Si Pairs Research Articles

Tin-vacancy acceptor levels in electron-irradiated n-type silicon

Si crystals (n-type, fz) with doping levels between $1.5\ifmmode\times\else\texttimes\fi{}{10}^{14}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ containing in addition $\ensuremath{\sim}{10}^{18}{\mathrm{S}\mathrm{n}/\mathrm{c}\mathrm{m}}^{3}$ were irradiated with 2-MeV electrons to different doses and subsequently studied by deep level transient spectroscopy, M\"ossbauer spectroscopy, and positron annihilation. Two tin-vacancy (Sn-V) levels at ${E}_{c}\ensuremath{-}0.214\mathrm{eV}$ and ${E}_{c}\ensuremath{-}0.501\mathrm{eV}$ have been identified ${(E}_{c}$ denotes the conduction band edge). Based on investigations of the temperature dependence of the electron-capture cross sections, the electric-field dependence of the electron emissivity, the anneal temperature, and the defect-introduction rate, it is concluded that these levels are the double and single acceptor levels, respectively, of the Sn-V pair. These conclusions are in agreement with electronic structure calculations carried out using a local spin-density functional theory, incorporating pseudopotentials to eliminate the core electrons, and applied to large H-terminated clusters. Thus, the Sn-V pair in Si has five different charge states corresponding to four levels in the band gap.

MBE growth kinetics of Si on heavily-doped Si(111):P: a self-surfactant effect

The influence of heavily-doped substrates on the homoepitaxial growth kinetics of Si on Si(111) is studied in real time by means of reflection high-energy electron diffraction as function of temperature and growth rate. The growth experiments were performed by means of molecular beam epitaxy. The obtained results were compared with the growth kinetics on undoped substrates within the framework of nucleation theory. It was found that in comparison with undoped Si(111), the nucleus density in the case of Si(111):P is significantly increased at the same growth conditions that can be attributed to a longer lifetime of the smallest stable islands consisting of a Si pair on the Si(111):P surface. It can be concluded, therefore, that phosphorus is always present on the growing surface during high-temperature preparation of heavily P-doped Si(111). There, the phosphorus atoms saturate dangling bonds at the island edges. For further island growth, phosphorus atoms have to be displayed before silicon adatoms can be incorporated into the island edges, demanding an additional energy. In this sense, phosphorus acts as a surfactant.

Low Temperature Si Direct Bonding by Plasma Activation

ABSTRACTChemical and physical effects of plasma exposed Si wafer pairs were investigated on Si wafer bonding. Oxygen plasma treated Si wafer pairs bonded more strongly at room temperature compared to chemically cleaned, hydrophilic and hydrophobic Si. After 50 hours of annealing at 105°C, the surface energy of the bonded Si pair reached the surface energy of bulk Si (100). X- ray photoemission spectroscopy (XPS) measurements indicated that the exposure of both hydrophilic and hydrophobic Si to oxygen plasma increased a SiO2 like state in the surface layer to a depth of 1.5 nm. Atomic force microscopy (AFM) study showed that plasma irradiation at 300 watts up to 30 seconds did not change surface roughness below 0.5 nm. Exposure to He plasma or N2 did result in enhanced bonding after annealing at 165°C, however, over smaller areas.

Theoretical studies of interstitial boron defects in silicon

We employ pseudopotential plane-wave calculations to study the interstitial B in Si in different ionic configurations and charge states. For all charge states the ground state is a B–Si pair in which the B atom is close to a substitutional site and the Si atom in a nearby tetrahedral position. The defect has negative-U property and exhibits a symmetry-lowering distortion. We also report several metastable configurations which are close in formation energy. The relation of the defects to B diffusion is discussed.

Ab InitioStudy of Acceptor-Donor Complexes in Silicon and Germanium

The electronic and geometrical structures, in particular the electric field gradients (EFGs), of $[\mathrm{Cd}D{]}^{\ensuremath{-}}$ ( $D\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{P}$, As, Sb) acceptor-donor pairs in Si and Ge are studied using the full potential Korringa-Kohn-Rostoker Green's function method. In addition, also neutral complexes ( $[\mathrm{Cd}D{]}^{0}$) and trimers ( $[\mathrm{Cd}{D}_{2}{]}^{0}$) in Si are investigated. The EFG depends very sensitively on the large lattice relaxations induced by the defects and can be understood by a simple hybridization model. Our calculations are in good agreement with experimental results and provide a consistent picture of acceptor-donor complexes in Si and Ge.

Wetting Silicon Carbide with Nitrogen: A Theoretical Study

In order to model the early stages of nitride growth on cubic silicon carbide, we have studied the deposition of atomic nitrogen on the Si-terminated SiC(001) surface, using first-principles molecular dynamics. For many configurations with coverages less than 1/2 monolayer, N is preferably adsorbed at zinc blende sites and forms nitridized Si pairs. At one monolayer, N forms instead hydrazinelike ${\mathrm{N}}_{2}{\mathrm{Si}}_{4}$ complexes, yielding a fully coated, inert substrate which inhibits growth. In this configuration, full nitridization of the surface is energetically favored. Interface mixing and formation of C-N bonds are observed when mimicking the deposition of positively charged nitrogen ions.

Sorption of Metal Ions on Clay Minerals: I. Polarized EXAFS Evidence for the Adsorption of Co on the Edges of Hectorite Particles

The local structural environment of Co sorbed on hectorite (a magnesian smectite) has been investigated by polarized EXAFS (P-EXAFS) spectroscopy on a self-supporting film of Co-sorbed hectorite. This sorption sample was prepared by contacting Co and hectorite at pH 6.5 and at high ionic strength (0.3 M NaNO3) to favor pH-dependent sorption reaction over cation exchange. A self-supporting film was elaborated after 120 h of reacting time, when apparent quasi-equilibrium conditions were attained. The half-width at half maximum of the orientation distribution of c* axis of individual clay platelets off the film normal was determined by quantitative texture analysis, and found to be equal to 18.9°. Co K-edge P-EXAFS spectra were recorded at angles between the incident beam and the film normal equal to 0°, 35°, 50°, and 60°; the 90° spectrum was obtained by extrapolation. Spectral analysis led to the identification of the two nearest cationic subshells containing 1.6 ± 0.4 Mg at 3.03 Å and 2.2 ± 0.5 Si at 3.27 Å. These distances are respectively characteristic of edge-sharing linkages between Mg and Co octahedra and of corner-sharing linkages between Co octahedra and Si tetrahedra, as in clay structures. The angular dependence of the Co–Mg and Co–Si contributions indicates that Co–Mg pairs are oriented parallel to the film plane, whereas Co–Si pairs are not. These results are interpreted by the formation of Co inner-sphere mononuclear surface complexes located at the edges of hectorite platelets, in the continuity of the (Mg, Li) octahedral sheet.

Surface Structural Study by Patterson Analysis of Thermal Diffuse Scttering.

A new direct structural analysis method, which is based on the vibrational correlation between the nearest neighbor atoms in crystal, is presented. A very simple oscillation has been observed in the three-dimensional data-set of electron thermal diffuse scattering in a low-to-medium energy range from a Si(001) 2×1 surface. The observed oscillation has been attributed to the coherent diffraction among the nearest neighbor atoms, in which the vibrational correlation is very large. We demonstrate that nearest neighbor pairs of Si(001) 2×1 dimer structure are directly reconstructed by using the Patterson analysis of the observed patterns. The present method is a promising new tool for determining surface structures.

Atomic-scale properties of the amphoteric dopant Si in GaAs(110) surfaces

The electronic properties of positively charged Si Ga donors and negatively charged Si As acceptors in different subsurface layers as well as Si donor–Si acceptor dipoles and uncharged Si pairs in GaAs(110) surfaces were investigated by scanning tunneling microscopy. The subsurface location of the dopant atoms is determined from the observed symmetry. Localized states of the dopant atoms are identified. The fraction of Si atoms incorporated in GaAs on As sites increases with increasing Si concentration. Si pairs are found to be incorporated substitutionally in GaAs and their contrast indicates the presence of half-filled dangling bonds. The Si Ga donor–Si As acceptor dipoles are surrounded by a height change indicating a dipole screening field.

Electron momentum distributions in elemental semiconductors probed by positrons

Momentum distributions of positron-electron pairs in diamond, Si, and Ge are systematically studied using state-of-the-art techniques in experiment and theory, i.e., the positron two-dimensional angular correlation of annihilation radiation (2D-ACAR) technique and two-component density-functional (TCDF) theory. It is experimentally examined that all samples are free from positron trapping. An interesting difference among the elemental semiconductors is then clarified, namely, a flat 2D-ACAR distribution of the [001] projection in the low momentum region is found in diamond, while a deep dip is observed at the origin in Si and Ge. These experimental results are compared with those of first-principles TCDF calculations within the local-density approximation based on the scheme by Puska, Seitsonen, and Nieminen [Phys. Rev. B 52, 10 947 (1995)] and the generalized-gradient approximation by Barbiellini et al. [Phys. Rev. B 53, 16 201 (1996)]. Good agreement between theory and experiment confirms the validity of the TCDF. Analysis of calculational results clarifies that the unique electron momentum distribution in diamond is due to the carbon $p$ orbital sharply localized in real space.

The LNLS soft X-ray spectroscopy beamline.

The soft X-ray spectroscopy beamline installed at a bending-magnet source at the LNLS is described. The optics are designed to cover energies from 800 to 4000 eV with good efficiency. The focusing element is a gold-coated toroidal mirror with an angle of incidence of 17 mrad. The UHV double-crystal monochromator has three pairs of crystals, Si (111), InSb (111) and beryl (101;0), that can be selected by a sliding movement. The UHV workstation is equipped with an ion gun, an electron gun, an electron analyser, LEED optics, an open channeltron and a photodiode array. This beamline is intended for photoemission, photoabsorption, reflectivity and dichroism experiments.

Changes of defect and active-dopant concentrations induced by annealing of highly Si-doped GaAs

We identified point defects and dopant atoms and measured their concentrations in as-grown and post-growth annealed highly Si-doped GaAs by scanning tunneling microscopy. The annealing under As atmosphere reduces the concentration of Si atoms incorporated into Si pairs and clusters by cluster dissolution, while the concentrations of Si donors, Si donor--Ga vacancy complexes, and Si donor--As vacancy complexes increase. For the dissolution of the Si clusters during heat treatment, a Ga-vacancy-mediated mechanism is suggested.

Polarity effect on failure of Ni and Ni2Si contacts on Si

Stability of contacts in shallow junction devices against high current density is a reliability issue for very large scale integration technology. We have observed a strong polarity effect on failure at nickel and nickel silicide contacts on both n- and p-type Si under high stress conditions. In a pair of cathode and anode contacts the Ni/n+-Si contact pair fails at the anode, while the Ni/p+-Si pair fails at the cathode. The Ni/Ni2Si/n+-Si and Ni/Ni2Si/p+-Si were found to fail preferentially at the cathode. Microbeam Rutherford backscattering spectrometry and Auger electron spectroscopy depth profiles show that a silicide reaction occurs between Ni and Si during current stressing, especially at the failed contacts. In situ resistance data indicate that the resistance of the failed contact increases with time while that of the other contact in the pair remains constant. Transmission electron microscopy shows that the silicide formation is not uniform at the damaged contacts. A mixture of dominant epitaxial NiSi2 and a minor amount of polycrystalline NiSi2 phases was identified. We have proposed mechanisms to explain the polarity effect on failure: wear-out mechanism for the damaged positive contacts of Ni/n+-Si, electromigration enhanced silicide formation for the damaged negative contacts of Ni/Ni2Si/n+-Si and electron-hole recombination mechanism for the damaged negative contacts of Ni/p+-Si and Ni/Ni2Si/p+-Si.

Ultrafast x-ray diffraction using a streak-camera detector in averaging mode

We demonstrate an apparatus for measuring time-dependent x-ray diffraction. X-ray pulses from a synchrotron are diffracted by a pair of Si(111) crystals and detected with an x-ray streak camera that has single-shot resolution of better than 1 ps. The streak camera is driven by a photoconductive switch, which is triggered by 100-fs laser pulses at a repetition rate of 1 kHz. The laser and the streak camera are synchronized with the synchrotron pulses. In the averaging mode, trigger jitter results in 2-ps temporal resolution. We measured the duration of 5-keV pulses from the Advanced Light Source synchrotron to be 70ps.

Photoreflectance study of etching and annealing effect on [formula omitted] heterostructure

We investigated the PR spectra of as-grown, annealed and etched modulation-doped AlGaAs GaAs heterostructure samples. As annealing time increases, the built-in electric fields are strengthened as a result of the diffusion of SiSi pairs. As the etching time increases the broad signal near GaAs band-gap energy and the single peak shape (SPS) signal near AlGaAs band-gap energy change to the step-like signal and the double peak shape (DPS) signal, respectively.

Quantum chemical study on p-doping effect of F-terminated silicon surface reaction with silane

The CVD reaction mechanisms of the variety of combinations of source gases with reaction surfaces have not been clarified so far. We have thus performed a quantum chemical study for the deposition of SiH 4 on the surface of F-terminated Si with its p-doping effect. We have clarified the tendency of F that is immersed into the bulk for chemical bonding in between a pair of Si for the p-doped case. This high stability of (SiFSi) + bonding structure in contrast to the unstable neutral SiFSi antibonding structure agrees qualitatively well with the experimental observation.

High-Temperature Damping in AlMgSi Industrial Alloys

The micromechanisms of high temperature damping in an AlMgSi industrial alloy are investigated by mechanical spectroscopy. The alloy in the as-received T6 state shows a relaxation peak at 483 K (1 Hz) with an activation energy H = 1.6 eV and a limit relaxation time τ0 = 1.3 × 10−18 s. It is also found in solubilized, quenched and overaged samples. The internal friction maximum is attributed to a mechanism in which: a) lattice dislocations are firmly pinned by β″ or β′ precipitates and b) lattice dislocation motion is controlled by the dragging of a complex Cottrell cloud composed by Mg and Si solute atoms. A good description of the experimental results is provided considering that the dislocation motion is controlled by transverse diffusion of MgSi pairs in the dislocation core. Relaxation mechanisms related with grain boundaries or precipitates can be discarded.

Recombination-enhanced Fe atom jump between the first and the second neighbor site of Fe–acceptor pair in Si

We studied the recombination-enhanced Fe atom jump between the first (1st) and second (2nd) neighbor sites of Fe–Al and Fe–B pairs in Si. We first annealed specimens at 80 °C to generate Fe–acceptor pairs after doping of Fe. Concentrations of the 1st and 2nd neighbor Fe–acceptor pairs were determined by electron-spin-resonance (ESR) measurement after annealing at around 150 K under optical excitation. The concentration of the 1st neighbor pair was decreased and that of the 2nd neighbor pair was increased by the above annealing. Activation energy for the above changes was about 0.11 eV in the case of the Fe–Al pair. This is much smaller than that (0.8 eV) of thermal annealing alone. In the case of the Fe–B pair, the ESR signal of the 2nd neighbor pair could be detected due to annealing at around 160 K under optical excitation.

Comparison of TOF-ERDA and nuclear resonance reaction techniques for range profile measurements of keV energy implants

A comparative study on the range measurements of keV energy implants by the Time-of-Flight Elastic Recoil Detection Analysis (TOF-ERDA) and conventionally used nuclear resonance reaction methods has been performed for 20–100 keV 15N + ions implanted into crystalline silicon. Range profiles of 15N atoms were chosen because they can be measured accurately using a very strong and narrow resonance at E p = 429.6 keV in the reaction 15N(p,αγ) 12C which provides a challenging test for other methods. The measured range profiles were simulated by molecular dynamics calculations where the interatomic NSi pair potential is deduced from first principles calculations. The electronic stopping power for 20–100 keV nitrogen ions in silicon is deduced from the comparison of the measured and simulated range profiles. The results are discussed in the framework of the applicability of the TOF-ERDA technique for keV energy ion range measurements.

Si/SiO2 resonant cavity photodetector

It has been shown earlier that GeSi/Si resonant-cavity photodiodes can achieve high speed without sacrificing quantum efficiency. In this letter, we report a Si-based resonant-cavity photodiode that utilizes a Si/SiO2 Bragg reflector. This structure is more compatible with standard Si processing technology than the GeSi/Si resonant-cavity photodiodes. The absorbing region is a 1-μm-thick polysilicon layer that has been annealed to enhance secondary grain growth and the bottom mirror consists of three quarter-wavelength pairs of Si and SiO2. After annealing the dark current was 9 μA at 1 V, the peak quantum efficiency was 44%, and the bandwidth was ≳1.4 GHz.