Au Si Interface Research Articles

Pattern formation via the oriented growth of Au-Si thin films on Si(001) substrate

The fabrication of patterned surfaces is integral to various device manufacturing processes. These techniques, developed over time, enable the precise manipulation of surface properties at the nanoscale. In this study, we introduce a self-driven approach for patterning metal and semiconductor surfaces by solidifying eutectic Au-Si thin films on Si(001) substrates. The resulting pattern encompasses meticulously aligned rectangular Au-Si eutectic structures and Si islands, the orientations of which are determined by the substrate. This exceptional alignment is attributed to the epitaxial growth of Si(001) and the heteroepitaxial growth of Au(001) and Au(011) within the films. This conclusion was corroborated through selected area electron diffraction and synchrotron radiation grazing-incidence wide-angle X-ray scattering (GIWAXS). We propose a 4:3 coincident site lattice (CSL) model to elucidate the Au-Si interface, explaining the dominant (001)[110]Au||(001)[110]Si orientation relationship that minimizes the Au/Si lattice mismatch to 0.2%. The other observed orientation relationships, (110)[001]Au||(001)[110]Si and (001)[010]Au||(001)[110]Si, entail a 6.3% mismatch in one or two directions, leading to contrasting elastic strains in the Au and Si lattices, accompanied by the accrual of elastic strain energy. This study underscores the practicality of substrate-oriented pattern formation, with potential applications in other phase-separated systems.

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.

Structural Properties of Nanometer‐Sized Gold Nanoparticles on a Silicon Substrate

Properties of gold nanoparticles (Au NPs) supported on Si substrates, which have been the focus of recent attention, depend sensitively on the size, structure, and coordination of Au NPs as well as on the processes of interdiffusion and chemical bonding at the Au–Si interface. There is a great need, therefore, for developing structural techniques capable of imaging such nanosystems in atomic detail. Here, it is discussed how the atomic structure of nanometer‐sized Au NPs deposited on a silicon surface can be determined by using high‐energy X‐ray diffraction measurements in combination with an analysis based on atomic‐pair‐distribution functions and advanced reverse Monte Carlo methods. This approach delivers simple and incisive pictures of the structure of Au clusters on Si and reveals direct structural evidence for the emergence of a Au‐silicide interface following room temperature deposition of Au NPs on Si. This study will contribute to designing Au/Si nanostructures with desirable properties.

ITO-based electro-optical modulator integrated in silicon-on-insulator waveguide using surface plasmon interference

In this paper, an innovative integrated Si photonic modulator based on electrically refractive index-tunable indium-tin-oxide (ITO) material is presented. The key component of the device consists of a thin Au layer embedded in a Si waveguide and a thin layer of ITO deposited on the Au layer through an adhesive HfO2 layer. Light coupled into the transverse-magnetic mode of the Si waveguide generates two surface plasmon modes propagating simultaneously at the Au-HfO2 interface and at the Au-Si interface. These waves recombine and interfere at the end of the Au layer. The modulation mechanism is activated by electrically tuning the free carrier concentration of the ITO layer, which changes the ITO refractive index, causing a difference in the group index between the two interfering modes. This complementary-metal-oxide-semiconductor-compatible integration of electro-optical modulation into a commercially available silicon-on-insulator platform provides great potential toward compact and efficient photonic integrated circuits for next-generation data communication.

Visible and near-infrared dual-band photodetector based on gold–silicon metamaterial

A photodetector with a controllable response spectrum and high responsivity shows great potential in practical photoelectric applications. Here, we design and experimentally demonstrate a visible and near-infrared (NIR) dual-band photodetector by integrating the out-of-plane Au gratings and n+-Si groove substrate metamaterial (ASGM). Simulation, experimental, and calculation results indicate that the absorption spectrum can be tailored by surface plasmon resonances (SPRs) of different interfaces and Si-groove resonance. Visible-light absorption is caused by Si-groove resonance, SPRs of the Au–air interface, and high-frequency mode SPRs of the Au–Si interface. NIR absorption is induced by the SPRs of the Au–Si interface. Moreover, out-of-plane coupling between upper and lower Au gratings gives rise to an enhanced electric field and broadened SPR absorption. The respective photoresponsivity in the visible and NIR band is more than 7 and 3 mA/W without an external bias, with a maximum responsivity of 13 mA/W at 535 nm. The photoresponse of such ASGM-based dual-band photodetectors can be modulated so that they may be used in various fields like medical imaging, optical communication, and aerography.

Tailoring the optical field enhancement in Si-based structures covered by nanohole arrays in gold films for near-infrared photodetection

We performed numerical simulations of plasmonic near-field enhancement in Si-based structures in near infrared region. Gold films perforated with periodic two-dimensional subwavelength hole arrays were used as the plasmonic couplers. The array periodicity was adjusted to excite the surface plasmon modes at the telecom wavelengths (between 1.3 and 1.55 μm). The field intensity enhancement factor and its spectral position, as a function of hole diameter, demonstrate the maximum at which the Bloch plasmon polariton waves propagating along the Au–Si interface change by a localized surface plasmon mode. The maximum peak wavelength and field intensity enhancement are reached at d/a = 0.5, where d is the hole diameter and a is the array periodicity. An over 14 times field intensity enhancement was obtained at λ = 1.54 μm for d = 200 nm and a = 400 nm. We found that the localized surface plasmon mode is confined mainly under the Au regions along the diagonals of the square lattice of holes. The lateral field distribution for propagating modes has either a hexagonal or square shape, reflecting in-pane symmetry of the grating. For structures with largest holes, the anticrossing of localized mode with the propagating one was observed implying coupling between the modes and formation of a mixed near-field state. The information acquired from the study is valuable for feasible device applications.

Plasmonic Field Enhancement by Metallic Subwave Lattices on Silicon in the Near-Infrared Range

The enhancement of the electric field in plasmonic nanostructures on a Si substrate in the near-infrared range is studied theoretically. Two-dimensional square arrays of circular holes of various diameters in a gold film serve as metasurfaces allowing the conversion of the external electromagnetic radiation into surface plas-mon modes at the Au-Si interface. It is established that illumination of nanostructures from the side of the Si substrate provides a higher field enhancement compared to the case of frontal illumination, since the incident light wave in the former case reaches the gold interface without passage through subwavelength holes for which the transmittance is extremely low. The plasmonic field enhancement, as a function of the diameter of holes of the array, shows the maximum at which the Bloch plasmon polaritons propagating along the Au-Si interface are transformed to localized surface plasmon modes. Two types of localized plasmons are detected. The short-wavelength plasmon is excited along the array diagonals and exists at any angles of incidence of light on the structure. The long-wavelength plasmon occurs only at nonzero angles of incidence and is located along orthogonal directions parallel to the sides of the square array. These results are directly related to the problem of fabricating effective silicon photodetectors with quantum dots.

Enhanced plasmon-mediated photo-assisted hydrogen evolution on silicon by interfacial modification

The superior catalytic activity of Pt towards proton reduction suggests application of Pt also in device architectures where hydrogen is produced by light-generated charge carriers. Large optical absorption cross sections of Pt nanoparticles, however, turn the attention to potential substitutes for Pt such as Au with more advantageous optical properties. In order to approach a functional Si/Au photocathode for hydrogen evolution, we report here on modifications of the Si–Au interface which result in improvements of charge transfer kinetics and optical properties of the device. After current-less deposition of Au nanoparticles onto silicon, these improvements are realized by chemical oxide exchange reactions at the Si/SiO_2/Au interface, i.e., dynamic etching of SiO_2 and re-oxidation of Si in NH_4F (40%). A chemical reaction route for the reformation of the SiO_2 layer in the presence of Au and the aqueous NH_4F solution is discussed. Simultaneous to the modification of the Si/SiO_2 interface, small Au nanoparticles form larger clusters with enhanced effective scattering cross sections. Thereby, improved electronic interface properties and enhanced forward scattering of light increase the saturation photocurrent density by about 9% from 32 to 35 mA cm^(−2). Improved stability of the device in acidic electrolytes, near the thermodynamic potential for evolution of hydrogen, is furthermore discussed.

Synchronization of charge carrier separation by tailoring the interface of Si–Au–TiO2 heterostructures via click chemistry for PEC water splitting

Electrochemical potential gradients that are established due to appropriate band edge alignment in heterostructures can synchronize the movement of electron and hole in opposite direction. Such synchronization is critical for efficient photoelectrochemical devices and can be achieved by tailoring the interface of heterostructures. To this end, we fabricate a tailored interface of Si–Au–TiO2 heterostructure over stainless steel ((Si–Au–TiO2)/SS) for effective charge carrier separation. The fabricated electrode possesses the following tailored interfaces to enhance the charge-carrier separation/transport: 1) gold nanoparticles form interface (sandwiched) with Si as well as TiO2; 2) Si, Au, and TiO2 form the interface with SS, 3) Si–TiO2, and Si–Au interface facilitates the quenching of h+ generated in Si (via electron transfer from TiO2 and Au to Si) which facilitates electron transport to counter electrode for hydrogen generation, and 4) TiO2 forms the interface with the electrolyte solution to facilitate hole transport. Photoelectrochemical (PEC) measurements of (Si–Au–TiO2)/SS heterostructure exhibits higher performance compared to other heterostructures ((Au–Si)/SS, (Au–TiO2)/SS, (Si–TiO2)/SS, TiO2/Au/Si/SS, and Si/Au/TiO2/SS), substantiating the importance of synchronized charge transport in PEC systems. Further, electrochemical impedance spectroscopy (EIS) studies also support the synchronized charge transport in the tailored heterostructure ((Si–Au–TiO2)/SS) compared to control samples. The calculated applied bias to photo-conversion efficiency (ABPE) of ~0.4%@~0.9V applied bias, intrinsic solar to chemical conversion efficiency (ISTC) of ~0.085 (or 8.5%) @1.66V vs. RHE and electrical and solar power-to-hydrogen (ESPH) conversion efficiency of ~3.3%@~1.0V applied bias.

Ion-beam-induced nanodots formation from Au/Si thin films on quartz surface

We report the synthesis of Si nanodots on quartz surface using ion irradiation. When a bi-layer of ultrathin Au and Si on quartz surface is irradiated by 500keV Xe-ion beam, the bi-layer spontaneously transforms into nanodots at a fluence of 5×1014ionscm−2. The spatial density and diameter of the nanodots are reduced with increase in applied ion fluence. The nanostructures exhibit photoluminescence in the visible range at room temperature where the intensity and wavelength depends upon ion fluence. The observed evolution seems to be correlated to ion beam mixing induced silicide formation at Au–Si interface.

Solid-State Synthesized Nanostructured Au Dendritic Aggregates Towards Surface-Enhanced Raman Spectroscopy

Micrometric Au structures, presenting a dendritic nano-structure, have been fabricated on a Si-based substrate. The fabrication method involves the deposition of a thin Au film on the substrate and a high-temperature annealing (1100°C) using fast heating and cooling ramps. The thermal process produces the growth, from the substrate, of Si micro-pillars whose top surfaces, covered by a crystalline Au layer, present a nanodendritic morphology. In addition to the micro-pillars, the sample surface presents a complex structural and chemical composition including Si3N4 regions due to the silicon–nitrogen intermixing during the heating stage. By studying the kinetic processes at the Au-Si interface during the thermal treatment, we describe the stages involved in the micro-pillars growth, in the dendritic morphology development, and in the Au atoms entrapment at the top of the dendritic surfaces. Finally, we present the analyses of the optical and surface enhanced Raman scattering properties of the Au dendritic aggregates. We show, in particular, that: (1) the Au dendrites aggregates act as effective scattering elements for the electromagnetic radiation in the infrared spectral region; and (2) the higher surface area due to the branched dendritic structure is responsible for the improvement in the sensitivity of the surface enhanced Raman scattering activity.

Anomalous Stagewise Lithiation of Gold-Coated Silicon Nanowires: A Combined In Situ Characterization and First-Principles Study.

Through a combined density functional theory and in situ scanning electron microscopy study, the effects of presence of gold (Au) spreading on the lithiation process of silicon nanowire (SiNW) were systematically examined. Different from a pristine SiNW, an Au-coated SiNW (Au-SiNW) is lithiated in three distinct stages; Li atoms are found to be incorporated preferentially in the Au shell, whereas the thin AuSi interface layer may serve as a facile diffusion path along the nanowire axial direction, followed by the prompt lithiation of the Si core in the radial direction. The underlying mechanism of the intriguing stagewise lithiation behavior is explained through our theoretical analysis, which appears well-aligned with the experimental evidence.

Study of faceted Au nanoparticle capped ZnO nanowires: antireflection, surface enhanced Raman spectroscopy and photoluminescence aspects

We report a single step growth process of faceted Au nanoparticles (NPs) on highly c-axis oriented ZnO nanowires (NWs) and report that a system with a lower antireflection coefficient also showed higher surface enhanced Raman spectroscopy (SERS) enhanced factors. Well-dispersed Au NPs are grown on silicon substrate using a thin film-in-air-annealing method (using 1 nm and 5 nm thick Au films on silicon and subsequent annealing in air at 800 °C) wherein enhanced oxide growth at the Au–Si interface was used to inhibit inter-diffusion to avoid Au–Si alloy formation (Au/SiOx/Si). These substrates are used to grow aligned ZnO NWs using a high temperature (≈900 °C) chemical vapour deposition method. Depending on the size and areal density of initial catalytic Au NPs, the resultant photoluminescence, reflectance characteristics, and effectiveness as SERS substrates of the faceted Au NP capped ZnO NWs coatings are systematically studied. The highly oriented and faceted Au NPs on ZnO NWs have been used as free standing SERS substrates to detect sub-micro molar crystal violet molecules with an analytical enhancement factor (AEF) of ≥104 and with high repeatability. The substrate with high-density Au–ZnO heterostructures (5 nm Au case) found to have larger AEF, very low reflectance (≈0.75%) and more green emission.

Metal assisted anodic etching of silicon.

Metal assisted anodic etching (MAAE) of Si in HF, without H2O2, is demonstrated. Si wafers were coated with Au films, and the Au films were patterned with an array of holes. A Pt mesh was used as the cathode while the anodic contact was made through either the patterned Au film or the back side of the Si wafer. Experiments were carried out on P-type, N-type, P(+)-type and N(+)-type Si wafers and a wide range of nanostructure morphologies were observed, including solid Si nanowires, porous Si nanowires, a porous Si layer without Si nanowires, and porous Si nanowires on a thick porous Si layer. Formation of wires was the result of selective etching at the Au-Si interface. It was found that when the anodic contact was made through P-type or P(+)-type Si, regular anodic etching due to electronic hole injection leads to formation of porous silicon simultaneously with metal assisted anodic etching. When the anodic contact was made through N-type or N(+)-type Si, generation of electronic holes through processes such as impact ionization and tunnelling-assisted surface generation were required for etching. In addition, it was found that metal assisted anodic etching of Si with the anodic contact made through the patterned Au film essentially reproduces the phenomenology of metal assisted chemical etching (MACE), in which holes are generated through metal assisted reduction of H2O2 rather than current flow. These results clarify the linked roles of electrical and chemical processes that occur during electrochemical etching of Si.

Schottky-contact plasmonic dipole rectenna concept for biosensing

Nanoantennas are key optical components for several applications including photodetection and biosensing. Here we present an array of metal nano-dipoles supporting surface plasmon polaritons (SPPs) integrated into a silicon-based Schottky-contact photodetector. Incident photons coupled to the array excite SPPs on the Au nanowires of the antennas which decay by creating "hot" carriers in the metal. The hot carriers may then be injected over the potential barrier at the Au-Si interface resulting in a photocurrent. High responsivities of 100 mA/W and practical minimum detectable powers of -12 dBm should be achievable in the infra-red (1310 nm). The device was then investigated for use as a biosensor by computing its bulk and surface sensitivities. Sensitivities of ∼ 250 nm/RIU (bulk) and ∼ 8 nm/nm (surface) in water are predicted. We identify the mode propagating and resonating along the nanowires of the antennas, we apply a transmission line model to describe the performance of the antennas, and we extract two useful formulas to predict their bulk and surface sensitivities. We prove that the sensitivities of dipoles are much greater than those of similar monopoles and we show that this difference comes from the gap in dipole antennas where electric fields are strongly enhanced.

Morphology of the Au-Si interface formed during solidification of liquid Au/Si(111) islands

Au islands grown on Si(111) substrates at substrate temperatures of 500 and 600 °C, both of which are greater than the bulk Au-Si eutectic temperature of 363 °C, are characterized using atomic force (AFM) and electron microscopy. Specific islands are imaged using AFM before and after Au dissolution using aqua regia to characterize the Au-Si interface formed as the islands solidify from the liquid phase while cooling from the growth temperature. Subsequent to Au dissolution, the islands present a craterlike morphology with a pit that may extend below the substrate surface depending on growth and annealing conditions. Craters formed beneath islands grown at a substrate temperature of 600 °C exhibit pits that penetrate below the substrate surface to a depth that is proportional to the area of the island footprint and possess a well-developed (111) facet at their base. Facets are also sometimes observed in the crater sidewalls and are more prevalent in samples slowly cooled through the solidus temperature than those that are radiatively quenched. Transmission electron micrographs of etched islands indicate the presence of segregated Au nanocrystals entrained in the crater lip.

Selective synthesis of SiO 2 NWs on Si substrate and their adjustable photoluminescence

Amorphous SiO 2 nanowires with diameters of 20–200 nm and lengths of several micrometers were selectively grown，at 1100 °C under ambient pressure，on the ultra-thin Au film coated silicon wafers，which were characterized by scanning electron microscope, Fourier transform infrared spectroscope, and high resolution transmission electron microscope. Using X-ray energy dispersion spectroscope equipped on the high resolution transmission electron microscope, the investigation was carried out on the micro-region chemical composition of the as-grown amorphous SiO 2 nanowire. The growth mechanism has been discussed, and it has been believed to be extended vapor-liquid-solid process. The SiO 2 layer, as the result indicates, can prevent the formation of Si–Au alloy from Si–Au interface. This work also discusses the photoluminescence emission from SiO x centers at room temperature, it is found that by changing the oxygen, the photoluminescence intensity can be adjusted.

Anomalous current–voltage characteristics and structural phenomena in an Au–Si interface

We report an abnormal current–voltage ( I–V ) characteristic observed on an Au–Si interface fabricated by a unique focused ion beam sputtering microdeposition method. The Au–Si interface shows neither Schottky nor ohmic behavior, but the I–V curve has a d I /d V peak at zero voltage. High-resolution transmission electron microscopy studies reveal the existence and unstable nature of an ∼10 nm thick interfacial layer at the Au–Si contact. The unusual electrical behavior is likely correlated with this metastable interfacial layer.

Interfacial Electronic Structure of Gold Nanoparticles on Si(100): Alloying versus Quantum Size Effects

Gold nanoparticles (Au NPs) were prepared on a native-oxide-covered Si(100) substrate by sputter-deposition followed by thermal annealing. The size of Au NPs could be controlled in the range of 8-48 nm by varying the sputter-deposition time and post-annealing temperature. The interparticle separation was found to be directly related to the size of Au NPs, with smaller separations for particles of smaller size. The surface morphology, crystal structure, and interfacial composition of the chemical states of these supported Au NPs were studied as a function of their average size by using scanning electron microscopy, glancing-incidence X-ray diffraction, and depth-profiling X-ray photoelectron spectroscopy (XPS), respectively. The new Au 4f7/2 peak found at 1.1-1.2 eV higher in binding energy than that for the metallic Au feature (at 84.0 eV) can be attributed to the formation of Au silicide at the interface between Au NPs and the Si substrate. Depth-profiling XPS experiments revealed no discernible change in the binding energies of the Au silicide and metallic Au 4f features with increasing Ar+ sputtering time, indicating that the Au-to-silicide interface is abrupt. Furthermore, the shift in the Au 5d5/2 valence band to a higher binding energy and the reduction of the Au 5d spin-orbit splitting with increasing Ar+ sputtering time also support the formation of Au silicide. No clear evidence for the quantum size effect was observed for the supported NPs. The finite density of state at the Fermi level and the fixed Au 4f7/2 peak position clearly indicate the metallic nature of the Au silicide at the Au-Si interface.

Fabrication and characterization of eutectic gold–silicon (Au–Si) nanowires

Hypoeutectic, eutectic and hypereutectic gold–silicon (Au–Si) alloys were synthesized using vacuum smelting. When Au and Si were smelted in a vacuum chamber near 1100 °C, Si (g), Si 2(g), Si 3(g), SiO 2(g), Au (g), SiO 2(s) and Au 2O 3(s) were formed rapidly; the quantities of Au and Si lost were estimated from thermodynamic formulas. The nano- and microstructures of the Au–Si eutectic compound were characterized with a SEM and a TEM, which showed a discontinuous phase of Si dispersed and mounted within a matrix phase of Au. Cracks that formed about the Au–Si interface made the Au–Si eutectic brittle. From an Au–Si alloy of superior quality as a bulk material, eutectic nanowires of average diameter 60 nm were fabricated after the alloy was melted and injected into an anodic aluminum-oxide (AAO) template.