Si Surface Layer Research Articles

Time dependent interface stability during rapid solidification

The destabilization of a planar solid/liquid interface during resolidification of the molten surface layer of Si–Sn alloys is investigated. Experimental data are compared with the results of a transient numerical stability analysis. In that analysis the time evolution of an infinitesimal sinusoidal perturbation of a planar interface is investigated. For this the concentration profile in the melt as well as the temperature distributions in the melt and solid are divided into a base state and a state due to the perturbation. The highly nonuniform Sn profiles implanted in (100) silicon samples serve as initial conditions for the numerical calculations. Deviations from thermal equilibrium at the solid/liquid interface are considered. The critical concentration for breakdown is calculated as a function of solidification velocity and compared with the results of the analytical analysis of Huntley and Davis [Acta Metall. Mater. 41, 2025 (1993)]. It is further investigated whether an analytical steady-state stability analysis can be used to approximately describe the experiments considered.

On the constant composition and thickness of the chlorinated silicon surface layer subjected to increasing etching product concentrations during chlorine plasma etching

The influence of etching products on the surface layer formed during chlorine (Cl2) plasma etching of unmasked crystalline p-type Si(100) was investigated using vacuum sample-transfer and angle-resolved x-ray photoelectron spectroscopy (XPS). Varying the Cl2 flow rate from 10.0 to 0.4 sccm at a constant pressure of 4 mTorr controlled the etching product concentration. Gas-phase Cl, Cl2, and SiCly (y=0–3) were monitored (∼1 cm above the wafer) by optical emission spectroscopy. For a positive ion density of 7×1010 cm−3 and an average ion energy of ∼140 eV, the Si etching rate decreased linearly with Cl2 flow from 2850 Å/min at 10.0 sccm to 1920 Å/min at 0.4 sccm. From these rates, mass balance, and the Si area, the ratio of product-to-etchant (SiCly-to-Cl) flux to the wafer varied from 0.078 to 11 at 10.0 and 0.4 sccm, respectively. After etching, Cl was present in the Si(100) surface layer as SiClx (x=1–3) at XPS Si (2p3/2) binding energies of 99.9, 101.0, and 102.0 eV, respectively, relative to Si at 99.1 eV. The amounts of the three silicon chlorides and the total Cl (derived from its 2p peak) were nearly independent of the product-to-etchant flux ratio. Depth profiles were obtained from an inversion of the observed take-off angle dependences of the XPS signals. For the Cl2 flow rates investigated, the chlorinated surface layer was ∼16 Å thick, with Cl falling off in a graded fashion. The Cl areal density, integrated throughout the layer, was similar for all experimental conditions and averaged 2.63±0.15×1015 Cl/cm2. The stoichiometry of the chlorosilyl layers was also independent of Cl2 flow rate and averaged [SiCl]:[SiCl2]:[SiCl3]=[1.0]:[0.45±0.09]:[0.33±0.02]. Reaction pathways are presented to interpert both the constancy of the chlorinated surface layer and the decrease in etching rate as the Cl2 flow rate was decreased from 10.0 to 0.4 sccm.

Fabrication of Nano-Scale Point Contact Metal-Oxide-Semiconductor Field-Effect-Transistors Using Micrometer-Scale Design Rule

In this paper, we report the novel fabrication technique of Si nano-scale point contact channels on the silicon-on-insulator substrate. In our method, the width of the point contact channel is determined only by the thickness of the surface Si layer and the nano-scale channels can be fabricated without using any fine resolution lithography. By applying the method to the point contact channel metal-oxide-semiconductor field-effect-transistor (MOSFET), the electrical properties of the nano-scale channels are investigated. The experimental results show that uniform nano-scale point contact channels are successfully formed.

Effects of the surface deposition of nitrogen on the thermal oxidation of silicon in O2

Nitrogen was deposited on the surface layers of Si(100) by ion implantation at a very low energy (approximately 20 eV), at fluences between 1 and 10×1014 cm−2. The samples were thermally oxidized in dry O2 at 1050 °C, and the areal densities and profiles of N and O were determined by nuclear reaction analysis and narrow nuclear resonance profiling, evidencing that: (i) the retained amounts of N just after ion beam deposition stayed in the range between 0.3 and 7×1014 cm−2; (ii) the oxide growth is influenced strongly by the presence of nitrogen, the thickness of the oxide films (which remained between 4 and 30 nm) decreased with the increase of the areal density of nitrogen; (iii) N is partially removed from the system as oxidation proceeds. These observations are discussed in terms of current models for the thermal growth of silicon oxide in the presence of N.

Surface Segregation Behaviors of B, Ga, and Sb during Si Molecular Beam Epitaxy: Calculation Using a First-Principles Method

The potential energies of B, Ga, and Sb dopant atoms in the three top layers of Si(100) surfaces were evaluated by accurate density functional calculations of the model clusters. The different behaviors of B and Ga in the surface segregation can be understood by considering the bond energies between the dopant and Si atoms as the driving force for segregation. The energy of the B–Si bond is greater than that of the Si–Si bond, and the incorporated state is more stable than the adsorbed state, which increases the segregation resistance to the Si surface. The incorporated state of the B atom is most stable in the second Si surface layer. The Sb atom has a higher potential barrier between the surface and the subsurface site than Ga, which causes less segregation of Sb to the Si surface under the same experimental conditions.

Arsenic solubility in single crystalline cobalt disilicide

The solid solubility of arsenic in CoSi 2 has been investigated at temperatures between 650°C and 950°C. The sample structure used was an epitaxial CoSi 2 layer buried in single crystalline Si substrate, a so called mesotaxy sample. Arsenic was implanted in the Si surface layer and a silicon dioxide cap-layer was subsequently deposited to prevent evaporation of the arsenic. The As profiles after annealing have been obtained by Secondary Ions Mass Spectrometry. Accumulation of As at both Si/CoSi 2 interfaces has been observed and is discussed in terms of precipitation and segregation. A transient enhancement of the As diffusion in the silicon substrate has also been found. The dependence of the As concentration on the annealing time in CoSi 2 has been examined carefully to determine the equilibrium solubility. The solubility of As is lower in CoSi 2 than in Si in the studied temperature range and arsenic in CoSi 2 can be described as a regular solution in the dilute limit (Henry's law) with an enthalpy of solution of 0.42 eV and a pre-exponential factor of 1.75 × 10 21 at/cm 3.

Third-nearest-neighbor carbon pairs in epitaxialSi1−yCyalloys: Local order for carbon in silicon characterized by x-ray photoelectron diffraction and Raman spectroscopy

The local order around carbon atoms in epitaxial ${\mathrm{Si}}_{1\ensuremath{-}y}{\mathrm{C}}_{y}$ alloys (with $y$ around 1%) grown by Si molecular-beam epitaxy and thermal decomposition of ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ on Si(001) or Si(111) has been studied as a function of growth temperature. To this end, information from local probes such as x-ray photoelectron spectroscopy C $1s$ binding energies, experimental and simulated C $1s$ core-level x-ray photoelectron diffraction (XPD) distributions, and Raman spectroscopy are compared. Between the growth conditions leading to a solid solution of substitutional C at lower temperature and those leading to SiC precipitation at higher temperature, original metastable and ordered C-rich alloy phases appear, which may be coherently embedded in Si without degrading crystal quality. The results for such phases probed by the two latter techniques are indicative of a crystalline order implying concentrated third-nearest-neighbor (NN) carbon pairs. The comparison of experimental XPD distributions recorded in different azimuthal planes with simulated ones with C either in substitutional or in interstitial sites is in favor of substitution with a local contraction of the first-neighbor Si-C bond length between 10% and 20%. If we admit that the surface ordering of the C atoms in the Si(001) surface layers is the extension of such particular bulk arrangements in third NN C pairs we are able to explain an experimentally observed $c\ensuremath{-}(4\ifmmode\times\else\texttimes\fi{}4)$ low-energy electron diffraction pattern.

Refined structure of (√3 × √3) [formula omitted

Refined structure of (√3 × √3) surface reconstruction induced by Sb adsorbed on Si(111) was obtained from X-ray diffraction, using a Patterson function and a least-squares fit. A wide range of reciprocal space was probed to get information for the lateral and the vertical displacements of adsorbate and the top layers of Si. The Patterson function confirms the “milk stool” trimer model, which, according to a least-squares fit, is centered on the T 4 site. The thermal vibration amplitudes of Sb ad-atoms and those of surface layers of Si were obtained and comparison with the values in other surfaces are made.

An embedded cluster study of dimer buckling on the Si(100) surface

Both Si9H12 and Si15H16 cluster models for the Si(100) surface were studied using the Hartree–Fock molecular-orbital method and density functional theory. Our investigation shows that the ground state of the Si(100) surface consists of buckled dimers, contrary to the results of a number of previous embedded cluster calculations, but in agreement with some recent slab calculations. The relaxation constraints used in previous embedded cluster studies probably do not model the relaxation of the Si(100) surface layers adequately.

Energetics and Equilibrium Properties of Thin Pseudomorphic Si1-xCx(100) Layers in Si

We investigate the structure of carbon enriched thin silicon films which involve large strains, using first-principles total energy calculations and Monte Carlo simulations. We identify the energetically most favored configurations of substitutional carbon atoms in the Si(100) surface layers, and obtain the equilibrium depth profile at various temperatures. The interplay between the reconstruction strain field and the solute-atom interactions leads to complicated structural patterns that are different from related weakly strained systems.

Controlled growth of SiO2 tunnel barrier and crystalline Si quantum wells for Si resonant tunneling diodes

Two methods for producing Si-oxide barriers upon which crystalline Si layers can be grown are presented. One method entails oxide island nucleation on a clean vicinal Si(001) surface. The second method makes use of void formation in ultrathin oxides on the Si(100) surface at elevated temperatures. Either method results in an oxide barrier which is porous and the exposed Si within these pores can serve as a way to seed c-Si overgrowth. We demonstrate that it is feasible to grow crystalline Si overlayers on top of such porous oxide barriers, while on the continuous Si-oxide surface, only amorphous or nanocrystalline Si layer overgrowth can be achieved. The controlled oxide growth and Si overgrowth on the oxide can find possible applications in Si-based resonant tunneling devices, optoelectronics, and other Si-based nanoelectronics.

Ellipsometric investigation of damage distribution in low energy boron implantation of silicon

As the scaling of silicon devices to 100 nm channel length requires the formation of ultra-shallow (< 60 nm) junctions, high depth resolution analytical techniques become necessary for the characterization of the dopant and damage distributions. In situ single wavelength Ellipsometric Etch Depth Profiling (EEDP) and non-destructive Variable Angle of incidence Spectroscopic Ellipsometry (VASE) have been used to obtain accurate and quantitative information on the depth profiles of radiation damage produced by low energy, room temperature ion implantation of B + into Si. Implantation energy ranged from 250 eV to 10 keV and the dose was in the range 5 × 10 14 B + cm −2 to 5 × 10 15 B + cm −2. EEDP has been applied to damage depth profiling of low energy implanted silicon for the first time. The range and shape of the damage distributions obtained from optical model calculations are in good agreement with TRIM calculations for the energy range investigated. However, for the 10 keV implant, EEDP results unambiguously show the presence of a 6 nm thick amorphous silicon layer at the surface which is not predicted by TRIM. In the case of the 250 eV implant the presence of an amorphous Si surface layer can not be inferred from VASE data analysis although the existence of a 1 nm thick amorphous Si surface layer is indicated by EEDP.

A flexible built-in gettering layer prepared by hydrophobic Si wafer bonding

The bonding interface of bonded hydrophobic Si pairs has been found to be a strong gettering layer. By thinning one wafer of the pair, the location of the gettering layer with respect to the defect-free Si surface layer where devices are fabricated can be varied from more than 500 μm to deep-submicrons. Secondary ion mass spectrometry and simulation results suggest that F atoms at the interface are primarily responsible for the gettering effect via Au-F ion pairing. It appears that a significant segregation of Au to the hydrophobic bonding interface is observed during annealing at 900 °C. These results are compared to gettering of Au by a hydrophilic bonding interface.

Copper Decoration Followed by TEM Observation Identifying Defects in the Buried Oxides of SOI Substrates

Copper decoration followed by transmission electron microscopy (TEM) observation is proposed for identifying defects in the buried oxides of silicon‐on‐insulator (SOI) substrates. This method comprises Si surface layer etching (i.e., exposure of the buried‐oxide surface), oxide‐defect location detection with copper decoration, and sample thinning for TEM observation and analysis. The advantage of copper decoration is that the electronic current needed for defect detection is very small, so the influence of the electronic current on the original oxide‐defect structure can be minimized. Defects observed are categorized into four groups from the viewpoint of size and shape.

STM investigation of 2- and 3-dimensional Er disilicide grown epitaxially on Si(111)

The surface atomic structure of 2- and 3-dimensional (D) Er disilicide epitaxially grown on Si(111) has been investigated by scanning tunneling microscopy (STM) and angle-resolved photoemission. The STM images reveal that highly ordered 2D and 3D silicide islands can be grown on the flat Si(111)7 × 7 terraces and atomic resolution scans clearly confirm that both silicides are terminated by a Si bilayer without vacancies. In the 3D case the outermost Si atoms exhibit an additional small buckling with √3 × √3R30° periodicity. The STM data imply a specific registry of the surface Si layer with respect to the vacancy net underneath which is found to be in nice agreement with the symmetry of the dangling bond states at \\ ̄ gG observed in polarization dependent photoemission.

Ultra-low Contact Resistivity by High Concentration Germanium and Boron Ion Implantation Combined with Low Temperature Annealing

AbstractA new formation method for an ultra-low resistivity contact and structural analysis are discussed. In this study, a minimum contact resistivity of 6.9× 10−9 Ω·cm2 has been successfully obtained in an AlSi(lwt%)Cu(0.5wt%) alloy / Si system by using heavy dose ion implantations of Ge and B combined with low temperature annealing as low as 550°C. Structural analysis on the Si surface layer has been carried out. As a result, it was found that the crystal lattice was expanded in-plane by 6% in the Si layer near the surface because of the high concentration of Ge and no relaxation of lattice strain at such a low temperature. It was also found that super-saturation of B was stably formed in this system. An ultra-low resistivity contact is considered to be a result of these combined effects.

Analysis of the XPS and optical reflectivity spectra of the chemically etched Si(111) surfaces

XPS and optical reflectivity (OR) studies were performed for Si (111) surfaces, finally treated by two alternative chemical etching processes: A - aqueous HF etch, B - diluted CP-4 etch - both followed by methanol bath. Surface deoxidation (A) resulted in low reflectivity coefficient, whereas removal of Si surface layer (B) revealed highly intensive OR spectrum. The core level XPS data showed similarity of the surface structure, irrespectively of the etch process. Contrary, comparison of valence band XPS- and OR spectra revealed qualitative correlation between them, indicating dominant influence of the subsurface bulk defects. More insight into Si oxidation was provided by additional angle- resolved XPS measurements. Two components were distinguished in O1s spectra: the main (E 1 = 532eV) and the minor one (E 2), shifted ∼1.5 eV to higher binding energies. Their relative intensity was found dependent on air exposure time and a take-off angle. The peaks were assigned to two different oxygen position: 1) O chemisorbed with methoxy group, 2) bridging O atom.

Dislocation Density Reduction in SIMOX (Separation by Implanted Oxygen) Multi-Energy Single Implantation*1

SIMOX (Separation by IMplanted OXygen) dislocation density in the surface-Si layer was reduced by a multi-energy single implantation process. In this process, implantation energy was reduced from 200 keV to 150 keV midway through a 1.6×1018/cm2 oxygen dose. After annealing at 1300° C for 6 h, threading-dislocation density was 1.5 orders of magnitude lower than under single-energy 200 keV implantation, and 3 orders of magnitude lower than under upward energy-change 150 keV→200 keV implantation. Under the new process, the oxygen-concentration depth profile was more diffuse than when upward-stepped implantation was used, as revealed by SIMS (secondary ion mass spectroscopy) and XTEM (cross-sectional transmission electron microscope) measurements.

Ab initio calculations of SiC(110) and GaAs(110) surfaces: A comparative study and the role of ionicity.

We report ab initio calculations of structural and electronic properties of \ensuremath{\beta}-SiC and of the nonpolar SiC(110) surface. The calculations are carried out self-consistently in the local-density approximation employing smooth norm-conserving pseudopotentials in separable form. Gaussian orbital basis sets are used for an efficient description of the wave functions. These are strongly localized at the carbon atoms, in particular. For the bulk crystal 40 Gaussians per unit cell with s, p, d, and ${\mathit{s}}^{\mathrm{*}}$ symmetry are found to be sufficient for good convergence. Our results for the SiC ground-state structural parameters and the bulk band structure are in excellent agreement with the results of previous plane-wave calculations and with experimental data. We scrutinize the character of the chemical bond in SiC by comparisons with diamond, Si, GaAs, and ZnS, which have been investigated on equal footing. The SiC(110) surface is described in a supercell geometry. The optimal surface relaxation is determined by eliminating the forces iteratively. We find a top-layer bond-length-contracting rotation relaxation in which the Si surface layer atoms move closer towards the substrate while the C surface-layer atoms relax only parallel to the ideal surface plane. SiC(110) exhibits an occupied and an empty dangling-bond band within the fundamental gap. The occupied band predominantly originates from the dangling bonds at the surface carbon atoms which behave like anions. The empty band mainly originates from the dangling bonds at the surface Si atoms which act as cations. We present and discuss our results for the SiC(110) surface in direct comparison with the GaAs(110) surface. Further comparisons with literature data on surfaces of more ionic II-VI compounds like ZnS or ZnO are given, as well. This allows us to address the physical origins of the surface relaxation behavior of these compound semiconductors and to identify characteristic differences and similarities in the relaxation which are related to the specific types of heteropolarity or ionicity of these systems.

Laser-assisted etching-like damage of Si

Experimental results are presented on localized Si damage induced by high repetition rate copper-vapor laser pulses under a liquid layer. The damage is confined to the laser spot and is supposed to be due to accumulation of defects in the Si surface layer and material fatigue. These defects are dislocations with lateral dimensions close to that of the laser spot. The time of damage increases exponentially with the decrease of the energy density of the laser pulse and does not depend on the crystallographic orientation of the Si wafer. The rate of etching-like damage in electrolytes increases with external polarization of the Si wafer. The laser writing of porous Si is observed in a certain range of anodic polarization of a Si wafer at scanning velocities of the laser beam of 30–300 μm/s with a spatial resolution of ∼ 20 μm.