dispersive spectrum (EDS). In addition, the phase composition of the diffusion layer obtained by the Al–Si alloy hot-dipping procedure was investigated by electron backscattered diffraction (EBSD), and the phase structure of the MAO-treated layer was studied by X-ray diffraction (XRD) analysis and X-ray photoelectron spectroscopy (XPS). The MAO method can make the hot-dipped Al–Si alloy layer in-situ oxidized to form a ceramic layer. Finally, a three-layer composite coating composed of a diffusion layer formed by the Ti–Al–Si interdiffusion, an Al–Si alloy layer and a ceramic layer was prepared on TA2 substrate. Compared with TA2 substrate, the TA2 sample with a three-layer composite coating has larger friction coefficient and less abrasion loss. The three-layer composite coating can significantly improve the wear resistance of TA2. A technical composite method was developed to the low cost in-situ growth of alumina-based ceramic wear-resistant coatings on TA2 substrate.

Thermal studies of individual Si/Ge heterojunctions — The influence of the alloy layer on the heterojunction

Phonon transport across an interface is of fundamental importance to applications ranging from electronic and optical devices to thermoelectric materials. The phonon scattering by an interface can dramatically suppress the thermal transport, which can benefit thermoelectric applications but create problems for the thermal management of electronic/optical devices. In this aspect, existing molecular dynamics simulations on phonon transport across various interfaces are often based on estimates of atomic structures and are seldom compared with measurements on real interfaces. In this work, planar Si/Ge heterojunctions formed by film-wafer bonding are measured for the interfacial thermal resistance (RK) that is further compared with predictions from existing simulations and analytical models. The twist angle between a 70-nm-thick Si film and a Ge wafer is varied to check the influence of the crystal misorientation. Detailed transmission electron microscopy studies are carried out to better understand the interfacial atomic structure. It is found that the alloyed interfacial layer with mixed Si and Ge atoms dominates the measured thermal resistance (RK). Some oxygen impurities may also help to increase RK due to the formation of glassy structures. Following this, RK reduction should be focused on how to minimize the interdiffusion of Si and Ge atoms during the formation of a Si/Ge heterojunction.