Silicon Crystal Structure Research Articles

Effect of High Pressure - Temperature on Structure of Silicon Crystals Implanted with Nitrogen / Silicon

Effect of High Pressure - Temperature on Structure of Silicon Crystals Implanted with Nitrogen / Silicon

Influence of high pressure and temperature on defect structure of silicon crystals implanted with N or Si ions

(111) oriented Fz-Si crystals were implanted with N + or Si + at 150 keV energy to a dose of 2×10 16 cm −2. After implantation the samples were treated at a high temperature (up to 1400 K) under hydrostatic argon pressure (up to 1.1 GPa) for 1–5 h. Structural properties of Si:N and Si:Si before and after the treatment were investigated by high-resolution X-ray diffraction, synchrotron topography, transmission electron microscopy and photoluminescence methods. The structural properties of Si:N and Si:Si treated under hydrostatic pressure at 1270 K are different. A buried layer of uniform thickness with smooth interface was formed in Si:N under HP=1.1 GPa; the buried layer/Si interface becomes less smooth with rising treatment time. The shape of interference fringes and their changing distance at rocking curves are explained as an effect of nitrogen diffusion to the buried layer during the treatment. For the treated Si:Si samples, the observed strain changes are related to the out-diffusion of interstitial silicon atoms from the implanted layer to the sample surface and to creation of numerous extended defects near the implanted atoms range. Prolonged treatment (for 5 h) at 1.1 GPa and 1400 K results in creation of dislocations for Si:N while for Si:Si samples the structure of treated samples is more complex, involving also creation of amorphous-like surface film.

Model, First-Principle Calculation of Ammonia Dissociation on Si(100) Surface. Importance of Proton Tunneling

The dissociation of an ammonia molecule on a cluster of Si atoms simulating the 100 silicon crystal structure with two Si dimers has been investigated by means of the DFT and an approximate instanton methods. The model corresponds to the low coverage limit of the surface. Absolute rate constants of two different dissociation paths are evaluated together with deuterium isotope effects. It is demonstrated that, even at room temperatures, the process is dominated by tunneling and that dissociation to a silicon atom of the adjacent dimer, rather than a silicon within the same dimer, is the prevailing mechanism. This leads to creation of a metastable structure which will slowly decay through a two-step hydrogen atom migration towards the absolute minimum on the potential energy surface corresponding to the NH2 group and the hydrogen atom residing in the same dimer.

Nanodimensional changes in the structure of ion-irradiated silicon crystals at a depth exceeding the ion range

Atomic force microscopy reveals changes in the surface micromorphology of ion-irradiated silicon crystals in the course of dynamic chemical polishing and selective etching to a depth significantly exceeding the range of 40-keV Ar+ ions.

In-situ investigation of the formation of eutectic alloys in the systems silicon–silver and silicon–copper

An in-situ electron microscopic investigation of the formation of eutectic alloys in the systems amorphous Si/particle Ag and amorphous Si/particle Cu showed that the formation of eutectics is preceded by metal diffusion into amorphous silicon with the formation of metastable amorphous metal silicide. Decomposition of the formed metastable amorphous metal silicide results in the evolution of polycrystalline silicon. An oriented formation of liquid eutectic alloy is observed in the systems crystalline (100) Si/particle Ag and (100) Si/particle Cu. In these cases, at the metal–silicon eutectic/silicon crystal interface the formation of silicide phases and a dislocation structure are observed. Investigations allowed us to propose a scheme for the formation of eutectic alloys in the studied systems. The scheme involves the diffusion of metal atoms into the silicon crystal structure, leading to the formation of a metastable silicide or solid solution, the occurrence of strain at the interface and, as a consequence, the formation of defects of different types. The subsequent process is accompanied by synchronous dissolution of metal atoms into the liquid eutectics and by diffusion from the liquid eutectics to the solid crystalline silicon phase, giving rise to metastable silicide, so that an intermediate diffusion zone is always present at the interface between the liquid eutectics and crystalline silicon.

X-ray study of silicon crystal structure changes due to implantation with fast nitrogen ions

An analysis of X-ray rocking curves and reciprocal space maps recorded for silicon single crystals before and after implantation with a 5×10 14-cm −2 dose of 3 MeV/n nitrogen ions is presented. Silicon single crystals of orientation (100) were obtained by the Czochralski method. X-ray investigations were performed by means of a high-resolution Philips diffractometer using the characteristic Cu Kα 1 radiation, and the integral reflectivities were measured with a double crystal diffractometer using the characteristic Ag Kα 1 radiation. It was stated that irradiation with fast nitrogen ions can create extended defects as well as formation of nano-areas of disordered structure in the silicon crystal. An increase in the surface roughness was also determined from the grazing incidence X-ray reflectivity measurements.

Effect of silicon crystal structure on spin transmission through spin electronic devices

Spin injection into and spin transport through silicon spacer layers in iron/silicon/cobalt structures has been investigated. Ultrahigh vacuum evaporated silicon spacers of varying crystal quality from amorphous to epitaxial of thicknesses from 10 to 200 Å were shown to improve their electrical conduction with increasing crystallinity, but no spin dependent transport was observed through the structure. Silicon and iron interdiffusion was also observed at the interfacial region. Device quality silicon was studied using 460 and 540 μm doped silicon wafers of resistivity 0.1 and 1 Ω cm, respectively, polished on both sides, onto which were deposited iron and cobalt layers. Sharp metal-semiconductor interfaces were achieved in this way, but no spin dependent transport, putting an upper limit on the spin diffusion length in device quality silicon wafers.

Structure Changes in Cz-Si Single Crystals Irradiated with Fast Oxygen and Neon Ions

The research of the surface and the near-surface region of Cz-Si wafers irradiated with fast oxygen and neon ions of energy 4 MeV/u and dose 10 14 particles/cm 2 is presented. In our study several methods based on the Bragg case of X-ray diffraction using Ag K« 1 , as well as reflection high-energy electron diffraction and Nomarsky optical microscopy were used. It was • shown that implantation with fast neon ions causes larger disturbances of silicon crystal structure than irradiation with oxygen ions.

Mechanical behavior of a μm-sized single crystal silicon structure with sharp notches

The mechanical behavior of μm-sized single crystal silicon structures is investigated in this paper. A new apparatus for microsample tensile testing is used to determine the mechanical properties of single crystal silicon microbars. Force and elongation are measured independently with high accuracy. Elastic constants as well as critical loads are determined. Failure of the sample occurs at the extremities of the microbar at a sharp notch. The stress field in the notch tip vicinity is therefore analyzed to characterize the load capacity of the structure. For this purpose, finite element computations are combined with approximate analytic solutions based on the Stroh formalism. Both the case of plane stress and two-dimensional displacements are considered. Comparison with the FEM solution shows that the plane stress assumption is better, leading to the determination of the critical stress intensity factor F cr for this case. Because F cr is dependent on the geometry, a new scalar failure criterion—i.e. a critical radius R cr—is introduced and derived from the calculated stress field at the notch. It is based on a comparison of stored elastic energy and surface energy at the critical location. Because it is a scalar criterion, it can be applied to single crystal microstructures with notches of arbitrary notch angle. For the case considered, experiments yield R cr = 0.8 nm. The proposed criterion can be considered as a step towards the definition of a design rule for sharp notches in single crystal structures, which is required for the optimized design of micromechanical devices.

The surface/bulk micromachining (SBM) process: a new method for fabricating released MEMS in single crystal silicon

This paper presents the surface/bulk micromachining (SBM) process to allow fabricating released microelectromechanical systems using bulk silicon. The process starts with a [111]-oriented silicon wafer. The structural patterns are defined using the reactive ion etching technique used in surface micromachining. Then the patterns, as well as sidewalls, are passivated with an oxide film, and bare silicon is exposed at desired areas. The exposed bare silicon is further reactive ion etched, which defines sacrificial gap dimensions. The final release is accomplished by undercutting the exposed bulk silicon sidewalls in aqueous alkaline etchants. Because {111} planes are used as etch stops, very clean structural surfaces can be obtained. Using the SBM process, 5-, 10-, and 100-/spl mu/m-thick arbitrarily-shaped single crystal silicon structures, including comb-drive resonators, at 5-, 30-, and 100-/spl mu/m sacrificial gaps, respectively, are fabricated. An electrostatic actuation method using p-n junction isolation is also developed in this paper, and it is applied to actuate comb-drive resonators. The leakage current and junction capacitance of the reversed-biased p-n junction diodes are also found to be sufficiently small for sensor applications. The developed SBM process is a plausible alternative to the existing micromachining methods in fabricating microsensors and microactuators, with the advantage of using single crystal silicon.

Study of resistivity and majority carrier concentration of silicon damaged by neutron irradiation

Working characteristics of silicon radiation detectors have been observed to change, when irradiated at very large neutron fluences (Φ>1012 n/cm2), due to generation of defect levels and capture of majority carriers on these levels. Direct study and confirmation of these phenomena were made by measuring the Hall effect constant RH and Resistivity, of the silicon material as a function of neutron irradiation of up to about 9x1015 n/cm2. It was found that, for fluences of about Φ≥5.95x1014 n/cm2, the sign of the Hall constant RH changes from negative to positive. Taking into account that, for Φ≥1.19x1014 n/cm2, the value of ϱ is independent on fluence, we assumed that the damage, craused at those neutron fluences, was high enough to create disordered silicon crystal structures. This disordered silicon may contribute to the positive sign of RH and make the ϱ of this material insensitive to further neutron irradiation. This can be explained by assuming that, for Φ>1.19x1014 n/cm2, the created disordered silicon material has an equivalent resistance in series with the single crystal Si resistance. The contribution to ϱ of the disordered silicon is large enough to have a strong influence on the silicon characteristics. The new defect structure was seen under a microscope on etched samples, irradiated at Φ>1013 n/cm2.

Fabrication of a new highly-symmetrical, in-plane accelerometer structure by anisotropic etching of (100) silicon

In this paper, we present a silicon bulk-microfabrication method which helps to overcome simultaneously several limitations of multi-axis micro-accelerometers. The method demonstrates an orginal solution to the building of a symmetrical structure by using double-side wet etching. This is a low-cost alternative to existing techniques for the fabrication of highly-symmetrical, single crystal silicon structures. The proposed approach provides low mechanical cross-sensitivities as well as the possibility of a batch fabrication process of the whole three-dimensional device without loss of accuracy due to assembly operation. For the fabrication of thin suspended beams with vertical sidewalls, a non-conventional alignment of from the wafer flat was used. This alignment allows one to fabricate two perpendicular devices on one wafer in the same etching step. The etching was performed with a simple standard wet etching process in a KOH solution. A number of structures were fabricated to demonstrate the feasibility of this method. Aspect ratios (beam height over beam thickness) of over 35 were easily achieved. Undercut directions were determined and design rules for the mask layout were established. To describe the mechanical behaviour of the fabricated structure, an analytical model was implemented and a finite-element simulation was performed. First measurements of the seismic mass displacement were performed with an optical comparator, and they agree with theoretically obtained results. The new design offers the possibility of a two-axis accelerometer system on one wafer, consisting of two sensor elements rotated by . A three-axis monolithic accelerometer system with intrinsic perpendicular alignment due to the rectangular symmetry of the (100) planes can be realized, by including a third sensor element sensitive to vertical accelerations.

Study of resistivity and majority carrier concentration of silicon detectors damaged by neutron irradiation up to 10 16 n/cm 2

Working characteristics of silicon radiation detectors have been observed to change, when irradiated at very large neutron fluences ( Φ > 10 12n/cm 2), due to the generation of defect levels and capture of majority carriers on these levels. Direct study and confirmation of these phenomena were made by measuring the Hall effect constant R H and resistivity of the silicon material as a function of neutron irradiation of up to about 9 × 10 15 n/cm 2. It was found that, for fluences of Φ ≥ 5.95 × 10 14n/cm 2, the sign of the Hall constant R H changes from negative to positive. Taking into account that, for Φ ≥ 1.19 × 10 14n/cm 2, the value of resistivity ϱ is independent of the fluence, we assumed that the damage caused by those neutron fluences was high enough to create disordered silicon crystal structures. This disordered silicon may contribute to the positive sign of R H and make the ϱ of this material insensitive to further neutron irradiation. This can be explained by assuming that, for Φ > 1.19 × 10 14n/cm 2, the created disordered silicon material has an equivalent resistance in series with the single crystal Si resistance. The contribution to ϱ of the disordered silicon is large enough to have a strong influence on the silicon characteristics. The new defect structure was seen under a microscope on etched samples, irradiated at Φ > 10 13n/cm 2.

Pendellösung fringes of silicon at low temperatures

The crystal structure of silicon has been examined by means of X-ray diffraction according to the Pendellosung method. Measurements of the fringe pattern were made for the 111, 220, 113, 004, 331, 224 and 333 diffractions in a temperature range 17–300 K. It is found that the value of the Debye temperature determined from the temperature dependence of the period of Pendellosung fringes is in good agreement with that in the literature except for the first three diffractions; for these with shorter scattering vectors, the value depends on specimen and, in some cases, is smaller than that in the literature and depends on the length of the scattering vector. It is also found that the fringe pattern is stress sensitive, and that the stress can be released at lowest temperatures. This phenomenon of a kind of aging is discussed in connection with a similar effect observed in the high-resolution measurement of the lattice spacing.

SCREAM MicroElectroMechanical Systems

A process called SCREAM for Single Crystal Reactive Etching And Metallization is used to make MicroElectroMechanical Systems (MEMS). The SCREAM process yields high-aspect-ratio (>50:1) released, single crystal silicon structures with micrometer-scale minimum features and a suspension span of greater than 5 millimeters. Such structures include picofarad sensing capacitors and high force actuators that generate milli-Newton forces (100mN/cm 2 at 40 Volts) and controllable, three dimensional motion and displacements. Examples of SCREAM devices and microinstruments include a materials testing or loading instrument, and micro-scanning tunneling microscopes.

The Si:Er crystal: model and excitation mechanism of the ErO center

Analysing the electronic structure of silicon crystals, both regular and doped with erbium, we conclude that the erbium ion tends to form bonds with three ligands. The role of oxygen in the passivation of the formed dangling bond is discussed. Studies of the electric, magnetic, and optical properties of Si:Er grown by the Czochralsky method show that the Er 2O complexes existing in these crystals become electrically and optically active only in the case when they locate near the shallow donor (phosphorus).

Depth Profiles of Medium Energy Phosphorus Implants into Silicon

AbstractSecondary ion mass spectrometry (SIMS) is used to produce depth profiles of ion-implanted phosphorus in silicon. The implant energies are 250, 500, and 750 keV, and there is a 0.06-μm thick oxide on the silicon. The experimental profiles are compared with predictions from a variety of simulation programs, most of which give larger projected ranges than the data. The silicon crystal structure needs to be included in the calculations to produce projected ranges and depth profiles that agree with the present experimental data and with data from the literature.

Thermally and electrically isolated single crystal silicon structures in CMOS technology

Thermally and electrically isolated single crystal silicon structures have been fabricated using a post-processing anisotropic tetramethyl ammonium hydroxide (TMAH) electrochemical etch. The process was carried out on CMOS circuits fabricated by a commercial foundry. Since the etch consists of a single micromachining step performed on packaged and bonded dice, this technique has the potential for cost-effective prototyping and production of integrated sensors and circuits. >

Asymmetry in Indentation-Induced Mechanical Damage on Si (111) Surfaces

Asymmetry in indentation-induced mechanical damage on Si (111) surfaces is investigated by using optical microscopy and X-ray diffraction topography. Cracks, residual strain fields and anneal-induced dislocation rosettes around the indentation are found to represent the asymmetries in the [1̄1̄2] and reverse [112̄] directions on the (111) surface. The crack formation along three <112> directions and the residual strain field distribution along <112> directions are discussed in terms of dislocation reaction and silicon crystal structure, respectively.

The Effect of Starting Silicon Crystal Structure on Photoluminescence Intensity of Porous Silicon

ABSTRACTIn previous work we reported that porous silicon (PS) films formed using a dilute HF:HNO3 chemical etch on polycrystalline, implant damaged single crystal, or amorphous starting material have luminescent characteristics that differ from PS fabricated on single crystal silicon1. Polycrystalline and implant damaged porous silicon exhibits brighter luminescence compared to single crystal silicon etched under identical conditions. No photoluminescence is detected from the porous amorphous silicon. In this work these effects are examined using HF:NaNO2 solutions with freely available NO2. The accelerated etching effects from work damage are reduced, and the PS from polycrystalline and implant damaged silicon luminesce with the same intensity as the PS from single crystal silicon. Again, etched amorphous silicon does not luminesce. TEM and EDX porosity measurements are used to determine the differences in structure and etching characteristics between the luminescent and non-luminescent materials.