Au In Si Research Articles

Pristine Graphene Insertion at the Metal/Semiconductor Interface to Minimize Metal-Induced Gap States.

Metal (M) contact with a semiconductor (S) introduces metal-induced gap states (MIGS), which makes it difficult to study the intrinsic electrical properties of S. A bilayer of metal with graphene (Gr), i.e., a M/Gr bilayer, may form a contact with S to minimize MIGS. However, it has been challenging to realize the pristine M/Gr/S junctions without interfacial contaminants, which result in additional interfacial states. Here, we successfully demonstrate the atomically clean M/Gr/n-type silicon (Si) junctions via all-dry transfer of M/Gr bilayers onto Si. The fabricated M/Gr/Si junctions significantly increase the current density J at reverse bias, compared to those of M/Si junctions without a Gr interlayer (e.g., by 105 times for M = Au in Si(111)). The increase of the reverse J by a Gr interlayer is more prominent in Si(111) than in Si(100), whereas in M/Si junctions, J is independent of the type of Si surface. The different transport data between M/Gr/Si(111) and M/Gr/Si(100) are consistent with Fermi-level pinning by different surface states of Si(111) and Si(100). Our findings suggest the effective way to suppress MIGS by an introduction of the clean Gr interlayer, which paves the way to study intrinsic electrical properties of various materials.

Evaluation of grating realized via pulse current electroplating combined with atomic layer deposition as an x-ray grating interferometer

In this work, the authors developed a simple and efficient two-step deposition process for the realization of an x-ray absorption grating: ALD (atomic layer deposition) of a conductive seed layer, followed by electroplating of the absorbing metal with a pulse current mode. An Si grating with a high aspect ratio of 1:40 was fabricated by deep reactive ion etching on an 8 in. Si wafer. In order to form a conductive seed layer on the Si grating with such a high aspect ratio over an area of 10 × 10 cm2, Ru was conformally deposited by a thermal ALD process with O2 reactant gas. The authors analyzed the results of electroplating performed in different bias modes to fill Au in a high aspect ratio Si grating structure. It was found that electroplating in the pulse current mode (duty cycle: 5%, current density: 1.7 mA/cm2) for 79 h allowed Au to uniformly fill the entire grating area, whereas in the direct current mode, severe step coverage on top of the grating was observed. The authors successfully tested the grating fabricated by the suggested two-step deposition process as an absorption grating (G2) for a high x-ray energy Talbot-Lau grating interferometer.

Gold-Hyperdoped Black Silicon With High IR Absorption by Femtosecond Laser Irradiation

Gold (Au)-doped-textured silicon (Si) material with a thermostable absorption below bandgap (>50%) is obtained by femtosecond laser irradiation. Although the concentration of Au impurity (1019 cm−3 ) in textured Si is at least four orders of magnitude greater than the solid solubility of Au in crystalline Si, the sheet carrier density (approximately 1010 cm−2) in Au-doped Si is very low due to a self-compensation effect of Au impurity in Si material. The infrared absorption of Au-doped Si is related to laser-induced-structural defects and sub-band absorption of deep energy levels of Au in Si, which is determined by temperature-dependent Hall Effect measurement. Besides supersaturated doping of Au, a gold silicide phase is formed at textured Si surface.

Negative-U properties for substitutional Au in Si

The isolated substitutional gold impurity in bulk silicon is studied in detail using electronic structure calculations based on density-functional theory. The defect system is found to be a non–spin-polarized negative-U centre, thus providing a simple solution to the long-standing debate over the electron paramagnetic resonance signal for gold in silicon. There is an excellent agreement (within 0.03 eV) between the well-established experimental donor and acceptor levels and the predicted stable charge state transition levels, allowing for the unambiguous assignment of the two experimental levels to the and transitions, respectively, in contrast to previously held assumptions about the system.

Detection of Deep-Levels in Doped Silicon Nanowires Using Low-Frequency Noise Spectroscopy

We report detailed characterization of electrically-active deep-levels in doped Si nanowires (SiNWs) grown using catalyst-assisted vapor-liquid-solid (VLS) technique. Temperature-dependent low-frequency noise (LFN) spectroscopy was used to reveal the presence of generation-recombination related Lorentzian-type peaks along with 1/f-type noise in these NWs. In Ni-catalyzed SiNWs, the correlated LFN spectroscopy detected electrically active deep-levels with ionization energies of 0.42 eV for the n-type and 0.22 eV for the p-type SiNWs, respectively. In Au-catalyzed n- and p-type SiNWs, the energies of the deep-levels were estimated to be 0.44 and 0.38 eV, respectively. These values are in good agreement with the known ionization energies of deep-levels introduced by Ni and Au in Si. Associated trap concentrations and hole and electron capture cross sections were also estimated. This paper clearly indicated the presence of electrically active deep-levels associated with unintentional incorporation of catalyst atoms in the VLS-grown SiNWs.

Atomic structure of two-dimensional binary surface alloys: Si(111)-√21 × √21 superstructure

The atomic structures of Au and Ag co-adsorption-induced √21×√21 superstructure on a Si(111) surface, i.e., (Si(111)-√21×√21-(Au, Ag)), where the Si(111)-5×2-Au surface is used as a substrate, have been investigated using reflection high-energy positron diffraction (RHEPD) and photoemission spectroscopy. From core-level spectra, we determined the chemical environments of Ag and Au atoms present in the Si(111)-√21×√21-(Au, Ag) surface. From the rocking curve and pattern analyses of RHEPD, we found that the atomic coordinates of the Au and Ag atoms were approximately the same as those of the Au and Ag atoms in other Si(111)-√21×√21 surfaces with different stoichiometries. On the basis of the core-level and RHEPD results, we revealed the atomic structure of the Si(111)-√21×√21-(Au, Ag) surface.

Heat Transfer Across Metal-Dielectric Interfaces During Ultrafast-Laser Heating

Heat transfer across metal-dielectric interfaces involves transport of electrons and phonons accomplished either by coupling between phonons in metal and dielectric or by coupling between electrons in metal and phonons in dielectric. In this work, we investigate heat transfer across metal-dielectric interfaces during ultrafast-laser heating of thin metal films coated on dielectric substrates. By employing ultrafast-laser heating that creates strong thermal nonequilibrium between electrons and phonons in metal, it is possible to isolate the effect of the direct electron–phonon coupling across the interface and thus facilitate its study. Transient thermo-reflectance measurements using femtosecond laser pulses are performed on Au–Si samples while the simulation results based on a two-temperature model are compared with the measured data. A contact resistance between electrons in Au and phonons in Si represents the coupling strength of the direct electron–phonon interactions at the interface. Our results reveal that this contact resistance can be sufficiently small to indicate strong direct coupling between electrons in metal and phonons in dielectric.

Java Simulation of Au Diffusion in Si Affected by Vacancies and Self-Interstitials: Partial Differential Equations

A Java program in a GUI environment has been developed for the numerical solution of basic partial differential equations and applied to Au diffusion in Si affected by vacancies and self-interstitials. Text fields of selected parameters for the calculation are set on the display, and the calculation starts by checking the start button after putting values on the text fields. The calculated results are plotted immediately after the finish of the calculation as the concentration profiles of substitutional Au, interstitial Au, vacancies and self-interstitials, and their diffusion can be presented immediately, resulting in the identification of the diffusion mechanism. By changing the values of the text fields, new results can be represented immediately. The diffusion of Au in Si can be simulated correctly and easily by this program. Results from the program for one set of conditions are shown, including images produced on the display.

Gold Electrodeposition into Mesoporous Silicon: The Effect of Solution Composition

Electrodeposition of Au in mesoporous Si prepared in a highly doped p-type Si was investigated. When the Au electrodeposition was carried out using the complex Au(S2O3)23-, the mesopores with 5 µm depth were filled with Au continuously from the bottom to the opening. When the Au electrodeposition was carried out using the complex AuCl4-, the openings of the mesopores were covered with an Au film. The solution composition has a strong influence on the electrodeposition behavior. The effect of the displacement deposition occurring at the initial few seconds before conducting the Au electrodeposition seems to be a key factor for the difference. The different behaviors of the electrodeposition with different supporting electrolytes were compared and discussed.

Self-diffusion in silicon - Change of a paradigm

Diffusion processes play a key role in the fabrication of semiconductor devices. For a long time the underlying mechanisms were thought to be analogous to those in metals, based on vacancies as thc dominant lattice defects in thermal equilibrium. From the mid-sixties onwards it became clear that this picture is invalid for Si, where strongly relaxed self-interstitials are dominant and responsible for self- and Group-III- diffusion. Inter alia, this change of a paradigm led to novel concepts and to the quantitative explanation of the diffusion of so-called hybrids such as Au, Pt, and Zn in Si by the so-called kick-out mechanism.

Study of annealing induced redistribution of implanted Au in Si: Fluence dependence

One hour 500 °C air annealing induced movement of implanted Au in Si have been studied for 32 keV Au implantation in Si, in the fluence range of 1 × 10 15 – 1 × 10 17 ions cm - 2 . Samples were characterized using Rutherford backscattering spectrometry and cross-sectional transmission electron microscopy. The results indicate that, depending on the initial state of Au in the matrix, there is a clear difference in the diffusion behaviour of Au in Si. When Au is precipitated as gold-silicide nanoclusters inside the Si matrix, annealing is found to cause diffusion of Au into the bulk Si. Compared to this, for a random atomic distribution of Au in an amorphous Si matrix, annealing is found to result in out-diffusion of Au towards the surface.

Distribution of Au atom number in the Au agglomerates generated during the out-diffusion process of supersaturated high-temperature substitutional Au in Si at 900 °C

Au atom number in the Au agglomerates generated during the annealing of supersaturated high-temperature substitutional Au in Si at 900 °C are measured by SIMS and their distributions have been investigated. The annealing time is chosen as 22.5, 90 or 360 h, which corresponds to initial, middle or near final stage of the annealing. Many “initial agglomerates” containing about 2.0 × 105 Au atoms are generated and the distributions show an abrupt one with the peak at the atom number in the initial and middle stages. The “initial agglomerates” have absorbed supersaturated Au atoms within 0.86 μm. The “initial agglomerates” are generated even in the near final stage and grow up to containing about 4 × 106 Au atoms by absorbing the Au atoms within 2.3 μm, finally. As the consequence, many agglomerates contain 5 × 104–1.3 × 106 Au atoms resulting in a broad distribution at the near final stage. Schematic models of agglomerations corresponding to each stage are proposed.

Motility of Metal Nanoparticles in Silicon and Induced Anisotropic Silicon Etching

AbstractThe autonomous motion behavior of metal particles in Si, and the consequential anisotropic etching of silicon and production of Si nanostructures, in particular, Si nanowire arrays in oxidizing hydrofluoric acid solution, has been systematically investigated. It is found that the autonomous motion of metal particles (Ag and Au) in Si is highly uniform, yet directional and preferential along the [100] crystallographic orientation of Si, rather than always being normal to the silicon surface. An electrokinetic model has been formulated, which, for the first time, satisfactorily explains the microscopic dynamic origin of motility of metal particles in Si. According to this model, the power generated in the bipolar electrochemical reaction at a metal particle's surface can be directly converted into mechanical work to propel the tunneling motion of metal particles in Si. The mechanism of pore and wire formation and their dependence on the crystal orientation are discussed. These models not only provide fundamental interpretation of metal‐induced formation of pits, porous silicon, and silicon nanowires and nanopores, they also reveal that metal particles in the metal/Si system could work as a self‐propelled nanomotor. Significantly, it provides a facile approach to produce various Si nanostructures, especially ordered Si nanowire arrays from Si wafers of desired properties.

Au agglomerates observed in the out-diffusion process of supersaturated high-temperature substitutional Au in Si

Out-diffusion profiles of supersaturated substitutional Au in Si annealed at 900 °C for 90 h have been measured by SIMS and ICTS methods over a distance of 80 μm from the specimen surface. The tendency of the profile of total Au atoms measured by SIMS and that of electrically active substitutional Au measured by ICTS agrees well, but the very small regions containing high-concentration of Au atoms, 3 × 1022 cm−3, are observed sporadically at the distance deeper than 10 μm from the surface by SIMS method. The size of the high-concentration region is estimated to be about 20 nm and its density is about 2 × 1010 cm−3. The small agglomerates contain 105–107 Au atoms and many of them contain about 2 × 105 atoms. The observed agglomerate is a new agglomeration of Au in Si. The state of the agglomeration (namely, precipitates of Au, agglomeration of substitutional Au in Si, or a new state) is not clear by SIMS measurement.

Mixing of Au in Si induced by secondary and high-order recoil implantation

The mixing of Au in Si induced by secondary and high-order recoil implantation was investigated using 350 keV Ar+ and 350 keV Kr+ ions to fluences from 1 × 1016 to 3 × 1016 ions/cm2 at room temperature. The thickness of the Au layer evaporated on Si substrate was ∼2400 Å.The ranges of the Ar and Kr ions were chosen to be lower than the thickness of the Au layer in order to avoid the ballistic mixing produced by the primary knock-on atoms. Rutherford backscattering spectrometry (RBS) experiments were carried out to study the effects induced by Ar and Kr irradiation at the interface of Au–Si system. We observed that in the case of the irradiation with Ar+ ions, a broadening of the Au–Si interface occurred only at the fluence of 3 × 1016 Ar+/cm2 and it is attributed to the surface roughening induced by ion bombardment. In contrast, the RBS analysis of a sample irradiated with 2 × 1016 Kr+/cm2 clearly showed, in addition to the broadening effect, the formation of a mixed zone of Au and Si atoms at the interface. The mixing of Au in Si atoms can be explained by the secondary and high-order recoil implantation followed by subsequent collision cascades.

High efficiency gettering of Au in Si(1 1 1) by MeV C implantation

A study of MeV C implantation induced effects on gettering of Au (2.2 × 10 15 cm −2), implanted into Si(1 1 1), has been carried out using Rutherford backscattering spectrometry. A 2 h anneal in Ar at 850 °C has been found to result in a gettering efficiency close to 55%. It increases beyond 80% with a further 2 h anneal at 900 °C. The C dose (0.3–1.5 × 10 16 cm −2) dependence of Au gettering is also presented and discussed.

Indiffusion and Out-Diffusion Profiles of Substitutional Au in Si Affected by Self-Interstitials and Vacancies

Precise indiffusion and out-diffusion profiles of substitutional Au in Si have been investigated by a numerical method for the nonlinear diffusion equation affected by self-interstitials and vacancies simultaneously. In contrast to usual approximate solutions, a small contribution of self-interstitials in the indiffusion results in a profile specific to a kick-out mechanism, and a small contribution of vacancies in the out-diffusion results in that to a dissociative mechanism. Therefore, both experimental profiles of indiffusion and out-diffusion are required to determine the contribution rate of these point defects. The experimental results indicate that the contribution rate of vacancies for the effective diffusion of Au is about 90% at 900°C, which is very different from previous result.

Monte Carlo Calculation of Correlation Factors and Concentration Profiles for Interstitial-Substitutional Diffusion Mechanism with Impurity-Vacancy Interaction in the fcc Lattice

Many diffusion systems present an anomalous fast diffusion (e.g. noble metals in lead; Zn, Pt, and Au in Si; Be in GaAs), which can be interpreted by means of the interstitial–substitutional exchange. In this work, the dissociative variant of this model, where the exchange between interstitial and substitutional configurations takes place with the interplay of vacancies, is simulated using the Monte Carlo method, that allows to avoid drastic simplifications used to solve analytically the set of reaction–diffusion equations describing this model. The interstitial–vacancy pair (I–V), where the binding energy is considered up to the second nearest neighbours, is considered. The results show, that the penetration profiles change their shape from two- to three-stages profiles depending on the magnitude of the binding energy of the pair (I–V). The variation of the impurity (or solute) correlation factor f versus the annihilation frequency of the pair (I–V) is found to be similar to that obtained analytically. The time evolution of the penetration profiles is also investigated.

Essential difference in concentration profile of Au in Si after annealing above or below 850°C

Indiffusion and out-diffusion profiles of Au in Si have been measured after a heat treatment at a higher or lower temperature than 850°C. The diffusion is slow and the profiles show a typical kick-out diffusion, in both heat treatments for the indiffusion above and below 850°C, and for the out-diffusion of Au supersaturated above 850°C. On the other hand, the out-diffusion of Au supersaturated below 850°C is very fast and the profile shows that it is flat. The essential difference in the concentration profiles are consistent with the change of the Au configuration supersaturated below 850°C from high-temperature substitutional Au to a low-temperature one.

Effect of Long-Term Heat Treatment on the Acceptor Behavior of Gold in Silicon

Data are presented on the electrical activity of Au in Czochralski Si heat-treated and diffusion-doped under various conditions. The results are interpreted in terms of force fields responsible for changes in the concentration of electrically active centers. Long-term heat treatment between 700 and 1050°C is shown to have a significant effect on the acceptor behavior of Au in diffusion-doped Si. The concentration of Au acceptors is sensitive to the surface condition in the course of heat treatment: a tungsten coating increases the Au concentration in the bulk of diffusion-doped Si.