Si Polytype Research Articles

Electronic, direct optical, and phonon-assisted optical properties of 4H Si from first principles

The cubic polytype of silicon (Si) is the most commercialized semiconductor material and finds applications in numerous electronic and optoelectronic devices, such as solar cells. However, recent reports on the synthesis of the hexagonal 4H Si polytype have attracted the attention of the scientific community to understand its functional properties. Here, we report the electronic, vibrational, and optical properties of the 4H Si polytype obtained with predictive first-principles calculations, with an emphasis of phonon-assisted absorption in the indirect regime. Compared to the cubic polytype, 4H Si shows a slightly narrower indirect gap by ∼0.05 eV. The calculated phonon-assisted optical spectra show that 4H Si exhibits a stronger absorption coefficient than cubic Si across the visible and IR spectral regions. We further evaluate the short-circuit current density of textured thin-films. We demonstrate that 4H Si can achieve the same short-circuit current density for a five times thinner film compared to the cubic polytype, which mainly resulted from absorption in the indirect gap regime. Our work demonstrates the advantages of 4H Si for thin-film silicon-based solar-cell applications.

First-principles insight in structure-property relationships of hexagonal Si and Ge polytypes

Hexagonal SiGe is a promising material for combining electronic and photonic technologies. In this paper, the energetic, structural, elastic, and electronic properties of the hexagonal polytypes ($2H, 4H$, and $6H$) of silicon and germanium are thoroughly analyzed under equilibrium conditions. For this purpose, we apply state-of-the-art density functional theory. The phase diagram, obtained in the framework of a generalized Ising model, shows that the diamond structure is the most stable under ambient conditions, but hexagonal modifications are close to the phase boundary, especially for Si. Our band structure calculations using the modified-Becke-Johnson--local-density-approximation (MBJLDA) and Heyd-Scuseria-Ernzerhof (HSE06) exchange-correlation functionals predict significant changes in electronic states with hexagonality. While Si crystals are always semiconductors with indirect band gaps, the hexagonal Ge polytypes have direct band gaps. The branch-point energies of the Si polytypes appear in the fundamental gaps, while for the Ge crystals they are below the valence band maxima. Band alignment based on the branch-point energy leads to type-I heterocrystalline interfaces between Ge polytypes, where electrons and holes can be trapped in the layer with the higher hexagonality.

Relative stabilities of Si polytypes under biaxial stress: A first-principles study

We explore the possibility of the production of Si polytypes via the total energies and free energies obtained in the density-functional theory and the density-functional perturbation theory. We first calculate the total energies of three polytypes of the tetrahedrally coordinated $s{p}^{3}$ Si atoms having the two-layer, three-layer, and four-layer stacking periods in the hexagonal unit cell (``$2H$'', ``$3H$'', and ``$4H$'') as a function of the in-plane lattice constant $a$ using the local-density approximation (LDA) and generalized gradient approximation (GGA) comparatively. In the LDA calculation, the $4H$ phase is energetically the most favorable among three phases in the range of 3.55 \AA{} $\ensuremath{\le}$ $a$ $\ensuremath{\le}$ 3.73 \AA{}. While the $4H$ phase is found to be more stable than the $2H$ phase at a relatively wide range of the $a$ value we studied, the $2H$ phase is found to be more stable than the $3H$ ($3C$) phase in the range of 3.57 \AA{} $\ensuremath{\le}a\ensuremath{\le}$ 3.61 \AA{}. In the GGA calculation, on the other hand, the $4H$ phase becomes the most favorable phase for almost the same range as the LDA case, 3.57 \AA{} $\ensuremath{\le}a\ensuremath{\le}$ 3.73 \AA{}. The $2H$ phase also becomes as stable as the $3H$ phase as in the case of the LDA around the small $a$ value of 3.62 \AA{}, although the $3H$ phase energy is always lower than that of the $2H$ phase in the GGA calculation. Considering the chemical vapor deposition (CVD) growth process, we calculate the biaxial stress values to reduce the lattice constant of the substrate Si crystal to realize the small $a$ values, which can lead to the homoepitaxial growth of the $4H$ phase. The values obtained are 4.69 GPa and 8.14 GPa for the LDA and GGA, respectively. Next, we study the thermal effect on the relative stabilities of the 3 phases. We calculate the phonon dispersion, bulk modulus and thermal expansion including the vibrational and thermal effect. From the comparison with experimental values of the $3C$ phase, it is found that the LDA results show much better agreement with experiment than the GGA, indicating that the LDA calculation is reliable to predict thermal properties of the real systems. Finally, we derive the free energy as a function of $a$ under several designated temperatures. The relative stabilities of the $2H$ and $4H$ phases with respect to the $3H$ phase are found to be enhanced. At around $a=3.65$ \AA{}, stabilities of $2H$ and $4H$ phases relative to the $3H$ phase become most prominent. Although stabilities of the $2H$ and $4H$ phases relative to the $3H$ phase are reduced with increasing temperature at around $a$ = 3.60 \AA{}, $2H$ phase is found to be as stable as $3H$ and $4H$ phases at higher temperatures, indicating the possibility of the production of the $2H$ phase via high-temperature CVD process with the biaxial stress.

Exotic silicon phases synthesized through ultrashort laser-induced microexplosion: Characterization with Raman microspectroscopy

Confined microexplosions induced in silicon by powerful ultrashort laser pulses can lead to new Si phases. Some of these have not previously been observed via near-equilibrium compression of silicon. In this study, confocal Raman micro-spectroscopy and Raman imaging of arrays of microexplosions have been conducted to search for Raman signatures of these novel allotropes of silicon. A microexplosion is generated at the interface between a thick silicon dioxide confinement layer and underlying silicon. It is characterised by a void at the interface above a region of compressed silicon. Raman data show a rich assembly of silicon phases within the modified silicon. Residual stresses up to 4.5 GPa in the modifications have been determined from the shift in the main diamond-cubic Si Raman peak. The computed Raman spectra for a number of Si allotropes show reasonable agreement with the experimental spectra. Two structurally similar tetragonal phases of silicon (the rhombohedral r8 and the body-centred bc8) phases as well as recently identified bt8-Si are all highly likely to be contained in Raman spectra from many laser-modified sites. Although the st12-Si phase, previously observed in our electron diffraction studies of the highly compressively stressed laser-modified regions, was not reliably identified from Raman data, we suggest this could be due to the possible difference in residual stress level in the sites analysed by the electron diffraction and Raman spectra. Several other unidentified Raman peaks were observed, suggesting the presence of other unknown silicon phases. All of these silicon phases are expected to have attractive semiconducting properties including narrow band gap that open up novel applications.

Novel Si Networks in the Ca/Si Phase Diagram under Pressure

The rich Ca/Si binary phase diagram is explored here theoretically and over a range of pressures, through four stoichiometries: Ca2Si, CaSi, CaSi2, and CaSi3. The calculations are calibrated by recovering the known P = 1 atm structures (even as they suggest metastable alternatives for some) and by predicting correctly the range of pressures at which CaSi3, which has a positive heat of formation at P = 1 atm, can be synthesized. Ca2Si, in accord with its Zintl compound nature, features isolated Si4– ions then chains until decomposition. CaSi, which begins as a Zintl phase (albeit metallic) chain structure evolves to rings and zeolitic-channel structures, while CaSi2 explores a variety of triply connected Si polytypes to Laves phases with increasing pressure. CaSi3 has a calculated narrow range of stability of 8–18 GPa. Interestingly, all the phases studied are calculated to decompose to the elements at P ∼ 250 GPa. This unusual behavior is traced to the special compressibility of elemental Ca and the backward charge transfer from Si to Ca. All the phases studied are metallic, except that Ca2Si is a semiconductor at 1 atm.

Atomic Structural Analysis of Nanowire Defects and Polytypes Enabled Through Cross‐Sectional Lattice Imaging

Correlated transmission electron microscopy imaging, electron diffraction, and Raman spectroscopy are used to investigate the structure of Si nanowires with planar defects. In addition to plan-view imaging, individual defective nanowires are imaged in axial cross-section at specific locations selected in plan-view imaging. This correlated characterization approach enables definitive identification of complex defect structures that give rise to diffraction patterns that have been misinterpreted in the literature. Conclusive evidence for the 9R Si polytype is presented, and the atomic structure of this phase is correlated with kinematically-forbidden reflections in Si diffraction patterns. Despite striking similarities between imaging and diffraction data from twinned nanowires and the 9R polytype, clear distinctions between the structures can be made. Finally, the structural origins of ⅓{422} reflections in Si [111] diffraction patterns are identified.

Formation of Si twinning-superlattice: First step towards Si polytype growth

We report about the formation of twinning-superlattice regions in Si epitaxial layers grown by multi-step molecular beam epitaxy on Si ( 1 1 1 ) ( 3 × 3 ) R 30 ∘ -B surfaces in which boron acts as a subsurfactant. Twinning-superlattice regions were formed by periodical arrangement of 180° rotation twins along the [1 1 1] direction separated by a few nanometers. The multi-step procedure consists of repeating several growth, boron deposition and annealing cycles on boron-predeposited undoped Si substrates. It is shown that the amount of subsurface boron and the growth mode influence the formation of twin boundaries. Only the nucleation of Si on the Si ( 1 1 1 ) ( 3 × 3 ) R 30 ∘ -surface covered by at least 1/3 ML subsurface boron results in the formation of 180° rotation twins. The presented technology should be suitable to prepare Si polytypes.

Formation of twinning-superlattice regions by artificial stacking of Si layers

We report about the formation of twinning-superlattice regions in Si epitaxial layers grown by molecular beam epitaxy on Si(1 1 1)( 3 × 3 )R30°-B surfaces. Twinning-superlattice regions were formed by periodical arrangement of 180° rotation twin boundaries along [1 1 1]-direction and are only separated by a few nanometers. The preparation method consists of repeating several growth, boron-deposition and annealing cycles on boron-predeposited undoped Si substrates. It is shown that the amount of subsurface boron and the growth mode influence the formation of twin boundaries. Only the nucleation of Si on the Si(1 1 1)( 3 × 3 )R30°-surface covered by at least 1 3 ML boron results in the formation of 180° rotation twins. The size of superlattice regions is restricted by surface morphology. However, the presented technology should also be suitable to prepare a new type of semiconductor heterostructure based on Si polytypes.

Epitaxial growth of non-cubic silicon

We report about the formation of a Si twinning-superlattice (TSL). Si layers containing 180° rotation twin boundaries arranged periodical along the [111] direction and only separated by 2.5nm (corresponding to TSL) have been prepared by molecular beam epitaxy. The method consists of repeating of several growth and annealing circles on boron(B)-predeposited undoped Si substrates. It is shown that the amount of subsurface B on the surface and the growth mode influence the formation of twin boundaries. Only the nucleation of Si on the (3×3)R30°-Si(111) surface covered by atleast 1/3 ML B resulted in the formation of a 180° rotation twin. The presented technology should also be suitable to prepare a new type of semiconductor heterostructures based on Si polytypes.

Enlargement of Crystal Grain Size and Significant Inclusion of Hexagonal Diamond Phase in Ultra Pure Microcrystalline Silicon

AbstractWe have shown significant effects of atmospheric impurities on crystal grain sizes of hydrogenated microcrystalline Si (μc-Si:H), the growth surface of which is totally covered by hydrogen. By reducing impurity concentrations, two growth modes emerge, bordered at 250°C. In particular, at higher temperature growth mode, strong (220) preferential growth takes place, resulting in increased grain size. Appearance of this growth mode seems to be related to surface monohydride. Sharp x-ray diffraction peak in lower angle than (111) is observed, which stems from (10-10) crystal plane of hexagonal diamond Si. Its formation is related to (220) oriented crystallites. Implication of these results is the possibility of Si polytype control.

Electronic band structure in hexagonal close-packed Si polytypes

A full-potential band structure calculation, within the density functional theory and the local density approximation, has been performed for the hexagonal close-packed polytypes 2H, 4H and 6H of Si. Calculated lattice constants are found to be in good agreement with experimental values. Energy band gap, crystal-field splitting and spin-orbit splitting have been determined and, furthermore, the effective masses have been calculated for the lowest conduction band and the two uppermost valence bands. Throughout the paper, comparison is made with corresponding polytypes of SiC and with diamond cubic Si.

Growth of SiC ingots with high ratea

The extended manufacturing of devices on the base of silicon carbide consumes big quantity of SiC substrates. Such tendency of development requires greater productivity of silicon carbide ingots growth. One solution to this problem can be an increase in the growth rate of SiC monocrystals. The maximum growth rate is 3.5 mm h −1. At the growth rate more than this value the crystals of silicon carbide were obtained with blocks and contained many inclusions. Ingots SiC of 4H and 6H polytypes with diameter up to 20 mm and length of 20 mm are obtained. It is necessary to note, that at growth on the plane (0001) C in 90% of cases polytype of the grown crystal was 4H, independently of substrate polytype. In the case of use for growth of the surface (0001) Si polytype of the ingot was 6H. Grown silicon carbide monocrystals had the following characteristics: the donor's concentration was 5 × 10 17 cm −3; the micropipes density was from 100 to 1000 cm −2.