Si-XII Phases Research Articles

Comprehensive structural changes in nanoscale-deformed silicon modelled with an integrated atomic potential

In spite of remarkable developments in the field of advanced materials, silicon remains one of the foremost semiconductors of the day. Of enduring relevance to science and technology is silicon's nanomechanical behaviour including phase transformation, amorphization and dislocations generation, particularly in the context of molecular dynamics and materials research. So far, comprehensive modelling of the whole cycle of events in silicon during nanoscale deformation has not been possible, however, due to the limitations inherent in the existing interatomic potentials. This paper examines how well an unconventional combination of two well-known potentials - the Tersoff and Stillinger-Weber - can perform in simulating that complexity. Our model indicates that an irreversible deformation of silicon (Si-I) is set in motion by a transformation to a non-diamond structure (Si-nd), and followed by a subsequent transition to the Si-II and Si-XII phases (Si-1→Si-nd→Si-II→Si-XII). This leads to the generation of dislocations spreading outwards from the incubation zone. In effect, our simulations parallel the structural changes detected experimentally in the deformed material. This includes both the experimentally observed sequence of phase transitions and dislocation activity, which - taken together - neither the Tersoff nor Stillinger-Weber, or indeed any other available Si interatomic potential, is able to achieve in its own right. Notably, the Si-XII phase was not discerned by any of the previous computational models, which points towards the effectiveness of our integrated approach to forecasting novel phenomena discovered by advanced structure examinations. Last not least, our method satisfies the demand for a quick means to construct potentials by opening up the huge library of existing models to new applications in various branches of materials science.

A microscopic TEM study of the defect layers in cast-mono crystalline silicon wafers induced by diamond-wire sawing

Slicing silicon ingots into wafers by diamond-wire sawing (DWS) is an important step in the material production chain in the semiconductor industry. It will induce defect layers that are highly related to the stress release process and further influence wafers’ mechanical properties. This work aims to investigate the stress release in brittle crystalline silicon via the behaviors of defect layers comprising a surface phase transformed layer and a subsurface crystalline defect layer in DWS silicon wafers from the microscopic perspective. The micro-characteristics of defect layers were mainly obtained by transmission electron microscopy (TEM). The grooves’ surface contains amorphous silicon (a-Si) and diamond-cubic silicon (dc-Si), whereas indentations also contain additional Si-III and Si-XII phases, which were confirmed by both characteristic Raman peaks and the diffraction spots in the TEM image. The subsurface crystalline defect layers were characterized with a similar depth distribution of ∼0.6 µm and possess high-density planar-like defects, which were confirmed as novel defects with a large number (typically 200–400) of compactly strung-together nano-stacking faults inside and believed to be more favorable for stress release. Results show that the stress in crystal silicon can be well relaxed via plastic ways. These findings provide in-depth insights for revealing the essential characteristics of the defect layers in DWS wafers and will be beneficial for the understanding of the plastically deformed mechanisms of brittle silicon crystals.

Phonon transport in multiphase nanostructured silicon fabricated by high-pressure torsion

We present a combined experimental and numerical investigation of phonon transport in multiphase nanostructured silicon. The sample was synthesized by high-pressure torsion with a nominal pressure of 24 GPa. Based on the x-ray diffraction measurement, we have identified the existence of three phases of silicon in the sample: Si-I, Si-III, and Si-XII, with volume fractions of 66%, 25%, and 9% and average grain sizes of 25, 14, and 11 nm, respectively. The measured thermal conductivities of the sample in the temperature range of 150–330 K are on the order of 5 W/(m K) and exhibit weak temperature dependence. A multiscale modeling that incorporates first-principles lattice dynamics, the Monte Carlo ray-tracing method, and effective medium theory was used to understand the mechanism of phonon transport in multiphase nanostructured silicon as well as the weak temperature dependence. We found that the thermal conductivity of single-phase nanostructured silicon decreases with decreasing average grain size and is about an order of magnitude lower than the corresponding bulk counterpart when the average grain size is O(10nm). The weak temperature-dependent thermal conductivity in the nanostructured silicon is attributed to the strong elastic phonon–boundary scattering at the grain boundary. The thermal conductivity predicted from the multiscale modeling matches reasonably well with the measurement. This work provides insights into phonon transport in multiphase nanostructured materials and suggests that the effective thermal conductivity of nanostructured silicon from high-pressure torsion can be further reduced by increasing the volume fractions of the Si-III and Si-XII phases.

Subsurface damage and phase transformation in laser-assisted nanometric cutting of single crystal silicon

Single crystal Si subsurface damage and phase transformation caused by laser-assisted nanometric cutting were investigated in this paper through the ultraprecision cutting experiments and molecular dynamics simulation. Post-cutting examination of a crystal's subsurface revealed a distorted SiI layer and an amorphous Si with embedded nanocrystalline Si-III and Si-XII. As a result of insufficient contact pressure during laser-assisted cutting, the amorphous Si was directly generated from the SiI through the collapse of the crystal lattice rather than from the intermediate high-pressure phase Si-II. The newly-formed amorphous Si crystallized partially during the laser-assisted cutting and transformed into metastable Si-III and Si-XII phases caused by the laser annealing effect. In comparison to machining without laser assistance, it was found that dislocation activity was increased by a factor of ~8 × 1014 when laser assistance was applied. This gave rise to enhancement of plastic deformability of the material, with the critical ductile-brittle transition depth of cut increasing from 150 nm to 395 nm and the thickness and extent of stress in the distorted SiI subsurface layer being reduced.

Crystal and electronic structural changes during annealing in severely deformed Si containing metastable phases formed by high-pressure torsion

Single crystalline p- and n-type Si(100) wafers were severely deformed by high-pressure torsion (HPT) under a nominal pressure of 24 GPa. The HPT-processed samples consisted of metastable phases of body-centered-cubic Si-III and rhombohedral Si-XII, as well as diamond-cubic Si-I and amorphous phases. The photoemission spectroscopy using synchrotron radiation revealed that the valence band edge of the HPT-processed p- and n-type Si samples was located at ∼0.3 eV. Successive annealing in vacuum up to 200 °C led to the valence-band edge shift by ∼0.4 eV for both p- and n-type Si, and the disappearance of Si-III and Si-XII. These results indicated that the Si-III and Si-XII phases in the HPT-processed samples have smaller band gap than that of Si-I, and suggested that the valence-band edge shift after annealing is due to the mixture of Si-I and Si-IV phases in addition to the mid-gap pinning of the Fermi level.

Fabrication of nanograined silicon by high-pressure torsion

This paper describes fabrication of Si nanograins through allotropic phase transformation by concurrent application of high pressure and intense straining using high-pressure torsion (HPT). Single-crystalline Si(100) wafers were processed by HPT under a pressure of 24 GPa at room temperature. X-ray diffraction and Raman analysis revealed that the HPT-processed samples were composed of metastable Si-III and Si-XII phases and amorphous phases in addition to the original diamond-cubic Si-I phase. It was found that nanograins formed because the Si-I diamond phase had transformed to high-pressure phases (Si-II, Si-XI, and Si-V) having metallic nature, and it then became easier to generate a high density of dislocations to form grain boundaries. The high-pressure phases were further transformed to the Si-XII and Si-III phases via the Si-II phase upon unloading and they existed as metastable phases at ambient pressure. Subsequent annealing at 873 K gave rise to reverse transformation to Si-I but with nanograin sizes. Although no appreciable photoluminescence (PL) peak was observed from the HPT-processed sample, a broad PL peak centered around 600 nm was detected from the annealed sample due to quantum confinement in the Si-I nanograins.

Phase transitions in metastable phases of silicon

Phase transitions in indentation induced Si-III/XII phases were investigated using a diamond anvil cell and nanoindentation combined with micro-Raman spectroscopy. The in situ high pressure Raman results demonstrate that the Si-III and Si-XII phases have very similar Raman spectra, indicating their relative amount cannot be determined if they are both present in a sample. The Si-III and Si-XII phases coexist in the indentations produced by a nanoindenter on a single crystalline silicon wafer as a result of the local residual compressive stresses near 1 GPa. High power laser annealing on the indentations can initiate a rapid Si-III/XII → Si-I phase transition. The newly formed polycrystalline Si-I phase initially has very small grain size, and the grains grow when the annealing time is extended. Si-IV phase was not observed in our experiment.

Comment on “The origins of pressure-induced phase transitions during the surface texturing of silicon using femtosecond laser irradiation” [J. Appl. Phys. 112, 083518 (2012)

Smith et al. [Appl. Phys. 112, 083518 (2012)] have reported that the irradiation of Si (100) with femtosecond laser pulses of a sufficiently high fluence, about 4 kJ m−2, results in the formation of the Si-III and Si-XII phases in the irradiated region. These authors have also suggested that the formation of the Si-III and Si-XII phases is due to the pressure-induced phase transitions in the silicon. It is argued here that Smith et al. [Appl. Phys. 112, 083518 (2012)] have incorrectly assigned the observed Raman peaks at 354 and 395 cm−1 to the Si-XII phase and that these peaks are due to the Si-III phase.

Phase transformation and nanograin refinement of silicon by processing through high-pressure torsion

Si(100) wafers were subjected to severe plastic deformation under a pressure of 24 GPa using high-pressure torsion (HPT). Si wafers were plastically deformed at room temperature. HPT-processed samples were composed of metastable body centered cubic Si-III and rhombohedral Si-XII phases in the initial cubic diamond Si-I. The volume fraction of metastable phases increased with increasing plastic strain. Successive annealing at 873 K led to the reverse transformation of metastable phases. A broad photoluminescence peak centered at about 650 nm appears due to the reverse transformation of Si-III/Si-XII nanograins and the reduction of number of defects in Si-I nanograins.

First principles study of phosphorus and boron substitutional defects in Si-XII

We present a first principles study of boron and phosphorus substitutional defects in Si-XII. Recent results from nanoindentation experiments reveal that the Si-XII phase is semiconducting and has the interesting property that it can be doped n- and p-type at room temperature without an annealing step. Using the hybrid functional of Heyd, Scuseria and Ernzerhof (HSE), we examine the formation energies of the B and P defects at the two distinct atomic sites in Si-XII to find on which site the substitutional defects are more easily accommodated. We also estimate the thermodynamic transition levels of each defect in its relevant charge states. The hybrid calculations also give an independent prediction that Si-XII is semiconducting, in agreement with recent experimental data.

Comment on ``Experimental evidence for semiconducting behavior of Si-XII''

Ruffell et al. [Phys. Rev. B 83, 075316 (2011)] have reported that within residual nanoindentations in films of amorphous silicon both Si-III and Si-XII phases exist in the ratio of about 20:80. It is shown here that, in the light of the existing experimental evidence, the material within the residual indentations cannot have the Si-XII phase in it, and instead the material is almost 100$%$ Si-III with possibly a very small fraction of the Si-I phase.

Raman Analysis of the Silicon Wafer Scratched by Single Point Diamond

This paper presents a Raman analysis of the monocrystalline silicon wafer scratched by single point diamond. Si-III and Si-XII phases are found to be existence in the scratched silicon surface, which is the result of the phase transformation. A mathematical model was developed to calculating the molar concentration of phase of the silicon. Based on the mathemathical model, the relationship between the molar concentrations of the Si-I phase and the applied load was analysied.

Experimental evidence for semiconducting behavior of Si-XII

The conventional diamond cubic phase of silicon, so-called Si-I, exhibits the desirable semiconducting properties on which the silicon chip industry relies and it is by far the most stable of all silicon phases. However, other phases of silicon can be formed under pressure and some of them are metastable at room temperature and pressure. Two such phases, Si-III (BC8) and Si-XII (R8), can be produced by indentation with a diamond tip but, despite an understanding of their structure, little is known about their electrical properties. As we demonstrate experimentally, such phases can have entirely different (electrical) properties to normal (diamond cubic) silicon, consistent with recent theoretical studies that predict Si-XII to be a narrow-band-gap semiconductor and Si-III to be a semimetal. We report here electrical measurements on the Si-XII phase and demonstrate that it is indeed a semiconductor. Furthermore, and somewhat surprisingly, both boron and phosphorus can be electrically activated in the Si-XII structure during its formation by indentation at room temperature.

Understanding pressure-induced phase-transformation behavior in silicon through in situ electrical probing under cyclic loading conditions

Cyclic indentation of crystalline silicon exhibits interesting pressure-induced phase-transformation behavior whereby sequential changes in the phase composition ultimately lead to a catastrophic (“pop-out”) event during subsequent cycles and complete transformation to high pressure Si-III and Si-XII phases. This study combines in situ electrical measurements with cyclic loading to monitor such phase-transformation behavior. We find that, if a pop-out is not observed on the unloading curve, the end phase is predominantly amorphous but a small and increasing volume of Si-III/Si-XII results with each cycle. At a critical Si-III/Si-XII volume, pop-out can occur on a subsequent cycle, whereafter Si-III/Si-XII dominates the indent volume.

Size effects in the nanoindentation of silicon at ambient temperature

The work reported here was undertaken to explore an interesting “size effect” in the nanoindentation of Si. Flat-topped wedges of Si were etched from single crystal wafers and indented with a three-sided pyramidal Berkovich tip with a 120 nm radius. The widths of the flats varied from 100 µm to 100 nm. Samples of width ≥1.6 µm were indented under load control to a load of 20 mN. Those with widths ≤1.6 µm were indented to a load of 1 mN. The indentation of thicker specimens is accomplished by transformation-induced plasticity, as reported in prior work. On indentation Si transforms to metallic Sn-II; during relaxation it transforms either to an ultrafine-grained mixture of the Si-III and Si-XII phases (with a pop-out in the load-deflection curve) or to amorphous Si (creating a knee in the curve). A small dislocated field appears at the interface between the metastable phase volume and the Si. As the sample thickness decreases to the indentation size, dislocation plasticity becomes more important. A pop-in appears in the loading curve, slip traces appear on the sample surface, and the dislocated region within the specimen becomes larger. At sufficiently small sample width (∼100 nm) phase transformations disappear and the sample deforms by classic dislocation plasticity (as we observed previously in in situ studies). The change in mechanism is due to the relative ease of nucleating dislocations in the thin sample, which is believed to be a consequence of the change in the indentation stress field.

Study on the Surface and Subsurface Integrity of Ground Monocrystalline Silicon Wafers

Wafer rotation grinding, as an important processing technology, is widely used in manufacturing and back thinning of the silicon wafer. However, the surface/subsurface integrity of the ground wafer, which has important influences on the surface quality and the output of the wafer in subsequent process, is becoming an attention-catching problem. This study is aimed at experimental investigation of the surface/subsurface integrity in wafer rotation grinding. The surface roughness, the subsurface crack configurations, the subsurface damage depth (SSD) and the phase transformations are evaluated by corresponding methods. The results show that the integrity of the ground wafer has a close relationship with the grit size of the grinding wheel. The surface roughness and the SSD increase with increasing of the grit size. The subsurface crack configurations of (100) silicon wafers are complicated. The material removal mechanism is different under different grinding conditions. Ductile grinding is accompanied by the phase transformations of diamond structure silicon (Si-I). The amorphous silicon (α-Si), the Si-XII phase (r8-rhombohedral structure) and the Si-III phase (bc8-body-centered cubic structure) exist on the near surface region of the wafer ground by #600 and #2000 grinding wheels.

Conducting atomic force microscopy study of phase transformation in silicon nanoindentation

We report the study of phase transformation in the nanoindentation of Si by conducting atomic force microscopy. Distinctively high current features with a smallest size of around 20nm have been observed and correspond directly to the generated conductive Si-III and/or Si-XII phases under pressure release. Local current-voltage relationships on the high current sites have also been obtained and found to follow the Fowler-Nordheim tunneling equation.

Laser-induced phase transformation in p-doped nanocrystalline silicon thin films

The results of a study of phase transformation in boron doped hydrogenated nanocrystalline silicon thin films prepared by PECVD and exposed to a powerful Ar + laser light (100 kw/cm 2) are presented. Through Raman spectroscopy, it was determined that the induced phase transformation is correlated with the boron concentration in the silane diluted with hydrogen used as reactive gases in the preparation of the films, on the grain size, and the crystalline volume fraction of them. Measurements of dark conductivity and atomic-force microscopy (AFM) have been made to corroborate the Raman measurements. The results show that the sequence of phase transformation in nanocrystalline thin films presents different characteristics from those, which are produced in silicon crystals. The occurrence of a Raman peak at 350 cm −1 is attributed to the formation of the Si-XII phase, which is attributed to the stress compression in the films. A model for hot-spot phase transformation in p-doped nanocrystalline silicon thin films is discussed.

Raman monitoring laser-induced phase transformation in microcrystalline silicon thin films prepared by PECVD

The results of a study of phase transformation in hydrogenated microcrystalline silicon thin films prepared from hydrogen-diluted silane and exposed to an Ar+ laser light are presented. Through Raman spectroscopy, it was determined that the induced phase transformation is interrelated with silane dilution in hydrogen used as a reactive gas in the preparation of the films and, therefore, depends on the grain size and crystalline fraction of the material. Dark conductivity, transmission electronic microscopy (TEM) and atomic-force microscopy (AFM) data have been used to corroborate the Raman measurements. The results show that the sequence of phase transformation in microcrystalline thin films presents different characteristics from those, which are produced in single-crystal silicon structures. The occurrence of a Raman peak at 350 cm−1 is attributed to the formation of the Si-XII phase. A thermal model to explain the phase transformation is proposed. To our knowledge, neither the presence of the phase Si-XII nor the phase transformation process in microcrystalline silicon thin films has been reported before.

High-resolution transmission electron microscopy study of metastable silicon phases produced by nanoindentation

Plan-view transmission electron microscopy (TEM) and Raman microspectroscopy were used to identify metastable silicon phases in nanoindentation. A mixture of metastable Si-III and Si-XII phases was observed by both selected area diffraction in TEM and Raman analysis. High resolution TEM observations provided detailed structural information about the metastable phases of silicon and the interfaces between different silicon structures. A mechanism of dislocation-induced lattice rotation that leads to a phase transition and distortion-induced amorphization in nanoindentation is proposed.